Field and Laboratory Measurements of Small Strain Stiffness of Decomposed Granites

Non-linear stress-strain characteristics and stiffness-strain relationships of sedimentary soils and sands at small strains have been reported by many researchers. Research work on the behaviour of weathered or decomposed granites at small strains, however, has rarely been reported. This paper compares some stiffness measurements of decomposed granites from field investigations involving crosshole seismic, self-boring pressuremeter (SBPM), and high pressure dilatometer as well as results from laboratory tests using bender element and internal transducers. The in-situ crosshole measurements show that the elastic stiffness of Moderately Decomposed Granite (MDG, approximately 7000 MPa) is about 25% greater than that (about 5500 MPa) of Highly Decomposed Granite (HDG), which is in turn approximately 18 times higher than that (about 300 MPa) of Completely Decomposed Granite (CDG). This is likely attributable to the materials' different bond strengths and structures. A new method has been adopted to interpret the SBPM data. Measured data from crosshole seismic and self-boring pressure meter tests for CDG are found to be consistent. Bender element laboratory tests on CDG indicate that the measured A-coefficient in the expression of G₀/p_r = A(p/p_r)ⁿ lies between the results from clay and sand as reported in the literature. However, the measured n-value for CDG is generally larger for clays and sands. The measured bender element results are consistent with data from internal transducers. Highly non-linear characteristics of CDG were observed in both the laboratory and field tests. Generally the elastic stiffness of CDG as determined by laboratory tests is about 50-80% of that from field tests. Some possible reasons are discussed.