Grillage on stilts: types and design features

Construction of a pile foundation with a monolithic grillage.

Pile-grillage infill foundation.
To save construction time, budget and at the same time give durability to the building, a variety of construction technologies are used. This applies to all elements of the building, from the foundation to the roof. Let's consider what a monolithic pile foundation is, made using a monolithic grillage. This type of foundation is used for the construction of low-rise buildings. A monolithic pile foundation is not as strong as other types, but this is compensated by the complete absence of excavation work. And one more advantage - you can start building it even in winter. A pile foundation with a monolithic grillage is used mainly on soft soils or with a high freezing depth.

You can build a pile monolithic foundation using the following steps:

Survey and earthworks.
Installation of supporting elements (piles).
Installation of grillage.
Construction of the basement.

Materials used in the construction of a pile foundation:

Scheme of pile foundation foundations.

Diagram of a pile-grillage foundation: 1 – bored pile made of monolithic concrete and frame made of reinforcement; 2-grillage made of monolithic concrete and frame made of reinforcement;3 – horizontal waterproofing; 4 – vent.

A monolithic pile foundation is made from various materials. Piles are made from concrete mixture. The structural system itself is made from prefabricated or monolithic reinforced concrete, as well as steel and wood.

In general, the entire process consists of the same stages as in the construction of any other foundation, but still, at each stage you can encounter its own unique installation operations, characteristic specifically for a pile foundation.

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After installing the screw piles, the upper part of the foundation must be tied together into a single rigid structure, that is, make a strapping - GRILLWAY, the strapping will also evenly distribute the weight of the structure being erected onto the base.

The grillage can be made of wood, metal, concrete.

Grillage made of timber 200x200

The strapping beam (grillage) is rigidly attracted to the pile head through a pin with a diameter of 1012 mm. In order to prevent rotting of the grillage, the timber must be treated with an antiseptic; also, waterproofing must be laid between the timber and the head of the pile, preferably based on non-woven materials, since condensation will form on the contact heel of the pile with the timber, which will lead to rotting of the wood and corrosion of the iron. . After completing all welding work, an anti-corrosion compound must be applied to all welded joints.

Option 1

Option 2

Metal grillage (channel)

Using a channel for tying piles:

When using a channel with a size of No. 24 or more, in order to evenly distribute the loads between the channel and the base of the pile, it is necessary to weld additional metal gussets to the upper base of the pile, this will additionally strengthen and also distribute the loads on the upper base of the screw pile. After completing all welding work, an anti-corrosion compound must be applied to all welded joints.

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Basic standard units for creating structural rigidity of a foundation made of screw piles.

Such fastening schemes are used in cases where:

With a base height of 0.7m-1m. Pile use: Single-bladed With a base height of 1m-1.5m. Pile use: Two-blade

Welding work must be carried out in accordance with SNIP 3.03.01-87 “Load-bearing and enclosing structures” SP 105-34-96 “Welding work and quality control of welded joints” Steel grade used for gussets (st 3sp thickness 5mm) Before starting welding work , it is necessary to thoroughly clean the piles in places of welding work from the anti-corrosion coating. After completing all welding work, an anti-corrosion compound must be applied to all welded joints.

Option 1

Option 2

Such fastening schemes are used in cases where:

With a base height of 1m-1.5m. Pile use: Single-bladed With a base height of more than 1.5 m. Pile use: Two-bladed

Welding work must be carried out in accordance with SNIP 3.03.01-87 “Load-bearing and enclosing structures” SP 105-34-96 “Work performance and quality control of welded joints” The grade of steel used for gussets (st 3sp thickness 5mm) Before starting welding work, It is necessary to thoroughly clean the piles in the places of proposed work from the anti-corrosion coating. After completion of all welding work, an anti-corrosion compound must be applied to all welded joints.

Option 1

Option 2

Excavation.

Boer.
Using a hand drill, you can drill holes for piles up to 5 m deep and no more than 30 cm in diameter.

During excavation work, an earth drill is used to construct a pile foundation. It is a device of two rods with a loosening and cutting cup. The drill makes small holes under the piles. You can use a hand drill, which drills holes up to 5 m. It is used when the diameter of the foundation support is no more than 30 cm. A motorized drill is used when it is necessary to drill a hole of a large diameter (from 50 cm). That is, when constructing a small pile foundation with a grillage, you can use manual labor, and if screw piles are used in the structure, then you can simply abandon the excavation work. There are several varieties of this tool: TISE-F, TISE-2F, TISE-3F.

On one side the drill has a soil accumulator, and on the other there is a crossbar with handles. A folding plow is mounted above the soil accumulator. When drilling, the plow cuts off part of the earth. Under the influence of its own weight, the plow cuts the soil, gradually lowering down. The cut soil falls into the storage tank and rises to the top. This drill weighs 11 kg. When drilling, direction should be taken into account.

If, for example, you need to drill a well deeper, then the drill must be rotated clockwise. If anchor extension is required, the tool is rotated counterclockwise.

Sod should be removed before drilling. Depending on the type of soil, drilling can be done either by one person or by two people. A partner is involved if drilling occurs on clay or dense soils.

Comparative review of different materials for grillages

The design of a pile foundation can be selected depending on the mass of the entire building and the materials used in construction and types of piles. It can be wooden, metal or concrete. Concrete can be monolithic or consist of concrete beams or panels. The connection between the strip part and the pile head has many design options. Depending on the needs and financial capabilities, a wide variety of pile foundation designs are used. But there are types of soil, for example swampy, on which it is possible to use only piles.

Pile grillage reinforcement scheme.

Most often, a foundation with bored supports and a monolithic concrete grillage is used. Other options for pile foundations cause various complications in operation. Wooden beams are relatively short-lived and their load-bearing capacity is significantly limited. Wood is susceptible to fungi and rot. Metal structures are subject to corrosion, and their installation requires the use of lifting equipment. A concrete foundation made of blocks requires a crane during installation. With a monolith of a bored pile foundation, the immobility of the connection between the grillage and the pile heads is ensured, which ensures high structural rigidity and the required stability of the entire pile foundation. When erecting a monolithic structure, you can do without lifting mechanisms.

Installation of grillage.

Steel grillage.
A grillage made from I-beams is highly expensive and difficult to install.

A grillage is a part of a pile foundation that combines load-bearing elements into one structure. They are made in the form of a strip or in the form of a slab. If a wooden structure is being erected, then the functions of a grillage are performed by the crown of beams located in the lower part. The steel system is based on I-beams. But such a pile monolithic foundation has several disadvantages. Firstly, a monolithic pile foundation, in which I-beam products are used, is highly expensive. Secondly, the installation process itself is labor-intensive and requires the use of special equipment. It is impossible to mount this part of the foundation manually. Thirdly, even steel treated with a special solution will soon corrode. Therefore, the only method left is using reinforced concrete. In this case, the grillage can be prefabricated.

The types of grillage designs vary, depending on the position in relation to the ground. The first type is tall. This is the most popular type and is located 0.1-0.15 m above ground level. This distance is quite enough to reduce the impact of the soil on the connecting system during heaving. Swelling of the soil due to weather changes is accompanied by raising the grillage and tearing out the pile elements. It is also possible to tear off the structures themselves from the piles. But a height of 0.1-0.15 m allows you to avoid such problems.

Pile foundations come with a low, raised or high grillage.

A monolithic pile foundation with a high grillage has one drawback - the free space between the floor covering and the soil. In this regard, additional thermal insulation is required.

Another type of grillage is an elevated one. The base of the system is located on the soil. Unlike the previous type, here there is no free space between the floor and the soil. To prevent the effect of heaving on the structure, part of the soil is removed from under it - approximately 0.1-0.15 m.

The next type of grillage is recessed. It is made below ground level and some soil is also removed from under the lower part.

The main task of the grillage is to connect all the piles into one system. The structure does not distribute any loads over the ground. It simply applies pressure to the piles. The grillages are not mounted directly on the ground so that sections of the base do not come off due to soil heaving.

Design features of a monolithic grillage

Installation of a grillage in a monolithic foundation.

To ensure high strength, the structure of a bored foundation does not allow communication channels to be laid in its body. Reinforcement is performed in at least two tiers. Each tier must contain at least two rods. The grillage experiences a load from above from the weight of the entire building, and on heaving soils in winter, loads from soil movement from below are added. For longitudinal bars of a pile foundation, reinforcement with a variable cross-section with a diameter of up to 12 mm is used. Transverse rods do not carry loads and serve only for the construction of the reinforcement frame. Their diameter can be half that size. The reinforcement frame must be recessed into concrete to prevent contact with the environment and to protect the rods from corrosion.

The technology for constructing bored foundations with a grillage requires paying special attention to the connection of the pile reinforcement with long rods in the tape frame. When preparing the head, the pile rods should protrude from the top. To create a good connection, these rods are bent and tied with tying wire to the grillage rods. To fill the monolithic reinforced concrete grillage of the pile foundation, wooden formwork is installed.

Diagram of vertical stress distribution under the grillage.

Depending on how the foundation body is located relative to the soil level, there are different design features. On swampy and other heaving soils, the body of the foundation is raised above the soil surface to a level of up to 150 mm. Such a high grillage is made in order to prevent the influence of forces from the ground when the soil swells in winter. As a result of soil swelling, the grillage may be torn off from the pile heads or pulled out of the soil. But this arrangement of the grillage has a major drawback. After construction is completed, a void remains between the floor of the house and the ground, which requires additional insulation of the floor below. The design of such a house does not have a basement.

A design in which the lower part of the tape is at the level of the ground surface or buried below this level avoids voids under the floor. But this option requires providing an air gap directly under the grillage. The recess is made to the same 150 mm.

To prevent the resulting space from being filled with collapsing soil, barrier plates made of slate or sheet metal are installed.

A fence is used to cover the space under the foundation. When installing a pick-up, the recess is filled with a material that is not subject to swelling and has looseness. Both recessing and additional fencing require additional costs, so a raised foundation has clear advantages in this regard.

Foundation reinforcement.

Construction of pile foundations.
Foundation reinforcement increases the strength of concrete.

Mandatory reinforcement of the foundation with a monolithic grillage solves a number of useful problems. The resistance of concrete to bending loads increases. Without reinforcing elements, the grillage may crack. All reinforcement work consists of creating two belts. The first chord is located at the top, and the second at the bottom, and each uses at least two reinforcements. The diameter of the reinforced rods should be within 1-1.2 cm. The rods neutralize the deformation effect.

Transverse reinforcement has nothing to do with the load. Its purpose is to knit the entire structure into a single system. Since there is no pressure here, reinforcement with a diameter of 0.6-0.8 cm is used. In addition, smooth reinforcement products can be used.

The reinforcement cage must be completely buried in the concrete mixture. Only the very top section should protrude 2-5 cm. Such belts are a kind of mesh, which consists of ribbed reinforcement. There should be a step of 0.2-0.4 m between the elements. Due to the vertical rods, the upper and lower sections are combined into one structure.

It is important to note that the construction of a foundation with a grillage requires more concrete mixture than for other types of foundations . Before pouring, formwork is installed, into which the reinforcement frame is mounted. To ensure that the steel structure is properly immersed in the concrete mixture, bars are placed under the lowest bars. All elements must be securely fastened.

Reinforcement is also used to reinforce piles. This is necessary to connect to the grillage and strengthen the concrete. To attach the grillage to the piles, surface rods are attached to a frame made of reinforcement. The width of the grillage is made larger than that of the walls, and the cross section is made in the form of a square.

Construction of the basement.

After the pile monolithic foundation is built, we proceed to finishing the base. The upper edge of the basement trim is attached to the bottom of the house and is protected from above by a special galvanized canopy (shimmer). From below, the finishing does not reach the ground by several centimeters. This is done so that during soil heaving the base is not subject to deformation. To remove precipitation, drainage is installed, and a concrete blind area about 70 cm wide is made. The best solution for a blind area around the entire perimeter of the house is paving slabs. It is durable, aesthetic and repairable.

Pile-grillage foundation - review of technology.

A pile-grillage foundation is not yet as popular a type of base for a building as a strip, pile or monolithic one. It began to be widely used relatively recently. But, as it turned out in practice, when building houses on loam, highly heaving or peat soils, this type of foundation is the most optimal. It takes into account and compensates for insufficient soil stability, thereby ensuring the stability of the building. An undoubted advantage is the savings of about 30% compared to the price of a traditional foundation.

Construction of grillage foundations.

Features of the foundation structure of the grillage type.

The grillage is essentially beams (slabs) through which piles located on the ground are connected to each other. These concrete lintels play the role of supporting structures for the structures erected on their basis.

Based on the amount of materials used, grillage systems are:

Classification of structures with grillage depending on load distribution:

a high foundation that takes the weight of the building through the piles and distributes it to the lower layers of the soil;
low, located below the soil surface and transmitting the load along the base.

Where is a pile foundation with a grillage used?

The construction of a grillage-type foundation base is carried out on soft soils. These include:

highly heaving;
peat and silt;
loess-like;
clayey mobile;
loams;
quicksand

This type is also used if:

a house is being built on a mountainside or on a site with complex terrain;
construction is carried out using frame technology;
the depth of soil freezing exceeds 1.5 m.

The grillage type is also ideal in the situation where the calculations show that the strip foundation for a brick house should have a width of more than 1.5 m.

Pile foundations with a monolithic grillage are used for the construction of small country houses, light brick buildings, houses made of central fiber board, laminated veneer lumber, foam concrete and other similar building materials. The grillage for the above buildings can be monolithic or precast reinforced concrete.

Foundation layout diagram.

Advantages and disadvantages of a pile-grillage foundation

The grillage type of pile foundation has a number of advantages and disadvantages, thanks to which this foundation is becoming increasingly popular:

A relatively simple installation technology that can be carried out by a small team of professional workers.
Thanks to the first advantage, construction costs are reduced.
Low consumption of concrete.
Since the foundation will not be in contact with frozen soil, the house will not lose heat.
The unique technology of connecting the grillage and piles prevents the house from vibrating even when located next to railroad tracks.
Work on installing such a foundation can be carried out at any time of the year, but not when the frost is below 10 degrees.
The construction of the foundation is not affected by the groundwater level.
This foundation, as mentioned earlier, is perfect for construction in areas with heterogeneous terrain.
There is no need to prepare the site in advance, for example, to remove the fertile layer or grass, which significantly saves construction time.
The cost of work, which depends only on the material used.

Like any technology, the equipment of a pile-grillage foundation also has its negative features:

It is not possible to create a basement or ground floor in the house.
Not suitable for large houses.
A clear calculation of the number of piles and the expected load in accordance with the type of soil is required.
It is mandatory to follow the reinforcement density in accordance with the brand of concrete used.
It is possible for piles to rupture when frozen soil puts pressure on them.
There is no grillage waterproofing.
You need to use a mixer and a concrete pump to pour concrete.

Installation stages.

The construction of a pile foundation with a grillage begins with driving the piles into the ground. A prerequisite when working is to accurately determine the level of soil freezing. The supports must be buried below this mark. Next, the piles are connected with grillages. By their nature, grillage joints can be made from the following materials:

Grillages protect against soil displacement. They counteract structural instability during freeze-thaw and provide rigid coupling of the building's structural elements. Piles are mounted at points where the heaviest load is expected; their length depends on the depth of soil freezing.

In practice, for the construction of low-rise buildings, supports from 1.5 to 3 m are used. The distance between adjacent elements is 1-1.5 m. The holes for the supports must be lined with roofing felt or similar material. It is possible to casing with asbestos-cement pipes.

4 reinforcing rods are vertically fixed and securely tied in the hole. The top of the pile must be above ground level. The reinforcing thread coming out of the pile support is tied to the grillage reinforcement. The holes are filled with concrete mixture.

Over the course of a week, the concrete will gain strength. After 7 days the indicator will reach 60%. After this, you can begin work on reinforcing the grillage. The process begins with formwork. Sand is poured into it or polystyrene foam is laid. Thus, the grillage will be raised above the ground (hanging on stilts).

Scheme of a pile-grillage foundation.

An important point before reinforcement is waterproofing the upper part of the pile pillars. This stage should not be skipped under any circumstances. Reinforcement is carried out according to the general scheme developed for a specific construction. This is due to the fact that the position of the supports and the distribution of loads on the grillage differs in different places of the structure. The backfill from under the grillage structure is removed after the concrete reaches half of its brand strength (no earlier than 7 days).

Taking into account the fact that for each specific case the position of the supports and the distribution of the load in the grillage is different, reinforcement is carried out on an individual basis. The reinforcement that comes out of the piles is connected to the grillage reinforcement. When the grillage reaches more than half of its brand strength, the bedding from under it is removed. The grillage structure hangs on stilts. The grillage is not allowed to rest on the ground between the piles.

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Pros and cons of a pile-grillage foundation

Like every other building structure, the pile-grillage structure has its own advantages, disadvantages and limitations in application.

It involves minimal time spent on creating a foundation, provided that the work is carried out by experienced craftsmen with their own special equipment. A pile foundation for a brick house (or other type of structure) is installed in the shortest possible time. The customer receives a ready-made turnkey project.

It involves the use of a small amount of concrete due to the fact that there is no need to build a cushion in the ground.

In the cold season, when the soil freezes, the area of interaction between the elements of the pile-grillage foundation and the frozen ground is minimal. Thus, the heat loss of the building is also small.

The presence of vibration-isolating properties in the structure.

The peculiarities of connecting piles with a grillage provide reliable protection of the structure from vibration associated with the close location of the building relative to large highways, railways or subways. A pile foundation without a grillage is less effective in such cases.

It should be noted that the construction of a pile-grillage structure can be carried out at almost any time of the year. Restriction – work cannot be carried out at temperatures below -10 °C. Another advantage of such a foundation is the affordability of the materials used.

Bored piles.

Disadvantages and disadvantages:

Use of special equipment.

It is impossible to build this type of foundation yourself. The technology requires the mandatory use of special equipment.

It is almost impossible to build this type of foundation with your own hands.

The difficulty of constructing basement floors.

Features and layouts of supports complicate the arrangement of basements and basements. Often the space between piles has to be filled, which significantly increases the final cost.

Low resistance to rollover.

Pile-grillage foundations on subsiding and swelling soils require special precision in adherence to technology.

Installation of driven supports.

Selecting the type of pile and the depth of the grillage.

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According to the recommendation of [3], we accept prismatic reinforced concrete piles of square cross-section as the most commonly used in mass construction. By cross section we take a pile m.

The length of the pile is determined based on engineering and geological conditions, taking into account the length of embedding the head of the pile into the grillage:

where is the depth of embedding the pile into the grillage, assumed to be 40 cm since the support of the grillage on the pile is rigid ( );

— depth of immersion of the lower end of the pile into the load-bearing soil, m (according to clause 9.10 [7]).

— the distance from the base of the grillage to the roof of the load-bearing soil layer, determined from the engineering geological section, is 1.14 m.

We assume the length of the pile is m.

The depth of the grillage is determined depending on the depth of seasonal freezing and on the design features of the structure being designed.

We take the height of the grillage to be 0.45 m.

The minimum height of foundations is assumed to be 1.5 m for columns of solid section [3].

In this course project, the depth of foundations according to design requirements is greater than the depth of freezing. Taking into account all the features, we take the depth of the monolithic grillage to be 1.95 m.

Determination of the bearing capacity of the pile.

The load-bearing capacity of the pile in terms of material in the course project is taken depending on the cross section. For the accepted cross-section of the pile

The load-bearing capacity of the pile on the ground is determined using tabular data in accordance with clause 10.55. [7] and Fig. 3.1.

where is the coefficient of operating conditions of the pile in the ground;

calculated soil resistance under the lower end of the pile, kPa;

area of support of the pile on the ground, , taken from the gross cross-sectional area of the pile or from the cross-sectional area of the camouflage widening along its largest diameter, or from the net area of the shell pile

outer perimeter of the cross section of the pile, m;

calculated resistance of that base soil layer on the side surface of the pile, kPa;

thickness of the soil layer in contact with the side surface of the pile, m;

coefficients of soil operating conditions, respectively, under the lower end and on the side surface of the pile, taking into account the influence of the driving method on the calculated soil resistance.

Figure 3.1 — Design scheme for determining the load-bearing capacity of a pile on the ground

; ; ; ; ;

; ;

Finally, the load-bearing capacity of the pile is taken to be less, i.e.

Determining the number of piles in a grillage, designing a grillage

The number of piles in the grillage of a free-standing foundation for columns is determined by the formula:

where the design load at the level of the base of the grillage can be taken without taking into account the weight of the foundation, grillage and soil on their ledges, i.e. .

reliability coefficient adopted according to [7].

The construction of a grillage with 2 piles (see Figure 3.2) is carried out in accordance with the design requirements [6].

Figure 3.2 — Scheme of grillage design

Checking the load-bearing capacity of the most loaded pile

For a centrally loaded foundation, the condition [7] must be met:

The value of the design load acting at the level of the grillage base:

where is the weight of the grillage, foundation and soil on its ledges, determined with a reliability coefficient, respectively;

Consequently, the load on the pile does not exceed its load-bearing capacity.

Calculation of pile foundation settlement

Settlement is deformation that occurs as a result of soil compaction under the influence of external loads and, in some cases, the soil’s own weight, not accompanied by a fundamental change in its structure.

The calculation of a pile foundation based on base deformations is carried out in the same way as for a foundation on a natural foundation using the layer-by-layer summation method in accordance with clause 5.14 [7].

The purpose of calculating foundations based on deformations is to limit the absolute or relative movements of foundations and superstructures to such limits that the normal operation of the structure is guaranteed and its durability is not reduced.

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Turnkey foundation cost.

Due to the difficulties of independently implementing the idea of arranging a foundation with a grillage, an important point is the price for the work of the team and the rental of special equipment. Another nuance that affects the final estimate is soil testing and design calculations. In both cases, a balanced approach is required from the customer. On the one hand, he cannot hire non-specialists, since the durability and reliability of the structure is at stake. But there is no need to pay an inflated price either. The optimal solution is to consult friends who have already gone through a similar stage of construction.

The construction price in the Moscow region averages 4,400 – 6,000 rubles per 1 linear meter. The total amount is affected by the number of piles, their quality and size, the total amount of materials, and installation technology.

Construction cost per linear meter.

What is a pile-grillage foundation?

Everyone probably imagines a pile foundation: it is a number of piles buried in the ground to the level of the load-bearing layer or below the freezing level. In its pure form, this type of foundation is rarely used. This is due to the peculiar design, which does not allow the load from the house to be redistributed between the piles. Therefore, pile foundations are mainly made for log buildings made of logs or timber, and sometimes for frame buildings. These types of building materials, due to their characteristics, themselves redistribute the load. They are not compatible with houses made of other materials.

But their improved form - a pile foundation with a grillage - is free of many disadvantages and can be used for both brick and block buildings. In them, all the supports are tied using a tape made of metal or reinforced concrete (concrete) into a single structure. This tape is called a grillage.

Pile grillage foundation.

This is what a pile-grillage foundation looks like when taken out of the ground.

A grillage is a part of the foundation that connects the pile heads and serves as a support for the walls. It is the grillage that receives and, due to the closed structure, redistributes the load, transferring it to the piles. It can be metal, wood, concrete or reinforced concrete. According to the type of execution, concrete (reinforced concrete) grillages are low and high.

Foundation grillage.
There are pile foundations with high and low grillages.

The high grillage is above ground level. Most often it is made of metal - large-section channels or square metal pipes. They also make such a grillage out of concrete, but its design is more complicated: you have to figure out how to fill the tape at a distance from the ground.

How does a grillage work and what does it provide?

Any house in different parts will have different loads: decoration, furniture, sanitaryware, other things are placed unevenly. Consequently, the load from its different parts will be different. The grillage takes on these uneven loads and redistributes them. The “leveled” load is already transferred to the piles.

Pile grillage foundation.

What is the difference between pile and pile-grillage foundations?

Why is this good? The fact is that with the same load on the piles, there is less chance that they will settle unevenly. And uneven shrinkage leads, as is known, to cracks in the foundation and walls. Therefore, the pile-grillage foundation is more stable. Although the main disadvantage of pile foundations remains: we cannot know what kind of soil is under each of the piles. Therefore, it is unrealistic to predict their behavior. That is why architects are not very fond of them: it is impossible to guarantee long-term operation of the house.

Strip foundation on piles.

Low grillages are more predictable in this regard. They usually start below ground level and are cast from reinforced (or not - depending on the project) concrete. Moreover, the reinforcement of the piles is connected to the reinforcement of the grillage.

In this case, the grillage is a shallowly buried strip foundation and it is made using the same technology. It differs in that it has a rigid connection with the piles, which significantly increases the reliability and stability of the structure. Such foundations are also called strip foundations on piles or pile-strip foundations. This design is almost ideal: it combines the advantages of pile and strip foundations, largely compensating for their disadvantages.

Pile grillage foundation.

Construction of a pile-strip foundation.

How does he work? The load from the house is transferred to the belt. Thanks to the presence of longitudinal reinforcement, it is redistributed over the entire area. Since the tape also rests on the ground, part of the load is transferred to it, the rest falls on the piles. In this case, the load and shrinkage are uniform: they are “evened out” by the tape.

In winter, when heaving forces begin to affect the foundation, all the advantages of a pile-strip foundation appear. If the house is located on heaving soils, their depth is below the freezing level, it is very difficult to imagine conditions under which the house will warp or it will shrink unevenly.

When heaving forces act on the tape, the “heels” of the piles, and they themselves, do not allow the soil to move the foundation. Therefore, strip-pile foundations are an excellent choice on highly heaving soils. The costs are much higher than when constructing a conventional pile foundation, but much lower than when constructing a strip below the freezing depth.

Description of a pile foundation with a grillage

The name of the pile foundation already suggests that it consists of piles that are buried in the ground to a certain level. Most often, piles are immersed to a dense load-bearing soil layer or below its freezing level. The finished pile structure cannot be used in its pure form, as it consists of disparate elements. The exception is frame buildings or log buildings made of timber or logs, in which the lower crowns are capable of taking on the load of the entire building.

Pile foundation with grillage

For the construction of houses from other types of material, a more advanced form is used - a pile grillage foundation. In this design, the above-ground parts of the piles are connected by a metal or concrete strip or grillage.

In professional language, a grillage is the part of the base that connects the pile heads and serves as a support for load-bearing walls.

When can it be used?

It is recommended to install pile-grillage foundations in the following cases:

When unstable and weak-bearing soils go to a sufficiently large depth. These are karst, forest, peat soils, quicksand, plant and fertile soils of great thickness (more than 1.4-1.5 meters). In this case, the load must be transferred to the dense soils located below with normal bearing capacity. It is not always possible to get to them, and even if it is possible, the foundation turns out to be too expensive. Therefore, transferring the load using piles is the best choice.
In areas with large elevation differences. In this case, it is often much cheaper to use piles of different heights than to carry out work on leveling the ground or pouring a deep tape that can compensate for height differences.

Pile grillage foundation.

At high groundwater levels. Pile foundations are the only ones for which the groundwater level does not matter. It is important that there is soil with load-bearing capacity under the heel. The level of water location affects only the type of grillage: if the water is close to the surface, the grillage is made high, if it lies deep, it can be made low.
During construction in hard soils. In this case, small volumes of earthworks (compared to strip or slab foundations) have a positive effect.
If you decide to build a house using frame technology. Making a tape for it is a waste of money: you get too much safety margin, which, in this case, is useless. In this case, a pile or pile-grillage foundation is the best choice.
With a large mass of the building (more than 350 tons). Then it turns out that the tape or plate must be very massive and, therefore, expensive. In this case, a pile-grillage foundation often turns out to be cheaper.

Sometimes it’s even simpler: they start from the lowest cost. But you should always remember that any type of pile foundation is less reliable than slab and strip foundations. And all because we cannot know exactly what kind of soil is under each of the piles. That is why, when calculating parameters, an increased safety margin is included in the design. Not 1.2, as is usually believed, but 1.4. And still, no one can guarantee anything.

What piles to use.

Any piles can be used in pile-grillage foundations. They are selected based on the soil and the planned load of the house. Piles are made of metal, concrete, and sometimes wood. They can have a round or square cross-section. They also differ in the installation method:

Hammers. They are installed without drilling or excavation. Usually they get clogged, that’s why they are called that. They are rarely used in private construction: special equipment is required.

Pile grillage foundation.
Driven reinforced concrete piles are used more often in the construction of multi-storey buildings.

Bored. First, a well is made in the ground, then formwork is installed and secured into it. It contains reinforcement that increases strength characteristics. Then the entire structure is filled with concrete.
Reinforced concrete drilling rigs. In this case, wells are also drilled, but ready-made reinforced concrete piles are installed in them (driven, but with less effort).
Screw. These are metal piles, pointed at the end and having screw blades that cut the soil. At greater depths, special equipment is required; small ones, up to 2-3 meters long, can be installed manually.

Pile grillage foundation.
One type of piles is screw piles. They can be used with grillages made of different materials for different houses.

In private construction, bored piles are most often used. They are especially popular in the construction of summer houses or baths. They can also be used in the construction of small houses. But if dachas and bathhouses can be built without calculations, then when building a house it is very advisable to order a project.

Different piles come in different shapes: square or triangular, round filled and round hollow, sometimes complex shapes are developed specifically for the project. Based on how piles transfer load to the ground, they are:

Pile grillage foundation.

How do piles differ by type of work?

Hanging piles have a large lateral surface and a significant part of the load (up to 60-70%) is transferred with its help. They are used if the bearing capacity of the soil at the achieved level is not enough to withstand the entire load, and further deepening is impractical. In this case, the presence of a “heel” changes almost nothing: the main load is transmitted by the surface, and not by the tip of the pile.

Rack piles, on the contrary, transfer most of the load through the tip. In this case, the side walls are undeveloped and smooth, and it makes sense to expand at the end of the pile. One type of this type is TISE piles. They have a cylindrical extension at the bottom, which is why they transfer the load to a larger area. Also, the heel prevents heaving forces from lifting the foundation.

Location of piles in the grillage.

In most cases, supports in the ground are placed vertically. Sometimes an inclined installation may be required, and several pieces in one place. This method is used if horizontally directed forces may arise. For example, during construction on sloping areas.

Pile grillage foundation.

Location of piles on the house plan.

Piles can be located pointwise - under supports (columns, racks). This arrangement is typical when constructing canopies or installing roofs over open areas. Then the supports are placed exactly under the columns holding the roof.

For private houses with a small number of floors, the piles are placed in the form of a strip. They are placed in one row, sometimes in two or three, shifting them relative to each other. Sometimes in the most loaded parts, for example, in corners, under areas with a large planned load, they are placed in bushes: several pieces at a short distance from each other.

How to place a house on a plan yourself.

When designing a pile foundation on your own, you usually proceed as follows. On a building plan drawn to scale, piles must be laid in the corners and at the intersections with the walls. If they are more than 3 meters apart from each other, intermediate supports are placed between them. It is advisable that the piles be located one from another at a distance of 2 to 3 meters.

Pile grillage foundation.

First, the piles are placed in the corners, then under the junctions of the load-bearing walls.

The principle is simple, but you still need to determine the required area of the piles. And it needs to be calculated (or rather, selected).

DIY pile foundation with grillage

Most often, the construction of houses is carried out on a bored pile foundation with a grillage in the form of a concrete strip, which is located above the ground at a distance of 0.2 meters or buried in the ground.

The construction of a pile foundation with a grillage is carried out in several stages.

Site marking

To build any foundation, correctly executed markings are required; a pile grillage foundation also cannot do without this stage. The site is marked as follows:

Marking the area

Along the line of location of one of the long sides of the house, two pegs are placed, the distance between which is equal to the length of this side. These will be the two corners of the house. A construction cord or rope is pulled between the pegs.
Two more cords are pulled perpendicular to the resulting line, on which the width of the house is marked, and pegs are set. Two other corners will be located here, they are also connected by a cord.
You can check the correctness of the angles in the following way: make a mark of 3 meters on one side of the corner, and 4 meters on the other. If the angle is right, then the distance between the marks will be 5 meters.
Then mark the places where the foundation supports will be located. It is important here that the piles are placed at the corners of the house and at the intersections of the walls.

Excavation

You can make holes for piles yourself using a hand drill. The depth of the wells is determined at the design stage, but most often it is 1.5-2 meters.

A layer of sand is poured onto the bottom of the drilled holes to create a cushion. The thickness of the layer must be at least 0.1 meters, it must be compacted very well.

An asbestos-cement pipe is installed on the cushion, strictly monitoring the vertical position of the support. You can use a building level to check.

Reinforcement of supports

To create a reinforcing frame, take four reinforcement rods with a cross-section of 12 mm and tie them with crossbars using soft wire. The height of the frame should be equal to the height of the support with a margin of 20 cm. This margin is necessary for further connection with the grillage.

Preparation and pouring of concrete

It is necessary to fill the piles with a high-quality concrete solution, for the preparation of which you will need 1 part of M400 cement, 3 parts of sand and 5 parts of crushed stone. More uniform and high-quality concrete is obtained by mixing it in a concrete mixer, but you can prepare the solution yourself. To do this, first mix the dry ingredients and then add water in small portions. The consistency of the solution should be medium thick.

Ready concrete is poured into reinforced pillars, constantly compacting it. This will help remove air bubbles from the solution.

Assembling the formwork for the grillage

Installation of formwork depends on the location of the grillage relative to the ground. If the grillage is located above the ground, then the formwork is built on supports, making a bottom of boards. Shields are made from wooden boards and the side walls of the formwork are made, connecting them with spacers inside the structure. The outside of the formwork should also be secured with supports.

When the grillage is recessed, the formwork is installed as follows:

Formwork for grillage

Dig a trench and fill the bottom with a layer of sand up to 10 cm.
Formwork is assembled from wooden panels installed vertically on both sides of the trench.
The panels are secured with spacers on the outside and spacers on the inside.

It is best to use a grillage raised above the surface of the ground; you do not have to worry about seasonal heaving of the soil. But construction in this case must begin only a week after pouring the concrete.

Grillage reinforcement

The frame for the grillage is also knitted from reinforcing bars with a cross-section of 12 mm, only the reinforcement is connected in two rows and laid along the entire length of the tape. It is recommended to make stands for the frame from pieces of bars 5 cm thick. The grillage reinforcement must be connected to the rods of the supporting pillars.

Calculation of a pile-grillage foundation.

In order to be at least partially confident in the reliability of the foundation, it is advisable to at least approximately calculate it. To do this, you will need to do several not very complex calculations.

The weight of the house is calculated (all materials and the approximate weight of the “filling” are taken into account).
Based on the weight and safety factor, the required supporting area is determined.
Since the piles have already been placed, we can count their number.
Next we select the cross-sectional area of the piles. Since we know their number, multiplying by the selected area, we find out the total.
We compare the obtained result with the reference area calculated earlier (point 4).
If the area found in step 4 is larger than that calculated in step 2, the selected pile parameters are suitable for us.
If it is less, we increase the planned cross-section of the piles, multiply again and compare. This is how we find the required area of the pile.

Sometimes it is more profitable not to increase the area of piles, but to install them more often. The calculation method does not change.

Load-bearing capacity of bored piles.

Load-bearing capacity of piles of different diameters in different soils.

The support area required for normal load distribution is calculated by the formula:

Where N is the bearing capacity of the soil (it can be taken from the table). The coefficient of 1.4 was applied due to the fact that pile foundations require a larger margin of safety: we do not know what kind of soil is under all the piles. Therefore, it is better to play it safe.

For more information on calculating the supporting area and calculating the mass of the house, watch the video.

Construction technology.

So, you have a project ready, or you yourself have decided on the location of the piles, their parameters, the type and height of the grillage. Construction can begin.

First, the entire fertile layer of soil is removed, taken out to the garden or stored in one of the corners of the site. The area for the foundation is leveled. Then, using pegs and cords, the boundaries of the house are marked. Moreover, two cords are pulled, marking the outer and inner surfaces of the walls. Be sure to check the geometry by re-measuring and comparing all the diagonals in pairs.

After the marking is completed, you can mark the installation locations of the piles. We have them drawn on the plan and calculated, the distances between them are determined.

Pile grillage foundation.

Schematic representation of the work procedure during the construction of a pile-grillage foundation with your own hands.

Excavation and pouring of piles.

How exactly the work will take place depends on the type of piles used. Most often, screw or bored piles are used when building a house.

Screws can be screwed in by hand. When twisting, their heads are aligned at the same level. It is more convenient to mark it with cords.

For drilling, you will need either special equipment or a hand drill of a suitable diameter. These devices drill holes in the right places. The selected formwork is lowered into the finished wells, and reinforcement is installed (four bars of ribbed reinforcement connected to each other). If the foundation is pile-strip, then the reinforcement must have an outlet of at least 60-70 cm. It will then be connected to the grillage. The last step in this stage is pouring the concrete.

Construction of foundations for pile grillage.

The pile foundation is ready for installation of formwork for the grillage. In this case, formwork made of roofing felt or roofing felt, rolled several times, is used.

Grillage arrangement.

If a metal grillage is selected, it is welded to the pile heads. When working, it is important to maintain strict horizontality: then the load will be transferred evenly.

To install a recessed reinforced concrete grillage (low), you will need to add crushed stone of a fraction of 20-60 mm and compact it with a vibrating platform. The minimum layer of crushed stone is 15 cm. To improve the bearing capacity of the soil under the belt, it is necessary to pour crushed stone in small layers (5 cm each) and thoroughly compact it with a vibrating platform. It will not be possible to compact well around the piles, but this is not so important.

Formwork is installed on this bedding. The width of the tape is slightly larger than the width of the walls, the height of the panels is made taking into account the required height of the base. Assembling the panels and installing stops exactly repeats the technology for installing formwork for strip foundations.

Pile strip foundation.

Scheme of reinforcement of a pile-grillage foundation with a reinforced concrete grillage (pile-strip).

Reinforcement is carried out in most cases in the same way as during the construction of a belt: two belts at the top and bottom made of ribbed reinforcement, transverse and vertical posts made of a smooth rod of smaller diameter. All reinforcement rules are preserved. Only the dressing with piles is added. The outlets of the reinforcement from the piles are bent: one row is tied to the lower belt, the second - to the top (do not forget to make the outlets of different lengths). They must be at least 50 bar diameters (if 12 mm reinforcement is used, you need to bend 600 mm (50 * 12 mm)). The reinforcement diagram for the pile abutment is shown in the figure.

Installation of embedded parts.

At this stage, it is necessary to leave ventilation ducts and lay pipes and ducts through which utilities will be supplied (plumbing, heating, electricity, sewerage).

Pile grillage foundation.

Don’t forget about laying pipes for vents and connecting engineering systems.

If this is not done, you will have to subsequently chisel the concrete. And this is a great evil for the foundation, because the integrity of the monolith is violated. So think through everything carefully: there will be no opportunity to redo it.

Pouring concrete.

Concrete is not just poured. It is also vibrated. Without this process, it is very difficult to achieve uniformity and high strength. Therefore, processing foundation concrete with a vibrator can be considered a mandatory process.

If the air temperature after pouring concrete is around +20°C, then the formwork can be removed after four days. During this time, the concrete will gain approximately 50% of its design strength, which is enough to carry out the following work. If the temperature was around +10°, you will have to wait 10-14 days. At lower temperatures, conditions are considered wintry and the poured concrete requires insulation or heating.

Pile grillage foundation.

Everything is ready for pouring concrete: the reinforced belt is tied, the formwork is set and secured (to enlarge the size of the picture, right-click on it).

For all their attractiveness, pile foundations have disadvantages:

As already mentioned, due to the fact that it is impossible to determine what kind of soil is located under each of the piles, its behavior is almost impossible to predict.
In such buildings it is impossible to install basements.
With high grillages, it is necessary to somehow close the gap: animals and insects like to live there.

Recommendation: A good review article, from it you can learn about the construction of a pile foundation with a grillage. Indeed, a pile-grillage foundation is not expensive. If it suits your building in all respects, then you can safely use it, but be careful when calculating the loads, you won’t be mistaken. This is an important point!

Technology for installing a pile-grillage foundation

Marking the construction site

It is best to remove the fertile layer of soil and take it out onto the plot or into the garden. Then it will be necessary to level the top surface. Using pegs and a cord, you need to determine the boundaries of the house. At this stage, not only external, but also internal wall boundaries must be outlined.

After the marking has been made, it is necessary to mark the places where the piles will be installed in accordance with the plan drawn up earlier.

Carrying out excavation work

In order to decide on the work on the ground that will be carried out, you need to clearly know which piles will be used. A house on a pile foundation is placed on screw or drilled piles. Screw-type piles can be twisted with your own hands, but only at one level, which is marked in advance by pulling the cord.

Drilled cast-in-place piles require the use of special equipment. With its help, you need to drill holes in the ground in marked places. Formwork is installed in these holes, and reinforcement is also made. Usually these are 4 pieces of ribbed reinforcement that are connected to each other. Next, the last step is to pour concrete into the prepared holes.

Grillage arrangement

When choosing a metal structure, the grillage must be welded strictly horizontally to the ends of the reinforcement bars.

In order to make a reinforced concrete grillage of a recessed type, you need to backfill it with crushed stone. To improve the quality of the foundation, crushed stone is poured in layers of 5 cm and each layer is processed with a vibrating platform. This must be repeated until the layer reaches the desired thickness, at least 15 cm.

Formwork is installed on the finished bedding. It should be slightly wider than the width of the walls, and the height depends on how high the base is intended. The installation of the support and the assembly of the panels is carried out using the technology of installing a strip foundation. Typically, boards nailed together or waterproof plywood are used for formwork. Each type of grillage has its own nuances of formwork construction, but the main thing remains that the formwork must be 5-10 cm higher than the height of the finished grillage.

The foundation is reinforced in the same way as when installing a strip foundation. That is, at the top and bottom there are two belts of ribbed reinforcement, connected to each other by rods of a smaller cross-section and a smooth surface. You only need to add a dressing with piles. The pieces of reinforcement coming out of the piles must be bent and one row tied at the bottom to the reinforced belt, and the second at the top. In order to avoid wasting time and nerves on pouring piles, you can purchase ready-made ones made of reinforced concrete.

Installation of additional elements and pouring concrete

This stage involves the laying of special pipes through which in the future it will be necessary to carry out communications such as heating, sewerage, water supply and others. Ventilation holes in the foundation are also required. If this stage is skipped, then the integrity of the grillage will definitely have to be violated, which is extremely undesirable and generally unacceptable for such a foundation.

To start concreting, you need to make the mixture yourself. A particularly high-quality solution will be obtained from the M200 or M500 cement brand. It must be diluted according to all the rules and mixed thoroughly. Concrete is poured, but it must be processed with a vibrator. Be sure to pour the entire mixture at one time to avoid layering. The top layer must be leveled using a rule and checked for horizontalness using a level. The top layer of concrete is covered with a film to prevent it from drying out and causing cracks on the surface. If the weather is hot outside while the concrete poured into the foundation is drying, then the top layer must be moistened.

After pouring, the formwork can be removed after a certain period of time:

If during the work of pouring concrete and the following days the air temperature did not drop below +20 degrees, then after 4 days the formwork can be removed. Under such conditions, the foundation and piles will gain more than half the strength, which will make it possible to use them for subsequent work.
If after pouring the temperature was around +10 degrees, then you will have to wait 14 days before removing the formwork panels.
At temperatures below +10 degrees, freezing conditions are already considered winter, and additional heating and insulation of the foundation will be required.

Before carrying out work on the installation of a pile-grillage foundation, it is imperative that you familiarize yourself with SNiP 2.02.03 - 85. This document officially specifies the requirements for the arrangement of this type of foundation, tolerances, calculation by coefficients, and pile materials. Also here you can find recommendations for construction on normal soils, as well as in areas that have suffered from landslides and karst. Pile foundations are fully regulated by SNiP.