Takato IITSUKA

Development of Evaluation Method for Low Tensile Strength Materials Such as Clay

Hirofumi TOYOTA

Tensile cracks in earth structures are induced by tensile stress and may cause serious geotechnical and environmental problems. Therefore, quantitative evaluation of tensile strength is essential. However, a standardized direct tensile test for soils with low tensile strength has not yet been established. Conventional direct tensile tests impose strong restrictions on specimen geometry, while indirect methods such as the Brazilian test have difficulty reproducing natural failure behavior.
 
This study examines a direct tensile testing method for cylindrical specimens using vacuum pressure, previously proposed by the authors. The objective is to evaluate the influence of testing conditions on measured tensile strength and to establish appropriate conditions for reproducibility. In addition, the effects of specimen properties are analyzed and compared with previous direct tensile test results.
 
The investigated parameters include vacuum pressure, membrane covering length, compressive force to ensure adhesion between the cap and specimen, membrane end treatment, and loading rate. Failure plane locations were statistically analyzed to assess stability of failure behavior.
 
Results indicate that vacuum pressure should be set higher than the specimen tensile strength within an appropriate range. Stable results were obtained when the membrane covering length was at least 20 mm for an 80 mm-high specimen. Adequate compressive force was required to prevent bonding failure, and rolled membrane ends improved reproducibility. A loading rate of 1 mm/min or less ensured stable measurements. More than 80% of failure planes occurred near the specimen center, demonstrating stable and natural tensile failure.
 
Tensile strength increased with decreasing moisture content and increasing dry density. Low moisture content caused rapid stress drop, whereas higher moisture content led to larger failure strain and gradual stress reduction. Similar trends were observed in previous studies, and Kasaoka clay showed greater sensitivity to these factors. Future work will investigate crack propagation and applicability to high-moisture-content specimens.