A Monnet & Y Iagolnitzer, Bachy Paris. This paper was first published in GE’s August 1994 edition.
Two construction techniques have evolved for buildings with deep basements under a phreatic water level:
- the construction of an impermeable concrete raft at the base of a structure. This approach applies as long as the building load can withstand the water uplift pressure, and where the sub-soil is pumpable, without disturbance, during the temporary stage when the base of the excavation has been reached but the raft is not yet constructed; and
- the creation of an area enclosed by side walls (usually diaphragm walls), and a grouted raft or a geological soil layer having a low permeability. Pumping rates are thus considerably reduced, but the need for pumping is permanent. This solution is chosen in cases where the first is technically not feasible, and also when permanent pumping proves more economical than a raft (for example, with very light structures).
It is the development of the latter concept that created the need for a new pumping test which, because permanent pumping has a cost (drainage layer, pumps, maintenance, etc.), makes it necessary for the client to know, prior to any excavation, what the pumping rate will be in the long term.