Investigation and assessment of pre-grouted rock mass

Helene Strømsvik,Bjørnar Gammelsæter

doi:10.1007/s10064-019-01722-9

Helene Strømsvik, Bjørnar Gammelsæter

Open Access

https://doi.org/10.1007/s10064-019-01722-9

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Abstract

Pre-grouting is a technique for reducing water ingress into tunnels and caverns by grouting fractures and joints prior to excavation. This study investigates pre-grouted rock mass to evaluate grout penetration in fractures and transmissivity of water in the rock mass surrounding the built tunnel, with the use for core drilling, OTV, high-precision water injection tests and core logging. The study was performed in three tunnel localities, in tunnels excavated in connection with the Follo Line project in Norway, where pre-grouting was performed using cement-based grouts. It was found less cement than expected in fractures with small apertures, compared with results of grout penetrability in laboratory studies of similar grouts. Further, it was found that fractures in coarse-grained rock types had rougher fracture surfaces and higher hydraulic apertures, than fractures in fine-grained rock types. It was also found that fractures with smoother surfaces had smaller hydraulic apertures in general. Hydraulic jacking was evidenced during the pre-grouting in this area, which is likely to have contributed to unnecessary high grout consumption.

Highlights

Tunnels have different requirements regarding allowable water inflow, depending on the use and location
It can be noted that in all three test locations, the majority of the observed fractures are subhorizontal. In this regard it is important to bear in mind that the test holes are all oriented with an E-W, or vertical direction, which would largely underestimate steep fractures with similar strike direction
In this view it is not possible to see the part of the thin fracture that intersects with the cemented fracture; it is possible to evaluate the approximate apertures of the visible parts of the fractures

Summary

Introduction

Tunnels have different requirements regarding allowable water inflow, depending on the use and location. The most important factor when determining the allowable water inflow is safeguarding of the environment above the tunnel. Groundwater flows through the rock mass in fractures, and this flow can be described in various ways. The water follows the path of least resistance, i.e. where the aperture is greatest, resulting in different flow distribution within each fracture. Transmissivity of water in a slot is proportional to the cube of the slot aperture and follows the cubic law. This implies that a small change in the fracture aperture will have great impact on the transmissivity of a fracture. One method for estimation of transmissivity is presented in “Method for high-precision water injection tests”

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