The Evaluation of Footing Bearing Capacity Rest on Top of Retaining Wall Reinforced by Geogrid in Saturated and Dry Soil

Reinforced soil is a common technique to improve the soil properties and can be used in design of foundations and retaining earth structures. Reinforced earth structures are embankments which are reinforced with reinforcing elements such as geogrids, steel straps, etc. This study evaluates the strip footing bearing capacity that rest on near the geogrid reinforced retaining walls in saturated and dry sandy soil conditions. The previous researches have indeed studied the effects of many different parameters on the strip footing bearing capacity including the number of reinforcing elements, reinforcement depth, vertical distance of the reinforcing elements, etc. However, the retaining walls behavior in saturated embankment conditions has not so far been studied up to now so the emphasis in this article was to study the effect of saturation condition on the footing bearing capacity near the reinforced walls. For this purpose small scale laboratory model tests were carried out to investigate the behavior of strip footing bearing capacity that rest on near the geogrid reinforced retaining walls. A steel frame model box with inner dimensions of 0.5 m × 0.5 m in plan and 0.5 m in height was used. One side of the test box was made of Plexiglas for observations during the tests. The strip footing was made of a steel plate 0.49 m in length, 0.05 m in width and 0.02 m in thickness. An aluminum plate with thickness of 3mm used as retaining wall model. A two-way geogrid sheet with a tensile strength of 20KN/m was used to reinforce the sand bed. The sand bed prepared by sand raining technique and a water tank placed on top of the frame to saturate the bed and Overall, 90 tests were conducted. To evaluate the effect of geogrid length on strip footing bearing capacity in dry condition, three different lengths (L/B=3, 4, 5) was used. The bearing capacity of the strip footing increases with an increase in the geogrid length. Increasing of geogrids lengths prevents expansion of the failure area and allows for wide distribution of applied loads. Based on the BCR diagrams reveals that in most diagrams, the slope of the first part (i.e. L/B=3 to L/B=4) is larger than the slope of the second part, which indicates that an increase in the length to the L/B=4 level causes a significant change in the strip footing’s bearing capacity. As a result, larger increases do not have significant effects on the strip footing’s bearing capacity. Therefore, in this study, the L/B=4 length ratio was recommended as the optimum ratio considering economic problems. To study the effect of the geogrid depth on the footing bearing capacity in the saturated embankment, the bearing capacity at four different depth ratios of u/B=0.25, u/B=0.5, u/B=0.75, and u/B=1.0 are used and that compared with dry condition. The results are showed that increasing the geogrid depth introduced a descending trend in the bearing capacity of the strip footing, so that the full capacity of the geogrid sheet could not be utilized. One reason for this is that increasing geogrid depth would compress the soil between the footing and the geogrid, leading to large settlements. Considering the conditions of the present study, the depth ratio of u/B=0.25 selected as the optimum ratio.