Columnar foundation with a high level of groundwater. What foundation is needed with a high level of groundwater

The construction of houses has to be carried out in different conditions, and sometimes designers and developers are faced with a situation where groundwater located close to the surface of the earth. If such a groundwater level is complicated by the composition of the soil, in which clay predominates, then the foundation structures have to withstand the significant impact of underground pressurized waters. When designing and erecting a foundation with a high level of groundwater, it is necessary to use additional measures to protect the structures of the house and underground premises from water coming from the ground. In our article, we will tell you which foundation for a house is better to choose if the groundwater level is high, and how to make it correctly.

Pragmatics of choice

If the groundwater is located close to or above the freezing point of the soil, then such construction conditions are considered difficult. It is even worse when clay and loamy soils prevail at this water level. In this case, in winter, the forces of frost heaving will act on the foundation of the house. Main question in such a situation, what kind of foundation should be poured in difficult construction conditions so that the house turns out to be strong and durable. To protect the structure of the building from moisture and heaving, it is better to make the appropriate preparation of the territory.

Important: if it is not possible to deepen the base below the GWL, it is better to make a "floating" foundation. Its cushion will absorb seasonal soil movements.

However, this is not the final answer to the question of which foundation is suitable for a high standing groundwater, because you can choose several types of floating foundation:

monolithic slab;
tape base;
columnar foundation.

Which type of base is better can only be said in each specific case, taking into account the weight of the house and other parameters. The only thing that can be said right away is that for swampy soils, the foundation on screw piles is definitely the best.

Impact of high GWL on the base

If the base of the house is close to groundwater, then it is not the effect of moisture itself that is destructive for it, but salt solutions and other chemicals contained in them. Therefore, it is so important to know the aggressiveness of groundwater. When a complex of corrosive substances in the composition of groundwater affects concrete structures, they begin to collapse. This can be seen by the following signs:

foundation structures are stratified;
a loose, light coating, like gypsum, forms on the surface of the concrete;
in addition, mold and fungi begin to develop on structures;
due to corrosion, the reinforcement begins to increase in size and break the concrete from the inside.

If the construction is carried out when the groundwater is close to the surface of the earth, then problems begin to arise even at the stage of digging the foundation pit. Water accumulates in the trench, the bottom becomes loose and fragile as a result of erosion. If the foundation of the house is installed in such a pit, then the destruction of concrete structures is inevitable. In this case, with closely located groundwater, it is necessary to equip a drainage system, with the help of which water will be diverted into a well, reservoir or city communications.

Attention: the main danger at high groundwater level is the processes of leaching of mineral substances from the soil, called ascending suffusion. This process leads to a decrease in the bearing capacity of the rock.

Errors in the construction of foundations in conditions of high GWL

Most often, if the construction is carried out in conditions of closely located groundwater, placing concrete in a semi-liquid mud composition at the bottom of the excavation becomes a common mistake. Even after pumping out the main volume of water from the pit, such laying can lead to negative consequences:

The foundation sole will turn out to be very loose, because it will consist of a mixture of earth lumps and concrete. There can be no question of any solid and even surface, as it should be.
If groundwater is constantly pumped out of the pit during the foundation, then the concrete monolithic surface will turn out to be porous, which will lead to a decrease in the bearing capacity.
Sometimes, in conditions of closely located groundwater, builders lay dry concrete mixture into the formwork. If you use this method, then instead of a solid monolith, you will get a stratified fragile surface.

Open water drawdown

In these conditions, it is also important what foundation for the house you decided to make, and what measures are used to drain the pit. The most convenient way to drain pits and trenches in private construction is open water lowering. The procedure is performed using pumping equipment in the following sequence:

The main thing is that you need to achieve such a lowering of the groundwater level so that the groundwater is 200-400 mm below the bottom of the pit. To do this, in the process of digging a pit, perforated drainage pipes are laid in such a way as to ensure the drainage of water outside the construction site.
The cheapest way for these purposes is to use sewer pipes with a diameter of 110 mm for external and internal networks.
In order not to wash out the rock at the bottom of the pit, the rock is removed so that the water flow is directed towards the bucket or shovel.
The process of open dewatering continues until the first signs of soil suffusion appear. At this point, the process is stopped so as not to reduce the bearing capacity of the rock.

Important: the first signs of suffusion will be rock particles washed away by water jets and deposited on the surface in the form of slugs, like around a volcano crater.

Drainage system

When building the foundation of a house on clay soil in conditions of high GWL, the rock behaves like quicksand. In this case, a drainage system should be made. For this, drainage pipes are installed on the site, connected to the storage and receiving well. Such a system should drain excess water not only from building structures at home, but also from the entire construction site.

To do this, do the following:

Amelioration canals are made along the perimeter of the site. With the help of a network of such structures, one can easily lower the GWL.
The principle of operation is based on the fact that water from the soil begins to collect in the network of trenches, since it does not meet the opposition of rocks on its way.
To prevent landslides of the ditch walls, metal or wooden shields are installed on them. Instead, rubble or gravel can be poured into the ditches.
In conditions of very high water levels, it is better to lay drainage pipes in trenches.

Drainage with pipes

The construction of the foundation with a high level of groundwater is done only after preliminary draining of the construction site. Moreover, the depth of laying the pipes is greater than with the installation of a storm. When carrying out work, the following rules are adhered to:

If the groundwater level is very high, then ring drainage is performed, and not wall drainage.
The depth of pipe laying depends on the type of rock in the area.
To correctly determine the distance at which the pipes should lie from the foundation, it is necessary to take into account the internal friction of the rock.

To independently determine the type of soil, you need to take a rock sample from a depth of 150-200 mm and from a meter depth. A dry sample is slightly moistened and rubbed between the palms, while wet samples, on the contrary, are slightly dried. Based on the results, conclusions are drawn:

if, after rolling, an elastic mass forms in the palms, then the rock is clay;
if, when rubbed, the sample breaks and cracks, then it is loam;
in the case when it is impossible to roll out the rock due to crumbling, the soil on the site belongs to sandy loam;
a rock with a high sand content will not roll at all.

Ring drainage device

Dig trenches 40 cm wide to the required depth.
The bottom of the trench is covered with a layer of sand 20 cm high and rammed.
After that, a gravel bed of the same height is made, followed by compaction. To exclude siltation after sand, you can lay a layer of geotextile.
Geotextiles are again laid on top of the gravel layer so that the edges of the strips form an overlap of at least 150 mm.
Drainage pipes are laid with the perforated side on the bottom.
From above, the pipeline is closed with a layer of geotextile.
We do backfilling: first a layer of sand 20 cm high, then a layer of gravel 150-200 mm high and then only the soil.

Base on "floating" cushion

The principle of the foundation on the "floating" cushion is the same for tape, pile and slab foundations. The sequence of actions is as follows:

After mounting the ring drainage system you can start digging a foundation pit or trenches. Since the base must rest on solid ground, the bottom is rammed.
Now a floating cushion of compacted sand is made. The height of the cushion is not less than 0.5 m. The filling is carried out gradually with layer-by-layer compaction.
Geotextile is laid on top of the pillow. This will protect the base from subsidence of small rocks.
Then a layer of crushed stone is made, which is also rammed. Layer height 150-200 mm.
After that, the roofing material is spread.
Now you can start installing the formwork, laying the reinforcement cage and pouring the concrete mixture.

In conditions of high GWL, the main danger lies in the high content of sulfates in the groundwater, which loosens concrete. Therefore, it is better to use sulfate-resistant Portland cement of the 500 grade for the solution.
The depth of the foundation in conditions when the groundwater level is less than 1.5 m below the freezing point should be within 0.7-1 m. This applies only to sandy loams and sands. For loams, the depth of laying should be lower design point by 200-300 mm or at the same mark with it.
When building on wet clay, the sole is made thicker than the foundation itself, and the walls of the structure are arranged with a slight slope. This will allow the base to better resist lateral heaving forces.
For the manufacture of concrete, only clean crushed stone and sand are used. The solution should not be fluid, but viscous. To increase plasticity, you can use plasticizers that are added to the water.
In order for the concrete solution to have water-repellent properties, it is possible to enter "Penetron Admix" into its composition. This dry mix can increase the strength of concrete by 15 percent.

The main task of the foundation is to take on the load from the house and distribute it evenly on the ground. Therefore, when laying the foundation, it is necessary to take into account the type of soil and the location of the groundwater table (GWL). The latter indicator may be too high, that is, the water is close to the soil surface.

It is difficult and costly to fill the foundation in such conditions. Therefore, the manufacturer of the work is faced with the question, what foundation is needed for a private house, if the groundwater is close? This issue affects not only the type of foundation structure, but also an additional choice, which concerns precisely the depth of the laying: below the groundwater level or above.

Influence of groundwater

In the construction environment, there is such a term - cement bacillus. This is when a concrete structure collapses under the influence of negative factors, which include groundwater. But one must understand that concrete is not negatively affected by the water itself, but by the salts and various chemicals dissolved in it. They penetrate the concrete body, where they interact with cement or fillers, breaking their bonds with each other. Hence the appearance of all kinds of plaque, dark spots and unpleasant odors.

High ground level is a problem even at the stage of excavation of the pit. Water collects inside, which softens the bottom, which leads to a decrease in the bearing capacity of the soil. If this happened, then you will have to first think over and lay a drainage system, with the help of which it will be possible to pump out water.

There are technologies that do not require land drainage. To lay the foundation, the pile method is used, with pillars driven into the depth of the earth to dense layers. Not a bad option, but costly and time consuming. Without the availability of special equipment, such a foundation cannot be built. For private housing construction, this technology is rarely used. Therefore, the construction of the foundation with a high level of groundwater is approached from the position of drainage.

And one more negative point. A dangerous time for such areas is winter with subzero temperatures. Freezing of the soil in winter turns into a situation dangerous for the foundation. Frosty heaving and high GWL can destroy the structure of the base of the house in one season.

Determine the level

The level of groundwater, that is, the indicator itself is fixed in a geological organization, a branch of which is in any big city... You can get information on GWL there. And you can measure the level yourself. To do this, you will have to dig a hole 3 m deep. Or drill a well to the same depth. A hole diameter of 20 cm will be normal, which requires only a garden drill.

Measurements should be taken in the spring, when the snow has melted. This is the period when the GWL is at its maximum peak point. Cover the pit with foil to prevent precipitation from falling into it. And after a day you check how much water has accumulated in it. You can check with the sixth: lower it to the end so that it touches the bottom. We pulled out and measured the dry area from wet to the mark that determines the ground level (the mark must be put when measuring in the pit). This is the UGV.

If this indicator is more than 2 m, it means that in the area of \ u200b \ u200bthe construction of the house there is a moderate level of groundwater. You can lay the foundation without additional measures.
If the dry section is less than 2 m long, the water level is high. We will have to think over the water drainage system, carry out its construction, provide the foundation structure with protective materials.
It is the second position - with high groundwaters - that will require the manufacturer to choose the type of foundation.

Freezing depth of soil and groundwater level

The presence of a high level of groundwater flow affects several foundations. They are clearly spelled out in SNiPs. And most often in the rules there is a ratio of GWL to the level of soil freezing. Because these two indicators are the main factors that reduce the strength of the concrete structure. Here are some positions.

If the water level is less than the freezing level, then the foundation is calculated according to the usual scheme, that is, only for the load from the house.
If the soil at the construction site is weak, soft and mobile, then the foundation is laid below the UPG. At the same time, a drainage system must be organized for the removal of groundwater.
If the water table is very high, then strip foundation it is not recommended to erect.
If there are frequent flooding in the building area, then the only option is acceptable - a house on stilts. In this case, the pillars are hammered into the ground below the level of its freezing.

If the groundwater level is high enough, and drainage of the site is organized, then there is a high probability of soil subsidence. This is especially true on sandy soils.

GWL closer than 0.5 m

In this situation, the only solution is piles. There are three options: monolithic finished, screwed from steel pipes and bored.

The ideal option is monolithic. They have been used for a long time in construction, they have an increased bearing capacity, they easily withstand frosty heaving... In addition, there is no need to think about soil drainage. True, this will require special equipment.
have become very popular today. In small private housing construction, such foundations for high groundwater are an optimal and cheap solution. Their only drawback is not the highest bearing capacity. Therefore, you will have to calculate the number of piles and the distance between them. It is recommended to install screw piles to a depth of no more than 3 m.
As for bored structures, this is a good option, which has a high bearing capacity. But this technology also has its disadvantage - it will have to carry out a large amount of drainage measures.

0.5 m and more

When choosing the type of foundation, preference should be given to the slab model. The thing is, if we are talking about a house, columnar structures in such a situation will not be able to provide the necessary bearing capacity of a large structure.

You can use it, but only shallow, which is usually built for small light buildings. In principle, it will withstand a frame cottage. In this case, it is recommended to build a foundation with an extended base.

To the question, Filling it to a depth of up to half a meter, one must understand that its thickness and the method of reinforcement will depend on the number of storeys of the building, as well as on the type of materials from which the walls will be mainly erected. In this case, you will have to think over the technology of thermal insulation. By the way, this is an important stage in the construction of the slab.

Replacement of soil will help to increase the bearing capacity of the slab. It is taken out to the groundwater level and instead sand or crushed stone is poured with careful compaction.

If the soil on the site is very weak, then houses for high groundwater are filled up until its materials displace excess moisture and cease to go into the depths.

1.5 m and more

Comparing the conditions described above, it should be noted that in this case it is possible to use strip-type and slab-type foundation devices on groundwater. But both structures should be shallow.

Options with a close location of groundwater

Above, options for foundations have already been given for close groundwater. Basically, the slab construction is the most commonly used construction in this case. There is no need for her to build drainage ditches, think over methods of protection, because everything is carried out according to standard technology.

High groundwater - building a house without a basement

Everything is quite simple here if a surface or shallow slab is used as a base. That is, a house without a basement is being built and the walls rise immediately from the slab. Here is the sequence of work being done:

A pit is dug up to the groundwater level;
A pillow of sand and crushed stone is poured with a rammer;
Waterproofing with roll material;
Reinforcement cage installation;
Pouring concrete.

If an insulated base is being built, then a heater is placed under the frame. In areas with high groundwater levels, this is an ideal foundation.

High GWL protection

In the construction of foundations on such soils, three types are used, with the help of which it is necessary to ensure the protection of the base structure and the house itself.

A coating, when bituminous mastics are applied to the foundation itself in several layers, due to which a smooth waterproof layer is formed on the surface of the structure.
Roll, when the structure is covered with waterproofing films or membranes in several layers.
Plastering, when cement mixtures are applied, which include waterproofing additives.

The thickness of the waterproofing layer is determined from the calculation of the occurrence of groundwater. The higher their level, the thicker the insulation coating.

Installation of a foundation on a floating cushion

What is a floating cushion? This is a thick layer of a substrate made of several materials, which are separated from each other by insulating films. For high groundwater, it is done very often. Here's the sequence:

Coarse-grained sand is poured onto the bottom of a pit or trench, which is well compacted.
Backfilling is done in layers with tamping of each layer. In this case, the final result is a layer 50 cm thick.
A waterproofing film is laid, roofing material is better.
Crushed stone is poured, rammed to a thickness of 30 cm.
Another layer of roll waterproofing.
Pouring a screed with a thickness of 10 cm.

After that, you can fill in the monolithic reinforced concrete structure of the foundation itself. The pillow creates conditions under which the foundation can move relative to it. Such a foundation is often called a floating monolithic foundation.

Tape

The strip foundation on soils with high GWL is poured in exactly the same way as described in the previous sections. It is important to understand here that a strong base in the form of a tape is a large material cost. They mainly relate to the consumption of concrete and reinforcement. At the same time, they try to construct the tape itself with an extended sole.

If you try to save on something, then the end result can lead to a weakening of the foundation structure, and, therefore, problems with the house itself will begin to arise. Therefore, one cannot deviate from the construction technology and from the exact sequence of the construction operations carried out.

Pile type

As already stated, pile foundation with a high level of groundwater - the optimal solution. The main thing is to choose the right type of laid elements.

As practice shows, when erecting a large heavy house, monolithic reinforced concrete pillars are used, which are made at precast concrete factories.

This technology is called TISE. It is true that it is not the cheapest pile technology of all, but it is the most reliable.

And although screw foundation is cheaper, it does not provide the necessary bearing capacity for a heavy structure. It should be borne in mind that waterproofing in a pile screw or other type device is a mandatory measure.

Conclusion on the topic

So, the question of what foundation to do if the groundwater is close, is very relevant today for many private developers. In the article, almost all situations that they may face were analyzed, and the types of foundation structures were also recommended. In fact, it is important to understand that the choice itself is based not only on the types of soil and groundwater level.

It is important, first of all, to realize what kind of house will be erected in an area with such a soil situation. If a large building of bricks or blocks is planned, then you will have to allocate a decent budget for its construction. And here it is impossible to save money, because a high GWL is a mobile unstable layer with a weak bearing capacity.

In contact with

The foundation with a high level of groundwater is one of the most complex and critical structures.

Such a foundation of a house must be erected, taking into account many of the most diverse factors, each of which must meet all the requirements associated with the danger of flooding and premature destruction of the building.

Accordingly, it is important to correctly determine the level of soil freezing, choose the most suitable base structure and ensure the availability of an effective drainage system.

Determination of the groundwater level and possible concerns

Ground water level

The construction of the foundation with a high level of groundwater must be stable and reliable. What is the degree of threat of subsidence and destruction of the building, they find out long before the start construction works... For this purpose, in the spring or autumn (at a time when the amount of moisture contained in the soil reaches its maximum level), in the place where, in accordance with the construction plan, the basement will be equipped, a hole should be dug with a depth of at least 3 m.

Dig a hole at least 3 m deep

To obtain accurate data, you will need to reliably protect the pit from the weather. After a few weeks, a certain amount of water will appear and settle at the bottom. Perhaps the bottom will remain dry, and then the foundation does not require additional protection.

If the water is at a distance of more than 2 m from the surface, it is necessary not only to calculate the depth at which the foundation will be built, but also to choose the right structure.

What should be the foundation with high groundwater, experts can say after geological surveys.

Piles will raise the level of the house to a safe height

Among the existing foundation structures on high-level groundwater, pile structures are especially popular and trusted by consumers.

Their arrangement will help to ensure high-quality and reliable protection of the base of the house from the negative effects of groundwater:

flooding of basements;
destruction of concrete structures;
the emergence and development of fungus and mold;
violation of the integrity of the foundation itself during freezing in the cold season.

With a high groundwater level, the walls of the pit may float

In addition, a high groundwater level becomes the reason for the sinking of the pit walls and a sharp reduction in the bearing capacity of the soil. This will require additional work to establish an efficient drainage system, including wells and catchments.

The process of leaching minerals from the soil is recognized as the most dangerous, which significantly worsens the strength characteristics of the soil and leads to a change in its structure. Installation of the foundation in such conditions has a number of limitations. Calculation of the depth at which it will be flooded support structure, is carried out taking into account the qualitative characteristics of the soil:

loam;
sandy;
clayey;
mixed.

The level of heaving and the depth of soil freezing depends on this. If the depth of freezing is less than the groundwater level, then when planning, there is no need to make an amendment for soil features.

The calculation is carried out taking into account the type of soil and the possible subsidence of soft soils.

The data obtained most often forces us to abandon the construction of a tape structure, since the work associated with this will be very laborious and require significant material costs.

Variety of foundations and the correct choice of the desired structure

Slab foundation is suitable for clay soils with high GWL in a shallow version

What foundations are needed for houses, if the groundwater is located close, is chosen depending on various features the very site on which the construction is being carried out. The foundation on the water is a structure that must ensure the stability of the building, its durability and reliability. For this, it is necessary to take into account both the quality of the soil and the forthcoming loads from the building.

The construction of a foundation on clay soils with a high groundwater level implies the construction of any type of foundation:

tape, the trenches of which are deeply buried;
pile;
slab (shallow).

The strip base requires the creation of a monolithic reinforced concrete structure located under the external and internal load-bearing walls.

The depth of the trench must exceed the freezing height

First of all, markings are made on the site, in accordance with which they dig trenches of the foundation pit for the strip foundation. Their depth should exceed the freezing height. The calculation is carried out taking into account the peculiarities of weather conditions (temperatures in winter) and soil.

If the groundwater is close, and construction is to be carried out on clay, the strip foundation will perfectly replace the "floating" monolithic slab. The weight of the building is evenly distributed over the entire surface of the slab, which is laid on a sand and gravel pad.

Before making such a foundation, it will be necessary to remove soil from the entire territory of the future foundation. The pit is dug to a depth of 50 cm greater than the thickness of the slab. The calculation is based on the index of soil freezing depth.

Pile foundation of the house - the best option creating a high-quality reliable foundation on clay soils.

By changing the parameters of the piles, it is possible to achieve the installation of supports on solid rocks that are not subject to destruction under the influence of groundwater.

To carry out work on a site with a high ground level, it is necessary to calculate the load on each individual pile.

Construction of various types of foundations

If the groundwater is close to the foundation site, then before proceeding with the construction of the slab base, it will be necessary to prepare ditches around the entire perimeter of the future building. It is better if it is a trench with a width of 20-30 cm and a height (depth) of at least 50 cm. The ditches will be filled with rain or melt water, and thus drainage will be carried out. For more information on the preferred type of foundation, see this video:

To protect the walls of the foundation, treat them with waterproofing mastics.

The "floating" slab does not lie on clay soil, but on a cushion made of sand and gravel. This type of foundation must be poured by erecting it on bulk soil. Before pouring, install the drainage system, laying the drains at a slope of at least 5 cm for each meter of the pipe. To protect the slab, it is necessary to expel the inner surface of the base with waterproofing materials. Roofing material is most often used, laying the canvases with an overlap of 10-15 cm wide. Fasteners are made with bitumen.

A reinforcing frame is laid on the waterproofing and poured with concrete, the filler for which is fine gravel. It is better to fill the entire base in one day.

The strip foundation requires careful preparation of the pit trenches. They must be deep enough and wide enough to exceed the depth of freezing of the ground and allow a high-quality assembly of the formwork structure.

A monolithic tape is poured, taking care of the correct filling of its bottom, high-quality tamping and arrangement of waterproofing. A frame connected from reinforcing bars of various sections is installed inside the formwork. The concrete is poured in layers with the obligatory compaction of each layer. Useful Tips when building a house on soil with a high groundwater level, see this video:

The pile grillage foundation is recognized as the most reliable for the construction of buildings in areas with high ground level. When making such a foundation, it is important to follow the soil indicators, depending on which the size of each of the piles used is established. Piles are used:

screw;
bored;
hammered.

On your own without involving heavy construction equipment mount screw structures. After installing all the piles, a grillage is assembled on them or a beam is laid, necessary in order to tie the entire structure together.

According to SNiP, the soils themselves contain a minimum percentage of silty and clay particles (read non-unguarded), and finances allow you to arrange a massive foundation to a depth below seasonal freezing. An almost ideal situation. Perfect big wooden house with a spacious and dry basement.

But what to do if the groundwater level (GWL) is observed at a shallower depth than the freezing of the soil. In addition, after all, the GWL is not constant - its level can change within certain values every year, but, say, once every 5 years, it seriously increases in the spring for literally a couple of weeks. And these very couple of weeks may be "in excess", and even with what.

Or the option when you bought (or you already have) a land plot on which a trailer stood on concrete blocks for many years, and suddenly you decided to build a wooden house, you have already started digging trenches for the foundation or drilled exploratory wells, maybe they started digging a well , and the water was already at a depth of about a meter, or even less, from the surface of the earth. That is, you are faced with the problem of constructing a foundation with a high level of groundwater.

Of course, the most justified act would be to sell such a plot and buy a more suitable one. But if such a possibility is not considered in principle, and besides, the neighboring houses seem to stand and do not fall. So you can? Sure.

But before rushing for answers on the Internet or shoveling a mountain of SNiPs, SP, GOSTs and other "standards" you need to fulfill the most important thing.

In any matter related to construction, you need to behave like a doctor in communication with a patient. Point number "one", read the guarantee of a correct diagnosis and the effectiveness of subsequent treatment, is an anamnesis or simply a "survey".

It's the same here. Firstly, if houses have already been built in the district, then everything is not so bad as it seems. The very first thing to do is go to a neighbor, talk, ask what and how. How he built, how he built, what materials he used and in what quantity. How did you arrange the foundation, to what depth. Is there a basement or not, is it dry, all year round or not. How many years of observations. Where did you take the excess water. Ask to show if suddenly there are cracks, white salt spots, or other unnatural plaque on its foundation. Pay attention to how the drain from the roof is organized, where. Are there drainage ditches around the site and how deep are they? Did he do his site in any form, or maybe someone from the other neighbors.

Don't hesitate to ask questions. After all, as you know, they don’t take money for questions, and it’s better to ask too much, which will not be useful at all in the future, than to miss something really important. And you can get to know your neighbors better. Nothing brings men closer together like the themes of cars and construction.

Influence of groundwater on the foundation

Great danger to concrete foundation represent not so much the groundwater itself, but the dissolved sulfates, salts and other chemicals. They can provoke the appearance of the so-called "cement bacillus", which loosens and dissolves concrete. This process can be easily detected visually, by the white gypsum coating that has appeared on the surface of the concrete and detached concrete, as if after freezing.

Also, the influence of groundwater directly on concrete leads to the appearance of mold, various fungi, the smell of dampness, the appearance of white or yellowish salt spots.

We build a foundation with a high level of groundwater

So, you already know for sure that the GWL on your site is high, but the decision to build was made irrevocably. What to do to achieve the goal.

High groundwater - building a house without a basement

To begin with, the problem of building a foundation with a high level of groundwater can be circumvented by trying to completely abandon the construction of a basement room. If you planned to use it exclusively for storing vegetables and fruits, then the best step would be to build a wooden house without a basement, and for storage purposes somewhere on the site to build a low storage, hydro- and heat-insulate it and fill it with earth “under the hill”.

In such a cellar it will always be dry, dark and cool. At least this is one of possible options that can be viewed. And the house itself should be built on a columnar foundation - as the most advantageous one with a great depth of freezing of soils and / or their strong heaving.

But there are situations when a columnar foundation is not suitable, for example, the construction of a house is heavy. Here a proposal will come to the rescue to build a strip foundation of shallow depth (MZLF), the so-called "floating foundation".

Floating foundation in high groundwater conditions

In dry pits and trenches

For its construction on an area with a depth of at least 70-80 centimeters and a width of 50 cm, at the bottom of which geotextiles or any other suitable material is lined that will prevent the bedding from mixing with the ground or going into it. In the trenches, a rectangular timber formwork is mounted - it will form a rectangular concrete shape when hardened.

Then they arrange a compacted sand cushion with a thickness of at least 15-20 centimeters, on top of which it is good to arrange a layer of gravel or crushed stone of the same thickness. Moreover, the thicker the crushed stone layer, the better. A large aggregate, due to a sufficiently large distance between each of its particles (read between stones), cannot raise water from the bottom up by capillary means. This will protect the base of the foundation from the direct effects of high groundwater levels and frost heaving forces.

For your information. Always compact the sand while wet. But it is wet, and not damp, since sand reaches its maximum density during compaction only at optimal moisture content. But since such accuracy cannot be achieved in "field" conditions, the main thing here is not to overdo it with water, from the excessive action of which we are trying to get rid of. When compaction (when hitting with a shovel or driving a vibrating plate), sand should not be splashed with water in different directions. If the water turned out to be too much, you should add another centimeter or two of sand.

After compaction of the underlying cushion, it is necessary to waterproof it and insulate the side surfaces of the formwork so that, after pouring the mixture into the formwork, the cement laitance does not seep through the sand cushion further into the ground or out through the cracks between the formwork boards.

Important. Monolithic foundation has one characteristic feature or, more precisely, even a "demand". You need to fill it all, completely and at once. It is also possible to fill in layers, of course, but the stronger the concrete structure will be, the fewer passes it will be erected.

For this reason, using a household concrete mixer is inconvenient. It is much safer and easier to order a ready-mixed concrete machine of the desired brand in any construction company... But you need to take into account in advance the possibility of such a car approaching the foundation under construction from different sides.

After the isolation of the formwork, a frame made of reinforcement with diameters of 10-12 mm is installed in it. It is made of pre-prepared rectangular (0.3 width and length for the length of the foundation) reinforcing mesh, twisted together with wire. Reinforcement profile numbers, their cross-sectional areas, as well as the mass of 1 kilogram suitable material you can find in table number 1 GOST 5781-82 *.

Then concrete is poured into the formwork.

When pouring concrete, it is important not to allow the appearance of air bubbles inside the mixture; for this, you need to constantly pierce the mixture to be poured with any available means (even with a shovel handle). The mixture must be constantly leveled and tamped. A shovel is fine, but having a construction vibrator will only speed up and simplify things.

In pits and trenches flooded by groundwater

It happens that the foundation pit seems to be dug, and in general is shallow, but moisture appears at its bottom, and now you do not have time to blink an eye, and its entire bottom is already in the water.

Someone will advise building the formwork and pouring the mixture with a low amount of water as it is, arguing that the concrete will take additional water from the groundwater as much as it still needs. This is, of course, wrong.

It is wrong because he will take something, but he will take only in his lower layer, and the upper ones will remain dry. In addition, the groundwater is dirty, and all this dirt and impurities, mixing with concrete mix, in the process of hardening.

In the same way, the soil itself at the bottom of such a trench will partially mix with the lower layers of the foundation under construction, and this is bad for him.

Firstly, in such places, after hardening, a loose, large-pored material of concrete and lumps of soil will turn out. Secondly, with this method, it will not be possible to achieve the evenness of the base of the foundation, and therefore the load from the building will be unevenly distributed to the ground, which will lead to local subsidence, cracks and partial, or maybe complete destruction of the foundation.

High groundwater - we arrange drainage

Open water drawdown by direct pumping of groundwater from trenches with a drainage pump. But this water must be constantly diverted somewhere, so the presence, for example, of a pond near the construction site will only be a plus;

But this method cannot be applied everywhere. More precisely, it is allowed only if there are no suffusion phenomena.

For reference. Suffusion is the removal of small mineral particles of the rock by water filtering through it. It leads to subsidence of the overlying strata of the earth and the formation of suffusion funnels and depressions. Another consequence may be a change in the granulometric composition of rocks, as a result of which the soil loses its bearing capacity, is capable of further subsidence and collapse.

In other words, if you decide to pump out water from trenches or a pit with a drainage pump, but in the process of work noticed that small springs of water appear at the bottom of the trenches, carrying soil particles upward, as if small volcanoes are lava, then the process of pumping water with a drainage pump must be stopped immediately , and raise the water level in the trenches until the suffusion phenomenon disappears. In this situation, the use of a drainage pump is no longer suitable and an alternative method of dewatering will need to be looked for.

Dewatering by a drainage device of the entire site.

We've all seen the ditches around on numerous occasions land plots, but not everyone knows what they are for. A network of drainage ditches around land plot and it just settles on it in order to lower the level of groundwater. After the ditch is dug, groundwater begins to flow into it from all sides as to the only place on the land plot where there is no ground resistance and there is no capillary rise of the liquid upward. For clarity and understanding of this process, you should pay attention to the following figure.

Additionally, you can take a whole range of measures to increase the foundation and lower the high level of groundwater. To do this, after the installation of drainage ditches, you can not just arrange a sandy base on the site for the house, but additionally raise (add) the level of the entire land plot by another half a meter. The event, of course, is costly, but it may well justify itself in the end.

In addition, you need to take into account that it is not enough just to dig ditches around the site, you also need to ensure the drainage of water from these ditches.

For your information. To prevent the walls of the ditch from sliding, it can be completely covered with gravel or natural crushed stone (for environmental reasons, it is highly discouraged to use blast-furnace slag for this purpose). Or fill the ditch with a sand and gravel mixture (ASM), having previously laid a drainage pipe on its bottom.

But, just like in the previous version, a situation may arise here when it is impossible to carry out drainage on the site. In this case, you will need a consultation with a highly qualified geologist or designer specializing in foundation calculations.

Protection of the foundation from groundwater

Since any foundation or building elements must be reliably protected from the influence of groundwater, then this must be done even at the initial stage of construction. The complex of protective measures may include different ways waterproofing and drainage.

In order to protect the foundation from the influence of surface groundwater (the so-called "top water"), a blind area or sidewalk is arranged around the building under construction.

If a wooden house is built without a basement or with it, but the groundwater is located below the basement floor level, then a waterproofing device against capillary moisture will be sufficient measures. For this, the outer walls of the foundation, even at the stage of its construction, immediately after its hardening, are insulated with any waterproof material, for example, mastics, resins or bitumen, in 2-3 layers.

If a basement is also being built, and the calculated groundwater level is above the level of the basement floor, then it is additionally necessary to arrange drainage.

But this is feasible only if the terrain and living conditions make it possible to drain excess water into a pond, river, along a natural slope, etc. Otherwise, it is necessary to arrange the basement waterproofing.

This is done even before pouring the basement floor, by laying roll waterproofing materials in several layers on the bottom of the pits and trenches. Depending on the pressure of groundwater, the appropriate grade of concrete is selected for pouring the floors, and sometimes it even becomes necessary to reinforce them.

High groundwater level and frosty heaving of soils

With a high level of groundwater exceeding the depth of seasonal freezing of the soil, one should not forget about another problem - this is a pronounced frosty heaving of soils.

In order for the foundation to not experience heaving forces at a high level of groundwater, measures must be taken to waterproof it. After all, when freezing heaving soil in winter and increasing it in volume, it acts on the foundation (as if pushing it out) not only from below, but also presses on the side surfaces of the base of the house.

Therefore, the foundation must be seriously protected from moisture and for this reason.

Be that as it may, with a high level of groundwater, it is impossible to rush to the construction site and one should resort to the advice of experienced builders, because in each specific case, the complex solutions to this situation will differ.

Often, when starting construction on summer cottage there is a problem associated with a high level of groundwater (GWL). GWL are water layers lying close to the surface. Their level directly depends on the season. It usually rises strongly in spring and autumn, when snow melts or heavy rains occur. The construction of a foundation with a high level of groundwater is also made more difficult by the presence of clay in the soil. In this situation, there can be no question of a cellar.
However, the foundation with a high level of groundwater can be laid if you adhere to the recommendations and the technological process.

Influence of GWL on the foundation

The foundation with high groundwater is more influenced by salts and substances dissolved in it. It is they who, reacting with concrete, gradually destroy it. From this, the base is gradually loosened and stratified. Visually, cracks, plaque, yellowish spots, fungus appear, and when you are close, you can smell dampness.
Problems begin to arise already in the process of digging trenches or a foundation pit. Rising water softens the bottom, washes away the soil, significantly impairing its physical condition, making it unable to withstand the pressure of concrete. In such a situation, drainage should be done immediately.

How to determine GWL

In river valleys, flood meadows, lowlands, they are determined by the naked eye. In the spring, the water stays there for a very long time, in the summer, if you go deeper with a couple of bayonets, the soil will be wet.
When starting work, you can contact the organizations dealing with these issues, if there are none, then you can do everything on your own:

in the immediate vicinity of the proposed foundation, make a pit, or preferably two in different places, with a depth of about 3 meters, a width of 1 m is recommended, but this value is not fundamental and depends to a large extent on the size of the one who will dig;
close it with something to avoid rainfall;
after about a day, the pits are opened and the water level is measured;
if the bottom is dry or the depth reaches below 2 m, then the GWL is low or moderate. In this case, there is no need to resort to any additional protective measures.

If the water level indicator is above two meters, then you will have to install drainage and take care of waterproofing.

Foundations for soils with high GWL

Consider what kind of foundation is needed for a house if groundwater is close.

On screw piles. Most suitable for wetlands, foundations on the water and in areas of constant flooding. In some cases, it can be used as foundations on fill soils. Installed quickly enough. A significant disadvantage is that it is not able to take high loads.
Plate. In this case, there is no need for a deep bookmark. This structure must be placed on a cushion of sand and gravel and insulated with plastic wrap or roofing felt. Otherwise, a crack may go. For the price, it is not economical.
Brick. Such a basis is good in that even winter ground movements do not have a significant effect. When working with a brick, you will have to take care of good waterproofing, and the presence of a high-quality blind area will also protect it from precipitation. The negative feature of such a foundation is the high cost, both in financial plan, and in terms of time and effort.
Floating is a strip foundation that is resistant to soil heaving. It is the most suitable for suburban construction. It does not sink much, therefore it is able to withstand the load of not very heavy buildings.

Water disposal when laying trenches and foundation pits

At the stage of digging pits or trenches under the foundation, it happens that the water level rises so quickly that it is difficult or impossible to carry out activities. To do this, it is necessary to drain the building area. For this, special drainage pumps or motor pumps are used.
Water should be pumped out until soil particles begin to be removed to the surface. If this starts, pumping is stopped.
For drainage, plastic sewer tubes with a diameter of 110 mm are used. Through them, the liquid will go by gravity into wells or water reservoirs, prepared in advance for this purpose, or into drainage ditches, reinforced with shields to avoid ground collapse.
Ideally, the groundwater level should be 200 - 400 mm below the level of the construction site.

Drainage equipment

If groundwater is close, then drainage is not a luxury, but one of the main guarantees of the strength and durability of both the foundation and the structure as a whole.
His device will require such materials:

sand;
crushed stone;
geotextile;
drains (drainage pipes).

For foundations with a high groundwater level, circular drainage is advisable. First, a trench about 400 mm wide is dug. The depth is calculated individually: the moat should be at the level of the sole, and preferably 200 - 300 mm lower.
It is recommended to carry out installation at a distance of no more than 20 m at a time, but with regard to the country house, outbuildings, baths, garages - this wish is most likely conditional. We must proceed from their real circumstances.
Sand is poured onto the bottom and carefully rammed. The thickness should be 200 mm. To prevent siltation, geotextiles are laid on the sand.
The next layer is crushed stone (gravel). Its thickness is also 200 mm. The choice of rubble is not the last. It is advisable to get washed. If not, you will have to sift at least. If liquid gets in, contaminated material impairs drainage: particles of dust, sand, earth in the water reduce the space between the gravel.

The gravel pad is covered with geotextiles, the pieces of which should overlap from 150 to 300 mm.
The next stage is draining. The perforation in the factory pipes is located on one side. She is laid on geotextiles. To save money, you can make drains yourself. In a conventional plastic sewer pipe, holes are drilled, the optimal diameter of which is 5 mm. The distance between the holes is no more than 10 mm.
When the pipeline device is completed, it is closed with geotextile, and backfilling is carried out. Sand layer not less than 200 mm, gravel (crushed stone) - 150-200 mm. To the top, the trench is filled with previously excavated soil.
At good quality drainage system, there will be practically no water under the foundation.

Construction of a strip floating foundation

Let's consider how to make a foundation. For building good home, if the groundwater level permits, a medium-deep view is chosen.

A trench is pulled out with a depth of 700-800 mm, a width sufficient for the device of the formwork and its subsequent dismantling.
The bottom is covered with waterproofing material.
The formwork is installed and strengthened, which is insulated from the inside with a film.
A sand cushion with a thickness of 200 mm is poured and compacted.
The next layer is gravel or crushed stone. The thickness is the same or thinner by 5-10 mm.
Insulate the pad from the concrete. Roofing material, plastic wrap is used.
The frame is made of reinforcement Ø12 mm and installed in the formwork.
The solution is poured. It should be noted that the concrete tape is continuous. This will help create a solid monolith. No need to rush, you need to fill in layers. Each is punctured by rebar to expel excess air and compact the concrete. When one layer has set enough, you need to fill in the next one.

To prevent the concrete from drying out, it must be watered daily, and covered with a film at night.
After the final hardening of the concrete, the formwork is removed and waterproofed with bitumen.

Monolithic slab foundation on a floating cushion

For drainage of swampy soils and leveling the relief, fill soil is used. If a person does it on his own, that's one thing. And if you buy a plot where these works were carried out several years ago, it is completely different.
The essence of the problem is that such soil does not have homogeneous structure, as it should not be compacted, as a result of which foundations on such soils can give uneven shrinkage. And if the GWL is still high, then problems cannot be avoided.
For these types of soils, a number of foundations can be used:

screw piles, but only if they will enter the "mother" established soil. To find out, an examination is being carried out. Do not forget that screw piles are not designed for heavy structures;
for a strip base, a qualitative analysis is also needed;
A monolithic slab, although an expensive measure, is most suitable for foundations on bulk soils and with a high GWL.

Stages of building a foundation on a floating cushion

A pit of a given size is being dug. You can do it yourself, you can use special equipment.
The bottom is carefully compacted. Here it is better to use a vibrating plate - a device that allows you to quickly and efficiently compact the soil.
Sand is used for the floating cushion. It is covered with layers, each of which is compacted. The thickness of the pillow should be at least half a meter.
The pillow is covered with geotextile (other waterproof material).
A layer of crushed stone with a thickness of 150-200 mm is poured.
Roofing material is laid on top of the crushed stone.
Formwork is formed and inserted inside. From the outside, it is carefully reinforced.
Reinforcing mesh with cells of 200 × 200 mm is welded. For it, a reinforcement with a diameter of 12 mm is taken. There is a single rule: the bottom row does not reach the sole by 50 mm, the top row does not reach 50-70 mm. Vertical bars are cut based on the thickness of the slab and staggered.
For pouring concrete, it is better to use a mixer to make it during the day.

Then everything follows the standard scheme: the plate is periodically moistened and sheltered from precipitation. When the mortar is completely dry, the formwork is dismantled, and the slab is treated with a waterproofing mixture.
The foundation built in this way will stand without fear of flooding and swelling of the soil. This is the most expensive option, but in this case it is the most suitable.
Your home requires both costs and the right attitude to the construction process. As a result, it will delight you for years to come.