Precompression design for secondary settlement reduction

A large water treatment plant is to be built on soft deltaic deposits. Prec ompression has been selected as the method for achieving the required groun d improvement. An instrumented preload test has been carried out to obtain reliable information on precompression performance. Distributions of displa cements throughout the foundation depth, obtained using sliding micrometer extensometers, have proved extremely useful for identifying the mechanisms of behaviour controlling ground deformation. The magnitudes of displacement s are largely dependent on the overconsolidation state of the soil. As prim ary consolidation settlements take place rather quickly, they can be largel y controlled by applying a preload over a limited period, The main design c riterion therefore concerns secondary settlements. Laboratory and field dat a indicate clearly that overconsolidating the soil, even in moderate amount s, significantly reduces the secondary compression rate. The performance of an unloading stage in the preload test provides crucial information in thi s regard. Therefore applying a preload surcharge larger than the final stru cture load is quite effective in controlling the magnitude of subsequent se condary settlements. The information collected during the site investigatio n and in the preload test provides the basis for the development of a groun d deformation model that can be used for computing settlement histories. Th e model is used as a design tool for the final proposal of the precompressi on treatment required for the various structures of the plant.