Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 270

C HAPTER D3
Underground Construction
1 PROBLEMS DUE TO GROUNDWATER
When an underground construction project passes through an aquifer, it
acts as a drain and noticeably alters the fl ow conditions in the aquifer. The
probability of encountering infl ows of water is high, especially if the project
is located under a thick cover of rock.
Two distinct consequences must then be considered:
• on the one hand, the role of groundwater on the construction conditions
and the stability of the project;
• on the other hand, the hydrogeologic impact of the project on its natural
and human environment.
2 IMPACT OF GROUNDWATER ON CONSTRUCTION
METHODS
Encountering groundwater in a tunnel is a frequent cause of more or less
serious diffi culties, both during the construction phase due to the sudden
intrusion of water or the unblocking of unstable terrain, and over the long
term under the effects of the hydrostatic pressure maintained around the
project, of the washing out of fi ne particles, or of the gradual alteration of
the concerned formations.
The problems encountered are, of course, a function of the nature and
characteristics of the rocks being traversed, but also of the magnitude of
the discharge collected during excavations. This discharge can vary from
a few tens to a few hundreds, or even a few thousands, of L·s -1 . Duffaut
(1981), for example, cites a few very sizeable discharges encountered below
ground (Table 22).
Extreme conditions, associated with inadequate preliminary study and
poorly adapted construction, can sometimes lead to real disasters: