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