Challenge

The long-awaited four/six-lanes of the Hyderabad-Vijayawada section of National Highway NH 65 was started in 2010, connecting the eastern parts of Andhra Pradesh with Hyderabad and reducing travel time significantly.

Typical cut and fill operations in highway construction result in a considerable number of embankments to maintain road alignment along its 117 km length. While the side slopes of these embankment are stable, the slope gradient is usually steeper than normal– typically 1:2 or 1:1.5 (v:h) – and subject to erosion if not mitigated.

Andhra Pradesh state suffers from very high rates of soil erosion by water due to its climate, soils and geography. The native red soils used in the embankment berms are of poor quality and if unprotected, are easily erodible and subject to scouring.

Guidelines for the National Highways Authority of India (NHAI) recommend surface stabilization for roadside slopes that may be subject to erosion. Soil bio-engineering is a preferred best method to achieve effective stabilization and limit environmental impacts.

Conventional Solution

Stone pitching (higher costs aside), is an intensive process requiring quarrying, hauling and mechanical handling equipment – and a high environmental footprint. Stone pitching is more suited to unstable slopes than for soil erosion control. It is also unsuitable for bio-engineered solutions and green landscapes. Coir mats are temporary solutions. They are difficult to keep anchored, are subject to uplift from water flow under the mats and easily damaged in torrential rainfall and winds.

PRS-Neoloy® Solution

Roadside embankments and slopes. Installation starts in a trench at the crest (infilled with gravel), and then the sections are expanded downslope to lengths of up to 16m and anchored with bamboo stakes. The bottom cells were buried in a toe trench. After infill with local topsoil and compaction with an excavator backhoe, native grass tufts were planted manually.

Collection drains along the crest and downslope spillways at regular intervals are an integral component of the design to channel heavy rain/water flows to preserve system integrity.

In this soil bioengineering solution the grasses (or other vegetation schemes) are important structural components in slope stabilization, not just aesthetic elements. Bioengineering mimics nature by using locally available materials and a minimum of heavy equipment and therefore lowers costs.