Experimental compression of loose sands: relevance to porosity reduction during burial in sedimentary basins

Abstract

This study examines the one-dimensional stress–strain behaviour of sand at effective stresses as high as 50 MPa. Experiments were performed on 22 sands (approx. 150 tests) with different grain size, uniformity coefficient, angularity, density, grain mineralogy, and clay content. The results show that minor grain corner crushing starts at stresses of 2–8 MPa. The point of maximum curvature (yield point) in the porosity (n) versus logarithm of vertical effective stress (σ'v) curve defines the initiation of marked particle crushing. The stress at the yield point varies between 3 and 31 MPa depending on sand characteristics. A low yield stress is indicative of high porosity loss in the interval of intermediate stress (5–25 MPa). The yield stress is low when the grain size is large, grains are angular, grain strength is low, and uniformity coefficient is low. The lowest yield stress value occurs in the coarser carbonate sand, and the highest in the chert-rich sands. The sands rich in clays are highly compressible up to 25 MPa. At stresses higher than ~10 MPa, the coarser biogenic carbonate sands maintain higher porosities than the other sands. This can be explained by the fact that coarser biogenic carbonate sands have low yield stresses due to high angularity and low grain strength and initially there is local grain crushing at grain contacts. This increases the area of the grain contacts, so the coarser carbonate sands become less compressible at higher stresses. Within the high stress range (25–50 MPa) the porosity loss differences related to grain size, grain shape, grain mineralogy, and sand uniformity coefficient are significantly reduced. Hence the greater compressibility of lithic and carbonate sands becomes less evident in the high-stress interval as the grain size increases.Key words: sand, grain crushing, grain size, high stress, compression.