Foundation soil resistance table. Determination of soil bearing capacity

Establishing the bearing capacity of the soil (tabular values) located under the designed or reconstructed foundation begins with geological exploration. To do this, soil samples are taken and examined at the construction site from wells or pits.

First, the soil is classified. The composition of the soil is determined using the granulometric and/or elutriation method and its name is determined.

Then the physical characteristics of the soil are examined. The density of the soil is determined by the cutting ring method, the moisture content is determined by the method of drying and weighing, and the consistency of the soil is determined by twisting the soil into a rope and testing with a balancing cone.

Next, additional laboratory studies of the soil are done or several more calculations are made to expand the number of physical characteristics of the soil.

If it is impossible to accurately determine the type of soil on your own, the presence of organic, frozen, bulk soils on the site, and if there are any other doubts about the classification of the soil, licensed geological organizations must be involved to determine the bearing capacity of the soil.

Building responsibility level

A building or structure must be classified as one of the following levels of responsibility: increased, normal and decreased (Article 4, paragraphs 7–10 of the current technical regulations on the safety of buildings and structures Federal Law No. 384-FZ).

TO increased level of responsibility includes: especially dangerous, technically complex or unique objects.

TO reduced - buildings and structures for temporary (seasonal) purposes, as well as buildings and structures for auxiliary use related to construction or reconstruction or located on land plots provided for individual housing construction.

All other buildings and structures belong to normal level of responsibility.

The wording for identifying buildings belonging to the third (lower) level of responsibility is vague. It is not clear whether two groups of buildings and structures are described: temporary and auxiliary or three groups - temporary, auxiliary and individual? Residential in Belarus individual houses with a height of no more than 2 floors are classified in the third group of responsibility, and in Russia, residential buildings up to 10 m in height were previously also classified in this group. In the new technical regulations there is no clarity on this issue. Apparently everyone will have to decide for themselves. The volume of responsibility depends on the choice of level of responsibility geological surveys and methods for calculating foundations.

Determination of the calculated base resistance R from tables

This method is used for preliminary and final calculation of foundations for buildings of the third level of responsibility located in favorable conditions. Or for a preliminary calculation of the foundations for buildings of the second level of responsibility located in any, including unfavorable engineering and geological conditions.

Conditions are considered “favorable” when the soil layers at the base lie horizontally (the slope of the layers does not exceed 0.1), and the compressibility of the soil does not increase to at least a depth equal to twice the width of the largest individual foundation and four widths of the strip foundation (counting from the level his soles).

For foundations with a width of b o = 1 m and a depth of d o = 2 m, the values ​​of the calculated foundation resistance (R o) are given in Tables 11–15. With an increase or decrease in the depth of the foundation, the bearing capacity of the foundation soil changes. In this case, the calculated base resistance (R) at various depths should be determined using the formulas:

R = R o (d + d o) /2d o at d< 2 м;

R = R o + k 2 γ"(d - d o) for d > 2m

where b is the width of the foundation, m; d - depth of the base, m; γ’ - calculated value of the specific gravity of the soil lying above the base of the foundation, kN/m³; k 1 - coefficient accepted for foundations composed of coarse soils and sands, k 1 = 0.125; for foundations composed of silty sands, sandy loams, loams and clays, k 1 = 0.05; k 2 - coefficient accepted for foundations composed of coarse sandy soils - k 2 = 0.25, composed of sandy loams and loams - k 2 = 0.2; clays - k 2 = 0.15.

Table 11

Table 12

Table 13

Table 14

Table 15

The calculated resistance of the foundation soil R o is such a safe pressure at which the linear dependence of the foundation settlement is maintained, and the depth of development of zones of local strength failure under its edges does not exceed 1/4 of the width of the foundation base.

An example of determining the design resistance of the foundation soil using tables

Determine the calculated resistance of the foundation base, having a base size of 2.5 × 2.5 m, a laying depth of 1 m; building without basement, class III. The foundation, to the entire explored depth, is composed of sand of medium coarseness, medium compaction (γ’ = 20 kN/m³). No groundwater was found. To determine the design resistance of the base, it is legitimate to use tabular values ​​of R o values. According to table. 2 R o = 400 kPa. Using the formula, we get: R = R o (d + d o) /2d o = 400 (1 + 2)/2×2 = 356 kPa.

Determination of the design resistance of the base R based on the physical characteristics of the soil

This method is used for the final calculation of foundations for buildings of the second level of responsibility.

The design resistance of the foundation soil is determined by the formula:

R = (m 1 m 2 / k) ,

where m 1 and m 2 are the operating conditions coefficients adopted according to the table. 16; k - coefficient, k = 1, if the characteristics of soil properties are determined experimentally, k = 1.1, if the characteristics are taken from reference tables; M 1, M 2, M 3 - coefficients accepted according to the table. 17; k z - coefficient, at b< 10 м - k z =1 при b >10 m - k z = z/b + 0.2 (here z = 8 m); b - width of the foundation base, m; γ is the average value of the specific gravity of soils lying below the base of the foundation (in the presence of groundwater, it is determined taking into account the weighing effect of water), kN/m³; γ’ - the same for soils lying above the base; c is the calculated value of the specific adhesion of the soil lying directly under the base of the foundation, kPa; d b - basement depth, i.e. distance from the planning level to the basement floor, m. For structures with a basement less than 20 m wide and more than 2 m deep, d b = 2 m is accepted, with a basement width greater than 20, d b = 0; d 1 - depth of foundation of basement-free structures from the planning level (m) or reduced depth of foundation from the level of the basement floor, determined by the formula: d 1 = h s + h cf γ cf / γ', here h s - thickness of the soil layer above the base of the foundation under the basement: h cf - thickness of the basement floor; γ cf - calculated value of the specific gravity of the basement floor material, kN/m³.

Table 16

1. Structures with a rigid structural design include structures whose structures are specially adapted to absorb forces from deformation of the foundations (subsection 5.9 SP 22.13330.2011).

2. For buildings with a flexible structural design, the value of the coefficient m 2 is taken equal to one.

3. For intermediate values ​​of L/H, the coefficient m 2 is determined by interpolation.

4. For loose sands, m 1 and m 2 are taken equal to one.

Table 17

An example of determining the design resistance of a foundation soil based on the physical characteristics of the soil

Determine the design resistance of the foundation base outer wall a two-story building without a basement, 10 m long. The foundation is strip, its dimensions are: width b = 1.0 m; depth d 1 =1.8 m, d b = 0.

Characteristics of soil properties were determined in the laboratory; the number of determinations allowed for statistical processing of the data. From the surface to the level of the base of the foundation lies bulk soil, its Specific gravityγ’ = 17 kN/m³. Under the base of the foundation to the entire explored depth (9 m) there is soft plastic loam (I L = 0.6). Calculated values: specific gravity γ = 20 kN/m³, angle of internal friction φ = 15°; specific adhesion c = 30 kPa.

According to the table 17 for the value φ = 15° we find the values ​​of the dimensionless coefficients: M 1 = 0.32; M 2 = 2.30; M 3 = 4.84.

According to the table 16 coefficient m 1 = 1.1 (I L > 0.5); coefficient m 2 = 1.0 (building L/H ratio more than 4).

Coefficient k z = 1, since the width of the foundation is b< 10 м.

For the given data we obtain: R = (m 1 m 2 / k) = (1.1 × 1 / 1) [(0.32 × 1 × 1.0 × 20) + (2.30 × 1.8 × 17 ) + (4.84 × 30) ] = 244 kPa.

Determination of the conditional design resistance of soils

1. This soil is silty sand, according to GOST 25100-95 “Soils. Classification", to dense sands. Considering that sand has an average degree of saturation with water (Sr = 0.79), we determine its design resistance from Table 2 of Appendix 3 of SNiP 2.02.01-83* “Foundations of buildings and structures”

R 0 = 400 kPa.

2. Clay. Taking into account the value of the porosity coefficient e = 0.71 and the fluidity index JL = 0.16, we determine the calculated resistance from Table 3 of Appendix 3 of SNiP 2.02.01-83 * “Foundations of buildings and structures”

R 0 = 400 kPa.

3. Considering that the porosity coefficient of this soil is e = 0.7 and the fluidity index JL = 0.11, according to Table 3 of Appendix 3 of SNiP 2.02.01-83 * “Foundations of buildings and structures” we determine

R 0 = 400 kPa.

Determination of specific gravity of soil

g = cg, kN/m 3

1. Sand, s=1.9 g/cm3=1.9 t/m3

g=1.9·9.8=18.62 kN/m 3

2. Clay, s=2.01 g/cm3=1.95 t/m3

g=2.01·9.8=19.7 kN/m 3

3. Loam, s=1.87 g/cm3=1.96 t/m3

g=1.87·9.8=18.326 kN/m 3

Design soil characteristics

• 1. Sand:
  • - clutch,

with I = 3/1.5=2, with II = 3/1=3;

Angle of internal friction,

q I = 28/1.15 = 24.35 0; q II = 28/1 = 28 0;

Specific gravity,

g I = g II = 18.62/1 = 18.62 kN/m3.

c I = 30/1.5 = 20 kPa, c II = 30/1 = 30 kPa;

ts I = 9/1.15 = 7.83 0, ts II = 9/1 = 9 0;

g I = g II = 19.7/1 = 19.7 kN/m 3.

3. Loam:

with I = 20/1.5 = 13.3 kPa, with II = 20/1 = 20 kPa;

c I = 20/1.15 = 17.39 0, c II = 20/1 = 20 0;

g I = g II = 18.326/1 = 18.326 kN/m 3.

Specified and calculated physical and mechanical characteristics of the soils composing construction site, tabulate

Table 1 Physical and mechanical properties of soil

Conclusion on the possibility of using soils as a foundation

The construction site is represented by the following types of soil:

• - from the surface to a depth of 0.4 m lies chernozem, which is not used in construction, is cut off and removed from the site;
• -then lies a layer of medium-sized sand, medium density, average degree of humidity with a thickness of 3.6 m, moderately compressible, conditional design resistance R 0 = 400 kPa, can be used as a natural base;
• -the next layer is brownish-gray clay, 4.0 m thick, in a semi-solid state, moderately compressible with a nominal design resistance R 0 = 400 kPa, can be used as a natural base;
• -the last layer is gray loam, 7.0 m thick, in a semi-solid state, moderately compressible with a conditional design resistance R 0 = 400 kPa, can be used as a natural base.

This is one of the most important parameters when building a foundation, as it allows you to determine the maximum possible values ​​of the mass of the overlying structure that the underlying surface can support.

If the permissible values ​​of the soil bearing capacity are exceeded, areas of limit equilibrium are formed under the base of the foundation. In other words, the soil located below cannot withstand the load and tends towards the direction of least resistance, that is, to the surface. The consequences are expressed in the form of mounds and shafts located near the boundaries of the foundation.

The most important danger in this case is the violation of the homogeneity of the underlying soil. The load from the structure begins to be distributed unevenly, the foundation loses its stability, deformation processes are activated and cracks soon begin to appear.

Calculation of soil bearing capacity

Determining the bearing capacity of the soil is a rather labor-intensive process that can be done using available means (manually/online) or using the services of geological and geodetic agencies!

We invite you to use our convenient online calculator for calculating the compressive/shear resistance of soil. At the end of the calculation, you will receive a calculated resistance value of four different units measurements (kPa, kH/m 2, tf/m 2, kgf/cm 2). In order to get the calculation result, you need to fill in several fields:

• Calculation type. Based laboratory tests or with unknown soil characteristics.
• Soil characteristics. Type, porosity coefficient and fluidity index, as well as the average calculated value of the specific gravity of soils.
• Foundation parameters. Base width and depth.

The last two soil characteristics are determined only for clay soils.

Foundation soil resistance calculator

First, we need to select the calculation type. The first option implies that you will receive a soil sample and send it to a specialized laboratory for testing. This method takes a large number of time and money. Therefore, if you do not have a complex area and you are sure that you can do everything on your own, we suggest using the second option and performing the calculation based on tabular data.

Soil classification

The next stage of work is related to determining the type of soil. According to SNiP 11-15-74, all types of soils are divided into two main groups:

• rocky;
• non-rocky.

How to determine the type of soil yourself?

There is a simple old-fashioned way to determine the type of soil that your parents and your parents’ parents used - it involves identifying the physical and mechanical properties of the rock.

To do this, it is necessary to take soil samples at the extreme points and in the middle of the site. Dig holes to the depth of the expected foundation level and take soil samples from each control point.

Prepare a work surface to conduct a science experiment.

• Wet the soil until it can be formed into a ball.
• Try to roll the ball into an oblong body (cord).
• If you were unable to do this, then in front of you sandy soil.
• If it sets a little, but still collapses - this is sandy loam.
• If the cord can be rolled into a ring, but breaks/cracks are observed - this is loam.
• If the ring is closed, but the body remains unharmed - this is clay.

For clarity, you can see the illustration below:

If you were unable to make anything from the soil sample, then the calculation of the bearing capacity of sandy soil is over for you. Select the appropriate item in the calculator and click " Calculate".

Soil bearing capacity - SNiP table

To determine the bearing capacity of clay soils, we need to obtain two more coefficients - soil fluidity index (I L) And porosity coefficient (e). The first indicator can be determined quite easily by eye; if the soil is frankly damp and viscous - choose I L = 1, if dry and rough - I L = 0. The second coefficient can only be obtained in tables from SNiP. Since all data is in the public domain, for your convenience we have copied tables of calculated soil resistance from SP 22.13330.2011.

Bearing capacity of clay soils

Insert value porosity coefficient e into the calculator, enter the foundation parameters and complete the determination of the calculated soil resistance.

Bearing capacity of sandy soil

These table values ​​of R 0 are valid for foundations with width b = 1 m and depth d = 2 m.

For other values ​​of b and d, formulas must be used. At d<= 2 м используется первое выражение, при d >2 m - second.

Calculated soil resistance (formula) #1: R = R 0 × × (d + d 0) / 2d 0

Calculated soil resistance (formula) #2: R = R 0 × + k 2 × γ" II × (d - d 0)

In order to save you from complex cumbersome calculations, we have added a fourth item to our calculated soil resistance calculator, in which you can indicate the estimated dimensions of the foundation. Use our service and save your time!

The possibility of using solutions from the theory of elasticity when calculating vertical deformations was substantiated by N.M. Gersevanov. However, this approach is valid within the limits of loads at which a linear relationship between stresses and strains is observed.

Designed according to dependency (8.29) foundations in many cases they turn out to be uneconomical due to underutilization of the bearing capacity of soils, especially sandy ones, as well as clayey ones (hard, semi-solid and refractory consistency) even in the linear stage of deformation. In this regard, SNiP 2.02.01-83* “Foundations of buildings and structures” recommends limiting the average pressure under the base of the foundation by the calculated resistance of the foundation soil R, which allows you to calculate foundation settlements based on the linear relationship between stresses and deformations. Thus, when calculating foundations based on deformations, it is necessary that the condition be satisfied

P ≤ R, (8.37)

Where R- average pressure along the base of the foundation; R- calculated resistance of the foundation soil.

Where γ с1 And γ с2- coefficients of operating conditions, respectively, of the soil foundation and the structure in interaction with the foundation, taken according to table 8.3; k- reliability coefficient accepted when determining the strength characteristics of soil by direct tests, k= 1.0; when using tabular calculated soil values k = 1,1; k z- coefficient taken equal to the width of the foundation base b≤10 m, k z= 1.0; at b≥10m - k z= Z 0/b + 0.2 (here Z 0= 8 m); M γ ; M q, M s- coefficients depending on the angle of internal friction of the load-bearing soil layer; b- width of the foundation base, m;

Table 8.3. Values ​​of operating conditions coefficients γ с1 And γ с2

Table 8.4. Coefficient values M γ , M q , M s

γ II And γ" II- averaged calculated specific gravity of soils lying respectively below the base of the foundation and within the depth of the foundation, kN/m3 (in the presence of groundwater, it is determined taking into account the weighing effect of water); d 1- depth of foundation from the basement floor; in the absence of a basement floor - from the planned surface, m; d b- basement depth, counting from the planning mark, but not more than 2 m (with basement width B > 20 m, db = 0 is accepted); c II- calculated value of the specific adhesion of the load-bearing soil layer, kPa (index II means that the calculation is carried out according to the second group of limit states).

Formula (8.38) is based on the solution of N.P. Puzyrevsky, which makes it possible to determine the pressure on the base at which in the massif under the edges foundation zones of limiting equilibrium are formed. Nevertheless, formula (8.38) differs in its structure from N.P.’s solution. Puzyrevsky additional coefficients ( γ с1 And γ с2), which increase the reliability of calculations and allow one to take into account, respectively, the influence of the strength and deformation properties of soils on the formation of zones of limiting equilibrium under the base of the foundation and the rigidity of the structure being built.

The additional term introduced into formula (8.38) is equal to ( Mq- 1), allows you to take into account the effect of everyday soil loading. When excavating a pit, the stressed state of the soil, caused by the action of everyday soil pressure, is preserved to a certain extent. At the same time, the maximum pressure increases, at which the zones of local disturbance under the edge of the foundation reach a value equal to 0.25 of the width of the foundation. However, the residual stress state depends on the depth of the excavation pit and its width. Then, with increasing depth of the pit, i.e. with increasing household load, there will be a greater residual pressure in the layer under consideration.

According to formula (8.38) the calculated soil resistance base is determined for the load-bearing layer where the base of the foundation lies. Sometimes in the depths Z less durable soil lies under the bearing layer ( rice. 8.8), in which plastic deformations can develop. In this case, it is recommended to check the stresses transmitted to the roof of weak soil according to the condition

(8.39)

Where σ zp- additional vertical stress; σ zg- stress from the soil’s own weight; Rz- calculated soil resistance at the roof depth of soft soil z.

Rice. 8.8. Conditional foundation diagram

Magnitude Rz is determined by formula (8.38), while the operating conditions coefficients γ с1 And γ с2 and reliability k, and M γ, M q, M s found in relation to a layer of weak soil.

Values b z And d z determined for a conditional foundation ABCD resting on soft ground.

In this case it is accepted that σ zp acts on the base of a conditional foundation ABCD (see fig. 8.8), then the area of ​​its sole is

Where N- load transmitted to the edge of the foundation.

Knowing the area of ​​the base of the conditional foundation, you can determine its width using the formula

(8.41)

Where a = (l- b)/2 (l And b- dimensions of the designed foundation).

Having determined by formula (8.38) the quantity Rz, check condition (8.39). If it is satisfied, shear zones do not play a significant role in the amount of developing sediment. Otherwise, it is necessary to accept large dimensions of the foundation base, at which condition (8.39) is satisfied.

Conditional design resistance of foundation soils R o

For appointment preliminary sizes foundations buildings And structures conditional design resistances of foundation soils Ro are used, which are given in table 8.5 - 8.8.

Examples

Example 8.2. Determine the conditional design resistance of fine sand if it is known: natural humidity ω = 0.07; natural density ρ = 1.87 t/m3, density of solid particles ρ S = 2.67 t/m3.

The “load-settlement” relationship for shallow foundations can be considered linear only up to a certain limit of pressure on the foundation (Fig. 5.22). The calculated resistance of the foundation soils is taken as such a limit R. When calculating foundation deformations using the calculation schemes specified in clause 5.5.1, the average pressure under the base of the foundation (from loads for calculating foundations based on deformations) should not exceed the design resistance of the foundation soil R, kPa, determined by the formula

where γ c 1 and γ c 2 - coefficients of working conditions, taken according to table. 5.11; k k= 1, if the strength characteristics of the soil ( With and φ ) are determined by direct tests, and k= 1.1, if the specified characteristics are taken according to the tables given in Chapter. 1; M γ , M q And M c— coefficients accepted according to table. 5.12; k z— coefficient accepted: k z= 1 at b < 10 м, k z = z 0 /b + 0,2 at b≥ 10 m (here b— width of the foundation base, m; z 0 = 8 m); γ II - the calculated value of the specific gravity of the soils lying below the base of the foundation (in the presence of groundwater is determined taking into account the weighing effect of water), kN/m 3 ; γ´ II - the same, lying above the sole; With II - calculated value of the specific adhesion of the soil lying directly under the base of the foundation, kPa; d 1 - the depth of laying the foundations of basement-free structures or the reduced depth of laying external and internal foundations from the basement floor, "determined by the formula

d 1 = h s + h cf γ cf /γ´ II

(Here h s— thickness of the soil layer above the base of the foundation on the basement side, m; h cf— thickness of the basement floor structure, m; γ cf- calculated value of the specific gravity of the basement floor material, kN/m 3); d b— basement depth — distance from the planning level to the basement floor, m (for buildings with a basement width IN≤ 20 m and depth more than 2 m is accepted d b= 2 m, with width fell IN> 20 and accepted d > 0).

Rice. 5.22.

Characteristic “load-settlement” relationship for shallow foundations d 1 > d If d(Where d- foundation depth), then d 1 is taken equal d b = 0.

,a Formula (5.29) applies to any shape of foundations in plan. If the base of the foundation has the shape of a circle or a regular polygon with an area A b, then it is accepted = . Calculated values specific gravity

soil and basement floor material included in formula (5.29) may be taken equal to their standard values ​​(assuming the reliability coefficients for soil and material equal to unity). With appropriate justification, the calculated soil resistance can be increased if the foundation design improves the conditions for its joint work with the foundation. For foundation slabs with corner cuts, the calculated resistance of the foundation soil can be increased by 15%. With 1 and γ With 2

I L

Notes: 1. Rigid structural schemes are structures whose structures are adapted to absorb forces from foundation deformations through the use of special measures. c 2. For structures with a flexible structural design, the value of the coefficient γ

2 is taken equal to one. 2 for structures with a rigid structural design when the ratio of the length of the structure or its compartment to its height 3. For intermediate values c coefficient γ

2 is determined by interpolation. TABLE 5.12. COEFFICIENT VALUES

When the calculated depth of foundations is taken from the level of the grading embankment, the design of foundations and foundations must include a requirement for the need to perform a grading embankment before applying the full load on the foundation. A similar requirement must be contained in relation to the installation of bedding under floors in the basement.

Odds M γ , M q And φ II,°, included in formula (5.29), are obtained based on the condition that the zones of plastic deformation under the edges of a uniformly loaded strip (Fig. 5.23) are equal to a quarter of its width and are calculated according to the following relations:

M γ= ψ/4; Mq= 1 + ψ; φ II,°= ψctgφ II,

Where ψ = π/(ctgφ II + φ II - π/2); φ II—calculated value of the internal friction angle, rad.

Rice. 5.23.

When calculating R values ​​of characteristics φ II, With II and γ II are taken for the soil layer located under the base of the foundation to the depth z R = 0,5b at b < 10 м и z R = t + 0,1b at b≥ 10 m (here t= 4 m). If there are several layers of soil from the base of the foundation to the depth z R weighted average values ​​of the specified characteristics are accepted. The same applies to the coefficients γ c l and γ c 2 .

As can be seen from formula (5.29), the value R depends not only on the physical and mechanical characteristics of the foundation soils, but also on the desired geometric dimensions of the foundation - the width and depth of its foundation. Therefore, the determination of the dimensions of the foundations has to be carried out in an iterative manner, having previously specified some initial dimensions.

Example 5.5. Determine the design resistance of the foundation soil for strip foundation width b= 1.4 m with the following initial data. The designed building is a 9-story large-panel building with a rigid structural design. The ratio of its length to height 2 for structures with a rigid structural design when the ratio of the length of the structure or its compartment to its height= 1.5. The depth of foundations from the planning level is accepted for design reasons d= 1.7 m. The building has a basement width IN= 12 m and depth d b= 1.2 m. Thickness of the soil layer from the base of the foundation to the basement floor h s= 0.3 m, thickness of concrete basement floor h сf= 0.2 m, specific gravity of concrete γ II = 23 kN/m 3. The site is composed of fine sands of medium density and low moisture content. Porosity coefficient e= 0.74, specific gravity of the soil below the base γ II = 18 kN/m 3 , above the base γ´ II = 17 kN/m 3 . Standard values ​​of strength and deformation characteristics are adopted according to the reference tables given in Chapter. 1:φ n= φ II = 32º, with n = c II = 2 kPa, E= 28 MPa.

Solution. To calculate the design resistance of the foundation soil using formula (5.29), we accept: according to table. 5.11 for fine, low-moisture sand and a building with a rigid structural design when 2 for structures with a rigid structural design when the ratio of the length of the structure or its compartment to its height= 1.5, γ With 1 = 1.3 and γ With 2 = 1.3; according to table 5.12 at φ II = 32º M γ = 1,34; Mq= 6.34 and M c= 8.55. Since the values ​​of soil strength characteristics are taken from reference tables, k= 1.1. At b= 1.4 m< 10 м k z = 1.

Reduced foundation depth from the basement floor according to formula (5.30)

d 1 = 0.3 + 0.2 · 23/17 = 0.57 m.

Using formula (5.29) we determine:

R= = 1.54 · 221 = 340 kPa.

Preliminary dimensions of foundations are assigned for structural reasons or based on the values ​​of the calculated resistance of the foundation soils R 0 given in table. 5.13. Values R 0 can also be used for the final determination of the dimensions of the foundations of class III structures, if the base is composed of horizontal (slope no more than 0.1) soil layers maintained in thickness, the compressibility of which does not increase with depth within the limits of double the width of the largest foundation below the depth of its foundation.

Double interpolation when determining R 0 according to table 5.13 for silt-clay soils with intermediate values with soil or filler fluidity index: And e it is recommended to follow the formula

Guidelines for the design of foundations of buildings and structures

SNiP 2.02.01-83. Foundations of buildings and structures

Where e 1 and e 2 - adjacent values ​​of the porosity coefficient in table. 5.13, between which lies the value of e for the soil in question; R 0 (1, 0) and R 0 (1, 1) - values R 0 in table 5.13 at coefficient, porosity e 1 corresponding to the values with soil or filler fluidity index:= 0 and with soil or filler fluidity index: = 1; R 0 (2, 0) and R 0 (2, 1) - the same, with e 2 .

TABLE 5.13. DESIGN RESISTANCES R 0 COARSE CLASTICAL, SANDY AND silty-clayey (non-subsidence) SOILS

Values R 0 in table 5.13 apply to foundations with a width b 1 = 1 m and depth d 1 = 2 m. When using the values R 0 according to table 5.13 for the final determination of the dimensions of foundations, the calculated resistance of the foundation soil R determined by the formulas:

at d≤ 2 m

;

at d> 2 m

,

Where b And d- respectively the width and depth of the designed foundation, m; γ´ - specific gravity of the soil located above the base of the foundation, kN/m 3; k 1 - coefficient accepted for coarse and sandy soils (except for silty sands) k 1 = 0.125, and for silty sands, sandy loams, loams and clays k 1 = 0,05; k 2 - coefficient accepted for coarse and sandy soils k 2 = 2.5, for sandy loam and loam k 2 = 2, and for clays k 2 = l.5.

Example 5.6. Determine the design resistance of clay with porosity coefficient e= 0.85 and fluidity index with soil or filler fluidity index:= 0.45 in relation to the foundation width b= 2 m, having a depth d= 2.5 m. The specific gravity of the soil located above the base is γ´ = 17 kN/m 3.

Solution. Using the values R 0 (see Table 5.13), using formula (5.32) we calculate:

Design resistance R foundation composed of coarse soils is calculated using formula (5.29) based on the results of direct determinations of the strength characteristics of soils. In the absence of such tests, the design resistance is determined by the characteristics of the aggregate if its content exceeds 40%. With a lower aggregate content the value R for coarse soils it is allowed to take according to the table. 5.13.

When artificially compacting foundation soils or constructing soil cushions, the design resistance is determined based on the design values ​​of the physical and mechanical characteristics of compacted soils specified in the project. The latter are established either on the basis of research or using reference tables (see Chapter 1) based on the required soil density. When calculating R The moisture content of silty clay soils is recommended to be equal to 1.2 ω p .

The design resistance of loose sand is determined by formula (5.29) at γ c 1 = γ With 2 = 1. Value R should be clarified based on the results of at least three tests of a stamp with dimensions and shapes possibly closer to the designed foundation, but with an area of ​​at least 0.5 m2. In this case, the value R no more than the pressure at which the expected settlement of the foundation is equal to the maximum is accepted (see further paragraph 5.5.5).

When constructing intermittent foundations, the calculated resistance of the foundation R is determined as for the original strip foundation according to formula (5.29) with increasing values R coefficient k d, accepted according to the table. 5.14.

If it is necessary to increase loads on the foundation of existing structures during their reconstruction (replacement of equipment, superstructure, etc.), the calculated resistance of the foundation should be taken in accordance with data on the state and physical and mechanical properties of the foundation soils, taking into account the type and condition of the foundations and superstructures of the structure , the duration of its operation and the expected additional settlement with increasing loads on the foundations. You should also take into account the condition and design features adjacent structures, which, once within the “sedimentary crater”, may be damaged.

TABLE 5.14. COEFFICIENT VALUES k d FOR SANDS (EXCEPT LOOSE) AND silty-clayey soils

Notes: 1. For intermediate values e And with soil or filler fluidity index: coefficient k d is accepted by interpolation.

2. For slabs with corner cuts, the coefficient k d takes into account the increase R by 15%.

If within the compressible thickness of the base at a depth z from the base of the foundation there is a layer of soil of lower strength than the strength of the layers above (Fig. 5.24), compliance with the condition must be checked

σ zp + σ zg ≤ Rz,

where σ zp and σ zg— vertical normal stresses in the soil at depth z from the base of the foundation, respectively, additional from the load on the foundation and from the own weight of the soil, kPa (see clause 5.2); Rz— calculated resistance of soil of reduced strength at depth z, kPa, calculated using formula (5.29) for a conditional foundation with a width b z, m, determined by the expression

;

When an eccentric load is applied to the foundation, it is necessary to limit the edge pressures under the sole, which are calculated using the eccentric compression formulas. Edge pressures under the action of moment in the direction of the main axes of the foundation base should not exceed 1.2 R, and the pressure at the corner point is 1.5 R. It is recommended to determine the edge pressures taking into account the lateral resistance of the soil located above the base of the foundation, as well as the rigidity of the structure resting on the foundation in question.

Current standards allow an increase of up to 20% of the design resistance of the foundation soil, calculated using formulas (5.29), (5.33) and (5.34), if the deformation of the foundation under pressure determined by calculation p = R do not exceed 40% of the limit values ​​(see further paragraph 5.5.5). In this case, the calculated deformations corresponding to the pressure p 1 = 1,2R, should be no more than 50% of the maximum. In this case, in addition, a check of the base for bearing capacity is required (see further paragraph 5.6).