I do not work on natural hazards, but am tempted to express layman's opinion.  

I think everyone knows that the root cause of landslides is gravity. That is, whenever gravitational pull exceeds the binding strength of the rocks or soil on a slope, sliding would take place. The binding strength gets weaker in rocks containing joints, fractures or which get affected by faulting, and the repose angle in the case of soil and boulders on a slope.Sliding may be triggered by earthquakes (i.e., movement along faults or simple shaking of unstable slopes), or weakening of the binding strength by rainfall or snow/ice melting (liquefaction), erosion of soil around loose boulders, dissolution of the binding material/cement between blocks, and so on.

Cheers,

Qasim

Dear Mr. Ahmad,

throughout  almost 40 years in geosciences I have been working in economic and mining geology, sedimentology, mineralogy, geography/geomorphology, pedology, field and structural geology, geochemistry, remote sensing, environmental and urban geology, age dating and applied geophysics. Therefore, I would like to direct your thoughts to some excellent textbooks in geography such as those published by Summerfield, on structural geology by Hobbs et al., applied sedimentology by Selley, applied geophysics by Parasnis etc....... Based upon these numerous books and reviews you will get a broad overviewof the topic and a fine introduction into this matter. For a first-hand information I attach a special file on this topic.

I wish you much success as you are going to browse these comprehensive studies.

Harald G. Dill

Good Morning Ijaz

I totally agree with Qasim and Harald. The other factors to consider drainage system management, removal of root-jacking trees on the slope, seepage management, removal of large boulders on the slope, compacting steep embankment to required density (eg.90% OMC or using moisture-density relationship ), avoidance of loading onto the slopes due to constructing large buildings and providing toe support (eg. retaining wall) to steep slope to minimise landslides, Drainage management is very important as soil looses strength when it is wet. During rainfall, sheet flow along the road can erode the embankment. Through the cracks and faults of rock, water can seep and cause instability of slope. Catch drain, batter drain,table drain and culverts within the road corridor need to be maintained without any blockages.

Best regards

Ravi

Dear Ahmad,

I agree with Qasim, Harald and Ravi. The other factors that important to consider in relation to landslide are lithology types, dip slope direction of stratified sedimentary rocks and alternation of stratified sedimentary rocks. 

Sliding would be easily occur if the are alternation between sand and clay on the bank, and the sediments have dip slope direction to open area.

Regards

Wahyu

Dear Mr. Ahmad,

an important failure mechanism is the so called progressive failure. This means that the failure develops more or less continuously until a point where the landslide results in a total failure. The failure will start in some finite places and will spread along weakening planes in the rock or soil. This has some implications on the characteristic strength parameters. Due to its progressive nature the landslide won't develop its full peak strength. The true peak strength will be lower and is more or less same to the residual strength.

If such progressive failure will develop depends on different factors like dip and dip direction, lithology etc. Generally you can say that progressive failures will develop in rocks and soils with strain dependent softening.

The factors mentioned by the other authors also play a significant role.

Best regards

Dear Ijaz,

re:    Landslides: failure mechanisms and triggering factors.

The scope of what you ask is vast and different disciplines approach them in different ways.  However, a landslide occurs due to two sets of factors :

1.         pre-disposing factors

2.         triggering factors

The best starting point, I suggest, is to read Varnes (1958).  His classification is sound and the different slides are well described.  It is clear that he divides landslides into falls, slides, flows and complex, and into bedrock and soils forms (p.21).  Brunsden (1979) is good for a more comprehensive approach.  The other literature I have appended gives an indication of the breadth of approaches to specific types of slide, e.g. Jaboyedoff (2011) gives a comprehensive study of rockfalls/slides, Campbell (1975) examines debris flows and Winter et al. (2005) gives the road engineer’s approach.

To return to pre-disposing factors, these include rock type, strike/dip of bedding planes, jointing patterns, susceptibility to weathering (both physical and chemical), soil characteristics, drainage patterns, groundwater movements, slope gradient, possibility of seismic activity, human interactions (mining activity/road or dam building in area), vegetation cover, climate, day-to-day weather experienced in area, among others.

Triggering factors include intense rainfall, excessive porewater pressures in rock/soil, gravity, seismic activity, undercutting by e.g. river or road building, overburden adding excess weight to slope

Models for landslide prediction will contain a selection of such factors relevant to the specific type of landslide involved.  I would assume that in Pakistan rockfalls, rockslides, debris slides and debris flows would loom large along with snow avalanches.

I hope this goes someway to answering your question.

George Strachan

BRUNSDEN, D. 1979. Mass movements. In Embleton C. & Thornes J. eds., Process in Geomorphology, Edward Arnold, p.130-186.

CAMPBELL, R. 1975. Soil slips, debris flows, and rainstorms in the Santa Monica Mountains and vicinity, Southern California. US Geological Survey Professional Paper, 851.

HENCHER, S.R. & LEE, S.G. 2010. Landslide mechanisms in Hong Kong. From Calcaterra D. & Parise M (eds.) Weathering as a predisposing factor to slope movements. GSL Engineering Geology Special Publications, 23, 77-103.

IVERSON, R.M. 1997. The physics of debris flows. Review of Geophysics (American Geophysical Union), 35, 245–296.

IVERSON, R.M. & LaHUSEN, R.G. 1993. Pore-pressure dynamics in debris-flow experiments. Abstract - Transactions (American Geophysical Union), 75, 274.

JABOYEDOFF, M. 2011. Slope tectonics. Geological Society Special Publication, 351.

Dear Ijaz,

My background is physics and groundwater hydrology. I think that all of the above answers that you have received pretty much provides you with quite a comprehensive set of mechanisms and triggers. I offer as an aside a contributing human element - poor construction practice  (roads, dams, buildings, and other structures) that fails to idenitfy potential landslip hazards that construction may exacerbate and even trigger.