Abstract: Suffering long from unloading and chemical weathering processes—especially the subtropical eluviation, not only clay mineral species transformation but also density decrease have taken place in eluvium of early Tertiary mudstone or sandy mudstone. Baise Basin (Guangxi municipality) swelling soil has a series of bad engineering properties such as low unit weight (about 19.02 kN/m 3), high void ratio (0.75～ 1.78 ), high water content (25%～41%),and high dispersibility (<2 μm grain>50%),and high contractility (bulk shrinkage>15%) which are typical characteristics of eluvial expansive soils. The unconfined compressive strength of this kind of soil is only 0.03～0.42 MPa, so it should not be categorized as “expansive rocks". The test results of scanning electronical microscope (SEM) and X ray diffraction (XRD) indicate that the main clay mineral is I/S (illite/montmorillonite) mixed layer mineral but not illite (different from existing opinions). Low safety factor resulted from defects in the discriminance of expansive potential, which is stipulated in the active official criterion. And this is one of the important causes of serious deformation and damage of roadbed. Engineering measures to deal with these roadbed defects are suggested in this paper.