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Buried Piping Frequently Asked Questions

Reference [1]: CAEPIPE User's Manual, v5.1J, 2003.

Q-1. How is buried piping modeled in CAEPIPE?

A-1. Soils in CAEPIPE are modeled based on Winkler's soil model of infinite closely spaced elastic springs. Soil stiffness is calculated for all three directions at each node. Pressure value in the load is suitably modified to consider the effect of static overburden soil pressure. Model is analyzed for operating (W+P1+T1) condition and the displacements in the three directions are noted. A check is made for whether skin friction is mobilized and the soil has attained the yield state. If true, then the spring is released in that direction indicating that soil no longer offers resistance in that direction. This modified model is again analyzed and checked for yield stage. The iterative process is continued till the percentage difference between displacement at each node for two successive iterations is less than 1%. The final stiffness is the true resistance offered by the soil to the pipe.

Q-2. What is the general Procedure to model buried piping?

A-2. One needs to do the following to model buried piping.

1. First define the types of soils you have (under Input>Misc menu>Soils),

2. Next, you tie these defined soils with pipe sections (in the Section dialog box got by Ctrl+L, Sections, double click on an empty row), you will see the item Soil in the bottom right corner.

3. Use this modified section for each row on the input screen that is buried with this soil around it.

Q-3. My piping model contains piping sections that are above ground and buried. Can I see only the buried sections of piping in CAEPIPE-graphics?

A-3. Yes. You can see only the buried sections by highlighting the section that is tied to the soil. You can use the Highlight feature in CAEPIPE under the Section List window and highlight the buried piping sections.

Q-4. Does CAEPIPE discretize the sections of buried piping?

A-4. Presently, no. Discretization is required near "regions of discontinuities." These regions are changes in directions (bends, tees), OR where piping exits to open space, OR piping enters the buried region. In these areas, the user needs to subdivide the model as finely as possible.

Q-5. Can you compare CAEPIPE's results with B31.1, Appendix VII?

A-5. There would be no point in performing a direct comparison with Appendix VII of B31.1 because CAEPIPE is not based on Appendix VII guidelines. CAEPIPE's method shares a few similarities with B31.1's "Non-mandatory procedures for the design of restrained underground piping."

CAEPIPE's method is based upon the two references mentioned later (Q-11) in this FAQ.

CAEPIPE does not use "pipe trench width" and "type of backfill" as given in Table VII-3.2.3. Soil density and Coefficient of friction are the same.

Q-6. What is a virtual anchor and how do I insert it in CAEPIPE?

A-6. The virtual anchor is a theoretical concept and need not be inserted as an anchor in CAEPIPE. It is "virtual." It is the point in the buried piping system where pipe is "virtually" restrained by the soil. To quote Appendix VII, it is the point "...where there is no relative motion between the pipe/soil interface."

Q-7. Is it possible to specify different soil characteristics for different portions of the pipe model?

A-7. Yes. It is possible to have different soils for different portions of the piping model. Here is how:

1. Define different soil types under Misc>Soils.
2. Then, create as many pipe sections as the number of soils.
3. Associate each soil type with a section (to be used for the different portions).
4. Model piping using the different sections for different portions of the model.

Note: For vertical sections of pipe with varying buried depths, you can use only one soil tied to one section and CAEPIPE will calculate appropriately.

Q-8. What is delta (units: degree)?

A-8. Delta is the standard physics definition for angle of friction, where tan (delta) = Friction coefficient.

Normal values for delta (angle of friction between soil and steel pipe) ranges between 25-45 degrees (for sand). Clean granular sand is 30 deg. With a mix of silt in it, the angle used is 25 deg.

Q-9. What is Ground Level?

A-9. Ground level is the height of the soil surface with respect to the global origin. It is NOT a measure of the depth of the pipe's centerline. For example, the height of the soil surface could be 8 feet above the model origin (see Buried Piping example in the User manual, pipe node 10 is defined at (0,-5,0) and ground level is +3'). So, the pipe is buried 8' deep into the soil.

The pipe centerline is calculated from the given data.

Q-10. What are some typical values for the coefficient Ks?

A-10. Ks can vary depending on the compaction of the soil [from 0.25 (for loose soil) to 1.0 (really compacted soil)].

Q-11. What references have been used for Buried piping formulations in CAEPIPE?

A--11. The Buried piping formulations in CAEPIPE are based on the following two works:

1. Tomlinson, M. J., Pile Design and Construction Practice. Fourth Edition. London: E & FN Spon, 1994.

2. Fleming, W.G.K., et al. Piling Engineering. Second Edition. Blackie Academic and Professional. (Chapters 4 and 5).

Q-12. Does CAEPIPE automatically subdivide long spans into shorter ones?

A-12. No. User should introduce intermediate nodes in his/her long pipe elements. This will also contribute to lowering vertical deflections. CAEPIPE does not presently break up long spans into shorter ones for buried piping. Download an illustrative model.

Q-13. Technical Notes on Buried piping

A-13. Soil in Buried piping analysis is modeled using bilinear restraints with an initial stiffness and an ultimate load. After the ultimate load is reached the displacement continues without any further increase in load, i.e. the yield stiffness is zero. The initial stiffness is calculated by dividing the ultimate load by the yield displacement which is assumed to be D/25, where D is outsided diameter of the pipe.

Nomenclature:

D = Outside diameter of pipe

Depth = Ground Level Avg. height of pipe element, (Depth > 0 for buried piping to have an effect)

Avg. height of pipe element = (Height of one end of pipe + height of other end of pipe)/2

Ks = Coefficient of horizontal soil stress which depends on the = relative density and state of consolidation of soil. Ks is empirical in nature and may be estimated from Nq/50.

Nq = Bearing capacity factor = 0.98414 exp (phi * 0.107311), phi is in degrees.

phi = delta + 5 (degrees)

delta = angle of friction between soil and pipe.

Sp = soil pressure = soil density * depth

Cs = Undrained cohesive strength (input for cohesive soil)

Af = Adhesion factor = 1.7012775 exp (-0.00833699*Cs). (Cs in = KN/M2) <= 1.0

kp = Coefficient of passive earth pressure = (1 + sin(phi)) / (1 - sin(phi))

bottom depth = depth + D/2; top depth = depth - D/2;

Nr = (Nq - 1.0) * tan(1.4*phi)

dq = dr = 1.0 + 0.1 * tan(PI/4 + phi/2) * depth / D for delta > = 10 deg otherwise dq = dr = 1.0

The ultimate loads (per unit length of pipe for axial and transverse = directions and per unit projected length of pipe for vertical direction) are = calculated as follows:

For cohesive (clayey) and cohesionless (sandy) soils different equations are used.

Axial direction:

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Cohesive soil:

Axial load = PI * D * Af * Cs;

Cohesionless soil:

Axial load = PI * D * Ks * Sp * tan(delta)

Transverse direction:

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Cohesive soil:

Transvers load = D * (2.0 * Cs + Sp + 1.5 * Cs * depth / D)

Cohesionless soil:

Transverse load = kp * kp * Sp * D;

Vertically downward direction:

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Cohesive soil:

Downward load = D * (5.7182 * Cs + Soil density * bottom depth)=20

Cohesionless soil:

Downward load = D * (Soil density * bottom depth * Nq * dq + 0.5*Soil density * D * Nr * dr)

Vertically upward direction:

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Cohesive soil:

Upward load = D * Soil density * top depth + 2 * Cs * topdepth

Cohesionless soil:

Upward load = D * Soil density * top depth

Buried pipes are defined by defining soils (under Misc menu) and specifying the soils for pipe sections under Sections.

The (soil) Name is used to identify the soil and to associate the soil with a pipe section. Soil density and Ground level are input for both cohesive and cohesionless soils. The ground level is used to calculate depth of the buried section. For cohesive soil, strength is the undrained cohesive strength (Cs). For cohesionless soil, Delta is angle of friction between soil and pipe and Ks is Coefficient of horizontal soil stress.