Establishing the bearing capacity of the soil (table values) under the designed or reconstructed foundation begins with geological exploration. For this on construction site soil samples are taken from wells or pits and examined.
First, the soil is classified. The composition of the soil is determined by the granulometric and / or elutriation method and its name is determined.
The physical characteristics of the soil are then examined. The soil density is determined by the cutting ring method, the moisture content is determined by the drying and weighing method, and the soil consistency is determined by twisting the soil into a bundle and testing with a balancing cone.
Further, additional laboratory studies of the soil are made or several more calculations are made that expand the number of physical characteristics of soils.
If it is impossible to accurately determine the type of soil on your own, if there are 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.
A building or structure must be assigned to one of the following levels of responsibility: increased, normal and reduced (Article 4, paragraphs 7–10 of the current technical regulation on the safety of buildings and structures federal law No. 384-FZ) .
To increased The level of responsibility includes: especially dangerous, technically complex or unique objects.
To lowered - buildings and structures for temporary (seasonal) purposes, as well as buildings and structures for auxiliary use associated with the construction or reconstruction or located on land plots provided for individual housing construction.
All other buildings and structures are normal the level of responsibility.
The wording of the identification of buildings belonging to the third (lowered) level of responsibility is vague. It is unclear whether two groups of buildings and structures are described: temporary and auxiliary, or three groups - temporary, auxiliary and individual? In Belarus residential individual houses no more than 2 floors high are referred to the third group of responsibility, and in Russia residential buildings up to 10 m high used to be also referred to this group. There is no clarity on this issue in the new technical regulation. It seems that everyone will have to decide on their own. The scope depends on the choice of the level of responsibility. geological surveys and methodology for calculating foundations.
This method is used for the preliminary and final calculation of the grounds for buildings of the third level of responsibility in favorable conditions. Or for a preliminary calculation of the grounds for buildings of the second level of responsibility located in any, including unfavorable engineering and geological conditions.
“Favorable” conditions are those under which the layers of soil in the foundation lie horizontally (the slope of the layers does not exceed 0.1), and the compressibility of the soil does not increase at least to a depth equal to twice the width of the largest single foundation and four widths of the strip (counting from the level his soles).
For foundations with a width b o = 1 m and a laying depth d o = 2 m, the values of the design resistance of the base (R o) are given in tables 11-15. With an increase or decrease in the depth of the foundation, the bearing capacity of the base soil changes. In this case design resistances bases (R) at various depths should be determined by the formulas:
R \u003d R o (d + d o) / 2d o at d< 2 м;
R \u003d R o + k 2 γ "(d - d o) for d\u003e 2m
where b is the width of the foundation, m; d is the depth of the sole, m; γ'- calculated value of the specific gravity of the soil lying above the base of the foundation, kN / m³; k 1 - coefficient taken 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 taken for bases composed of coarse sandy soils - k 2 \u003d 0.25, composed of sandy loam and loam -k 2 \u003d 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 unwetted subsiding soils with a degree of moisture S r ≤ 0.5; in the denominator - R o values related to the same soils with S r ≥ 0.8, as well as to wet soils.Table 15
| Embankment characteristics | R o , kPa (kg/cm²) | |||
|---|---|---|---|---|
| Sands are coarse, medium and fine, slag, etc. at the degree of humidity S r | Silty sands, sandy loam, loam, clay, ash, etc. at the degree of humidity S r | |||
| S r ≤ 0.5 | S r ≥ 0.8 | S r ≤ 0.5 | S r ≥ 0.8 | |
| Embankments, systematically erected with compaction | 250 (2,5) | 200 (2,0) | 180 (1,8) | 150 (1,5) |
| Soil dumps and production wastes: with seal without seal |
250 (2,5) |
200 (2,0) |
180 (1,8) |
150 (1,5) |
| Soil dumps and production wastes: with seal without seal |
150 (1,5) |
120 (1,2) |
120 (1,2) |
100 (1,0) |
The design soil resistance of the base R o is such a safe pressure at which the linear dependence of the settlement of foundations is maintained, and the depth of development of zones of local strength failure under its edges does not exceed the size of 1/4 of the width of the base of the foundation.
Determine the design resistance of the base of the foundation, having sole dimensions of 2.5 × 2.5 m, laying depth of 1 m; basement building, III class. The base for the entire explored depth is composed of sand of medium size, medium compaction (γ' = 20 kN/m³). Groundwater has not been found. To determine the design resistance of the base, it is legitimate to use the tabular values of R o . According to Table. 2 R o = 400 kPa. According to the formula, we get: R \u003d R o (d + d o) / 2d o \u003d 400 (1 + 2) / 2 × 2 \u003d 356 kPa.
This method is used for the final calculation of the bases for buildings of the second level of responsibility.
The design soil resistance of the base is determined by the formula:
R \u003d (m 1 m 2 / k) ,
where m 1 and m 2 - coefficients of working conditions, taken according to table. 16; k - coefficient, k = 1, if the characteristics of soil properties are determined empirically, k = 1.1, if the characteristics are taken from reference tables; M 1 , M 2 , M 3 - coefficients taken according to the table. 17; k z - coefficient, at b< 10 м - k z =1 при b >10 m - k z \u003d z / b + 0.2 (here z \u003d 8 m); b - width of the base of the foundation, 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 sole; c is the calculated value of the specific cohesion 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 over 2 m deep, d b = 2 m is taken, with a basement width of more than 20, d b = 0; d 1 - the depth of the foundation of the basement-free structures from the planning level (m) or the reduced depth of the foundation from the level of the basement floor, determined by the formula: d 1 \u003d h s + h cf γ cf / γ', here h s is the thickness of the soil layer above the base of the foundation under the basement: h cf - basement floor thickness; γ cf - calculated value of the specific gravity of the basement floor material, kN/m³.
Table 16
| soils | Coefficient m 1 | The coefficient m 2 for structures with a rigid structural scheme 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-clastic with sandy filler and sandy, except for fine and silty | 1,4 | 1,2 | 1.4 |
| The sands are fine | 1,3 | 1,1 | 1,3 |
| Sands are silty, low-moisture and wet | 1,25 | 1,0 | 1,2 |
| Sands saturated with water | 1,1 | 1,0 | 1,2 |
| Dusty-clayey, as well as coarse-grained with silty-clayey filler with a soil or aggregate flow 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 |
1. Structures with a rigid structural scheme include structures whose structures are specially adapted to the perception of forces from the deformation of the bases (subsection 5.9 of SP 22.13330.2011).
2. For buildings with a flexible design scheme, 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
| Angle of internal friction, φ, deg | Odds | ||
|---|---|---|---|
| M1 | M2 | M3 | |
| 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 |
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 tape, its dimensions: width b = 1.0 m; laying depth d 1 \u003d 1.8 m, d b \u003d 0.
Characteristics of soil properties are determined in the laboratory; the number of determinations made it possible to perform statistical data processing. From the surface to the level of the base of the foundation, bulk soil lies, its Specific gravityγ' = 17 kN/m³. Under the base of the foundation to the entire explored depth (9 m) soft-plastic loam (I L = 0.6). Calculated values: specific gravity γ = 20 kN/m³, internal friction angle 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 \u003d 1.0 (the L / H ratio of the building is more than 4).
The coefficient to z = 1, since the width of the foundation b< 10 м.
For the given data, we get: R \u003d (m 1 m 2 / k) \u003d (1.1 × 1 / 1) [(0.32 × 1 × 1.0 × 20) + (2.30 × 1.8 × 17 ) + (4.84 × 30) ] = 244 kPa.
The calculated electrical resistivity of the soil (Ohm * m) is a parameter that determines the level of "electrical conductivity" of the earth as a conductor, that is, how well the electric current from the ground electrode will spread in such an environment.
This is a measurable value that depends on the composition of the soil, dimensions and density.
adhering to each other of its particles, humidity and temperature, the concentration of soluble chemicals in it (salts, acid and alkaline residues).
| Priming | Resistivity, average value (Ohm*m) | ZZ-000-015 Ohm | Earth resistance for kit ZZ-000-030 Ohm | Earth resistance for kit ZZ-100-102 Ohm |
| Asphalt | 200 - 3 200 | 17 - 277 | 9,4 - 151 | 8,3 - 132 |
| Basalt | 2 000 | |||
| Bentonite (clay grade) | 2 - 10 | 0,17 - 0,87 | 0,09 - 0,47 | 0,08 - 0,41 |
| Concrete | 40 - 1 000 | 3,5 - 87 | 2 - 47 | 1,5 - 41 |
| Water | ||||
| sea water | 0,2 | 0 | 0 | 0 |
| pond water | 40 | 3,5 | 2 | 1,7 |
| Plain river water | 50 | 4 | 2,5 | 2 |
| ground water | 20 - 60 | 1,7 - 5 | 1 - 3 | 1 - 2,5 |
| Permafrost (permafrost) | ||||
| Permafrost - thawed layer (near the surface in summer) | 500 - 1000 | - | - | 20 - 41 |
| Permafrost soil (loam) | 20 000 | Special measures are required (replacement of soil) | ||
| Permafrost (sand) | 50 000 | Special measures are required (replacement of soil) | ||
| Clay | ||||
| Clay wet | 20 | 1,7 | 1 | 0,8 |
| Clay semi-hard | 60 | 5 | 3 | 2,5 |
| Decayed gneiss | 275 | 24 | 12 | 11,5 |
| Gravel | ||||
| Clay gravel, heterogeneous | 300 | 26 | 14 | 12,5 |
| gravel homogeneous | 800 | 69 | 38 | 33 |
| Granite | 1 100 - 22 000 | Special measures are required (replacement of soil) | ||
| granite gravel | 14 500 | Special measures are required (replacement of soil) | ||
| gra f itite chips | 0,1 - 2 | 0 | 0 | 0 |
| Grass (fine gravel/coarse sand) | 5 500 | 477 | 260 | 228 |
| Ash, ashes | 40 | 3,5 | 2 | 1,7 |
| Limestone (surface) | 100 - 10 000 | 8,7 - 868 | 4,7 - 472 | 4,1 - 414 |
| Limestone (inside) | 5 - 4 000 | 0,43 - 347 | 0,24 - 189 | 0,21 - 166 |
| Il | 30 | 2,6 | 1,5 | 1 |
| Coal | 150 | 13 | 7 | 6 |
| Quartz | 15 000 | Special measures are required (replacement of soil) | ||
| Coke | 2,5 | 0,2 | 0,1 | 0,1 |
| Loess (yellow soil) | 250 | 22 | 12 | 10 |
| Chalk | 60 | 5 | 3 | 2,5 |
| Marl | ||||
| Marl ordinary | 150 | 14 | 7 | 6 |
| Clay marl (50 - 75% clay particles) | 50 | 4 | 2 | 2 |
| Sand | ||||
| Sand heavily moistened with groundwater | 10 - 60 | 0,9 - 5 | 0,5 - 3 | 0,4 - 2,5 |
| Sand, moderately wet | 60 - 130 | 5 - 11 | 3 - 6 | 2,5 - 5,5 |
| The sand is wet | 130 - 400 | 10 - 35 | 6 - 19 | 5 - 17 |
| The sand is slightly damp | 400 - 1 500 | 35 - 130 | 19 - 71 | 17 - 62 |
| Sand dry | 1 500 - 4 200 | 130 - 364 | 71 - 198 | 62 - 174 |
| Sandy loam (sandy loam) | 150 | 13 | 7 | 6 |
| Sandstone | 1 000 | 87 | 47 | 41 |
| Garden land | 40 | 3,5 | 2 | 1,7 |
| Saline | 20 | 1,7 | 1 | 0,8 |
| Loam | ||||
| Loam heavily moistened by groundwater | 10 - 60 | 0,9 - 5 | 0,5 - 3 | 0,4 - 2,5 |
| Loam semi-solid, forest-like | 100 | 9 | 5 | 4 |
| Loam at a temperature of minus 5 C ° | 150 | - | - | 6 |
| Sandy loam (sandy loam) | 150 | 13 | 7 | 6 |
| Slate | 10 - 100 | |||
| Slate gra f itite | 55 | 5 | 2,5 | 2,3 |
| Sandy loam (sandy loam) | 150 | 13 | 7 | 6 |
| Peat | ||||
| Peat at 10° | 25 | 2 | 1 | 1 |
| Peat at 0 С° | 50 | 4 | 2,5 | 2 |
| Chernozem | 60 | 5 | 3 | 2,5 |
| rubble | ||||
| Wet rubble | 3 000 | 260 | 142 | 124 |
| Crushed stone dry | 5 000 | 434 | 236 | 207 |
Ground resistance for kits ZZ-000-015 and ZZ-000-030, indicated in the table can be used
with various configurations of the earth electrode - both point and multi-electrode.
Together with a table of approximate values of the calculated resistivity of the soil, we offer you
use the geographic map of previously installed grounding switches based on ready-made ZANDZ grounding kits
with the results of measurements of ground resistance.
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Page 33 of 34
CALCULATED RESISTANCE OF BASE SOILS
1. Calculated soil resistance of the base R 0 given in Table. 1-5 are intended for preliminary sizing of foundations. Scope of values R 0 and R/ 0 for the final determination of the dimensions of the foundations is indicated in paragraph 2.42 for Table. 4, in clause 8.4 for table. 5 and in paragraph 11.5 for table. 6.
2. For soils with intermediate values e and I L(Table 1-3), p d and S r(Table 4), S r(Table 5), as well as for foundations with intermediate values g(Table 6) values R 0 and R/ 0 are determined by interpolation.
3. Values R 0 (Tables 1-5) refer to foundations having a width b 0 = 1 m and laying depth d 0 = 2 m.
When using values R 0 for the final assignment of the dimensions of the foundations (clauses 2.42, 3.10 and 8.4) design soil resistance of the base R, kPa (kgf / cm 2), is determined by the formulas:
at d£2m (200cm)
R= R 0x( d + d 0)/2d 0 ; (1)
at d> 2 m (200 cm)
R= R 0 + k 2 g /II( d-d 0), (2)
Where b and d- respectively, the width and depth of the designed foundation, m (cm);
g / II - the calculated value of the specific gravity of the soil located above the base of the foundation, kN / m 3 (kgf / cm 3);
k 1 - coefficient taken for foundations composed of coarse and sandy soils, except for silty sands, k 1 \u003d 0.125, silty sands, sandy loams, loams and clays k 1 = 0,05;
k 2 - coefficient taken for foundations composed of coarse and sandy soils, k 2 = 0.25, sandy loam and loam k 2 = 0.2 and clays k 2 = 0,15.
Note. For structures with a basement width AT= 20 m and depth db³ 2 m, the depth of laying of external and internal foundations taken into account in the calculation is taken equal to: d = d 1 + 2 m [here d 1 - the reduced depth of the foundation, determined by the formula (8) of these standards]. At AT>20m accepted d=d 1 .
Table 1
Design resistances R 0 coarse soils
| Coarse clastic soils | Meaning R O, kPa (kgf / cm 2) |
| Pebble (gravel) with filler: | |
| Sandy | |
| I L£0.5 | |
| 0,5 < I L£0.75 | |
| Gravel (grid) with filler: | |
| sandy | |
| dusty-clay with a fluidity index: | |
| I L£0.5 | |
| 0,5 < I L£0.75 |
table 2
Design resistances R 0 sandy soils
| Values R O, kPa (kgf / cm 2), depending on the density of the sands |
||
| medium size | ||
| low humidity | ||
| wet and saturated with water | ||
| Dusty: | ||
| low humidity | ||
| saturated with water | ||
Table 3
Design resistances R 0 silty clay (non-subsidence) soils
| Dusty clayey | Coefficient Porosity e | Values R O, kPa (kgf / cm 2), with an indicator of soil fluidity |
|
| loams | |||
Table 4
Design resistances R 0 subsiding soils
| R O, kPa (kgf / cm 2), soil |
||||
| Natural build with dry density p d, t/m 3 | compacted with dry density p d, t/m 3 |
|||
| 300 (3) | 350 (3,5) | |||
| loams | 350 (3,5) | 400 (4) | ||
Note: The numerator shows the values R O , relating to unwetted subsiding soils with a degree of moisture S r£0.5; in the denominator - values R O related to the same soils with S r³ 0.8, as well as to soaked soils.
Table 5
Design resistances R 0 bulk soils
| R O, kPa (kgf / cm 2) |
||||
| Characteristics | Sands are coarse, medium and fine, slag, etc. at the degree of humidity S r | Silty sands, sandy loam, loam, clay, ash, etc. at the degree of humidity S r |
||
| S r£0.5 | S r³ 0.8 | S r£0.5 | S r³ 0.8 |
|
| Embankments, systematically erected with compaction | ||||
| Soil dumps and production wastes: | ||||
| with seal | ||||
| without seal | ||||
| Soil and industrial waste dumps: | ||||
| with seal | ||||
| without seal | ||||
Note: 1. Values R O in this table refer to bulk soils containing organic matter I om£0.1.
2. For non-compacted dumps and dumps of soils and industrial wastes, the values R O are accepted with a coefficient of 0.8.
Table 6
Design resistance of backfill soils R 0
for pull-out foundations
overhead power lines
| Values, kPa (kgf / cm 2) |
||||
| Relative foundation depth l = d/b | Dusty clay soils with a fluidity index I L£ 0.5 and backfill soil density, t / m 3 | Sands of medium size and small, low-moisture and wet at the density of the backfill soil, t / m 3 |
||
Notes: 1. Values R O for clays and loams with a flow index of £0.5 I L£0.75 and sandy loam at 0.5< I L£ 1.0 are accepted according to the column "silty clay soils" with the introduction of reducing factors of 0.85 and 0.7, respectively.
2. Values R O for silty sands are taken as for medium-sized and fine sands with a coefficient of 0.85.
| Content |
|---|
The "load-settlement" dependence for shallow foundations can be considered linear only up to a certain pressure limit on the foundation (Fig. 5.22). As such a limit, the design soil resistance of the base is taken R. When calculating the deformations of the base using the design schemes specified in clause 5.5.1, the average pressure under the base of the foundation (from loads for calculating the bases by deformations) should not exceed the design soil resistance of the base 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\u003d 1.1, if the indicated characteristics are taken according to the tables given in Ch. one; M γ , M q and M c- coefficients taken according to table. 5.12; kz- coefficient accepted: kz= 1 at b < 10 м, k z = z 0 /b + 0,2 at b≥ 10 m (here b— width of the base of the foundation, m; z 0 = 8 m); γ II - the calculated 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 / m 3; γ´ II - the same, lying above the sole; With II - the calculated value of the specific cohesion of the soil lying directly under the base of the foundation, kPa; d 1 - the depth of laying the foundations of non-basement structures or the reduced depth of laying the external and internal foundations from the basement floor, "determined by the formula
d 1 = h s + h cf γ cf /γ´ II
(here hs- the thickness of the soil layer above the base of the foundation from the side of the basement, m; hcf- thickness of the basement floor structure, m; γ cf- the calculated value of the specific gravity of the basement floor material, kN / m 3); db- basement depth - the distance from the planning level to the basement floor, m (for structures with a basement width AT≤ 20m and depth over 2m is accepted db= 2 m, with a tan width AT> 20 and accepted d > 0).
Rice. 5.22. Characteristic load-settlement dependence for shallow foundations
If a d 1 > d(where d- the depth of the foundation), then d 1 is taken equal d, a db = 0.
Formula (5.29) is applied for any form of foundations in the plan. If the base of the foundation has the shape of a circle or a regular polygon with an area BUT, then it is accepted b= . Estimated values specific gravity soil and basement floor material included in formula (5.29) may be taken equal to their normative values (assuming the reliability coefficients for soil and material equal to one). The design resistance of the soil, with appropriate justification, can be increased if the design of the foundation improves the conditions for its joint work with the foundation. For foundation slabs with corner cuts, the design soil resistance of the base 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 scheme with a 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 The sands are fine Dusty sands: damp and wet saturated with water Coarse-clastic with silty-argillaceous filler and dusty clay with the index of soil or aggregate fluidity: I L ≤ 0,25 0,25 < I L ≤ 0,5 I L > 0,5 |
1,4 1,3 1,25 |
1,2 1,1 1,0 |
1,4 1,3 1,1 |
Notes: 1. Structures whose structures are adapted to the perception of forces from deformations of the bases through the use of special measures have a rigid structural scheme.
2. For structures with a flexible design scheme, the value of the coefficient γ c 2 is taken equal to one.
3. At 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 ,° | Mγ | M q | Mc | φ II ,° | Mγ | M q | Mc |
| 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 design depth of foundations is taken from the grade level by the bedding, the design of foundations and foundations shall require that a graded embankment be completed before the full load is applied to the foundation. A similar requirement should be contained in relation to the installation of bedding under the floors in the basement.
Odds M γ , M q and Mc, included in formula (5.29), are obtained based on the condition that the plastic deformation zones under the edges of a uniformly loaded strip (Fig. 5.23) are equal to a quarter of its width and are calculated using the following relationships:
Mγ= ψ/4; M q= 1 + ψ; Mc= ψctgφ II ,
where ψ = π/(ctgφ II + φ II - π/2); φ II is the calculated value of the angle of internal friction, rad.
Rice. 5.23.
When calculating R values of characteristics φ II , With II and γ II are taken for the soil layer under the base of the foundation to a depth z R = 0,5b at b < 10 м и z R = t + 0,1b at b≥ 10 m (here t= 4 m). In the presence of several layers of soil from the base of the foundation to the depth z R accepted weighted averages the specified characteristics. The same is done with 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 base soils, but also on the desired geometric dimensions of the foundation - the width and depth of its laying. Therefore, the determination of the dimensions of the foundations has to be carried out in an iterative way, having previously given some initial dimensions.
Example 5.5. Determine the design soil resistance of the base for strip foundation width b= 1.4 m with the following initial data. The building being designed is a 9-storey large-panel building with a rigid structural scheme. The ratio of its length to its height L/H= 1.5. The depth of foundations from the level of planning for structural reasons is accepted d= 1.7 m. The building has a basement with a width AT= 12 m and deep db= 1.2 m. The thickness of the soil layer from the base of the foundation to the basement floor hs= 0.3 m, basement concrete floor thickness h cf\u003d 0.2 m, specific gravity of concrete γ II \u003d 23 kN / m 3. The site is composed of fine, medium-density, low-moisture sands. Porosity coefficient e= 0.74, the 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 from the reference tables given in Ch. 1: φ n= φ II = 32º, with n = c II = 2 kPa, E= 28 MPa.
Solution. To calculate the design soil resistance of the base according to the formula (5.29), we accept: according to table. 5.11 for fine, low-moisture sand and a building of a rigid structural scheme with L/H= 1.5, γ With 1 = 1.3 and γ With 2 = 1.3; according to the table 5.12 at φ II = 32º Mγ = 1,34; M q= 6.34 and M c= 8.55. Since the values of the strength characteristics of the soil are taken from the reference tables, k= 1.1. At b= 1.4 m< 10 м kz = 1.
The reduced depth of the foundation from the basement floor according to the formula (5.30)
d 1 \u003d 0.3 + 0.2 23/17 \u003d 0.57 m.
According to the formula (5.29) we determine:
R= = 1.54 221 = 340 kPa.
The preliminary dimensions of the 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 assignment of the dimensions of the foundations of class III structures, if the base is composed of horizontal (slope not more than 0.1) layers of soil sustained in thickness, the compressibility of which does not increase with depth within the double width of the largest foundation below the depth of its laying.
Double interpolation when defining R 0 according to the table. 5.13 for silt-clay soils with intermediate values I L and e it is recommended to follow the formula
Guidelines for the design of foundations for 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 is the value of e for the soil under consideration; R 0 (1, 0) and R 0 (1, 1) - values R 0 in the 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. DESIGNED RESISTANCES R 0 LARGE-CLASTIC, SANDY AND DUTY-CLAY (NON-SETTLEMENT) SOILS
| soils | R 0 , kPa |
| coarse clastic | |
| Pebble (crushed stone) with aggregate: sandy silty clay Gravel (grid) with aggregate: sandy silty clay |
600 450/400 500 |
| Values R 0 at flow rate I L≤ 0.5 are given before the line, at 0.5< I L≤ 0.75 - below the line. | |
| Sands | |
| Large medium size Small: low-moisture wet and saturated with water Dusty: low-moisture wet saturated with water |
600/600 500/400 400/300 300/250 |
| Values R 0 for dense sands are given before the line, for medium-density sands - below the line. | |
| Dusty clayey | |
| Sandy loam with porosity coefficient e
: 0,5 0,7 Loams with porosity coefficient e : 0,5 0,7 1,0 Clays with a porosity coefficient e : 0,5 0,6 0,8 1,0 |
300/300 250/200 300/250 600/400 |
| 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 the table. 5.13 apply to foundations having a width b 1 = 1 m and laying depth d 1 = 2 m. When using the values R 0 according to the table. 5.13 for the final assignment of the dimensions of the foundations, the design soil resistance of the base 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 a coefficient of porosity e= 0.85 and flow index I L= 0.45 in relation to the foundation width b= 2 m, having a laying depth d\u003d 2.5 m. The specific gravity of the soil located above the sole, γ´ \u003d 17 kN / m 3.
Solution. Using the values R 0 (see Table 5.13), by formula (5.32) we calculate:
Design resistance R foundation, composed of coarse-grained soils, is calculated by 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 content of aggregate, the value R for coarse soils it is allowed to take according to Table. 5.13.
In case of artificial compaction of base soils or arrangement of soil cushions, the design resistance is determined based on the design values of the physical and mechanical characteristics of the compacted soils specified in the project. The latter are established either on the basis of studies or with the help of reference tables (see Chapter 1) based on the required soil density. When calculating R the humidity of silty clay soils is recommended to be taken equal to 1.2 ω p .
The design resistance of loose sands is determined by the formula (5.29) with γ c 1 = γ With 2 = 1. Meaning R should be clarified based on the results of at least three tests of a stamp with dimensions and shape that are possibly closer to the designed foundation, but with an area of at least 0.5 m 2. At the same time, the value R no more than the pressure at which the expected settlement of the foundation is equal to the limit is taken (see further paragraph 5.5.5).
When constructing discontinuous foundations, the design resistance of the foundation R is determined as for the initial strip foundation according to the formula (5.29) with an increase in the value R coefficient k d taken according to Table. 5.14.
If it is necessary to increase the loads on the foundation of existing structures during their reconstruction (replacement of equipment, superstructure, etc.), the design resistance of the foundation should be taken in accordance with the data on the state and physical and mechanical properties of the foundation soils, taking into account the type and condition of the foundations and above-foundation structures of the structure , the duration of its operation and the expected additional sediment with increasing loads on the foundations. It should also take into account the condition and design features of adjacent structures, which, once within the "sedimentary funnel", may be damaged.
TABLE 5.14. COEFFICIENT VALUES k d FOR SANDS (EXCEPT LOOSE) AND SILTY-CLAY SOILS
Notes: 1. At intermediate values e and I L coefficient k d taken by interpolation.
2. For slabs with corner cutouts, 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 lying above (Fig. 5.24), it is necessary to check compliance with the condition
σ zp + σ zg ≤ Rz,
where σ zp and σ zg- vertical normal stresses in the soil at a 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- design resistance of soil of reduced strength at depth z, kPa, calculated by formula (5.29) for a conditional foundation with a width bz, m, determined by the expression
;
When an eccentric load acts on the foundation, the edge pressures under the sole should be limited, which are calculated using the eccentric compression formulas. Edge pressures under the action of a moment in the direction of the main axes of the base of the foundation 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 pressure of the soil located above the base of the foundation, as well as the rigidity of the structure resting on the foundation in question.
The current standards allow an increase of up to 20% of the design soil resistance of the base, calculated by formulas (5.29), (5.33) and (5.34), if determined by the calculation of the deformation of the base under pressure p = R do not exceed 40% of the limit values (see further section 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 limit. In this case, in addition, it is required to check the base for bearing capacity (see further paragraph 5.6).
The possibility of applying the solutions of the theory of elasticity in the calculation of vertical deformations was substantiated by N.M. Gersevanov. However, this approach is valid within the limits of such 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 the underutilization of the bearing capacity of soils, especially sandy, as well as clay (solid, semi-solid and stiff-plastic 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 design soil resistance of the base R, which makes it possible to calculate foundation settlements by a linear relationship between stresses and strains. Thus, when calculating the bases by deformations, it is necessary that the condition is satisfied
P ≤ R, (8.37)
where R- average pressure along the base of the foundation; R- design soil resistance of the base.
where γ c1 and γ c2- coefficients of working conditions, respectively, of the soil base and structure in interaction with the base, taken according to tab. 8.3; k- reliability factor taken in determining the strength characteristics of the soil by direct tests, k= 1.0; when using tabular design values of soils k = 1,1; kz- coefficient taken equal to the width of the base of the foundation b≤10m, kz= 1.0; at b≥10m - kz= Z0/b + 0.2 (here Z0= 8 m); M γ ; M q , M s- coefficients depending on the angle of internal friction of the bearing layer of soil; b- width of the base of the foundation, m;
Table 8.3. Values of coefficients of working conditions γ с1 and γ c2
| soils | γ с1 | γ c2 for buildings with a rigid structural scheme with a ratio of the length of the structure (compartment) to its height L / H, equal to |
|
| 4 or more | 1.5 or less | ||
|
Coarse clastic with sandy |
1,25 |
1,2 1,1 1,0
|
1,4 1,1 |
| Notes. 1. Structures of structures with a rigid structural scheme are adapted to the perception of forces from deformations of the bases. 2. For flexible buildings γ c2 is taken equal to 1. 3. For intermediate values of L/H, the coefficient γ c2 determined by interpolation. |
|||
Table 8.4. Coefficient values M γ , M q , M s
| φ | Mγ | M q<.SUB> | M s | φ | Mγ | 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); d1- the depth of the foundation from the basement floor; in the absence of a basement floor - from the planned surface, m; db- basement depth, counting from the planning mark, but not more than 2 m (with a basement width B> 20 m, db = 0 is taken); cII- the calculated value of the specific adhesion of the bearing layer of soil, kPa (index II means that the calculation is carried out for the second group of limit states).
Formula (8.38) is based on N.P. Puzyrevsky, which makes it possible to determine the pressure on the base, at which in the array under the edges foundation zones of limiting equilibrium are formed. Nevertheless, formula (8.38) differs in its structure from N.P. Puzyrevskii by 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 ultimate equilibrium under the base of the foundation and the rigidity of the structure under construction.
An additional term introduced into formula (8.38), equal to ( M q- 1), allows you to take into account the effect of household soil surcharge. During the excavation of the pit, the stressed state of the soil is preserved to a certain extent, due to the action of the domestic pressure of the soil. In this case, the limiting 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 excavated pit and its width. Then, with an increase in the 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 the foundation is determined for the bearing layer, where the sole of the foundation lies. Sometimes deep Z under the bearing layer lies a less durable soil ( rice. 8.8), in which plastic deformations can develop. In this case, it is recommended to check the stresses transmitted to the roof of soft soil according to the condition
(8.39)
where σzp- additional vertical stress; σzg- stress from the own weight of the soil; Rz- calculated soil resistance at the depth of the roof of weak soil z.
Rice. 8.8. Conditional foundation scheme
Value Rz is determined by formula (8.38), while the coefficients of working conditions γ с1 and γ c2 and reliability k, as well as M γ , M q , M c found in relation to a layer of weak soil.
Values bz and dz determined for a conditional foundation ABSD resting on soft ground.
In this case, it is assumed that σzp acts on the sole of the conditional foundation ABSD (see fig. 8.8), then the area of its sole
where N- the load transferred to the edge of the foundation.
Knowing the area of the sole of the conditional foundation, you can determine its width by the formula
(8.41)
where a \u003d (l-b) / 2 (l and b- dimensions of the designed foundation).
Having determined by formula (8.38) the value Rz, check the condition (8.39). When it is satisfied, shear zones do not play a significant role in the magnitude of the developing settlement. Otherwise, it is necessary to accept large dimensions of the base of the foundation, under which condition (8.39) is satisfied.
To assign preliminary dimensions of foundations buildings and facilities conditional design soil resistances of the base Ro are used, which are given in tab. 8.5 - 8.8.
Example 8.2. Determine the conditional design resistance of fine sand, if known: natural humidity ω = 0.07; natural density ρ = 1.87 t/m3, density of solid particles ρ S = 2.67 t/m3.
