4. EFFECT OF STRUCTURE RIGIDITY. Computed differential settlement is less
accurate than computed total or average settlement because the interaction
between the foundation elements and the supporting soil is difficult to
predict. Complete rigidity implies uniform settlement and thus no
differential settlement. Complete flexibility implies uniform contact
pressure between the mat and the soil. Actual conditions are always in
between the two extreme conditions. However, depending on the magnitude of
relative stiffness as defined below, mats can be defined as rigid or
flexible for practical purposes.
a. Uniformly Loaded Circular Raft. In the case where the raft has a
frictionless contact with an elastic half space (as soil is generally
assumed to represent), the relative stiffness is defined as
[retrieve Equation]
R = radius of the raft, t = thickness of raft, subscripts r and s refer to
raft and soil, [upsilon] = Poission's ratio and E = Young's modulus.
For K+r, < /= 0.08, raft is considered flexible and for K+r, < /= 5.0
raft is considered rigid.
For intermediate stiffness values see Reference 13, Numerical
Analyses of Uniformly Loaded Circular Rafts on Elastic Layers of Finite
Depth, by Brown.
b. Uniformly Loaded Rectangular Raft. For frictionless contact between
the raft and soil, the stiffness factor is defined as:
[retrieve Equation]
B = width of the foundation.
Other symbols are defined in (a).
For K+r, < /= 0.05, raft is considered flexible and for K+r, >/= 10,
raft is considered rigid.
For intermediate stiffness values see Reference 14, Numerical
Analysis of Rectangular Raft on Layered Foundations, by Frazer and Wardle.
Section 6.
METHODS OF REDUCING OR ACCELERATING SETTLEMENT
1. GENERAL. See Table 6 for methods of minimizing consolidation
settlements. These include removal or displacement of compressible material
and preconsolidation in advance of final construction.
2. REMOVAL OF COMPRESSIBLE SOILS. Consider excavation or displacement of
compressible materials for stabilization of fills that must be placed over
soft strata.
7.1-241
Previous Page
