Building land, on which the building is founded, takes over the weight of the building through the foundations. Building land is rarely a homogeneous layer, usually forming stratifications of different types of soil with different properties.
Safe foundation of the building requires a detailed examination of the ground. Before proceeding with the design of foundations, it is necessary to determine:
• Type of soil
o The best foundations are solid rocks. There is practically no ground subsidence in rocks,
o Loose mineral soil is a good foundation for building foundations. Stony and gravel soils are not very compressible and there is almost no hairline rising of water in them. Sandy soils are well permeable to water, but the capillary rise of water increases all the more, the smaller the grain size of the soil. Loose soils settle immediately after the foundations are fully loaded – subsidence ceases after construction is complete,
• Cohesive mineral soils – The danger in cohesive soils is rapid and easy dampening. As a result of high moisture, the soil swells significantly and the building is lifted up. Cohesive soils are heave soils - they increase in volume after the water contained in them freezes. Subsidence of cohesive soils is long-lasting (up to several years) and consists in displacing the water contained in them. Osfedation of cohesive soils can be uneven.
• Organic soil is a bad foundation for building buildings. They are characterized by large and uneven settlement, therefore, only light structures can be placed on them. The water contained in these soils contains acidic humic compounds harmful to concrete and other building materials.
• Load capacity is the ultimate load, which can be taken by a unit of land area without disturbing the state of equilibrium. Due to the safety of the structure, the permissible load is determined (less than full capacity). Soil loading causes deformation called settlement (soil compaction, grain deformation, displacement of water, pushing the ground up and to the sides),
• Depth of ground frost. Soil freezing is the freezing of water molecules in the ground. Freezing causes the volume of water to increase, as a result, the ground begins to swell and blow up the elements resting on it, causing damage to floors on the ground and displacement of foundations and basement walls. In Poland, the depth of ground freezing is from 0,8 do 1,2 m.
* Natural soil moisture. For well-draining non-cohesive soil (gravel or sand) it can be assumed that there is no groundwater under pressure. Basements should only be protected against natural ground moisture.
* Water conditions – groundwater level and the presence of groundwater. Execution of foundations below the groundwater level requires protection of excavations against flooding and execution of heavy waterproofing and drainage, thus significantly increasing costs. Groundwater is particularly dangerous in the presence of cohesive soils and the location of the building on a slope. When these waters are piled up – walls and floors in the basement are exposed to pressurized water.
With predominance of impermeable or low permeable soils (clay, loam, margiel) should be assumed, that surface and aquifer waters seep into the trench backfill, and then they pile up. Then, the possibility of using band drainage around the building should be considered.
• Directions of groundwater flow and the degree of its aggressiveness. Groundwater may contain aggressive chemicals (np. carbonic acid and sulfate acid), which destroy materials in the basement.
The geotechnical opinion is the basis for the selection of materials and the design of the foundations of the building, foundation depth, type of waterproofing, itp.