GEOGUIDE 1 GUIDE TO RETAINING WALL ... - HKU Libraries

44testing can be obtained from trial pits (GCO, 1987). Trial pits are particularly useful forassessing the mass characteristics of the ground, as the fabric of the saprolite as well as anyrelevant features in the ground can be examined at the time of the pit excavation. It istherefore recommended that trial-pitting is carried out wherever possible, especially for thepurpose of foundation design.Saprolitic soils are easily disturbed by sampling and specimen preparation procedures.The effects of any disturbance should be taken into account when analysing the test results.An estimate of the lower bound strength of a test specimen may be obtained by examiningthe portion of the stress-strain data in a triaxial compression test which corresponds to thecritical state. Theoretically, the approach of such a state is indicated by the effective stressratio reaching a constant value in an undrained test, or the dilation rate approaching zero ina drained test. However, it has been found in practice that the critical state is often notachieved even at high strains for the saprolitic soils commonly encountered in Hong Kong.For such soils, an estimate of the critical state angle of shearing resistance may be moreconveniently obtained by carrying out shear tests under high confining pressures onspecimens which have been remoulded to a loose state. Care is required to interpret the testresults, which may be influenced by the effects of any relict structures or fabric not destroyedby the remoulding process and by the effects of grain crushing. Knowledge of the criticalstate strength, if considered together with the results of laboratory shear tests on'undisturbed 1 intact samples, can assist in the selection of soil shear strength parameters fordesign (Figure 5).5.5.5 ColluviumColluvium, the product of the downslope movement of earth materials essentiallyunder the action of gravity, can be very heterogeneous even within a small area. Incharacterizing a colluvial deposit, consideration should be given to the age of the deposit, thenature of the matrix material (including soil type and degree of cementation) and theproportion, distribution and nature of the coarse ifragments. Where the coarse fragments incolluvium can be characterised with confidence, the mass strength of the colluvium can beassessed according to Brand (1992). Otherwise, the shear strength of the matrix only shouldbe used in a design.A large number of tests are required to evaluate the shear strength parameters of avariable colluvial deposit, for which a c' -