Dewatering

Dewatering

Prof. Jie Han, Ph.D., PE

The University of Kansas

Outline of Presentation

• Introduction

• Applications

• Design

Purposes for Dewatering

 For construction excavations or permanent

structures that are below the water table and are not waterproof or are waterproof but are

not designed to resist the hydrostatic pressure

 Permanent dewatering systems are far less

commonly used than temporary or construction dewatering systems

Permanent Highway Below

Groundwater Table

Common Dewatering Methods

 Sumps, trenches, and pumps  Well points

Sumps, Trenches, and Pumps

 Handle minor amount of water inflow

 The height of groundwater above the excavation

bottom is relatively small (5ft or less)

 The surrounding soil is relatively impermeable

Wet Excavations

 Sump pumps are frequently used to remove surface

water and a small infiltration of groundwater

 Sumps and connecting interceptor ditches should be

located well outside the footing area and below the bottom of footing so the groundwater is not allowed to disturb the foundation bearing surface

 In granular soils, it is important that fine particles not

be carried away by pumping. The sump(s) may be lined with a filter material to prevent or minimize loss of fines

Dewatering Open Excavation by

Ditch and Sump

Well Point Method

 Multiple closely spaced wells connected by pipes

to a strong pump

 Multiple lines or stages of well points are required

for excavations more than 15 to 20 ft below the groundwater table

Well Point Installation

Staged Ring Well Point System

Effect of Foundation Depth and

Impermeable Stratum

Deep Wells with Submersible Pumps

 Pumps are placed at the bottom of the wells and

the water is discharged through a pipe connected to the pump and run up through the well hole to a suitable discharge point

 They are more powerful than well points, require

a wider spacing and fewer well holes

Deep Well

 Bore a hole with rotary boring method

 Insert a temporary outer casing if necessary

 Place a perforated well liner  Plug the well bottom

 Place layers of filter material around the casing

Deep Wells with Submersible Pumps

Inappropriate Recharging

Applicability of Dewatering Systems

Permanent Groundwater Control

System

Buoyancy Effects on Underground

Structure

Recharge Groundwater to Prevent

Settlement

Sand Drains for Dewatering A Slope

Grout Curtain or Cutoff Trench

around An Excavation

Design Input Parameters

 Most important input parameters for selecting

and designing a dewatering system:

- the height of the groundwater above the base of the excavation

- the permeability of the ground surrounding the excavation

Depth of Required Groundwater Lowering

 The water level should be lowered to about 2 to

5 ft below the base of the excavation

Methods for Permeability

 Empirical formulas

 Laboratory permeability tests  Borehole packer tests

 Field pump tests

Accuracy

Darcy’s Law

Average velocity of flow

L

h

k

ki

v

=

=

Rate (quantity) of flow

A

L

h

k

kiA

q

=

=

Actual velocity of flow

n

v

Typical Permeability of Soils

Soil or rock formation _{Range of k (cm/s)}

Gravel _{1 - 5}

Clean sand ₁₀-3 _{- 10}-2

Clean sand and gravel mixtures Medium to coarse sand

Very fine to fine sand Silty sand Homogeneous clays Shale Sandstone Limestone 10-3 _{- 10}-1 Fractured rocks 10-2 _{- 10}-1 10-4 _{- 10}-3 10-5 _{- 10}-2 10-9 _{- 10}-7 10-11 _{- 10}-7 10-8 _{- 10}-4 10-7 _{- 10}-4 10-6 _{- 10}-2

Permeability

vs.

Effective

Grain Size

Army TM 5-818-5

Effective Grain Size (D₁₀) of Soil, mm

C oe ffi c ie nt of H or iz onta l pe rme a bi li ty of S oi l (k h ) x10 -4 cm /sec

Note: k_h based on field pumping tests

h

Q

A

Soil

L

Constant Head Test

hAt QL k =

Falling Head Test

Soil

A

Valve

h

₁

h

₂

At t=t

₁

At t=t

₂

dh

a

L



_









∆

=

2 1

h

h

t

A

aL

k

ln

Field Pumping Test

h₂ h1 r₂ r₁ r dr dh h Phreatic level before pumping Phreatic level after pumping Test well Observation wells Impermeable layer q

Observation

Wells

Permeability from Field Pumping Test

Permeability

(

2

)

2 2 1 2 1

h

h

r

r

q

k

−

π













=

ln

Dupuit-Thiem Approximation for Single Well

h₀ h_s r₀ Influence range, R r h_w Phreatic level

before pumping Phreatic level_{after pumping}

Impermeable layer Q Original GWL s

(

)

(

)

(

(

₀

)

)

2 w 2 0 2 w 2 r / R log h H k 366 . 1 r / R ln h H k Q = π − = − H y

(

)

k r / r ln Q h y2 2_w 0 π = −

Height of Free Discharge Surface

(

)

H h H C h_s 2 0 − =

Influence Range

(

H h

)

k ' C R = − _w Sichardt (1928) C = 3000 for wells

or 1500 to 2000 for single line well points H, h_w in meters and k in m/s

Forchheimer Equation for Multiwells

(

)

n 2 1 2 2 x ... x x ln ) n / 1 ( R ln y H k Q − − π = Forchheimer (1930) x₁ Well 1 Well 2 Well 3 Point P x_{2 x} 3 a

(

)

a ln R ln y H k Q 2 2 − − π =

0 1 2 3 _{4 5} R/H 0 0.4 0.2 0.6 0.8 1.0 h_s/H h_s h₀ h Q H R y Army TM 5-818-5

Height of

Free

Discharge

Surface

Estimation of Flow Rate

– Darcy’s Law

r₀ h₀ H H-h₀ R

(

)(

)(

)

0 0 0 0 r R r R h H h H k 571 . 1 Q − + + − =

Estimation of Flow Rate

– Well Formulas

r₀ h₀ H H-h₀ R

(

₀

)

2 0 2 r / R log h H k 366 . 1 Q = − Cedergren (1967)

Estimation of Flow Rate

– Flow Nets

r₀ h0 H H-h₀ R

(

)(

)

d f 0 0 n n r R h H k 14 . 3 Q = − + Flow net

n_f = number of flow channels n_d = number of head drops

Capacities of Common Deep Well

Pumps

Mansur and Kaufman (1962) Min. i.d. of well

pump can enter (in.)

Preferred min. i.d. of well (in.) Approximate max. capacity (gal/min) 4 5 5/8 6 8 10 12 14 16 5 6 8 10 12 14 16 18 90 160 450 600 1,200 1,800 2,400 3,000

Rate of Flow into A Pumped Well

or Well Point

Approximate formula 0 0 44 kr h Q = k = permeability, ft/min

r₀ = effective radius of the well, ft h₀ = depth of immersion of well, ft

Typical Well Point Spacing in Granular Soils

Spacing of Deep Wells

 The spacing of deep wells required equals to the perimeter of the excavation divided by the

number of wells required  Sichardt recommended:

Head vs. Discharge for Pump

Bottom Stability of Excavation

Settlement of Adjacent Structures

' vo ' vo c 0 log C e 1 H σ σ ∆ + σ + = δ w hγ ∆ = σ ∆

Plan View &

Cross-Section

Design Requirement

Lower the groundwater table to 5ft below the bottom of the excavation

Equivalent Radius and Influence Range

Equivalent radius of excavation

ft 357 ft 500 ft 800 r₀ = π × =

Height of water level in well

(

)

(

)

ft 1130 m 340 10 7 . 4 3 . 0 85 140 3000 k h H ' C R _w 5 = = × × × − × = − = − Influence range h₀ = 160 – 70 – 5 = 85 ft

Rate of Flow in Wells

Using Darcy’s law

Single well formula

(

)

(

)

(

(

)

)

min / gal 2350 min / ft 313 357 / 1130 log 85 140 00925 . 0 37 . 1 r / R log h H k 37 . 1 Q 3 2 2 0 2 0 2 = = − × × = − =

(

)(

)(

)

(

)(

)(

)

min / gal 2592 min / ft 346 357 1130 357 1130 85 140 85 140 00925 . 0 57 . 1 r R r R h H h H k 571 . 1 Q 3 0 0 0 0 = = − + + − × × = − + + − =

Flow Rate into Wells using Flow Net

(

)(

)

min / gal 1782 min / ft 238 30 3 ) 357 1130 ( ) 85 140 ( 00925 . 0 14 . 3 n n r R h H k 14 . 3 Q 3 d f 0 0 = = + × − × × = + − = R=1500’

Pump Test

A pump test indicates that the field permeability k = 9.2 x 10-4 _{cm/sec and the radius of influence}

R = 2200ft. The new solutions based on the pump Test results are

Method Darcy’s law Well formula Flow net

Multiple Wells

(

)

(

)

min / gal 290 min / ft 7 . 38 357 ln 2200 ln 85 140 00181 . 0 14 . 3 a ln R ln y H k Q 3 2 2 2 2 = = − − × × = − − π =

290/8 = 36.3 gal/min per well Deep well size:

4” dia. for 36.6 gal/min

Discharge pump:

6” dia. Pump for 290 gal/min Header pipe: