- Features of the use of pile foundations
- The principle of operation and construction of the pile foundation
- Construction of a pile foundation
- Immersion of finished piles
- Manufacturing of rammed piles
- Screw piles
The article will focus on the technology of erecting pile foundations. Let’s consider the main types of piles, find out what elements they consist of, how they are mounted, for what conditions they are designed. We will understand the mechanics of the work of pile foundations, we will comprehend all the pros and cons.
If you are interested in other types of foundations, we suggest studying our other articles on this topic:
- How to drain the site
- What type of foundation to choose
- Strip foundation. Part 1: types, soils, design, cost
- Strip foundation. Part 2: preparation, marking, earthwork, formwork, reinforcement
- Strip foundation. Part 3: concreting, final operations
- Strip foundation. Part 4: assembling concrete block structures
- Column foundation
- Slab foundation
The great migration of people, drawn to nature, is taking its course – suburban construction is now experiencing a real boom. Residents of megalopolises confidently and systematically develop suburban areas, building them up more and more densely. Unfortunately, we do not always get a site we like with an acceptable topography and “good” soils. It turns out that all the tidbits either already belong to someone, or there is no need to expect stability from the land in this area – peat, floaters or, even worse, some kind of permafrost. But, of course, there is always a way out, in this case the technologies of industrial and multi-storey civil construction came to the rescue – these are pile foundations, which are increasingly used in the construction of private houses.
Features of the use of pile foundations
The idea of installing structures on piles is not new, the technology has been known to people for several centuries, but it entered a new stage of development in the second half of the nineteenth century, when instead of wooden racks, they began to use driven, rammed and screw structures made of a combination of steel and concrete. The pile foundation has its own clear specialization, it would be incorrect to talk about its advantages over other options for organizing the foundation of the house and compare them according to any characteristics. In the conditions for which it was developed, the pile foundation has no competitors, it cannot be reliably replaced with another structure. Piles can be used on any type of soil, except, perhaps, rocks and foundations with very strong horizontal movements, however, it is technologically and economically feasible to use them:
- for the construction of buildings in areas with a high level of groundwater;
- where the upper layers of the natural base are characterized by a weak bearing capacity (peat bogs, floaters), while dense soils are located quite deep;
- at a great depth of freezing (northern latitudes, permafrost);
- on steep slopes;
- if the mass of the building is too large (for example, multi-storey buildings made of heavy materials).
If we talk about the technological and functional features of pile foundations, then we can formulate the following points, many of which logically follow one from the other:
- Versatility and variability (a huge number of technical solutions, there is always a choice for any conditions).
- Large bearing capacity.
- Practically no manual labor is needed (complete mechanization of processes is acceptable).
- Industrialization (it is possible to use elements that are completely manufactured in the factory, the “human factor” is minimized).
- High construction speed.
- The ability to build at any time of the year.
- Complete absence or minimization of earthworks.
- durability (especially applicable to reinforced concrete structures).
- relative efficiency (low material consumption – reduced consumption of concrete, there is no need to remove and transport large masses of earth).
The principle of operation and construction of the pile foundation
A pile foundation is a group of piles located under the entire building in the form of a “pile field”, or under specific building elements (rows, “bushes”, one by one).
In most cases, in the upper part, the piles are combined into a single system using a grillage – a set of beams (crossbars), a lattice (frame) or a solid slab. In addition to stabilizing individual foundation pillars, the grillage is designed to evenly distribute the building loads, therefore it is a load-bearing element. Depending on whether it rests on the ground (perhaps even buried), or there is a gap between the surface and the beam / slab, the grillage is divided into: low and high. A low grillage can also participate in the transfer of pressure to the ground (albeit to a small extent), but it is subject to heaving forces and cannot be used on soils that are dangerous in this regard, or complex cushions are arranged under it. A high grillage does not directly participate in the soil / building interaction, therefore it is not pushed out by expanding soils and does not move “to tear” in relation to the piles. It cannot be said that the grillage is an integral part of the pile foundation, since there are structures with single heads for each pile..
Any pile is a long rod-shaped element (sometimes with extensions in various parts), through the heel and side surface of which loads from the above-ground part of the house are transferred to the ground. Of course, here you can draw a certain parallel with the posts of the columnar foundation, only the piles are laid an order of magnitude deeper, and due to their length (read: the large total area of the side surfaces) they act on the base, using also a significant frictional force.
It is interesting that the piles are installed not only strictly vertically, some of them can be located at certain angles to the plumb axis, which makes it possible to increase the spatial rigidity of the entire foundation, since some of its elements work in different ways in compression, bending, stretching. According to the mechanics of interaction with soils, piles are divided into:
- racks – cut through unstable layers and with the bottom rest on a dense, low-compressive layer with good bearing capacity (gravel, rocks);
- frictional (friction piles, hanging) – when driving, they compact the soil in the working area and transfer the load to a weak base only due to friction forces, the supporting effect of the lower end of the pile is insignificant.
Piles are deepened in various ways, therefore, according to the technology of immersion in the ground, piles are divided into several classes, which can cover, so to speak, the main assortment of choice in civil engineering:
- Driven piles – ready-made elements that are installed in a natural foundation without excavation, by pressing them, vibrating, hammering.
- Driven piles are made at the construction site by filling the well with concrete. The well can be either drilled or driven (displacement and compaction).
- Screw piles are steel pipes with blades at the bottom. They are screwed into the ground, after which they are filled from the inside with a concrete mixture..
- Pillars – finished products are immersed in a drilled hole.
- Support piles – finished products are installed at the bottom of the pits, in pits or trenches, after which they are fixed with backfilling.
- Injection piles are obtained by pumping fine-grained concrete into the soil under high pressure.
- Caisson (submerged, well) piles are sunk into the base under their own weight or with the use of additional loading. In parallel with the immersion, the soil is selected from the inside of the shell.
Some pile driving methods involve the use of different types of shells that can fulfill a specific role. So, for shallow bored piles, non-retrievable pipes made of roofing material are actively used, which are both the waterproofing of the underground part and the formwork at the top. In vertically movable pipe technology, the shell acts as a formwork, it is removed from the cut in parallel with the monolithic work. Concrete and pile-pipes have a shell that performs a load-bearing function. Most of the pile shafts are not equipped with casings – we have direct contact “concrete / soil”.
The most traditional materials are used for the manufacture of piles:
- Wooden piles were among the first to be used, and it must be said that they were quite successful. For example, in the construction of bridge supports in St. Petersburg, coniferous logs with a diameter of 20–40 cm were used, some of which have been well preserved to our time. On dry soils, wooden piles can normally perform their function for a long time, especially with new types of processing, but, of course, they are not able to compete in durability with reinforced concrete products.
- Reinforced concrete piles are the most popular at the moment. These are finished products with a length of 3 to 12 meters, most often they have a solid square section (from 20×20 to 40×40 cm), although there are other options – round, prismatic, triangular, complex. The reinforcement cage for the finished concrete product is necessary mainly to withstand bending loads, but for driven immersion, the metal also helps withstand shock loads, therefore, the number of transverse structural reinforcement at the ends of the pile is increased.
- Concrete piles do not have full body reinforcement, but steel frames are applied at the top.
- Rubble concrete piles can contain in their mass up to 30% rubble stone.
- Metal piles are round or rectangular pipes.
- Combinations of different materials are quite common, for example, a steel pipe of a screw pile is filled with concrete, or a metal tip with blades is placed on a wooden log.
The specific type of pile and the technology of its immersion are chosen depending on many factors, the main of which are:
- the mass and structure of the building;
- bearing capacity of each element of the pile foundation;
- characteristics of the natural base;
- working conditions on the construction site (high building density, tightness …).
Despite the fact that not all of the piles listed above are used in private construction, most often several options for pile foundations are suitable for solving a certain problem, then the most economically profitable one is chosen from the available ones. We will talk about them further..
Construction of a pile foundation
The construction of a pile foundation begins with the same preparatory operations as when creating any other. The working area is cleared of vegetation and the sod layer is removed, access roads are prepared, drainage and dewatering operations are performed, materials are imported in full and other logistic issues are resolved. Further, the marks of the axes of the building and the location of each well are taken into nature. We described in detail the preparatory stage of the zero cycle in the article “Strip foundation. Part 2: preparation, marking, earthworks, formwork, reinforcement “.
Naturally, preliminary engineering-geological surveys are carried out on the site, on the basis of which all the main properties of the existing soils are revealed – their layer-by-layer composition, bearing capacity (compressibility), degree and nature of moisture (water balance). In parallel with geological exploration, the operational load from the aboveground part of the building, which will be applied to the soil, is calculated.
Special attention is paid to the working conditions. Nuances such as the presence of other buildings nearby that could be damaged, or the cramped space on a construction site, can dramatically affect the choice of a particular technology..
The obtained data are considered in a complex, they will become the starting point in the development of the project of the pile foundation. The design of the pile foundation will consist of the following points:
- determination of the bearing capacity of the soil and loads from the structure;
- selection of the type (section, material, construction) of piles;
- rational choice of immersion technology (including the specifics of equipment);
- calculation of the required number of piles and the nature of their location;
- diving depth calculation;
- grillage design (height / depth, material, section);
- modeling the work of the foundation – comparing possible deformations with acceptable ones;
- economic justification.
As the series of articles developed, we repeatedly touched upon the problems of developing foundations and, where possible, gave practical recommendations on this matter: “Strip foundation. Part 1: Types, Soils, Design, Cost “or in the Column Foundation Design section of the previous Column Foundation publication. In the case of pile foundations, the situation is much more complicated, primarily due to the fact that, since we have chosen and started calculating piles, it means that we are, by definition, dealing with very problematic soils and a relatively heavy house. Therefore, it is better to entrust the creation of a working project to organizations specializing in this, which use modeling computer programs for calculations and are guided by the current GOSTs.
Immersion of finished piles
We have already noted that piles that are manufactured in the factory (as a rule, these are concrete products) and are already in finished form immersed in the base are called driven piles. This version of the pile foundation is characterized by increased bearing capacity, since the soil near the shaft is compacted. A side effect of soil displacement can be considered the emerging dynamic stress in the foundation, as a result, a number of restrictions on the conditions of work. In general, the industrial nature of factory piles (as opposed to rammed ones) ensures all-weather and high rate of foundation construction, convenient logistics, low dependence on water saturation of soils, high bearing capacity (due to compaction) and low material consumption..
The process of driving piles into the ground is carried out with the help of massive hammers (the mass of the striking part is from 1.5 to 9 tons), installed on heavy self-propelled vehicles. Commonly used are hydraulic and cable excavators, crawler cranes, which have a rotary guide mast and a diesel, mechanical or hydraulic hammer moving along it. Vibratory hammers and vibratory hammers, pressing installations can also be used as piling equipment..
The driving cycle of the driven pile looks like this: the machine grabs the pile and pulls it up – the pile rises and is driven into the copra frame – the impact part of the hammer acts on the product and drives it into the ground. There are several ways to influence.
If a diesel hammer is used, then the main work is carried out cyclically, according to the principle of the internal combustion engine (the striker is dropped from a height, the fuel mixture ignites, the hammer is raised due to recoil).
Lowering and raising of the working part of the hydraulic hammer occurs due to hydraulics – there is no dropping and recoil, which allows fine adjustment of the frequency and driving force.
Heavy vibrators are attached to the pile head and give it vibrations directed along the axis of its trunk. The work of vibrating equipment is carried out by a hydraulic station or an electric motor with an eccentric mechanism.
In the design of SVU pressing machines, the main elements are: hydraulic cylinders of the working stroke and the reverse stroke, as well as a cargo frame for hanging the anchor load. The principle of operation of such installations is the application of constant static pressure.
Impact pile driving is rightfully considered the fastest and most inexpensive method of driving (from about 300 rubles per linear meter), however, it is accompanied by a significant level of noise and vibration of the soil, therefore it cannot be used in dense buildings or, for example, in landslide zones. Another important point – you need a fairly large site (about 15×35 meters).
The deepening of piles by indentation is used if the construction site is located in the historical part of the city, near dilapidated or emergency buildings, on moving soils. The undoubted advantage of indentation is the high accuracy of driving the pile, maintaining the integrity of its head, the ability to measure the bearing capacity of each foundation element in real time, compactness (you can work in cramped conditions – a site measuring 10×10 meters, to the nearest structure – from 1 meter). Corner and side piles can also be driven in by pressing. This technology is somewhat more expensive – from about 800 rubles per running meter.
We examined the principles of power immersion of the finished pile; this is, so to speak, the main action. However, it should be noted that before the start of the mass immersion of wells, test piles are installed on the site, which, after holding for several days (from 1 to 10, depending on the characteristics of the soil), are subjected to experimental tests. The pile is either hit with a hammer (dynamic method) or loaded in a stepwise manner (static method), after which the settlement is measured. The purpose of this stage is to determine the actual bearing capacity of each element of the pile foundation. Based on the results obtained, either the design calculations are confirmed, or the length, section, quantity, and system of the pile field are adjusted..
Leader drilling can be used to optimize the driving process of finished piles. This procedure allows to reduce the load on the ground, reduce noise and vibration, use longer boreholes, and pass sand layers more than 2 meters thick. The boreholes have a diameter smaller than the cross-section of the reinforced concrete products, they do not reach the design elevation of the pile sinking by 0.5-1 meter in depth.
If the pile stops during immersion (the phenomenon is called “failure”), then the excess of its aboveground part is removed, and the buried part is tested for compliance with the design bearing capacity. If unsatisfactory results are obtained, an additional, duplicate pile is installed nearby.
After creating the pile field, the concrete of all piles is cut at the same height, the elements of the reinforcing cage can be saved and folded back for ligation with the grillage structure.
When choosing an organization for the construction of a pile foundation, it is imperative to take into account the characteristics of the contractor’s equipment – weight (specific pressure of the undercarriage on the base), features of the hammer.
Manufacturing of rammed piles
Rammed piles are made from concrete directly on the construction site. Such foundations are more difficult to manufacture and more expensive (from 23,000 rubles per cubic meter); during construction, they are highly dependent on weather conditions and soil water balance. The advantage of monolithic piles can be considered the possibility of construction near buildings, as well as inside buildings (for reconstruction, strengthening the foundation). In addition, there are options not to use heavy equipment on the site – sometimes you can do with a motorized or even hand drill (if the design diameter is up to 30 cm). We add that when organizing the expansion of the heel, it is possible to significantly increase the bearing capacity of the pile and the foundation as a whole. In principle, the process of erecting a common bored pile consists of the following operations:
- With the help of a screw with a borehole, wells are made in the ground. There are many options for producing a well by a percussion method – by displacing the soil by vibratory ramming, hammering of retrievable / non-retrievable shells.
- A reinforcement cage is placed in the well.
- With jacks, a concrete pipe is brought into the cavity, on top of which a receiving funnel is installed.
- Concrete is fed into the well, the pipe is removed as the well is filled.
- The concrete is compacted by mechanical compaction, pneumatic or hydraulic compaction.
- The head of the pile is formed (inventory conductor).
- The pile is settled until the concrete mixture has completely hardened.
- If necessary, the pile heads are cut to the design height.
For most stages of the production of rammed piles, SNiP 3.02.01-87 “Earthen structures, foundations and foundations” and SNiP 3.03.01-87 “Bearing and enclosing structures” can be applied. You can find a lot of useful information in our article “Column foundation”, as well as “Strip foundation. Part 3: concreting, final operations “.
This type of pile stands alone, as it is a full-fledged class in terms of construction and immersion method. The screw pile is a steel pipe (diameter 50–300 mm, wall thickness from 3–6 mm, length of a single piece up to 12 meters) with a pointed end, to the body of which a special blade is welded, and a metal head to the end of the above-ground part. Many manufacturers coat their products with anti-corrosion compounds based on epoxy resins; the service life of high-quality screw piles is declared by them as “150 years or more”.
The technological and functional advantages of foundations on screw piles are considered to be:
- high speed of construction;
- shockless immersion;
- the possibility of manual installation;
- installation accuracy;
- lack of earthworks;
- seismic resistance;
- high resistance to frost heaving forces (smooth side surface with a small total area).
Screw piles are mounted by screwing in, and the use of an additional vertically acting load is not necessary. Therefore, the immersion of the screw pile can be done with a motorized tool or even manually. If necessary (sandy layers, greater depth), pilot drilling of auxiliary wells can be used. Like driven piles, this method of submersion allows the soil to be compacted around the shaft. Depending on the product diameter, blade area, wall thickness, each pile can withstand pressure from 1 to 30 tons.
Installed piles are poured from the inside with concrete with a strength of M300 and higher. To improve the bearing capacity, the cavity of a large pipe can be reinforced with a steel frame.
After installing the entire pile field, the bodies of the screw piles are cut in height, the heads of the required design are mounted and welded on them, which depends on the type of grillage (for a bar or the first crown log, for a reinforced concrete lintel, for a monolithic slab).
This is approximately how we see modern pile foundations. Of course, it was possible to describe far on all types of piles, but in order to understand in which direction to move and, as a result, make the right choice, there should be enough information. Next in line we have a slab monolithic foundation.