Sustainability Issue #3 June 2010

The way a tunnel will behave as regards groundwater infiltration and stability can seldom be measured directly; one solution is to use supplemenary parameters, proxy parameters that predict how the system will perform in reality. It is thus a matter of identifying measurable physical parameters that are important for the behaviour of the system.

We consider that there is a great need for a framework to interpret, describe and communicate geological and hydrogeological information in both the early and late stages of a tunnel project. In accordance with Eurocode 7 or the Observational Method, both the most common and disadvantageous but possible rock conditions should be described, so that a design can be proposed that will make it possible to deal with the expected problems. The design may be envisaged to deal with water infiltration and grouting, of which we will give examples here, and also stability and environmental consequences.

Soil, rock and water

From a grouting perspective and at an early stage of a tunnel project, the behaviour of the system soil – rock - water should be described in a way that facilitates communication regarding what may be expected in the future tunnel construction. If a lot of water infiltrates into the tnnel, it may be necessary to lower the groundwater level.

We consider three principal elements as especially important: rock conditions in the area; estimates of infiltration into the tunnel; soil stratification. Rock conditions describe the environment where grouting is to be performed and are an important basis for the choice of method. Estimates of infiltration must be related to the tunnel’s need for inflow and give an indication of the extent of grouting. Soil stratification determines how sensitive the area may be expected to be to groundwater lowering, with settlements and dry wells as possible consequences  Much of the information needed to make an assessment is available from e.g. Geological Survey of Sweden SGU.

If it is necessary to reduce inflow to the tunnel, grouting prior to blasting is a common method. This is employed during tunnel construction and involves drilling holes into the rock volume where the tunnel will be blasted and injecting grout into the holes.The grout spreads into the fissures in the rock and seals the rock against infiltration. However, it is not always so simple. It is necessary to know what rock will be grouted, which can be done by appropriate characterisation, and application of grout must be adapted to the size of the fissures to be sealed. Grout must be injected so well that the groundwater cannot erode and break up the grout before it solidifies.  Special problems such as dripping and icicle formation may ensue if the procedure is not the right one. Research relating to both characterisation of the rock, grouting procedure, grout erosion and drip sealing, and the grout material itself, has been carried on at Chalmers for many years. Several of the projects have been funded by Formas. For characterisation, research funds from the firm Swedish Nuclear Fuel Handling (SKB) has also been very sigificant.

The mass of rock can be divided into blocks of rock (in the photograph wall rock or protolith) and major faults/fracture zones which generally consist of a core and a damaged zone.

Blocks of rock and fracture zones

The rock mass can in principle be divided into blocks of  rock and major fracture zones or faults. There are often a few main fracture groups in the blocks of rock and the faults generally comprise a core with metamorphosed material of variable thickness and a damaged zone that is often waterbearing. A description of the fracture system, both all fractures and the waterbearing ones, is essential information for rockworks and the design of grouting.  On the basis of general descriptions of the blocks and fracture zones and how they may be expected to be distributed, an assessment of the degree of difficulty can be made at an early stage. The prediction and description of rock conditions must be updated with the help of data from surveys of boreholes and tunnels

In the early stages of planning the borehole archive of SGU is a good source of information. On the basis of data from boreholes and their siting in the terrain, an idea of the problems due to infirtration of water can be formed at an early stage.  In this case also, a division into blocks of rock and fracture zones should be made, and the topopgraphy casn help in making an early (preliminary) division, because valleys and depressions are often a result of faults and fracture zones. This can be updated with the help of tests in boreholes.These may be exploratory boreholes for investigating a section in front of the tunnel or grout holes prior to the actual grouting operation. This work is carried out in parallel with geological mapping.

Identified rock conditions, estimated infiltration (in relation to specified requirements) and the sensitivity of the soil strata give an idea of what may be expected in the future tunneling operation. Illustration: Karin Holmgren.

Infiltration into the tunnel

Grouting prior to blasting is often carried out in a routine manner, but it is better to estimate the need for grouting along the individual tunnel sections. It is an advantage that there are relatively simple tests that can provide relevant information. It is necessary to know whether grouting is needed, and how the design of any grouting can be suited to the rock volume.

The extent of grouting may be affected by inflow requirements. It is therefore essential to make an estimate of the expected inflow at an early stage.

In order to estimate the expected inflow into a tunnel, the specific capacity for a borehole, Q/dh, can be used as a proxy parameter. This describes how much water the rock leaks, Q, due to the existing pressure, dh. The equations that are to be used for estimating inflow and the way the proxy parameter, in this case specific capacity, is to be handled is associated with the described waterbearing fracture system. In a rock body with several waterbearing fracture groups, flow is linked more easily to surrounding fractures. This is done, for instance, when there is a risk that the water will be displaced as a result of grouting.

Better tunneling!?

The working procedure described above should make is possible to make improvements in tunnel projects straight away. This shows preliminary results. To make it into an accepted procedure takes time, however. Simply by compiling and visualising rock conditions so that decision makers, clients and contractors may jointly receive an idea of the conditions increases the opportunity to make a good decision concerning the siting of tunnels and the choice of method. Both the tunnel at the Halland project and the tunnel at the Botnia Railway are examples of tunnels where the geology has caused problems. We believe that in the case if future projkects there will be both environmental and economic gains to be made if conditions are described early and design is then continually updated.

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