Tunnel inflow, prognosis, and results - Evaluation of inflow to rock tunnels in the Gothenburg
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Examensarbete för masterexamen
Modellbyggare
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Grouting design has been shifted from being based on a rather empirical basis, to
having more focus on calculations of fracture apertures and having the design being
carried out in accordance with the observational method. In the literature, limited
data is available regarding follow-up on prognoses of grouting and tunnel inflow from
projects based on either of the two grouting design methods. This thesis therefore
aimed at illuminating how the theoretical method used to estimate inflow coincides
with the empirical results from four tunneling projects in the Gothenburg region.
The aim of the thesis also included to analyse how the parameters used in the inflow
equation affected the estimation of inflow, and in particular the choice of skin factor.
Collected data from hydraulic tests performed during the construction of the four
tunneling projects, e.g. through probe drillings and preinvestigation boreholes, were
used to evaluate the hydraulic conductivity of the rock mass. Since different grouting
designs were used, the evaluation of the hydraulic conductivity had to be treated
differently depending on the information available. Predictions of the inflow were
performed using selected best fit values of the input parameters for the inflow equation
based on collected data, which were then compared to the actual measured
inflows.
Through a sensitivity analysis, it was found that the main parameters affecting the
inflow was the hydraulic conductivity of both the grouted and ungrouted rock mass
and the hydraulic head. These three input parameters (K, Kgr, and H), also had the
largest uncertainties in the estimation of the parameter values. It was also found
that the ratio of K and Kgr influenced the results to a quite large extent. It was
concluded that when a small sealing effect is expected (for example when the rock
mass already have a low hydraulic conductivity), more care should be taken when
choosing the skin factor.
Generally, the inflow equation overestimated the predicted inflow (approximately
with 30%). It was also concluded that the inflow equation worked well in conditions
of "normally" fractured rock mass. When the hydraulic conductivity of the rock
mass was very low, the equation did not perform as good.
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grouting, followup, water loss measurement, hydraulic conductivity, inflow