Design of Soldier Piles 1

[dik]

I have a pit wall that will be constructed using soldier piles at about 8'-9' o/c.

The height of the wall is about 10' high.

The native soil is a highly plastic clay that will be excavated about 2' from the wall location and extend upwards about 4'. The remaining 6' of soil will be excavated at a 1:1 slope, that is, the native soil will be cut away from the wall by about 8'.

The H soldier piles will be embedded in concrete caissons/piles at the base of the wall.

The space between the native soil and the wall will be backfilled with compacted granular stone.

Is there a relatively easy manner to determine the soil pressure?

Is it reasonable to assume that the pressure would be caused by the granular material only? will little or no contribution by the native cohesive soil?

Is the native cohesive soil too close and it will provide an additional component?

Is there a rule of thumb about the proximity of native soil to the wall, assuming there is a granular backfill material?

thanks, Dik

 

[fattdad]

Use the properties of the native soil and ignore any perceived beneficial value from the aggregate backfill (other than for drainage). If it's cantelevered, use a triangular active earth pressure (well other than a surcharge component) and for the passive reaction you can multiply the passive earth pressure on the embedded depth by Cp. If there are tiebacks then you will use a recgangular earth pressure distribution as shown in the Terzaghi and Peck textbook.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

With no disrespect intended to fatdad, as he is a geotech, if you or your firm is going to have to stamp the wall design, you really should hire a local geotech and get his input in a written report.

Mike McCann
MMC Engineering

 

[hokie66]

I agree with fattdad if this is a permanent structure. If temporary, I would take advantage of the cohesion of the clay and design only for the pressure of the granular backfill.

 

[fattdad]

I agree on hiring a local geotech. I'm not sure about complete faith in the cohesion. Actually if the clay is stiff enough to stand vertical, you may als have fissures. For the case of stiff fissured clay, you may not even need lagging, but the support calculated for the soldier piles needs to consider some sliding wedge. Local geotech should have the correct approach.

Regarding the support to the gravel backfill, we had a thread a few months (or so ago) on the methods used to calculate the contribution of a granular fill of this nature. My approach is to use the friction angle of the granular material, take a angle of 45+phi/2 and where that angle (measured horizontally from the back of the soldier pile wall) intersects the ground surface you have the maximum pressure contribution from the granular backfill. At that point strike a vertical line to form the pressure rectangle for this backfill. You'll see that it's just not much for the case that you have a 2-ft wide prism of granular backfill on a 10 ft tall wall.

It's a fun discussion, but further input would require boring data, classification testing and boundary conditions for the pit and neighboring land (heck, what if there's some historic church next to this?).

f-d

¡papá gordo ain’t no madre flaca!

 

[dik]

Thanks, gentlemen... your caution about retaining a geotech is appreciated. It's intended for a permanent structure and my prelim design uses cohesive soil.

Dik

 

[DRC1]

If you go to SlideRule Era's website,

http://www.slideruleera.net

look sheeting & H-piles, which has the US Steel Sheet Piling Design Manual. I think there maybe a procedure for the condition you describe.

 

[dik]

DCR1:
Thanks, I have a copy of the USS Design Manual. The problem is far less onorous than I thought. At the end of the day, the soldier piles are attached to a slab at the top, there is 4' of backfill in the base (the wall is only 8' high, now) and there is no backhoe running around the edge to excvate the slag.

Dik