Uplift pressure, also known as hydrostatic uplift, is a force that pushes a structure upward and has the potential to elevate it relative to its surroundings. This occurs when the pore water pressure is greater than the overburden pressure of the structure. Uplift pressure can become problematic if it exceeds the weight of the structure exerted downwards. To address this, ASCE 7-10 recommends taking into account the full hydrostatic pressure applied over the entire area and measuring the hydrostatic load from the underside of construction.
To prevent risks associated with water pressure, it is essential to design the structure to withstand uplift forces effectively. This involves considering uplift pressure during the design stage since rehabilitating and maintaining the foundation can be challenging in the future. It is crucial to assess the possibility of this upward force on the foundation when the groundwater table is higher than the bottom of the foundation level. As hydraulic uplift pressure acts in an upward direction, it reduces the downward weight of the structure and poses a threat to its stability.
How to Check for Uplift Pressure?
To comply with relevant codes, the weight of a substructure must be greater than the uplift pressure. This means that the sum of downward load per given area must be larger than the vertical load per given area. Specifically, the downward load should be at least 25% greater than the upward load caused by hydrostatic pressure.
To determine if uplift force is present below the structure being considered, the following equation can be used:
Equation 1: where: Ø represents the mass reduction factor and can be taken as 0.90, W represents the mass of the structure, w represents the specific weight of water, h represents the height of water from the water table to the bottom of the foundation, and 1 represents the factor of safety.
Consequences of Uplift Pressure
Strategies for Prevention of Uplift Pressure
The use of dewatering procedures is helpful in directing water away from construction sites, however, it is not a sustainable solution for the entire lifespan of the construction due to its lack of economic viability. As such, alternative methods must be employed to mitigate the impact of uplift pressure.
Various methods can be employed to avoid the influence of uplift pressure. These methods are designed to prevent the water table from rising to an extent where it negatively affects the construction. Techniques such as the use of watertight membranes or grout curtains can help to prevent water from infiltrating the construction site. Additionally, the installation of drainage systems can help to divert excess water away from the construction area. The use of prefabricated vertical drains can also help to accelerate the drainage of excess water.
Overall, while dewatering procedures may be effective in controlling water in the short-term, more sustainable solutions must be sought for the entire lifespan of a construction project. By employing alternative methods, the negative impact of uplift pressure can be mitigated, ensuring the long-term durability and stability of the construction.
1. Toeing in the Foundation into the Surrounding Ground
When constructing a substructure within an open excavation or temporary cofferdam, it may be necessary to extend the base slab or raft foundation beyond the perimeter in order to enhance its resistance to uplift. By adding weight to the tied-in foundation through the use of backfill material, the weight of the structure is increased, thereby strengthening its resistance against uplift pressure. However, it should be noted that this method is not a viable option in situations where a diaphragm or secant pile wall is used as a permanent retaining structure. In such cases, alternative methods must be employed to ensure the structure’s stability and integrity.
2. Increase Self-weight of the Structure
It is possible to increase the self-weight of a building by increasing the size of its structural members. This method can be used to add more weight to the lower levels of a high-rise complex. Another way to increase the dead weight of the low-rise podium is by creating a rooftop garden with a thick layer of soil. This will not only add weight but also provide additional benefits such as insulation, absorption of rainwater, and a pleasant aesthetic view. Incorporating a rooftop garden with a thick soil fill can be an effective way to increase the weight of the lower levels of a high-rise building.
3. Ground Anchors
Ground anchors can be utilized as a temporary solution to address the issue of uplift forces by employing pre-stressed anchors. However, the long-term performance of permanent ground anchors is limited due to concerns regarding their susceptibility to corrosion.
4. Piling Method
The piling method is widely recognized as the most effective approach to counteracting uplift pressure. Depending on the soil conditions and the specific requirements of the site, various types of piles can be utilized for this purpose. These include end bearing piles, friction piles, un-reamed piles, sheet piles, tension piles, and compaction piles. Table 1 provides a comprehensive overview of the different types of piles commonly used to withstand uplift pressure, taking into account the characteristics of the soil at the site.
Table 1 Types of Piles Chosen Based on Soil Conditions to Withstand Uplift Pressure
| Types of Pile | Suitable Types of Soil |
| End bearing pile | Used when bedrock or rocklike material is present at a site within a reasonable depth. Piles are extended to the rock surface. The ultimate bearing capacity of the pile based on the underlying material. |
| Friction pile | It is used in cohesionless soil in which end bearing pile would be very long and uneconomical. The load on pile is resisted mainly by skin/friction resistance along the side of the pile. Capacity of the pile is a function of the shaft area in contact with the soil. Friction piles are often used to increase the density and thus the shear strength of soil. |
| Unreamed pile | It is used in black cotton soils, filled up ground and other types of soils having poor bearing capacity. |
| Sheet pile | Sheet piles, which made of thin plates of concrete; timber; or steel, driven into the ground for separating members or stopping seepage of water. it is not constructed for carrying any vertical load. |
| Tension pile | Tension piles uplift piles anchor down the structures subjected to uplift pressure. |
| Compaction pile | It is driven in loose granular soil. |