Sample methods for settlement evaluation
F.1 Stress-strain method
(1) The total settlement of a foundation on cohesive or non-cohesive soil may be evaluated using the stress-strain calculation method as follows:
- computing the stress distribution in the ground due to the loading from the foundation; this may be derived on the basis of elasticity theory, generally assuming homogeneous isotropic soil and a linear distribution of bearing pressure;
- computing the strain in the ground from the stresses using stiffness moduli values or other stress-strain relationships determined from laboratory tests (preferably calibrated against field tests), or field tests;
- integrating the vertical strains to find the settlements; to use the stress-strain method a sufficient number of points within the ground beneath the foundation should be selected and the stresses and strains computed at these points.
F.2 Adjusted elasticity method
(1) The total settlement of a foundation on cohesive or non-cohesive soil may be evaluated using elasticity theory and an equation of the form:
Em is the design value of the modulus of elasticity
f is the settlement coefficient
p is the bearing pressure, linearly distributed on the base of the foundation
and the other symbols defined in 1.6.
(2) The value of the settlement coefficient f depends on the shape and dimensions of the foundation area, the variation of stiffness with depth, the thickness of the compressible formation, Poisson's ratio, the distribution of the bearing pressure and the point for which the settlement is calculated.
(3) If no useful settlement results, measured on neighbouring similar structures in similar conditions are available, the design drained modulus Em of the deforming stratum for drained conditions may be estimated from the results of laboratory or in-situ tests.
(4) The adjusted elasticity method should only be used if the stresses in the ground are such that no significant yielding occurs and if the stress-strain behaviour of the ground may be considered to be linear. Great caution is required when using the adjusted elasticity method in the case of non-homogeneous ground.
F.3 Settlements without drainage
(1) The short-term components of settlement of a foundation, which occur without drainage, may be evaluated using either the stress-strain method or the adjusted elasticity method. The values adopted for the stiffness parameters (such as Em and Poisson's ratio) should in this case represent the undrained behaviour.
F.4 Settlements caused by consolidation
(1) To calculate the settlement caused by consolidation, a confined one-dimensional deformation of the soil may be assumed and the consolidation test curve is then used.
Addition of settlements in the undrained and consolidation state often leads to an overestimate of the total settlement, and empirical corrections may be applied.
F.5 Time-settlement behaviour
(1) With cohesive soils the rate of consolidation settlement before the end of the primary consolidation may be estimated approximately using consolidation parameters obtained from a compression test. However, the rate of consolidation settlement should preferably be obtained using permeability values obtained from in-situ tests.