Abstract:
The design of footings and the accurate prediction of settlements is inherently difficult as the parameters involved vary considerably. The initial stage of the design consists of three steps. The first step is to define all the parameters required. The second step is to give quantitive values to these parameters by performing tests and calculations. The third and last step should be the investigation of changes in external or local conditions on these parameters. The importance of this step is brought to the fore when considering collapsing soils and heaving clays. The next stage of the design would be to consider the stress changes at either an average point or a grid of points below the footing within the soil mass. To do this the engineer makes use of conventional methods of analysis. Factors such as horizontal strata and variations of properties with depth must be considered. Once the engineer has defined all the stress changes he must then decide which method of settlement prediction would be appropriate. The deformations of the soil mass do not only depend on the change in stress, but also on the previous stress-strain history of the soil and the actual variation of stresses with time during the settlement process. (e.g. see the error involved when considering the Skempton-Bjerrum method. Refer to Chapter 7). As the stress path accurately reflects all the stress changes the stress path method is an excellent tool for describing the stress and strain variations. There are numerous methods all of which have their merits and disadvantages. These will be discussed, and the influence of the stress path on settlement prediction is also analysed. Although a stress path method of settlement prediction might not always be essential, an analysis of a typical stress path for the particular problem would appear to be an excellent initial approach to deciding on the validity of any method of settlement prediction. The final stage is the accumulation of data and the prediction of the settlement value using one or more of the following methods: 1) Find the field penetration using the Dutch Cone Penetrometer {or any accepted penetrometer). This value is then used in conjunction with predetermined design curves to find the settlement. 2) Make use of settlement formulae which are directly derived from elastic theory. These do not take into consideration any of the variations of properties within the soil mass (see Chapter 7). 3) Define an average point or a number of grid points below the foundation (see Chapter 5). For each point evaluate a stress variation and hence calculate a corresponding strain value. The settlement of the footing is then calculated using the strain values. The above estimated settlement values are also to be regarded with caution, as foundation size and rigidity do influence predicted values. There are however methods to diminish these inaccuracies (see Chapters 3 and 7). The engineer should be able to predict the stress variations with reasonable accuracy if he has a knowledge of the influences of all these factors on the idealised elastic case (e.g. foundation size, rigidity, existing horizontal stresses etc.). It therefore becomes apparent that the predicted settlement is not a 'wild estimate', but a carefully analysed value which can be forecast with confidence, providing the proper tests have been made.
Reference:
Errera, L. 1977. Stress paths and collapsing soils. University of Cape Town.
Includes bibliography.