ABSTRACT

ABSTRACT: Although under the majority of stress states found underground, saltrock yields and deforms viscoplastically and does not fail in a brittle manner, brittle failure does occur in the immediate vicinity of underground openings as a result of the high stress and strain rates induced by excavation. This paper examines the brittle failure behavior for saltrocks and evaluates the ability of several proposed failure criteria to predict this behavior.

1. INTRODUCTION

It has long been recognized that the relationship between the shear stress and the normal stress for failure under compressive loading is linear only for relatively low normal stress levels. Nevertheless, the commonly-used Mohr-Coulomb and Drucker-Prager failure criteria are linear. While one or other of these criteria may be acceptable at low stresses, a criterion is also required t hat reasonably represents t \he non-linear locus obtained at higher stress levels. In addition, strength data obtained from true triaxial testing has shown that failure envelopes derived from conventional laboratory triaxial testing, where s1> s2= s3, do not represent failure in the more general case where s1>s2>s3. However, with a general failure criterion expressed in terms of the Lode parameter, µ , the material parameters could be obtained from conventional triaxial testing rather than using the less-common true triaxial testing. (It would be wise, however, to obtain the failure stresses for a few true triaxial loadings to verify the validity of the material parameters for general stress states.)

Although masses of saltrock generally deform in a viscoelastic/viscoplastic manner, brittle failure does occur in the immediate vicinity of underground openings as a result of the dynamic stresses and strains caused by excavation. To evaluate the stability of pillars between openings in saltrock or to select pillar dimensions that will result in stable, but not overly conservative, pillars, an analyst must be able to predict the extent of this failure zone. To do so requires strength data for the saltrock of concern and failure criteria that cover the range of expected stress states.

In the recent past, several general failure criteria have been proposed in an attempt to overcome the limitations of the conventional Mohr-Coulomb and Drucker-Prager criteria. The Mohr-Coulomb criterion neglects the influence of the intermediate principal stress (s2), a situation that is unimportant when characterizing the behavior in conventional triaxial compressions tests( where s2=s3) but which leads to incorrect predictions in the more general case of three unequal principal stresses. An extended Mohr-Coulomb criterion has been developed (Zienkiewicz, 1975; Senseny et al., 1983) that does account for general triaxial stress. However, the extended Mohr-Coulomb and the Drucker-Prager criteria each postulate a linear relationship between the octahedral normal and shear stresses at failure, a condition that exists only at relatively low confining stresses.

A complication that cannot be overlooked in the case of saltrocks is the fact that the behavior depends on the strain rate as well as the confining stress.

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