ANALYSIS OF SOIL NAILED SLOPES UNDER SEISMIC CONDITIONS

Abstract

Vertical nailed cuts and nailed slopes subjected to seismic loading were analyzed using the modified pseudo-dynamic approach, assuming different failure surface geometries. Estimating the maximum support force and corresponding active zone length requirement of each reinforcement layer based on the limit equilibrium framework remains a statically indeterminate problem. Considering seismic loading further increases its complexity. The present study addresses it by proposing a novel calculus-based methodology to analyze the internal stability of nailed cuts/slopes. The behavior of nailed cut is captured in terms of the spatial distribution of horizontal and vertical inertial forces; maximum total support force required; critical failure surface; layer-wise maximum support force demands, active zone nail length requirements, and line of maximum tension. The influence of depth of cut to wavelength ratio, nail orientation, surcharge, wave amplitudes, soil‘s shear strength parameters, and damping ratio are reported in this study. The lower angle of shearing resistance, stronger horizontal base acceleration, and closeness of frequency level to fundamental shear wave frequency claimed additional nail lengths in both active and passive zones highlighting their significance in the stability of nailed cuts/slopes. Both possible directions of initial vertical acceleration must be considered with and without surcharge for designs to arrive at each nail layer's most extensive length requirement. Though the deeper nail layers were observed to require more support forces, their active zone nail length requirement was found lesser in this study. This trend, combined with the higher nail-soil bond strengths due to higher vertical stresses, suggests that the bottommost nail layer need not be the longest. An increase in nail inclinations puts more demand on nail length embedded within the passive zone. However, the resulting geometry of LMT reduces the nail length requirement in the active zone. The most common range of nail inclination (10o  20o ) seems to be justified. Nails should not be inclined more than o 20 unless encountered with site-specific situations like buried obstacles. Steeper soil slopes demand exponentially more support force for each nail layer. Practicing engineers can use proposed closed-form expressions by employing inexpensive spreadsheet software rather than costly finite element or slices-based commercial software. It has the advantage of being fully explicit. Amongst analytical studies, hardly any study focused upon the support force requirement as an analysis output parameter. The factor of safety is the output parameter preferred by most of the studies. This warrants a presumed nail layout and configuration so that its safety factor can be estimated by computing nail support force capacity (minimum of pull-out capacity and tensile strength). Hence practitioners often need to iterate the nail configuration till a target safety factor is achieved. The nail level maximum reinforcement force demand and associated locus of maximum tension presented in the present study eliminate the necessity of an iterative approach.