Pipeline Drilled Pier Support Design

[JFMPE]

I am designing the pipeline and support system for a 20" diameter hot water (315 psig, 215 deg. F) pipeline. The pipeline will be above ground on 8" diameter pipe piers in 36" diameter drilled holes to various depths. This is a standard design for this type of application in this plant, with thousands of feet of piping on hundreds of drilled pier supports. However previous detailed calculation documents are almost non-existent.

I've been using the Embedded Pole Footing equation for allowable support loading calc's; (non constrained)
d= A/2(1+(1+4.36h/A)^.5), where A=2.34P/S1/b

This is a Seismic Zone 4 area. (Dynamic response loads for the pipeline design have been calculated for these conditions using CAESAR II.)

Hand calculations by others have used a Load Duration Factor of 1.33 for similar designs in the general area. This factor was used to multiply the allowable lateral soil loading, increasing the calculated support loads. How can I verify if this is OK?

[The soils report says for Drilled Pier Foundations;
-Soil Density; 120 pcf
-Allowable vertical bearing @ 5 ft depth; 2000 psf
-Rate increase per foot; 150 psf
-Max. not to exceed @ 20 ft; 4000 psf
-Allowable Positive skin friction per foot; 300 psf/ft
-Allowable Negative skin friction per foot; 200 psf/ft
-Friction angle of soil; 20 deg.
-Undrained shear strength of soil; 1000 psf
-Allowable lateral soil pressure per
foot of depth (UBC formula); 150 psf/ft.

According to the soils report, liquidfaction will not a problem for the project area.]

Thanks for your help.

JFMPE

 

[VoyageofDiscovery]

Hi JFMPE,

One should not be using a generic equation for a situation where soil conditions change. That equation may have been for a certain situation only. Typically an engineered solution would be to approach a geotechnical engineer on this.

With regard to the 1.33 you mention, I believe that is an allowable overload for a certain load combination. So I would not say that the loads can be increased and assumed for every loading situation, but it seems right for earthquake.

Regards

VOD

 

[VoyageofDiscovery]

Ah yes, I remember this equation, but the embedment cannot be over 12 ft for the purpose of computing lateral pressure.

Another caution is deflection limits for your piping system.

VOD

 

[JFMPE]

Thanks for you comments.

This equation was used by a geotechnical engineering firm (now gone)for a similar project very close to the project site, with identical soils reports, so it looked like a decent start. They were the ones who used the 1.33 factor to the loading conditions.

The pipeline deflections and related stresses are well accounted for, as the design is modeled for all load combinations on CAESAR II. It is a B-31.1 Code Piping system.

JFMPE