LAYERED CLAY-SAND SCHEME OF LAND RECLAMATION

Abstract

A layered clay-sand scheme is proposed as an alternative method for land reclamation projects where sufficient sand is not available as a fill material. In this scheme, hydraulically placed soft marine clay is sandwiched between thin layers of sand. Sedimentation of clay slurry, construction of a sand layer on the surface of sedimented clay and consolidation of very soft clay fills are identified as three major stages in the successful achievement of the above scheme and these stages are studied in detail. A field test was carried out to study the feasibility of the proposed scheme. Alternating clay and sand layers were successfully constructed during the field test. Borehole samples and Cone penetration tests confirmed the layered format ion. The seepage force was found to assist in the sedimentation of the clay slurry. The sedimented clay slurry supported the sand which was sprinkled 5 days after the end of clay pumping. The layered fill which consisted about 0. 5m thick clay layers consolidated and gained strength in approximately 3 weeks. Laboratory tests were conducted to study the effect of various factors such as the initial water content and seepage force on the sedimentation of marine clay slurry. Seepage force is found to assist in the sedimentation process whereas the initial concentration affects the p-S curve. A modified Kynch model is proposed to predict the clay-water interface during sedimentation. These prediction compare favourably with the laboratory observations. The rheological property of marine clay slurry was measured in the laboratory and found to follow Bingham behaviour. Non-destructive ?-ray technique was used to find the depth of sand penetration into clay sediments. Submerged clay sediments having water content less than 210¾ were found to support suddenly dropped 50 mm thick sand at its surface. An analytical model based on the laboratory experiments is proposed to predict the depth of penetration of a sand particle in clay sediments. A finite element computer program which analyses one dimensional finite strain consolidation problems is modified to incorporate initial void ratio variation with depth, single or double drainage condition and time dependent surcharge and seepage effects in the analysis. A graphical output module is also incorporated in the program. The results obtained from this program are supported by various experimental results published elsewhere under similar conditions. The prediction of settlement and pore pressure of the field test are in good agreement with the corresponding field measurements