Abstract: The five-star-shaped concrete pile is a new type of pile with a special shaped cross-section. It is formed by cutting five circular arcs in a circle pile. The main parameters of the cross section include the radius of the circumcircle R and the corresponding circumcircle central angle θ, the radius of inward cutting circular arc r and the corresponding circumcircle central angle 2x. Through the optimization analysis of the section size, the maximum of the circumference of the five-star-shaped piles and a maximum ratio of perimeter to the area of five-star-shaped piles are derived. To further explore the vertical bearing capacity of five-star-shaped piles, experimental studies are carried out on the comparison of four single piles in the dry sand by the sand pouring method. These four single piles are the maximum ratio of perimeter to the area of the five-star-shaped pile F1, the maximum of the circumference of the five star-shaped pile F2, the round pile C1 with the same perimeter of F2, and round pile C2 with the same cross-sectional area of F2. The following conclusions are drawn from this study. The side surface area of F2 gets close to that of F1, and the ultimate bearing capacity is approximately the same. The concrete dosage of F2 is 0.75 times that of F1, the bearing capacity per concrete is higher, and thus F2 is optimal cross-sectional pile. Compared with C2, the side surface area of F2 is 1.53 times that of C2, and the ultimate bearing capacity of F2 is 2.4 times that of C2. It shows that F2 has better bearing capacity than that the same amount of concrete, and the expansion effect of cross-section is obvious. Compared with C1, F2 pile cross section area is 0.44 times that of C1, the ultimate bearing capacity is 0.96 times that of C1, and the unit volume concrete bearing capacity of F2 pile is 2.21 times that of C1, which exhibits higher load cost performance. At all levels of load, the ratio of the lateral friction of the five star-shaped piles is more than 80%, especially F2, and the proportion of side friction resistance is above 90%, which shows the characteristics of friction pile. This study provides theoretical support for the engineering application of five-star-shaped piles.

Key words: five-star-shaped pile, section optimisation, bearing mechanism, model test, sand pour method