Elsevier Encyclopedia of Geology

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The Walk-Over Survey

The walk-over survey is a detailed inspection of the

site. It is often done in stages, with an initial visit for

familiarization, photography, and checking of the

current site conditions, and subsequent visits to

check out features noted on historical maps and

photographs. Features should be sketched at an ap-

propriate scale on a base plan for inclusion in the

desk-study report.

Derelict, Reused, and Contaminated Land

The desk study may reveal that a site has a history of

previous use. Industrial use is likely to have left a

legacy of pollution, which will require identification

and possibly treatment depending on the intended

reuse of the site. Guidance on the investigation of

contaminated ground on the basis of ‘source-path-

way-receptor’ is given in the key reference documents

listed. The desk study will form an important element

of the ‘phase 1 risk assessment’ that is required by

contaminated-land legislation. An environmental

specialist will need to be consulted whenever contam-

ination is suspected, and appropriate safety precau-

tions should be taken at all stages of the investigation

work.

Reporting the Desk Study

On completion of the desk study, the results are for-

mally presented in a report that brings together details

of site topography, geology, geotechnical problems

and parameters, groundwater conditions, existing

construction and services, previous land use, expected

construction risk, and proposed ground-investigation

methods. A hazard plan is a good way of presenting

a summary of the data accumulated during the desk

study and walk-over survey.

Properties to be Determined

The prime purpose of a site investigation is gener-

ally to determine the ground profile and the continu-

ity and properties of the various strata. This will

include ranges of values and assessments of probabil-

ity and risk, so that construction may proceed within

a defined framework of knowledge.

Ground Investigation

Ground Investigation Design

The objective of a ground investigation is to ensure

economical design and construction by reducing to an

acceptable level the uncertainties and risks that the

ground poses to the project.

The initial appraisal (desk study) will identify what

is already known and will facilitate a preliminary

understanding of the ground and its behaviour. This

provides a basis for assessing the nature, location,

extent, and duration of subsequent fieldwork and

for preparing a programme of laboratory tests of

samples obtained during the fieldwork. It is the geo-

technical model against which every piece of acquired

data can be checked. As the field programme pro-

gresses, so the model will be either confirmed or

amended.

The design of the ground investigation can com-

mence only once the following information has been

obtained and client approval given:

1. a clearly defined purpose for the investigation;

2. an assessment of what information is required

and when;

3. the areas and depths of ground to be investigated;

4. the time required for the investigation; and

5. an estimate of the cost.

These items can be conveniently covered by the

Procedural Statement, as discussed previously.

Ground Investigation Methods

The ground investigation can include many different

activities such as:

trial pits with descriptions of the material exposed;

boreholes with sampling for later laboratory

testing;

tests in boreholes using simple or sophisticated

instruments;

probing from the ground surface;

loading tests at the surface or in excavations;

geophysical testing; and

geochemical sampling.

Ground-contamination investigations are often re-

quired, especially on derelict sites; appropriate ex-

pertise is essential as there are important health and

safety considerations.

How Many? How Deep?

The scope and size of the ground investigation will

depend both on what is known about the site and on

the nature of the project. The ground investigation

may, therefore, vary from a few trial pits dug by an

excavator in one day, for a small housing project, to a

major undertaking lasting many months, for a large

earth dam. There are often alternative approaches

that are equally acceptable technically, but sometimes

one method is preferable to the other because of

plant availability or access restraints. Boreholes

may to some extent be replaced with trial pits, cone

ENGINEERING GEOLOGY/Site and Ground Investigation 585

penetrometer probes, or geophysical surveys to pro-

vide similar or complimentary information. Cost is

always important, but the designer should prepare

the ‘right’ investigation for the project rather than

working to a fixed budget provided by the client.

Teamwork is important and involves pooling ideas

and suggestions and taking account of restraints im-

posed by the client, structural designers, auditors, or

environmental specialists, who may have valuable

input that can be used to support the geotechnical

engineer in preparing the investigation. Again, the

Procedural Statement is a useful vehicle for conveying

proposals to all interested parties.

Time should be allowed for innovative design and

review of proposals. ‘Sleeping’ on ideas for a day or

two can often lead to the development of better alter-

natives. Checklists are useful to remind the designer

of aspects that should be covered.

Guidance on spacing and depth of and sampling in

exploratory holes may be found in the publications

of the Site Investigation Steering Group and other key

texts.

There may be a temptation to believe that every-

thing about a site should be discovered at the investi-

gation stage. It may be more realistic to accept that

certain local features (for example swallow holes in

chalk, mine workings beneath a highway cutting, or

precise founding levels in variable strata) are best

picked up and reviewed during the construction or

remediation phase. This could be more cost-effective

than attempting to detect every void and strata

variation during the investigation, but it does require

an element of flexibility and the application of

appropriate expertise during the construction work.

Such flexibility is more readily available in the ‘design

and construct’ type of construction contract and in

the use of ‘observation methods’, which are gaining

popularity with clients.

Ground Investigation Fieldwork

The various approaches that might be considered for

gaining information on ground conditions are briefly

reviewed in this section.

Trial Pitting

Trial pitting is a relatively cheap and efficient method

of exploring the ground. Techniques vary from dig-

ging by hand – useful where services may be present

or access by machine is difficult – and using mini

excavators – where access is restricted – to using

large-tracked back-hoe type excavators, which can

reach depths of 6 m or more (Figure 2A).

Trial pits enable the stratification and the true

nature of each soil horizon to be logged and represen-

tative samples selected. Block samples can be cut for

subsequent laboratory testing, and localized samples

can be taken from shear surfaces. However, access to

the pit for close visual inspection is permitted only

where temporary side support (shoring) is provided

and a careful assessment of the risk of instability and

the presence of gas has been made.

When inspecting the ground, a small wedge of soil

is frequently sheared away from the side of the pit to

reveal the true nature of the soil behind the zone of

smear resulting from excavation. A moisture-content

profile using close centres (typically 75 mm) can pro-

vide much information about the consistency of the

soil. This is easily and accurately undertaken using

the ‘moisture in the bag’ technique. The moisture

Figure 2 Two common methods of exploration. (A) The trial pit, with shoring and ladder for safe access and egress. (B) The

percussion boring rig.

586 ENGINEERING GEOLOGY/Site and Ground Investigation

samples are sealed in a lightweight polythene bag

(Figure 3). They are then weighed and dried in the

bag, avoiding the need for separate moisture-content

containers and reducing the error due to dehydration

during sample storage.

Observation of the sides of the pit will provide an

indication of the stability of the ground. The nature of

the excavated material will provide information on

possible reuse during earthworks. If a trial pit can be

safely protected and left overnight, drying of the soil

surface often reveals fissures, discontinuities, and

possible existing slip surfaces within the strata.

Groundwater conditions and their effect on ground

stability may be closely observed in the pit. Instability

due to groundwater flow may prevent safe advance-

ment of the pit in finer-grained granular soils. Per-

meability testing (by pumping or by water addition)

may be carried out to give an indication of the ground

permeability.

On completion of the excavation, strata should be

replaced and compacted back to the depth excavated.

The location (grid coordinates) of the pit must be

carefully recorded as the ground is now disturbed

and could present a hazard to future construction.

For this reason, pits are generally excavated outside

the planned area of the proposed foundations. The

positions of exploratory holes are readily established

by modern handheld global positioning systems.

Boring Techniques – Soft Ground

The light cable tool percussion (‘shell and auger’)

technique of soft-ground boring at a diameter of

150 mm is perhaps the best-established, simplest,

and most flexible method of boring vertical holes,

particularly in the UK (Figure 2B). It generally allows

data to be obtained for strata other than rock. A

tubular cutter (for cohesive soil) or a shell with a

flap valve (for granular soil) is repeatedly lifted and

dropped using a winch and rope operated from

a tripod frame. The soil that enters these tools is

regularly removed and laid to one side for backfill-

ing. Steel casing is used where necessary to prevent

collapse of the sides of the borehole.

The technique can determine conditions to depths

in excess of 30 m under suitable circumstances and

usually causes less surface disturbance than trial

pitting.

Small disturbed samples are taken on encountering

each new stratum and at regular intervals within it.

Larger samples of the materials are taken typically at

1 m intervals in the top 5 m of the borehole and

subsequently at 1.5 m intervals. In cohesive or ‘fine

grained’ soils, sampling is by nominal 100 mm diam-

eter 0.45 m long open-tube drive sampling (U100),

and the sample is often referred to as a (relatively)

‘undisturbed’ sample. These are sealed with wax to

preserve moisture and capped at each end prior to

transport to the laboratory for testing.

In granular soils a standard penetration test (SPT)

is typically undertaken at 1 m to 1.5 m intervals,

depending on depth, and a bulk sample is removed

from the tested length for subsequent description and

classification. The SPT involves driving a 50 mm

diameter tube or cone into the ground using a 65 kg

weight falling a distance of 760 mm and controlled

using a trip mechanism. Blows are counted for

successive penetrations of 75 mm for a distance of

450 mm. The SPT value is the number of blows re-

quired to penetrate the final 300 mm. This test is

sometimes also used in firm clays. The results of the

SPT are correlated empirically with the density and

angle of shear resistance in granular soils and with

shear strength in cohesive soils.

Alternative techniques that might be considered

for advancing boreholes include the less reliable

wash-boring method, where the soil is washed out

of the cased hole, and flight auger boring, where

disturbed samples are brought to the surface by the

rotation of the auger and, where hole stability per-

mits, the auger is removed for undisturbed sampling.

Figure 3 ‘Moisture in the bag’; an efficient method of determin-

ing moisture contents.

ENGINEERING GEOLOGY/Site and Ground Investigation 587

The hollow-stem flight auger is a more reliable

system, which supports the hole whilst undisturbed

samples are recovered through the central stem.

For shallow holes where access is difficult, hand

augering may be employed. The use of a lightweight

mechanical hammer associated with dynamic probing

has led to the recent development of the ‘window’

sampling technique. This involves driving a tube with

a section of the side wall removed to allow the soil

sequence to be logged and samples to be selected from

the side for classification and, increasingly, for con-

tamination testing. Sample-tube lengths are typically

1 m or 1.5 m. Deeper sampling is possible by pro-

gressively reducing the tube diameter to reduce side

friction from the strata above the sampling level.

Boring Techniques – Hard Ground

Rotary drilling produces rock cores by rotating an

annular diamond-impregnated tube or barrel into

the ground. This is the technique that is most ap-

propriate for making investigation holes in rock or

other hard strata. It may be used vertically or at any

angle.

Core diameters of less than 100 mm are most com-

mon for site-investigation purposes. A flushing fluid

such as air, water, mist, or foam is used to cool the bit

and carry cuttings to the surface.

Examination of rock cores allows detailed descrip-

tion and generally enables angled discontinuity sur-

faces to be observed. However, it does not necessarily

reveal the presence of near-vertical fissures or joint

discontinuities. The core can be tested in the field or

in the laboratory. Core recovery depends on rock type

and the techniques employed.

Where open-hole rotary drilling is employed, de-

scriptions of the strata depend on the examination at

the surface of small particles ejected from the bore-

hole in the flushing medium. Consequently, no indi-

cation of fissuring, bedding, consistency, or degree

of weathering can be obtained. Depths in excess of

60 m can be investigated using rotary techniques with

minimal surface disturbance.

Dynamic and Static Probing

Dynamic probing involves driving a rod with a fixed or

detachable cone into the ground by allowing a falling

weight to impact on an anvil attached at the top of

the rod and counting the number of blows required

to achieve successive penetrations, typically of 100

mm. The apparatus can be handheld (for example the

Mackintosh probe), or a machine-mounted weight and

guide can be used for larger-diameter rods (for example

the Pennine probe) (Figure 4).

Figure 4 Examples of dynamic probing (A) by lightweight drilling and (B) by hand. (A) A dynamic probe rig (photograph courtesy of

Soil Mechanics). (B) Lightweight dynamic (Mackintosh) hand probing to obtain an indication of soil density.

588 ENGINEERING GEOLOGY/Site and Ground Investigation

Whilst the number of blows may be correlated

empirically with the shear strength or density of

granular soils, these are generally profiling tools best

used for interpolating between boreholes or trial pits.

Samples are not normally taken, but the technique

has been extended by using the hammer apparatus to

take ‘window’ samples, as described above.

The static cone technique (Figure 5) involves push-

ing an instrumented cone and sleeve into the ground

at a defined rate and recording the point resistance

and skin friction using electronic means. More so-

phisticated instruments can now monitor the pressure

of pore water just behind the cone tip. By considering

the ratio of end resistance to shaft friction resistance,

an indication of the soil type and its density may be

deduced.

The apparatus can measure changes in soil type

over very short distances and avoids the problems of

disturbance, particularly of granular soils, below the

water table. It is a very rapid technique, often used as

a profiling tool between boreholes, and is capable

of electronic interpretation. It has been used exten-

sively in North Sea oil and gas structure foundations

and is now used widely for land-based investigations.

Figure 5 The static cone apparatus. (A) ‘Dutch’ cone penetrometer testing truck with (B) control console and (C) electric static cone

(photographs courtesy of Fugro Ltd).

ENGINEERING GEOLOGY/Site and Ground Investigation 589

As with all probing techniques, the presence of large

cobbles and boulders can prevent penetration in cer-

tain soils and in made ground. Although linked with

boring techniques, the static and dynamic cones are

regarded as a form of in situ test.

In Situ Testing

The main objectives of a ground investigation are

generally to identify and classify the soil types into

groups of materials that exhibit broadly similar en-

gineering behaviour and to determine the parameters

that are required for engineering design calculations.

Some soils, such as certain clays, may be readily

sampled and transported to the laboratory for quality

testing under controlled conditions. Other soils, such

as very soft or sensitive clays, stony soils, sands, and

gravels, and weak, fissile, or fractured rock are not

easily sampled in an intact ‘undisturbed’ state, and

therefore in situ testing is required to obtain the

necessary engineering parameters. In situ tests may

take the form of geophysical tests (as described in the

following section), in situ soil-testing techniques, or

links with field instrumentation.

In situ tests fall into three typical groups.

1. Empirical tests, where no fundamental analysis is

possible and stress paths, drainage conditions, and

rate of loading are either uncontrolled or inappro-

priate (example is SPT).

2. Semi-empirical tests, where a limited relationship

between parameters and measurements may be

developed (examples are cone penetrometer tests,

California Bearing Ratio (CBR) and borehole per-

meability tests).

3. Analytical tests, where stress paths are controlled

(but strain levels and drainage often are not)

(examples are vane shear strength, pressuremeter

tests, pumping tests and packer permeability).

Details of the tests and their application are

given in British Standards 1377 and 5930 and other

references listed in the further reading section. The

main point to note with in situ testing is that the

drainage conditions are almost impossible to

control, and, therefore, there is uncertainty as to the

drained/undrained nature of the test. However, the

very act of testing in situ provides a good indication

of the actual ground response provided that scale

factors are taken into account. For example, a plate-

load test having a plate width of B will affect the

ground to a depth of approximately 1.5B. The actual

foundation may be many times the width of the test

plate, and the depth of influence will therefore be

much deeper.

The SPT is a simple cost-effective means of assess-

ing granular soils. More analytically correct tools

such as the pressuremeter are expensive and their

results are not always straightforward to interpret.

Judgement must be used to determine the level of

sophistication required for a particular project.

Geophysical Investigation

Measurement of the geophysical properties of the

ground may provide an indication of the location of

strata boundaries and anomalous ground conditions.

Such surveys may include measurements of ground

conductivity, magnetic and gravity fields, electrical

resistivity, and seismic responses (see Analytical

Methods: Gravity, Seismic Surveys).

The use of geophysical techniques is generally

non-intrusive, with little site disturbance, and is

therefore sometimes carried out at the desk-study

stage. However, the main benefit comes when some

exploratory hole data are available to permit more

accurate calibration, correlation, and processing of

the geophysical data. The ability of the current

generation of computers to process large amounts

of data rapidly has led to a resurgence of interest in

the potential value of geophysical investigation

techniques.

Groundwater and Instrumentation

The presence of groundwater leads to many engineer-

ing problems, and it is vital that all water observa-

tions are carefully recorded. Whenever water is struck

in a borehole or trial pit the point of entry must

be noted. It is normal practice to cease the advance-

ment of the hole for 20 min whilst any rise in the

water level is observed. Longer-term observations of

groundwater fluctuations may be made by instal-

ling a standpipe or standpipe piezometer. This

typically comprises a porous filter at the base of a

19 mm diameter plastic tube inserted to an appropri-

ate depth in a borehole (Figure 6). Water enters

through the porous filter, and its level in the plastic

tube can be monitored over a period of time using a

dip meter that emits an audible signal when an elec-

trical circuit is completed as it meets the water in the

borehole.

Water-level records taken over an appropriate

period of time will reveal the likely range of water

conditions to be allowed for in the design and to be

encountered during the project construction.

More sophisticated equipment such as hydraulic,

electrical, or pneumatic piezometers with trans-

ducer systems and automatic recording can be used

590 ENGINEERING GEOLOGY/Site and Ground Investigation

where the information obtained is critical to the

project.

Other instrumentation, such as settlement gauges,

plates, and extensometers to measure vertical move-

ment, and slip indicators and inclinometers to meas-

ure lateral movement at depth, are available for use

in conjunction with the ground investigation or

with advance trial constructions and main works

constructed by ‘observational’ methods.

Laboratory Testing

It should be remembered that soil is an assemblage

of solid particles, which may contain organic matter.

The voids between the particles are filled with gas –

usually air – and water. The geotechnical engineer

will need to determine the composition of the

ground in situ and also explore the changes that

will occur to the soil as a result of the proposed

construction.

Laboratory tests tend to be divided into two main

classes.

Classification tests involve grouping soil types into

categories of soils that possess similar properties. For

coarse-grained soils the particle size distribution is

often the key property, whereas for fine-grained soils

the moisture content and mineralogy, as reflected in

the Atterberg limits (index properties), most often

hold the key to grouping soils of similar characteris-

tics. For general classification purposes, the moisture

content and index properties (liquid limit, plastic

limit, and sometimes shrinkage limit) are used to-

gether with the particle size distribution, density of

the soil, and, most importantly, an accurate descrip-

tion carried out in accordance with the standards in

the code of practice.

Engineering tests assess the engineering properties

of the soil, such as routine shear strength, compress-

ibility (see Figure 7), compaction characteristics, and

permeability. Where more specific soil properties are

required for complex design functions, ‘advanced’

laboratory tests may be used, in which high-quality

undisturbed samples are carefully prepared and tested

under precise laboratory control and stress and small

strains are measured.

Full details of laboratory tests and their relevance

to engineering design are given in the appropriate

British Standards and the key references in the further

reading section.

It is important that the testing regime is planned in

advance of the fieldwork in order that suitable

samples are recovered from the ground. The Proced-

ural Statement will help with this planning process,

as the reasons for the sampling and subsequent

Figure 7 Commercial laboratory testing for shear strength and consolidation. (A) A 100 mm triaxial test cell. (B) A bank of consoli-

dation (oedometer) test machines. (Photographs courtesy of CSL Ltd.)

Figure 6 Schematic of standpipe piezometer (courtesy of

Geotechnical Instruments UK Ltd).

ENGINEERING GEOLOGY/Site and Ground Investigation 591

laboratory testing can be logically considered to-

gether with the significance of the anticipated results.

Reporting

The reporting of site and ground investigation work is

most important. The factual report is generally the

only tangible output from the investigation and

represents a considerable financial investment. The

factual report should include:

a statement as to the purpose and rationale of the

investigation;

a description of the work carried out, including

references to the specification and standards

adopted and any deviations from them;

exploratory hole logs (including location and

ground level);

in situ test records;

laboratory test results; and

a site location plan.

The development by the Association of Geotech-

nical and Environmental Specialists of a standard

format for geotechnical data files in digital form has

led to the efficient collection, transfer, processing,

presentation, and storage of the data for all parties

involved in the investigation process. Data are ideally

input manually to the computer only once or are

collected directly in electronic form from monitoring

and testing devices. Specialist software houses have

developed programs to help the investigator collect,

store, and process the data and to present it in the

form most suited to the requirements of a particular

project (Figure 8).

The exploratory hole log can perhaps be regarded

as the heart of the factual report because it records

all the information obtained at that particular loca-

tion. An example log prepared from digital data is

presented in Figure 9.

Interpretation of the geotechnical data to relate

them to the proposed design and construction will

be required for each project and is normally the re-

sponsibility of the Geotechnical Advisor. Formal in-

terpretive reports (also known as assessment reports

or appraisal reports) may be prepared as required by

the design team. It is good practice to prepare a

Figure 8 Use of digital techniques for efficient handling of site investigation data (image courtesy of Key Systems and Geotechnical

Developments).

592 ENGINEERING GEOLOGY/Site and Ground Investigation

f0045 Figure 9 Example exploratory hole log (Courtesy Soil Mechanics).

ENGINEERING GEOLOGY/Site and Ground Investigation 593

geotechnical design brief to assist the construction

team on site and, at the end of the project, to prepare

a feedback (maintenance) report on any issues from

which lessons can be learned and on which future

scheme development can be based.

The various stages of quality control and reporting

that help to ensure maintenance of the standards and

accuracy of the output from a site and ground investi-

gation are described in the publications of the Site

Investigation Steering Group.

Concluding Remarks

Site investigation involves establishing facts concern-

ing the ground; the excitement of discovery is

tempered by the professional responsibility for much

of the financial risk and safety associated with each

engineering project.

The combination of appropriately experienced in-

vestigators working to agreed team procedures within

a sensible permitted time frame with limited compe-

tition is likely to produce the best solutions to engin-

eering challenges presented by particular ground

conditions. This article has touched on some of the

important issues relevant to site investigation but is in

no way intended to be a complete guide. A good

investigator will set out to define the investigation

but to expect the unexpected.

Elsevier Encyclopedia of Geology - vol I A-E

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