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Subsidence investigation, remedial design and control
Subsidence - ground and foundation movement with associated
damage
Subsidence has in recent years become very much associated
with buildings insurance claims. In particular since 1971
when cover for subsidence within household policies was generally
introduced.
Buildings insurance policies cover a number of insured events,
which together with subsidence should be defined within the
terms of the insurance policy. Those associated with subsidence
normally include heave, landslip and water damage (typically
leakage from below ground drainage or water supply services).
These events may be all encompassed as events or causes of
ground and foundation movement with associated damage affecting
the building or structure.
In London and the south east of England subsidence has been
generally associated with problems caused by clay soil movement
and the affect of trees, because of the prevalence of outcropping
clay.
However, it should be appreciated this is by no means the
only or principal cause, nor the only possible cause in these
areas.
Problems of major foundation movement and damage can be due
to other causes.
For example these may include subsidence in mining areas,
collapse and movement of solution features in chalk, adverse
groundwater affects upon loose on granular / sandy soils,
settlement or failure of made ground, landslip or retaining
wall failure.
For further guidance in respect of household and or low -
rise buildings you may refer to The Institution of Structural
Engineers guide - Subsidence of low-rise buildings.
We regularly advise upon all aspects of subsidence, including
initial appraisal, investigation, recommendations, design
and control of remedial measures. Not only where insurance
claims have been made, please refer below in this respect
for further information.
Initial appraisal - advise and diagnose
- initial visual inspection
Where requested to advise and diagnose problems of suspected
subsidence, in the first instance we would usually carry out
an initial visual inspection to assess the extent and seriousness
of the movement and damage, general conditions prevailing,
probable causes, further investigation and immediate remedial
or temporary works required.
Extent of movement and damage
Not all movement and damage is abnormal or of such seriousness
that it requires remedial measures to be undertaken.
The majority, if not all buildings and structures are subject
to movement and some associate damage, not necessarily caused
by ground or foundation movement, for example thermal or shrinkage
movement.
The key issues to determine are whether the movement and
associated damage is unacceptable and or is likely to continue
or deteriorate without remedial measures.
Having assessed the seriousness of the movement and damage
and provided any immediate remedial works or temporary works
to maintain integrity and safety as far as practicable, proper
investigation as necessary should be undertaken to fully assess
the problem.
Initial appraisal report
Following initial inspection and appraisal we would normally
provide a brief but well detailed initial appraisal report.
This would include:
· Brief and reasons for the report
· General description of the building or structure
· The extent of movement and associated damage
· The related structure and implications generally
· Initial advice and opinion upon cause/s of movement
and associated damage
· Further investigations including monitoring of any
movement necessary
· Remedial measures and or need for temporary support
· Insurance and appropriateness of making a claim
· Summary, way forward and recommended course of action
Investigation
Subsidence investigation will often involve accurate monitoring
of the building or structure movement.
This will typically take the form of precise datum monitoring
of principal fractures and where appropriate precision level
survey monitoring of datums fixed to the building or structure
close to the foundations.
PC controlled level and tilt monitoring of structures can
be carried out for more sensitive or critical structures e.g.
historic or structurally suspect structures.
Whilst monitoring of fractures should normally identify the
extent of continued movement and damage it will not necessarily
indicate the extent and nature of the foundation movement,
which is important to ascertain as far as possible.
Precision level survey monitoring can provide useful information
in this respect. However, in many instances it can be hampered
by lack of a stable reference datum and may only provide an
indication of relative rather than total movement.
Proper investigation of the ground and foundations conditions
should be undertaken.
This should include detailed investigation with trial pits
to expose foundations and adjacent ground and boreholes to
ascertain the ground and soil parameters and ground water
conditions as far as practicable.
The location and condition of below ground drainage and services
should be ascertained. There are a number of non-invasive
techniques available for this other than excavation works.
The influence of environmental factors including the influence
of trees, often important on clay soils, should be determined.
Please refer to section Ground and Foundation investigation
for further information.
Investigation would normally include such exploratory and
opening up works of the existing building construction or
structure, necessary to have a proper understanding of the
structure and its sensitivity to foundation movement and vulnerability
to associated damage.
Report following investigations
Following completion of investigations we would normally
provide an appropriately well detailed report. This would
include:
· Synopsis
· Brief and instructions received
· General description of the building or structure
· The extent of movement and associated damage
· The related structure and implications generally
· Investigations undertaken
· Detailed advice and opinion upon cause/s of movement
and associated damage
· Likelihood of further movement and damage
· Any further investigation necessary
· Remedial measures required with options as appropriate
· Insurance
· Approvals required including party wall agreement
where remedial works might affect adjoining properties
· Summary, way forward and recommended course of action
Remedial Measures
Having conducted sufficient investigation to enable appropriate
decisions to made we would then make recommendations and provide
advice in respect of appropriate remedial measures, normally
in the form of our Report following investigations.
This would include the need for foundation stabilisation
works. This may simply be measures, not directly affecting
the foundations, to avoid or limit ground movement to tolerable
limits and prevent unacceptable ground movement. For example
these may include tree management in clay soil areas and avoidance
of undue groundwater by preventing water leakage from defective
below ground drainage or water supply services.
Underpinning - foundation stabilisation
Where direct measures are considered necessary to stabilise
the building and structure foundations, these may include
underpinning of the foundations and ground bearing floors
etc. or ground improvement perhaps by injection grouting techniques.
Underpinning may take various forms from more traditional
mass concrete underpinning, concrete pad and reinforced concrete
beams or slabs to piled underpinning or other more specialist
techniques. Each case should be fully considered and the form
of underpinning decided on its merits.
The extent of and any effect underpinning might have upon
adjacent buildings or structures should be carefully assessed
and understood.
Where ground movement affecting the building or structure
is caused by slope instability, creep, landslip or retaining
wall failure, careful consideration will need to be given
to appropriate remedial and or strengthening works, where
practicable.
Numerous recent modern advanced techniques are available
for retaining wall or ground slopes strengthening. Which subject
to the prevailing conditions may be carried out rather than
major ground works reconstruction.
Remedial works - design and control
Having ascertained and agreed the scope or remedial works
required we can design the appropriate remedial scheme and
prepare all necessary drawings, specifications, tender and
contract documents.
Dependent upon the circumstances remedial schemes may only
be repair of damage to the building, superstructure and finishes.
However, they might extend to foundation stabilisation, underpinning
or major structural repairs perhaps including specialist groundworks,
retaining wall ground anchorages etc.
We may then control the remedial scheme through selection
of contractors, tendering, cost analysis and the building
contract, providing necessary site inspections, contract administration
and control of contractors payments until successful conclusion
of the works and the building contract.
On large projects we can provide on site control with site
inspections carried out by our resident engineer.
On most small scale or domestic remedial projects, for economic
reasons it is unlikely a resident engineer will be used and
site control will usually be limited to appropriate site inspections
at key stages and or when valuation inspections are undertaken.
For insurance claim projects, unless these are of large size
or special circumstances warrant it is unlikely Insurers will
agree to fund the cost of resident engineer control.
For further information please refer sections including Building
and structure design, Foundation design and Geotechnical
Engineering
Insurance claims
We have considerable experience and regularly advise Insurers,
Insurers appointed loss adjusters and the Insured regarding
technical and engineering aspects of building or structure
damage and or subsidence claims.
We can advise upon all aspects of insurance claims within
our expertise as consulting structural engineers.
In all cases it is important and we value highly close liaison
and communication with the respective parties, seeking agreement
to the way forward, investigations and remedial measures at
all key stages.
Provided the cause of damage comes within the scope of an
insured event, professional fees for necessary investigation
and remedial works are normally included within the claim
by Insurers, subject to any policy excess applicable and the
particular terms of each policy.
The Insured should be aware that professional fees are not
covered by Insurers, for pursuing the claim beyond that required
for necessary investigation and remedial works and appropriate
associated liaison with the parties involved.
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Building
and structure design