Chapter (5) Allowable Bearing Capacity and Settlement
Foundation Engineering Allowable Bearing Capacity and Settlement
Introduction As we discussed previously in Chapter 3, foundations should be designed for both shear failure and allowable settlement. So the allowable settlement of shallow foundations may control the allowable bearing capacity. The allowable settlement itself may be controlled by local building codes. For example; the maximum allowable settlement for mat foundation is 50 mm, and 25 mm for isolated footing. These foundations should be designed for these limiting values of settlement (by calculating the allowable bearing capacity from the allowable settlement). Thus, the allowable bearing capacity is the smaller of the following two conditions:
q (to control shear failure Ch.3) FS q = smallest of
q , (to control settlement)
In this Chapter, we will learn how to calculate the allowable bearing
capacity for settlement (q , ), but firstly we want to calculate the total settlement of the foundation. The settlement of a foundation can be divided into two major categories:
a) Immediate or elastic settlement ( ): Elastic or immediate settlement occurs during or immediately after the application of the load (construction of structure) without change in the moisture content of the soil.
b) Consolidation Settlement ( ): Consolidation settlement occur over time, such that pore water is extruded from the void spaces of saturated clayey soil submerged in water. Consolidation settlement comprises two phases: Primary and secondary.
To calculate foundation settlement (both elastic and consolidation), it is required to estimate the vertical stress increase in the soil mass due to the net load applied on the foundation (exactly as discussed previously in soil mechanics course “Ch.10” ). Hence, this chapter is divided into the following three parts: 1. Calculation of vertical stress increase (Ch.10 in soil mechanics course).
Page (82) Ahmed S. Al-Agha Foundation Engineering Allowable Bearing Capacity and Settlement
2. Elastic Settlement Calculation (Main topic of this chapter). 3. Consolidation settlement calculation (Ch.11 in soil mechanics course).
Vertical Stress Increase in a Soil Mass Caused by Foundation Load Stress Due to a Concentrated (Point) Load:
We calculate the vertical stress increase at any point at any depth due to the applied point load as following: Consider we want to calculate the vertical stress increase at point A in figure below:
3. P. Z ∆σ , = , and the same at any point. 2π(r + Z )
r = X + Y X , Y and Z are measured from the point of applied load as shown in figure above. Note: If there are more than one point load applied on the soil profile at
different positions , you should calculate ∆σ for each load and then : ∆σ , = ∆σ , + ∆σ , + ∆σ , + ⋯ + ∆σ ,
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Stress Due to a Circularly Loaded Area:
If we want to calculate the vertical stress below the center of the circular foundation or at any point at distance (r) from the center of the foundation, do the following: B Let R = = Radius of circular area 2 r = distance from the center of the foundation to the required point z = depth of point at which the vertical stress increas is required ∆ We can calculate the value of from (Table 5.1 P. 226) which gives the ∆ variation of with and For 0 ≤ ≤ 1 / / /
column load q = the stress at the base of the foundation foundation area