Treatment and Mitigation of Chronic Land Slide

Border Roads Organisation (BRO) is actively involved in road construction in Upper Subansiri District of Arunachal Pradesh and is responsible for construction and maintenance of Roads Tama Chung Chung (TCC) – Taksing and others in terrain that is characterised by steep mountains and deep valleys. The young mountain slopes are not stable; resulting in frequent landslides, shooting rocks and erosion of soil along river banks. This article elaborates on the steps that can be useful for mitigation and treatment of destabilised hill slopes, chronic landslides, arresting activated slides, erosion control and longevity of slopes.

Alignment of roads in Upper Subansiri areas leading to TCC, Taksing and onwards to the LAC, traverses through a number of Zigs/Hairpin bends. In May 2008, when road construction commenced in this sector, a massive landslide was triggered. This severely affected the link road and Road TCC-Taksing at multiple locations. Over a period of time, this impacted area grew bigger and became chronic resulting in repeated expenditure due to commitment of more resources, like deployment of earth moving machines for clearance of frequent falling debris as well as works to restore the intermittent disruption to line of communication. The local populace, troops deployed
in forward areas and BRO karmyogis also suffered due to this problematic zone.

To address this recurrent problem of landslides, a new technology was identified by BRO that involved multiple steps to address the problem in a holistic manner.

(a) Vegetation Clearance

Affected vegetation, debris and loose soil with other deleterious matter were scaled off & cleared manually from the crest to the bottom of the slide in a gentle manner so as to ensure safe and easy access for placement of stores and implementation of treatment procedures.

(b) Anchoring of hill slope

A self drilling anchor (SDA) of 32mm diameter with sacrificial bit of 51mm diameter continuously threaded with ultimate load of 220 KN was driven in stages with the help of a drilling mast. The SDA is of hollow section for grouting and extendable with the help of coupler for getting the desired length. Grouting was done using OPC grade 43, duly adding anti shrinkage admixture and having water cement ratio of 1:1 till fresh grout started leaking from the hole. The anchor was required to protrude outside the surface for a minimum length of 200 mm.

(c) Placing of Erosion Mat            

A non woven coir mat blanket made with fully biodegradable coir fibres reinforced with polypropylene netting, having a design life of 36 months was unrolled along the slope and pinned down with 6-8mm diameter steel u¬-pins. A 300mm overlap with the adjacent mat was ensured and the mats were duly covered with minimum 100mm of soil, without stretching them.

(d) Placing of Double Twisted Hexagonal Shaped Wire Mesh

Soft tempered steel, double twisted, zinc coated wire mesh was used. It was made up of mechanically woven hexagonal mesh with an opening of maximum 100mm and diameter of wire 2.7 to 3.7mm. It was rolled down from top anchoring system. Overlap of 200 mm was maintained and gap rising due to surface topography was wired and closed with lacing with a steel wire of similar grade and diameter.

(e) Placing of High Tensile Rolled Cable Net           

High Tensile Rolled Cable Net with mesh size 300 X 300mm having 100 KN/m load resistance in both horizontal and vertical, capable of 1770 N/mm2 pressure and breaking load of 45 KN was installed from the top corners of the area to be protected. A lacing wire rope was weaved in a spiral manner to join the cable net followed by placing of heavy galvanized base plate of 200x200mm size, tightened with a lock nut.

(f) Construction of Gabion Wall

In the reinforced soil zone, Gabion Walls were erected along the road side longitudinally, duly anchored with 5m length SDA with 200mm protrusion for nut & plate fixing with H-frame. Each and every gabion box was fixed with vertical MS channel horizontally, forming an H-frame to enable fixing with the hill slope.

(g) Planting of Mulching Seeds

For longer durability of the treatment of slopes, seeding with specific vegetation was undertaken on the stabilised hill slope. These seeds, are capable of growing even in rocky strata.

Present Status of Mitigated Slide

The slide is now fully stabilised and the road is now open to traffic even during heavy rains and a perpetual slide point has been conquered through the use of technical solutions.