Lagging - long vs short term

[InDepth]

Does anyone have any references or discussions on long vs short term lagging pressures? I am specifically looking for experiments, fem analysis, or field tests/monitoring discussions.

What I am after: Is lagging pressure the same for long term and short term?

I believe that this depends on the stability of soil arching; which is related to soil type and creep. ie would you design permanent lagging/facing for full soil pressure without considering arching?

 

[irawanfirmansyah]

Two extreme conditions are available for geotechnical analyses: short term and long term analyses. The 1st one should use undrained strength parameters and the 2nd one has to use drained strength parameters.
For excavation works (unloading case), long term analysis will result in the worst. Hence, long term analysis should be carried out for excavation works.

Conclusion: the two sets of soil parameters are different, consequently, lagging pressures for short and long terms are different.

Recent advances, people come to the analysis which consider the stage between the two extremes, to accommodate fast excavation work, which soils have not reached drained condition.

 

[csd72]

agree with last post but with regards to soil arching, I would expect that consolidation of the soil would negate this in the long term.

 

[wmta]

Does not sound like anyone has any idea about this. There has not been must testing doing to find real loading conditions. The arching for sand would not be time dependent. The arching of soil is a complex system that is not easily modeled by FEA or other means.

 

[csd72]

wmta,

I would disagree that there is no time dependent variable with sand though I agree that it would be much smaller than for clay.the consolidation of sand over time would depend on the amount of vibration and the extent of drying out.

I would suggest that you google this in many different ways as I have seen a few different research papers that cover similar topics but I dont believe that I saved them.

 

[irawanfirmansyah]

There is no correlation between soil pressure on wall / lagging and arching effect. Arching effect will work on unsupported part of excavation, for example the space between soldier piles.

I attach two figures which show that drained analysis will result in higher pressure / higher deformation (more critical). The 1st figure was prepared by Prof. Tuncer B. Edil of the Univ. of Wisconsin-Madison. The figure shows that for unloading case (excavation), drained analysis will yield more critical result in compared to undrained analysis. For example if you run slope stability analysis for excavation slope using drained strength parameters, the SF will be lower than the one obtained using undrained strength parameters.

The 2nd figure was cited from a paper by Prof. S.L. Lee of the National Univ of Singapore, some years ago. The figure shows that undrained analysis resulted in smaller deflection than drained analysis. Consolidation analysis in the middle means the analysis utilizing soil parameters in between undrained and drained parameters, and this result in deflection in between the two extremes.