Rain and melt water can seriously swamp the soil and thereby cause irreparable harm to buildings and soil. To avoid damage due to moisture, a good drainage system should be installed. The laying depth of storm or closed type storm sewers must comply with the established norms of SNiP. Otherwise, communication will be useless.
The importance of correct calculations when designing a storm sewer system
It is extremely important to adhere to the recommended SNiP norms for deepening and rainfall slope. If you neglect the exact calculations, this poses the following problems:
- poor outflow of rain / snow water and, as a result, waterlogging of the site;
- formation of stagnation of moisture and debris in the form of silt, sand, earth in specific sections of the sewer;
- ejection of laid pipes due to the tendency of the soil to heave;
- the need for complex repair work to clean the drainage line;
- freezing of pipes or sewage trays in frosts, and hence the possible rupture of drains;
- the risk of dismantling the entire system and its new installation.
At the time of compiling the design scheme for storm sewers, it is advisable to familiarize yourself with SNiP 2.04.03-85 or with the updated print version of SP 32.13330.2012.
Determination of the depth of storm sewers
In order for the installed communication to work properly for more than a dozen years, it is important to correctly determine the depth of laying the stormwater in a private house. SNiP 2.04.03-85 regulates such minimum standards:
- for gutters / pipes of the drainage system with a cross-section up to 500 mm, the recess is from 30 cm;
- for trays / pipes with a diameter of 500 mm and more, the stormwater depth starts from 70 cm.
For a closed, laid drainage system, drainage pipes should be 20 cm below the groundwater level. This will allow moisture to freely fall and go through the pipes during the spring snowmelt.
It is important to consider the level of soil freezing in winter. The more porous the soil in the area, the smaller the thickness of the ice in the ground at minus temperatures. In addition, pay attention to the thickness of the snow in the winter. If it is 10 cm or more, then it is a kind of pillow that reduces the thickness of freezing of the earth. Thus, knowing the level of freezing of the soil and taking into account the thickness of the snow, it is possible to derive the minimum indicators for laying drainage pipes in the area: approximately 30 cm (with pipes with a cross section up to 50 cm) and 50 cm (with a cross section) are taken from the existing level of ice in the soil in winter drains from 50 cm).
Example: the level of soil freezing in the area is 1200 mm. The thickness of the snow layer in the region is 10 cm. The pipe diameter is 50 cm. Therefore, we subtract 30 cm from the permitted ones and get a minimum depth of 90 cm. It is important to remember that 90 cm is the depth along the upper edge of the already laid pipe with all the sand and gravel bedding.
If you lay a closed system at the level of soil freezing or higher, the ice in the pipes will melt slowly during the spring flood season. During this time, meltwater will flood buildings and garden crops.
If for any reason the installation of drainage pipes at the recommended depth is not possible, you will have to do their quality insulation. It is necessary to provide for the level of dynamic and static load on the system with a decrease in the depth of the drain.
The value of the minimum slope of storm sewers according to SNiP
An important criterion for the device stormwater is the slope of the system. If it is respected, rainwater flows by gravity towards the receiver. Minimum slopes of storm sewers according to SNiP 2.04.03-85 vary depending on the internal section of the pipe. The regulation looks like this:
- For a drainage system with pipes / gutters with a diameter of 20 cm or more, the permissible slope for each linear meter of stormwater is 7 mm.
- If trays / drains of smaller diameter are used for the installation of the sewer, the slope increases from 8 mm to 1.5 cm. Since the smaller diameter of the pipes forms a greater resistance to free outflow of liquid.
In SNiP 2.04.03-85, the maximum level of rainfall inclination is also prescribed - it is 1.5 cm / m. With an arbitrary increase in the slope, the system will become clogged with sand and earth, which, due to the greater specific gravity, will not be able to move along with the flow of water at the same speed.
The slope of open drainage systems depends on the level of roughness of the internal walls of the communication. The regulations set such indicators:
- drainage ditches and trays on asphalt roads - 0.003;
- crushed stone and cobblestone road surfaces - 0.004;
- cobblestone coatings - 0.005.
As a rule, in the private sector they make open ditches covered with broken bricks and cobblestones.
Calculation of minimum rainfall slope
To calculate the minimum slope of the drainage system, you need to take into account the following storm storm parameters:
- type of drainage system (open / closed);
- diameter of used pipes / gutters;
- the material from which the drains are made (water glides faster on plastic than on concrete);
- 1 meter storm deviations according to SNiP depending on the drain section.
It is important to choose the right diameter of the pipes for the volume of water discharged on the site. It is calculated by the following formula:
- Q = q20 × F × Ψ
All values in the formula are interpreted as follows:
- Q is the total amount of rainwater that will go through the drains.
- q20 - coefficient of intensity of annual precipitation (calculated in liters per second per 1 ha of land area). The coefficient is taken at the local environmental organization in the region.
- F is the total area of the site, taking into account the roof from which water will be discharged (calculated in ha).
- Ψ - correction factor, which takes into account the absorption capacity of the soil.
The coefficient Ψ is equal to such indicators:
- open soil - 0.35;
- crushed stone - 0.4;
- concrete - 0.85;
- asphalt - 0.95;
- roofing - 1.
According to the displayed value, a pipe section is selected for the drainage system or stormwater. According to practical experience, the diameter of the tubes for private wastewater disposal is often 110-150 mm.
If, due to the relief of the construction site or for other objective reasons, it is impossible to comply with the recommended SNiP slope standards, it may be reduced to 5 mm for pipes with a cross section of 20 cm and up to 7 mm for pipes with a cross section of 15 cm or less.
It is important to consider: the larger the diameter of the pipes in the system, the smaller the slope; the rougher the surface of the inner walls of the drains, the stronger you need to tilt the collector for free outflow of water from the site.
When installing a closed drainage system, a security zone must be observed. It implies indents from each pipe wall by 3-5 meters. It is forbidden to build, plant trees, organize landfills in this place. It is also undesirable to block the approaches and entrances to the revision / overflow wells.
