Apparent earth pressure envelope

[jdonville]

All,

Everyone here is, I trust, familiar with Peck's apparent earth pressure envelopes for braced cuts. These seem excessively punitive (in terms of the lateral forces required) for braced cuts up to say, 18 feet or so in depth, using a single level of lateral restraint (strut, tieback, whatever).

Is there any research or papers that suggest alternate apparent earth pressure envelopes that provide a middle ground between the unbraced condition (generally triangular earth pressure distribution) and the Peck criteria?

Jeff

 

[PEinc]

Precast concrete lagging panels are OK to use for fill walls or for lower cut walls where an open cut can be safely made so that the panels can be stacked from the bottom up. Taller walls may prevent making a safe open cut. If the wall has to be built from the top down, as should all tiedback walls, then it can get very hard and sloppy to try placing heavy concrete lagging panels beneath previously installed panels. Some people think that the panels should be placed at the top of the wall and then be slid down as the excavation progresses. This is wrong. If you can slide down the panels, then you do not have good contact between the lagging and the dirt behind. Therefore the wall will deflect backwards and the panels will crack when the tiebacks are tensioned. Additionally, depending where the tiebacks are located, they may interfere with sliding the panels down. Also, if you make a big, sloped open cut to stack lagging, then you can't backfill the panels until the tiebacks are installed and stressed but you can't stress the tiebacks without the panel backfill being in place. This then requires that the contractor install very heavy bracing or cribbing behind the soldier beam so that the ties can be tested prior to backfill. Then, how do you remove the cribbing without excesive pile movement ot detensioning the ties? Not a good way to build tiedback walls. it's better to install the soldier beams, install temporary but L-I-P lwood lagging, install ties, continue to subgrade, and then attach a C-I-P or precast cancrete facing.