Lateral soil pressure on box SOE

[beagleboy]

hey Engineers!

Looking for some input on the lateral soil pressure on the box SOE system. I'm designing a box SOE. The box is about 18ft square x 25ft deep. Sheet piles are running vertically with walers all along the perimeter with vertical spacing of 6-8ft. On the sheet piles what the lateral soil pressure distribution would look like? Would it be a triangular or any other shape such as trapezoidal? Would the pressure distribution change because walers are experiencing bending as well as axial loads from the soil? Please note the soil pressure on one waler induces the bending in that waler and axial into the walers that it is connected to at both ends. Any thoughts, experiences would be helpful. Thanks.

 

[DaveAtkins]

It sounds like you are designing a "braced cofferdam," so the recommended soil pressure diagram is the Terzaghi trapezoidal distribution.

 

[geotechguy1]

There are trapezoidal type / rectangular type distributions if you're doing it that way, alternatively it sort of becomes an FE problem.

 

[HTURKAK]

Common practice is trapezoidal soil pressure distribution. What about GWL ?
( The following excerpt from Joseph E. Bowles - Foundation Analysis and Design )

 

[beagleboy]

Thanks all. Really helpful. Any reason why soil pressure changes from triangular to trapezoidal? Is it due to lateral restraints (due to strut or walers). It has GWL for last 4-5 ft.

Other option is box SOE with corner posts connected with timber laggings spanning horizontally. Would it experience the same trapezoidal lateral pressure? The site has several constraints (existing structures, utilities, etc) and want to explore every option that would give me smallest Box SOE. Thanks again.

 

[geotechguy1]

The struts / walers cause load re-distribution would be the theoretical explanation for the non-triangular distributions. The origin of them was Terzhagi / Peck / others measurements of loads behind actual walls and finding the measurements were non-triangular, although I think with modern FE and constitutive models you can also produce realistic models now.

Box SOE's are interesting. I have seen them designed using those same pressure distributions, although in practice they often get installed so that there is a gap between the box and the soil for a significant amount of the depth. I'd be very hesitant about doing a 25 foot depth excavations with 5 feet of water at the bottom with that type of approach though.