A Study of Direct Shear Testing using a Large Shear Box

Dr Stephen Fityus, The University of Newcastle

Reinforced soil walls are finding increasing application in Australian civil engineering infrastructure. The commonly applied Q181C ‘Test Method’ of Direct Shear Testing to estimate the ‘Effective angle of internal friction at constant volume conditions for granular (coarse grained) materials’ is commonly applied to assess the suitability of backfill soils. This presentation presented the outcomes of a comparison between the results of three series of shear box tests on a typical ripped rock material that might be considered as a possible backfill material for a reinforced earth wall. One set of tests was performed using a 300mm shear box, on a soil sample containing only sub-19mm fractions. The second set of tests was also carried out using a 300mm shear box, but on a sub-4.75mm sample, derived from the sub-19mm sample by removal of the 4.75-19mm size fractions. The third set of tests was performed using a standard small 60mm shear box on the same sub-4.75mm fraction of the soil. Tests were performed at a range of shearing rates. The effect of sample preparation (by pre-testing of samples or by using different consolidation stresses) was also considered. The results showed that accurate measurements of effective friction parameters for coarse grained, granular backfill soils require:

• the use of fresh soil specimens for each shearing test;
• the use of a large shear box that can accommodate soils with relatively large particles;
• careful selection of shearing rates so that shearing takes place under drained conditions.

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