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Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 1
Temporary Structures
Construction
Dewatering
Lecture 11
Temporary Structures
2
Construction Dewatering
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.
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 2
Temporary Structures
3
Dewatering: CAISSONS Excavation 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 cohesionlesssoils.
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4
Dewatering: 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.
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 3
Temporary Structures
5
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.
Permeability and SeepageFlow of Water in Soil
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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 = qt
Apparent velocity of the flow
A
qv vAq or
q
Soil area, A
Q
Permeability Test (Constant Head Test)
HL
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 4
Temporary Structures
7
Permeability (Cont’d)
In 1856, Darcy published a simple equation for the discharge velocity of water through saturated soils:
kiv
L
Hi
v : the apparent velocity
k : the coefficient of permeability (aka: Hydraulic
conductivity a material’s constant)
i : hydraulic gradient
By definition:
H : the head causing flow over the distance L.
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8
Permeability (Cont’d)
Q = qt => Q = (vA)t => Q = (ki) At
Q = k ( ) At
Solve for k :
L
Hi
L
H
HAt
QLk
vAq kiv
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 5
Temporary Structures
9
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
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10
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. 10175.0
25.24
18
10031.0 a. 2
2
HAt
QLk
in./min 10315.010
18 10175.0 b. 22
kiv
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 6
Temporary Structures
11
Permeability in the Field by 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
2
1
2
2
ln
R
R
HHkq
1
2
2
1
2
2
lnR
R
HH
qk
WW R
R
HH
qk ln
22
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k Determined from Pumping Tests
For D10 = 0.3 mm,
k = 2000 x 10-4 cm/sec
= 0.2 cm/sec
0.3
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 7
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13
Example
Consider the case of pumping from a well in an unconfined permeable layer underlain by an impermeable stratum. Given:
q = 26 ft3/min
H1 = 15.7 feet at R1 = 100 feet
H2 = 18.0 feet at R2 = 200 feet
Calculate the hydraulic conductivity (in feet/min) of the permeable layer.
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Example (Cont’d)
14
Natural
GWT
q = 26 ft3/min
15.7’
100’
200’
Ground Surface
Impermeable
Stratum
Permeable
Layer 18’
ft/min 074.0
7.1518
100200ln26ln
22
1
2
2
1
2
2
R
R
HH
qk
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 8
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15
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 - Wellpoints
Effective lifts of 15 feet are quite common at sea level, and under certain circumstances, lifts can be increased to as much as 25 feet.
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 9
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17
Dewatering Methods - Wellpoints
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Dewatering Methods - Wellpoints
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 10
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19
Dewatering Methods - Wellpoints
Multistage Wellpoint System
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20
Dewatering Methods - Wellpoints
Temporary Structures
Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 11
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21
Dewatering Methods – Wellpoints
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22
Dewatering Methods – WellpointsTypical Wellpoint System
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Construction Dewatering
Professor Kamran M. Nemati
Spring Quarter 2018 12
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Dewatering Methods – WellpointsGroundwater control by Pumping
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Questions?
Kamran M. Nemati
Architecture Hall
Room 130J