Experimental study on the hardening behavior of compacted cohesive soil and its rapid evaluation method

Abstract

Shield tunnelling cutterheads are prone to mud cake formation in cohesive strata, which significantly reduces tunnelling efficiency. Current research on mud cake formation and risk prediction primarily focuses on adhesion behavior and largely overlooks the evaluation of the degree of mud cake hardening. Investigating the hardening behavior of cohesive soil under pressure is crucial to elucidate the development process of shield clogging and improve the methodology for assessing clogging risk. In this study, a Shore durometer is proposed for rapid assessment of the hardening level of compacted cohesive soil. The effects of clay mineral composition, initial water content, and pressure on the compaction characteristics, Shore hardness, and unconfined compression strength (UCS) of soil are systematically analyzed using two manually prepared cohesive soil samples: Sample I (illite-dominant) and Sample M (montmorillonite-dominant). The results indicate that under pressure, the dry density and UCS increased more significantly in Sample I than in Sample M due to efficient pore water drainage. The initial water content and compaction pressure jointly determine the peak UCS, with both dry density and UCS reaching their maxima at the plastic limit. Although low-plasticity muck exhibits low adhesion, it compacts and hardens easily under high pressure. Therefore, shield clogging risk assessment should comprehensively consider both adhesion and compaction properties. Shore hardness in low-hardness regions is affected by the puncture effect, whereas data distortion limitations became dominant in high-hardness regions. Overall, Shore hardness effectively reflects the degree of hardening of compacted cohesive soil and can serve as a rapid indicator for shield clogging assessment.