The development of the effective method for the reinforcement of existing foundations is an urgent problem to improve the stability of urban buildings. Micropile, which is a cast in place mortar pile of small diameter, is one of the most promising methods for reinforcing foundations and improving their bearing capacity. The mechanism which enhances the bearing capacity of foundations reinforced with micropiles under the static loading condition was the subject of this study. A series of loading tests were carried out on the model micropile-foundations located on loose sand. Micropiles of 6mm and 200mm in diameter and length, respectively, were prepared. Rigid circular foundation with a diameter of 100mm was reinforced with micropiles. Various arrangements of micropiles were tested; the number and inclination angle of micropiles were parametrically changed. The micropile foundations placed on sand grounds with low relative density were loaded. From the comparative examination of the observed behavior of micropile foundations, the influence of micropiles' arrangement on the mechanism and improvement of bearing capacity of foundation was discussed. To assess quantitatively the degree of improvement in the bearing capacity of surface foundations reinforced with micropiles, an index R called "Network Effect Index" was introduced in this study. The index R of unity means that the bearing capacity of foundations reinforced with micropiles is simply equal to the summation of the individual value of the surface foundation and that of the micropile group. There is an upward trend in the rate of index R when the number of micropiles is increased. On the other hand, in high numbers of micropiles used to reinforce the foundation, index R declines with increasing the inclination angle. In the case of micropiles with low-inclination-angle being implemented, bearing capacity is improved remarkably; An index R of 1.997 is achieved in this study where 8 micropiles inclined at an angle of 15° were used to reinforce the foundation.
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