7 Buried Pipe Modeler

SECTION 7

Buried Pipe Modeler Buried Pipe Modeler or Input > Underground takes an unburied layout and buries it. The modeler performs the following functions: 

Allows the direct input of soil properties. The modeler contains the equations for buried pipe stiffnesses. These equations are used to calculate the stiffnesses on a per length of pipe basis and then generate the restraints that simulate the discrete buried pipe restraint.



Breaks down straight and curved lengths of pipe to locate soil restraints using a zone concept. Where transverse bearing is a concern near bends, tees, and entry/exit points, soil restraints are located in close proximity.



Breaks down straight and curved pipe so that when axial loads dominate, soil restraints are spaced far apart.



Allows the direct entry of user-defined soil stiffnesses on a pipe-length basis. Input parameters include axial, transverse, upward, and downward stiffnesses, as well as ultimate loads. You can specify stiffnesses separately or in conjunction with CAESAR II’s automatically generated soil stiffnesses.

The Buried Pipe Modeler is designed to read a standard CAESAR II input data file that describes the basic layout of the piping system as if it was not buried. From this input, the software creates a second input data file that contains the buried pipe model. This second input file typically contains a much larger number of elements and restraints than the first job. The first file that serves as the pattern is called the original job. The second file that contains the element mesh refinement and the buried pipe restraints is called the buried job. CAESAR II names the buried file by appending the letter B to the name of the original job. The original job must already exist. During the process of creating the buried model, the modeler removes any restraints in the buried section. Any additional restraints in the buried section can be entered in the resulting buried model. The buried job, if it exists, is overwritten by the successful generation of a buried pipe model. It is the buried job that is eventually run to compute displacements and stresses. Typical buried pipe displacements are considerably different than similar above-ground displacements. Buried pipe deforms laterally in areas immediately adjacent to changes in directions, such as those found in bends and tees. In areas far removed from bends and tees, the deformation is primarily axial. The optimal size of an element, that is, the distance between a single FROM and a TO node, is dependent upon which of these deformation patterns is to be modeled. Because there is no continuous support model, the software must locate additional point supports along a line to simulate this continuous support. These additional point supports can also be user-defined. For a given stiffness per unit length, one of the following must be added: 

Several closely spaced, low stiffness supports



A limited number of distant and high stiffness supports

Where the deformation is lateral, smaller elements are needed to properly distribute the forces from the pipe to the soil. The length over which the pipe deflects laterally is called the "lateral bearing length" and can be calculated using the following equation:

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Buried Pipe Modeler Lb = 0.75(π) [4EI/Ktr] 0.25 Where: E

=

Pipe modulus of elasticity

l

=

Pipe moment of inertia

Ktr =

Transverse soil stiffness on a per length basis

CAESAR II places three elements in the vicinity of this bearing span to properly model the local load distribution. The bearing span lengths in a piping system are called the Zone 1 lengths. The intermediate lengths in a piping system are called Zone 2 lengths, and the axial displacement lengths in a piping system are called the Zone 3 lengths. To properly transmit axial loads, Zone 3 element lengths are computed using 100 x Do, where Do is the outside diameter of the piping. The Zone 2 mesh consists of four elements of increasing length; starting at 1.5 times the length of a Zone 1 element at its Zone 1 end, and progressing in equal increments to the last which is 50 x Do long at the Zone 3 end. CAESAR II views a typical piping system element breakdown or mesh distribution as shown below. All pipe density is set to zero for all pipe identified as buried so that deadweight causes no bending around these point supports.

CAESAR II automatically puts a Zone 1 mesh gradient at each side of the pipe framing into an elbow. You must tell CAESAR II where the other Zone 1 areas are located in the piping system. A critical part of the modeling of an underground piping system is the proper definition of Zone 1or lateral bearing regions. These bearing regions primarily occur: 

On either side of a change in direction.



For all pipes framing into an intersection.



At points where the pipe enters or leaves the soil.

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Buried Pipe Modeler 

Using any user-defined node within or near Zone 1.

Data Conversion CAESAR II converts the original job into the buried job by meshing the existing elements and adding soil restraints. The conversion process creates all of the necessary elements to satisfy the Zone 1, Zone 2, and Zone 3 requirements, and places restraints on the elements in these zones. All elbows are broken down into at least two curved sections, and very long radius elbows are broken down into segments whose lengths are not longer than the elements in the immediately adjacent Zone 1 pipe section. Node numbers are generated by adding “1” to the element’s FROM node number. The software checks a node number to make sure that is unique in the model. All densities on buried pipe elements are zeroed to simulate the continuous support of the pipe weight. A conversion log is also generated, which details the process in full.

See also Buried Pipe Modeler Window (on page 496) Soil Models (on page 501)

Buried Pipe Modeler Window To start the Buried Pipe Modeler, click Underground Pipe Modeler displays:

. The following window

Alternatively, you can click Input > Underground. The Buried Pipe Modeler window is used to enter the buried element descriptions for the job and allows you to define: 

Which part of the piping system is buried



Mesh spacing at specific element ends



Soil stiffnesses

The first two columns of the data input grid contain element node numbers for each piping element included in the original system. The next three columns allow you to describe the sections of the piping system that are buried and to define any required fine mesh areas.

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Buried Pipe Modeler A finer mesh area is necessary for buried areas that will need to undergo lateral displacements. The remaining eight columns are used to define soil stiffnesses and ultimate loads.

Buried Pipe Modeler Toolbar The Buried Pipe Molder toolbar displays icons for commonly-used commands. Open - Opens an input data file that will serve as the original job. Save - Creates an input data file that contains the buried pipe model. By default, the software appends the filename of the original job with the letter B to create the second input data file (the buried job). Print - Prints the data input from the Buried Pipe Modeler window. Soil Models - Opens the Basic Soil Modeler dialog box in which you specify soil properties for the CAESAR II buried pipe equations used by the software to generate one or more soil restraint systems. For more information, see Basic Soil Modeler Dialog Box (on page 507). Convert - Converts the original job into the buried job by meshing the existing elements and adding soil restraints. Find - Activates the search feature.

Change the Name of a Buried Pipe Job 1. Click File > Change Buried Pipe Job Name. 2. In the Change Job Name dialog box, type a new name for the buried pipe job and click OK. The software updates the name of the job.

From Node Displays the node number for the starting end of the element

To Node Displays the node number for the end of the piping element.

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Soil Model No. Defines which of the elements in the model are buried. 

If you enter 0, the element is not buried.



If you enter 1, then specify the buried soil stiffnesses per length basis in columns 6 through 13.



If you enter a number greater than 1, the software points to a CAESAR II soil restraint model generated using the equations outlined in Soil Models (on page 501). You can specify soil properties, such as buried depth, friction factor, undrained shear strength, using the Basic Soil Modeler dialog box (on page 507). The software uses these properties to calculate the buried soil stiffnesses on a stiffness per length basis. Because the soil properties can change from point-to-point along the pipeline, several different soil models can be entered for a single job. Each different soil model is given a unique soil model number starting with 2. Consider the following example: From Node

To Node

Soil Model No.

5

10

0

10

15

0

15

20

1

20

25

1

25

30

1

30

35

2

35

40

2

The pipe from nodes 5 through 15 is not buried. From nodes 15 through 30, you will specify your own stiffnesses (using columns 6 through 13 of the data input area). From nodes 35 through 40, the software will use the property values indicated in the corresponding soil model number to generate stiffnesses.

From/To End Mesh Indicates a fine mesh is needed at the From or To element end. Long, single elements that you enter need to be broken down into smaller elements to properly distribute the soil forces. The software performs this breakdown automatically. If the particular end of an element will undergo lateral displacement, it must have a finer mesh than an element end that only undergoes axial displacements. Axial displacement ends are at the end of a virtual anchor length. Element ends undergo lateral displacements wherever there is a bend at the end of the element. In this case, the software automatically places a fine mesh along the element entering the bend

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Buried Pipe Modeler and along the element leaving the bend. At all other locations, you must tell the software where the fine meshes must go. These locations include: 

1 - Element ends that frame into intersections.



2 - Element ends that enter or exit from the soil.



3 - Element ends where there is any change in direction not defined by a bend.

Follow the rule that too many mesh elements will never hurt the solution, whereas too few may produce incorrect results. Thus, always check the appropriate box if you are uncertain. Consider the following example:

CAESAR II places a fine mesh at the 5 end of the element because the pipe enters the soil at 5 and there are probably some displacements there. The software automatically places fine meshes at element ends where there are bends, so checking the FROM END MESH/TO END MESH boxes is not needed on the 10-15 element. A fine mesh is also placed at each element end that frames into the intersection at 20. Finally, a fine mesh is placed at the terminal points 35 and 30.

User-Defined Lateral "K" Specifies the soil stiffness perpendicular to the pipe axis on a stiffness per length basis. This stiffness value acts in both directions perpendicular to the pipe. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5.

Ultimate Lateral Load Specifies the ultimate lateral load carrying capacity of the soil on a force per length basis. It is at this point in the loading where the soil behavior becomes perfectly plastic. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5.

User-Defined Axial Stif Specifies the soil stiffness along the axis of the pipe on a stiffness per length basis. This stiffness value acts in both directions along the axis of the pipe. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5. To simulate a rigid, perfectly plastic soil for axial pipeline deformation, enter 1.0E12.

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Ultimate Axial Load Specifies the ultimate axial load carrying capacity of the soil on a force per length basis. It is at this point in the loading where the soil behavior becomes perfectly plastic. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5.

User-Defined Upward Stif Specifies the soil stiffness in the upward direction on a stiffness per length basis. The value that you enter is the stiffness that will resist upward displacement of the pipeline. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, you can leave this option blank. The smallest allowable non-zero value is 0.5. 

If the upward and downward stiffnesses are equal, then you need only enter a value for one--the stiffness value that is not entered defaults to the stiffness value that is entered.



If both User-Defined Upward Stif and User-Defined Downward Stif (on page 500) are set to 0 or left blank, a fatal error results.

Ultimate Upward Load Specifies the ultimate upward load carrying capacity of the soil on a force per length basis. The value you enter is the maximum resistance of the soil to an upward displacement of the pipeline. It is at this point in the loading where the soil behavior becomes perfectly plastic. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5. 

If the upward and downward ultimate loads are equal, then you need only enter a value for one. The other load defaults to the entered value.



If both Ultimate Upward Load and Ultimate Downward Load (on page 501) are set to 0 or left blank, a fatal error results.

User-Defined Downward Stif Specifies the soil stiffness in the downward direction on a stiffness per length basis. The value that you enter is the stiffness that will resist downward (-Y) displacement of the pipeline. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, you can leave this option blank. The smallest allowable non-zero value is 0.5. 

If the upward and downward stiffnesses are equal, then you need only enter a value for one. The other stiffness defaults to the entered value.



If both User-Defined Upward Stif (on page 500) and User-Defined Downward Stif are set to 0 or left blank, a fatal error results.

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Ultimate Downward Load Specifies the ultimate downward load carrying capacity of the soil on a force per length basis. The value you enter is the maximum resistance of the soil to a downward (-Y) displacement of the pipeline. It is at this point in the loading where the soil behavior becomes perfectly plastic. This option is required if Soil Model No. (on page 498) is set to 1; otherwise, leave this option blank. The smallest allowable non-zero value is 0.5. 

If the upward and downward ultimate loads are equal, then you need only enter a value for one. The other load defaults to the entered value.



If both Ultimate Upward Load (on page 500) and Ultimate Downward Load are set to 0 or left blank, a fatal error results.

Soil Models Only use the following procedures for estimating soil distributed stiffnesses and ultimate loads when you do not have better available data or methods suited. The soil restraint modeling algorithms used by the software are based on the following: 

CAESAR II Basic Model - “Stress Analysis Methods for Underground Pipelines,” L.C. Peng, published in 1978 in Pipeline Industry. For more information, see CAESAR II Basic Model (on page 502).



American Lifelines Alliance - "Appendix B: Soil Spring Representation" from the Guidelines for the Design of Buried Steel Pipe by the American Lifelines Alliance (http://www.americanlifelinesalliance.org/pdf/Update061305.pdf). For more information, see American Lifelines Alliance (see "American Lifelines Alliance Soil Model" on page 503).

Soil supports are modeled as bi-linear springs having an initial stiffness, an ultimate load, and a yield stiffness. The yield stiffness is typically set close to zero. After the ultimate load on the soil is reached, there is no further increase in load even though the displacement may continue. The axial and transverse ultimate loads must be calculated to analyze buried pipe. Many researchers differentiate between horizontal, upward, and downward transverse loads, but when the variance in predicted soil properties and methods are considered, this differentiation is often unwarranted. The software allows the explicit entry of these data if it is necessary to your specific project. After the axial and lateral ultimate loads are known, the stiffness in each direction can be determined by dividing the ultimate load by the yield displacement. Researchers have found that the yield displacement is related to both the buried depth and the pipe diameter. The calculated ultimate loads and stiffnesses are on a force per unit length of pipe basis.

See also Basic Soil Modeler Dialog Box (on page 507)

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CAESAR II Basic Model The following recommendations apply when you select CAESAR II Basic Model as the Soil Model Type in the Basic Soil Modeler dialog box. For more information about the dialog box and the available soil properties, see Basic Soil Modeler dialog box (on page 507). Either FRICTION COEFFICIENT or UNDRAINED SHEAR STRENGTH may be left blank. With clays, the friction coefficient is typically left blank and is automatically estimated by CAESAR II as Su/600 psf. Both sandy soils and clay-like soils can be defined here.

The soil restraint equations use these soil properties to generate restraint ultimate loads and stiffnesses. Defining a value for TEMPERATURE CHANGE is optional. If entered the thermal strain is used to compute and print the theoretical “virtual anchor length. These equations are: Axial Ultimate Load (Fax) Fax = μD[ (2ρsH) + (πρpt) + (πρf)(D/4) ] Where: μD = Friction coefficient, typical values are: 0.4 for silt 0.5 for sand 0.6 for gravel 0.6 for clay or Su/600 ρs= Soil density H = Buried depth to the top of pipe ρp= Pipe density t = Pipe nominal wall thickness ρf= Fluid density D = Pipe diameter Su = Undrained shear strength (specified for clay-like soils) Transverse Ultimate Load (Ftr) Ftr = 0.5ρs(H+D)2[tan(45 + φ/2)]2OCM If Su is given (that is, the soil is clay), then Ftr as calculated above is multiplied by Su/250 psf.

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Buried Pipe Modeler Where: φ = Angle of internal friction, typical values are: 27-45 for sand 26-35 for silt 0 for clay 

OVERBURDEN COMPACTION MULTIPLIER (OCM) is an artificial CAESAR II term that allows you to take a conservative approach when modeling uncertain soil response. Because a higher stiffness generally produces conservative results, you may wish to increase the transverse soil stiffness. CAESAR II uses the OCM to serve this purpose.



You can reduce the OCM from its default of 8 to values ranging from 5 to 7, depending on the degree of compaction of the backfill. There is no theory which suggests that the OCM cannot equal 1.0.



For a strict implementation of Peng's Theory as discussed in his articles (April 78 and May 78 issue of Pipeline Industry), use a value of 1.0 for the OCM. Yield Displacement (yd): yd = Yield Displacement Factor(H+D) The Yield Displacement Factor defaults to 0.015(suggested for H = 3D). Axial Stiffness (Kax) on a per length of pipe basis: Kax=Fax / yd Transverse Stiffness (Ktr) on a per length of pipe basis: Ktr=Ftr / yd

American Lifelines Alliance Soil Model The following information references "Appendix B: Soil Spring Representation" in the American Lifelines Alliance document Guidelines for the Design of Buried Steel Pipe (http://www.americanlifelinesalliance.org/pdf/Update061305.pdf). This document provides bilinear stiffness of soil for axial, lateral, uplift and bearing. Each stiffness term has a component associated with sandy soils (subscripted q) and a component associated with clays (subscripted c). Data can be entered for pure granular soils and pure clays. Soil stiffness for both clay and sand (cohesive and granular soils, respectively) are defined through the following user-defined parameters: c = soil cohesion representative of the soil backfill H = soil depth to top of pipe (this is converted by C2 to depth to pipe centerline in ALA calculations)  = effective unit weight of soil  = total dry unit weight of fill Ko = coefficient of earth pressure at rest (can be calculated based on internal friction angle of soil)

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Buried Pipe Modeler f = coating-dependent factor relating the internal friction angle of the soil to the friction angle at the soil-pipe interface φ = internal friction angle of soil

Elastic range of soil is either fixed or a function of D & H with limits based on D. Yield Displacement Factor

Entry

Limited by

Δt (dT) – Axial

Length units

―

Δp (dP) – Lateral

Multiple of D

0.04(H+D/2)

Δqu (dQu) – Upward

Multiple of H

Minimum

Δqu (dQu) – Upward

Multiple of D

Δqd (dQd) – Downward

Multiple of D

―

Axial

Tu = peak friction force at pipe-soil interface maximum axial soil force per unit length that can be transmitted to pipe)

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Buried Pipe Modeler D = pipe OD  = adhesion factor (for clays only)

c = soil cohesion representative of the soil backfill (undrained shear strength) H = depth of cover to pipe centerline = effective unit weight of soil Ko = coefficient of earth pressure at rest The ratio of the horizontal effective stress acting on a supporting structure and the vertical effective stress in the soil at that point. At rest indicates the pipe does not move for this calculation. δ = interface angle of friction for pipe and soil,  = f f = coating-dependent factor relating the internal friction angle of the soil to the friction angle at the soil-pipe interface Pipe Coating

f

Concrete

1.0

Coal Tar

0.9

Rough Steel

0.8

Smooth Steel

0.7

Fusion Bonded Epoxy

0.6

Polyethylene

0.6

 = internal friction angle of soil Δt = axial displacement to develop Tu = 0.1 inch for dense sand, 0.2 inch for loose sand, 0.3 inch for stiff clay, and 0.4 inch for soft clay

Lateral Pu = maximum horizontal soil bearing capacity (maximum lateral soil force per unit length that can be transmitted to pipe) Nch = horizontal soil bearing capacity factor for clay (0 for c=0)

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Buried Pipe Modeler Nqh = horizontal soil bearing capacity factor for sand (0 for =0°)

Factor

j

x

a

b

c

d

e

Nch

0°

H/D

6.752

0.065

-11.063

7.119

--

Nqh

20°

H/D

2.399

0.439

-0.03

1.059E-3

-1.754E-5

Nqh

25°

H/D

3.332

0.839

-0.090

5.606E-3

-1.319E-4

Nqh

30°

H/D

4.565

1.234

-0.089

4.275E-3

-9.159E-5

Nqh

35°

H/D

6.816

2.019

-0.146

7.651E-3

-1.683E-4

Nqh

40°

H/D*

10.959

1.783

0.045

-5.425E-3 1.153E-4* *

Nqh

45°

H/D*

17.658

3.309

0.048

-6.443E-3 1.299E-4* *

*CAESAR II limits the height/diameter (H/D) ratio to a maximum of 20 for angles at 40 to 45 degrees. The software calculates any values specified that result in a ratio that is greater than 20 as equal to 20. **The American Lifelines Alliance standard lists the horizontal soil bearing capacity factor for sand (N qh) as a negative value for both 40 and 45 degree angles. This results in negative yield load values. CAESAR II calculates these values as a positive value, as shown in the previous table.

Nqh can be interpolated for φ between 20°and 45°.

Vertical Uplift

Qu = maximum vertical upward soil bearing capacity (maximum vertical uplift soil force per unit length that can be transmitted to pipe) Ncv = vertical upward soil bearing capacity factor for clay (0 for c=0)

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Nqv = vertical upward soil bearing capacity factor for sand

= 0.01H to 0.02H for dense to loose sands < 0.1D = 0.1H to 0.2H for stiff to soft clays < 0.2D

Vertical Bearing

Qd - maximum vertical bearing soil force per unit length that can be transmitted to pipe. Nc, Nq, N = vertical downward soil bearing capacity factors

 = total dry unit weight of fill qd = vertical displacement to develop Q d = 0.1D for granular soils = 0.2D for cohesive soils

Basic Soil Modeler Dialog Box Soil Models specifies options for the soil model method to use and defines basic soil properties, such as undrained sheer strength, friction angles, and so forth. The modeler uses the values that you define to compute axial, lateral, upward, and downward stiffnesses, along with ultimate loads. Each set of soil properties is identified by a unique soil model number, starting with the number 2. The soil model number is used in the buried element descriptions to tell CAESAR II in what type of soil the pipe is buried. You can enter up to 15 different soil model numbers in any one buried pipe job. 

Soil model number 1 is reserved for user-defined values.



The soil models you enter do not have to be used in the current job. This provides a convenient mechanism for soil property range studies.

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Soil Model Type and Classification Select the soil model method on which the software will base its calculations. Three different soil model methods are available, each with its own set of soil properties. 

American Lifelines Alliance (Sand/Gravel) - This is the default model is that is presented for granular soils in "Appendix B" of the America Lifelines Alliance document Guidelines for the Design of Buried Steel Pipe. This model was developed jointly by the American Society of Civil Engineers and the Federal Emergency Management Agency in July 2001 (addenda through February 2005.



American Lifelines Alliance (Clay) - This model is for clay soils and from the same document as American Lifelines Alliance (Sand/Gravel).



CAESAR II Basic Model - A modified implementation of the method described by L.C. Peng in his two-part article "Stress Analysis Methods for Underground Pipe Lines", published in Pipe Line Industry (April/May 1978). For more information, see Soil Models (on page 501).

ALPHA - ADHESION FACTOR Specifies the soil adhesion factor. This option displays only when you select American Lifelines Alliance in the Soil Model Type list and Clay as the Soil Classification. If no value is defined, the soil adhesion factor is calculated using C - SOIL COHESION OF BACKFILL based upon the following equation: Alpha = 0.608-0.123C-0.274/(C**2+1)+0.695/(C**3+1) Where C is in kips/sq.ft. Possible values are listed in Figure B.2, "Appendix B: Soil Spring Representation" from the Guidelines for the Design of Buried Steel Pipe by the American Lifelines Alliance

C - SOIL COHESION OF BACKFILL Specifies the soil cohesion representative of the backfill. This option displays only when you select American Lifelines Alliance in the Soil Model Type list and Clay as the Soil Classification. Typical values for cohesive soils are between 2.5 and 20 psi (18 and 140kPa).

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dP - YIELD DISP FACTOR, LAT, MAX MULTIPLE OF D Specifies the value of the soil displacement at which the ultimate lateral restraint load is developed. This is calculated using as the following equation: dP = 0.4 (H + D/2) However, the calculated value must be limited to a maximum multiple for the pipe outer diameter (D). Typical values are between 0.1 and 0.15.

dQd - YIELD DISP FACTOR, DOWN, MULTIPLE OF D Specifies the value of the soil displacement at which the ultimate downward restraint load is development. This value is calculated as a multiple of the pipe outer diameter (D). Typical values are as follows: 

Granular soils - 0.1



Cohesive soils - 0.2

dQu - YIELD DISP FACTOR, UP, MAX MULTIPLE OF D Specifies the value of the soil displacement at which the ultimate upward restraint load is developed. This value is calculated as per the following equation: dQu = MIN (MULTIPLE OF H) * H, (MULTIPLE OF D) * D) The maximum multiple of the pipe outer diameter (D), must be entered here. Typical values are as follows: 

Sand - 0.1



Clay - 0.2

dQu - YIELD DISP FACTOR, UPWARD, MULTIPLE of H Specifies the value of the soil displacement at which the ultimate upward restraint load is developed. This value is calculated as per the following equation: dQu - MIN (MULTIPLE OF H) * H, (MULTIPLE OF D) * D The maximum multiple of the pipe buried depth (H) must be entered here. Typical values are as follows: 

Dense Sand - 0.01



Loose Sand - 0.02



Stiff Clay - 0.1



Soft Clay - 0.2

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dT - YIELD DISP FACTOR, AXIAL Specifies the value of the soil displacement at which the ultimate axial restraint load is developed. This option displays only when you select American Lifeline Alliance in the Soil Model Type list. Typical values are as follows: 

Dense Sand - 0.1 in. (2.5 mm.)



Loose Sand - 0.2 in (5.0 mm.)



Stiff Clay - 0.3 in. (7.5 mm.)



Soft Clay - 0.4 in. (10 mm.)

GAMMA - DRY SOIL DENSITY Specifies the dry density of the soil on a per unit volume basis. This option displays only if you select American Lifeline Alliance in the Soil Model Type list and Sand/Gravel as the Soil Classification. Typical soil densities are listed below: Soil

Dry Density 4.33E-2 lb/cu.in.

Clay

Very Loose Sand

=

4.35E-2 lb/cu.in.

>=

1.206E-3 kg/cu.cm.

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F - COATING FACTOR Specifies the coating dependent factor that relates the internal friction angle of the soil to the friction angle at the soil-pipe interface. This option displays only if you select American Lifeline Alliance in the Soil Model Type list and Sand/Gravel as the Soil Classification. Typical values for external pipe coatings are: 

Concrete - 1.0



Coal Tar - 0.9



Rough Steel - 0.8



Smooth Steel - 0.7



Fusion Bonded Epoxy - 0.6



Polyethelyne - 0.6

FRICT. ANGLE Specifies the internal friction angle of the soil. Typical values are: 

Clay - 0



Silt - 26-25



Sand - 27-45



For the American Lifelines Alliance soil model, this entry must be between 20- and 45-degrees.



For the CAESAR II basic soil model, this entry is used in the soil restraint equations to generate restraint ultimate loads and stiffnesses.

FRICTION COEFFICIENT Specifies the coefficient of friction between pipe and soil. If the undrained shear strength (on page 514) is entered, the friction coefficient may be left blank. The friction coefficient is calculated using the following equation: Friction Coeffecient = Su/0.4167E + 1 Typical friction coefficient values are: 

Silt - 0.4



Sand - 0.5



Gravel - 0.6



Clay - 0.6 or Su/ 0.4167E + 1

This option displays only when you select CAESAR II Basic Model in the Soil Model Type list.

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H - BURIED DEPTH TO TOP OF PIPE Specifies the buried pipe depth to the top of the pipe. This option displays only when you select American Lifelines Alliance in the Soil Model Type list. The American Lifetime Alliance method actually defines H as "depth to pipe centerline". CAESAR II automatically converts this based upon the individual pipe sizes. Upward soil stiffness calculations are considered to be applicable for H/D ratios of 10 and below.

KO - COEFFICIENT OF PRESSURE AT REST Specifies the coefficient of earth pressure. This option displays only if you select American Lifeline Alliance in the Soil Model Type list and Sand/Gravel as the Soil Classification. Typical values are on the order of 1.0. If left blank, K0 defaults to the following: K0 - 1.0 - sin(internal friction angle of the soil) The internal friction angle of the soil is defined by FRICT. ANGLE (Sand=27-45; Silt=26-35; Clay=0) (deg.) (see "FRICT. ANGLE" on page 512).

OVERBURDEN COMPACTION MULTIPLIER Specifies the factor by which the transverse ultimate load is multiplied. This option displays only when you select CAESAR II Basic Model in the Soil Model Type list. This value is used in the soil restraint equations to generate restraint ultimate loads and stiffnesses. The default value is 8. This number can be reduced depending on the degree of compaction of the backfill. Backfill efficiency can be approximated using the proctor number, defined in most soils text books. Standard practice is to multiple the proctor number by 8 and use the result as the compaction multiplier.

SOIL DENSITY Specifies the weight of the soil on a per unit volume basis. This value is used in the soil restraint equations to generate restraint ultimate loads and stiffnesses. This option displays only when you select CAESAR II Basic Model in the Soil Model Type list.

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TEMPERATURE CHANGE Specifies the installed to operating pipe temperature difference. The thermal expansion coefficient and the temperature change inputs are required if you want CAESAR II to calculate and display the virtual anchor length.

THERMAL EXPANSION COEFFICIENT Specifies the pipe thermal expansion coefficient multiplied by 1E06. The thermal expansion coefficient and the temperature change inputs are required if you want CAESAR II to calculate and display the virtual anchor length.

UNDRAINED SHEAR STRENGTH Specifies the undrained shear strength. This option displays only when you select CAESAR II Basic Model in the Soil Model Type list. You can leave this option blank if Friction Coefficient is defined.

YIELD DISPLACEMENT FACTOR Specifies the value used to calculate the soil restraint stiffness. This value must be greater than 0.0. This option displays only when you select CAESAR II Basic Model in the Soil Model Type list. The yield displacement factor is inversely proportional to the soil restraint stiffness. By default, the yield displacement depth of 1.5% of the buried depth is used, which translates to a yield displacement factor of 0.015.

Model an underground piping system The recommended workflow for using the Buried Pipe Modeler is outlined in the steps below. A buried pipe example problem is provided to illustrate the features of the modeler. This example should not be considered a guide for recommended underground piping design. For more information, see Buried Pipe Example (on page 515). 1. Click Underground Pipe Modeler Underground to open the modeler.

on the CAESAR II toolbar or click Input >

2. Click File > Open on the Buried Pipe Modeler main menu and select the original unburied job. The original job serves as the basis for the buried pipe model. It must already exist and need only contain the basic geometry of the piping system. The modeler will remove any existing restraints in the buried portion. 3. Click Soil Models

on the Buried Pipe Modeler toolbar.

4. In the Basic Soil Modeler dialog box, select a Soil Model Type. The software populates the dialog box with soil data properties specific to the soil model you select.

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Buried Pipe Modeler 5. Enter the necessary soil data and click OK to exit the dialog box. To enter additional soil models, click Add New Soil Model. The software saves the soil data in a file with the extension SOI. 6. In columns 1-5 of the buried element data input area, describe the sections of the piping system that are buried and define any required fine mesh areas and click Save . User-defined soil data can be entered in columns 6-13. 7. On the Buried Pipe Modeler toolbar, click Convert to convert the original model into the buried model. This step produces a detailed description of the conversion. By default, the software appends the name of the job with the letter B. For example, if the original job is named UndergroundPipe, the software saves the second input file with the name UndergroundPipe B. If the default name is not appropriate, click File > Change Buried Pipe Job Name and rename the buried job. 8. Click File > Exit to return the CAESAR II main window. From here, you can use Input > Piping to review and edit the buried model, add any additional underground restraints (such as thrust block) to the buried model, and perform the analysis of the buried pipe job. 

A buried pipe example problem is provided to illustrate the features of the modeler. This example should not be considered a guide for recommended underground piping design. For more information, see Buried Pipe Example (on page 515).

Buried Pipe Example The following buried pipe example problem is provided to illustrate the features of the modeler. This example should not be considered a guide for recommended underground piping design. Consider the following example:

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Buried Pipe Modeler The following input listing represents the unburied model shown above.

Terminal nodes 100 and 1900 are above ground. Nodes 1250 and 1650 (on the sloped runs) mark the soil entry and exit points.

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Buried Pipe Modeler Using the Basic Soil Modeler dialog box (on page 507), Soil Model Number 2 properties for a sandy soil is defined.

Elements 1250-1300 through 1600-1650 are buried using soil model number 2. Zone 1 meshing is indicated at the entry and exit points.

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Buried Pipe Modeler Clicking Convert model.

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on the Buried Pipe Modeler toolbar begins the conversion to a buried

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Buried Pipe Modeler The screen listing can also be printed.

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Buried Pipe Modeler The original unburied model is shown along with the buried model below. Restraints have been added around the elbows and along the straight runs.

Bi-linear restraints have been added to the buried model. The stiffness used is based upon the distance between nodes.

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Buried Pipe Modeler The first buried element, 1250-1251, has no density.

You can now analyze the buried job.

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