ABSTRACT
There can be many uncertainties in the application of soil mechanics in geotechnical engineering practice. Soil is a natural (not a manufactured) material, therefore some degree of heterogeneity can be expected within a deposit. A ground investigation may not detect all the variations and geological detail within soil strata, so the risk of encountering unexpected conditions during construction is always possible. Specimens of relatively small size, and subject to some degree of disturbance even with the most careful sampling technique, are tested to model the behaviour of large in-situ masses which may exhibit features which are not included in the test specimen (e.g. fissures in a heavily overconsolidated clay). Results obtained from in-situ tests can reflect uncertainties due to heterogeneity (e.g. values of Nk in Figure 7.22). Consequently, judgements must be made regarding the characteristic soil parameters which should be used in design. In clays, the scatter normally apparent in plots of undrained shear strength against depth is an illustration of the problem of selecting characteristic parameters (e.g. Figure 5.38). A geotechnical design is based on an appropriate theory which inevitably involves simplification of real soil behaviour and a simplified soil profile. In general, however, such simplifications are of lesser significance than uncertainties in the values of the soil parameters necessary for the calculation of quantitative results. Details of construction procedure and the standard of workmanship can result in further
uncertainties in the prediction of the performance of geotechnical constructions. Section 13.2 discusses the interpretation of ground investigation data, and the selection of characteristic values for use with the design techniques outlined in Chapters 8-12. In most cases of simple, routine construction, design is based on precedent/experience and serious difficulties seldom arise. In larger or unusual projects, however, it may be desirable, or even essential, to compare the actual performance to that predicted during design. Lambe (1973) classified the different types of prediction. Class A predictions are those made before the event. Predictions made during the event are classified as Class B, and those made after the event are Class C: in both these latter cases no results from observations are known before predictions are made, though further independent ground data may be available at these later stages to develop more reliable characteristic values of the soil parameters. If observational data are available at the time of prediction these types are classified as B1 and C1, respectively, with the observational data usually being used to infer what the values of the soil parameters must be to give the observed response (this procedure is also sometimes referred to as back-calculation). Studies of particular projects (case studies), as well as showing whether or not a safe and economic design has been achieved, provide the raw material for advances in the theory and application of soil mechanics. Case studies normally involve the monitoring over a period of time of quantities such as ground movement, pore water pressure and stress. Comparisons are then made with theoretical or predicted values, e.g. the measured settlement of a foundation could be compared with the calculated value. If failure of a soil mass has occurred and the profile of the slip surface has been determined, e.g. in the slope of a cutting or embankment, the mobilised shear strength parameters could be back-calculated and compared with the results from laboratory and/or in-situ tests. Empirical design procedures are based on in-situ measurements from case studies, e.g. the design of braced excavations is based on measurements of strut loads in different soil types (see Figure 11.37). The measurements required in case studies depend on the availability of suitable instrumentation (described in Section 13.3), the role of which is to monitor soil or structural response as construction proceeds so that decisions made at the design stage can be evaluated and if necessary revised. The use of measurements to continuously re-evaluate design assumptions (Class B1 analyses) and refine the design or modify/control construction techniques is known as the Observational Method (Section 13.4). Instrumentation can also be used at the ground investigation stage to obtain information for use in design (e.g. details of groundwater conditions, as outlined in Chapter 6). However, instrumentation is only justified if it can lead to the answer to a specific question; it cannot by itself ensure a safe and economic design and the elimination of unpredicted problems during construction. It should be appreciated that a sound understanding of the basic principles of soil mechanics is essential if the data obtained from field instrumentation are to be correctly interpreted. Section 13.5 will introduce a set of case histories covering a range of different geotechnical constructions. A detailed evaluation is not given in the main text, but each may be found as a self-contained docu ment which may be downloaded from the Companion Website. In these cases, the basic ground investigation data will be given, from which characteristic values will be interpreted using the methods outlined in Section 13.2. The appropriate limit states will then be verified to Eurocode 7 as fully worked examples, and the results compared with the observed performance to demonstrate that the limit state design procedures described within this book produce designs which are acceptable.
