Excavation-induced hydraulic conductivity reduction around a tunnel – Part 1: Guideline for estimate of ground water inflow rate

doi:10.1016/j.tust.2010.03.006

Tunnelling and Underground Space Technology

Volume 25, Issue 5, September 2010, Pages 560-566

https://doi.org/10.1016/j.tust.2010.03.006 Get rights and content

Abstract

Field observations indicate that current engineering practice does not consistently estimate ground water flows into unlined rock excavated tunnels due to various factors that analytical solutions do not take into account. These factors include significant geological features, groundwater drawdown, inadequate estimates of hydraulic conductivity from packer tests, and stress-induced rock-mass permeability reduction in the vicinity of tunnel (lining-like zone). A key variable that is not properly accommodated in current practice, is the hydro-mechanical interaction within the joints in the surrounding rock mass. The significance of this variable is discussed in the 1st part of the paper which presents an analytical solution assessing ground water inflow rate into a tunnel using a mathematical derivation that takes into account the excavation-induced rock permeability reduction in the vicinity of a tunnel based on hydro-mechanical coupling effect. In the 2nd part of the paper, results from numerical analysis are presented to verify the proposed analytical solution for estimating ground water inflow rate into a tunnel. Further studies are currently underway to identify other key variables and their impact on the behavior of unlined tunnels and hydrological flow regime in the surrounding fractured rock mass using a distinct element method program which can fully consider hydro-mechanical coupled behavior of joints.

Introduction

Groundwater control is a significant issue during most underground construction in a jointed rock mass. More than any other single factor, lack of groundwater control can be the major cause of extra cost and construction delays in underground construction. A proper estimate of the rate of inflow is critical in selecting tunnel alignment, determining the potential need for ground treatment (i.e. grouting) and/or lining installation, and obtaining an adequate schedule and cost estimate.

However, field observations and measurements indicate that the approach used in current engineering practice is not consistently accurate and can result in significant construction delays and cost increase. The inadequate estimation of water inflow rate is in part due to the lack of understanding of the hydro-mechanical interaction within the joints of the surrounding rock mass. Thus, it is essential to develop an analytical framework to incorporate this interaction and assess its impact on ground water inflow into unlined rock tunnels.

The main purpose of the research is to provide guidelines to incorporate the impact of key variables that are not properly accommodated in current practice to estimate water flow rate into unlined rock tunnels. As the first step, this paper presents the significance of excavation-induced rock permeability reduction due to large joint closure in the vicinity of a tunnel and proposes an analytical solution considering the effect.

The first part of the paper presents modified analytical solutions for ground water inflow rate and pore-water pressure distribution around a tunnel in a jointed rock mass considering the effect of excavation-induced hydraulic conductivity reduction in the vicinity of tunnel. In the second part of the paper, the proposed analytical solutions will be supported by field observations and numerical modeling study.

Section snippets

Hydro-mechanical joint behavior

In general, the characteristic of discontinuities dominates the mechanical and hydraulic behavior of a jointed rock mass. Discontinuities are a major source of deformability and flow in a fractured rock mass. The correlation between joint closure and joint effective normal stress is generally characterized as nonlinear and hysteretic as observed by many researchers (Snow, 1972, Goodman, 1974, Iwai, 1976, Bandis et al., 1983, Gale and Raven, 1980, Barton and Bandis, 1982). The deformation

Evaluation of hydraulic conditions around an unlined tunnel

The analytical approach developed to estimate the amount of inflow and the pore-water pressure distribution in the rock mass around an unlined tunnel includes two steps. First, an evaluation was made for the unlined tunnel in a homogeneous, isotropic porous medium. Then, the impact of joint closure in the vicinity of the tunnel was evaluated.

Discussion and conclusions

In summary, generally 20–50% of excavation-induced joint closure takes place within a zone in the vicinity of tunnel due to joint effective normal stress increase, and the equivalent hydraulic conductivity is reduced to 0.1–0.5 of the initial equivalent hydraulic conductivity in the lining-like zone which is ½–1 tunnel-radius thick. A significant pressure drop takes place across the lining-like zone, and thus the actual raise of pore-water pressure in the surrounding rock mass is steeper than

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Excavation-induced hydraulic conductivity reduction around a tunnel – Part 1: Guideline for estimate of ground water inflow rate

Abstract

Introduction

Section snippets

Hydro-mechanical joint behavior

Evaluation of hydraulic conditions around an unlined tunnel

Discussion and conclusions

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

A comparison of rock mass disturbance in TBM and drill and blast drivages at the Doonkin Mine, Nova Scotia

Geotech. Geol. Eng.

Seepage-induced effective stresses and water pressures around pressure tunnels

J. Geotech. Eng.