Screw pile foundations are suitable for permanent structures and even unstable soils. When the project timeline is limited, construction must continue even in adverse weather conditions, and the budget is tight, this is the optimal option. The technology itself is quite simple, requiring minimal machinery and equipment. Therefore, all the work can be done by hand.
Planning and calculation
Work can be carried out in any weather. However, the primary criterion for stability is anchoring the screws to a stable soil layer. Therefore, before beginning work, it's important to find the appropriate geodata or conduct field tests. Experts recommend conducting a trial screw-in to save time.
In general, the process of constructing such a foundation consists of the following stages:
- design and marking;
- pile driving;
- filling;
- installation of headwalls;
- binding.
It's practically impossible to install screw piles yourself; you'll need to hire a professional. You can then pour the foundation yourself by first calculating the required materials.
For a standard house, a pile 89-114 millimeters thick, with a 3-5 millimeter wall and a blade up to 30 millimeters thick, is suitable. The pile's length should be greater than the frost line, at least 1.5 meters for temperate climates. The total weight of the building should be divided by the load-bearing capacity of each pile, plus 20 percent for wind and snow loads, and the number of supports should be determined based on this.
Screwing and pouring
To ensure even screwing without special equipment, pits must first be dug. They should be 0.5 to 0.7 meters deep. This will facilitate the blades' entry into the ground at the desired angle. Then, the piles are set in place and turned slowly using a system of levers. This requires only two people. A third worker must ensure there is no tilt. Screwing is stopped once the depth is below the frost line and a significant increase in mechanical force is felt.
All screw piles are then measured with a level and brought to a uniform alignment. They are all trimmed to a uniform height, which is necessary for the subsequent tying and pouring of the grillage. Then, the pile caps are installed if the building is to be framed, paneled, or log. Plates of the selected size are welded to the pile body.
Before installing the pile cap, pour concrete. This will increase strength, extend the service life, and protect the pile body from the damaging effects of corrosion. The following can be used:
- dry mix of sand concrete M300;
- ready-mixed concrete;
- sand concrete M400.
The most reliable mixture is M400. It is void-free due to its fine aggregate fraction, cements without moisture inside the support, and achieves its final hardness within the pile in just a few days. The mixture is simply poured into the pile through a funnel. Significant expansion will not occur, so it is necessary to completely fill the void before it flows back out. Then, the excess is removed flush to avoid bulges, which interfere with the welding of the pile caps.
Construction of the grillage
This is a mandatory step. Screw piles must be tied together into a single structure to prevent movement of the entire foundation. The most complex technology is monolithic shoring, which is impractical for ordinary residential cottages. Wood or metal grillages are optimal.
A wooden grillage is well suited for frame, panel, and log buildings, requiring the installation of flat square caps. The bottom crown of the future house often serves as the grillage in this case. Wooden beams are fastened directly to the caps with screws. The optimal number of stories in the finished building is one full story and an attic. Logs, if used, are joined at half-stacks.
For the metal grillage, channels are used. They are laid with the flange facing down and cut at a 45-degree angle. Welding is used to secure them to the caps; tacks at 4-5 points are sufficient. Once the entire area is assembled, a double weld is performed.
A pile-and-screw foundation is a cost-effective solution for DIY timber house construction. You can increase the number of floors and the overall load by simply adjusting the spacing of the supports. The final strength will depend on the accuracy of the initial calculations, the quality of the concrete used for pouring, and even the correct type of grillage.