CROSS Safety Report
Adjacent excavations in Australia
This report is over 2 years old
In parts of Australia, says a reporter, deep excavations in clay and sandy materials have caused damage to adjacent property and to existing buildings founded on high level footings.
Key Learning Outcomes
For the construction team:
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all excavations are carefully considered and planned
Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
For civil and structural design engineers:
The condition and stability of adjoining structures should be determined to make sure that the methodology of deep excavations does not endanger the stability of adjoining property
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In parts of Australia, says a reporter, deep excavations in clay and sandy materials have caused damage to adjacent property and to existing buildings founded on high level footings. Additional regulations have been introduced, but these difficulties continue. There have been cases, says the reporter, where building owners have been involved in civil actions against developers or builders to try to recover the costs of rectification to their damaged property.
This is a potential risk for structural and geotechnical engineers and builders. Under the Australian regime, when a local Building Surveyor consents to a new project there are standard regulations intended to reduce the risk of damage to adjacent buildings, and there are also legal obligations under various statutes.
However, continues the reporter, reliance is placed on the builder to repair any obvious and immediate damage. This is an unsatisfactory situation when the consequences of any damage can include latent defects which may not become apparent for many years.
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This situation is addressed explicitly by BS EN 1997-1:2004 (Eurocode 7) - see 9.2(1)P, 5th bullet point. The clause is a principle for which there is no alternative and must be followed. A designer working in the UK, and using currently applicable codes (EC7 is the the only standard applicable now), will have to address this situation appropriately. Personally, this is one of the good things about EC7 - it provides comprehensive check lists telling designers what they shall and should consider - not necessarily analyse - and this seems to have been lost in the discussion that's focussed more on the issues of design cases, partial factors and analysis.
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This is an example where different customs, practices, regulations and legal principles may apply in different regions. In general, however any redevelopment next to an existing building needs to take into consideration the implications on the adjoining structure. A potential major implication always relates to foundations where overloading existing foundations or causing settlement during construction or in the longer term are potential issues.
These may be related to serviceability as well as to potential collapse but in every type of scheme it is incumbent upon the structural engineer to consider the wider consequences of the design. Proving that adjacent excavations are or are not the reason for distress to adjacent properties is easier if there is site monitoring of movements during and after the works.
In the UK, regulations in the form of the CDM (Construction Design and Management) Regulations and the Party Wall Act are there to ensure that consequential risk is mitigated to a satisfactory level. In particular The Party Wall Act 1996 provides a framework for preventing and resolving disputes in relation to party walls, boundary walls and excavations near neighbouring buildings.
Particular problems associated with protection work in Australia are considered to be as follows: