Most popular FAQs
The CIRIA C580 relationships have mostly been derived from ground movements measured during installation of piles and excavations that have been carried out in London Clay. Please refer back to CIRIA C580 to review whether the case studies used reflect the construction that is proposed to be modelled in Xdsip. The CIRIA relationships are not appropriate for sub-surface displacements.
There are two potential methods to model shafts in Xdisp. Both have limitations/drawbacks.
The first method is to define a shaft in the ‘embedded wall excavation’ function. An approximate shaft shape is defined in the excavation details tab. The user then specifies an appropriate relationship for surface and sub-surface ground movement. This could be based upon measured data, for example New and Bowers (1994), or from carrying out other analyses.
The alternative method is to specify a number of very short tunnel sections (say 1m length) stacked vertically on top of each other to the proposed depth of the shaft. The total volume of the tunnel sections should equal the total volume excavated for the shaft. Reasonably close matches have been achieved between measured settlement data from New and Bowers (1994) and the results from Xdisp. Generally a high k value and low volume loss has been used to achieve these correlations. No validation has been carried out to check whether sub-surface displacements or horizontal displacements are appropriate.
- New B. M. and Bowers K. H. (1994). Ground movement validation at the Heathrow Express Trial Tunnel, Proc. IMM Tunnelling ’94, Chapman and Hall, pp 301-327
Where soil is influenced by more than one excavation, the displacement may be greater than the sum of the calculated displacements. Soil stiffness is strain dependent, therefore later elements of work may give rise to larger displacements. See Nyren et al (2001).
- Nyren R J, Standing J R and Burland J B (2002). Surface displacement at St James’s Park Greenfield reference site above twin tunnels through the London Clay. Chapter 25 of CIRIA publication, Building response to tunnelling. Case studies from construction of the Jubilee Line Extension, London, Vol. 2 case studies.
Consider the corner of an excavation whose adjoining walls have different stiffnesses. Wall A is stiffer than Wall B.
The horizontal component of surface ground movement alongside Wall A is 10mm towards the excavation. The same movement alongside Wall B is 100mm towards the excavation. The area in the arc of the corner, shown grey in the plan below, experiences horizontal movements influenced by Walls A and B. What are the directions and magnitudes of those horizonal movements? CIRIA C580 gives no guidance. Xdisp 19.4 Build 6 uses a different approach to that of 19.4 Build 5.
Plan of Corner of Excavation
Xdisp 19.4 Build 5
Xdisp 19.4 Build 6
|Magnitude (of horizontal displacement)||Magnitude (of horizontal displacement)|
|Direction (of horizontal displacement)||Direction (of horizontal displacement)|
The output from Xdisp shown below illustrates the differences in the resulting horizontal displacements around such corners for both methods. In this example the north and west walls are stiffer than the south and east.
Xdisp 19.4 Build 5
Xdisp 19.4 Build 6
|Horizontal X Displacements||Horizontal X Displacements|
|Horizontal Y Displacements||Horizontal Y Displacements|
|Vertical Z Displacements (no difference)||Vertical Z Displacements (no difference)|
|Horizontal Displacements||Horizontal Displacements|
|Resultant Displacements||Resultant Displacements|
Note the discrete bulbs of movement around the south-west corner (for horizontal X displacements) and the north-east corner (for horizontal Y displacements) shown in the results from Xdisp 19.4 Build 5. These are the result of horizontal movements due to the less stiff walls influencing the results in the area of the corner. They do not occur in the revised method used in Xdisp 19.4 Build 6.
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.