Mechanical analysis using the unconfined penetration test on the tensile strength of Q3 loess around Lanzhou City, China

• The mechanism of the unconfined penetration test (UPT) was deduced and explained.
• The constant K in UPT was found to be larger than that proposed previously.
• The tensile strength of the loess by UPT had good correlations with UCS (R2 = 96.6%).
• The K value was suggested to be 2.19 for the loess in this study.

The unconfined penetration test (UPT) is one of the indirect methods to measure the tensile strength of soils. The objective of this paper is to explain the mechanism of the unconfined penetration test and to investigate the correlations of the tensile strength obtained from the unconfined penetration test with other mechanical parameters of Q3 loess collected around Lanzhou City, China. First, the mechanical principle of the unconfined penetration test was deduced from the perspective of energy balance. Next, a series of unconfined penetration tests were conducted for evaluations of the correlations. In addition, the constant K, which depends on specimen-punch size and soil type, was also considered. The results indicated that the tensile strength of the loess has high correlations with the unconfined compressive strength (UCS), with a coefficient of correlation of 96.6%. However, for cohesion, the coefficient of correlation obtained was slightly lower than that of the UCS, with a coefficient of correlation of 85.0%. The calculated constant K considered in the unconfined penetration method was found to be higher than the value proposed by a previous study (Fang et al., 2006). The calculated tensile strength using the higher K also has good correlations with the unconfined compressive strength and cohesion. However, due to the limited number of tests, some further studies are required.