Pile foundations are a type of deep foundation used to support large superstructures. They are characterized by their slender and columnar structures, which are designed to transfer mainly compressive loads. These foundations are typically used in areas where the soil is weak, compressible, or where there is a strong rock stratum. Pile foundations are classified as such if they have a depth greater than three times the breadth of the structure. In areas where there is a risk of the piles being extracted from the ground, tension piles, also known as uplift piles or anchor piles, are constructed to resist uplift forces.
Tension Piles Under Uplift Forces
Large structures like transmission towers, tall buildings, jetties, and chimneys are subject to various forces that can cause uplift. Some of these forces include seismic activity, hydrostatic pressure, and overturning moments. Skyscrapers, in particular, are vulnerable to high wind loads or seismic forces, which can lead to overturning and the development of uplift forces. To prevent uplift, the foundation (pile) must be designed to safely transmit the compression and tension forces to the ground. Tension piles are constructed to transfer the uplift force safely, using the frictional force developed along the length of the pile due to under-reaming. Sheet piling walls are used to resist any horizontal forces, and the vertical piles must be of sufficient depth to provide shaft friction and resist uplift. Modifications in the design of piles can significantly reduce uplift forces.
Uplift Resistance of Piles and Tension Piles
It is often advised to have a substantial depth of pile in order to effectively counteract uplift loads through shaft friction. However, in certain cases where there is a hard rock layer beneath the soil, achieving the desired depth may be challenging. In such situations, to make up for the reduced depth and still harness the necessary frictional resistance, additional dead weight can be added to the pile. This extra weight helps to overcome uplift forces. Alternatively, another solution to address this issue is to anchor the pile to the rock stratum, ensuring stability and adequate uplift resistance.
Fig.1. Piles Provided Under the Floor for a Shipbuilding Dock
In certain scenarios, adding dead weight to piles is not considered cost-effective. For example, in shipbuilding (as depicted in Figure-1), the piles may need to withstand compressive loads and uplift forces in alternative ways. However, increasing the dead weight in such cases would significantly raise construction costs and subject compression rakers to excessive loads. To address these concerns, anchors can be installed for the piles, extending down to the rock layer, as a viable solution.
Design of Piles for Uplift Pressure
The design of tension pile foundations aims to ensure that they can withstand extreme loading conditions without experiencing complete overturning, collapse, or significant displacement. The maximum allowable displacement for piles is determined based on the codes and specifications of the country where the foundation is being built. The ultimate resistance of the pile and pile group is determined based on the design criteria that do not take into account the concern for displacement. To ensure safety, a factor of safety is added to the computed load when calculating the allowable loads for uplift pressure in the pile foundation design.
Analysis of Tension Piles
The analysis method used for evaluating the uplift resistance of piles is the Limiting Frictional Approach. When piles are subjected to uplift pressure, a failure surface may form, and this is studied as one mode of analyzing pile failure. Alternatively, empirical relations can be derived through experimental investigations. In analysis, piles are treated as cylindrical shafts. The context provided offers a brief overview of the analysis of a single pile laying in a specific scenario.
1. Clayey Soil
The equation “The Ultimate Uplift Resistance Qu = caAs + Wp” represents the relationship between the uplift resistance (Qu) of an embedded pile and various factors. In this equation, “ca” represents the average adhesion along the pile shaft, while “Wp” represents the weight of the pile. The embedded pile is assumed to have a surface area denoted as “As”. Additionally, the undrained cohesion value is also considered in the equation. This equation provides a mathematical expression for calculating the uplift resistance of a pile, taking into account factors such as adhesion, weight, surface area, and undrained cohesion.
Fig.2. Relationship between Ca/Cu & the Cu (Undrained Shear Strength)
In Figure-2, which was developed by Sowa in 1970, a graph is shown depicting the relationship between the ratio of Ca (adhesion forces) to Cu (undrained shear strength) for piles under downward loading. The values on the chart appear to be mostly accurate, with higher values of Ca/Cu indicating softer clays, while lower values indicating stiffer clays. This graph illustrates the trend between Ca and Cu for different types of clay soils, with the ratio Ca/Cu being higher for softer clays and lower for stiffer clays.
2. Sandy Soil
The gross uplift Qu of piles placed in sandy soil is influenced by the skin resistance that develops between the pile shaft and the soil. This skin resistance is crucial in determining the overall uplift capacity of the pile. The diameter of the pile, denoted as ‘d’, and the embedment length, denoted as ‘L’, are important factors that impact the skin resistance and consequently the gross uplift Qu of the piles. The skin resistance is the result of the interaction between the pile and the surrounding sandy soil, and it plays a significant role in the overall performance of the pile in resisting uplift forces. Therefore, understanding the relationship between skin resistance, pile diameter, and embedment length is crucial in designing and analyzing pile foundations in sandy soils.
The coefficient of earth pressure, denoted by Ks, represents the relationship between lateral earth pressure and vertical effective stress in a soil mass. It is a key parameter used in geotechnical engineering to analyze the stability of soil structures such as retaining walls and sheet piles. Another important factor in soil mechanics is the soil-pile frictional angle, which characterizes the resistance to sliding between the soil and the pile. The soil-pile frictional angle is influenced by the effective unit weight of the soil, which takes into account the density and moisture content of the soil. By considering the interplay between Ks and the soil-pile frictional angle, engineers can make informed decisions about the design and construction of soil structures to ensure their stability and performance.
