Development of sounding equipment for the assessment of the time- settlement characteristics of recent alluvial deposits when subjected to embankment loads.
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Date
1992
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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 The piezometer cone performed so well that it superseded the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
Description
Thesis (Ph.D.)-University of Natal, Durban, 1992.
Keywords
Soil penetration test., Soils--Testing., Piezometer., Embankments., Theses--Civil engineering.