The foundation is the basis of any building. It takes on the total load from the wall, ceiling, floor and roof. The service life of the building will depend on its strength. The cost of its construction can reach 40% of the cost of the building. Pile foundation will help reduce construction costs. Installation of piles with a grillage is an excellent replacement for the classic strip foundation.
Differences of the pile foundation
The main function of any foundation is to create a fixed support for the building. Depending on the type of soil, the layer that takes on the load of the structure may be at different depths. Loose and sandy soils require significant deepening to create a reliable foundation. Conversely, rocky and rocky soils do not require deep foundations.
In addition to soil stability, the average winter temperature in the construction region is taken into account. The heaving that occurs in winter pushes the foundation to the side with forcesurface.
In order to avoid the construction of a strip foundation with a large depth (more than 2 m), piles are being installed that take the load of the house and transfer it to deep soil layers.
The following types of piles 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. Twisted into the ground.
- Reinforced concrete. This variety is driven into the soil with a pile driver.
- Vibrosubmersible. These piles can be both steel and reinforced concrete. They are distinguished by their high strength. Originally designed for low density soils.
Installation of bored piles is the optimal solution for building a foundation on low density soils, as well as on shifting and heaving soils. At the drilling stage, geodetic exploration is carried out in parallel. As the drill moves deeper, soil samples are taken. This helps determine how deep you need to drill.
To avoid collapse of the well, a casing pipe is installed in it. In addition, it serves as a formwork for pouring concrete.
Preparation of wells for installation of piles
Drilling under piles should be strictly perpendicular to the surface of the earth. This condition is difficult to comply with due to the error in the operation of the drill. Soits diameter is chosen with a margin, a few centimeters more than the casing. This will allow it to be accurately correlated with the axis of the future wall.
If the well is up to 200 mm in diameter, then you can use a manual motor drill. To install bored piles of larger diameter, a tractor installation is used, because the resistance of the drill is so great that it is unrealistic to hold a hand tool.
If the equipment is rented and paid by the hour, it is advisable to prepare the access roads to the working area in advance, as well as pre-make all the markings.
When drilling, it must be taken into account that the depth of the tool immersion in the soil will not correspond to the calculated depth. When lifting the drill, 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 covered with dense material (gravel, crushed stone), then rammed.
The main task of these elements is to protect the well from soil shedding. In addition, they protect concrete from the dangerous effects of moisture: freezing wet concrete leads to its rapid destruction.
In hard rocks that are not subject to collapse, instead of a casing pipe, waterproofing can be made from roofing felt rolled into a ring into several layers. It is necessary to choose the diameter so that it exactly matches the well. Otherwise, the concrete will crush the ring during pouring, and the pile will remain without waterproofing.
Casing materialserves 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 margin is taken for the subsequent alignment of all casing pipes in a single plane.
Before installing piles under the house, their bottom is covered with a dense film so that the poured concrete does not flow from below.
The casing pipe is installed immediately in the prepared well, then it is leveled in level. The gap between the soil and the pipe must be filled immediately to avoid displacement.
Concrete is great at handling compressive loads. But the narrow pile does not tolerate the transverse effect well. Such loads can occur during ground movements and can lead to the destruction of the foundation.
In order to increase the bending strength, the concrete pile is reinforced with a steel reinforcement frame before installation.
The frame is made separately from rods tied with 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 from the concrete.
For the frame, reinforcement with a thickness of 12-14 mm is taken. It is connected or welded with a wire with a diameter of 5 mm. If welding is used, the fixation pitch is 0.4 m. To ensure the same strength for the knitted frame, the pitch is halved.
The rebar is installed exactly in the center of the casing. It should protrude above the surface to the height of the futuregrillage.
There are two ways to concrete: order ready-made concrete in a mixer or make it yourself at the construction site. In this regard, the problems that arise during the pouring process are solved 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 the concrete arrives. If you fill with a mixer, large volumes will not allow you to manually level the frame. Therefore, it is necessary to strengthen the reinforcement in a vertical position in advance, using wedges and spacers.
In order to remove air bubbles from the concrete mixture, after pouring it is necessary to compact the solution with a submersible vibrator or a long pole.
Reinforced concrete piles
For the construction of objects with great weight and high pressure on the soil, the installation of reinforced concrete piles is used for the foundation. With many advantages - high strength, load uniformity, large immersion depth, this technology is expensive for private construction due to the use of specific equipment.
Before the installation of reinforced concrete piles, the marking of the territory is carried out, and the soil is also examined. After that, a test driving is done to clarify the mobility of the soil.
The work is done with the help of pile driving. The weight of the working hammer is in the range from 0.3 to 10 tons. The position of the pile must be strictlyvertical. Permissible deviation - no more than 1 degree.
The difference between the installation of piles of this type is that they are driven to the stop. With each blow of the hammer, the soil is compacted, and a moment is reached when the pile is not able to move deeper. This is called the failure level.
The main advantage of installing reinforced concrete piles is the absence of subsequent shrinkage under the weight of the building, as well as resistance to longitudinal loads arising from soil movement.
For light weight structures, such as frame or log houses, screw piles are a good substitute for concrete piles. The fundamental difference between them is that a bored pile is not limited by the weight of the perceived load, while a screw pile can withstand 6 tons. Therefore, before installation, it is necessary to calculate the total weight of the building and determine the pile installation step.
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, podzolic soils or peatlands are highly corrosive, so steel pipes as a foundation are not the best option.
Screw piles are more often used for the construction of houses with timber strapping. Rarely install grillage.
Driving screw piles by hand is hard work, requiring great physical endurance from workers. The denser the soil and the larger the screw diameter, the more difficult it is to drive the pile.
Toppart of the pipe has a hole for mounting. A pipe is inserted into it, the length of which must provide the necessary torque. Two workers rotate the pile, holding the ends of the pipe, and the third makes sure that there are no deviations from the vertical.
For the mechanical installation of piles under the foundation, a tractor with a screwdriving device is used. A special head, suspended on the boom, captures the pipe and transmits torque to it through the hole. By placing the pile at the right point and adjusting the correct position, the device wraps the pipe into the ground to the desired depth.
Reinforced concrete grillage
The culmination of pile installation technology is tying them into a single structure using a grillage. It serves as the basis for walls and floors.
The width of the grillage is equal to the width of the wall plus 10 cm. The height depends on the weight of the building, 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 the rigidity of the structure, the rebar bundles in the middle of the spans are also welded together.
In order to fill the grillage with concrete, you need to make a formwork. Concrete has a high density, so the bottom must support its weight. The support under the formwork must be reliable. You can use bricksexpanded clay blocks. The higher the grillage, the greater the weight of the concrete, which means that the supports must be placed at a closer distance from each other.
Formwork is knocked together from boards 20-25 mm thick in the form of a box. Its side walls in the upper part must be interconnected. This will prevent collapse under the weight of the concrete.
The reinforcing cage is laid inside the box 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 bottom formwork boards should have a small overhang for installing additional cuts. The box inside is lined with a film that prevents concrete from leaking through the gaps between the boards.
For pouring the grillage, it is necessary to use high-strength concrete that can withstand harsh climatic conditions.
Concrete is gaining strength within 28 days. Therefore, it is not advisable to start laying before this time. 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 method of checking 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 grade the concrete corresponds to:
- 1 cm - strength class M75;
- 0.5 cm - M150;
- less than 0.5cm - M200-250
If there is almost no chipping after impact, then the strength corresponds to the M350 brand.