• Which type of properties suffer most from subsidence? *
• What are the most common subsidence problems facing local authorities and how should they be addressed? *
• How should an evaluation to decide whether structural repair is possible take place? *
• What advancements in repair techniques have been made in the past couple of years and how do they contribute to solving the problem of subsidence? *
• When undertaking an engineered foundation project, what are the major steps to consider? *
• plus member?">What are the benefits of using an ASUCplus member? *

These items have been extracted from an article which first appeared in the May issue of LABM ( Local Authority Building and Maintenance) published by Hamerville Magazines http://www.hamerville.co.uk 
 

Geographically, the south-east of England tends to receive the most publicity associated with subsidence and this is generally linked with the geology of the area  - i.e. the presence of shrinkable clay subsoil.  Predominantly, domestic residential properties are the types of structures that suffer from subsidence and they generally are conventional brick built buildings with foundations that generally do not conform to today's minimum building regulation standards.

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What are the most common subsidence problems and how should they be addressed?

I suspect that the problem that for most it is establishing is at what stage does a property become damaged to an extent that superstructure repairs, on their own, are not adequate to continue the serviceability of a property.

Various guidelines have been prepared over the years that refer to crack damage and how this type of defect can be quantified.  In addition, domestic residential properties often suffer from associated building fabric distortions which, if allowed to develop over a considerable period of time, will also seriously affect the serviceability of individual properties.

The most publicised cause of subsidence problems is associated with the effects of trees on shrinkable clay subsoils and how this, in turn, can affect the stability of conventional shallow foundations.  However, on granular type subsoils, defective drains can also cause considerable structural distress, resulting in major remedial works being required.

Ultimately the only guaranteed long term solution to solving an ongoing subsidence problem is to carry out an underpinning remedial repair solution.

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How should an evaluation to decide whether structural repair is possible take place?

As soon as evidence is available that a structure is suffering from subsidence, various methods of monitoring can be adopted.  These include measuring the movement and lengths of superstructure cracking, and/or monitoring the level of an individual structure utilising a fixed deep stable datum.

If it is proved that ongoing movements are being recorded, then a site investigation should be carried out in order to confirm the nature and condition of the existing foundations supporting the structure, combined with subsoil retrieval and analysis to varying depths around an individual structure.  This should generally reveal the cause of a subsidence problem and an Engineer/Surveyor should then decide upon the correct course of action.  Wherever possible, experts should be consulted including specialist contractors.

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What advancements in repair techniques have been made in the past couple of years and how do they contribute to solving the problem of subsidence?

The most significant advancement has been the ongoing development of piled foundation technique involving mini pilling, plant and equipment.  However, there are still many situations that require more conventional traditional underpinning techniques or beam and base methods of underpinning construction.

The final underpinning construction technique to be utilised is often dependent upon the actual cause of the subsidence damage and the access available in order to carry out the sub structure repairs.

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When undertaking an engineered foundation project, what are the major steps to consider?

If monitoring and investigation works clearly indicate the underpinning works will be necessary in order to stabilise a structure, the following needs to be given detailed consideration:  

• What type of underpinning techniques is likely to be technically successful in stabilizing the subsidence movement problem?  
• When taking into account the nature of the existing substructure and the existing subsoil conditions, what underpinning technique is likely to be most cost effective? 
• What are the access restriction enabling underpinning remedial works to be carried out to an individual structure when taking into account surrounding site conditions.  
• Will there be a requirement for the tenant of the property to vacate their residence? and
• What could the effects of the underpinning technique be on either adjoining properties or adjacent structures?

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plus member?" id="What are the benefits of using an ASUCplus member?">What are the benefits of using an ASUCplus member?

Members are financially and technically audited on a continuous basis.  They also undergo health and safety audits to ensure that they are complying with current legislation and best practice.  All member companies are authorised to offer their clients the ASUCplus 12 year insurance backed guarantee.  This is issued on the property and is fully transferable and stay in place whatever happens to the contracting company.

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