Now we will list the most frequently encountered types of piles. Note that almost all pile types we will mention below are made for resisting all of the first three of the loading types we just mentioned above, which were compression, tension and lateral from its top, but some of them can also be used to resist the fourth, lateral earth pressure, to act as a retaining wall.
In all of the pile types mentioned below, the installation method depends on the pile material, length, cross section area, the soil type, installation depth, and the level of groundwater.
The installation depth, number of piles, cross section area and material of piles are determined during design, by considering all load requirements, ground conditions, seismicity of the area, available budget, available time, available equipment and so on.
Driven piles are installed by “driving” them into the ground by hamering, vibration or simply pushing them into their place. Driving of piles is a very old construction technique which dates back to centuries ago.
For penetrating dense soils, before driving the pile, pre-drilling may be necessary, to soften the soil, to avoid damage to pile driving equipment and the pile itself.
Driven piles can be steel H piles, steel pipe piles, precast concrete or even timber. In addition to slender piles, steel sheet piles can also be pushed into the ground.
When these piles are driven into the ground, it pushes the soil around them. This is a good thing, as it compacts that soil and provides a stronger resistance at the side face of the piles. Because of this pushing of the earth around the pile, driven piles are also called “displacement piles”.
During placement of piles by hammering and vibration, the neighboring structures can experience negative effects of vibration and noise. Especially is these piles are installed too close to an existing structure, any possible negative effects from impacts and vibration must be studied. Pile driving by hammer is one of the noisiest activities in construction, which is not only loud but acts as an impact noise.
Although during design, it is possible to specify an approximate length of installation, the actual installation length of each these piles are determined in the field, because it is impossible to know the precise soil conditions at every pile location beforehand. The actual pile driving length is determined according to the energy required to proceed the pile further. At first, this energy to push the pile is low, as the soil is generally weaker in upper layers and there is less pile length that provides side friction with surrounding soil. But as the pile penetrates deeper, gradually more energy is required, which means that the pile will be able to support more load. For example when driving by hammer, the number of blows are continuously counted. When the number of blows to proceed the pile for a certain length exceeds a certain number, it is considered that the pile has reached sufficient depth and strong enough to support its future design loads. This point is called “refusal” (as the ground “refuses” to receive the pile further, as there is enough resistance of pile now from its sides, and pile has reached its design capacity). This number of blows per length is determined by engineers, based on the pile driving equipment and the hammer weight, and the dropping height of the hammer, from energy equations.
Especially in soft ground, when piles are driven, a slight shift of piles may occur to any side. The pile contractor’s skill plays an important role to prevent this. Predrilling holes can remedy this but of course it adds to the cost. Predrilling holes will considerably decrease pile driving stresses. But this must not be overdone, because it may have negative effect on skin friction capacity. For this reason, a hole that is smaller than the pile diameter is predrilled.
After installation of piles are complete, the pile heads will remain higher than the ground surface. These can be cut off down to a required height, so that the foundation or pile caps can be placed on them at the required elevation.
Steel H- Piles are more suitable for difficult ground conditions or where splicing is required to produce longer pile lengths as steel piles can be spliced easily by welding end to end. Splicing is needed when the required installation depths are large, or when the underlying bedrock is at varying depths. If you wonder why we need splicing, but not just make piles as very long single pieces, think about transporting of these long piles to the project site on trucks, on the streets. Transportation of piles larger than a certain length is simply not possible sometimes. Not only that, but the pile rig (pile driving equipment) may not be able to work with piles longer than a certain length. Steel piles are also able to develop considerable end bearing resistance, but for timber piles this is sometimes not possible.
Precast concrete piles are also used as driven piles often. They are especially useful in locations that has water, such as marine environments. They are reinforced with prestressed tendons. They are generally square but sometimes also circular or other shapes in cross section. As far as individual pile compressive capacities, steel and concrete piles have higher compressive capacities than timber piles.
One main disadvantage of driven piles is that, driven piles can not be installed as tangent/secant piles (touching each other side by side, to act as retaining structure). For that purpose, piles in the other main category, which is, cast-in-place type of piles are needed, such as drilled shafts or cast in place concrete piles, which we will cover in the following pages.
Caissons have two meanings.
One meaning is, especially in North America this term is used interchangibly with drilled shafts, which we will cover next.
Now let’s divert from the piles for a minute and look at the other meaning…
Caisson also means retaining structure, which is watertight, to allow construction work to take place underwater or below water surface and unlike cofferdams caissons permanently become part of the finished structure. They can be used when building bridge pillars or underwater foundations at river crossings, when an underwater shaft is built, when building a ship repair area, or when building dam foundations.
Types of caissons are:
- Pneumatic Caissons: These have compressed air inside, to keep water and mud out during construction. They are closed on top but open at the bottom. Workers are exposed to high air pressures here.
- Box Caissons: A box shaped caisson which is open at top. These are put into place and then filled with concrete, which can also serve as foundation to other structures. Until filling with concrete, anchoring them at the bottom can be necessary.
- Open Caissons: These do not have top or bottom. These are driven into the ground.
- Excavated Caissons: These are mainly made to support excavations
- Floating Caissons: These are precast caissons that can be put in place as needed
Even if a caisson is built off site and then put in place, still some of the construction work will be underwater, so building caissons is challenging, slow and complicated work which also requires divers. Sometimes a cofferdam can also be used to clear the area of construction, and provide a dry area, so that the caisson is installed.
Now let’s get back to discussing types of piles…
Drilled Shafts / Drilled Piers / Cast in Place Concrete Piles / Caissons:
Also called drilled piers, cast in place concrete piles, cast in drilled hole piles, bored piles or caissons, (different than the meaning in the previous item above), drilled shafts are built by drilling a hole to the ground by augers, removing that soil, then placing steel reinforcement in that drilled hole, and then filling the hole with concrete.
Drilled shafts’ diameter and length (especially diameter) can be much more than driven piles, and they can resist very high loads, for all load directions possible on a pile that we talked about. They are the common or sometimes the only choice to support piers of large bridges, very high and heavy towers such as communication towers, large cantilevered signs, very high buldings for example.
The installation of drilled shafts do not produce vibration as in pile driving, so in vibration and noise sensitive areas, they can be preferred simply for that reason too. Another advantage is that, drilled shafts usually require less height clearance for installation in comparison to driven piles, because we do not have to align the pile with its full height on the ground and on top of that need space for the hammer as in driven piles. The maximum height we will need is when we will lower the reinforcing cage, to the drilled hole but that requires less height than the same length of pile. Plus, for drilled shafts, the same pile capacity can be achieved with shorter piles because larger diameter piles can easily be dug, where, for driven piles diameters can not be so much.
The holes for drilled piles are dug by drilling rigs. A wide variety of diameters can be achieved by different size rigs, plus depending on the soil and groundwater conditions, the rig can be completed with drilling buckets, augers, core barrels, and other tools as applicable.
They can be built as isolated columns to support compressive or uplift or lateral forces, or tangent (secant) walls that touch each other for retainage of earth.
To install a drilled shaft, first a hole with the required diameter and depth is drilled to the ground with augers. If the ground easily caves in after drilling a hole, such as sandy soil, one of the methods is to use a temporary steel casing to prevent that and to act as a formwork. Then the reinforcing steel cage, which was previously prepared outside, is lowered into the hole, which goes full length of the pile. This steel cage is more heavily reinforced towards the top where it will connect with pile cap, as lateral loads and bending moments are higher there. After that the hole is filled with concrete and the steel casing that acted as formwork is pulled out. Other than using casing, another method is to fill the drilled hole with slurry, which is a dense liquid material with high consistency, to prevent the walls of the hole from caving in. Then the rebar cage is lowered into the slurry filled hole, and then concrete is poured starting from the bottom, by lowering the tremie hose all the way to the bottom of the hole and gradually pulling it up, and as concrete fills the hole, the slurry will be pushed out from the top of the hole. When pouring with tremie, its end must remain in wet concrete to prevent from slurry to mix with concrete. For dry and stable soil conditions, tremie pouring may not be necessary, and concrete can be poured with free fall too, but tremie is the usual choice regardless.
As always in concrete construction, no matter what is being built, sufficient concrete cover must be maintained for the rebar. To achieve this and to hold the rebar cage in central position with respect to the drilled hole, so that the rebar cage will remain in center of the concrete shaft, small blocks of stone or concrete is placed all around the outside of the rebar cage. This cage is usually fabricated on site, due to the posible difficulty and cost of transporting lare cylindrical cages with truck (a truck can bring the necessary rebar to the site for a lot of cages in unassembled form, but when one cage is finished, due to its size, far fewer cages would fit at the back of a truck). Note that instead of rebar cage, sometimes steel columns can also be used, although that is not often.
During concrete placement, the way to make sure that the pile is being poured as it is supposed to be, the height of the tremie is compared with the volume of the poured concrete, as we know how much concrete is needed for a certain height of pile from simple cylinder volume calculation. This is done to ensure that something unforeseen did not happen underground, such as soil caving in from sides or at the bottom and concrete is not just escaping to surroundings. This is not much of a concern when casing is used, as the concrete can not escape anywhere.
Continuous Flight Auger (CFA) Piles / Auger Cast Piles:
As in cast in place concrete piles, for CFA piles the hole is drilled by an auger, but then, while slowly raising the auger at a controlled rate, the concrete or grout is pumped into the hole from the hollow stem of the auger. While being raised, the auger also removes earth material from the hole by its rotation. This auger is called “Continuous Flight Hollow Stem Auger”. The volume of the pumped grout or concrete vs. the rise distance of the auger is carefully monitored to make sure that the mix is not flowing into surroundings underground. After the dug cylindrical hole is completely filled with grout or concrete, and the auger is completely removed, while the mix is still wet, the rebar cage is lowered into it.
This type of pile also has the ability to resist all types of forces and act as retaining structure. These piles can not be as large as drilled shafts, but still very commonly used in many applications. They do not produce vibration and noise as driven piles, and can be installed in locations with narrower access. Indeed this is a very quiet method of piling as far as noise considerations. One advantage of these piles with respect to drilled shafts is that, since the grout is instantly pumped after drilling the hole, the caving risk of the hole is eliminated, and therefore a casing is not needed as was in drilled shafts. This means faster production and cost efficiency. Especially in soft and wet soils where caving risk is high, this can be a serious advantage. Of course by not putting casing, the volume of concrete and the actual shape of the pile is harder to control. In addition to being installed as isolated piles, these piles can also be made as a retaining structure as secant walls. (Other similar types of production are called interlocking / contiguous pile installation).
Pressure Injected Footings (Franki Piles):
To install these piles, first a steel casing is filled with dry concrete and then from inside the casing, a weight is dropped repeatedly. This action advances the casing & concrete assmebly downwards. When adequae level is reached, by using extra weight drop, the concrete is ejected from the bottom of casing. Afterwards as in cast in place piles, rebar cage is lowered and concrete is poured, while the casing is removed.
Mini Piles (Micro Piles/Needle Piles/Pin Piles/Root Piles):
Micropiles are very small diameter (3-10 inches) piles, mostly steel threaded rods or casings to be filled with concrete.
If a casing is used, after it is in place, reinforcing steel cage is inserted into it, and concrete is poured, and casing is removed. They can also be precisely installed by coring from the top of an existing foundation, and installing the pile by a small pile driving equipment. Micropiles can be installed in sections, adding them end to end. This also helps to reduce installation headroom requirement in tight access areas with low headroom.
These piles can also resist loads from all directions. This method is used when the loads are not very high and the access is restricted, even in low headroom interiors and when there are a lot of underground utilities present which would make using larger piles not possible. Micropiles also have the advantage of causing very minimal noise or vibration. They can be used for all types of soil. Micropiles are mostly for strengthening existing foundations in challenging locations, which is also called underpinning or foundation support. Other than this, they are also used for slope stabilization. Micropiles are also used for earth retention purposes, for instance when used as soldier piles over timber, see more on next section for retaining structures.
Helical Piles / Screw Piles:
This method is used especially as an existing foundation capacity increase method, when just mass pouring below the footing is not enough (as will be described in underpinning and foundation strengthening section), if the strong strata is at deeper levels. A common situation is when part of the foundation settles excessively, in relation to the rest.
These are basically large steel screws that are screwed into the ground and advanced deeper by twisting action, and upon reaching the desired depth, the top of them are connected to the foundation of a structure. The desired depth is determined by measuring the torque required to advance the screws. This value is predetermined in design and related to soil strength, as a target value to reach, during driving screws. Compared to previous pile types, the loads here are lighter and this is mainly used for strengthening existing foundations.
To increase support for an existing foundation with this method, first a trench near the existing foundation is dug, so that at least some portion of the foundation hangs out in the air. After that, with special equipment, steel screws are penetrated into the ground by rotation. If one piece is not enough for the required depth, more than one piece can be used, and added to each other. After the desired torque level is reached (the torque effort which is required to rotate and advance the screw deeper), the drilling stops and the top of the screw is connected to the bottom of the existing foundation for future support. Because of using brackets when connecting to the foundation, this method is also called bracket piles method. There are usually at least few piles installed near each other, spaced several feet apart, and after all of them are installed, though using hydraulic jacks, all of the brackets are raised, brining the part of the foundation back to the desired level. Finally the trench is backfilled.
This method is suitable when site access is restricted because even a small crew can install these piles using equipment such as a small excavator and a jack, and all they need to do is to advance the piles by applying torque, which doesn’t cause any vibration problem as in the case of driven piles. In addition the speed of installation is fast.
The important factor to know here is determining what amount of torque effort means how much bearing capacity, and geotechnical engineers make calculations to convert this torque effort into the required bearing capacity, by considering properties of the subsoil.
Pushed Piers (Jacked Piers):
Push piers are very similar to helical pier system, except, the brackets are attached at the foundation first, and then, the piles are pushed into the soil though hydraulic jacks, until they reach to a strong soil layer. Here the weight of the building itself helps the pushing. The jack hydraulic pressure is measured to determine if the strong layer is reached.
Battered (inclined) Piles / Anchor Piles:
Piles can not only be vertical but inclined too, which is called “battered piles”, in order to provide better lateral or inclined load support, especially when the soils immediately below the structure are soft and offer little or no lateral resistance. In the case of using battered piles, the vertical load carrying capacity per pile is slightly reduced (a vertical pile can support vertical load better than an inclined pile), but, still this is hardly an issue because in this case the total load is distributed over a larger area, which means more overall pile group capacity and less settlement. The amount of tilt can usually be from a few degrees to as much as upto 15 degrees, and the batter piles can also be driven with ordinary pile driving equipment. Battered piles can also be called as anchor piles.