Question #1:

What is an adequate spacing required for piping points (nodes) and soil points?

Answer:

There is no one answer to this question. See AutoPIPE's online help for suggested soil spring spacing based on Zone 1, Zone 2, or Zone 3 piping. These are only suggestions may need it increase/decrease soil points as required for convergence issues.

Question #2:

The more the nodes, some of the load cases (temperature, pressure and user-defined) do not converge

Answer:

If a model does not converge, please see the following AutoPIPE help section:
Help > Contents> Search Tab> enter "Failure to Converge" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information. 

Review the *.LOG file to understand where the failure is occuring and address the location. See omline help for details about the *.LOG file.

Question #3:

What is the most soil deformation that AutoPIPE can handle?

Answer:

Soil Deformations
This can be a difficult problem, since the settlement is such a large soil deformation of 1600mm All beam theory  piping analysis programs such as Autopipe, Caesar, pipeplus are based on small deformation theory and large displacements can give an ill-conditioned solution and erroneous results.

As an example, large deformation across a large span of 200m. The maximum slope of the pipe may be of the order of 0.025 radians and the solution should be ok

As a check that Autopipe can handle these large deformations use the following rule of thumb. Check that the maximum slope angle in radians of the deformed pipe = approx. sin (slope angle) then the solution should be ok.

One method to simulate subsidence of known amount e.g 25mm see below. Set the yield P1 = 25 as shown below and K1 (initial) value = 0.001 and calculated soil stiffness = K2 (final)

                     SOIL PROPERTIES
   Soil                     Initial K   yield P       Final K    Yield disp
    ID           Dirn       (N/mm/mm )  (N/m    )     (N/mm/mm )   (mm    )
  ------      -------      ----------  ---------  ----------  ---------- 
  SOIL1         Horiz.        6.540   699861.00      0.000       07.0118 
                Long         17.540    43859.00      0.000        2.5005
                Vert Up       2.790   139303.00      0.000       49.9291
                Vert Dn       0.001       25.00     23.030       24.9999

One method:
If the maximum settlement is unknown, then  model the soft waterlogged soil over the known span and known firmer soil at  ends. Apply the over-burden sand weight as UDL load or vertical wind load (to simulate the pressure of the 4m sand) on the pipe and observe the deformation and check the resulting maximum slope.

Note: ensure the final soil stiffness values K2 is not = 0 but a finite value e.g  0.001. The vertical down K2 value should be known (ie final soil conditions after settlement) and the P1 (soil yield ) value can be 'tweeked' to  achieve the distance between the soft and final soil condition. Check that the actual pipe movement reaches the final soil condition otherwise the solution may become ill-conditioned.
 
Maybe the sand has a influence on calculating the upward soil stiffness but this direction should not be important..