Chapter 7: Settlement of Shallow Foundations

1 Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 1 Chapter 7: Settlement of Shallow Foundations Introduction The Settlement of a Shallow foundation can be divided into two major categories: (a) elastic, or immediate Settlement and (b) consolidation Settlement . Immediate, or elastic Settlement Immediate, or elastic Settlement of a foundation takes place during or immediately after the construction of the structure.

2 Consolidation Settlement Consolidation Settlement occurs over time. Pore water is extruded from the void spaces of saturated clayey soils submerged in water . The total Settlement of a foundation is the sum of the elastic Settlement and the consolidation Settlement . Consolidation Settlement comprises two phases: primary and secondary. The fundamentals of primary consolidation Settlement were explained in detail in Chapter 2. Secondary consolidation Settlement occurs after the completion of primary consolidation caused by slippage and reorientation of soil particles under a sustained load.

3 Primary consolidation Settlement is more significant than secondary Settlement in inorganic clays and silty soils. However, in organic soils, secondary consolidation Settlement is more significant. Elastic Settlement of Shallow Foundation on Saturated Clay ( = ) The average Settlement of flexible Foundations on saturated clay soils (Poisson s ratio, = ). Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 2 Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr.

4 Yasser M. Almadhoun Page 3 Example See example in textbook, page 340. Elastic Settlement in Granular Soil Settlement Based on the Theory of Elasticity Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 4 Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 5 To calculate Settlement at the centre of the foundation, use: Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr.

5 Yasser M. Almadhoun Page 6 To calculate Settlement at a corner of the foundation, use: The elastic Settlement of a rigid foundation can be estimated as: Due to the nonhomogeneous nature of soil deposits, the magnitude of Es may vary with depth. For that reason, Bowles (1987) recommended using a weighted average of Es: Example See example in textbook, page 309. Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr.

6 Yasser M. Almadhoun Page 7 Settlement of Sandy Soil: Use of Strain Influence Factor The recommended variation of the strain influence factor Iz for square (L/B =1) or circular Foundations and for Foundations with L/B = 10. Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 8 Note that the maximum value of Iz [ that is, Iz(m) ] occurs at z = z1 and then reduces to zero at z = z2.

7 The maximum value of Iz can be calculated as: where qc = cone penetration resistance Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 9 Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 10 Substitute in the equation of Se in order to get the elastic Settlement of sandy soil.

8 Settlement of Foundation on Sand Based on Standard Penetration Resistance (Meyerhof Method) Meyerhof proposed a correlation for the net bearing pressure for Foundations with the standard penetration resistance, N60. The net pressure has been defined as: According to Meyerhof s theory, for 25 mm (1 in.) of estimated maximum Settlement : In English units: Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 11 In SI units: where Se = Settlement , in mm.

9 N60 = the standard penetration resistance between the bottom of the foundation and 2B below the bottom. Later, Meyerhof (1965) suggested that the net allowable bearing pressure should be increased by about 50%. Bowles (1977) proposed that the modified form of the bearing equations be expressed as: In SI units: where B is in meters and Se is in mm. Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr. Yasser M. Almadhoun Page 12 Example See example in textbook, page 328.

10 Effect of the Rise of water Table on Elastic Settlement Terzaghi suggested that the submergence of soil mass reduces the soil stiffness by about half, which in turn doubles the Settlement . In most cases of foundation design, it is considered that, if the ground water table is located to 2B below the bottom of the foundation, it will not have any effect on the Settlement . The total elastic Settlement (S e) due to the rise of the ground water table can be given as: Civil Engineering Department: Foundation Engineering (ECIV 4052) Engr.