Expansive soils are of a problematic soil due to its sensitivity behaviour in changes in water content and pressure. Aside from its swelling and shrinkage behaviour, sometimes the soils also exhibit degradation of its shear strength over time. This study observes, measures and analyse the degradation of expansive soil shear strength on a retaining wall structure constructed using concrete sheet piles that have undergone shifting progression over 13 years. The analysis was made by doing back-calculation using the finite element method (FEM), with models developed as accurately as possible to reflect the actual sheet pile inclination, as well as the rigid pavement heaving at its lower part. Measurement and analysis results indicate that the soil's shear strength parameters decrease over time, particularly in its active zone. At the upper layer, the soil's internal friction angle, ϕ reduced from 4.24° to 2.75°, while the cohesion, c declined from 13.04 to 8.48 kN/m2, and the modulus of elasticity, E dropped from 2,945.65 to 1,914.67 kN/m2. These reductions of soil shear strength need to be anticipated in the future design and construction of retaining walls on expansive soils.