Effective shear strength parameters c' and φ' are obtained from the back-analysis of an approximate 180 m high slope considering different water table depths. The back-analysis considers multiple slips in a uniform material to obtain all the possible combinations of c' and φ' for a high, medium and low water table position. The geometry (depth and length) of the failure surface that has a factor of safety equal to 1.0 is obtained for any pair of c' and φ' values for a given water table position. These dimensions are compared to the failure surface obtained from inclinometer data. Consequently, a possible range for representative values of c' and φ' is evaluated. Additionally, two in situ direct shear tests on 750x750mm samples are performed under different confining pressures to obtain the strength parameters c' and φ'. One triaxial test under different confining pressures was also performed on an undisturbed sample. A comparison between laboratory testing, in-situ testing, and the back analysis revealed a good agreement between the values obtained, offering confidence in the material strength parameters to be used in a future slope stabilization design.