Publicat: 14 Feb 2010 00:00
Laboratory testing is part of the physical survey. As an integral part of site investigation, the need for laboratory tests will often dictate the type and frequency of sample to be taken, and will therefore control the method of forming boreholes.
Thus the type of sampling requires a precognition of the soil conditions on site; this has had the effect of leading some writers to recommend at least two stages of field work, with the bulk of laboratory testing being carried out after specific sampling in the second phase of investigation. For routine work such a programme is impractical and rarely used, because of the increases in cost and time that it causes. If two phases of site and laboratory work cannot be included then the investigation must be more carefully planned. With provision for changes during field work, with close engineering supervision and with a knowledge of soil conditions on site based on a first-class desk study, it should be possible to avoid the use of two field investigations.
Soil mechanics, although a -branch of engineering, is often imprecise. Since many problems cannot be solved with accuracy, either as a result of imperfect analytical techniques or complex ground conditions, the use of refined sampling and testing techniques has been questioned. Terzaghi and Peck (1948) have commented ‘ ... On the overwhelming majority of jobs no more than an approximate forecast is needed, and if such a forecast cannot be made by simple means it cannot be made at all’. But is this attitude always justified?
Certain classes of structure are so costly and the consequences of their failure so serious that, whatever the soil conditions, no effort should be spared in making as accurate a prediction of performance as possible. Where routine jobs are concerned, individual judgement based on low-cost sampling and testing may well suffice in the majority of cases, but such a method has a serious drawback; it does not allow extension of engineering knowledge based on observation and comparison with good quality data. Routine jobs are much more numerous than those for which the cost and time required for accurate and specialist testing can be justified, but can an engineer afford not to develop his experience and can he now afford the consequences of failure? Brunel and Stephenson could do so, for in their day experimental data were almost non-existent in the field of soil mechanics and it could be expected that the almost exclusive use of personal judgement would inevitably lead to some failures. We can no longer enjoy such luxury.
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