THE ENGINEERING CHARACTERISTICS OF IMPORTANT SOUTHERN AFRICAN ROCK TYPES WITH EMPHASIS ON SHEAR STRENGTH OF CONCRETE DAM FOUNDATIONS.
Report No 433/1/00
The stability along joint planes is one of the most important characteristics of a rock mass forming the foundation of a concrete dam. The shear strength of discontinuities within the foundation rock is probably the most important characteristic.
Objectives and Purpose of the study
The objectives of this research project were:
(a) to determine and to analyse the shear strength of joints in a number of rock types, sampled at different locations, and to link these strengths to the condition of the foundations and, in particular, the condition of the surfaces of the rock joints. The information so obtained can then serve as a databank for the design of new dams and for the evaluation of the safety of existing dams, and
(b) to determine the characteristics of a number of southern African rock types to serve as preliminary design parameters to allow safer and more economical designs for the foundations of concrete dam walls.
Stages of investigation
The study was carried out in four identifiable phases. The first phase that took place during
1992 and 1993 was to do a literature study in order to determine the engineering characteristics of different rock types world-wide and in southern Africa. During this stage a visit was undertaken to the UK, Norway and the USA to study shear apparatus and the rock testing methods in these countries. The second phase was to determine the general characteristics of important southern African rock types. During the period 1993 to 1995 the shear apparatus and surface-scanning device to be used in the third stage were designed and constructed. The third phase (1994 to 1999) comprised of shear tests on NX-size borehole core samples (base shear and direct shear) and the testing and characterisation of large shear surfaces. The last phase (1999 to 2000) was used to compile the report.
Several delays were encountered mainly due to the following reasons: (a) the late delivery of the large shearbox and subsequent problems with the computer controlling the shearbox, (b) resignation of the technician working full-time on the project and (c) illness of the researcher during 1996.
Format of the report
The text of the report start by stating the problems to be investigated followed in Chapter two by the findings of a literature study. Chapter three describes the determination of the engineering characteristics: methods and equipment, and Chapter four the presentation and discussion of results. This is followed in Chapter five by a description of the estimation of shear strength using a geotechnical characterization of the joint surface followed in Chapters six, seven and eight by conclusions, recommendations and references.
The Compact Disc (CD) contains the appendixes in electronic form as reports, graphs and photo' s.
A comprehensive literature study on test methods and engineering characteristics of different rock types was conducted. It was found that although engineering characteristics of rock material have been investigated on a regular basis for civil and other engineering applications, this information is not readily available to the engineering community at large. It is often regarded as confidential information by clients and filed for possible use in claims situations. This document is probably the most comprehensive source of engineering characteristics of southern African rock types available today.
This report describes the strength, deformation and general characteristics of quartzite, shale, sandstone, dolerite, mudstone, granite, rhyolite and tillite. Chapter 4 describes each of these rock types in detail. The results are too comprehensive to describe here. These rock types were selected because they cover a very large portion of the surface area of southern Africa, and as such many dams and other civil engineering structures have been built on them.
Emphasis was placed on the shear strength parameters of joints, especially the angle of friction. Two types of joints are recognised in nature: (a) joints with no or little fill material where the shear strength is determined by the characteristics of the rock material and (b) joints with fill material where the shear strength is determined by the characteristics of the fill material. The major part of this research concentrated on (a) joints with no or little fill material.
The three major characteristics determining the shear strength parameters of this type of joint are (i) the base shear strength of the rock material, (ii) the roughness profile along the joint surface and (iii) the hardness of the material on the joint surface.
The base shear strength parameters of the different rock materials were determined as part of the determination of rock material characteristics. The angle of friction obtained for the different materials corresponds very well to those published in the literature. The values for cohesion obtained through testing is zero to very small.
As part of this research project a laser-scanning device was developed and built in association with the Department of Civil Engineering of the University of Natal. This device measure x, y and z co-ordinates on a rock joint surface on a grid pattern. This information can be analyzed with software on a computer to produce a contour diagram of the joint surface area. From this contour diagram, joint roughness profiles were obtained. These, as well, as profiles obtained with a carpenter's comb, were compared with typical roughness profiles as published by Barton.
Because of the importance of joint roughness in connection with the shear strength of joints, a further attempt was made to obtain a method of quantifying this phenomenon. The volume of material (asperities) above the lowest point on a joint surface was determined for each specimen tested with the large shear apparatus. The volume was then divided by the surface area of the specimen to obtain a value called the volume-area ratio. This was then correlated to the shear strength (in particular to the angle of friction) of a joint plane. Although no good correlation could be found, it is believed that this ratio could be a method of expressing joint roughness. Further investigation will be needed to verify this.
This study provides a useful guide to engineering parameters of several important rock types in southern Africa for planning and preliminary design purposes. It is probably the most comprehensive document describing the rock material, the testing procedure, and the engineering characteristics of so many rock types in southern Africa.
This research project was the first attempt to determine the shear strength characteristics of joints in southern African rock types with a large shear apparatus.
This study also contributes to the knowledge on shear strength of southern African rocks, in particular on (i) the sampling and preparation of specimens for testing in the large shear apparatus, (ii) the measurement of the roughness of the joint surfaces and (iii) the testing procedure. The shear strength characteristics of the rock joints of southern African rocks are described and an attempt was made to classify joints using a geotechnical description of the joint surface. Geotechnical parameters include rock type, roughness, hardness, and description of fill joint material. The joint fill material could be a clay or secondary mineral like smectite, it could be staining or it could be clean. This classification is a first attempt and further work still needs to be done in this regard.
It is recommended that a project be initiated to investigate the shear strength of southern African rock types in further detail in a systematic manner. Such an investigation can build on the knowledge obtained in this investigation. It is important to keep the variables such as rock type, weathering, and hardness be kept a minimum to investigate influence of joint roughness. An appropriate rock type to start with could be mudstone from the Qeduzisi Dam area near Ladysmith. This is a relative soft rock with smooth joints that gave low shear strength results during testing. These results should be confirmed. This could then be extended to other rock types once the influence of roughness has been established.
The author whishes to express his gratitude to the Water Research Commission for funding this project and hope that the report will contribute to dams and other civil engineering structures be built more cost effectively and safely.
It could be noted that this project was the first project at a Technikon to be funded by the Water Research Commission. As such it contributed to the building of capacity at the Department of Civil Engineering at Technikon Pretoria: Two technicians had the opportunity to gain experience in testing rock specimen on the newly built shear apparatus belonging to the Department of Water Affairs and Forestry. Staff of the above-mentioned department were trained on the use and procedures of the machine. The researcher had the opportunity to gain experience and knowledge on many aspects of rock mechanics with the emphases on shear strength. This abundant source of information established during the project will lead to publication in of articles in scientific and engineering journals.