Module 5: Mitigation, Monitoring & Repairs Crack stitching, resin, underpinning

Once cause is confirmed and mitigation works are done, the building is repaired. Substructure (if needed) and superstructure repair methods.

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CORE Subsidence · Module 5 of 6

Content

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Subsidence Repairs

Once the cause of damage is confirmed as subsidence and any mitigation works are completed (drain repair, tree management works, etc) the building needs to be repaired:-

Substructure Repairs – If mitigation works cannot achieve ground stability, substructure reinforcement is required. Methods include:-

Traditional Underpinning – constructing a new mass concrete foundation at a stable depth

Pad & Beam – Excavating intermittent concrete “pads” down to firm ground and constructing a continuous concrete beam across the pads.

Piling – using an auger to form a pile down to a designated depth and placing reinforcement and concrete into the augered hole

Resin Injection – Used to stabilise ground and lift structures

Superstructure Repairs – Once ground stability is achieved repairs the cracks in the building will need repairing. The type of masonry will dictate the repair method. Crack stitching methods may also be used

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  • Resin Injection
  • Underpinning
  • Stitching Bars

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Resin Injection

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Trainer note:

The resin uses an injection delivery method to raise, re-level, and re-support structures that have suffered the effects of ground subsidence.

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Underpinning

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Stitching Bars

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Trainer note:

Crack stitching is the masonry repair method for repairing cracked walls and ensuring that they are reinforced. It involves retrofitting a number of crack stitching bars which are grouted across cracks in walls in order to reconnect them and provide stability to the masonry

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Geotechnical

In the vast majority of cases, it will be necessary to carry out a geotechnical (or soils) investigation.

Although a geological map or database will identify the likely nature of the naturally occurring sub soils at depth, they will not tell you depth or extent of made-ground or the exact nature of man’s influence on the topographical features.

In simplistic terms there are basically two types of soil - cohesive and non-cohesive. Cohesive soils, such as clays and silts, vary in their ability to change their volume when wetted and dried, whilst non-cohesive soils, such as sands and gravels, do not share this propensity to change in volume but are susceptible to the wash out of the finer particles when subjected to flowing water.

A house built on clay soils will rise in the winter when the soil expands and fall in summer when the soil shrinks. This has no structural significance if the house moves uniformly. However, if a local factor such as a tree or a broken drain upsets the equilibrium then differential movement and structural damage can occur.

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  • Subsidence Investigation
  • Monitoring
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Remote Monitoring

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Trainer note:

Monitoring procedure - it is important to fit a suitable number of studs to the correct cracks and to take the readings at appropriate intervals. Too many monitoring studs fixed to a property can cause confusion. If studs are fixed to the wrong cracks, they may measure movements unrelated to subsidence damage. A sufficiently experienced engineer should be able to identify the cracks which are most likely to provide representative readings and in most instances there should be no need to fix more than six monitoring stations unless damage is widespread and thought to be due to more than one cause. The monitoring interval should generally not be more than three months. However, there may be instances of rapid movement with potentially dangerous consequences eg to retaining walls, where readings will need to be taken on a weekly basis. However, these are the exceptions and usually three monthly readings should be sufficient to confirm a trend. Consistency is all important throughout the monitoring process, not only regarding the operative taking the readings but also time periods so that truly meaningful results can be obtained.

Cross-references

Knowledge check

Pass mark: 80%. You'll get immediate feedback with the correct answer + rationale on each question. Wrong answers can be retaken without penalty.

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