Pile is a construction element used for the foundation of works, which allows loads to be transferred to a resistant stratum of the soil, when it is at such a depth that it makes a more conventional foundation using footings "Footing (foundation)") or slabs technically or economically unfeasible.
It has the shape of a column "Column (architecture)") placed vertically inside the ground on which the element that transmits the loads (pillar, capping, slab...) rests and which transmits the load to the ground by friction of the shaft with the ground, supporting the tip in more resistant layers or by both methods at the same time.
Types of piles
First piles
It is the oldest type of pile, usually made of wood, and was invented to make foundations in areas with wet soil, high water tables, or flooded areas. They were made of wood, trunks simply stripped of bark, and their bearing capacity was highlighted either by reaching a sufficiently resistant layer of the ground, or by friction of the pile with the ground. A well-known example is the city of Tenochtitlan, part of present-day Mexico City, built on the bed of Lake Texcoco.
Replacement piles
The name is applied when the construction method consists of making a hole in the ground which, once finished, will be reinforced inside and later filled with concrete.
Sometimes, the material on which it is being laid is a frictional soil (such as sand, coarse materials and silt, which can be considered frictional materials since having such a fragile cohesive structure, any movement such as that produced by the drill bit or tool when drilling or the simple presence of water in the soil, among others, causes said cohesion to break and the material works as a frictional soil); This is why collapses occur inside the walls of the perforation. This phenomenon is called "falling", and therefore various methods are used to prevent it from occurring.
Due to the way the casting is carried out, two types of piles are basically distinguished: extraction piles and displacement piles. An extraction pile is made by extracting the ground, while the displacement pile is made by compacting it. In both cases, different techniques are used to maintain the stability of the excavation walls.
Deep Foundation Risk Assessment
Introduction
Pile is a construction element used for the foundation of works, which allows loads to be transferred to a resistant stratum of the soil, when it is at such a depth that it makes a more conventional foundation using footings "Footing (foundation)") or slabs technically or economically unfeasible.
It has the shape of a column "Column (architecture)") placed vertically inside the ground on which the element that transmits the loads (pillar, capping, slab...) rests and which transmits the load to the ground by friction of the shaft with the ground, supporting the tip in more resistant layers or by both methods at the same time.
Types of piles
First piles
It is the oldest type of pile, usually made of wood, and was invented to make foundations in areas with wet soil, high water tables, or flooded areas. They were made of wood, trunks simply stripped of bark, and their bearing capacity was highlighted either by reaching a sufficiently resistant layer of the ground, or by friction of the pile with the ground. A well-known example is the city of Tenochtitlan, part of present-day Mexico City, built on the bed of Lake Texcoco.
Replacement piles
The name is applied when the construction method consists of making a hole in the ground which, once finished, will be reinforced inside and later filled with concrete.
Sometimes, the material on which it is being laid is a frictional soil (such as sand, coarse materials and silt, which can be considered frictional materials since having such a fragile cohesive structure, any movement such as that produced by the drill bit or tool when drilling or the simple presence of water in the soil, among others, causes said cohesion to break and the material works as a frictional soil); This is why collapses occur inside the walls of the perforation. This phenomenon is called "falling", and therefore various methods are used to prevent it from occurring.
The types of in situ piles are included in the Building Technology Standards.[1].
Usually as shallow piling working by tip, supported on rock or hard layers (firm stratum) of terrain, after crossing soft layers. Also as piloting working by shaft and tip in medium or loose granular terrain, or in terrain with alternating coherent and granular layers of some consistency.
This type of pile is executed by excavating the ground and using a sleeve (metal tube as a formwork), which prevents the excavation from collapsing. Once the pouring is completed, and as the pile is concreted, the sleeve is gradually removed, which can be reused again.
Usually as shallow piling, working from the tip, supported by rock. Also as piloting working by shaft on coherent terrain of firm consistency, practically homogeneous.
It is executed using the same system as the in situ extraction type with recoverable tubing, with the difference that the metal jacket is not extracted, but remains permanently attached to the pile.
Usually as piling working by tip supported on rock or hard layers of terrain and whenever layers of fine incoherent terrain are crossed in the presence of water, or there is water flow and in some cases with layers of soft coherent terrain; when there are aggressive layers to the fresh concrete. The jacket will be used to protect a section of the piles exposed to the action of an aggressive soil, fresh concrete or a flow of water. The length of the tube that constitutes the jacket will be such that, suspended from the mouth of the drilling, it goes two diameters below the dangerous layer.
It is an extraction pile, in which the stability of the excavation is entrusted to the action of thixotropic sludge.
Usually as piloting working from the tip, supported on rock or hard layers of terrain. When soft layers are crossed, they must be maintained without detachments due to the effect of sludge.
Usually as piloting working from the tip, supported by a layer of hard coherent terrain. Also as piloting working by shaft on coherent terrain of practically homogeneous firm consistency or coherent terrain of medium consistency in which no detachments from the walls occur.
Usually as piloting working from the tip, supported on rock or hard layers of terrain. Also as piloting working by shaft and tip in terrain of compactness or medium consistency, or in terrain with alternating coherent and granular layers of some consistency.
These are piles by displacement of the earth by means of a continuous auger. Subsequently, concreting is carried out by pumping through the central tube existing inside the auger.
This system is appropriate for soft and unstable soils and the presence of water. The reinforcement is introduced once the pile has been drilled and concreted, which creates the disadvantage that due to the density of the concrete, the reinforced length does not exceed 7.00-9.00 m.
driven piles
It consists of introducing prefabricated concrete elements or metal sections using pilers into the ground.
These elements are placed vertically on the surface of the ground and subsequently "driven" into the floor by means of "hammer" blows, this causes the element to descend, penetrating the ground, a task that continues until the depth of the resistant stratum is reached and the "rejection" of the soil occurs in case of a pile that works by "tip", or reaches the design depth, in case of a pile that works by "shaft" (floating pile).
prefabricated piles
Precast piles belong to the category of deep foundations, they are also known by the name precast piles; They can be built with ordinary reinforced concrete or prestressed concrete.
Conventional reinforced concrete piles are used to work in compression; Prestressed concrete ones work well in traction, and are used for sheet piles and when they must be submerged under water. These piles are driven into the ground by means of blows made by a hammer or with a metal shovel equipped to drive the pile.
Its section is usually square and its dimensions are normally 30 × 30 cm or 45 × 45 cm. They are also built with hexagonal sections in special cases. They are composed of two reinforcements: a longitudinal one with a separation of 5 cm over a length of one meter. The tip is reinforced with a special metal piece to facilitate driving.
Eccentric piles
Eccentric piles are those that are located outside the axes of the columns and those that can be contracted in urban buildings with a reticular structure, offering substantial advantages over the traditional ones installed in the open before the building is laid out, cast on site or prefabricated ones driven with hammer blows and coinciding with the axes, which in other words means that the eccentric piles can be installed after the construction of the building has begun. When it already has some weight, it is used as free ballast to give the driving reaction to the hydraulic equipment, which is compact, silent, without vibrations, clean and with greater capacity than that given with hammer blows.
The substantial financial advantages in favor of the owner and engineering advantages in favor of the participating technicians, due to the simple change in the location of the piles, are the following:
• - Savings in the total construction time of the building by eliminating from the work program what would correspond to the traditional installation of the piles done before the start of construction.
• - The verticality of the piles is guaranteed thanks to the type of drilling in the subsoil, which guides the tip of the pile according to the line of gravity until it reaches the support layer.
• - The piles can be of any type of operation, namely: tip-supported, floating, or negative friction, as decided by the Soil Mechanics Study.
• - All of the piles supported on a hard stratum rebound to the test load when the tip reaches the stratum and the lateral friction is negligible, guaranteeing the immobility of the piles under any load request and at no additional cost to the owner. Immobility is used in specific cases to control the stresses and deformations of the "soil-building-pile" assembly, installing modern, very energetic earthquake-proof mechanisms, both in new works where the concept "build before, drive later" has originated, or in buildings that, already in operation, require re-cementing without ceasing to function, to recover the lost verticality because it is risky that the safety factors of the structure have been reduced, consumed by the inclination, which puts at risk the safety of the users, that of the building itself and that of the adjacent buildings, as well as the public facilities under the sidewalks and also to recover the correct levels when the buildings have apparently "emerged" with respect to the level of the sidewalks on the public road.
• - When problems arise in buildings generated by a behavior of the subsoil that is different from that expected by us, as happens in lake areas subject to a consolidation process due to loss of humidity, overload or the harmful effect of high-energy earthquakes, the eccentricity of the piles allows at any time, if necessary or convenient, to change their type of originally designed operation.
Micropiles
The main differences between a conventional in situ concrete pile and a micropile are found in the execution process and the size of the element. Micropiles are usually metallic with a diameter of no more than 350 mm. Its advantage over conventional piles is that the equipment necessary for its execution is much lighter and allows access to places where the machines necessary to make piles could not reach. In addition to this, micropiles are poured with a concrete injection process through a tube that goes from the top to the bottom of the micropile, and a pile can be poured with simpler processes such as introducing concrete from the top and, due to the effect of gravity, descending to the bottom. This is achieved because the diameter of the pile is larger and there is less chance of the concrete becoming clogged.
Working principle
The piles transmit the loads they receive from the structure to the ground through a combination of lateral friction (shaft resistance) and penetration resistance (tip resistance). Both depend on the characteristics of the pile and the terrain, and the ideal combination is the object of the project. For a circular pile, concreted in situ and supported whose lower tip is on a stratum of appreciable resistance, the subsidence load is given by:.
Where:.
It should be noted that, as in all work related to geotechnical engineering, there is a certain degree of uncertainty in the final capacity of a pile. This is why a good part of the research that is being developed in this field has to do with methods that allow low-cost quality control of the piling before applying the loads. The most obvious but most expensive method is to do a load test. As alternative methods we can mention: resonance tests, Osterberg hydraulic press, wave analysis tests, seismic tests.
In many cases the theories that allow estimating shaft resistance and tip resistance are empirical. That is, they are the result of a statistical analysis of the behavior of certain piles in certain ground conditions. Therefore, it is extremely important to know the origin and conditions under which certain calculation formulas are valid.
Construction precautions
In-situ concrete placement
The minimum distance between the pile driver and the concrete placement must be specified. Tests have been carried out showing that vibrations from the pile driver have no adverse effects on fresh concrete, and a criterion of an open pile between drilling and pouring operations is considered satisfactory.
The jacket, shell, tube or pipe must be inspected just before filling with concrete and must be free of foreign material and contain no more than four inches of water, unless the tremie method of placing concrete is used. Concrete must be poured into each hole or casing without interruption. If it is necessary to interrupt the concrete pouring process for a time interval such that the concrete hardens, steel segments must be placed in the upper concreted area of the pile. When pouring is discontinued, all burrs must be removed and the concrete surface must be washed with a flowable grout.
Concreting with the tremie method
The tremie method, of reverse flow filling, is used to pour concrete through water, when the borehole becomes flooded. The concrete is loaded by hopper or pumped, continuously, into a pipe called tremie, sliding to the bottom and displacing water and impurities to the surface. The bottom of the tremie must be closed with a valve to prevent the concrete from coming into contact with water. The tremie reaches the bottom of the hole before the concrete is poured. At first, it must be raised a few centimeters to start the flow of concrete and ensure good contact between the concrete and the bottom of the hole.
As the tremie is high during pouring, it must be kept within the volume of the concrete, avoiding contact with water. Before removing the tremie completely, enough concrete must be poured to displace all the water and diluted concrete.
To empty the water from the tremie you can use a rubber ball or a cork stopper.
Integrity tests for piles
In addition to direct methods (visual inspection and geotechnical survey), there are various indirect techniques to detect possible anomalies in deep foundations (mainly piles and shell modules).
• - Sonic transparency test in deep foundations (sonic sounding, sonic test, cross-hole or crosshole). Detects and precisely locates anomalies (washouts, contamination, inclusion,..., etc.). The propagation of ultrasound between pairs of metal tubes embedded in concrete is studied by introducing probes into them.
• - Echo test (conventional hammer). It allows the length of the element to be evaluated by measuring the time elapsed between an impact and its reflection. An accelerometer is placed on the head and hit with a conventional hammer. Requires hitting sound concrete.
• - Mechanical impedance test in deep foundations (hammer with force sensor). In addition to the length, by measuring the impact and the vibration response, it assesses the interaction with the ground and changes in section and/or quality of the material. A geophone is placed on the head and struck with an instrumented hammer. Requires hitting on healthy, smoothed concrete.
Pile repair
In some cases, problems occur in the piles due to corrosion or structural causes. Sometimes they can be repaired by casing, using formwork shaped like the pile but with a larger diameter. In the remaining space you can install rods to reinforce the structure and then pour mortar "Mortar (construction)"). In the case of underwater repairs, there are special high-density mortars that do not disintegrate.
Due to the way the casting is carried out, two types of piles are basically distinguished: extraction piles and displacement piles. An extraction pile is made by extracting the ground, while the displacement pile is made by compacting it. In both cases, different techniques are used to maintain the stability of the excavation walls.
The types of in situ piles are included in the Building Technology Standards.[1].
Usually as shallow piling working by tip, supported on rock or hard layers (firm stratum) of terrain, after crossing soft layers. Also as piloting working by shaft and tip in medium or loose granular terrain, or in terrain with alternating coherent and granular layers of some consistency.
This type of pile is executed by excavating the ground and using a sleeve (metal tube as a formwork), which prevents the excavation from collapsing. Once the pouring is completed, and as the pile is concreted, the sleeve is gradually removed, which can be reused again.
Usually as shallow piling, working from the tip, supported by rock. Also as piloting working by shaft on coherent terrain of firm consistency, practically homogeneous.
It is executed using the same system as the in situ extraction type with recoverable tubing, with the difference that the metal jacket is not extracted, but remains permanently attached to the pile.
Usually as piling working by tip supported on rock or hard layers of terrain and whenever layers of fine incoherent terrain are crossed in the presence of water, or there is water flow and in some cases with layers of soft coherent terrain; when there are aggressive layers to the fresh concrete. The jacket will be used to protect a section of the piles exposed to the action of an aggressive soil, fresh concrete or a flow of water. The length of the tube that constitutes the jacket will be such that, suspended from the mouth of the drilling, it goes two diameters below the dangerous layer.
It is an extraction pile, in which the stability of the excavation is entrusted to the action of thixotropic sludge.
Usually as piloting working from the tip, supported on rock or hard layers of terrain. When soft layers are crossed, they must be maintained without detachments due to the effect of sludge.
Usually as piloting working from the tip, supported by a layer of hard coherent terrain. Also as piloting working by shaft on coherent terrain of practically homogeneous firm consistency or coherent terrain of medium consistency in which no detachments from the walls occur.
Usually as piloting working from the tip, supported on rock or hard layers of terrain. Also as piloting working by shaft and tip in terrain of compactness or medium consistency, or in terrain with alternating coherent and granular layers of some consistency.
These are piles by displacement of the earth by means of a continuous auger. Subsequently, concreting is carried out by pumping through the central tube existing inside the auger.
This system is appropriate for soft and unstable soils and the presence of water. The reinforcement is introduced once the pile has been drilled and concreted, which creates the disadvantage that due to the density of the concrete, the reinforced length does not exceed 7.00-9.00 m.
driven piles
It consists of introducing prefabricated concrete elements or metal sections using pilers into the ground.
These elements are placed vertically on the surface of the ground and subsequently "driven" into the floor by means of "hammer" blows, this causes the element to descend, penetrating the ground, a task that continues until the depth of the resistant stratum is reached and the "rejection" of the soil occurs in case of a pile that works by "tip", or reaches the design depth, in case of a pile that works by "shaft" (floating pile).
prefabricated piles
Precast piles belong to the category of deep foundations, they are also known by the name precast piles; They can be built with ordinary reinforced concrete or prestressed concrete.
Conventional reinforced concrete piles are used to work in compression; Prestressed concrete ones work well in traction, and are used for sheet piles and when they must be submerged under water. These piles are driven into the ground by means of blows made by a hammer or with a metal shovel equipped to drive the pile.
Its section is usually square and its dimensions are normally 30 × 30 cm or 45 × 45 cm. They are also built with hexagonal sections in special cases. They are composed of two reinforcements: a longitudinal one with a separation of 5 cm over a length of one meter. The tip is reinforced with a special metal piece to facilitate driving.
Eccentric piles
Eccentric piles are those that are located outside the axes of the columns and those that can be contracted in urban buildings with a reticular structure, offering substantial advantages over the traditional ones installed in the open before the building is laid out, cast on site or prefabricated ones driven with hammer blows and coinciding with the axes, which in other words means that the eccentric piles can be installed after the construction of the building has begun. When it already has some weight, it is used as free ballast to give the driving reaction to the hydraulic equipment, which is compact, silent, without vibrations, clean and with greater capacity than that given with hammer blows.
The substantial financial advantages in favor of the owner and engineering advantages in favor of the participating technicians, due to the simple change in the location of the piles, are the following:
• - Savings in the total construction time of the building by eliminating from the work program what would correspond to the traditional installation of the piles done before the start of construction.
• - The verticality of the piles is guaranteed thanks to the type of drilling in the subsoil, which guides the tip of the pile according to the line of gravity until it reaches the support layer.
• - The piles can be of any type of operation, namely: tip-supported, floating, or negative friction, as decided by the Soil Mechanics Study.
• - All of the piles supported on a hard stratum rebound to the test load when the tip reaches the stratum and the lateral friction is negligible, guaranteeing the immobility of the piles under any load request and at no additional cost to the owner. Immobility is used in specific cases to control the stresses and deformations of the "soil-building-pile" assembly, installing modern, very energetic earthquake-proof mechanisms, both in new works where the concept "build before, drive later" has originated, or in buildings that, already in operation, require re-cementing without ceasing to function, to recover the lost verticality because it is risky that the safety factors of the structure have been reduced, consumed by the inclination, which puts at risk the safety of the users, that of the building itself and that of the adjacent buildings, as well as the public facilities under the sidewalks and also to recover the correct levels when the buildings have apparently "emerged" with respect to the level of the sidewalks on the public road.
• - When problems arise in buildings generated by a behavior of the subsoil that is different from that expected by us, as happens in lake areas subject to a consolidation process due to loss of humidity, overload or the harmful effect of high-energy earthquakes, the eccentricity of the piles allows at any time, if necessary or convenient, to change their type of originally designed operation.
Micropiles
The main differences between a conventional in situ concrete pile and a micropile are found in the execution process and the size of the element. Micropiles are usually metallic with a diameter of no more than 350 mm. Its advantage over conventional piles is that the equipment necessary for its execution is much lighter and allows access to places where the machines necessary to make piles could not reach. In addition to this, micropiles are poured with a concrete injection process through a tube that goes from the top to the bottom of the micropile, and a pile can be poured with simpler processes such as introducing concrete from the top and, due to the effect of gravity, descending to the bottom. This is achieved because the diameter of the pile is larger and there is less chance of the concrete becoming clogged.
Working principle
The piles transmit the loads they receive from the structure to the ground through a combination of lateral friction (shaft resistance) and penetration resistance (tip resistance). Both depend on the characteristics of the pile and the terrain, and the ideal combination is the object of the project. For a circular pile, concreted in situ and supported whose lower tip is on a stratum of appreciable resistance, the subsidence load is given by:.
Where:.
It should be noted that, as in all work related to geotechnical engineering, there is a certain degree of uncertainty in the final capacity of a pile. This is why a good part of the research that is being developed in this field has to do with methods that allow low-cost quality control of the piling before applying the loads. The most obvious but most expensive method is to do a load test. As alternative methods we can mention: resonance tests, Osterberg hydraulic press, wave analysis tests, seismic tests.
In many cases the theories that allow estimating shaft resistance and tip resistance are empirical. That is, they are the result of a statistical analysis of the behavior of certain piles in certain ground conditions. Therefore, it is extremely important to know the origin and conditions under which certain calculation formulas are valid.
Construction precautions
In-situ concrete placement
The minimum distance between the pile driver and the concrete placement must be specified. Tests have been carried out showing that vibrations from the pile driver have no adverse effects on fresh concrete, and a criterion of an open pile between drilling and pouring operations is considered satisfactory.
The jacket, shell, tube or pipe must be inspected just before filling with concrete and must be free of foreign material and contain no more than four inches of water, unless the tremie method of placing concrete is used. Concrete must be poured into each hole or casing without interruption. If it is necessary to interrupt the concrete pouring process for a time interval such that the concrete hardens, steel segments must be placed in the upper concreted area of the pile. When pouring is discontinued, all burrs must be removed and the concrete surface must be washed with a flowable grout.
Concreting with the tremie method
The tremie method, of reverse flow filling, is used to pour concrete through water, when the borehole becomes flooded. The concrete is loaded by hopper or pumped, continuously, into a pipe called tremie, sliding to the bottom and displacing water and impurities to the surface. The bottom of the tremie must be closed with a valve to prevent the concrete from coming into contact with water. The tremie reaches the bottom of the hole before the concrete is poured. At first, it must be raised a few centimeters to start the flow of concrete and ensure good contact between the concrete and the bottom of the hole.
As the tremie is high during pouring, it must be kept within the volume of the concrete, avoiding contact with water. Before removing the tremie completely, enough concrete must be poured to displace all the water and diluted concrete.
To empty the water from the tremie you can use a rubber ball or a cork stopper.
Integrity tests for piles
In addition to direct methods (visual inspection and geotechnical survey), there are various indirect techniques to detect possible anomalies in deep foundations (mainly piles and shell modules).
• - Sonic transparency test in deep foundations (sonic sounding, sonic test, cross-hole or crosshole). Detects and precisely locates anomalies (washouts, contamination, inclusion,..., etc.). The propagation of ultrasound between pairs of metal tubes embedded in concrete is studied by introducing probes into them.
• - Echo test (conventional hammer). It allows the length of the element to be evaluated by measuring the time elapsed between an impact and its reflection. An accelerometer is placed on the head and hit with a conventional hammer. Requires hitting sound concrete.
• - Mechanical impedance test in deep foundations (hammer with force sensor). In addition to the length, by measuring the impact and the vibration response, it assesses the interaction with the ground and changes in section and/or quality of the material. A geophone is placed on the head and struck with an instrumented hammer. Requires hitting on healthy, smoothed concrete.
Pile repair
In some cases, problems occur in the piles due to corrosion or structural causes. Sometimes they can be repaired by casing, using formwork shaped like the pile but with a larger diameter. In the remaining space you can install rods to reinforce the structure and then pour mortar "Mortar (construction)"). In the case of underwater repairs, there are special high-density mortars that do not disintegrate.