SAMPLE QUALITY OF COHESIVE SOILS: LESSONS FROM THREE SITES, ARIAKE, BOTHKENNAR AND DRAMMEN

HIROYUKI TANAKA

ABSTRACT: Sample quality was studied at three sites; Ariake (Japan), Bothkennar (Britain) and Drammen (Norway), using various types of samplers. It was found-that the unconfined compression strength (q_u) is significantly affected by the sample quality. Sample quality obtained by the Japanese standard (JPN) sampler is the same as that by the Laval or the Sherbrooke samplers, which have a worldwide reputation for their ability to get high quality samples. However, there is a remarkable difference in q_u for the lean clay layer of the Drammen clay between the JPN and the Sherbrooke samplers. It is concluded from a series of soil investigations that the reduction of the q_u value may be caused by two different reasons: loss of the residual effective stress and destruction of the soil structure. In most of the previous studies, these two phenomena were thought to take place concurrently. When the structure is kept unchanged and only the residual effective stress is lost, the recompression technique, where a specimen is consolidated under the same stress level as in situ, is effective in producing the in situ soil behavior. However, if the soil structure is destroyed and its e-log p curve is different from the original one, the recompression technique is not capable of providing real soil behavior.

Key words: cohesive soil, consolidation, sample disturbance, sampling, shear strength, soil structure, triaxial compression test, unconfined compression test (IGC: C6)