required for wharves and jetties where fossil-fired stations often receive their fuel from colliers and tankers. Steelwork, concrete and building techniques are discussed and the chapter concludes with architec­ tural considerations, statutory regulations and civil engineering contract strategy.

2Geotechnical investigations

2.1General and desk studies

The extent and scope of geotechnical investigations vary greatly depending on the object of the exercise. These can range from initial site selection studies, through the investigation of preferred sites to feasibility and design stages. The financial implications vary accordingly from tens of thousands to millions of pounds. There is often advantage in phasing such inves­ tigations so that the earlier findings can be utilised to focus later attention on those matters likely to prove crucial to the development.

Geotechnical investigations are often undertaken to deduce the availability and extent of constructional materials from local sources, and in the case of nuclear stations special considerations (usually related to safety against defined hazard criteria) need additional geo­ technical evaluation. As all power station construction is undertaken against a background of political over­ tones it should be recorded that constraints are often imposed, particularly in terms of timing, granting of planning permissions, consents, etc., which can radi­ cally affect the nature, liming and even sometimes loca­ tion of geotechnical investigations relative to the ideal investigatory plan. '

It is important to state the principles behind the plan­ ning of all site investigations and the first element to consider in this context is a desk study. It is essential that all investigation planning commences with a desk study to examine all the available information on topics such as mapping geology, hydrology, ground condi­ tions, site usage (both present and past), geography, etc., so that maximum use can be made of such existing information and the risk of unnecessary duplication is minimised. In the case of a preliminary site selection study this desk exercise may be all that is warranted, but usually it is the forerunner to further work, much of which is site orientated.

It is normal for a desk study to cover an appreciable area around the potential site as well as the site itself. Judgement is required in assessing the area to be included in a desk study, and certain proposals will warrant a larger area or degree of study than others.

For instance a site may be adjacent to a river, whose water levels have been recorded over many years at or near to the site. But to equate those levels to volu­ metric flows may mean obtaining data from gauging stations many kilometres up or down stream and then interpolating or extrapolating the gauging data to produce equivalent data for the site. If this issue is

deemed of particular importance, or if a high degree of accuracy is required, it may be prudent to consult a hydrologist or the Water Authority regarding the figures produced.

A desk study should be set up with a list of objectives and these must include such items as the examination of potential problems in developing the site, making economic use of investigation techniques and the selection of techniques appropriate to the anticipated ground conditions and geotechnical requirements. Such studies will establish guidelines for the scope, depth "&nd layout of investigation measures required in sub­ sequent site investigations.

Unless the desk study is of a very preliminary nature, such as basic site appraisal, a walk-over survey is highly desirable. This may have to be confined to roads, tracks and other public rights of way, but even so limited a walk-over is often most valuable in confirming desk study impressions and gaining further information to supplement published data. A walk-over survey is essential before committing expenditure on items such as physical sub-surface investigations or geophysics. As with the desk study, the area of coverage might be appreciably larger than the site itself. A list of study aspects Io be recorded should be compiled prior lo a walk-over survey and the possibility that such a survey may warrant extension into mapping exercises (topo­ graphical, geological and/or geomorphological) should be borne in mind, particularly for large or complex sites.

In these very early stages of site evaluation, recourse should also be made to the more sophisticated forms of aerial surveillance. As well as vertical and oblique photography using standard film, infra red photo­ graphy and satellite imagery can highlight surface and near surface features which can be difficult to distin­ guish at close range. Such methods can be especially valuable where access is difficult in terms of either geography or wayleave permissions, or where struc­ tural geology can play an overriding role, such as in the location of the safety related elements of a nuclear station layout.

z

2.2 Geophysical investigations

A wide variety of methods of geophysical investigation is available. These methods measure the variations in selected physical properties of the ground such as wave velocity, resistivity, density and magnetic susceptibility. These parameters are generally related to geotechnical or geological characteristics of the ground. The methods work best if there are strong contrasts in the measured parameters either with depth or laterally across the site.

The accuracy of geophysical techniques can be very variable and often cannot be predicted in advance. The results always therefore require to be checked against or correlated with subsurface geotechnical or geological

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

vieuieciiiiicdi investigations

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 bb 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