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T-shaped foundation at high angle. High groundwater level and foundation. Floating foundation in conditions of high groundwater

The high level of groundwater on the site causes many problems for the developer, primarily in relation to foundation structures. After all, a foundation with a high groundwater level (GWL) requires increased investment in its installation, and the entire construction technology becomes significantly more complicated. What should be taken into account during the construction phase in order to calculate all the necessary costs.

What are the dangers of groundwater?

Groundwater (soil) water refers to moisture that collects in the upper layers of the soil above the natural hydraulic barrier, which is usually clay. The level of these waters is most often unstable; it can reach maximum values ​​in the spring (flood) or autumn (heavy rainfall).

The main danger of high GWL lies in the following factors:

How to determine the groundwater level

Before starting construction, it is necessary to determine the groundwater level on the site. This should be done at the time of maximum water inflow (autumn or early spring). In some cases, they simply monitor the water level in the well available on the site.

But when deciding what kind of foundation to make if groundwater is close, you should not rely on this information. The fact is that traditionally wells are installed in places with maximum water inflow, so the information received may be distorted due to this factor.

Therefore, it would be correct to define the ground level as follows:

Relationship between groundwater level and soil freezing depth

When choosing foundations for high groundwater, the depth of soil freezing should also be taken into account. After all, it is the combined effect of these two factors that causes the so-called frost heaving of the soil. Moisture in the soil turns into ice under the influence of negative temperatures, and its volume increases significantly, which leads to soil displacement. The resulting forces can in some cases even destroy the foundation.

Building codes offer the following options for developing projects and performing work, taking into account the sum of these two factors:

The drainage of groundwater from the foundation must be carried out in accordance with the developed project; carrying out work without a clearly developed plan will not bring tangible results.

Pile type foundations

The best option for such conditions is considered to be a pile-type foundation. In practice, reinforced concrete, bored, screw and other types of piles are used. With their help, you can install a foundation not only in areas with high levels of soil (ground) water, but also in wetlands. Quite often, such structures are used for the construction of pier structures, and even buildings on the water.

When choosing which foundation to build when the groundwater level is high, you should definitely consider the possibility of using pile technology. It is in this way that the foundation can be supported on stable layers of soil, while installation does not require a large amount of excavation work, which is significantly complicated by the presence of a high groundwater level.

It is worth considering the fact that the use of a pile foundation may make it impossible to construct basements or basements. Therefore, in private housing construction, another technology for constructing a foundation in such conditions is often used.

Prefabricated block foundation

It’s worth saying right away that this technology is quite expensive in conditions of high soil water levels, but in many cases it has no alternative:

• First of all, it is necessary to construct a pit under the entire future building. In this case, groundwater must be constantly pumped out, and its access is limited by installing enclosing structures made of polymer and metal piles of a special shape (locking connections). Such a wall can almost completely block the access of groundwater to the pit.
• After draining the pit, the soil is thoroughly compacted and a footing is made (a thin concrete screed that allows the base to be leveled).
• Any foundation below the groundwater level must be carefully waterproofed; only this and the installation of drainage networks can guarantee reliable protection of the structure.
• Two layers of roofing felt or other modern built-up materials are laid on the footing. In this case, a certain reserve must be left, which will then be placed on the foundation walls.
• First of all, the reinforcement of the site is carried out; the diameter of the rod and the pitch of its laying are determined using calculations using a special technique. The fittings must be connected only by viscous connections; welding is not allowed.
• The slab should be poured in one go, this will ensure the solidity of the structure. Therefore, it is best to resort to delivering ready-mixed concrete of the required type to the construction site.
• After the base slab receives the required strength (the period depends on many indicators), you can begin to build a foundation from reinforced concrete blocks.
• The outer surface of the foundation must be covered with coating or roll waterproofing.
• Before backfilling the pit, it is necessary to install a drainage system. The most effective is considered to be a design made of perforated polymer pipes, crushed granite stone and protective geotextile fabric.

The cost of constructing foundations in areas with high groundwater levels can be several times higher than the price of conventional structures. But, unfortunately, these conditions do not imply alternative options.

If groundwater lies high on loam, sandy, clayey or mixed soils, the foundation must be built to a depth exceeding the freezing level of the soil. At the same time, groundwater is drained; this event is considered mandatory in the data.

In the case when the depth of soil freezing is less than the water level, it is possible not to make an adjustment for the type of soil, but to plan the foundation, taking into account only the load of the building being erected. When carrying out calculations, they make an adjustment to the groundwater level (GWL) from half a meter to a meter in the direction of increasing the actual level. For this reason, a strip foundation is excluded from possible designs, since its price will be very high, and construction work will be cumbersome and costly.

When calculating the foundation on soils with high groundwater level, an allowance is also made for subsidence of weak soils. If there is a possibility of seasonal flooding, it is recommended to construct buildings using monolithic reinforced concrete structures. In this case, the piles should be below the level of possible soil erosion.

Foundation construction technology at high groundwater level

First, to the depth of the foundation, after which the site is leveled and drained. To do this, a ditch is dug along the perimeter of the site with an indentation of 50 cm, its depth is 30 cm, and its width is about 20 cm. The ditch is designed to accumulate and drain moisture. Foundations are not constructed in groundwater, as there is a high risk of mortar being washed out and the slab being constructed being weakened.

Then soil compaction and concrete preparation are carried out at the construction site. This layer is necessary to level the site and place waterproofing. After drying, it is covered with two layers of roofing felt with an overlap of each layer of 10-15 cm. Sand and crushed stone are not used, and fixation is done using ordinary bitumen, brought to working condition.

The foundation will be located below the groundwater level, so concrete slabs, walls and ceilings must be insulated from moisture. When laying roofing felt, the material must be moved beyond the screed to protect the slab, and also overlap the foundation walls being built.

After this, a structure made of iron mesh products and fittings is installed; it will need to be filled with mortar. It is recommended to create a slab with a thickness of 15, 20 or 30 cm. The thicker this slab is, the better it will hold the foundation and walls of the building. After the slab has stood, you can begin building the foundation. It is best to use reinforced concrete blocks that form a monolithic well. When the work is completed, all that remains is to protect the foundation from groundwater using bitumen-based mastic or film waterproofing.

• Date: 05/13/2015
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• Rating: 70

Construction of a house can be carried out on almost any type of soil, but for complex soils saturated with water, the type of foundation should be carefully selected. This concerns not only the question of how deep to dig the foundation to ensure its strength and stability, but also what exactly it should be. Today, only a few types of base are used for such conditions. These are concrete blocks, screw piles, floating monolithic foundations, while the latter option and blocks are often combined. What to prefer, what types of foundation are better suited if groundwater comes close to the soil surface?

A pile foundation for a house is suitable for complex, heaving soils.

Foundation options for close proximity to groundwater

What kind of foundation is needed on the site if the groundwater is close to the surface? There is only one answer - this is a special floating type of foundation. Such a foundation is intended for laying on weak-bearing soils, on bulk soil, on heavy heaving soil and under conditions where groundwater is low, that is, there is a possibility of flooding of the foundation, basement, and ground floor structure, and this has a negative impact on the entire structure.

If groundwater lies close to the soil level, then it is the floating foundation that can reliably protect the entire structure from destruction and possible deformation under the negative influence of constant moisture.

A trench is dug under the slab, at the bottom of which a cushion of sand and crushed stone is poured.

The construction of such a foundation for a frame house or any other structure is quite simple; it consists of pouring a monolithic concrete slab, which is reinforced with steel rods. The technology for constructing such a foundation for a frame house of a small area, when groundwater lies close to the surface, is simple: you need to dig a trench with a depth of 60 cm, at the bottom of which a 10 cm layer of crushed stone and a 50 cm layer of sand are poured. After this, the resulting cushion is soaked water, sits down and fills up to the required level. Next, at ground level, you should lay out pillars of brick, which are one and a half to two bricks wide (concrete blocks can also be used), covered on top with roofing felt, a simple wooden board protected from rotting (this layer should be 40 mm).

In this case, it is not recommended to install a strip foundation, since moisture under the base of the house should be distributed evenly, and not squeezed out from under some areas and flow into others. A strip foundation, despite all its advantages, when installed on this type of soil creates excessive hydrostatic pressure, that is, there is a danger of its distortions, deformations, and cracks will begin to appear along the walls of the structure. You can use a strip foundation when the groundwater level is high enough; in other cases it is not recommended to use it. Sometimes such a structure can be installed on loam, but it is necessary to ensure proper waterproofing. If you decide to use a strip foundation, conduct preliminary geological studies that will show how realistic it is.

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Is it possible to use a pile foundation?

Options for using screw piles for foundations.

A pile foundation is recommended for complex soils, heaving soils; it can be used for nearby groundwater and quicksand. For example, cities such as St. Petersburg and Venice have buildings built on pile foundations, which perfectly protect houses from moisture, giving them strength and reliability.

Most often, a pile foundation is made using special screw piles, characterized by the highest load-bearing capacity. This is due to the fact that when going into the ground, it does not loosen it, as when using conventional driven piles, but compacts it, that is, the earth between the steel screws is compacted, making the support reliable and stable. The question of which pipes are best for such a foundation is easily resolved. These are steel, which have a zinc coating to protect against corrosion (if necessary, other types of protection are additionally provided). At the end of the pipe, which goes deep into the soil, there are screw blades that allow you to securely screw in the pile.

Determining a place for construction is quite simple, since such a base option can be installed in almost any conditions. Constructing a foundation using screw piles is cost-effective; heavy construction equipment is completely unnecessary.

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This type cannot be used only for rocky soils.

How to make a floating foundation with your own hands?

If groundwater is close, building a foundation involves creating several layers.

• If groundwater lies close to the surface, work can be done to install the so-called. A pie of this base includes the following layers (starting from the bottom):
• sunken soil;
• compacted sand and gravel bed;
• concrete monolithic base;
• vapor barrier layer;
• thermal insulation layer;
• a layer of waterproofing (usually a polyethylene film);

reinforced concrete screed.

The technology itself for building a foundation for a house has certain features for each individual case. It depends on what type of soil the house is being built on, what features of the terrain are observed, what kind of house is being built (frame, more massive).

The process of constructing a floating foundation for a house includes the following steps:

1. Digging a pit, the dimensions of which correspond to the size of the future house. The depth must be precisely determined by the project; it depends not only on how close the groundwater lies, but also where the freezing point of the soil is located. The bottom of the pit must be compacted, this must be done evenly. Do not leave areas or pits with sand filled with moisture or loosened.
2. Next, the construction of the foundation for the house involves laying a sand and gravel cushion. If the groundwater in the area is too close, it is recommended to use geotextiles first, which are laid along the entire bottom of the pit. This preliminary layer will provide effective protection against sand washout. The thickness of the fill for the foundation of the house should be 20-30 cm, each layer of fill must be moistened, then compacted and filled to the required level. You can use a vibrating plate for this.
3. Next, the foundation of the frame house (or any other structure) is poured with concrete. The solution usually used is brand M100. It is not difficult to determine the thickness of the fill; usually 10 cm is enough; the dimensions of such a screed should slightly exceed the dimensions of the foundation slab for the house itself. Before pouring, it is necessary to install formwork from ordinary edged wooden boards.
4. After the concrete has dried, a layer of waterproofing film is laid on top, overlapping the walls, all seams of which are carefully sealed. The edges of such a film for the foundation of a house are wrapped. This provides excellent waterproofing for the foundation and the house itself. Laying thermal and vapor barriers may depend on the project, but most often it is recommended to do it, especially if the work is carried out on soils where groundwater comes close to the surface. What material should I choose for this? Polymer films and solid insulation in the form of slabs that do not shrink are best suited. The project must contain information about what materials are selected and what their thickness is, so that a pit can be dug to the required depth.
5. After the insulation is laid, a layer of plastic film is also covered on top.

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Queue for reinforced concrete structure

Steel reinforcement for a monolithic slab is laid in two belts.

Now work is underway to construct a monolithic reinforced concrete slab. Blocks are not used; first, reinforcement is performed using steel reinforcement in two belts, after which the entire space is filled with concrete. For this, formwork is required; concrete with grade M300 (no less) is used for the solution; its class should be B22.5. After pouring the foundation for an area where groundwater comes close to the surface, you must wait the required time for drying.

If necessary, all communications are laid along with the construction of the foundation. All pipes and so on are laid simultaneously with the construction of each layer; if necessary, special protective corrugated pipes are used. Filling such a base is done at one time. If you break this rule, the strength will be extremely low, and it is important when laying is carried out on loams or on complex and weak soils, where groundwater lies close to the surface.

In some cases, it is recommended to use additional concrete blocks as a floating foundation, which are mounted on a thin concrete base in the form of support pillars, but this is rarely done. Using such blocks for the base, you only increase the cost of the work, since their installation is associated with some difficulties. And the strength of the foundation for which concrete blocks are used is not too great. It is best to use an option in which the one for which blocks are used is completely filled with concrete after completion of work, then covered with a layer of liquid waterproofing rubber.

But a similar option, in which blocks are used, is comparable in cost to a conventional monolithic one, and their strength is almost the same. That is why, when the question arises whether reinforced concrete is monolithic or for which concrete blocks are used, followed by pouring and waterproofing, preference is given to the first.

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

The influence of ground water on the foundation

When groundwater is high, the foundation is more affected by salts and substances dissolved in it. It is they who react with concrete and gradually destroy it. As a result, the base gradually loosens and delaminates. 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 pits. Rising water softens the bottom, washes away the soil, significantly worsening its physical condition, making it unable to withstand the pressure of concrete. In such a situation, drainage should be done immediately.

How to determine groundwater level

In river valleys, water meadows, and lowlands, they are visible to the naked eye. In the spring, the water stays there for a very long time; in the summer, having gone a couple of feet deeper, the soil will be wet.
When starting work, you can contact the organizations involved in 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, about 3 meters deep, the recommended width is 1 m, but this value is not important and depends largely on the size of the one who will dig;
• cover it with something to prevent precipitation from entering;
• 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 is higher than two meters, then you will have to install drainage and take care of waterproofing.

Foundations for soils with high groundwater level

Let's consider what kind of foundation is needed for a house if groundwater is close.

• On screw piles. Most suitable for wetlands, foundations on water and those in areas of constant flooding. In some cases it can be used as foundations on bulk soils. Installs quite quickly. A significant disadvantage is that it is not able to take high loads.
• Slab. In this case, there is no need for deep laying. This structure must be placed on a bed of sand and crushed stone and insulated with plastic film or roofing felt. Otherwise, a crack may occur. In terms of price, it is not economical.
• Brick. This foundation is good because even winter soil movements do not have a significant impact. When working with brick, you will have to take care of good waterproofing, and the presence of a high-quality blind area will protect it from precipitation. A negative feature of such a foundation is its high cost, both financially and in terms of effort and time.
• A floating foundation is a strip foundation that is resistant to soil heaving. It is the most suitable for dacha construction. It does not go very deep, so it can withstand the load of not very heavy buildings.

Water disposal when laying trenches and pits

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

Drainage system 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.
Its device will require the following materials:

• sand;
• crushed stone;
• geotextiles;
• drains (drainage pipes).

For foundations with high groundwater levels, circular drainage is advisable. First, a trench about 400 mm wide is dug. The depth is calculated individually: the ditch should be at the level of the base, or 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 a country house, outbuildings, bathhouses, garages - this wish is most likely conditional. We must proceed from their real circumstances.
Sand is poured into the bottom and compacted thoroughly. 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 crushed stone is not the least important. It is advisable to purchase washed. If not, you will have to sift at least. When liquid gets in, contaminated material impairs drainage: particles of dust, sand, and earth in the water reduce the space between the crushed stone.

The gravel bed is covered with geotextiles, the pieces of which should overlap from 150 to 300 mm.
The next stage is laying the drains. Perforation in factory pipes is located on one side. This is used to lay it on geotextiles. To save money, you can make drains yourself. Holes are drilled in a regular plastic sewer pipe, the optimal diameter of which is 5 mm. The distance between holes is no more than 10 mm.
When the pipeline installation is completed, it is covered with geotextiles and backfilled. A layer of sand of at least 200 mm, gravel (crushed stone) - 150-200 mm. The trench is filled to the top with previously removed soil.
If the drainage system is of good quality, practically no water will get under the foundation.

Construction of a strip floating foundation

Let's look at how to make a foundation. To build a good house, if the groundwater level allows, a medium-depth type is chosen.

• A trench is dug 700-800 mm deep and wide enough for the installation of formwork and its subsequent dismantling.
• The bottom is covered with waterproofing material.
• The formwork is installed and strengthened, which is insulated with film on the inside.
• A sand cushion 200 mm thick is poured and compacted.
• The next layer is gravel or crushed stone. The thickness is the same or 5-10 mm thinner.
• The pillow should be isolated from the concrete. Ruberoid and polyethylene film are used.
• A frame is constructed from Ø12 mm reinforcement and installed in the formwork.
• The solution is poured. Care should be taken to ensure that the concrete strip is continuous. This will help create a durable monolith. There is no need to rush, you need to fill it in layers. Each is pierced with reinforcement to force out excess air and compact the concrete. When one layer has set sufficiently, you need to pour the next one.

To prevent the concrete from drying out, it must be watered daily and covered with film at night.
After the concrete has completely hardened, the formwork is removed and waterproofing is done with bitumen.

Monolithic slab foundation on a floating cushion

To drain swampy soils and level the terrain, bulk soil is used. If a person does it on his own, that’s one thing. But if you buy a plot where this work was carried out several years ago, it’s completely different.
The essence of the problem is that such soil does not have a uniform structure and is not properly compacted, as a result of which foundations on such soils can shrink unevenly. And if the ground level 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 go into the “mother” established soil. An examination is being carried out to find out. Do not forget that screw piles are not designed for heavy structures;
• for the strip base, qualitative analysis is also needed;
• Although a monolithic slab is an expensive undertaking, it is most suitable for foundations on bulk soils and at high groundwater levels.

Stages of building a foundation on a floating cushion

• A pit of a given size is dug. You can do it yourself, or 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 filled in layers, each of which is compacted. The thickness of the pillow should be at least half a meter.
• The pillow is covered with geotextile (another waterproof material).
• A layer of crushed stone 150-200 mm thick is poured.
• Roofing felt is laid on top of the crushed stone.
• The formwork is formed and inserted inside. It is carefully strengthened from the outside.
• A reinforcing mesh with cells 200×200 mm is welded. For it, reinforcement with a diameter of 12 mm is taken. There is a single rule here: the bottom row does not reach the sole by 50 mm, the top row by 50-70 mm. Vertical rods are cut based on the thickness of the slab, and are arranged in a checkerboard pattern.
• To pour concrete, it is better to use a mixer to do it during the day.

Then everything follows the standard scheme: the slab is periodically moistened and protected from precipitation. When the solution 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 own home requires both costs and the right attitude to the construction process. As a result, it will delight you for many years.