Comparison of frozen soil strength characteristics by cone penetration and triaxial compression testing

Abstract

The paper provides comparison of field and laboratory tests of permafrost soil. Electrical resistivity and temperature piezocone penetration tests (RTCPTu) were carried out in the area of sporadic distribution of permafrost. RTCPTu detected islands of permafrost at depth more than 10 meters. Based on this information, undisturbed samples of warm permafrost soils with massive cryogenic structure and thick ice lenses (up to 10 cm) were collected using CPT equipment with a direct push soil sampler (MOSTAP) mounted instead of cone. The samples were kept in frozen condition and transported to the soil lab, where physical properties and triaxial compression tests were carried out. The CPT results included the characteristics of long-term frozen soil strength such as σc – the long-term cone resistance and σn – the net long-term cone resistance. The results of triaxial tests provided the characteristics of peak strength: σ1 – the maximum vertical stress and σ1-σ3 – deviator stress at failure. The frozen soil samples were cleared from ice lenses since it is not possible to test them with thick ice lenses. The results of CPT and triaxial testing showed fairly similar values, when the cone measured resistance in frozen soil with massive cryogenic structure. When the cone was located near ice lenses, the net long-term cone resistance appeared to be much lower compared to deviator stress. This observation is in line with conventional theory on physics of ice which states that ice has near-zero long-term strength. The obtained results clearly show the value and advantage of CPT in permafrost (in-situ testing) compared with triaxial compression testing (laboratory). The properties of frozen soil in-situ may significantly differ from properties of frozen soil in a sample. This must be considered for designing civil structures on permafrost.