Subsidence

Reduction is understood in the building and the earth sciences, the slow reduction of a building or a rock body by gradual compaction ( compaction ) of the substrate. Cause of the sinking is usually the weight of the overlying masses about it, but can the volume reduction by dehydration, for example, due to a lowering of ground water and its causes other shrinkage processes.

For structures, much of the reduction on the yielding of fanned or padded soil or the subsoil is due. Your prospective extent can be estimated if one knows the soil structure and the possible building load.

When setting the reduction of dumped or built bottom is referred to under its own weight, which can be frequently observed in inadequately compacted backfill, backfilled trenches, or even very common in dams.

Course of the legislative process

In aufgegrabenem and again filled- bottom of a pit, a grave and the other setting at the beginning is very rapid, it is usually accelerated by rain and disappears within some weeks. Only then is construction makes sense (otherwise would be leaps and imbalances arise ), but may be preferred by some sturdy frame systems.

The duration of the legislative process - which consists mainly in a reduction of porosity - can be substantially shortened by artificial compaction of the soil by masher or construction vehicles. It is important that uniform compression - because uniform subsidence under a building cause in this generally no damage.

In excavations by building authorities or other official bodies a " calming period " of half a year is common to achieve a sufficient reduction of the backfill and prevent subsequent damage. In road construction, therefore, is possible any disturbance of the soil structure avoided ( excavating only to Bausohle ) of clashing hold a subsequent reduction of parts of the roadway.

In buildings, the ground A., is so fast that the settlements have been completed during the building shell. The less the bottom of the pit was broken up under the metalling or the foundation, the more favorable is the situation. For larger structures are residual subsidence is also possible after completion, which can cause cracks only with uneven settlement, however.

Larger construction projects

For larger construction projects, the setting is analyzed by longer-term settlement measurements and / or pressure tests with Rammsonden or load boards. In the latter - for example with a loaded truck, or vibration - artificial replacement process is determined by measuring its velocity in short time intervals modeled ( pressurization diagram). The measured reduction is related to the capacity and the degree of compaction of the soil. For known values ​​of Verfüllböden there is abbreviated test specifications. In addition, can be a hedge by cutting out a cylindrical soil samples.

Technical projects such as large buildings, heavy or high bridges, power stations and their dams are usually monitored by longer-term settlement measurements. From geodesics periodic height measurements, or leveling be performed, the place from geologically stable points. In dams, these units are several points in the bedrock, but at a greater distance from the abutments. The measurements can also be done automatically ( computer science with total stations ) and - such as the risk of landslides - be coupled to a warning or alarm system.

About strain gauges, temperature and electrical sensors - - In addition, geotechnical sensors are integrated into the building in large buildings tend to be small changes to automatically detect. In dams and periodic measurements in the control transitions are required. For active and abandoned mines can be expected with any ground subsidence above the cavities and tunnels, even if these several hundred meters below the earth's surface. On mining and engineering colleges these phenomena are studied in the fields of Mining Subsidence Engineering and Mine Surveying.

Reduction areas in sediments

Occur naturally settlements almost everywhere, where there is deposition of sediments. From loose rocks such as gravel, sand, or silt occurs by the superposition ( pressure of the upper layers of the " hanging wall " ) is a significant compaction of a by decreases in the pore volume. In deeper sedimentary basins with initial densities of about 1.5 to 2 g / cc in the course of a few million years, a density of 2.2 to 2.5 g / cm ³, which finally ends in a solidified into sandstone ( about 2.6 ). In contrast, especially the pore water is squeezed in clay, which, for example, slate occurs in the metamorphosis.

With these " geological " Settlements sinking of the land is accompanied by the for example in the Rhine Graben and in the Vienna Basin is currently an average of 1 mm per year. However, these reductions are uniform and effect - except on some fault lines - no more damage.