【正文】 nalysis considering the mechanical behavior of the soil until failure may be thought of as a possible method. However, such an elastoplastic analysis is rarely used in practice due to the plexity of the putations. From a practical standpoint, the primary focus of a stability problem is on the failure condition of the soil mass. Thus, practical solutions can be found in a simpler manner by focusing on conditions at impending collapse. Stability problem of natural slopes, or cut slopes are monly encountered in civil engineering projects. Solutions may be based on the slipline method, the limitequilibrium method, or limit analysis. The limitequilibrium method has gained wide acceptance in practice due to its simplicity. Most limitequilibrium method are based on the method of slices, in which a failure surface is assumed and the soil mass above the failure surface is divided 29 into vertical slices. Global staticequilibrium conditions for assumed failure surface are examined, and a critical slip surface is searched, for which the factor of safety is minimized. In the development of the limitequilibrium method, efforts have focused on how to reduce the indeterminacy of the problem mainly by making assumptions on interslice forces. However, no solution based on the limitequilibrium method, not even the so called “rigorous” solutions can be regarded as rigorous in a strict mechanical sense. In limitequilibrium, the equilibrium equations are not satisfied for every point in the soil mass. Additionally, the flow rule is not satisfied in typical assumed slip surface, nor are the patibility condition and prefailure constitutive relationship. Limit analysis takes advantage of the upperand lowerbound theorems of plasticity theory to bound the rigorous solution to a stability problem from below and above. Limit analysis solutions are rigorous in the sense that the stress field associated with a lowerbound solution is in equilibrium with imposed loads at every point in the soil mass, while the velocity field associated with an upperbound solution is patible with imposed displacements. In simple terms, under lowerbound loadings, collapse is not in progress, but it may be imminent if the lower bound coincides with the true solution lies can be narrowed down by finding the highest possible lowerbound solution and the lowest possible upperbound solution. For slope stability analysis, the solution is in terms of either a critical slope height or a collapse loading applied on some portion of the slope boundary, for given soil properties and/or given slope geometry. In the past, for slope stability applications, most research concentrated on the upperbound method. This is due to the fact that the construction of proper statically admissible stress fields for finding lowerbound solutions is a difficult task. Most previous work was based on total stresses. For effective stress analysis, it is necessary to calculate porewater pressures. In the limitequilibrium method, porewater pressures are estimated from groundwater conditions simulated by defining a phreatic surface, and possibly a flow , or by a porewater pressure ratio. Similar methods can be used to specify porewater pressure for limit analysis. The effects of porewater pressure have been considered in some studies focusing on calculation of upperbound solutions to the slope stability problem. Miller and Hamilton examined two types of failure mechanism: (1) rigid body rotation。 and (2) to check the accuracy of Bishop’s simplified method for slope stability analysis by paring Bishop’s solution with lowerand upperbound solution. The present study is an extension of previous research, where Bishop’s simplified limitequilibrium solutions are pared with lowerand upperbund solutions for simple slopes without considering the effect of porewater pressure. In the present paper, the effect of porewater pressure is considered in both lowerand upperbound limit analysis under planestrain conditions. Porewater pressures are accounted for by making modifications to the numerical algorithm for lowerand upperbound calculations using linear threenoded triangles developed by Sloan and Sloan and Kleeman. To model the stress field criterion, flow of linear equations in terms of nodal stresses and porewater pressures, or velocities, the problem of finding optimum lower and upperbound solutions can be set up as a linear programming problem. Lower and upperbound collapse loadings are calculated for several simple slope configurations and groundwater patterns, and the solutions are presented in the form of chart. LIMIT ANALYSIS WITH POREWATER PRESSURE Assumptions and Their implementation Limit analysis uses an idealized yield criterion and stressstrain relation: soil is assumed to follow perfect plasticity with an associated flow rule. The assumption of perfect plasticity expresses the possible states of stress in the form F( 39。ij? ) = yield function。ij? = effective stress tensor. Associated flow rule defines the plastic strain rate by assuming the yield function F to coincide with the plastic potential function G, from which the plastic strain rate pij? can be obtained though 39。pij ij ijGF? ? ??????? （ 2） where ? = nonnegative plastic multiplier rate that is positive only when plastic deformations occur. Eq. (2) is often referred to as the normality condition, which states that the direction of plastic strain rate is perpendicular to the yield surface. Perfect plasticity with an associated with very large displacements are of concern. In addition, theoretical studies show that the collapse loads for earth slopes, where soils are not heavily constrained, are quite insensitive to whether the flow rule is associated or nona