Estimating Shear Strength Parameters of Clay and Weak Rocks Using Needle Penetration Tests

ABSTRACT Weak and clay-bearing rocks and soils are highly abundant worldwide. They are currently being investigated for their suitability for deep geological repositories for high-level radioactive waste. For this purpose, uniaxial compressive strength, tensile strength, and shear strength parameters are required. However, preparing suitable core samples is not always possible. Moreover, determining these strength parameters at various locations is often time-consuming and scale dependent. To overcome these difficulties, a portable and non-destructive penetration test can be used both in the laboratory and in the field. The objective of the current study is to develop a method for predicting the shear strength parameters of clay and weak rocks. Weak and clay-bearing geological materials were collected from various locations in Europe and Türkiye. In situ needle penetration tests were also conducted to make a preliminary assessment of the mechanical properties of the collected samples at their natural moisture content. Shear strength parameters of selected samples were determined using a triaxial compression test device (Hoek cell) and a modified direct shear box apparatus. The latter was specially developed for performing shear tests on soil-type materials. The results of the experimental studies suggest that the penetration resistance and shear strength parameters of the studied materials are statistically significantly related.