What are Piles in Construction?
Definition of Piles in Construction
Piles are structural supports used to support the foundations of a building or other structure. The use of piles is normally required due to the inability of the soil to support the designed foundations of a project. Piles have allowed the construction of significant projects, on sites that would not support the structures, unless piles were used. There are various types of piles, designed for different conditions. The end bearing pile is exactly that, the pile is driven down into the ground until the designed strength is reached. The design of a end bearing pile, is normally a pile that is driven to what is referenced as refusal. Refusal means that the pile driver is incapable of driving the pile any deeper into the soil without the pile collapsing. All piles have a yield point, at which time, further driving of the pile will compress the pile and it will fail. The driving of a pile to refusal is an end bearing pile by design.
A friction pile, is a pile that achieves its strength from the friction that develops between the sides of the pile and the earth, or whatever material the pile is being driven into. The load bearing strength is determined by the number of blows per foot that the pile driver achieves. To identify the proper depth of a friction pile, the number of hammer blows per foot are counted, as the hammer blows increase, this indicates that the friction between the piles sides and the medium being driven into is increasing. It is by this calculation that the depth of the pile is determined to be sufficient for the loading require The blows per foot are very elementary to calculate. The piles are physically marked with a can of paint and a measuring taper prior to being driven at one foot increments. As the pile is driven into the ground the observer witnesses the piles one foot markings and literally counts the number of hammer blows required to drive the pile each foot of its length. As the blows increase, per foot of penetration, the friction increases. The structural engineer has noted that a certain amount of blows per foot for a specific type pile driving hammer results in a load bearing capability that is calculated. The engineer will inform the inspector that once the pile exceeds a certain amount of blows per foot, the structural integrity of the pile has been reached and the pile driver can move onto the next pile.
A thermal pile, is a specialized pile that is used in areas of permafrost, which is ground that is permanently frozen. A thermal pile is a friction pile that achieves its strength from the bond of the frozen material around its circumference to the pile surface. This type of pile is not driven into the ground. This pile is set in a drilled hole, that is drilled larger than the pile itself. The setting of this pile is maintained within the center of the hole, to ensure that there is an equal spacing around the circumference of the pile, for a medium such as sand to be placed. The pile itself is designed with a thermal radiator assembly installed within the center of the pile itself. This assembly is basically a piping system, that transfers a fluid up and down the interior of the pile. This fluid must be engineered to vaporize at a temperature below the freezing point of the medium placed around the outside of the pile. The intent of the system, is to freeze the material around the pile solid, which is where the friction strength is created. This freezing is maintained by the fluid’s physical characteristics that allows it to vaporize at a temperature below freezing, rise up within the piping system into a radiator type assembly which is mounted on the top of the pile. This radiator assembly cools the vapor down to the point that it becomes a liquid again and runs down the piping located within the piles, to the bottom of the system. This repeated vaporizing and condensing, is the process that transfers energy, enabling the material surrounding the pile to remain frozen and the friction capabilities maintained.
Helical piles, the term helical piles are piles that are basically screwed into the earth. The helical piles are engineered in such a fashion, as to have the bearing capacity of the pile created by the friction of the helical fins, or screw threads against the soil, producing a bearing strength. There are various types of helical piles from various manufacturers. Depending upon the location of the project, as well as the types of soils, will determine the proper types of helical piles to be used. The use of helical piles is becoming more common, especially along the shoreline and in sandy and silty soils.
Compacted gravel piles, the term is used for the compaction and vibration of gravel or a mix of products in a drilled hole. This drilled hole or excavation as well as the pattern for these piles is designed and identified by the structural engineer. These compacted piles basically consolidate the entire site to a greater extent than prior to the procedure, due to the vibrations and compaction of each individual pile location. The transfer of forces created due to the vibration and compaction of the material throughout the site, has a tendency to consolidate all the areas. There are various different methods of compacted gravel piles as well as grout injected piles. Each process, strengthens the soil to accept an increase in bearing capacity beyond the limits of the original soil.
Piles that are driven, are normally driven by a pile driver. The pile driver will have different weight ram and are hydraulically driven, to produce tremendous impact forces on the top of the pile. The normal identification of how a pile is being driven is either the end bearing pile that is driven to refusal, or the friction pile that is driven to a limited blows per foot. During the installation of friction piles, the yield point of the pile must be carefully engineered to prevent failure of the pile. If a failure occurs, the pile is either pulled from the ground and reset, or abandoned, and another pile is driven in its place.