The foundation is the foundation of any building. It assumes the combined load of wall, ceiling, floor and roof. The durability of the building will depend on its strength. The cost of its construction can reach 40% of the cost of construction. A pile foundation will help reduce construction costs. The installation of piles with grillages is an excellent replacement for the classic strip foundation.
Pile foundation differences
The main function of any foundation is to create a fixed support for the building. Depending on the type of soil, the layer taking on the load of the structure may be at different depths. Loose and sandy soils require significant deepening to create a solid foundation. Conversely, rocky and rocky soils do not require a foundation of great depth.
In addition to soil stability, the average winter temperature in the construction region is taken into account. The foulness that occurs in the winter, with force pushes the foundation towards the surface.
To avoid the construction of a strip foundation with a large depth (more than 2 m), the installation of piles is carried out, which take the load of the house and transfer it to the deep layers of the soil.
Piles of the following types are used in construction technology:
- Bored. Concrete is poured into the prepared well, forming a pillar inside the soil.
- Screw. They are metal pipes with blades in the form of a screw. Twist into the ground.
- Reinforced concrete. This variety is driven into the soil with the help of a pile driver.
- Vibrating submersible. These piles can be either steel or reinforced concrete. They are distinguished by their high strength. Originally developed for low density soils.
Bored piles
Installation of bored piles is the optimal solution for the construction of foundations on soils of low density, as well as on quick and heaving soils. At the drilling stage, geodetic exploration is conducted in parallel. As the drill moves deeper, soil samples are taken. This helps to determine: to what depth drilling is needed.
To avoid collapse of the well, a casing is installed in it. In addition, it serves as a formwork for concrete pouring.
Well preparation for piling
Drilling under piles should occur strictly perpendicular to the surface of the earth. This condition is difficult to observe due to errors in the drill. Therefore, its diameter is selected with a margin of a few centimeters more than the casing. This will allow it to accurately correlate with the axis of the future wall.
If the well is up to 200 mm in diameter, then a manual motor drill can be used. A tractor unit is used to install larger bored piles, because the resistance to the drill is so great that it is impossible to hold a hand tool.
If the equipment is leased and paid by the hour, then it is advisable to prepare access roads to the working area in advance, as well as pre-complete all markings.
When drilling, you need to take into account that the depth of immersion of the tool in the soil will not correspond to the calculated depth. When the drill rises, the earth from the auger will crumble, which will partially reduce the depth of the well. Therefore, you need to remove the soil with a margin of 10-20 cm.
To avoid shrinkage of the pile and subsequent cracking of the grillage, the bottom of the well must be filled up with dense material (gravel, crushed stone), then rammed.
Casing
The main task of these elements is to protect the well from soil crumbling. In addition, they protect concrete from the dangerous effects of moisture: freezing of wet concrete leads to its rapid destruction.
In hard rocks not prone to collapse, instead of a casing pipe, waterproofing can be made from a rolled up ring in several layers of roofing material. It is necessary to choose a diameter so that it exactly matches the well. Otherwise, the concrete will crush the ring during pouring and the pile will remain without waterproofing.
The material for casing is asbestos cement or galvanized steel. Pipes are taken with a margin. The length is calculated based on the depth of the well and the height of the surface part. In addition, a reserve is taken for the subsequent alignment of all casing pipes in a single plane.
Before installing piles under the house, their bottom is closed with a thick film so that the concrete poured does not flow out from below.
The casing is installed immediately in the prepared well, then it is leveled according to the level. The gap between the soil and the pipe must be filled up immediately to avoid displacement.
Pile reinforcement
Concrete copes well with compressive loads. But the transverse impact of a narrow pile is poorly tolerated. Similar loads can occur during soil movements and can lead to destruction of the base.
In order to increase bending strength, before installation, the concrete pile is reinforced with a steel reinforcement frame.
The frame is made separately from rods connected by steel wire. The diameter of the structure is selected 40 mm less than the thickness of the future pile. This is done so that the metal does not protrude out of the concrete.
For the frame, reinforcement 12-14 mm thick is taken. It is bonded or welded with a 5 mm diameter wire. If welding is used, the fixing step is 0.4 m. To ensure the same strength for a knitted frame, the step is halved.
The reinforcement skeleton is installed exactly in the center of the casing. It should protrude above the surface to the height of the future grillage.
Concreting process
Concreting can be done in two ways: order ready-made concrete in a mixer or make it yourself at a construction site. In this regard, the problems that arise during the pouring process are resolved in different ways.
Concrete entering the casing can move the steel frame to the side. If the mixture is made by hand, then its portions are small, and the reinforcement can be adjusted manually, as concrete arrives. If you fill with a mixer, large volumes will not allow you to align the frame manually. Therefore, it is necessary to pre-strengthen the reinforcement in an upright position, using wedges and struts.
In order to remove air bubbles from the concrete mixture, after pouring, it is necessary to compact the solution with an immersion vibrator or a long pole.
Reinforced concrete piles
For the construction of facilities with high weight and high pressure on the soil, the installation of reinforced concrete piles is used for the foundation. Having many advantages - great strength, uniformity of load, great depth of immersion, this technology is expensive for private construction due to the use of specific equipment.
Before installing reinforced concrete piles, the territory is marked, and the soil is also examined. After that, a trial driving is done to clarify the mobility of the soil.
The work is done using a piling installation. The weight of the working hammer is in the range from 0.3 to 10 tons. The position of the pile should be strictly vertical. Permissible deviation - not more than 1 degree.
The difference in the installation of piles of this type is that they are clogged to the stop. With each blow of the hammer, the soil is compacted, and the moment is reached when the pile is not able to move deeper. This is called the failure rate.
The main advantage of installing reinforced concrete piles is the lack of subsequent shrinkage under the weight of the building, as well as the resistance to longitudinal loads that occur when moving soil.
Screw piles
For buildings with low weight, such as frame or timber houses, screw piles will be a good substitute for concrete. The fundamental difference between the two is that the bored pile is not limited by the weight of the perceived load, while the screw pile can withstand 6 tons. Therefore, before installation, you need to calculate the total weight of the building and determine the installation step of the piles.
The choice of type will be based on the economic component, and will also depend on the type of soil on which the foundation is being built. For example, in podzolic soils or peat bogs there is high corrosion activity, therefore steel pipes as a foundation are not the best option.
Screw piles are more often used for the construction of houses with timber. Less commonly installed grillage.
Installation
Manually screwing screw piles is hard work requiring great physical endurance from employees. The denser the soil and the larger the diameter of the screw, the more difficult it is to tighten the pile.
At the top of the pipe there is a hole for installation. A pipe is inserted into it, the length of which should provide the necessary torque. Two workers rotate the pile, holding the ends of the pipe, and the third ensures that there are no deviations from the vertical.
For mechanical installation of piles under the foundation, a tractor with a screwdriver is used. A special head suspended on the boom captures the pipe and transfers torque to it through the hole. Piling at the right point and adjusting the correct position, the device turns the pipe into the ground to the desired depth.
Reinforced concrete grillage
The culmination of the technology of installing piles is to bind them into a single structure using a grillage. It serves as the basis for walls and floor ceilings.
The width of the grillage is equal to the width of the wall plus 10 cm. The height depends on the weight of the structure, as well as on the distance between the piles. The longer the span, the higher the grillage should be. For light houses, this value does not exceed 0.3 m.
Before pouring the grillage with concrete, a reinforcing frame is installed. It is made of steel reinforcement with a diameter of 16-20 mm and is welded to the pile frame. In order to increase structural rigidity, reinforcing beams in the middle of spans are also welded together.
Formwork
In order to fill the grillage with concrete, it is necessary to make the formwork. Concrete has a high density, so the bottom must support its weight. Formwork support must be reliable. You can use bricks, expanded clay blocks. The higher the grillage, the greater will be the weight of concrete, which means that the supports must be placed at a closer distance from each other.
The formwork is knocked together from boards with a thickness of 20-25 mm in the form of a box. Its side walls in the upper part should be interconnected. This will prevent destruction under the weight of concrete.
The reinforcement cage is laid inside the duct so that 0.2 m remains to the bottom. This will allow the reinforcement to remain inside the grillage and protect it from corrosion.
The lower formwork boards should have a small overhang for installing additional cuts. The box inside is lined with a film that prevents the leakage of concrete through the cracks between the boards.
For grill pouring, it is necessary to use high-strength concrete that can withstand severe climatic conditions.
Concrete gains strength within 28 days. Therefore, it is impractical to start masonry earlier than this period. Before starting the next stage of construction, you need to make sure that the concrete has reached the desired strength. It is not always possible to send a sample to a laboratory for testing. Therefore, you can use the simple verification method with a hammer and chisel.
The blow must be applied with a hammer weighing 300-400 gr. By the size of the spall, you can determine which brand corresponds to concrete:
- 1 cm - strength class M75;
- 0.5 cm - M150;
- less than 0.5 cm - M200-250
If the chip after the impact is almost absent, then the strength corresponds to the brand M350.