Mechanical behaviour of compacted decomposed granite soil
The purpose of this research was to determine the mechanical behaviour of decomposed granite soil, especially when it is used as a construction material. The soil tested was a completely weathered granite soil (grade V) taken from Seoul, Korea and was a well graded silty sand and gravel. Triaxial compression tests have been carried out on compacted samples and special attention has been given to the determination of the effects of the moisture condition on the behaviour. The test results show that the soil conforms to the basic features of critical state soil mechanics, although particle crushing takes place during loading. A unique normal compression line, critical state line and the state boundary surface have been identified. This verifies the application of critical state soil models to the behaviour of crushable soils, which has been already revealed by the work on carbonate sands (Coop, 1990). In detail, however, the soil shows a number of unusual features. The soil exhibits very high peak and ultimate strengths. Peak friction angles range from 390 to 51° and the ultimate friction angle is 390• The main source of these high strengths seems to be the soil's mineral composition, in particular the high proportion of feldspars. Particle crushing has been observed during the test. The behaviour of the soil is considerably affected by this phenomenon. The effects of particle crushing appear in a high value of the ratio A/ic and in the location and shape of the state boundary surface. The amount of particle crushing has also been found to be dependent on the sample moisture condition as well as the maximum stress applied. Less particle crushing occurred in dry soils giving rise to a greater peak strength and higher ultimate specific volume for the dry soils. However, the ultimate strength of the soil is not affected by the moisture condition. This water sensitive behaviour has been clearly demonstrated by flooding tests. A comparision was also made between the behaviour of compacted and truly over—consolidated samples which had been unloaded from higher stress levels. Compacted samples usually exist on the dry side of critical, however, their behaviour is different from that of the truly over—consolidated samples. Compacted samples are less stiff and do not show distinct yield points. Soils may also be densified by flooding, in which case the peak and ultimate strengths are comparable to mechanically compacted soils but the stiffness is not much improved by flooding. Samples compacted in a wet state and later air—dried show very high strengths and stiffnesses. These high strengths and stiffnesses are due to the suction between the soil particles and once flooded they follow the paths of the saturated samples.