L C Peng – Assuming values for Longitudinal Friction Force, f, created by the soil for a buried pipe

[auzie5]

Reference:
L C Peng’s “Pipe Stress Engineering” book, Chapter 10 (Transportation Pipeline and Buried Piping), Section 10.2.3 (Potential Movement of Free Ends).

Equation 10.16:
L = F/f
Where,
L = virtual anchor length or active length
F = The anchor force
f = the longitudinal friction force per unit length created by the soil for a buried pipeline

Question:
I am curious what typical values for f (the longitudinal friction force per unit length created by the soil) designers would use for firm-to-stiff clay or clay till applications.

Background:
NPS 8 x 8.2mmWT pipeline.
Burial depth = 1.2m.
Design pressure = 9930 kPa.
Design Temp. = 40 deg C.
Install Temp. = -5 deg C.

Ceaser II calculates a virtual anchor length of ~90m. If I model an anchor before the riser, the anchor load is ~550 kN.

If I follow the L C Peng formulas, I calculate an anchor force of 751 kN. However, I’m not too sure what value to use for the longitudinal friction force in order to calculate the virtual anchor length using the L C Peng formula listed (above).

I have some research from a colleague that provides the following values for soil-to-pipe interaction parameters:

​

For a mineral soil type of firm to stiff clay or clay till with no peat; use 7 to 8 kN/m.

​

For a mineral soil type of firm to stiff clay or clay till with 1/3 peat; use 5 to 7 kN/m.

​

For a mineral soil type of any type with all peat; use 1 to 3 kN/m.

These values seem to align with the Ceaser II model since 550kN/90m = 6.1 kN/m (i.e. Ceaser II is effectively assuming a soil friction force of 6.1kN/m for clay).

Furthermore, if I assume a longitudinal friction force of 7 kN/m to use in my L C Peng formula, I would calculate a virtual anchor length of
L = F/f
L = 751 kN / 7 kN/m
L = 107.3m
This is not that far off from the Ceaser II virtual anchor length.

However, I have colleague playing Devil's Advocate who references the soil force formula recommended in the “Pipeline Rules of Thumb Handbook” (page 108, equation 5) which I am not familiar with:

F(soil) = 80*(Do/12)^2 lbf./ft
Where F(soil) is the soil resistance [lbf./ft] and Do is the pipe outside diameter [inch]. This formula is referenced from Wilbur, W. E., “Pipe Line Industry, February 1963".

For an NPS 8 line with 8.2mmWT, this Wilbur formula gives me a soil resistance, F(soil) of 0.6kN/m. This would lead me to calculate a virtual anchor length of ~1,240m (which is a far cry from the ~90m Ceaser II calculates).

Can anyone with familiarity with the L C Peng or the Wibur formulas please comment? Or can anyone familiar with typical values for soil resistance for firm-to-stiff clay applications please share or point me to a reference resource where some values are published? All other comments are also welcome.

Thanks in advance for any advice or insight.

 

[BigInch]

Corps of Engineers has the most experience with burial of pipes.
You can compare with Corps of Engineers EM 1110 2902 and with American Concrete Pipe Association method.
You'll probably get 15,000 N/m or so.

These American Lifelines and ASCE methods consider only an equivalent hydrostatic load from the dirt, like water. Nonsense.
As pipe moves through soil grains of soil nearby are forced into adjacent grains just ahead. That causes a wedging action that increases the friction force even more, which is not accounted for in water models.

Newer research by the University of British Columbia, among others, that are based on full scale load tests and models wedging effects tends to agree with the older Corps of Engineer methods, especially in sandy soils where good particle wedging action is observed.

You might also refer to Pipeline Rules of Thumb page 227 (attached) where I get something like 5000-8000 N/m.

 

[auzie5]

Thanks BigInch. The formula from the Pipeline Rules of Thumb is very similar to the LC Peng formula I found after posting the original request. I am now on the hunt for Corps of Engineers EM 1110 2902.

As always, very much appreciate your insight.

 

[BigInch]

Here,
link