Effects of Heavy Rainfall on The Landslide Probability

Introduction

A massive landslide wiped out the village of Malin located at 110 km from Pune city, in the Western Ghats on July 30. Mound of mud and debris that came down from a nearby hillock, swallowed up almost the entire tribal village of around 50 families. Due to the remoteness of the village, the tragedy came to light only when the driver of the first state transport bus reached the village a little after 7.30 am to find it uprooted. Final death toll was 153 when the rescue operation was stopped and around 100 people were missing. This tragedy has brought the focus back on the management –or mismanagement – of the vulnerable hills of India especially Western Ghats, Himalayas and North Eastern states There are many speculations to what caused such massive landslide. In this paper we are analyzing geotechnical aspects of a catastrophe. Some of the key facts to note are, heavy rainfall before the landslide (10.8 cm on July 29) and heavy downpour throughout the following day. This calamite is a lesson for sustainable geotechnical planning in future to avoid massive loss of human life and property. Detailed analysis of geotechnical facts is carried out and an attempt is made to pinpoint the cause and preventive measures are suggested.

Effects of a Very Heavy Rainfall

The region was receiving particularly very heavy rainfall in the week between 25th to 31st July. The data suggests that as much as 600 mm of precipitation may have affected this area prior to the landslide occurring. Cumulative rainfall in the last week of July as recorded by NASA’s (The National Aeronautics and Space Administration of US) TRMM (Tropical Rainfall Measuring Mission, was more than 600 mm. In fact on the 29th July, the region including Malin was shown purple in 24 hr rainfall map, which signifies the highest range of rainfall, exceeding 175 mm. There was continuous heavy rainfall since the last three to four days in the area, rainwater saturated the soil along the slope of the village. This developed as loose mud and eventually flowed down after gaining momentum, sweeping terraces, walls and ultimately the houses in the village

Deforestation and Levelling for Cultivation

Environmentalists say that the area around Malin has been deforested extensively. According to the Hindustan Times, official data indicates that close to 28,000 trees were cut, but unofficial figures put the count at 300,000. The main purpose of deforestation was to make way for farming, construction and mining. Government in its tribal employment project Padkai scheme implemented under Mahatma Gandhi National Rural Employment Guarantee Act (MNREGA), initiated flattening of hill slopes and trees were also cut down to develop cultivable plots. More recently JCB machines were employed on the hillside to developing adivasi land. Land was levelled by uprooting trees, which in turn loosened the soil, stone bunds were not built to contain erosion and nullahs were not cut into the soil to allow drainage. The slope of the hill was flattened almost halfway for agricultural purpose to such an extent that the hilltop had become unstable

Effects of a Landslide

In this study, a rainfall induced landslide which occurred along the upslope of Malin village, Ambegaon Tehsil in Pune district of Maharashtra (India) is examined. The landslide occurred in the early hours of July 30th 2014 claiming 151 human lives and damaging 45 houses. Heavy rainfall was reported in the area on that day and over the previous one week. Hence rainfall infiltration is assumed to be the direct cause of landslide in our analyses. Back analysis of the failed slope is performed to determine the shear strength parameters and other conditions at the time of failure. Generally, back analysis is an effective approach to provide an insight into the underlying failure mechanism and improve the understanding regarding the factors controlling the stability of slopes. One of the advantages of back analysis is that it can account for important factors that may not be well represented in laboratory and in-situ tests such as the presence of cracks and pre-existing shear planes within the soil mass. Finally, the cause of slope failure/landslide is explained based on the basic concepts of unsaturated soil mechanics.

Precautionary Measures

The best option to prevent the occurrences of such landslides would be to create awareness among the people by organizing programs that explain the basics of good hill slope development and management practices in the simplest and generalized way. Some of the very important practices to be followed are enumerated below:

1. The natural drainage shouldn’t be disturbed or blocked on the slope. Water shouldn’t be allowed to accumulate or percolate on the slope.
2. For support wall, wider base and slightly incline (towards hill) are to be provided. Weep holes are must and they should weep. This can be achieved with proper backfilling i.e. coarser material at the bottom and successive finer layer.
3. Care should be taken for anything that may move/fall downward.
4. Exposing a very high vertical face should be avoided.
5. Avoid adding load very close to the edge of the slope.

Slope Stabilization and Development

Slope failures, or landslides, typically occur where a slope is over-steep, where fill material is not compacted, or where cuts in natural soils encounter groundwater or zones of weak material. In most excavation and embankment work, relatively flat slopes, good compaction, and adding needed drainage will typically eliminate routine instability problems. Install slope drainage such as deep cutoff trenches or dewater with horizontal drains. Drainage control is fairly effective in reducing landslide risk. But in the case of intense amount of rainfall, a drainage system can prevent storm water from infiltrating and saturating the ground. This can be achieved above the surface, by installing drains to perform the same functions as gutters that remove water from the roofs of houses, or by covering slopes with an impermeable layer such as soul-cement or plastic. For drains that divert water away from unstable slopes, drain control improvement can be one of the most cost efficient means of reducing the probability of landslide. If there is a lot of rainfall in a short span of time, then the soil is not able to absorb the moisture, which makes it vulnerable to erosion and slope instability, eventually leading to landslides.