Soils and foundation types
The soil is a fundamental part of the structure, taking into account the different types of soil, it depends on what type of foundation should be used. And if the soil fails, the structure, home or building will too.
If the land was made up of rocks, you could begin to lay foundations without any problem since this soil is very resistant, but this situation is very rare, you have to dig until you find soil suitable for the foundation. The professionals, in this case, when carrying out the excavations at different types of height, analyze in their laboratory the characteristics of the type of soil according to its level, until reaching the level or foundation plane, which is the perfect level where the soil resists the loads demanded by the building or home.
Foundation level or plane: type of soil suitable for unloading the force of the structure on it.
These data are obtained through a geotechnical report that allows us to know at what depth the foundation plane is located and what will be the admissible stress that the ground will have, meaning what weight of the structure that ground will support, if this stress is exceeded it can lead to a break in the ground causing the structure to collapse.
Another piece of information that the soil study allows us to know is the presence of layers, that is, groundwater due to the presence of rain.
They are those that rest on the superficial or shallow layers of the soil, because they have sufficient bearing capacity or because they are constructions of secondary importance and relatively light. In this type of foundation, the load is distributed on a horizontal support plane.
In important structures, such as bridges, foundations, even shallow ones, are supported deep enough to ensure that no deterioration occurs. Surface foundations are classified as:
In cohesive soils where the trench can be made with vertical faces and without landslides, the cyclopean concrete foundation is simple and economical. The procedure for its construction consists of filling the trench with stones of different sizes while pouring the concrete mixture in a 1:3:5 ratio, trying to perfectly mix the concrete with the stones, in such a way that continuity in their joints is avoided. Cyclopean concrete is made by adding more or smaller stones as it is concreted to save material. Using this system, smaller stone can be used than in concrete masonry foundations. The cyclopean concrete technique consists of throwing stones from the highest point of the trench onto the mass concrete, which will be deposited on the foundation.
Precautions:
Isolated footings are a type of superficial foundation that serves as a base for specific structural elements such as pillars; so that this shoe expands the support surface until the ground supports the load transmitted to it without problems. The term isolated footing is because it is used to support a single column, hence the name isolated. It is the simplest type of footing, although when the bending moment at the base of the column is excessive, it is not suitable and instead combined footings or strip footings in which more than one column rests must be used. The insulated footing does not need to be jointed because as it is embedded in the ground it is not affected by thermal changes, although in structures it is normal as well as advisable to place a joint approximately every 30 m. In these cases the footing is calculated as if only a single pillar rested on it. A variant of the isolated footing appears in buildings with an expansion joint and in this case it is called "footing under column in fingerboard joint".
In the calculation of the pressures exerted by the footing, in addition to the weight of the building and the overloads, the weight of the footing itself and the earth resting on its flights must be taken into account; these last two loads have an unfavorable effect with respect to subsidence. On the other hand, in the overturning calculation, where the own weight of the footing and the land on them have a favorable effect. The following formulas can be used to calculate the subsidence pressure for small eccentricities (, where is the vertical load on the ground and is the maximum bending moment):
where A is the area of the isolated footing and P its weight. To build an isolated footing, the foundations and structures of buildings located on terrain of a heterogeneous nature, or with discontinuities, must be made independent so that the different parts of the building have stable foundations. It is advisable that the building facilities be on the plane of the foundations, without cutting footings or braces. For all types of footings, the support plane must be embedded 1 dm in the ground layer.
The depth of the support plane is set based on the geotechnical report, without altering the behavior of the ground under the foundation, due to variations in the water table or due to possible risks due to frost. It is advisable to reach a minimum depth below the surface level of 50 or 80 cm in those areas affected by these variables. In the event that the building has a structural joint with duplicate support (two columns), a single footing is made for the two supports. It is advisable to use concrete of plastic consistency, with aggregates of around 40 mm size. During execution, and before pouring the concrete, place a layer of lean concrete approximately 10 cm thick (cleaning concrete) at the bottom, before placing the reinforcement. The footings have concrete.
Strip footings are used to foundation load-bearing walls, or rows of pillars. Structurally, they function as a floating beam") that receives separate linear or point loads.
They are foundations of great length compared to their cross section. Strip footings are indicated as the foundation of a longitudinally continuous structural element, such as a wall, in which we intend to seat the ground. This type of foundation also acts as "bracing", it can reduce the pressure on the ground and can bridge defects and heterogeneities in the ground. Another case in which they are useful is when many isolated footings close together would be required, making it easier to make a continuous footing.
Strip footings are normally applied to walls. They can have a rectangular, stepped or conically narrowed section. Their dimensions are in relation to the load they must support, the compressive strength of the material and the admissible pressure on the ground. For practicality, a minimum height for concrete foundations of approximately 3 dm is adopted. If the heights are greater, they are given a staggered shape taking into account the angle of pressure distribution.
In the event that the earth tends to crumble or the foundation must be staggered, formwork will be used. If the foundations are made of rammed concrete, they can be concreted without the need for them.
If the foundation work must be interrupted, it is recommended to cut the vertical joint into steps to achieve a correct connection with the following section. Likewise, placing some reinforcing irons will reinforce this union.
Striped Footings are, according to the Technical Building Code (CTE), those footings that include more than three pillars. He considers them different from combined footings, which are those that support two pillars. This distinction is the subject of debate since a combined footing can perfectly support four columns.
A combined footing is an element that serves as a foundation for two or more columns. In principle, isolated footings take advantage of the fact that different columns have different bending moments. If these are combined into a single foundation element, the result can be a more stabilized element subjected to a lower resulting moment.
A foundation slab is a floating plate supported directly on the ground. As a slab, it is mainly subjected to bending forces. The thickness of the slab will be proportional to the bending moments acting on it. The relationship between the thickness of the slab, the bending moments of the plate, the external loads and the elastic properties of the concrete of the slab is given by the following expression:.
Where:.
The foundation piles or also known as piles are structural elements corresponding to the foundation, these consist of three fundamental parts, the head which is the one that opens the way for the construction slab to be supported, the other part is the cylinder or pile and this is buried in the ground, the length of these is dependent on each project but they must touch rocky ground so that they do not suffer displacements in their different axes. These structural elements must also have counter beams or foundation beams, these are the ones that connect the entire assembly of the heads and provide rigidity to the structural system, they are vital as they help mitigate the movement that may be caused within the foundations due to the change in the soil. This foundation system is mainly made up of two materials, concrete and steel.