On the Liquid Limit of Diatomaceous Soils: Complex Behavior of a Non-Standard Material

During the liquid limit (LL) testing on diatomaceous soil from the fall cone and Casagrande cup tests, some eccentric behaviors have been observed: (1) There are inconsistent LL results of diatomaceous soil using the fall cone (BS 1377) and Casagrande cup (ASTM D4318) method. Based on the current criteria of these standards, the LL for diatomaceous soil tested with the fall cone is much higher (approximately 30%) than that from the Casagrande cup. (2) With the Casagrande cup test, diatomaceous soil slides towards the groove instead of flowing plastically as is assumed in the slope stability framework used for data interpretation. This may be related to the underpredicted LL from the Casagrande cup test. (3) Both tests expect the specimen to be tested from dry state to wet state, which leads to little water content change during the whole test for diatomaceous soil. Hence, there is a distinct difference between running the tests from dry to wet and wet to dry for diatomaceous soil, resulting in different LL and flow indices. In the current work, we perform laboratory vane shear tests to accompany fall cone tests and Casagrande cup tests and adopt the well-accepted theory that the undrained shear strength of fully remolded fine-grained soils at their LL is around 2 kPa (typically 1.7-2.3 kPa). We also consider modifications to the test procedures and equipment: coat the Casagrande cup with sand to force the flow failure mechanism and run both tests from wet state to dry state. Preliminary results from testing are presented and discussed.