Slope stabilisation and rockfall protection in railway project

Excavation is an inevitable part of every major railway project along hilly and mountainous terrains which induce potential instability along the resulting cut slopes. In such scenarios arises the need to stabilise and protect these slopes from impending slides and rock falls. The primary concern of any remedial and preventive measures for slope instability is to ensure global stability to the slope and also to provide surficial stability to prevent local wedge failures. The present case study focuses on slope stabilisation and rock fall protection of cut slopes from Nadikudi to Srikalahasti new broad-gauge line in the state of Andhra Pradesh.

Introduction

The Nadikudi-Srikalahasti section falls under the administration of Guntur and Guntakal railway divisions of the South-Central Railway zone. This serves as an alternative route to Delhi-Chennai and Howrah-Chennai routes. 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 stabilisation and rockfall protection measures to ensure smooth working of the adjacent railway line upon completion. The mesh made for these purposes is made from high tensile steel wire with a minimum tensile strength of 1770 N/mm2.

Problem definition

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 stabilisation and Rockfall Protection works are required.

Protection measures

To determine adequate slope stabilisation and rockfall protection measures, it is viable to conduct appropriate stability analyses. For the present project, two types of analyses are carried out for the slope to check its stability. Initially, kinematic analysis is conducted to predict the probable failure modes using stereo net representation. Based on the results of kinematic analysis, numerical analysis is performed using finite element software Phase 2. The solution for stabilisation and rockfall protection is suggested as follows:

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

Stabilisation of slope: Based on the result of numerical analysis for the present slope, 25 mm diameter anchors of length 9 m at a spacing of 4 m c/c horizontally and 2 m c/c vertically is to be provided.

Rockfall protection system: The reinforcement system adopts a scientific active design concept which combines a rhomboidal cable net or ring net with a double twisted hexagonal net for small gravel-sized materials. The flexible system is anchored by an anchor bolt to cover unstable slopes and rocks to prevent the weathered rock from spalling, collapsing, toppling, or sliding.

Completely weathered sections

Stabilisation of slope: 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.

Rockfall protection system: For a 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)

Stabilisation of slope: The stabilisation 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 and 9 m anchors shall be provided at 4 m c/c horizontally and 2m c/c vertically for half of the slope.

Rockfall protection system: The 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)

Stabilisation of slope: Stabilisation 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.

Rockfall protection system: 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 mat with self-drilling anchors or rhomboidal cable net along with coir mat with steel wire mesh netting (with rock bolts) for weathering prone zones and rhomboidal cable net and reinforced double twisted hexagonal mesh with longitudinal ropes (and self-drilling anchors) for other sections.

For planar failure sections

The joint orientation changes due to folding causing planar failures at the site. Numerical analysis for such sections is also carried out by changing the joint pattern of the concerned joint plane.

Stabilisation of slope: The stabilisation is carried out using 9 m long 25 mm diameter anchors at a spacing of 1 m c/c horizontally and 1 m c/c vertically for the bottom bench and 3 m (32 mm if SDA) long anchors for a top bench at 4 m c/c horizontally and 2 m c/c vertically.

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

Stabilisation of slope: 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.

Rockfall protection system: For slope sections prone to weathering, 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 (with rock bolts).

Shotcrete

Along Chainage km. 20+900 to km. 21+200, stable slope sections, and where blocky strata is available, fiber-reinforced shotcrete with steel wire mesh reinforcement shall be provided to prevent exposure to weathering. Shotcrete is a pneumatically applied, fine aggregate mortar that is usually placed in a 50-100 mm layer and is often reinforced for improved tensile and shear strength.

Installation

Loose and unstable rock particles that could detach easily from the slope surface have been removed. The Coir mat is rolled out over the soil profile ensuring good ground contact. The top row of anchors has been installed initially and then the primary net viz. reinforced double twisted hexagonal mesh or three-dimensional reinforced erosion control mat was rolled down. Once the reinforced double twisted hexagonal mesh is spread over the slope surface, the surface anchors shall be installed over the surface. Bottom anchoring was carried out to restrain the blocks at slope toe. After that GPS-2 (rhomboidal net) was placed at the slope surface.

Concluding remark

The stabilisation of slopes and rockfall protection system is suggested based on geotechnical data obtained from the testing. The protection system successfully stabilises the slopes and mitigates the rockfall on the passing commuters.

– Ashish Kumar

Asst. Manager (Technical), OST Slope Protection Eng. India

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