Serious flaws have been found in the 'soil nailing' technique used during the construction of National Highway 66 in Kerala, according to a team of expert geotechnologists who recently inspected the project sites.

The team comprised Dr A Bhoominathan, former head of the Geotechnical Department at Madras IIT, Dr N Unnikrishnan, former head of civil engineering at the Thiruvananthapuram College of Engineering, and Dr G Hari, Managing Director at CGL Engineers, Kochi. 

The team said that the contractors had failed to consider the varied nature of the soil in the state and did not seem to have carried out the requisite testing and proper design.

What is soil nailing?
Hills are sliced vertically in many areas for constructing the national highway, and ‘soil nailing’ is carried out to stabilise the remaining earth. The process involves testing the nature of the soil and inserting long anchor rods into the ground. The rods are then connected together by fixing a metal mesh between them. Subsequently, a cement mixture is applied to the mesh. The experts said that soil nailing is effective in preventing the earth from collapsing if carried out in a proper manner on suitable soil.

Several defects noticed
The team found that there was excess distance between rods at many spots on NH 66. In certain areas, the rods were hardly used. Even the basic principles of nailing were violated in some locations, leading to landslides even after the first rain, said the team. They warned that unless the faults were repaired, more landslides would occur along NH 66 in the state.

The major cause of the failure of soil nailing in northern districts of Kerala, Kasaragod, Kannur, Kozhikode and Malappuram, was the acutely vertical slicing of the hills. Steep cutting of hills was reportedly carried out to minimise land acquisition, and the angle of soil nailing at some spots was 90 degrees. The experts were of the opinion that soil nailing would be more effective at slopes that are less steep.

How is soil nailing carried out, and what caused its failure?

Soil testing is conducted to decide factors such as the depth of the rods in the ground, the distance between the rods, and the amount of shotcrete (cement mixture) to be applied. All this work would be ineffective if there were errors in the findings of soil testing, leading to the failure of soil nailing. The experts said that it was doubtful whether soil testing was conducted in many areas of NH 66 in Kerala.

The team noted that the standard spacing of one metre between rods in laterite soil was not followed, with gaps wider than recommended and rods missing at several locations.

According to soil nailing principles, if one rod fails to hold the soil, the adjacent rod provides support. However, the team found that this redundancy failed due to an insufficient number of rods. They also discovered that in some sections, a cement mixture had been applied without inserting any iron rods.

Yet another lapse was the failure of the contractors in fixing the rods by pumping cement mixture at high pressure into holes on the ground. The method of anchoring the soil nailing by fixing rods with a thick end into the earth was also not followed. In addition, the metal plates fixed over the rods were smaller than the required specifications.

After the rods are inserted, their strength has to be tested by pulling them from the soil. Such tests were not carried out at the worksites. The collapse of soil at the spots where nailing was carried out during the start of the rainy season itself is a proof of its defects and is a major warning sign, said the experts.