c2112_siteinvestigation

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SITE INVESTIGATION AND EXPLORATION

1. GENERAL

1.1 Objectives

The practice of site investigations is undoubtedly one of the most critical aspects of

the geotechnical engineers work. The broad objective of site investigation is to

accumulate all data for the site in question so that the proposal being considered may

be satisfactorily carried out. It follows that the site in question must be examined

thoroughly so that consequent analyses and design procedures are appropriate to and

representative of the proposal being considered.

The type of information required will depend on the proposal as indicated below.

(a) Investigation for new works: access, rights of way, other rights and convenants,

location of services, presence of mine workings, likely effect of works on

adjoining properties, soil conditions, records of performance of adjacent

structures, topographic and hydrologic and climatologic data (for dams),

environmental considerations such as land use, population etc.

(b) Investigation of failures (and possibly remedial design measures): observation of

mode of failure, past or continuing movements, specialized investigations such as

soil conditions or structural data etc., depending on nature of failure.

(c) Reports on existing works: Perhaps on likely adverse effects from new works, or

the possibility of enlargement or modifications. All available records concerning

design and performace of original structures, observations of movements and/ormovement records, soil conditions relevant to the problem.

(d) Investigation of availability of materials: for fills, road bases, concrete aggregate,

dam core, filters or fill etc. Largely reconnissance and geologic data, drilling to

estimate extent of deposits and possibly laboratory tests to assess suitability for

ground treatment.


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1.2 Reasons

(a) To establish site suitability

(b) To enable effective and economic design

(c) To anticipate immediate and/or future problems and apply appropriate measures.(d) To give confidence in the design assumptions.

(e) To give insurance against legal actions.

(f) To enable efficient and economic construction planning.

1.3 Sources of Information

Interpreted in conjunction with all plans and data for the proposed works. The

chronological order is as follows:

(a) Experience, general knowledge of locality. (Basis for scientific investigation -

may give warning that otherwise might be overlooked, etc.)

(b) Existing records: eg. geological survey, maps and memoirs; ordinance survey;

municipal and public authority plans; meteorological records; borehole records from

adjacent or significant areas; areial photos.

(c) Personal on-site investigations: Inspection of adjoing areas, collection of hand

specimens to correlate with geological map, topographical features especially

instability, drainage, fault evidence. Talking with locals, especially land owners,

farmer, local builders, etc. Terzaghi maintained that this was the absolute

minimum site investigation - to sit and stare.

(d) Site exploration (with field and laboratory testing). This is the investigation of the

subsurface conditions and forms the main part of this course. Unfortunately, its

practice is still very much neglected particularly on structural projects. In the past,

a site investigation is often not undertaken and engineers used results from

previous investigations of nearby sites instead. The revised Building Control

Act has now changed that.

In these instances, problems only become apparent on construction with the result that

delays for remedial measures and/or redesign operations increase costs quickly, often


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overtaking, by considerable amounts, the capital which would have been required for

site exploration.

In other instances, the site exploration may have been of poor quality with the resultthat soil stratification and parameters are not representative of the sub-surface

conditions. This situation can lead to two extremes:

(I) Gross over-estimation of suitability of the sub-surface soils resulting in excessive

settlements, bearing capacity failures in extreme cases, ground water problems,

pile lengths, etc. One very serious aspect of this extreme can occur if the depth of

exploration does not extend to a depth which is significantly stressed by the

proposed construction eg. pile group settlements.

(2) Gross under-estimation of the suitability of the sub-surface soils resulting in very

conservative, hence costly, foundation design.

It follows from the above, that some form of site exploration is desirable for almost

every form of development. The scope and extent of the exploration will depend on

many variables, but its absolute exclusion can prove to be a very false economy.

More important, it is not just sufficient to undertake any old site exploration as this

can lead to further excessive expenditure; whether of the form of remedial measures

for over-estimation of site suitability or costly foundations for under-estimation of site

suitability. The site exploration should be carefully planned, undertaken and

supervised in relation to the proposed development and the sub-surface conditions

encountered.

Scott, in a review paper to the 1966 Sydney Symposium on Site Investigation said

Every structure that is built, because of Newtons 3rd Law, transfers the dynamic

and static reactions it creates into the earths crust by some path or other and this

makes the foundation a working part of every design which the engineer should

review just as critically as any other part. Now Engineers, by training, always require

a very precise examination of any material they utilise in a structure. A sample of

steel or concrete is tested at all stages of its processing and the complete history is

fully recorded before its use is condoned as a stress carrying part of structures. If an


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inspector brough a sample of material and reported it was untold millions of years

old; the original composition was unknown; it had been stressed and strained, sheared

and failed many times in its life under conditions of immense stress and heat; it could

never by statistically representative of the whole; it was nonhomogeneous; it wasintersected in all directions by imperfections; its properties were continually changing

with time; and its was subject to a residual stress field of unknown value; then most

engineers would instantly reject the material. This description and worse applies to

many foundations which are accepted with the most cursory examination.

2. SITE EXPLORATION - OBJECTIVES

The precise objectives of the site exploration will be determined by the nature and

scope of the geotechnical engineers brief - what answers are required. It should be

noted however, that ant unusual conditions encountered and not necessarily in the

brief, should also be reported if considered relevant.

In general, the objectives would be as follows:

(a) To determine the nature, thickness, dip and variability of strata to be affected by

the proposed development.

(b) To determine the physical properties of the strata relevant to design.

(c) To determine the ground water level, variation and chemical composition.

(d) To obtain the above data in the most economic and technically accurate way. This

requres flexibility in planning and a good knowledge of all available techniques.

3 SITE EXPLORATION - MAGNITUDE AND EMPHASIS

3.1 General

The aim must always be to provide adquate reliable information. If compromise is

necessary (inadequate funds) obtain reliable information at the expense of adequacy.

Some site exploration is essential in all civil engineering and building works.

Magnitude depends on information available, relevant variables, consequences of


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failure, complexity of foundation, magnitude of loads, geology of site and ease of

interpretation, overall job economics, etc.

3.2 Variables

Apart from economics, the major considerations governing the magnitude and

emphasis of the exploration are the type of soil profile and the type of development.

(a) Profile:

(i) Very variable mixed soil: much field investigation, simple laboratory tests -

little undisturbed testing.

(ii) Very uniform soil: Considerable lab testing.Minimum field work.

(b) Type of development.

(i) Very extended eg. subways, roads, transmission lines, tunnels: mainly shallow

diffuse exploration using SPTs or CPTs but with some deep boreholes.

(ii) Limited areas eg building sites: deep, concentrated, sufficient number to allow

reasonable assurance of locating any variations in foundation conditions.

3.3 Phases of Exploration

It is sometimes advisable to carry out the exploratory operations in 3 phases

especially for major projects.

(1) Preliminary explorations - to assess the general suitability of the site for the given

development or to select the most suitable site from alternatives. This phase and

its results will enable a preliminary assessment of soil and ground water

characteristics so that subsequent phases of exploration can be planned - location

of boreholes, depth, likely size of boreholes, testing techniques and equipment,

necessary. Often in areas which are well document, this phase of exploration is

not always necessary.

(2) General exploration - to select areas of a given site with the most suitable ground

conditions. To obtain the required samples and conduct lthe appropriate testing.

Usually this phase is sufficient for general construction.

(3) Detailed explorations - generally for use in major heavy or unusual developments.

Special techniques of testingand exploration may be used to examine specific soil

behaviour when consitions require it. This may include plate bearing tests,


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