FIELD GUIDE FOR THE IDENTIFICATION AND …extras.springer.com/2018/978-3-319-57774-6/PDF...

FIELD GUIDE FOR THE IDENTIFICATION AND ASSESSMENT OF LANDSLIDE AND

EROSION FEATURES AND HAZARDS

Part of a GNS Science Short Course on Landslides and Erosion Hazards

Revision 2.1

October 2007

Chris Massey, Graham Hancox and Mike Page

GNS Science, Lower Hutt

1 Fairway Drive, Avalon

Lower Hutt 5010 New Zealand

Tel:+64 4 570 1444 Fax:+64 4 570 4600

PO Box 30368 Lower Hutt 5040 New Zealand

Short Course -Field Guide i

GNS Science, Lower Hutt November 2007

LIST OF CONTENTS

1. Introduction ...................................................................................... 1

1.1 Field Guide Structure and Contents .................................... 1

2. Identifying and Recording Landslide and Erosion Hazards ........ 1

2.1 Some definitions .................................................................... 1

2.2 Landslide activity ................................................................... 5

2.3 Identification of Landslide and Erosion Features .............. 6

2.3.1 Typical Landslide and Erosion Features ................................. 6 2.3.2 Aerial Inspections ................................................................. 13 2.3.3 Ground Inspections ............................................................... 14

2.4 Recording Landslide and Erosion Information................. 15

2.4.1 Plotting locations of landslide features .................................. 15 2.4.2 What to Record ..................................................................... 17

3. Classification schemes ................................................................. 19

3.1 Landslides ............................................................................ 19

3.1.1 Open slope flows .................................................................. 24 3.1.2 Channelised flows................................................................. 25 3.1.3 Slides .................................................................................... 27 3.1.4 Rockfalls ............................................................................... 30

3.2 Erosion .................................................................................. 30

3.2.1 Sheet erosion, rills and gullies .............................................. 32 3.2.2 Stream bank and bed erosion ............................................... 33 3.2.3 Tunnel gullies ....................................................................... 35 3.2.4 Wind erosion ......................................................................... 36 3.2.5 Coastal erosion ..................................................................... 37

3.3 Material types ....................................................................... 38

4. Hazards and their impacts ............................................................ 40

4.1 Open slope flows ................................................................. 40

4.2 Channelised flows ............................................................... 42

4.3 Slides ..................................................................................... 43

4.4 Rock falls .............................................................................. 45

4.5 Erosion .................................................................................. 46

4.5.1 Sheet, rill and gully erosion ................................................... 46

Short Course -Field Guide ii


4.5.2 Stream bank and bed erosion ............................................... 46 4.5.3 Wind erosion ......................................................................... 49 4.5.4 Coastal erosion ..................................................................... 49

4.6 Other hazards and indicators of potential hazards .......... 50

4.7 Hazard assessment ............................................................. 53

4.7.1 Site assessment ................................................................... 53

5. Mitigation Measures ....................................................................... 56

5.1 Stabilisation techniques ...................................................... 56

5.2 Hazard assessments ........................................................... 58

5.2.1 Case Study – The Waikorora Bluff landslide ........................ 58

5.3 Design measures (landslides, slopes and erosion) ......... 60

5.3.1 Retaining/protective structures ............................................. 60 5.3.2 Earthworks ............................................................................ 63 5.3.3 Drainage ............................................................................... 63 5.3.4 Bioengineering (erosion control) ........................................... 67 5.3.5 Monitoring and maintenance ................................................ 71

5.4 Design measures (river crossings) .................................... 72

5.5 Mitigation conclusions ........................................................ 74

6. Conclusions .................................................................................... 80

Landslide Short Course -Field Guide 1


LANDSLIDE AND EROSION FIELD GUIDE

1. INTRODUCTION

This Field Guide has been prepared by GNS Science as part of a GNS

Short Course on the Identification and Assessment of Landslide and

Erosion Hazards aimed at pipeline overseers and technicians. .

The guide is designed essentially as a reference for pipeline technicians to

use in the field. It contains much of the information presented in the

Landslide Short Course, including: some definitions; a short glossary of

relevant landslide terms; check-box sheets for field observation and

recording of information in the field; a landslide and erosion classification

scheme developed for technicians; and a review of mitigation measures

typically used for landslide and erosion hazards. Some key references for

further reading are also included. The content of the Landslide Short

Course and this Field Guide are intended to provide information that fulfils

and exceeds the requirements set out in the NZQA Unit Standard – Control

erosion and Erosion Planting on pipelines in a petrochemical environment.

1.1 Field Guide Structure and Contents

Initially, the context and purpose of the Field Guide are introduced

(Section 1). The guide then defines landslides and erosion features, and

next focuses on how to identify and record these features in the field from

both aerial and ground reconnaissance (Section 2). The other parts of the

guide deal with landslide classification schemes, including a simplified

scheme for use by technicians (Section 3). The impacts on the pipeline

are then discussed (Section 4) and the different mitigation measures

typically used to stabilise landslide and erosion features are reviewed

(Section 5). Conclusions, along with a proforma, which could be used to

record landslide and erosion features during aerial and field

reconnaissance, are then presented (Section 6).

2. IDENTIFYING AND RECORDING LANDSLIDE AND EROSION HAZARDS

2.1 Some definitions

Short Course -Field Guide 2


(a) Landslide: – Landslides are one type/form of erosion and for the

purposes of this guide are being dealt with separately. A landslide is a

gravitational movement of rock or soil down a slope as a mass along

discrete shear surfaces, owing to failure of the material. Soil includes earth

(material smaller than 2 mm) and debris (material larger than 2 mm). Rock

is a hard or firm intact mass and in its natural place before movement.

Landslides are most often triggered by heavy rainfall or strong

earthquakes, but also occur ‘spontaneously’ without an obvious triggering

event. Such failures are often caused by undercutting slopes by natural

erosion, or slope modification by man, together with long-term weathering

and weakening of slopes. Strong earthquake shaking of Modified Mercalli

(MM) intensity MM7 can cause small failures (<103 m

3), but MM8 or

greater is generally required for larger landslides (≥103–10

6 m

3) –see

Appendix 1.

Landslides are usually classified or described in terms of: (a) the type of

material involved (rock, earth, debris, or sometimes sand, mud etc.), and

(b) the type of movement – fall, topple, slide, flow, spread, which are

distinct modes of movement. Combining these two terms gives a range of

landslide types such as: rock fall, rock slide, rock topple, debris fall, debris

slide, debris flow, earth flow etc. Landslides involving soils and bedrock

are often called slips or landslips, while small failures with rotational slide

surfaces are generally referred to as slumps. Small landslides often do

little damage, but very large failures of thousands or millions of cubic

metres moving downslope can runout and bury buildings and roads, or

cause foundation collapse at the tops of slopes. Effects of landslides can

range from minor deformation of foundations and structural failures to total

destruction of sites and all buildings, lifelines and infrastructure above or

below slopes. General definitions of the main landslide types are shown in

Table 1 and illustrated in Figure 1, and briefly discussed below, these

landslide and erosion types will be discussed in detail in Section 3.



Table 1 Definitions and characteristics of the main types.

Landslide Type (Based on movement)

General Characteristics (after Cruden and Varnes, 1996)

Falls Falls are masses of rock, soil, or debris that move rapidly down very steep slopes (>40°) by free fall, bounding or rolling. Disrupted soil and debris falls most common.

Slides Slides are masses of rock, soil, or debris that slide down planes of weakness (bedding, joints, faults) and other surfaces. Rotational slides (or slumps) in soft rocks and soils move on curved failure surfaces. Disrupted soil and debris slides are most common. Landslides are also referred to (non-specifically) as slips, landslips, or slippages.

Avalanches Rock and debris avalanches are very rapid, long run-out failures on steep slopes (>35-40°) more than 150-200 m high. They may start as falls or slides, and transform into flows (wet or dry). Occur mainly on hill country and high mountain slopes.

Debris Floods and

Debris Flows

Debris floods are rapid hyper-concentrated flows in streams, of water charged with sediment, often coarse gravel and sand. Debris flows are a type of landslide: they have much higher sediment concentrations (like wet concrete) than debris floods, and are potentially much more hazardous and destructive. Objects impacted by debris floods are surrounded or buried by gravel, but are often largely undamaged.

Figure 1 Main types of landslides (generalised after Varnes, 1978).



Debris flows and debris floods: These are both hydrological mass-transport

phenomena but have different hazard and risk implications. Debris floods

are rapid hyper-concentrated flows in stream channels of water charged

with sediment. Debris flows are a type of landslide: they have higher

sediment concentrations than debris floods, with a consistency rather like

wet concrete. Debris flows have the ability to transport large boulders, and

are therefore potentially much more hazardous and destructive. A debris

flood is not a landslide and is less hazardous, with destructiveness similar

to that of water, but less than debris flows; both generally occur on alluvial

fans. Objects impacted by debris floods are surrounded or buried by flood

debris but are often largely undamaged (Hancox, 2003), as seen during

the 2003 Paekakariki storm (Figure 2).

Figure 2 Debris flood gravels buried buildings and cars at this motel during a

rainstorm at Paekakariki in 2003, but caused little serious damage to the buildings (the cars were written off however).

(b) Erosion: – Erosion is a term referring to those processes of denudation

which wear away the land by the mechanical actions of erosional agents

such as rivers, streams, waves on the shores of the sea and lakes,

glaciers, and wind. The process of erosion thus involves transportation of



soil and rock debris by the various agents, and must be distinguished from

(chemical) weathering, in which no transportation is involved. Gravity is

regarded as a prime factor in erosional processes, along with abrasion and

corrosion (the mechanical erosion of a rock surface by material (debris)

being transported across it by streams, rivers, glaciers, waves, and wind.

Landsliding (gravitational downslope mass movements) is therefore a

significant erosional process.

Typical erosional features and geomorphic landforms include: erosional

terraces and banks in stream and river channels, and water-eroded

channels and gullies formed on slopes and other geomorphic surfaces by

rapid runoff during rainstorms. Steep cliffs along streams, rivers, glaciers,

and the shores of lakes and coastal areas are also erosion features,

formed by progressive erosional under cutting and collapses of these

oversteepened slopes. Typical erosion features are illustrated in Figure 3.

Figure 3 Erosion gully and debris flood gravel deposits (a), and erosive effects of flood water on road-edge fill (b) at Paekakariki, October 2003.

2.2 Landslide activity

Landslides are often described by terms relating to their activity, or timing

of the landslide movements, which generally reflects their hazard potential.

Active landslides are those that are currently moving, while those that have

moved within the last year, but are not moving at present, are said to be

suspended (UNESCO, 1993). Inactive landslides last moved more than

( a) ( b )



one year ago, and are currently not moving. Inactive landslides are

subdivided into: Dormant – the causes of (and potential for) movement

remain. Stabilised– remedial measures have stopped landslide movement.

Relict– landslides that developed under different geomorphological or

climatic conditions, and may be several hundreds or thousands of years

old (prehistoric, see Figures 4 and 9). A landslide that has been inactive

but has started to move again is called reactivated.

Figure 4 Very large (~30 million m3) relict landslide near Martinborough.

Several thousand years old, its main scarp (MS) is eroded, and its toe (lt)

trimmed by the river when the terrace (t) was formed >1,000 years ago.

Landslide ponds (p), an active earth flow (efl) with hummocky ground, and

shallow soil slides (sl) formed on the main scarp in February 2004, both

superficial reactivations on the slide mass, are also apparent.

2.3 Identification of Landslide and Erosion Features

2.3.1 Typical Landslide and Erosion Features

Landslides have geomorphic features that make them identifiable as mass

movement landforms. Typical landslide features and their internationally-

recognised names are shown in Figure 5, which are unchanged from

M S

M S

p

p

p

e f l

s l

s l

s l



Varne’s original classification (1978), apart from the addition of some minor

features (landslide pond, hummocky ground, springs, and seeps). These

features are important as their recognition makes it possible to identify

landslides, from aerial and ground inspections. Important landslide

features, their significance, and some simple ways to recognise both active

and inactive landslides, and erosion features are summarised in Table 2

and Figure 5.

Figure 5 Block diagram of an idealised complex rotational earth slide and

earth flow showing typical landslide features (after Varnes 1978). ‘Real life’ examples of similar landslides are shown in Figures 6, 8, and 9.



Table 2 Landslide features and criteria for field recognition.

Landslide Features Description of features

Active landslides (and recently active or dormant landslides)

Landslide scar Includes the source area and debris trail.

Source area The area at the head of the landslide (zone of depletion) where the landslide mass (debris) is derived from.

Landslide debris Material (rock, soil, vegetation) displaced from the source area and transported down-slope by gravity.

Main scarp The main scarp is the steep slope in undisturbed ground at the head of the slide (head scarp) – the visible part of the failure surface. Minor (secondary) scarps may be present within the displaced material of the landslide mass

Tension cracks Often located upslope of the landslide main scarp and tend to be aligned in an arc, and can be continuous or discontinuous, but are essentially linear. These indicate horizontal (pull-apart) movement, but may also show vertical and shear movement.

Hummocky ground Ground surface irregular, often formed of low amplitude hummocks, resulting from differential (compressional and shear) deformations within the displaced material – a feature of many landslides (active and inactive).

Ponds (un-drained) Ponds formed in depressions, which are often un-drained, are present within the displaced material of many landslides, especially at the slide head; they may be filled by seepage from springs, or by rainfall.

Springs, seepages Give rise to areas of swampy or boggy ground; seepage water may accumulate in ponds.

Trees with curved trunks or leaning backwards

Wind, steep topography and ground movement can all give rise to non-vertical tree trunks, so care is required in their interpretation so additional supporting evidence of landslide movement is required.

Notes: Relative positions of features referred to in this table are shown in Figure 5. Actual examples of some of the landslide illustrated in Figures 4, 6, 8 and 9. (Table 2 contd. next page)



Table 2 - Landslide features and criteria for field recognition (contd.).

Landslide Features Description of features

Disruption of natural drainage

May be seen directly or inferred from seepages. Also, where landslide debris may have totally/partially blocked a drainage line, or where the drainage line has been forced to alter its course.

Cracking to structures and paved surfaces and dislocation of drainage structures

These can also be related to local settlement of fill and foundations, so additional supporting evidence is required, e.g. presence of a source areas/landslide debris, tension cracks, trees leaning backwards

Relict landslides (inactive old landslides with little potential for reactivation)

Relict landslides typically have eroded, rounded and subdued features, with no sharp features or bare scarps visible. The main scarp is generally eroded and well vegetated. The displaced landslide mass often has ponds and hummocky and irregular ground. Generally, no cracks or indications of movement are visible. Trees and established vegetation show no evidence of tilting, non-vertical trunks, or disturbance

Notes: Relative positions of features referred to in this table are shown in Figure 5. Examples of some of the landslide illustrated in Figures 4, 6, and 7.

Typical erosion features and geomorphic landforms include: erosional

river terraces, river/stream banks and bed, and water-eroded rills and

gullies formed on slopes and other geomorphic surfaces by rapid runoff

during rainstorms. Steep cliffs along streams, rivers, glaciers, and the

shores of lakes and coastal areas are also erosion features, formed by

progressive erosional under cutting and collapses of these over-steepened

slopes. Typical erosion features are illustrated in Figure 3.



Figure 6 This large, recently active rotational slide in mudstone formed a small

landslide-dammed lake in the Whangaehu valley in February 2004. Other earth slides and flows formed at the same are also visible.

Areas of bare ground including planar/disk-shaped paddocks, unsealed

roads and tracks, areas of heavy stock concentration, landslide scars and

landslide debris trails are all prone to sheet erosion. If sheet erosion is left

unchecked rills can develop, and once established, rills can develop into

gullies and ultimately landslides and washouts. Different erosion processes

and their impacts are discussed in more detail in Sections 3 and 4.

Figure 7 This typical erosion

gully at Paekakariki was formed in periglacial gravel deposits in the valley head during the October 2003 rainstorm. No such features had formed in the area in the last 150 years (since the native bush was cleared), which suggests that the rainfall (c. >125 mm in 4 hours) during the storm was exceptional.



Figure 8 Oblique aerial photo of the large (c.5,000 m3), rotational slide (sl) and

earthflow in mudstone, which threatened several houses (H1, H2) in Hunterville in July 2006. The landslide, which formed on the headscarp of a ‘relict’ (prehistoric) landslide, is c. 75 m wide and 3–12 m deep at its head (left). It transformed into a slow-moving earthflow, which extended c. 265 m down a gentle slope towards the houses. Other features seen here are: tension cracks around the slide head (tc); shallow soil (earth) slides formed during the Feb. 2004 storm; old landslide debris (ols); and a bund (b) and channel (c) to direct further debris and protect the houses.

Figure 8 shows that the July 2006 (active landslide) has fresh, sharply

defined features (scarps, cracks debris) that are devoid of vegetation. In

sharp contrast, the ‘relict landslide’ feature has typically rounded and

subdued features; although the former headscarp (main scarp) and

mounds of old landslide debris are still clearly recognisable. In this

example, the active (2006) landslide could also be described as

reactivated because it has formed on the former main scarp of the relict

landslide.



Figure 9 Multiple shallow earth slides and a large rotational slide (s l ) and earth flows occurred in the Mangawhero valley

during the February 2004 rainstorm. The large (>c.100 million m3) prehistoric ‘relict’ landslide (r s l ) opposite, with its vegetated,

hummocky topography and landslide ponds (p) was apparently unaffected by the storm.

r s l

p s l



The typical landslide features described above can be used to identify the

existence of landslides. The next two sections explain how this should be

done on aerial and ground inspections, along with the benefits of both

approaches.

2.3.2 Aerial Inspections

Aerial reconnaissance using either helicopter or fixed-wing aircraft provides

an efficient means to inspect long sections of pipeline. The latter is

cheaper and can fly faster, but is not as good for accurate observation.

Fixed wing aircraft usually fly at higher altitude, which not only affects the

quality of the observations, but also any photos and video shots taken on

the flight. Helicopters are more expensive, but are much better for

observing landslide and erosion effects as they can fly slowly along and a

few hundred metres above pipelines, hovering over critical areas for close

observation and photography. As well as routine inspections, aerial

inspections should also be carried out following severe rainstorms/flooding

events and high magnitude earthquakes.

For clarity, and to avoid reflections, photographs should be taken with the

door open, or it should be taken off before the flight. If possible, always

observe and take photographs looking away from the sun, so flight

directions should be planned with this in mind. Sunny days are better than

overcast days, as the greater contrast and shadows present on sunny days

make landslide features stand out more clearly. Avoid aerial inspections

and photography in the rain, which reduces visibility and safety.

Accurate locations of pipeline observations and photographs are critical during

aerial inspections. Topographic maps (1:50,000) or pipeline route maps allow

good locations to be obtained, as do hand-held GPS units. Some cameras (for

example the Nikon D200) allow photographs to be linked to a GPS unit, with grid

references added to meta file data for photographs taken on a flight. Photographs

taken using normal digital cameras can also be linked to grid references using

GPS-photo link software. This works by linking the date and time of the hand held

GPS unit to the date and time of the camera (usually by photographing the screen

of the GPS with the camera). The software then links the time stamp of the photos



to the time stamp in the GPS track log. Any new landslide features that affect the

pipeline, or have the potential to affect or damage the pipeline, should be

photographed and noted for future ground inspection. On future flights photos

should be taken from a similar distance and direction to monitor any changes in the

size and activity of the feature.

2.3.3 Ground Inspections

Ground inspections are also used by technicians to inspect sections of

pipeline route, especially in areas where erosion or landslide features were

observed on aerial inspections. Ground inspections are often carried out

during fine weather, or when a storm has passed. There is also, however,

value in inspecting pipeline sites during bad weather. Although this may be

difficult, and possibly hazardous, because of flooding and track wash-outs,

it does allow direct observation of the erosional effects of rapid runoff on

slopes, streams, and rivers in the vicinity of the pipeline. If sites of known

landslides and erosion features cannot be inspected during a storm, they

should be inspected as soon as the weather clears to determine the effects

of the storm, and especially to observe and record any changes in the

known features in relation to the pipeline.

The main activities and data collected during a typical ground inspection

may include:

Precisely locate new landslide or erosion features, or changes in old

features, on topographic or pipeline route maps, or by GPS.

Inspect and take photos of any known features from various angles.

Take both close-up and distant photos, and if possible from sites used

to take previous photos. Photos should show the slopes above and

below the pipeline, as well as the areas immediately adjacent to the

pipeline.

Map the extent of the features using GPS, tape measure, or by pacing

out the features. Draw a rough sketch of the features and annotate

with brief descriptive notes.

Note the nature of the materials present in the area (e.g. silt, clay,

sand, gravel, cobbles/boulders or rock) and strength (soft, strong, very

strong etc).



Note areas of water seepage and ponding and whether the materials

are dry, moist or wet.

Note any changes in stream flows in the area near the pipeline, and

any changes in erosive effects.

Details and methods of recording and plotting landslide and erosion

information obtained during pipeline inspections is described in greater

detail in the next section.

2.4 Recording Landslide and Erosion Information

2.4.1 Plotting locations of landslide features

Landslide and erosion features can be plotted on:

(a) Published topographic maps (1:50,000);

(b) Pipeline route maps (based on Topomaps);

(c) Vertical aerial photos;

(d) Oblique aerial photos (taken on aerial inspections);

(e) GPS (NZMG coordinates).

Figures 10a-c show examples of a topographic map and vertical and

oblique aerial photos in relation to a gas pipeline.



Figure 10a Topographic map (1:50,000) showing pipeline near Paekakariki at the

Ohariu Fault crossing, where the pipeline is above ground on ‘skids’ to allow for future fault movement. The fault crossing is more clearly shown on vertical and oblique aerial photos (see Figures 10b and 10c).

Figure 10b Vertical aerial photo of a gas pipeline (p) where it crosses aFault (F).

Although the pipeline at this site is located on a ridge, and is not particularly exposed to landslides, an erosion gully (eg), formed in October 2003 below the pipeline to the east, could possibly affect it in the future.

( a)

( b )

p

e g

F



Figure 10c Oblique aerial photo of a gas pipeline (p) where it crosses a fault (F).

2.4.2 What to Record

A variety of information should be recorded by technicians on any landslide

and erosion features that are impacting, or could impact the pipeline. The

main points that should be recorded are described briefly below.

(a) Topographic location: The positions of all landslide and erosion

features in the immediate vicinity (within c.10-20 m) of the pipeline should

be recorded, especially landslides upslope of the pipeline, or gullies and

drainage features that cross it. Locations of features should be marked on

a sketch map and GPS locations used where possible.

(b) Site details: Site details that should be recorded include:

Position on slope (upper, middle, lower).

Slope steepness (measure or estimate slope angle),

Distance from pipeline, streams, rivers, tracks etc.

Nature of slope above features and pipeline

(c) Size of landslide and erosion features:

From the air the size of any landslide and erosion features can be

p

( c )

F



estimated in relation to the pipeline dimensions, or the size of

known features such as access tracks, roads, farm gates etc.

On the ground the size of any features should be measured using

GPS (also records location), tape measure, or by pacing (a last

resort, but better than nothing). Record vertical limits (height

difference) across feature (m), and also estimate the slope or

gradient (i.e. the horizontal distance (m) for 1 m vertical rise) in two

or three locations above and below the pipeline.

(d) Material type:

The nature of the materials present in and around the feature

should be recorded and their locations marked on the sketch map.

The strength and moisture conditions of the materials should also

be noted using the details contained in Section 3.3.

(e) Activity:

The current activity and rates of movement of landslide features

should be estimated (use terms outlined above). Note whether the

feature is currently active and is still moving, or shows signs of

recent movement, or signs of the potential for possible future

movements or slope failures (such as ground cracking above the

landslide scar or failure scarp, stream erosion at toe etc.).

(f) Potential for Future Activity:

Consider (your best guess) what might happen to the landslide or

erosion feature in the future, based on the visible features (and

using the information contained in Appendix 4). Provide

photographic or sketch record of the evidence.

(g) Photographic Record:

It is always valuable to record and show the location of features.

Use both aerial and ground photos.

Aerial photos: Take close up (high resolution, at least 5 mega pixel)

photos of the entire feature, and also more distant shots showing

the pipeline and terrain above and below the feature and pipeline.

Record the date and time (digital photos make this easy); repeat

photo from the same height and angle on future inspections; take

several photo over several weeks or months to monitor feature.

Ground photos: Use the same approach as for aerial photos. Take



a series of shots to show details of the feature at its most active

location, and also shots showing its position relative to the pipeline.

Repeat photos on subsequent inspections.

This information should be recorded using the proforma contained in

Appendix 2 (Pipeline Threat Investigation proforma). A sketch map

showing the fey features identified on site should also be produced using

the symbols contained in Appendix 3.

3. CLASSIFICATION SCHEMES

3.1 Landslides

When carrying out a landslide assessment for a pipeline or other key

network it is usual to establish a series of landslide-hazard models, which

best describe the landslides that either have occurred or could occur along

the alignment. Landslide-hazard models are landslides that have been

grouped together on the basis of:

Material type – nature of the displaced material – rock, debris or earth

Type of movement – how the debris from the landslide is transported

e.g. by falling, toppling, rolling and bouncing, sliding, flowing or as a

combination, e.g. a slide that develops into a flow etc.

Additional descriptions can also be used regarding: topographical location

– where the landslide is located on the slope e.g. open slope (where the

landslide and debris remains totally on the open hillside and is not

channelised along a stream course), or channelised (where the landslide

debris is channelised along a stream course); and velocity of the debris –

extremely rapid (typical velocity 5m/second) to extremely slow (typically

>16mm/year).

These different descriptions are combined to then classify the landslide.

Many different types of landslide have been defined and these are shown

in Table 3, which summarises the main landslide hazards.



Table 3 Classification of landslide type after Varnes, 1978, DoE., 1990 and BGS, 1999.



Table 3 Continued



Table 3 Continued



Table 3 shows that there are many different types of landslide hazard,

which although important, are difficult to identify in the field and many are

in fact variations on a theme.

For the purpose of this course the landslide hazards contained in Table 3

have been simplified into those landslides, which predominantly occur in

New Zealand and more importantly, occur along the pipeline alignment. It

is recommended that when trying to classify landslides the following main

hazard types are used, the details of which are presented in Table 4.

Table 4 Landslide hazard types typically encountered in New Zealand along

pipeline alignments.

Hazard type Description

FLOWS These types of hazard usually originate as shallow slides, however, the failed mass tends to break down, becoming saturated and remoulded to move as a flow.

1) Open slope – FLOW

These types of hazard are slow to rapid (typically 3 m/minute to 13 m/year), with the debris remaining wholly on the open hillside and not channelised along a stream course.

2) Channelised – FLOW

These types of hazard are rapid to extremely rapid (typically 5m/second to 3m/minute), with the debris becoming channelised along a stream course. Channelised debris flows generally have much greater mobility than open slope flows, and normally develop when debris from one or more landslides enters a stream course, and becomes mixed with stream water. Deposition of the debris tends to occur once it reaches low angle open slopes, forming debris fans.

3) SLIDES

(deep-seated)

Movement of an intact mass by sliding along a basal rupture surface. These types of hazard are generally slower moving (1.6 m/year to 16 mm/year) and have deeper rupture (slip) surfaces than the type of shallow



Hazard type Description

slides that lead to flow-type landslides. The displaced material tends to move along the surface of rupture as a series of discrete intact blocks (these types of landslide are often referred to as translational block-slides).

4) ROCK FALLS/TOPPLES

This type of hazard results from one or more rock fragments being transported initially by free-falling but may include sliding, rolling and bouncing.

It is relatively straightforward to identify these different landslide hazard

types in the field as each type has a series of distinctive features, which

relate specifically to that type.

3.1.1 Open slope flows

Open slope flows (shown schematically in Figure 11) are landslides where

the debris stays unconfined, remaining on the open-slope. As the

landslide debris remains unconfined the runout (defined as the distance

from the landslide crown to the toe of the debris) tends to be limited, due

to frictional drag, as the debris is allowed to spread out over the ground

surface.

These landslides tend to originate as slides, with the debris breaking

down and mixing with water to become a slurry (flow). Blocks or rafts of

intact material tend to be transported on this slurry. The Hunterville

landslide (Figure 8, Section 2.3) is one such example of an open slope

flow. Debris from this landslide was recorded moving at a velocity of 80m

over 30 minutes (0.04 m/sec).



Figure 11 Schematic diagram of an open slope flow

3.1.2 Channelised flows

Channelised flows (as shown schematically in Figure 12) are landslides

where the debris becomes confined (channelised) along a stream course.

As the landslide debris is confined runout of the debris tends to be

significant, as it mixes and becomes diluted with additional water from

along stream course.

These landslides also tend to originate as slides and could be in part open

slope flows, before being channelised. These types of landslide are

extremely rapid and can runout over several kilometres, and so the source

area could be located some distance away from where the debris

eventually stops.



Figure 12 Schematic diagram of a channelised flow

As a result, these types of landslide tend to cause the most deaths world

wide. The Eastbourne landslide, Wellington (Figure 13) is one such

example of a small channelised flow. Debris from this landslide was

recorded moving at a velocity of approximately 12 m/sec).



Figure 13 Aerial view of two channelised flows at Eastbourne, which closed

the road and damaged two houses at the bottom of the stream course. The channelised flows originated as shallow slides in an area of recently milled pine forest (visible at the top of the photograph) some distance upslope from the affected road and houses.

3.1.3 Slides

Slides (as shown schematically in Figure 14) are landslides where the

failed material remains as a series of intact blocks (or rafts), which slide

along a basal rupture surface (slip plane). These landslides tend to pose

the greatest hazard to road, rail and pipeline networks as they are

relatively large and deep-seated, requiring detailed investigations and

costly designs to mitigate against. These landslides should be avoided if

possible.



Figure 14 Schematic diagram of a slide.

These types of landslide are typically slow, and only tend to affect the

infrastructure located on or within the sliding mass. The Waikorora Bluff

slide (Figure 15), located at the northern end of the Whitecliffs State

Forest is a good example of a slide. At this location an approximate 400m

length of pipeline passes around the head and lateral margins of an active

slide.



Figure 15 Oblique aerial photograph of the Waikorora Bluff slide



3.1.4 Rockfalls

Rockfalls (as shown schematically in Figure 16) are hazards associated

with steep cliffs, and usually involve either individual rock blocks, or

multiple rock blocks (avalanche), which free fall, bounce, roll and slide

down slope. These hazards are extremely rapid and can involve small to

large volumes of material.

Figure 16 Schematic diagram of a rockfall

3.2 Erosion

Other main forms of erosion which affect pipelines and associated

infrastructure (e.g. access roads to and from the pipeline) are described in

the Table 5. The defining aspects of each of these processes are

highlighted in bold italics.



Table 5 Erosion hazards typically encountered along gas pipe alignments in

New Zealand.

Erosion type Description

Sheet erosion The removal of surface material by non-channelised overland flow of water.

Rills Typically less than 50cm deep and less than 100cm wide, near linear features, which occur either on their own or as multiple features. They are features that can be smoothed out/removed by cultivation using normal farm equipment. Rills typically develop as sheet erosion becomes more established.

Gullies Unlike rills, these are large permanent features that cannot be removed using normal farm equipment. Like rills, gullies are formed by the channelised flow of water, including headword migration of the channel. Gullies tend to develop once rills become established.

Stream bank and stream bed erosion

Stream bank and bed erosion refers to the removal of material from the banks and bed of a stream during periods of high water flows

Wind erosion Refers to the removal and transportation of particles (soil, sediment etc) by wind action.

Coastal cliff erosion

Coastal cliffs and escarpments (not necessarily located on the coast) can retreat (erode backwards) due to removal of material from the slope toe (for coastal cliffs caused by wave action) and exposed (bare) slope surfaces (through rock falls, rills, gullies etc).

Tunnel gullies (aka: pipe; shaft erosion, tomos, under runners)

Caused by the subsurface flow and concentration of water, resulting in the removal of material by water, forming narrow conduits, tunnels, voids or pipes.

Deposition of sediment/debris

Deposition refers to sediment (including vegetation) that has been eroded, transported and deposited by running water. This material may be deposited in channels, on terrace surfaces by overbanking of streams or rivers, or on fans (possibly deposited by landslide processes and not just water).



Descriptions and photographic examples of the different types of erosion

discussed in Table 5 are contained in the following sections.

3.2.1 Sheet erosion, rills and gullies

Sheet erosion is caused by a combination of raindrop impact dislodging

fine soil particles, and overland flow, which transports the soil particles

away. Where overland flow concentrates and where the velocity of the

flow increases, rills may develop. Rills can develop into gullies as a result

of ongoing incision and headword migration caused by cutting down due

to concentrated flow of water. The main difference between rills and

gullies are that rills can usually be removed by farm machinery, while

gullies cannot.

Figure 17 Photograph showing the development of rills on a de-vegetated

slope.



3.2.2 Stream bank and bed erosion

Stream bank and bed erosion is caused by the flow of water along a

stream or other water course. Mechanisms of erosion include bed and

bank scour, which removes support and leads to the toppling of the bank.

For stream bank erosion this generally occurs on the falling stage of a

flood event, when the strength of the material forming the bank is

decreased due to uptake of water from the river and the support of the

river is removed, while stream bed erosion is most severe during peak

flows (e.g. associated with storm events).

Figure 18 Photograph showing stream bank erosion due to undercutting and

scour of the bank by water flowing along the stream course (Awakino River).



Figure 19 Photographs showing the change in river course before (A) and

after (B) a storm event, as a result of bed erosion. Note where material from the river bed has been eroded and new material deposited.

A

A

B



3.2.3 Tunnel gullies

Tunnel gullies form in weathered or weak materials where subsurface

water concentrates above a relatively impermeable layer (e.g. landslide

debris overlying rock). Land susceptible to tunnel gully erosion includes

moderately steep hill country where soft materials such as loess and

tephra deposits (volcanic materials) overlie stronger sandstones and

mudstones.

Figure 20 Photograph of a tunnel gully, which resulted in failure of the entire

road carriageway.

Like tunnel gullies, sinkholes (also known as a sink, shake hole, swallow

hole, swallet, doline or cenote) are natural depressions or holes in the

surface topography. Sinkholes may vary in size from less than a meter to

several hundred meters both in diameter and depth and can form

gradually or rapidly. Mechanisms of formation typically include: gradual

http://en.wikipedia.org/wiki/Cenote



removal of slightly soluble bedrock (such as limestone) by percolating

water; collapse of a cave roof; or a lowering of the water table.

Sinkholes typically form in areas where limestone rock if present at or

near ground surface.

3.2.4 Wind erosion

Wind erosion is the detachment, and transportation (by suspension, or

bouncing along the surface) of particles by wind action. Wind erosion

rates depend on wind velocity and currents, particle size and moisture

content of the material, surface roughness and vegetation cover. Wind

erosion may occur on flat or sloping ground and is the dominant erosion

process on sand country, increasing in severity towards the coast, where

there is little soil development and on recently formed volcanic soils (e.g.

central plateau, NZ). In general wind erosion refers to the removal of

material; however, the eroded material is transported and deposited

elsewhere, often resulting in the formation of new sand dunes some

distance from the area eroded.

Figure 21 Photograph of an area susceptible to wind erosion (Spence Road,

Kakaramea).

http://en.wikipedia.org/wiki/Limestone

http://en.wikipedia.org/wiki/Subsidence

http://en.wikipedia.org/wiki/Cave

http://en.wikipedia.org/wiki/Water_table



3.2.5 Coastal erosion

Coastal erosion is the wearing away of land or the removal of beach or

dune sediments by wave action (generated by storms or fast moving

motor craft), tidal currents, wave currents, or drainage. Coastal erosion

typically results in the long-term loss of sediment and rocks from one area

and the temporary redistribution of material to another, therefore erosion

in one location may result in deposition nearby. Dramatic formations can

be formed in areas where the coastline contains alternating layers of rock

or fracture zones (causing variations in material strength). Softer materials

are eroded much more readily than stronger materials and so these

variations can result in the formation of tunnels, caves, arches, bridges,

stacks, columns and pillars.

Figure 22 Schematic diagram showing the typical landforms associated with

coastal erosion

fracture

http://en.wikipedia.org/wiki/Beach

http://en.wikipedia.org/wiki/Dune

http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Wave

http://en.wikipedia.org/wiki/Tide

http://en.wikipedia.org/wiki/Erosion

http://en.wikipedia.org/wiki/Rock_%28geology%29



Figure 23 Aerial photograph of the Twin creeks area (North Taranaki), note

the actively retreating sea cliffs, caves and arches. A pipeline follows the crest of these cliffs, and because of ongoing erosion, was realigned using horizontal directional drilling technology.

3.3 Material types

In most cases, it is the nature of the material, whether soil, rock or man

made fill, which usually controls the susceptibility of the ground to erosion.

Recording material properties in the field is essential to understanding the

nature of the problem. Although many variations in materials exist, there

are two essential and relatively straightforward properties that should be

recorded: material type; and material strength. By using the details

contained in Tables 6a and 6b it should be possible to classify the

different materials in the field, based on a few simple observations and

tests.



Table 6a Material type descriptions

Material Type Description

Organic (peat) Contains much organic vegetable matter; often has

noticeable smell and changes colour on oxidation.

Clay Grain size < 0.002mm, not visible to eye. Plastic

(cohesive), sticks to the fingers and dries slowly;

shrinks appreciably on drying, usually showing

cracks.

Silt Grain size 0.06 to 0.002 mm, not visible to eye.

Dilatant; slightly granular or silky to the touch;

disintegrates in water, lumps dry quickly; can be

plastic but can be powdered between fingers.

Sand 2 to 0.06 mm, grains visible to the eye. Contains

little or no cohesion; grading can be described, well-

grade or poorly-graded.

Gravel 60 to 2 mm. Shape and grading can be described

Cobbles/boulders > 60 mm. Shape and grading can be described

Rock (include type

e.g. greywacke etc)

Describe rock: strength; colour; texture; and name if

possible; also include a description on weathering

Table 6b Material strength descriptions

Strength Description

Soil – sands, gravels and boulders

Loose Obvious Voids between grains

Dense No obvious voids between grains

Soil – clays, silts and organics

Very soft Exudes between fingers

Soft Moulded by light finger pressure

Firm Moulded by strong finger pressure

Stiff Cannot be moulded by finger pressure

Rock

Weak Easily crumbled by hand



Strength Description

Moderately weak Broken with difficulty in two hands

Strong Firm blows with point of pick causes only superficial

damage

Very strong Many hammer blows required to break specimen

In addition to material properties, the moisture content of a material

should also be recorded (e.g. dry, moist and wet), as soft wet materials

tend to be more prone to erosion than dry materials.

4. HAZARDS AND THEIR IMPACTS

This section looks at the possible impacts each of the main hazard types

discussed in Section 3 could have on the pipeline alignment. Impacts to

the pipeline alignment (or access roads) from most types of erosion

hazard will depend upon the topographic position of the feature with

respect to the at-risk facility.

4.1 Open slope flows

If the landslide source area is located upslope of the pipeline it is possible

that debris from the landslide may runout and impact the pipeline. Open

slope flows do not tend to runout long distances and in most cases (if the

landslide is relatively small) may have little impact on the pipeline itself.

However, debris from the flow may block access along the pipeline. If the

landslide is of significant size and debris (several meters in thickness)

were to be deposited on top of the pipeline, then this could place

additional load on to the pipeline. In this situation the debris should be

removed and if deemed necessary pipeline integrity should be checked.

If the source area is located down slope of the pipeline it is unlikely that

debris from the landslide will have any effect. However, the source area

may undercut the pipeline and in the worst case this could lead to the

pipeline being exposed. It is also possible that a source area located



immediately down slope of the pipeline could retrogress (erode

backwards) undercutting the pipeline. Figure 24 shows the main hazards

associated with open slope flows.

Figure 24 Schematic diagram showing the hazards (1 and 2) associated with

open slope flows.

Hazard 1 – Slide movement in the landslide source area could cause the

pipe to shear at the flanks of the landslide, or undercut the pipeline if

located above the main scarp.

Hazard 2 – Debris sourcing from open slope flows on the steep slope

could deposit material on top of the pipeline, placing additional load on the

pipe.



4.2 Channelised flows

Unlike open slope flows, debris from channelised flows can travel long

distances. If the debris remains channelised along the stream course it

can cause significant erosion. Some channelised flows may have only

small source areas, but can bulk to many times their original size due to

erosion and entrainment (inclusion) of additional material from along the

stream course. Debris from these types of landslide tends to deposit if:

the debris overtops the stream course (becoming non-channelised); or

when the channel flattens out. If the latter occurs the debris typically

forms distinctive fan-like features. A catchment prone to debris flows

usually has a distinctive debris fan located at its mouth. When assessing

the hazard from channelised flows it is important to look further-a-field

(than for open slope flows and slides) at entire drainage catchments

rather than individual slopes. If the pipeline crosses a potential debris-

flow path it is important to assess whether the crossing is located at a

channelised or fan section of the stream course

For CHANNELISED sections (where the debris is channelised along a

stream course) the pipeline is at greatest risk. Channelised flows are high

in energy and can easily scour, expose and damage buried pipelines. At

aerial pipeline crossings it is also possible that debris could impact the

pipeline if the debris overtops the channel. Figure 25 shows the main

hazards associated with channelised flows.




channelised flows.

Hazard 1 – Stream course prone to channelised flows. Potential exists

for the pipeline to be scoured (buried crossing) or impacted (aerial

crossing).

Hazard 2 – Potential exists for future debris flows to either scour out the

pipeline or place additional load on top of the pipeline.

4.3 Slides

These landslides tend to pose the greatest risk to pipelines as they are

often misidentified or assumed to be inactive. In most cases the slip

plane usually corresponds to a change in material type or zone of

weakness. Many of the slides observed in the hill country around

Taihape, Wanganui and New Plymouth have slip planes which



correspond to the boundary between weathered soil and rock

(mudstone/siltstone) and tend to be relatively shallow (slip plane <5 m

below ground surface). In most cases open slope and channelised flows

originate as slides. Other types of slide, e.g. the Waikorora slide (Figure

15, Section 3.1.3) have slip planes that are relatively deep (>10 m below

ground surface), which correspond to weak zones (clay seams) within the

rock. Figure 26 shows the main hazards associated with slides.


slides.

Hazard 1 – Retrogressive failure (eroding backwards) of the landslide

main scarp could undercut the pipeline. Evidence of potential

retrogression is shown by the presence of tension cracks (located upslope

from the main scarp).



Hazard 2 – As the pipeline is within the sliding mass, shear forces can

develop on the pipe at the flanks of the landslide, leading to deformation

or even rupture of the pipe.

4.4 Rock falls

Rockfalls are triggered by earthquakes and more frequently by periods of

high rainfall and frost shattering (in areas of extreme cold). Areas

particularly at risk from rockfalls are those located at the toe of steep

slopes, e.g. coastal cliffs etc. Coastal cliffs are particularly vulnerable to

rockfalls as they are constantly being eroded by wave action, leading to

retrogression (eroding backwards) of the cliff top. Figure 27 shows the

main hazards associated with rockfalls.


rockfalls.

Hazard 1 – Continued erosion of the steep cliff by either: sea erosion (if



forming a sea cliff, e.g. Tongaporutu, North Taranaki); rainfall; and even

earthquakes, could lead to undercutting of the pipe as a result of cliff-line

regression.

Hazard 2 – If the pipeline is located along the toe of an actively eroding

cliff (evidence in the form of scree/talus or boulders at the toe of the

slope), it could be susceptible to impact loading from rockfalls.

4.5 Erosion

4.5.1 Sheet, rill and gully erosion

Sheet, rill and gully erosion can lead to undercutting and exposure of the

pipeline. Once established, rills and gullies can incise and enlarge

relatively quickly and in some cases lead to the development of

landslides.

4.5.2 Stream bank and bed erosion

At river crossings and where the pipeline alignment is adjacent to

rivers/streams, it is extremely vulnerable to stream bank and bed erosion.

The banks of active stream courses are prone to collapse as a result of

undercutting by stream bank erosion, therefore where the pipeline

alignment is located next to a stream/river course, bank collapse could

lead to undercutting of the pipeline (Figure 28). Stable stream banks can

be made unstable by changes in the stream course. For example a tree

stump washed downstream (at times of peak flow) can form a constriction

(at times of low flow), leading to concentrated erosion and stream bank

collapse (Figure29).

Materials forming the bed of a river are constantly being eroded,

redistributed and deposited, and during periods of peak flow (during

storms), streams and rivers can laterally migrate, often altering their

courses. Bed erosion can result in exposure of the pipeline where it

crosses a river. If the pipeline is exposed it becomes vulnerable to impact

by material being transported by the river. Figure 30 shows the main

erosion hazards and their potential impact on the pipeline.



Figure 28 Photograph showing erosion on the outside bend of a meander,

possibly caused by deposition of debris into the main river channel from a smaller stream, leading to the development of a bar, which has in turn caused undercutting and collapse of the stream bank (Awakino River).

Figure 29 Photograph showing stream bank erosion initiated by the deposition

of a tree stump (Araheke Stream).



In addition to stream bank and bed erosion, old river terraces (areas of flat

ground adjacent to large streams and rivers) can form a hazard to the

pipeline. These terraces are usually formed of soft, organic, clays and

silts, which are highly compressible. If these soils are loaded by e.g.

landslide debris, fill material, or over-bank deposits from flooding of the

river, the increased load can lead to differential settlement, and ultimately

to deformation of the pipeline. Old river terraces are usually easy to

identify, as they tend to be located next to large rivers, are flat, with

hummocky surfaces and ponded water apparent.


stream bank erosion.

Hazard 1 – Stream bank erosion and incision can cause bank collapse

leading to undercutting of the pipeline. Stream bank and bed erosion can

also remove cover material from the pipeline, leaving it exposed at the

surface and vulnerable to impact from boulders and vegetation washed

down the stream/river.



Hazard 2 – Gully erosion on stream banks (especially where smaller

streams and gullies join the main river), can lead to undercutting of the

pipeline, and failure of the banks.

4.5.3 Wind erosion

Wind erosion usually develops when surface vegetation covering old sand

dunes is removed (by farming practices or excavations). High winds

associated with coastal areas transport material away from bare exposed

areas, and over time and if left un-managed, can lead to exposure of the

pipeline.

Figure 31 Pipeline exposed in an old sand dune, due to ongoing removal of

sand by wind erosion.

4.5.4 Coastal erosion

Coastal erosion caused by wave action can lead to cliff retreat. The rate

of cliff retreat depends upon several factors, but is mainly controlled by



the nature (strength) of the materials forming the cliff, and presence of

dominant geological structures (e.g. faults and joints). In areas where the

pipeline is located along the top of actively retreating sea cliffs, it can be

vulnerable to severe undercutting, especially in locations where it

traverses across dominant geological structures, as these areas tend to

erode more readily.

Figure 32 Coastal cliff erosion at Tongaporutu, North Taranaki. A pipeline passes

behind the crest of the actively eroding cliffs. Rates of sea cliff regression are relatively high in this area where the material comprises weak, siltstones and sandstones.

4.6 Other hazards and indicators of potential hazards

As well as landslide and erosion hazards, other potential hazards along

the pipeline alignment that should be assessed are listed in Table 7.



Table 7 Other potential hazards affecting the pipeline.

No. Potential Hazard Description

Ground vulnerable to landslides

1 Steeply sloping ground in colluvium (typically

>25 in angle)

Typically slope angles >25 in colluvium (old landslide debris) tend to be unstable and if possible these slopes should be identified and routinely inspected. Things to look for when identifying these features are: presence of relict landslide features; hummocky ground; ponding of water; and tension cracks (see table 2 for detailed descriptions).

2 Slopes where water can be seen to collect and/or seeping

Slopes usually fail as a result of locally high groundwater levels. Therefore areas of sloping ground which appear to have seepages, springs or ponded water on them, could be either large relict slides, or ground susceptible to future failure.

3 Slopes formed in adverse geological structures and materials (e.g. the Tertiary materials of North Island NZ.

Certain material types are more prone to landslides and erosion, due to the properties of the materials, mainly: strength; and composition (clay, silt, sand etc). For example the Tertiary sediments, (through which a large section of the Maui pipeline traverses), are low strength materials formed of sand and silt, which are prone to wide-scale landslides.

4 Slopes adjacent to active fault zones

Geological faults cause materials to become sheared and fragmented, usually leading to the material having a lower strength and therefore more prone to landslides.

5 Slopes likely to be prone to river or stream scour at their base

Undercutting of slopes by rivers/stream courses can cause the slopes to become over-steep, and unstable. This process can also lead to reactivation of old relict landslides, as material from the landslide toe is removed.

6 Sheet Erosion

(including areas of rilling/gullying)

Bare surfaces such as: paddocks; unsealed roads and tracks; areas of heavy stock concentration; landslide scars; and landslide debris trails. Once sheet erosion becomes established it can lead to the development of rills and gullies. Therefore once identified it should be addressed immediately.

7 Areas devoid of vegetation

Including areas of recently felled forest (also refer to number 10).

8 Stream banks As number 10.

9 Landslide flow-paths

Landslides tend to leave scars on the hillside due to removal of vegetation. As a result, the bare ground is prone to erosion, which can lead to further landsliding.



No. Potential Hazard Description

Human interference

10 Slopes modified by man, e.g. slope over steepening, undercutting of toe and removal of vegetation.

Steep man-made cuts are prone to landslides, as slope cutting either leads to the over-steepening or undercutting of slopes. If not done correctly this can lead to landslides. Removal of vegetation can also lead to shallow landslides, as vegetation intercepts rainwater and the roots tend to hold the near surface materials together, increasing their strength. It should be noted that vegetation has little stabilising effect on large deep-seated slides, as the slip planes tend to be below the depth of root penetration.

11 Excavations along the pipeline alignment (unrelated to slope modifications)

Unauthorised excavations along the pipeline right of way can lead to exposure and damage to the pipeline. It is assumed that there are procedures in place to deal with these types of issues. Current code requirements state that there must be a minimum of 800 mm of cover.

12 Placement of fill along the pipeline alignment

Placement of fill material (or any other material) on top of the pipeline increases the vertical earth load acting on the pipeline. This is primarily a consideration for non-operating conditions of buried steel pipelines (when the pipeline is under no internal pressure). Under most operating conditions, the external earth pressures are insignificant in comparison to the internal pipe pressures. Vertical earth load is an important consideration when designing pipe casings used for rail and road crossings. Placement of additional load onto the pipeline can lead to settlement of the ground and deformation of the pipe. Areas susceptible to settlement tend to be soft (saturated) ground (e.g. river flood plains and old terraces, valley bottoms and areas of landslide debris).

13 Creation of unauthorised access routes across the pipeline alignment

In addition to supporting dead loads imposed by earth cover, buried pipelines can also be exposed to superimposed concentrated or distributed live loads. Large concentrated loads such as those caused by truck-wheel loads can place additional load onto the pipe and if the pipeline has not been designed to take these loads (in the case of unauthorised access routes over the pipeline) problems could occur.

It is assumed that a procedure is in place to deal with unauthorised access across buried pipelines.

14 Changes in land use

Removal of vegetation (as numbers 6 and 8), changes in farming practices and water course deviations caused by man could all impact the pipeline by changing the local conditions making some areas more prone to landslides/erosion.



4.7 Hazard assessment

Figure 33a is a schematic diagram showing how the different hazards

discussed in this section interact in the landscape. Typically an

assessment of the hazards along the pipeline alignment is carried out

prior to route selection and construction. The purpose of this assessment

is to identify the locations of the different hazards so that mitigation

measures can be designed and constructed to safeguard the pipeline,

Figure 33b. The main tasks which form a typical hazard assessment are

discussed in detail in Section 5.2.

The hazards discussed in this section relate to those hazards more

frequently encountered along the pipeline. This discussion does not

include hazards such as earthquakes, Tsunami, flooding and volcanic

eruptions, as well as any health and safety hazards (as detailed in the

Health and Safety in Employment Act 1992).

4.7.1 Site assessment

A site assessment form (contained in Appendix 4) has been created to

assess the severity (threat) a particular hazard may have on the pipeline.

This form brings together the information contained in Sections 2, 3 and 4

to allow the hazard (threat) type and risk to the pipeline to be determined.

The risks have been classified into three categories:

Low – No risk to the pipeline at present or in the future (years) – hazard

assessed as inactive

Intermediate – No immediate risk to the pipeline, however, ongoing

development of the hazard could impact the pipeline in the future

(months) – hazard assessed as active.

High – High risk to pipeline, where the pipeline has been exposed at the

ground surface, or where the hazard is highly active and will lead to failure

of the pipe.



Figure 33a Schematic diagram showing the different landslide and erosion hazards discussed in Section 4.



Figure 33b Schematic diagram showing the impacts from the different hazards shown in Figure 26a.

1 Aerial crossing over river – potential for streambank erosion and impact from channelised flows

2 Pipeline at toe of steep cliff, with rock fall debris on surface – potential for rockfalls.

3 Pipeline traverses debris fan – potential for burial or scour by channelised flows.

4 Pipeline traverses the main scarp of an active landslide with tension cracks visible on surface – potential for the landslide main scarp to retrogress, undercutting the pipeline.

5 Pipeline passes near the head of actively incising erosion gully – potential that the gully could retrogress, exposing the pipe.

6 Pipeline traverses the toe of a steep slope, with evidence of historic open slope flows – potential for the debris from future open slope flows impacting and burying

the pipe.



5. MITIGATION MEASURES

Landslide and erosion mitigation measures should ideally be designed on

a site specific nature by an engineering geologist/geotechnical engineer.

However, there are several prescriptive techniques which could be used

to maintain the pipeline ROW and limit the severity of the

landslides/erosion hazards.

This section will look at the approaches typically adopted by engineering

geologists/geotechnical engineers when designing landslide/erosion

mitigation measures. This section will also include some prescriptive

designs, which could be adopted to either limit or protect the pipeline or

other areas of critical infrastructure related to the pipeline (e.g. access

roads etc) form landslide /erosion hazards.

5.1 Stabilisation techniques

Four main approaches are typically used for the design of soil/rock slopes,

landslide and erosion mitigation measures, these are: 1) avoid the

problem; 2) reduce the driving forces; 3) increase stabilising forces by

application of an external load; and 4) increase stabilising forces by

increasing the internal strength. These approaches can be used either

individually or in combination depending on the nature of the particular

hazard. The effectiveness of these measures also depends upon the

material forming the slopes, as some techniques are best applied to soil

slopes while others are specifically for rock slopes.

Table 8, provides comments on the applications and limitations of the four

main approaches for stabilising landslides, slopes and reducing the

effects of erosion.



Table 8 Typical mitigation techniques

Approach Technique Discussion

Avoid Problem

Realign pipeline HIGH COST; could create similar problems; slow to implement.

Completely or partially remove unstable material

Moderate cost; only feasible for shallow, small slips; could create further instability

Construct structures to prevent debris from impacting the pipeline

HIGH COST; slow to implement; must be capable of containing debris

Reduce driving forces

Re-grade slope Unlikely to be feasible in steep terrain; requires earthworks to remove overburden/rock blocks, which can be costly.

Drain surface, re-direct stream courses/preferential flow paths

Will prevent surface water from infiltrating, but need to be combined with other measures e.g. bioengineering, spring tapping etc.

Drain subsurface (sub-soil drains)

Should be used as cut off-drains to prevent near-surface water from entering landslide; or at the landslide/slope toe to promote drainage of water from wet areas.

Increase stabilising forces by application of an external load

Construction of retaining walls/buttresses

Moderate cost to HIGH COST; walls must be founded beneath slip plane; buttresses should be keyed into rock slope with dowels. These should be combined with other techniques.

Construct shear key at toe

Requires large working area at toe of landslide.

Install anchors HIGH COST, only used for rock slopes; specialist installation equipment and ongoing monitoring required.

increase stabilising forces by increasing the internal strength

Drain sub-surface (sub-soil drains)

If the slip plane is deep then sub-soil drains can have little effect as water needs to be removed from near the slip plane.

Soil nails HIGH COST, typically only used for soil slopes that have not failed; specialist installation equipment and ongoing monitoring required.

bioengineering Not suitable for steep slopes and deep-seated slides. Vegetation type is critical.



5.2 Hazard assessments

When assessing what stabilisation measure(s), are applicable to a

particular hazard type the engineering geologist/Geotechnical engineer

usually carries out a detailed investigation and hazard assessment, which

includes some or all of the following tasks:

Desk study (review of all existing information, at both the regional

and site-specific scales), including: geology and geomorphology

maps, aerial photographs; results from any previous investigations;

rainfall records etc.

Field Mapping (to field verify the findings from the desk study),

including: site-specific geology and geomorphology, material index

testing etc.

Ground investigation (to quantify the materials and conditions on

site, e.g. depth to slip plane, groundwater conditions etc; and to

collect samples for laboratory testing), including: drilling of boreholes,

excavation inspection trenches, in situ testing, geophysics etc.

Laboratory testing (to determined the strength characteristics of the

materials derived from the ground investigation)

Analyses (to analyse the information collated from the desk study,

field mapping, ground investigation and laboratory testing stages), this

may include: setting up critical cross sections through the

slope/landslide; numerical stability analysis; identification of

movement triggering factors (in the case of landslides)

Design: the findings from the hazard assessment are then used to

determine the most appropriate mitigation strategy.

5.2.1 Case Study –landslide

Detailed investigations were carried out for th landslide, Figure 15,

Section 3.1.3. In 1994 the movement rate of the landslide increased

rapidly and so a detailed assessment of the landslide was undertaken to

design the most appropriate mitigation measures. The assessment

included: desk study; ground investigation; laboratory testing; analysis;

and design of mitigation measure, comprising: 1) removal of material from



the main scarp of the landslide (to reduce the driving forces); 2) re-

directing of the stream course away from the landslide (again to reduce

the driving forces); and 3) installation of sub-soil drains (to increase the

stabilising forces by increasing the internal strength of the landslide

debris).

Figure 34 shows the movement history of several markers installed on the

surface of the landslide. From 1981 to 1996 the movement rate of the

landslide was up to 1.5 m/year. Following installation of the mitigation

measures this movement rate dropped to approximately 0.003 m/year,

indicating that the mitigation measures were working.

Figure 34 Graph showing the cumulative movement (displacement) of the

landslide. The movement of several marker installed on the landslide has been plotted against rainfall patterns to determine the links between movement and rainfall.



The landslide is a large, deep-seated slide and is a very complex

landslide, requiring a detailed hazard assessment by suitably qualified

people. Other smaller landslides or areas suffering from erosion may not

require such detailed analysis and design. The following sections discuss

some prescriptive mitigation measures which could be used l to reduce

the impacts of such hazards on the pipeline and associated infrastructure.

5.3 Design measures (landslides, slopes and erosion)

Typically any design measures used for landslide and erosion hazards fall

into five main categories:

1. Retaining/protective structures (rock/soil slopes)

2. Earthworks

3. Drainage

4. Bioengineering (biological erosion control/plant materials)

5. Monitoring and maintenance

5.3.1 Retaining/protective structures

Some of the more common techniques for treating rock slopes are

illustrated in Figure 35a, 35b and 35c. These techniques mainly

comprise: rock slope reinforcement; rock fall control; rock removal. The

majority of these techniques require detailed design by an engineering

geologist/geotechnical engineer. Some techniques such as creating rock

catch ditches (at the toe of steep rock slopes) can be done without

detailed design and could prevent rock falls from impacting the pipeline.



Figure 35a and 35b Rock slope control and reinforcement methods

commonly used for rock slopes.

Figure 35c Rock slope control – removal of potentially unstable rock blocks.



For soil slopes there are a range of retaining walls available. These are

illustrated schematically in Figure 36. For most types of retaining wall,

detailed design is required by an engineering geologist/geotechnical

engineer. In some cases, however, it could be possible to install

gabion/concrete block retaining walls without detailed design, if: the wall is

temporary (to prevent ongoing deterioration prior to a detailed

investigation); the size of the wall is relatively small; (i.e. <1.5 m high); and

if the asset is low risk (i.e. access road). Gabion (bolster) walls (typically

one row of gabions installed as a line on a slope/landslide) are particularly

useful as they allow water to pass through them, acting like large drains,

as well as trapping sediment, therefore preventing rill and gully

development. Gabion walls can also accommodate settlement and

movement without being compromised, while pile walls and crib walls

cannot take even minor movements without cracking and failing.

Figure 36 Typical retaining wall design details



5.3.2 Earthworks

Earthworks to stabilise slopes/landslides typically comprise: the regrading

(cutting back) of the slope to a more stable angle; removal of overburden

(to reduce the driving forces) and creation of a shear key at the toe of the

slope (to increase the stabilising forces). In most cases, debris from a

landslide (depending upon the size of the landslide) should be removed

from the landslide source area, to prevent it from re-mobilising in

response to rainfall.

Earthworks tend to be costly and require planning, permits and consents

to be obtained. Disposal of spoil can also pose a serious issue, as

placing spoiled material on terrain susceptible to landslides/erosion could

accelerate these processes and in some cases lead to the development of

landslides on originally stable ground. Placement of spoil should be in a

stable area: on level ground; in dry valleys; on the tops of spurs; at

locations protected by bedrock; as far away from the pipeline and stream

courses as possible and where property or public safety are not affected).

Bare slopes (whether newly formed cut slopes or spoil slopes) are prone

to erosion once exposed to the elements. Any bare slopes should have

erosion protection measures installed on them soon after formation.

These may comprise drainage and/or bioengineering measures. To

prevent erosion in spoil areas it is also preferable to compact the spoil

material and re-vegetate therefore increasing the material strength of the

spoil.

5.3.3 Drainage

Drainage control is critical to the stability of slopes. Uncontrolled runoff

can create major erosion problems within very short periods of time.

Slope drainage as described in Table 9, can often result in a marked

improvement in the stability of a slope.



Table 9 Typical drainage details commonly used for landslides and erosion

control.

Function Type Description Hazard Type/Location Used

Interception of surface runoff

Unlined cut-off drain

Shallow trench used to intercept surface water, preventing sheet erosion, rills and gullies from developing. Can also be used to prevent water from entering landslides.

All landslide hazard types and areas prone to sheet erosion including constructed slopes. Typically installed upslope of landslide main scarps or at the crest of newly formed slopes. The prevent water from entering the landslide, debris or slope. May create line of instability beyond crest; may be prone to erosion.

Lined cut-off drain.

Same as above but lined with concrete, masonry or plastic.

Same as above, but Less prone to erosion and leakage Requires frequent inspection for damage/blockage.

Interception of high/perched water table.

Herringbone drain.

Able to intercept water up to a max of approx. 1.5m depth below surface; good for intercepting surface seepage of springs and point sources of water; can accommodate some slope movement.

All landslide hazard types and areas prone to sheet erosion including constructed slopes. Typically installed in the landslide source area, newly formed slopes, or active landslide debris. May only have limited effect on slope stability for deep-seated slides as the drains do not intercept deep groundwater.

Counterfort drain. Able to intercept water up to approx. 3m depth below slope face.

Slides and constructed slopes Typically used to drain the toe and debris of active slides or potentially unstable slopes. They can act as a shear key if founded below landslide slip plane. Difficult to construct in bouldery material.

Interception of deep water table.

Horizontal/subsoil drains.

Only feasible method of intercepting deep groundwater. Should be installed

Deep-seated slides Highly effective in stabilising deep-seated slides provided they can be constructed to an adequate depth. Moderately



Function Type Description Hazard Type/Location Used

in the landslide debris, with the base just above the slip plane.

costly requires drilling rig (horizontal drain) or excavator with shield for subsoil drains; may not always work.

Diversion or improvement of stream course/gully.

Lined channel or cascade.

May be necessary if existing stream course is direct cause of instability.

All landslide hazard types and areas prone to sheet erosion including constructed slopes Anywhere where water from the stream course is thought to be the main driving force for instability/erosion. Usually very expensive and difficult to construct and can result in moving the problem elsewhere.

Erosion control in gully/stream course.

Check dam. Relatively cheap, removes energy (erosion potential) from water.

Streambank erosion Good for stream courses affected by channelised flows or streambank erosion, as they reduce the velocity and therefore erosion potential of the flow.

The most common forms of drainage are open drains or trench drains

used as cut-off drains. These are relatively cheap and easy to construct

and can improve the stability of a slope markedly by preventing surface

water from entering the landslide or area of erosion. Open drains can

typically be used prevent further deterioration of problem areas. Subsoil

and horizontal drains on the other hand are expensive; require specialist

equipment to install and proper design by an engineering

geologist/geotechnical engineer.

Drainage should be used in conjunction with other mitigation measures.

Figure 37 shows standard design details for the main drainage types as

discussed in Table 9.



Figure 37 Typical drainage design details for landslide and slope

mitigation works.



Figure 37 Continued

5.3.4 Bioengineering (erosion control)

Bioengineering measures (vegetation) can be installed to prevent erosion

from developing, or to stabilise slopes and reduce sediment generation

where erosion already exists. Bioengineering measures can be used on

their own, or in combination with other engineered solutions (e.g. retaining

walls drains etc).

How bioengineering/vegetation works

Soils lose strength when saturated and plants, especially trees, help to

maintain soil strength and reduce soil moisture. In general terms, the

mechanisms whereby vegetation influences slope stability can be

classified as either mechanical or hydrological. Mechanical mechanisms

are provided by the root system which acts to reinforce the soil, and

include the root density (the extent of the root mass), the lateral spread



and depth of the roots, and the tensile strength of the roots. Hydrological

mechanisms modify the soil moisture condition through interception of

rainfall by the foliage which leads to absorption and evaporation thus

preventing infiltration into the soil, and through roots extracting water from

the soil, which is ultimately returned to the atmosphere via transpiration

from the foliage (Figure 38).

Figure 38 Slope – vegetation interactions affecting stability

The specific vegetation types and their spacing should be based upon the

rooting medium (soil type, rock type), climatic conditions, erosion type,

topographic position and the presence of animal pests (refer to Table 9 for

1. Foliage intercepts rainfall 2. Roots and stems increase the

permeability of the ground promoting infiltration

3. Roots extract moisture from the soil 4. Roots reinforce the soil increasing

shear strength 5. Tree roots anchor into firm strata

providing slope support 6. Roots bind the soil particles at the

ground surface reducing susceptibility

to erosion



details). When planting, look for places where soil is sufficiently deep and

moist for a tree to establish. Poplars and willows have been bred for

specific characteristics such as fast growth, large/deep root mass, disease

resistance, unpalatability to possums, drought resistance etc. These

different types of poplars and willows are called “clones” Poplars and

willows are typically planted as “poles” i.e. no roots. Other species are

usually planted as seedlings.

Table 10 Plant types and their use.

Type Description Location Used Hazard Type

Trees

Poplars .

Typically open planted 10 to 12 m apart to ensure root interlocking

Usually used on slopes when the erosion/landslides are relatively shallow (typically 1 to 2 m in depth)

Landslides flows

Typically planted 5 m apart (closed canopy)

Closed canopy planting should be used when erosion/land sliding is severe or deeper-seated.

Landslides flows Gullies

Willows Typically planted in rows along the streambanks or within gullies

Typically used to prevent erosion along stream courses (including areas of gully erosion) Areas with high water tables/poor drainage

Gullies Streambank erosion

Gums and Acacias

Typically open planted on slopes.

Used in drought environments, but do not grow as quickly as poplars or willows

Landslides

Pines Typically planted in blocks as closed canopy

Used in areas of sever erosion or land sliding.

Flows Gullies

Shrubs Typically closely planted in blocks.

Often more appropriate in sub-alpine or exposed areas. The choice of shrubs over trees is usually dictated by climatic conditions and/or soil types, e.g. where thin soil exists

Sheet erosion Wind erosion

Hydroseed Planted as continuous surface cover

Used to prevent sheet erosion or in areas of bare ground to prevent sheet erosion developing into rills.

Sheet erosion



When determining the most appropriate species or clone for a particular

problem you must consider the following issues:

Is stock protection required (sleeves)

Climatic conditions (summer dry or moist, frost prone or mild, humid

or dry, sheltered or windy;

Location (coastal or inland, valley or slope)

When considering bioengineering methods, you need to look further a

field for example to prevent surface water from inundating the pipeline

ROW, planting may be required upslope some distance from the ROW,

and would require larger areas of planting.

Animal pests

Many plants, especially certain poplar and willow clones, are palatable to

possums, hares, rabbits and goats. Poplar and willow poles should be

fitted with sleeves to protect from browsing and rubbing against the bark.

A smooth sleeve will also deter climbing possums. Other forms of

protection such as cyclone netting, drums, or electrified wire/fencing may

be necessary to protect against livestock, especially cattle. Stock may

need to be excluded for several months to a year.

Maintenance

Once the selected species have been planted, it is important that they be

maintained. Not all plants will survive, and in harsh environments survival

rates may be less than 50% in the first few years, especially if there has

been a severe drought or cattle damage or possum browse has occurred.

On going silviculture (pruning and thinning) is a good idea to develop

good tree form and maximise the erosion mitigation benefits. Trees need

to have developed enough foliage to survive adverse conditions before

pruning. Generally pruning should not start until at least three years after

planting.

Where to get advice

Given all of the requirements outlined above for achieving successful

planting, it is strongly recommended that unqualified personnel seek



expert advice. Many local Regional Councils have land management

officers with experience in which species and varieties of plant materials

to use in specific sites, recommended spacing, pest control and

silviculture. Regional Councils are also a source of plants, particularly

poplar and willow poles. They can also advise on obtaining plants from

commercial nurseries.

5.3.5 Monitoring and maintenance

Monitoring has two main purposes: 1) to maintain the effectiveness of

mitigation measures following installation; and 2) to assess any ongoing

deterioration of a hazard.

Once mitigation measures have been installed, whether retaining walls,

drainage or bio-engineering measures, they should be monitored routinely

for any deterioration. Retaining walls should be checked for cracking,

deformation (e.g. bowing of gabion walls) including subsidence, and if

identified should be reported. In areas where earthworks have taken

place the newly formed slopes (including spoil areas), should be

monitored for erosion. If erosion occurs then mitigation measures

(bioengineering, drainage, stock control etc) should be installed. Drains

should be inspected regularly and routinely cleaned and cracks repaired.

Outlets for subsoil drains, horizontal drains or trench drains should be

kept clear of vegetation (weed spraying) and should be marked clearly so

that other personnel (with less knowledge of the site) can find them. If

bioengineering is adopted the planting should be inspected, dead plants

removed and new plants installed. Details of any area requiring

maintenance should be recorded and routinely inspected.

Once a potential hazard has been identified along the alignment (or

access road) it is important that the location of the hazard is recorded

correctly (i.e. marked on a map with a GPS location using the proforma

contained in Appendix 2), assessed (using the Site Assessment From

contained in Appendix 4), and routinely monitored to record any change

(increase or decrease in the threat risk). Photographs of the hazard



should be taken routinely, preferably from the same location each time,

therefore allowing changes between subsequent photographs to be

identified. Measurements of key features, e.g. if a landslide source area is

down slope below the pipeline and if retrogression (eroding backwards) is

thought to be a problem, then the distance from the landslide main scarp

to the pipeline should be routinely measured and any changes identified.

In some cases (e.g. when assessing streambank erosion at pipe bridge

abutments) it may be difficult to assess the actual rate of erosion etc, in

such situations any deformation (cracking, tilting etc) of the pipe bridge

abutments as well as the pipe itself should be recorded and routinely

inspected for changes. Knock-out pins (metal or wooden stakes) driven

into the ground adjacent to erosion features can be used to measure

erosion rates.

Inspections of any known hazards along the pipeline alignment should be

carried out following either periods of high rainfall or earthquakes.

5.4 Design measures (river crossings)

Typical design measures for river crossings fall into similar categories as

those for landslides, slopes and erosion hazards. However, river

crossings require specific attention due to the continuously changing

nature of the hazard. The main design measures used to prevent erosion

as stream crossings are:

1. Avoid the problem – via realigning the pipeline using horizontal

directional drilling techniques. These tend to be highly expensive

and depend upon the magnitude of the problem. At Twin Creeks,

North Taranaki, a 600m long section of pipeline was re-aligned

due to the hazard posed by coastal sea cliff retreat. In some

instances it may be possible to open cut a trench through the river

and re-install the pipe. This option will depend on the size and

flow rate of the river.

2. Construction of protective measures – Concrete/rock mattresses



can be used to armour the pipeline, preventing it being impacted

from material transported by the river. In addition, rock groins ,

wooden retaining walls, rock walling, wire mesh walls (Figure 39)

and sand bagging can all be used to reduce the effects of stream

bed and bank erosion.

3. Bioengineering – River bank stabilisation using bioengineering

methods (Figure 40); refer to Section 5.3.4 for details.

Figure 39 Wire mesh walls used to prevent river bank erosion (Mangatainoka

River No.2).



Figure 40 Planting combined with geo-fabric rock-filled socks and rock riprap

for stream bank erosion control (Martinborough).

5.5 Mitigation conclusions

In most cases it is a combination of these different mitigation measures

which are used to stabilise slopes, landslides and areas of erosion, based

on a detailed assessment of the hazard, as outlined in Section 5.2. An

example of how these different mitigation measures discussed in this

section combine are discussed in the case study below

Case Study - Access road to Gibbs Fault crossing

The landslide is located along the access road leading to the aerial

pipeline crossing of the Ohariu fault, locally referred to as Gibbs fault

crossing (Figure 41). The landslide occurred in 2006 and blocked the

main access road. An engineering geological assessment of the landslide

was carried out, including, field mapping and topographic survey, the

results of which can be found in Figure 42a.



Although several factors are thought to have contributed to the

development of the June 2006 landslide; it is likely that erosion caused by

the surface flow of water from the two ephemeral drainage lines (located

upslope above the landslide) was the main contributory factor. Evidence

from field mapping indicates that future instability is also likely, as tension

cracks were apparent on the slope above (southeast) the June 2006

landslide scar (Figure 42a). Although the access road is currently clear it

is likely that any future instability will lead to further blockages of the road.

Based on this assessment the following mitigation measures were

proposed:

1. Increase stabilising forces by constructing a gabion retaining wall at

toe of the landslide to prevent further landslide debris from affecting

the road;

2. Remove the driving forces by Installing surface drainage measures

(cut-off drains, flexible drainage pipe and controlled discharge of

surface water away from the problem area) to reduce the effects of

erosion which could lead to further instability.

A plan and cross section showing the layout of the proposed mitigation

works are contained in Figures 42b and 42c respectively. Following

installation of the mitigation measures it will be important to routinely

inspect them for maintenance reasons. The catch pit and drainage pipe,

along with the culverts and toe drain should all be inspected for blockages

and the gabion wall should inspected for bowing. Any signs of further

instability (e.g. appearance of tension cracks, bare ground etc up-slope

above the landslide main scarp, or bowing of the gabion wall) should also

be looked for and recorded routinely. This may mean that the person

carrying out the inspection needs to walk around the landslide, even if the

weather is bad.



Figure 41 Photograph of the landslide on the access road to Gibbs Fault.

Note the fresh spoil material on the down slope side of the road. At the time this photograph was taken, tension cracks were starting to develop in this fill material.



Figure 42a Engineering geological map of the landslide.



Figure 42b Map showing the layout of the proposed mitigation measures.



Figure 42c Cross section A-B through the landslide, showing the layout of the mitigation measures.



6. CONCLUSIONS

The majority of the mitigation designs used to stabilise the landslide

erosion hazards discussed in this report require detailed assessment

and design by a suitably qualified person. However, the design phase

for these types of hazard is the end product of a process that begins

with Pipeline Technicians and overseers. Routine aerial

reconnaissance (both fixed wing and helicopter) currently carried out by

耳身耳 sonnel forms the main way by which these types of hazard are

identified, reported and monitored. This booklet has been designed to

try and standardise the way in which landslide and erosion hazards are

identified, recorded, classified, monitored and reported.

Some of the mitigation designs discussed in Section 5, can be applied

by the non-specialist (prescriptive measures), e.g. installation of cut–off

drains, small gabion walls and bio-engineering measures to prevent

minor landslide or erosion hazards from deteriorating further. Any

design whether carried out by a specialist or non-specialist should be

based on the following procedures:

Identify the hazard

Classify the hazard

Identify the cause of the hazard

Target the cause with the mitigation design.

Monitor for change

Appendix 2 contains a proforma, which should be used in the field to

record the main features associated with the different hazards. The site

assessment form (in Appendix 4) brings together the information in

Sections 2, 3 and 4, and with field observations will allow the hazard to

be classified. The purpose of these two documents is to standardise the

way in which erosion hazards are recorded and classified, therefore

allowing the most appropriate course of action to be determined,

whether this be: do nothing, monitor, install prescriptive measures; or

refer the problem to an experienced consultant.



APPENDIX 1 — LANDSLIDE AND ENVIRONMENTAL CRITERIA FOR THE NEW ZEALAND MODIFIED MERCALLI INTENSITY SCALE 4

MODIFIED MERCALLI (MM) INTENSITY SCALE

– Landslide and Environmental Criteria –

MM 6 Trees and bushes shake, or are heard to rustle. Loose material dislodged on some slopes, e.g. existing slides, talus and scree slope.

A few very small (<103 m3) soil and regolith slides and rock falls from steep banks and cuts.

A few minor cases of liquefaction (sand boil) in highly susceptible alluvial and estuarine deposits.

MM 7 Water made turbid by stirred up mud.

Very small (<103 m3) disrupted soil slides and falls of sand and gravel banks, and small rock falls from steep slopes and cuttings common.

Fine cracking on some slopes and ridge crests.

A few small to moderate landslides (103 –105 m3), soil/rock falls on steep slopes (>30°) on coastal cliffs, gorges, road cuts/excavations etc.

Small discontinuous areas of minor shallow sliding and mobilisation of scree slopes in places. Minor to widespread small failures in road cuts in more susceptible materials.

A few instances of non-damaging liquefaction (small water and sand ejections) in alluvium.

MM 8 Cracks appear on steep slopes and in wet ground.

Significant landsliding likely in susceptible areas.

Small to moderate (103-105 m3) slides widespread; many rock and disrupted soil falls on steer slopes (terrace edges, gorges, cliffs, cuts etc).

Significant areas of shallow regolith landsliding, and reactivation of scree slopes.

A few large (105-106 m3) landslides from coastal cliffs, and possibly large to very large (>106 m3) rock slides and avalanches from steep mountain slopes.

Larger landslides in narrow valleys may form small temporary landslide-dammed lakes.

Roads damaged and blocked by small to moderate failures of cuts and slumping of road-edge fills.

Evidence of soil liquefaction common, with small sand boils and water ejections in alluvium, and localised lateral spreading (fissuring, sand and water ejections) and settlements along banks of rivers, lakes, and canals etc.



MM 9 Landsliding widespread and damaging in susceptible terrain, particularly on slopes steeper than 20°. Cracking on flat and sloping ground.

Extensive areas of shallow regolith failures and many rock falls and disrupted rock and soil slides on moderate and steep slopes (20°-35° or greater), cliffs, escarpments, gorges, and man-made cuts.

Many small °to large (103-106 m3) failures of regolith and bedrock, and some very large landslides (106 m3 or greater) on steep susceptible slopes.

Very large failures on coastal cliffs and low-angle bedding planes in Tertiary rocks. Large rock/debris avalanches on steep mountain slopes in well-jointed greywacke and granitic rocks. Landslide-dammed lakes formed by large landslides in narrow valleys.

Damage to road and rail infrastructure widespread with moderate to large failures of road cuts slumping of road-edge fills. Small to large cut slope failures and rock falls in open mines and quarries.

Liquefaction effects widespread with numerous sand boils and water ejections on alluvial plains, and extensive, potentially damaging lateral spreading (fissuring and sand ejections) along banks of rivers, lakes, canals etc). Spreading and settlements of river stop banks likely.

MM 10 Landsliding very widespread in susceptible terrain (3).

Similar effects to MM9, but more intensive and severe, with very large rock masses displaced on steep mountain slopes and coastal cliffs. Landslide-dammed lakes formed. Many moderate to large failures of road and rail cuts and slumping of road-edge fills and embankments may cause great damage and closure of roads and railway lines.

Liquefaction effects (as for MM9) widespread and severe. Lateral spreading and slumping may cause rents over large areas, causing extensive damage, particularly along river banks, and affecting bridges, wharfs, port facilities, and road and rail embankments on swampy, alluvial or estuarine areas.

NOTES:

(1) “Some or ‘a few’ indicates that threshold for an effect or response has just been reached at that intensity. At less

than MM 6 shaking, landslide effects are insignificant in NZ and are therefore not included in this scale.

(2) Intensity is principally a measure of building damage. Environmental damage (response criteria) also occurs, mainly on susceptible slopes and in certain materials, hence the effects described above may not occur in all places, but can be used to reflect the average or predominant level of damage (or MM intensity) in a given area.

(3) Environmental response criteria have not been suggested for MM11 and MM12, as those levels of shaking have not been reported in New Zealand. However, earlier versions of the MM intensity scale suggest that environmental effects at MM11 and MM12 are similar to the new criteria proposed for MM9 and 10 above, but are possibly more widespread and severe.

(4) Appendix 3 is based on Hancox et al. 1997, 2002.



APPENDIX 2 – PIPELINE THREAT INVESTIGATION PROFORMA



APPENDIX 3 – SKETCH MAP SYMBOLS



APPENDIX 4 – SITE ASSESSMENT FORM

Date post:	05-Jul-2018
Category:	Documents
Upload:	truongthien
View:	216 times
Download:	0 times

FIELD GUIDE FOR THE IDENTIFICATION AND …extras.springer.com/2018/978-3-319-57774-6/PDF...

Documents