Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 188

Table 5-4 Forms of water ingress into the cavity [160].

Form Properties

Sweating water appearing at the perimeter of the cavity; dripping from the crown (similar

to condensation from the rock)

Percolating water water emerging from cracks and joints (isolated or distributed around the entire

perimeter)

Water ingress
(strained water)

water spraying in jets from individual cracks (Fig. 5-2); in thin jets, this is de-
scribed as springing; concentrated flows which can have particularly large flow
sections in karst, are described as joint water

Water inflow massive, often sudden inflow of water, often carrying suspended particles, mud

or broken rock

All these forms can continue for a long time or reduce after a short time or even stop. The
intensity, with which the water can enter the tunnel, is a result of the high pressure heads
and flow gradients that are possible in a deep tunnel but is primarily due to the fact that the
permeability of the rock mass in certain directions can often be eight to twelve times as
high as that of the solid rock. C. Louis gives water permeability coefficients for selected
rocks, which are shown in Table 5-5.

Figure 5-2 Water
inflow at the face of the
Schönrain Tunnel near
Gemünden.

The quantity and point of appearance of the inflowing groundwater can also change with
time, if the tunnel is only being driven partially below the groundwater table or if the
groundwater table fluctuates with time [130, 160].