Technical Note Influence of Normal Load on Joint Dilatation Rate By B. Va ´sa ´rhelyi Technical University of Budapest, Department of Engineering Geology 1. Introduction Seidel and Haberfield’s (1995) work was based on the analysis of the regular triangular asperities and assumed that the asperities were rigid. They used normal stiffness shear stress (CNS) to measure the joint dilatation rate and extended the Ladanyi and Archambault (1970) approach. The aim of this research is to investigate the dependence of the constant normal load (CNL) on the rate of the dilatation. Three points were chosen on the bilinear failure envelope: the first in the first linear part, the second in transition stress, and the third in the second part of the bilinear failure envelope. Twelve regular triangular cement mortar specimens were used to carry out this research. These cement mortal specimens were investigated by Tisa and Kova ´ri (1984) before, so all the material and shearing constants were well-known. The other purpose of this research was to observe the influence of the normal stress on the dilatation-displacement curves. 2. Theoretical Background Patton (1966) performed a series of constant load stress direct shear tests with regular teeth inclination, i, at varying normal stresses. From these tests he established a bilinear failure envelope – failure from an asperity sliding and asperity shearing mode. The equations for the two portions of the failure envelope are given below: t ¼ j n tan ðf m þ i Þ ð1Þ t ¼ c j þ j n tan f m ; ð2Þ where c j ¼ cohesion; f m ¼ basic friction angle; i ¼ regular teeth inclination. Rock Mech. Rock Engng. (1998) 31 (2), 117–123 Rock Mechanics and Rock Engineering  Springer-Verlag 1998 Printed in Austria