Piles are long and slender structural elements that are driven or bored into the ground to support various types of structures, such as buildings, bridges, offshore platforms, etc. Piles can transfer loads to deeper and stronger soil layers or rock formations by end bearing or skin friction, or both.
Sometimes, a single pile is not enough to carry the load of a structure or to provide adequate stability against lateral forces. In such cases, piles are arranged in groups or clusters to increase the load-carrying capacity and resistance of the foundation system. However, when piles are combined in a group, their behavior becomes more complex and different from that of a single isolated pile. This phenomenon is known as group action of pile.
Group action of pile refers to the interaction between piles in a group and the surrounding soil due to their close proximity and collective load transfer. Group action can affect various aspects of pile performance, such as bearing capacity, settlement, load distribution, efficiency, stiffness, etc.
The main factors that influence group action of pile are:
- The number and arrangement of piles in a group
- The spacing between piles
- The length and diameter of piles
- The type and properties of soil
- The mode and magnitude of loading
Depending on these factors, group action can have positive or negative effects on pile performance. For example:
- Group action can increase the bearing capacity of end-bearing piles on rock or dense sand by creating a composite block effect.
- Group action can reduce the bearing capacity of friction piles in clay by reducing the effective stress and increasing the pore water pressure around the group.
- Group action can reduce the settlement of pile groups in sand by compacting the soil during driving or by mobilizing passive resistance from adjacent piles.
- Group action can increase the settlement of pile groups in clay by increasing the consolidation time due to reduced drainage paths or by inducing negative skin friction from overlying soil layers.
- Group action can affect the load distribution among piles in a group depending on their relative stiffness and position within the group.
To analyze and design pile groups considering group action effects, various methods have been proposed based on empirical formulas, theoretical models, numerical simulations, field tests, etc. Some common methods are:
- Equivalent raft method:
This method assumes that a virtual raft exists at some depth below which all piles act together as one unit. The settlement of this raft is calculated using conventional methods for shallow foundations.
This method assumes that a virtual raft exists at some depth below which all piles act together as one unit. The settlement of this raft is calculated using conventional methods for shallow foundations.
- Interaction factor method: This method considers that each pile in a group interacts with other piles through soil deformation. The interaction between two piles is quantified by an interaction factor that depends on their relative position and spacing. The total load on each pile is obtained by superposing its own load with those transferred from other piles through interaction factors.
- Elastic continuum method: This method treats both soil and pile as elastic materials with certain modulus values. The deformation and stress distribution within this elastic continuum are calculated using analytical solutions or numerical methods such as finite element analysis.
In conclusion, group action of pile is an important phenomenon that affects the performance of pile foundations. It should be taken into account during analysis and design to ensure safety and economy of structures supported by pile groups.
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