Strip foundation. Part 1: types, soils, design, cost

Recommendation points

• Types of strip foundations
• Calculation of the strip foundation
• How to independently determine the type of soil
• We count the loads from the building
• Reinforcement belt calculation
• How much is a strip monolithic foundation

In this article, we will discuss the features of the main types of strip foundations, consider their disadvantages and advantages. Today we will learn to independently determine the types of soils on the site, calculate the specific load on the natural foundation from the building – in other words, design simple foundations. We will also talk about how their value is formed..

• How to drain the site
• What type of foundation to choose
• Strip foundation. Part 2: preparation, marking, earthwork, formwork, reinforcement
• Strip foundation. Part 3: concreting, final operations
• Strip foundation. Part 4: assembling concrete block structures
• Column foundation
• Pile foundation
• Slab foundation

The strip foundation is the most versatile and most common type of foundation for the construction of a private house. It is the tape design that first comes to mind for most people when it comes to foundations..

Strip foundations are a continuous strip – “tape”, which is located under all load-bearing external and internal walls, or under the rows of support columns. Throughout its length, the “wall in the ground” structure has the same shape and cross-section. Most often, a rectangular section is used, but if it is necessary to reduce the pressure on a weakly bearing and easily deformable soil, that is, options with a trapezoidal shape (wide base, inclined walls) or an inverted letter “T”.

Being a single whole, the strip foundation redistributes well the loads from the house, therefore it can be used at any depth of freezing, on almost any soil, with any building in terms of mass and configuration. The strip foundation cannot be called simple and cheap, it is not always economically justified, and in some cases an inexpensive and less labor-intensive columnar foundation can be used instead. However, in most cases there is no alternative..

The strip foundation is irreplaceable in the following cases:

• when erecting heavy houses (stone, concrete, brick) with massive slab floors;
• if the building will have buried premises (basement, garage, basement);
• when uneven soil bedding is found under the building;
• in an area where there is a high level of groundwater;
• the site has a significant slope.

Types of strip foundations

According to the method of construction, tape, like any other foundations, are divided into prefabricated and monolithic. A monolithic foundation is made directly at the construction site by pouring concrete into a special formwork, in which a belt of reinforcement is laid. Prefabricated foundations are made from piece elements. These are not necessarily concrete blocks and pillows such as FBS and FL, the foundation can be assembled from brick, rubble stone, small blocks 200x200x400 mm.

Reinforced concrete or rubble concrete monolithic foundation is preferable to precast, if it is necessary to protect the building from high groundwater, since it does not have vulnerable joints. The monolithic structure does not have any spatial restrictions, it can be used for a house of any configuration, whereas, for example, FDS blocks are difficult to apply on roundings and oblique corners, and the height of the foundation is cyclical to the dimensions of piece elements. When preparing concrete directly on the site, it is often possible to do without the use of technology; it is quite possible to limit ourselves to electric concrete mixers and other means of small mechanization. Moreover, a monolithic foundation is cheaper than a precast foundation, although it takes longer to build, it is more complicated (reinforcement work, formwork installation) and is more time consuming. The service life of reinforced concrete belts is about 150 years, while foundations from factory blocks work up to 75 years.

Prefabricated strip foundations are used practically only in the post-Soviet space, Western designers prefer monoliths, relying on the integrity of the structure and cost advantages. In our country, precast foundations made of FBS are no less popular than monolithic ones, and the reason for this is the low level of construction culture – reinforcement workers, concrete workers, engineers and technicians do not have sufficient qualifications, almost no technical means are used to move and compact concrete, not all teams are equipped with high-quality formwork. All these problems are accompanied by a large number of very important hidden works, which is why our developer is ready to overpay for industrialization. In the next article on the installation of strip foundations, we will definitely consider prefabricated bases from blocks.

Rubble foundations are rarely used, since they are very time consuming, everything has to be done manually, in addition, such structures are not used on clay and heterogeneous soils. But on rocky and sandy foundations, if rocks are being developed in your area, a rubble foundation can significantly reduce the cost of construction. Rubble foundations dry (in layers) are assembled from flat rubble stones up to 30 cm wide, after which all the gaps between the piece elements are filled with cement mortar. Another way is to sink the stones into the laid mortar. Large gaps are filled with small pebbles (chipping).

Brick foundations are also infrequent guests on our construction sites, since brick strongly absorbs moisture and can collapse when freezing. Only well-fired clay bricks can be used. The underground part of the brick foundation is necessarily covered with a layer of waterproofing, but often this building material is used only for the above-ground part, in tandem with other more practical structures, for example, “brick on buta”, “brick on concrete”. The service life of brick foundations is limited to 30-50 years.

By the nature of the location relative to the ground level, the strip foundation can be buried or shallowly buried. This or that option, first of all, is chosen according to the nature of the loads. So a large stone house must be built only on a buried foundation with a high bearing capacity, which can withstand its mass, while the total frost heaving forces pushing the building up will not exceed gravity. The second indication for the choice of a buried foundation is underground rooms, the walls of which will be formed by the body of the foundation. Such a foundation is irreplaceable if there are large elevation differences on the site. Buried foundations must be laid lower than the freezing depth, on average, this figure is from 1.2 to 2 meters.

Shallow foundations are used for the construction of lightweight buildings made of solid wood, frame, stone one-storey, on stable and slightly heaving soils. With a depth of about 50–70 cm from the level of the site, such a structure is much cheaper than a buried one, since less materials are needed, and the volume of earthworks is reduced. However, this kind of foundation cannot be applied on heaving grounds, on a slope, or if there are basements. A shallow strip foundation has a small area of ​​side surfaces, so the tangential forces of frost heaving cannot push out a light building (it exerts too little loads) as if it were built on a buried foundation with a large area of ​​side surfaces.

Sometimes, to combat heaving soils, non-buried foundations are used, which are installed almost on the surface of the site, however, elastic cushions are arranged under the monolithic tape (material – sand, slag, crushed stone, non-heaving soil …), which do not freeze and do not deform.

In fact, this is a lattice version of a slab foundation, which can significantly reduce the consumption of concrete and reinforcement. Usually it is used for light houses with a flexible frame, but on hard rocky and coarse soils, an unburied foundation is used for the construction of stone houses. Due to the large amount of work associated with the installation of the formwork, there are known options for using islands made of slab insulation as permanent elements, between which a reinforced concrete tape is formed.

Calculation of the strip foundation

In the previous article “What type of foundation to choose” we have already touched upon the topic of professional design of foundations for large houses and focused your attention on the need to attract specialists in hydrogeological surveys and the development of building structures. We also found out what information the engineer needs to provide to create a project. Let’s try to independently calculate the strip foundation for a small country house.

The key questions, the answers to which we must get are the size of the foundation (section) and the depth of its laying. To solve the assigned tasks, it will be necessary:

• determine the type of soil
• calculate the load from the building

How to independently determine the type of soil

The depth of the foundation and the area of ​​the base – the lower part, which rests on the ground – will depend on the characteristics of the natural foundation (first of all, its bearing capacity). It is best to order geological exploration of the site of a specialized organization, but if the building is relatively small, then almost all soils are able to withstand its weight and can be examined independently. The only exceptions are, perhaps, only weak organic soils – silty, peat bogs; as well as soils with special properties – saline, swelling.

To determine the type of soil on the construction site, you should dig several wells up to two meters deep (at least in the area of ​​the corners of the house and in the center). Passing every half a meter deep, it is necessary to take soil samples, which we will investigate.

The most important thing for the developer is not to lose sight of the concentration of clay in the soil, since it is this that causes severe frost heaving. It is also worth paying special attention to weak sandy-silty soils (quicksand). Inspection and tactile research of the soil is the fastest and most effective way to determine the type of soil. We clean the samples from debris and grind them. We will study the soils dry and moist – rolling a cord with a diameter of 1 cm to the minimum possible. Trying to squeeze the ball into a cake:

1. Sandy soil – sand particles are clearly visible under a magnifying glass, dry sample is free-flowing, a moistened lump does not roll into a sausage and is not plastic.
2. Sandy loam – large grains of sand prevail, but there are inclusions of clay particles. The lump easily disintegrates, does not roll into a sausage (or disintegrates into pieces up to 5 mm in size), is not plastic when moistened.
3. Silty sandy loam – crumbly with a predominance of dust, there is a feeling of a powdery mass, when moistened, “dirt” appears, the ball easily turns into a cake, does not roll into a cord.
4. Light loam – in dusty particles, inclusions of clay and sand are visible, lumps are easily crushed. There is a slight stickiness when moistened, the plasticity is moderate – a long cord does not work.
5. Silty loam – powder is visible against the background of sand and clay particles. The sample is plastic and sticky, but the sausage breaks into small pieces when bent.
6. The loam is heavy – among the sand particles there are hard lumps that cannot be crushed by hand, the stickiness and plasticity are good, you can roll a long cord with a diameter of up to 2 mm. The ball cracks at the edges when squeezed.
7. Clay soil – sand is not felt and not visible, lumps are practically not crushed by hands. The structure is homogeneous with particles up to 0.25 mm in diameter. When moistened, the mass is very sticky, rolls into a cord up to 1 mm in diameter. The crushed ball does not crack.

Another research option is a longer one. The soil sample is placed in a glass jar (by? Volume) and filled with water until? volume. A teaspoon of dishwashing liquid is added to the vessel. The can is closed and thoroughly mixed for 8-12 minutes, after which it is placed for some time to stratify the mass. The sand settles in about a minute, in two to three hours a layer of silt will settle (this is dust), several days will be required for the formation of a clay sediment (the main sign is that the water will become transparent). We measure the thickness of the layers of sand, dust and clay, and calculate their percentage. Further, using the Ferret Triangle, we can determine the type of soil on the site.

If we managed to deal with the soils on the site on our own, then we can easily get data on the resistance of the natural foundation to the loads transmitted from the building. Here we take into account the weight (mass) acting on one square centimeter of soil..

For sandy soils, it is important to take into account the degree of its density and moisture:

• coarse sand – 4.5 kg / cm² (dense) and 3.5 kg / cm² (medium density)
• medium sand – 3.5 and 2.5
• fine, low-moisture sand – 3.0 and 2.0
• fine sand saturated with water – 2.0 and 2.5
• low-moisture silty sand – 3.0 and 2.5
• silty sand saturated with water – 1.0 and 1.0

The resistance of silty-clayey soils is influenced by their porosity (simplified – density / looseness) and fluidity (simplified – plasticity and stickiness):

• Dense sandy loam – 3 kg / cm² (non-plastic) and 3 kg / cm² (plastic)
• porous sandy loam – 2.5 and 2.0
• dense loams – 3 and 2.5
• porous loams – 2.0 and 1.0
• dense clay – 6.0 and 4.0
• clay of medium density – 3.0 and 2.5
• porous clay – 2.5 and 1.0

Resistance indicators for crushed stone, pebbles, gravel, coarse soils are practically unchanged from extraneous factors, they are about 5-6 kg / cm².

We count the loads from the building

The main condition that the foundation must meet (more precisely, the area of ​​its base): the pressure of the base must not exceed the nominal soil resistance of the base. So, it remains for us to determine the load that the building has on the ground. It is necessary to take into account:

1. The mass of all building structures (walls, ceilings, roof elements, joinery, finishing materials, insulation, communications …). Do not forget about the foundation itself – for now we take the average version, since we are only looking for its characteristics. Note that the width of the strip foundation is not less than 300 mm, and its height directly depends on the depth of burial (for non-porous soils with a freezing depth of up to 1 meter – not less than 50 cm; 1.5 m – 75 cm; up to 2.5 m – 100 cm and more).
2. Operational loads (weight of furniture, equipment, people).
3. Snow cover weight.

To determine the weight of a house, you will have to separately calculate the area of ​​all its structural elements. To do this, you need to get the volume of each object by multiplying its length, width and height. The specific gravity of the main building materials can be found in publicly available tables for estimators.

To calculate the snow load, it is necessary to multiply the roof area by the snow cover mass for a particular area. So for Russia in the middle zone, it is about 100 kg / m², for the north – 190 kg / m², for the south – 50 kg / m².

Operational, payload (furniture, people, equipment) is accepted with a certain margin, at the rate of 150-180 kg / m².

Now all loads (snow, all house structures, payload) should be summed up and applied to the total footprint of the foundation. The resulting figure (kg / cm²) should be less than the resistance of the soil, and it is better if there is a margin of safety up to 15%. If the specific pressure is too high, then it is necessary to increase the area of ​​the sole (do not forget to recalculate the mass of the foundation, it will increase).

Reinforcement belt calculation

For low-rise private buildings erected on strip foundations, reinforcement with a cross section of 10 to 14 mm is most often used. As for the number of rods, it should be taken into account that for a monolithic tape up to 40 cm wide, at least four “threads” are used – these are horizontally located rods, two in the upper and lower tier. They are positioned in the corners, five centimeters from the outer walls of the foundation. For high recessed foundations, another one is added – the middle tier, then the total number of rods on the cut will be six. When calculating the required molding of rods, keep in mind that the longitudinal connection of the reinforcement is made with an overlap of 250-300 mm.

Vertical and short transverse elements of the reinforcing cage are used to stabilize the spatial position of metal in concrete. They do not take direct part in the fight against transverse deformations, therefore they can be of a much smaller section, including smooth ones. The “short ones” and “racks” are located approximately at a distance of 30-50 cm from each other. Sometimes they are harvested according to a pattern in the form of a rectangle, inside which, in the corners, the main threads pass.

More detailed information on the design of the foundation can be found in SNiP 2.02.01–83, or TSN MF-97 MO (for shallow foundations of buildings). If you have any doubts about the composition of the soil, or you cannot calculate the loads, then still contact the specialists for help. Mistakes made when designing foundations can be very expensive.

How much is a strip monolithic foundation

The cost of any foundation will consist of material costs and labor costs. In some cases, equipment rental is a separate cost item..

Having a project on hand, even a primitive one, developed by you yourself, you can always more or less accurately calculate the amount of required materials:

1. High-quality factory concrete will cost about $ 50-70 per cubic meter, depending on the distance of the mixing unit. We calculate the volume by multiplying the cross-sectional area of ​​the foundation by its total length.
2. A ton of reinforcement will tighten by $ 600-900. We already know how many horizontal threads are needed for a monolithic foundation; he also adds vertical racks, as well as short transverse elements. The mass of a running meter of reinforcement of a specific diameter can be specified in specialized reference books. Don’t forget about the knitting wire.
3. Sand (river) is needed to organize the pillows. Depending on the thickness of the layer, width and length of the foundation, we obtain the required volume of sand. A ton of this material costs on average about $ 10-15, here a lot depends on the distance of the object.
4. A square meter of formwork (for example, edged board or OSB) with spacers and fasteners will cost about $ 5-7. The formwork is placed on the entire height of the foundation, on both sides.
5. When erecting a prefabricated foundation, the price consists of the cost of purchasing FBS blocks and slabs, the cost of delivery and installation (crane rental), the price of cement mortar. It is often necessary to calculate the cost of a monolithic belt, this is done similarly to calculating the price of a monolithic foundation.

As for the foundation work, the invited builders can ask for a fee for the following items:

• digging trenches, cleaning up slopes after equipment, compaction of the base;
• sand cushion device;
• assembly, installation, dismantling of formwork;
• binding and installation of the reinforcing cage;
• pouring, leveling, concrete compaction;
• unloading and assembly of blocks;
• preparation of mortar / concrete.

In the next article, we will talk about the technology for the construction of monolithic and prefabricated strip foundations (from FBS), we will try to pay attention to all the main nuances of installation. We plan to provide you with practical material collected on the basis of the experience of professional builders. We want you, if you wish, to be able to independently build a durable, correctly working foundation, or competently control the work of contractors.