Estimates of ground deformations and liner stresses in a tunnel are usually obtained from empirical correlations or from past experience on similar tunnels. These correlations account for only a few of the significant factors, and extrapolation to other cases is questionable because similitude conditions are not generally fulfilled. In this paper, complete analytical solutions for a shallow tunnel in saturated ground are obtained. Two different drainage conditions have been considered: full drainage at the ground-liner interface, and no drainage. The solutions cover different construction processes and soil conditions: (1) dry ground; (2) saturated ground with and without air pressure; (3) with and without a gap between the ground and the liner; and (4) applicability for short term analysis (i.e., undrained excavation and liner installation) and for long term analysis. Since the ground and the liner are assumed to behave elastically, the solutions obtained are restricted to cases where ground deformations are small, such as stiff clays and rocks, or when the excavation method prevents large deformations of the ground.

Analytical Solutions for Shallow Tunnels in Saturated Ground

A discussion of a paper with the aforementioned title by Gabriel Fernandez, published in this journal (Volume 120, Number 10, October 1994), is presented. Discusser Swiger discusses the two rules of thumb for determining length of steel liners in pressure tunnels. Swiger provides examples of undesirable behavior of pressure tunnels, supporting the author's purpose in establishing a valid analytical framework to extrapolate precedent. Discussion is followed by closure from the author.

Closure of "Behavior of Pressure Tunnels and Guidelines for Liner Design"

Numerous cases of pressure-tunnel failures have occurred in recently commissioned projects. The cost of the remedial measures, particularly the lost revenue during the time taken for the implementation of these measures, is significant. This paper presents a framework to identify and evaluate the variables that control tunnel behavior and thus determine the key decisions made in the design of a pressure tunnel. An approach is also given for incorporating the hydraulic and mechanical liner-ground interaction in the evaluation of the water tightness of various types of liners under operating conditions, as well as for evaluating the capability of these liners to control the pore-water pressure in the surrounding rock mass. This approach also permits an evaluation of potential benefits derived from consolidation grouting of the rock mass surrounding the tunnel. Finally, specific liner design recommendations and guidelines are given to accommodate a wide combination of rock mass characteristics, topography, ground-water levels, and operating conditions.

Behavior of Pressure Tunnels and Guidelines for Liner Design

Indeed, determination of groundwater inflow into tunnels, especially in discontinuous rock masses, is still one of the most challenging issues faced by tunneling experts, which may have destructive consequences and result in impediments to tunneling operations. Hence, it is necessary to predict the location and amount of inflow in the tunneling designation phase. In order to quantify the groundwater inflow rate into tunnels, various analytical, empirical, and numerical methods exist in the literature. Analytical solutions are of great interest since they provide desirable approximations quickly and without the necessity for advanced computers. However, there are no comprehensive and unique databases concerning analytical methods. Therefore, the attempt of this study was to present the various analytical methods that exist in the literature, their application domain, and their validation.

https://link.springer.com/content/pdf/10.1007%2Fs10064-018-1348-9.pdf

Water flow into tunnels in discontinuous rock: a short critical review of the analytical solution of the art

This study deals with the investigation of analytical solutions for the seepage forces acting on the lining in a drained circular tunnel under the steady-state groundwater flow condition. Two diffe...

Seepage force in a drained circular tunnel: An analytical approach

This article refers to the design studies and work done in constructing the pressure shaft in the Capivari-Cachoeira hydro-power scheme, which has an installed output of 250,000 kW. The shaft operates under pressures of up to 85 kg/cm2. Its diameter is 3.0 m in the upper unlined portion and 2.90 m in the lined portion below it. The article begins with a theoretical study of stresses in indefinite porous ground out of which a cylindrical cavity has been hollowed out and filled with water under a constant pressure. The ground thus contains a flow, the resulting loss of head being simply considered as an additional volume force acting at all points in the ground. The calculation result confirms the Bratz theorem which states that the forces acting at any point are defined simply by adding the percolating flow pressure and the stresses in the ground, assuming pressure to be only applied on the surface. The discrepancies between the resulting conclusion and the results of two other investigations are explained by calculation : the strain assumptions differ in each case, but each has its own particuar field of application. A theoretical study of the effect of grouting on the percolating flow characteristics is then described. Next, the results of pressure tests on a cavity under conditions which are seldom if ever attained in practice are discussed. The cavity was 15 m long, with a diameter of 2.20 m and was subjected to a pressure of 65 kg/cm2. The test was also appllied to the cavity with unlined sides, then with a concrete lining grouted under high pressure. The load with stood by the rock was found to he practically the same in both cases, with little difference between the respective leakage rates. The analysis of losses over a period of time confirmed the results of a theoretical interpretation of these effects, it having been found that the filling of the ground with water appeared to reduce the losses, i.e. an effect which is generally ignored in standard tests on small-diameter wells. These tests were completed by tests in which small-diameter boreholes were put under pressures of up to as much as 100 kg/cm2 by the classical Lugeon test method. The results obtained are compared with the experimental cavity data. This enabled the upper limit of the lined shaft portion to be determined, which was found to be at a point at which the maximum static head was 400 m of water. Finally, the calculations whereby the lining thickness in the lower shaft portions were determined are discussed. These calculations as conventiona1 as regards internal pressures, but an original solution was found to the outside pressure problem which consisted in sinking a shaft running parallel to the lined shaft and providing drainage borings to eliminate dangerous water pressure in the ground when emptying the system.

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Aménagement Caprivari-Cahoeira étude en charge