4 - HKU Libraries

-15-S - c 1 + a s tan 0 f . (2)where c ! and 0 ? are termed the effective strength parameters. These parametersshould be used for earth pressure calculations in Hong Kong soils- It isimportant to note that the design strength parameters must be those determinedin the laboratory for the range of effective stress which is appropriate to thefield situation.Laboratory triaxial tests or shear box tests are commonly used todetermine the strength envelope of a soil. Guidance on these methods ofstrength measurement and on the interpretation of test results can be obtainedfrom Lambe & Whitman (1969) and from the Geotechnical Manual for Slopes(Geotechnical Control Office, 1979).The following two types of triaxial tests can be used :(a) Consolidated-undrained tests with pore pressure measurement(CU tests) carried out on specimens saturated using backpressure.(b) Drained tests (CD tests) on saturated specimens.Shear box tests are simpler to carry out than triaxial tests butonly drained tests can be conducted on Hong Kong residual soils. Care shouldbe taken to ensure that test specimens are soaked for a sufficient period priorto testing and that submergence is maintained during shear«The shear strength of a backfill material depends on its density*and laboratory strength tests should be carried out on specimens compactedto the density that will exist insitu. Where inadequate shear strengthinformation is available at the time of preliminary design 9 the followingvalues may be taken as guidance to the properties of compacted Hong Kongsoils :For decomposed volcanics, c f « 0, 0 f « 35°,Ya 88 1750kg/m 3For decomposed granite, c' « 0 9 0 ? « 39°,Yd ** 1850kg/m 3