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Stability Index for Underground Structures in Granitic Rock

John R. Ege
John R. Ege
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January 01, 1968

More than 2400 ft of exploratory core drilled for an underground installation was logged in terms of joint frequency, degree of weathering, relative hardness, core loss and broken core. A statistical regression analysis was computed relating weathering, hardness, core loss and broken core to joint frequency. A significant correlation existed between these parameters within practical limits. The correlation broke down when joint frequency exceeded 8 joints per foot, and when core loss and broken core was greater than 30 percent for a given core interval.

The range of joint frequency between no joints and 8 joints per foot was arbitrarily divided into 10 equal divisions and designated as grades 1 (poor) to 10 (good). The corresponding values for weathering, hardness, core loss, and broken core were assigned to the 10 grades. A grade of φ was reserved for faults.

A regression analysis was computed relating the rock grades to laboratory-determined physical and mechanical properties of core samples previously determined by logging. Significant correlations existed between the rock grades and dry bulk density, total porosity, Young’s modulus, shear modulus, and bulk modulus. Poisson’s ratio, however, did not show any significant correlation with rock grade.

Underground mapping confirmed that rock of grades 8 to 10 presented no construction or stability problems. Rock of grades 5 to 7 was acceptable but tended to have moderate overbreak and required occasional steel sets for support. Rock of grades 3 and 4 was unstable and had to be supported. Rock of grades φ to 2 was incompetent.

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GSA Memoirs

Nevada Test Site

Edwin B. Eckel
Edwin B. Eckel
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Geological Society of America
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Publication date:
January 01, 1968




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