GEOTECHNICAL ENGINEERING

ECG 503

CHAPTER 2: SLOPE RISK ENGINEERING

Learning outcomes:

At the end of this chapter, the students should be able to:

Differentiate between natural slope and made engineered soil slope

Discuss the role of suction in slope stability

List effective stress method in slope stability analysis

Explain the factors that influence the slope stability

Discuss the types of mass movement

Describe the indication of slope movement and slope monitoring system

Explain slope face protection method

Describe slope rehabilitation works

Differentiate between guniting and grouting

Describe the advantages of using geotextile in slope improvement works

TOPIC TO BE COVERED

Slope rehabilitation works

Post mortem analysis, rehabilitation technique such as soil nailing, grouting, engineered slope stability structures

FORENSIC WORKS

SITE INVESTIGATION (S.I.)

SLOPE FAILURE TYPES

SLOPE FAILURE CAUSES

SLOPE REHABILITATION WORKS

SITE INVESTIGATION

Slope Mapping Site survey by visual aspect Ground water observation Indication of geological materials Drainage system observation Water ponding Site Testing

SITE INVESTIGATION cont’

Laboratory Testing Preparation of S.I. Report

SLOPE FAILURE TYPES

Sliding Fall

Erosion Toppling Planar

Wedge

SLOPE TREATMENT Short-Term

a. Tarpaulin Long-Term

a. Turfing f. Masonary Wall

b. Guniting g. Sheet Pile

c. Gabions h. Rock Bolts

d. Cribwall i. Netting

e. Concrete Wall j. Soil Nails

Tarpaulin

Soil Nailing In Slopes Stabilization Proven cost, used to cut slopes or to support

deep excavation by reinforcing the insitu ground. (Drilling, grouting + inserting bar, and nail head construction/ facing/ guniting.)

Pull-out resistant of the reinforcing element. Practice : estimated based on soil data with

reference to empirical observations.On site : pull-out test result derived from full

scale verification/ sacrificial tests.

Soil Nailing In Slopes Stabilization Some slope movement is required to mobilize

load tension in the soil nail (up to 30mm).

Lateral displacement due to stress-relief of excavated steep soil nailed slope/wall is about 0.1-0.3%H.

When lateral deformation exceed 0.5%H, excessive bending and shear in soil nails may happen, resulting in excessive creep and tension cracks in upslope or eventual slope failure, if left unattended.

Soil Nailing In Slopes Stabilization

Soil nails need to extend to sufficient length beyond the active zone or any plane of weakness to overcome external stability including overturning, sliding, bearing and overall slope instability, modes of failure.

Three internal failure modes must be checked to ensure an adequate factor of safety, i.e. nail pull-out resistance, nail material tensile capacity and nail head/ facing capacity.

Drilling Method

Characteristics: Constant straight diameter Stable drill hole Drilling debris wholly Cleanly removed are achievable.

The rotary percussion method Using top hammer or down-the-hole hammer Capable of completing the drill hole within a short time

(<1hour) should normally used.

Drilling Method

Observation of the drill log Location Time/ duration Soil type/ strata Changes in penetration rate/ sound Flushing characteristics (wetness, color, nature and

sizes of cutting…)

SOIL NAILING AND ROCK BOLT

Appropriate in many cohesive soils or fragmented rock Process:

1. Holes are drilled into the excavated face. Typically, holes are angled at 15 degrees below horizontal.

2. The hole is pumped full of ready-mixed grout soon after drilling to ensure the hole remains open.

3. Nails, generally continuously threaded steel bars, are long enough to penetrate the failure plane of the excavation, and are inserted immediately following grouting. The nails are equipped with centralizers to ensure central placement in the grouted hole.

4. The concept is to stabilize the soil by creating a grouted mass that the surrounding soil will act upon in friction. The grout also provides corrosion protection for the nail.

5. Once the grout sets, the protruding nails are fitted with a steel plate to transfer forces from the wall to the nail.

Soil Nailing in Slopes Stabilization

Installation Of Reinforcement Bar

For permanent works, rebars should be protected against corrosion by hot-dip galvanizing (BS729) with a minimum coat thickness of 85microns or 610gm/m2.

Rebars only need to be protected in corrugated HDPE stealth for cases of proven aggressive soil (pH value <4.5; or sulphate >200ppm; or chlorite content >100ppm.)

Installation Of Reinforcement Bar

Centralizers should be made from galvanized steel or other suitably high quality material dimensioned to fit the rebar at the centre of the hole whilst not obstructing passage of cement grout pipe.

Nail Head

Guniting facing

HDPE Geocell slope

protection system

Grid beam system

For green vegetative slope landscape

Different between grouting and guniting Grouting: Chemical or cement grout is useful for

embankments which is made up of relatively loose material and by the introduction at the surface soils to fill the void will have lowered the permeability sufficiently that water is subsequent excluded from that area giving further improvements in strength and stability.

Guniting is protecting the loose soil surface with a thin layer of reinforced concrete which is to protect the slope surface totally from erosion and infiltration of water.

CASE STUDY

SLOPE FAILURE AT KOMPLEKS IMIGRESEN,

PENGKALAN HULU, PERAK DARUL RIDZUAN

WHAT WOULD YOU DO?

?

SITE INVESTIGATION

INTRODUCTION

The proposed site of construction for Kompleks Imigresen Pengkalan Hulu is located at the boundary of Thailand and Malaysia, about 60 km north of Grik town in the upper Perak region, through access road to Betong, Thailand.

The proposed site area is experienced slope failure and high water table problems. The landslide had occurred at the slope, close to proposed construction of Kompleks Imigresen Pengkalan Hulu

OBJECTIVE

The objectives of works to be carried out are : To understand subsurface condition

and geology at study area To understand type of slope failure. To determine probable causes of failure. To propose mitigation measures. To obtain soil type, soil strength

parameter for slope stability analysis.

SITE LAYOUT

GEOMORPHOLOGY OF LANDSLIDE AREA

The original geomorphology of Pengkalan Hulu and surrounding area is characterized by mountainous terrain. Hills and valley in this area can be clearly seen from the digital terrain model (DTM) generated from the topographic map of the area

GEOTECHNICAL PARAMETERS

LOCATION OF BOREHOLES AND PROBE MACHINTOSH TO DETERMINE THE SOIL

PROFILE AND SOIL STRATA

DETERMINATION OF FOS

MOST PROBABLE CAUSE OF FAILURE

Presence of Geological Weak Material The presence of Carbonaceous Shale probably mainly due to high organic carbon content. The Carbonaceous Shale are argillaceous deposits containing carbonaceous matter that induces a dark colour, between grey and black depending on the concentration of the carbonaceous matter. This is an extremely fine-grained rock consisting of quartz, sericite, chlorite, feldspar, clays and carbonaceous materials. This rock seems brittle, slickenside at the plain , dull luster and its recumbent fold.

Increase in negative Pore Water Pressure

Based on the data obtained from the investigation and analysis it was observed that the most probable cause of failure is due to extensive increase in negative pore water pressure in the soil. The increase in this negative pore water pressure will reduce the shear strength of the soil thus weakening the integrity and stability of the cut slope. The indication of seepage water from the ground can be seen after the failure which resulted in flow of water from the slope surface.

Improper Drainage System

Improper drainage system on the cut slope area may resulted in the penetration of runoff water thus increasing the groundwater table in this area. Some of the drainage system in this area seems to be not functioning as required to channel the runoff water to its outlet. Proper subsoil drainage has not been installed on site to lower down the groundwater table in this area.

Characteristic and Weathering Process of Subsoil

The occurrence of weak zone in the subsoil material which is highly decomposed as in contact with water may have its contribution to the cut slope failure. High groundwater table has weakening the subsoil material thus creating a weak layer to trigger the slip circle failure. Completely Weathered (Grade V) of interbeded shale and mudstone layer are very sensitive with water. The occurrence of water in this layer may weaken the characteristic strength of this material.

MECHANISM OF FAILURE Initially the cut slope was cut at a gradient of 1V : 1H with

a 6m high slope and a 1.5m benches. Berm drain and cut off drain was constructed to channel out runoff water in this area. Hydroseeding was implemented to the slope surface however less than half of the surface areas were vegetated. This is due to the nature of the subsoil material which unable vegetation to growth on the soil.

As the drainage system in this area are poorly constructed some of the drains has collapsed which enable runoff water to penetrate into the subsoil material.

Circular type of slip failure occurred which then moves in retrogressively starting from the crest of the top slope downward to the toe of the slope. The increase in negative pore water pressure, which leads to weakening of the soil strength characteristic, has lead to the slip failure on the cut slope.

REMEDIAL AND MITIGATION MEASURES

• Short Term Measures

Short term measures shall be carried out immediately to prevent slope failure area from getting worst. Tarpaulin sheet should immediately installed at all failure area to prevent further increase in negative pore water pressure to ensure the failure from propagate.

• Long Term Measures

Soil Nail, Allan Block 12 Wall and Green Terramesh Reinforced Earth Slope

This comprises of installation of soil nail made up of a 12m long Y25 bar inserted into a 100mm diameter drilled hole which in turn will be grouted to the full length. The spacing of the soil nail is proposed to be at 1.5m c/c interval. The soil nail will increase the existing factor of safety of the cut slope by providing resistance to the overturning forces. Horizontal drain of 75mm diameter perforated UPVC pipe of 12m length shall be installed to reduce the negative pore water pressure thus reducing the groundwater along this area.

Green Terramesh system was to be installed on the front face of the cut slope which has been installed by soil nail. The installation of Green Terramesh will increase the internal stability and provide aesthetical features to the finishes of the cut slopes. Allan block wall will also be installed on the front face of the cut slope at the bottom portion as will increase the internal stability of the cut slope. The drainage system within the area shall be upgraded to ensure the surface erosion will not take place after the treatments implemented. A new drainage system will be introduced to integrate with the existing drainage system.

ALLAN BLOCK

GREEN TERRAMESH

UPGRADING DRAINAGE SYSTEM

ALLAN BLOCK

GREEN TERRAMESH

GREEN TERRAMESH

TUTORIAL for test 1