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Review_2 - Review Test#2 Summary of Schmertmann Strain...

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Review Test #2
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Summary of Schmertmann Strain Factor Method (CPT) where: C 1 = correction factor for depth of embedment = C 2 = correction factor for secondary creep settlement Δσ Z = net foundation pressure increase at footing bottom = (q - σ 0 ) σ 0 = effective stress at footing bottom before any excavation I Zi = Strain influence factor at mid-height of each sub-layer from idealized distribution shown below E i = Young’s modulus for each sub-layer (estimated from CPT) = 2.5 q Ci for axisymmetric footings (L/B = 1) = 3.5 q Ci for strip footings (L/B > 10) q Ci = cone tip bearing (average assigned to each sub-layer) Δ z i = height of each sub-layer t yr = time in years after placement of footing =
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Summary of Schmertmann Strain Factor Method (CPT) Idealized I Z Distribu0on
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Use the Schmertmann strain factor method to find the expected se;lement (inches) of the embedded [email protected] shown below. Use the CPT q c (bearing) sand profile shown below and a @me period of 25 years. The plan [email protected] size is 10 K x 20 K and it will be embedded 5 K below the ground surface. No soil will be backfilled on top of the [email protected] and the load shown (Q) includes the weight of the [email protected] GWT is at 6.6 K. q (tsf) 50 100 10.56 30
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Notes on Schmertmann Method Interpolate idealized I Z distribution for footings with 1< L/B < 10: initial I Z 0.1 0.2 depth of I ZP 0.5B B maximum depth of I Z influence 2B 4B Interpolate modulus, E, for footings with 1< L/B < 10: E = 2.5q C 3.5 q C Developed for NC sands - generally conservative if sand is preloaded, compacted, etc. Applies to static loading only, dynamic loads can create large pore pressures and significantly decrease effective stresses leading to large deflections Can use N SPT to estimate q C if no CPT data available. This is a common and generally conservative procedure, but there is significant uncertainty due to the variability of the SPT (some prefer to use q c to estimate N). The ratio (q C / N SPT ) increases as the mean grain size increases.
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BC Failure Modes General shear failure (q u ) - abrupt, sudden. Failure surface extends to ground surface (dense sand). Local shear failure (q ls ) - occurs slowly, with substantial settlement. Failure surface does not extend to ground surface. Progresses to general shear failure (medium compacted sand, clayey soil) Punching shear failure (q ps ) - continuous punching failure and settlement with gradual increase in q s due to compaction (loose sands), more likely to occur at depth.
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Vesic (1973) Recommended for Sands 7 [email protected]@on of failure modes based on plate load tests (circular plate) in sand • D/B < 2: can fail in any of the modes, depending on Dr • D/B > 4: always punching shear • Only apply to circular [email protected] in sand • Gives a general [email protected] between the mode of failure, Dr and D/B [email protected]
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Terzaghi Bearing Capacity Equation for general shear failure: strip: square: circular: bearing factors: ( φ in radians) , N c = 5.70 for φ = 0 (Coduto) (rigorous [email protected] for N γ in chart below or table on next page )
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