Development of sounding equipment for the assessment of the time- settlement characteristics of recent alluvial deposits when subjected to embankment loads.

Abstract

Many embankments on the soft, highly variable, recent alluvial deposits along the South African coast have suffered large settlements necessitating ongoing costly repairs. Due to the soft variable soils, borehole sampling is difficult and laboratory testing requires to be extensive for adequate subsoil modelling; cone penetration testing was considered to be a potential means to overcome these problems. Twenty five years ago in South Africa, as elsewhere, cone penetration testing equipment was relatively crude and the methods of interpretation were simplistic. The application of cone penetration testing to recent alluvial deposits therefore required improvements to both the equipment and the derivation of soil parameters. The equipment was upgraded by introducing strain gauge load cells capable of measuring cone pressures in soft clays with adequate accuracy. Hence, correlations of cone pressures with compressibility and shear strength became possible. Predictions of settlement times and magnitudes are of equal importance and a consolidometer-cone system was developed to assess both of these. A piezometer was incorporated into a cone to ascertain whether the settlements were due to consolidation. The piezometer cone performed so well that it superseded the consolidometer-cone and by 1977 a field piezometer cone was in regular use. Developments in piezocone interpretation have taken place concurrently with those in equipment; coefficients of consolidation are evaluated from pore pressure dissipations, and soils identified from the ratio of pore and cone pressures. These developments have been validated in two recent research projects, by comparing measured and predicted settlements at eleven embankments monitored for up to fifteen years. The data shows that for embankments on the recent alluvial deposits the constrained modulus coefficient, am is : am = 2,6 ± 0,6 The data also shows that coefficients of consolidation from piezometer cone dissipation tests are correlated with those from laboratory tests and back analysed embankment performance as follows : Embankment c = 3 CPTU c = 6 Lab cv It is concluded that piezometer cone penetration testing is particularly suitable for the geotechnical investigation and the subsequent design of embankments on recent alluvial deposits and should be considered as complementary to boreholes with sampling and laboratory testing. The existing database of embankment performance should be expanded with particular emphasis on long term measurements and on thorough initial determination of basic soil parameters