Pile-screw foundation: pros and cons, tips for choosing. Reviews of a screw foundation - review of technology Brick house on stilts, pros and cons

A pile-screw foundation has objective advantages and disadvantages associated with the operational capabilities of the structure. The base is characterized by low cost and high degree of strength. There is no need to carry out labor-intensive earthworks. Installation is carried out by 3 workers within 3 days.

Advantages (pros) of screw pile foundations

Let us highlight the main advantages of structures that make the foundation in demand in most cases:

• the possibility of erecting buildings in relief terrain - it is allowed to carry out work in the forest without cutting down or uprooting roots;
• construction of the foundation is allowed on weak soils with high-lying aquifers;
• ease of construction of extensions to already constructed buildings;
• increased load-bearing capacity of screw piles;
• natural compaction of the soil during the process of deepening the supports - achieved by preserving the structure;
• load distribution over a large area is ensured by blades that additionally resist heaving caused by a sharp drop in temperature and crystallization of groundwater.

Screw pile design

Operational advantages of a screw foundation

The tangible operational benefits are:

• There is virtually no shrinkage, which allows you to immediately begin building walls.
• It is possible to carry out work in any season.
• The erected structure will last for at least 100 years.
• The foundation may be installed close to utility lines.

It is recommended to construct a pile-screw foundation on soils with high levels of moisture and increased fluidity. The base is in demand in conditions of severe freezing of the ground, when other types of soles are not able to withstand heaving.

Disadvantages (cons) of pile-screw foundations

Naturally, there are certain disadvantages that are not associated with design defects, but rather are expressed in limited operation. Many problems are completely solvable if you follow technology and methodological recommendations in the process of building a foundation. The main disadvantages are the following:

• material of manufacture - the product is made of steel, susceptible to corrosion. The problem is solved by treating the surface layer with epoxy resin;
• the impossibility of carrying out construction work in rocky soil conditions - soil dotted with layers of hardened limestone and rocky rocks is not suitable for pile-screw type foundations;
• impossibility of creating a basement - it is technically possible to dig a basement, but insulating such an underground room will cost incredible costs.

The corrosive effect is especially noticeable in wet soil conditions. If the piles were made without preliminary sandblasting prior to applying an anti-corrosion coating, then the destruction will make itself felt within several years. Modern screw piles from Ustanovkasvay will allow you to avoid corrosive destruction of piles thanks to a special coating and cast pile structure.

Factors accelerating corrosion and destruction of screw piles

Pronounced operational deficiencies appear in the event of a violation of the technological production process. The following factors determine the quality of products:

• thickness and grade of steel;
• technology for creating a weld;
• quality of seams of screw blades.

External aspects that affect the duration of operation are the mineral composition and soil moisture level.

Prohibited operating conditions for screw piles

The service life of pile supports is limited if they are used under the following conditions:

• the presence of an electrical substation at a slight distance from the house - around 50 meters;
• location in close proximity to cell towers;
• proximity to railway tracks;
• increased soil saturation with moisture.

Attention! High humidity can increase the leakage of electricity into the ground and will significantly shorten the service life of the foundation due to electrochemical corrosion.

A screw pile is a welded single-seam steel pipe with a rolled tip, to which a special blade 1.25 times larger than the trunk itself is welded. Piles must be coated with a composition based on epoxy resin and enamel, which provides an anti-corrosion effect. The ideal material for piles is galvanized iron.

Screw pile foundations are widely used and differ in diameter:

• 57 mm is used for installing chain-link mesh or installing fences. The design can withstand a relatively light weight - 1-2 tons, and the thickness of the barrel wall is 2.5 mm;
• 76 mm is ideal for corrugated fences. The load can reach up to 3 tons with a pipe wall thickness of 2.5 mm;
• 89 mm is laid under brick fences, summer gazebos, terraces, verandas with an overhang of 3 to 5 tons and a metal wall thickness of 3 mm and blades of 4 mm;
• 108 mm is effective when constructing houses from timber, logs with a wooden or metal frame and is designed for a weight of 5-8 tons with a metal thickness of 4 mm, blades 5 mm;
• 133 mm is used for the construction of heavier houses made of brick, foam block, gas silicate, expanded clay and they are designed for a weight of 8-11 tons with a wall and blade thickness of 5 mm.

IMPORTANT! The main disadvantage of screw piles is their susceptibility to corrosion, so you should purchase them only from professionals who will not save a penny on a high-quality coating, will weld the seam correctly and will give you a guarantee that the structure will last for decades.

It should be noted that installation in hard rock or rocky soil is very difficult and will require additional costs. All the disadvantages of the building material more than offset their advantages:

• wide range of applications: construction of power lines, bridges, piers, bathhouses and other low-rise buildings;
• Possibility of installation at any time of the year;
• relatively low price for installation and the material itself;
• possibility of repair, replacement and strengthening with concrete;
• no additional time is required for shrinkage. Immediately after installation, you can continue construction of the facility;
• there is no need to prepare and level the ground surface;
• convenient installation on swampy, watery, clayey soil and scree.

An undoubted advantage is the speed of installation. It is quite possible to complete a full cycle of work in one day.

Foundation calculation

First, they draw up a preliminary design and evaluate the soil under the foundation. To make it durable, you need to take the calculation seriously and carefully, determining the number of pieces and the distance between the screw piles.

The foundation calculation consists of two parts:

1. The total weight of the structure and the load on the foundation, taking into account the actual weight of the material from which the structure will be built, plus the payload (during operation, add 150 kg per square meter for residential premises and 200 kg per square meter for an administrative or office building). Don't forget about the additional snow load in winter;

1. Soil characteristics determine the possible weight of the structure without the risk of subsidence, and are calculated only on the basis of geological surveys. If research is not possible, it is necessary to use the help of specialists and apply minimum design load schemes for each unit. The length of the piles also depends on the installation location and can range from 1650 mm to 7000 mm. If the length is not enough, then it can be increased.

Repair and restoration of foundations with screw piles

Over time, many finished buildings require repairs. The reasons can be very different, for example, over a long period of operation, some elements of the foundation may deform (settle or rise), which will inevitably lead to misalignment of the entire house. Restoring the foundation with screw piles allows you to extend the life of the building. Modern replacement technology is as follows :

1. the piles are screwed in around the perimeter of the house at a distance of 30 cm from the walls;
2. the structure is lifted with jacks or other lifting equipment;
3. the foundation is being replaced with screw piles;
4. all piles are connected by strapping and form a new support on which the house is lowered.

This method of reconstructing an old foundation is the most effective and inexpensive. If the house is in good condition, but the foundation has become unusable, then it makes sense to carry out such a restoration. There is a second option for saving the house. You can make a new foundation nearby, strengthen it and move the structure.

Materials for tying screw piles

After installing the screw piles, it is necessary to tie them. The following materials are used for this:

• channel;
• I-beam;
• corner;
• beam;
• concrete.

The choice of material depends on the weight of the structure. Channels are usually used for heavier structures. The diameter of the piles must be no less than 108 mm, and the height no less than 2500 mm. If the soil on your site is soft or watery and the seasonal height difference can reach up to 100 cm (for example, the water under the foundation freezes in winter and pushes the house up, and in the spring it melts and the house sits), then tying it with a channel or corner will protect your structure from cracks and distortions. An I-beam is a high-strength profile whose cross-section resembles the letter “H”. It is made of low-grade and carbon steel and is resistant to high longitudinal stress.

The most popular material for building a foundation remains a wooden beam, because it is easy to work with (if the screw piles are not aligned at the same level, they can be leveled by cutting out the required depth in the beam itself at the junction with the support). An additional advantage is the affordable price and durability of the structure.

It must be taken into account that strapping from any material is made only if the screw piles are 60 cm higher from the ground. Concrete blocks BF are also effectively used to strengthen the foundation.

Installing a pile-screw foundation with your own hands

This project will take a little time and effort. You can install a new foundation from screw piles with your own hands. First, take a calculator and calculate how much and what material you will need. Of course, you can weld a screw pile yourself. To do this, you can purchase one piece and make the rest in its likeness, or use ready-made calculations.

Invite your friends and screw the piles into the ground using pre-prepared drawings. The process itself is similar to how you would screw in a giant self-tapping screw. To make installation easier, you can drill a hole in the top of the pile and insert a rod, holding on to which together from different sides you can quickly screw in one support after another.

1. Prepare pegs in advance and mark the places where the piles will be installed. The corners must be even, and the dimensions and diagonals must match the drawing;
2. Under each peg you need to dig a small hole, 10-15 cm deep and 2-3 cm wider than the blade. You don’t have to prepare the holes, but this will make it a little more difficult to screw the pile into the ground;
3. Using a rod, screw the pile into the ground, constantly ensuring that it is installed vertically. The depth depends on the weight of the structure and the level of soil freezing;
4. Check and level the top of all piles. Cut off the excess with a grinder;
5. Concrete can be poured into the pile to increase strength;
6. A metal plate must be welded to the top of each so that it protrudes a couple of centimeters on all sides;
7. Next, the strapping is performed from the previously selected material.

The straighter you install the screw piles, the stronger and more reliable your structure will be.

Appearance of the foundation

Upon completion of all construction work, the question arises - how to cover the screw foundation in order to give the house a beautiful and finished look. The most popular method is finishing the base with a special masking. Typically, decorative panels and plinth siding are used.

When cladding, do not forget to leave a gap between the panel and the ground (approximately 5-7 cm). It can be covered with a metal mesh to prevent pets from getting under the house. The distance is necessary for natural ventilation and stability of the structure in loose soil.

Warmth and comfort

The main disadvantage of a pile-screw foundation is the cold floor in the room due to the free flow of cold air under it. When the room is cool, there can be no talk of any comfort. Insulation must be carried out comprehensively inside and outside, and it is divided into the following stages:

• waterproofing of piles and piping;
• erection of a structure that will replace the base;
• Sprinkling with expanded clay or soil.

Build a structure similar to a plinth and attach a decorative panel to it. This device will be the main insulation material. Additionally, foam plastic or any other insulation can be glued to the decorative panel from the inside, and the joints can be sealed with polyurethane foam.

Screw pile- this is a steel pipe with a blade welded at the end, essentially a drill, which is screwed into the ground like a screw. The main element of a screw pile is a screw tip with a blade, which is both a support for the pile and transfers the load of the weight of the building to the ground. Also, the screw tip simultaneously serves as an anchor that prevents the pile from being pulled out of the ground by the forces of frost heaving when the soil layer freezes. The problem of winter heaving especially often arises during the construction of light wooden structures on clayey, water-saturated soils. In this case, deepening the foundation of the house below the frost line may not give a positive result, because the tangential forces of frost heaving acting on the lateral underground part of the foundation are quite large and are not balanced by the low weight of the building, as a result of which the foundation or part of it is squeezed upward. If the foundation support has an extension (anchor) in the lower part, then the pile does not extrude, because the anchor is located below the frost line and is held in place by a significant layer of dense soil. In this case, the steel pile itself does not threaten to rupture, because tangential heaving forces reach 0.1-0.2 MPa, and the tensile strength of steel is 330-600 MPa.

Main purpose of screw piles– installation of prefabricated and economical foundations for light outbuildings and wooden housing construction in individual construction. The category of the structure determines the standard size of the screw pile. The main standard sizes for private housing construction: SV89x250, SV108x300, SV133x350, where the first number means the diameter of the barrel (mm), and the second is the diameter of the propeller blade. Approximate distribution of the purpose of a pile depending on the diameter of its trunk: 76mm – light fences, garden structures; 89 mm – brick fence posts, country verandas, gazebos and canopies; 108mm – small log, beam and frame houses; 133mm – heavy log houses and structures made of foam concrete blocks (aerated concrete). It is better to check the feasibility of using a particular standard size by calculating it using the screw pile calculator provided here.

Basic advantages of foundations on screw piles: speed and all-season installation of screw piles, increased resistance to winter heaving of soils, the ability to install on complex terrain and bulk soils, as well as serial factory production of piles, which reduces the cost compared to traditional types of foundations - all this makes this type of foundation extremely popular , and in some cases – simply irreplaceable.

Disadvantages of screw piles they consider a shorter service life, compared to concrete ones, due to increased corrosion in wet soil and an objective likelihood of manufacturers using low-quality materials, welding, and anti-corrosion treatment. The service life of screw piles is significantly influenced by the grade and thickness of steel, the quality of the weld of the screw blade, the presence and composition of the anti-corrosion coating, humidity and mineral composition of the soil. The use of steel piles for the foundation of a house is justified only if there is documentary evidence of the quality of the piles: certificates, passports, factory test reports, etc. A decrease in service life of up to several years can also occur due to improper operation of a steel pile, for example, due to its simultaneous use as a grounding conductor in the power supply of a house: if the neutral conductor of the power supply line breaks, the operating current will flow through the steel grounded support, which will cause its accelerated rusting , especially in the area of ​​welds. The presence near the house (up to 50m) of electrical substations, cellular communication structures, railway tracks, in the case of water-saturated soil, will also reduce the service life of the pile-screw foundation, due to possible leaks of electricity into the ground and an increased level of stray currents in the soil, contributing to electrochemical corrosion.

Types of screw piles

There are two main types of screw piles: with welded tip and with cast tip. The first are an economical option made from a pipe with a drill blade welded on the pointed end. The quality of such piles is often quite low; they are often made at home, although they are inexpensive. Almost all negative reviews about screw piles refer to welded piles of poor quality. If the blade is not welded accurately enough, then when screwing in manually, the pile may deviate from the vertical and move away from the axial marking of the foundation. Also when twisting into dense soil or the presence of stones, large roots, etc. in the ground. deformation and even separation of the weakly welded blade from the body of the pile often occurs, as a result of which the pile ceases to be a proper support and the house may warp or settle. The separation of the pile blade is easily felt when tightening it manually: the pile suddenly begins to rotate easily. It’s even worse when, after 5-8 years of operation of a house on stilts, a blade comes off from some pile support (for example, due to corrosion or a defective weld). At the same time, the load on the neighboring piles sharply increases, the foundation grillage above the problematic support begins to deform, and the house quickly becomes unsafe.

The best use of inexpensive welded screw piles is in the construction of fences and temporary utility structures. When building a house, you must always check with the contractor what type of screw piles he is going to use to build the foundation.

A more reliable option is a pile with a cast screw tip. Cast tips for piles are manufactured in a special production facility using precision casting in a vacuum environment from steel ST-25, ST-35. For a solid-cast tip, the thickness of the blade at the base reaches 13 mm for a diameter of 300 mm (for welded ones it is usually 5 mm), and the geometric parameters are highly accurate. The shaft of the pile support is made of a steel pipe, into the end of which a cast screw tip is inserted and welded. For a trunk diameter of 108 mm, a wall thickness of 4 mm and a trunk length of 2.5 m, the weight of the pile will be about 35 kg. The bearing capacity of such a pile will be up to 6 tons (depending on the characteristics of the soil and the torque of the pile). Screw piles with a cast tip cost about 25% more than welded ones. Also, when choosing piles, you should give preference to the option of a galvanized pipe, or even better - with a polymer coating of the trunk. Anti-corrosion treatment of the cast screw tip is optional, because firstly, the friction with the ground when twisting the pile is very high and the coating of the tip can be worn down to the metal, and secondly, the oxygen content in dense layers of soil is insignificant and corrosion of steel can last for decades. Piles with a cast tip serve as a reliable support for the house; they are screwed without damage even into soil with construction debris or small stones. The service life of such a foundation in favorable soil conditions can exceed one hundred years.

However, when purchasing piles with cast tips, you also need to be careful, because... There are quite a lot of fakes on the market today, pseudo-cast tips or simply defective ones. The quality of the cast tip is greatly influenced by the accuracy of adherence to factory technology, for example, mandatory heat treatment of the product after casting, compliance with the steel grade, etc. A low-quality cast screw tip can be brittle and lose strength under load over time. Factory passports and certificates for piles with cast tips and the use of special equipment for installing a pile foundation can serve as a certain guarantee of quality. Therefore, it is better to buy screw piles separately from the manufacturer, and during installation use equipment that controls the force torque of screwing the pile supports or, after screwing, conduct pressure tests on the support.

It is especially necessary to highlight welded piles from thick-walled pump-compressor (tubing) pipes. The quality and reliability of such piles is not inferior to piles with a cast tip. In the production of such piles, thick-walled pipes with a wall thickness of 6.5–7.5 mm are used for the pile shaft, used in the gas and oil industries and capable of withstanding high pressures. Special steel pipes are designed to work without a protective coating in aggressive environments. In comparison with the water-gas pipe with a wall of 3.5-4 mm usually used by other manufacturers, the tubing pile is 5 times higher in strength and up to 7 times higher in corrosion resistance. The blade of such a pile is made of structural steel, which is 35% stronger than the standard version. The use of thick-walled screw piles can significantly improve the main problem of steel piles - insufficient corrosion resistance, which reduces the service life of a screw foundation.

It should be noted that the price of high-quality screw piles is often comparable to the cost of reinforced concrete bored piles, incl. performed using TISE technology with expansion at the bottom of the pile (to counteract frost heaving of the soil). In this case, the choice of a foundation on screw piles will be justified only by the need for urgent construction of a house or the presence of difficult soil conditions (for example, embankments, peat bogs) or weather conditions.

Installation of screw piles

The main stages of work on installing screw piles are as follows:

• carrying out topographic survey and marking of the pile field;
• clearing and preparing the pile field site;
• manual or mechanized screwing of piles;
• cutting piles to level; -filling pile shafts with concrete;
• welding of channel or fastening plates.

The cost and timing of the work depends on the screwing method: manual or mechanized.

Piles with a blade diameter of up to 300 mm and up to 3 m in length can be screwed manually. Let's consider a typical technology manual installation of screw piles. Before starting work, you need to clear the construction site, be sure to uproot stumps and large roots, and remove large metal and stone debris. For manual screwing, they usually first make holes with a diameter slightly smaller than or equal to the diameter of the pile shaft and a depth of no more than half a meter to ensure accurate installation of the piles along the foundation line and vertical. A large depth of preliminary drilling is undesirable, since this somewhat weakens the load-bearing capacity of the support and also leads to greater “loosening” of the above-ground part of the pile. To work with piles 108/300 (barrel diameter 108mm, blade diameter 300mm), you will need a team of 3-4 people and two crowbars or cutting large reinforcement 1.5-2 m each. When screwing in a pile of this size, it is necessary to create a torque up to 450 kg.m, depending on the size of the blade, the length of the pile and the characteristics of the soil. First, two workers insert a crowbar into the holes in the head of the pile (at an angle of 45 degrees to the shaft) to form levers. Then one installer applies a bubble level to the shaft of the pile (it is more convenient to use a level with a magnet) and holds and aligns the pile vertically, while the other two begin to twist the pile clockwise, resting against the installed levers. When the pile drops to approximately 1.5 m from the ground to the tip, the crowbars are removed, and a three-meter rod of thick reinforcement is threaded through the through holes and the twisting continues. To provide the necessary torque when deepening the pile, additional pieces of extension (up to 4 m) pipes can be attached to the ends of the reinforcing arms. During the screwing process, one installer constantly adjusts the shaft of the pile to the vertical level; the permissible angle of deviation of the pile from the vertical is no more than 2 degrees.

Manual technology requires a lot of physical labor, and most importantly, high-quality piles, because Imprecisely welded blades will cause the screwed-in pile to move away from its exact location. It would be a mistake to screw all the piles to the same depth, because... the bearing capacity and composition of soils may differ even within the construction site; for example, there may be areas with bulk soil. The correct technology is to screw the piles to a certain torque, with control using an appropriate measuring device (dynamometer). In practice, sufficient torque is achieved by the efforts of 2 (4) workers with 4 (2.5) meter levers on each side of the pile shaft, until the lowering of the pile stops (to the “stop”) and the lever holes of the pile shaft begin to deform ( for pipe wall thickness - 4mm). This force approximately corresponds to 350-450 kgf*m, which gives 4.0-6.0 tons of load capacity for a pile 108/300. In any case, the depth of twist of the pile blade must be at least 1.7 m (for the Moscow region) so that the blade is below the soil freezing zone and serves as an anchor for the foundation support. During the dry season, driving the pile (especially into clay soil) can be particularly difficult, so it is recommended to periodically fill the installation hole with water, as The screw pile penetrates into wet soil much easier. After screwing in the pile, a pre-drilled hole around the pile shaft is filled with soil and compacted, and the above-ground parts of the pipes are cut using a level (laser level). In cases of loose upper soil layers (chernozem, bulk soil, etc.), as well as when piles are trimmed higher than 50 cm from the ground, screw piles should be connected with a welded frame with braces to increase their stability. Manual screwing of piles, in addition to problems with installation accuracy and verticality, has low productivity: a team of 4 workers usually installs up to a dozen screw piles per shift.

A better way to screw in screw piles is mechanical installation using hydraulically driven machines. Usually these are mini-excavators with a hydraulic drill or self-propelled pit drills with a hydraulic drive on a boom, developing torque in the range of 7500-10000 N/m. Some models are equipped with retractable telescopic booms up to 14 m, which allows the installation of piles in remote and inconvenient areas.

The main advantages of installing screw piles using this technique:
• ensuring the accuracy and verticality of pile installation;
• creating a large torque force for the required load capacity of the support;
• the ability to install piles of large diameter and to considerable depth;
• the ability to install the entire pile field without moving the machine due to the retractable boom;
• checking the load capacity of the installed pile by applying pressure with the weight of the excavator;
• performance on hard soils and areas with rocky and root obstacles;
• speed of production of a pile field (up to 30 piles per shift).

After screwing in, the screw piles are cut to the design level and filled with concrete. Concreting increases the strength of the support and prevents corrosion from inside the pile shaft. For piles made of water-gas welded pipes with a wall thickness of up to 4 mm, concreting the shaft is mandatory, because If water gets in and freezes, such a pipe may collapse. A grillage - a wooden beam or a steel channel - is installed on top of the pile supports. The grillage connects the individual supports into a single structure, increasing lateral stability and eliminating loosening of single screw piles.

Construction of a base for a foundation on screw piles

Usually, plinth for a foundation on screw piles it is made of decorative false panels on a wooden or metal profile sheathing. These can be polymer foundation siding panels (like brick or stone) or cut-off sheets made from fiberglass boards. Moreover, the plinth panels are suspended on the sheathing with a gap of 10 cm to the ground level in order to avoid their deformation and tearing off during frost heaving of the underlying soil. The 10 cm gap left below can be closed from the outside with a blind area of ​​paving slabs on a sand backfill. In this case, when the adjacent soil freezes, seasonal rise/lowering of the tiles will occur without deformation of the base panels. For the same reason, you should not make a blind area from monolithic concrete, because, unlike tiles, concrete slabs will be cracked or skewed after winter. The best solution is to use effective insulation of the base and blind area with extruded polystyrene slabs, which almost completely eliminates the problem of frost heaving of the soil. To insulate the base, it is enough to use polystyrene slabs 5 cm thick, and to insulate the blind area, polystyrene sheets 10 cm thick and 1.5 m wide from the base line are placed under the sand backfill of the tiles.

Calculation of screw piles

A detailed calculation of the bearing capacity of a screw pile is given in SP 24.13330.2011 “Pile foundations” (updated edition of SNiP 2.02.03-85). However, carrying out this calculation will require special education and experience.

To quickly calculate the number of screw piles, you can use a simplified method, which takes into account only three groups of initial data:

• type and size of building;
• predominant type of load-bearing soil;
• standard size of the pile.

The optimal (in terms of labor costs and price) standard size of the pile is calculated automatically, but you can check other standard sizes for different loads and soils. In the calculator, the load of a house on the ground is calculated by the type of structure and its size (taking into account operational and snow loads), the load capacity of the soil is determined by its type, and the standard size of the pile determines the support area. For determination of soil type Test drilling to the calculated depth, excavation and analysis of soil layers adjacent to the pile support will be required. You can also get similar information from neighbors who have already dug foundations or wells nearby. The lack of data on the type of soil (sand, clay, loam, etc.), degree of humidity and density makes it impossible to correctly calculate the load-bearing capacity of a screw pile. The depth of screwing in the pile must be no less than the soil freezing level (for the Moscow region - at least 1.7 m), therefore, the optimal length of the pile was chosen for calculation - 2.5 m, taking into account the basement above-ground part. The maximum distance between piles in the calculation does not exceed 3 meters.

Determination of soil type and parameters

Sand It is easy to determine - a wet lump of sand crumbles when squeezed, but when dry it is free-flowing. If the grains of sand are up to 5 mm in size, then the sand is considered large and has a bearing capacity of 5-6 kg/cm2, up to 1 mm - medium (4-5 kg/cm2), up to 0.1 mm - fine (3-4 kg/cm2). Coarse and medium sand practically do not change the load capacity when wet, but for fine sand it drops to 2 kg/cm2. Coarser sand does not retain moisture well and is not susceptible to frost heaving.

Sandy loam It is a mixture of sand and a small amount (up to 10%) of clay. In a dry state, sandy loam has a lumpy structure and does not have the flowability of pure sand. When wet, the lump of sandy loam falls apart when pressed. The bearing capacity of sandy loam varies in the range of 2-3 kg/cm2, depending on humidity and density.

Loam– sandy-clay soil with clay content from 10 to 30%. A lump molded from wet loam is compressed into a cake, cracking at the edges due to the high sand content. The bearing capacity of dry loam is 2-3 kg/cm2, depending on the density; for wet loam it is reduced to 1 kg/cm2. Loams absorb more water than sandy loams and are susceptible to winter heaving.

Clay- This is a soil made of very fine clay particles and a low sand content. The clay is very plastic, does not form cracks when pressed, and is easy to shape. The load-bearing capacity of clays most of all depends on the water content - for dry dense clays it is 6 kg/cm2, and for water-saturated and loose clays it drops to 1 kg/cm2. Raw clay is more prone to heaving—swelling in volume when frozen.

Humidity or the water saturation of any soil directly depends on the groundwater level. If, during test drilling to a depth of 2 m, water begins to seep into the well, and the soil being removed is wet, then this soil should be considered water-saturated.

Density soil strongly depends on its porosity and is different at different well depths. If the soil at the construction site is not bulk, but natural sandy or clayey, then at a depth of 1.5-2 m it can be considered quite dense. The exceptions are areas with complex terrain, slopes, and lands with a thick fertile layer (chernozem). The screw pile enters dense dry soil with maximum force.

The calculator calculates the price of screw piles with installation based on average market prices for piles with a welded tip and a pipe shaft with a wall thickness of 4 mm.

This calculation allows you to avoid typical mistakes when an insufficient number of piles are installed under a house or a non-optimal standard size of piles is chosen.

Pile-screw foundations have long been used by the military for the construction of various structures.

However, now they have begun to be actively used in civil engineering, although the pros and cons of pile-screw foundations are still actively discussed by specialists and amateur builders.

The load-bearing element of a pile-screw foundation is a screw pile. It is a steel pipe with a pointed tip and a screw blade welded onto it.

The tip may be cast or molded from the body of the pipe. The tip with the blade - the screw - transfers the load from the above-ground part of the structure to the ground and at the same time plays the role of supporting the pile.

In addition, the screw tip prevents the pile from being squeezed out of the ground when it freezes. This function is especially important when constructing lightweight structures (for example, wooden) in water-saturated clay areas.

From the above we can conclude that the screw tip is the most critical part of the pile, therefore all the pros and cons of a screw foundation depend on its characteristics.

Advantages of pile-screw foundations

The main advantages of pile-screw foundations are:

• installation speed: you can install piles under a medium-sized house in just one day;
• undemanding to the topography of the construction site: even on hilly terrain it is possible to build a screw foundation without excavation work, and therefore without disturbing the landscape. The height difference of the site is compensated by correctly selected pile lengths;
• possibility of construction on any type of soil;
• ease of installation of piles: in most cases they can be screwed into the ground manually using a special key;
• low cost: a pile-screw foundation is 70% cheaper than a strip foundation;
• independence of construction work from the time of year: screw piles are perfectly screwed into the ground at any degree of freezing;
• the possibility of starting construction of the above-ground part of the building immediately after installing the foundation.

Reducing the volume of auxiliary work is also important: during the installation of a pile-screw foundation, no construction waste is generated.

Disadvantages of pile-screw foundations

You can build a house on screw piles if you have evidence of their good quality (certificates, passports, factory test reports, etc.).

The disadvantage of a pile-screw foundation is the susceptibility of the metal to electrochemical corrosion.

If there is a railway within a radius of 50 meters from the house, an electrical substation or cellular communication facilities, the service life of the pile will be reduced due to the increased level of stray currents in the soil (if it is water-saturated).

A house built on screw piles does not have a basement, which can also be attributed to the disadvantages of this type of foundation. You can surround the piles around the perimeter with brickwork, but in winter such a room is unlikely to be warm.

Calculation

The purpose of calculating a pile-screw foundation is to determine the minimum number of screw piles

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Step 1: Determine the total load on the foundation

The total load includes:

• building weight: it is defined as the sum of the weights of all building materials that make up the walls, roof and other parts of the building;
• payload arising during the operation of the premises. According to SNiP 2.01.07-85*, for every square meter of a residential building there is 150 kg of payload. For office premises this parameter is 200 kg/sq. m;
• snow load: for the III snow region of the Russian Federation it is taken as 180 kg per square meter of roof surface.

The resulting amount increases by 1.1 - 1.2 (safety factor).

Step 2: determining the load-bearing characteristics of the soil

This parameter determines the maximum possible load of the screw pile. To calculate it, geological research data is required.

Private developers often do not have such documents on hand; in this case, you can use the value of the minimum permissible load on the pile, which is indicated by manufacturers in the accompanying documentation.

The table shows approximate values ​​of the permissible load on the pile and recommendations for use depending on the diameter of its shaft:

Note: the table shows the average permissible load of piles. This parameter depends on the steel grade, wall thickness and some other features of the product’s production.

Step 3: Determining the minimum number of piles

The minimum number of piles is determined using a simple arithmetic operation: the total load is divided by the minimum permissible load of the selected pile.

The resulting value is adjusted based on the requirement: the distance between the piles should be 1.5 - 3 meters.

Lifetime

Almost all manufacturers of screw piles promise high durability of their products - up to 150 years. One could agree with this if the metal were in ideal conditions. But given the many factors that contribute to an increase in the rate of corrosion development, one can argue with this statement.

If we take into account that the pipe is subject to corrosion from the inside and outside, the results of the analysis are disappointing: the service life of a pile-screw foundation is only 30 years (if the metal does not have a protective coating).

The life of piles can be extended to approximately 100 years if the pipe is coated inside and outside with bitumen and hot or cold galvanizing is used.

But these measures will only help on slightly corrosive soils. In addition, after several decades (60 or 70) inside the metal, destruction processes may begin that are invisible from the outside.

The most realistic estimate of the service life of screw foundations is 40-70 years.

Video about the pros and cons of a pile-screw foundation