Piling platform over very soft clay with stiff desiccated crust.
Piling platform over very soft clay with stiff desiccated crust.
(OP)
I'm looking at a working platform design. There is a stiff desiccated crust (conservative 1.50m thick, Cu 75 to 100kPa) over low strength alluvium (Cu 20kPa). The rig is a heavy one and the loads are quite high (330kPa under extreme loads). The track width is 900mm.
My gut feeling is that the working platform plus the underlying stiff crust should be of sufficient thickness / stiffness to prevent overstressing the underlying soft clay. If I run the calcs using the Cu of the crust I get a platform thickness of the minimum (third track width = 300mm). Feels too thin to me for that kind of load. If I use the Cu of 20 for the underlying slop, I get a platform thickness of 1250mm !
The actual thickness needed will obviously be somewhere between the two. Question is how do I calculate it using the conventional BRE 470 approach. I can only use one Cu value, and I can't just pluck on out of thin air without justification.
One approach I am thinking of is to look at the reduction in stress beneath the tracks and use the design pressure that woudl be imposed at the top of the slop.
This must have been looked it many times before, most areas of soft alluvial soils have a desiccated crust and need piled foundations.
Thanks
My gut feeling is that the working platform plus the underlying stiff crust should be of sufficient thickness / stiffness to prevent overstressing the underlying soft clay. If I run the calcs using the Cu of the crust I get a platform thickness of the minimum (third track width = 300mm). Feels too thin to me for that kind of load. If I use the Cu of 20 for the underlying slop, I get a platform thickness of 1250mm !
The actual thickness needed will obviously be somewhere between the two. Question is how do I calculate it using the conventional BRE 470 approach. I can only use one Cu value, and I can't just pluck on out of thin air without justification.
One approach I am thinking of is to look at the reduction in stress beneath the tracks and use the design pressure that woudl be imposed at the top of the slop.
This must have been looked it many times before, most areas of soft alluvial soils have a desiccated crust and need piled foundations.
Thanks





RE: Piling platform over very soft clay with stiff desiccated crust.
http://www.soilstructure.com/
RE: Piling platform over very soft clay with stiff desiccated crust.
RE: Piling platform over very soft clay with stiff desiccated crust.
http://www.soilstructure.com/
RE: Piling platform over very soft clay with stiff desiccated crust.
Mr Ratty : your BRE approach is certainly right. I tried the SOFFONS (french equivalent of FPS) and found 1.4 m or 1.1 m with a 100KN strength geotextile. Your 1.25 m is in the same order of magnitude, hence keep it there because in France as in UK people don't like to see piling rigs overturning ! (or leave it to a competitor ! :) )
RE: Piling platform over very soft clay with stiff desiccated crust.
RE: Piling platform over very soft clay with stiff desiccated crust.
Maybe you can switch to rotary piles or drivent piles but it won't be the same price otherwise don't try to make savings on the Platform thickness !
RE: Piling platform over very soft clay with stiff desiccated crust.
RE: Piling platform over very soft clay with stiff desiccated crust.
RE: Piling platform over very soft clay with stiff desiccated crust.
Might use a secondary higher layer of geogrid as these only generally have any effect of the first 300mm to 400mm anyway. Will increase punch shear resistance as well as provide much greater stiffness to the platform.
I'm not overly familiar with Button or Fang, a quick bit of research on the former suggests you reduce Nc based on ratios of Upper Cu/Lower Cu and Crust thickness/FND B. I then assume you then calculate bearing capacity using the reduced Nc and Cu of the upper layer?
RE: Piling platform over very soft clay with stiff desiccated crust.
RE: Piling platform over very soft clay with stiff desiccated crust.