When you are constructing a new foundation near to the old building at that time because of vibrations and other forces which generate during construction can affect the stability of the existing structures. The depth of the foundation which is going to construct and the Foundation distances between the two buildings (old and New) is very important and necessary to reduce the failures.
The intensity of damage caused to the old structure depends upon two factors
1- Distance between the new and old structures
2- Depth of new foundations
In order to eliminate the chances of any damage to the old structure, the following precautions should be taken
1-The clear horizontal distance between adjacent faces of old and new foundations should be at least to the larger of the widths of foundations.
2- If the foundation of the new structure is to be laid touching the old structure, its foundation should not be carried to the depth greater than the depth of the old foundation.
If however, a new foundation has to be carried for large depth, the old structure should be adequately protected by giving lateral support to it and also by giving suitable support of the foundation.
3- For deciding the depth of new foundation up to which it can be excavated without affecting the old structure, draw an inclined line from the outer bottom-most edge of the old foundation. The depth of the new foundation should not cut this line. The inclination of this line with horizontal for soft soils is kept 30° and for average soils 45°.
4- The foundation of the new structure with reference to that of the old structure should be located after making a careful analysis of bearing capacity and settlement of soil.
