Bearing capacity )BC( test results are presented for bounded and unbounded twin circular footings on 17 unreinforced and geocell-reinforced )GCR( sand. Analysis of the results demonstrate material, scale and size effects 18 on the BC for a given combination in materials (sand-GCR), footing (single-twin) and the problem geometric 19 dimensions. The significance of these combinations on BC and settlements is used to arrive at suitably modified BC 20 factors for design that could be generalized. Plots given relative to reference cases for which BC design solutions are 21 available provide correction factors to modify classical BC equations. Values of the BC and BC factors represent the 22 lumped effect of all or separate problem variables including scale and any experimental limitations. Compared with 23 previous works, these results give deeper critical depths for twin footings on unreinforced and GCR sand and BC 24 higher than 4 times the reference case.25 26 27All classical bearing capacity (BC) relations were originally derived for a single footing on a soil 29 layer of infinite thickness. Modifications appeared subsequently to arrive at more realistic BC 30 and settlement measures through a set of factors concerning base confinement [1], interaction 31 with adjacent foundation [2], shape [3], scale [4], and so on. Circular footings received some 32 attention due to their applicability and axi-symmetric nature which approximates 3-D 33 foundations without the effects resulting from multi-edged footings. Some studies on the 34 behavior of circular footings on sand [5][6][7][8][9] resulted in determination of dimensionless values 35 such as N γ (BC) and S γ (shape) factors. This foundation type is commonly employed for 36 axisymmetric structures such as silos, cooling towers, tall tanks, etc. 37 In practice, soils are bounded and experience interference unless foundations are constructed far 38 from each other on a deep layer. The presence of an adjacent foundation [2], reinforcement [10], 39 layering or a rigid base [3] change entirely the shear failure mechanism, footing BC and 40 settlement behavior. These constraints impose complexities that cannot be easily resolved 41 theoretically, but, for design purposes it can be approximately approached experimentally at 42 different scales and relative dimensions using lumped modified factors for shape (S γ * ) and BC 43 (N γ * ). By choosing the small scale next to the earlier reported larger scale tests [11], and referring 44 to published results on single footing on bounded layer [12-16] or twin footings on unreinforced 45 and geogrid-reinforced stratum [17-22], as reference cases, one can recalculate modified factors 46 and assess their applicability in design. 47 This study investigates and quantifies: (1) BC from laboratory tests on two sets of circular model 48 footings of different diameters; (2) the separate and coupled influence of different confinement 49 3 types on the BC and BC factors from second neighboring footing, geocell reinforcement, and a 50 rigid ...
