Discussion of “Interference of Two Closely Spaced Strip Footings on Sand Using Model Tests” by J. Kumar and M. K. Bhoi

This discussion presents the effect of interference of two closely spaced strip footings on sand on ultimate bearing capacity. The authors used model footing test results to determine the efficiency factor. This was then related to footing spacing to assess the degree of footing interference. The authors also compared their results with available information obtained using different methods of analysis, including limit equilibrium method Stuart 1962, method of characteristics Kumar and Ghosh 2007a, and upper bound limit analysis Kumar and Ghosh 2007b; Kumar and Kouzer 2008. Separately, their test results were compared with the experimental data of Saran and Agarwal 1974, Das and LarbiCherif 1983, and Kumar and Saran 2003. Considerable discrepancy between the authors’ results and the available information is seen. All of the available data show that footing interaction diminishes when the ratio of footing spacing to footing width reaches about 5 or 6, whereas the authors’ results reveal that footing interaction continues to over 12. The authors attributed the discrepancy to the conditions of their model tests, including 1 both footings were loaded to failure simultaneously, 2 footings were not allowed to tilt under loading, and 3 experimental results were based on small-scale, 1 g model tests. Such conditions rarely exist in common practice. Therefore, the authors’ findings have limited engineering applications. A similar study on the interaction between two parallel equalwidth strip footings was also conducted by Jao et al. 2002. They employed finite-element analysis to assess the behavior of both surface and embedded strip footings parallel to each other. The foundation soil was a compacted cohesive soil having 158.5 kPa cohesion, 8.0° angle of internal friction, 15.75 kN/ m 3 moist unit weight, and 19,860 kPa initial tangent modulus. In the analysis, the soil was characterized as a nonlinear elastic-plastic material. The footing behavior analyzed was footing settlement, footing rotation, and plastic yielding of the soil. Based on the results of the analysis, the ratio of noninteracting to interacting footing settlements under ultimate load was plotted against the ratio of footing spacing to footing width. As would be expected, the settlement ratio decreased as the footing spacing-width ratio increased, indicating that the degree of footing interaction decreased with footing spacing. The curve for embedded footings was slightly higher than that for surface footings, and the two curves merged at a footing spacing to width ratio of about 6. Meanwhile, the curve obtained from the experimental results of Das and Larbi-Cherif 1983 was bracketed between the curves of surface and embedded footings. All three curves revealed that footing interaction diminished around the footing spacing-width ratio of about 6. The study results of Jao et al. 2002 also showed that footing interaction caused the footings to tilt toward each other. As would be expected, the tilting angle increased with increasing footing pressure, and under a given footing pressure, the tilting angle decreased with increasing footing spacing until the footing spacing to footing width ratio reached around 6. Based on the results of preceding studies, the engineering significance of footing interaction in terms of foundation analysis and design deserves further evaluation and discussion. Since footing interaction will increase the ultimate bearing capacity as well as the allowable bearing capacity, the needed footing size for a given column load will decrease owing to interaction. In other words, footing interaction will enhance the safety factor against bearing capacity failure under a given column load. On the other hand, footing settlement increases as a result of footing interaction. This, in turn, will decrease the safety factor against excessive settlement. Furthermore, footing interaction may result in significant footing tilting, which may potentially cause structural damage. Thus, in the design of two closely spaced strip footings, the footing stability should be carefully analyzed with respect to bearing capacity failure, excessive settlement, and footing rotation with due consideration of footing interaction.