III. GEOTECHNICAL ENGINEERING

A. COMPLETED PROJECTS
4. Investigation and Remedial Measures for the Stabilisation of Slopes at km.42 on NH-150, Mizoram
Duration

May 2005
(i) Central Road Research Institute, New Delhi (R)
(ii) PWD, Mizoram (S, I)

 

Present Status and Progress
Completed
 
Findings/ Conclusions

The objective of the study is to investigate the causes of failure and to suggest suitable remedial measures, for the landslide at km 42 on NH-150, which is located about 87 kilometres away from Aizawl city. The landslide is located at Khawruhlian village in the north-eastern side of Aizawl city. The slide is located at the centre of the village and it is on a ridge. A Bailey bridge has now been constructed to keep the traffic moving over the landslide portion. The Bailey bridge is resting on damaged slope on both sides. The soil cut below the Bailey bridge is almost a vertical cut and there is a continuous erosion of soil.

The objectives of this project are:
(1) Field investigations of landslide area,
(2) Evaluation of material properties in the laboratory and
(3) Analysis of slope failure and other field data to evolve suitable remedial measures for slope stabilisation.

 
Limitations / Remedial Measures
The remedial measures for preventing slope failure are given below:

Based on field and laboratory investigations, suitable remedial measures are being proposed to be adopted to prevent shallow movement and to control percolation of water into the slope. Broadly remedial measures suggested have been categorized into two groups as alternative one and alternative two.

Alternative One – Soil nailing coupled with Reinforced Earth wall

Soil nailing is one of the methods developed in recent times for in-situ stabilisation for natural slope. The fundamental concept of soil nailing consists of driving closely spaced inclusions (which are called soil nails) into the ground so that overall shear strength of the in-situ soil increases. The total height of the vertical cut is 10 m below the bridge. Since the width of ridge in the top 2 m portion of the vertical soil slope is very much limited (an average of about 2 m thickness) this section is proposed to be treated with Steel Fiber Reinforced Shotcrete (SFRS). Remaining 8 m height of the soil slope. The conventional Soil Nailing system has three components:

(1)Soil nails,
(2) Facing elements,
(3) Arrangements for drainage

Alternative Two- Construction of RE wall
The stability analysis of the RE wall was carried out using computer software.
Towards the demolished church side the exposed rock surface has not suffered weathering to the extent seen on the other sides. However if erosion and water seepage problems are not checked, the slope may suffer instability in future. The dumping of household waste by the villagers is preventing growth of vegetation. So it is proposed that dumping of household waste should be immediately discontinued and the waste collected should be removed.

Remedial measures for the left flank (High school side) of landslide
The bridge should be properly rested on concrete footing and to minimize vibrations, Neoprene bearings should be provided in between bridge and concrete footing. It is proposed to construct a stone masonry retaining wall to support the soil mass below the bridge and shot Crete the exposed rock surface on the left flank.

Application of jute/coir mesh for erosion control
Application of jute or coir mesh on the exposed slopes promotes the growth of vegetation due to which stability of slopes is increased and thus controls slope erosion.

Remedial measures for the left flank (High school side) of landslide
The bridge should rest on concrete footing such that Neoprene bearings should also be provided in between bridge and footing to minimize the vibrations. Along with this a stone masonry wall should be provided to support the soil mass below the bridge and shotcrete the exposed rock surface on the left flank.

 
Recommendations

• The portion of the slope below the vertical cut is recommended to stabilize with series of Reinforced Walls.

• Instead of adopting a combination of RE wall, soil nailing and shotcreting, RE wall itself can be used as a retaining structure from the toe of the embankment up to the bridge level.

• The exposed soil surface on the right flank should be treated with jute geotextile and mulching is to be undertaken.