Module 6 Lecture 37 Evaluation of Soil Settlement - 3 Topics

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1 Module 6 Lecture 37 Evaluation of Soil Settlement - 3 Topics Settlement Prediction in by Empirical Correlation Calculation of Immediate Settlement in Granular Soil Using Simplified Strain Influence Factor Settlement Prediction in by Empirical Correlation Based on several field load s, Terzaghi and Peck (1967) suggested that for similar intensities of load q on a footing where S e is the settlement of a footing with width B and s e(1) is the settlement of a smaller footing with width B 1. The value of B 1 is usually taken as 1 ft. S e = SB B+B 1 2 Se(1) (30) Table 6 Young s modulus for vertical static compression of from standard penetration number (After Mitchell and Gardner 1975). Reference Relationship* Soil types Basis Remarks Schultze and Dry Meizer (1965) E s = vσ o kg/cm 2 v = log σ o ± < σ o < 1.2 kg/cm 2 σ o = effective overburden pressure Webb (1969) E s = ton/ft 2 E s = 10/3 + 5 ton/ft 2 Penetration s in field and in shaft. Compressibility based on e, e max, and e min (Schultze and Moussa. (1961) Screw plate s Correlation coefficient = for 77 s Below water table Farrent (1963) Trofimenkov (1974) E s = 40 + C 6 kg/cm 2 > 15 E s = C + 6 kg/cm 2 > 15 Silt with to gravel with Used in Greece E s = 350 to 500 log kg/cm 2 U.S.S.R. practice Table 7 Equivalent Young s modulus for vertical static compression of -static cone resistance (After Mitchell and Gardner 1975). Reference Relationship Soil type Remarks Buisman (1940) E s = 1.5q c s Overpredicts settlements by a factor of about 2 Trofimenkov E s = 2.5q c Lower limit Dept. of Civil Engg. Indian Institute of Technology, Kanpur 1

2 (1964) E s = q c Average De Beer (1967) E s = 1.5q c Overpredicts settlements by a factor of 2 Schultze and Meizer (1965) E s = Dry Based on field and lab vσ m o v penetration s-compressibility Bachelier Parez (1965) and v = log q c 382.3σ o ± 60.3 ± 50.3 σ o = effective overburden pressure E s = αq c α = 0.8 to 0.9 α = 1.3 to 1.9 α = 3.8 to 5.7 α = 7.7 Pure Silty Soft clay based on e, e max and e min Correlation coefficient = for 90 s valid for σ o = 0 to 0.8 kg/cm 2 Thomas (1968) E s = αq c α = 3 to 12 3 s Based on penetration and compression s in large chambers. Lower values of α at higher values of q c : attributed to grain crushing Webb (1969) E s = 1 2 q c + 30 ton/ft 2 Vesic (1970) Schmertmann (1970) Bogdanovic (1973) E s = 1 2 q c + 15 lb/ft 2 E s = 2(1 + D R 3 )q c D r = relative density below water table below water table Based on screw plate s: correlated will with settlement of oil tanks Based on pile load s and assumptions concerning state of stress E s = 2q c Based on screw plate s E s = αq c q c > 40 kg/cm 2 α = < q c < 40 α = 1.5 to < q c < 20 α = 1.8 to < q c < 10 α = 2.5 to 3.0 Schmertmann (1974) E s = 2. q c, y gravels Silty saturated s silts with silty and silty saturated s with silt C s C s L/B = 1 to 2, axisymmetric L/B 10, plane strain Dept. of Civil Engg. Indian Institute of Technology, Kanpur 2

3 E s = 3.5q c De Beer (1974) E s = 1.6q c 8 Bulgarian practice E s = 1.5q c, q c > 30 kg/cm 2 Greek practice E s = 3q c, q c < 30 kg/cm 2 E s > 1.5q c, or E s = 2q c Italian practice E s = 1.9q c E s = 1 q 2 c k/m 2 ) Fine to South African practice Trofimenkov (1974) E s = 1 2 q c k/m 2 ) E s = αq c, 1.5 < α < 2 E s = 3q c E s = 7q c medium s, PI < 15% s Clays U. K. practice U. S. S.R. practice Table 8 Values of β from various case studies of immediate settlement (After Appolonia, H. G. Poulos, and C. C. Ladd 1971). o. Location of structure Plasticity index Clay properties Sensitivity Overconsolidation E field, ton β ratio /m ,600 1,200 CIU 1 Oslo: ine-story building 2 Asrum I: Circular load ,000 1,200 Source of S u CIU 3 Asrum II: Circular load ,000 1,100 CIU 4 Mastemyr: Circular load ,300 1,200 1,700 Bearing capacity 5 Portsmouty: Highway embankment ,000 2,000 1,700 Bearing capacity CK o U 6 Boston: Highway embankment ,000 13,000 1,600 1,200 7 Drammen: Circular load ,200 1,400 1,100 CK o U 8 Kawasaki: Circular load 38 6± , CIU 9 Venezuela: Oil tanks 37 8± CIU 10 Maine: Rectangular load 33± to to to 160 UU and Bearing capacity Dept. of Civil Engg. Indian Institute of Technology, Kanpur 3

4 Equation (30) can be rewritten in the form S e S e(1) = 4 (1+B 1 /B) 2 (31) D Appolonia et al. (1970) compared the above equation with several field experiments conducted by Bjerrum and Eggstad (1963) and Bazaraa (1967). The results of the comparison are shown in Figure It appears that the relationship gives the general trend; however, there appears to be a wide scattering of points. Figure 6.15 Comparison of field results with equation (31). (After D. J. D Appolonia, E. D Appolonia, and R. F. Brisette, discussion on Settlement of Spread Footings on, J. Soil Mech. Found. Div., ASCE, vol. 96, 1970) Using the standard penetration resistance obtained from field explorations, Meyerhof (1965) proposed the following relationships for settlement calculations in : S e = 4q for B 4 ft (32a) And S e = 6q B B+1 2 for B > 4 ft (32b) Where q = intensity of applied load, kip/ft 2 B = width of footing, ft S e = settlement, in Dept. of Civil Engg. Indian Institute of Technology, Kanpur 4

= standard penetration number Figure 6.16 shows a comparison of the observed settlements to those obtained through equation (32). It appears that the predicted settlements are rather conservative.

5 = standard penetration number Figure 6.16 shows a comparison of the observed settlements to those obtained through equation (32). It appears that the predicted settlements are rather conservative. Bowles (1977) suggested that for a more reasonable agreement equation (32) can be modified as S e = 2.5q for B 4 ft (33a) And S e = 4q B B+1 2 for B > 4 ft (33b) In a later work, based on the analysis of the field data of Schultze and Sherif (1973), Meyerhof (1974) gave the following empirical correlations for settlement of shallow foundations: Figure 6.16 Comparison of observed settlement to that calculated from equation (32). (After Meyerhof 1965) S e = S e = q B 2 q B 2 (for and gravel) (for silty ) (34a) (34b) Where S e = settlement, in Dept. of Civil Engg. Indian Institute of Technology, Kanpur 5

6 q = intensity of applied load, ton/ft 2 B = width of footing, in Calculation of Immediate Settlement in Granular Soil Using Simplified Strain Influence Factor The equation for vertical strain ε z under the center of a flexible circular load was given in equation (5) as where I z is the strain influence factor. ε z = q(1+v) [ 1 2v A + B ] E Or I z = ε ze q = 1 + v [ 1 2v A + B ] (35) Figure 6.17 shows the variation of I z with depth based on equation (35) for v equal to 0.4 and 0.5 also. According to this simplified strain-influence factor method, the immediate settlement of a foundation can be calculated as where C 1 is the correction factor for the depth of embedment of foundation, and C 2 is a correction factor to account for the creep n soil. The factors C 1 and C 2 are given by the following equations: S e = C 1 C 2 q I z 2B E s z 0 (36) C 1 = q o q (37) Where q o = effective overburden pressure at foundation level q = net foundation pressure increase = q 1 q o q 1 = average pressure of foundation against soil C 2 = log t 0.1 (38) Where t is time, in years. Below is an example for using equation (36) which was given in Schmertmann s 1970 paper. Dept. of Civil Engg. Indian Institute of Technology, Kanpur 6

7 Figure 6.17 Theoretical and experimental distribution of vertical strain influence factor below the center of a circular loaded area. (after J. Schmertmann,1970) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 7

Chapter 7: Settlement of Shallow Foundations

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.