DECOMPOSED GRANITE AS AN EMBANKMENT FILL MATERIAL: MECHANICAL PROPERTIES AND INFLUENCE OF PARTICLE BREAKAGE. FINAL REPORT. VOLUME 1

The suitability of decomposed granite as an embankment fill material has been investigated. Using a sample from the Shasta Bally batholith in Northern California, important mechanical properties were studied in oedometer and triaxial tests. In addition, the feasibility of using geogrid reinforcements in decomposed granite embankments to enable steepening of side slopes was investigated through direct shear and pullout tests. Experimental results show that breakage is primarily controlled by the applied strain level rather than the stress level. Shear-induced breakage under conventional triaxial conditions is greater than under oedometer conditions, because of the greater shear stress/strain component under triaxial conditions. The friction angle of compacted decomposed granite decreases significantly with increasing stress level. In dense triaxial specimens, under confinements ranging from 100 to 1500 kPa, the reduction in peak angle of shear resistance value was about 25%. The pullout coefficient of interaction of a geogrid embedded in dense decomposed granite decreased by more than 50% when normal pressure was increased from 70 to 700 kPa. In direct shear (decomposed granite-geogrid) interface strength test, the residual angle of shear resistance values were nearly equal to those from direct shear tests of the soil alone. Settlement and hydrocompression in oedometer specimens were not large under axial pressures as high as 1600 kPa, probably because breakage in these specimens was small. However, decreasing the compaction water content significantly increased the hydrocompression.