Page 186 - Buried Pipe Design

160 Chapter Three

test whether they are on the primary loading curve or on the unloading-
reloading curve during the first iteration of the previous loading incre-
ment. If the results of the first iteration indicate that the soil element
is changing from one curve to another, the element condition is
flagged and the second iteration follows the same logic as the first,
except that the correct modulus is used to evaluate the elastic para-
meters based on the stresses from the final iteration of the previous
loading increment. The third iteration that follows uses the average
stresses to compute new element properties and responses to the cur-
rent loading increment. It appears that at least three iterations are
required if soil unloading-reloading is to be included. However, since
the results of the analysis reflect an equilibrium condition, neigh-
boring elements to those that changed their stress condition at the
first iteration may not have come to “equilibrium” at the end of the
third iteration, particularly if the resulting stresses of the second (or
later) iteration indicate that an element should change from one soil
model to another. Changing soil models is only permitted on the first
iteration. This may cause some difficulties in the strain compatibili-
ties of the solution.

Soil Input Properties. Execution of PIPE5 requires the user to prepare
a data file that contains all the mesh information and material prop-
erties. The data that are required in the input file include nodal coor-
dinates, element data, structural material and properties, soil
material properties (Table 3.10 lists the parameters required for the
soil model), construction sequence information, preexisting element
stresses, strains, displacements, and external loading information.
The data on the input file must be prepared according to specific for-
mats as given in the user’s manual.

Preexisting stresses. A convenient feature of PIPE is the specification
of preexisting elements. These elements may be soil, structure, or
interface elements. The preexisting elements are elements already in

TABLE 3.10 Summary of Required Soil Properties for the Hyperbolic Soil Model
Parameter Name Function
Modulus number
K, K ur Relate E i and E ur to
3
n ur Modulus exponent
c Cohesion intercept Relate (
1
3 ) f to
3

,
Friction angle parameters
Failure ratio
R f Relate (
1
3 ) ult to (
1
3 ) f
K b Bulk modulus number Value of B/P a at
3 P a
m Bulk modulus exponent Change in B/P a for 10-fold increase in
3
SOURCE: After Duncan et al. 6