In construction, the area of the ground that is important for the construction of a structure is referred to as building ground . By definition, subsoil is “soil or rock including all constituents (e.g. groundwater and contamination) in and on which structures are to be or are embedded, or which is influenced by construction measures” ( DIN 4020 , Section 3.1 ).
The properties of the subsoil are particularly important with regard to the foundation (foundation) of a structure. An essential property is the load-bearing capacity, i.e. its ability to absorb loads from the structure without significant subsidence or frost-related uplift (see also Eislinse ) or a ground failure occurring. As a rule, the subsoil is made up of different soil layers and types of soil, and groundwater can also accumulate .
The properties of the subsoil are primarily determined by the soil types and soil classes. These properties are regionally - depending on the geological origin - very different. Sometimes they vary greatly locally. Therefore, before the start of the construction work, the soil must be sufficiently examined as part of a subsoil investigation to determine its suitability as a subsoil.
A basic distinction is made between organic and inorganic soils. The organic soil consists, for example, of humus , peat or lignite and is not suitable as building ground, as strong subsidence is to be expected. Inorganic soils consist, for example, of sand , gravel or rock and represent a useful building site.
The soils cannot only be differentiated according to the content of organic material, but also according to the type of soil . Based on DIN 1054 , the following soil types can be defined:
- Rock (solid rock)
- is very solid in the unweathered condition and therefore sufficiently stable. The preparatory measures necessary for the erection of structures can, however, be very time-consuming, as rock is very difficult to remove. For example, explosions may therefore be necessary
- Grown soil (loose rock)
- was created by geological processes such as weathering and deposition. The load-bearing capacity can be low to very high, depending on the soil.
- Heaped soil
- was created by flushing or pouring. Depending on the degree of compaction, the fill has a low to high load-bearing capacity.
Grown or poured soils can be divided into cohesive and non-cohesive soils with regard to their nature.
- Cohesive soil
- is a soil with a high proportion of clay or silt (colloquially known as loam ). Cohesive soils deform relatively strongly over a long period of time under pressure. Compared to non-cohesive soils, they settle very slowly, so there may still be residues after the building has been completed, which can lead to damage . The behavior of cohesive soils depends on the water content. Depending on the proportion of clay and silt, these soils are poorly water-permeable. Water may accumulate, reducing the capacity and accumulates on the building exteriors. In addition, the soil is sensitive to frost , as the pore water freezes and uplifts occur. Clay minerals also tend to swell or shrink under the influence of water.
- Non-cohesive soil
- is a soil with a low proportion of fine grain . This type of soil includes sand and gravel in various grain sizes and mixtures. Contrary to the phrase “built on sand ”, this is mostly good building ground, provided it is not loosely stored. This is due, among other things, to the fact that its mechanical behavior does not depend on the water content and, on the other hand, to the fact that the grain structure is relatively stable. The relatively low compressibility of sand means that settlement remains relatively low. The settlement also occurs immediately when the loads are applied and are therefore largely completed when the shell is completed. In the case of low storage density or cohesive or humic proportions, subsidence can also occur here. In the case of non-cohesive soils, frost damage usually does not occur because the change in volume of the water can be absorbed by the air voids in the grain structure. Non-cohesive soil is also known as rolling soil.
An important prerequisite for the planning and construction of excavation pits or foundations is knowledge of the pending subsoil. For this purpose, soil examinations are to be carried out. Their type and scope depends on the difficulty of the structure and the expected subsoil conditions.
According to DIN 4020 , a distinction is made between three geotechnical categories :
- simple structures on level, stable ground that neither affect the environment nor the groundwater
- Construction projects that do not belong to either Category 1 or Category 3
- Construction projects with difficult structures and difficult subsoil conditions that require advanced geotechnical knowledge
The building site expert defines the investigation program and must use the results to assess the building site with regard to its load-bearing capacity. Depending on the results of the subsoil investigation, he recommends a foundation concept and provides information and parameters for dimensioning the foundation. The aim of these recommendations is that the foundation is selected and dimensioned in such a way that its failure can be ruled out with a certain degree of certainty, that it is suitable for use and that the foundation measure is economical.
An important part of a subsoil investigation is the determination of information about the groundwater. The groundwater level and its fluctuation ranges are particularly important when planning a structure.
Methods for examining the subsoil on site are, for example, natural outcrops (stream bed or slope). Furthermore, statements about the subsoil conditions can be obtained from geological maps and on-site surveys with little effort. If these measures are not sufficient, prospects , boreholes or soundings (such as ram core sounding ) must be carried out.
With the help of a scrape, the stratification of the soil can be easily recognized. In addition, undisturbed, i.e. unchanged, soil samples can be taken for analysis in the laboratory. When creating the dig, ensure that the slope wall is adequately secured. The economic limit is around 4 to 6 meters. For greater depths, holes are suitable, with which soil samples can also be taken. In the course of these investigations, the current groundwater level can also be measured. In the construction industry, soundings are often carried out in the form of pile-driving soundings ; other methods are pressure sounding and standard penetration tests . The storage density of non-cohesive soils or the consistency of cohesive soils is determined with the ramming and pressure probing .
From the exploratory results and his regional geological knowledge, the subsoil expert ( subsoil expert ) can calculate the permissible loads on the subsoil and the expected settlement. The evaluation of the subsoil investigation is usually carried out in accordance with DIN 1054. Here, typical soil parameters are specified for general and unambiguous cases.
The building ground is deformed according to its compressibility and shear strength due to the action of structural loads. If the subsoil is loaded by perpendicular loads, it will initially settle because the soil layers are more compressed. This process is generally to be regarded as uncritical and in most cases as unavoidable. Depending on the nature (cohesive or non-cohesive), however, different settlements occur in relation to the duration. Cohesive soils settle slowly and distinctly as the pore water is slowly pushed out of the soil. In addition, the properties of the soil change with high pore water pressure. Non-cohesive soils, on the other hand, settle more quickly and less because there is no pore water and the grains are in direct contact. Settlements in the subsoil are problematic if they occur unevenly on the foundation base. The structure is tilted, which creates tension on the structure. Uniform subsidence, on the other hand, generally does not damage the structure.
If the load on the subsoil is increased so that the critical breaking load is reached, the soil is suddenly displaced to the side. The foundation sinks down or to the side and a ground break occurs. The risk of a ground failure increases the smaller the width and embedment depth of the foundation and the lower the shear strength of the soil. Furthermore, the inclination and eccentricity of the load increase the risk of ground failure.
If the subsoil does not meet the required properties, appropriate technical measures to improve the subsoil (including soil improvement) must be carried out. These measures improve stability and reduce the extent of subsidence. The following measures can be used:
- In the case of soil replacement (soil replacement method), the unsound soil is replaced in whole or in part by more suitable soil types. This method is economical when soil layers with a relatively small thickness have to be replaced and suitable replacement soil is cheaply available. When installing the replacement floor, ensure that it is sufficiently compacted.
- Soil compaction by surface compaction or deep compaction methods. Special methods in special foundation engineering are, for example, vibratory plug compaction or deep compaction with piles.
- In soil consolidation , the addition of binding agents such as cement or lime improves the stability of unsound soils. Examples are chemical soil consolidation, various injection methods ( ground injection ) such as high-pressure injection and soil freezing . When the ground freezes, the pore water in the ground is frozen with the help of a coolant, thus creating sufficient stability. The process is very costly and is therefore only used for temporary securing or sealing of construction pits.
If the load-bearing capacity of the subsoil is not sufficient for a shallow foundation and no changes are to be made to the subsoil itself, deep foundations provide the option of creating a stable foundation for the foundations. Here, for example, the pile foundation is to be mentioned as the most famous representative.
Norms and standards
- DIN 1054 - Subsoil - Evidence of safety in earthworks and foundations
- DIN 4019 - Subsoil - settlement calculations (three parts, two supplements)
- DIN 4020 - Geotechnical investigations for structural purposes - Supplementary regulations to DIN EN 1997-2 (with a supplement)
- DIN 18300 - VOB procurement and contract regulations for construction works - Part C: General technical contract conditions for construction works (ATV) - Earthworks
- ÖNORM B 4402 - Earthworks and foundation engineering - Geotechnical investigations for structural purposes
EN 1997 - Eurocode 7: Design, calculation and dimensioning in geotechnical engineering
- Part 1 General rules
- Part 2 exploration and investigation of the subsoil
ISO 22282 Geotechnical exploration and investigation - Geohydraulic tests
- Part 1 General rules
- Part 2 Water permeability tests in a borehole using open systems
- Part 3 water pressure test in the rock
- Part 4 pumping tests
- Part 5 infiltrometer experiments
- Part 6 Water permeability tests in the borehole using closed systems
- Günther Schalk: The legal significance of the building site as a building material. ( Memento of the original from September 2, 2009 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF file, 0.1 MB).
- Hansjörg Frey: Structural engineering - specialist knowledge of construction . Europa-Lehrmittel, Haan-Gruiten 2003, p. 213 . ISBN 3-8085-4460-0 .
- Balder Batran: Expertise in construction . Craft and technology, Stuttgart 2002, p. 23, 24 . ISBN 3-582-03503-4 .
- Martin Mittag: Building construction theory . Vieweg, Braunschweig 2000, p. 12 . ISBN 3-528-02555-7 .
- Klaus Kirsch , Alan Bell (Ed.): Ground Improvement, CRC Press, 3rd edition 2012
- Wolfgang Sondermann , Klaus Kirsch: Construction site improvement, in: Karl Josef Witt (Ed.), Grundbau-Taschenbuch , Volume 2, 7th edition, Ernst and Son 2009.