Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains

Abstract

a b s t r a c tCyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, γ c , around the threshold shear strain for cyclic pore water pressure buildup, γ tp E 0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small γ c below γ tp where there is no buildup of cyclic pore water pressure, Δu N , with the number of cycles, N, the cyclic secant shear modulus, G SN , initially increases with N for 10-20% of its initial value G S1 and then levels off or just slightly decreases, (ii) at small γ c between γ tp E0.01% and 0.10-0.15%, Δu N continuously increases with N while the modulus G SN first increases for up to 10% of G S1 and then gradually decreases, and (iii) at γ c larger than approximately 0.15%, relatively large Δu N develops with N while the modulus G SN constantly and significantly decreases. This means that at γ c between γ tp and 0.10-0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of γ c , the pore water pressure Δu N can reach up to 40% of the initial effective confining stress before G SN drops below G S1 . The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening.

Undrained Seismic Compression of Unsaturated Sand

“…As there was no effect of unsaturated conditions on the interparticle stresses between the particles in dry and near-saturated conditions, these two extreme conditions resulted in lower initial secant shear modulus and higher initial damping ratios compared with those for the unsaturated sand specimens, as shown in Figures 6(a) and 6(b), respectively. The secant shear modulus decreased sharply in the first 30 cycles due to the rapid increase in pore water pressures and the resulting decrease in mean effective stress, which agrees well with the findings in the evolution of secant shear modulus for saturated sands at larger cyclic shear strain amplitude by Vucetic and Mortezaie (2015). Despite the decrease in mean effective stress observed in Figure 4(d), the secant shear moduli for the unsaturated sand specimens were observed to increase slightly during cyclic shearing.…”

Excess pore pressure generation and post-cyclic loading settlement of geofiber-reinforced sand

“…The larger the geofibre content is, the greater the post-cyclic loading settlement becomes. The limited settlement below r u = 0.4 may be attributed to cyclic degradation characteristics of fully saturated sand in undrained conditions as reported by Vucetic and Mortezaie [15] where sand was found to undergo cyclic degradation only after r u = 0.35 is reached. Kenan Hazirbaba, Maksat Omarow greater than that of clean sand specimen.…”

Threshold Shear Strains for Cyclic Degradation and Cyclic Pore Water Pressure Generation in Two Clays

“…In the cyclic tests on dry sand, Drnevich and Richart (1970) obtained an increase in G SN at γ c smaller than γ c necessary to generate the permanent volume change, i.e., at γ c smaller than the threshold shear strain for cyclic compression (the volumetric cyclic threshold shear strain). In cyclic tests on fully saturated sands, the changes of modulus G SN also start occurring at γ c smaller than γ tp (Vucetic and Mortezaie 2015).…”

“…The level of γ td in sands and its relation to cyclic pore water pressure have been investigated in a systematic manner just recently (e.g., Mortezaie 2012;Vucetic and Mortezaie 2015), while γ tp and γ td in clays have been evaluated on a relatively small number of soils (Hsu and Vucetic 2006;Tabata and Vucetic 2010). A systematic investigation of γ td and γ tp in OC clays has not been conducted, while a number of factors affecting γ td and γ tp in both NC and OC clays still need to be studied.…”