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BUILDING

CONSTRUCTION

During construction, excavations must be kept

free of standing water. Such water may come

from :

-PRECIPITATION

IT MAY COME FROM GROUND-WATER SEEPAGE

originating from any

of a number of sources, such as

-surface water percolating through the Soil

-underground streams,

-perched water moving over impervious soilstrata

-adjacent permanently saturated soil areas

where the excavation extends below the water

table.

DEWATERING

The most common method of dewatering is to run water by pumping as it accumulates in pits, called sumps, created at low points

in the excavation.

This can be done either by pumping water from the surrounding soil to depress the water table below the level of the bottom of theexcavation or by erecting a watertight barrier, such as a slurry wall, around the excavation

Most excavations require some form of

dewatering, or extraction of water from

the excavation or surrounding soil.

ATUL ANAND JHA

TAUSEEF AHMED

B.ARCH. III A

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Well points are commonly used to depress the water table. These are vertical

sections of pipe with screened openings at the bottom that keep out soil particles

while allowing water to enter.

Closely spaced well points are driven into the soil around the entire perimeter of

the excavation. These are connected to horizontal header pipes leading to pumps

that continually draw water from the system and discharge it away from the

building site. Once pumping has drawn down the water table in the area of the

excavation, work can continue in the dry.

For excavations deeper than the 20 feet (6 m) or so that cannot be drained by

a suction pump stationed at ground level, two rings of well points may be

required, the inner ring being driven to a deeper level than the outer ring, or

a single ring of deep wells with submersible force pumps may have to be

installed.

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In some cases, well points may not be practical:

they may have insufficient capacity to ensure that an

excavation remains dry;

restrictions on the disposal of groundwater may limit

their use;

reliability due to power outages may be a concern; or

lowering of the water table may have serious adverse

effects on neighboring buildings by causingconsolidation and settling of soil under their

foundations or by exposing untreated wood

foundation piles, previously protected by total

immersion in water, to decay.

In these cases, a watertight barrier wall may be used as

an alternative (Figure 2.26).

Slurry walls and soil mixed walls can make excellent

watertight barriers.

Sheet piling can also work, but it tends to leak at the

joints.

Soil freezing is also possible. In this method, an array

of vertical pipes similar to well points is used to

continuously circulate coolant at temperatures low

enough to freeze the soil around an excavation area,

resulting in a temporary but reliable barrier to

groundwater.Watertight barriers must resist the hydrostatic pressure

of the surrounding water, which increases with depth, so

for deeper excavations, a system of bracing or tiebacks is

required. A watertight barrier also works only if it

reaches into a stratum of impermeable soil such as clay.

Otherwise, water can flow beneath the barrier and rise

up into the excavation.

Slurry wall and tieback construction used

to support historic buildings around a

deep excavation for a station of the Paris

Metro.

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Installing tiebacks. (a) Drilling

through a slurry wall for atieback. The ends of

hundreds of completed tiebacks

protrude from the wall.