Engineering and Construction

     a. Structures supported by shallow foundations or short piles may have to be underpinned if located near an excavation.  Techniques for underpinning are depicted in figure 11-4.  The most widely accepted methods are jacked down piles or piers, which have the advantage of forming positive contact with the building foundation since both can be prestressed.  The use of drilled piers is of more recent vintage and is more economical where it can be used.  In sandy soils, chemical injection stabilization may be used to underpin structures by forming a zone of hardened soil to support the foundation.

Figure 11-4.  Methods of underpinning.

     b. In carrying out an underpinning operation, important points to observe include the following:

          (1) Pits opened under the building must be as small as possible,  and survey monitoring of the building must be carried out in the areas of each pit to determine if damaging movements are occurring.
          (2) Care must be taken to prevent significant lifting of local areas of the building during jacking.
          (3) Concrete in piers must be allowed to set before any loading is applied.
          (4) Chemically stabilized sands must not be subject to creep under constant load.

     c. The decision to underpin is a difficult one because it is hard to estimate how much settlement a building can actually undergo before being damaged.

The values given in tables 5-2 and 5-3 may be used as guidelines.

Table 5-2.  Value of Angular Distortion (δ/ϑ)  That Can Be Tolerated Without Cracking

Table 5-3.  Empirical Correlations Between Maximum (∆) and Angular Distortion (δ/ϑ)