The coal mined out area in sandstone is an outstanding problem for Fukuoka arch Dam's abutment stability. To determine the concrete replacement depth of coal mined out area in abutment, this paper conducted numerical simulation by employing finite element method, and compared in detail on the dam stress and deformation characteristics of Dukouba dam under operation condition of original foundation and three reinforcement conditions, whose concrete replacement depths of coal mined out area are 50m, 80m, 100m respectively during normal storage period. The comparison shows that the dam stress and deformation are strongly affected by the coal mined out area in abutment, reinforcement measures must be taken. Compared with the original foundation condition, when the replacement depth reaches 80m, the distribution of dam stress and deformation have been significantly improved, the tensile stress on upstream dam surface is decreased remarkably. The overload capacity of the dam and foundations are increased to meet the design requirements.
Dukouba dam is an arch dam of parabola double curvature located in Fengjie County of Chongqing, China, with dam base at el.470m and dam crest at el.578.5m. The normal water level is 575.0m, total reservoir volume is 9.254×108m3, and designed generating capacity is 129MW. The reservoir serves mainly for power generation, but also for tourism, flood control and irrigation as well. The valley at dam site  is precipitous, approximately symmetric and typically V-shaped, with the ratio of width to height to be 2.0. Geologically it lies at the south limb of Damuya anticline, and the rock stratum dips 38°~ 45° toward downstream. 4 series of joint fissures develop, and densely distribute at some spots. Sandstone is stiff rock, marlite is of medium hardness, argillaceous dolomite, marlite, shale and coal seam are soft~very soft rocks. There are 4 coal mined out areas in dam abutment because of small scale exploit, of which Md2, RJ16 series and RJ10 coal mined out zones are the biggest threaten to dam safety. Md2 series coal mined out areas has a width of 0.4~0.6m, extending horizontally more than 300m from both banks into mountain, vertically at el.483. 2~650.0m; RJ10 coal mined area has the same width range, extends more than 100m horizontally and distributes at el.510 m~600m in both abutments (Fig.1). This will greatly decrease stiffness in abutments, leading to excess stress and deformation , so concrete replacement measure  is adopted to treat the coal mined out areas downstream near the dam. The aim of the paper is to optimize the concrete replacement depth, so that the dam deformation and stress can meet specifications on an economical basis. Considering the geological conditions of Dukouba dam site, 3D finite element model is built in detail and the main aims of performing 3D-FEM calculation are as follows: 1. Give quantitative assessment [4–6] to the influence degree of depth of concrete replacement (hereinafter short for DCR) to the stress and deformation of dam and abutment, optimum DCR would be picked out according to the results. 2.