Variation of Jointed Rock Permeability Induced by Tunnel Excavation
Date Issued
2012
Date
2012
Author(s)
Yang, Chou-Han
Abstract
In various underground excavations, groundwater issue is an often encountered problem, among which massive water inflow is frequently reported. The deductions of flow field and mechanical field of fractured rock mass are always the key to safe rock engineering. In this study, a coupled hydro-mechanical analysis procedure is proposed and used to probe into the variation of hydraulic conductivity of jointed rock mass induced by tunnel excavation.
In this study, a numerical model of tunnel excavation and the discrete fractured networks (DFN) proposed by Zong-Xun Yang (2010) is used to explore the changes of jointed rock mass hydraulic conductivity after tunnel excavation. Through selecting different observation ranges of the rock mass and evaluating statistics of the hydraulic conductivity, we deduce the size of representative volume element (RVE). Scan line method is used to compute the average RQD value of the RVE. According to RMR
classification, the mechanical parameters of the jointed rock mass is estimated, and a
numerical tunnel seepage model is created. The iterative computing method is proposed
in this study to consider the coupling hydro-mechanical effect during the stress
equilibrium process though tunnel excavation. The iterative computation stops until the
hydraulic conductivity stabilized, that is, the hydraulic conductivity of the jointed rock
mass after disturbance.
Results of numerical simulation indicate that the joint dilation has a significant
impact on the jointed rock mass hydraulic conductivity. On the joint plane which is
tangential to the tunnel wall, the normal stress on joint plane decreases and shear stress
increases. Such behavior causes the hydraulic conductivity around tunnel surface surge
a hundred times larger than its initial value. It is also found that the joint dilation would
III
influence the jointed rock mass hydraulic conductivity remarkably. Different joint dip
angles and the coefficient of lateral earth pressure cause different distribution of jointed
rock hydraulic conductivity as well. Taking the coefficient of lateral earth pressure 0.5
for example, when joint cross angle equals to 30 degree, the hydraulic conductivity of
the jointed rock mass is low at tunnel roof and floor; on the contrary, it is high at tunnel
sidewalls. This distribution is caused by the effect of tunnel excavation, during which
the stress tends to decrease in radial direction and increase in tangential direction. As the
included angle of joints increases, the distribution of hydraulic conductivity changes too.
When the included angle of joints equals to 90 degree, the permeability of the jointed
rock mass falls at tunnel sidewall, and rises at tunnel roof and floor. As to the two cases
with lateral earth pressure coefficients 0.5 and 1.5, the rock mass deformation in the
former one concentrate on roof and floor of the tunnel, while it localizes at sidewalls in
the later one, causing subsequent increase in coefficient of hydraulic conductivity within
these areas. For cases with lateral earth pressure equals to 1.0, the stress distribution and
deformation is uniform, thus the variation of rock mass hydraulic conductivity is the least of the three.
Subjects
jointed rock mass
hydro-mechanical coupling
tunnel
discrete fractured networks (DFN)
representative volume element (RVE)
Type
thesis
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