Building Subsidence

Causes of building subsidence, its effects and how to fix a sunken building

Causes of Building Subsidence

Building subsidence can be caused in many different ways. Below are some of the main subsidence problems that are suited to rectification by Uretek

Loss of Ground Moisture Causes Building Subsidence

Almost two decades of drought conditions in eastern and southern Australia caused many houses and other buildings to subside.

The lack of moisture in ground that has some content of reactive clay causes the clay to shrink. With the weight of the house on it, the foundation ground subsides and the house goes down. The soil contraction under the various parts of the building are almost never uniform. So with loss of support varying across the structure wall cracking often occurs.

Nearby trees with extended root systems can also remove water from the foundation ground, as can variations in the level of the water table.

The action of this clay is sometimes called “shrink-swell effect”.

What happens when reactive clay swells with new moisture content? As the clay expands it seeks the path of least resistance. It is generally easier for it to expand sideways rather than upwards, because there is a whole building  or major structural element on top of it. However there may be some narrowing of wall cracks.

Wide cracks will sometimes close up partially. Very narrow cracks may close up sufficiently to satisfy home-owners. However without ground compaction all wall cracks are prone to re-open with the next extended dry period.

In many cases, the resulting damage occurs over very long periods of time, making it difficult to establish the exact cause.

Our experienced Area Managers can advise best in regard to your particular circumstance.

Leaking water pipes, sewers and drains cause building subsidence

Another common cause of subsidence is washout of the foundations caused by leakage from buried pipes.

Liquids flowing in areas not even immediately adjacent to the building can saturate the foundation ground and reduce of its mechanical resistance. The foundation ground is unable to uniformly support the load above leading to differential subsidence. Infiltration of uncontrolled surface water, even in small quantities, can increase the plastic qualities of the foundation material, lowering soil bearing capacity.

Building subsidence can also be caused by excavations

Adjacent to buildings, excavations, especially in non-cohesive ground (sand and gravel), can cause subsidence unless proper precautions are taken. Uretek injections can help to form a cohesive conglomerate to prevent migration of material.

Inadequate foundations are often the cause of a sunken building

Structural settlement can be caused by an inadequate relationship between the pressure imposed on the foundations and the acceptable ground load.

Slow consolidation of cohesive soils is another factor. Cohesive ground reacts quite slowly to the imposed pressure in reaching a state of equilibrium over long periods. When determining cause of settlement, care must be taken not to underestimate slow ground movements and not to overestimate the possible short-term elements.

Replaced soil also often the cause of a sunken building

Ground disturbed during construction of a building can lose its supporting properties. Differential settlement resulting from this can cause cracking which can often take years to become apparent.

Other causes of building subsidence

• Earthquake and seismic tremors.
• Vibrations caused by heavy road traffic or by machinery.
• Absence of an organised footing system - in very old buildings or in buildings with additions or alterations.
• Overloading of ground adjacent to the building, for example during the construction of ornamental gardens.
• Different geological strata in the ground.
• Changes in the volume and direction of water flows due, for example, to works near the affected area.
• Differences in densification of the foundation ground caused by different levels of consolidation.

Uretek Correction of Building Subsidence

Uretek rectifies building subsidence by injecting structural resins into the foundation ground under the footings of the building. The resins expand together chemically creating pressure.

That pressure is at first directed so as to lift the building back to its correct level.

Then, if there are weak layers in the ground, continuing injection at deeper levels compacts the ground, densifying and strengthening it to increase its bearing capacity.

This way, in some soil conditions, the bearing capacity of weak strata can be increased by as much as 500%.

Other ways to treat shrink/swell in clay soils.

• Control water sources: All piped services in the ground should be checked by a plumber to ensure that there are no breaks or leaks leading to periodic local wetting of the subsoil.  Check sanitary drainage, storm-water and water supply.  Outside water taps and downpipe should discharge into drains not onto the ground.

  Irrigation systems that maintain a reasonably constant moisture content in the soil may be acceptable however those that are used only periodically may exacerbate wetting and drying cycles.  Generally it is preferable not to have planting or irrigation systems immediately adjacent to walls of buildings in reactive clay soils.

  Flower beds should be separated from the building by paved areas about 2 m wide, if possible.

• Tree roots: trees near the building (eg within a distance of 2 x tree height from the walls) can greatly exacerbate shrinkage of clay soil as moisture is sucked out. Options include installing cut-off walls for root control with root pruning (an arborist’s advice is essential to ensure tree health) or removal of trees within the influence zone of the building.  This zone varies with tree size and type, soil type and landform (again an arborist’s advice is required). Works to trees generally require Council approval.

• Building perimeter moisture control: this strategy aims to minimise changes in moisture content below the footings by site grading, drainage and moisture control devices.   The objective is to maintain stable water content in soil below the footings by creating an increased path length for evaporation to the surface.

• Subsoil drains : Subsoil (or agricultural) drains can be useful in preventing excessive saturation of the soil. But they should not be used immediately adjacent to external walls.  Free drainage adjacent to the walls can have the effect of increasing the rate of wetting and drying.  So subsoil drainage is only recommended at the perimeter of paving and generally at least 2 m away from external walls.  Site grading and paving should grade away from the walls towards such drains.  The drains should not be too deep in case they dry out the soil excessively.

Once a wall has cracked it can be difficult to prevent recurrent cracking at the same location. If moisture control methods have been adopted, without Uretek treatment, then future movement is likely to occur.  

More information:

• Soil Stabilisation & Ground Compaction

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