geotech report question

[par060]

I recieved a soils report for a particular project. This report gave an allowable soil pressure and a frictional resistance for drilled piles. In discribing the project in the report they did not mention a particular significant point load. I responded mentioning that this load was present. I did this only because the report mentioned a change in scope might effect the report.

Today I recieved an e-mail saying I should make the geotech aware of any loads that are additional to the main thrust of the project.

I don't quite understand why he needs this information. I am designing the foundations to not exceed the allowables he has given me....what difference does it make to the soil as long as the allowables are not exceeded?

 

[UcfSE]

I believe they use the anitipated maximum loads to arrive at allowable soil pressures based on settlement criteria.

 

[jdonville]

par060,

UcfSE is correct. The allowable capacity of foundations - and most usually deep foundations - is related to the expected settlement of the foundation element or group.

Drilled shaft research indicates that the percentage of the ultimate capacity in both side friction and end bearing vary with respect to the ratio of the settlement to the shaft diameter. The side friction mobilizes at relatively low displacement while the full ultimate tip capacity takes a relatively larger displacement to mobilize. As a result of the strain compatibility issues (non-linear), the allowable capacity of a drilled shaft will depend on the allowable/assumed settlement ratios and the theoretical ultimate capacity of both side friction and end bearing.

Unfortunately, this means that the allowable values presented in a geotech report are only accurate over a given range of loading, so please be nice to your geotech - he or she is trying to simplify a somewhat complicated design issue.

Jeff

 

[TDAA]

jdonville is correct. Additionally, your load may represent a load in excess of what should be bearing on the soil – ie it should bearing on rock. I had a structural designing drilled piers carrying 1800K, decide he would like to shorten the piers because he could get the load through the skin friction we provided, while ignoring the fact we told him to bear in the rock.

Sure, saving 20 to 30 feet off of 4 to 5 foot diameter shafts sounds great, until you have excessive settlement.