Tunnel Cut Cover

STRUCTURAL DESIGN OF CUT AND COVER TUNNEL

STRUCTURE:

LOAD ESTABLISHMENT: Depending upon site conditions, a cut and cover concrete conduit, in addition to its self weight, is subjected to the forces listed below. The structural design of the conduit shall be based on the most unfavourable combination of all loads and effects listed below acting simultaneously: a) Loads due to backfill, b) Internal water pressure, c) External water pressure, d) Loads due to concentrations and surcharge including effects of live load, and e) Seismic effects. The most critical loading condition would be : Load due to backfill+Self weight of the structure+Tunnel is in empty condition The design is done for the maximum backfill of

18.75 m say H=

The diameter of the tunnel(equivalent)=

6m

Minimum thickness of the tunnel in soft strata is = Thickness of the tunnel adopted=

18.75 m

(As per IS : 4880 ( Part V ) )

Thickness of tunnel

72 cm 1.00 m 100 cm

Diameter/width of the tunnel(inside)

6m

Add working area

0.5 m

Side slopes

1 H:1V

Unit weight of saturated soil=

1950 kg/cum

Moist density of soil

1850 kg/cum

SBC of soil at the base=

35 t/sq.m

Assuming minimum over burden above the crown=

15 m

Net SBC at the bottom of foundation(assuming the height of soil 15m)=

78.475 t/sq.m

MWL at inlet of tunnel

737 m

Invert Lvel of exit

732 m

Head of water at the center of tunnel=

8 m of water

The soil load load according to the Russian practice depends upon the degree of firmness. The strength factors after Protodyakonov are given in Table below With cover-depth sufficiently deep for arching action the rock load will be defined by the area enclosed by the arch as shown in fig and assumed to act over the diameter of the tunnel. ananda d s

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STRUCTURE:

CATEGORY

STRUCTURAL DESIGN OF CUT AND COVER TUNNEL

STRENGTH GRADE

DENOTATI ON OF ROCK

UNIT WEIGHT t/cum

CRUSHIN STRENGT G H FACTOR STRNGTH kg/sq.cm

1

Highest

Basalt & Other solid massive rocks

2

Very High

Granite

2.6

2.7

1500

15

3

High

Quartz, conglomerat es

2.5

2.6

1000

15

High

weathered granite, Lime stone, marble

2.5

2.5

800

8

Sandstone

2.4

2.4

600

6

sandstone shales

2.3

2.3

500

5

Clay shales

2.4

2.6

400

4

2.4

2.8

300

3

2.2

2.6

200

2

2.2

2.4

1.5

3a

4 4a

Moderately strong Moderately strong

2.8

3

2000

20

5

Medium

5a

Medium

6

Moderately Loose

6a

Moderately Loose

7

Loose

Claye ground

2

2.2

1

7a

Loose

Loam,Loess , gravel

1.8

2

0.8

8

Soils

Peat, wet sand

1.6

1.8

0.6

9

Granular

sand, fine gravel

1.4

1.6

0.5

10

Plastic Soils

Saturated soils

1.4

1.6

0.3

Shales & slate cemented gravel &boulders Gravelly ground

EL: 755.00m

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STRUCTURE:

STRUCTURAL DESIGN OF CUT AND COVER TUNNEL 18.75m

1.00m

EL: 736.25m

3.00m 8.500m 1.00m

invert: 3.50m

EL: 729.75m EL: 728.75m

8.00m

1.00m

As per IS : 4880 ( Part V ) pp 22

For soils and soft rocks, the unit pressure may be assessed by using Terzaghi’s theory:

2C   B   2H  K tan   B   B  Pv   1 e   2 K tan   

Where,

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b    B  2  m tan  45 o    2 2   

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=

15.815 m

STRUCTURE:

STRUCTURAL DESIGN OF CUT AND COVER TUNNEL 1.950 t/cum

y=unit weight of soil=

19.250 m

H= height of overburden above crown b=width of tunnel

6.000 m

H/B=

1.217

C=cohesion of soil

0.000

m=height of tunnel

8.500 m 30.000 

=angle of internal friction K= an emphirical factor=1 for H/B=1 and 2.5 for H/B=1.5

1.652 14.584 t/sq.m

Pv=

7.479 m

Therfore the equivalent height of overburden=

As per Protodyakonov method the the effective load on the arch may be obtained from the formulae

 45o    B  b  2m tan   2   where b=

8.00 m

=

30 

m=

8.50 m

Therefore B=

10.24 m 30 

Angle of internal friction of the soil= Height of over burden=H=

18.75 m

Total width=(outer)

8m

Width at the crown=Bt=

(As per IS 7563 the width B)

25 m

Considering the maximum for design purposes, the width B= 

The height of the overburden to be considered=h= Where f =strength factor,

25.00 m B 2f

for saturated soils f=

0.30

Therefore h=

41.67 m

But actual overburden=

18.75 m

Hence design overburden =

18.75 m

Unit weight of saturated soil=

=

1.95 t/cum

Intensity of vertical load-=

36.56 t/sq.m Lateral pressure=KaYH at crown

 ananda d s

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

STRUCTURAL DESIGN OF CUT AND COVER TUNNEL   where K=rankines earth pressure coefficient= K a  tan 2  45o   = 2  STRUCTURE:

0.33

Lateral load Intensity of lateral pressure at crown: at crown height of over burden h1=

18.750 m

Intensity of pressure=pc=

24.688 t/sq.m

at Bottom of the tunnel=

27.250 m

Intensity of pressure=pb=

35.879 t/sq.m

H

FILL MATERIAL 40.00t/sq.m Bt

24.69t/sq.m h or m Bc

35.88t/sq.m 43.9t/sq.m

Intensity of base pressure: Self weight of the tunnel= Soil weight

top arch

3.4 t/m

Side members=

2.2 t/m

Reaction of overburden= Soil wt between arch & crown= Total vertical reaction=

Weight of contained water in tunnel

1.7 t/m 43.9 t/m 4.9 t/m

Total vertical reaction with water=

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36.6 t/m

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48.7 t/m