Meena Rezkallah, P.Eng.
Updated: Dec 20, 2020
CAESAR II provides an automatic underground pipe modeler, accessed from the main menu, which markedly simplifies this modeling process. The underground modeler provides two services to the user:
If soil properties are not known, or if a good mathematical model of the soil is not available, CAESAR II provides a default soil model that may be used to approximate "typical" soil support characteristics.
Given soil support stiffnesses either from user input or from the default model, CAESAR II distributes the buried restraint stiffnesses over the buried part of the piping system. This is probably the most useful part of the buried pipe modeler. Properly breaking down the model into a finer element mesh and distributing restraints over the piping system is a very time consuming task to do accurately by hand, which the buried pipe modeler can do in seconds. The distribution of restraint stiffnesses over lateral bearing lengths, transition lengths, and over axial bearing lengths is described in detail in the CAESAR II user's manual.
Seldom are soil properties known very accurately. Often there is absolutely no quantitative data available on the soil at the site. In these situations, the default soil model will probably provide as good an estimate of the actual soil properties as any. This model is based on a combination of data available from a variety of sources (many from research on driven piles). After evaluating this data, it was determined that there was insufficient accuracy to differentiate between the horizontal and vertical soil restraint stiffnesses.
Intuitively, it seems plausible that the downward stiffness of a buried pipe will be greater than both the lateral and upward stiffnesses, and the lateral stiffnesses will be greater than the upward stiffness, which will vary according to the buried depth. When there is good soil data and a good soil model available, it should be used in place of CAESAR II's default soil model. These numbers can be input directly to the buried pipe modeler. This improved model can take into account the differences among lateral, upward, and downward distributed soil stiffnesses.
The underground piping modeler provides the user the opportunity to enter the following soil properties:
FRICTION COEFFICIENT
SOIL DENSITY
BURIED DEPTH TO TOP OF PIPE
FRICTION ANGLE
UNDRAINED SHEAR STRENGTH
OVERBURDEN COMPACTION MULTIPLIER
THERMAL EXPANSION COEFFICIENT
TEMPERATURE CHANGE (Installed to operating)
Note that the overburden compaction multiplier is a degree of compaction of the backfill, and is calculated by multiplying the Proctor Number (a measure of backfill efficiency defined in most soils textbooks as a ratio of unit weights) by 8. The underground pipe modeler defaults to a value of the overburden compaction multiplier of 8; this results in somewhat conservative restraint stiffnesses. Under common practice, this value is often reduced to somewhere in the range from 5 to 7.
From this data, the underground pipe modeler calculates the axial friction loads, the transverse ultimate load and elastic stiffness, and automatically inserts the appropriate restraints, adding additional nodes as necessary. A diagram of a pipe for which soil restraints were generated by the underground piping modeler is shown in Figure 3-120.
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