information from a limited number of exploratory excavations or boreholes.

If ground conditions are suitable, the use of an appropriate geophysical method can prove very econ­ omical in establishing some of the more important

of expensive boreholes to be employed later. Alterna­ tively, anomalies revealed by geophysical investigations can be used to select the optimum locations for the more expensive subsurface investigation techniques. For example, low seismic velocity zones may indicate very fractured bedrock associated with faulting, while magnetic anomalies can indicate the presence of basic igneous intrusions.

Care and experience are required in selecting appro­ priate geophysical techniques. If methods inappro­ priate to the ground conditions are adopted they can be of dubious accuracy or even misleading. Thus it is essential to have some idea of the likely ground condi­ tions from desk studies or other preliminary investiga­ tions before employing any geophysical investigation technique. Expert interpretation of the results of geo­ physical surveys is essential. The primary data are often produced in a form that is largely unintelligible to the practising engineer, but which are capable of remark­ ably accurate assessment by the relevant professional.

The most common methods of geophysical surveying and their uses are listed in Table 3.1.

2.3 Trial excavations and boreholes

Trial excavations and boreholes arc traditional methods of direct subsurface exploration. The former permit visual examination of the ground in-situ. Both methods allow samples to be taken for description and laboratory testing. In-situ tests can also be carried out below the ground surface in either trial excavations or boreholes.

Trial excavations allow the most thorough in-situ examination of soil or rock masses since a relatively

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large volume of material can be inspected and a large area of exposed surface can be examined. This is important in common circumstances where the ground possesses a network of fissures or fractures or contains a complex pattern of seams or lenses of materials of varying composition. In these cases the behaviour of the ground may be critically controlled by this large scale fabric rather than the characteristics of small laboratory specimens which do not contain a represen­ tative amount of the fabric.

Large trial excavations may also be used to advan­ tage where geological conditions are very complex. Trenches may be particularly useful to locate the position of faults or other linear features known to be present at shallow or even at reasonable depths. Adits can reveal three-dimensional geology of rock masses pertinent to the design of underground excavations or abutments of dams in complex formations. However adits of deep trenches may represent substantial engineering works in their own right.

Machine-excavated trial pits and trenches can prove very economical down to depths of about five metres although shoring or battering of the excavation sides will be necessary if personnel are to descend into them. Trial excavations may prove impractical or costly in ground with a high water table. Rapid groundwater inflow may cause problems in permeable ground. And in less permeable soils such as silts and fine sands groundwater inflow may destabilise the sides and base of the excavation. Base instability can also be trouble­

a more cost-effective method of investigation as the depth of exploration increases or where groundwater conditions arc troublesome.

Investigation by means of boreholes will inevitably form a major part of any investigation for a power station site. Boreholes will certainly be adopted for any overwater investigations associated with cooling water or jetty studies.

Civil engineering and building works

In British practice, methods of forming site investiga­ tion boreholes fall into two categories — cable tool methods and rotary drilling methods. The former, often referred to as shell and auger methods, are normally used for investigating uncemented or weakly cemented materials such as sands, silts and clays. They may also be used to advance the borehole in the upper weathered zones of rock, but the rate of progress becomes too slow as more solid rock is encountered. > Frequent samples have to be taken from each bore­ hole to enable a record of the ground conditions to be compiled and for possible laboratory testing. In-situ tests are often performed to provide information about the mechanical properties of the ground and the improvement in these properties with increasing depth.

Figure 3.1 shows a light cable tool boring rig. Disturbed samples are obtained during shell and

auger boring from the tools used to advance the bore­ hole, but these have suffered severe disturbance and are only of use for recognition of the materials pene­ trated. More representative and less disturbed samples can bfe obtained in cohesive strata by hammering or pushing a sample tube into the soil at the base of the borehole. The sample recovered in this way is often referred to as ‘undisturbed’, but its true quality is determined by the details of the boring and sampling ’ procedures, particularly in either very hard or soft

materials. Only thin-walled samplers employed in a carefully drilled borehole produce samples really suit­ able for testing for deformation or strength properties. Piston samplers and special sampling procedures need to be employed in very soft soils. It is always very difficult and often impossible to obtain samples of cohesionless materials (sands and gravels) which retain the original'soil structure. In-situ testing is generally relied upon in such soils.

In foreign practice, boreholes in soils are frequently advanced by rotary drilling methods using hollow steam augers or non-coring drill bits. Sampling and in-situ testing can be performed through the hollow stem auger or through the base of the borehole.

Boreholes in rock strata are formed by rotary core drilling methods. These are intended to produce a con­ tinuous core of the materials penetrated. The core can be described and tested to establish respectively the geological succession and geotechnical characteristics of the ground. Again, in-situ tests can be carried out in the borehole.

Figures 3.2, 3.3 and 3.4 show a large rotary drilling rig and its application to an upwardly inclined bore­ hole.

In some instances boreholes intended primarily for in-situ testing or instrumentation may be drilled with­ out recovering cores or samples.

Improvements in rotary core drilling methods have now enabled good quality cores to be recovered in many types of uncemented soils with appropriate drill­ ing techniques. These methods yield continuous cores

Chapter 3

Fig. 3.1 Light cable tool boring rig

(see also colour photograph between pp 242 and pp 243)

of the material penetrated and thus give a more complete record of ground conditions than is obtained by intermittent sampling in shell and auger holes. They may therefore be used to supplement the findings of shell and auger boreholes on important investigations.

Rotary core drilling in strata consisting of solid, strong rocks is normally a fairly straightforward pro­ cess, but in weaker or fractured rocks or in soils the drilling process may fail to recover a complete core of the material penetrated. Alternatively the condition

Fig. 3.2 Large rotary drilling rig

(see also colour photograph between pp 242 ancj pp 243)

of the recovered core may be unsatisfactory. Prudent assessment of zones of core loss or poor quality core may well lead to unacceptable uncertainties about ground conditions and hence to unduly pessimistic designs. Measures can be taken in most cases to maximise the quality and quantity of core recovered. These include using larger ditimeter drilling tools, changing the flushing medium used to remove drill cuttings from the borehole or using more sophisticated drilling tools. However, the ultimate success of all

Geotechnical investigations

drilling techniques is critically dependent on the skill of individual rig operators.

A few other methods of direct exploration are also available, although only a few companies own and operate the more specialised items of equipment. Probably the most important such method is the Delft sampler, which is restricted in use to softer, mainly cohesive, alluvial soils. It yields a continuous small diameter core which can be very useful for establishing the presence of a fabric of thin seams of silt and sand which is often present in such materials and can have an important influence on their geotechnical behaviour.-

2.4 In-situ tests

As noted, it is extremely difficult to take undisturbed samples of cohesionless materials, and samples of cohesive materials have inevitably suffered some dis­ turbance which may affect the results of laboratory tests. These factors have led to the development of tests which attempt to measure the characteristics of the ground in-situ.

In-situ tests range from routine simple procedures to very complicated and expensive methods. They aim to measure many different geotechnical parameters. Tests may be performed in exploratory excavations, bore­ holes, or even in some cases from the ground surface. The more common tests are listed in Tables 3.2 to 3.4.

It must not be thought that in-situ tests are a panacea for all problems of measurement of geotechnical para­ meters. The behaviour of the majority of geological

all the relevant conditions and thus provide an incon­

still depends strongly on engineering judgement and experience.

Other more sophisticated tests may be carried out in adits for example. These may include cross-tunnel jack­ ing tests, which are a variant of place bearing tests, or measurements, by a variety of methods, of the in-situ state of stress in the rocks surrounding the adit. To obtain useful engineering data from such sophisticated methods normally requires the employment of appro­ priately specialised firms or university research staff.

in certain circumstances there may be advantages in carrying out large scale field tests designed to examine specific aspects of geotechnical behaviour. These may occur where, because of the prevailing ground condi­ tions, unacceptable geotechnical design uncertainties or contractual risks remain even after a full conven­ tional investigation. Alternatively, large scale field tests may be warranted where their cost is justified by the potential savings to a major project resulting from a particularly accurate assessment of geotechnical para­ meters.

Examples of large scale field tests which can be used are instrumented trial embankments or excavations, pumping tests, pile tests and trials of ground improve­ ment processes.

2.5 Groundwater investigations

Groundwater is the most frequent cause of problems in geotechnical engineering. It is therefore essential that the geotechnical investigation provides a sufficiently thorough understanding of the groundwater regime and

the consequent pore pressure distribution. Not only does the existing groundwater regime influence the construction of excavation for foundations and the long term performance of a development, but a large devel­ opment may itself alter that regime. These potential hazards can only be evaluated from a knowledge of the initial hydrogeological conditions. -

The first aspect of groundwater investigation is to establish and characterise the spatial variation of water pressures in the ground. They probably vary laterally across the site and may also exhibit a non-hydrostatic distribution with depth. Groundwater observations

should be made routinely as drilling proceeds in each borehole, but such measurements may not be suffi­ ciently reliable on their own because the duration of drilling is too shost for equilibrium levels to be measured. A proper distribution of piezometers or observation wells is needed to establish reliable groundwater pressure information. It should be noted that various types of piezometer installation exist, and it is important to select a type of instrument appro­ priate to the permeability of the strata encountered and the rate of variation of groundwater pressure which is expected.

Observation of groundwater pressures using piezo­ meters need to be made over a significant period (preferably at least a year) because they are influenced by seasonal and climatological factors. Abstraction of groundwater by pumping can also be a major influence on groundwater conditions. On coastal sites tidal influences need to be examined.

For a complete understanding of the groundwater regime, which will be needed, for example, for the design of a dewatering scheme, it will be necessary to establish the coefficient of permeability of the various strata. This will entail in-situ testing, possibly supple-