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STABILISING & LIFTING - The URETEK Method

CAUSES OF SUBSIDENCE & OTHER PROBLEMS

The need to raise, relevel or simply stabilise a concrete slab or other building component, can originate from a number of causes.

Most problems can be solved, efficiently, with a URETEK solution.

• Voids filled.
• Slabs, foundations and buildings lifted.
• Surfaces relevelled
• Ground compacted.

Whilst URETEK cannot solve every problem, situations like those described below have been corrected by URETEK hundreds of times.

In certain cases, there may be no practical answer to the problem and programmed, preventative maintenance may be the only solution. For example, with decaying vegetable matter in ground.


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Inadequate sub-base preparation relative to slab thickness

A floor slab or road pavement on ground, provides a functional surface (in terms of appearance and wear characteristics) and is designed to distribute concentrated, applied loads over a wider area, bridging any localised weak spots in the underlying sub-base or subgrade.

It is the sub-base under the slab that supports the applied load (and the weight of the slab). Therefore the adequacy of the sub-base is often more important than the strength of the concrete itself.

If the sub-base is inadequately prepared before the concrete is laid, it can compact over time, settling initially to form voids and eventually leading to slab failure.

A sub-base comprising a sand layer under the concrete often presents as a cause.

In the case of buildings, loose soil left in excavated trenches or at bottom of pier holes before concreting can later crush, causing damage to walls.

Inadequately compacted back-fill

Settlement can often be attributed to filled ground, inadequately compacted at time of construction. (This occurs commonly where ground level has been raised or in vicinity of a retaining wall.)

At times, even where little settlement is evident, voids exceeding ten centimetres (4 inches) are discovered right under the concrete.

Vibration caused by traffic, machinery or construction activities.

Vibration due to machinery or passing traffic can cause ground densification. That is, the ground compacts and shrinks, leaving the structure above, unsupported.

Such problem also presents on properties adjacent to building excavations where there is rock excavation, or in the vicinity of underground tunnelling.

Earthquake

Similarly, the forces unleashed during an earth tremor or earthquake, very often cause ground densification of inadequately compacted soils - especially in non-cohesive material (e.g. sand).

Overloading

If the applied loads imposed on a slab exceed design, compression of the subgrade can occur, with attendant settlement. A perhaps surprising benefit, however, is that the ground immediately below the slab will have become stronger, providing a good sub-base from which to lift and support additional load.

Loading along slab edges

The weakest areas of the sub-base and concrete slab dependency, are at corners and along edges. Where the sub-base is inadequate, crushing can occur under applied loads, a void forms and the slab deflects. Continual traffic loading causes a hammering effect onto the sub-base, leading to greater voiding - and eventual slab failure.

Erosion

Whilst many insurance claims arise from a burst water pipe, which results in massive washout of the sub-base, even a minor leak in a sewer or stormwater line can weaken the soil or activate reactive clay.

Reactive clay

Many clays are affected by the addition or removal of water. They expand and contract.

Whilst volume changes due to manmade influences are readily treatable, expansion and contraction due to seasonal climatic changes need special consideration.

It is sometimes difficult to decide whether the cause of settlement is due to the weakening a clay base due to over-wetting or reactive clay shrinkage.

"Pumping" action on roads and driveways

A major cause of road failure is water ingress through joints between concrete pavement slabs. This weakens the sub-base and allows the slab to deflect. Deflection pumps out dirty water. That is, water containing fine particles from the sub-base. The sub-base is consequently eroded. More deflection occurs and the cycle continues.

Mine subsidence

Long-wall mining (being total extraction of resources) below urban areas can see the effects excavation at great depth, transmitted to the surface, causing settlement of entire suburbs.

Lowering of water table

Dewatering of abandoned mine shafts, tunnelling, open-cut mining, building excavations and the like, can all lead to lowering of the water table. As pore pressure is reduced, so settlement begins.

Exceed design life

Concrete pavement restoration or CPR has become an increasingly important area of the Public Works sector, as more and more streets and highways are approaching, or have already exceeded, their design lives.

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