Lecture 9 - s1 SETTLEMENT IN/ON SAND Considerations: o o o...

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ρ s1 SETTLEMENT IN/ON SAND Considerations: o Settlement occurs very rapidly o Settlement is typically small o Creep normally not a factor o Sand stiffness strongly dependent on stress, both vertical and horizontal o Sand stiffness increases with depth (stress) and classical linear elastic models do not perform well. o Insitu properties can vary dramatically (violent deposition environment) o Sands difficult to sample and test undisturbed in the lab o Need method which measures stiffness insitu Possible Settlement Methods for Sands: o Use immediate settlement method from clays - relatively poor results o Plate Load Tests - must transfer to larger footing area o Standard Penetration Methods (SPT) - lots of variability, unreliable precision of field data, need correlation of modulus to N SPT (blow count) o Dilatometer Test (DMT) - flat plate, direct measurement of modulus, Boussinesq stress increase, calculate strain - good method, relatively new o Cone Penetrometer Test (CPT) - measurement of bearing on tip of cone, correlate with modulus, use strain factor method, can be conservative but good field data precision Schmertmann Strain Factor Method (CPT): o Widely used in practice today to estimate settlement under center of footing. o Mixture of rational & empirical basis o Typically conservative (overestimate settlement) o Backed by field and model observations o Originally proposed 1970, based on UF research o Revised 1978 for spread & strip footings, time effects, and updated E vs. q c correlation o Method uses linear-elastic theory for a half- space: ( ) [ ] Y X Z Z E 1 σ + σ μ - σ = ε (Hooke’s law) where : ε Z = strain in the z direction E = Young’s modulus μ = Poisson’s ratio σ X = stress increase in the x direction σ Y = stress increase in the y direction σ Z = stress increase in the z direction σ x , σ y , σ z , ε z due to surface load P Sand
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ρ s2 For surface loading, Hooke’s law may be rewritten as: E I q Z Z = ε where: q = surface contact pressure I Z = strain influence factor I Z varies only with Poisson’s ratio and the location of the point for which the strain is to be evaluated (similar to previous immediate settlement analyses). The theoretical depth distribution of I Z is shown below at left. Strain distributions for cohesionless soils
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Lecture 9 - s1 SETTLEMENT IN/ON SAND Considerations: o o o...

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