Abstract
Plain strain model tests were performed on beds of sands with different particle size distribution (Coarse, Medium and Fine) prepared at loose state (Relative density Dr. of 30%). A strip footing model with skirt was placed on the bed of sand and loaded vertically up to failure at different ratios of skirt depth to width D/B of (0.5, 1.0, 1.5, 2, and 3). The applied stress increments and the corresponding settlements were measured. The improvement ratio due to different skirt depth and the behavior of bearing capacity parameters Nγ and Nq at each depth were evaluated and compared with some theoretical approaches. The test results revealed that the improvement ratio increased linearly up to D/B of 1.5 then reduced. Two factors were introduce into the general bearing capacity equation where used to evaluate bearing capacity of skirt footing, there values are about 1.6 for skirt ratio ranged between 0.5 to 1.5, and 1.25 for skirt ratio more than 1.5. Also, it is found that the Nγ parameter for D/B=0 were very close to Vesic proposal for fine and medium grain size distribution, while it’s close to Biarez proposal for coarse sand. The behavior of Nq parameter with different skirt ratio shows slight increase up to D/B of 1.5 then decrease with increasing D/B ratio for different grain size distribution. While the behavior of theoretical Nq parameter (depending on angle of internal friction values) shows a linear increase with skirt ratio for different grain size distribution.
Highlights
Ultimate bearing capacity theories for shallow footings are available since the proposals of Terzaghi in (1948), and its modifications by Meyerhof (1951, 1963), Hansen (1970), Vesic (1973) etc
Skirt foundation can be used to increase the depth of footing increase the length of failure surface that would develop under central vertical loading condition and improve the bearing capacity of the foundation
5.2.2 Variation of Nq : Figure (13) shows the variation of Nq versus angle of internal friction (φ) for coarse, medium, and fine graded sand determined from the ultimate bearing capacity of experimental model tests for different skirt ratio (D/B) of (0.5, 1.0, 1.5, 2.0 and 3) by adopting equation no. (1) and Vesic proposal for Nq
Summary
Ultimate bearing capacity theories for shallow footings are available since the proposals of Terzaghi in (1948), and its modifications by Meyerhof (1951, 1963), Hansen (1970), Vesic (1973) etc. In 2004, Aghbari and Mohamedzein[6] performed laboratory model test on skirted strip foundation on dense sand and found that the use of structural skirts can improve the bearing capacity by a percent of 1.5 to 3.9 depending on the geometrical and structural properties of the skirts and foundations, soil characteristics and interface condition of soil skirt foundation system as in figure(2). They suggested adding a skirt factor, Fγ, to Terzaghi bearing capacity equations (see equation 7). Figure (3) shows the grain size distribution curve, according to the Unified Soil Classification System (USCS), the sand is classified as poorly graded sand with symbol (SP)
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