Each building and engineering structure can be erected only on such a foundation, the strength of which ensures their durability and stability. Settlement of soil under foundations is inevitable, but it should not lead to deformations of buildings. Uneven settlement is especially dangerous, causing cracks and deflection of the walls of buildings..
The upper layers of the soil are influenced by a number of physical factors (wetting and drying, weathering, freezing and thawing), which change the condition of the soil, its building properties and reduce its bearing capacity. Therefore, the structures of buildings and structures must be located on reliable underground elements – foundations, which serve to transfer loads from the structure to the ground located at a certain depth from the earth’s surface..
The layer of soil that bears the weight of the structure with all external loads acting on it is called the base of the structure.
Foundations are distinguished: a) natural, when the soil under the foundation remains in its natural state, and b) artificial, when, due to insufficient soil strength, measures are taken to increase its bearing capacity.
Soils: types and properties
On the basis of building qualities, soils are divided into rocky, coarse-grained, sandy and clayey (including loess-like loams).
Rocky, and coarse soils in construction practice are very rare. Most of the soils are igneous, metamorphic and sedimentary rocks with a rigid bond between grains (soldered and cemented), occurring in the form of a solid massif or fractured layer. Such soils serve as a reliable foundation for buildings, provided that the soil layer under the rock is completely stable and is not washed out by water. The main soils at our construction sites are sandy, clayey and their varieties..
Sandy soils – product of destruction of rocks. Sands have their characteristic flow properties, since there is no cohesion between the individual grains. As a result, sandy soil has good water permeability and does not swell when freezing..
According to the grain size, sands are gravelly (25% of particles are larger than 2 mm), large, medium-sized (50% of particles by weight are larger than 0.25 mm), small and silty.
Dry clean (especially coarse) quartz sand can withstand heavy loads and is a reliable foundation for structures. Fine sand liquefied by water, especially with admixtures of clay and silt, is unreliable as a base.
Clay soils formed as a result of physicochemical processes that occurred during the destruction of rocks. Their characteristic property is the adhesion of the smallest particles of soil to each other. Due to its impermeability, clayey soils always contain water (from 3 to 60%, usually 12 to 20%). When freezing, moisture, increasing the volume of clay soil, causes its strong heaving.
Dry, densely packed clayey soils with a high layer thickness can withstand significant loads from structures if there are stable underlying layers under them.
The most common sandy and clay soils of the foundations are very diverse both in particle size and in physical and mechanical properties..
Soils in which clay particles less than 0.005 mm in size are contained in the range from 10 to 30% are called loams; with a content of up to 10% clay particles, soils are called sandy loams.
Specific properties have loess loam, containing a significant amount of dusty particles (0.005 – 0.05 mm) and water-soluble limestone, etc. In a dry state, such soils have significant strength, but when moistened, the soil softens and sharply compresses. As a result, significant precipitation occurs, severe distortions and even destruction of structures erected on it, especially from bricks.
Thus, in order for loess-like soils to serve as a reliable base for structures, it is necessary to completely eliminate the possibility of soaking them. For this, it is necessary to carefully study the regime of groundwater and the horizons of their highest and lowest standing.
Geological survey of the construction site
To obtain data characterizing the composition and properties of soils serving as the base and underlying layers, geotechnical and hydrogeological studies are carried out. To do this, on the site planned for development, at several points along the perimeter of the foundation, a system of boreholes and pits is laid, from which soil samples are taken. A pit is a round or rectangular well, the walls of which in sandy and bulk soil are reinforced with plates and boards to protect against collapse. Well drilling is carried out using a drilling tool, immersed in the ground by blows or rotation.
During the period of drilling a well or opening a hole, a log is kept, according to the records in which they make up geological sections of the soil and judge the bedding of the underlying layers, their thickness and the level of groundwater. Based on these data and samples of the undisturbed structure taken in the pits, in special laboratories the physical and mechanical properties of the foundation soils and the hydrogeological regime of the site are determined.