An example of a house on bored piles. Calculation of bored piles. Bearing capacity of a bored pile
Before proceeding with the design and, even more so, the construction of a pile foundation, it is necessary to go through a number of preparatory stages, which include surveys and calculations of various types. The result of correctly carried out preliminary measures will be a solid, economical, and, most importantly, reliable foundation. One of the key characteristics that affect the profitability of a particular type of pile is the geometric parameters of the pile columns.
Correctly determining the dimensions of the cross section, the depth of the foundation, the number of wells and other parameters means building a reliable foundation for the future building.
Typology of bored pile foundations
Bored pile foundations are one of the few structures that do not lend themselves to strict classification. Typical sizes presented in various assortments, codes of practice and state standards are only rough guidelines. While mass-produced products must pass a series of rigorous quality control tests, bored piles are virtually impossible to test because they are made in the field and placed directly into the ground.
Concreted directly at the construction site, bored piles are distinguished by high strength indicators, which can only be calculated empirically. Tests carried out on prototypes show the operation of these experimental products only. Since manufacturing conditions such as type of soil, groundwater level, water saturation of the working soil layer, the characteristics of the reinforcement and concrete used cannot be predicted. All available strength and geometric data are approximate and are presented only as an example.
Construction of bored piles
For typification of bored piles, a division is used according to geometric features and technological features of production and operation. SNiP 2.02.03-85 is an updated version of the building code of 1983 and proposes to classify bored piles according to the manufacturing method as follows:
- Bored continuous section:
- with extensions and without them;
- without fixing the walls;
- with strengthening of the side walls of wells with clay mortar or casing pipes (when the pile column is dislocated below the groundwater level)
- Bored with the use of continuous hollow auger technology;
- Berets - drilling, manufactured using a flat grab or soil cutter;
- Bored with a camouflage heel, arranged with the subsequent formation of broadening with the help of an explosion (including an electrochemical one).
Their final cost and, most importantly, the maximum and minimum dimensions pile columns. It is important to consider the type of bored piles prior to construction, since different production technologies require a different set of specialized equipment, as well as allowable borehole dimensions.
Preliminary preparation for calculation
Certain geometric characteristics of the pile column are not just a whim of the contractor and designer, but a need due to the need to select the most rational volume of the foundation that can not only withstand the expected load of the future building, but also save the customer's budget. In each individual case, before sizing and arranging the foundation, it is necessary to carry out a number of the following studies and surveys:
- geological exploration of the area - drilling of control wells at strategic points of the site to determine the type and size of soil strata, the bearing capacity of the soil and other characteristics of the base;
- hydrogeological surveys - determination of the level of groundwater, soil water saturation;
- calculation of the total mass of the building and determination of the maximum design load per linear meter of the foundation slab;
- final calculation of the geometric parameters of the bored pile and the required number of piles of the selected section.
The result of the calculation will be a summary table of the dimensions of pile columns, and a diagram of the most rational foundation, taking into account the selected type of bored piles. The calculation of the dimensions of the piles can be entrusted to the design department of a construction company or carried out independently. It is not recommended to use geological survey data obtained on neighboring land plots. Information on the depth of soil freezing can be found in SP 22.13330.2011.
Pile field calculation
After the geological surveys you can start calculating the pile field. Given the type of soil, as well as the location of the groundwater level, one can get an idea of the estimated depth of the wells. The table below shows approximate recommendations for the depths of wells that are safe under the specified conditions in weakly subsiding soils:
Wet, subsidence, high heaving and other unreliable types of soil bases are not recommended for use in them for bored piles.
Groundwater layout
Soils with a level groundwater higher than 1000 mm are considered water-saturated and the device pile foundations on such grounds is strictly contraindicated by technology. The high level of groundwater can be lowered by carrying out measures for draining, laying drainage flows, etc. Reliable weakly heaving soils are those in which the GWL is at least 1 meter below the freezing depth.
The data given in the table will help to get a general idea of the dependence of the depth of the pile column on the characteristics of the soil. To obtain more accurate and reliable indicators, a simple mathematical calculation should be carried out. The principle of calculation is to take one of the indicators (for example, diameter) as a standard and calculate the rest based on these data. The comparison method selects the most appropriate configuration of piles, from which a pile field is subsequently formed.
Calculation of the length of hanging piles
Pile poles that do not rest on the bearing layer of soil are considered hanging. This means that the main load is taken by the side walls of the well, and not by the supporting soil layer. Such foundations are preferably installed in areas with a deep rocky layer. The bearing capacity of such piles does not differ from racks of the same diameter.
If you have access to the geology of the area, and the type of soil is suitable for the installation of bored hanging pile columns, you can begin to calculate the length. The proposed calculation scheme is as follows:
- We accept a certain average width of the cross section of the pile n=60 mm.
- We calculate the load of the house per linear meter of the foundation slab:
To calculate the load per linear meter of the foundation, you need to divide the total load by the perimeter. You can calculate the total load of the house in accordance with the instructions of SNiP 2.02.01-83 * or SP 22.13330.2011 - in the relevant sections you can find the calculation algorithm, the required values of the wind and snow load coefficients and other necessary information.
The resulting value in kg / m will be the desired value. The average weight of a one-story brick house is 50 tons. Therefore, for a house with a perimeter of 20 meters (10 × 10), the load per linear meter will be 2500 kg / m.
- We accept a column pitch of at least three diameters and no more than two meters - a step of 1.5 meters is suitable for the selected diameter. The total number of piles will be 13.
- We calculate the load on one pile: for this, we divide by the value of the pile pitch the load perceived by the running meter of the foundation. We get a value approximately equal to 1700 kg / m. Such a necessary tensile strength must be laid in one pile.
- For a pile with a cross section of 0.28 m2, this strength value will be equal to:
F=R∙A+u∙Eycf∙fi∙hi;
Where F is the bearing capacity; R is soil resistance, the calculation formula for which can be found in SNiP 2.02.01-83 *; A is the sectional area of the pile; Eycf,fi and hi are coefficients from the same SNiP; u is the perimeter of the pile section divided by the length.
For the two-meter-long pile considered in the example, the ultimate load in clay soil will be 32.3 tons, which makes it possible to reduce the number of piles by increasing the pitch of pile columns, or to reduce the cross-sectional area of each individual pile, which will save money spent on concreting wells.
The depth of such piles will depend solely on the characteristics of the topsoil, the relative level of groundwater and the depth of freezing. It should also take into account data on soil freezing and the position of the groundwater level. Detailed examples calculation of the depth of laying of hanging piles are given in SNiP 2.02.01-83 * in section 2, paragraph 5 or in SP 50.102-2003.
Rack length calculation
Bored piles of increased depth can work as racks. And although drilling types are usually hanging, there are designs based on a solid layer of soil. The calculation of the length of such piles should be made taking into account the depth of the location of a strong bearing layer.
There are many services on the Internet for automatically calculating the size and number of bored piles. The use of such services imposes a certain risk on the user, since the algorithm does not always take into account all the necessary parameters, and the owners software are not responsible for the result.
All related calculations of the bearing capacity and geometry of the pile are made in accordance with the technology for calculating the pile-pillars and are similar to the example given earlier. Additional information about the calculation can be obtained in the above documents.
The dependence of the pile diameter on the type of installation
The cross-sectional area of a bored pile corresponds to the area of the bore hole, adjusted for the plasticity of the soil. The shape of the cast-in piles is close to ideally cylindrical, although it has slight broadening due to involuntary lateral punching of the weak spots of the soil by the concrete mixture. Also, in the process of pouring the concrete mixture, by increasing the supply pressure, intentional widening of the pile body can be created to give additional strength. Such actions are especially relevant for hanging piles.
Among other things, the average diameter of a bored pile is determined based not only on the calculated indicators, but also on the capabilities of the equipment intended for the installation of one or another type of pile. Approximate values of diameters depending on the design features of the installation:
The device of barets is supposed in the presence of highly heaving unstable soils. To make such a foundation for the average foundation is irrational. The design of the drill involves the construction of only wells with a diameter of either 300 mm or 400 mm.
The diameter step is determined by a set of drills used for the construction of wells of one type or another. The design features of each of the types of drilling rigs do not allow arranging wells of a larger or smaller diameter than those indicated in the specifications for the work. You can get acquainted with the operating parameters of drilling rigs from the supplier or lessor.
When constructing a pile field and determining the size of pile columns, the recommended pile spacing should be taken into account, on which the frequency of wells and load distribution will depend. Watch the video on the correct installation of piles:
To evenly distribute the pressure of the mass of the future building on the foundation slab, the following rules must be observed:
- the maximum distance between bored piles should not exceed two meters;
- the minimum pitch of pile columns should be within three to four pile diameters - in order to prevent the collapse of the walls of adjacent wells in loose soils, the minimum limit should be increased;
- the layout of the pile field should be made taking into account the location of the piles at the corner points of the foundation;
- according to the results of the calculation of the geometric characteristics, after the layout, the total number of piles must correspond to the recommended pitch values - in case of exceeding the maximum pitch of the piles, increase the number of wells and reduce the diameter of the piles to the maximum possible;
- the maximum and minimum diameters of the wells should not exceed those allowed for the selected type of installation.
By following these recommendations, you can design the most efficient and rational foundation without worrying about its reliability. If necessary, you should seek help from specialists, but all calculations can be done independently, without much difficulty.
“To dig or not to dig” - this Hamlet question is solved unambiguously during the construction of a house: to dig. It gives rise to several counters: what foundation to choose, to what depth to fill it, how to make everything reliable and not too expensive?
Trench strip foundation- a variant of the supporting part of the building familiar to developers. Except positive qualities it has serious flaws. The main ones are high material consumption and labor intensity.
The sole of the concrete "tape" has to be poured below the freezing mark of the soil. AT middle lane Russia is at least 1.2 meters. In more severe climatic conditions, dozens of “cubes” of concrete have to be driven even deeper to protect against frost heaving.
If the construction is carried out on soft ground, then deepening below the freezing horizon will not save the building from precipitation. It is not always possible to reach a dense base on which the reinforced concrete "tape" will reliably lie. In this case, the only way out is the foundation on bored piles.
At cost, it is cheaper than a belt one, does not require the involvement of powerful earth-moving equipment and is built faster. We will talk about what such a design is, how it is calculated and built, in this article.
Getting to know the bored foundation
The idea of a bored base is very simple: where it is impossible to get to the bottom of dense soil at minimal cost, long posts can be used. To connect them into a common structure, a grillage is used - a monolithic reinforced concrete tape that connects the pile heads.
It is useful to know that piles are very different from conventional massive foundations in terms of the nature of their interaction with the soil. The pile transfers the load in two ways: through the bottom end (heel) and through the side surface due to friction forces between the wall and the ground.
Depending on which 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 suspended structure holds the load only due to the force of contact with the surrounding soil. Since the dense natural base lies deep enough, a significant part of the bored structures is of the hanging type.
Classification, calculation and other important parameters, without which it is impossible to perform the installation of 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 bearing capacity of the support posts. Knowing it and determining the weight of the building, you can choose the right number of piles.
The data shown 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;
- soil mechanical strength.
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 this design 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 not true. Today, thousands of brick nine-story buildings stand on piles and no one doubts their reliability.
The strength of a field-made drilled shank is slightly lower than that of a fully factory-machined design. Nevertheless, it is enough with a margin for the construction of a brick house.
The main condition for quality in this case is the correct calculation and exact observance of the technology, which includes several stages:
- Drilling a well for bored piles (hand-held motor drill or a more powerful mobile unit).
- Installation of a casing pipe (in loose and damp soils).
- Installation of reinforcing cages.
- Well concreting.
- Backfilling of a sand and gravel 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 the calculation of the pile foundation
The first step from which the calculation of the pile field begins is to determine the weight of the building. It will depend on him how many piles, what diameter and at 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 regulation requires the distance between the centers of adjacent supports was at least 3 pile diameters. With a decrease in this distance, the bearing capacity of the racks decreases.
Reinforcement of piles is performed with vertical rods of a periodic profile (diameter 12-14 mm). Their number depends on the diameter of the rack and can be from 3 to 8 pieces. Between themselves, the vertical reinforcement is connected by horizontal segments of rods with a diameter of 6-8 mm. The pouring of bored piles must be carried out with concrete grade not lower than 100.
For a simpler calculation of the cost of materials and the 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. Vertical reinforcement 12 mm, transverse - 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 for a house weighing 60 tons. The table shows that one rack can withstand a maximum weight of 1884 kg. Dividing 60,000 kg by 1884 kg, we get 31.84 pieces. We round up to a 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 rubles.
Of course, the final cost of your foundation will be much higher, since its cost will include many other costs: excavation, delivery of building materials, installation of a grillage, services of workers and equipment. However, if you wish and an objective assessment of 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 corners and intersections of the walls. In this case, the pitch of the piles will depend on overall length walls.
Any novice builder knows that the basis for the strength of the house is its foundation. But installing a good foundation is a rather laborious procedure that requires knowledge, experience and a large number time, especially when it comes to pile foundation. It will be necessary to correctly calculate the bored piles and their bearing capacity. After all, the strength and service life of the erected building will depend on this. This article will discuss how to correctly calculate the bearing capacity of piles on the ground and what data will need to be used for this.
Methods for determining the bearing capacity of a pile
There are several methods for making such calculations. These include:
- Calculation method. It is not very effective, but it is used quite often, because, unlike others, it is quite simple.
- Trial static loads. An extremely effective technique, but it requires a lot of time and effort. Quite often used by professionals.
- Dynamic test. It is carried out by means of several blows of a hammer on the installed piles, after which the sediment is fixed. The advantage of this method is that it can be used directly on the construction site, but unlike the previous method, it is not as effective.
- Sounding. This method involves a combination of static and dynamic methods. It is produced by registering data on the bearing capacity of the basis surface from pre-installed special sensors. The equipment is quite expensive, so such calculations are often performed only by specialists.
The calculation method is often used by ordinary people, since this does not require special equipment or a lot of experience. It will only be necessary to collect certain data that will be useful for calculations. Other methods can also be used, but their implementation will require knowledge and devices that beginners in the building trade often do not have.
To increase the amount of knowledge on the topic of calculating the bearing capacity of piles, it is recommended to watch the following video.
Studying the parameters of bored piles for calculations
When installing a pile base, it is necessary to take into account such a characteristic as the bearing capacity of a bored pile, as it affects the material consumption for their installation and the quality parameters of the base and the entire building.
This parameter largely depends on the diameter of the column used. For example, a bored pile with a diameter of 300 mm can withstand a pressure of 1.7 tons, and a pile with a diameter of 500 m can withstand even 5 tons. solid foundation. In addition, the consumption of materials for building a house depends on this characteristic.
Based on this, the calculation of the number of piles and the distance during their installation is part of the overall calculations that must be performed to erect a strong building.
Production material
Pile size is not the only factor to consider. When calculating, it is also necessary to take into account the material from which the product was made. The variety and brand of concrete used during the pouring of the site greatly affects the wear resistance and service life of the foundation, and, consequently, the entire building.
As an example, a pile filled with concrete M 100 can withstand pressure up to 100 kg per 1 cm². This is a pretty good indicator, since a pile with a base of 20 cm and an area of 400 cm² can hold up to 40 tons.
In addition, it is necessary to consider not only the load that will be exerted on the pole, but also the strength characteristics of the soil itself. This is due to the fact that with a possible shortage of poles and high blood pressure on the ground, the foundation may be damaged due to the fact that some piles go too deep into the ground. If this happens, it will be quite difficult to carry out repair work, and it will no longer be possible to do without the help of specialists.
The higher the strength of the underlying soil, the fewer supports will be required to create a solid foundation. You will also need to take into account the depth of soil freezing, the level of groundwater, the quality of reinforcement and other factors.
Calculation of the bearing capacity of piles
A beginner will be able to cope with such calculations, so the involvement of specialists is not required. Determining the bearing capacity of piles consists of the following steps:
- Preparation for the procedure, collection of information, soil analysis.
- Calculation according to the prepared formula.
Preparation for calculations
The data that will be used to calculate the bearing capacity of piles is obtained after carrying out geological procedures and calculating the planned pressure on the building. The collection of these data is extremely important work, since the correctness of the calculation result depends on them.
When calculating, it is necessary to take into account a large number of various soil characteristics. Information on these data can be found in the SNiP, where it is divided into climatic zones and presented in different forms.
The determination of the bearing capacity of piles cannot be based on data collected on neighboring plots. Even within the same land area, geological indicators can vary quite a lot. Several wells along the perimeter of the site will allow you to collect detailed information about the quality of the soil. An error in data collection can lead to rather unpleasant consequences.
The calculation of the mass of the building is carried out taking into account the climatic factor, the placement of the building on the surface relative to the direction of flows, the amount of precipitation in winter, the weight of building materials and equipment.
Formula calculation
The bearing capacity of a pile on the ground, which affects the applied load, depends on the characteristics of the material from which it was made and the strength parameters of the soil. For calculations, the minimum indicator is selected, since it sometimes increases.
The bearing capacity of the pile is calculated by the following formula: P=k o *R n *F+U*k p *F in *L i , where P is the direct bearing capacity; k o - indicator of soil homogeneity; R n - possible resistance of the soil relative to the foundation; F - base area on piles, cm²; U – site perimeter, m; k p - working coefficient; F in - allowable soil resistance on the sides of the used piles; L i is the thickness of the soil adjacent to the side surface of the column, m.
All the necessary soil data must be sought in the annexes of SNiP in the section intended for this. If the soil is multi-layered, then the possibilities of surface resistance are calculated for each layer separately, after which the indicators are added together. Also, when calculating the existing bearing capacity for pressure, you will need to add the mass of the piles themselves and the grillage.
After the bearing capacity of the piles has been calculated, their required number is calculated to create the basis of the building. It should be borne in mind that the largest interval between piles is a mark of 2 m, and the smallest is the sum of 3 borehole diameters.
When all the necessary calculations are carried out, the filling is carried out. Concrete for this is made directly at the site where construction work is carried out, which saves on delivery. You can use different brands of solution, but you need to monitor its quality and expiration date. If low-quality concrete is used, this will significantly affect the life of the building.
As can be seen from the article, building a pile foundation on your own is quite difficult, but possible. The main procedure is the calculation of the bearing capacity of the pillars. If all calculations are performed correctly, then the result will be high level, and the building will last a large amount of time. There are special tables in which many data are already collected. With them, you can skip the time-consuming process of collecting a large amount of data for calculations.
Foundations are an extremely important part of any building. Whether cracks appear on the walls, whether the house will sag over time - it all depends on how well the dimensions and materials for the supporting part are chosen. In order to correctly design a bored pile-grillage foundation, it will be necessary to calculate its bearing capacity.
The bearing capacity of the foundation is the load that it can withstand without destruction, deformation or other unpleasant processes. When designing a bored base, you will need to find out the following information:
- element section;
- length;
- distance between individual piles.
The calculation of piles for bearing capacity is often performed with a pre-known section of the foundation. This characteristic depends on the available technology. As initial data it is necessary to prepare:
- composition of soils on the site;
- collection of loads on the support of the house.
Collection of initial data for calculation
Before calculating a bored pile-grillage foundation, it will be necessary to study the properties of the soil at the construction site. This can be done in two ways: extracting pits (deep holes) or drilling with a hand tool. The study of the soil is carried out a little deeper than the intended sole (about 50 cm). When performing work, it is necessary to analyze each soil plate, determine its type.
To get an idea of what soils are, how to distinguish them correctly, it is recommended to read. Annex A deserves special attention, which gives the main definitions.
The next stage in the calculation of a bored pile and grillage is the collection of loads. It is easier to do it in tons. To do this, you need to know the volume building structures and density of the materials from which they are made. To calculate the mass of a building, you need to remember a simple formula from school physics: "We can easily find the mass by multiplying the density by the volume." The collection of loads on foundations includes:
- own weight of the supporting part (appointed approximately);
- a lot of ceilings, walls, partitions (it is better not to subtract openings from the total volume);
- payload on floors (for residential buildings, this load is assigned 150 kg / m 2 of the floor, taken on each floor);
- weight of the roof;
- snow load (depends on the climatic area of construction, the calculation is performed according to).
Advice! To simplify the task, the snow load can be assigned according to a special map or table. That is, without performing complex calculations.
The found mass of each element must be multiplied by the load safety factor. The value of this coefficient depends on the material from which the structure is made. For snow and payloads, the coefficients are constant and are 1.4 and 1.2, respectively.
More information about collecting loads on foundations can be found in the article ".
reference Information
To correctly calculate the bored pile foundation, you will need to know the strength characteristics of the soil. Information about this can be found in VSN 5-71. For convenience, adapted tables from this document are presented below for each type of soil separately.
Table 1. Bearing capacity of clay soils, depending on the consistency and porosity in the bearing area of the pile, t/m2.
Table 2. Bearing capacity of clay soils along the length of a bored pile, t/m2.
Table 3 Bearing capacity of sandy soils, t/m2.
Table 4 Bearing capacity of coarse soils, t/m2.
To calculate the cross-section and distance between piles, one or two (for clays) values must be selected from those given in the table, depending on the results of excavating pits or drilling.
Calculation procedure
After a careful study of all the previous paragraphs for the calculation of the pile-grillage foundation, the following information should be available:
- the mass of the house in tons and the load per linear meter of the grillage;
- bearing capacity of soil in tons per m 2.
To find the load per linear meter of the foundation, you need to divide the mass of the house by the total length of the grillage.
The bearing capacity of one pile is found by the formula:
P = (0.7*R*S) + (u*0.8*fin*li), where
P is the bearing capacity of each foundation pile;
R - soil strength, found in Table. 1, 3 or 4;
S - sectional area of the pile at the end (the formula for finding is given below);
u - pile perimeter;
fin - soil resistance on the side surface of the bored pile foundation, found from Table. 2;
li is the thickness of the soil layer that resists the side surface;
0.7 and 0.8 are coefficients that take into account the homogeneity of the soil and the working conditions of the pile.
For a pile of circular cross section, the area is found through the diameter or radius: S = 3.14 * D 2 /4 = 3.14 * r 2 /2. Here D and r are the diameter and radius, respectively.
l is the distance between the piles of the bored foundation;
P is the bearing capacity of one pile, found earlier;
Q - load per linear meter of the foundation (weight of the house divided by the length of the grillage).
Advice! Before starting the calculation, you must familiarize yourself with. The minimum diameter of the pile foundation with an element length of less than 3 meters is 30 cm. To find the most rational solution, it is recommended to consider 2-3 options for the geometric dimensions of the piles. For each case, find the distance between the supports and estimate the cost of construction. Choose the most economical option.
A detailed calculation of the distance between piles, considering several examples, can take a long time. But here, the future owner of the house is faced with a choice of what to save: time or money.
Reinforcement of a bored pile
Working reinforcement is located vertically along the pile. As it is used rods of class A400 (Alll) with a diameter of 10-16 mm. The transverse piping is made of smooth reinforcement A240 (Al) with a diameter of 6-8 mm. Each pile must have at least four working vertical bars.
Calculation of the grillage
The calculation of the pile foundation grillage is performed in approximately the same way as the calculations for the tape type of the supporting part of the house. To calculate the width of the tape, you will need to use the formula:
B \u003d M / L * R, where
B - the required width of the grillage;
M is the mass of the house (minus the mass of the piles);
L - grillage length;
R is the bearing capacity of the soil (layer near the surface).
This calculation is suitable for a tape located directly on the ground or with a slight depth. For a hanging grillage, the calculation will be more complicated, it is problematic to carry it out on your own.
Reinforcement grillage
Having chosen the width of the grillage of the bored foundation, it is necessary to correctly reinforce it. You can use the requirements for steel bars from .
As a material for reinforcement, rods of class A400 (Alll) are chosen. The most admissible diameter of working rods - 40 mm. The minimum values are given in the table.
An example of calculating a piled bored foundation
Initial data for calculation:
- one-story brick house with attic, wall thickness 380 mm;
- dimensions in terms of 7 by 9 meters, no internal load-bearing walls (only partitions), floor height 3 m;
- mansard rafter roofing with a metal tile coating;
- soils on the site - semi-hard clay with a porosity coefficient of 0.6, lies at 3 m, R = 72 t/m2, fin = 3.5 t/m2 (value taken for a depth of 1 m).
It is more convenient to collect loads in tabular form. It is necessary not to forget the coefficients for reliability.
The grillage is preliminarily accepted with a width of 0.4 m and a height of 0.5 m. The length of the bored pile is preliminarily 3 m, the cross section is 40 cm in diameter, and are installed in increments of 1.5 m.
Number of piles = 32 m (L, grillage length) / 1.5 m (pile spacing) +1 = 22 pcs. (round down to the nearest whole number). S \u003d 3.14 * 0.42 / 4 (area formula in terms of diameter, see earlier) \u003d 0.126 m 2.
Grill weight: 0.4 m * 0.5 m * 32 m (length) * 2500 kg / m3 (density of reinforced concrete) * 1.3 (coefficient) = 20800 kg.
Pile weight: 22 pieces * 3 m * 0.126 m2 * 2500 kg / m 3 * 1.3 = 27030 kg.
The total mass of the whole house = 235830 kg = 236 tons.
Load per linear meter = Q = 236 t/32 m = 7.36 t/m.
Pile calculation
Pile calculation option 1.
Bearing capacity of one pile = P = (0.7*R*S) + (u*0.8*fin*li) = (0.7*72 t/m2*0.126 m2) + (1.26 m*0 .8 * 3.5 t / m 2 * 3 m (pile length)) \u003d 16.93 t.
u = 3.14*D = 3.14*0.4 = 1.26 m, where D is the pile diameter.
The distance between the piles = l = P / Q = (16.93 t) / (7.36 t / m) = 2.3 m. The step is large enough, you can reduce the length of the pile to 2 m.
Pile calculation option 2.
In the calculations for the previous case, only one value needs to be replaced. Bearing capacity of one pile \u003d P \u003d (0.7 * R * S) + (u * 0.8 * fin * li) \u003d (0.7 * 72 t / m 2 * 0.126 m2) + (1.26 m * 0.8 * 3.5 t / m 2 * 2 m (pile length)) \u003d 13.41 t.
Distance between piles = l = P/Q = (13.41 t)/(7.36 t/m) = 1.82 m.
Pile calculation option 3.
Consider another option with a pile diameter of 50 cm and a length of 2 m.
S \u003d 3.14 * 0.52 / 4 \u003d 0.196 m 2;
u \u003d 3.14 * D \u003d 3.14 * 0.5 \u003d 1.57 m.
Maximum load of one pile \u003d P \u003d (0.7 * 72 t / m2 * 0.196 m 2) + (1.57 m * 0.8 * 3.5 t / m 2 * 2 m (pile length)) \u003d 18, 67 tons
Distance between supports = l = P/Q = (18.67 t)/(7.36 t/m) = 2.54 m.
It is recommended to choose a pile spacing close to 2 m. In this case, option 2 with foundations of small cross section and length will be optimal. For a more accurate result, you can calculate the material consumption in all cases and compare it.
Since it is planned to build a heavy brick house, we assign larger rods with a diameter of 14 mm as working reinforcement. For the manufacture of transverse clamps, 8 mm reinforcement is used.
Calculation of reinforced concrete grillage
From the mass of the house used in the previous calculations, it is necessary to subtract the mass of the piles. We get a load of 208800 kg = 209 tons.
Grillage width \u003d B \u003d M / L * R \u003d 209 t / (32 m * 72 t / m 2) \u003d 0.1 m. The required grillage width is less than the width of the building wall. We assign a structural value of 0.4 m. The overhangs of the wall from the grillage should not be too large, the maximum value is 0.04 m. We also select the height of the grillage structurally 0.5 m. It remains to assign reinforcement:
- The working is taken 0.001 * 0.6 m * 0.5 m \u003d 0.0003 m2 \u003d 3 cm 2. According to the assortment, 4 rods with a diameter of 10 mm are suitable, but according to the requirements of the joint venture, the minimum value for a grillage side length of 6 m is 12 mm. We accept 4 rods with a diameter of 12 mm (two above and two below).
- Cross reinforcement with a diameter of 6 mm.
- Vertical reinforcement with a diameter of 6 mm (because the tape height is less than 0.8 m).
Performing the calculation will allow optimal use of materials and labor force at the construction site.
A characteristic indicator of the strength of a pile foundation is the bearing capacity of a single pile. This characteristic affects the total number of piles in the perimeter of the foundation - by adjusting the frequency, you can increase the load limit that the foundation will be able to withstand. The number of bored piles and the bearing capacity of a single pile column are interrelated characteristics, the optimal ratio of which is determined by simple calculations.
Preparation for calculation
The initial data that will be needed to calculate the bearing capacity of a bored pile is obtained as a result of geological surveys and the calculation of the total expected load of the building. These are the mandatory stages of the calculation, the implementation of which is justified by the theory of calculating the strength characteristics of bored foundations.
Indicators such as freezing depth, groundwater level, soil type and its mechanical characteristics are very important for obtaining an accurate result. Information on the depth of soil freezing is in SNiP 2.02.01-83 *, the data is divided into climatic regions, presented cartographically and in the form of tables.
Do not rely on geological and hydrogeological survey data obtained in neighboring areas. Even within the perimeter of one land allotment, the state of foundation soils can change dramatically. Three to four control wells at the control points of the perimeter will give accurate information about the condition of the soil.
The calculation of the mass of the building is carried out taking into account the climatic region, the location of the building relative to the rhumb of the winds, the average amount of precipitation in the winter, the mass of building structures and equipment. This indicator is most significant in the design of the foundation - the data for this part of the calculation, as well as the scheme and calculation formulas can be found in SNiP 2.01.07-85.
Conducting geology
Conducting geological surveys is a responsible event, and in mass production construction, this is done by geologists. In individual housing construction, an independent assessment of the state of the soil is often carried out. Without experience in conducting surveys of this level, it is very difficult to assess the real state of affairs. The work of a competent specialist for the most part consists in a visual assessment of the state of the strata.
To begin with, suffers are arranged on the site - vertical excavations of soil of rectangular or circular cross-section, with a depth of two meters and a width sufficient for visual inspection of the base of the walls of the pit. The purpose of the shuffer is to open the soil in order to access the layers hidden under the top layer of soil. Geologists measure the depth of the layers, take a soil sample from the middle of each layer, and subsequently monitor the accumulation of water at the bottom of the face. Instead of shufers, round wells can be arranged, from which a core is taken using a special device or local samples are taken.
Shufry shelter for a while - two or three days - limiting the ingress of precipitation. After that, the water level that has risen in the well cavity is estimated - this mark, counted from the upper boundary, will be the level of groundwater occurrence.
All the data obtained are entered into a summary table. In addition, a profile of the soil section is compiled, which makes it possible to predict the state of the soil at points where drilling has not been carried out. When self-assessing the bases, one should be guided by the information provided in SNiP 2.02.01-83 * and GOST 25100-2011, where the relevant sections present soil classifications with descriptions, methods for visually determining soil types and characteristics in accordance with types.
How to use geological exploration data
After the geology of the area has been carried out - independently or by hired specialists - you can begin to determine the initial geometric characteristics of the piles.
We are interested in the type of soil, the coefficient of soil heterogeneity, the depth of freezing and the level of groundwater. The scheme for calculating the bearing capacity of a bored pile for various types of soil is in the appendices of SP 24.13330.2011.
The depth of the pile should be at least half a meter below the freezing depth in order to prevent the impact of frost heaving of the soil on the supporting part of the column. The average freezing depth in the central strip of Russia is 1.2 meters, which means that the minimum length of the pile should be 1.7 meters in this case. The value varies for individual regions.
Not only relative humidity, but also the relative position of the lower mark of soil freezing and the depth of groundwater. In the cold season, high-lying frozen groundwater will exert strong lateral pressure on the body of the pile column - such soils are highly deformed and are considered heaving.
Some soils, characterized as weak, high heaving and subsidence, are not suitable for pile foundations - strip or slab foundations are more suitable for them. To determine the type of soil, as well as the type of compatible foundation, means to exclude the rapid destruction of structures. Soil heterogeneity indicators indicated in the tables of the above normative documents are used in further calculations.
Calculation of the total load
The collection of loads allows you to determine the mass of the building, which means the force with which the building will act on the foundation as a whole and on its individual elements. There are two types of loads acting on the supporting structure - temporary and permanent. Permanent loads include:
- Mass of wall structures;
- The total mass of floors;
- The mass of roof structures;
- Mass of equipment and payload.
You can calculate the mass of structures by determining the volume of structures, and multiplying it by the density of the material used. An example of calculating the mass for a one-story building with reinforced concrete floors, a roof made of ceramic tiles and with walls of 600 mm reinforced concrete, dimensions 10 by 10 meters in plan, floor height 2 meters:
- We calculate the volume of the walls, for this we multiply the cross-sectional area of \u200b\u200bthe wall by the perimeter. We get V walls = 20 ∙ 2 ∙ 0.6 = 24 m3. We multiply the obtained value by the density of heavy concrete, which is equal to 2500 kg / cm3. The total mass of wall structures is multiplied by the safety factor, for concrete equal to k = 1.1. We get the mass M of the wall = 66 tons.
- Similarly, we consider the volume of floors (basement and attic), the mass of which, with a thickness of 250 mm, will be equal to Mpc = 137.5 tons, taking into account a similar safety factor.
- We calculate the mass of roof structures. The mass of the roof for 1 m2 of metal tiles is 65 kg, for soft roofs - 75 kg, for ceramic tiles - 125 kg. The area of a gable roof for a building of such a perimeter will be approximately 140 m2, which means that the mass of structures will be Mcr = 17.5 tons.
- The total size of the permanent load will be equal to Mpost = 221 t.
Reliability factors for various materials are in the seventh section of SP 20.13330.2011. When calculating, the mass of partitions, facade cladding materials and insulation should be taken into account. The volume occupied by window and door openings is not subtracted from the total volume for ease of calculation, since it is an insignificant part of the total mass.
Calculation of live loads
Rostverk on screw piles
Live loads are calculated in accordance with the climatic region and the instructions of the set of rules "Loads and effects". Temporary loads include snow and payloads. The payload for residential buildings is 150 kg per 1 m2 of floor, which means that the total payload will be Mpol = 15 tons.
The mass of equipment that is supposed to be installed in the building is also summed up in this indicator. For a certain type of equipment, a safety factor is applied, located in the above set of rules.
Exist different types special loads that must also be taken into account in the design. These are seismic, vibrational, explosive and others.
where ce is the snow drift coefficient equal to 0.85;
ct is a thermal coefficient equal to 0.8;
m - conversion factor, for buildings in terms of less than 100 m, taken according to table D of the above joint venture;
St is the weight of the snow cover per 1 m2. Accepted according to table 10.1, depending on the snow area.
The indicators of temporary loads are summed up with constant ones and a quantitative indicator of the total load of the building on the foundation is obtained. This number is used to calculate the load per pile column and compare the tensile strength. For the convenience of calculation and clarity of the example, we will take the temporary loads Mvr = 29 t, which, in total with the constants, will give Mtotal = 250 t.
Determination of the bearing capacity of the pile
The geometrical parameters of the pile and the tensile strength are interrelated quantities. AT this example, the load per meter of foundation will be 250/20 = 12.5 tons.
The calculation of the limit of the load limit on a single bored pile is carried out according to the formula:
where F is the limit of the bearing capacity; R - relative soil resistance, an example of the calculation of which is in SNiP 2.02.01-83 *; A is the sectional area of the pile; Eycf, fi and hi are the coefficients from the above SNiP; y is the perimeter of the section of the pile column, divided by the length.
Watch the video on how to check the bearing capacity of a pile using professional equipment.
For a pile of one and a half meters in length with a diameter of 0.4 meters, the bearing capacity will be 24.7 tons, which allows increasing the pitch of pile columns to 1.5 meters. In this case, the load on the pile will be 18.75 tons, which leaves a fairly large margin of safety. By changing the geometric characteristics, as well as the pitch of the pile columns, the bearing capacity is regulated. This table, presented below, shows the dependence of the bearing capacity of a one and a half meter pile on the diameter:
The dependence of the bearing capacity on the width of the pileThere are many services that allow you to calculate the bearing capacity of a pile online. You should use only trusted portals with good reviews.
It is important not to exceed the allowable load on the pile and leave a margin of safety - few services are able to plan load distribution, so you should pay attention to the calculation algorithm.
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