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Earth Pressure Distribution Models for Flexible Wall Systems

Earth Pressure Distribution Models for Flexible Wall Systems

Earth Pressure Distribution Models for Flexible Wall Systems



This is my 1st post and I'm new to this, therefore I hope it makes sense and I can get some great feedback from you guys...............

I would like to hear some expert opinion about the way in which earth pressures should be modelled when it comes to internally-braced and cantilever flexible sheet piled wall systems.

I have been designing multi-braced sheet piled cofferdams for a number of years now (for various in-house design teams) and I have heard various peoples takes on whether Triangular (eg. Rankine) OR Apparent Earth Pressures models (eg. T & P) should be adopted. Researching around on the web and using educational databases on this topic I have found there to be probably hundreds of documents and guidance documents from various states, countries, government establishments, CIRIA's, piling handbooks etc. all giving design examples and guidance in one way or another which contradicts somebody / somewhere else.

I am from the UK and the Rankine method is used extensively in almost every situation. Most temporary cofferdams are cantilever sheet pile jobs or are internally strutted jobs that are built in a staged dig down then prop sequence using simplified load sharing support analysis methods.

My own thoughts on this subject are that the Rankine analysis was done around 1860 (Coulomb 80 years even earlier) and considered only an active pressure acting on a solid non-flexible retaining systems and materials (such as masonry retaining walls) which was around at that time. I believe (from courses lectures), that Rankine underestimates the passive pressure by as much as 100% when compared to modern methods. Hardly suitable for a cantilever support! Therefore this method seams to not really be suitable for flexible; cantilever, single-propped or multi-propped embedded retaining walls which yield in different places depending where struts are located (if any) and require a passive toe-in? On the other hand T & P measured strut loads in various ground condition within different support types and came up with pressure envelopes. So this was for propped walls, therefore not really suitable for cantilever walls.

Can someone help with explaining something I might be missing or not understanding as it seams non of these methods are really appropriate in every circumstance.

I am interested to hear comments for these classic analysis methods and not going down the route of complex finite element analysis (unless they help explain my woes).

Thanks in anticipation


RE: Earth Pressure Distribution Models for Flexible Wall Systems

There are many, differing methods for designing these walls (T&P, Teng Simplified, Teng Conventional, Tschebortarioff, Schnabel, FHWA, etc. All of theses methods have worked for many, many years and projects. I assume, also, that all have had failures. I believe that wall problems are most often associated with construction mistakes and, less frequently, with changed ground conditions. I don't believe that design method used is anywhere near the main cause of wall failures.


RE: Earth Pressure Distribution Models for Flexible Wall Systems

IMHO, there is good reason to use Coulomb-Rankine for a typical cofferdam but a more modern analysis for a typical retaining wall. It all has to do with the water table.

Cofferdams where I work are almost entirely below the water table. Water pressure is the major component of the design pressure... typically over 2/3 of the total. The remainder is the equivalent liquid pressure (ELP) based on the submerged weigh of soil - usually an ELP less than 30 PSF. Any error in design pressure is confined the small submerged soil component. Water pressure, based on 62.4 PCF or 64 PCF for sea water, is accurately predictable. The actual water pressure and the calculated ELP for submerged soil are easily combined.

I'm assuming that typical retaining wall backfill is mostly above the water table. Therefore soil pressure has a much greater influence on design pressure. In that case, well worth the effort to predict soil pressure as accurately as possible.

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RE: Earth Pressure Distribution Models for Flexible Wall Systems

I defer to T&P for braced or tied-back construction bracing. As you likely know this returns an earth pressure that's 30 percent greater than the Rankine earth pressure - i.e., if you integrate the stresses with depth the former is 30 percent greater. If the wall is cantilevered, I'd use Rankine. Now you just have to decide whether you want the upper extent of the wall to move 1-in laterally for every 10 ft of height. If you don't use at-rest earth pressures, which are (typically) 50 percent greater than Rankine active.


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RE: Earth Pressure Distribution Models for Flexible Wall Systems

The best guidance out there for simple analysis is the USS Sheet Piling design manual.

RE: Earth Pressure Distribution Models for Flexible Wall Systems

CIRIA C580 (replaced by a new version but still a good guidance) for embedded retaining walls and classic Rankine and Coulomb theories for gravity retaining walls. The classic theories are generally inappropriate for embedded retaining wall design due to soil arching effects. Embedded retaining walls include sheet pile wall, soldier pile wall, diaphragm wall, etc that rely on wall bending and shear and/or internal bracing or external anchorage to balance the earth/water pressure due to deep excavation in front of the wall.

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