Worth knowing

iage Uncategorized March 03, 2025 Accessed: 1183

Practical knowledge

The load of a structure generates additional compressive stresses in the subsoil, which are associated with settlements. Under the foundation, the stress distribution takes the form of an onion, which is referred to as a compression onion. The stresses are greatest directly below the base surface. They gradually decrease with increasing depth.

According to the rule of thumb, the effect of the pressure bulb is largely reduced at a depth corresponding to twice the width of the foundation. Consequently, the settlements primarily occur in this zone and the measures to reinforce the subsoil should concentrate on this depth range.

Indications of building settlements

The following observations indicate subsidence:

Cracks in the masonry
Stepped cracks in walls
Diagonal wall cracks
Stress cracks in the building structure
Crack width increases
Dilatation joints expand increasingly and tear
Connection joints open, the higher the further
Sticking windows and doors
Self-slamming doors
The marble rolls independently across the floor
Noticeable inclination: a sloping floor can be felt when walking from a gradient of around 3 - 5%
House corner sags
House starts to lean

Damage potential of building settlements

Settlements that are evenly distributed over the base area are usually not relevant for buildings. However, if the settlements are differential, deformation stresses occur. Small differences in settlement are absorbed by the building structure without damage. However, the potential for damage increases with increasing settlement differences. As a rule, the following evaluation applies:

Value	Damage potential
Δs/ℓ < 1/500	Settlements are hardly harmful
Δs/ℓ > 1/300	Architectural damage is possible
Δs/ℓ > 1/150	structural damage is likely

Δs = Settlement difference between two points
ℓ = Distance between two points

Causes of building subsidence

Several causes often interact to lead to the settlement damage observed. Typical causes are pressure overlay and a heterogeneous subsoil (see diagrams).

Other causes of subsidence are

Softening, washing out or washing away of the subsoil as a result of leaking sewer pipes and shafts, seepage of roof water, burst water pipes, etc.
Load increase due to building extension, garden fill, etc.
Shrinkage of cohesive soils due to drying out during long periods of heat
Drying out and decay of peat soils
Lowering of the groundwater level
Landslide

Polyurethane in geotechnics

Polyurethane (abbreviation: PUR) is a synthetic resin that is used in countless products in modern everyday life, such as household sponges, mattresses, shoes, assembly foam, ....

PUR has been used for soil stabilization in geotechnical engineering for many decades. For example, subsoil injections with PUR were used as early as the 1960s in German coal mining to secure fractured zones and seal off water-bearing layers. During the construction of the Furka tunnel (1973-1982), PUR injections made a significant contribution to solving rock mechanical problems. Since then, PUR injections have become established in tunnel construction and are also used successfully and frequently in geotechnical engineering.

Impact of polyurethane on the environment

Before the injection system presses the two-component synthetic resin system into the injection lances, the two components are mixed in a 1:1 ratio to form polyurethane in the mouth of the injection gun, which is mounted on the injection lance. If the delivery pressure of the individual components differs, the injection system stops delivery automatically. This ensures that only synthetic resin with the correct mixing ratio of 1:1 is injected. Correctly applied polyurethane has no negative impact on the environment, which is confirmed by test reports from accredited laboratories.

If the soil material injected with PUR is excavated and disposed of in the distant future, it can be disposed of in inert building material landfills, just like brick or concrete waste.