Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 405

Many building codes and foundation authorities place restriction on differential settle-
ment. Terzaghi and Peck (1948) specify a permissible differential settlement of 20 mm be-
tween adjacent columns and recommend that foundations on sand be designed for a total
settlement of 25 mm. Skempton and MacDonald (1956) specify that the angular rotation or
distortion between adjacent columns in clay should not exceed 1/300, although the total settle-
ment may go up to 100 mm. Sowers (1957) recommends, in his discussion of the paper by
Polshin and Pokar (1957) a maximum differential settlement of 1/500 for brick buildings and
1/5000 for foundations of turbogenerators. Bozozuk (1962) summarised his investigations in
Ottawa as follows:

Angular rotation Damage

1/180 None

1/120 Slight

1/90 Moderate

1/50 Severe

The I.S.I. (IS: 1904-1961) recommends a permissible total settlement of 65 mm for iso-
lated foundations on clay, 40 mm for isolated foundations on sand, 65 to 100 mm for rafts on
clay and 40 to 65 mm for rafts on sand. The permissible differential settlement is 40 mm for
foundations on clay and 25 mm for foundations on sand. The angular distortion in the case of
large framed structures must not exceed 1/500 normally and 1/1000 if all kinds of minor dam-
age also are to be prevented.

Maximum and differential settlements as specified in IS: 1904-1978 ‘‘Code of Practice
for structural safety of Buildings: Shallow foundations (Second revision)’’ are shown in
Table 11.4.

Opinions on this subject vary considerably and were discussed by Rutledge (1964).

11.6.3 Remedial Measures Against Harmful Settlements

Settlement of soil is a natural phenomenon and may be considered to be unavoidable. How-
ever, a few remedial measures are possible against harmful settlement (Jumikis, 1962):

1. Removal of soft soil strata, consistent with economy.


2. The use of properly designed and constructed pile foundations (chapter 16).


3. Provision for lateral restraint against lateral expulsion of soil mass from underneath
   the footing of a foundation.


4. Building slowly on cohesive soils to avoid lateral expansion of a soil mass and to give
   time for the pore water to be expelled by the surcharge load.


5. Reduction of contact pressure on the soil; more appropriately, proper adjustment be-
   tween pressure, shape and size of the foundation in order to attain uniform settlements
   underneath the structure.


6. Preconsolidation of a building site long enough for the expected load, depending upon
   the tolerable settlements; alternatively, any other method of soil stabilization (chapter
   17).