Comparative Study on the Influence of Pile Length and Diameter on Bearing Capacity and Efficiency of Root Piles, Straight-Shaft Piles and Pedestal Piles

In order to popularize the application of root pile (RP), which is a miniaturized application after the simplified construction procedure of root foundation and a promising new type of noncircular section pile, and to verify the bearing capacity and efficiency advantages of RP over traditional straight-shaft pile and pedestal pile (SP and PP, respectively), on the basis of validation of the numerical model, numerical simulation of the three pile types with different lengths and diameters and root sizes was carried out under vertical (including uplift and compressive) and lateral static loading. The results show that both the vertical and lateral bearing capacities and efficiency of RP are higher than those of SP and PP, and the larger the pile length and diameter are, the greater the advantages are. Specifically, compared with the bearing capacity of SP, vertical capacity could be improved greatly only by RP instead of PP when the pile length is no less than 15 m, and not the enlarged base of PP but the roots arranged on the upside of RP could improve lateral capacity. Besides, increasing pile length could not improve lateral capacity when the pile length is no less than 10 m, and both the vertical and lateral bearing capacities could be improved by increasing pile diameter. Two efficiency comparison methods were put forward, and their application condition, advantage and disadvantage were analysed to compare the efficiency of pile length and diameter in improving capacity, according to which the economic way to improve vertical and lateral capacities is to increase pile length and diameter, respectively. Enlarged base of PP and roots of RP could be played more fully when the displacement is large enough; therefore, the difference of capacity and efficiency of the three pile types with failure criterion of small displacement (10 mm lateral and 30 mm vertical displacement) is smaller than that with large displacement (30 mm lateral and 60 mm vertical displacement). Furthermore, in order to facilitate the application of RP, it is suggested that roots length should not exceed 0.5 times of pile diameter, and the height and width should not greater than the spacing between stirrup bars or main steel bar, respectively. In addition, roots annulus spacing of 0.4 ~ 0.5 m is appropriate, and the spacing of root layers should not be less than 2.4 m. On the whole, it is of great significance to popularize root piles in practice.