| Supporting sides of excavated soil close to pre-existing footings, particularly in crowded cities, or building new footings adjacent to pre-existing retaining structures is commonly encountered in Civil Engineering applications. In such circumstances, retaining structures are responsible in supporting foundation/backfill soils and footings. Most current retaining structure design methods are limited in estimating the ultimate bearing capacity of adjacent footings due to not considering the non-linear interaction of footing-soil-retaining structure system. Currently, there is no method available to calculate the ultimate bearing capacity of a finite footing near a discontinuous retaining structure like a spaced pile-row. Under such scenarios, footing bearing capacity will decrease since the pile lateral stiffness is significantly lower than that would be generated by the soil before excavation. This study investigates the destabilizing effects of placing a concrete footing near a spaced concrete pile-row, which is supporting excavated soil sides. The inter-dependent effects among pile width, pile spacing, footing geometry, pile- footing distance, and soil type and their subsequent impact on footing bearing capacity and lateral pile-row stability were investigated using both two- and three-dimensional finite element analyses (ANSYS13.0 2010). Three foundation/backfill soil deposits were selected: compacted kaolin (stiff clay), compacted silty clay (soft clay), and compacted 10% kaolin with 90% sand (kaolin sand; dense sand). Each component of the footing-soil-pile system was idealized as nonlinear elastic-perfectly plastic and rate-independent material with an upper bound function of Drucker and Prager (1952) yield criterion. A total of 288 unique finite element runs were analyzed for the present footing-soil-pile system. The two- and three-dimensional finite element models show a good agreement with published data. Footing bearing capacity was determined through the footing pressure-displacement curve using an intersecting bi-tangent criterion. The footing bearing capacities in the footing-soil-pile system were compared with a footing-infinite soil system. The destabilizing effects of the footing-soil-pile system were quantified using reduction factors of footing bearing capacity. One unique empirical equation describing the reduction factor of footing bearing capacity was derived using non-dimensional analysis and multiple linear regressions (Minitab16.0 2010). Soil yielding patterns and pile lateral deflection were also studied; active and passive earth pressure zones were highlighted to understand the inter-dependent behavior of footing-soil-pile system. The three-dimensional soil arching effect was also investigated to show its effects on the pile lateral pressure. The results of footing-infinite soil system indicate that the two-dimensional (plane strain) finite element analyses underestimate footing bearing capacity in comparison with the three-dimensional finite element analyses. The results of footing-soil-pile system demonstrate that destabilization of this system is influenced by soil type, footing aspect ratio, footing-pile distance, and pile width and spacing. The effect of pile spacing is disappeared with increasing footing-pile distance. The system of footing-soil-pile has no destabilizing effects when the footing-pile distance is approximately as follows: 6 and 7 times footing width for an infinite footing resting on clayey and sandy soils, respectively; 5 2/3 and 6 2/3 times footing width for a moderate finite (rectangle) footing resting on clayey and sandy soils, respectively; and 4 and 6 times footing width for a very finite (square) footing resting on clayey and sandy soils, respectively. Local and general shear failures are observed in the failure mode of clayey and sandy soils, respectively, in the system of footing-soil-pile. Long piles always bend rather than rotate; and soil arching forms in case of a finite footing adjacent by long piles, irrespective soil type. |
