an engineering geological characterisation of tropical clays - GBV

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Plate 7.7a. Shear testing showing preconsolidation
and shearing of specimens.
Plate 7.7b. Shear testing procedure
showing automatic data acquisition.
In this study, slow consolidated-drained (CD) direct shear tests (according to ASTM 3080;
DIN 18 137) were carried out on undisturbed specimens of soil samples previously collected
from the field using 100 mm diameter U-tubes. The consolidated-drained conditions were
provided for by allowing soil specimens to completely consolidate under selected normal
pressure before the shearing process to allow for complete dissipation of excess pore
pressures. Further drainage was provided for during shear by adjusting the motor speed to
give a suitably slow rate of displacement and thereby dissipate any additional pore water
pressure which could have further developed. In practice, the drained shear strength
parameters (Cd, φd) differ only slightly from effective shear strength parameters (c´, φ´ )
obtained from undrained tests in which pore water pressures are measured (Head, 1984). For
many purposes therefore, the two sets of parameters are considered to be equal so that this
study adopts the symbols c´, φ´; for the drained shear strength parameters of soils involved.
The shear tests served to provide the angle of internal friction, φ (degrees), as well as amount
of cohesion, c (kN/m²), of the soils investigated. The relative displacement (mm) of the two
portions of soil specimen as well as the applied shearing force (kN) were measured with time
(t ); and the results used to plot a shear stress/ displacement curve. The vertical movement
(mm) of the top surface of specimen, which indicates changes in volume, was also recorded
and enabled changes in density and voids ratio during shear to be evaluated.
Up to three tests at different normal loading conditions were carried out on specimens of the
same soil sample and the appropriate shear stress/ displacement curves drawn. The maximum
drained shear resistance and/ or shear strength offered by the soil at a given normal loading
condition was given by the peak value of the respective shear stress/ displacement curve. The
peak or maximum shear stress from each curve was read off and plotted against the
corresponding normal stress to give the failure envelope, i.e. an approximately straight linegraph
whose inclination to the horizontal axis represents the angle of shearing resistance, φ.
The cohesion, C, was given by the intercept of the line on the vertical stress axis. The straight
line relationship of the Coulomb or failure envelope is defined by Coulomb´s law (Coulomb,
1773), i.e.
Tf = C + σntanφ (7.16)
where Tf = maximum shearing resistance (kN/m²)
σn = normal stress (kN/m²)
C = cohesion (kN/m²)
tanφ = frictional component