🕑 Reading time: 1 minutePile foundation that is constructed to resist the uplift forces can be called as Tension Piles. These are also called as uplift piles or anchor piles. In those areas where there are chances of extraction of piles from the ground, the uplift piles will work well. Foundations are broadly classified as deep and shallow foundations based on the load carrying capacity and the properties of the soil at the site. The most widely used type – Pile foundations, come under the category of deep foundations. Any foundation structure that have a depth greater than three times the breadth of the structure can be categorized as pile foundations. Pile foundations form slender and columnar structures intended to transfer mainly compressive loads from large superstructures. The below-transferring medium may be either weak soil or a stratum that is compressible or a strong rock stratum like structure.
Tension Piles Under Uplift ForcesDifferent reasons for the development of the uplift forces are the:
- Seismic Forces
- Overturning Moments
- Hydrostatic Pressure
Uplift Resistance of Piles and Tension PilesIt is recommended to have a long pile in depth to take up the uplift load in the shaft friction. In some situation, it may be difficult to incorporate the long depth due to the presence of a hard rock stratum below the soil. So, to mobilize the remaining frictional resistance, it must be augmented through the addition of dead weight to the pile. This will help overcome the uplift forces. Another solution for this issue is the anchoring of the pile to the rock stratum.
Fig.1. Piles Provided Under the Floor for a Shipbuilding DockIn the other side, the addition of dead weight for the piles is found uneconomical. In some applications like shipbuilding (Figure-1), it is necessary for the piles to take up alternative compressive loads and uplift forces. An increase in dead weight in such situations will tremendously increase the construction costs and the compression rakers will be subjected to high compression loads. These issues can be solved by providing anchors for the piles that will be taken down to the rock layer.
Design of Piles for Uplift PressureThe design of the pile foundation (Tension Piles) is performed such a way that no complete overturning or collapse has to occur under extreme conditions of loading or undergo a displacement at any severe combinations of the working loads. This consideration will not affect the functioning of the foundation under severe load conditions. The country and the followed codes and specifications will define the maximum or the permissible displacement possible for the piles. The ultimate resistance of the pile and the pile group is determined for design when displacement is not a concerned factor. The allowable loads can be calculated by adding a factor of safety to the computed load in the design of pile foundation for uplift pressure.
Analysis of Tension PilesThe universal approach followed for the analysis is the Limiting Frictional Approach. The method will evaluate the uplift resistance of the piles. The piles under uplift might be subjected to uplift pressure that will result in the failure surface. The formation of failure surface is one mode of studying or analyzing the failure in the piles. Another method is to derive empirical relations through experimental investigations. In analysis, piles are considered as cylindrical shaft. Given below is a brief idea of analysis of single pile laying in
1. Clayey Soil
The Ultimate Uplift Resistance Qu = caAs + WpHere, ca = average adhesion along the pile shaft Wp = Weight of the Pile, The embedded pile will possess a surface area As and the undrained cohesion value is cu.
Fig.2. Relationship between Ca/Cu & the Cu (Undrained Shear Strength)The figure-2 below shows the graph between Ca/Cu and Cu developed by (Sowa, 1970). These values in the chart is developed almost correctly for a pile under downward loading. The ratio Ca/Cu is more for soft clays and the stiffer clays will have lesser value.