hi does anybody has info. about the subgrade modulus coefficient, k (t/m3) can be used for the settlement amount? if the mat foundation system is rigid and the layer below it is sandy silty clay; as far as i know it can not be used for the settlement calculations. it is only used for differencial deformations.? empric correlations recommended for the k coefficient is 1500-2000 t/m3 for this soil structure. But if you calculate that from the formula (Braja Das. "princ. of foun. engng.)it will be app. 150-200 t/m3.
and consequently: if you use emprical values the settlement value is 1-2 cm. and if the other, the sett. is 12-13 cm. if i calculate the settlement by classical stress distribution method, it will be 15 cm.
i use this modulus as a soil parameter to the SAP2000 programme.
would you tell me that which value is reliable? and can i calculate the settlement amount by this way?.. thanks..
Well, this has been subject of discussion for some time. What kind of data do you have to assess the stiffness of the ground? You can represent the ground stiffness by an equivalent elastic modulus (E'), or alternate shear modulus (G') whereby E' = 2G(1+u') both in tonnes/m2, or by spring constant k' or subgrade coefficient (kh in tonne/m), or by subgrade reaction modulus (Kh in tonnes/m3). Only the E' (and G') are true soil parameters (see recent article by Briaud in Geotechnical News). The other k', kh, and Kh values are derived from soil + foundation geometry.
For example Kh (tonnes/m3) you want comes from elastic continuum theory for deflection (w) of circular foundation of diameter (d) on a semi-infinite elastic medium:
w = q d (1-u2)/E where u = Poisson's ratio and q = applied surface stress. For finite layer and nonhomogeneous E with depth, as well as flex vs. rigid footings, see Mayne and Poulos (ASCE JGGE June 1999).
Then the definition of Kh = q/w which from above elastic solution, give Kh = E/[d(1-u2)] for simple case (has units of tonnes/m3). Thus depends on foundation size & soil deformation characteristics.
thank you Paul, i have two things that i need more info? 1st as far as i understand, the subgrade reaction coefficient Kh (t/m3) can not be used for the setllement analysis. it is only used for deflection of the foundation.? is settlement only determined by E (t/m2) layer by layer?
2nd if i derive Kh (t/m3) by Kh=E/b(1-u2) it is app. 100-150 t/m3 but emprical correlations give 1500-2000 t/m3. in addition, 1st value represents very soft and slime soil condition? but it is not really. what is the difference betw. them.?
Dear sir My name is Maryam Akbarzad and I am doing an MS in Geotechnical Engineering at the University of Tabriz. My subject of thesis is Investigaion about equations and the methods of estimation of the coefficient of subgrade reaction and analysis of the sensitivity of it in design of foundations and retaining walls. that would be very kind of you to provide me with any more lectures, books and etc. best regards Akbarzad
University of Tabriz - IRAN Faculty of Civil Engineering - Department of Geotechnical Engineering Maryam Akbarzad
I think the subgrade modulus of soil should be 1500-2000 t/m3 .