Differential settlement

[debpriya1984]

“The differential settlement can be computed as the difference in settlement between two adjacent points. It may be estimated as three-quarter (3/4) of the computed maximum total settlement.”

The problem is:

Sometimes we have cut and fill operation and when you have areas of fill we need to estimate de immediate settlement induced by live load (before construction). Long term and settlements after construction could be estimate. Finally, differential settlement could be expected between the cut and fill area…
Some people uses the settlement “after construction” (immediate + total settlement) to estimate the differential settlement!!!

I have used the maximum settlement computed and took ¾ or just the difference between two adjacent points…

I’ve checked this methodology for all sites and the results are satisfactory. Anyway…

what do you think?

 

[msucog]

here's my thoughts. you've got multiple things to consider and may be different for different soil types, geologies, etc.

"long term" may be a lot or may be insignificant depending on the soils. "long term" is also rather relative to the structure being built and expected performance. be sure to consider what is significant to your project.

for differential settlement, some use 1/2 some use other factors for the tolerances. i try not to estimate differential from total depending on the situation. i would rather calculate the estimated settlement at multiple points to "better" approximate what settlement potential exists. you might even try this using a three pronged look (best case, "average" and worse case) and then mix and match for the specific project. also it's sometimes good to check for things like significant drops in groundwater (due to long term drought, draining of an adjacent lake, rerouting nearby drainage swale, etc) even though we often have no idea what curve ball mother nature will throw us.

i would suggest that fill induced settlement be allowed to essentially stop prior to continuing construction. depending on the underlying soils this could take a long time...maybe not. sometimes, the schedule presses to move forward and the assessment can become difficult to state it nicely. either way, i attempt to calculate settlement based on the settlement that the structure (be it wall, building, road, etc) may experience. so if a 30' fill and 20' cut will be executed and building immediately constructed, it's highly possible that the differential settlement will be high. fill area settlement will be the fill induced settlement plus extra building loading plus secondary where the cut area settlement will be the settlement due to building load (maybe zero due to removal of overburden) plus secondary (again, maybe nothing). also, some of the fil induced settlement will be "immediate" but may need to be considered in the analysis (again, depends on the particular scenario as to whether the structure will ever see this movement).