Shallow foundations on natural foundations These are the foundations that are built in open pits more than 5-6m deep. Basic requirements for foundations– their sufficient strength, durability, frost resistance, resistance to the aggressive effects of groundwater.

The foundation must be of such dimensions that the average pressure at the base of the foundation does not exceed the calculated resistance of the foundation soil.

In addition, the calculated values ​​of absolute settlement and settlement differences between individual foundations of one structure should not exceed the limit values ​​​​established by design standards.

Classification of shallow foundations

Individual glass-type foundations include foundations for columns. Typically, such foundations are used in industrial buildings. With not too large loads on the ground, with sufficiently strong and low-compressible soils, as well as with a flexible operating scheme for the above-ground part of the building, when columns and crossbars or columns and trusses are hinged.

There are different ways of attaching a foundation to a column:

a) embedding (?small, cold?)

1 - Fine aggregate concrete is not lower than the concrete class of the foundation itself (not lower than B20).

2 - glass

b) large columns are installed without a glass

hard joint - welding and joint are sealed with concrete

Typically, individual column foundations are made in combination with rand beams (or foundation beams).

Columnar glassless foundations for a brick wall

They are used for one-story buildings with good soil conditions for private individual construction.

Strip foundations

Under brick walls they are sometimes prescribed continuous.

They are used for uniform loads from walls to ground and constant loads along the wall in ground conditions. (l/b≥10).

Changing the dimensions of the laying depth is possible only in individual sections of limited length. Areas of different sizes are separated by sedimentary seams. They are used under significant loads and fairly weak soils. They do not significantly change the rigidity of the structure; they almost do not work on bending in the longitudinal direction (with high rigidity of the walls).

Parallel strip foundations for columns are used when the column spacing is no more than 6 m and in the presence of weak soils. Such foundations reduce the uneven settlement of individual columns.

Lecture 7 – 05/10/12

Cross strip foundations for columns

They are used for small column spacing, heavy loads and weak soil. Cross tapes allow you to level out settlements not only of individual columns in a row, but also of the building as a whole.

Solid foundations

Foundations in the form of a solid slab for both columns and brick walls are installed under the entire structure or under part of it in the form of reinforced concrete slabs under a grid of columns and walls. Such foundations bend in two mutually perpendicular directions, have a small uniform settlement, they are not afraid of being soaked by surface water, and they also protect the basement parts of the building. The dimensions of such foundations are determined by the dimensions of the structure in plan.

Lecture plan.

1.1. Soil work under load.

1.2. Natural grounds. Types of soils and their most important characteristics.

1.3. Artificial foundations.

2. Foundations of low-rise residential buildings.

2.1. Classification of foundations

2.2. Constructive solutions for foundations.

1. Foundations and their characteristics.

1.1. Soil work under load

Soils are geological rocks located in the upper layers of the earth's crust, consisting of solid particles (grains) of different sizes (soil skeleton) and pores filled either completely with air or partially with water. And the soil that is under the foundation in a stressed state due to the load from the building is called foundation base .

The base of the foundation is a mass of soil located under the foundation and directly receiving loads from a building or structure through it.

These loads cause a stressed state in the foundation (Fig. 7.1), which, when reaching a certain level, can lead to deformations, both the base itself and the foundation.

Due to the pressure exerted by the building on the foundation, the soils under the foundation experience significant compressive forces. Under the influence of these forces, the soils are evenly compacted. Such uniform deformations are called soil settlement, which causes settlement of foundations.

Uneven soil deformations resulting from compaction and, as a rule, a radical change in the structure of the soil under the influence of external loads, the soil’s own mass and other factors (soaking of subsidence soil, thawing of ice lenses in the soil, etc.), are called drawdowns. They can cause foundations to rotate, etc. up to destruction. Drawdowns of the foundations are unacceptable.

To ensure that precipitation does not have dangerous effects on structures operating under load, and also does not affect the operating conditions of buildings, Limit values ​​for foundation deformations and stresses in the soil have been established, arising under the base of foundations.

1.2. Natural grounds. Types of soils and their most important characteristics.

If the soils are motionless and capable of bearing loads without preliminary reinforcement, then they can be used as natural foundations .

The quality of the natural foundation depends on many factors, but first of all, it is determined by the type of soil, its humidity, groundwater level and freezing conditions.

Natural foundations are soils that in their natural state have sufficient bearing capacity, low and uniform compressibility, not exceeding permissible values.

According to their structure, soils consist of particles that are kept from mutual displacement in various ways: by a rigid connection between grains (cohesion) - in cemented soils that constantly retain their structure; friction force – in loose soils; adhesive force – in cohesive soils.

Soils, used as foundations of buildings and structures, are divided depending on geological characteristics into rocky and non-rocky.

TO rocky soils include: igneous, metamorphic and sedimentary rocks with rigid connections between grains (welded and cemented), occurring in the form of a continuous or fractured massif. Such rocks include, for example, granites, basalts, sandstones, and limestones. Under load from buildings and structures, these rocks do not compress and are most durable natural basis.

TO non-rocky soils include coarse-clastic, sandy And clayey.

Coarse clastic Based on their structure (grain composition), soils are divided into crushed(the weight of particles larger than 10 mm is more than half) and woody(the weight of particles measuring 2 - 10 mm is more than 50%). If rounded particles predominate in these soils, they are respectively called pebble or gravel.

Sands in a dry state represent their mass loose priming. By size particles distinguish between sands: gravelly, large, medium coarse, small and dusty with a corresponding particle ratio from 2 mm to 0.05 mm as a % of the weight of the air-dry soil. Sandy soils made from gravelly, coarse and medium-sized sands are slightly compressible and, with sufficient layer thickness, serve as a strong and stable foundation for buildings and structures.

Clayey soils belong to the category messengers soils with flat particle sizes not exceeding 0.005 mm and thickness less than 0.001 mm. Clay particles are held together by internal cohesion forces, the magnitude of which depends on soil moisture. Clay soils are plastic, i.e. When moistened, they are capable of transitioning from a solid to a plastic and even fluid state. Clay soils that are in a hard, dry state serve as a solid foundation.

Clay soils also include loams and sandy loams, which contain sand impurities along with clay particles. The content of these impurities is characterized by the so-called “plasticity number”. For sandy loams this value ranges from 0.01 to 0.07, for loams – from 0.07 to 0.17.

If clay soils contain up to 15 - 25% (by weight of particles larger than 2 mm, the terms “with pebbles” (“with crushed stone”) or “with gravel” (“with gruss”) should be added to the indicated names); if the content of particles is 25 - 50% (by weight) the terms “pebble” (“crushed stone”), “gravelly” (“woody”) are added. If there are particles larger than 2 mm, more than 50% (by weight) the soils are classified as coarse-clastic.

Depending on the degree of humidity or the degree of filling of pores with water distinguishes soils low moisture, wet And rich water. Coarse-grained and sandy soils with particle sizes above average are slightly compressible when moistened and can serve as a stable foundation. Moistening of fine-grained sandy soils reduces their bearing capacity the more, the smaller the soil particle size. Moistening of silty sands with clay and silty impurities has a particularly strong effect on reducing the bearing capacity of the soil. Such soils in a water-saturated state become fluid and are called quicksand . The construction of buildings on such soils requires additional measures to strengthen the foundation.

In construction practice, there are bulk soils - artificial embankments formed as a result of cultural and industrial activities of humans. Such soils are formed when filling ravines, dried-up reservoirs, on the site of landfills and industrial waste, etc.

The density of bulk soils often depends on the nature of the underlying layer and the composition of the embankment (the presence of debris, slag, etc.). The issue of using bulk soils as a foundation for buildings and structures is considered in each individual case, depending on the nature of the soil and the age of the embankment. For example, sandy embankments, which basically contain sand, self-compact after 2-3 years, and clay embankments after 5-7 years, after which they can be used as a natural base. The bearing capacity of clay soils when they are moistened is significantly reduced. When wet clay soils of the foundation freeze, water freezes in the pores: so-called “heaving” occurs, which often causes deformation of foundations and buildings. Therefore, the depth of foundations from ground level on clay soils should, as a rule, be 15–20 cm below the winter freezing depth.

Clay soils (for example, loess And loess-like), which in their natural state have large pores (macropores) visible to the naked eye, are called macroporous soils. When moistened, such soils, due to the content of water-soluble lime, gypsum and other salts, lose cohesion, quickly become wet and at the same time become compacted, forming subsidence. These soils are called subsidence and to ensure the necessary strength and stability of buildings and structures erected on such soils, special measures must be taken to strengthen the foundation soils and protect them from moisture.

Groundwater is formed as a result of precipitation penetrating into the soil. Having reached a waterproof layer (“aquiclude”), for example a layer of clay, water flows down its slope, seeping through permeable layers (coarse-grained, etc.). The level of drained water depends on the proximity of the aquitard to the surface, on seasonal fluctuations in water levels in the area’s reservoirs, etc. This level, called groundwater level, can also change from the penetration of water from above - the so-called high water during melting snow, rain and the presence of layers of clay soils that retard the movement of water.

Depending on hydrogeological conditions, soil layers can be saturated with groundwater to varying degrees. Coarse-grained soils contain it if water-resistant layers lie below them. Fine-grained soils may contain groundwater partially or completely, and clayey soils, due to their high moisture capacity, most often have only capillary (cohesive) water.

Groundwater containing dissolved impurities of salts and other substances that destroy foundation materials is called aggressive.

To protect against aggressive groundwater, special structures are created that are capable of operating in an aggressive environment and protecting foundations from destruction (SNiP 3.02.01-83).

Soils containing ice are called frozen. Soils that freeze only during one winter season are called seasonally frozen; maintaining a frozen state continuously for many years - permafrost. Seasonally frozen soils in winter, under the influence of zero or negative temperatures in the construction area, freeze to a certain depth.

Freezing of some of these soils can cause them heaving. Soils in which there is a significant amount of clay (sandy loam, loam and clay) are called freeze-swelling. The remaining soils (sand, gravelly, etc.) make up the group of soils that do not expand when frozen. Heaving forces are always directed from bottom to top; during the process of freezing or thawing, individual sections of the surface shift relative to each other. According to the degree of heaving, soils are divided into highly heaving, heaving and non-heaving. Clay soils are the most susceptible to heaving. When saturated with water, fine sands swell to a small extent. Coarse-grained and sandy soils of large fractions do not heave even when saturated with water. In rocks and coarse soils, ground deformations that develop during freezing are insignificant or completely absent.

Foundations on a natural foundation differ: by design - into separate, strip, solid and massive; according to material - concrete and reinforced concrete (prefabricated and monolithic), brick, rubble, sawn stone, etc.; for its intended purpose - on foundations for buildings (residential, industrial, etc.), structures, equipment.

Individual foundations are pillars with a developed supporting part that transfer concentrated loads from columns, corners of buildings, frame supports, beams, trusses, arches and other elements to the ground. To install columns, recesses - “glasses” - are arranged in the upper part of individual foundations. Such foundations are usually called separate glass-type foundations.

Strip foundations are used to transfer loads from extended elements of building structures - walls of buildings, structures, equipment support frames, etc. According to their location in plan, they differ into intersecting and parallel.

Solid foundations are constructed under the entire area of ​​the building. According to their design solutions, they are divided into slab and box-shaped. Slab foundations, in turn, can be ribbed (caisson) and smooth.

Massive foundations are arranged for towers, masts, columns, heavily loaded supports of artificial structures (bridge supports), for cars, machine tools and other equipment.

The classification of foundations on natural foundations by design is shown in Fig. IV-1, and for the materials used - in table. IV-1.

Rice. IV-1.

Table IV-1

Classification of foundations on natural foundations according to the materials used

Foundation type	Material
	concrete and reinforced concrete		bottle	brick	sawn stone
	made	monolithic
1. Separate: Glassless Glass 2. Tape 3. Solid 4. Massive	+ + + – –	+ + + + +	+ – + – +	+ – + – +	+ – + – +
Note. The + sign marks the materials used for the listed foundations.

The content of the article

FOUNDATION, the underground or underwater part of a structure that transfers to its soil foundation the static load created by the weight of the structure, and additional dynamic loads created by the wind or the movement of water, people, equipment or vehicles. A properly designed foundation transfers all loads to the ground in such a way that the possibility of unacceptable settlement and destruction of the structure is eliminated. As a rule, this is achieved by distributing the load over a sufficiently large area, excavating the soil to the level of strong rocks lying at greater depths, using piles immersed in a layer of weak rocks up to a layer of stronger ones, or strengthening the surface layer of soft soil. If the entire support area is formed by rocky soil, then the settlement will be negligible. Difficulties arise when a structure needs to be erected on soil with high compressibility, especially if it changes.

The main types of foundations are: foundation on a natural foundation, floating solid foundation and pile foundation with driven and cast-in-place piles. Special underwater foundations occupy a special place.

Foundations on natural foundations.

Such foundations can be solid slab (made of reinforced concrete slabs) or cross-shaped (in the form of a lattice made of reinforced concrete, steel, and sometimes wood). The contact area of ​​the foundation with the soil must correspond to the load, taking into account the expected resistance of the soil. The maximum resistance (reactive pressure) of the soil is determined experimentally based on the principles of soil mechanics, and state building codes provide tables of permissible soil resistance for certain geographic zones. The foundation must be properly designed to resist bending and shear. The base of the foundation should be below the maximum freezing depth of the soil to prevent swelling of the soil when freezing. The safe depth depends on annual temperature variations, the type and range of local soil variations, and the normal groundwater level. In addition, seasonal changes in the volume of clay soils are sometimes observed, which should not be allowed under a foundation laid on a natural foundation.

In very cold regions, such as the Arctic, the soil freezes to a great depth and thaws only in the upper layer 0.5–3 m thick. In such “permafrost” conditions, a special approach to building a foundation on a natural foundation is required. Typically, thermal insulation is provided between the top of the structure and the base of its foundation, preventing the melting of the subsoil and the subsequent swelling of the subgrade when it refreezes.

Floating foundation.

In deep layers of soil with high compressibility, expanded solid foundations are used, which support the structure as if “afloat” in plastic soil. If a solid foundation is properly designed, then settlements and distortions are evenly distributed throughout the entire structure and no serious deformations occur in the upper part of the structure.

It is believed that a solid foundation will be floating if its mass, taking into account all loads, is approximately equal to the mass of the displaced soil (or water); then equilibrium is achieved and large settlement does not occur. This rule places somewhat higher demands on depth. Due to internal friction, the soil can withstand a greater load than the weight of the excavated soil, although at a slightly higher settlement. To uniformly distribute the load transmitted to the soil foundation by columns, prestressed concrete slabs and beams, inverted arches with concrete slabs, distribution foundation grids, inverted arches with ribs and shells are used. The foundation must be properly designed to resist bending, shear and normal forces.

Driven piles.

In the case of weak soils, foundations are used in which the main elements that transfer loads from the structure to the foundation are piles immersed in the ground. Loads are transmitted not only due to support pressure, but also due to lateral friction against compacted soil. Due to partial unloading by the surrounding soil, the piles of the pile “bush” are less loaded than free-standing piles.

Driven piles can be wood, concrete or steel. A wooden pile (sleeper) is a processed log with a diameter of about 30 cm at the head (butt) and a length of 3–15 m. The logs must be straight, sanded, with knots cut off at the root. To increase friction on the side surfaces, wooden piles are sometimes equipped with wooden or metal hoops. Concrete piles can be manufactured either on site or in a factory. Prefabricated piles must be well reinforced with steel so that they are not afraid of loading and unloading and impacts when driving. The steel pile can be extended up to ~90 m and is usually an I-section or pipe of suitable length. A steel casing pipe with a diameter of 20–60 cm, after immersion in the ground, is filled with concrete. Thick-walled steel pipe piles with a steel core at the end, corrugated from the surface, are used to reduce the impact when entering the ground. Such shell piles are also filled with concrete. To increase strength, a steel I-section is inserted into pipe shell piles of both types. Sometimes the inner concrete is knocked outwards from the bottom end of the pile, thereby creating an extended support. The immersion of piles into the ground is carried out by driving, pressing, vibrating and screwing. Pile driving is carried out using pile drivers with steam-air and diesel hammers. The process of immersing a pile into sandy and gravel soil is greatly facilitated and accelerated if the soil under the lower end of the pile is washed away by a strong stream of water, for which a channel can be left in the body of the pile or a pipe can be installed to supply water (under a pressure of about 0.7 MPa).

Driven piles.

Driven piles are used in cases where heavy structures have to be installed on strong soil, covered on top with a thick layer of weak soil. To do this, in soft soil, a well is drilled to a layer of rock, mandrel or gravel and filled with concrete. Suitable for moderately strong soils is the so-called. Chicago method: the soil is removed sequentially in sections of 1.5 m, securing each with wooden side formwork before starting to excavate the soil of the next section. The cast-in-place pile thus constructed transfers the loads from the column support directly to the solid soil. Sometimes, to increase the support area, it is expanded at the lower end if it does not reach the rock. Part of the load is transferred to the soil due to friction on the side surfaces of the pile.

Caisson driven piles are made by driving a wide steel casing cylinder, open at the ends, into the ground with a steam driver. Then the soil is removed from the submerged cylinder and the vacated space is filled with concrete, having previously inserted an I-beam steel profile inside for reinforcement, if necessary. Steel casing left in the well increases the strength of the pile in proportion to its cross-sectional area and elastic modulus.

Underwater foundations.

To provide a safe space for workers and equipment, the construction of an underwater foundation begins with the construction of a sheet piling or sinkhole. These waterproof devices allow you to remove water and soil from the location of the future foundation, clear it and perform the necessary work with the precision possible on dry soil.

Sheet pile fencing.

Sheet piling is most suitable for shallow water depths, although they have been known to be used in water depths of up to 30 m. Sheet piling is constructed from wooden or steel sheet piles installed in one or two rows and fastened together to withstand the pressure of the water. The gap between the piles of the double-row fence is filled with compacted soil, which prevents water from flowing through. Cellular sheet piling is made of closed cylindrical steel cells filled with soil. Water is pumped out from the fence area by pumps.

Caisson.

An open sink well is a hollow cylindrical shell, the size of which corresponds to the foundation and is well reinforced inside with transverse walls. Typically, a drop well is used to construct deep supports that transfer pressure to lower, more durable layers of soil. The well is lowered to the bottom, its inner ridge is filled with stone, and a caisson driven pile is set up on top. The soil is removed through wells: silty soil by pumping, and dense soil by a lift with a multi-jaw grab dredge bucket. A submerged well and caisson piles, formed by filling excavation wells with concrete, serve as the foundation for the abutment - the support of the upper part of the structure. Concrete for laying on this foundation is supplied through a metal concrete pipeline with a diameter of at least 20 cm, lowered from above under water. The concrete pipe can also be lowered directly to the bottom.

Caissons.

Caissons are used at great depths, which do not allow the installation of sheet piling. A caisson is a large, shallow glass-like shell that sinks upside down to the bottom of a reservoir. The dimensions of the caisson are determined by the area of ​​the soil base corresponding to the full design load for a given permissible resistance of the bottom soil. If the caisson lies on rocky ground, then its diameter can only slightly exceed the support of the abutment or other supporting element of the structure attached to it. The height of the caisson is determined by the level of the soil foundation and the level of high waters. Therefore, it is first necessary to obtain data on the level and nature of the soil foundation. Caissons are usually made on land, towed on pontoons to the foundation site and attached to bush piles. If the water depth is not sufficient for towing afloat, then the caisson can be assembled on piles in the right place and then lowered to the bottom.

The working chamber is provided over the entire area of ​​the caisson; its height is about 2 m. Compressed air is continuously supplied to the chamber under pressure, eliminating the possibility of water leakage. Workers enter and exit the pressurized chamber through an airlock, which also serves to unload excavated soil and supply construction materials. The soil is developed at the bottom and under the sharp edges of the walls, so that the caisson gradually lowers under its own weight and the weight of the adjustable abutment. At the same time, the pressure in it increases in accordance with the external pressure. When the caisson reaches the solid ground on which it is to rest, its working chamber is filled with compacted concrete, which serves as a foundation for an abutment or other support.

The caisson is usually bulky and inconvenient to operate. Waves make it difficult to install, and uneven lateral soil pressure makes it difficult to accurately guide it by excavating under the sharp edges of the walls. Depending on the strength of the soil and operating conditions, the rate of immersion of the caisson into the ground can range from 3 cm to 2.5 m per day. The maximum known depth of immersion of the caisson under water is about 40 m. Excessive pressure at such a depth (3.5 times higher than atmospheric pressure) is at the limit acceptable for the human body.

People who work in conditions of high air pressure for a long time are susceptible to two specific diseases. One, less serious, has symptoms similar to a cold (“stuffy nose”) and can develop into pneumonia. Another – decompression sickness (air embolism) – often causes paralysis with a fatal outcome.

Bridge supports.

Bridge supports (piers and piers) are elements intermediate between the foundation and the upper part of the bridge structure. However, they are often referred to as the foundation. Abutments, which are usually concrete walls that support the bridge ends and retain the soil fill of its entrance, are integral with their foundation and transfer the load directly to the soil foundation. The bulls, like columns, rest on their foundations and support the upper part of the structure. The foundations of bridge supports can be on a natural foundation, piled or caisson and are designed so that they can withstand all loads and protect the structure from washing out the soil by water flow.

Temporary foundations.

When it is necessary to replace or strengthen the foundation, it is replaced or strengthened in parts, using side supports and support beams if necessary.

Replacement in parts.

In short areas, at certain intervals, the soil under the old foundations is removed to a new soil base. In the resulting pits, sections of the new wall with corresponding foundations are built and connected to the lower part of the old wall. When these wall sections are completed, they support the old wall until the remaining intermediate sections are excavated and new wall extensions are constructed.

In another option for strengthening the foundation, metal pipes are driven into the ground under the wall at certain intervals. When the pipes reach the new soil base, they are cleared of soil from the inside and filled with concrete up to the bottom edge of the wall. These pipe piles support the wall during the construction of wall additions and new foundations.