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GEOTECHNICAL INVESTIGATIONS OF TUNNEL SITES

NOTICE TO USERS OF THIS LESSON

The sole purpose for this reproduction is for nonprofit educational/teaching purposes; the lesson

forming only a very small part of a complete course of study on Tunneling and Underground

Construction Techniques.

A portion of this lesson is reproduced from "Tunnel Engineering Handbook", (Bickel, Kuesel &

King, 1996), to which all rights are reserved. No part of this lesson covered by their copyright

hereon may be again reproduced or used in any form beyond this lesson or by any means--

graphic, electronic, or mechanical, including photocopying, recording, taping or information

storage and retrieval systems beyond this lesson--without again obtaining the written permission

of the publisher, Kluwer Academic Publishers (formerly Chapman & Hall copyright).

(Tunnel Engineering Handbook, 1996, Editors, J.O. Bickel, T.R. Kuesel, & E.H. King, 2ND

Edition, Chapman& Hall(acquired by Kluwer Academic Publishers), pp 544.

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GEOTECHNICAL INVESTIGATIONS OF TUNNEL SITES

(Most of this lesson were taken from Geotechnical Investigations , H.W. Parker, Tunnel

Engineering Handbook, Chapman and Hall, 1996, pp 46-79.)

Why a Geotechnical Investigation

The dominance of Geology/Hydrology in influencing the site selection and the overall cost of

driving a tunnel cannot be over emphasized. Geology/Hydrology will affect every aspect of the

tunnel design, rate of development and cost. Yet the Geology/Hydrology of every project is

different. Therefore, just as in mining, we must do a proper job of exploration before we design

and develop a mine, in tunneling; we must do a proper job of exploration before we design and

develop a tunnel. The Geotechnical Investigation for tunnel construction could be a semester

course by itself. The areas of Geology/Hydrology that must be understood are:

Developing sufficient understanding of regional geology and hydrogeology for project

design and construction;

Defining the physical characteristics of the materials that will govern the behavior of the

tunnel;

Helping define the feasibility of the project and alerting the engineer and contractor to

conditions that may arise during construction for the preparation of contingency plans;

Providing data for selecting alternative excavation and support methods and, where

project status permits, determining the most economical alignment and depth;

Providing specific rock, soil and hydrogeologic design parameters;

Minimizing uncertainties of physical condition for the bidder;

Predicting how the ground and groundwater will behave when excavated and supported

by various methods;

Establishing a definitive design condition (geotechnical basis for the bid) so a changed

condition can be fairly determined and administered during construction;

Improving the safety of the work;

When project funds permit, providing experience working with the specific ground at the

project site through large-scale tests or test explorations, which in turn will improve the

quality of design and field decisions made during construction;

Providing specific data needed to support the preparation of cost, productivity, and

schedule estimates for design decisions, and for cost estimates by the owner and bidders.

Designing and building anything underground is infinitely more complex than something on the

surface. Why? Because your building material changes, sometimes every foot as you excavate

through it. On the surface, we can sample the quality of our material before we ever use it; we

can design it or make it to meet the specifications required for the design. But underground, we

take what the good Lord put there and try to use it as it is or reinforce it to meet the requirements

of our design. Usually we are successful: but sometimes we fail and our failures are never gentle.

Rather, they are often catastrophic.

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Thus there is always a degree of uncertainty in underground construction that is not present in

surface construction. But to those of us who have made this our lifes work, these are the

challenges that motivate us. Some of these challenges are:

There is vast uncertainty in all underground projects;

The cost and feasibility of the project is dominated by geology;

Every feature of geologic investigation is more demanding than traditional surface

foundation engineering projects;

The regional geology must be known;

Engineering properties change with a wide range of conditions, such as time, season, rate

and direction of loading, etc., sometimes drastically;

Groundwater is the most difficult condition/parameter to predict and the most

troublesome during construction;

Even comprehensive exploration programs recover a relatively minuscule drill core

volume, less than 0.0005% of the excavated volume of the tunnel;

It is guaranteed that the actual stratigraphy, groundwater flow, and behavior encountered

during construction will be compared with the geotechnical teams predictions.

When Do You Do The Geotechnical Work and When Do You Need the Information?

The Geotechnical work starts just as soon as someone gets serious about driving a tunnel. The

sooner the better. Furthermore, the Geotechnical work will continue throughout the project, and

even after the project is completed. These activities are shown in Table 4-2 p. 49*.

There is no intent here to try to train you to perform all of these task: rather simply it is to make

you aware of the fact that they must be done, and you as an engineer, must be able to take the

results of the geotechnical information and use it to design the tunnel and predict what progress

and cost will be the result of such studies.

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* In this lesson, when a page number is indicated in the lesson text, this is referring to your

Tunnel Engineering Handbook which is the text for this course.

Soil and Rock Classifications for Tunnels

Soil Classifications:

I will only go into the various classifications enough so that you can become familiar with the

nomenclature that is used and what it means.

Soils in the U.S. have adopted the Unified Soil Classification (UC) System, which is based on

the description of the soil particles themselves. (Table 4-4, p.52).

The classifications in itself doesnt give you any engineering parameters. You must determine these. For example, the parameters that are important for tunneling, are:

Soil mass strength;

Soil mass modulus and;

Soil mass permeability.

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The first of these, soil mass strength can be determined by the Standard Penetration Test (SPT),

which is a standardized method to estimate the relative density by driving the SPT sampler into

the soil deposit with a 140 -lb hammer. The relative density of the soil is roughly correlated to

the N value or blows per foot of the SPT as shown in Table 4-5, p53. Soil Modulus is the

stiffness of the soil mass, but unfortunately, it is not so easily obtained since any soil disturbance

in the sampling process affects the moduli.

Permeability is also a difficult parameter to measure since the results are also strongly dependent

on the non-disturbance of the sample. Therefore, sometimes it must be obtained by estimates

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based on grain size or other means, or measured in situ by simple water test between boreholes.

An approximate correlation between permeability and soil type and grain size is given in

Figure 4-1, p.53. But none of this is important unless we understand how the classification

shows up as a reaction to the soil to tunneling.

Heuer modified the original Tunnelmans Ground Classification system, first developed by

Terzaghi, then by Proctor and White, to what is shown in Table 4-6, p 54. This Table in itself is

useful, since it now gives us some terms that describe what we can predict will be the action that

will take place when a tunnel is put into these material.

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However, for dense silty sands, above the water table, Table 4-2, p. 49, better describes the

ground behavior.

Deere developed a classification (Figure 4-3, p. 54) which covers both above and below the

water table.

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To relate all of this to the strength of fine-grained soils, Schmidt developed the Figure 4-4, p. 55.

This will be used later in the course when we discuss tunneling in clay soil with a shield.

Rock Classifications:

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I will skip the geologic rock classifications that relate to Sedimentary, Igneous and Metamorphic

rocks, simply because you have had them before, and the classification in themselves dont lend

information to the later classifications which will help us design tunnels. However, two

classifications which will at least help us identify the rock that we are considering and how it

relates to other similar rocks are shown in Tables 4-10 and 4-11, p 55. These classifications

attach names to the conditions of the rock that should be followed in all geotechnical report

writing. i.e. if a rock has a UCS of 12,000 psi, you refer to it as a medium high strength rock.

Likewise, if the rock is totally discolored by weathering, but none of it is decomposed, it should

be referred to as slightly weathered.

When you are considering rock, you must realize that it isnt just the small sample that is a

measure of the strength of the material that you will be dealing with in a tunnel. You must

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learn to consider the rock fabric: from the smallest discontinuity, such as schistose

foliation, to jointing patterns, to major faults in the area. The significance of the

discontinuities in the rock mass is that they have an enormous effect on the engineering

properties of the rock mass and they all must be taken into consideration when designing

the tunnel. An attempt to put a name on the spacing and roughness of discontinuities is shown

in Table 4-12, p.56.

One must always be aware of the size effects of the sample or test as it relate to the full size of

the tunnel. A core cannot represent the discontinuities that are found in the tunnel. Likewise, a

10-foot pilot tunnel will not behave as a 30-foot final tunnel in highly jointed rock. Still each

test, no matter what the size gives some information. These tests along with geologic mapping

and good common sense will allow us to use other rock classifications that have been developed

through the history of tunneling, to go forward with a design that will probably work, most of the

time.

The first one to come up with a Rock Mass Description was Terzaghi in 1946. His original table

had the rock descriptions related to the size of steel [passive] supports that would be needed. But

while the steel supports that he related too have been somewhat replaced by other more affective

ground supports,[active] his rock descriptions are still worth studying (Table 4-13, p. 57).

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The single, most widely used term to relate rock mass strength is Don Deeres Rock Quality

Designation (RQD). It is used by nearly everyone, worldwide. I can remember when it was like

pulling a tooth to get geologic core loggers to record this simple piece of information so that the

engineer might have some information on rock mass strength. The minimum standard for

obtaining RQD are

Good drilling practices are carried out;

Minimum NX size core;*

Drilled with double-tube core barrel, generally no greater than 5-foot long;*

Count only pieces of core that are at least 4-inches long;

Count only pieces of core that are hard and sound;

Count only natural joints and fractures; ignore core breaks from drilling;

Log RQD in the field immediately after recovery, (preferably before the core is

placed in the core box) and before any deterioration.

*RQDs obtained without these minimum standards can still be useful. While they may exhibit

lower quality, they will demonstrate relative rock mass strength to each other.

An example of Deeres core logging procedure is given in Figure 4-6, p. 58. In addition, if the

rock is weathered, Moderately Weathered (W-3) should be marked with an *, and Highly

Weathered and worse should not be counted at all. A table which Deere assigns rock quality

names to equivalent RQDs is given in Table 4-14, p. 58.

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But as useful and as universal as RQD rating is, it falls short in many ways of truly quantifying

the rock mass. Others have developed other Rock Mass Classification systems, which use RQD

as one of the basic parameters, but also consider many other things. One of these is Bieniawskis

Rock Mass Rating (RMR) system.

RMR is a reasonable system, but one that is more universally excepted, particularly in the

tunneling business is Bartons Quality or Q-System (sometimes known as the Norwegian or

NGI system (Barton, et al, 1974). To obtain the overall Rock Mass Quality (Q), six parameters

to which he has assigned a numerical range, depending upon the observed condition of the rock,

are combined into an equation, the result of which may give a range of Q from 0.001 to 1,000.

This equation is:

Q = (RQD/Jn) x (Jr / Ja) x (Jw / SRF)

Where: Jn Number of Joint Sets

Jr Joint Roughness Number

Ja Joint Alteration Number

Jw Joint Water Reduction Factor

SRF Stress Reduction Factor

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Each of the terms in the equation relate to the physical significance of the basic components

listed. For example:

RQD/ Jn - Is the approximate relationship to the physical block size;

Jr/Ja - Is the effect of minimum inter-block shear strength;

Jw/SRF - Is the effect of active stress (i.e., effect of external forces on rock

mass, such as groundwater flow and in situ stresses).

The range of values for each of the parameters are shown in Table 4-20, p 62.

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Barton goes on to implement this relationship of Q to an effective span[in some of his

writings, he calls it Equivalent Dimension (De)] .

In order to relate their Tunnelling Quality Index Q to the behaviour and support requirements

of an underground excavation, Barton, Lien and Lunde defined an additional quantity which

they call the equivalent dimension De of the excavation. This equivalent dimension is obtained by dividing the span, diameter or wall height of the excavation by a quantity called the

excavation support ratio ESR.

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Hence:

De = Excavation span, diameter or height(m)/ Excavation Support Ratio (ESR) The ESR

values are shown in the Table below:

Table for ESR Values

The excavation support ratio is related to the use for which the excavation is intended and the

extent to which some degree of instability is acceptable. Suggested values for the ESR are as

follows:

Excavation category ESR

A. Temporary mine openings 3- 5

B. Vertical shafts,,

1. Circular section 2.5

2. Rectangular/square section. 2.0

C. Permanent mine openings, water

tunnels for hydro power (ex-

cluding high pressure penstocks)

pilot tunnels, drifts and head-

ings for large excavations. 1.6

D. Storage rooms, water treatment

plants, minor road and railway

tunnels, surge chambers, access

tunnels. 1.3

E. Power stations., major road and railway

tunnels, civil defense

chambers, portals, intersections. 1.0

F. Underground nuclear power sta-

tions, railway stations, sports

and public facilities, factories. 0.8

The ESR is roughly analogous to the inverse of the factor of safety used in the design of rock

slopes.

The relationship between the Tunnelling Quality Index Q and the Equivalent Dimension De of an

excavation which will stand unsupported is illustrated in figure 7. [Fig. 2-1 on a separate CD

prepared for this Course]

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The effective span is Bartons way of applying a safety factor to the application. He does this by

assigning an Excavation Support Ratio (ESR) number for various types of underground

construction, and then dividing the actual span by this number to obtain what he calls an

effective span. He relates his Q number to a Rock Quality name in Table 4-22 and finally, to

ground support requirements in Figure 4-7 p. 63. A similar Barton chart is shown in Fig. 2-2

[on a separate CD].

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The value of this classification is not just in being able to decide on the ground support method

that you will need when you tunnel through ground that has a given Q rating, but it is also a

means of establishing and agreeing in advance, between a client and a contractor what ground

support will be used in certain sections of the tunnel, providing of course that those conditions do

actually appear as predicted. The Fig 2-3 (Fig. 10.5.7, Barton et al, 1974)(on separte CD)

shows how it is applied to bolt spacing and shotcrete thickness. [It should be noted that Barton,

nor the NGI no longer recommend the use of WWF under any conditions, rather they recommend

steel fiber shotcrete.] If other conditions appear, then you have a changed condition which

warrant different ground support application, which warrant a different cost per foot of advance.

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Rock Borings, Sampling and Testing

This is not a course in Rock or Geological Engineering, so I dont intend to go into a great

amount of detail on this broad subject. But a few things to remember are worth going over.

Remember, unlike mining, most tunnels are close enough to the surface that good, detailed

geologic mapping of the surface, supplemented with aerial photos and satellite imagery can go a

long way in defining some of the structures that are likely to be encountered. However, the more

information that you get from core holes, the better. But spend your money wisely. This is

illustrated by Figures 4-8 and 4-9, p63.

The drill holes dont all have to pass through the same horizon of the tunnel level. It is more

important to identify and characterize all of the rock types that the tunnel will pass through, with

emphasis on the portal areas and any area on the surface that contains a valley (likely to be a

softer rock or a fault area).

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Another consideration, once the core is in the box, most of the expense is over. Now think what

information can be obtained from the remaining hole. Some of the things you might consider

are:

Bore-hole directional surveys.

Groundwater head or flow measurements, permeability tests;

Groundwater temperature and salinity measurements;

Rock structure studies using bore-hole or television cameras;

Rock physical property measurements using down-hole seismic velocity probes or

bore-hole deformation pressure cells;

Geophysical logging to locate permeable strata, changes of material in core loss

zones, voids, etc., using electrical resistivity , or gamma or neutron density logs;

Hole diameter calipering used in conjunction with core calipering;

There are various amounts and types of geotechnical information needed for different purposes

for which the tunnel will be used, different geological settings where the tunnel will be placed

and for different types of tunnel construction methods. These are given in Tables 4-25, 4-26 and

4-27, pp 66 and 67.

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22

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To go on with this, for planned TBM tunnel construction, there are even more than what is listed

above, Table 4-28, p 68, gives special requirements for more information.

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Geotechnical Work Monitoring the Tunnel Advance

Sometimes it may be advisable to install instrumentation to monitor and control tunnel

construction. Some of the purposes for this monitoring are:

To observe the behavior of the ground and ground water to confirm design

assumptions;

To confirm that specified influences have been achieved by the contractors;

To provide documented safety of construction methods and early warning of

potentially adverse behavior;

To provide data that point to likely cause(s) of adverse behavior, so that remedial

measures may be implemented.;

To provide data to assure adjacent property owners and the general public of

satisfactory construction behavior;

To confirm safety of innovative construction methods;

To provide facts on which future designs can be based to obtain greater safety and

cost-effectiveness;

To provide factual data for planning future phases or extensions of the project or

to provide direction for research;

To provide factual data for legal claims management;

To control construction, such as for NATM type contracts.

How Much Will Geotechnical Services Cost

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Investigations of past work have found that for reasonably deep or somewhat complex tunnel

jobs, when only 1% of the overall tunnel cost is spent for exploration, then the cost overruns

resulting from contractor claims ran about 12% of the original basic cost of construction. Data

have been plotted from studying 89 projects, 49 projects claimed large amount money after the

fact. These results are shown in Figures 4-14 and 4-15, p71. It was concluded from these

studies that an amount of 3% of the total construction cost was the most prudent amount to spend

to obtain the amount of information to adequately plan and design the tunnel for the geologic

conditions that exist.

The United States National Committee of Tunneling Technology provides the following

recommendations for site investigation prior to tunnel planning and construction (Table 4-31,

p 72.

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