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Settlement Criteria

Settlement Criteria. for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation) in this module SANDS (including gravelly sands, silty sands and non-plastic silts) are considered. The concern?.

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Settlement Criteria

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  1. Settlement Criteria for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation) in this module SANDS (including gravelly sands, silty sands and non-plastic silts) are considered

  2. The concern? in most cases the maximum allowable settlement will not be reached before shear failure at a factor of safety of 3 the main concern is with narrow footings settlement in sands is rapid, occurring almost entirely during construction and initial loading \ dead load + max. live load are considered to estimate settlement

  3. Maximum Allowable Settlement Footing on Sand~25 mm this makes it likely that any differential settlement between footings will be less than 20 mm Raft on Sand~50 mm corresponds to differential settlement between footings less than 20 mm

  4. Pre-Construction Treatment for loose sand deposits, compaction prior to construction is recommended (vibroflotation, for example) for clays, if possible, surcharging with fill and vertical drains for several years prior to construction will reduce the ultimate settlement of the structure as most of the consolidation will have taken place

  5. Plate Bearing Test • a 1.5 m square test pit is dug used to simulate a foundation Sowers & Sowers, 3rd Ed.

  6. Plate Bearing Test (Cont’d) • then a 1 foot square (300mm x 300mm) steel plate is loaded in increments and the corresponding settlements measured Peck, Hanson, Thornburn, 2nd Ed.

  7. a load-settlement curve is produced Plate Bearing Test Results Sowers & Sowers, 3rd Ed.

  8. Modulus of Vertical Subgrade Reaction, Kv • Kv is taken from straight line portion of this curve McCarthy, 6th Ed. Splate = Plate settlement Qplate = Plate Load Aplate = Area of plate

  9. Design Kv Values B *These are for the case where the water table is at a depth greater than 1.5B. If the water table is at the base of the foundation use 0.5Kv. Use linear interpolation for intermediate locations of the water table. D Dw 1.5B McCarthy, 6th Ed.

  10. For cohesionless soils where D < B < 6.1m: Settlement Calculation where S = expected foundation settlement (m) Q = column load (kN) B = footing width (m) Kv = modulus of vertical subgrade reaction (kPa/m or kN/m3)* *Remember, when calculating Kv from plate test data, plate area (Aplate) is 0.09m2!

  11. The Plate Bearing Test results are extrapolated for the design of the foundation! Beware! Craig, 6th Ed.

  12. part of a standard bore hole investigation • split barrel sampler is advanced by dropping a 64 kg hammer 760 mm • N-Values are the number of blows (hammer drops) to advance sampler 300 mm Standard Penetration Test McCarthy, 6th Ed.

  13. Split Barrel Sampler Peck, Hanson, Thornburn, 2nd Ed. • N-Value = “Standard Penetration Resistance” McCarthy, 6th Ed.

  14. Sample Bore Hole Log McCarthy, 6th Ed.

  15. for B < 1.2m for B > 1.2m North American Equations where Sa = allowable settlement (mm) qa = allowable bearing pressure (kPa) B = footing width (m) N = average corrected standard penetration resistance • a number of corrections are applied to N-Values (pore water pressure, overburden stress…see Craig)

  16. we will be using average corrected N-Values • Terzaghi and Peck proposed a correction, Cw to the allowable bearing pressure, qa to reflect the depth of water table Water Table Correction B D Dw Dw = D + B B water table correction:

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