The handy and practical Schmidt Hammer (SH) has been used for both in-situ and laboratory measurements of hardness and estimate of the Unconfined Compressive Strength (UCS) of rocks. Geological features such as structure, texture and mineralogy influence the mechanical properties of rock masses and these factors should be carefully evaluated for each testing location. In the work presented here, a case study was chosen to demonstrate and discuss the effectiveness of particular geological circumstances on in-situ SH test results concerning different carbonate rocks. Three different types of limestones, chalky, marly and dolomitic are available in the Tertiary carbonate sequences at the studied area which is Jabal Hafit, south of Al Ain city, United Arab Emirates (UAE). The results clearly show that rapid changes in carbonate rocks structure, texture and even mineralogy always create problems in generalizing the mechanical properties which are essential parameters in geological and engineering practices.

1 Introduction

The measurement of rock strength and characteristics of discontinuities (mainly fractures) are essential tasks in engineering constructions. In general, rock strength is defined as the ability to withstand stress or deformation; the quality of bodies by which they endure the application of force without breaking. In engineering terms, rock strength may be defined as the inherent strength of an isotropic rock under specific conditions – notably wet or dry. However rock masses are anisotropic and as a consequence, the strength of rocks is influenced both by the presence of impurities, weak zones and/or discontinuities (Hawkins 1998). Carbon- ate rocks represent large exposure of rock masses in the United Arab Emirates (UAE) and are targeted in the study presented here.

Rapid (on meter scale) changes in carbonate rocks structures (layering, fractures, faults, folds), texture (grain size, fossil content, cementation, dissolution) and even mineralogy (dolomitization, percent of clay minerals, silicification) have always created problems in generalizing an overall bulk physical properties such as rock strength, porosity and permeability. Thus, mass homogeneity may only exists at small scale (a few decimeter cubes) and rock failure problems are faced by the oil companies, construction engineering companies and environmental agencies when the size of the rock mass gets larger. The results of such rapid change in the physical properties cause unexpected rock failure or formation of karst (surface and subsurface caves and sinkholes) that are sometimes difficult to predict. Rock mass changes in homogeneity become even worse when it is combined with water. All these factors suggest that it is essential to measure the rock strength. In-situ measurements of rock strength when possible can provide a relatively accurate and simple estimate of changes in rock physical properties.

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