Site investigations

All civil engineering works and structures induce stress changes in soils and rocks by virtue of their con¬tact with the ground. Unless there is pre-knowledge of the interaction between these factors, the design can be nothing more than a shot in the dark resulting in over- design, or, equally likely— and potentially far more dangerous—underdesign. The problem is basically that of designing the best foundation to prevent move- . ment of the structure during and after construction. "The determination of the effect that ground conditions will have on civil engineer¬ing and building works is therefore the very essence of site investigation and a sine qua non for a safe and eco¬nomic design," is the view of Soil Mechanics Ltd, one of the many specialist companies in the field of geotechnics. So, what basically should you expect from a site investigation? Given that it is impossible to design a structure without knowing what it will stand on, then in the extreme you could expect a complete and seemingly infallible analysis obtained by taking boreholes over the entire site. In practice, you will generally get some¬one's opinion of what is there. How¬ever, not only do economic considera¬tions invariably rule out the possi¬bility of such a thorough investigation ever being made, but the desirability of it occurring is even questionable. Such an operation would interfere with the ground conditions to such an extent that the results would no longer be representative.

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Poor quality

•This means that there is no such thing as the perfect site investigation, al¬though there is such a thing as one which is perfectly adequate. The real danger is being saddled with one which is inadequate. No one conversant with site investiga¬tion practice in the UK is happy about current standards. When faced with the question "Speaking generally, are site investigations satisfactory?" the response is consistently "No, definitely not." But before exploring the reasons for this inadequacy, it is necessary to see who does the work. "Broadly speaking, there are three ways in which a site investigation con¬tract will come to us," explains Geoff Child, principal engineer with Soil Mechanics. There is the non- specialist enquiry where someone says 'I want to build on this site, and this is the sort of building I have in mind' — they will produce detailed or vague plans — 'will you design and conduct a site investigation to give me the information I require?' "At the other end of the scale, an experienced engineer will come along specifying completely what he wants and we are then purely a contractor. In between comes the consultant who knows a fair amount, and here there is scope for a joint effort." Good or bad? Well there is no prob¬lem with the first approach, other than that of choosing a company which is competent to do the work. The second is probably the most ideal but, needless to say, the exception, whilst the third way—well, this can be equally as good, but in practice it is in this middle ground that the majority of bad work occurs. On a road where the pitfalls are mani¬fest, and accepting that good site investigation companies do exist, it is possible to say that the weakest link is invariably the consulting engineer. Nevertheless, his problems frequently stem from constraints imposed by his client.

Returning to the second approach mentioned by Child—that of an experienc¬ed engineer in complete control — it is interesting to look at the way the consulting engineer Ove Arup & Partners operates. Arup has always considered prevailing British practice in this field to be unsatis¬factory, and has its own geotechnics section which is quite capable of undertaking the site investigation for any job. The engineers in the section are in full con¬trol throughout, only going outside the practice to get someone to provide the equipment and operators who actually drill the boreholes or conduct the laboratory tests on the samples — under supervision. Arup does not ask for anyone outside to interpret results, saying, "We don't expect anything more from a site investigation contract than an honest attempt to meet our specifications."