Pile foundation with grillage: DIY device

It is far from always and not in all situations that traditional foundations can be applied - tape and slab. In some situations, they become too expensive: their arrangement can take up to 40-50% of the cost of the entire building. This usually happens during construction in zones with a large freezing depth (below 2 meters) or on unstable, weakly bearing soils. In such cases, a pile-grillage foundation is often the best solution.

What is a pile-grillage foundation

The pile foundation is probably imagined by everyone: it is a certain number of piles buried in the ground to the level of the 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, the pile foundation is mainly made for log cabins from logs or beams, sometimes for frame buildings. These types of building materials, due to their characteristics, redistribute the load themselves. They are poorly compatible with houses made of other materials.

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

It looks like a pile-grillage foundation taken out of the ground

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

There are pile foundations with high and low grillages

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

How does the grillage work and what does it give

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

What is the difference between pile and pile-grillage foundations (to increase the size of the picture, right-click on it)

Why is this good? The fact that with the same loading of piles, there is less chance that they will shrink unevenly. And uneven shrinkage, as you know, leads to cracks in the foundation and walls. Therefore, the pile-grillage foundation is more stable. Although the main drawback 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 the 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 with the reinforcement of the grillage.

In this case, the grillage is a shallow 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 tape-based on piles or pile-tape. This design is almost ideal: it combines the advantages of the pile and strip foundations, largely compensating for their shortcomings.

The device of the pile-strip foundation (to increase the size of the picture, right-click on it)

How does he work? The load from the house is transferred to the belt. Due 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 "aligned" by the tape.

You can read about monolithic strip foundations in this article.

In winter, when heaving forces begin to affect the foundation, all the advantages of the pile-strip foundation are manifested. If the house stands on heaving soils, their depth is below the freezing level, it is very difficult to imagine the conditions under which the house will be flexible or it will give uneven shrinkage.

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. At the same time, the costs are much higher than in the construction of a conventional pile foundation, but much lower than in the construction of a tape below the freezing depth.

When can it be used

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

• When unstable and weakly bearing soils go to a sufficiently large depth. These are karst, forest, peat soils, quicksands, plant and fertile soils of great thickness (more than 1.4-1.5 meters). In this case, the load must be transferred to dense soils located below with normal bearing capacity. It is not always possible to get to the bottom of them, and if it is possible, then 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 level the ground or fill in a deep tape that can compensate for the height difference.
  

  One of the types pile-rostrum foundation - with TISE piles

• With a high level of groundwater. Pile foundations are the only ones for which the level of groundwater does not matter. It is important that there is a load-bearing soil under the heel. The level of the location of the waters 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.
• When building in hard ground. In this case, small amounts of land work (compared to strip or slab foundations) have a positive effect.
• If you decide to build a house using frame technology. To make a tape for it is a waste of money: it will turn out to be too large a margin of safety, which, in this case, is useless. In this case, a pile or pile-grillage foundation is the best choice.
• With a large building mass (more than 350 tons). Then it turns out that the tape or slab must be very massive, and, therefore, expensive. In this case, the pile-grillage foundation is often cheaper.

Sometimes it's even simpler: they proceed from the lowest cost. But you should always remember that any type of pile foundation is less reliable than slab and strip. And all because we cannot know exactly what kind of soil is under each of the piles.That is why, when calculating the parameters, an increased safety margin is laid in the design. Not 1.2, as is commonly believed, but 1.4. And still, nobody can guarantee anything.

What piles to use

In pile-grillage foundations, any piles can be used. They are chosen based on the soil, the planned load of the house. Piles are made of metal, concrete, sometimes wood. They can be round or square. They also differ in the installation method:

• Driving. They are installed without drilling or digging. Usually - they are hammered, that's why they are called so. They are rarely used in private construction: special equipment is required.
  

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

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

  One of the types of piles is screw. They can be used with grillages from different materials for different houses (to increase the size of the picture, right-click on it)

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

Different piles are in shape: square or triangular cross-section, round filled and round hollow, sometimes complex shapes are developed specifically for the project. By the way piles transfer the load to the ground, they are:

• hanging;
• piles-racks.
  

  What is the difference between piles by type of work (to increase the size of the picture, right-click on it)

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 sufficient to withstand the entire load, and further deepening is impractical. In this case, the presence of the "heel" changes almost nothing: the main load is transferred by the surface, and not by the pile tip.

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 kind of this type - TISE piles... They have a cylindrical expansion at the bottom, which is why they transfer the load over a large area. Also, the heel prevents the heaving forces from raising the foundation.

The location of the piles in the grillage

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

The location of the piles on the plan of the house

Piles can be located pointwise - under the supports (columns, racks). This arrangement is typical for the construction of awnings or roofing over open areas. Then the supports are placed exactly under the columns holding the roof.

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

How to place on a house plan yourself

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

First, piles are placed in the corners, then under the abutment of the bearing walls

The principle is simple, but you still need to determine the required pile area. And it needs to be calculated (rather, picked up).

Calculation of the pile-grillage foundation

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

1. The mass of the house is calculated (all materials and the approximate weight of the "filling" are taken into account).
2. Based on the weight and safety factor, the required support area is determined.
3. Since the piles have already been placed, we can count their number.
4. 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.
5. The obtained result is compared with the previously calculated (point 4) support area.
6. If the area found at step 4 is larger than that calculated at step 2, the selected pile parameters are suitable for us.
7. If less, we increase the planned cross-section of the piles, again multiply and compare. So we find the required pile area.

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

Bearing capacity of piles of different diameters in different soils (to increase the size of the picture, right-click on it)

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

Where H is the bearing capacity of the soil (it can be taken from the table). The coefficient 1.4 was applied due to the fact that the pile foundations require a greater 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 support area and calculating the mass of the house, see 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. You can start building.

Markup

First, they remove the entire fertile layer of the earth, take it out to the garden or fold it in one of the corners of the plot. The foundation area is leveled. Then, using pegs and cords, mark the boundaries of the house. Moreover, they pull two cords marking the outer and inner surfaces of the walls. Be sure to check the geometry by measuring and comparing all the diagonals in pairs.

After the marking is completed, you can mark the pile installation locations. They are drawn on the plan and calculated, the distances between them are determined.

Schematic representation of the order of work during the construction of a pile-grillage foundation with your own hands (to increase the size of the picture, click on it with the right mouse button)

Excavation and pouring of piles

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

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

For bored ones, either a special technique or a hand drill of a suitable diameter will be required. Holes are drilled with these devices in the right places. The selected formwork is lowered into the finished wells, reinforcement is installed (four bars of ribbed reinforcement connected to each other). If the foundation is pile-strip, then the reinforcement should have an outlet of at least 60-70 cm. It will then contact the grillage. The last step in this step is pouring the concrete.

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

Grillage device

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

For the device of a buried reinforced concrete grillage (low), it will be necessary to add crushed stone with a fraction of 20-60 mm and its compaction 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 fill the crushed stone in small layers (5 cm each) and carefully 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 backfill. The width of the tape is slightly larger than the width of the walls, the height of the boards is made taking into account the required height of the basement. The assembly of the shields and the installation of stops exactly repeats the installation technology formwork for strip foundations.

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

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

Read how the strip foundation is reinforced here.

Installation of embedded parts

At this stage, it is necessary to leave the ventilation ducts and lay pipes and ducts through which engineering communications (water supply, heating, electricity, sewage) will be supplied.

Do not forget about laying pipes for air vents, connecting engineering systems

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

Read about the choice of concrete for the foundation here.

Pouring concrete

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

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

Everything is ready for pouring concrete: the armopoyas is connected, the formwork is exposed and secured (to increase the size of the picture, right-click on it)

For all its attractiveness, pile-bed foundations have disadvantages:

• As already mentioned, due to the fact that it is impossible to determine which soil is under each of the piles, its behavior is almost impossible to predict.
• In such buildings, it is impossible to equip basements.
• With high grillages, it is required to somehow close the gap: animals and insects like to settle there.