CAESAR II Basic Model - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

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.

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.

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