Drilled Piers/Lateral

[Ibeam]

I'm looking at evaluating lateral load capacity for drilled piers w/ respect to soil support (not RC design). I've used in the past a paper published by PCA titled "Design of Concrete Foundation Piers" by Frank Randall, May 1968. Is anyone familiar with this paper? or have opinions on the accuracy of the procedure? Specifically I've used it for short piers with free-head. (situations such as light poles and pipe supports), the "R" value is the crux and I take it from the geotech report. Also, are there reliable approximations of R (lateral earth press. coeff.) available per soil type/blow count when not found on the report?

 

[Focht3]

Never heard of that paper. Is a free copy available on-line?

...the "R" value is the crux and I take it from the geotech report. Also, are there reliable approximations of R (lateral earth press. coeff.) available per soil type/blow count when not found on the report?

In short, "No." The paper must use a modulus of subgrade reaction, k, approach; there is no unique k-value for a specific soil and site - forget about "universal" values.

What are you designing? Where? Soil and site conditions?




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[Ibeam]

Site soil is silty sand (sm), medium dense to dense for upper 4 ft., dense clayey sand (sc) for another 3-4 ft. below that and finally light grey caliche in varying densities.

The items are 35' light poles. Will try a drilled shaft diameter of 2 ft.

Site conditions are dry, low water table, subject to hot temperatures and no freezing. Nevada, Zone 3.

Any help is always welcome but my questions were about the paper and the accuracy of the procedure therein.

I don't have a copy of the paper unfortunately.. but I do have a hard copy of the mathcad spreadsheet I made from it. I mis-spoke when referring to the "R" value. Now that I've been thinking about it, it probably came from a table in the paper (which I know is a big no-no in the geotech realm). My only guess is that it was suppose to be similar or the same as the lateral modulus (KH). The units are pcf (or psf/ft of depth).

Some of the Equations:

E = M/H
a = (4(E/L)+ 3) / (6(E/L)+4)
Ho = H/D
Mo = M/D
p = (1.18(4Mo + 3HoL)^2)/ (L^2(3Mo + 2HoL))
s = 9.42(2Mo + HoL)/ (L^2)
p critical = greater of p/ (a/2), or s/L in psf/ft.
f(R) = L^3 - L(14Ho/R) - (19Mo/R) , find min. R

Where:
E = Height above grade where Lateral Force is Applied.
M = Applied moment
H = Applied lateral force
L = Length of pier below grade
a = Point of Zero soil pressure along length of Pier
Ho = Lateral Force per unit pier diameter
Mo = Applied Moment per unit pier diameter
p = Max. soil pressure against upper portion of pier
s = Max. soil pressure at bottom of pier

 

[Focht3]

Some parts of your MathCad spreadsheet look familiar; others don't. Modulus of subgrade reaction k-values have units of FL^-3 - so I'm pretty sure that was the basis of the paper. Can you find the paper's citation?

I've done a fair amount of transmission and distribution line design, and a lot of experience with laterally loaded piles and piers. You have tough site conditions - cohesionless or low cohesion/brittle soils for the first 8 feet or so. No real soil resistance to overturning until you reach a depth of 4 feet - and those soils may be brittle.

I don't know what the area practice is, but I'd be reluctant to use an embedment that gave me a pole penetration into the caliche of less than 3 feet. That's about 11 feet - pretty deep for a 35 foot pole, but you are in an urban environment. (The poles aren't tall enough for a transmission line.) You have to worry about sharing the easement with water lines, fire lines, natural gas lines, sewer lines, sprinkler systems - some of which could be at least 6 feet deep and within 2 feet of the pole. And you have to worry about seismic forces, too.

How will the excavations be backfilled - cuttings, gravel, concrete or Polecrete?



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