In deep excavation construction, improved soil layers consisting of overlapping cement-admixed columns formed by deep mixing method or jet grouting are often used to stabilise an excavation in soft soils. The purpose of such soil layers is to resist lateral compression generated by movement of the retaining wall. Cement-admixed soils are well known to have high heterogeneity in strength. In this paper, the heterogeneity in strength and Young's modulus are studied using random finite-element analyses, considering three sources of variation: namely, a deterministic radial trend in strength and Young's modulus; a stochastic fluctuation component due to non-uniform mixing; and positioning errors arising from off-verticality of the mixing shafts. The results show that positioning errors have the largest effect on the strength of the slab as a whole, whereas the radial trend has the smallest effect, when normalised by the volume-average strength. Based on the results obtained, methods are proposed which allow equivalent homogeneous mass strength and modulus of the improved slab to be determined for a chosen percentile of exceedance or reliability index, which can be used in deterministic finite-element analyses.