Soil Nailing

Soil Nailing

Project Name : ADDITIONAL OFFICE COMPLEX FOR SUPREME COURT OF INDIA, PRAGATI MAIDAN, NEW DELHI

The additional office complex for Supreme Court of India is being constructed near Pragati Maidan, New Delhi on a land area measuring 12.19 Acres. The building complex is designed to be a state of art, environment friendly, centrally air conditioned office complex with all modern facilities. This new building complex will have five functional blocks of building and one service block. These blocks will be 4 to 9 storey high. Three level basements are provided covering all the blocks, with a car parking capacity of about 1800 cars. The foundation provided is raft foundation, with soil anchors provided below the raft to counter uplift due to water pressure. SITE FEATURES - The site is located near Pragati Maidan where “Appu Ghar ”was functioning earlier. The site is close to the Yamuna River and natural water table at the site is 2.5m below existing ground level. The depth of excavation is 15 m below ground level. The soil profile is indicated in Fig.1.Since the water table was very high and the excavation was very deep, to find suitable solution for retaining the earth for a height of 15 m was challenging.

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88430 Lakh

Project Budget

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01 Jan 2015

Start Date

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15 Sep 2018

Completion Date

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NEW DELHI

Location

Project Team / Contact Us

S.No. Name(Mr/Mrs) Designation Office Address Office Number Mobile Number Email ID
1 B.B. Makkar CPM, SCPZ Supreme Court Project Zone, CPWD, Gate No-9, Pragati Maidan, New Delhi 01123370796 9871600317 cpmscp@gmail.com

TECHNOLOGY

Technology
Soil Nailing
Description

Soil nailing is a tempory earth retaining structure used for supporting vertical soil during excavation with limited setback conditions. This technique uses the pull out the strength of the reinforcement which should be more than the weight of the slip surface of the earth. Soil is reinforced with closely spaced steel nails()TMT bars and a surface is shot-created using steel wire mesh. The nailing is done at an angle as per design. Drainage pipes are also provided to act as weep holes.

Elements of Soil nailing system -

Nail Installation - 

Nails are installed by driven process using pneumatic rammers with threaded end of the nail protruding outside the excavated surface. The threaded end is provided with steel plates which are tightened against the soil surface using steel nuts. The steel plates transfer bearing loads from the soil mass to soil nails and distributes the force at nail end to temporary shortcrete facing. 

Fixing of Wire- Mesh -

Wire-mesh of  appropriate gauge and size is fixed against the exposed  surface of the slope and held in position by U- pins with the rear threaded part of the nail left outside.

Shotcrete -

Shotcrete is  applied perpendicularly into the exposed earth surface by means of pneumatic pressure over the weld mesh. The high speed shooting mortar produce self compacted cementious mortar at the facing. The ground surface is conditioned by trimming the slope surface reasonably smooth before receiving the shotcrete. 

Subsurface Drainage System -

The drainage network consists of perforated drain pipe inserted into the soil mass to facilitate good drainage through soil mass and expedite quick disposal of rain water or sub- soil water. The drainage network is installed prior to shotcreting. Drainage pipes perforated at top and duly wrapped with geotextile is provided at suitable intervals to reduce the build of pore water pressure. 

Major Gains

Cost comparison -

The methods available for temporary earth retaining are, (a) Diaphragm wall (b) H pile and wooden lagging (c) Sheet piling and (d) Touch piles & (e) Soil nailing

Approximate cost comparison (for depth 12 to 15 m) tabulated below- 

Method

Rate per sqm(approx.)

Diaphragm wall

25000-30000

H pile and wooden lagging

12000-15000

Sheet piling

20000-22000

Touch piles

20000-25000

Soil nailing

8000-10000

 

Implementation Strategy

S.No. Description Images
1

Design Details -

Soil at site is cohesion less soil. Pull out tests at different depths were carried out to assess frictional properties of soil. Average pull out resistance was found to be 0.50 t/m. Based on the pull out test results, the design for the same was developed.    

Following design details have been adopted for soil nailing -

Nails -                         25 mm dia TMT bar

Nail length -                7 m up to 12 m depth & 5 m for 12 to 15 m depth.

Nail spacing -             First row of nail is at 0.4 m  from top with 0.40 m c/c  horizontal spacing and subsequent  nails are at 0.80 m c/c horizontally and vertically.

Slope of cutting -       10° with vertical.

Welded wire mesh -  18 gauge ,aperture size 50 mm x 50 mm

Bearing plate-           150 mmx150 mmx10 mm.

Shotcrete -                75 mm thick with  ratio 1 cement : 2 coarse sand: 2 pea size aggregate with w/c ratio 0.45 to 0.50 on sloped surface and 50 mm thick on horizontal berm.

Drainage -               75 mm dia PVC pipe 7 m long with top half perforated wrapped with geo textile. One drainage pipe covers an area of 10 sqm of vertical surface.
2

Installation procedure -

First the excavation was carried out up to the depth of 0.80 m with the required slope and is dressed removing all loose pockets. Then the first row of nails was installed at 0.40 m below the existing ground level. The nail is pushed in to the soil using pneumatic rammers .The nails are inserted perpendicular to the slope using guiding channels. Channels are placed at required slope to guiding pneumatic rammers while pushing the nails into the soil. After installation of nails over sufficient area, wire mesh is placed against the slope surface and fixed to the surface using U-Pins. After fixing the wire mesh bearing plate is inserted through the threaded portion of the nail & pressed against the wall by tightening the nut. After tightening the bearing plate, 3 inches thick shotcreting is done by shotcreting machine to give a uniform coating over the exposed surface of soil.

After the shotcreting is set, subsequent rows of nailing is done and the process is repeated. Finished nailed wall surface is shown in Plate-4.

To protect the toe of the excavation against failure, additional strengthening has been done by providing nails at closer interval of 0.30 m c/c vertically and horizontally.

Drainage system consists of 3” Φ PVC pipe of 7 m length, perforated at top and wrapped with geo-textile was installed by boring with hand augur and pushing the pipe through the bored hole.

Lessons Learnt / Take Aways

S.No. Description Images
1

Soil nailing is an accepted technology, but practical guidelines are not available that offers experience-based insight into the construction considerations. Soil nailing application is a fast and economical as compared to other methods of slope protection.  The system has been successfully used in the deep excavation in the construction of additional office complex for Supreme Court of India. 
2

While dealing with sandy soil strata, proper sequencing is to be followed strictly, otherwise sand particles may come out creating loose pockets behind nailed wall. It is advisable to mobilise all the resources beforehand and work to be completed as per specified sequence. In case of high water table utmost care should be taken to keep water table at desired level because seepage of water through sandy strata may cause sand blows, affecting the stability of the nailed wall. Water coming out on the slope surface is to be completely stopped, before carrying out the shotcreting. 

 
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