78_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

78_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 - o...

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ps1 SETTLEMENT INION SAND Considerations: o Settlement occurs very rapidly o Settlement is typicaliy smali o Creep normally not a factor '" \~\.,~ ... '}" t~., ~t. £-. . I.NJ i,l", ~,'(JI,I;I" o Sand stiffness strongly dependent on stress, both verticai and horizontal a Sand stiffness increases with depth (stress) and classical linear elastic models do not perform weli. o Insitu properties can vary dramaticaliy (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 NSPT (blow count) o Dilatometer Test (DMT) - flat plate, direct measurement of modulus, Boussinesq stress increase, caiculate strain - good method, relatively new
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Unformatted text preview: 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 ..... I ! ax, (Jy, a z, E z P i due to surface load Sand • 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 Typicaliy 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, correlation o Method uses linear-elastic theory for a half-space: where: €z E 11 Ox Oy °z = strain in the z direction = Young's modulus = Poisson's ratio = stress increase in the x direction = stress increase in the y direction = stress increase in the z direction...
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