Experimental Lnvestigation of Geometry and Number of Helices on the Tensile Bearing Capacity of Helical Piles

Document Type : Original Research

Authors
F. Gharib
E. Seyedi Hosseininia
Ferdowsi University of Mashhad
10.22034/22.5.107
Abstract
Nowadays, helical piles as tensile anchors are widely used in industry. A helical pile is consisted of one or several bearing plates which are attached to the central shaft. A helical pile is installed in the ground by applying a torque on the shaft top together with a thrust force. The main advantages are the ease and rapidity in the installation, instant loading after the installation and no use of soil drilling. In this research paper, the tensile bearing capacity of helical piles in sandy soils is investigated experimentally in small scale. In most cases, the geometry of the bearing places is circular. Considering other geometries such as square or hexagonal may reduce the materials used in the pile fabrication and also, the effect of bearing capacity is not known in the engineering practice. The main aims of this study are to find the effect of the plate geometry as well as the number of bearing plates on the bearing capacity. To do so, helical piles were made up of steel with one, two and three plates with circular, square and hexagonal shapes. The cross section area of all the plates are considered the same. The piles were installed in a reservoir filled with dry medium-dense Firoozkooh sand by rotating manually the pile top. The pile top was drawn vertically under a uplift force. The loading process was performed according to quick method explained in ASTM D3689 standard. Having obtained the variation of force against the pile displacement, the bearing capacity of the pile were assessed. Four different methods were considered here to assess the average bearing capacity by considering different criteria mentioned in the technical literature. As the first step to find the best match theatrical formula, the pile bearing capacity with single circular plate was studied. By investigating several formulas, it is found out that the formula presented by Veesaert and Clemence (1977) matches well with the experiment and thus, this formula was used hereafter in all the analytical investigations. By comparing the experimental results with theoretical formulations, it can be said that the shear cylinder is the dominant mechanism which was observed in all the multi-plate piles. The results also show that the geometry of the plates clearly influences on the tensile bearing capacity. The piles with hexagonal and circular plates have the greatest and the lowest capacity, respectively. Based on the results, a shape factor is defined for the pile with hexagonal plates. The shape factor of the piles with square plate is obtained 1.20 which is consistent with other previous studies. For the pile with hexagonal plate, the shape factor is assessed as 1.34. It is also shown that this bearing capacity is correspondent to an equivalent circumferential circular plate. By using the values of the shape factors (for both square and hexagonal) and the analytical method, the bearing capacity of the piles with two three plates were assessed. A good match is observed between the results of the experiment and analytical method. Again, the results implicitly confirm the dominancy of cylindrical shear mechanism. Results also show that the increase in the number of plates does not necessarily increase the bearing capacity, but it depends on the plate geometry. The reason can be explained by the differences in the perimeter of the helices and accordingly, the effective soil weight which is considered as the generated wedge against the uplift. This can be justified by the results of analytical methods.

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Modares Civil Engineering journal
Volume 22, Issue 5
November and December 2022
Pages 107-123