Lecture8A(Flow&Seepage)

8/13/2019 Lecture8A(Flow&Seepage)

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Lecture 8A

Flow and Seepage Around Tunnel


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Steady State Flow Condition follow the Laplaces Equation (2-D):

headtotalHwhere

0y

H

x

H2

2

2

2

=

=+

Solution is sought by two families of curves intersecting at right angles such

that:

Equipotential function (x,y)=constant (equipotential lines)

Stream function (x,y)=constant (flow lines)

Hydraulic Gradient, i is given as:

l

hi=

e

f

n

nkHq=Darcys law, flow quantity q is given as:k=coefficient of permeability

H=total head change

nf=number of flow channels

ne=number of equipotential drops


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nf=10

ne=4.8

H=9m

k=10-8 m/sec

q=1.9x10-7 m3/sec/m length of tunnel

Fitzpatrick et. al. (1981)


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Drawdown

(consolidation) and

Recharge



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Effect of Lining Permeability


ks=permeability of soil

kL=permeability of

lining

d=18m

No lining


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Effect of Layered Soil

(Tunnel intersecting sand layer)



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8Fitzpatrick et. al. (1981)


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Allowable

Infiltration Rate

ORourke (1984)


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Allowable

Infiltration Rate


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Spectacular Water Problems Experienced elsewhere:

Tanna Tunnel Japan (1925) 2000 l/s inflows

Eklutna Tunnel Alaska (1951) 1200 l/s in a fault zone

Kurobe Tunnel Japan (1965) 700 l/s in a fault zone

Seikan Tunnel Japan (1980) 1200 l/s in a fault zone

SSDS Tunnel C Hong Kong (1999) ??? l/s in a fault zone


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Strategic Sewage Disposal Scheme Stage 1

DC/96/17 - Completion works for Transfer System from CW to KT and from TKO to KT

Water Inflow in Tunnel C

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

23

-Jan-

99

14

-Mar-

99

03

-May-

99

22

-Jun-

99

11

-Aug-

99

30

-Sep-

99

19

-Nov-

99

08

-Jan-

00

27

-Fe

b-

00

17

-Apr-

00

06

-Jun-

00

26

-Ju

l-00

14

-Sep-

00

03

-Nov-

00

23

-Dec-

00

11

-Fe

b-

01

02

-Apr-

01

22

-May-

01

11

-Ju

l-01

Date

Waterpum

pedouttosurface(L

iter/min)

0

1000

2000

3000

4000

5000

Chainage(m)

Water Inflow Reading Tunnel Excavation In-situ concrete l ining construction

Tunnel Excavation

1st stage

Lining

Invert

concrete

Final stage in-situ

concrete lining

Grouting and repairing of the

in-situ concrete lining

Crossing w ater

features at Ch. 1440

Crossing Rennie's

Mill Fault


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Drawdown (consolidation) modelling


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Prediction of Inflow of Rock Tunnel under Sea (Freeze and Cherry, 1979):

For homogenous, isotropic and steady

state condition where,

k=hydraulic conductivity

h=depth of tunnel

hr=thickness of rock cover

r= tunnel radiusQ=inflow rate per unit length of

tunnel

=

r

2hlog

kh7318.2Q

r

Prediction of Inflow (Goodman, 1965):


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( , )

For Homogeneous Isotropic Aquifer with

constant head:

Q=Flow per unit length of excavation

K=hydraulic conductivity of jointed rock

mass

D=average depth of excavation below ground

level

H=hydraulic head (depth below groundwater

table)

r= tunnel radius

=

r

2Dln

KH2Q

This equation predicts that inflow (Q) into the

excavation is proportional to H/ln(D)(increases with depth) if K is constant


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Observations show

that hydraulic

conductivitydecreases with depth

due to joint closing

under higher

confining stresses

Prediction of Inflow (Zhao, J.): Inflow decreases with depth due to joint


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closing with increase in effective normal stress (waste depository design)

where,

g=acceleration due to gravity

B=ratio of conductive joints to total joint

number (0.1-0.3 for jointed crystalline rock

mass)

=joint frequency (fracture index) of rockmass

ei=joint aperture (estimate from core logging,

varies from 10 to 100 m) at shallow depth

Di

=kinematic viscosity of fluid (0.0101cm2/sec for pure water at 200C)

A=0.14-0.22, a parameter indicating the

decrease of joint permeability with anincrease in effective stresses

2

i

2i

DDAln1

r

2Dl6

HgBQ

=

n

e

This equation takes into account of thevariation of hydraulic conductivity with

stress (or depth) and the effect of

decreasing hydraulic conductivity may

counterweigh the effect of increasinghydraulic head


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2

i

2

i

D

DAln1

r

2D

l6

HgBQ

=

n

e

=

r2Dln

KH2Q

K=1.0x10-7 m/s

H=Di=50m


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Artificial

Recharge to

reduce

Groundwater

Lowering

due to

Dewatering

by Tunnel


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Zone of Influence

( )

( )( )

( )

( )trenchorflowline2000to1500flowradial3000factorempiricalC

m/stypermeabilik

mdrawdownh

kChRInfluenceofRadius 0

=

==

=

=

=

Somerville (1986)

Types of Aquifers affecting quantity of flow


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Types of Aquifers affecting quantity of flow


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Measurement of Discharge

By meter

By estimation

(V-notch or pipe flowing)

Somerville (1986)


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Non-symmetrical

drawdown

Symmetrical

drawdown

Rock

Soil

Impermeable Layer

Uniform

Rock Mass

Conductive

Joints

Soil


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Impermeable Layer

For the same rock

mass joint

distribution, inflow

control by tunnel

intersecting discreteconductive joints


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Recharge isreliable in

homogenous soil

Recharge is unreliable

in fractured rock,

depending onintersecting correct

flow path


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Water Control Specification:

During tunnelling In Settlement SensitiveArea, groundwater inflow to be reduced if:

Any probe hole recording more than 20

l/min, or

Inflow recorded more than 50 litre/min at

the tunnel face and within 25m of the

tunnel face or over any 50m length of

tunnel

Water tightness of permanent linings Inflow

of water shall not exceed 5 litres per 24 hours

per square metre of internal surface of lining

measured over any 100m length of completed

tunnel or shaft (SSDS Stage I tunnels contract)


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Documents

Lecture8A(Flow&Seepage)