Knowing the resistance and load capacity of our Helicas, when applied to the ground, is something essential that our technical team performs daily. Capacity may be limited by geotechnical parameters or by structural parameters. In this article we will only address the geotechnical capacity to vertical loads of the Helicas model P.
Helicas may have one or more helix. In the case where there is only one helix, the resistance of the element is given by the tip resistance conferred by the helix. This state-of-the-art resistance, considering only the geotechnical parameters, is determined using the cohesion of the soil, the maximum permissible stress of the soil, its mass and its internal friction angle. In addition to the tip resistance, in cases where there is only one helix there is also a contribution of the friction of the tube with the terrain. Thus one can obtain the vertical capacity of the Helicas using an expression of the type:
Pu = q x Na + α x H x (π x d)
Where the first part of the expression represents the tip resistance of the helix and the second part the friction generated between the tube and the soil.
In cases where we have more than one helix along an Helica, the capacity calculation has some differences. When the helixes are too far apart from each other, the tip resistance method can be used by adding up the tip resistance conferred by each helix along the element. In cases where the helixes are close to each other, the method of tip resistance may not be the most appropriate. The rupture can be given by the creation of a surface delimited by the outer contour of the helixes, reducing the capacity of the element compared with the method of the sum of the tip resistances of the helixes.
Image obtained by the study Numerical study of single helical piles and helical pile groups under compressive loading in cohesive and cohesionless soils
Thus, in the case where more than one helix is used, in order to increase the capacity of the element, it is always best to approximate as closely as possible the values obtained by the method of the sum of the tip resistance of each helix and the method of cylindrical rupture by the contour of the helixes. To achieve this, one of the very important things is to know the minimum distance between helixes to be used where a change in the rupture mode occurs. Although this distance depends on several factors, such as the size of the helix, soil density, soil capacity, existence of water in the soil and depth, the value of 2 to 3 times the diameter of the helix can be used as a pre-design rule.
One might also think that the area of the tip of the tube could be considered as a resistant area in the tip capacity. The tubes of our Helicas model P are opened at their tip and what happens during the installation is that the soil enters the pipe up to a certain maximum height that depends on the type of soil. The soil inside the tube achieves a high degree of compaction and as such, the existence of the same inside the tube contributes positively to the vertical resistance of the element. As the determination of the increase in capacity conferred by this soil inside the pipe is not easy to account, in a simplified and conservative way its contribution to the capacity of the element can be disregarded.
During the installation of the Helicas, the rotation of the pipe in the ground causes the soil in contact with the pipe to become looser and can even lead to the creation of a vertical hole with a diameter slightly larger than the tube. This also happens due to the coupling elements that have a larger diameter than the pipe and generate a loss of cohesion of this soil. As a consequence, the friction of the soil with the Helica tube can be disregarded conservatively, although it is known that this has some contribution in the capacity of the element.
The load tests that Helica carries out continue to provide useful information to improve the calculation methods and justify the formulation.
If you need more information, do not hesitate to contact Helica.
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