Dewatering

Prof. Jie Han, Ph.D., PEThe University of Kansas

Outline of Presentation

• Introduction• Applications• Design• Examples

Introduction

Purposes for Dewatering

For construction excavations or permanent structures that are below the water table andare not waterproof or are waterproof but arenot designed to resist the hydrostatic pressure

Permanent dewatering systems are far lesscommonly used than temporary or constructiondewatering systems

Permanent Highway Below Groundwater Table

Highway

Common Dewatering Methods

Sumps, trenches, and pumps

Well points

Deep wells with submersible pumps

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 (such as clayey soil)

Sump Arrangement

Harris (1994)

Wet Excavations

Sump pumps are frequently used to remove surfacewater and a small infiltration of groundwater

Sumps and connecting interceptor ditches should belocated well outside the footing area and below thebottom of footing so the groundwater is not allowedto disturb the foundation bearing surface

In granular soils, it is important that fine particles notbe 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

Army TM 5-818-5

Well Point Method

Multiple closely spaced wells connected by pipesto a strong pump

Multiple lines or stages of well points are required for excavations more than 15 to 20 ft below thegroundwater table

Well Point

Harris (1994)

Well Point Installation

Harris (1994)

Single Stage Well Point System

Caltrans

Single Stage Well Point System

Typical Multiple Well Point System

Johnson (1975)

Staged Ring Well Point System

Harris (1994)

Effect of Foundation Depth and Impermeable Stratum

Harris (1994)DDC = Draw-Down Curve

Deep Wells with Submersible Pumps

Pumps are placed at the bottom of the wells andthe water is discharged through a pipe connectedto the pump and run up through the well hole toa suitable discharge point

They are more powerful than well points, require a wider spacing and fewer well holes

Used alone or in combination of well points

Deep Well

Harris (1994)

Bore a hole with rotary boringmethod 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

Harris (1994)

Recharge Water from Trench

Harris (1994)

Inappropriate Recharging

Harris (1994)

Recharge Water from Wells

Harris (1994)

Applicability of Dewatering Systems

Army TM 5-818-5

Applications

Permanent Groundwater Control System

Army TM 5-818-5

Deep Wells with Auxiliary Vacuum System

Army TM 5-818-5

Buoyancy Effects on Underground Structure

Xanthakos et al. (1994)

Recharge Groundwater to Prevent Settlement

Army TM 5-818-5

Sand Drains for Dewatering A Slope

Army TM 5-818-5

Grout Curtain or Cutoff Trench around An Excavation

Army TM 5-818-5

Design

Design Input Parameters

Most important input parameters for selectingand designing a dewatering system:

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

- the permeability of the ground surroundingthe excavation

Depth of Required Groundwater Lowering

The water level should be lowered to about 2 to5 ft below the base of the excavation

2 to 5ft

Methods for Permeability

Empirical formulas

Laboratory permeability tests

Borehole packer tests

Field pump tests

Accuracy

Cost

Darcy’s Law

Average velocity of flow

Lhkkiv ==

Rate (quantity) of flowA

LhkkiAq ==

Actual velocity of flow

nvva =

Typical Permeability of Soils

Soil or rock formation Range of k (cm/s) Gravel 1 - 5Clean sand 10-3 - 10-2

Clean sand and gravel mixturesMedium to coarse sandVery fine to fine sandSilty sandHomogeneous claysShaleSandstoneLimestone

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 5-818-5Effective Grain Size (D10) of Soil, mmC

oeffi

cien

t of H

oriz

onta

l per

mea

bilit

y of

Soi

l (k h

) x10

-4cm

/sec

Note: kh based on field pumping tests

h

QA

SoilL

Constant Head Test

hAtQLk =

Falling Head Test

Soil

AValve

h1h2

At t=t1

At t=t2dh

a

L

∆

=2

1hh

tAaLk ln

Field Pumping Test

h2h1

r2

r1

r

dr dh

h

Phreatic levelbefore pumping

Phreatic levelafter pumping

Test well

Observation wells

Impermeable layer

q

Observation Wells

NAVFAC (1982)

Permeability from Field Pumping Test

Permeability

( )22

21

21

hhr

rqk

−π

=ln

Dupuit-Thiem Approximation for Single Well

h0

hs

r0

Influence range, Rr

hw

Phreatic levelbefore pumping

Phreatic levelafter pumping

Impermeable layer

QOriginal GWL

s

( )( )

( )( )0

2w

2

0

2w

2

r/RloghHk366.1

r/RlnhHkQ −

=−π

=

Hy

( )k

r/rlnQhy 02w

2

π=−

Height of Free Discharge Surface

( )H

hHChs

20−

=

Ollos proposed a value of C = 0.5

Influence Range

( ) khH'CR w−=

Sichardt (1928)

C = 3000 for wells or 1500 to 2000 for single line well points

H, hw in meters and k in m/s

Forchheimer Equation for Multiwells

( )n21

22

x...xxln)n/1(RlnyHkQ

−−π

=

Forchheimer (1930)

x1

Well 1

Well 2

Well 3

Point P

x2 x3

a( )

alnRlnyHkQ

22

−−π

=

Circular arrangement of wells

0 1 2 3 4 5R/H

0

0.4

0.2

0.6

0.8

1.0

hs/H

hs

h0h Q H

Ry

Army TM 5-818-5

Height of Free

Discharge Surface

Estimation of Flow Rate – Darcy’s Law

r0 h0H

H-h0

R

( )( )( )0

000

rRrRhHhHk571.1Q

−++−

=

Cedergren (1967)

Estimation of Flow Rate – Well Formulas

r0 h0H

H-h0

R

( )0

20

2

r/RloghHk366.1Q −

=

Cedergren (1967)

Estimation of Flow Rate – Flow Nets

r0h0

HH-h0

R

( )( )d

f00 n

nrRhHk14.3Q +−=

Cedergren (1967)

Flow net

nf = number of flow channelsnd = number of head drops

Capacities of Common Deep Well Pumps

Mansur and Kaufman (1962)

Min. i.d. of wellpump can enter

(in.)

Preferred min. i.d. of well

(in.)

Approximate max. capacity

(gal/min)

45 5/8

6810121416

5681012141618

90160450600

1,2001,8002,4003,000

Rate of Flow into A Pumped Well or Well Point

Bush (1971)

Approximate formula

0044 hrkQ =

k = permeability, ft/minr0 = effective radius of the well, fth0 = depth of immersion of well, ft

Typical Well Point Spacing in Granular Soils

NAVFAC (1982)

Typical Well Point Spacing in Stratified Soils

NAVFAC (1982)

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:

Min. spacing = 10rwπ ≈ 32r0

Well Point Pump

Carson (1961)

Head vs. Discharge for Pump

Carson (1961)

Head vs. Discharge for Pump

Carson (1961)

Bottom Stability of Excavation

γz > γw (h +z) Caltrans

Settlement of Adjacent Structures

'vo

'vo

c0

logCe1

Hσ

σ∆+σ+

=δ

whγ∆=σ∆

∆h = reduction of groundwater level

Examples

Plan View & Cross-Section

Xanthakos et al (1994)

Design Requirement

Lower the groundwater table to 5ft below the bottomof the excavation

Equivalent Radius and Influence Range

Equivalent radius of excavation

ft357ft500ft800r0 =π×

=

Height of water level in well

( ) ( )ft1130m340

107.43.0851403000khH'CR 5w

==×××−×=−= −

Influence range

h0 = 160 – 70 – 5 = 85 ft

Rate of Flow in WellsUsing Darcy’s law

Single well formula

( )( )

( )( )

min/gal2350min/ft313

357/1130log8514000925.037.1

r/RloghHk37.1Q

3

22

0

20

2

==

−××=

−=

( )( )( )

( )( )( )

min/gal2592min/ft3463571130

3571130851408514000925.057.1

rRrRhHhHk571.1Q

3

0

000

==−

++−××=

−++−

=

Flow Rate into Wells using Flow Net

( )( )

min/gal1782min/ft238303)3571130()85140(00925.014.3

nnrRhHk14.3Q

3

d

f00

==

+×−××=

+−=

R=1500’

Pump Test

A pump test indicates that the field permeability k = 9.2 x 10-4 cm/sec and the radius of influenceR = 2200ft. The new solutions based on the pumpTest results are

Method Darcy’s law Well formula Flow net

Q (gal/min) 370 290 360

Xanthakos et al (1994)

Layout of Deep Wells

Multiple Wells

( ) ( )

min/gal290min/ft7.38357ln2200ln

8514000181.014.3alnRln

yHkQ

3

2222

==−

−××=

−−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:6” dia. for 5 x 36.6 gal/min = 181 gal/min