This article was written in 2010 and has since been only slightly edited. Over the years, the foundation on screw piles has gained literally national recognition. Already a few engineering plants produce cast-iron tips for screw piles, but the demand is so great that during the season it is sometimes necessary to queue up for shipment.
A low percentage of complaints, high reliability, excellent conditions for the ventilation of wooden floors - and this is not all the advantages of pile-screw foundations.
In industrial construction, screw piles have been used since the 19th century. In individual construction, the active use of screw piles has occurred in the last fifteen years. Therefore, the cautious attitude to this "novelty" is natural (it is written in 2010 and it seems no longer relevant). Basically, all three questions concern: the life of the foundation on the screw piles, its reliability and ways of its exterior decoration. In this article we will discuss these problems.
The screw pile (screw anchor) is a steel pipe with a pointed tip and a spiral blade that allows the pile to be turned into the ground like a screw:
But this is not the main thing. The blade of the screw pile performs the same function as the broadening of the lower part of the columnar foundation made using TISE technology: reduces the pressure of the loaded pile on the ground and prevents its pulling out by the forces of frost punching of the soil. Therefore, the screw tip (auger) is the main element of the pile, which determines its reliability as a support.
Thanks to the blade, the screw pile provides a fixed support point. This is very important for the Canadian house, which has a relatively small weight, because the soils in the suburbs are mostly puffy. And the forces of frost heaving can push out the usual pile from the ground.
Under standard conditions, the pile is screwed into a dense (bearing) layer of soil below the depth of freezing (1.5 meters in the Moscow region).
In individual construction screw piles with a barrel diameter of 108 mm and a blade diameter of 300 mm are used.
The bearing capacity of such a pile depends on the soil and is based on the results of field tests:
The bearing capacity F of the screw pile is linearly dependent on the twist moment:
where M (kg * m) is the torque, k (1 / m) is the experimentally determined coefficient, the value of which depends on the ground and lies in the range of 6-12.
This property allows you to determine the resistance of piles twisting as reliably in terms of bearing capacity, this pile is installed.
Piles with a 300 mm blade can be screwed manually with the help of levers. The simplest set of devices: a scrap and two pieces of a pipe in length of 3 meters. In cramped conditions, a special device will be required. To create the necessary torque of about 300-500 kg * m, 4 people are enough.
The main drawback of the manual method is that it is often impossible to ensure accurate installation of the piles. It depends not so much on the skill of the installers, as on the coincidence of circumstances (geo-conditions at a particular point). We resort to manual screwing only in two cases: either there is no access for machinery, or piles are so small that the delivery of equipment is too expensive.
Builders for screwing piles use various devices and equipment. The most accurate, high-quality and reliable installation of the piles is provided by the excavator-driven hydroborus due to the more complex and flexible control of the multi-hinged boom.
An experienced excavator driver has the opportunity to test the bearing capacity of a pile by pressing on it with the weight of an excavator (see below for the necessity of field testing of piles for bearing capacity):
The extensive use of excavators with a hydraulic drill for installing screw piles is hampered by the high price for the rent of the excavator and its delivery, since for the transportation of an excavator special vehicles are required:
Self-propelled Yamogurs (hydraulic drive on a crane boom) are more common and available. Crane to operate a little more complicated, but experienced specialists cope with the task confidently:
After screwing, the piles are cut to the level. The barrel of the pile is poured with concrete. This protects the trunk from internal corrosion. From the top on the pile is dressed the headband for fastening the wooden strapping bar. Sometimes metal channels are used instead of the heads. Under the Canadian house, in most cases, the piling of the piles is not needed by the channel.
For a Canadian home, the foundation on screw piles is an ideal solution, but its device is not always possible. Stony soils, muddy soils of fluid consistency, deep peat bogs - possible contraindications to the use of screw piles in the Moscow region. But such cases are rare. In our practice, and this is a lot of hundreds of foundations, only in a few cases had to abandon the screwing of the piles. For example, one building site was in place of the former swamp. To the bottom (before the bearing soil), then they did not reach it. With stony soils, we have been able to cope so far thanks in large part to the cast tip, but about this a little.
The advantages of the foundation on the screw piles are many. The most pleasant ones are relative cheapness (cheaper than high-quality ribbon shallow concrete foundation) and high speed of installation (on average it takes one or two days for the whole cycle of work).
The more complex the relief of the site, the more important these advantages of screw piles. Very popular are foundations on screw piles during construction on slopes with a large height difference (a cottage on Nikolina Mountain can be seen). Another cottage from the CIP panels on a fairly steep slope (April 2013):
Unlike "floating" foundations (ribbon shallow or slab) foundation on screw piles, you can at any time increase for the construction of an extension to the house. This is very important, since most people think about an extension to the house after a couple of years of operation.
Excellent ventilation of the underground ensures a long service life of the zero wooden floor. In the case of a strip foundation, this ventilation is hampered by internal bridges.
Easy access from outside facilitates repair and other work underground. In the case of the strip foundation, it is necessary to make hatches in the zero overlap.
Screw piles are perfectly screwed in the winter, which allows you to build a Canadian house from scratch in winter during seasonal discounts on building materials and work:
Of shortcomings (removable) screw foundation, we note the relatively low stability of the individual screw piles in relation to the lateral load. When the soil is weak or the soil is loosened around the pile (most often by pre-drilling), one can even feel the rocking of individual piles. Over time, the soil around the pile is compacted, and the pile acquires the necessary lateral stability.
A grillage made of a wooden beam or channel makes the foundation piles work together, which gives the necessary lateral stability of the screw foundation as a whole:
If the top soil is weak, if the piles are cut high above the ground, if the building looks like a tower (large height with a small footprint), then the reinforcement of the pile foundation can be strengthened by additional connections (steel corner, rectangular pipe, etc.):
On a stretch with a large slope, if the piles are cut high (0.5 meter norm) and (or) clearly weak upper soil (for example, loose soil, chernozem or peat bog), then it is better to order the strapping immediately. Experts on screw piles can accurately predict the need for amplification. But it also happens that the problem is revealed after the house is built. There is nothing terrible in this, since the strengthening (welding) of screw piles with metal profiles is a matter of several hours.
Life time
History The screw pile covers almost two centuries. The main application of screw piles are such responsible structures as beacons, bridges, moorings, lighting masts, power transmission towers, cellular repeater towers.
The oldest building (a quay on screw piles) stood in seawater for almost a century and a half and was dismantled because of a decrease in the water level in the ocean. The screw pins of the first piles are kept in museums.
In Russia, screw piles were mainly used by the military, for which the decisive importance is the shortest time for installation and dismantling of the foundation for rapid deployment or change of positions.
The Russian experience of using screw piles as pillars of an individual house does not exceed fifteen years. During this period, no complaints were raised regarding the corrosion resistance of the piles. There are claims to the quality of piles manufactured by handicraft, to the quality of anti-corrosion coating and to the qualification (conscientiousness) of workers. No foundation has failed due to corrosion of the metal. Single cases of subsidence are noted, which is a consequence of a technology violation, according to which the pile blade must enter a dense bearing soil. The cases of tearing off the pile blade during screwing due to poor quality welding are documented. Molded tips remove this problem.
It is alleged that with proper manufacture, proper anti-corrosion coating and competent installation, the screw pile will last at least 100 years. It is clear that it is impossible to confirm or deny this statement 100% now. The two-hundred-year history of the screw pile and the absence in the sector of low-rise construction during 15 years of alarm signals about the rapid exit of screw piles due to corrosion indirectly confirm the long life of the screw foundation.
For comparison, the facts of failure of "eternal" reinforced concrete foundations in the first years of operation in individual construction are many. A common case - reinforced concrete tape after the first winter because of the poor quality of concrete has come into complete disrepair:
Therefore, the service life of 150 years for ribbon concrete and 30-50 years for columnar concrete foundation is in the ideal case. In individual construction, there is a clear tendency to reduce the cost of the foundation due to such potentially dangerous solutions as reducing the cross-sectional area of the tape, poles and grillage, reducing reinforcement density, etc.
It is difficult to say what will happen with a ribbon shallow foundation or "floating" slab in 30-50 years. Experiences of continuous operation, as in the case of screw piles, no. Adopted in 1998, the territorial building standards of the Moscow region "Design and construction of shallow foundations of low-rise residential buildings in the Moscow region (TSN MF-97 MO) (TSN 50-303-99)" were recognized in 2008 as invalid.
It is quite easy to monitor the condition of screw piles. The zone of increased risk of corrosion is at the surface of the ground. Periodic revision of the pile state does not present technical complexity.
Restoration of the screw pile is possible in principle. One option is to install a steel clutch on the top of the pile stem. It is also possible to reconstruct the foundation: unlike stone houses, a frame-panel house can be raised with jacks without the risk of destroying the structure.
In December 2010, our company demolished an old house in the Kaluga region. In akkurat under the outer wall of the house at a depth of about 15-20 cm found the details of the old scales. Judging by the age of the house, the piece of iron remained in the ground for at least 70 years. The state of the metal surprised. In such conditions the steel pile will last for 200 years without any anti-corrosion protection.
Load bearing capacity
The bearing capacity of the pile with a diameter of 108 mm, with a blade diameter of 300 mm, on average in our experience in the Moscow region is about 4 tons (often much larger: 6-9 tons). This is not determined by calculation, but by field tests. The weight of a two-storey house of 150 m2 from SIP panels is about 25 tons. And the foundation of such a house according to SNIP 2.02.03-85 "Pile foundations" is calculated for a load of about 70 tons (taking into account the normative operational and snow load and the load reliability factor). Thus, according to SNiP, 17 piles are enough to build a foundation. Screw piles are twisted in increments of 2-3 meters for the device grillage from the wooden beam. Under a two-storey house with an area of 150 m2, about 25 piles are usually screwed in. Therefore, the screw foundation for the Canadian house in most cases turns out to be "extremely reliable" in terms of bearing capacity.
Important! Calculation and design of screw foundations is carried out in accordance with SP 50-102-2003 "Design and construction of pile foundations." One of the prerequisites is the engineering-geological survey to a depth of not less than 5 m below the projected depth of the lower ends of the piles. It is long and expensive for an individual house. Therefore, with rare exceptions, all violate this requirement.
This practice in the sector of individual construction has become the norm not only for pile foundations, but also for tape, etc. This is not insanity. An individual house is not a skyscraper. Loads on the foundation are relatively small. For many thousands of years, mankind has been building houses, and it has long been known to the experts about the behavior of foundations on various soils. The accumulated experience protects against errors. If you do not find specialists, then a good way is to look at the neighboring houses.
There is always some information about the ground. Someone was drilling a well, someone was digging a well, etc. A lot of valuable information is given by the process of screwing. In the Moscow suburbs, mostly clastic soil is clayey sandy loam and loam. They are perfect for the installation of a screw foundation. Without foundation information, it is risky to erect a foundation.
Sometimes engineering and geological surveys are conducted for the entire country or cottage community. If you are lucky with a construction or design company, then according to the engineering and geological survey you will calculate the foundation for the bearing capacity with respect to the main combinations of loads and impacts. For an individual house, this is already good. Only there is one problem - the formula from SP 50-102-2003 for calculating the bearing capacity of the screw pile on the parameters of the ground gives a greatly understated value. Moreover, the rules require you to divide it into a safety factor of 1.4! As a result, the estimated number of piles exceeds by several times what is actually needed. This is beneficial for the builders. The designer does not care. The more the stock, the more relaxed the dream of the builder and the designer. For the Customer this is unacceptable for financial reasons. In this case, SP 50-102-2003 provides field testing. Fundamental brigade of LLC "Hotwell", when there are doubts about the ground, performs a test of piles with the weight of an excavator (5-7 tons):
For many Clients, a limited budget does not allow to pay for engineering and geological surveys, field trials and related SNiP calculations and design. Therefore, all the screw foundations for individual houses are now installed with a formal violation of SNiP. In such a situation, the more experienced and responsible the construction organization, the less risk.
According to the experience of Hotwell, who built many times more SIP houses than any other company, the percentage of complaints on screw foundations is negligible. In the entire 10-year history of Hotwell, problems arose only in isolated cases. And during this time the specialized brigades Hotwell installed more than 1500 screw foundations. That is, we are talking about tenths of a percent. And in all cases, the defects are eliminated. This is not only the merit of the Hotwell engineers, but also a useful feature of screw foundations.
If necessary, we can not only strengthen the screw foundation, but completely replace it with the already assembled house.
Complaints in the construction of reinforced concrete foundations only because of the poor quality of concrete is an order of magnitude greater. And the consequences of errors in the construction of reinforced concrete foundations are often unavoidable.
Therefore, the foundation on screw piles is a reliable foundation, especially if you trust its installation to experienced and responsible specialists. It is best if the foundation is established by the company that will build your house. High financial responsibility is disciplined.
The transfer of this stage of construction to third parties offering a favorable price is a big risk. The low price forces the contractor to take risks and save on everything, including the wages of their workers. Practice is the manual screwing of piles by guest workers, etc. For the same reason, such contractors do not have the physical ability to compensate you for even a part of the possible damage. It's not a secret that such "organizations" risk consciously, because they do not plan to do anything in case of problems. There are thousands of examples.
The erection of any building begins with a foundation. It is very important to choose the right type, as the estimate for this type of work can reach 30% of the value of the entire facility. In addition, everything is complicated by the fact that during operation, access to these structures is limited, and conducting a survey or repair is quite a costly exercise. In the article, we will talk about the construction of a pile screw foundation.
Types of foundations
The most commonly used structures are of three types:
- tape;
- plate;
- pile.
Piles allow to save considerably compared to others and can perform their functions on soils with poor characteristics. They, in turn, are divided into:
- bronabivnye, made of monolithic reinforced concrete directly on the construction site;
- piling, installed in the design position by means of a special installation;
- screw, metal piles, immersed in the ground by screwing.
The driven piles were used quite often a couple of decades ago. If there was construction in the area of kilometer, you could hear strong claps that arose during the impact of a heavy load on the pile. This method has limited application, for example, in conditions of dense building it can not be used, since the works will have a destructive effect on neighboring buildings. For a small building they also do not fit.
To build individual houses, it is rational to use bored or screw piles. In the first case, it is necessary to install a reinforcing cage in the casing and pour all with concrete. The screw foundation is much easier to mount and in most cases cheaper.
What are screw piles?
- This type is a long metal pipe with a screw at the end. The pipe has sufficiently thick walls. Screw blades allow the basement to become embedded in the ground and catch on there. In this case, the screw prevents pulling the structure out of the ground and preventing the building from moving.
- When installing, the pile is rotated with pressure. You can use special settings or physical strength. The more often the blades are placed, the better will be the adhesion to the ground.
- The diameter of the pipes is usually in the range of 90-140 mm, the wall thickness is assumed to be not less than 4 mm. If there is a need to install products with a diameter of more than 140 mm, they must be made to order.
Types of screw piles
There are two types, depending on the type of construction:
- cast tip;
- welded tip.
Piles with cast tip
- Such piles are an integral structure consisting of a pipe and a screw tip. They are made in special forms according to standard sizes. They are more expensive, but due to the integrity of the construction they have increased strength and a long service life. The cast construction is completely made of steel, which provides the foundation with high reliability. This pile screw foundation receives only positive feedback.
Piles with welded end
- This is a metal pipe, sharpened from below. On the sharp end of the welded blades, which are made by cutting metal. The tip is treated with anticorrosive compounds.
- The diameter of the used pipe can be different. This method of manufacturing allows the use of products of a sufficiently large diameter. There are several types of blades:
- single blades;
- double blades.
- A double screw prevents skewness and ensures maximum reliability of the structure.
Scope of pile screw foundation
The foundations are selected based on the geology of the site. Features of the ground that force to resort to the use of screw piles are as follows:
- weak soils (bulk, marshy);
- high groundwater table;
- waterlogging of the territory;
- location of the construction site in the water;
- a large degree of uneven terrain.
In all these cases, the use of a concrete foundation is undesirable, since it can not provide a reliable basis for the house. In addition, a serious waterproofing is required to protect the material from moisture penetration, since concrete is still a more porous material than steel.
Screw piles are an ideal solution for such types of construction as:
- low-rise individual housing construction;
- small buildings requiring short construction dates;
- strengthening of slopes and slopes;
- supports under power lines;
- support for lampposts and billboards.
Products can be used in the construction of a private house from timber, logs or frame house construction.
Advantages and disadvantages of pile screw foundation
Screw piles are now widely used in the construction of private houses, but before choosing a house on a pile screw foundation, it is necessary to study their positive and negative sides. The pluses are:
- strength due to the use of steel as raw materials;
- durability, the service life of such a substrate can reach 150 years with the observance of manufacturing and installation technology;
- low cost of the product itself and its installation;
- simplicity and high speed of installation;
- the possibility of using on terrain with a difficult terrain, for example, on the slopes;
- ensuring reliable operation on poor soils, such as sedimentary rocks, wetlands;
- possibility of reuse if necessary.
Simplicity and speed of installation are possible due to the fact that:
- lack of preliminary soil development (digging trenches or pits);
- there is no need for complex special equipment;
- simplicity of technology does not require special skills and knowledge;
- in contrast to concrete foundations, it is not necessary to wait 4 weeks until the construction is completely ready.
All these positive characteristics made it possible to obtain a mass distribution of screw piles, but do not forget about the negative sides. Especially if you can prevent their consequences in time.
Pile screw base of video
Pile screw foundation:
- corrosion of metal;
- limited bearing capacity;
- impossibility of installation close to the walls of the existing structure;
- the impossibility of building a basement in the house.
Corrosion can be avoided by treating the material with special compounds. With individual construction, there is no heavy load on the foundation, so the second point does not become significant. With an impressive list of advantages, not a few flaws, which are rather operational features and are completely preventable, screw piles are an excellent support option for a private house.
Calculations of pile screw foundation
Basically, calculations are made to determine the diameter, length and pitch of the piles. These values are affected by the type of soil and the weight of the structures of the future house.
- The type of soil can be determined by geological studies. In case of individual construction, it is easier: a pit is dug out, approximately 50 cm deep, and soil analysis is performed. If layers with sufficient strength have already begun at this depth, the length of the piles can be used about 2.5 m.
- If there are lenses of poor soil, quicksand or up to a layer of soil with good characteristics on the site, it is better to take the length with a margin to prevent possible problems. On sufficiently bad grounds, the required length can reach 3-4 m.
- For private construction, piles with a diameter of 108 mm are most often used. Calculation of the pile step depends on their required number. There is a need to collect loads from home.
- In order to calculate all loads it is necessary to determine the materials used, the dimensions of the building and the main dimensions of the load-bearing elements. The load is calculated from:
- overlapping with the construction of the floor;
- walls with decoration;
- partitions;
- roofing taking into account the roofing;
- snow cover, which depends on the construction site;
- furniture and equipment, 150 kg per one square meter of the total area of the building is accepted for residential buildings.
- To find these values, you need to calculate the planned volume and multiply it by the density of the materials used. To ensure the stock is allowed not to subtract from the volume of the walls window and door openings. The found value is multiplied by 20-30%, which will provide a safety margin for the load.
- Depending on the characteristics of the base screw pile diameter of 108 mm is able to withstand from 4 to 9 tons. The number obtained in the collection of loads is divided by the amount of bearing capacity (in case of doubt in the characteristics of the ground, it is better to take the minimum value) and find the necessary number of products for the foundation of the house. Further, this amount is evenly distributed around the perimeter of the building.
Technology pile screw foundation
Screwing piles can be carried out by special technique or manually. Regardless of the method chosen, the substrate is first prepared. The preparation work is carried out in the following order:
- selection of the starting point;
- removal of the location of piles on the terrain;
- marking with twine.
The work is carried out with the utmost care, the dimensions must match up to a millimeter. Otherwise, there will be problems with the device grillage and the normal distribution of loads.
Screwing with special equipment
- It is better to assign such work to professionals. The installation process is carried out using a heavy-duty machine equipped with a special installation. First, the technique is brought to a predetermined position and secured to the outriggers, then the position accuracy is checked and the screwing process begins. The machine moves to the next position. The method allows you to perform the work as soon as possible.
Manual screwing
With an independent foundation, it is difficult to rent special equipment and master it, so the most competent solution will be the use of physical strength. Twisting the piles with your own hands requires the following tools:
- shovel for leveling the surface;
- building level for compliance with work accuracy;
- measuring tape;
- scrap metal with two pipes 50 mm in diameter as a lever for screwing;
- a hammer or a small sledgehammer;
- garden drill;
- bulgarian for cutting the top of the pile;
- gloves and overalls.
The garden drill is used with a smaller diameter than the selected piles.
Stages of work
- Drilling holes with a smaller diameter than piles with the help of a garden borer. The depth should be shorter than the length of the pile. The well in this case serves as a guide and ensures the accuracy of the installation. Screwing starts from the corners;
- installation of a pile in the hole, fixing the level to it and passing scrap, which will serve as a lever;
- the screwing process is performed under constant vertical control. For this, the level was set. Several people press on the levers, but it is uneven, so the pipe can get away from the design position;
- as they go deeper into the ground, the pipes are put on the scrap, thereby increasing the arm of the lever, which makes it possible to apply less force;
- after screwing in one corner go to others;
- completing the work at the corners and checking the accuracy of the installation, proceed to screw the intermediate pipes;
- cutting metal pipes to get one level;
- device for cutting piles of grillage.
Grillage for this type of foundation depends on the material of the house and can be made of:
- timber;
- glued board;
- a metal corner;
- metal channel bars;
- t-beam;
- in the house of timber or logs, the first row of wall fencing can become the harness.
Arrangement grillage is necessary for the well-coordinated work of individual piles. Thanks to the strapping, even if one of the pipes fails, the house will continue to stand firmly.
- The pile will not enter the ground strictly vertically, it will be necessary to constantly level it and return it to the design position, so it is better to have at least three people running the work: two act on the lever and one checks the accuracy. To prevent the shift, sometimes use special devices to fix it.
- If an obstacle or a layer of dense soil is found in the path of the pile as the screw is screwed in, analyze the depth of entry and location. If it is not located on the corners of the house and it turned out to enter its ground for a meter or more, you can just cut off the top and leave it as it is. At a depth of less than a meter there is the possibility of a skewed house. At the corners of the piles should be installed at full length, as these are the most critical areas.
- The concluding stage of the installation can be concreting. It will provide additional reliability, but this process is not necessary in the case of screw foundations.
Cost of pile screw foundation
- The total price of a pile screw foundation depends on the dimensions of the building. The most common piles 108 diameter can be purchased at a price of 1900-3000 rubles. depending on the required length. The cost of installing such a pile is about 2000 rubles. When installing with your own hands, you will need to add the price of tools to the total cost. It turns out that for screwing one pile will have to pay from 1900 to 5000 rubles, depending on the method of installation.
- Also it is necessary to take into account the cost of grillage, if there is one. For example, you can quote the price for a pile foundation "turnkey" for a house measuring 6 to 6 m. With good soil characteristics, it will be less than 30,000 rubles. If the question is, how much is the pile screw foundation, then for example, you can calculate the building size of 10 to 10 m, in this case it will be from 70,000 rubles, about the same amount will cost and pile 12 at 12 m.
Screw piles - an excellent solution for the construction of lightweight private houses, they will provide a reliable and solid foundation for a small building in the shortest possible time with low costs. The choice of design and installation method gives them another advantage over other types of foundations. Nevertheless, it is important to remember that for a brick or stone heavy house such supports will not work, since their load capacity is not sufficient to withstand such loads.