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Horizontal Directional Ground Investigation – Reducing Tunnelling Risks by Minimising

Geological Uncertainty

CHRISTOPHER BROOK Geotechnical Engineer, Mott MacDonald Singapore Pte Ltd, Singapore

Christopher.brook@mottmac.com

Abstract

Geological uncertainty represents the key risk in tunnelling projects. Ground investigation for these schemes has

traditionally focussed in the sinking of vertical and inclined exploratory holes along the tunnel alignment allowing for

the interpretation of the geology between these points. The spacing of the exploratory holes is principally determined

by assessing the perceived geological variation between holes and the project specific tunnelling risks while bearing in

mind both programme and cost.

Recent advances in drilling technology have enabled an alternate method of ground investigation to be considered for

tunnels in rock. Horizontal Directional Coring (HDC) allows for a continuous core to be obtained along a particular

path at, or adjacent, to the axis of the future tunnel. A specially designed steerable core barrel using a system of

pressure controlled packers and adjustable drilling bits are used to either drill from ground level at an inclined angle

turning to horizontal or directly horizontally depending on topographic conditions. HDC is a highly accurate drilling

technology which can reduce the need to interpolate in ground conditions which are complex, highly varied or not

well understood. The remote nature of the drilling also negates potentially difficult logistical, safety and

environmental constraints which may obstruct or restrict a conventional ground investigation from being undertaken.

The use of HDC in tunnelling projects, whilst initially a seemingly expensive outlay, is considered likely to result in

more meaningful baselines being produced in GBR/GIBRs (where used), reduced contractor assumptions and

contingencies resulting in lower tunnelling costs, reduced contractors claims and minimised problems during

construction. In addition, the increased certainty in the ground may also reduce conservatism in the design.

This paper presents an overview of Horizontal Directional Coring, highlights the benefits and limitations of the

investigatory method, and provides several case studies from around the region with particular experiences from a

recent project in Singapore. Project consideration and challenges relating to HDC are also shared and the future of

ground investigation for tunnelling schemes discussed.

Key Words: Horizontal Directional Coring; Ground Investigation; Tunnelling, Measurement-Whilst-Drilling (MWD)

Introduction

The importance of proper and adequate ground

investigation in construction projects is no greater

than in underground construction and tunnelling

projects where the most significant risks are

associated with uncertainty in the ground.

The use of Geotechnical Baseline Reports (GBR), or

Geotechnical Interpretative Baseline Reports (GIBR),

is becoming more regular and owners of projects are

gradually moving away from the traditional allocation

of risk for unforeseen ground in preference of a

process where a contractor is able to submit a more

precise and competitive bid without contingency

factors. With these contractual statements regarding

the anticipated ground conditions there is therefore

an ever increasing need to be provide meaningful,

reasonable and realistic interpretations (ref 1).

Until recently, ground investigation for tunnelling

projects has been generally limited to vertical and

inclined boreholes above and adjacent to the future

tunnel alignment. In other areas, topography

precludes any substantial investigation being

undertaken along the alignment. Elsewhere,

horizontal coring (without directional control) has

been undertaken either from the side of a hill or from

a shaft ahead of tunnel construction although this has

been fairly limited in length as it has not been

possible to control the orientation and inclination of

the hole. Notwithstanding the limited length

horizontal core holes, generally the existing nature of

investigations provide only a pin-hole view of the

ground and significant interpretation of the geology is

required.

The amount of ground investigation that is required

ahead of construction stage of a tunnelling project

can vary hugely depending on the complexity of the

geology, tunnelling construction method, site setting

(e.g. urban, greenfield etc) and procurement route.

There are no set guidelines for the spacing of

exploratory holes on tunnel projects and each project

requires consideration on a case by case basis. More

often than not a phased iterative ground investigation

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is the most cost effective approach in order to obtain

adequate ground information. The ground

investigation must aim at accurately defining the

ground conditions and highlight and define the keys

ground related risks to the project

Horizontal Directional Coring enables a continuous

core (effectively a pilot tunnel) to be obtained along a

future tunnel alignment. This, in combination with a

reduced scope vertical borehole investigation and

testing, allows for a more comprehensive

understanding of the ground to be obtained. The use

of this technique in underground construction

projects, whilst initially expensive, is thought to be

hugely beneficial to a project and likely to result in

more meaningful GBR, GIBR baseline being produced,

reduced contractor assumptions and contingencies

resulting in reduced tunnelling costs, claims and

minimised problems during construction. In addition,

the increased certainty in the ground conditions can

reduced conservatism within design.

This paper gives on overview of the key aspects of

Horizontal Directional Coring and its application in

tunnelling projects. The key benefits and limitations

are highlighted and broad guidance on the design and

supervision of the works are given. Several case

studies are presented and discussion is given on the

potential advances in ground investigation for

tunnelling projects in the future.

Geological Uncertainty in Singapore

The geology of Singapore is highly variable over a

small area. Triassic granitic rocks of the Bukit Timah

Granite occupy the central spine and basement of

Singapore while the sedimentary rocks of the Jurong

Formation dominate the west of the isle with the

younger Old Alluvium in the east (Figure 1).

Figure 1. Geological Map of Singapore (DSTA, 2009)

It is widely appreciated that there is still some

considerable uncertainty regarding the geological

history of Singapore, the structural geology and

stratigraphy in relation to the Malaysian Peninsula.

Notably, the stratigraphy and structure of the folded

and faulted sedimentary rocks of the Jurong

Formation is poorly understood as are the geometries

and location of igneous intrusions and faulting within

the Bukit Timah Granite.

Singapore is now densely populated, with significant

tunnels associated with transportation, power, water

and sewerage etc, which are located at various levels

in near sub-surface. Singapore continues to develop

and new trenchless infrastructure must be positioned

at deeper levels of strata. This leads to new

challenges in tunnelling and indeed challenges for the

ground investigation industry. As technology

improves, ground investigation techniques must

evolve to allow for a better understanding of the

ground prior to underground construction.

Figure 2. Conceptual Ground model and a selection of risk

features in tunnelling in sedimentary rocks

Figure 2. shows an indicative ground model for a

tunnel project in layered highly weathered and

faulted sedimentary rocks including karst features. A

conventional borehole investigation for a tunnel is

shown with boreholes placed at set intervals along

the alignment and targeted boreholes at known

geological features of concern. The benefit of using

HDC in this situation as a supplementary investigation

is clear as a full appreciation of the risks associated

with the ground would not have been gained

otherwise.

Horizontal Directional Coring

Overview of Technology

HDC is an adaption to the conventional borehole

coring process and utilises much of the same

equipment as that to core a deep non-directional

borehole. The key to the technique is a specially

designed steerable core barrel mounted to a

conventional drill string attached to a powerful

drilling rig capable of drilling.

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HDC makes it feasible to control the direction of

coring in sub-vertical or horizontal boreholes along a

set alignment and investigate a linear tunnel scheme

within rock along or adjacent to the actual future

tunnel (See Figure 3 and 4).

Figure 3. Overview of HDC trajectory from ground level (HK AGS,

Ref 4)

For inclined holes, the works can be carried out from

a remote location negating potentially difficult

logistical and environmental constraints which may

obstruct or restrict a conventional vertical or even

inclined borehole from being undertaken.

Figure 4. Overview of HDC trajectory in topography

Horizontal Directional Coring should not be confused

with Horizontal Directional Drilling which is an

alternative technology that was born in the oil & gas

industry. Directional drilling is a relatively complex

technology and there are a number of ways to drill a

deviated hole (e.g. mudmotors with bent sub etc).

Horizontal Directional Drilling is the construction of a

drillhole without a core sample being taken - typically

the materials in which the hole has been constructed

through has been ground up during destructive

drilling often being recovered in the drilling mud as

chippings. HDD techniques are available for use in soil

and rock conditions and are frequently used in the

installation of pipelines where trenchless solutions

are required.

Directional Core Barrel

The first Horizontal Directional Core barrel was

developed in the 1980s by a Norwegian firm who

have continually improved the technology since and

lead the market in the development, manufacture

and use of HDC technology.

The directional core equipment consists of a long

wireline operated core barrel that replaces the

standard core barrel in steering sections. The core

barrel is operated under the same parameters as a

standard core barrel and requires no additional

equipment or significant adjustments to the drill rig

or drill string. The steerable core barrel was designed

to operate in hard, competent rock conditions.

The drilling trajectory is controlled by the toolface

angle while dog leg angle controls the curvature of

the corehole (Ref 4). A robust inflatable packer at the

rear of the core barrel maintains the toolface position

during steering. As with conventional wireline

systems the inner barrel of core can be removed from

the end of the borehole by lowering an overshot on

the end of a wireline into the hole. The overshot

attaches to the back of the core barrel inner tube and

the wireline is pulled back with the inner tube

disengaging itself from the barrel.

Figure 5. Schematic and photograph showing the working of the

proprietary HDC core barrel. (HK,AGS, Ref 4)

A corehole is typically undertaken using a

combination of conventional wireline drilling with

non-directional coring tools for straight line sections

with the specially design core barrel used in steerable

sections. When a change in direction is required as

determined through borehole survey (see below), the

directional barrel is preferentially modified. After the

steering correction is completed, normal straight line

drilling is resumed until another correction is

required.

The steerable core barrel yields a DV size core of

31.5mm in diameter (Photograph 1) and produces an

N sized (76mm diameter) hole. A conventional

wireline NQ core barrel obtains core of 47mm in

Planned tunnel trajectory

Exploration bore hole

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diameter in comparison (Photograph 2). The max.

bending angle is 9 deg/30m i.e. a radius of 180m. This

limit is partially due to limitations in the drilling

method but also in view of the flexibility and strength

of the drill string and the potential for breakage or

shearing to occur.

Photograph 1. DV size (31.5mm) core obtained in a deviated

section of the HDC

Photograph 2. NQ size (47mm) core obtained in a straight line

sections of the HDC corehole

It is noted that the proprietary HDC system detailed

above is not the only option for directional coring

investigation as a similar core (albeit non-continuous)

can be obtained using a combination of non-coring

directional downhole tools and conventional wireline

drilling technology.

Surveying

Locating the corehole is key to accurately controlling

the directional coring works and this must be

carefully managed throughout the operation.

Downhole survey tools are typically placed at the rear

of the steerable core barrel during drilling allowing

real time inclination and orientation data to be gained

which is feedback to the HDC engineer controlling the

works at the surface.

The corehole is typically surveyed at set intervals, and

as and when required, using a magnetic downhole

instrument. The survey tool measures inclination,

azimuth and orientation of the hole and allows for the

accurate determination of the ‘as drilled’ corehole

alignment. Other non-magnetic surveying tools, also

of high accuracy, are available for use in situations

where there may be background magnetic

interference. These tools are calibrated by the

manufacturing in a test hole of known position and on

site using conventional survey methods.

Corehole Considerations and Challenges

Technical Considerations

The following is a broad overview of the technical

considerations which should be given by the HDC GI

Drilling Contractor and the Engineer in the design,

specification, planning and construction of a long

directional corehole. For each project it is important

the aim of the investigation is clearly defined and

actions taken to ensure that these aims are, so far as

possible, met with during the investigation.

Alignment

The alignment of the HDC will depend on the project

specific requirements, the environmental settings,

the geology and the alignment of the future tunnel

alignment to which the investigation will relate.

As outlined earlier in Figure 3 & 4, depending on

topographic conditions an HDC corehole may either

commence on sidelong ground horizontally or from

ground level at an inclined angle in which the

corehole will need to drill through the soil overburden

before turning to horizontal in rock to meet the HDC

alignment at depth.

The position of the HDC corehole relative to the

tunnel alignment must be decided. As above, this will

depend on a number of factors but will focus on

addressing the major geological features and

potential construction risks associated with a

particular tunnel project. For example, should a

corehole be positioned at the axis level of the future

tunnel then it might be most representative of the

weathering conditions at this level while a corehole at

the invert of a tunnel could investigate the possibility

of encountering hard abrasive ground or cavities

which could impact the tunnelling works. Elsewhere,

for a mined tunnel a corehole in the crown of the

tunnel might be the most beneficial area to

investigate.

Whilst a corehole within the future tunnel face may

give the most representative view of the ground

conditions that the tunnel will be constructed through

it is recommended that this is avoided and the

corehole is offset from the tunnel face. There is a

potential for the HDC drill string to snap in the

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borehole which might lead to additional obstruction

risks being added to the tunnel project, further, if not

properly grouted upon completion, the corehole may

create a man-made pathway for water ingress to the

tunnel face.

Figure 6. Schematic potential HDC alignments relative to future

tunnel face

Drilling Tolerance Envelope

After designing the HDC alignment a drilling tolerance

envelop must be specified in which the Contractor

must stay within during the drilling. The tolerance

envelope is varied depending on a specific projects

aims and objectives although this must be balanced

with programme and cost. Typical tolerance

envelopes vary between 2m and 7m depending on

the size of the tunnel and the area of ground which is

of interest.

The tighter the drilling envelope the more frequent

the survey and steerable sections will be and the

slower the progress of the hole. The more relaxed the

tolerance envelope, the faster the drilling works will

be and the less frequent the surveys (Figure 7).

Figure 7. HDC design alignment and tolerance envelope and

schematic to demonstrate the typical working procedure for a

HDC corehole (modified after Ref 4)

The contractor is required to carefully manage the

corehole alignment within the tolerance envelope to

minimise tight radius turns which could stress the drill

string, increase the possibility of the breakage of the

drill string and increase the torque which may limit

the overall length of the hole.

The works need close supervising and real time data

should be available to all parties during the works.

Drilling Rig

The selection of an appropriate drilling rig is

important in a successful drilling operation. The

choice of rig depends on a number of factors

including:-

- Rig capacity

The drilling rig selected must be powerful enough to

reach the scheduled termination depth bearing in

mind the alignment and drilling tolerance

requirements. For very long coreholes the rig will

need a very high torque to deal with additional forces

put on the elongated drill string. For efficient drilling

the maximum speed of the drilling rig should be

appropriate to the most efficient RPMs to core in the

prevailing rock conditions. Other key rig parameters

include the capacity of the main hoist and wireline.

- Rig footprint

In some situations the environmental constraints will

dictate the use of a more compact rig. For

underground projects such as tunnelling, should

investigation be carried out during construction

compact underground rigs are available which can

have a footprint of as little of about 8m by 3m.

Elsewhere where site conditions allow more efficient

larger footprint rigs can be used.

- Pump capacity

A pump with sufficient volumetric capacity must be

used in order to provide flush down the drill string

and core barrel and remove the cuttings at a

sufficient velocity in the annulus between the hole

side walls and the drill string.

- Crew and training

An experienced drilling crew familiar with a particular

drilling rig will limit any learning curve where

programme is critical. A number of service and

training agreements are usually available from the

drill rig manufacturing. The efficiency of a drilling

team usually led by a drilling manager will have a

direct impact on progress, quality and health and

safety.

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- Support

It is paramount that the rig is properly maintained

and crucial spares are on hand during the drilling

works. This can reduce downtime in case of

breakages.

Drilling Tools

Drilling tools must be carefully selected. There are a

range of products for different ground conditions. A

carefully selected core bit can increase productivity,

the quality of core obtained and reduce wear on the

downhole tools For varied ground conditions many

difference tools are carried to optimise the works.

Drill String

A key risk in drilling operations is the possibility of

breaking or having drilling rods disconnect from each

other. Threads must be carefully looked after and

once damaged drill rods should not be used again.

Drilling Flush

The flush fluid is primarily for the cooling of the drill

bit but also to remove the cuttings from the base of

the hole back to the surface in the annular area

available between the wall of the borehole and the

drill string. The drilling mud used in operations will

vary from section to section depending on the

materials cored and other factors. It is paramount

that an adequate circulating system is in place to mix,

filter and recirculate drilling mud. In tricky drilling

situations it is important for the contractor to consult

with a suitable qualified mud engineer to optimise

the works.

Measurement-While-Drilling (MWD)

For a more complete geological record, a

Measurement-Whilst-Drilling (WMD) system should

be used. Through a drilling console (Photograph 3) or

data logger this enables real time drilling parameter

data to be viewed on site and the conditions in the

borehole more accurately determined. This is

particularly important when drilling in weak, faulted

or friable ground where it may be difficult to

reasonable obtain 100% core recovery. Multiple

drilling parameters are measured; thrust, torque,

flush pressure, rotation speed and drilling rate. These

results can be graphically presented against the

borehole records for further interrogation (Figure 8)

should it be required and allow for the best

interpretation of the ground to be made. The raw

data can be transferred to a PC via a USB which

downloads from the drilling console or data logger.

Photograph 3. Drilling console displaying RPM, fluid pressure,

rate of penetration and other drilling parameters

The data recorded, particularly the drilling rate, is a

direct reflection of the nature of the ground being

drilled. It is influenced by several factors including

strength, type and structure of the material being

drilled, the flushing media and pressure and other

drilling parameters. The widths of penetration rates

peaks and trough are particularly diagnostic and can

assist with the identification of, for example, voids

(e.g Karst landforms), fracture zones etc.

Figure 8. Graphical display of drilling parameters to assist in

interpretation of coreloss and other drilling events

In coring investigation drillers are required to balance

productivity with quality and wear on the drill bit they

are using and may adjust the drilling parameters

accordingly. For instance, in soft rock formations

where a harder matrix tool with large diamonds

should be used low RPMs may be most appropriate.

The recording of drilling parameters via MWD data

loggers is not only for rotary coring investigations,

they can also be used in boreholes using destructive

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methods, using a drag or tri-cone bit, or percussion. In

these applications a continuous recording can be

undertaken with parameters. When parameters are

kept constant, only the penetration rate will fluctuate

which will provide an indication of the ground that is

being drilled through.

Attempts have been made to correlate the specific

energy of drilling (Ref 8 & 9), a product of the main

drilling parameters and defined as the energy needed

to drill a determinant volume of rock, with rock

properties. This is outside the scope of this paper

however the author will be following further research

and development in this area as MWD systems

become more frequently used.

Insitu Testing

Insitu testing may be undertaken within a HDC

corehole like any other borehole with special

considerations required for effective insertion of the

tools.

Typical insitu in HDC coreholes for tunnels comprises

of permeability testing in the form of:-

- Lugeon packer tests, and

- Inflow tests.

It is also possible to gain core orientation information

either through using a core locking system, core

marking system or downhole impression packers.

Grouting

Extensive grouting of the borehole both during the

construction of the hole and upon completion are

likely in long directional coreholes where lost

circulation or borehole instability is a problem (see

more below). The grout mix and method will vary

depending on the situation, and the depth of

borehole.

Photograph 4. Set type grouting packer – a sacrificial packer

equipped with a non-return valve for pressure grouting

Impact of small diameter core

In steerable sections, the diameter of the core is

31.5mm, which is smaller than the recommended

minimum size for standard rock laboratory tests. Non-

standard laboratory tests can be undertaken but

consideration with respect to scale affects and

suitable correlations/ corrections are required.

The smaller diameter core and difference in diameter

of core between steerable sections and straight-line

sections also requires core logging to depart from the

norm. Deere (1967) stated that a core diameter of NX

size (54.7mm) is the minimum which Rock Quality

Designation (RQD) should be recorded. This was to

discourage the use of excessively small core

diameters which could yield artificially low core

recovery through drilling induced core breakages.

Since Deere’s original definition, ASTM indicate that a

minimum core size of NW is acceptable, presumably

to account for the improvements in drilling

technology at smaller diameters since 1967 and

acknowledgement that in good ground conditions

with good drilling technique high quality core

recovery can still be obtained. In general the larger

the diameter of core the higher the core recovery and

less disturbance is achieved in the recovered core.

Core obtained from horizontal directional coring

ground investigation is below the minimum core size

recommended for recording RQD. It is therefore

imperative that logging must be more carefully

carried out particularly in weak friable rock conditions

by experienced geologists. Drilling induced fractures

may be greater in DV size core and the core recovery

could be poorer than if a larger diameter core barrel

was used in the same ground conditions.

Challenges

During the course of a horizontal directional corehole

geological conditions may challenge a drilling

contractor from achieving his objectives of reaching

the termination criteria and recovering representative

samples of the ground. The following section outlines

these key challenges:

Lost Circulation

Lost circulation, more frequently known in the

geotechnical drilling industry as waterloss, is the loss

of drilling fluid to discontinuities or pore space in the

rock being cored resulting in reduced or complete loss

of drilling fluids and a lack of flush and cuttings

returning through the circulation system.

Zones through which lost circulation can occur are

coined thief zones and may be highly fractured rock

with persistent wide aperture joints such as shear or

fault zones or highly porous mediums such a clean

coarse grained sandstones. In very long holes

cumulative lost circulation may occur and it may be

very difficult to identify the key thief zones which

require treatment.

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Lost circulation is a serious problem in the drilling of a

deep or long boreholes and the detection and

treatment of this problem is difficult and requires

considerable experience to effectively and correctly

address. Without circulation, cuttings from the

borehole cannot be expelled from the bottom of the

hole or the annular space along the borehole. This

can lead to core sticking events at the core barrel or

increased torque on the drill string which if significant

could stick the drill string and even contribute to

breakages. Although coring can be still be undertaken

if sufficient water supply is available the lack of flush

return may also lead to an increased deterioration in

the down hole assembled. All the above can also

reduce the quality of core gain and the rate of

progression of the corehole - lost circulation is a

costly problem and must be mitigated.

Minor losses can sometimes be mitigated by

modifying the drilling mud composition either by

increasing the density of the bentonite or drilling

polymer being used at the time.

Major losses are more difficult to deal with and

attention must be given to the likely cause of the

losses, the width of thief zone, the aperture of

fractures or porosity of the material to correctly

implement the right mitigation which could comprise

of :-

- The use of Lost Circulation Material (LCM), or

- Grouting.

The decision with respect to the way forward must be

a combined decision between the senior geologist on

site and the drilling manager who together have the

necessary skills to accurately define the problem and

choose the most appropriate solution.

Lost circulation materials to mitigate lost flush

generally come in two forms; fibrous and granular

materials. These are added to the drilling flush and

pumped into the hole to plug the gaps in the fractures

and/or pores. Many products are available on the

market which are environmentally friendly and

suitable for various applications. Some granular

hydrophilic LCMs are available which swell in size

when in contact with water.

In significant thief zones where it is a necessity to seal

the hole completely and LCMs are ineffective,

grouting of a particular section of corehole may be

required.

Borehole Collapse / Borehole Closure

A serious concern to the completion of horizontal

directional core holes is the progression of the

corehole through soil, or weak and highly fractured

ground which may squeeze onto the drill string or

collapse into the hole leading to re-drilling or, at

worst, having the core barrel and drill rods stuck in

the borehole.

In areas of weak ground pressure grouting using a

downhole packer introduced earlier may assist with

stabilising the hole and enable the drilling to continue

safely. As above, casing is not an option and should

soil conditions be encountered significant problems

may occur.

Loss of Steering

HDC technology is only possible in competent rock

conditions. In soil it is not possible to steer the HDC

core barrel with any accuracy.

HDC Case Studies

An Undersea Directional Corehole in Singapore

Recently a 1193m long horizontal directional corehole

was completed within the Triassic Jurong Formation

in southwest Singapore in an area of restricted access

which precluded other means of investigation. This

enabled a continuous core to obtained along and

adjacent to a future tunnel project where limited

information was available from conventional ground

investigation techniques in an area where undersea

docking of TBM’s may be undertaken in the future.

The investigation targeted a portion of the tunnel

alignment where is was suspected that the ground

could be faulted and there was high potential for

deep tropical weathering profiles to exist.

The corehole was designed to follow on the outside

and at the axis level of the tunnel. This alignment was

chosen to gain the most representative information

without adding to future construction risks and to

ease the drilling works themselves. A 5m tolerance

envelope was specified – a product of the tunnel

diameter and perceived geological variation balanced

with achieving a very tight programme for the works.

The corehole was launched at an angle of 27 degrees

and was steered to horizontal to meet the design

alignment with an elevation of ~70m below Chart

Datum. The ground conditions typically comprised of

slightly to highly weathered rocks of the Jurong

Formation composed of sandstones, conglomerates,

mudstones and siltstones.

The works were undertaken on a 24hr basis in shift

patterns and were completed within a 3.5 month

period. The average advance rate was ~5m per shift

inclusive of alignment corrections, grouting events

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and in the treatment of lost circulation. The maximum

advance for a single shift was 22m. Core runs were a

maximum of 3m in length.

Photograph 3. HDC coring works in progress during a night shift

The drilling works were undertaken by an

experienced geotechnical GI contractor with

appropriate experience in the drilling of long length

directional coreholes. An Atlas Kopo U8 drilling rig

with an in-built Measurement-Whilst-Drilling system

and a wireline core-recovery system was used. The

works were carried out by a team of experienced

drillers. Specialist HDC engineers were on hand

throughout the investigation to control the hole

orientation and steerable sections. A combination of

DV size and NQ2 size core was obtained. This

approach balanced productivity rates, quality and the

wear on the tools. A total of 371m was cored with the

steerable barrel while 742m with a conventional

coring barrel, the balance was open hole drilled in the

soil overburden.

Surveying was undertaken at a maximum of every

20m with additional surveys undertaken as and when

required to ensure the hole stayed within the

tolerance envelope.

Zones of lost circulation were identified during the

drilling works. Both grouting and use of lost

circulation products were utilised to progress the hole

and keep the drillhole in a suitable condition for

obtaining high quality representative samples. The

works were closely monitored by the Engineer.

Faults/ shear zones were encountered during the

investigation and drilling parameters obtained using

the Monitoring –Whilst –Drilling system were

paramount in the best interpretation of faulted zones

where core loss events occurred.

As with many tunnel projects, a project risk register

was developed for the HDC borehole (effectively a

micro tunnel in itself) prior to the investigation which

was updated and kept live throughout the project.

During construction this was owned by the Contractor

and was regularly reviewed by the team. This enabled

a clear understanding of risks and control measures

to be developed between the various parties on the

project and to ensure that best practice was being

followed.

In addition to the GI Contractors conventional

engineering geological descriptions in accordance

BS5930:1999, the consultant also undertook detailed

sedimentological logging of the core samples to gain a

better understanding of the structural geology along

the future tunnel alignment. Geological features

within and at the top and bottom of bedding were

noted which enable for a comprehensive

understanding of the stratigraphy and structure to be

developed which otherwise would not have been

possible from the standard engineering geological

logs.

The investigation allowed for a detailed

understanding of the ground conditions along the

tunnel alignment to be developed, shear zones to be

identified and the overall structure of the prevailing

geology to be ascertained with accuracy.

The completed borehole trajectory is shown in Figure

9.

Figure 9. As drilled profile of the HDC corehole

Other selected case studies

Table 1 provides a summary of selected HDC projects

undertaken within the region with details available in

the public domain.

Tunnelling and Underground Construction Society (Singapore) Hulme Prize Competition 2013

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Table 1. Selected summary of details of completed HDC ground

investigation projects within the region.

Key Advantages and Limitations

Advantages

The advantages of using HDC for ground investigation

of tunnel route are clear, providing a continuous

record of the ground conditions along the tunnel

alignment prior to construction. Gaining this

information will reduce the geological uncertainty

and risks associated with unforeseen ground

conditions by negating the need to interpolate

between stratum boundaries. HDC can be undertaken

remotely which is particularly advantageous in areas

where there is limited or no surface access to

conventional borehole positions such as beneath

environmental sensitive areas, busy waterways and

roads or even beneath buildings.

As consequence to the increased certainty in the

geology conditions it is considered that this could

further result in:-

- More meaningful and accurate GBR & GIBR

baselines being prepared;

- Reduced contractor assumptions and

contingency resulting in lower tunnelling costs;

- Reduced contractor claims;

- Minimised problems during construction;

- Greater confidence in risk, programme and

costs, and

- Reduced conservatism in design.

Limitations

The drilling and steering of small diameter rock coring

bit has certain inherent limitations which must not be

overlooked when giving consideration to the possible

use of the method. These limitations relate to the

geological conditions in which the coring may feasibly

take place and some initially reasonable

understanding of the likely ground conditions is

required.

A key limitation in the use of HDC is the expense and

difficulties in being able to demonstrate tangible cost

benefits from the use of HDC. A key challenge for HDC

is the changing of mindsets from what was done in

the past and worked to align with the improved

technology and the possible benefits.

As an emerging technique unfamiliar to many it is a

necessity for the works be specified and supervised

by experienced consultant and undertaken by a

suitably qualified GI Contractor with past experience

in similar works. Should an inexperienced contractor

undertake the works and with poor or no supervision

there is a considerable risk that the challenges and

considerations described in this paper might not fully

be appreciated. This could lead to early termination

of the corehole hole or worse, poor un-

representative, core samples being recovered which

could mislead consultants and contractors into

thinking the ground is worse than it actually is.

Conclusions

The use of Horizontal Directional Coring in association

with Measurement-While-Drilling technology,

undertaken by an experienced drilling contractor with

careful supervision and recording can enable for a

comprehensive picture of the ground conditions

along a particular tunnel route to be gained even in

the most varied rock conditions. Geological risks

which would have been unlikely to have been

uncovered in conventional investigations could be

identified.

It is in an owner’s best interest to invest in an

effective and adequate ground investigation – in

certain situations the technical benefit of horizontal

directional coring in complex, varied or constrained

environments in rock are clear and it is the author’s

opinion that these might outweigh the added cost at

the outset of the project.

Future Visions / Improvements

Tunnelling technology and practices continue to

improve and develop in order to mitigate and reduce

risks, make cost savings, and speed up construction.

The ground investigation industry must also respond

by moving with technology and in so doing potentially

having a major positive impact on the tunnelling

industry.

As underground space in urban settings becomes

more constrained the need to go deeper is inevitable,

which will likely mean that underground works will be

Project Name Description Length(s) Ground

Conditions

Comments

Eagles Nest Road

Tunnel comprises of

two, three-lane

tunnels – Hong Kong

2No directional

coreholes from

each side of the

mountain

516m &

1151m

Granite Coreholes

positioned at

axis level of

tunnels in-

between each

running tunnel

Jurong Island Rock

Cavern – Singapore

6No directional

coreholes

Total length

of 2400m

Longest

singular HDC

~600m

Siltstones,

Mudstones

and

Sandstones

HATS2A deep sewer

project – Hong Kong

6No directional

coreholes

Total of

5000m

Longest

singular HDC

~1250m

Understood

to be of

Granite,

Tuffaceous

To identify

location and

width of fault

zones

Tunnelling and Underground Construction Society (Singapore) Hulme Prize Competition 2013

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located in rock conditions. It is the author’s opinion

that in Singapore and elsewhere around the world, as

has occurred in Hong Kong, deeper tunnel projects

will utilise HDC as a way to better understand and

define ground related risks. This in turn will force

improvements in the existing technology.

One area of development which requires further

consideration by the industry is the investigation of

ground along tunnel alignments in mixed face or soil

conditions. In the US, horizontal directional drilling

and soil samplers have been successfully used in

tandem in geo-environmental projects (Ref 5). For

geotechnical applications there will be always be

problems in gaining representative and undisturbed

samples in all but the softest of ground conditions.

The use of destructive (non-sampling) horizontal

directional investigatory solutions may also have their

applications both in soil and rock through increased

use of Measurement-Whilst-Drilling technology and

the possibility of correlations to define rock and soil

strengths and other properties. These technologies

could also be used together with Horizontal

Directional Coring in tunnel alignments where there is

both rock and soil.

Improvement to Ground Investigation for Tunnels in

Singapore

In Singapore, it is hoped that the ground investigation

industry for tunnelling projects may gain significantly

from the application of HDC to supplement and

enhance geotechnical data for complex tunnelling

projects. By proactive consideration and management

of the challenges associated with deep/long

coreholes, it is considered that risks from tunnelling

works can be better evaluated.

References

1. J, B, Longbottom. Geotechnical Baseline

Reports – Their Use & Abuse in Hong Kong.

ADR Digest Issue 13 Spring 2011.

2. R, Lindhjem & C, Tai. Directional Core Drilling in

Tunnel construction. World Tunnel Congress

2008 – Underground Facilities for Better

Environment and Safety – India.

3. M, P, Chan. The Use of Horizontal Directional

Coring Technique for Ground Investigation of

Tunnelling Projects in Hong Kong. Masters

Dissertation. June 2007.

4. Association of Geotechnical &

Geoenvironmental Specialists (Hong Kong).

Ground Investigation Guidelines – 04.9 –

Horizontal Directional Coring.

5. E, N Allouche, S, T, Ariaratnam, K, W Biggar & J,

Mah. Horizontal Sampling: A new direction in

characterisation. Trenchless Technology. Res.

Vol.12, Nos. 1-2, pp. 17-25, 1998

6. B, B, Bramble & M, T, Callahan, Construction

Delay Claims, Third Edition, pages 2-46

7. Underground Technology Research Council

(UTRC), 2007, Geotechnical Baseline Reports

for Construction, Technical Committee on

Geotechnical Reports of the UTRC, American

Society of Civil Engineers.

8. R, Teale, 1964, The concept of Specific Energy

in Rock Drilling. Rock Mechanics Mining

Science, Vol 2, pp 57-73.

9. B, Celada. The use of the Specifc Drilling Energy

for Rock Mass Characterisation and TBM

Driving During Tunnel Construction.