presentation on tunnels
TRANSCRIPT
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DEFECTS IN TUNNELCAUSES AND RECTIFICATION
By
Chief engineer/C/North
Northern railway
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This presentation covers following topics.
• Brief on method of tunneling.
• Salient features of tunnel no T8, T9 and T10.
• Type of defects noticed in tunnel no T8, T9 and T10.
• Rectification of Defects.
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• Methods of Tunneling
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• Conventional method of tunneling is adopted.
• Done in two phases i.e. heading and benching.
• Heading
• Firstly excavation in heading done and rib comprising of ISHB-150 is provided @ 500 to 750 mm C/C as per condition of strata.
• The heading ribs are supported on the SPL beam provided longitudinally in the tunnel.
• Space between two ribs is covered by 50 mm thick precast RCC slabs called legging.
• Space between excavation line and ribs/leggings is back filled with M-10 concrete.
Heading
Benching
Method of Tunneling
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• Benching
• After sufficient length is achieved in heading then excavation in benching is taken up.
• Following the same pattern as in heading, one by one vertical ribs are provided under the SPL beam.
• Thereafter lagging slabs provided and back filled with concrete.
• The vertical beam connected with invert beam embedded in the floor of the tunnel.
• When erection of rib in both the heading and benching is complete inner lining 30cm consisting of MCC is done.
• Thus ISHB 150 ribs are embedded in concrete.
Heading
Benching
Method of Tunneling
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Leggings Precast Concrete Blocks 50 mm Thk
RIBSISHB 150
Plain Concrete Lining
Typical section of Tunnel
A A
Section A-A
5.2 m on straight
5.53 m on curves
Side Drains Track
3.55m on straight
3.7 m on curve
Springing Level
Crown
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Arrangement at Springing Level
Springing Beam ISHB 150
Ribs ISHB 150Elevation at springing Level
Cross-section of Tunnel wall
Plain concrete Lining
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Salient features of tunnel.
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T-8&9 2/2004 to 3/2006T-10 5/2003 to 3/2006
Year Construction.
Geology.
Alignment passes through loose soil mixed with small pebbles and boulder. Soft sand stone also encountered during construction of tunnel.
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Length of Tunnels
DATA FOR TUNNEL NO. T-8 &9 AND T-10.
S.No.
Tunnel No.
ChainageLength
( In Mtr)
Length in
straight
Length in curveStart End
1 T-8 26/500 27/275 775 313.752 461.248 2 T-9 27/275 27/983 708 463.078 244.922 3 T-10 28/288 29/200 912 96.161 743.582
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Max. Over Burden
Sr no Tunnel no At chainage Height of overburden
1 T-8 26/835 31.174
2 T-9 28/425 58.167
3 T-10 28/650 190.620
Cavity Formation Sr no
Tunnel no At chainageHeight of
overburdenover break
1 T-8 nil -
2 T-9 27/433 to 27/445 12 Mre 38 Mtr
3 T-1028/728 to 28/862
134 Mtr186.165 Mtr at 28/750
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Sr no Tunnel no At chainage Height of overburden over break
1 T-8 nil -
2 T-9 27/433 to 27/445 12 Mre 38 Mtr
3 T-10 28/728 to 28/862 134 Mtr186.165 Mtr at
28/750
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• Defects in tunnel
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For the purpose of simplicity tunnel structure will be represented by a single line representing centre line of ISHB 150
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Defect in Tunnel-Deformation/Bulging of Ribs at Springing Level
Causes
•Bulging of ribs in benching into the tunnel.
•Due to excessive cutting and improper backfilling.
Effects
•Dislocation of leggings.
•Excessive displacement causes legging leaving their place-resulting cavity formation
•Cross-section of the tunnel is deformed
•SOD is infringed.
•In some cases extent of deformation is so large it is visually noticeable.
•In other cases it required measurement.
•Reduced thickness of lining.
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Bulging of vertical ribs in benching into the tunnel.
Lateral pressure is so much that horizontal struts provided have buckled.
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Defect in Tunnel-Deformation of Ribs at crown
causes.
1. Excessive pressure on crown due to improper arch action.
2. Rib for heading consist of two pieces of quarter circle joined at crown by bolts.
3. Weak point at crown due to insertion of joint in rib at crown.
Effects
1. Leggings are displaced.
2. Excessive deformation causes legging loosing their support.
3. Displacement of legging from their position causes discontinuity in tunnel often leading to falling of the overburden- cavity formation.
4. Loosening of overburden disturbs the arch action in strata and initiation of chain reaction of tunnel collapse.
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Deformation of Ribs at crown
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Defect in Tunnel – failure of beam at springing level
Causes
1. Excessive reaction on the springing beam.
2. Buckling of the web of the beam.
3. Low shear capacity of beam in comparison to the reaction.
Effects
1. In some cases results in complete collapse of the web.
2. Ribs in benching may remain unaffected.
3. Ribs in heading gets deformed, requiring replacement of ribs.
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Defect in Tunnel – Heaving of floor beam.
causes
1. Excessive surcharge/overburden.
2. Poor strata at foundation level of tunnel.
3. Seepage of water into the tunnel from bottom due to natural hydraulic gradient on account of strata.
Effects
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Defect in Tunnel-Deformation/Bulging of Ribs in heading portion.
Causes
•Bulging of ribs in benching into the tunnel.
•Due to excessive excavation and improper backfilling.
Effects
•Excessive displacement causes legging leaving their place-resulting cavity formation
•Cross-section of the tunnel is deformed
•SOD is infringed.
• if left unattended reduced thickness of lining.
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A
Lateral displacement of rib at A into the tunnel is visible w.r.t position at B
B
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Remedial measures
• Loose strata over the tunnel lining is stabilized by injecting cement grout.
• After the loose strata is hard enough to withstand the weight of overburden.
• One rib at a time is dismantled.• Excavation is done and new rib is placed.• Leggings are placed.• Space between excavation line and legging is Backfilled with
concrete.
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Typical sketch of a D shape Tunnel
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Modified elliptical section
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Remedial measures• Stretches where effected length is considerable elliptical section is being adopted instead of D shape.
• Benefits of elliptical section– Because of the double curvature has better load carrying capacity.– Elliptical shape has been used and proved better then conventional D shape.– Springing beam is avoided, defects of springing beam are eliminated.– Bottom curved beam, better resist the heaving action.– Vertical curved member resist the lateral pressure better than the vertical member of D shape.– Lateral girder facilitate tunneling operation in two stage i.e. heading and benching.– Vertical joint at crown is avoided by suitable designing the heading rib.
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• Thank You