A New Direct Tension Test Method for Soils and Soft Rocks

Abstract

Tensile strength plays a crucial role in many engineering activities involving soils and soft rocks.Currently, several methods for tensile strength determination exist, the most common of which include the direct tension method, Brazilian test, ring test, axial fracturing test, bending test, and hydraulic fracturing test. However, outstanding problems associated with these methods, e.g., undesirable eccentric forces and damage of specimen ends, significantly affect obtained tensile strength values. To overcome these problems, we propose an alternative direct tension test method together with the newly developed apparatus. The proposed method uses an annular specimen that is tension-loaded on the inner hole. The method was evaluated through a series of tests on undisturbed soil and remolded gypsum-sand mixture specimens. For validation, comparison of results between the proposed method and the International Society for Rock Mechanics (ISRM)-suggested method was performed. Based on the results of the experimental program, the proposed method is considered more capable of and more suitable for determining the tensile strength of soils and soft rocks than the ISRM-suggested method.

Comparison of Test Methods for Determining the Tensile Strength of Soil and Weak Rocks

“…IHFT is the direct tensile test we developed as an alternative to UDTT (Li et al, 2020). Figure 3E shows the IHFT apparatus, of which the most important feature is the drawbar device consisting of movable and fixed pull seats.…”

“…Results obtained from direct tension test is fundamentally sound because the stress field developed within the specimen depends only on the applied load and boundary conditions, and not on specimen properties (Luong 1990). However, because of the high cost and difficulties in sample preparation and test operation, direct tension method is less used in laboratories (Li et al, 2020). By contrast, specimen preparation, experimental setup and data reduction are much simpler in indirect tensile tests.…”

“…For instance, maintaining the applied force along the vertical diametrical plane of the specimen is an outstanding challenge in the Brazilian (BT) test. Problems in both tensile test methods become more serious when dealing with soil and weak rocks (Li et al, 2020).…”

“…As emerges clearly from the foregoing, both classes of tensile tests have important advantages and disadvantages, and in recent years, there have been efforts among the scientific community to improve them and address their limitations. As such, new methods have been developed not only for testing soil and rocks (e.g., Tamrakar et al, 2007;Li et al, 2020) but also other geomaterials like concrete (e.g., Sarfarazi et al, 2015;Sarfarazi et al, 2018;Liao et al, 2020;Resan et al, 2020), the validity of which was investigated both experimental and numerically (e.g., Blümel 2000;. However, most of these methods, in particular those developed for concrete, require complicated specimen shapes and therefore inapplicable to soil and weak rocks.…”

“…However, most of these methods, in particular those developed for concrete, require complicated specimen shapes and therefore inapplicable to soil and weak rocks. In light of this, we recently developed new direct and indirect techniques for tensile strength determination of these materials, namely, the Inner Hole Fracturing Test (IHFT) and Horizontal Compression Test (HCT), respectively (Li et al, 2020;Guan et al, 2022). In this study, a detailed comparison of these new methods and the three most commonly used tensile tests, the UDTT, BT, and TPBT (Three Point Bending Test), was conducted.…”