Example of calculation of bored piles. Detailed calculation of the foundation on bored piles Bored piles calculation of materials

“To dig or not to dig” - this Hamlet question when building a house is resolved unambiguously: to dig. It gives rise to several challenges: what foundation to choose, to what depth to pour it, how to do everything reliably and not too expensively?

A trench strip foundation is a common option for developers for the supporting part of a building. In addition to its positive qualities, it has serious disadvantages. The main ones are high material consumption and labor intensity.

The base of the concrete “ribbon” must be poured below the freezing mark of the soil. In central Russia this is at least 1.2 meters. In more severe climatic conditions, to protect against frost heaving, dozens of “cubes” of concrete have to be driven even deeper.

If construction is carried out on soft soil, then digging below the freezing horizon will not save the building from settlement. It is not always possible to reach a dense base on which the reinforced concrete “ribbon” will rest securely. In this case, the only option left is a foundation on bored piles.

In terms of cost, it is cheaper than belt construction, does not require the use of powerful earth-moving equipment, and is built faster. We will talk about what such a structure is, how it is calculated and built, in this article.

Getting to know the bored foundation

The idea of a bored foundation is very simple: where it is impossible to get to dense soil with minimal effort, you can use long posts. To connect them into a common structure, a grillage is used - a monolithic reinforced concrete tape connecting the pile heads.

It is useful to know that piles are very different from conventional massive foundations in the nature of their interaction with the soil. The pile transmits the load in two ways: through the lower end (heel) and through the side surface due to friction forces between the wall and the soil.

Depending on what part of the structure is included in the work, all bored piles are divided into two types:

Racks.
Hanging.

The pile-rack rests on a dense soil layer. The hanging structure holds the load only due to the force of contact with the surrounding soil. Since the dense natural foundation lies quite deep, a significant part of the bored structures are of the hanging type.

Classification, calculation and other important parameters, without which it is impossible to install bored piles, are contained in SNiP 2.02.03-85– a reference book for all designers and contractors. The developer can be guided by ready-made tables from this standard. They indicate the load-bearing capacity of the support posts. Knowing it and determining the weight of the building, you can select the required number of piles.

The data indicated in the table is indicative. The exact value of the bearing capacity of a bored pile is calculated using a formula that takes into account several parameters:

diameter;
brand of concrete;
type of reinforcement;
drilling depth;
mechanical strength of the soil.

After all that has been said, the question arises: for which buildings is the construction of a bored foundation with a grillage justified? Some developers believe that such a structure is not able to withstand heavy loads, so they use it only for light frame buildings, as well as houses made of timber, gas or foam concrete. This is wrong. Today thousands of nine-story brick buildings stand on stilts and no one doubts their reliability.

The strength of a drilled rack manufactured in the field is slightly lower than that of a structure that has gone through a full cycle of factory processing. However, it is sufficient to build a brick house.

The main condition for quality in this case is correct calculation and strict adherence to technology, which includes several stages:

Drilling a well for bored piles (manual motor drill or more powerful mobile unit).
Installation of casing pipe (in loose and damp soils).
Installation of reinforcement cages.
Well concreting.
Filling of a sand-crushed stone cushion under the grillage (thickness 10-15 cm), compensating for the rise of the soil as a result of frost heaving.
Installation of formwork above the ground, installation of reinforcement and pouring of the grillage connecting the piles.

Features of calculating a pile foundation

The first step from which the calculation of a pile field begins is determining the weight of the building. It will depend on him how many piles, what diameter and to what depth we will have to install. The heavier the house, the denser the piles are placed under the walls.

At the same time, the standard requires that the distance between the centers of adjacent supports was at least 3 pile diameters. As this distance decreases, the load-bearing capacity of the racks decreases.

Reinforcement of piles is carried out with vertical rods of a periodic profile (diameter 12-14 mm). Their number depends on the diameter of the rack and can range from 3 to 8 pieces. The vertical reinforcement is connected to each other by horizontal sections of rods with a diameter of 6-8 mm. Bored piles must be filled with concrete of grade no lower than 100.

To more easily calculate the cost of materials and load-bearing capacity of piles, you can use the table below.

The table shows the calculation of bored piles with a length of 2 meters and a diameter of 15 to 40 cm. The vertical reinforcement is 12 mm, the transverse reinforcement is 6 mm in increments of 1 meter.

As an example, let’s determine how many piles with a diameter of 20 cm are required for the foundation of a house that weighs 60 tons. The table shows that one rack can support a weight of no more than 1884 kg. Dividing 60,000 kg by 1884 kg, we get 31.84 pieces. We round up to the nearest whole number and get 32 piles. To fill them (without siege pipes), you need to buy reinforcement and concrete with a total cost of 32x428.68 rubles. = 13,717 rub.

Of course, the final cost of your foundation will be much higher, since its cost will include many other costs: excavation work, delivery of building materials, grillage installation, services of workers and equipment. However, if you wish and objectively assess your strengths, all or part of the work can be done with your own hands.

The resulting number of pile supports must be evenly distributed under the load-bearing walls and partitions of the building, as well as at all angles and intersections of the walls. In this case, the pitch of the piles will depend on the total length of the walls.

A bored foundation is a foundation that transfers the load from a building to the ground through individual concrete piles, which are subsequently covered with a reinforced concrete slab. Piles for this type of foundation are made in specially drilled channels, right on the construction site.

Bored foundation: concrete, waterproofing, reinforcement.

The advisability of choosing a bored foundation is most often justified on soft, weak or heaving soils, the incompressible layer of which, capable of absorbing the loads from the building, is located very deep, and all other types of foundations are not able to transfer the loads from the weight of the building to them. Such soils are found in wetlands, ravines, peat soils, hillsides, etc.

Design calculation

The frame of a bored foundation is regulated by GOST.

To perform the calculation, it is necessary to take into account the bearing capacity of each individual pile and their number. It is clear that the load-bearing capacity directly depends on its dimensions. Moreover, as we will see during the calculation, a very small difference in the diameter of the pile significantly increases its load-bearing capacity. For example, with d=300 mm it will withstand a load of 1700 kg, and if you increase its diameter by 200 mm, then its load-bearing capacity will increase sharply and it will be able to withstand a weight of up to 5000 kg.

When constructing a bored foundation on your own, it is very difficult to understand whether the level of soil incompressibility has been reached during drilling. Therefore, experts advise, even after calculations, to drill to a depth of one and a half to two meters to be on the safe side. This depth ensures that the freezing depth remains significantly higher, the groundwater level has already been passed, and the bearing capacity of the soil at such a depth is quite large and will certainly be greater than the calculated one by a large margin (approximately 6 kg/cm2)

Another point directly related to calculations is the choice of drill size. Modern equipment makes it possible to drill very deep wells of different diameters from 15 to 40 cm. And the so-called foundation drills allow, with a very small drilling diameter, for example 20 mm, upon reaching the bottom, the diameter of the base can be doubled or even tripled. This expansion provides bearing area for the pile and increases its ability to resist buckling.

When making calculations, we are guided by the regulatory document - Code of Practice SP 24.13330.2011 Pile foundation. Updated version of SNiP 2.02.03-85.

Calculation formulas

To calculate the number of piles in the foundation, two parameters are needed - the total weight of the building and the load-bearing capacity of each individual pile.

The calculation is made using the formula.

Φ = m R F, where

R – design soil resistance accepted.

F – support area.

m is the coefficient of operating conditions of the pile in the ground (we assume m=1).

The calculated soil resistance is accepted for sands with any moisture level 3-4.5 kgf/cm2, 1-6 kgf/cm2, 5-6 kgf/cm2.

In practice, the calculation of the strength of a pile is determined by the grade of concrete that was used in its manufacture. The number in the concrete grade shows how much load the pile can withstand per square cm of its weight. For example, a pile made of M100 concrete with a cross section of 200x200 mm = 400 cm2 will withstand a load of 40,000 tons.

Summary data

For convenience, we will summarize the data in a general list:

Reinforcement of a bored foundation: reinforcing bars.

With a support diameter of 150 mm, the support area will be equal to 177 cm2, the load-bearing capacity of the pile is assumed to be 1062 kg.
With a support diameter of 200 mm, the support area will be equal to 314 cm2, the load-bearing capacity of the pile is assumed to be 1884 kg.
With a support diameter of 250 mm, the support area will be equal to 491 cm2, the load-bearing capacity of the pile is assumed to be 2946 kg.
With a support diameter of 300 mm, the support area will be equal to 707 cm2, the load-bearing capacity of the pile is assumed to be 4242 kg.
With a support diameter of 400 mm, the support area will be equal to 1256 cm2, the load-bearing capacity of the pile is assumed to be 7536 kg.
With a support diameter of 500 mm, the support area will be equal to 1963 cm2, the load-bearing capacity of the pile is assumed to be 11775 kg.

Data on the diameter of the pile reinforcement is given in the section “Brief information on the technology for constructing a bored foundation.”

A 150 mm pile is a frame of 3 rods, with a ribbed profile reinforcement consumption of 6 m, a smooth reinforcement – 0.75 m.
200 mm - frame of 4 rods, with a consumption of ribbed profile reinforcement 8 m, smooth - 1 m.
250 mm - frame of 4 rods, with a consumption of ribbed profile reinforcement 8 m, smooth - 1.26 m.
300 mm - frame of 6 rods, with a consumption of ribbed profile reinforcement 12 m, smooth - 1.51 m.
400 mm - frame of 8 rods, with a consumption of ribbed profile reinforcement of 16 m, smooth - 2.01 m.
From 500 mm - a frame of 10 rods, with a consumption of ribbed profile reinforcement of 20 m, smooth - 2.05 m.

Calculation of the total number of foundation piles

Calculation of piles: number of piles, dimensions.

Taking into account the total weight of the house and its load on the bored foundation, the required number of piles is calculated. It is proportional to the weight of the house, which depends on what material it will be made of. If it is aerated concrete, the pressure will be lighter; if it is brick, then its weight will be much higher. The greater the weight of the house, the greater the load on the foundation, the more piles will need to be installed. Accordingly, the installation pile step will decrease. Here it is worth remembering one important point that will help to avoid errors in calculations. There is a limitation on the minimum distance between the axes of the piles. It should not exceed three diameters of the pile. Failure to maintain this distance will have the opposite result - it will reduce their load-bearing capacity, thereby weakening the foundation of the house.

For example, with a pile diameter of 500 mm, which is most often used in the construction of a brick house with a bored foundation, the minimum distance between the axes should be more than 150 cm.

In this way and using these recommendations, you can independently calculate the bored foundation for your brick house. However, if you have doubts about whether you have chosen the right foundation, or about the correctness of the calculation, then you should contact specialists who can more accurately determine the type of soil and its characteristics, and also check your calculations.

Didn't find the answer in the article? More information

The construction of any foundation begins with design. Calculations and drawings can be performed without the involvement of specialists, independently. Of course, these calculations will not be highly accurate and will represent a simplified version of the calculation, but they can give an idea of how to ensure the bearing capacity of the foundation. The following discusses bored piles and an example of their calculation.

Design work is performed in the following order:

study of soil characteristics;
collection of loads on the foundation;
calculations of bearing capacity, determination of the distance between piles and their cross-section.

About each point in order.

Geological surveys

During mass construction, geologists prepare characteristics for calculations. They take soil samples, conduct laboratory tests and give accurate values of the bearing capacity of a particular layer, the location of soils with different characteristics. If bored piles are used for private housing construction, it is not economically profitable to carry out such activities. The work is done independently in two ways:

pits;
manual drilling.

Important! The characteristics are studied at several points, all of which are located under the building's development footprint. One is necessarily in the lowest part of the earth's surface. When studying soil characteristics, the depth of soil development is set at 50 cm below the expected level of the base of the foundation.

A pit is a rectangular or square hole; the soil is studied by analyzing the soil of the walls of the open pit. When drilling, soil analysis is performed on the drill blades. After familiarizing yourself with the soil type, determine the type of soil. For some types of substrates, consistency or moisture content will need to be determined. Table 1 will help with this question.

External signs and methods	Consistency
Clay bases
If the soil is compressed or hit, it crumbles into pieces	Semi-solid or hard ground
The sample is difficult to knead; when you try to break the bar, it bends strongly before breaking into two parts	Tight plastic
Retains its sculpted shape and is easy to sculpt	Soft-plastic
It creases with your hands without difficulty, but does not retain its sculpted shape.	Fluid-plastic
If a sample is placed on an inclined surface, it will slowly slide (drain) along it.	Fluid
Sandy bases
Disintegrates when squeezed in the hand, has no external signs of moisture	Dry
The test is carried out using filter paper; it should remain dry or damp after a period of time. When squeezed in the palm of the hand, the sample gives a cooling sensation	Low moisture
The sample is placed on filter paper and the damp spot is observed. When compressed, a feeling of moisture is created. Able to retain its shape for some time	Wet
Shake the sample in the palm of your hand, it should turn into a cake	Saturated with moisture
Spreads or spreads without external mechanical influence (at rest)	Waterlogged

Having determined the type and consistency of the base by external signs using the table, we begin to determine the standard resistances. These values are needed to calculate the bearing capacity of the foundation and calculate the distance between piles.

Bored piles place a load not only on the layer of soil on which they rest, but also along the entire side surface. This increases their effectiveness.

Table 2 shows the standard resistance of the foundations, in places where the base of bored piles rests on them.

Priming	Standard resistance taking into account additional tests, t/m 2
Clay bases
—	Porosity coefficient	Solid consistency		Semi-solid		Resistant		Soft-plastic
Sandy loam	0,50	47		46		43		41
	0,70	39		38		35		33
Loam	0,50	47		46		43		41
	0,70	37		36		33		31
	1,00	30		29		24		21
Clay	0,50	90		87		78		72
	0,60	75		72		63		57
	0,80	45		43		39		36
	1,10	37		35		28		24
Sandy bases
—	Dense				Medium density
	wet		low moisture		wet		low moisture
Major faction	70		70		50		50
Middle faction	55		55		40		40
Small fraction*	37		45		25		30
Dusty*	30		40		20		30
Coarse foundations
Crushed stone with added sand	90
Gravel formed from crystalline rocks	75
Gravel formed from sedimentary rocks	45

Soil porosity coefficient is the ratio of the volume of voids to the total volume of rock. To calculate the pore sizes of cohesive rocks (clayey), such quantities as specific and volumetric gravity are used.

Also, when calculating the bearing capacity of bored piles, it is necessary to take into account the resistance along the lateral surface. Values for clayey rocks are presented in Table 3.

Having found out all the necessary data related to soil resistance, we proceed to the next point of calculation for the bearing capacity of the foundation.

Load collection

Here it is necessary to take into account the mass of all structures. These include:

walls and partitions;
floors;
roof;
temporary loads.

The first three loads are considered constant. They depend on what materials the house will be built from. To calculate the mass of walls, ceilings or partitions, take the density of the material from which they are planned to be made and multiply by the thickness and area. When calculating the roof, everything is a little more complicated. Need to consider:

filing;
lower and upper sheathing;
rafter legs;
insulation (if any);
roofing covering.

You can give average values for the three most common types of roofing:

weight of 1 m2 of roof pie covered with metal tiles - 60 kg;
ceramic tiles - 120 kg;
bitumen (flexible) tiles - 70 kg.

Temporary loads include snow and useful loads. Both are accepted by . Snow depends on the climatic region, which is determined according to the joint venture “Construction Climatology”. Useful is assigned depending on the purpose of the building. For residential - 150 kg/m² of floors.

It is not enough to calculate all the loads; each of them must be multiplied by a safety factor.

the coefficient for calculating permanent loads depends on the material and method of manufacturing the structure and is taken according to table 7.1;
coefficient for snow load - 1.4;
coefficient for utility in a residential building is 1.2.

All values are added up and the calculation of bored piles based on bearing capacity begins.

Formulas for calculations

P = Rosn + Rbok. again,

where P is the load-bearing capacity of the pile, Rosn is the load-bearing capacity of the pile at the base, Rbok. surface - bearing capacity of the side surface.

Rosn = 0.7 * Rn * F,

where Rн is the standard bearing capacity from Table 2, F is the base area of the bored pile, and 0.7 is the coefficient of soil uniformity.

Rbok. surface = 0.8 * U * fiн * h,

where 0.8 is the operating conditions coefficient, U is the cross-sectional perimeter of the pile, fin is the standard soil resistance at the side surface of the bored pile according to Table 3, h is the height of the soil layer in contact with the foundation.

Q = M/Uhome,

where Q is the load per linear meter of the foundation from the building, M is the sum of all loads from the building structures, calculated earlier, Uhouse is the perimeter of the building.

Important! If the house has a large area and the installation of internal walls under which the foundation will be built is planned, their length is added to the perimeter to calculate the distance between the bored piles of the foundation.

where P and Q are the previously found values, and L is the maximum distance between the piles.

The calculation to calculate the distance between foundation piles is usually carried out several times. In this case, different sections and depths are selected.

Important! Due to the fact that not only the supporting part of the bored foundation works, the bearing capacity increases with increasing depth in most cases (depending on the characteristics of the base for the foundation). When designing a support for a future home, it is recommended to consider several examples, changing the cross-section and depth. The distance between the piles and their number are calculated. After this, the estimate is “pretended” (exact calculations can be labor-intensive, so approximate values are sufficient), and the most economical option is selected.

Before calculating, you need to familiarize yourself with. According to the requirements of this standard, bored piles up to 3 meters long are recommended to be provided with a diameter of 30 cm.

Calculation example

Initial data:

Geological conditions of the area: at a depth of 2 meters from the soil surface there are tight-plate loams, then hard clays with a porosity coefficient of 0.5 are located throughout the entire depth of the study.
It is required to design a foundation for a one-story house with an attic. The dimensions of the house in plan are 4 by 8 meters, the roof is covered with hipped metal tiles (the height of the outer wall is the same on all sides), the walls are made of bricks 0.38 m thick, the partitions are plasterboard, the ceilings are reinforced concrete slabs. The height of the walls within the first floor is 3 meters; on the attic floor the external walls are 1.5 meters high. There are no internal walls (only partitions).

Collection of loads:

wall mass = 1.2 * (24 m (perimeter of the house) * 3 m (ground floor) + 24 m * 1.5 m (attic)) * 0.38 m * 1.8 t/m³ (density of brickwork) = 88.65 t (1.2 - load safety factor);
mass of partitions = 1.2 * 2.7 m (height) * 20 m (total length) * 0.03 t/m² (weight per square meter of partitions) = 2 tons;
weight of floors including cement screed 3 cm = 1.2 * 0.25 m (thickness) * 32 m² (area of one floor) * 2 (first floor floor and attic floor) * 2.5 t/m² = 48 tons;
roof mass = 1.2 * 4 m * 8 m * 0.06 t/m² = 2.3 tons;
snow load = 1.4 * 4 m * 8 m * 0.18 t/m2 = 8.1 tons;
payload = 1.2 * 4 m * 8 m * 0.15 t/m² * 2 (2 floors) = 11.5 tons.

Total: M = 112.94 t. Building perimeter Uhouse = 24 m, load per linear meter Q = 160.55/24 = 6.69 t/m. We first select a pile with a diameter of 30 cm and a length of 3 m.

Using formulas for determining the distance between piles

All the necessary formulas are given earlier, you just need to use them in order.

1. F= 3.14 D²/4 (round pile area) = 3.14 * 0.3 m * 0.3 m / 4 = 0.071 m², U = 3.14 D = 3.14 * 0.3 m = 0.942m; (perimeter of the pile in a circle);

2. Psn = 0.7 * 90 t/m² * 0.071 m2 = 4.47 t;

3. Rbok. surface = 0.8 * (2.8 t/m² * 2 m + 4.8 t/m² * 1) * 0.942 = 7.84 t;

In this formula, 2.8 t/m² is the calculated resistance of the side surface of the pile in hard-plastic loam, 2 m is the height of the loam layer in which the foundation is located. Resistance is found from Table 3. The values are presented there for suitable depths of 50, 100 and 200 cm. We take the minimum into account in order to ensure a margin of bearing capacity.

4.8 t/m² is the calculated resistance of the side surface of the pile in semi-hard clay, 1 m is the height of the foundation located in this layer. The last number in the formula is the perimeter of the pile found in the first paragraph. The values 0.7 and 0.8 in paragraphs 2 and 3 are the coefficients from the formulas.

4. P = 4.47 t + 7.84 t = 12.31 t (full load-bearing capacity of one pile);

5. L = 12.31 t/6.69 t/m = 1.84 m - maximum value of the distance between piles (between centers).

We assign a distance of 1.8 m. Because the length of our walls is a multiple of 2 m, it is more convenient for the distance between the piles to be 2 m, for this you need to slightly increase the load-bearing capacity of the pile, for example, by increasing its diameter. If the resulting step value is large enough, it is more reasonable to find the minimum, since the greater the distance between the piles, the larger the grillage cross-section will be needed, which will lead to additional costs. Calculations are performed using the same principle for a reduced diameter. Calculate the applicable amount of material for several options and select the optimal value.