Water and ease of excavation are common issues for most underground excavations of any great size as considerable costs can be incurred if predictions relating to either of these are wrong.
Water and Underground Excavations
Water flowing into the excavation can be not only a nuisance but also a hazard. Water pressures reduce effective stress and subsequently the shear strength of the ground.
Water pressures and flow may also cause material to be flushed into the excavation causing overbreak and instability (Fig. 1).
Flowing water can prevent the application of shotcrete from good contact with the ground. Therefore, water flowing into an excavation in large quantities has normally to be prevented. Draining is feasible if permeability of the ground is not too high or if sealing (impermeable) layers are present that limit the quantity of water to be drained. If the rock mass is highly permeable drainage will lower the groundwater table above an excavation.
This is normally unacceptable for social, environmental, and geotechnical reasons. Under these circumstances the excavation has to be sealed locally with, for example, by grouting.
Even if the water inflow is no geotechnical problem, it is not regarded as good engineering to have large water inrushes into an excavation.
It is not easy to estimate water inflow into an underground excavation before the excavation has been made. The theoretical background can befound in many books, but to determine the nature of the groundwater regime and the permeability of the strata is very difficult. In a homogeneous mass without discontinuities, the water will flow via the pores between the grains. For this, reasonable estimates can be made based on theoretical calculations. Most masses are, however, not homogeneous and not with out discontinuities. Reliable estimations of the quantity of water can then only be made from boreholes to locate these zones, packer tests in these zones and from pilot tunnels.
Portals of tunnels may in particular be at risk because these are near to the surface and the mass near portals is often more permeable because of a higher number of discontinuities. Rainfall may then directly flow into the portal area. This problem can be reduced by building the portal in a dry season if such a season exists.
Excavations can be made in many different ways, from digging by hand to using highly mechanised tunnel boring machines (TBMs). Excavation can be made by any type of method whatever the ground; prisoners made escape tunnels in rocks using their bare hands and spoons, however in commercial practice the excavation method chosen has to be suited to the ground, available skilled labour, and the constraints on the project from time and economics.
Generally, two types of excavation method can be distinguished: mechanical and blasting (Fig. 2); both methods can be further divided (Table 1).
Blasting techniques were used extensively in the past, but nowadays mechanical methods are more popular, having various advantages over blasting methods especially for smaller works and for reducing vibrations at ground level. ‘Specials’ noted in Table 1 include methods that make use of the expansion characteristics of wood or chemicals, the force of water under high pressure, used for jetting to erode the rock mass, or use of sawing techniques.
They are seldom used in underground excavations. The method of excavation has a considerable influence on the quality of the perimeter of an excavation especially if high levels of stress exist.
Table 2 gives values for the damaging influence of methods of excavation on a rock mass. Natural, handmade, and bored excavations show fewer new mechanical discontinuities than excavations made by blasting in the same rock mass, as they have not been subject to the transient loading blasting imparts to the ground. Large excavations may not be stable long enough for support to be installed; in these cases it is often necessary to complete the excavation in sequence starting with a small part of the excavation where loads are least, at the top, or crown, of the tunnel (Fig. 2 bottom).