Keywords: Foundations, subsoil (foundation soil), earthworks, deep of foundations, excavations, shoring, underground walls

The foundation engineering is engaged in designing and establishing the foundations manner. The foundations are load-bearing components of objects that provide the load carrying structure into the subsoil. The foundations must be designed to safely transmit all loads with minimal distortion and without breaking the subsoil. According to the way the load is transferred, are distinguished shallow foundations and deep foundations.

Subsoil is a functional part of the building. The footing bottom is an area where the foundations meet the subsoil.

Soil is unpaved or slightly hardened rock

Rock is a heterogeneous mixture of various minerals, sometimes organic compounds, volcanic glass or a combination of these components.

Topsoil is the upper thin layer (100-300 mm) on the surface with plant and animal residues. Topsoil is rake off before work and later is thrown back around the building.
Mud is clay soil mixed with a considerable amount of silica sand, mica, calcium, iron and organic matter. If it contains more than 40% sand, it is referred to as skinny mud. At a sand content below 40%, it is a greasy mud. Holding greasy mud in hand, it sticks and holds together, while the skinny mud does not stick and decay. These include brick clay, fireproof mud and kaolin

Clays are siliceous sediments, consisting of 25-30% clayey earth and 65-70% more silicon dioxide. They are always very fine, without sand or mixed with fine sand, very colloidal and water impermeable. The water gets on the volume, shrinking by drying. A special kind of clay is bentonite, which is very fine, so it has properties of colloidal substances. It receives plenty of water - up to seven times its own weight.

Marl or marlstone is clay-mud containing 25-60% calcium carbonate and magnesium carbonate. Marlstone soil have tended to sliding. There are very dangerous.

Fusible mud containing a mixture of alumina or lime clay, sand and mica. Contains 10-40% lime. Water-tight. It is slightly softer than clay, and in nature it has a slate structure. This group also includes shale or claystone, often containing coal.

Loess is a fine, sandy, dusty wind. It consists of a higher content of calcium compounds and up to 50% of dust, mostly silica. It has less ductility than clay and marl.  Loess is yellow to light brown, so is often confused with mud. If we put it between our fingers, it is finer than clay, since it contains grains of sand less than 0.1 mm. It draws water and its water permeability is very considerable because it is penetrated by the hair channels. The unpleasant nature of the loess is its great tenderness: up to 5-6 m above the ground water level. However, if it is dried up thoroughly and properly, it is relatively small for the water.

There are 3 classes according to soil exploitation:

• Class I is defined by mining by conventional excavation mechanisms (bulldozers, excavators) or by hand.
• Class II is defined by mining with special mechanisms - rippers, rock spoon, hammers
• Class III is defined by mining by blasting works

Depth of foundation affects the size of the building's settlement. Greater depth reduces the overall settlement construction. The depth of foundation is the difference between the level of the footing bottom and the closest terrain point. The depth of foundation is determined with respect to stability and settlement construction, climatic conditions (freezing, drying out of the soil) and geological and hydrogeological soil profile.

The minimum depth of the foundation is determined by climatic conditions - winter temperature and the type of soil. In the case of freezing of footing bottom under the foundations, there is a real risk of increasing the volume of soil under the foundations (water changes in the state of ice to increase its volume) and thus the formation of stresses and consequently faults. Depending on the soil, we choose the depth of foundation:

• 500 mm for rock and weak rocks soil and under the interior walls
• 800 mm from landscaped terrain (loose soil outside the mountain range)
• 1000 mm from landscaped terrain (cohesive soils outside mountain areas)
• 1200 mm in cohesive soils with ground water depth less than 2 m deep

Depth of foundation in mountain conditions always depends on local climatic conditions. The type of soil is always determined on the basis of the site survey results. In the case of inappropriate soil types, the soil can be improved by replacing with other soil (cushions), compaction, drainage, soil admixtures (grout, lime + polit) or by drying.

On cohesive soils, due to the load, the water is exuded from the pores and thus partially bursts and consequently decreases the foundations. That is why rough sand, gravel or gravel is used as drainage under the foundation. The height of the embankment must secure the isobar under the foundations so that the stress is less than the bearing capacity of the foundation soil.

Earthworks in civil engineering are divided into preparatory earthworks, major earthworks and finishing earthworks.

The main types of earthworks are clearances, embankment and backfills. Theclearances eliminate terrain inequalities. It also includes rake off the topsoil. The topsoil is surface organic soil with a thickness of 150 to 300 mm. The embankments are poured structures built on the surface of the territory. The embankments are formed over thin layers (150 - 700 mm) which are compacted. The backfills are sprinkled structures that fill the space below the terrain level and around the building structure. The bulk material is non-frost-free, stable and low compressible materials (eg gravel). The backfills need to be compacted. The most important earthworks are excavations.

Excavations are carried out by excavating below ground level. The area in which excavations are made is called the excavation site. Exhausted soil is called a borrow material.

According to the shape and dimensions of the excavation, there is a pit, trench and shaft. The pit is an excavation whose length and width is greater than 2 meters. The furrow has a predominant length dimension and a maximum width of 2 meters. The shaft has a predominant depth dimension and a maximum floor area of 36 square meters.

The lifting of the soil is carried out by various types of earthmoving machines. Hand excavations are limited to clearing work. The method of excavation is chosen according to the volume and type of rock.

The footing bottom must not be broken during excavations. It must also be protected from climatic effects (rain, flooding, drying and freezing). The soil layer (approx. 200 - 500 mm) is retained at the bottom of the excavation as a protection layer, which is removed just before the realization of the foundations.

Excavation walls must be secured against landslides. The choice of method depends on the excavation depth, physical-mechanical properties of the soil, the loading of the edges of excavations and the time the excavation remains open.

Vertical walls can be excavated in cohesive soils with a depth of no more than 1.5 meters. In other cases, excavation walls must be provided with one of the following options:

• Sloping walls of excavations: The slope of the excavation walls should be as steep as possible because the cubes of earthworks and the excavation area are increasing. At the same time, minimal slope, defined primarily by the angle of internal soil friction and the coefficient of soil cohesion, should be respected ((eg sandy gravel 1:1, clayey sand 1:0.50, dust 1:0.25). In excavations deeper than 3 meters, slopes are interrupted by field benches with a minimum width of 500 mm.
• Shoring of excavation walls: Shoring is a temporary building structure that protects sloping walls against landslide during excavation work. Timbering must be done directly with the excavations. Timbering consists of sheeting and bracing. Sheeting is flat part of shoring which is in direct contact with the soil. Sheeting consists of wood or steel planks laid vertically, horizontally or obliquely. Soil pressure acting on sheeting is intercepted by horizontal and diagonal struts. Depending on the construction and the method of implementation, we distinguish:
  • Shoring with attached sheeting: Attached sheeting is used in cohesive and incoherent soils. According to the coherence of the soil, the struts are laid either at a meeting or with spaces, horizontally or vertically
  • Piles shoring: Piles shoring consists of a piles rammed into the subsoil. Horizontal sheeting is triggered between pilots. Bracing pilot induces high strength sheeting. This method can be used in wide construction pit and up to 20 m depth. Piles shoring cannot be established in boulder soils where the defects cannot be pulled to the required depth or at the necessary distances.
  • Weft shoring: Weft shoring is used in construction pit and grooves. It may be vertical or oblique.
  • Driven shoring: Driven shoring is carried out in cohesive, cohesive and incoherent soils where we can get secure enclosed space in which we can work. It is the costly and hardest way of shoring.
  • Triggered shoring: Triggered shoring is used in less cohesive soil at excavation depth of up to 6 m. Carved frame from round logs, columns, vertical shoulder and wedge.
• Underground walls: Underground walls are used to secure the walls of deep excavations, in blank space or at a great load on the edges of excavations. Depending on the building material used, we distinguish the underground walls of clay, clay-cement and concrete. Underground walls can fulfil not only the function of armor and sealing, but also the function of construction and foundation for the peripheral load-bearing masonry. Milano's underground walls are made up of a continuous groove with a depth of up to 40 meters, into which prefabricated concrete panels are launched or they are concreted in a width of 0.6 - 1.0 m and at the same time serves as the load-bearing wall of the underground part of the building.
• Pile walls: Pile walls can be used in soils and rocks with low strength. The individual piles overlap each other below the groundwater level. Piles simply touching above the water level and the axial distance is less than 2 m. Non-anchored piles are used up to 6 m if the span is larger, they are anchored or bracing.
• Sheet pile walls: Sheet pile walls are used in cohesive to solid and non-cohesive soils (outside the boulders). They can be used below the level of ground water. The locks are connected to each other to ensure water tightness. The best known type is Larssen sheet piling which can be used up to a depth of 20 m. After finishing the work, it is possible pull out and re-use them.