24-10-2014, 04:42 PM
Construction Dewatering
and Ground Freezing
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Introduction
The control of groundwater is one of the most common and complicated problems
encountered on a construction site. Construction dewatering can become a costly issue if
overlooked during project planning. In most contracts, dewatering is the responsibility of the
contractor. The contractor selects the dewatering method and is responsible for its design and
operation.
The purpose of construction dewatering is to control the surface and subsurface hydrologic
environment in such a way as to permit the structure to be constructed “in the dry.”
Dewatering means “the separation of water from the soil,” or perhaps “taking the water out of
the particular construction problem completely.” This leads to concepts like pre-drainage of
soil, control of ground water, and even the improvement of physical properties of soil. If
ground water issues are addressed appropriately at the investigation and design stage,
construction dewatering, which involves temporarily lowering the ground water table to
permit excavation and construction within a relatively dry environment, is rarely a problem.
Construction dewatering has existed as a specialty industry for a long time. Consequently, a
number of well-established techniques have been developed to lower the ground water table
during excavation. The geology, ground water conditions, and type of excavation all
influence the selection of dewatering technology. The most common methods for dewatering
include sumps, wells and wellpoints.
• Sumps provide localized, very shallow dewatering (less than 3 feet) and consist of
pumping from perforated drums or casings in a gravel-filled backhoe pit. Sumps work
best in tight, fine grained soils, or very coarse, bouldery deposits.
• Wells are large-diameter (greater than 6 inches) holes, drilled relatively deep (greater than
10 feet), and contain slotted casings and downhole pumps. Wells work best in soils
consisting of sand, or sand and gravel mixtures, and can dewater large areas to great
depths.
• Wellpoints are small-diameter (less than 6 inches), shallow wells, and are closely spaced
(2 to 10 feet apart). Wellpoints effectively dewater coarse sands and gravels, or silts and
clays. They have a wide range of applications. However, wellpoints use a vacuum
system and their depth is limited to about 25 feet. Wellpoint systems generally cost more
than either sumps or wells, and require near-continual maintenance.
Underwater Excavations
In special cases where the soil is very pervious or when it is not possible or desirable to lower
the groundwater table, underwater excavations can be considered. If underwater excavation is
to be performed, the work area must be enclosed with an impervious structure. Once the
impervious structure is in place, the excavation is performed within the structure. Once the
desired excavation level is achieved within the structure, it is sealed with an impervious layer,
such as concrete, in order to prevent water from sipping into the work area. After the
impervious seal has been constructed, the water remaining within the structure is pumped out
and construction is completed.
Permeability in the Field by Pumping from Wells
When a well is pumped, the groundwater surface in the surrounding area is lowered which
depends on the pumping rate, the size of the well, the permeability of the soil, and the
distance from the well. In the field, the average hydraulic conductivity of a soil deposit in the
direction of flow can be determined by performing pumping tests from the well. Figure 4
shows a well in an open aquifer being pumped at a pumping rate of q. (An aquifer is a
permeable geological stratum or formation that can both store and transmit water in
significant quantities).
Ground Freezing
The principle of ground freezing is to change the water in the soil into a solid wall of ice.
This wall of ice is completely impermeable. Ground freezing is used for groundwater cutoff,
for earth support, for temporary underpinning, for stabilization of earth for tunnel excavation,
to arrest landslides and to stabilize abandoned mineshafts. The principals of ground freezing
are analogous to pumping groundwater from wells. To freeze the ground, a row of
freezepipes are placed vertically in the soil and heat energy is removed through these pipes
(Figure 10). Isotherms (an isotherm is a line connecting locations with equal temperature)
move out from the freezepipes with time similar to groundwater contours around a well.
Once the earth temperature reaches 32 °F (0 °C), water in the soil pores turns to ice. Then
further cooling proceeds. The groundwater in the pores readily freezes in granular soils, such
as sands. For instance, saturated sand achieves excellent strength at only a few degrees below
the freezing point. If the temperature is lowered further, the strength increases marginally. In
cohesive soils, such as clays, the ground water is molecularly bonded at least in part to the
soil particles. If soft clay is cooled down to freezing temperature, some portions of its pore
water to begin to freeze and it causes the clay to stiffen. With further reduction in
temperature, more pore water freezes and consequently more strength gain is achieved. When
designing for frozen earth structures in cohesive soils, it may be necessary to specify
substantially lower temperatures to achieve the required strength, than in cohesionless soils.
A temperature of +20 °F may be sufficient in sands, whereas temperatures a low as –20 °F
may be required in soft clays.
The design of a frozen earth barrier is governed by the thermal properties of the underlying
soils and related response to the freezing system. Formation of frozen earth barrier develops
at different rates depending on the thermal and hydraulic properties of each stratum.
Typically, rock and coarse-grained soils freeze faster than clays and silts (Figure 11).
When soft clay is cooled to the freezing point, some portion of its pore water begins to freeze
and clay begins to stiffen. If the temperature is further reduced, more of the pore water
freezes and the strength of the clay markedly increases. When designing frozen earth
structures in clay it may be necessary to provide for substantially lower temperatures to
achieve the required strengths. A temperature of +20 °F may be adequate in sands, whereas
temperatures as low as –20 °F may be required in soft clay.
Planning Dewatering Operation
The analysis of a dewatering system require knowledge of the permeability of the soil to be
dewatered. The dewatering methods discussed are applicable to certain specific soil
conditions and excavations sizes. Figure 17 shows their suitability for various soil types. As
seen from Figure 17, methods involving well and wellpoint systems are used where the soil to
be pumped are predominantly sand and gravel. Freezing may be used in the same soils.
Summary
Today’s improved well equipment and well construction techniques make possible the
dewatering of many projects with wells and wellspoints. Other methods of groundwater
control that have been developed and used such as ground freezing, slurry trenches, cast in
situ diaphragm walls, etc. have had some degree of success in the specific job conditions to
which they are suited. Though construction dewatering has not been reduced to an exact
science yet, the selection of the dewatering system should hinge on the experience and
professional judgement of the engineer based on the soil materials, the source of water, and
the demands of the project.
With this lesson, this course come to an end. I hope that the materials presented in the
preceeding eight lessons will be of value in your professional career. The M.K. Hurd book is
a classic concrete formwork book and I am sure you will be referring to it any time you
design formwork.