GGU-SETTLE: Limiting depth

Of great importance for the size of settlements is the limiting depth. The program allows definition of limiting depth in three different ways:

• "Limiting depth = profile base"
  The settlements are calculated to the base of the defined soil profile.

• "Limiting depth = x * b"
  The settlements are calculated to a depth of x * foundation width, but not deeper than the profile base.

• "Limiting depth at p %"
  The settlements are calculated to a depth at which the overburden stress * p from the soil weight corresponds to the foundation stress.

The last possibility is also described in DIN 4019 (see at http://www.din.de). Here, a percentage value p of 20 % is given. This is the program default value. If you have selected this type of limiting depth calculation, the program calculates the overburden stress due to the soil weight for each foundation and compares this value to the average stress due to the foundation load at the characteristic point. The depth distribution of the unit weights for analysis of the overburden stress is determined in the foundation centre. Any excavation unloading for the foundation can be subtracted from the foundation stress, which the usual case.

For settlement analyses, only the stress from the foundation base to the limiting depth will be considered. Thus, for all points outside of the foundation, the limiting depth of the appropriate foundation is valid. Figure 3 shows, in a vertical section, how the settlements of a point, A, are composed if two foundations are defined.

If you divide a foundation into several smaller sub-foundations you should, according to theory, get the same settlement values. However, a problem then occurs when calculating limiting depths. The limiting depths of the sub-foundations is smaller due to the smaller widths. Because of the differing limiting depths you will also get different settlement values. To solve this contradiction the GGU-SETTLE program offers the possibility of calculating the limiting depth from the stress distribution of all foundations. If you select this possibility the stress distribution at the characteristic point is calculated from the stresses of all foundations. In this case you will get larger settlements and, depending on the system, more appropriate results.