CASE STUDIES OF IN-SITU STRUCTURE OF NATURAL SEDIMENTARY CLAYS

SATORU SHIBUYA TOSHIYUKI MITACHI and SENG CHUL HWANG

ABSTRACT Natural sedimentary clays exhibit ageing effects on their behaviour when subjected to both consolidation and shearing. Attempts to characterising the in-situ 'structure' of soft marine clays have been made in case studies performed in Ariake (Japan) and in Bangkok (Thailand), each showing a typical profile of aged clay-structure with depth reflecting its site-specific geological consolidation history. In each site, profiles of Atterberg limits, natural water content, overconsolidation ratio (OCR), quasi-elastic shear modulus G_max and the undrained shear strength c_u with depth were attained from various laboratory tests, together with the a comparable profile of quasi-elastic shear modulus from in-situ seismic survey G_f. Quantitative assessment of the aged clay-structure was made by using two different measures, metastability index MI(G) and the soil constant reflecting structure S (Jamiolkowski et al., 1994). The profiles of MI(G) and the S-value with depth were both determined based on the corresponding G_max behaviour of the reconstituted sample. "Disturbance" of laboratory samples retrieved by using two different samplers (i.e., Laval and Japanese thin-walled samplers) with two different sampling techniques (i.e., pre-boring and displacement methods) was discussed. A new approach to estimate c_u "in the ground" , termed by the authors the MILK (Metastability Index coupled with Laboratory K_o test) method, is proposed A trial of the MILK-method is demonstrated in soft Bangkok clay, the texture of which is similar to "soft cheese ."

Key words: ageing, case study, clay, elastic shear modulus, in-situ test, laboratory test (IGC: D7/D6/C6/E8/ C2/B1)