EireChch
Geotechnical
- Jul 25, 2012
- 1,336
Hi all,
Our client is proposing to construct a new house on a sloping site. House is to be set back from a 20 degree slope by approximately 3m. We have a hand auger borehole drilled in the footprint. Loess soils (silty sand) to at least 4.5m depth. (Refer attached sketch).
I haven’t done a stability analysis yet. Given the dense soils and absence of any groundwater (or potentiall for saturation) I assume a FOS will be above 1.5. This will be confirmed.
Ground creep is common in Loess soils in our area. Typically to 1-1.5m depth. Given the dense soils we are assuming the upper 1m may be prone to creep. To mitigate this, it is proposed to incorporate protection piling on the downslope perimeter to 3m depth. Upslope piles will reduce in length to provide a smooth transition to shallow foundations.
The structural was querying me on what forces should he be designing for. I know (well at least I think I know) ground creep is a difficult thing to assess and my only be 50mm of movement over 10 years for example. Would it be inappropriate to provide the structural engineer with an amount of displacment so he can design is piles to be suitably stiff to resist the bending moment of 50mm horizontal displacement over 1m height.
From discussion with some engineers in our office I understand we normally recommend the structural engineer to design the pile as a laterally loaded pile i.e. the pile is retaining 1m of soil on the upslope side while there is no passive resistance on the downslope side to 1m depth. The soils from 1m depth to 3m depth would be providing passive resistance to resist movement. The piles would be spaced at 3D centres with soil arching effect supporting the soils between piles.
What are your thoughts on this? Any help is greatly appreciated.
Our client is proposing to construct a new house on a sloping site. House is to be set back from a 20 degree slope by approximately 3m. We have a hand auger borehole drilled in the footprint. Loess soils (silty sand) to at least 4.5m depth. (Refer attached sketch).
I haven’t done a stability analysis yet. Given the dense soils and absence of any groundwater (or potentiall for saturation) I assume a FOS will be above 1.5. This will be confirmed.
Ground creep is common in Loess soils in our area. Typically to 1-1.5m depth. Given the dense soils we are assuming the upper 1m may be prone to creep. To mitigate this, it is proposed to incorporate protection piling on the downslope perimeter to 3m depth. Upslope piles will reduce in length to provide a smooth transition to shallow foundations.
The structural was querying me on what forces should he be designing for. I know (well at least I think I know) ground creep is a difficult thing to assess and my only be 50mm of movement over 10 years for example. Would it be inappropriate to provide the structural engineer with an amount of displacment so he can design is piles to be suitably stiff to resist the bending moment of 50mm horizontal displacement over 1m height.
From discussion with some engineers in our office I understand we normally recommend the structural engineer to design the pile as a laterally loaded pile i.e. the pile is retaining 1m of soil on the upslope side while there is no passive resistance on the downslope side to 1m depth. The soils from 1m depth to 3m depth would be providing passive resistance to resist movement. The piles would be spaced at 3D centres with soil arching effect supporting the soils between piles.
What are your thoughts on this? Any help is greatly appreciated.