General information on tunnelling and underground work is given in the preceding chapters, mainly in part three. The case histories developed in this chapter concern the Kariba South Bank and the Kariba North Bank underground works, excavated in igneous rock and the Waldeck II machine house in sedimentary rock. In all three cases the rock required support. The methods used were very different in each case. It is interesting to compare designs, excavation techniques and rock support.

The Kariba South Bank power scheme (fig. 16.1)

The geology of the site (figs. 16.2 and 16.3)

The most impressive element of the Kariba power scheme is the high double-curvature arch dam impounding water in the Kariba Reservoir. Most of the geological investigation on the site was concentrated on the dam foundations and on the rock abutments on both sides (see section 11.4.3) and on the reinforcement work on the South Bank.

The geology of the Kariba area was recorded and interpreted by the late Dr Francis Jones and Dr Dubertret (Paris). Dr J. L. Knill and Dr K. S. Jones have published an extensive geological description of the area, including the areas where the South Bank and the North Bank underground power houses had to be located. They reported that:

Introduction

Tunnelling engineering techniques are very closely related to rock mechanics and mining. It is not possible to summarize here the vast amount of information now available on this subject, therefore only specific aspects will be discussed.

First to be mentioned among tunnels is the Thames tunnel built between 1826 and 1843 by Brunei. The excavation of the 12 849-m-long Mont Cenis tunnel between France and Italy, in 1870–1, was one of the major undertakings at the end of the last century. It was rapidly followed by others in America (Hoosac tunnel, 1873, Canadian tunnels on the Canadian Pacific Line, 1875) and in England with the Severn tunnel in 1873 and the Blackwall tunnel in 1891. The first very large Alpine tunnel, the 14·98-km-long St. Gotthard tunnel was opened in 1882, followed shortly by the Arlberg tunnel (10 240 m) in 1884. The longest are the two Simplon tunnels both of 19 803 m, built in 1906 and 1922. The twentieth-century underground railway systems and hydro-power developments gave fresh impetus to tunnelling techniques. More recently, large road and power tunnels have been built through mountain ridges. The Gotthard Road tunnel (Switzerland), now under construction, will be 16·32 km long and the Orange–Fish power and irrigation tunnel in South Africa, 82·45 km.

Considerable progress has been made during the past twenty years in drilling techniques, the quick removal of soil and in lining methods. Roof bolting has been introduced on a large scale.

General information

The Vajont reservoir is located in the lower reaches of the river bearing the same name, not far from its confluence with the River Piave. The Vajont dam, a dome-shaped shell, built by the Società Adriatica de Elettricità (Chief Engineer Carlo Semenza), is the highest arch dam in the world, being 265 m high with a 160 m chord, which spans a very deep and narrow gorge. Several plaster models were built for testing the dam and its deformations under full load. The rock on both sides of the gorge had been skilfully reinforced (sections 6.6.1(3) and 11.4.1(1)).

During the night of 9 October 1963, a mass of rock estimated at 200–300 million m3, broke loose from the sides of Mount Toe and crashed into the reservoir below in which the water level was about 700 m above sea level. (The dam was designed for a water storage level of 722·5 m a.s.l.) Almost 2500 lives were lost when a wave overtopped the dam and flooded the Piave Valley, destroying the small town of Longarone. The Vajont dam was not damaged and it still stands as the highest arch dam in the world, proof of the remarkable work done by the Italian engineers when reinforcing the fractured rock abutments on both sides of the gorge.

The slide, mainly rocky formations which, though strongly fractured, have maintained many of their original morphological features, extended from the close vicinity to the dam to as far as 1800 m upstream.

The geologist's approach to rock mechanics

An American geologist, Professor Kiersch (1963), submitted to the annual Congress of the Austrian Society of Rock Mechanics in Salzburg (1962), a most interesting paper on the trends of engineering geology in the United States and on the modern teaching methods introduced at several American universities for training the new generation of engineering geologists. At the same meeting Professor Bjerrum, from Norway, speaking on behalf of A. Casagrande (President of the International Conference on Soil Mechanics) brilliantly explained how rock mechanics should be integrated into soil mechanics as a mere chapter of a wider, more advanced technical science. The Congress reacted by deciding to expand their own efforts and to form the nucleus of an International Society of Rock Mechanics.

Rock mechanics adopts many of the techniques developed in soil mechanics based on the simple law of Coulomb which related shear strength of elastic materials to the friction factor and the normal stress. But the behaviour of rocks is far more complex than the behaviour of soils, and in many cases rock mechanics uses techniques unknown in soil mechanics. The two are parallel but distinct chapters of the science of discontinuous spaces as opposed to the classical theory of strength of materials assuming a continuous space. The links between rock mechanics, engineering geology and classical geology are even more intricate and complex. It is not possible to think about rock mechanics without examining and discussing these links.

General remarks

Tests carried out on samples have shown the complexity of rock behaviour when under strains and stresses. It has been proved that deformations, strength and rupture of rock material, depend on the degree of fissuration of the sample, the porosity and water content and the dispersion of the test results. Griffith's theory is at the root of the problem of rupture of rock material.

Rock material can be used as small-scale models of rock masses. But microfractures and geological faults, bedding and stratification in the mass intensify the defects observed in it. Measurements of the modulus of deformation largely depend on the fractures and faults observed in the mass to such an extent that in addition to a modulus of elasticity E, a modulus of total deformation Etotal including non-elastic, non-reversible deformations of the mass must be defined, and differ from the modulus of elasticity of the rock.

Chapter 5 described how the strength of rock material depended upon microfissures and microfractures; on voids, their density and shape. It will now be seen that the strength of rock masses depends on microfractures, joints and faults.

The scale effect observed on rock material is intensified in tests on rock masses. It is of great importance when discussing test methods and results to be fully aware of the actual volume of rock mass under examination. For example, the Talobre tests with a 50-ton jack on a small bearing plate may involve a far smaller volume of rock than a large size test with 4 × 300-ton jacks.

At the time the first edition of Rock Mechanics and Engineering was being printed, important progress was being made both in theory and practice of rock mechanics. Some new advances were analysed in an ‘Appendix’ to the book, which is now incorporated, with the necessary additions, in the relevant chapters of the second edition. New developments of the new Austrian tunnelling method (NATM) and similar methods caused the important chapter on underground power-stations to be rewritten, and several new chapters to be added.

The problem of bridging the gap between scientific research in rock mechanics and practical engineering has become more acute. Such bridging has recently been achieved in Fluid Transients (Jaeger 1977); it is also vital to applied rock mechanics, as explained in the Preface to the first edition. Many geologists suggest that the rock quality designation (RQD) of Deere is the most reliable parameter for an engineering classification of jointed rock masses. Some geophysicists did not agree and recently introduced their own more complex classification, based on the combination of several parameters describing rock characteristics. Engineers in charge of the construction of large tunnels and underground works were not convinced by these efforts and base their own designs on the rock deformations they expect to occur.

The second edition deals with these problems in several new chapters. There is no better method to deal with them than the close analysis of some case histories.

The first attempts at investigating the mechanical and physical properties of rocks go back to the second half of the last century; but systematic efforts to develop this into a real science of rocks are recent.

One of the more important results of these efforts has been to show the paramount importance of voids, fissures and faults at the level of rock crystals or grains of the rock material and of large masses of rocks in situ. Any theory of rock mechanics which considers rocks to be no more than homogeneous masses is just a first approximation. All the properties of rock and rock constants: strength, elasticity, plasticity, perviousness and reactions to sound and to seismic waves depend on the gaps, voids or fissures of the rock as much as on the skeleton of the solid material which builds it up. It is from this point of view that this book has been written.

One of the main purposes of the book is to stress the many links between rock mechanics and engineering. Mining engineers and civil engineers cannot excavate mines or build structures without a knowledge of rock mechanics. This is now accepted by these professions, which include discussions on rock mechanics at their international congresses and symposia. On the other hand, the benefit that rock mechanics gets from the development of modern engineering techniques has seldom been adequately appreciated.