Calculation of the depth of the foundation under the house: how to calculate

Depth calculation principle

In most cases, a strip foundation is calculated for buildings. Therefore, the question often arises: how to determine the depth of the foundation? You can answer it by calculating all the loads from the projected house. Therefore, before starting the calculation, you will need:

draw a plan of the house and all adjacent buildings;
decide on the presence of a basement and its geometric shape;
calculate the level of the base and choose the material for its manufacture;
take the necessary materials for the insulation of the house.

After completing the above steps, you can begin the calculation. First, you need to draw up a table, in it you need to enter the specific weight of all materials used, then you need to break down the project work into the following stages:

assignment of the height of the insertion of the tape;
calculate the load on the belt;
calculate the geometric parameters of the tape;
correct the data, taking into account the soil structure.

Types of foundations for the house

Before calculating the foundation of the house, you need to decide what type of foundation you will use for your structure.

By type of construction, they are divided into:

tape;
slab;
columnar;
pile.

The strip foundation is a reinforced concrete cavity, sufficiently strong, durable and simple

A slab is called a monolithic reinforced concrete slab laid in deepened and leveled soil. The rare use of this type of base, despite the possibility of using it on unfavorable soil, is due to its high cost.

Columnar is a structure of pillars connected by beams to each other. Although this option is the cheapest, it is recommended to use it for soils that are not subject to temperature changes. Moreover, it is only able to withstand a small wooden house.

The foundation on piles can be used on soft soils or in the construction of a multi-storey building. However, the need to use a lot of technology raises the cost of the entire building significantly.

Calculation of the width of the sole

Each foundation has two horizontal planes. The upper, in contact with the walls, is called the edging, and the lower, in contact with the soil, is called the sole. Determining the size of the sole is the main thing in its calculation. In the video you can see the process of laying the lower part of the foundation.

For an error-free calculation of the width of the sole, in addition to the weight of the structure, you also need to know the type of soil on which the construction is carried out. The bearing capacity of each type of soil is different. It is determined by the table:

We found out the mass / weight of the building. Now, knowing the type of soil, you can determine the width of the base of the monolith.

We will increase the total mass of the structure by the weight of the furniture and the supporting platform, which will also exert pressure on the soil, the possible weight of precipitation - snow.

The total mass of the house, taking into account the additional loads, was 200 tons. We choose the width of the sole equal to the width of the block material - 50 cm. The length of the house is 3000 cm. 3000 is multiplied by 50, the total is 150,000 sq. Cm. - the area of the house, which will exert pressure on the ground.

Our house is built on wet clay. On average, it perceives a weight of 2 kg / cm2.

We multiply 150,000 by 2, we get 300,000 kg / cm2 - the maximum weight that this soil can withstand. Our house weighs 200 thousand kg.

300,000 - 200,000 = 100 thousand kg - safety factor.

The soil will easily withstand the construction, even additional loads are possible. The width of the base of the tape will be 50 cm.

Determine the level of groundwater

To do this, in the territory where the house will stand, they dig a ditch about 3 m deep. Or, if there is a well on the site, thanks to it, you can find out the level of groundwater. The most accurate information can be determined in the off-season (spring, autumn), when the water table is highest.

You can also find out the composition of the soil.The top layer is fertile; it is removed during construction. It is easily recognizable by its color - it is darker. Under the top layer is the main one. He will be the carrier, the foundation and the house will exert a load on him.

If, after all the calculations, it turned out that the soil would not withstand the mass of the house, then you need to increase the area of the foundation, or use other materials in the construction of the house. After each change, you will have to recalculate the dimensions of the foundation again.

Set of rules

The rules for determining the depth to make the foundation are established by SP 50-101.2004. They include the following basic requirements:

It should be selected taking into account the main influencing factors (the purpose of the structure, design features, geological and hydrological factors, seasonal freezing).
The depth of soil freezing is taken taking into account the results of observation over a period of at least 10 years, as an average annual value.
In the absence of tabular data for a specific calculation, a calculation is carried out taking into account the normative freezing of a certain type of soil.
Determined separately for external and internal walls.
Corrected taking into account the coefficient of influence of the operational characteristics of the room.
The bottom of the buried foundation should be located below the freezing depth of the soil, but above the groundwater level. The minimum depth of a shallow foundation is set taking into account the properties of the soil and takes into account the depth of freezing.
When designing the foundation, water-lowering (drainage) measures should be provided.
When erecting a foundation on a slope, a stepped deepening is used, which ensures the formation of a horizontal base of the floor.
You should not dig according to the principle: the deeper, the stronger and more reliable. So you can simply uselessly bury a lot of money in the ground. The foundation must be optimal for the type of building.

General estimate

Summing up, so that the whole theory outlined becomes a little clearer, we will give an example of calculating the foundation for a one-story house.

The building measures 6x10 m, with an internal six-meter wall. At the same time, the height of the first floor is 3 m, and the height of the attic is 2 m.

The foundation is tape, made of reinforced concrete, 1.5 m deep, 0.5 m wide. The gable roof is made of slate.
Wall surface area = (6 + 10 + 6 + 10 + 6) * 3 + (1/2 * 6 * 2) * 2 = 126 sq.m.
Overlap surface area = 6 * 10 = 60 sq.m.
Roof surface area = 4 * 10 * 2 = 80 sq.m.
Foundation load = 126 * 270 + 60 * 300 + 60 * 200 + 80 * 50 = 68020 kg.
Foundation base area = Exterior area - Interior area + Internal wall area =
= 6 * 10 - (6-2 * 0.5) * (10-2 * 0.5) + 0.5 * (6-2 * 0.5) = 17.5 sq.
Foundation volume = 17.5 * 1.5 = 37.5 sq.m.
Foundation weight = 37.5 * 2500 = 93,750 kg.
Load per 1 sq. Cm. soil = (93750 + 68020) / (17.5 * 10000) = 0.9244 kg / sq. cm.
Such a load is permissible for the weakest soils - for sands saturated with water.
Concrete volume = Foundation volume = 37.5 sq.m.
Ribbed reinforcement = (2 * (6 + 10) +6) * 3 * 2 = 228 m
Number of reinforcement connections = (2 * (6 + 10) +6) / 0.5 + 1 = 77
Smooth reinforcement for 1 connection = (0.5-2 * 0.05) + (1.5-2 * 0.05) * 3 = 4.6 m
Rebar is smooth, total = 4.6 * 77 = 354.2 m
Knitting wire = 0.3 * 3 * 4 * 77 = 277.2 m

As you can see, the calculation of foundations is not such a difficult science as to refuse construction on your own, and this example of calculating the foundation is given here as the main proof.

Choosing the type of foundation

Before starting the calculations, it is necessary to select the type of foundation used according to the circumstances:

The strip foundation is the most versatile type, since it is reliable and durable, suitable for buildings of any height, a basement can be arranged in the building. The downside is the high consumption of materials and the bulkiness of the structure. It is a tape of reinforced concrete buried in the ground, poured on a cushion of sand and crushed stone.

Column foundation - represents concrete pillars buried in the ground at a certain distance from each other, connected by beams. Suitable for the construction of low-rise (1-2 floors) timber houses, from vulture panels, or log cabins. This type of foundation is suitable for soils where there are no sharp temperature fluctuations.

Tiled - requires preliminary deepening of the soil, and the unevenness of the soil under it is leveled by adding sand, gravel or concrete.

Pile - consists of piles connected by beams and reinforced concrete slabs. It is used in conditions of shaky soils, for the construction of light multi-storey buildings. The installation of piles requires a lot of construction equipment and is expensive.

What is the depth of the foundation

Before you start planning your house, you must decide where on the lot you want to place the house. If there is already geological research, take into account their results: in order to have fewer problems with the foundation, it has the lowest cost, it is advisable to choose the "driest" area: where the groundwater is as low as possible.

First of all, you must decide on a place for a house on the site

Further, in the selected place, geological studies of the soil are carried out. For this, pits are drilled to a depth of 10 to 40 meters: it depends on the structure of the layers and the planned mass of the building. Wells are made at least five: in those points where the corners are planned and in the middle.

The average cost of such a study is about $ 1000. If a large-scale construction is planned, the amount will not greatly affect the budget (the average cost of a house is 80-100 thousand dollars), but it can save you from many problems. So, in this case, order the research from professionals. If you want to build a small building - a small house, a summer cottage, a bathhouse, a gazebo or a barbecue area, then it is quite possible to do the research yourself.

An example of calculating the weight of a house

To determine what size the tape will be, it is necessary to calculate the mass of the future structure.

We calculate the approximate mass of the future structure, you will need the value of the area of the walls, the surface of the floor and ceiling, as well as the roof.

Consider an example, a house is being built with walls 6 m and 5 m long, with one main transverse wall inside, the length of which is 5 m. The height of the walls is 3 m. The length of the outer walls is 22 m plus a transverse wall of 5 m, we get 27 m. the length of the walls is 27 m to their height of 3 m - we determine the total wall area of 81 sq. m.

The floor and ceiling area will be 30 sq. m.

Next, we calculate the roof area. We measure the height of the pediment using geometric formulas, calculate its area, then calculate the area of the roof.

We multiply each resulting area by the specific gravity of 1 sq. m. of the corresponding material. We add the numbers, we get the approximate weight of the future house. To it you need to add the weight of the attic and basement floors.

What determines the depth of the laying

The correct calculation of the deepening of the base of the building is a guarantee that the structure will stand for many years without shrinkage and cracks.

When planning the size of the foundation deepening, the following factors play an important role:

The load on the base from the building and the presence of underground structures (basements, garage)

It is necessary to pay attention to the building plan and take into account the severity of the materials from which the house will be built.
Composition of soil components, level of freezing of the ground.
The degree of relief of the construction site.
The level of occurrence of underground sources, depending on the season and weather.
What communications will be laid under the building (they should not be located below the base of the foundation).

How does the composition of the soil affect the depth of laying the foundation

Sandy soil does not require much deepening of the base

It is good if there is a homogeneous soil composition on the site. The stability of the structure directly depends on this factor, it will be strong and durable. You can choose a fairly small depth of the base of the building on rocky, stony ground, after removing the top layer of soil.

A base with a depth of more than 500 mm is poured on "cartilaginous" soil, consisting of gravel and large rocks. The degree of freezing of the ground in this case will not affect the base of the house.

But at the same time, it is also important to correctly determine the level of occurrence of groundwater and the load from the building.

The foundation at a depth of 500-700 mm is laid on sandy soil, taking into account the amount of soil freezing.If the soil is very shallow and dusty, then it freezes to a much greater depth. On the basis of which, the foundation must be laid deeper.

From 700 mm to 1 meter, the depth of the foundation on clay-sandy soil (sandy loam) should be assigned. It is imperative to take into account the depth of freezing.

On clay soil, it is necessary to go deeper below the soil freezing line, since the soil swells at sub-zero temperatures and contracts from gravity, as if pushing everything inside it outward. To determine the composition of the soil, you can contact the construction organization or make several holes yourself with a drill and determine the prevailing soil.

Influence of groundwater level

The groundwater level can significantly affect the deepening of the base of the house.

The depth of the foundation depends on the groundwater level in the construction area. The foundation of the building is laid by 500 mm when underground sources are deeply buried, but the amount of freezing of the soil is also taken into account.

The base of the building must be arranged below the freezing line of the layers of the earth, if it is not located at the same point with the groundwater level. If these values coincide, then the foundation level must be chosen below the freezing line by more than 100 mm.

If underground sources are located close to the surface, it is recommended to invite specialists who can professionally drain around the building. This type of work will save money on laying the foundation. The service life of the structure depends on the professionalism of the workers performing the drainage work.

Influence of soil freezing

In the presence of heaving soil, the foundation is laid below the freezing line, which in temperate climates ranges from 1 to 1.5 m. The depth increases to the north and east, and decreases to the south and west. The freezing point in a certain region can be found in large construction organizations.

The depth of soil freezing

To roughly determine to what level the soils freeze through in your region, just look at the map below.

On this map, you can roughly determine the level of soil freezing in the region

But this is averaged data, so that for a specific point, the value can be determined with a very large error. For inquisitive minds, we present a calculation method depth of soil freezing anywhere. You only need to know the average temperatures for the winter months (those in which the average monthly temperature is negative). You can calculate it yourself, the formula and calculation example are laid out below.

The formula for calculating the depth of freezing

D_fn- the depth of freezing in the region,

Do - coefficient taking into account soil types:

for coarse soils, it is equal to 0.34;
for sands with good bearing capacity 0.3;
for loose sands 0.28;
for clays and loams, it is 0.23;

M_t - the sum of the average monthly negative temperatures for the winter in your area. Find metrology statistics for your area. Choose the months in which the average monthly temperature is below zero, add them, find the square root (there is a function on any calculator). Substitute the result into the formula.

For example, we are going to build on clay. Average winter temperatures in the region: -2 ° C, -12 ° C, -15 ° C, -10C, -4 ° C.

The calculation of soil freezing will be as follows:

M_t= 2 + 12 + 15 + 10 + 4 = 43, we find the square root of 43, it is 6.6;
D_fn= 0.23 * 6.6 = 1.52 m.

We got that the estimated freezing depth for the given parameters: 1.52 m. That's not all, take into account whether heating will be needed, and, if so, what temperatures will be maintained in it.

If the building is unheated (a bathhouse, a summer residence, construction will go on for several years), a multiplying factor of 1.1 is used, which will create a margin of safety. In this case, the depth of the foundation is 1.52 m * 1.1 = 1.7 m.

If the building is heated, the soil will also receive a portion of its heat and will freeze less. Therefore, in the presence of heating, the coefficients are decreasing. They can be taken from the table.

Coefficients that take into account the presence of heating in the building. It turns out that the warmer it is in the house, the shallower the foundation needs to be buried.

So, if the temperature in the premises is constantly maintained above + 20 ° C, the floors are insulated, then the depth of the foundation will be 1.52 m * 0.7 = 1.064 m.This is already less cost than going deeper by 1.52 m.

The tables and maps show the average level for the last 10 years. In general, it is probably worth using in the calculations the data for the coldest winter that has been over the past 10 years. Abnormally cold and snowless winters occur with approximately the same frequency. And when calculating, it is advisable to focus on them. After all, it will not calm you down if, after having defended 9 years, on the 10th your foundation will crack due to too cold winter.

Calculation of the depth of the foundation

This indicator is directly related to the depth of soil freezing and the level of groundwater.

Provided that the groundwater is deep relative to the level of freezing, which means that the soil in your area is dry and the foundation can be laid at a shallow depth. After watching the video, you will learn many important points that need to be considered when laying the foundation.

If the groundwater is closer than 2 m to the level of freezing of the earth, then the soil is moist, and in winter it will be susceptible to such a phenomenon as frost heaving

In this case, it is important that the base of the foundation is below the freezing mark of the earth.

For dry soils - sand, hard clay, the foundation is arranged no higher than 0.7 m.

If the soil is wet - plastic clay, loam, sandy loam, the minimum laying depth should be increased to 1.2 m.

When a house with a basement is being built, the depth of the bookmark should be at least 0.4 m below the floor level in the cellar.

Calculation of concrete for the foundation

A professional house project, namely such documents will be official when it is put into operation, also shows the amount of building materials. On it, determine how much of what you need to have, starting to lay the foundation. Finding out how much concrete is required is the simplest thing in these calculations.

The concrete grade for the base must be at least 100; when using M400 cement, its proportion with sand will be 1: 4. But it will not be durable concrete, it is hardened with four to five parts of gravel and steel reinforcement. The weight of a cubic meter of reinforced concrete will be 1.8-2.5 tons.

We present an approximate calculation of the types of foundations and loads. Using this information, even an inexperienced builder will be able to understand how much material is required for work, and how much it will cost.

The soil

The choice of foundation depends on the correct determination of the type of soil.

The very first factor that should be carefully examined is the soil on the site, which is chosen for the construction of the house. Much depends on its type:

type of foundation;
the depth of its occurrence;
selection of the type of waterproofing;
the possibility of arranging a basement.

In order to properly assess the soil, it is necessary to dig holes in several places or drill wells. The distance between them must be at least one meter. Soils on the same site can be different, and, therefore, their properties are different.

It is very important not to focus on the properties of the soil of the neighboring area and ignore the examination of your own. The well is drilled to a depth of 2 meters

This depth is sufficient to get an idea of what type of soil prevails.

The well is drilled to a depth of 2 meters. This depth is sufficient to get an idea of the prevailing soil type.

Further, they are determined with the type of foundation and the depth of its occurrence.

Here are the characteristics of the most common types of soil and options for solutions regarding the calculation of the base of the house.

Stony and semi-stony soils have a very high bearing capacity. Based on this, it is possible to carry out work on the construction of any type of foundation, except for the pile foundation.

Choosing the type of foundation

Depending on what the values of the calculated area of the basement base turned out to be (with reference to the terrain), a specific type of base for the house is chosen. For the above calculation example, 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 such reasons as:

tape;
slab;
MZLF;
columnar;
columnar-tape;
pile;
pile-grillage

Calculation of the parameters of the base

Based on the obtained value of the area of the base of the foundation and the distribution of loads, the area of its individual structures is calculated. So, using the example of the above calculation (the minimum area of the foot is 7.2 m2 for a house of 6 × 9 m), you can lay a tape 0.4 m wide.Then the resulting area of the foundation will be: 9 × 0.4 × 2 + (6-0.8 ) × 0.4 × 3 = 7.2 + 6.72 = 13.44 m2
This is more than enough for building 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 foot area of each support will be: 3.14 × 0.5 × 0.5 / 4 = 0.2 m2
Such piles will require 7.2 / 0.2 = 36 pieces.

When does it change stepwise?

When building a house on a site with a slope, it is necessary to stepwise change the depth of the foundation. In such cases, tape and columnar structures are used.

The most reliable is the concrete tape. This type of foundation can be built on slopes up to 28-35 degrees.

The essence of the stepped foundation is that the lower wall is a retaining wall and has an increased depth of laying.

Its scheme looks as follows: first, a trench is being prepared for the upper wall with a depth equal to the design depth.

A trench of the same depth is dug under the lower retaining wall, but the formwork rises to such a height that its upper boundary is strictly in the same horizontal plane with the upper cut of the upper wall formwork.

The side wall of the strip foundation under consideration has a stepped structure. Its entire length is conventionally divided into sections of approximately 2 m. At each section, a horizontal trench bottom is formed. The starting point is the deepening of the upper wall.

The structure of the side wall of the foundation on a slope can be explained with an example. For example, a house with a width of 6 m is being built on a slope with a slope of 15 degrees. We conditionally select 3 steps of the side wall 2 m long.

The steps must compensate for the slope, and the sole on each of them must be strictly horizontal. The starting point is located on the top, and the end point is on the bottom wall, where the depth of the foundation is 0.8 m.

However, due to the slope, the end point is located below the upper one by 6xsin15 = 1.55 m. The height of each step will be 1.55: 3 = 0.53 cm.

The upper step at the starting point will have a deepening of 0.8 m, and at the end point - 0.8-2sin15 = 0.28 m. The lower step: at the end point - 0.8 m, and at the initial point - 0.8 + sin15 = 1.32 m. The middle step has a similar value.