Pile foundation is the most common type of deep foundation used to transmit structural loads, namely axial load and lateral load into the deeper layers of firm soil. It is required to understand the types of loads on piles and their transfer mechanism in order to select and design a type of pile.
Axial loads create compressive or tensile forces that act parallel to the axis of the foundation. If the pile is vertical, then the axial load is equal to the ly applied load. Lateral loads generate moments and shear and subsequent lateral deflection in the pile foundation. The lateral deflection activates lateral resistance in the adjacent soil.
1. Axial Loads
An axial load may be either compressive (downward) or tensile (uplift). When it is compressive, deep foundations resist the load using friction resistance and toe bearing resistance, as shown in Fig. 1.
However, when the load is tensile, the resistance is caused by side friction and the weight of the foundation as shown in Fig. 1. In deep foundations with an enlarged base, uplift loads are also resisted by bearing along the ceiling of the enlarged base. Axial loads are comprised of dead loads, live loads, snow and ice loads which are transferred from superstructure to the pile foundation.
Dead and Live Loads
Dead loads can be calculated after the structural designer has provided all the details about the design of the superstructure. As for the live loads, applicable codes are used to compute live load based on the type and function of each space within the building.
If no such information is made available to you, it is possible to decide an initial estimate of loading for each floor in case of high-rise buildings which ranges from 10 to 15kPa/storey. Self-weight of a pile foundation is based on the raft thickness, dimension and number of piles, and concrete's unit weight.
2. Lateral Loads
Lateral loads produce both shear and moment in a deep foundation, as shown in Fig. 2. These shears and moments produce lateral deflections in the foundation, which in turn mobilize lateral resistances in the adjacent soil.
The magnitudes of these lateral deflections and resistances, and the corresponding load-bearing capacity of the foundation depend upon the stiffness of both the soil and the foundation.
Pile foundations usually find resistance to lateral loads from passive soil resistance on the face of the cap, shear on the base of the cap, and passive soil resistance against the pile shafts. The latter source is usually the only reliable one.
Wind loads create a significant eccentric loading on the foundation plan as shown in Fig. 3. As a rule of thumb, wind load on a structure can be considered as 1.5% of the dead load or 2kPa pressure for structures up to 200m. If the structure is higher than 200m, then wind tunnel test is used to compute wind pressure. Different standards have provided procedures for the estimation of wind loads such as ASCE7 and AS1170.2–2011.
Similar to wind loads, earthquake loads generate a large eccentric loading on the foundation plan. This type of load is mostly horizontal and needs to be considered during pile design.
The designer should consider the inertial effects due to loads applied to the pile by the supporting structure like kinematic effects due to ground movements generated by the earthquake acting on the pile, possible loss of soil support during the earthquake due to liquefaction or partial loss of soil strength. Earthquake loads are computed using response spectra and dynamic structural analysis.
Loads Due to Earth Pressures
Loads due to earth pressure are particularly related to basement walls and substructure system. From a very early stage of the design, earth pressure theory can be used to compute loads due to earth pressure. However, soil-structure interaction is employed for a and final design.
Loads Due to Ground Movements
Ground movement is another cause for the lateral loads acting on a pile foundation. It is able to consider the interaction between the foundation system and the source of ground movement via the magnitude of the ground movements, rather than trying to directly convert the ground movement to an equivalent force.
3. Other Loads
Other sources of loading that may need to be considered include snow, ice, thermal effects, major impacts and explosions. Requirements to consider such loads are set out in the relevant standards that govern the structural design of buildings.