Conditional design soil resistance table. Determination of soil bearing capacity. Condition Dependencies

The possibility of using solutions from the theory of elasticity in 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 makes it possible 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 - design resistance 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

Soils	γ с1	γ с2 for structures with a rigid structural design when the ratio of the length of the structure (compartment) to its height L/H is equal to
		4 or more	1.5 or less
Coarse clastic with sandy filler and sandy, except for fine and dusty Sands are fine Dusty sands: - low moisture and damp - saturated with water Clayey, as well as coarse-clastic with clay filler with soil or filler fluidity indicator: JL≤ 0,25 0,25≤ JL <0,5 JL > 0,5	1,25 1,2 1,1	1,2 1,1 1,0 1,0 1,0 1,0 1,0	1,4 1,1 1,1 1,0
Notes 1. The structures of structures with a rigid structural design are adapted to absorb forces from deformations of the foundations. 2. For buildings with flexible design γ с2 is taken equal to 1. 3. For intermediate values ​​L/H coefficient γ с2 determined by interpolation.

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

φ	M γ	M q<.SUB>	M s	φ	M γ	M q	M s
	0,00	1,00	3,14	23	0,69	3,65	6,24
1	0,01	1,06	3,23	24	0,72	3,87	6,45
2	0,03	1,12	3,32	25	0,78	4,11	6,67
3	0,04	1,18	3,41	26	0,84	4,37	6,90
4	0,06	1,25	3,51	27	0,91	4,64	7,14
5	0,08	1,32	3,61	28	0,98	4,93	7,40
6	0,80	1,39	3,71	29	1,06	5,25	7,67
7	0,12	1,47	3,82	30	1,15	5,59	7,95
8	0,14	1,55	3,93	31	1,24	5,95	8,24
9	0,16	1,64	4,05	32	1,34	6,34	8,55
10	0,18	1,73	4,17	33	1,44	6,76	8,88
11	0,21	1,83	4,29	34	1,55	7,22	9,22
12	0,23	1,94	4,42	35	1,68	7,71	9,58
13	0,26	2,05	4,55	36	1,81	8,24	9,97
14	0,29	2,17	4,69	37	1,95	8,81	10,37
15	0,32	2,30	4,84	38	2,11	9,44	10,80
16	0,36	2,43	4,94	39	2,28	10,11	11,25
17	0,39	2,57	5,15	40	2,46	10,85	11,73
18	0,43	2,73	5,31	41	2,66	11,64	12,24
19	0,47	2,89	5,48	42	2,88	12,51	12,79
20	0,51	3,06	5,66	43	3,12	13,46	13,37
21	0,56	3,24	5,84	44	3,38	14,50	13,98
22	0,61	3,44	6,04	45	3,66	15,64	14,64

γII 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 make it possible 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.

Introduced into formula (8.38) additional member, 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; R z- calculated soil resistance at the roof depth of soft soil z.

Rice. 8.8. Conditional foundation diagram

Magnitude R z is determined by formula (8.38), while the operating conditions coefficients γ с1 And γ с2 and reliability k, and also 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 conventional 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

To assign preliminary dimensions of foundations buildings And structures conditional design resistances are used foundation soils Rо, 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.

Establishment bearing capacity soil (tabular values) located under the designed or reconstructed foundation begins with geological exploration. To do this on construction site Soil samples are taken and examined 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) is 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

The numerator shows the values ​​of R o relating to unsoaked subsidence soils with a moisture degree S r ≤ 0.5; in the denominator are the values ​​of R o related to the same soils with S r ≥ 0.8, as well as to soaked soils.

Table 15

Calculated resistance R o of bulk soils

Embankment characteristics	R o , kPa (kg/cm²)
	Large, medium-sized and fine sands, slags, etc. at humidity degree S r		Silty sands, sandy loams, loams, clays, ash, etc. at humidity degree S r
	S r ≤ 0.5	S r ≥ 0.8	S r ≤ 0.5	S r ≥ 0.8
Embankments systematically constructed with compaction	250 (2,5)	200 (2,0)	180 (1,8)	150 (1,5)
Dumps of soil and industrial waste: with seal without seal	250 (2,5) 180 (1,8)	200 (2,0) 150 (1,5)	180 (1,8) 120 (1,2)	150 (1,5) 100 (1,0)
Dumps of soil and industrial waste: with seal without seal	150 (1,5) 120 (1,20)	120 (1,2) 100 (1,0)	120 (1,2) 100 (1,0)	100 (1,0) 80 (0,8)

1. R o refer to bulk soils with organic matter content I om ≤ 0.1.
2. For unpacked dumps and dumps of soil and industrial waste, R o is accepted with a coefficient of 0.8.

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 - averaged value of the specific gravity of soils lying below the base of the foundation (in the presence of groundwater 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

Values ​​of coefficients m 1 and m 2

Soils	Coefficient m 1	Coefficient m 2 for structures with a rigid structural design with a ratio of the length of the structure or its compartment to the height L/H equal to
		4 or more	1.5 or less
Coarse clastics with sandy filler and sandy ones, except for small and silty ones	1,4	1,2	1.4
Sands are fine	1,3	1,1	1,3
Silty sands, low moisture and wet	1,25	1,0	1,2
Sands saturated with water	1,1	1,0	1,2
Silty-clayey, as well as coarse-clastic with silty-clayey filler with a soil or filler fluidity index I L ≤ 0.25	1,25	1,0	1,1
The same at 0.25< I L ≤ 0,5	1,2	1,0	1,1
The same for I L > 0.5	1,1	1,0	1,0

Notes:

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

Values ​​of coefficients M

Angle of internal friction, φ, degrees	Odds
	M 1	M 2	M 3
0	0	1,00	3,14
1	0,01	1.06	3,23
2	0,03	1,12	3,32
3	0,04	1,18	3,41
4	0,06	1,25	3,51
5	0,08	1,32	3,61
6	0,10	1,39	3,71
7	0,12	1,47	3,82
8	0,14	1,55	3,93
9	0,16	1,64	4,05
10	0,18	1.73	4,17
11	0,21	1,83	4,29
12	0,23	1,94	4,42
13	0,26	2,05	4,55
14	0,29	2.17	4.69
15	0,32	2,30	4,84
16	0,36	2,43	4,99
17	0,39	2,57	5,15
18	0,43	2,73	5,31
19	0,47	2,89	5,48
20	0,51	3,06	5,66
21	0,56	3,24	5,84
22	0,61	3,44	6,04
23	0,69	3,65	6.24
24	0,72	3,87	6,45
25	0,78	4,11	6,67
26	0,84	4,37	6,90
27	0,91	4,64	7,14
28	0,98	4,93	7,40
29	1,06	5,25	7,67
30	1,15	5,59	7,95
31	1,24	5,95	8,24
32	1,34	6,34	8,55
33	1,44	6,76	8,88
34	1,55	7,22	9,22
35	1,68	7,71	9,58
36	1,81	8,24	9,97
37	1,95	8,81	10,37
38	2,11	9,44	10,80
39	2,28	10,11	11,25
40	2,46	10,85	11,73
41	2,66	11,64	12,24
42	2,88	12,51	12,79
43	3,12	13,46	13,37
44	3,38	14,50	13,98
45	3,66	15,64	14,64

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 base of the foundation of the outer wall of a basementless two-story building 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 gravity γ’ = 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 get: 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.

The design resistance of a foundation made of non-rocky soils to axial compression is determined by the formula

Where - conditional soil resistance, kPa;

,
- coefficients accepted according to Table 11;

- width (smaller side or diameter) of the foundation base, m;

- foundation depth, m;

- calculated value of the specific gravity of the soil averaged over layers,

located above the base of the foundation, calculated without taking into account

suspended action of water;

allowed to accept =19.62 kN/m3.

When determining the design resistance, the depth of the foundation should be taken for intermediate bridge supports - from the soil surface at the support at the cutting level within the foundation contour, and in riverbeds - from the bottom of the watercourse at the support after lowering its level to the depth of the general and half of the local erosion of the soil during estimated flow rate. Design resistances calculated using formula (24) for clays and loams in the foundations of bridges located within permanent watercourses should be increased by an amount equal to 14.7
, kPa,
- water depth from lowest level low water to the bottom of the watercourse

Values ​​of conditional soil resistances are determined according to SNiP 2.05.03-84 (Tables 9, 10) depending on the type, type and variety for sandy soils and the type, value of the porosity coefficient e and turnover rate for silty-clayey soils. For intermediate values e And quantities determined by interpolation. At plasticity number values within 5-10 and 15-20 average values ​​should be taken , given respectively for sandy loam, loam and clay. For dense sands should be increased by 60% if their density is determined based on the results of laboratory tests of soils. For loose sandy soils and silty clay soils in a fluid state ( > 1) or with porosity coefficient e > e max (where e max – maximum tabulated value of the porosity coefficient for a given soil type) conditional resistance not standardized. These soils are considered weak soils, which cannot be used as a natural foundation without special measures.

Table 1.3.1. – Extract from table 1 appendix 24 SNiP 2.05.03-84

	Coefficient porosity e	Conditional resistance R 0, silty-clayey (non-subsidence) foundation soils, kPa depending on the fluidity index
Arrogance at ≤5
Loams at 10 ≤ ≤ 15
Clays at ≥20

Table 1.3.2. – Extract from table 2 appendix 24 SNiP 2.05.03-84

Sandy soils and their moisture content	Conditional resistance R 0 sandy soils of medium density at the base, kPa
Gravelly and large regardless of their moisture content
Medium size: low moisture wet and saturated with water
Small: low moisture wet and saturated with water
Dusty: low-moisture saturated with water

Table 1.3.3. – Extract from table 4 appendix 24 SNiP 2.05.03-84

	Odds
	, m -1	, m -1
1. Gravel, pebbles, gravelly sand, coarse and medium-sized
2. Fine sand
3. Sandy sand, sandy loam
4. Loam and clay: hard and semi-hard
5. Loam and clay: hard-plastic and soft-plastic

Example 1.3.1. Determine the design resistance to axial compression of a foundation made of low-moisture medium-sized sand under the base of a shallow foundation for an intermediate support of a road bridge, if given: foundation width
its depth
the calculated value of the specific gravity of the soil located above the base of the foundation, averaged over layers, =19.6 kN/m3.

Solution. For low-moisture sand of medium size according to table. 1.3.2 we find R 0 =294 kPa, and according to Table 1.3.3 - coefficient values =0.10 m -1 and
=3.0 m -1 .

The calculated resistance of the soil foundation is determined by the formula

Example 1.3.2. Determine the design resistance to axial compression of a base made of refractory loam under the base of the foundation from the sinkhole of the intermediate support of a road bridge located in a permanent watercourse, if given: width of the foundation
its depth
loam fluidity index
plasticity number =0.12, porosity coefficient =0.55, calculated value of the specific gravity of the soil located above the base of the foundation, averaged over layers, = 19.6 kN/m 3, water depth from the lowest low-water level =5 m.

Solution. From the table 1.3.2 by interpolation we find the conditional resistance refractory loam with
And =0,55.

From Table 1.3.3 – coefficient values =0.02 m -1 and
=1.5 m -1.

Taking into account the loading of the loam layer with water, the calculated resistance of the soil foundation will be determined by the formula

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 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

If d 1 > d(Where d- foundation depth), then d 1 is taken equal d, a d b = 0.

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, then it is accepted b= . The calculated values ​​of the specific gravities of soil and basement floor material included in formula (5.29) can 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 cutouts, the calculated resistance of the foundation soil can be increased by 15%.

TABLE 5.11. VALUES OF COEFFICIENTS γ With 1 and γ With 2

Soils	γ With 1	γ With 2 for structures with a rigid structural design when the ratio of the length of the structure or its compartment to its height L/H
		≥ 4	< 1,5
Coarse clastic with sand filler and sandy, except for small and dusty Sands are fine Dusty sands: low moisture and damp saturated with water Coarse-clastic with silty-clayey filler and silt-clay with soil or filler fluidity index: I L ≤ 0,25 0,25 < I L ≤ 0,5 I L > 0,5	1,4 1,3 1,25 1,2 1,1	1,2 1,1 1,0 1,0 1,0	1,4 1,3 1,1 1,1 1,0

Notes: 1. Rigid structural schemes are structures whose structures are adapted to absorb forces from foundation deformations through the use of special measures.

2. For structures with a flexible structural design, the value of the coefficient γ c 2 is taken equal to one.

3. For intermediate values L/H coefficient γ c 2 is determined by interpolation.

TABLE 5.12. COEFFICIENT VALUES M γ , M q , M c

φ II,°	M γ	Mq	M c	φ II,°	M γ	Mq	M c
0	0	0	3,14	23	0,69	3,65	6,24
1	0,01	0,06	3,23	24	0,72	3,87	6,45
2	0,03	1,12	3,32	25	0,78	4,11	6,67
3	0,04	1,18	3,41	26	0,84	4,37	6,90
4	0,06	1,25	3,51	27	0,91	4,64	7,14
5	0,08	1,32	3,61	28	0,98	4,93	7,40
6	0,10	1,39	3,71	29	1,06	5,25	7,67
7	0,12	1,47	3,82	30	1,15	6,59	7,95
8	0,14	1,55	3,93	31	1,24	5,95	8,24
9	0,16	1,64	4,05	32	1,34	6,34	8,55
10	0,18	1,73	4,17	33	1,44	6,76	8,88
11	0,21	1,83	4,29	34	1,55	7,22	9,22
12	0,23	1,94	4,42	35	1,68	7,71	9,58
13	0,26	2,05	4,55	36	1,81	8,24	9,97
14	0,29	2,17	4,69	37	1,95	8,81	10,37
15	0,32	2,30	4,84	38	2,11	9,44	10,80
16	0,36	2,43	4,99	39	2,28	10,11	11,25
17	0,39	2,57	5,15	40	2,46	10,85	11,73
18	0,43	2,73	5,31	41	2,66	11,64	12,24
19	0,47	2,89	5,48	42	2,88	12,51	12,79
20	0,51	3,06	5,66	43	3,12	13,46	13,37
21	0,56	3,24	5,84	44	3,38	14,50	13,98
22	0,61	3,44	6,04	45	3,66	15,64	14,64

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 M c, 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; M c= 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 L/H= 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 strength and deformation characteristics are taken 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 hard building design diagram at L/H= 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 silty-clayey soils with intermediate values I L 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 I L= 0 and I L = 1; R 0 (2, 0) and R 0 (2, 1) - the same, with e 2 .

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

Soils	R 0 , kPa
Coarse clastic
Pebble (crushed stone) with filler: sandy silty-clayey Gravel (wood) with filler: sandy silty-clayey	600 450/400 500 400/350
Values R 0 for turnover rate I L≤ 0.5 are given before the line, at 0.5< I L≤ 0.75 - beyond the line.
Sands
Large Medium size Small: low moisture wet and saturated with water Dusty: low moisture wet saturated with water	600/600 500/400 400/300 300/200 300/250 200/150 160/100
Values R 0 for dense sands are given before the line, for medium-density sands - behind the line.
Silty-clayey
Sandy loam with porosity coefficient e : 0,5 0,7 Loams with porosity coefficient e : 0,5 0,7 1,0 Clays with porosity coefficient e : 0,5 0,6 0,8 1,0	300/300 250/200 300/250 250/180 200/100 600/400 500/300 300/200 250/100
Values R 0 at I L= 0 are given before the line, with I L= 1 - beyond the line. At intermediate values e And I L values R 0 are determined by interpolation.

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 I L= 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 LOSS) AND silty-clayey soils

Notes: 1. For intermediate values e And I L 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 ≤ R z,

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); R z— 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 in corner point — 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).