Strength-permeability-disintegration evolution and microstructural coupling mechanisms of oil-contaminated silty sand at natural moisture content

Petroleum contamination can alter particle contacts and fluid distribution, thereby affecting the engineering behaviour of soil. This study investigates oil-contaminated silty sand under natural moisture conditions using unconfined compression, static-water disintegration, falling-head permeability, SEM–EDS, NMR, image analysis, and apparent water-droplet contact-angle tests. The UCS decreased continuously from 31.95 kPa at 0% oil content to 12.63 kPa at 12%, representing a 60.5% reduction. In contrast, resistance to static-water disintegration increased with oil content: the uncontaminated specimen disintegrated within approximately 50 s, whereas the 8% and 12% specimens remained largely intact after 24 h. Hydraulic conductivity and time to first water discharge showed non-monotonic responses, reaching measured maxima of 13.52 × 10 − 3 cm/s and 120 s, respectively, near 5% oil content. This response is regarded as an empirical turning feature under the present test conditions rather than a universal threshold. The microscopic results suggest changes in local particle-surface appearance, fluid-relaxation behaviour, visible interparticle spaces, and composite-surface wetting response. Overall, oil contamination weakened compressive resistance while delaying static-water disintegration, and the hydraulic response resulted from the combined effects of initial water entry, fluid distribution, local flow-path changes, and possible preferential flow.