The choice of foundation technology depends on a variety of technical and operational requirements that are described in the design solution, taking into account the characteristics of the construction site. Along with the traditional well-established technologies of capital construction, lightweight methods of creating the foundation for a building are increasingly being used today. Pile foundations can also be included in this category. This is a shallow construction that does not require excavation, but is limited in bearing capacity. For a wide range of engineering structures and buildings, this is a rational option for basing on the ground, but large houses and cottages, for example, cannot be built on such a basis.
Technology overview
The use of columnar structures as load-bearing elements has long been practiced in the construction industry. But if before such foundation methods were applied mainly to outbuildings andfunctional buildings, today, thanks to the optimized technology of load distribution, this method finds its place in the projects of residential buildings. Another thing is that the imposed loads must correspond to the bearing potential of the platform. This discrepancy often becomes the main limiting factor in the use of technology. The parameters of the system device are regulated by document SP 24.13330 on pile foundations and the updated version of SNiP 2.02.03-85. According to technical standards, the main purpose of piles is to provide cutting through the soil from the surface and transfer the loads acting from above to the underlying layers. In other words, pile elements bear the weight of the structure, providing stability under static and dynamic loads.
Foundation structure
In terms of the structural device, the technology is quite simple, which, in particular, contributes to its prevalence. At the heart of the foundation is a group of piles that are installed in the soil layers around the perimeter of the construction site. The methods and formats for performing this part can be different, for example, one of the simplest in technical execution is considered to be a pile-driven foundation, which is installed manually without machines and special equipment. The grillage forms the upper part of the structure. On its reinforced concrete or concrete base, the load is distributed over the entire area. Connection nodes are made with jumpers and fasteners of a suitable format - depending on the type of piles and the configuration of the grillage.
Materials of pile elements
Metal piles are mainly used as the most reliable and durable. They withstand high voltage, exhibit dynamic resistance under seismic loads and are easily mounted in soils of various types. Concrete can be called the closest alternative to metal in terms of technical and physical qualities. Such piles are not subject to corrosive processes, do not require the use of special equipment for penetration into the ground and are durable. However, concrete is not recommended for use in places with seismological activity or in other areas of unstable operation of pile foundations. This is dangerous due to high dynamic loads that have a destructive effect on the concrete structure. But in the context of using other types of piles, problematic soil is precisely the target place for using columnar foundations in principle. This is due to the fact that traditional strip and monolithic concrete structures can deform under such conditions.
The construction of a wooden pile foundation from a bar of fir, larch or pine is also practiced. Of course, only specially selected lumber with protective impregnations, flame retardant and biological coatings is allowed for construction work. On such a base, you can build baths with sheds and a utility block, and one-story residential buildings.
Types of foundations on piles
There are many structural designs of such foundations, the differences between which are mainlylie in the area of classification directly piles. But the basis of the carrier base in terms of interaction with the frame of the target object is still largely formed by the grillage, from which it is worth starting in determining the types of pile foundations. SP 24.13330 notes that a grillage is a structure designed to combine pile heads in order to transfer the load evenly.
By the nature of the mechanical interaction of the bearing sole with the retained structure, it is possible to distinguish the main types of foundations with a grillage on piles. A flexible or rigid hitch is possible, depending on the technology of piling and fastening to the lower floor of the house. Under conditions of high dynamic loads (natural oscillations and vibrations of the ground), it is recommended to use a flexible or soft hitch, which will have a damping effect and levels out the influence of loads. A rigid bond performs better under static loads without vibrations.
Material for making grillage
Concrete, reinforced concrete, metal and wooden structures are used. Most often, preference is given to reinforced concrete structures as the most reliable, although they cannot be combined with wooden piles, and due to the high cost, reinforced concrete is replaced with metal elements.
Location of the sole relative to the ground surface
We are talking about the height of the grillage above the ground. This value depends entirely on the local topography, soil characteristics and requirements for the bearing capacity of the structure.
Pile installation methods
According to the principle of the device in the soil mass, the following types of piles are distinguished:
- Drive-in. The technology of driving the bearing rod is used, which, as already noted, can be implemented by a small team of workers. If we are talking about large-format piles, then it will not be possible to do without special equipment. By the way, a wooden pile foundation can only be built by driving piles. Further, the technique of belt reinforcement of the carrier base can be used as a reinforcement.
- Printed. A special type of piles that are installed in a tubular base. First, holes are formed in the ground by a mechanized method, after which cylindrical tubular pillars of concrete or asbestos concrete are immersed in them. Then concrete with reinforcing bars is poured into them.
- Drilling. Mostly screw piles, which are twisted into the ground by hand or machine. Usually thin rods are used, but in large quantities.
Foundation design
All technical parameters and nuances of the pile foundation structure are described in the design solution. Materials of geodetic and hydrometeorological studies, as well as the results of geological surveys are used as initial data. In combination, this information makes it possible to draw up a technological map of the soil and choose the best way to arrange an engineering structure for it. As noted in the joint venture on pile foundations, the characteristics of load-bearing elements withdimensional parameters are determined taking into account possible engineering-geological and hydrogeological changes. In assessments and forecasts, the factor of the future impact of the load from the constructed building on the ground is also taken into account.
Instructions for load calculation
The main design parameter is the planned load on the piles, which must correspond to the bearing capacity of the foundation. The efforts from the own mass of the piles, which is determined by the coefficient of dynamism, should also be taken into account:
- 1, 25 - when calculating the opening and formation of a crack.
- 1, 5 - when assessing the strength value.
At the same time, from the point of view of the perception of general loads, pile foundations are a group of columnar support elements, each of which takes its own measure of pressure. In this sense, experts recommend starting from the quantitative index of pile units per unit area. On average, a site of 10 x 10 m2 requires a basic set of 25 piles, provided that the depth of occurrence is 2.5-3 m. a metal pile can withstand up to 1,000 kg. It would seem that it remains rational to distribute a sufficient number of rods over the entire area, but the difficulty lies in the impossibility of accurately predicting the mass of the house, taking into account the arrangement of equipment, furniture and structural elements in the future.
Installation technology
The procedure for installing the foundation on a pile base is as follows:
- Marked upconstruction site in accordance with the design decision, in the scheme of which the pile installation points are indicated.
- Mounting of bearing rods is carried out - by driving, screw, mortgage or vibration method with or without equipment connection.
- If necessary, strapping is done. It should be remembered that pile foundations are a structure essentially held on poles, so additional reinforcement on the surface makes sense if it does not overload the heads of the mounting shafts.
- Arrangement grillage. Today, optimized technologies use the joining of the upper parts of the piles with ready-made grillage structures, which are superimposed on the heading system in one configuration or another and closed with fasteners.
Conclusion
The use of load-bearing pillars in the construction of the foundation eliminates many technological operations, many of which are mandatory, for example, when constructing tape and monolithic foundations. It is enough to note the absence of the need for formwork installation. Therefore, the primary advantages of the pile foundation include economic accessibility and technological ease of execution. A significant advantage of such foundations is their versatility. If the possibility of using concrete piles is excluded in advance, then theoretically houses on such a base can be built on a variety of problematic soils, including sandy and clayey ones. If you pay attention to the shortcomings, then the main one will be the limitation on the bearing capacity,which does not allow the construction of multi-storey and simply large buildings with massive structures.
