Pile-grillage foundation

The foundation in the form of short piles, united by a monolithic grillage, is becoming increasingly popular in low-rise construction. It is recommended by the Code of Practice 22.13330.2011. And this is primarily due to the reliability of its operation under the uncertainty of engineering and geological conditions..

Pile-grillage foundation

The scope of application of short piles is not limited to frame buildings, they can be successfully used in buildings with load-bearing walls, including those on soft soils.

Consider the principles of the device and design of short piles-pillars, made by pouring concrete in advance prepared wells and united by a monolithic beam grillage.

The length of the pile depends on the value of the load falling on it, the calculated resistance of the soils it crosses and is limited by the available opportunity for drilling wells. Determined so that the heel of the pile is in a layer of non-freezing soil.

Pile-grillage foundation

By the nature of work in the ground, piles can be:

  • racks resting on practically incompressible soil;
  • hanging, surrounded by compressible soils and transferring the load to the soil due to the friction forces arising under the heel and along the surfaces.
Town Freezing depth depending on the type of soil, m
clay soils sandy loam and fine sands gravelly soils, coarse sand
Arkhangelsk 1.56 1.90 2.04
Vologda 1.43 1.74 1.86
Yekaterinburg 1.57 1.91 2.04
Kursk 1.06 1.29 1.38
Moscow 1.10 1.34 1.44
Nizhny Novgorod 1.45 1.76 1.89
Eagle 1.10 1.34 1.44
Permian 1.59 1.93 2.07
Pskov 0.97 1.18 1.27
Rostov-on-Don 0.66 0.80 0.86
Ryazan 1.36 1.65 1.77
Samara 1.54 1.88 2.01
St. Petersburg 0.98 1.20 1.28
Saratov 1.19 1.44 1.55
Yaroslavl 1.43 1.74 1.86

Table. Background information on the depth of soil freezing in some cities.

The elevation of the bottom of the grillage depends on the presence of a basement or technical underground, as well as on the features of the relief of the site.

For the convenience of servicing communications located in the technical underground, its height is usually assigned at least 70 cm.

Principles of designing pile-grillage foundations

The required number of piles is installed in accordance with the requirements of SP 24.13330.2011. However, since geological surveys and calculations of foundations in the construction of low-rise buildings are usually neglected, in order to ensure reliable operation of the foundation, a reserve of its bearing capacity can be foreseen by placing the piles at the minimum permissible distance from each other. The distance between the pile posts in the clear must be at least 1 m.

Pile-grillage foundation

The location of the piles in the corners and at the intersections of the walls is mandatory. The centers of the piles must be aligned with the alignment lines of the building. The diameter of the posts can be equal to 150-400 mm.

The width of the grillage is assigned more than the diameter of the piles by 100-150 mm, the center of which also coincides with the alignment axis of the building.

Pile-grillage foundation

Reinforcement of piles and grillage is performed with spatial knitted or welded frames with longitudinal reinforcement of class A-III (A400). The diameter of the working rods of the grillage must be at least 12 mm, piles – 8 mm. Transverse rods and clamps are made of steel of class A-I (A240) O 6-10 mm, placing them in increments of 50-80 cm. Clamps are wrapped around the longitudinal rods and connected to them using a knitting wire.

Pile-grillage foundation

In soft soils and with eccentrically applied loads on the walls, it is recommended that the norms ensure a rigid coupling of the piles with the grillage by embedding the outlets of the pile reinforcement for the length of the anchoring (usually at least 0.4–0.5 m). You can bend the rod outlets and connect to the grillage frame with a knitting wire.

Pile-grillage foundation 1 – outlets of reinforcement from the pile frame; 2 – reinforcement cage of the pile; 3 – compacted sand and gravel mixture 100 mm; 4 – reinforcement cage of the grillage

Brief algorithm for the construction of foundations

1. Carrying out the center lines of the building on the ground: installing stakes and pulling the cord between them. Determination of the dimensions of the pit (in buildings with a basement / technical underground) or the location of trenches for the grillage (in buildings with floors “on the ground”).

When setting out the axes on an area with a large slope, it is necessary to make corrections to obtain their projection onto an inclined plane.

2. Cutting off the vegetation layer of the earth and digging a foundation pit to the specified mark of the bottom of the grillage or tape – to the mark of the bottom of the pillow for the grillage. In this case, the walls of the pit are made with small slopes..

A piece of tape with a width equal to the width of the grillage will allow you to avoid formwork, but in loose moist soils there is a high probability of crumbling of the trench walls before concreting.

After digging the foundation pit, the alignment axes should be transferred to its bottom, along which you can perform manual processing of the soil with a depth of 10-15 cm to the width of the grillage and outline the locations of the pile centers.

3. Well development. Drilling of wells for pile-piles with a length of 1-3 m can be carried out in one of the following ways:

  • using a hand-held earth auger, the disadvantages of this method are high labor intensity, limited diameter (usually 100-200 mm) and the length of the drilled wells (on average up to 1 m);
  • using drilling devices (motor-drill, electric or hydraulic drill) – the most preferable option, which allows you to get wells up to 3 m deep;
  • with the involvement of special drilling equipment on a road trip.

4. The device under the bottom of the grillage sand or sand and gravel cushion 10–20 cm thick. In order to reduce the negative effect of frost heaving forces, such a filling should be performed from fine gravel, coarse sand or a mixture of sand with gravel. If it is necessary to increase the bearing capacity of the base soil, the pad should consist of a mixture of coarse sand with crushed stone or gravel in a ratio of 2: 3.

Pile-grillage foundation

5. Manufacturing of reinforcement products.

Pile-grillage foundation

6. Installation of the grillage formwork. It is convenient to take the height of the grillage as a multiple of the width of the boards used for its manufacture (usually at least 40 mm thick), for example, equal to 40 cm.Having knocked the boards together in height, they are installed in the design position, controlling the level of the horizon using a building level.

Pile-grillage foundation

The opposite walls of the formwork are connected to each other with special bolts or boards, providing a given width of the grillage.

Pile-grillage foundation

For smoother grillage surfaces and for the possibility of reusable formwork boards, you can cover them with foil.

7. Installation of reinforcing cages. Provide a protective layer of concrete (15–20 mm) under the grillage frames using special clamps or other available means.

Pile-grillage foundation

In a similar way, the device of pile-pillars is made for the posts.

Pile-grillage foundation

8. Concreting. Optimal is continuous concreting of piles and grillage with breaks for compacting the concrete mix. Wells can be poured first, and the grillage tape is the second stage. In any case, the less work seams are formed, the better..

Sealing is done with vibratory rammers or manual bayonets. After concreting, the surface of the grillage is smoothed, if possible during hardening, it is wetted.

Pile-grillage foundation

9. Dismantling the formwork. The formwork boards can be dismantled after the concrete has hardened (usually not earlier than 3 days). On the dry surfaces of the grillage, layers of coating or pasting waterproofing are applied.

Some mistakes when constructing pile foundations:

  • errors in assigning the width of the grillage, for example, equal to the diameter of the piles, which will not allow a straight line of the grillage in the case of errors made during drilling;
  • concreting at negative temperatures without providing the necessary measures to insulate the formwork;
  • installation of piles in pipes, the smooth walls of which reduce the friction forces along the side surfaces, which, in turn, negatively affects the bearing capacity of the hanging piles;
  • incorrect location of the working reinforcement, for example, in the center of the pile;
  • the use of formwork boards up to 40 mm thick;
  • neglect of thorough compaction of concrete mix.

The construction of foundations of any type is one of the most important stages in the construction of a building. A professionally made foundation for your home will guarantee its long-term trouble-free operation..

Rate article
Tips on any topic from experts
Add comment

By clicking the "Submit comment" button, I consent to the processing of personal data and accept privacy policy