Abstract
In this paper, the stress–strain method for the elastic settlement analysis of shallow foundations is revisited, offering a great number of strain influence factor charts covering the most common cases met in civil engineering practice. The calculation of settlement based on strain influence factors has the advantage of considering soil elastic moduli values rapidly varying with depth, such as those often obtained in practice using continuous probing tests, e.g., the Cone Penetration Test (CPT) and Standard Penetration Test (SPT). It also offers the advantage of the convenient calculation of the correction factor for future water table rise into the influence depth of footing. As is known, when the water table rises into the influence zone of footing, it reduces the soil stiffness and thus additional settlement is induced. The proposed strain influence factors refer to flexible circular footings (at distances 0, R/3, 2R/3 and R from the center; R is the radius of footing), rigid circular footings, flexible rectangular footings (at the center and corner), triangular embankment loading of width B and length L (L/B = 1, 2, 3, 4, 5 and 10) and trapezoidal embankment loading of infinite length and various widths. The strain influence factor values are given for Poisson’s ratio value of soil, ranging from 0 to 0.5 with 0.1 interval. The compatibility of the so-called “characteristic point” of flexible footings with the stress–strain method is also investigated; the settlement under this point is considered to be the same as the uniform settlement of the respective rigid footing. The analysis showed that, despite the effectiveness of the “characteristic point” concept in homogenous soils, the method in question is not suitable for non-homogenous soils, as it largely overestimates settlement at shallow depths (for z/B < 0.35) and underestimates it at greater depths (for z/B > 0.35; z is the depth below the footing and B is the footing width).
Highlights
Schmertmann et al.’s [1] strain influence factor method for immediate settlement analysis is among the most popular ones worldwide
The method in question incorporates the use of a bilinear strain influence factor Iz versus z/B relationship (z is the depth measured from the foundation level and B is the width of foundation); the area under this relationship multiplied by the net footing loading and divided by the elastic modulus of soil, E, gives the footing settlement
The problem of calculating the elastic settlement of footings relying on the stress–strain method has been revisited, offering a great number of strain influence factor charts covering the most common cases met in civil engineering practice
Summary
Schmertmann et al.’s [1] strain influence factor method (or stress–strain method) for immediate settlement analysis is among the most popular ones worldwide. It is mentioned that independent studies [2,12,13,14,15,16,17,18,19] comparing the measured settlement of structures or full-size test footings with the respective settlement calculated with Schmertmann’s method indicate great deviation This deviation can be attributed to the replacement of the actual curved Iz –z/B relationship with a simplistic bilinear one, the way the embedment depth is taken into account, the plastic response of the ground and the fact that Schmertmann’s method was developed considering Poisson’s ratio values in the order of 0.4 to 0.5.
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