Determination of Prestress in Cohesive Soils Using AE

In an attempt to develop an acoustic emission (AE) field procedure to directly determine preconsolidation pressures, laboratory feasibility tests on granular soils (published earlier as a companion paper; Ref. 7) and on cohesive soils (this study) have been conducted. Consolidation test specimens were prepared, stressed to known values and then unloaded. Upon restressing, acoustic emission and deformation data were taken for use in estimating the previously applied stress level, i.e., the preconsolidation pressure. Six soils, each at three different water contents were tested, and all 18 were evaluated at four different stress levels, i.e., 72 tests were performed. It was found that the average error between the known and estimated values of preconsolidation pressures Varied from +2.5%--4.4% for direct deformation (dial gage) monitoring, and from +5.5%--7.0% for AE monitoring. This variation was very similar to the results found in the prior study on granular soils. While not quite as accurate as direct deformation monitoring, the data suggests that AE monitoring is within engineering accuracy for such determination. Its strength, however, lies in the fact that the AE method can be readily used in field measurements, e.g., by adapting a pressuremeter into an "acoustic pressuremeter." This should enable one to augment (possibly in a very significant manner) the downhole deformation, or volume, measurements necessary in the field during standard pressuremeter testing.