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

Integrated Publishing, Inc. |