Preconsolidation Pressure of Soft, High-plastic Clays
In the early days only the ground that was best from a geotechnical point of view was used for foundations, but now, owing to the growth of cities and to the rising prices of land, softer and more compressible soils, less suitable for foundations, are also often used.
The choice of foundation method is both economically and technically an important matter, and thorough testing of the soil is often needed to determine its geotechnical properties, and thus, to furnish necessary information.
The most important parameter in a discussion of fou:ndation methods on clay is the preconsolidation pressure, which is usually determined from the results of oedometer tests.
For the last 30 years, the loading procedure suggested by Terzaghi has been used, where the sample is loaded in increments of 24-hour duration each. The Casagrande construction is then used for determining the preconsolidation pressure. Lately other methods have been suggested by Leonards (1962) and Bjerrum (1973), where reduced increments of shorter duration are used so as to give a better defined pressure/compression curve.
The introduction of routines where the sample is continuously loaded made it possible to carry out more detailed studies of the stress/strain characteristics of clays in the laboratory.
Carefully performed full-scale tests are then needed to enable a comparison of the stress/strain behaviour in situ with that found in the laboratory.