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Rock Mechanics and Tunnel Support

RQD: Time to Rest in Peace[1.17 MB]

It is shown that a key facet of the definition of RQD is ignored in many parts of the world, and it is noted that there are several inherent limitations to the use of RQD. The introduction of a look-up chart for assessing GSI has effectively removed the need to measure, or estimate, RQD. It has been found that GSI values derived from the look-up chart are as valid as those derived by calculation from the original component parameters, and are satisfactorily consistent between professionals from diverse backgrounds. The look-up charts provide a quick and appropriate means of assessing GSI from exposures. GSI is, in turn a useful rock mass strength index; one new application is presented for assessing potential erosion of unlined spillways in rock. Incorporation of RQD within the RMR and Q classification systems was a matter of historical development, and its incorporation into rock mass classifications is no longer necessary.

By P J Pells, Z T Bieniawski, S R Hencher and S E Pells

Assessing Parameters for Computations in Rock Mechanics[1.39 MB]

Estimation of rock mass stiffness and shear strength parameters is fundamental to the validity of most applied mechanics calculations for rock mechanics design purposes. Estimation of such parameters is very difficult and has to be based on rock mass characterisation. Currently the Hoek-Brown approach using GSI is widely used for assessing stiffness and shear strength parameters. However, there appear to be few test data to support the parameters determined using this method. This paper presents case studies for both rock mass modulus and rock mass shear strength in an attempt to assess the validity of the approach of using GSI and the Hoek-Brown criterion. It is concluded that for some rock masses the method works well, but in other cases can give very misleading assessments of stiffness and shear strength parameters. It is also concluded that further work is required in collecting and analysing good case studies in order to provide guidelines for the profession.

By P Pells

Limit State Tunnels and Tunnelling[24.6 MB]

It is inappropriate, if not meaningless, to attempt to apply limit state design methods, such as those encapsulated in Eurocode 7, to the design of primary support for tunnels in rock, particularly weak rock or any rock under high stress. By reference to some 41 case histories of major tunnel failures, it is shown that more than 85% were the result of unexpected geology or hydrogeology. It is submitted that the unknowns and uncertainties in geology and hydrology cannot be properly dealt with by applying codified factors to geotechnical parameters. It is concluded that working stress methods should be retained for tunnel support design, and that such design would benefit, far more, from competent geological and hydrogeological understanding, than from faith in Code factors.

By P Pells

Limitations of Rock Mass Classification Systems[316 KB]

The writers are of the view that tunnel design should be done by methods of applied mechanics, like any other structural design. Classification systems are good for communication, and in some cases good for producing correlations in particular geological environments. However, on the basis of the critique by Palmstron and Broch [8] and the experience set out in this paper they should not be used as the primary tool for the design of primary support. A final point to note is that Q and RMR values are not factual data in respect to the engineering geology of a rock mass. Firstly, they include a significant a significant degree of interpretation. Secondly, they relate to a particular structure at a particular depth. Therefore, in the writers opinions they should not appear on engineering geological logs of boreholes or on records of line mapping of excavations. These records should be restricted to data as set out in References 18, and 19.

By P Pells and R Bertuzzi

What Happened to the Mechanics in Rock Mechanics and the Geology in Engineering Geology?[5.61 MB]

A good thing is becoming a bad thing. Rock mass classification systems, that are so excellent for communications between engineers and geologists, and that can be valuable in categorizing project experience, are emasculating engineering geology and rock mechanics. Some engineering geologists have been sucked into thinking that Q and RMR values are all that is needed for engineering purposes, and seem to have put aside what can be learned from structural geology and geomorphology. Many rock mechanics engineers seem to have forgotten the scientific method. This paper attempts to redress the situation by showing how mechanics can be used in rock engineering to design with a similar rigour to that used in the fields of structural engineering, hydraulics and soil mechanics. It also attempts to remind practitioners of what can be achieved by skilled engineering geology.

By P Pells

Permanent Rockbolts – the Problems are in the Detail[417 KB]

Permanent rock bolts in civil engineering are taken as having to serve their design purpose for at least 50 years. In many projects the design life is specified as greater than 100 years. A review of recent Australian tunnelling projects shows that substantial differences exist in the level of corrosion protection provided for the permanent steel shafted rock bolts. This paper reviews the current state of practice and sets out what the authors consider to be appropriate requirements for corrosion protection.

By P Pells and R Bertuzzi

Design of Roof Support of the Sydney Opera House Underground Parking Station[562 KB]

The paper presents the method for design of primary roof support for the large donut shaped cavern constructed to house the Sydney Opera House parking station. The cavern was constructed with 7m of rock cover beneath Sydney’s Royal Botanic Gardens. Design of rock reinforcement using a combination of fully grouted rock dowels and Macalloy bars was based on control of horizontal shear movement along bedding features in the roof. A combination of linear arch analyses and non-linear jointed finite-element analyses were used in the design study, together with an analysis of the shear resistance offered by fully grouted bolts under shear deformation.

By P J N Pells, R J Best and H G Poulos

Developments in the Design of Tunnels and Caverns in the Triassic Rocks of the Sydney Region[1.79 MB]

Details are given of the analytical methods used to design rockbolt and shotcrete support for tunnels and large span caverns under relatively low cover in the near horizontally bedded Triassic sandstones of the Sydney region. The paper provides a concise description of the engineering geology of the Sydney sandstones, because it is fundamental to tunnel support design that a valid geological model be the basis of any analytical design. Equations are provided which allow calculations of the lengths, density and capacity of rockbolts for support in this geological environment. The paper also discusses the design approach adopted for support of tunnels and caverns which are at sufficient depth to generate compressive and shear failure of the rock mass, so-called True Rock Pressure.

By P J N Pells

The Sydney Opera House Underground Parking Station[695 KB]

A summary of the key features of the underground cavern excavated for construction of the double helix carpark to serve the Sydney Opera House. This 18m span donut shaped cavern is probably the widest span, low cover excavation in the world that is supported by permanent rockbolts.

By P J N Pells

Underground Excavations in Near Horizontally Bedded Sedimentary Rocks[2.50 MB]

In 1993 Philip Pells was invited to present the EH Davis Memorial Lecture of the Institution of Engineers Australia. The paper was in three parts: the first part describes the development of design methods for flat topped tunnels in near horizontally bedded rock: the second part sets out what constitutes a proper engineering geological model for tunnel design purposes: and the third part is a thumbnail engineering geology of the Sydney basin. The first part of the lecture was published in the Australian Geomechanics Journal, the second and third parts are in the following two documents which were handed out at the time of the lecture but were never formally published.

By P J N Pells

Geotechnical Models for Underground Works[618 KB]

This is Part 2 of the EH Davis Lecture of 1993. Part 1 was published in the Australian Geomechanics Journal. This part was handed out at the time of the lecture but not formally published. It sets out the components of a proper engineering geological model for tunnel design purposes.

By P J N Pells

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