Elsevier Encyclopedia of Geology

Подождите немного. Документ загружается.

54 mm in diameter, with a thickness that is approxi-

mately equal to the radius of the specimen. The tensile

strength of the specimen is obtained as follows:

¼ 0:636P=DH ½9

where H is the thickness of the specimen. The Brazil-

ian test is useful for brittle materials but for other

materials it may give erroneous results.

In the point load test the specimen is placed be-

tween opposing cone-shaped platens and subjected to

compression. This generates tensile stresses normal to

the axis of loading and the indirect tensile strength

) is then derived from:

¼ P=D

½10

Loading can take place across the diameter of the

specimen, as in Figure 12, or along the axis. Rocks

that are anisotropic should be tested along and paral-

lel to the lineation. Irregular-shaped specimens can

also be tested, but at least 20 tests should be made

on the same sample material and the results averaged

to obtain a value. The point load test is limited to

rocks with uniaxial compressive strengths exceeding

25 MPa (i.e., point load index above 1 MPa).

The effect of the size of specimens is greater in

tensile than compression testing because in tension,

cracks open and give rise to large strength reductions,

whilst in compression the cracks close and so disturb-

ances are appreciably reduced. This is especially the

case in the axial and irregular lump point load tests.

Accordingly, a standard distance between the two

cones of 50 mm has been recommended, to which

other sizes should be corrected by reference to a

correction chart. Once determined, the point load

index can be used to grade the indirect tensile strength

of rocks, as shown in Table 4.

Finally, in the flexural test a cylindrical specimen of

rock is loaded between one lower and two upper

supports until the sample fails. The flexural strength

gives a higher value of tensile strength than that

determined in direct tension.

Durability of Rocks

Durability refers to the resistance that a rock offers to

the various processes that lead to its breakdown and

therefore durability tests can be used to provide a

general impression of how a rock will behave in rela-

tion to weathering, especially mechanical weathering.

Durability tests most frequently are used to assess the

behaviour of suspect rocks, that is, those that tend

to breakdown more readily such as mudrocks, some

chalks, and certain basalts and dolerites. There are a

large number of tests that have been used to assess the

durability and many of them are used to determine

the durability of rock as a material for building pur-

poses, for aggregate, or for armourstone. The latter

types of test are not dealt with here.

Some of the more simple tests include the water

absorption test, the wet and dry test, the freeze-thaw

test, and soak tests. The water absorption test in-

volves oven drying a rock specimen at 105



C until

it has attained a constant weight and then saturat-

ing it under vacuum. The percentage saturation is

determined and reflects porosity. As rocks break

down, their porosity increases and so the water ab-

sorption test has been used to indicate the degree of

Figure 11 (A) Graph derived from shear test, showing curves derived for both peak and residual strength. (B) Other forms of shear

strength tests.

ENGINEERING GEOLOGY/Rock Properties and Their Assessment 575

weathering a rock, especially a crystalline rock such

as granite, has undergone. In other words, is it fresh,

slightly weathered, moderately weathered, or high-

ly weathered. However, some moderately and highly

weathered rocks may break down before becoming

saturated. Similarly, in the wet and dry test the rock

specimen is first dried and then saturated, but this

time for a given number of cycles. The effect that

cyclic wetting and drying has on the specimen is

recorded (i.e., no effect, softening, minor spalling,

minor hairline cracking, severe hairline cracking,

breakdown before last cycle noting cycle number).

In the freeze-thaw test the specimen is saturated and

then frozen for 24 h. The specimen is subjected to a

given number of cycles and the effects are recorded in

the same way as for the wet and dry test. The freeze-

thaw test has been used to assess the frost resistance

of building stone. However, it is no longer used for

this purpose in Britain, it being regarded as unsatis-

factory primarily because of the difficulty of inter-

preting the results in relation to a period of time

over which a rock will perform as required. Soak

tests are used to assess the breakdown of rocks as a

result of swelling brought about by the absorption of

water, especially of those rocks that contain swelling

minerals. The rock specimen either may be soaked in

water or ethylene glycol (CH

OH)

for a given

number of days. Ethylene glycol is much more effect-

ive than water as far as assessment of those rocks

that contain swelling minerals are concerned. The

soak test allows five classes of rock disintegration

and the time when the worst condition occurs to be

recognized:

Degree of disintegration:

Class 1 : No obvious effects, or only very minor

spalling of sand-sized particles.

Class 2 : Flaking and/or swelling.

Class 3 : Fracturing without extensive spalling.

Class 4 : Fracturing with extensive spalling.

Class 5 : Complete disintegration.

Time required to reach worst condition:

Class 6 : 0–1 day

Class 5 : 2–3days

Figure 12 Point load test apparatus.

Table 4 Point load strength classification

Description

Point load

strength

index (MPa)

Equivalent uniaxial

compressive

strength (MPa)

Extremely high strength Over 10 Over 160

Very high strength 3–10 50–160

High strength 1–3 15–60

Medium strength 0.3–1 5–16

Low strength 0.1–0.3 1.6–5

Very low strength 0.03–0.1 0.5–1.6

Extremely low strength Less than 0.03 Less than 0.5

576 ENGINEERING GEOLOGY/Rock Properties and Their Assessment

Class 4 : 4–10 days

Class 3 : 11–15 days

Class 2 : 16–20 days

Class 1 : 21–30 days

Class 0 : Over 30 days

An index value can be derived from the integration

of the degree of disintegration and time taken to reach

the worst condition. In other words, a scale of relative

durability values can be developed, ranging from 1 in

which specimens are more or less unaffected to 11

when they are totally disintegrated (Table 5). The

ethylene glycol soak test has been used to distinguish

certain unsound basalts and dolerites, commonly

described as slaking types, from sound types,

particularly in relation to their use as road aggregate.

Slaking refers to the breakdown of rocks, especially

mudrocks, by alternate wetting and drying. If a frag-

ment of mudrock is allowed to dry out, air is drawn

into the outer pores and high suction pressures de-

velop. When the mudrock is next saturated the en-

trapped air is pressurised as water is drawn into the

rock by capillary action. This slaking process causes

the internal arrangement of grains to be stressed.

Given enough cycles of wetting and drying, break-

down can occur as a result of air breakage, the pro-

cess ultimately reducing the mudrock to gravel-sized

fragments. The slake-durability test estimates the re-

sistance to wetting and drying of a rock sample, and is

particularly suitable for mudrocks and shales. The

sample, which consists of 10 pieces of rock, each

weighing about 40 g, is placed in a test drum, oven

dried, and weighed. After this, the drum, with

sample, is half immersed in a tank of water and

attached to a rotor arm, which rotates the drum for

a period of 10 min at 20 revolutions per minute

(Figure 13). The cylindrical periphery of the drum is

formed using a 2 mm sieve mesh, so that broken down

material can be lost whilst the test is in progress. After

slaking, the drum and the material retained are dried

and weighed. The slake-durability index is then

obtained by dividing the weight of the sample retained

by its original weight, and expressing the answer as a

percentage. The following scale of slake-durability is

used:

under 25%, very low

25–50%, low

50–75%, medium

75–90%, high

90–95%, very high

over 95%, extremely high.

However, it has been suggested that durable mudrocks

may be better distinguished from non-durable types

on the basis of compressive strength and three-cycle

slake-durability index (i.e. those mudrocks with a

compressive strength of over 3.6 MPa and a three

cycle slake-durability index in excess of 60% are

regarded as durable). In fact, the value of the slake

durability test as a means of assessing mudrock dur-

ability has been questioned as the results obtained

frequently do not compare well with those of other

durability tests. This has led to a number of adapta-

tions being made to the test, for example, ethylene

glycol has been used instead of water, the time taken

to carry out the test has been extended and the number

of cycles has been increased.

Failure of weak rocks occurs during saturation

when the swelling pressure (or internal saturation

Table 5 Ethylene glycol soak test index values

Degree of

disintegration class

Time class

0–1 day

2–3 days

4–10 days

11–15 days

16–20 days

21–30 days

Over 30 days

1 765432 1

2 876543 2

3 987654 3

4 1098765 4

5 11109876 5

Figure 13 Slake-durability test apparatus.

ENGINEERING GEOLOGY/Rock Properties and Their Assessment 577

swelling stress, s

) developed by capillary suction pres-

sures, exceeds their tensile strength. An estimate of s

can be obtained from the modulus of deformation (E):

E ¼ s

½11

where e

is the free swelling coefficient. The latter is

determined by a sensitive dial gauge recording the

amount of swelling of an oven-dried core specimen

per unit height along the vertical axis during saturation

in water for 12 h, e

being obtained as follows:

change in length after swelling

initial length

½12

A durability classification has been developed based on

the free-swelling coefficient and uniaxial compressive

strength (Figure 14).

When the free expansion of rocks liable to swell

is inhibited, stresses of sufficient magnitude to cause

damage to engineering structures may develop. A

number of tests may be conducted that provide an

indication of the swelling behaviour of such rock.

These include the swelling strain index test performed

under unconfined conditions, and the swelling pres-

sure index test, carried out under conditions of zero

volume change. Swelling strain measurements can

also be undertaken on radially confined specimens

under various conditions of axial loading. The swell-

ing strain represents maximum expansion of an un-

confined rock specimen when it is submerged in

water. Test specimens may either be oven-dried or

retain their natural moisture content. As a test speci-

men is submerged in water it is advisable to prevent

rock prone to slaking from collapsing before the ul-

timate swelling strain has developed by wrapping the

specimen in muslin. During the test the specimen is

supported by a frame between a fixed point and a

measuring point. The latter may consist of a dial

gauge or a deformation transducer. The specimen

and frame are placed in a container that is filled

Figure 14 Geodurability classification chart (After Olivier HJ (1979) A new engineering-geological rock durability classification.

Engineering Geology 4: 255–279).

578 ENGINEERING GEOLOGY/Rock Properties and Their Assessment

with water just above the level of the specimen. Strain

is measured in all three perpendicular directions. No

strain is allowed to develop in the test specimen

during the swelling pressure test. This is accom-

plished by tightly mounting a cylindrical specimen

inside a rigid ring that provides radial constraint.

The specimen absorbs water through porous end

plates. Any axial strain that develops is monitored

and compensated for by increasing the axial load.

The stress required to prevent expansion when equi-

librium conditions are established is equal to the

swelling pressure index.

Permeability

Permeability considers the ability of a rock to allow the

passage of fluids into or through it without impairing

its fabric. In ordinary hydraulic usage, a substance is

called ‘permeable’ when it permits the passage of a

measurable quantity of fluid in a finite period of time

and ‘impermeable’ when the rate at which it transmits

that fluid is slow enough to be negligible under existing

temperature-pressure conditions. The permeability of a

particular rock is defined by its coefficient of permea-

bility or hydraulic conductivity. Grades of permeability

are given in Table 6.

Determination of the permeability of many rock

types in the laboratory is made by using a falling-

head permeameter (Figure 15A). The sample is placed

in the permeameter, which is then filled with water

to a certain height in the standpipe. The stopcock is

then opened and the water allowed to infiltrate the

Table 6 Grades of permeability (IAEG, 1979)



Class

Permeability

1

Description

1 Greater than 10

2

Very highly

210

2

–10

4

Highly

310

4

–10

5

Moderately

410

5

–10

7

Slightly

510

7

–10

9

Very slightly

6 Less than 10

9

Practically impermeable



International Association of Engineering Geology.

Figure 15 (A) Falling head permeameter. (B) Radial percolation test apparatus.

ENGINEERING GEOLOGY/Rock Properties and Their Assessment 579

sample, the height of the water in the standpipe

falling. The times at the beginning, t

, and end, t

of the test are recorded and these, together with

the two corresponding heights, h

and h

, the cross-

sectional area of the standpipe, a, and cross-sectional

area, A, and length, l, of specimen are substituted in

the following expression to derive the coefficient of

permeability, k:

k ¼

2:303al

Aðt

 t

ðlog

 log

Þ½13

Variations of permeability in rocks under stress

can be obtained by using a radial percolation test.

A cylindrical specimen, in which an axial hole is

drilled, is placed in the radial percolation cell. The

latter can either contain water under pressure where

the axial hole is in contact with atmospheric pressure;

or water can be injected under pressure into the hole

(Figure 15B). The flow is radial over almost the whole

height of the sample and is convergent when the

water pressure is applied to the outer face of the

specimen, and divergent when the water is under

pressure within the axial hole. Porous rocks remain

more or less unaffected by pressure changes. On

the other hand, fissured rocks exhibit far greater

permeability in divergent flow than in convergent

flow. Moreover, fissured rocks exhibit a continuous

increase in permeability as the pressure attributable

to divergent flow is increased.

Further Reading

Anon (1975) Methods of Sampling and Testing Mineral

Aggregates, Sands and Fillers, BS 812. London: British

Standards Institution.

Anon (1982) Standard TestMethods for Absorption and Bulk

Specific Gravity of Natural Building Stone, C93–117.

Philadelphia: American Society for Testing Materials.

Bell FG (ed.) (1992) Engineering in Rock Masses. Oxford:

Butterworth-Heinemann.

Bell FG (2000) Engineering Properties of Soils and Rocks.

Oxford: Blackwell Science.

Brown ET (ed.) (1981) Rock Characterization, Testing and

Monitoring. Oxford: Pergamon Press.

Farmer IW (1983) Engineering Behaviour of Rocks, 2nd ed.

London: Chapman and Hall.

Goodman RE (1989) An Introduction to Rock Mechanics,

2nd ed. New York: Wiley.

Hudson JA and Harrison JP (1997) Engineering Rock

Mechanics: An Introduction to the Principles. Oxford:

Pergamon.

Site and Ground Investigation

J R Greenwood, Nottingham Trent University,

Nottingham, UK

Introduction and Terminology

The procedure of ‘investigation’ is fundamental to

any project or activity involving the ground. The

historical records need to be reviewed, current condi-

tions need to be established, and the consequences of

the proposed activity, works, or construction need to

be carefully considered.

Investigation is an on-going process of establishing

and reviewing the facts and processing the informa-

tion to assist our future activities. With respect to

construction works the following definitions are used:

‘Site investigation’ is a continuous process, as the

construction project develops, involving both the

site under consideration and the interaction with

the surrounding areas. It is not confined to obtaining

information on geotechnical aspects but may in-

clude hydrological, meteorological, geological, and

environmental investigation.

‘Ground investigation’ is more site-specific and

aims to investigate ground and groundwater con-

ditions in and around the site of a proposed

development or an identified post-construction

problem.

The term ‘site characterization’ is now also used; it

stems from the environmental specialist’s study of

contaminated sites but is equally applicable to any

site. ‘Characterization’ perhaps implies the results of

‘investigation’.

This article reviews the procedures necessary for

quality site investigation to be carried out and de-

scribes some of the ground investigation techniques

580 ENGINEERING GEOLOGY/Site and Ground Investigation

commonly applied in advance of construction or

remedial works.

Responsibilities

Investigation work must be entrusted to competent

professionals with appropriate geotechnical engin-

eering or engineering geology experience. Advice on

the qualifications and experience required of such a

professional is given in the publications of the Site

Investigation Steering Group. The Geotechnical Ad-

visor, who is an appropriately experienced Chartered

Engineer or Chartered Geologist in the UK, will be a

key figure in ensuring the correct geotechnical input

to the project and should be appointed at an early

stage and continue to work with the project team

throughout the life of the project. Projects proceeding

without the benefit of specialist geotechnical advice

are more likely to encounter unforeseen problems,

resulting in delay and budget overspend.

The Investigation Process

Investigation Stages

The investigation work for most projects can be

divided into stages, as illustrated in Table 1. The Geo-

technical Advisor will ensure appropriate geotechnical

input at each stage (see Engineering Geology: Codes of

Practice).

The Procedural Statement

The key to successful investigation lies in the plan-

ning process. If all aspects of the investigation work

are considered in advance, together with necessary

actions relating to the probable findings, then the

outcome is likely to be satisfactory for all parties

involved.

A convenient way to bring together and record the

proposals for each stage of site and ground investi-

gation is by preparing a ‘Procedural Statement’ or

‘Statement of Intent’. This approach was formally

introduced for the United Kingdom by the Depart-

ment of Transport/Highways Agency in the 1980s

and has now become widely accepted as good prac-

tice. An example of the topics covered in a Procedural

Statement is given in Table 2. Headings and content

will change slightly for each phase of the investigation

process as more information is accumulated. The

Procedural Statement is usually prepared by the geo-

technical engineer or specialist responsible for the

work and should be agreed by all interested parties,

and in particular the client, before the investigation

proceeds.

The Procedural Statement encourages the designer

to consider relevant aspects of the proposed investi-

gation and to seek authority to proceed. It forms a

valuable document within a quality management

system, and it becomes a base reference as the investi-

gation proceeds in case changes are needed in the light

of the findings.

The Desk Study

The desk study, sometimes referred to as the ‘initial

appraisal’ or ‘preliminary sources’ study, is vital for

gaining a preliminary understanding of the geology of

the site and the likely ground behaviour. The term

‘desk study’ can be misleading because, in addition to

collection and examination of existing information, it

must include a walk-over survey. The study will de-

termine what is already known about the site and

how the ground should be investigated.

Before embarking on groundwork, much valuable

information may be readily gleaned from existing

Table 1 Stages of an investigation (developed from Clayton CRI, Matthews MC, and Simons NE (1995) Site Investigation: A Handbook

for Engineers

, 2nd edn. Oxford: Blackwell Scientific)

Construction phase Investigation work

Definition of project Appointment of Geotechnical Advisor for advice on likely design issues

Site selection Preliminary sources study (desk study) to provide information on the relative geotechnical merits of

available sites

Conceptual design Detailed preliminary sources study (desk study) and site inspections to provide expected ground conditions

and recommendations for dealing with particular geotechnical design aspects and problems

Plan ground investigation (Procedural Statement)

Detailed design Full ground investigation and geotechnical design; (additional ground investigation if necessary for design

changes or for problematic ground conditions)

Construction Comparison of actual and anticipated ground conditions; assessment of new risks (additional ground

investigation if necessary)

Performance/

maintenance

Monitoring, instrumentation, feedback reporting

ENGINEERING GEOLOGY/Site and Ground Investigation 581

sources, such as geological and Ordnance Survey

maps, aerial photographs, and archival material.

Such documents can yield significant information

about site conditions and, following the walk-over

survey, a geotechnical plan of the site may be pre-

pared. A checklist of the type of information to be

sought in a desk study is given in Table 3.

The desk study is often the most cost-effective elem-

ent of the entire site-investigation process, revealing

facts that cannot be discovered in any other way. The

preliminary engineering concepts for the site are

prepared and developed at the desk-study phase,

based on the acquired information. The ground

investigation in the field is then designed to confirm

that the conditions are as predicted and to provide

ground information for the detailed design and

project construction.

Figure 1 illustrates the types of ground-related

problem that might be investigated. It is important

to identify the zone of influence relating to each

example. Each project is unique and therefore requires

a specially designed ground investigation.

Table 2 Example of a Procedural Statement’s contents to be prepared before the ground investigation phase (HD 22/02)

The Procedural Statement (sometimes referred to as ‘statement of intent’ or the ‘ground investigation brief’) should be prepared by

the responsible Geotechnical Advisor and agreed by the client and interested parties

1. Scheme

Details of scheme and any alternatives to be investigated; key location plan

2. Objectives

(For example) to provide information to confirm and amplify the geotechnical and geomorphological findings of the desk study as

reported separately and to obtain detailed knowledge of the soils encountered and their likely behaviour and acceptability (for

earthworks). To ascertain groundwater conditions and location of any underground workings (work limits to be defined)

3. Special problems to be investigated

Location of structures. Subsoil conditions below high embankments. Aquifers and likely water-bearing strata affecting the proposed

works. Rock-stability problems. Manmade features to be encountered. Effects on adjacent properties

4. Existing information

List of all relevant reports and data

5. Proposed investigation work

Fieldwork

– Details of exploratory work proposed for specific areas with reasons for choice of investigation methods selected.

Proposed sampling to match laboratory testing

Laboratory work – Details of proposals with reasons for choice of tests and relevance to design

6. Site and working restrictions

Assessment of risk associated with proposals. Site safety, traffic management, difficult access, railway working

7. Specialist consultation

Details of specialist needed to support proposals

8. Programme, cost, and contract arrangements

Anticipated start date, work programme, contract arrangements, cost estimates, specification and conditions of contract.

Arrangements for work supervision

9. Reporting

Responsibility for factual and interpretive reporting. Format of reports and topics to be covered

Table 3 Some sources of information for desk study work (preliminary sources study)

Topic Possible sources

Site topography Topographical maps, aerial photographs

Geology and soil conditions Geological maps, regional guides and publications (British Geological Survey, sheet memoirs),

learned society journals

Geotechnical problems and

parameters

Published technical journal articles, civil engineering and geological journals, newspapers,

previous ground investigation reports, local authorities

Groundwater conditions Topographical maps, aerial photographs, well records, previous ground investigation reports,

water authorities (flood records)

Meterological conditions Meterorological Office

Existing site use and services As-built drawings, utility information, mining records, construction press, land-use maps,

commercial records, contamination records

Previous land use Aerial photographs, old maps, archaeological records, agricultural records, mining records, local

resident knowledge, local library records

582 ENGINEERING GEOLOGY/Site and Ground Investigation

Figure 1 Construction zones to consider within site investigation. (Reproduced from Site Investigation Steering Group (1993) Part 2

Planning, Procurement and Quality Management.

London: Thomas Telford.)

Continued

ENGINEERING GEOLOGY/Site and Ground Investigation 583

Figure 1 Continued

584 ENGINEERING GEOLOGY/Site and Ground Investigation

Elsevier Encyclopedia of Geology - vol I A-E

Подождите немного. Документ загружается.