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The present case study focuses on slope stabilization and rockfall protection of cut slopes from Nadikudi to Srikalahasti New Broad-Gauge line in the state of Andhra Pradesh, India. To lay the railway tracks, the stretch along the Chainage km. 17+300 to km. 17+800 and along Chainage km. 19+900 to km. 21+200 which consisted of hilly topography was excavated using a blasting technique. This has caused a section of unstable and rockfall vulnerable slopes which require stabilization and rockfall protection.

The area is mainly composed of highly weathered quartzite rocks and shale with bedding planes subjected to folding at some sections. Drilling and blasting were carried out to break rocks for excavation. This has caused severe deterioration of rock mass strength subjected to the development of new fractures and shearing along with joint sets. The chainages detailed of the vulnerable locations are (17+300 to km. 17+800 (LHS and RHS) and along with Chainage km. 19+900 to km. 21+200 (LHS and RHS) where slope stabilization and Rockfall Protection works are required.

Chainage km. 17+300 to km. 17+800 (RHS section)

Based on the result of numerical analysis for the present slope, 25 mm diameter anchors of length 9m at a spacing of 4 m c/c horizontally and 2 m c/c vertically is to be provided and the reinforcement system adopts a scientific active design concept which combines a Rhomboidal cable net with a double twisted hexagonal net for small gravel-sized materials.

Completely weathered sections

Anchors of 25 mm diameter and length 9 m at a spacing of 4 m c/c horizontally and 4 m c/c vertically are to be installed along with the half-height of the slope from the top. Also, for toe protection, it is recommended to install 9 m rock bolts of 25 mm diameter at a spacing of 2 m c/c vertically and 2 m c/c horizontally and for highly weathered section, erosion control measures over the slope need to be provided in addition to rockfall protection. Since, exposure to weathering being the primary concern at the site, rockfall protection with Rhomboidal cable net is combined with Coir mat with double twisted hexagonal mesh is to be provided.

Chainage km. 17+300 to km. 17+800 (LHS section)

The stabilization consisted of anchors with 25 mm diameter of length 3 m anchors at a spacing of 4 m c/c horizontally and 2 m c/c vertically. For improved stability 25 mm diameter, 9 m anchors shall be provided at 4 m c/c horizontally and 2m c/c vertically for half of the slope and for rockfall Protection secured drapery system (active protection net) using Rhomboidal cable net and double twisted hexagonal mesh shall be Provided for the Particular section.

Chainage km. 20+900 to km. 21+200 (RHS section)

Stabilization was carried out using 25 mm rock bolts of 9 m length at top 4 m layers at 4 m c/c horizontally and 2 m c/c vertically, remaining 3 m anchors of 25 mm diameter (32 mm if SDA) at 4 m c/c horizontally and 4 m c/c vertically and the rockfall protection can follow the scheme of the RHS section of the current chainage using Rhomboidal cable net along with Three-dimensional erosion control.

Chainage km. 20+850 to km. 21+200 (LHS section)

The rock bolting is carried out using 25 mm diameter rock bolts of 9 m length at a spacing of 4 m c/c horizontally and 2 m c/c vertically. The bottom half of the slope shall be of anchors 9 m and 3 m in length (32 mm if SDA) alternatively (2 x 2 m c/c). The choice of 9 m long anchors may be also site-specific owing to changing joint orientation caused by folding and for rockfall protection a system consisting of a Rhomboidal cable net along with a Three-dimensional erosion control mat with self-drilling anchors or Rhomboidal cable net along with coir mat with steel wire mesh netting.

CONCLUDING REMARK:

The Stabilization of slopes and rockfall protection system is suggested based on geotechnical data obtained from the testing and analysis based on numerical modeling. The Protection system successfully stabilizing the slopes and mitigating the rockfall on the passing commuters.

Contact for further details:

OST SLOPE PROTECTION ENGINEERING INDIA PVT. LTD.

JMD Megapolis, Unit No. 1237, 12th floor,

Sector — 48, Sohna Road, Gurugram (H.R.)

Web: www.ostindia.co.in