The economically sound foundation design of the same wooden house will differ significantly from each other depending on the type of foundation soil. Let’s illustrate this with examples and calculate the foundation of the same wooden house, the reconstruction of which is described on our website, on non-heaving, slightly heaving and excessively heaving soils. See respectively the pages of this section Correct foundation, Calculation of the foundation base and the following:
Foundations of low-rise buildings of other types, with the exception of slab ones, can be calculated similarly. Examples of foundation calculations taking into account the rigidity of the building structure are given in the currently valid OSN APK 2.10.01.001-04 “Design of shallow foundations of low-rise rural buildings on heaving soils.”
Foundation loads
The values of the main combination of loads for calculating the foundation base of a reconstructed wooden building in accordance with 5.2.1 with the accepted load safety factors γ f in accordance with , are equal to
F=F 1 -G f,rec =88.12-16.72=71.49 kN.
The load on the foundation from the foundation for calculating foundations and foundations under the influence of forces of frost heaving of soils with the accepted load reliability coefficient γ f = 0.9, according to , is equal to
F m =F 2 -0.9×G f,rec =88.21-0.9×16.72=73.16 kN.
Characteristics of the foundation soil
Let us assume that, based on testing of foundation soil samples, it has been established that at a depth of 0.2-6.0 m there is a layer of yellow-brown clay, which, in accordance with classification [X], is classified as heavy (Table B.16), soft-plastic clay (Table B.19), having the following characteristics:
- soil density ρ= 19.9 kN/m 3,
- density of dry soil ρ= 15.2 kN/m 3,
- natural humidity W=31%,
- humidity at the yield point W L =37,
- humidity at the rolling boundary W p =16%,
- plasticity number I p =21,
- turnover rate I L =0.71,
In accordance with classification [X], the base soil belongs to heavy (Table B.16), soft-plastic clay (Table B.19). underground water at a depth of 1.69 m from the surface.
For the construction site under consideration (Dmitrov), the standard freezing depth is equal to
- where d 0 is the value taken equal to 0.23 m for loams and clays;
- M t - dimensionless coefficient, numerically equal to the sum of the absolute values of average monthly negative temperatures for the year in a given area, adopted according to SP 131.13330
Depth of seasonal soil freezing
The standard depth of seasonal soil freezing d df , m, is taken to be equal to the average of the annual maximum depths of seasonal soil freezing (according to observation data for a period of at least 10 years) on an open horizontal area bare of snow at a groundwater level located below the depth of seasonal soil freezing .(5.5.2 SP 22.13330.2016) The depth of seasonal thawing is determined by the largest vertical distance per year from the ground surface (excluding vegetation cover) to the roof of permafrost. (4.1.1 GOST 26262-2014) seasonal soil freezing df, m, determined by formula (5.4) is:
d f = k h d fn = 1 1.35 = 1.35 m.
For external and internal foundations of unheated buildings k h =1.
Degree of frost heaving of soil
Relative heaving strain ε fh = 0.123, characterizing the degree of frost heaving of the soil, was determined according to Figure 6.11 using the calculated parameter R f = 0.0154 and the fluidity index of the foundation soil I L = 0.71. The parameter Rf was calculated using formula (6.34).
R f = 0.67 1.99 =0.0153
When calculating the parameter Rf, we used the calculated values of the total moisture capacity of the soil W sat = 29.1% and the critical moisture content W cr = 20.5% determined from Fig. 6.12, .
Using the parameter R f = 0.0153 (Fig. 6.11), we determine the degree of frost heaving of the soil ε fh = 0.123. The foundation soil in accordance with Table B.27 [X] refers to overly heaving.
Specific soils, which according to SP 22.13330.2016 include heaving soils, which have a decisive influence on the design decisions of the foundations of wooden houses, have the III (complex) category of complexity of engineering and geological conditions in accordance with Table A.1 SP 47.13330.
When laying foundations above the calculated freezing depth of heaving soils (shallow foundations), according to 6.8.10, it is necessary to carry out calculations based on the frost heaving deformations of the foundation soils, taking into account the tangential and normal forces of frost heaving.
Columnar foundation on a sand cushion
We preliminarily assign the dimensions of the concrete foundation pillar: a×b×h=0.25×0.25×0.9 m, area of the base of the pillar S st =0.25×0.25=0.0625 m 2, laying depth d=0.5 m. Weight foundation column made of fine-grained concrete with a volumetric weight of γ = 21.7 kN/m 3 is equal to G f = 0.0625 × 0.7 × 21.70 = 1.22 kN. Let us determine the calculated value of the clay soil resistance R using the tabulated (Table B.3, e=0.8, I L =0.71) resistance values R 0 =229 kPa:
R = R 0 (d+d 0)/(2d 0)=229 kPa××(0.5m+2.0m)/2×2.0m=156.5 kPa (B.1, II)
The values of rise S u and relative deformation ΔS/L u of the unloaded base are less than the permissible limits(Table 3):
- S u =0.925≤ =5 cm
- ΔS/L u =0.947/154=0.0053≤S u,max = 0.006
Checking the strength of the underlying layer
According to 5.6.25, if there is, within the compressible thickness of the foundation at a depth z from the base of the foundation, a layer of soil of less strength than the strength of the soil of the overlying layers, the dimensions of the foundation should be assigned such that the condition is ensured for the total stress σ z
σ z =(σ zp -σ zγ)+σ zg ≤R z (5.9)
- where σ zp, σ zγ and σ zg are vertical stresses in the soil at depth z from the base of the foundation (see 5.6.31), kPa;
- R z - design resistance of soil of reduced strength, kPa, at depth z, calculated using formula (5.7) for a conditional foundation with width b z, m, equal to:
- b z = √(A z 2 + a 2) - a, (5.10)
- where A z =N/σ zp ,
- a=(l-b)/2.
The coefficient α p =0.0675 is determined by interpolation according to Table 5.8 with a relative depth ξ equal to 2z/b=2×0.65/0.25=5.2;
Vertical load on the base from the foundation N=P/S st =123.52×0.0625=7.72 kN.
The width of the conditional foundation will be
b z =√(7.72/8.34) 2 =0.926 m.
The specific gravity of the soil located above the base is equal to
γ"=(γ gr d hr +γ"d)/(d hr +d)=(12×0.2+19.94×0.5)/(0.2+0.5)=17.67 kN /m 3
The vertical stress from the soil's own weight is calculated using the formula (5.18), while the coefficient α γg is determined according to Table 5.8 with a pit width b=2δ×0.65+b=1.55 m for a relative depth ξ=2×0.65/ 0.926=1.404.
σ zγ =α γg σ zg0 =αγ"d n =0.8387×17.68×0.7=9.65 kN. (5.18)
Vertical effective stress from the soil’s own weight σ z,g, kPa, on the roof of clayey soil z=0.65 m is calculated using formula (5.23)
σ z,g =γ"d n +Σ i=1 n γ i h i +γ 1 (z-z i-1)+q=17.68×0.7+Σ 6 1 19.94×0.1+19.94 (0.65-0.6)+2.4=25.32
We calculate the stress values on the roof of the clay layer using the formula (5.9)
σ z =(8.34-9.65)+25.33=24.02 kPa.
We determine the calculated resistance of clay soil under a conditional foundation using formula (5.7) with d b =0. We take the coefficients M according to table 5.5 at φ=13°
R= γ c1 γ c2 /k =1.1×1×[ 0,26 ×1.1×0.926×19.94+ 2,05 ×1.15×17.78+ 4,55 ×38.5]/1.1=221.61 kPa.
Condition (5.9) is satisfied:R=221.61>σ z =24.02 kPa.
Calculation of foundation settlement
- base settlement s=0.08≤s u =20 cm,
- relative difference in precipitation Δs/L=0.00045≤(Δs/L) u =0.006.
Pile foundations
4.6 Pile foundations should be designed based on the results of engineering surveys carried out in accordance with the requirements of SP 47.13330, SP 11-104 and section 5 of SP.
Design of pile foundations without appropriate sufficient data from engineering and geological surveys is not allowed.
According to 7.1.15, piles and pile foundations should be calculated based on the strength of the material and the stability of the foundations should be checked under the influence of frost heaving forces if the foundation is composed of heaving soils (Appendix G).
Screw piles
Let's consider the possibility of using screw steel piles as a foundation with a barrel diameter d0 = 57 mm, blade diameter d = 200 mm, length L0 = 5000 mm. Pile weight 24 kg. Design load on the pile N= /11=6.56 kN, here 11 is the number of piles.
A pile as part of a foundation and a single pile in terms of the bearing capacity of the foundation soil should be calculated based on the condition
γ n N≤F d /γ c.g , (7.2 pile)
- where N is the design load transferred to the pile from the most unfavorable combination of loads acting on the foundation, determined in accordance with 7.1.12;
- F d - ultimate soil resistance of the base of a single pile, hereinafter referred to as the load-bearing capacity of the pile, which is determined in accordance with subsections 7.2 and 7.3;
- γ n - reliability coefficient for the responsibility of the structure, adopted according to GOST 27751 [V], but not less than 1;
- γ c.g - ground reliability coefficient, taken equal to
- 1.4 - if the load-bearing capacity of the pile is determined by calculation using tables of the set of rules, including the results of dynamic tests of piles performed without taking into account elastic deformations of the soil;
F d = γ c , (7.15)
- where γ c is the coefficient of operating conditions of the pile, depending on the type of load acting on the pile and soil conditions and determined according to Table 7.9;
- F d0 - load-bearing capacity of the blade, kN;
- F df - bearing capacity of the trunk, kN.
F d0 = γ c (α 1 c 1 + α 2 γ 1 h 1)A, (7.16)
- where α 1, α 2 are dimensionless coefficients taken according to Table 7.10 depending on the calculated value of the angle of internal friction of the soil in the working zone φ (the working zone is understood as a layer of soil adjacent to the blade with a thickness equal to d);
- c 1 - calculated value of specific soil adhesion in the working area, kPa;
- γ 1 - averaged calculated value of the specific gravity of soils lying above the pile blade (for water-saturated soils, taking into account the weighing effect of water), kN/m 3 ;
- h 1 - the depth of the pile blade depending on the natural topography, and when planning the territory by cutting - from the planning level, m.
- A is the projection of the blade area, m2, counting along the outer diameter, when the screw pile is operating under a compressive load, and the projection of the working area of the blade, i.e. minus the cross-sectional area of the trunk, when the screw pile is operating under a pull-out load.
F d0 =uf 1 (h-d), (7.17)
- where f 1 is the calculated soil resistance on the side surface of the screw pile shaft, kPa, taken according to Table 7.3 (averaged value for all layers within the immersion depth of the pile);
- h is the length of the pile shaft immersed in the ground, m;
- d - diameter of the pile blade, m;
F d = 0.8××0.0314+0.179×5.3×(4.0-0.2)=15.33 kN
The bearing capacity of a single screw pile for indentation load is greater than the design load transmitted to the pile, condition (7.1) is satisfied!
γn×N= 1×5.9 =15,33 (7.1 )
Stability of pile foundations under the influence of tangential forces of frost heaving
The stability of pile foundations under the influence of tangential forces of frost heaving of soils should be checked according to the following conditions:
τ fh A fh - F ≤ γ c F rf /γ k , (Х1, )
- where τ fh is the calculated specific tangential heaving force, kPa, the value of which, in the absence of experimental data, can be taken according to Table G.1, depending on the type and characteristics of the soil.
- A fh - area of the lateral freezing surface of the pile within the estimated depth of seasonal freezing-thawing of the soil or layer of artificially frozen soil, m 2
- F is the design load on the pile, kN, taken with a coefficient of 0.9 for the most unfavorable combination of loads and impacts, including pull-out ones (wind, crane, etc.);
- F rf - the calculated value of the force that keeps the pile from buckling due to friction of its side surface with thawed soil lying below the calculated freezing depth, kN, taken according to the instructions of Zh.4;
- γ c - operating conditions coefficient, taken equal to 1.0;
- γ k - reliability coefficient, taken equal to 1.1.
- f 1 - calculated resistance of the soil on the side surface of the screw pile shaft to thawed soil, kPa, determined according to Table 7.3 (averaged value for all layers within the immersion depth of the pile);
- h is the length of the pile shaft immersed in thawed soil, m;
F τfh =τ fh A fh =0.8×0.9×110 kN/m 2 ×0.024 m 2 =19.18 kN.
Here, the surface area of the screw pile shaft located in the soil freezing zone is equal to
A fh =πd 2 d f =π×0.057 2 ×1.35=0.024 m 2 .
We calculate the value of the holding force by substituting the corresponding values into formula (7.15)
F d =0.7×(×0.0288+0.179×7.8×(4.6-1.35-0.2))=
14.23 kN. (7.15)
We check the condition (Х1, )
The intensive development of natural resources in various regions of our country raises urgent questions about the reliability and durability of buildings and structures erected on heaving and permafrost soils.
8.1.Features of designing foundations on heaving soils.
Frost heaving of soils refers to physical and mechanical processes, as a result of which freezing soil acquires a stress-strain state under the influence of thermodynamic changes.
The stresses arising during soil heaving are so significant that they can cause:
Deformations of industrial buildings and structures;
Displacement (and curvature) of railway tracks, bridge supports and power lines;
Destruction of road surfaces, airfields, etc.
8.1.1. General information.
Heaving And frost-hazardous are soils that increase in volume when they freeze.
Figure 2 shows the heaving forces that arise during seasonal freezing of soils.
Fig.2. Heaving forces acting on the foundation when the soil freezes:
- σ - normal;
- τ – tangents.
Heaving soils include fine and silty sands, loams and clays, as well as coarse soils with clay filler, containing more than 30% (by weight) of particles less than 0.1 mm in size and freezing under humid conditions.
According to the degree of frost hazard (depending on the particle size distribution, natural humidity, freezing depth and groundwater level), heaving soils are divided into 5 groups, given in Table 8.1.
Heaving soils are characterized by frost heave deformation h f , equal to the height of the rise of the surface of the frozen soil layer, as well as relative heave f, determined by the ratio
d f - a layer of freezing soil subject to frost heaving.
The R f parameter is used to evaluate whether a clay soil belongs to one of the above groups, and determines
where is the humidity within the layer of freezing soil, corresponding to the natural one, at the boundary of fluidity and rolling out, a fraction of units; - the calculated critical humidity, below which the redistribution of moisture in the freezing soil stops in fractions of units (determined according to Fig. 8.2); - dimensionless coefficient, determined in the same way as and (according to SNiP.2.27).
4) 9.5.4. Climatic factors.
Under the influence of annual freezing and thawing, drying and wetting, the soil can change its volume. Many soils experience heaving when they freeze. Heaving, as noted in paragraph 3.3.3, is often accompanied by the formation of lenses and layers of ice due to the migration of moisture to the freezing front; This phenomenon can also develop when the soil under the foundation freezes. However, some soils are heaving-hazardous and non-heaving-hazardous. Heaving-hazardous soils include all silty-clayey soils, as well as silty and fine sands. Medium-sized, coarse and gravelly sands, gravel, pebbles and rocks are not hazardous.
To determine the possibility of soil freezing under the foundation, it is necessary first of all to know the standard freezing depth d f.n. Its value is taken according to observational data as the average of the annual (at least 10 years) maximum depths of seasonal freezing under a surface bare of snow or according to the SNiP map 2.01.01. – 82, or according to formula (9.2).
df.n. =d o √Mt,(9.2).
Where Mt – a dimensionless coefficient equal to the sum of the absolute average monthly negative temperatures during the winter period in the construction area; d o – freezing depth at Mt = 1, taken equal to 23 cm for clays and loams, 28 cm for sandy loams and silty and fine sands, 30 cm for medium-sized sands, large and gravelly, 34 cm for coarse soils (for pits with significant development them beyond the outer edge of the wall d o taken depending on the backfill soil).
Table 9.1. Depth of foundation base d depending on the estimated freezing depth d f.
Since the heaving of soils depends on the position of the groundwater level and the condition of the soil in terms of fluidity, the depth of laying the foundations of external walls is established according to table. 9.1 depending on the calculated freezing depth d f, which is determined by the formula:
df =kh V c d f.n,(9.3)
Where kh– coefficient of influence of the thermal regime of the building on soil freezing near the external walls; Vc– coefficient of working conditions; taking into account climate variability in the construction area.
Size k h determined for the most unfavorable conditions, which include soil freezing on the north side of the building and near protruding corners. It is more correct to find k h thermal engineering calculations, however, you can take k h according to SNiP 2.02.01 - 83. In this case, the extension of the foundations beyond the outer edge of the wall should be taken into account.
Introduction to formula (9.3) of the coefficient V c an adjustment is made for the depth of freezing in cold winters. Magnitude df.n ensures that the soil under the foundation does not freeze in only 50% of cases. When constructing capital structures, such provision cannot be considered sufficient. Therefore, in areas where the sum of average monthly negative air temperatures for a single severe winter is 1.5 times or more higher than the average long-term observations, it is advisable to take V c> 1. Currently in these areas it is recommended to take V c = 1.1.
5) 8.1.2.Types of foundations.
Foundations erected on heaving soils are shown in Fig. 3. Shallow foundations include those whose ratio of height to width of the base does not exceed 4.
Shallow * They call foundations a depth of 0.5...0.7 from the standard freezing depth.
Non-buried foundations made of monolithic (prefabricated monolithic) slabs are used for buildings with a length-to-height ratio of less than 4. Foundation slabs are laid on bedding (pillows) made of non-heaving materials. The material used to construct the cushion is coarse and medium-sized sand, fine crushed stone, boiler slag or heat-treated copper material.
As measures against frost heaving, laying the foundation below the calculated freezing depth is currently widely used. However, such a solution not only leads to a significant increase in construction costs, but also does not exclude (for lightly loaded foundations) large uneven movements of the foundation, which leads to damage to the structural elements of the building. This is due to the fact that the loads transmitted to the foundations of low-rise buildings are, as a rule, significantly less than the tangential forces of frost heaving acting on the side surface of buried foundations.
*Shallow and (not buried) foundations are recommended for one and two-story buildings.
When designing foundations on heaving soils, in addition to foundations on natural foundations (column, strip) and pile foundations, foundations on locally compacted foundations have been used. They are represented by foundations made of driven blocks and foundations in rammed pits (FTK).
The peculiarity of the FTC method is that the pits for individual foundations are not torn off, but are compacted to the required depth, followed by filling them with monolithic concrete or installing prefabricated elements.
The choice of foundation design should be made based on the specific conditions of the construction site based on the results of a technical and economic comparison of possible foundation options.
As soon as the owner of a land plot has an idea for land development, most often he begins to choose a project, calculate the area and quantity of materials. But before construction begins, it is important to know what kind of soil your foundation will support. There are many types of soils that builders classify: rocky, coarse-grained, clayey, sandy, quicksand, etc. And each type has its own construction method.
A type of soil that is subject to constant deformation when weather conditions vary, contributing to a change in the aggregate state of groundwater, is called heaving soil. It is very difficult to design a future building on such land, since its features will require additional measures from the builder to strengthen the foundation and accuracy in calculations. Silty soils, which usually contain clay, gravel, and pebbles, are most susceptible to heaving. Dispersed soils (with free moisture) and sandy soils are less prone to this process. The concept of the degree of heaving determines measures to combat it. We will describe in this article how to resist the process of unwanted deformation of buildings under the influence of the phenomenon described above.
What does the term “frost heaving” mean?
Frost heaving (a. frost heaving) is the process of uneven raising of the soil and decompaction of mineral particles in it (the skeletal structure of the earth) when the aggregate state of groundwater changes. The moisture in the soil expands during the phase transition and thus breaks the soil structure from the inside. Building anything on such land is not only not economically feasible, but also dangerous.
The process of frost heaving itself is divided into:
- Seasonal - occurs after thawing of frozen layers of earth after winter;
- Perennial - occurs when frozen rocks are layered.
In the first case, the soils are covered with so-called “heavens” - mounds, a couple of tens of centimeters thick and about 1 meter in diameter. Sometimes huge areas of mounds are formed, up to 10 meters in diameter.
In the second case, long-term layers already become part of the soil mesorelief and, to some extent, are not as dangerous for the foundation as frequent deformations during seasonal heaving.
The degree of heaving can also be determined using the approximate formula:
E = (H-h)/h,
E– degree of soil heaving;
h– average height of the soil before freezing;
H— average height of the soil after swelling.
If this value exceeds 0.01, it means that the earth is heaving.
But to start construction, you need to know exactly what degree of heaving your site belongs to.
There is a certain classification of different types of earth according to the degree of susceptibility to heaving.
- With medium heaving. This group includes wet soils, the main composition of which is clay with a high level of natural humidity, loam, and dusty sands (with a significant excess of the normal groundwater level).
- With slight heaving. In this group, the soil is filled with silty sands, loams and low-moisture clay (with a significant excess of the normal groundwater level)
If you decide to lay a foundation on such land, but are not confident in your knowledge, a professional builder can give a more accurate classification. This information will help in calculating the necessary measures to design a structure taking into account heaving. But in general, if the calculated coefficient is not large, then you can start from the degree of humidity and the level of stagnation of groundwater in the period before the beginning of winter and in the spring.
Methods for designing a foundation on heaving soils
1. Using drainage
But to obtain the desired effect, you need to do deep drainage. The drainage process includes several stages: This method of combating heaving is based on the principle: no water - no problems. In addition to the fact that after drainage you can easily build on heaving soil, it will also provide an additional bonus in the form of protection from seasonal flooding of walls and floors with groundwater. This method is especially useful on plots of land located above mine communications or on heavily flooded soil.
The advantages of this method of combating soil heaving include additional protection of the house from the unpleasant consequences of watery soils, such as:
- flooding of basements and cellars;
- moldiness of the premises;
- dampness of walls and floors.
2. Laying the foundation below the freezing level
If you accurately determine the nature of the soil and its physical properties, you can use a method such as laying the foundation below the freezing level. Usually, this method ends up not being the most effective and expensive, but if you are planning to build a stone house, or the house will have a very strong frame, then such measures will prevent the direct impact of heaving on the structure. The indirect impact will still remain, since the lateral friction of the heaving soil against the walls of the building can cause inconvenience in the form of displacement of the level of the walls, jamming of doors and windows, etc. But if the frame is calculated correctly, and the force of the deforming layers will be insufficient to move the walls, then these phenomena can be prevented.
3. Insulation
If you want to build a wooden house, then insulating its base is just right as a way to combat soil heaving. Briefly, at the stage before pouring the foundation itself, insulating material is placed in the pit, equal in thickness to the height of the soil freezing layer. You can learn how to calculate the insulation parameters from reference materials, or take the advice of a professional. When the foundation is laid and concreted, it is insulated from water, after which it is also insulated.
4. Replacement of soil
The last and most expensive method is to change the type of soil on the site. By the name itself, the process of implementing the method is already clear. Despite the radical nature, this method is very effective. At the beginning, the first stage of the second method is performed - digging out a layer of soil subject to deformation. Next, the excavated pit is filled with material that can be selected from construction manuals, focusing on the lowest degree of heaving. Most often, coarse river or quarry sand is used, the main thing is that it has a high level of filtration. After compaction, you will have a ready-made base for pouring the foundation. But due to the high cost of excavating and removing soil, this method is not very popular.
Having set himself the task of building a country house with his own hands, an individual developer must be prepared to independently solve a huge number of problems. Having decided on the house design, you should pay increased attention to the “zero cycle” - the construction of the foundation. But before ordering all the necessary building materials, it is necessary to conduct a thorough calculation of the foundation. In this article we provide an example of foundation calculations in exactly the sequence that is recommended to be followed.
Working with soil
Let's assume that you have become the happy owner of ten acres outside the city. The site is, as they say, empty, with only trees and shrubs growing here and there. Before deciding on the location of the future construction site, it is necessary to conduct a soil assessment. To do this, we dig holes in different places of the site to a depth of about 2 meters. If the soil sections are the same, then you are lucky - the soil layers lie evenly. If not, then you will have to choose the lesser evil - bet on the most favorable option. An ideal case: you have many neighbors who have built their houses a long time ago - then calculating the foundation is significantly simplified. You can consult with them about the soil, the type of foundation and its “behavior,” and even ask for documentation on geological soil research if an expert assessment was carried out before construction.
UGV
The groundwater level (GWL) is an important indicator of the soil of the site on which it is planned to build a house. It is nothing more than the distance from the surface of the earth to the first aquifer. It is he who determines what the depth of the foundation will be. The groundwater level changes seasonally: in winter it is minimal, in spring, when the soil absorbs a huge amount of moisture, it reaches its maximum level. In our foundation calculation example, we We recommend measuring groundwater level in the spring, because one way or another, the foundation of the house will be exposed to groundwater, and it is better to carry out calculations based on critical indicators. It is believed that if surface waters lie at a depth of 2 meters or more, then this is normal water level (low) for building a house. If water appears already in the hole dug for soil research, this will mean that the groundwater level is high, based on which, when constructing the foundation, you will have to rely on certain types of foundations. For example, it turned out that the groundwater level is only 1 m. In this case, depending on the load on the soil foundation, preference is given to either a slab foundation or a shallow strip foundation, because the higher the groundwater lies, the lower the bearing capacity of the soil.
Soil heaving
The surface layers of the soil represent a fertile layer. It does not play a special role - during the construction of the foundation it is simply cut off over the entire area of the construction site. But everything that lies deeper needs to be assessed. There may be a layer of clay, loam, sandy loam, and if you're lucky, coarse sand or even rock. It is obvious that each type of soil is characterized by its bearing capacity and resistance to external load (design soil resistance, R). We wrote about how to assess the nature of the soil in this article. You will be able to determine the soil foundation of the construction site and draw a conclusion about the heaving of the soil. Heaving is nothing more than the ability of wet soil to expand due to the freezing of water in winter. This indicator depends on the groundwater level and soil type, and largely determines the choice of foundation for a house.
PPG
GPG or soil freezing depth is an indicator that characterizes the impact of heaving phenomena on the soil thickness. You should be afraid of it if the soil is heaving and the groundwater level is high. Measures to “fight” heaving phenomena:
- insulation of the soil base along the perimeter of the building - thereby we reduce the gas flow rate and level out heaving phenomena;
- installation of a drainage system, thanks to which the soil base under the foundation remains dry and not subject to expansion due to freezing water
To summarize the above
Soil heaving, GGL, groundwater level - all these indicators need to be considered in one complex, because they are interconnected. Thus, a high groundwater level may be the cause of excessive heaving of the soil base due to the large GGL. If we give an example of calculating the foundation for a construction site with ideal indicators: shallow soil freezing depth, low groundwater level, non-heaving foundation, you can choose any type of foundation. But in most cases the situation is the opposite, then the developer:
- or relies on “floating” foundations, which include slab or shallow strip foundations;
- or eliminates the shortcomings of the site by replacing part of the heaving base, insulating the soil under the base of the foundation, draining the sub-foundation area
Relief of the site
Not everyone can be lucky enough to acquire a perfectly flat plot of land. As you know, relief is one of the decisive factors when choosing a specific type of foundation. Thus, the presence of a significant slope at a construction site can cause equally impressive investments in leveling it and the subsequent installation of a strip or slab foundation. Another option is to leave everything as is, but rely on a columnar or pile foundation. Below we will give examples of calculations and such foundations too.
Calculation of the required area of the foundation base
Choosing a foundation type
Depending on the values of the estimated area of the base of the foundation (with reference to the terrain), a specific type of foundation for the house is selected. For the calculation example above, a recessed strip foundation is best suited. If you have to build a house almost in a swamp, then it is safer to fill the slab. In general, there is a choice between the following reasons:
- tape;
- slab;
- MZLF;
- columnar;
- columnar-ribbon;
- pile;
- pile-grillage
Calculation of base parameters
Based on the obtained value of the area of the foundation base and the distribution of loads, the area of its individual structures is calculated. So, using the example of the above calculation (minimum base area 7.2 m2 for a house 6x9 m), you can lay a strip 0.4 m wide. Then the resulting foundation area will be: 9x0.4x2+(6-0.8 )×0.4×3=7.2+6.72=13.44 m2
This is more than enough to build a house, because the area of the foundation is almost 2 times the calculated value!
You can go in the other direction - install bored piles with an expansion at the bottom with a diameter of 0.5 m. In this case, the area of the base of each support will be: 3.14 × 0.5 × 0.5/4 = 0.2 m2
You will need 7.2/0.2 = 36 such piles.
Calculation of building materials
At the next stage, it is necessary to estimate the volume of building materials that will be required to build the foundation of the house: the amount of concrete mixture, reinforcement, formwork - in some cases, it is even necessary to calculate the bricks for the foundation. A competent approach will allow you to avoid unnecessary transportation costs and significantly save time on building the foundation.
Armature
We described the specifics of calculating reinforcement for the foundation in the corresponding article. There you will also find a detailed description of calculations for different types of reinforced concrete foundations. For a strip foundation, a frame is usually used of two belts of longitudinal reinforcement, 2 rods each, with a step of transverse (horizontal and vertical) reinforcement of 0.3-0.5 m. As an example of calculating the foundation, consider the same foundation of a house 6 × 9 m with one internal wall, we take the height of the tape equal to 1.5 m, width – 0.4 m.
The cross section of the tape has an area of: 0.4×1.5=0.6 m2=6000 cm2. Of this, 0.001% should be occupied by reinforcement, which is 6 cm2. Using the table below, we determine the required diameter of the rods - 14 mm.
The number of meters of such reinforcement is approximately equal to: (6×3+9×2)×4=144 m
Smooth reinforcement, which, in fact, only plays the role of a connecting link for longitudinal bars, with a step of 0.5 m will require: (36/0.5) × (0.4 × 2 + 1.5 × 2) = 273, 6 m, where (36/0.5) is the number of connections of smooth reinforcement, (0.4×2+1.5×2) is the perimeter of a rectangular element formed by smooth reinforcement.
Concrete
It doesn’t matter whether you plan to order a concrete mixture from a manufacturer, or are thinking about preparing it yourself - it’s simply necessary to estimate the volume of concrete! This is very easy to do using simple mathematical formulas and taking into account the geometry of the foundation.
We talked about how to calculate the volume of concrete mixture in one of the articles, but just in case, we will give an example of calculation for our case: a 6x9 house with one internal wall, tape width - 0.4 m, height - 1.5 m .
The volume of our foundation, also known as the volume of concrete, will be: (9 × 0.4 × 2 + (6-0.8) × 0.4 × 3) × 1.5 = 20.16 m3 or 21 cubic meters of solution.
The same applies to situations in which you decide to prepare concrete yourself. In this case, information on the characteristics of the concrete mixture for the foundation will help you, as well as an article on how to calculate the amount of cement for concrete. They describe the work procedure in a simple and accessible way and present all the necessary calculations.
Calculation of formwork for the foundation
Of course, if you are going to pour concrete into pipes - use a bored pile foundation, then the issue with formwork will be resolved by itself. But when constructing a strip or slab reinforced concrete foundation, it is problematic to do without formwork. You can rent construction formwork kits, but it is expensive, especially if the construction time frame is unclear. Therefore, in some cases you have to make the formwork yourself - from lumber. Moreover, it must be done in such a way that the boards after stripping can be used, for example, for a subfloor or scaffolding. The cheapest option is to buy ordinary one-inch boards, which can be knocked down into fairly reliable panels. In an article devoted to calculations of formwork for foundations
The holding forces are equal
The tangential heaving forces are equal
The tangential forces of frost heave far exceed the holding forces and the foundation will bulge.
In order to reduce the tangential forces of frost heaving, the cross-section of the foundation should be reduced by 2 times, leaving the size of its base the same.
It is also possible to reduce the tangential forces of frost heaving by using thermochemical measures, such as an insulated blind area, which reduces the estimated depth of soil freezing, or by covering the side surface of the foundation with a polymer film, which reduces τ
n 2 times.
3.328 (9 appendix 6). For the foundations to perceive the holding force Q n, determined by formulas (3.109) or (3.110) [(2) or (3) adj. 6], it is necessary to ensure adequate tensile strength of the cross-section of the foundation body and the corresponding connections of individual elements of precast foundations.
3.329 (10 app. 6). If there is a possibility of freezing of heaving soils under the base of the foundation, the stability of the foundation must be checked under the combined action of tangential and normal forces of frost heaving.
The check is performed using the formula:
Where n 1 ,N n, n,τ n, F— the designations are the same as in formula (1) of this appendix [(3.108) Manual];
F f
- area of the foundation base, cm 2;
h 1
- depth of soil freezing, counting from the base of the foundation, cm 2;
σ
n
— standard value of normal frost heave pressure created by 1 cm 3 of frozen soil layer, determined experimentally, kgf/cm 3 ; in the absence of experimental data for medium- and low-heaving soils, the value σ
n can be taken equal to 0.06 kgf/cm 3 , and for highly heaving ones - 0.1 kgf/cm 3 .
3.330. To select protective technological measures that prevent emergency freezing of the soil under the base of the foundation, it is necessary, based on formula (3.111) (4 appendix 6), to determine the thickness of the soil layer, the limiting condition for maintaining the stability of the foundation.
The check should be carried out for the construction period before filling and compacting the sinuses with soil and after backfilling, but before heating the building, as well as for the period of operation of the building.
3.331. A verification calculation of the forces of the pressure of the frozen layer of heaving soil normal to the plane of the base of the foundation is of great importance in the design of foundations and foundations of all types of buildings and structures, regardless of their number of storeys, erected on heaving soils.
These calculations will make it possible to clarify the prescribed measures to prevent freezing of the soil under the base of the foundations, leading to deformations of the designed buildings and structures.
It is recommended to take into account in these calculations that the weaker the clay soil (the greater its consistency), the greater the size of the foundation is required for the same load on the foundation. At the same time, with a higher consistency, the normal forces of frost heaving are significantly higher (both specific per unit area of the foundation base, and especially total for the entire foundation).
Examples, checking the stability of foundations in case of emergency freezing of heaving soil underneath them
Example 1. The building is designed on strip foundations with a laying depth of 1.6 m.
Within the standard freezing depth there is loam characterized by the following values: e= 0.75 and I L = 0.20.
The groundwater level is located at a depth of 3.5 m. Standard freezing depth H n = 1.8 m and calculated H= 1.5 m.
According to the consistency of the soil and the position of the groundwater level, the soil is slightly heaving and the values of the tangential and normal heaving forces are allowed [according to paragraphs. 3.323 and 3.329 (5 and 10 appendix 6)] taken equal τ
n = 0.6 kgf/cm2 = 6 tf/m2 and σ
n = 0.06 kgf/cm3 = 60 tf/m3.
The width of the foundation is assigned based on the magnitude of the load on it and the value of the conditional design pressure on the foundation soils R 0 according to clause 3.204 (clause 1 appendix 4).
By table 3.24 (2 app. 4) for loam having e= 0.75 and I L = 0.20, value R 0 = 24 tf/m2. n = 23 tf/m. With foundation width b= 1 m the pressure along its base will be equal to R= 23 tf/m2, which satisfies the condition p<R 0 .
Base area 1 m of foundation F f = l m 2, side surface (on both sides) within the calculated freezing depth F= 2×1×1.5 = 3 m2.
Check for the construction period when the load is N n 1 = 12 tf/m and the sinuses of the foundations are not filled with soil, shows that a violation of the stability of the foundations (their rise) will occur when the soil layer freezes with a thickness exceeding the maximum - h 1:
A check for the period when the main work is completed and the sinuses are backfilled and compacted with soil, as well as for the period of operation, shows that the limiting value of the thickness of the frozen layer of soil under the base of the foundation in these cases will be:
Limit values h 1 in all cases are small and therefore reliable heat protection measures are necessary.
Example 2. The building is designed on columnar foundations with a depth of h= 1 m.
Within the standard freezing depth there are clays with the following characteristic values: e= 0.5 and I L = 0.1. In the upper layer 0.2 m thick, the soils are non-heaving.
Conditional design pressure R 0 on the foundation composed of these soils, with foundations with a depth h= 1 m, will be according to paragraphs. 3.204 and 3.206 (1 and 2 adj. 4) equal
R 0 = 0.75·58 = 43 tf/m2.
The groundwater level is located at a depth of 3 m. Standard freezing depth H n = 1.2 m, calculated H= 0.8 m. According to the consistency and position of the groundwater level, the soil is slightly heaving, as a result of which τ
n = 6 tf/m2 and σ
n = 60 tf/m3.
The foundations are designed without ledges, square in plan, size 0.8x0.8 m, area F f = 0.64 m2. n = 27 tf, which, with the chosen size of the foundation, satisfies the condition p<R 0 .
Since during planning the top layer 0.2 m thick is made of practically non-heaving soil, then in case of emergency freezing of the base below the calculated freezing depth N= 0.8 m for at least 0.2 m tangential heaving forces will act along the side surface of the foundation with an area F= 4×0.8(1-0.2) = 2.55 m2.
The maximum thickness of the frozen soil layer under the base of the foundation according to the stability condition h 1 during construction when N n 1 = 10 tf and the foundations are not covered with soil:
Same value h 1 for the end of construction at full load and emergency freezing of the soil under the base of the foundation:
In both cases, in order to avoid emergency freezing of the soil by more than 20 cm, reliable heat-protective measures are required.