The design of foundations is one of the most difficult issues of designing of buildings and structures. In the design an engineer makes a decision about the choice of materials from which the construction will be implemented. In the design of foundations an engineer should take into consideration existing soil on the construction site and use its construction quality in order to take a rational solution. A foundation of a good quality is the key to solidity and durability of the building, hence a person should get serious about its construction. Even a light country house will serve for many years, if it is built on a solid foundation.
A deep foundation is a foundation which base is located in a well with a depth greater than the depth of freezing of the soil surface. The main purpose of laying deep foundations is to eventually create a reliance on dense layers of soil in a more load capacity. Obviously, the deep foundation requires a large amount of land works, because at least a trench under a foundation needs to be performed at a greater depth, respectively – consumption of sand, cement and gravel increase. The main advantage of this method of laying the foundation is that carrying capacity greatly increases. These foundations involve the construction of big facilities, mostly of brick (or concrete), in particular for high-rise residential and commercial buildings. In the case of the high water level, namely, when the depth of its location is more than 2 meters, laying of the deep foundation does not make sense for the reason that then it will be based on a wet soft ground. A high degree of plasticity and mobility of such soil, as well as its low bearing capacity, excludes the benefits of deep foundations.
There are several types of deep foundations: drilled, pier and strip foundations. “The common deep foundation systems for buildings are caissons and piles”. Each of them is also divided into several types. Strip foundations can be built of reinforced concrete, rubble, stone, brick or concrete. Pier foundations can be built of stone, brick or concrete posts and wooden chairs (cuttings of logs). The main disadvantage of strip foundations is the high cost of materials and work performance.
Frost resistance of deep foundations is provided by the fact that its base is located below the ground freezing. The impact of shear forces of frost heave is less than the same action on the base with the other areas. However, these forces are large enough and can reach values of 5 tons per square meter of the side surface. Thus, deep foundations should be used in the construction of stone and concrete houses of big weight below the freezing surface and ground water, but no more than 2 meters from the surface. There are many examples of the use of deep foundations nowadays. In the UAE, Dubai, there is the tallest building in the world to date — Burj Khalifa. It was built with the use of the deep foundation. Depth of the pile of tall buildings is usually from 40 to 120 m, depending on the depth of a strong soil layers.
The shallow foundation with sandy base is used for the construction of light suburban low-rise buildings. This technology allows the optimal use of building materials and leads to significant savings. There is no need to use foundation gravel for this type of foundation. This type of foundation is effective for building on heaving soils with a high water level. It is simple and cheap. The construction of the foundation shallow is essentially a rigid frame. This frame is a strip foundation of concrete or reinforced concrete. It is better if the design of the foundation is made of reinforced concrete. The foundation is laid on a cushion of rocks, gravel, broken bricks or coarse sand. This will reduce the degree and inequality of shifts of the foundation. In the performance of this design, the consumption of concrete is reduced by 50-80% comparing to the deep foundation. Labor expenditures on the construction of a zero cycle are reduced by 40-70%. Basic principles of the shallow foundation are as follows:
- A wall area of the foundation, on which a force of the frost heaving can affect, is minimized;
- In the most difficult cases, the shallow foundation must redistribute the load and provide the uniform lift of a house.
Sand in the design of the shallow foundation plays several roles. First of all it does not retain moisture, allowing it to go to the depths. Secondly, even if sand is wet, then all the moisture is evenly distributed over the entire area of the base. Thus, the swelling of soil acts uniformly, without destroying the foundation. A concrete strip perfectly smooths uneven ground pressure that remained. Shallow foundations are useful in the construction of bridge piers on dry land or on the ground, covered with a thin layer of water. With the wrong calculation of the project or the wrong choice of the shallow depth of the basement at the base of the house, cracks can develop due to the ground freezing.
As it is seen, due to modern technologies, significant savings of time, finances and resources can be achieved. The emergence of an entirely new equipment (dredging and special types of grabs) permitted extensive development of a new design of deep and shallow foundations, work on the construction of which is highly mechanized.