دانلود رایگان مقاله انگلیسی ارزیابی ظرفیت باربری لرزه ای پی های کم عمق واقع در خاک های آهکی دو لایه - اشپرینگر 2018

Abstract

The seismic bearing capacity of shallow strip footings located on homogeneous soils is studied extensively. But the effect of building height and layering is not included in those studies. In this paper two-layered cohesive soils are studied along with building height effect. The formulation of the problem is derived by extending the limit equilibrium method developed by Merlos and Romo (J Soil Dyn Earthq Eng 26(2):103–114, 2006) for two-layered soils. A log-spiral potential failure surface is assumed, and the inertia forces are applied directly to the building and the failure block soil mass. The position of the failure surface is obtained by a minimization process for every earthquake shaking acceleration. Results show that the potential failure surface moves upward and its length shortens as the peak ground acceleration (PGA) increases, reducing the bearing capacity. Also, the effects of building height, PGA, ratio of cohesion of the layers and the thickness of the top layer on seismic bearing capacity factor (Nce) are studied.

4 Conclusions

Using the limit equilibrium method and pseudo-static method of analysis, an extension of Merlos and Romo model is developed for considering the effect of soil layering and height-to-width ratio of building on the seismic bearing capacity of shallow strip foundations. These were not considered in earlier studies. The material is assumed to be rigid-perfect-plastic. The model was verified by comparing with the results of homogeneous soils. Also, the effects of some effective parameters on the seismic bearing capacity factor are studied and compared to experimental results. The results emphasize that the bearing capacity is reduced mainly due to the horizontal force of earthquake under seismic loading, which drives the failure surface upward. The results, also, clearly show that the failure surface position and size and the seismic safety of the foundations depend on the acceleration magnitudes developed throughout the seismic event as shown in Figs. 7, 8 and 9. This is supported by the experiments presented in Boulanger and Idriss (2007), Knappett et al. (2006) and Vesic et al. (1965). In both cases of homogeneous or twolayered soils, increasing the PGA of an earthquake or the height of the structure reduces the seismic bearing capacity factor. The cohesion ratio has a complex effect on the seismic bearing capacity factor. In general, the failure surface tends toward the weaker layer. The failure surface would confine in the top layer if this layer is weaker, while if the stronger layer is on the top, the failure surface would penetrate deep into the lower weak layer. If the thickness of the top layer, H1, is greater than 0.5B, then the lower layer is not contributing, and the bearing capacity factor remains constant if the top layer is weaker, while if the top layer is stronger, thicker top layers increase the bearing capacity factor as the contribution of lower weak layer reduces. Also, it was concluded that increasing the foundation depth, Df/B, increases the effect of the lower layer as the failure mechanism penetrates more in the lower layer. A weak top layer increases the seismic bearing capacity factor, while a stronger top layer reduces it. If both the thickness and foundation depth ratios increase equally, the seismic bearing capacity factor remains constant.