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• The failure cause of a composite wall during reservoir excavation is reported. • Three-dimensional FEM is utilized to verify the failure cause. • The primary failure cause is the unexpected soft clay thickness. • The rehabilitation was performed by revising the original composite wall configuration. • The factor of safety against global failure during excavation is 1.60. In this paper, the failure cause of a composite soil-cement (CSC) retaining wall is examined during reservoir excavation in the soft clay area of Bangkok, Thailand, by field investigation and three-dimensional finite element analyses. A rehabilitation method is also reported. The CSC wall consisted of precast reinforced concrete walls, soil-cement walls, and soil-cement columns encased with a stabilized mat. The original CSC wall configuration was designed based on the maximum soft clay thickness of 4 m found in the excavation areas from seven soil boreholes, providing an FS global of 1.61. However, a 50-m-long section of the composite wall failed unexpectedly. The field observation revealed that the wall collapsed from the loss of internal wall stability, namely, broken wall structures caused by bending failure. Continuous circular failure was observed due to the shear failure of the surrounding clays. This failure's primary cause was the unexpected soft clay thickness of 7 m induced by insufficient previous soil explorations. Finite element analyses confirmed that the unanticipated 7-m-thick soft clay facilitated significant failure since the FS global value was lower than 1.00. The soil and wall bodies exhibited a collapse because the shear and tensile failure points were fully developed in whole soil bodies and wall structures, respectively. Rehabilitation was performed by revising the original CSC wall configuration for supporting 7-m-thick soft clay, namely, increasing the embedded wall structure lengths and soil-cement rows to strengthen the CSC wall. The revised wall configuration improved the FS global during excavation to a value of 1.78, resulting in no visual failure after implementing the rehabilitation. During reservoir operation, the water storage level was kept at a level of −1.00 m to obtain a high FS global of 2.33. [ABSTRACT FROM AUTHOR]