Scale effect on shear strength behaviour of ISRM roughness profile

1994. Scale effect on shear strength behaviour of ISRM roughness profile / by Mario Alvarez Grima. - Delft : ITC, 1994. - 200 p. ; 30 cm. - ITC MSc-thesis. ABSTRACT, supervisor: Hack, thesis availability: ITC Library

ABSTRACT

This thesis investigates the scale effect and the influence of the surface roughness on shear strength behaviour on artificial closely matched joints. In this study the term scale effect is related not only to the selected testing point but also to the dimensions of the volumes involved in the test within the same discontinuity plane.

To execute this research three different discontinuity profiles (Rough Undulating, Rough Planar, and Rough Stepped) from ISRM Suggested Methods, 1978 were selected. The selected profiles were scanned at the John Brown Company in The Netherlands. The joint mould was made of Aluminum at the Central Work Shop of the Delft University of Technology by means of Wire Cutting Electrical Discharge Machine. In those moulds several samples with dimensions of 40*60, 80*60 and 120*60 mm2 were made of LODUR-500.

Testing was carried out by using the Golder Shear Box Machine. A direct single shear test was performed under a low and constant normal stress of 279 Kpa. In this work low normal stress means no complete asperity damage during the test.

An attempt was made to evaluate and quantify the contact area ratio (this is the ratio of the area upon which the shearing effect occurs to the area upon which the frictional effect occurs) by using the New Laser Profile Meter (THE TWIN) at the section of Engineering Geology of TU - Delft. For that purpose some assumptions were made, after Peek, 1981 & Lee, et al 1993. For the calculation of the contact area ratio a program called Rough.c was written

To evaluate the scale effect a New Shear Model of Shear Strength on Simulated discontinuities formulated by Lee, et. al, 1993 was used.

The Analysis of the results show the strong influence of the discontinuity type controlling the type of mode of failure. It is presented a transition from a classical brittle behaviour on a rough stepped profile controlled by the steps to a more plastic one as the effective dilation angle decreases.

It ascertains the validity of the New Shear Strength Criteria, but it must be considered that the present research constitutes the first attempt of the use of the New Shear Criteria to study the scale effect, hence further research is required on both artificial and natural samples.