ESTIMATING IN SITU MAXIMUM PAST (PRECONSOLIDATION) PRESSURE OF SATURATED CLAYS FROM RESULTS OF LABORATORY CONSOLIDOMETER TESTS

Abstract

The Casagrande method of estimating the in situ maximum past pressure (p sub c) from the results of laboratory consolidometer tests is presented and the various mechanisms that can cause P sub c are discussed. The principal factors influencing laboratory compression curves, recommended consolidometer test procedures, and specific recommendations for determining P sub C are presented. This discussion is restricted to the estimation of P sub c from incremental consolidometer tests on undisturbed samples of medium to soft saturated clays obtained from below the water table. A figure is used to illustrate the estimation of the value of P sub c and a 4-step procedure is set forth. Mechanisms (tabulated) by which clay might acquire P sub c greater than the existing vertical effective stress are: change in total stress, change in pore water pressure, change in soil structure, and change in strain rate on loading. The 3 most important factors influencing the determination of P sub c from laboratory consolidometer tests are sample disturbance, load increment ratio, and load increment duration. The equipment employed for incremental consolidometer tests should generally conform to the recommendations of ASTM D 2435-70. Particularly important freatues of such equipment are noted and comments are made on the test procedures and the presentation and interpretation of test data. The relative merits of incremental consolidometer tests and continuously loaded sample tests are also noted.