Temporary dewatering

Water control during the construction phase

During construction, when building below the water table or in the presence of significant water flow, temporary water-control devices must be installed. Three main families of techniques are used depending on the needs.

Water collection

Gravity collection is the simplest technique when the water inflow is moderate. It consists of gathering surface or excavation-bottom water into sumps or channels, then evacuating it by pumping towards an outlet (network, watercourse, infiltration).

Typical implementation:

Creation of sumps at the bottom of the excavation (low-point pits)
Submersible pumps (from a few m³/h to several tens)
Peripheral channels to collect runoff
Discharge tank, control of suspended matter

Diagram of gravity water collection — Principle of gravity collection: water runs off the inclined faces (watertight cover) then is gathered by the V-shaped channels and directed towards the downstream outlet.

Sub-horizontal drains

Sub-horizontal drains are drilled into the ground to capture water from a perched water table or a permeable horizon and lower it by gravity without resorting to pumping. They are particularly effective in unstable slopes and hillsides.

Characteristics:

Drilled from the toe of the slope with a slight upward gradient (~5°)
Screened casing in PVC or metal
Typical length 10 to 50 m, sometimes more for large structures
Spacing of 5 to 20 m depending on the permeability of the ground
Gravity drainage, no energy required

Widely used for the stabilization of saturated slopes, the drying of sliding hillsides, or in retained walls when one wants to avoid uplift pressures without pumping continuously.

Slope stabilization by sub-horizontal drains — Stabilization of a sliding hillside by drilled sub-horizontal drains. The drains capture the water trapped between the slip surface and the stable ground, bringing the soil back to a moisture state compatible with its stability. Visible are the scarp, the profile before and after the slide.

Groundwater lowering

When the expected flow is large or the excavation must be completely dried out, a groundwater lowering system is installed. Several techniques depending on the permeability of the ground and the targeted depth:

Layout plan of lowering wells — Plan view of a layout of filter wells around an L-shaped excavation. The wells are arranged in a peripheral ring so as to lower the water table within the excavation perimeter. The spacing is tighter on the upstream hydraulic side to intercept the incoming flow.

Well-points

For permeable soils (sands, gravels) and moderate lowering (3 to 6 m). A series of filter points (perforated lances of 50-75 mm) is driven at the toe of a bank, connected to a header and drawn by a vacuum pump. Widely used for building excavations in sandy soils.

Detailed diagram of a well-point — Composition of a well-point: the perforated point is driven down to the water table, surrounded by a filter sand that prevents fines from being washed in, sealed at the top by a watertight plug. Water is drawn through the riser pipe to the suction header connected to the pumping unit at the surface. The curve of the lowered water table forms around the pumping point.

Deep pumping wells

For larger lowering (beyond 6 m) or in less permeable soils. Boreholes fitted with screened casing and submersible pumps, spaced 5 to 20 m around the excavation. In practice, allows the water table to be lowered by several metres.

Precautions

Settlement by hydraulic consolidation of nearby compressible ground → piezometric and topographic monitoring of neighbouring structures
Washout of fines which can loosen the ground and cause subsidence
Environmental impact on the aquifers, neighbouring wells, aquatic environments (Water Act regulations)
Management of discharge water (release, possible treatment of laden water)

The design relies on the prior estimation of the flow rate (formulas of the Forchheimer, Theis, Jacob type) from the measured hydrogeological parameters (Lefranc tests, preliminary pumping test).

Temporary water control (construction phase)