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Transitional behavior in well-graded soils: An example of completely decomposed granite

Elkamhawy, Elsayed, Zhou, Bo, Wang, Huabin
Engineering geology 2019 v.253 pp. 240-250
fabrics, granite, granite soils, isotropy, microstructure, sand fraction, soil mineralogy
This study presents the results of a series of isotropic compression tests and drained and undrained triaxial shearing tests on naturally graded and reconstituted completely decomposed granite soils to investigate their compressive response and transitional behavior. Naturally graded soils exhibited unique isotropic compression and critical state lines within the stress levels studied. Soils reconstituted by increasing the sand content by 5% and 10% revealed non-unique isotropic compression lines; however, unique critical state lines were present in the e-log p' space. The degree of non-convergence increased as the sand content increased. The value of the m parameter, which is used to quantify the transitional mode and is defined as the ratio between the changes in the final and initial specific volumes (i.e., m = ∆v2/∆v1; see Fig. 1), increased from 0 in the case of naturally graded soil to 0.3 and 0.73 for reconstituted soils with increased sand content of 5% and 10%, respectively. For naturally graded soils, the initial void ratio affects only the initial compression response, and its influence does not extend to the normal compression line. In the case of reconstituted soils, the fabric that results from various compaction efforts exerts a significant effect on the soil′s compressive response. However, this fabric cannot withstand large strain during the shearing path and is completely destroyed, producing a unique fabric in the critical state. Both natural and reconstituted soils revealed isotropic responses during the loading and unloading compression paths with unrecoverable volume change. The transitional behavior is no longer limited to gap-graded soils, and grading is no longer the only dominant factor in this behavior. The transitional mode occurs due to relatively inconsiderable changes in soil mineralogy, grading, and microstructure.