Strip foundation: laying depth, tables and calculation

Despite the fact that the depth of the strip foundation is not the only indicator of reliability and durability, it plays a huge role in the integrity of the entire house during its operation. Reinforced concrete tape of any size and brand of concrete may burst over time if it is incorrectly placed in the ground, not taking into account its features.

In order not to get confused in all types of foundations and soils, let's try to understand everything in order. First, we will analyze the types of monolithic tapes, and then, specifically for each type of strip foundation, we will determine the depth of the foundation.

Factors affecting the depth of strip foundations

It is probably worth starting with the fact that the strip foundations themselves are divided into three main types:

1. Unburied
2. Shallow
3. Buried

Each of these types is laid to a certain depth, which depends on several main factors:

• Soil freezing depth
• Soil type
• Ground water level

It is worth noting that strip foundation depth- this is the distance from the soil surface to the base of the foundation, and not the depth to which the trench is dug. In the trench, in addition to the foundation, there may be a pillow.

Now let's see how these factors affect each type of strip foundation separately.

Unburied strip foundation

An unburied strip foundation is extremely rarely used in the construction of private houses, because it is a very weak support for the future structure. As a rule, it is all located on top of the ground, and inside there is only a sand or sand and gravel cushion.

I won’t write much about the non-buried strip foundation, especially since a whole article was already devoted to it earlier. And in general, the very concept of the depth of laying in such a foundation is absent.

Calculation of the depth of laying strip shallow foundations

This is the most capricious, in terms of the depth of the foundation. Firstly, it is not as reliable as buried, and secondly, in order for such a strip foundation to withstand the load of the structure, and also to restrain all the heaving forces transmitted from the ground, its calculation must be approached with special responsibility.

How to fill in, I have already described in detail in one of the previous articles. Therefore, we will not go into details.

Such a strip foundation is laid to a depth that is much higher than the depth of freezing of the soil, which is why it is called shallow. It, in contrast to the buried one, can be largely affected by the forces of heaving of the soil.

Also, an important difference between shallow foundations is that it must be made monolithic not only below the ground level, but immediately, after putting up the formwork, pour the above-ground part of the foundation - the basement. This will greatly strengthen the entire strip foundation.

The depth of laying a shallow foundation directly depends on all three factors described above. In order not to get confused, let's look at the table.

Table No. 1: Depth of laying a strip shallow foundation (minimum), depending on the type and depth of soil freezing

Table No. 2: Depth of soil freezing in some regions

Note: In addition to the fact that the depth of freezing and the type of soil affect the depth of the strip foundation, one more very important factor should not be discarded - the level of groundwater, which we will talk about later.

The dependence of the depth of laying the strip foundation on the groundwater level (GWL)

There are two options for the location of groundwater - when they are located below the freezing depth of the soil, and when - above.

The groundwater level is below the freezing depth of the soil

This can be considered a good indicator, and in this case, groundwater in most types of soils does not have a particular effect on the depth of the installation of a monolithic reinforced concrete tape.

The only limitation, in this case, is that in such soils as loams, clays and the like, the tape must be laid at least half the freezing depth of such soil. In other, “good” soils, this factor does not affect the laying of the foundation.

In other words, if the freezing depth in your region is, for example, 1.5 meters, then the tape shallow foundation must be arranged at least 0.75 meters.

The groundwater level is above the depth of soil freezing

If groundwater is high, then the depth of digging a trench for strip foundation does not depend on their level only on rocky soils, sandy coarse-grained, gravel and the like.

On any other types of soils with a high GWL, the monolithic tape will have to be buried below the freezing depth by 10-20 cm (table No. 2). In this case, it will become a deep foundation.

Buried strip foundation

A recessed strip foundation is considered the most reliable of all tapes. It is laid below the freezing depth of the soil by 10-20 cm. Another condition for its construction is that the soil under its sole must be more or less solid.

In the case of swampy soils, peat bogs and the like, the strip foundation is laid to a depth that is below these layers. In some cases, it is enough to dig a trench to hard ground, and then arrange a sand or sand and gravel cushion to a level that is slightly below the freezing depth of the soil in your area.

When the soil at the construction site is very bad for laying a strip foundation, or its installation is very expensive, you can try to calculate another type of foundation, for example, slab. Perhaps it will be both cheaper and more reliable.

How to reduce the depth of the strip foundation

After carrying out all the calculations on the depth of laying the strip foundation, it often happens that, taking into account the soil and the region, it must be laid very deep. This raises the question of how to cut costs and reduce depth.

There are several ways to reduce the depth of strip foundations, all of which are based on reducing the importance of the main factors affecting the foundation.

Reducing the depth of soil freezing

Of course, we will not be able to change the climate in the region, but we will be able to change the depth of freezing, specifically under the base of the foundation, by insulating the foundation itself and the soil adjacent to it from the outside.

In this way, we can reduce the depth of the foundation, as well as reduce the cost of it.

Groundwater drainage from the strip foundation

Another effective way to reduce the depth of the strip foundation is to drain water from it.

This is done with the help of a good drainage system, which will divert a significant part of the water from the foundation and prevent it from adversely affecting it.

Sand or sand and gravel cushion under the foundation

In the case when heaving layers of soil lie deep enough on the site, strip foundation will also have to be laid to a greater depth. You can reduce it by replacing the heaving soil with a sandy or sandy-gravel cushion.

In other words, it is necessary to dig a deep trench to hard ground rocks, and then arrange a massive sand and gravel cushion there, which will distribute the load from the foundation and the house to the ground evenly and prevent heaving forces from adversely affecting the foundation.

It is advisable to make a pillow not only under the sole of the foundation, but also next to it, as shown in the diagram.

It should be noted that the most reliable method for reducing the depth of the strip foundation is the combined method, i.e. and a pillow device, and insulation, as well as a drainage device, if necessary.