The total settlement of a foundation comprises three parts as follows
Immediate settlement, Se, is that part of the total settlement, 5 , which is supposed to take place during the application of loading. The consolidation settlement is that part which is due to the expulsion of pore water from the voids and is time-dependent settlement. Secondary settlement normally starts with the completion of the consolidation. It means, during the stage of this settlement, the pore water pressure is zero and the settlement is only due to the distortion of the soil skeleton.
Footings founded in cohesionless soils reach almost the final settlement, 5, during the construction stage itself due to the high permeability of soil. The water in the voids is expelled simultaneously with the application of load and as such the immediate and consolidation settlements in such soils are rolled into one.
In cohesive soils under saturated conditions, there is no change in the water content during the stage of immediate settlement. The soil mass is deformed without any change in volume soon after the application of the load. This is due to the low permeability of the soil. With the advancement of time there will be gradual expulsion of water under the imposed excess load. The time required for the complete expulsion of water and to reach zero water pressure may be several years depending upon the permeability of the soil.
Consolidation settlement may take many years to reach its final stage. Secondary settlement is supposed to take place after the completion of the consolidation settlement, though in some of the organic soils there will be overlapping of the two settlements to a certain extent.
Immediate settlements of cohesive soils and the total settlement of cohesionless soils may be estimated from elastic theory. The stresses and displacements depend on the stress-strain characteristics of the underlying soil. A realistic analysis is difficult because these characteristics are nonlinear. Results from the theory of elasticity are generally used in practice, it being assumed that the soil is homogeneous and isotropic and there is a linear relationship between stress and strain. A linear stress-strain relationship is approximately true when the stress levels are low relative to the failure values. The use of elastic theory clearly involves considerable simplification of the real soil.
Figure 13.6 Overburden pressure and vertical stress distribution
Some of the results from elastic theory require knowledge of Young's modulus (Es), here called the compression or deformation modulus, Ed, and Poisson's ratio, μ, for the soil.
Seat of Settlement
Footings founded at a depth D, below the surface settle under the imposed loads due to the compressibility characteristics of the subsoil. The depth through which the soil is compressed depends upon the distribution of effective vertical pressure p'0 of the overburden and the vertical induced stress Δp resulting from the net foundation pressure qn as shown in Fig. 13.6.
In the case of deep compressible soils, the lowest level considered in the settlement analysis is the point where the vertical induced stress Δp is of the order of 0.1 to 0.2qn, where qn is the net pressure on the foundation from the superstructure. This depth works out to about 1.5 to 2 times the width of the footing.
The soil lying within this depth gets compressed due to the imposed foundation pressure and causes more than 80 percent of the settlement of the structure. This depth DS is called as the zone of significant stress. If the thickness of this zone is more than 3 m, the steps to be followed in the settlement analysis are
1. Divide the zone of significant stress into layers of thickness not exceeding 3 m,
2. Determine the effective overburden pressure p'o at the center of each layer,
3. Determine the increase in vertical stress Ap due to foundation pressure q at the center of each layer along the center line of the footing by the theory of elasticity,
4. Determine the average modulus of elasticity and other soil parameters for each of the layers.