Shear strength of planar joint surfaces in rock

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Objectives of the study

The engineering characteristics of rocks and particularly, the shear strength of joints (discontinuities) in rock masses playa key role in civil engineering and specifically in the design and safety evaluation of dams. Civil engineers are confronted with the problem of shear strength when designing excavations in rock masses for structures such as dam foundations, cuttings in rock for roads (slopes), tunnels etc. To evaluate the stability of a dam foundation, the shear strength of those joints with the most unfavourable orientations relative to the applied loads is required. Determination of the orientation of joints in a foundation by means of a joint survey is relatively easy. From this, the most unfavourably oriented joints can be selected. The design parameters for shear strength of these joints are usually not available during the early stages of the design and it is thus necessary to estimate these. It is therefore the aim of this investigation to provide a guideline for the estimation ofthese shear strength characteristics as accurately as possible. The objective of this research project was to determine and to analyse the shear strength of joints in a number of rock types, sampled at different locations and to classifY these strengths in accordance with joint surface parameters. The information so obtained serves as a data bank of shear strength parameters for the design of new dams and for the evaluation of the safety of existing dams in South Africa.

Motivation

A large proportion of South Africa’s economically most active population lives in the Gauteng Province, which is situated on a watershed. Furthermore, South Africa is a relatively dry country that necessitates that water storage dams have to be built in suitable riverbeds from where the water has to be transferred to the end users by means of pump stations, pipelines, tunnels and canals. A number of major water schemes have been constructed, e.g. the Drakensberg Pumped Storage Scheme, the Orange-Fish River Scheme and the Lesotho Highlands Water Project currently under construction. Increasing the capacity of existing schemes will become necessary and further, similar schemes will have to be constructed in the future to satisfY the ever-increasing demand for water in the RSA. A number of dams in our country have reached ages of 40 years and more with the result that their safety and stability will have to be re-evaluated. The stability of a dam depends on its design, on the materials and methods used during its construction and on the stability of the foundations on which it is built.
The characteristics of the rocks and particularly the shear resistance of the joints in the rocks are very important design parameters. The latter parameter has generally not received the necessary attention, mainly because it cannot be determined quickly and cheaply. An additional explanation is that obtaining representative rock samples is very difficult and often only the more competent materials survive the sampling processes. A large number of rock types occur in southern Africa. These include igneous, sedimentary and metamorphic rocks. A database including the widest range is thus preferable. The types of rocks tested include in this investigation were: sandstone of the Cape Supergroup; postKaroo dolerite; mudstone, sandstone, of the Karoo Supergroup; and granite of the Basement Complex.

The history ofthe study conducted

The duration of the study was ten years, from 1992 until 2002. This study formed part of a more extensive study to determine the engineering characteristics of important southern African rock types with the emphasis on the shear strength of concrete dam foundations (Geertsema, 2000). The Water Research Commission (WRC) was the main funder of this project. The study has been executed in five distinct phases. The first phase, took place between 1992 and 1993 and consisted of a literature survey with the aim of collecting and studying data on the engineering properties of different types of rock from southern Africa and elsewhere, as well as of worldwide origin. The literature study was updated during 2002-2003.
During this phase the United Kingdom, Norway and the United States were visited by the author, to study inter alia their methods of determination of the engineering characteristics of rocks and particularly also their equipment for shear testing. During the same period, the Department of Water Affairs and Forestry (DW AF) designed a large shear box. The shear box was built during 1993 – 1995. This shear box was used for testing oflarge samples during this research project. During the second phase a sampling programme was undertaken and the general engineering properties of the sampled southern African types of rocks were determined. During 1995 a scanning apparatus was developed and built. This apparatus scans the surface of a rock specimen and can produce a contour map of the scanned surface, which can be used to describe quantitatively the surface topography and thus the roughness of the rock joints to be tested.

Shear strength of rough joint surfaces in rock

A natural discontinuity surface in hard rock is never as smooth as a sawn or ground surface of the type used for determining the basic friction angle. The undulations and asperities on a natural joint surface have a significant influence on its shear behaviour . Generally, this surface roughness increases the shear strength of the surface, and this strength increase is extremely important in terms of the stability of excavations in rock. Patton (1966) demonstrated this influence by means of an experiment in which he carried out shear tests on ‘saw-tooth’ specimens such as the one illustrated in Figure 2.7. Shear displacement in these specimens occurs as a result of the surfaces moving up the inclined faces, causing dilation (an increase in volume) of the specimen.

Determination of joint wall hardness_

Researchers such as Barton and Choubey (1977), Szwedzicki (1998), Katz et al (2000) and van Loon (2003) have studied joint wall hardness. Barton and Choubey (1977) studied hardness of joint surfaces and stated that the measurement of this parameter is of fundamental importance in rock engineering since it is largely the wall characteristics that control the strength and deformation properties of the rock joints. They describe hardness of joint surfaces as joint wall compressive strength (JCS). The importance of the parameter is accentuated if the joint walls are weathered, since then the JCS value may be only a small fraction of the uniaxial compressive strength of the rock material associated with the majority of the rock mass, as typically sampled by borehole core. The depth of penetration of weathering into joint walls depends on the rock type, in particular on its permeability. A permeable rock will tend to be weakened throughout; while impermeable rocks will just develop weakened joint walls leaving relatively unweathered rock in the interior of each block.

CONTENTS :

• I. Introduction and motivation
  • l.l Objectives of the study
  • 1.2 Motivation
  • 1.3 The history of the study conducted
  • 1.4 Outline of the thesis
• 2. The shear strength of joints in rock
  • 2.1 Introduction
  • 2.2 Discontinuities in rock
  • 2.3 Principals of shear
  • 2.4 Shear strength of planar joint surfaces in rock
  • 2.5 Shear strength of rough surfaces in rock
  • 2.6 Determination of joint wall hardness
  • 2.7 Joint matching
  • 2.8 Infilling of joints
  • 2.9 Shear strength equations
• 3. The experimental stage of the study
  • 3.1 Rock types tested
  • 3.2 Apparatus used in testing
  • 3.2.1 Shear boxes
  • 3.2.2 Laser apparatus
  • 3.3 Tests methods
  • 3.3.1 Testing basic friction angles with small shear box
  • 3.3.2 Testing shear strength with the large shear box
  • 3.3.2.1 Phase I testing
  • 3.3.2.2 Phase 2 testing
  • 3.3.2.3 Shear strength of joints in Granite – Phase
• 4. Results of the investigation
  • 4.1 Basic friction angle
  • 4.2 Shear strength of rock types tested
  • 4.3 Maximum post-peak shear strength – Phase I
  • 4.3.1 Basalt
  • 4.3.2 Dolerite
  • 4.3.3 Granite
  • 4.3.4 Sandstone
  • 4.3.5 Mudstone
  • 4.4 Minimum post-peak shear strength – Phase
  • 4.4.1 Basalt
  • 4.4.2 Dolerite
  • 4.4.3 Granite
  • 4.4.4 Sandstone
  • 4.4.5 Mudstone
  • 4.5 Shear strength of joints in Granite – Phase
  • 4.5.1 Granite IC
  • 4.5.2 Granite 2C
  • 4.5.3 Granite 3C
  • 4.6 Discussion of test results
  • 4.6.1 Discussion of test results of Phase I and
  • 4.6.2 Discussion of test results of Phase
  • 4.7 Relationships investigated
  • 4.7.1 The relationship between shear displacement and joint roughness
  • 4.7.2 The relationship between friction angle and joint roughness
  • 4.7.3 Field estimate of shear strength of joint surfaces in rock
  • 4.7.4 The influence of true cohesion, rock bridging and waviness on shear strength
  • 4.8 Further research and conclusion
• 5. Classification of shear strength of joints in rock
  • 5.1 Introduction
  • 5.2 Classification of joints according to this study
  • 5.2.1 Classification of joint wall compressive strength
  • 5.2.2 Classification of roughness profiles
  • 5.3 Shear strength classification based on roughness and hardness of joint surfaces
  • 5.3.1 Joints in hard rock filled with clayey material of more than 2mm thickness
  • 5.3.2 Joints in hard to very hard rock with stained surfaces
  • 5.3.3 Smooth, planar bedding joints with unweathered surfaces in moderately hard rock
  • 5.3.4 Rough planar tectonic unweathered joint surfaces in hard rock
  • 5.3.5 Rough irregular tectonic joints in unweathered hard rock
  • 5.4 Proposed classification of joints according to roughness and hardness
  • 5.5 Application in shear strength in the design of concrete dam foundations
• 6. Conclusions and recommendations
• 7. References
  • Appendices

GET THE COMPLETE PROJECT
The shear strength of rock joints with special reference to dam foundations