Piling platform over very soft clay with stiff desiccated crust. 1

[MrRatty]

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

 

[FixedEarth]

You mentioned two cohesive soil layer problem, then a slope. Can you attach a section?

http://www.soilstructure.com/

 

[MrRatty]

Sorry, colloquial English, no slope. Slop. The very soft clay,

 

[FixedEarth]

O.k. Thanks for the explanation. You can use the attached to get a starting point of bearing capacity but I suspect settlement and stress at the top of the compressible will govern. You may try Westergaard stress distribution and see the stress at the top of the soft clay and also the induced settlement. I agree your crust is too thin and may need a suspended slab with a deep foundation support.

http://www.soilstructure.com/

 

[BigHarvey]

Fixedearth, I guess Mr Ratty is trying to build a working Platform for a piling rig, it might not be practical to pile it
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 ! )

 

[MrRatty]

I'm reluctant to ignore completely the contribution to resistance that a layer of stiff clay would provide. I will conservatively assume a 1m thick layer of 50kPa clay that would need to be sheared by punch through into the underlying soft clay. I'll also see if the contractor can use a lighter rig!

 

[BigHarvey]

Is it CFA or rotary ?. I guess CFA because it is more demanding during the extraction phase, most accidents happen during this phase where the front of the tracks punch through the platform. In the Soffons approach I took into consideration the 1.5 m crust layer but the stress level is high.
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 !

 

[MrRatty]

It's driven piles, I'm not sure why they are using such a large rig, they're only 200mm pre-cast sections. The first blow will probably fire the fist section down to bedrock!

 

[BigH]

Have you looked into the layered bearing capacity papers (Button, for example)? You will find this in Fang's Foundation Engineering Handbook. This would tell you if your crust is thick enough without a platform or not. As BigHarvey confirms, a geotextile or geogrid under a man-placed fill of likely a minimum thickness (say 500mm?) should work. FYI, we placed 4 m of sand fill for a RE retaining wall on 20 kPa clay with no crust without failure. Of course, had we gone to 5 or 6 m, it might have been a different story.

 

[MrRatty]

Thanks for the tip. I wouldn't feel comfortable without a platform. It'll need a running surface at the very least. Common sense tells me a 58 tonne rig needs more than just 1.50m of stiff clay between it and oblivion.

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?

 

[BigH]

Pretty much it. If you don't have Fang:

http://www-personal.umich.edu/~rlmich/index_files/references/ICSMGE Osaka Zhu&Michalowski.pdf

 

[BigHarvey]

Check the stress level. I could understand a stress concentration on the tip of the tracks for a CFA rig but not for a driven pile rig. You have a load of 60 tons distributed over about 6 m2 which leads to 0.1 MPa and you have 0.33 MPa ! Recently we did a job in the same conditions (slop with Cu < 15 kPa, and 1 m dessicated crust with Cu = 30 to 40 kPa), we had a Juntann PM20 with 800 mm tracks and worked with 1 m of Platform.