In current geotechnical seismic design practice, the empirical correlation between equivalent number of uniform cycles (Neq) of shaking and earthquake magnitude (Mw) forms an integral part of liquefaction potential evaluation. This relationship, in turn, is used to derive the magnitude scaling factors that are commonly used in field-based liquefaction evaluation procedures. The Neq versus Mw relationship for liquefaction assessment was examined for fine-grained soils using time-histories in the range 5 < Mw ≤ 9, especially including strong ground motion time-histories from the latest subduction zone earthquakes with Mw > 8.0. The experimental database available from cyclic direct simple shear tests conducted on natural fine-grained soils retrieved from undisturbed soil sampling was used to obtain the cyclic shear resistance weighting curves for the study. The work presented herein has contributed to further improving the current models used to represent magnitude scaling factor (MSF) values for large earthquake magnitudes and the functional dependency of this parameter on soil type. The MSF–Mw curve derived for low-plastic Fraser River Delta silt lies in-between the MSF curves derived for clean sand and clay, resonating with the inferences that have been made that the silt behavior can neither be considered sand-like nor clay-like.
The direct simple shear (DSS) device is commonly employed to characterize the shear behavior of soil while representing a plane strain condition. In DSS devices that use cylindrical specimens, the plane strain condition is mimicked by constraining the lateral boundaries of the specimen against radial deformations during consolidation and shear loading. The current test standard recommends using either wire-reinforced rubber membrane or unreinforced rubber membrane enclosed with stacked rigid rings as suitable modes of confinement to constrain the lateral boundaries of DSS test specimens. Only limited studies have been performed to assess the effects arising because of these different ways of lateral boundary constraints on the test results, particularly when DSS tests are conducted at relatively higher vertical effective confining stress (σ′v0) levels. An experimental program was undertaken to study the effect of lateral boundary constraint mode during constant-volume monotonic and cyclic DSS tests. The constant-volume tests were conducted on reconstituted cylindrical DSS specimens prepared from a natural silty soil under the following three modes of lateral boundary constraints: (a) steel wire–reinforced rubber membrane only (Mode 1); (b) steel wire–reinforced rubber membrane enclosed with a set of thin, low-friction stacked rigid rings (Mode 2); and (c) unreinforced rubber membrane enclosed with a set of thin, low-friction stacked rigid rings (Mode 3). The findings from these tests, conducted on specimens initially consolidated to σ′v0 level up to 900 kPa, indicate that there was no significant difference in the shear response derived from the three cases of lateral boundary constraints. This suggests that the commonly used approach of simply confining the DSS test specimen using steel wire–reinforced rubber membrane alone (i.e., Mode 1) would be effective and adequate for use in monotonic and cyclic direct simple shear testing, for σ′v0 ≤ 900 kPa.
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