Waiting period for MSE walls 1

[dfranks]

I understand that in some states they allow MSE designs with a bearing pressure greater than the bearing capacity, IF the contract agrees to build the wall to a height where the pressure and capcity are equal and then wait 30 days. From what I understand the assumed capacity is then increased by the weight of the material that was placed in stage 1 allowing the new overall bearing capacity to accomodate stage 2 or even stage 3 of wall height.

Has anyone else heard of this process?
Does this process make sense, if so why?

Thanks!

 

[fattdad]

it makes sense that the undrained shear strength will increase under the new confinement stress. It makes sense that it takes time.

What doesn't make sense if the notion of bearing capacity related to MSE walls. Fundamental to the design of MSE walls is internal, external and global stability. You must show appropriate safety factors for these three failure modes at the instant of completion. Global stability is actually the purer form of bearing capacity. I mean what process is used to determine the "bearing capacity" of an MSE wall? You can't use the typical rational equations, 'cause they are based on boundary conditons that are not available on the MSE wall. Would you use "L" as the foundation width (i.e., the length of the reinforcement strips)? O.K. that'd be fine. Would you use the equations for an infinitly long footing? O.k. that makes sense too. What's your depth of burial? 20 ft on one side and zero feet on the other (e.g., for a 20 ft tall MSE)?

I don't buy it. Terzaghi's equiations (Meyerhof?) just don't fit the boundary conditons. Why force the square peg into the round hole?

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

I trust that you meant bearing pressure than the "allowable" bearing capacity. Agree with fattdad that this is possible and the "wait" time is important. To use a "30" day rule doesn't make sense to me unless it is founded on geotechnical engineering principals. In India, we had MSE (RE) walls that were being placed to heights of 10 m on ground with an ultimate bearing capacity of only about 120 kPa. Clearly the wall would not stand up. As a result we used pvd (prefabricated wick drains) and installed them at about 1.5 m centres to decrease drainage path of the very soft clay. We built the retaining walls in stages whereby the increase in undrained shear strength due to consolidation was used as a measure to permit the whole construction - for us it was about 60 days between stages. Had mostly success except in one instance where there was pressure to put on the stage loading early. Settlements of up to 1100 mm occurred during construction. Suggest you might want to look at BS 806 (I think that is the correct British standard for MSE wall design and construction).

 

[Doctormo]

BigH - BS 8006 is the reference.

I tend to agree with fattdad on MSE wall bearing capacity but that is a long discussion. Regardless of method, there can be short term and long term strength characteristics to deal with soft foundations and constructing a wall too fast has the same problem as constructing a levee too fast and risking a quick foundation failure on a soft foundation. The concept is pretty basic and piezometers are usually installed to measure pore pressures and allow the surcharge to continue when the pressures stabilize.

This method may be overdone at times as I have seen it prescribed without piezometer monitoring and as just a "wait 30 days" criteria. Some of the foundation soils do not appear to be that sensitive in my opinion but it is a conservative way to avoid a possible situation without doing all the work necessary to define the problem. Just the way it goes in real life occasionally.