Reply to the discussion by Jefferies, Been, and Olivera on “Evaluation of static liquefaction potential of silty sand slopes”1 1Appears in the Canadian Geotechnical Journal, 49(6): 746–750 [doi: 10.1139/t2012-039].

In regular beach sand, the direct sand grain to sand grain structure creates a strong framework that will tend to dilate, unless the sand structure is made unreasonably loose by moist tamping in which the sand grains are lightly wetted before deposition and tamping, a condition that rarely occurs in nature (Wood et al. 2008). The structure of silty sand may be stronger or weaker than clean sand depending on the location of the silt particles. If the silt particles are located in the voids between the larger sand grains, the effect of the silt is to increase the strength and decrease the liquefaction potential of the structure. But if the silt particles are located between the sand grains so as to separate the sand grains, then the structure is much weaker because forces act to crush the silt particles or to make them roll or slide out into the voids. Either way, the sand structure will contract and this will result in increasing pore pressures under undrained conditions. Thus, the mechanism of contraction observed in silty sand during plastic straining is based on the initial location of the silt particles between the sand grains so that the silt particles separate the larger sand grains. How much of the silt is located in the voids and how much is separating the sand grains is an important question for silt contents below approximately 30%. Above 30%, the silt particles are located both in the voids and between the sand grains and they all act to separate the sand grains, as discussed by Lade et al. (1998) and as shown in Fig. R1. Thus, above approximately 30% silt, the sand grains are floating in a matrix of silt particles and the larger sand grains are not in contact, which would form a stronger soil. As the silt content increases the compressibility increases (Lade et al. 2009), while the permeability decreases. Thus, the mechanism of soil structure collapse is likely different from that occurring at lower silt contents. The amount of silt that participates in producing a weaker structure has been addressed by Rahman et al. (2008) and Yamamuro et al. (2008). In addition, the type of silt, in terms of gradation, grain size, and shape, also influences the liquefaction potential of silty sands (Monkul and Yamamuro 2011). When drained tests are performed on loose silty sand and the stress ratio and volume change are plotted versus the axial strain, the resulting curves appear to show no influence of confining pressure, as indicated in Fig. 14 of the paper under discussion. Very similar behavior is obtained from drained tests on normally consolidated clays, as indicated by the SHANSEP concept (Ladd and Foot 1974). These results for normally consolidated (NC) clays, as well as results for any granular material (see e.g., Lee and Seed 1967), are plotted with reference to their initial void ratio at low confining pressures. Only if further analyses are made may the void ratio after consolidation be considered (see e.g., Lee and Seed 1967; Seed and Lee 1967). Thus, the plot in Fig. 14 is not unusual, and only for special purposes is the void ratio after consolidation employed in presentation of experimental results. In fact, preparing specimens at low confining pressures with the intent of reaching a certain void ratio at a given higher confining pressure is difficult. To circumvent this difficulty, Lee and Seed (1967) performed their analyses to predict the strengths under undrained conditions from the drained test results in terms of void ratio after consolidation as interpolated or extrapolated from tests performed on specimens prepared at four different initial void ratios.