Equivalent fluid pressure for residential foundation 1

[CelticJam]

Hello All,

I am wondering about typical ranges of equivalent fluid pressures for stiff clays. I have viewed many reports on similar soils and can't seem to calc the values the majority seem to be finding.

I am looking for typical values. Using the Rankine calc, I seem to get values that are substantially higher than the ones presented in the typical soils report. Any help would be greatly appreciated!

 

[fndn]

Equivalent fluid pressure=Ka x Bulk unit weight of the soil. This formula is valid only for soils with phi only like Sands. In your case you have Stiff Clays, so you have to approximate it as half of the Bulk unit weight. That would put us in the 65 pcf range.

For residential walls, tallest wall would be 9 ft, sticking out of the ground at least 1 foot. So you have soil and may be hydrostatic pressure for 8 ft + footing depth or agin back to 9 ft. What I have done is to look at the excavated soils during basement excavation and if there are no organic soils, but clays, to require the bottom half of of the backfill depth be with coarse sand, or peagravel, and the top half be capped with the excavated native soils.

In addition, if you have a brick facade, or 10" thick walls, to require them to add two #5 rebar near the top and bottom, of all the foundation walls.

Lastly, most of the problems with residential basement walls, are either in the winter, when while concrete is curing, the temperatures may drop more than 20 degrees Fehrenheit overnight & you get cracks. Or that you have an exterior wall that spans more than 38 ft without a beam tying into the wall, and you get an inside bowing of the wall near the center.

 

[CelticJam]

fndn,

Thanks for the info. What you have said makes perfect sense and seems to fall in line with the calcs I have been coming up with. However, many ( almost all) of the soils reports I have seen are coming up with eq fluid pressures of 50 to 55 pcf. Any guess as to why?

The soil parameters seem to fall within a fairly typical range for slightly sandy clay of (-200 @ 75 to 95)bulk unit weights of about 130 pcf and spt blow counts (with a California Barrel) of 12 to 25 for 12 inches.

I have reviewed soil reports from five different respected geotechnical engineering firms.

 

[fndn]

CelticJam;

I agree, the 50-55 pcf is a little low for stiff Clays. It is possible that if you have predominantly Sandy Clay soils in one locale, that everyone uses by default those equivalenet pressures, regardless of actual site soils.

As to why most firms have similar values, I think it is because people change firms, and with them they take past experiences and soil parameters, so after a while we all end up using the same rule of thump numbers.

In the end, go with your own calculated numbers. I congratulate you for questioning the cookie cutter values.

 

[CelticJam]

IS it possible these engineers are using the percent of recommended compaction as the unit bulk weight as opposed to the lab evaluated data or possibly even the dry density unit weight? Sounds strange to me but their values come much closer.

 

[DRC1]

The Ka for clay is 1, assuming short term condition of phi = 0. As stiff clays drain, c will reduce and phi increase so that for stiff clays, the drained condition may control. However, the cohesion (c) provides tensile capacity in the soil which crates a zone of soil near the top that does not have lateral load. This depth is equal to 4c/gama* sqrt Ka
where gama equals total unit weight and Ka equals active earth pressure which for the short term case equals 1.
However all that being said, ussually foundations are not backfilled with clay. Usually the founadtion is over excavated and backfilled with a free draining granular material This material would have a unit wt of about 110 to 120 pcf. Ko (at rest coefficent) is typically .45 to .5. So 110pcf bacfill, the equivilent fluid pressure is 50 to 55.
Therfore I suspect the values you are seeing are for the granular backfill, not the clay.

 

[eric1037]

We generally recommend that granular backfill will be used and proper drainage will be utilized. Therefore, the equivalent fluid pressure is based on this assumption.

Re-read your geotech reports. I would bet they assume the same thing.

 

[CelticJam]

I have reread the reports and they are actually talking about using the clay removed from the excavation and in some cases on-site claystone recompacted to 90%. There is no recommendation for use of granular fill, however adequate surface drainage is specified. These reports are from several different firms and are about 5 years old.

 

[Focht3]

Are these values for restrained or unrestrained walls? Where are the sites?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[CelticJam]

How do you define restrained vs. unrestrained? If restrained means reinforced with a #5 steel grid with approximate 18 inches between bars (horizontal and vertical),foundation walls approximately 8 foot into the soil 8 inches thick and the foundations supported by approximate 8 to 10 inch diameter caissons 25 to 30 feet deep with counterforts approximately every 25 feet, then they would be restrained. Is that what you mean? These sites are located just west of Denver, Colorado in a fairly localized area.