Development of a Methodology for Predicting the Strength of Intact Bentonitic Shale and Gypsiferous Siltstone Using Fragments and Micro-Indentation Modulus Testing

ABSTRACT: In many geologic and geotechnical investigations, obtaining sufficient intact rock core samples for uniaxial compression or point load testing can be difficult in weak sedimentary rock that spans into the realm of an Intermediate Geo-Material (IGM). These IGM materials have sufficient strength for infrastructure construction, but are difficult to core, sample, and test with sufficient volume to meet conventional ASTM or other test standards. Samples that are obtained, have often been fractured in the rock coring process. Thus, it is needful to develop alternative laboratory test methods to characterize the strength of these IGM soil/rock formations for engineering design. In the case of IGM bentonitic shales and gypsiferous siltstone materials of the Cretaceous Pierre Shale and Jurassic Spearfish formations of Western South Dakota, micro-indentation equipment was herein proposed as an alternative to uniaxial compression or point load testing. Fractured specimens of these materials were immobilized in polymer resin and tested using a micro-indentation apparatus to obtain the micro-indentation modulus. Complimentary intact and remolded specimens were tested in uniaxial compression to obtain parallel data. Coupling of the data from testing of fractured material and intact material and regression analyses show that a strong correlation exists between micro-indentation modulus and intact or remolded IGM strength. The method proposed in this paper is portable and extractable to other formations and validates the work of others around the world who have proposed this alternative approach for characterizing the strength of rock masses that are difficult to sample and test using conventional methods. 1. INTRODUCTION Nearly 80% of the near surface geology west of the Missouri River and east of the Rocky Mountains in the Dakotas, Montana, Wyoming, Colorado, Nebraska and into Canada is composed of weathered residual soil, weathered rock, or weak sedimentary rock that spans from mechanically behaving as soil, Intermediate Geomaterial (IGM) and weak rock. IGM may be defined as cohesive or cohesionless geomaterial with unconfined strength ranging from 0.5 to 5-10 MPa or penetration blowcounts at 60% energy for a standard split-spoon sampler of 50-1501. The transition between soil and IGM may be noted by increased compressive strength at the 0.5 MPa threshold, while the transition between IGM and weak rock may be governed by an uncertain strength range of 5 to 10 MPa but also by a Rock Quality Designation (RQD) of 20 or less2. Highly fractured weak sedimentary rock with RQD < 20 may behave similar to IGM3 in foundation engineering.

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