The Seventh James K. MitchelL Lecture: Characterization of silt/sand soils

The term silt/sand or M/S is proposed as an inclusive abbreviation of soils that can span from sand with very little silt, silty sand to pure silt. It is generally believed that sands with fines (particles passing #200 sieve) tend to be more compressible. Because of their low permeability, cone penetration tests (CPT) in sands with fines can be partially drained. High compressibility makes the soil more contractive and thus showing lower resistance in undrained shearing. Significant ground subsidence can also be associated with the high compressibility of M/S/ soils. For CPT in granular soils with similar density and stress states, the high compressibility and partial drainage can both contribute to lower cone tip resistance. Natural granular soils are more likely to contain fines than being clean (fines content < 5%). Studies on M/S soils are far less than those on clean sands. Because of the unique geological setting, the author had the opportunity to work with a local M/S deposit in Central Western Taiwan in the past 25 years. Procedures for laboratory soil element tests as well as CPT calibration tests using reconstituted specimens have been developed and a series of tests performed. Practical undisturbed sampling techniques in M/S soils were experimented and applied. Methods to correlate cyclic strength, fines contents and cone tip resistance in M/S deposit were proposed. The concepts of equivalent granular void ratio and state parameter were experimented in compiling the test data. The paper describes the new characterization techniques developed and lessons learned in the interpretation of test data for the studied M/S soils.