Shear strength of the Bremanger Sandstone: “Determining the basic friction angle using a Golder Direct Shear Box”

Shear strength of the Bremanger Sandstone: “Determining the basic friction angle using a Golder Direct Shear Box”

Bootsma, M.T.

Ngan-Tillard, D.J.M. (mentor)
Van Paasen, L.A. (mentor)
Mulder, A. (mentor)

Civil Engineering and Geosciences

Geotechnology

Geo-Engineering

2010-09-03

This project focuses on the shear strength of Devonian sandstone cobbles that will protect the area between the sea-water breaker and the beach and will be used to construct a runway for a large crane for the new Maasvlakte 2. In rock slope design the shear strength of sliding interfaces is often based on Coulomb’s model in which shear strength (?) is expressed as a function of cohesion (c), normal load (?) and the friction angle (?). A specific model on shear strength behaviour of rockfill is proposed by Barton (Barton, 2008). Barton’s model can be used to predict shear strength of rock joints and rock fill when basic rock properties are known. One main factor of influence on shear strength of rockjoints and rock fill is the basic friction angle. A Golder direct shear box was used to obtain the basic friction angle of the Bremanger sandstone. First the basic friction angle was determined (using flat saw-cut surfaces). In the next phase fresh tensile cracked rock discontinuities were tested. Measured stresses were corrected using measured dilatancy to estimate the basic friction angle. The influence of different rock properties such as roughness, rock strength and visible layering was studied. Finally natural non-matching surfaces were researched to see how the shear strength changes due to the weathering and smoothening of the rock. The following main results and conclusions were drawn from the research project: Flat saw-cut surfaces: - The basic friction angle of the Bremanger (flat saw-cut surfaces) was not measured correctly. Since the sample halves were polished in order to make them match, the measured friction angle is rather an indication of the polishing process than the rock properties. Tensile-cracked surfaces: - For the tensile cracked samples, the average residual friction angle is 1111 degrees. The residual friction angle was used as the basic friction angle rather than the peak friction angle since high peak shear stress values are caused by asperities at the edges of the tested samples. The shear strength dropped massively as soon as these asperities broke off. - Residual friction angles do not depend on wet or dry test conditions. - There is a direct link between UCS and shear strength. The higher the UCS value, the higher the residual stress value. - The Bremanger sandstone is a Metasandstone. Layering is still macroscopically visible but cleavage does not occur along this layering. Natural non-matching surfaces: - These surfaces were weathered and smoothened. This caused the residual friction angle to drop to 1111degrees. 3D Leica pictures confirmed that the surface of this tested sample was smoother than the fresh tensile-cracked ones.

Bremanger
friction angle
Barton
rock joints
rock fill
basic friction angle
residual friction angle
shear stress
golder shear box
maasvlakte

http://resolver.tudelft.nl/uuid:10f1516c-2282-41f9-9f00-4eed1bcd8add

Student theses

bachelor thesis

(c) 2010 Bootsma, M.T.