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CE-632 Foundation Analysis and Design

CE-632 Foundation Analysis and Design. Settlement of Foundation. Settlement. Immediate Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil)

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CE-632 Foundation Analysis and Design

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  1. CE-632Foundation Analysis and Design Settlement of Foundation

  2. Settlement • Immediate Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil) • Primary Consolidation: Due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change. (Important for Inorganic clays) • Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils) For any of the above mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the foundation

  3. Elasticity

  4. Stress Distribution: Concentrated load Boussinesq Analysis

  5. Where, Stress Distribution: Concentrated load Boussinesq Analysis

  6. 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Vertical Stress: Concentrated load Influence Factor for General solution of vertical stress

  7. Vertical Stress: Uniformly Distributed Circular Load Uniformly Distributed Circular Load

  8. Vertical Stress: Uniformly Distributed Circular Load Rigid Plate on half Space

  9. Vertical Stress: Rectangular Area

  10. Vertical Stress: Rectangular Area

  11. Pressure Bulb Strip Footing Square Footing

  12. Pressure Bulb for Square Foundation

  13. Pressure Bulb for CircularFoundation

  14. Newmark’s Chart Influence Value This Model is good for normally-consolidated, lightly overconsolidated clays, and variable deposits

  15. Newmark’s Chart Point of stress calculation • Determine the depth, z, where you wish to calculate the stress increase • Adopt a scale as shown in the figure • Draw the footing to scale and place the point of interest over the center of the chart • Count the number of elements that fall inside the footing, N • Calculate the stress increase as: Depth = z2 Depth = z1

  16. Westergaard’s Method • Provided solution for layered soils • Point Loads • Assumption:Elastic soil mass is laterally reinfrced by numorous, closely spaced, horizontal sheets of negligible thickness but infinite rigidity, that allow only vertical movement but prevent the mass as a whole from undergoing any lateral strain. This Model is specially good for pre-compressed or overconsolidated clays

  17. Westergaard’s influence Chart

  18. Fröhlich Chart with concentration factor m‘ = 4 This Model is specially good for Sands

  19. Simplified Methods (Poulos and Davis, 1974) Circular Foundation: Square Foundation:

  20. Simplified Methods (Poulos and Davis, 1974) Strip Foundation: Rectangular Foundation:

  21. Approximate Methods Rectangular Foundation: Square/Circular Foundation: Strip Foundation:

  22. Contact Pressure and Settlement distribution Cohesive Soil - Flexible Footing Granular Soil Flexible Footing Cohesive Soil - Rigid Footing Granular Soil - Rigid Footing

  23. Modulus of elasticity Thickness of soil layer Poisson’s ratio of soil Elastic settlement of Foundation Elastic settlement: Elastic settlement for Flexible Foundation: = influence factor: depends on the rigidity and shape of the foundation = Avg elasticity modulus of the soil for (4B) depth below foundn level

  24. Elastic settlement of Foundation

  25. Elastic settlement of Foundation E in kPa

  26. Elastic settlement of Foundation

  27. Elastic settlement of Foundation Soil Strata with Semi-infinite depth

  28. Steinbrenner’s Influence Factors for Settlement of the Corners of loaded Area LxB on Compressible Stratus of m = 0.5, and Thickness Ht

  29. Strain Influence Factor Method for Sandy Soil: Schmertmann and Hartman (1978) Correction factor for foundation depth Correction factor for creep effects For square and circular foundation: For foundation with L/B >10: Interpolate the values for 1 < L/B < 10

  30. Example For square and circular foundations For rectangular foundations Correlation with SPT data:

  31. Burland and Burbidge’s Method for Sandy Soils Depth of Stress Influence (z'): If N60'‘ is constant or increasing with depth, then If N60'‘ is decreasing with depth, use smaller of Elastic Settlement (Se): 0.0047 for NC sand0.0016 for OC sand with qna≤ po‘0.0047 for OC sand with qna ≤ po‘ Compressibility Index: for NC sand for OC sand for NC sand and for OC sand with qna≤ po‘ for OC sand with qna ≤ po‘

  32. Settlement due to Primary Consolidation For NC clay For OC clay For OC clay Average effective vertical stress before construction Average increase in effective vertical stress Effective pre-consolidation pressure Initial void ratio of the clay layer Compression Index Swelling Index Thickness of the clay layer

  33. Settlement Correction for Effect of 3-D Consolidation

  34. Fox’s Depth Correction Factor for Rectangular Footings of (L)x(B) at Depth (D) Rigidity Factor as per IS:8009-1976

  35. Time Rate of Settlement Assumption of pore pressure distribution under the given stress conditions For open clay layer with two way drainage use curve for V=1

  36. Settlement Due to Secondary Consolidation Void Ratio, e Secondary Compression Index Void ratio at the end of primary consolidation Time, t (Log scale) Thickness of Clay Layer Secondary consolidation settlement is more important in the case of organic and highly-compressible inorganic clays

  37. Total Settlement from SPT Data for Cohesionless soil Multiply the settlement by factor W'

  38. Total Settlement from CPT Data for Cohesionless soil • Depth profile of cone resistance can be divided in several segments of average cone resistance • Average cone resistance can be used to calculate constant of compressibility. • Settlement of each layer is calculated separately due to foundation loading and then added together

  39. Plate Load Test – IS:1888-1982

  40. Plate Load Test – IS:1888-1982 Bearing Plate: • Rough mild steel bearing plate in circular or square shape • Dimension: 30 cm, 45 cm, 60 cm, or 75 cm. Thickness > 25 mm • Smaller size for stiff or dense soil. Larger size for soft or loose soil • Bottom of the plate is grooved for increased roughness. • Concrete blocks may be used to replace bearing plates.

  41. Plate Load Test – IS:1888-1982 Test Pit: Plan Section Usually to the depth of foundation level. Width equal to five times the test plate Carefully leveled and cleaned bottom. Protected against disturbance or change in natural formation

  42. Plate Load Test – IS:1888-1982 Procedure: • Selection of Location • Based on the exploratory boring. • Test is carried out at the level of proposed foundation. If water table is below the foundation level but the depth is less than width of plate then the test is carried out at the level of water table. If the water table is above the foundation level then the water level is reduced to proposed foundation level by pumping out the water during the test; however, in case of high permeability material perform the test at the level of water table. • In case the soil is expected to have significant capillary action and the water table is within 1 m below the foundation, it becomes necessary to perform the test at the level of water table in order to avoid the effect of higher effective stresses due to capillary action resulting in lower values of interpreted settlements. • Reaction supports should be at least (3.5 x width of plate) away from the test plate location, and loading arrangement should provide sufficient working space. • Test plate should be placed over a 5 mm thick sand layer and it should be centered with the loading arrangement.

  43. Plate Load Test – IS:1888-1982 Procedure: (Contd.) • A seating pressure of 7 kPa is applied and then released after some time before the test. • Loads are applied in the increments of approximately 1/5th of the estimated ultimate safe load. (Or, one may choose to increase the load at an increment of 0.5 kN.) • At each load settlement is recorded at time intervals of 1, 2, 4, 6, 9, 16, 25 min and thereafter at intervals of one hour. • For clayey soil, the load is increased when time settlement curve shows that the settlement has exceeded 70-80% of the probable ultimate settlement or a duration of 24 Hrs. • For the other soils, the load is increased when the settlement rate drops below 0.02 mm/min. • The minimum duration for any load should, however, be at least 60 min. • Dial gauges used for testing should have at least 25 mm travel and 0.01 mm accuracy. • The load settlement curve can then be platted from settlement data.

  44. Plate Load Test – Load-Settlement Curve Zero Correction: The intersection of the early straight line or nearly straight line with zero load line shall be determined and subtracted from the settlement readings to allow for the perfect seating of the bearing plate.

  45. Plate Load Test – Load-Settlement Curve Terzaghi and Peck (1948): Settlement of a foundation of width Bf (cm) Settlement of the test plate of width Bp (cm) at the same load intensity as on the foundation Bond (1961):

  46. Plate Load Test: Some Considerations • The width of test plate should not be less than 30 cm. It is experimentally shown that the load settlement behavior of soil is qualitatively different for smaller widths. • The settlement influence zone is much larger for the real foundation sizes than that for test plate, which may lead to gross misinterpretation of expected settlement for proposed foundation. Soft soil layer • The foundation settlements in loose sands are usually much larger than what is predicted by plate load test. • Plate load test is relatively short duration test and gives mostly the immediate settlements. In case of granular soils the immediate settlement is close to total settlements. However, due to considerable consolidation settlement in case of cohesive soils, the plate load test becomes irrelevant in such case. Although the following relationship is suggested for interpreting the settlements in cohesive soils, it can not be used seriously for design.

  47. Plate Load Test: Bearing Capacity • In case of dense cohesionless soil and highly cohesive soils ultimate bearing capacity may be estimated from the peak load in load-settlement curve. • In case of partially cohesive soils and loose to medium dense soils the ultimate bearing capacity load may be estimated by assuming the load settlement curve so as to be a bilinear relationship.

  48. Plate Load Test: Bearing Capacity • A more precise determination of bearing capacity load is possible if the load-settlement curve is plotted in log-log scale and the relationship is assume to be bilinear. The intersection point is taken as the yield point or the bearing capacity load. For cohesioless soil  For cohesive soil 

  49. Modulus of Sub-grade Reaction

  50. L d D D • = maximum settlement • = differential settlement • /D = angular distortion Differential Settlement Terzaghi’s recommendation: Differential settlement should not exceed 50% of the total settlement calculated for the foundation. Considering the sizes of different footing, the following criteria is suggested for buildings: For raft foundation the requirements shall be more stringent and they may designed for the following criteria Allowable maximum and differential settlements as prescribed by IS:1904-1986 are given on the next slide

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