4. SANITARY LANDFILL. Foundations on sanitary landfills will undergo
extensive settlements, both total and distortional, which are extremely
difficult to predict. Settlements result not only from compression of the
underlying materials, but also from the decomposition of organic matter.
Gases in landfill areas are health and fire hazards. A thorough study
isnecessary when utilizing sanitary landfill areas for foundations. Further
guidance is given in DM-7.3, Chapter 3.
5. PEAT AND ORGANIC SOILS. Settlements in these soils are computed in a
similar manner as for fine-grained soils. However, the primary
consolidation takes place rapidly and the secondary compression continues
for a long period of time and contributes much more to the total settlement.
TOLERABLE AND DIFFERENTIAL SETTLEMENT
1. APPLICATIONS. For an important structure, compute total settlement at a
sufficient number of points to establish the overall settlement pattern.
From this pattern, determine the maximum scope of the settlement profile or
the greatest difference in settlement between adjacent foundation units.
2. APPROXIMATE VALUES. Because of natural variation of soil properties and
uncertainty on the rigidity of structure and thus actual loads transmitted
to foundation units, empirical relationships have been suggested to estimate
the differential settlements (or angular distortion) in terms of total
settlement (see Reference 11, Structure Soil Interaction, by Institution of
Civil Engineers). Terzaghi and Peck (Reference 8, page 489) suggested that
for footings on sand, differential settlement is unlikely to exceed 75% of
the total settlement. For clays, differential settlement may in some cases
approach the total settlement.
a. Criteria. Differential settlements and associated rotations and
tilt may cause structural damage and could impair the serviceability and
function of a given structure. Under certain conditions, differential
settlements could undermine the stability of the structure and cause
structural failure. Table 4 (Reference 12, Allowable Settlements of
Structures, by Bjerrum) provides some guidelines to evaluate the effect of
settlement on most structures. Table 5 provides guidelines for tanks and
b. Reduction of Differential Settlement Effects. For methods of
reducing or accelerating consolidation settlements, see Section 6.
Settlement that can be completed during the early stages of construction,
before placing sensitive finishes, generally will not contribute to
structural distress. In buildings with light frames where large
differential settlements may not harm the frame, make special provisions to
avoid damage to utilities or operating equipment. Isolate sensitive
equipment, such as motor-generator sets within the structure, on separate
rigidly supported foundations. Provide flexible couplings for utility lines
at critical locations.