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An analysis of some 200 tunnel case records has revealed a useful correlation between the amount and type of permanent support and the rock mass qualityQ, with respect to tunnel stability. The numerical value ofQ ranges from 0.001 (for exceptionally poor quality squeezing-ground) up to 1000 (for exceptionally good quality rock which is practically unjointed). The rock mass qualityQ is a function of six parameters, each of which has a rating of importance, which can be estimated from surface mapping and can be updated during subsequent excavation. The six parameters are as follows; theRQD index, the number of joint sets, the roughness of the weakest joints, the degree of alteration or filling along the weakest joints, and two further parameters which account for the rock load and water inflow. In combination these parameters represent the rock block-size, the interblock shear strength, and the active stress. The proposed classification is illustrated by means of field examples and selected case records. Detailed analysis of the rock mass quality and corresponding support practice has shown that suitable permanent support can be estimated for the whole spectrum of rock qualities. This estimate is based on the rock mass quality Q, the support pressure, and the dimensions and purpose of the excavation. The support pressure appears to be a function ofQ, the joint roughness, and the number of joint sets. The latter two determine the dilatency and the degree of freedom of the rock mass. Detailed recommendations for support measures include various combinations of shotcrete, bolting, and cast concrete arches together with the appropriate bolt spacings and lengths, and the requisite thickness of shotcrete or concrete. The boundary between self supporting tunnels and those requiring some form of permanent support can be determined from the rock mass qualityQ.

Summary Geology is an important determinant affecting the planning, design and construction of a road tunnel. A road tunnel has to satisfy certain geometrical criteria and the achievement of the best economic route for a tunnel may be a complex operation, where the related geology offers any considerable choice. In this paper examples are given of the principal characteristics of three recent road tunnels in Britain, explaining the degree to which the wide variations in tunnelling cost depends upon the nature of the ground. It has generally been assumed that, where topography so allows, a road may be set underground by cut-and-cover construction at less cost than in a bored tunnel. In ground particularly favourable for economic tunnelling this is no longer necessarily true on direct engineering costs and, in an urban setting, other indirect costs tend to favour the case for a bored tunnel. Reference is made to the pattern of variations of cost with depth below surface for each of these types of tunnel. The site investigations for a tunnel should be designed to determine the properties of the ground relevant to the selection of tunnelling method and the estimation of constructional difficulties. A highly mechanised system of tunnelling is particularly sensitive to geological variation and calls for an appropriate effort to eliminate, within reasonable limits, the risk of encountering an insuperable or disproportionately costly difficulty. The problem of tunnel design relates directly to the competence of the ground. The concept of a competence factor is introduced in this respect. In soft ground, a principal problem is one of the adequacy of immediate support. In somewhat more competent ground, the extent of the need for immediate support depends on the knowledge of the stress/strain characteristics of the ground. For competent rock, the main problem concerns the means for determining the behaviour of the ground en masse. The method of excavation is also a factor affecting the behaviour of the rock around a tunnel. Over the last 15 years or so there has been considerable experience in the use of rock bolts and shotcrete in providing support to the rock as an alternative to more traditional methods. The behaviour of such expedients is discussed in relation to the nature of the rock and of its discontinuities. In soft ground, surface settlement may be important in determining the scheme for an urban tunnel. Settlement is not only largely dependent on the system of tunnelling but also upon the use of special expedients in relation to tunnelling in poor ground.

The Upper Seti (Damauli) Storage Hydroelectric Project has a capacity of 128 MW, the storage type scheme, and includes 1000 m long horse shoe headrace tunnel, 140 m high concrete gravity dam, two diversion tunnels of lengths 712 m and 881 m and an underground powerhouse. The study was carried out to identify stability and stress conditions for the headrace tunnel to suggest the required tunnel support. The project area extensively covers dolomite and minorly covers slate. The rock mass classification showed fair to good quality of dolomite and poor to fair quality of slate. The surface wedges would form in intake portal and powerhouse site. In the headrace tunnel, structural wedges would be formed due to underground excavation and would be stabilized with the help of shotcrete and rock bolting. doi: 10.3126/bdg.v10i0.1419 Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 10, 2007, pp. 45-54

Diversion of the Pend Oreille River at Boundary Arch Dam site was accomplished by constructing two rockfill cofferdams and a 42-ft horseshoe tunnel in the left abutment. Model tests determined construction procedures for the cofferdams. Excavation of the tunnel was performed from the inside of the arch dam abutment via an adit from the underground power plant. Opening of the portals and rerouting of the river was by a series of singular rock blasts. The tunnel floor was left unlined and gunite was applied to the side walls and roof. The scheme accommodated river discharges during the low flow periods only, and flood flows were discharged over the partly completed arch dam. The diversion tunnel was finally closed by lowering a counterbalanced, arched, steel gate into the flow at the tunnel portal. Concrete plugs closed the access tunnel during diversion and the diversion tunnel after its closure.

A support technique characterized by reliance on the carrying capacity of the rock or soil surround rather than on heavy wood, steel, or concrete support systems has evolved during the last decade in Europe. Carrying capacity is mobilized by the carefully timed and dimensioned provision of certain protective measures such as thin, seimirigid linings adhering directly to the surround (i.e., usually shotcrete) and system anchoring. Frequent measurements, experience, and a thorough theoretical understanding of time-deformation-stress relations of the rock or soil surround, of the measures to be provided, and their interaction are required to achieve optimum rock loads, made possible by the most advantageous stress distribution within the mostly self-carrying surround. Recent theoretical and experimental advances, and recent site experience at two large tunnel projects, are described.

The use of shotcrete is described as a structural support in tunnels in soft, waterbearing, semi-consolidated soil-rocks with an unconfined compressive strength ranging about 27 psi. The tunnels were 20′–25′ span at springline, widening to 56′ in a transition section, 20′–30′ in height. Continuous water inflows of 25 litres/sec with surges to 50 litres/sec were experienced. No interior steel reinforcement was used in the transition section, though 3 m grouted bolts were used for rock reinforcement of the arch. Shear and tensile stresses slowly developed in the walls about the transition section, which were then bolted. In the face of waterinduced wall cavitations occurring in subsequent (20′) tunnelling steel wire mesh was introduced in the shotcrete below springline. Abandoned for 12 months and left under water, floor heave of 20–30 cm developed, without damage to the shotcrete arch and walls. The inverts had been left open without lining.

To evaluate methods for the economical construction of shotcrete personnel shelters, shotcrete arches were cast over various forms: a Quonset arch, a flexible plywood shell, a pneumatic airform, and a compacted earth mound. Salvageable Quonsets were found to be the most economical. Wet-mix shotcrete containing coarse aggregate was placed which permitted use of conventional concrete mixes.

The objective was to develop an economical, arch-shaped shelter, utilizing pneumatically placed mortar, as an alternate to existing standard types. Criteria and plans for a 100-man shotcrete shelter are presented which will provide protection against an overpressure of 100 psi and concomitant effects from nuclear weapons. Shotcrete is recommended because of the economic advantages gained from using a single lightweight form as opposed to the heavy double form required for conventionally placed concrete. Effort was made to provide a balanced and versatile design which may be adapted to the specific needs of various commands. Methods for the design of the basic structural components of the shelter are given, including a method for estimating the relative displacement between the floor and the foundation when the structure is subjected to blast loading. Simple yet adequate design procedures are given which are suitable for use in the design office.

The report is a listing of magnetic tape containing computer programs listed in Volume 2 of the final technical report generated under contract H0110238. A computer model of the overall hard rock tunneling process which considers both performance and costs of the operation was developed. Segments of the tunneling process modeled include geology, fragmentation methods, muck removal, ground support, and environmental considerations. A three-dimensional stratified geology model, functions as a data file to represent geological conditions in the area surrounding the tunnel. The user has a choice of simulating rock fragmentation by drill and blast, boring machine, high velocity water jet, and projectile impact. He may also simulate either rail, truck, or conveyor belt haulage systems. Steel sets, shotcrete, and rock bolts are offered as a choice of ground support methods. Environmental factors considered include water removal, ventilation, and cooling. (Modified author abstract)