DEFECTS IN TUNNELCAUSES AND RECTIFICATION

By

Chief engineer/C/North

Northern railway

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.

• Methods of Tunneling

• 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

• 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

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

Arrangement at Springing Level

Springing Beam ISHB 150

Ribs ISHB 150Elevation at springing Level

Cross-section of Tunnel wall

Plain concrete Lining

Salient features of tunnel.

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.

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

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

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

• Defects in tunnel

For the purpose of simplicity tunnel structure will be represented by a single line representing centre line of ISHB 150

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.

Bulging of vertical ribs in benching into the tunnel.

Lateral pressure is so much that horizontal struts provided have buckled.

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.

Deformation of Ribs at crown

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.

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

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.

A

Lateral displacement of rib at A into the tunnel is visible w.r.t position at B

B

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.

Typical sketch of a D shape Tunnel

Modified elliptical section

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.

• Thank You