The 12 th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India Effect of Cavity on Bearing Capacity of Shallow Foundations in Geotextile-Reinforced Soil M. Ghazavi Dept. of Civil Engineering, K N Toosi University of Technology, Tehran, Iran Y. Soltanpour Dept. of Civil Engineering, K N Toosi University of Technology, Tehran, Iran Keywords: shallow foundation, bearing capacity, underground cavity, FLAC, geosynthetics ABSTRACT: Underground cavities near shallow foundation may decrease the bearing capacity. On the other hand, the bearing capacity increases when the foundation soil is reinforced with geotextiles. The use of geotextiles may compensate the reduction of bearing capacity due to the presence of cavities. In this paper, numerical investigation of such phenomenon is presented using FLAC-3D software. Mohr-Coulomb failure criterion is assumed to govern the soil failure. Interface elements are used to allow slip between the soil and geotextiles. Strip, circular, and square foundations have been used and their bearing capacities compared. The ratio of bearing capacities of reinforce upon unreinforced soil is determined and compared for each type of foundation. The results have shown that with the presence of cavity at a specific depth, the bearing capacity ratios (BCR) are 1.004, 1.1, and 1.22 for strip, square, and circular foundations respectively. It is generally found that the reinforcement of the soil above the cavity crest can compensate the reduction of bearing capacity of footings. 1 Introduction In some situations, the construction of shallow foundations above the underground cavities is inevitable. It has been found that in these situations, the bearing capacity of foundations decreases (Baus, 1978; Baus and Wang, 1983; Badie and Wang, 1984; Wang and Badie, 1985; Wang and Jao, 1998). A solution to compensate this reduction of the bearing capacity may be reinforcement of the soil above the cavity, hereafter referred to as cavity ccrest, using geosynthetics. In this paper, attempt has been made to investigate numerically the influence of reinforcement of the cavity crest on the bearing capacity of shallow foundations constructed above underground cavities. Accordingly, FLAC 3D software (version 3) was used to simulate the ‘cavity-reinforcement-footing’ system. In the analysis, a comparison was made with the case where no reinforcement layer was used. 2 Modeling procedure The footings were assumed to be square, strip, or circular. First, a strip footing having a width of 3 m is assumed and simulated. A value of 5 kN/m 2 was assumed for the soil cohesion. The soil friction angle was assumed to be 35 o . These values are kept the same in all analyses. For simplicity, no embedment depth has been assumed for footings. The depth and radius of hole are constant. The hole has a circular shape and its axis is parallel to the length of foundations. The distance of the first geotextile layer from the ground surface is represented by U. The width of geotextile is shown by b. The thickness of the reinforced soil is expressed by d and is kept 2 m in all anayses. The depth of the cavity is 7 m from the surface (Fig. 3). The boundary of the soil zone is assumed to be 20 and 7 m in the lateral and vertical directions, respectively. To verify the numerical simulation, first, the non-reinforced soil between footing and the cavity cret is modelled and the results are compared with those given by Shin and Das (2000), as shown in Figure 1. In their experimental studies, Shin and Das (2000) reinforced the soil by one layer of reinforcement located at depths ranging 0.35-0.4 times the foundation width below the footing. 3210