Modern technologies for the construction of barrier and enclosing structures make it possible for each project to select the optimal solution. Pile construction methods are considered the most flexible and versatile in terms of installation, but they have significant drawbacks such as low strength. The technology of installing bored piles just allowed us to minimize the negative factors of operation of columnar fences, increasing the reliability of their design.
The device of the cutting elements
The basis of the pile is formed by a cylindrical trunk with a cavity designed for pouring concrete. There are different formats of technical execution, which ultimately determine the purpose of the design. In the simplest versions, it is a thick trunk with a diameter of 50 cm, along the entire line having the same formation. For the construction of foundations, long 40-meter structures can be used, the thickness of which can reach 150-200 cm. Moreover, the lower part is sometimes performed with expansion. Actually, in the basic device, bored piles can resemble bored rods used as casing pipes. The principal difference of the bore cutting element is the continuous placement of casing pipes so that one concrete rod subsequently mates with the neighboring one. This forms a dense fence around the entire perimeter of the pit.
Required equipment and special equipment
In the process of fencing, two main types of equipment are used - earth moving and concrete mixing. Also, in some cases, hoists with cranes are used for manipulating casing structures. As for earthmoving equipment, the wells under the piles are made by drilling machines or stationary installations. The option of manual development of holes in the soil is also allowed, but this applies only to small diameter wells. Further, a concrete mixer will fill the bored piles with concrete mortar. The mixture itself eventually forms a supporting or enclosing barrel with reinforcement elements. After solidification, the frame will remain, and the casing is removed. This task will be performed by a jib crane or a lifting block structure - again, the choice of a particular tactic for solving a problem at a particular stage depends on the size of the project and the requirements for loads.
Calculation of bored piles
At the stage of project development, the chief engineer must determine the number and parameters of the piles used. The initial data constitute a wide list of indicators from the type of soil to external operating conditions. On average, it is believed that a monolithic enclosing structure along the perimeter of the pit will be able to withstand about 100 tons. The bearing capacity of a single cutting pile with a diameter of 150-200 mm on medium-density sandy soil is more than 1000 kg. In other words, 80-100 piles will be enough for an average building load. Other parameters will also affect the final numbers - for example, soil resistance, determined by the results of geodetic studies. For example, on clay soil, when the piles are deepened by 5 m, the load will be 650 kPa. From here you can determine the optimal diameter and area of the base (shoe) of the pile. Thus, an optimum area of 300-320 cm2 will correspond to a thickness of 200 mm.
Technological stages of construction
After preparatory activities with clearing the work platform, the following operations are performed:
- Wells are being drilled at control points. Casing pipes are immersed in them, the structure is strengthened.
- The bottom of the pipe is checked for leaks. An indicator of reliability will be the lack of groundwater in the shoe.
- Filling with concrete. Using the hose of a concrete mixing machine, the solution is poured pointwise into the casing niche. Also, the technology of the device of bisecting piles provides for the possibility of immersion of the reinforcing cage. This should be done before the mixture has frozen.
- Removing the supporting structure. The casing is raised by a tap after the first signs of hardening. It is important that the solution is not crystallized to the end, otherwise the pipe will not be removed.
- Standing up. The formation of concrete piles with a set of strength.
- Strapping. After the device of the group of piles, they are strapped with a reinforcing metal belt.
Concrete Pile Filling Technique
The most crucial stage in creating a bore cutting structure, so it should be considered separately. Initially, the concrete solution is directed down the casing tunnel to the very bottom of the well. The lower shoe is removed, after which the filling of the annulus should begin. Next, the barrel is filled to the very head. In this case, the walls must be cleared, otherwise defective zones will form on the surface of the structure. After the final filling with the solution, the frameworks of the bore-cutting piles from metal reinforcement are mounted. Reinforcing structures should be initially prepared for laying along the vertical channel.
It is important to emphasize several nuances, compliance with which will rely on the durability of the structure. Firstly, the mixture is laid without interruptions. Not even minute delays associated with moving equipment are allowed. This point should be considered before starting to fill. Secondly, some wells in conjunction with casing pipes can give the effect of jerky suction, which violates the stability of the flow of the solution. Eliminate this factor will allow small swaying of the pipe by applying reciprocating-rotational movements within the allowable radius (as clamps like clamps allow).
Concrete Compaction Technique
Not in every case, but after pouring concrete, it may be necessary to ram it. This is the operation of getting rid of air bubbles in the structure of the solution, as well as intensive mixing of the mass. Tamping is carried out with the help of a vibration compactor, the mechanical vibrations of which contribute to the subsidence of concrete. The operation lasts 30-40 minutes, after which the tool is smoothly removed from the solution and immediately cleaned. At this point, cement milk may be on the surface and should be removed.
Monitoring the work done
Design verification is carried out by the master or foreman, as well as the surveyor. Reinforcing frames, casing elements, parts of piles at the top point are controlled, compliance with standard sizes, etc. is checked. Typically, these activities are carried out by visual inspection, taking into account design or passport data from the casing manufacturer. Operational control begins with the manufacture of piles and is carried out at every stage from drilling to installation of the strapping belt. At these stages, it is important to control the accuracy of the barrel position. In particular, before concrete hardens, the verticality of the bored pile should be checked using levels and levels. Correctly installed casing is fixed and reinforced by supporting elements. As for the tasks of the surveyor at the control stage, he performs a breakdown of the pile field, after which he compiles a technological map of the construction site with the designation of the points of pile placement.
The use of bored piles
Pile bearing and building envelope technologies are used in cases where it is not possible to use simpler solutions. For example, on sandy or clay soils, construction of strip foundations is usually excluded - both for the same foundation pit fences and for the grillage of the future building. Moreover, the construction industry - although this is the main area of use of this design, is not the only one. In areas of point building, cutting sections of piles can be useful as a means of preparing a small but solid engineering structure. This applies to technological critical structures, for which there is no need for a solid foundation, but it is also impossible to deprive an object of a reliable bearing base. Similar tasks can be solved with more familiar columnar elements like screw piles, but, again, in the conditions of problem soil, these rods are not always allowed.
Conclusion
In itself, the specificity of the technical structure of the cutting structures both during the construction process and during operation is manifested both from positive and negative sides. The indisputable advantages of the technology include a high degree of reliability of the formed frame, the ability to conduct work in the winter, low financial costs and the versatility of the method. As for the shortcomings of bored piles, the technology involves the use of complex organizational schemes in the preparation and production of work. This is due to the connection of equipment serving the concrete mortar at several stages, as well as the need for alternate use of equipment at the construction site with minimal time intervals. There are options for replacing special equipment with manual methods for performing individual tasks, but this only applies to a narrow category of projects.