Abstract

The measurement of the natural and/or induced stress state in rock masses excavations is generally carried out thanks to techniques able of providing results both in the two-dimensional (2D) and in the three-dimensional (3D) environment, coded in the "Suggested Method" of the ISRM. The stress relief method is one of the most consolidated measurement methods, based on the differences in strain intensity evaluated in selected directions before and after making a continuous notch around the sensors involved in the measurement. With a special miniaturized data-logger applied to the sensors used for the evaluation of the rock stress in a 2D (doorstopper) or 3D (HI cell, CSIRO) environment, the execution of the measurement on site can be considerably simplified, and the complete strain-time diagram acquired due to over-coring can be obtained. The paper presents an example of measurement of the state of stress induced in some abandoned pillars in an underground quarry.

Introduction

The evaluation of the state of natural and/or induced stress of rock masses is one of the most demanding problems that must be faced during the design and realization of excavations for civil and/or industrial purposes.

In the last twenty years, despite the amount of measurement techniques available, a growing attention has been focused on "stress relief method". This method involves the rigid connection of a sensor to a free surface of the rock volume of which the state of stress is to be determined, and the execution of a preliminary measurement of a physical quantity detected by the sensor, called "zero" measurement. The procedure therefore provides the execution of a continuous notch around the sensor, with a minimum depth of about two times the largest size of the sensor itself. The difference between the final displacement recorded by the sensor after the "stress relief" and the "zero" measurement, indicates the effect of the stresses around the on-site rock core examined. The International Society of Rock Mechanics, in the Part II of its Suggested Method [1], indicates the "doorstopper strain cell" developed at the Council for Scientific and Industrial Research (CSIR) [2] and the "drill hole deformation gauge" developed at the United States Bureau of Mines (USBM) [3], as the most widely used instruments for determining the plane stress state (2D). Similarly, the I.S.R.M. indicates the CSIR 3D gauge, the "Hollow Inclusion cell" developed in the laboratories of the Commonwealth Scientific and Industrial Research Organization (CSIRO) [4] and the Borre probe studied at the Division of Land and Water Resources Engineering of the Royal Institute of Technology (KTH) of Stockholm [5], as the most widely used instruments for the determination of the complete state of stress (3D), using a single borehole. In this work, reference is only made to the relief-method based on the instrument called HI cell or CSIRO gauge, and on its modification made at the Laboratory of Rock Mechanics of the Institute of Geosciences and Georesources of the National Research Council (IGG-CNR) of Turin, Italy.

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