Causes of Lanslide

[ngangey]

We have a pipeline project on a 60 degree slope mountain. We hired a specialist to evaluate the factor of safety which they provided equal to 2.31. Based on my site inspection the soil is dominantly composed of loose gravel. Thickness of soil is 3-6m from boreholes. Bedrock consiting of DIORITE. Climate is tropical. traces of landslide were visible near the area is near the river.
QUESTION:
1. Based on experience is the contractor correct?
2. What causes landslide?
3. Circular and Planar failure was used for evaluation?
4. References?

thanks

 

[BigH]

Be careful of working on mountainous slopes. A 60 deg slope giving a factor of safety that high? How deep will you pipeline be buried - into rock or within the "gravel". If you take "loose gravel" - infinite slope (shallow translational slides) would be the most critical.
see:

http://www.cedd.gov.hk/eng/publications/guidance_notes/doc/tgn10.pdf

 

[ngangey]

BigH the pipeline is proposed to be exposed outside..pipeline slippers or its foundation is approx. 2m deep at 250 bearing capacity, pipeline dia is 2.0.

 

[mconti]

with 60° slope angle it is difficult that material is loose.
it is better foundation with collar and (no excavation) and micropile or similar for staibility. the value of 2.31 was performed from geotecnical data?

 

[oldestguy]

Question Number 2 as to causes can have several answers. Using your geotechnical info may provide some answer, such as weak layer. However, did the evaluation consider you weather and possibly some saturation of the site at times? In my experience saturation is generally in the picture when failure occurs.

Also, is that 60 degree a measurement or someone's guess? Slopes may seem steeper than the measurement says.

Slides in the area would tell me that 2.31 is wrong.

 

[ngangey]

i agree with you oldestguy since the climate is tropical and the type of soil is loose difinitely it will surely fail. they are using "geoslope" software to evaluate the stability based on the boreholes and soil parameters they gathered on site (cohesion, angle of internal friction, grading, water table etc) thats why we are currently hiring another company to conduct another slope stability on the area. thanks

 

[BigH]

Don't forget seismic effects in your translational sliding model.

 

[ngangey]

for more information this is the area and the pictures i had taken. including the geology....