Shear Strength for Temporary Support of Excavation

[DirtPlayer]

Over years, I have been trying to find good resources regarding reasonable soil strength used for temporary support of excavation design, such as soldier piles with wood lagging. Most textbooks and researches have discussed permenant retaining structures, which are quite different in terms of drainage condition, loading conditions, etc.

The question is for short term excavtions of a project in several weeks, what kind of shear strength value should be used to check the stability? Drained v.s. undrained?, UU or CU? Other factors, such as normally consolidated and over conconsolidated clay, saturated and unsaturated, have made the things more compalicated.

Any inputs? Thanks

 

[dgillette]

Depends on soil, water conditions, and, at least in theory, rate of excavation. The way your question is asked suggests that you are dealing with saturated fine-grained soils, presumably with little potential for dewatering.

In saturated clay with low OCR, there are times when what you need to assess is the UU condition - undrained shear strength based on existing effective stresses, as might be measured by vane shear or CPT. If it's a big job (with time and budget and enough at stake that you can't economically cover the uncertainty by a large FS) a SHANSEP approach may be best. UU lab tests can give unrealistically low strength because of sampling disturbance.

Bon chance!

DRG

 

[DRC1]

If you are designing for temporary shoring 90% of the time you will have little data, most of it marginally useful.

Typically, sands and gravels are evaluated on drained conditions and should not make a difference if the design is long or short term. Normally consolidated and under consolidated clays typically gain strength with time, and for temporary shoring are generally evaluated based on undrained strength. Overconsolidated clays generally loose strength as they drain, thus shoring duration and conditions come into play, but for short durations a conservate undrained strength may be okay as theese clays tend to have lower permiabilities, those longer drainage times. Silts and muds are a jump ball.
As for testing, If I can get a couple of SPT's I am estatic. In grad school we used to do CU & UU with pore pressure measurements, which if done correctly gave you a lot of both drained & undrained strength information. I think doing a U-U vs. C-U test would hinge on how disturbed the sample was and how sensitive it is. For steel sheet piling the failure surface tends to be more of a plane strain, where as soldier piles are a triaxial strain condition.
The premier issue of the DFI Journal has an article on designing drilled shafts for sound walls. In that they quote some recent corelations of SPT data to sand & clay properties. It is availble as a download from their website. Google DFI. For the record, I am not a big fan of using SPT's for clays.
Hope that helps.

 

[dgillette]

"For steel sheet piling the failure surface tends to be more of a plane strain, where as soldier piles are a triaxial strain condition."

That's interesting. How does it happen? Is it because the lagging between soldiers is softer and gives a little, in contrast to the "relatively" uniform stiffness of the sheet pile? I never thought about it before.

 

[DirtPlayer]

Thanks guys for the input. Yes, I am talking about fine grain soil soils, such as CL, CH, ML, MH. As a matter of fact, local geotech guys here in DC have been trying to convince others about whether a peak strength or residual strength values should be considered for the SOE design. Again, different factors affect a lot of the considerition depanding which angle you are looking at.

I prefer use UU at peak strength because the residual strength can only start to kick on at 15% of strain, which will result in disaster for a shoring design.

Agree?

 

[BigH]

I know I will sound like a dinosaur - but if you can get a copy of the Pan-American Conference (mid-1960s), there is an article on soldier pile and lagging design used for the Toronto subway - author was Golder and Seychuk (the last for sure).

 

[DRC1]

dgillete,
Since soldier pilr theory is based on the resitance of a 3-D wedge, Iwould assume triaxial conditions. For sheet piling, the failure surface is continous as are the sheets, so I would assume plane strain conditions. I actually don,t have a reference for this, but always how I saw it in my own mind.

 

[SmokeyBear]

DirtPlayer,
Assumming we are not looking at partially saturated soils, which seems to be a new kid on the block, low strength cohesive materials are probably best considered as undrained for "normal" construction times. We tend to use vane shears to assess Su, applying Bjerrum's correction for PI.
UU's, CU's etc generally exhibit large strength variations due to sampling and preparation disturbance. As you say, peak strengths (as measured by vanes) are appropriate. The large strains required to reach residual values means the wall has probably failed.
We look at the Su/p ratio to develop a stress history and find for well behaving cohesive materials, we can get a sensible strength plot.
However, we are never sure what the soil considers a "reasonable" construction time and therefore run a drained analysis to see what happens. We use PI relationships to get a phi'and guess a c' (often taking c'=10% of Su for a starting point). It is very unusual to get any drained triaxial results. Fortunately,there is a growing use of high capacity CPTu and DMT testing in Oz which really helps profiling. This combined with Vane Shear tests is an efficient and very cost effective SI tool - especially for the foundation contractors.

 

[BigH]

SmokeyBear - better inform our American friends what you mean by "Oz"!!! (Australia!!)