The behaviour of a soft recent sediment under embankment loadings

Abstract

Summary The behaviour under load of a thick deposit of normally consolidated recent sediments in north-west Malaya was studied by means of full-scale trial embankments. Two parallel embankments of 9 ft and 11 ft in height, with slopes averaging one vertical to two horizontal and lengths of about 200 ft were constructed. The soft foundation soils were investigated and instrumented, and provided the following information: (1)The in-situ three-dimensional permeability of the stratum was some two to three orders of magnitude higher than the vertical permeability computed from laboratory tests. (2) Consolidation settlements beneath the embankments were therefore extremely rapid and largely completed within two or three months of the end of construction, in contrast to periods of years predictable from oedometer tests. (3) There appeared to be a reduction in the permeability of the soft foundation soils under-going settlement as well as an increase in strength of the soils, the latter occurring largely during the early consolidation stages. (4) The major part of the settlement is believed to be provided by secondary consolidation of the soft soils, a process which is still continuing at the time of writing. Normal consolidation theory for settlement predictions is shown to be applicable only for loads below a certain threshold level. Some laboratory and field values of the coefficient of secondary consolidation, C α , are quoted and the application of this parameter is briefly discussed. (5) The normally consolidated sediments showed several characteristics typical of over-consolidation. The results of the trial were invaluable in the design of further embankments in the area, and in avoiding installation of unwarranted measures such as sand drains.