Laboratory testing of unconfined compressive strength of cement stabilised soil cores

Abstract

Deep cement mixing has become widely used in reclamation works especially for mega projects in Hong Kong. According to Interim Guidelines on Testing of Unconfined Compressive Strength of Cement Stabilised Soil Cores in Hong Kong, unconfined compressive strength test will be carried out to verify the quality of the reclamation work. Cement stabilised soil cores drilled from the reclamation site usually cannot satisfy the length to diameter ratio of 2 which ensure a more uniform stress distribution during loading compared with the cores with the length to diameter ratio less than 2. As is suggested in ASTM C42-03, correction factors are recommended to be applied to the unconfined compressive strength of the specimen with length to diameter ratio from 1 to 2. To find the correction factors for different length to diameter ratios of cement stabilized soil core, Hong Kong marine deposit, Portland blast-furnace cement and seawater from Tung Chung is prepared to mould specimen on which unconfined compressive strength test following the recommended test method in Interim Guidelines is performed. Kaolin as a substitute soil of marine deposit are also prepared. 155 kaolin specimens and 60 marine deposit specimens of different binder dosage and curing days have been tested to determine the correction factors. There are old apparatus and new apparatus for specimen preparation in Public Works Central Laboratory (PWCL). Before preparation of the specimens for unconfined compressive strength test, Kaolin specimens are moulded by old apparatus to standardize the laboratory mixing specimen preparation procedures for Hong Kong soils. Dry density of specimens cured for 7 days is regarded as the standard indicator of the uniformity. The preparation for kaolin specimens and marine deposit specimens by new apparatus have some difference in pretreatment and the mixing frequency. More kaolin and marine deposit specimens are needed to determine the correction factor.