- Supporting capacity of the pile
- Load calculation
- Why you need to drill holes
- Screwing in piles
- Installation of caps and grillage
The installation of screw piles is a complex process, but it is mainly due to physical exertion during installation. The main work, the calculation of the parameters and the number of piles, can be done in a comfortable chair. Today we will give you a guide on load calculation and practical use of screw piles – screwing in with a hand tool.
Supporting capacity of the pile
If you intend to conduct an independent calculation of the pile foundation for drawing up a project, it will be extremely difficult to do this without understanding the mechanics of the screw pile. Unlike strip foundations and concrete piles, the products considered in this review have a rather specific structure, which is expressed in a more complex principle of distribution of loads in the bearing soil layer..
Let’s start with the pile blade being the main supporting element. Despite its almost perpendicular position relative to the main load vector, the inclination of the support platform plays a role. According to the code of SP 50-101-2004, the inclined plane of the support should be compensated for by a correction factor by analogy with the compensation for the roll of a reinforced concrete foundation.
Since there is no single standard for the production of screw piles yet, one has to be content with approximate values of the coefficient in the range of 0.85–0.95. A more accurate value depends not only on the angle of attack of the blade, but also on the type of soil on which it rests. In the simplest case, the support area can be calculated from the section of the blades in a horizontal projection without applying a correction factor.
The design bearing capacity of the pile foundation is determined by analogy with all other types of foundations – by two types of limiting states. The first is determined solely by the characteristics of the soil, the second – by the ability of the pile to deform. In the first case, everything is quite simple and complicated at the same time. Here you need an expert report on a geological section, which determines the bearing capacity of the soil in kg / m2, support area at the depth of each of the blades.
Applying a correction factor to compensate for an inclined support, it is necessary to determine the maximum support area of a specific type of pile and, based on the obtained value, calculate the minimum required number of them according to the installed curb weight of the building. If piles have two or more blades at different levels, their support area cannot be calculated simply as the sum of their components. In addition to the fact that the upper blades exert pressure on the already loaded soil layers, the bearing capacity of the latter at different depths can differ significantly. It is better to entrust this kind of calculation to the pile manufacturer, it should take into account the features of the product shape, the difference in heights and the data of the geological section..
The curb weight of a building is defined as the sum of three components. The first and main one is the mass of building structures. For foundations on screw piles, their own weight is not taken into account, since the calculation of the bearing capacity does not include the frictional forces that indirectly affect the pile post. Building structures mean a grillage, walls, semi-capital partitions, finishing materials, glazing, ceilings and roof.
The second component is the operational (payload) load. It is calculated for each floor individually. According to SNiP 2.01.07-85, the average value for residential buildings is taken as 150 kg / m2, for industrial – 200 kg / m2 and higher. It is not the useful area that is taken into account, but the total area of the building in the horizontal section without taking into account the load-bearing walls. This is due to the fact that the configuration of partitions without bearing capacity can change over time..
The third part of the curb weight is snow loads. They are determined in accordance with the regional table from the above set of SNiP. The Russian climate is characterized by loads from 120 to 200 kg / m2 roof sections in horizontal projection.
When these three types of loads are combined into the total mass of the building, a safety factor is applied to the result. It is determined individually, but for residential buildings it cannot be less than 1.2. Values of this coefficient close to 3–3.5 are usually taken for regions with high seismic activity or on weak water-saturated soils..
Why you need to drill holes
Let’s move on to describing the installation process. The first necessary step, which is often neglected, is drilling holes. First of all, it is an excellent marking tool that allows you to accurately position the piles and achieve an even load distribution over the grillage. But holes provide other benefits too..
When tightening piles, especially by hand, the most difficult task is to maintain their vertical position. Even with a small deflection, a large proportion of the bearing load can be lost, usually due to the risk of deformation of the pile column.
The holes can be drilled with an ordinary garden drill. Their diameter does not have to correspond to the width of the blades, but it should not be less than the thickness of the pile pipe. It is necessary to drill to a depth exceeding the thickness of the upper alluvial soil layer (chernozem) by 25–30 cm. Such deepening into a denser soil layer will be sufficient for confident screwing and maintaining the direction for further immersion.
Screwing in piles
The technology provides for two methods of screwing in screw piles. The most reliable and productive – using small-scale mechanization. These can be both specially equipped machines and installations driven by the power take-off shaft of a tractor or walk-behind tractor. The advantage of the method is that the customer does not have to participate in the process directly, it is enough only to control the correctness of the work and monitor the correct spatial location of the piles.
The manual screwing method is more time consuming. It involves the use of a headstock, which is fixed by crimping with screws and rotated by several long adjustable levers. If there are holes, there is no need to apply additional axial force, otherwise the clamp is carried out due to the own weight of the workers. Recently, more and more motorized headstock is used, which does not require the transfer of rotational force by workers. They only need to be supported by two or three elongated rods, setting the direction of twisting..
There are situations when piles can only be screwed in manually. This usually happens during the construction of attached buildings, when the equipment cannot drive up close to the wall. In this case, the lever is constantly thrown only to one side. The disadvantage of the method is that during normal screwing, workers hold on to opposite ends of the lever, counterbalancing each other and supporting the pile in an upright position.
The correctness of the installation is checked with a plumb line, which is tied to the headstock every 5–8 turns, until the pile confidently takes direction. To level the pile when it is blocked, it is enough that the workers provide an advantage in the direction opposite to the blockage. It is preliminarily recommended to unscrew the pile by 3-5 turns. To facilitate screwing under the pipe, you can add water.
Installation of caps and grillage
A number of special products are used to reliably connect the piles to the rest of the building. The most common are flat flanged heads, which provide a square or round steel platform. There are also special forms of platforms, for example, remote and reinforced. Sometimes the head has the shape of two horns with holes designed for laying a grillage from a channel beam, fixed with a massive steel pin.
Regardless of the shape of the head, there is only one technique for fixing it. In the lower part of the head there is a casing sleeve with a clamping bolt. It is twisted, achieving reliable temporary fixation, and carefully leveled with a rack level.
The main attachment of the head to the pile is performed by arc welding. First, you need to make potholders at four diametrically opposite points and check if the position of the site is not lost. As a result, a one-piece weld is applied, slag is knocked off and a protective coating is applied, for example, alkyd enamel. Upon completion of the installation of all heads, a grillage is installed.
There are two main types of grillage for screw piles. For frame buildings and log cabins, the grillage is arranged with a crown log of rectangular cross-section or in the form of an obapola. The logs are laid flat on the head platforms, spliced together with an undercut, and then rigidly fixed to the piles through holes in the steel plates. For fixing, bolts or studs, long pins or ragged crutches can be used.
The second type of grillage is reinforced concrete. For its device, the installation of a formwork of a rather complex type is required. Usually it is arranged on an unpaved embankment aligned in the plane of the head platforms. The side parts are arranged according to the principle of panel formwork made of boards or OSB. The lower plane of the formwork can be formed without dumping the soil, in this version the wide board is tied with hot-rolled wire to the platforms through the holes. For this version of the device, the step of the piles should be quite frequent; you can also install intermediate supports made of bricks or chocks.