Methods for evaluating sample disturbance of deep clayey soil in Shanghai

To accurately understand the in situ mechanical properties of soils, it is necessary to evaluate the degree of sample disturbance. The existing sample disturbance evaluation methods mostly focus on shallow soils, while the counterpart for deep clayey soil is needed. This work investigates the applicability of the existing methods for evaluating sample disturbance of deep clayey soil, via conducting laboratory one-dimensional consolidation and shear wave velocity tests on Shanghai deep clayey soil, performing constitutive modeling of structured clay, and comparing test data in the literatures. A new sample disturbance index is proposed, which is based on the shear wave velocity and excludes the influences of stress state and void ratio. The performance of the disturbance index is assessed against the test results from this study and the literatures. The results show: (1) There is a significant difference in sampling disturbance between the block samples and borehole samples of Shanghai deep clayey soil. (2) For the soil samples with the same disturbance, the existing index ∆e/e₀ increases with the increase of the sampling depth, which may overestimate the sampling disturbance of deep clayey soil, and cannot reflect the above-mentioned difference in the disturbance degree. (3) The disturbance index V_{s, lab}/V_{s, in situ} (i.e., the ratio of shear wave velocity measured in the laboratory to that in situ), cannot reflect the difference in the disturbance degree of Shanghai deep clayey soil caused by different sampling methods, and its application to other clays gives a relatively large dispersion along depth. (4) The proposed disturbance index can overcome the influences of sampling depth, and for the clayey soil in the same region, it can uniformly quantify the disturbance degree caused by sampling.