Correlation between Shear Modulus and Shear Strength of Bentonite Gels

Abstract

The hydrogel of Wyoming bentonite shows characteristic slip patterns under a critical shear stress i.e. the shear strength. The critical shear stress, or the shear strength, and the shear modulus have been measured by various methods over a wide range of concentration.
The shear modulus increases with increasing bentonite concentration, as is shown by Curve I in Fig. 5. This is similar to the general feature of polymer gels. Curve II in Fig. 5 shows the relation between the shear strength and the concentration. Curve I and Curve II suggest that a proportional relationship holds good between the shear modulus G and the shear strength σ.
Fig. 6 shows that the shear strength of bentonite gels lie between G/10 and G/30 over the whole concentration range examined. This result accords approximately with the value (G/30) derived theoretically for the shear strength of crystals by Mackenzie⁸⁾ without assuming the effect of internal flaws. Buchdahl¹⁰⁾ has pointed out also that the ratio G/σ for various polymers takes the value nearly equal to that predicted by Mackenzie.
It appears that bentonite gels have a network structure analogous to polymer gels, and their shear strengths are related to the shear moduli, the critical shear strain taking values between 1/10 and 1/30 although it may not always be explained by assuming an atomistic mechanism as proposed by Mackenzie.
Thus, the following mechanism is proposed for the slip fracture of bentonite gels. Under the critical shear stress or strain, the card-house of the bentonite gel breaks down one upon another starting from the point of stress concentration; then the slipping is induced by the orientation of bentonite leaflets in the direction of maximum shear. The orientation of bentonite particles is confirmed partly by the birefringence on shearing the bentonite gel between the two transparent plates.³⁾ Below the critical stress, the gel creeps or yields very slowly.