The bearing capacity of footings on sand with a weak layer

Abstract

Minor details of the ground, such as thin weak layers, shear bands and slickensided surfaces, can substantially affect the behaviour of soil-footing and other geotechnical systems, despite their seeming insignificance. In this paper, the influence of the presence of a thin horizontal weak layer on the ultimate bearing capacity of a strip footing on dense sand is investigated by single-gravity tests on small-scale physical models of the soil-footing system. The test results show that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than about four times the footing width. It is found that the presence of a thin weak layer can cause decreases of the ultimate bearing capacity of up to 80%. Numerical simulations, by finite-element analysis, of the behaviour of the reduced-scale models are able to capture the failure mechanism and the ultimate bearing capacity correctly, only if the mean equivalent constant value of the secant angle of shearing resistance used in calculations is selected, taking into account the curvature of the shear strength envelope of the sand within the very low normal stress range existing in the tested models.

Centrifuge tests on strip footings on sand with a weak layer

“…The results of tests are summarised in Tables 4 and 5 Results of test C01 on homogeneous sand bed is also reported for comparison B m = 40 mm, footing model width; z i , depth of the top surface of the weak layer; t 0 , thickness of the weak layer; q lim , ultimate bearing capacity; q lim,0 = 3869·7 kPa, ultimate bearing capacity for the homogeneous sand case; r m,lim , settlement of footing model corresponding to ultimate bearing capacity; l m , maximum lateral extent of failure mechanism; z m , maximum depth of failure mechanism; q L and q R , emersion angles of the failure surface on the left and right sides of the footing, respectively The photograph of test B63 performed at 1g (Valore et al, 2017) is included in Figure 8 for comparison.…”

“…In order to compare the results of 1g and centrifuge tests and to study the scale effects, sand B was also used in 1g tests on 40 mm wide footings (Valore et al, 2017), according to a wellestablished practice (Altaee and Fellenius, 1994;Kimura et al, 1985;Schofield, 1980;Toyosawa et al, 2013;Yamaguchi et al, 1977). It is worth noting that the ratio of the footing width to the mean particle size, B/d 50 = 40/0·45 = 88·9, is larger than 50, which is the minimum value beyond which the particle size effect can be considered negligible, as suggested by many researchers (e.g.…”

“…The material used for the weak layer is the 'CM3' dry talc powder that was also used in tests at single gravity (Valore et al, 2017). The angle of peak shear strength, f 0 2p , of this material was determined by direct shear tests and is equal to 27°.…”

“…The settlements of the footing are uniform. The peak angle of shearing resistance, d 0 , of the interface between the silica sand and the sandpaper was 42° (Valore et al, 2017). As the average peak shearing resistance angle, f 0 1p , of silica sand was about 48°at the stress level relevant to the model tests, the sand-footing contact can be considered perfectly rough (Hansen and Christensen, 1969;Kumar and Kouzer, 2007) (Fioravante, 2002;Garnier and König, 1998;Kishida and Useugi, 1987;Lings and Dietz, 2005).…”

“…The simplest of such details is probably exemplified by a thin horizontal weak soil layer interbedded in a mass of stiffer soil. This problem was recently investigated with reference to the ultimate bearing capacity of strip footings by 1g small-scale model tests discussed in a companion paper (Valore et al, 2017).…”