ABSTRACT
In Chapter 5, laboratory tests for determining the constitutive behaviour of soil (strength and stiffness properties) were described. While such tests are invaluable in quantifying the mechanical behaviour of an element of soil, there remain a number of disadvantages. First, to obtain high quality data through triaxial testing, undisturbed samples must be obtained, which can be difficult and expensive in some deposits (e.g. sands and sensitive clays, see Chapter 6). Second, in deposits where there are significant features within the macro-fabric (e.g. fissuring in stiff clays) the response of a small element of soil may not represent the behaviour of the complete soil mass, if the sample happens to be taken such that it does not contain any of these features. As a result of such limitations, in-situ testing methods have been developed which can overcome these limitations and provide a rapid assessment of key parameters which can be conducted during the ground investigation phase. In this chapter the four principal in-situ testing techniques will be considered, namely:
the Standard Penetration Test (SPT);●● the Field Vane Test (FVT);●● the Pressuremeter Test (PMT);●● the Cone Penetration Test (CPT).●●
In each case the testing methodology will be briefly described, but the focus will be on the parameters which can be measured/estimated from each test and the theoretical/empirical models for achieving this, the range of application to different soils, interpretation of constitutive properties (e.g. φ′, cu, G), stress history (OCR) and stress state (K0) from the test data, and limitations of the data collected. The worked examples in the main text and problems at the end of the chapter are all based on actual test data from real sites which have been collated from the literature. In these examples/problems extensive use will be made of spreadsheets to perform the required calculations, and digital data for this purpose are provided on the Companion Website The four techniques listed above are not the only in-situ testing techniques; a Dilatometer Test (DMT), for example, is similar in principle of operation and in the properties which can be measured to the PMT, involving expanding a cavity within the soil to determine mechanical properties. This common test will not be discussed herein, but references are provided at the end of the chapter for further reading on this topic. Plate Loading Tests (PLT) are also in common usage – these involve performing a load test on a small plate which is essentially a model shallow foundation, and are most commonly used to derive soil data for foundation works due to the close similarity of the test procedure to the ultimate construction. Soil parameters are then back-calculated using standard techniques for analysing shallow foundations, which are described in detail in Chapter 8. It should also be noted that geophysical methods for profiling which use seismic methods (e.g. SASW/MASW, seismic refraction) measure the shear wave velocity (Vs) in-situ from which G0 can be determined and are therefore in-situ tests in their own right (these methods were previously described in Chapter 6). The data collected from in-situ tests should always be considered as complementing rather than replacing sampling and laboratory testing. Indeed, three of the tests that will be discussed (SPT, FVT, PMT) require the prior drilling of a borehole, so a single borehole may be used very efficiently to gain visual identification of materials from spoil (Section 6.4), disturbed samples for index testing and use of subsequent empirical correlations (Section 5.9), undisturbed samples for laboratory testing (Chapters 5 and 6) and in-situ measurements of soil properties (this chapter). These independent observations should be used to support each other in identifying and characterising the deposits of soil in the ground and producing a detailed and accurate ground model for subsequent geotechnical analyses (Part 2 of this book).
