Xdisp – How might I create a sub-surface ground movement curve from a surface ground movement curve, with movements proportioned linearly from the ground surface to the base of an excavation?
Warning: This answer provides methodology which should be used with careful engineering judgment. It will result in calculations of soil movements below the ground surface merely as a proportion of those at the surface and is unlikely to be accurate in many situations. It is recommended that sub-surface curves are derived instead from field data or from finite element model results as discussed in the tutorial Guidance on how to create a sub-surface curve using 2D FE Analysis.
If, after careful consideration, it is thought valid to model sub-surface ground movements as a simple proportion of the surface movements, then the following steps may be taken to create sub-surface ground movement curves from surface ground movement curves.
(1) Copy the chosen surface movement curve to a new user-defined curve.
(2) Select the new curve in the Ground Movement Curves dialog’s droplist and set its type to “Surface and sub-surface movements” and its curve fitting method to “Linear interpolation”
(3) Add a series of points representing zero movement at full wall/excavation depth. This is most simply done by copying the existing surface points (by highlighting the range of points in the data table, then right-click “Copy”, then right-click “Paste” into the first blank field at the end of the table.
(4) Modify the new values in the second column (“Depth/wall depth or max. excavation depth (y)”) to “1” (or to another value which represents the depth at which you wish movements to converge to zero, and the new values in the third column (“Settlement/wall depth…” or “Horizontal movement/wall depth…”) to “0”
(5) … and “Apply” the changes.
(5) Review the graph
With reference to the warning above, note the difference between the graph of a typical sub-surface movement curve created in this way (above) and the graph of a sample sub-surface movement curve created by a finite element analysis with more detailed knowledge and specification of the soil stratigraphy (below). A sample of the latter is provided with the program. Its derivation is described in the help file (see section “Sample Sub-surface Ground Movement Curve”). The comparison of these graphs shows that maximum movements may, in reality for some situations, occur below the ground surface, whereas the method described above will produce maximum movements at the ground surface.
Hence the need for caution when generating sub-surface movement curves using this generic method, and without more detailed data on which to base ground movement curves.