construction dewatering lecture 11 - university of...

Temporary Structures

Construction Dewatering

Professor Kamran M. Nemati

Spring Quarter 2018 1


Construction

Dewatering

Lecture 11


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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.






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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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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.






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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






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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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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






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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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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






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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






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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.


Example (Cont’d)

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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






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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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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.






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Multistage Wellpoint System


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Dewatering Methods – Wellpoints


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Dewatering Methods – WellpointsTypical Wellpoint System






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Dewatering Methods – WellpointsGroundwater control by Pumping


Questions?

Kamran M. Nemati

[email protected]

Architecture Hall

Room 130J

mailto:[email protected]

construction dewatering lecture 11 - university of...

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