Slope Failures and Remedial Measures for Strategic Highway Construction in Hilly Terrain

Dr. S.S. Seehra, Senior Consultant (Roads & Highways), LEA Associates South Asia Pvt. Ltd. (Former Director – Grade Scientist & Area Adviser, CSIR-CRRI, New Delhi)

Introduction

Gravitational forces are always acting on a mass of soil or rock beneath a slope. As long as the strength of the mass is equal to or greater than the gravitational forces, the forces are in balance, the mass is in equilibrium and movement does not occur. An imbalance of forces results in slope failure and movement in the forms of creep, falls, slides, avalanches, or flows. Failure occurs when driving forces exceed resisting forces.

Slope failures are a major issue for the highway safety and stability. Such slope failures as landslides are predominant in warm, humid climates and occur during years of unusually heavy precipitation or during period's of heavy concentration of rainfall or during wet years. However, prediction of landslip failure is often uncertain and slope stability is an important element depending upon slope geometry, inherent soil strength and ground or pore-water pressure characteristics.

The paper is related to the slope failure in the mountainous terrain where natural and manmade slope failures such as landslides or landslip usually occur.

A landslide is triggered if the shearing (tangential) stresses appearing in a soil mass exceed the magnitudes that the soil is able to resist. An increase of active shearing force can be due to erection of an engineering structure on the slope, increase in the weight of soil mass, higher gradients etc. A reduction of resisting forces can be due to removal of lateral support, say, when excavating a cut across the slope. In such a case, the prediction of landslide failure is often uncertain and slope stability becomes frequently costly.

Erosion is the natural process of removing soil particles by external agents such as wide or water. This involves rainfall which is responsible for the removal of surface layer, resulting in gullies of about 10-60 cm depth. However, over time the sills and gullies deepen further and cause slope to overstepped, thus precipitating slope instability. the slope protection generally means the engineering feature composed of suitable material constructed as a selectively thin layer on a slope otherwise vulnerable to erosion.

Vegetation is an important slope stabilizer. Planting the slope with thick native vegetation serves to strengthen the shallow soils with root systems, prevents erosion, deters infiltration and increasing seepage pressures. Vegetation also discourages desiccation which causes fissuring. Deep fissures provide channels for rain water to enter the slide mass, increasing seepage pressure within the mass as well as applying hydrostatic pressure against the walls of the fissure or crack.

Seeding or soading can also be used for slope treatment. Asphalt mulch technique can be used in which the slopes are prepared into vast seed beds. Asphalt mulch is then spread by a sprayer. The asphalt film gradually disintegrates, its place being gradually taken up by a carpet of green vegetation. The carpet of grass, that supplants the asphaltic film, acts as an immediate cover for the slopes till the more deep rooted species of shrubs and trees develop and take root.

NBM&CW March 2012