Dewatering

September 26, 2017 | Author: nasrullahk_24 | Category: Permeability (Earth Sciences), Groundwater, Civil Engineering, Hydrology, Liquids
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De watering excavation. Well points...

Description

Dewatering Prof. Jie Han, Ph.D., PE The 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 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

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 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

Army TM 5-818-5

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

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

DDC = Draw-Down Curve Harris (1994)

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  Used alone or in combination of well points

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

Harris (1994)

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 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

2 to 5ft

Methods for Permeability  Empirical formulas  Laboratory permeability tests Accuracy  Borehole packer tests Cost  Field pump tests

Darcy’s Law Average velocity of flow

h v = ki = k L Actual velocity of flow

v va = n Rate (quantity) of flow

h q = kiA = k A L

Typical Permeability of Soils Soil or rock formation Gravel Clean sand Clean sand and gravel mixtures Medium to coarse sand Very fine to fine sand Silty sand Homogeneous clays Shale Sandstone Limestone Fractured rocks

Range of k (cm/s) 1-5 10-3 - 10-2 10-3 - 10-1 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

Coefficient of Horizontal permeability of Soil (kh) x10-4 cm/sec

Permeability vs. Effective Grain Size Note: kh based on field pumping tests Effective Grain Size (D10) of Soil, mm

Army TM 5-818-5

Constant Head Test

h

L

Soil Q A QL k= hAt

Falling Head Test a At t=t1 dh h1

At t=t2

h2 Soil

A

aL  h1  k= ln   A∆t  h2 

L Valve

Field Pumping Test r1 r

r2

Observation wells

q dh

dr

Phreatic level before pumping Phreatic level after pumping

h

h2

Test well Impermeable layer

h1

Observation Wells

NAVFAC (1982)

Permeability from Field Pumping Test

Permeability

r1   q ln   r  2 k= π h12 − h22

(

)

Dupuit-Thiem Approximation for Single Well Influence range, R r

r0 Q

Original GWL

s

Phreatic level before pumping

Phreatic level hs after pumping

y hw

H

h0 Impermeable layer

(

)

(

)

πk H 2 − h 2w H 2 − h 2w Q= = 1.366k ln (R / r0 ) log (R / r0 )

y 2 − h 2w =

Q ln (r / r0 ) πk

Height of Free Discharge Surface C (H − h0 ) hs = H

2

Ollos proposed a value of C = 0.5

Influence Range Sichardt (1928) R = C' (H − h w ) k

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 Well 1

Forchheimer (1930)

(

)

πk H 2 − y 2 Q= ln R − (1 / n ) ln x1x 2 ...x n

Point P Well 2 x1 x2 x3 Well 3

Circular arrangement of wells

(

πk H 2 − y 2 Q= ln R − ln a

)

a

1.0

hs 0.8 h0

H

Q

h

y

Height of Free Discharge Surface

R

0.6 hs/H

0.4

0.2

0

0

1

3

2 R/H

4

5

Army TM 5-818-5

Estimation of Flow Rate – Darcy’s Law R H-h0 r0

H

h0

( H − h 0 )(H + h 0 )(R + r0 ) Q = 1.571k R − r0

Cedergren (1967)

Estimation of Flow Rate – Well Formulas R H-h0 r0

h0

H

H 2 − h 02 Q = 1.366k log (R / r0 ) Cedergren (1967)

Estimation of Flow Rate – Flow Nets R H-h0 r0

H

h0 Flow net

nf Q = 3.14k (H − h 0 )(R + r0 ) nd nf = number of flow channels nd = number of head drops Cedergren (1967)

Capacities of Common Deep Well Pumps Min. i.d. of well pump can enter (in.) 4 5 5/8 6 8 10 12 14 16

Preferred min. i.d. of well (in.)

Approximate max. capacity (gal/min)

5 6 8 10 12 14 16 18

90 160 450 600 1,200 1,800 2,400 3,000

Mansur and Kaufman (1962)

Rate of Flow into A Pumped Well or Well Point Approximate formula

Q = 44 k r0 h0 k = permeability, ft/min r0 = effective radius of the well, ft h0 = depth of immersion of well, ft

Bush (1971)

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 σ 'vo + ∆σ H δ= C c log 1 + e0 σ 'vo

∆σ = ∆hγ w

∆h = reduction of groundwater level

Examples

Plan View & Cross-Section

Xanthakos et al (1994)

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

Equivalent Radius and Influence Range Equivalent radius of excavation r0 =

800ft × 500ft = 357ft π

Height of water level in well h0 = 160 – 70 – 5 = 85 ft Influence range R = C' (H − h w ) k = 3000 × (140 − 85)× 0.3 × 4.7 ×10 −5 = 340m = 1130ft

Rate of Flow in Wells Using Darcy’s law ( H − h 0 )(H + h 0 )(R + r0 ) Q = 1.571k R − r0

( 140 − 85)(140 + 85)(1130 + 357 ) = 1.57 × 0.00925 ×

1130 − 357 = 346ft 3 / min = 2592gal / min Single well formula

(

)

(

1.37 k H 2 − h 02 1.37 × 0.00925 × 140 2 − 852 = Q= log (R / r0 ) log (1130 / 357 ) = 313ft 3 / min = 2350gal / min

)

Flow Rate into Wells using Flow Net

R=1500’

nf Q = 3.14k (H − h 0 )(R + r0 ) nd 3 = 3.14 × 0.00925 × (140 − 85) × (1130 + 357) 30 = 238ft 3 / min = 1782gal / min

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 Q (gal/min)

Darcy’s law Well formula Flow net 370

290

360

Layout of Deep Wells

Xanthakos et al (1994)

Multiple Wells

(

)

(

πk H 2 − y 2 3.14 × 0.00181× 140 2 − 852 = Q= ln R − ln a ln 2200 − ln 357 = 38.7ft 3 / min = 290gal / min

)

290/8 = 36.3 gal/min per well Deep well size: 4” dia. for 36.6 gal/min Header pipe: 6” dia. for 5 x 36.6 gal/min = 181 gal/min Discharge pump: 6” dia. Pump for 290 gal/min

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