Evaluation of a sand-mat system in a reclamation

Abstract

One of the major problems encountered in using dredged seabed clay and waste soil from construction as fill materials for land reclamation is the long consolidation time needed if there is no soil improvement. The idea of the clay-sand mat scheme, incorporating a horizontal drainage sand mat at the bottom of the reclamation pond is to accelerate the consolidation process in such a clay fill during the long filling time, allowing for more such materials to be used as fill. The objective of this study is to establish an understanding of the consolidation behaviour of the clay-sand mat system and specifically to provide some quantitative evaluation of the performance of horizontal drainage sand mat during selfweight and surcharge consolidation. This is to facilitate the development of a guideline for evaluating the performance of such a clay-sand mat scheme. The study has to take into consideration of large strain that will develop and non-linear stress-strain relationship of soil properties as the soil settlement is expected to be large when soft clay is used as fill material. The main driving impetus is that in a reclamation project, the area is usually very large and thus any saving in the thickness of the sand will mean substantive saving. This is also related to the fact that the cost of placing such sand mat is usually much higher than normal dumping of sand.The first phase of the study involved the conduct of centrifuge model tests to investigate the process of selfweight and surcharge consolidation of clay fill under nearly ideal single and double drainage conditions, which represent the two extreme cases in such clay-sand mat scheme. Comparisons between the centrifuge results and numerical results show that the progress of consolidation predicted by non-linear finite strain consolidation theory are much better than those using infinitesimal strain and constant average soil properties. The time ratio (TSD/TDD), is defined as the ratio of time for clay layer to reach the same average degree of consolidation under singly drained consolidation over that under doubly drained consolidation. It provides valuable information on the maximum time saving achievable on the application of such a clay-sand mat scheme. When the time ratio is small, the benefit of applying the scheme is less attractive. Centrifuge and finite strain results confirmed that the time ratio varies with the type and thickness of clay. This value is very much dependent on the non-linearity in the consolidation properties of soil and could be much lower than the value predicted by conventional Terzaghi consolidation theory.Another series of centrifuge model tests have been conducted in the second phase of the study to investigate the inter-relationship of the parameters influencing the drainage efficiency of such a horizontal drainage sand mat. The experimental results confirmed that the characteristic factor (=(ks/kc)*(hs/L)*(hc/L)), which was first introduced by Tan et al. (1992), is a fundamental parameter that controls the drainage behaviour of sand mat. A single curve is obtained when the drainage efficiency is plotted against this characteristic factor. The same drainage efficiency curve is applicable for the assessment of the performances of both the selfweight and surcharge consolidation of clay-sand mat scheme.