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Transcript of Construction Dewatering- Lecture
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 1
Temporary StructuresCM 420 CM 420 CM 420 CM 420
Temporary StructuresCM 420 CM 420 CM 420 CM 420
Construction Dewatering
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Construction DewateringThe 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.
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Dewatering: CAISSONSExcavation from within the permanent structure.If the site is on land, the structure is built in place.If the site is offshore, the structure is floated into position.To reduce the frictional resistance between the caisson and the surrounding ground:
Add weight Bentonite clay slurry is injected at the soil-structure interface.Jetting is used in cohesionless soils.
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 2
Temporary StructuresCM 420 CM 420 CM 420 CM 420
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
CAISSONS (Cont’d)During unwatering a caisson in cohesionless soils, the upward flow from the surrounding groundwater induces a quick condition which results in loss of strength at the bottom of excavation.
To prevent quick condition, the head difference causing flow should be kept low.Caissons should not be used in the vicinity of existing structures that can be damaged due to loss of ground from beneath their foundations.
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Permeability and SeepageFlow of Water in Soil
Soils have interconnected voids through which water can flow from points of high energy to points of low energy.It is necessary to estimate the quantity of underground seepage for investigating problems involving the pumping of water for underground construction, and making stability analysis of earth dams and earth-retaining structures that are subjected to seepage forces.
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Temporary StructuresCM 420 CM 420 CM 420 CM 420 Permeability Test
(Constant Head Test)ASTM D2434
q = Water flowing through the soil at a constant rate
Q = Amount of water collected in a given time period, t
Then: Q = qtApparent velocity of the flow ⇒
Aqv = vAq =or
H
q
Soil area, A
Q
L
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 3
Temporary StructuresCM 420 CM 420 CM 420 CM 420
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Permeability (Cont’d)In 1856, Darcy published a simple equation for the discharge velocity of water through saturated soils:
ν : the apparent velocityk : the coefficient of permeability (aka: Hydraulic
conductivity − a material’s constant)i : hydraulic gradient
By definition:
kiv =
LHi =
H : the head causing flow over the distance L.
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Permeability (Cont’d)
Q = qt => Q = (vA)t => Q = (ki) At
Q = k ( ) At
Solve for k : ⇒
LHi =
LH
HAtQLk =
vAq = kiv =
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Range of Permeability for Various Soils
Gravels are 1 million times more pervious than clays
SoilPermeability Coefficient, k
(cm/sec)Relative
Permeability
Coarse gravel Exceeds 10-1 High
Sand, clean 10-1 to 10-3 Medium
Sand, dirty 10-3 to 10-5 Low
Silt 10-5 to 10-7 Very low
Clay Less than 10-7 Impervious
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 4
Temporary StructuresCM 420 CM 420 CM 420 CM 420
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Temporary StructuresCM 420 CM 420 CM 420 CM 420 Example for the Constant Head Test
For a constant head laboratory permeability test on a fine sand,the following values are given:
Length of specimen = 10 in.Diameter of specimen = 2.5 in.Head difference = 18 in.Water collected in 2 minutes = 0.031 in.3
Determine:a. Hydraulic conductivity, k, of the soil (in./min.)b. Discharge velocity
( )( )( ) ( )
in./min. 10175.025.2
418
10031.0 a. 2
2×=
⎟⎠⎞
⎜⎝⎛
==πHAt
QLk
( ) in./min 10315.01018 10175.0 b. 22 −− ×=⎟
⎠⎞
⎜⎝⎛×== kiv
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Temporary StructuresCM 420 CM 420 CM 420 CM 420 Permeability in the Fieldby Pumping from Wells
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.
( )
⎟⎟⎠
⎞⎜⎜⎝
⎛−
=
1
2
21
22
ln
RR
HHkq π
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−
=1
22
122
lnRR
HHqk
π
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−
=WW R
RHH
qk ln22π
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
k Determined from Pumping Tests
For D10 = 0.3 mm,
k = 2000x10-4 cm/sec
= 0.2 cm/sec
0.3
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 5
Temporary StructuresCM 420 CM 420 CM 420 CM 420
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ExampleConsider the case of pumping from a well in an unconfined permeable layer underlain by an impermeable stratum. Given:
q = 26 ft3/minH1 = 15.7 ft at R1 = 100 ftH2 = 18.0 ft at R2 = 200 ft
Calculate the hydraulic conductivity (in ft/min) of the permeable layer.
( )( ) ( )
( ) ft/min 074.07.1518100200ln26ln 22
1
22
122
=−
=⎟⎟⎠
⎞⎜⎜⎝
⎛−
=ππ R
RHH
qk
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Dewatering Methods - Wellpoints
Small pipes, up to 2.5 inches in diameter, connected to screens at the bottom and to a vacuum header pipe at the surface constitute a wellpoint system.
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Dewatering Methods - WellpointsEffective lifts of 15 ft. are quite common at sea level, and under certain circumstances, lifts can be increased to as much as 25 ft.
Construction Dewatering
Professor Kamran M. NematiWinter Quarter 2007 6
Temporary StructuresCM 420 CM 420 CM 420 CM 420
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Dewatering Methods - Wellpoints
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Dewatering Methods - Wellpoints
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Temporary StructuresCM 420 CM 420 CM 420 CM 420
Dewatering Methods - Wellpoints
Multistage wellpoint system