Drainage Efficiency of Sand Layer in Layered Clay‐Sand Reclamation

In the layered clay‐sand scheme of land reclamation, horizontal sand layers are sandwiched between hydraulically placed marine clay to provide shorter drainage paths for the consolidation of the clay. The rate of consolidation of the fill will depend on the drainage efficiency provided by the sand layers. From the governing equation of the problem, it is shown that the drainage efficiency of the sand layer is controlled by a characteristic factor called $\lambda [\lambda =(k_s/k_c\left)\text{.}\right(H/L)\text{.}{H}_s/L)]$. This factor is composed of permeability and flow‐length ratios of the sand to clay, together with the ratio of the sand thickness to its horizontal length. When the drainage efficiency is defined on the basis of its retardation to primary consolidation settlement, a unique curve for the efficiency value can be obtained as a function of λ for the point of 50% and 90% degree of settlement. The theory is then tested against two case histories of land reclamation projects using the layered clay‐sand scheme. It is shown that the drainage performance of the systems in the field agreed very well with that predicted by the present study. It is observed that for the sand layer to be fully effective as a drainage filter, the characteristic factor λ of a given system must be above 20, giving an efficiency greater than 95%. However, a system with a λ value of 6 is sufficient to achieve a satisfactory drainage efficiency of 80%.