The choice of foundation technology depends on many technical and operational requirements, which are described in the design decision taking into account the characteristics of the construction site. Along with the traditional well-established technologies of capital construction, lighter 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 the development of a foundation pit, 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 this basis.
Technology Overview
The use of columnar structures as load-bearing elements has long been practiced in the construction industry. But if earlier such foundation methods were applied mainly to household buildings and functional structures, today thanks to the optimized technology of load distribution, this method finds its place in residential projects. Another thing is that the assigned loads must correspond to the carrying potential of the platform. This discrepancy often becomes the main limiting factor when using 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 regulations, the main purpose of piles is to provide ground penetration from the surface and transfer loads acting on top to the underlying layers. In other words, the pile elements take on the mass of the structure, providing stability under static and dynamic loads.
Foundation construction
In terms of structural design, the technology is quite simple, which, in particular, contributes to its prevalence. The foundation is based on a group of piles that are installed in soil layers around the perimeter of the construction site. The methods and formats for the implementation of this part may be different, for example, one of the simplest in technical performance is considered to be pile-driving foundation, which is installed manually without machines and special equipment. The top of the structure forms a grillage. On its reinforced concrete or concrete base, the load is distributed over the entire area. Connection nodes are made out of jumpers and fasteners of a suitable format - depending on the type of piles and the grillage configuration.
Pile element materials
Metal piles are mainly used as the most reliable and durable. They withstand high voltage, exhibit dynamic stability under seismic loads and are easily mounted in soils of various types. The closest alternative to metal in terms of technical and physical qualities can be called concrete. Such piles are not susceptible to corrosion processes, do not require the use of special equipment for implementation in the soil 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, which destructively affect the structure of concrete. 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 tape and monolithic concrete structures in such conditions can be deformed.
The construction of a wooden pile foundation from 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 mainly lie in the field of classification of piles directly. But the basis of the bearing base from the point of view of interaction with the frame of the target object is still largely formed by the grillage, from which it is worth starting from the definition of the types of pile foundations. In SP 24.13330 it is noted that the grillage is a design designed to combine pile heads in order to uniformly transfer the load.
By the nature of the mechanical interaction of the carrier sole with the retained structure, one can distinguish the main types of foundations with grillage on piles. Flexible or rigid coupling is possible depending on the technology of piling and fastening to the lower floor of the house. In conditions of high dynamic loads (natural vibrations and soil vibrations) it is recommended to use a flexible or soft coupling, which will have a damping effect and levels the influence of loads. A rigid ligament performs better under conditions of static loads without vibrations.
Production material 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 their high cost, reinforced concrete is replaced with metal elements.
The location of the sole relative to the dirt surface
We are talking about the height of the grillage above the ground. This value completely depends 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 massif, the following types of piles are distinguished:
- Clogged. The technology used to drive the carrier rod, which, as already noted, can be implemented by a small team of workers. If we are talking about large-format piles, then without special equipment can not do. By the way, a wooden pile foundation can only be built by driving piles. Further, the technique of belt reinforcement of the bearing base can be used as reinforcement.
- Stuffed. A special kind of piles that are installed in a tubular basis. First, holes are formed in the soil by a mechanized method, after which cylindrical tubular columns of concrete or asbestos concrete are immersed in them. Further, concrete with reinforcing rods is poured into them.
- Drilling. Mostly screw piles that are screwed into the ground manually or by machine. Thin rods are usually used, but in large quantities.
Foundation design
All technical parameters and nuances of the device pile foundation construction are described in the design solution. As the initial data, materials of geodetic and hydrometeorological studies are used, as well as the results of geological surveys. Together, this information allows you to draw up a technological map of the soil and choose the optimal method for constructing an engineering structure. As noted in the joint venture on pile foundations, the characteristics of load-bearing elements with dimensional parameters are determined taking into account possible engineering-geological and hydrogeological changes. Estimates and forecasts take into account the factor of the future impact of the load from the constructed building on the ground.
Instructions for calculating the load
The main design parameter is the planned load on the piles, which should correspond to the bearing capacity of the foundation. Efforts from the own mass of piles, which is determined by the dynamic coefficient, should also be taken into account:
- 1.25 - when calculating the opening and formation of a crack.
- 1.5 - when assessing the strength value.
Moreover, 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 indicator of pile units per unit area. On average, a basic set of 25 piles is required for a site of 10 x 10 m 2 , provided that the depth is 2.5-3 m. Another principle of load calculation indicates that a typical 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.
Mounting technology
The order of installation of the foundation on the pile base is as follows:
- The construction site is marked in accordance with the design decision, the scheme of which indicates the installation points of piles.
- The mounting of the supporting rods is carried out - driven, screw, embedded or vibration method with the connection of equipment or without it.
- If necessary, strapping is carried out. It should be remembered that pile foundations are a structure that is essentially supported on poles, so it makes sense to additionally strengthen the surface if it does not overload the heads of the mounting trunks.
- Arrangement of grillage. Today, in optimized technologies, docking of the upper parts of piles with ready-made grillage designs is used, which are superimposed on the head system in one configuration or another and closed by fastening devices.
Conclusion
The use of load-bearing pillars in the construction of the foundation eliminates many technological operations, many of which are mandatory, for example, with the installation of tape and monolithic foundations. It is enough to note the lack of need for the installation of formwork. Therefore, the primary advantages of the pile foundation include economic affordability and technological ease of execution. A significant advantage of such foundations is 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 clay ones. If you pay attention to the shortcomings, then the main one will be a restriction on the bearing capacity, which does not allow the construction of multi-storey and simply large buildings with massive structures.