Route to Successful
Underpinning
Contents:
It is the Association of Specialist
Underpinning Contractor's (ASUC) objective to introduce quality standards, standards in
health and safety and codes of conduct for the industry. This article is intended as a
practical guide for supervising officers and for contractors alike.
The effects of a dramatic rise in the
underpinning market in 1989/90 were particularly emphasised by the recession and seized
upon as a lifeboat by professionals and contractors alike. Some tried hard to get it
right, others realised they were in the market for a quick killing and understandably
standards dropped to an all time low. A similar lowering of standards took place in the
down-turn of 1983, where again, professionals and contractors normally engaged in other
activities turned to underpinning as a source of revenue.
This inflow of non-experts resulted in
many projects being either highly over-designed and therefore, significantly more
expensive or under-designed leading to failures. There are even examples of schemes which
have been completely over-designed without acknowledging the cause of the original problem
producing the combination of a very expensive job and subsequent failure.
Following this, we have seen the long
awaited publication on the subject "Has your House got Cracks" from the
Institution of Civil Engineers and the "Subsidence of Low Rise Buildings" from
the Institution of Structural Engineers. Both publications are extremely welcome. However,
neither recognises that underpinning is possibly the most difficult construction technique
to manage and monitor the quality of workmanship. Nor do they recognise that the
underpinning techniques which have been developed and practised by specialist underpinning
contractors and consulting engineers are highly advanced and as such, the most cost
effective scheme would undoubtedly be designed and constructed by an underpinning
specialist.
The progression of activities involved
in carrying out an underpinning project may be listed as follows:
This has been well covered in the
publications described above, however, the industry is still suffering from the activities
of some contractors and even professionals who are not "expert" in the field. No
doubt, this trend will be gradually reversed.
A proper site investigation should
allow the expert to predict the potential for further subsidence. Long term monitoring,
the continual repairing of cracks and redecoration work is sometimes a needless expense.
The expert should be in a position to
instruct on the necessity for underpinning and repair/decoration work or repair and
redecoration work only. Whilst it is acknowledged that monitoring is appropriate in some
circumstances, there is a tendency for the "expert" to absolve himself/herself
of responsibility by monitoring the property for a period of years. It is ASUC's view that
monitoring is generally required only when the site investigation indicates that movement
is unlikely to occur.
Another common problem with the
investigation phase of the work is the method of borehole construction and the limit in
number of boreholes and their depth. For example, the ground conditions themselves would
determine the depth of a hand augured hole, i.e. unstable ground will collapse and small
obstructions will prevent progress. The majority of underpinning schemes which run over
budget do so unnecessarily and often due to the lack of site investigation information.
This point is made abundantly clear in the Institution of Structural Engineers
publication. The interpretation of information is very much the job of the
"expert", however, it is often the case that there is not sufficient information
to interpret rather than the information misinterpreted.
Here skills of the specialist
underpinning engineers and contractors should be sought. Such specialists are designing
various types of underpinning schemes on a daily basis. They are aware of the limitations
of available equipment and will inherently produce the most cost effective scheme.
Notwithstanding the above, there are
still too many contracts awarded purely on price without taking into consideration the
potential for additional costs, other associated problems such as noise, vibrations, and -
most importantly - the likelihood that the scheme will work. The attitude that the
contractor is "guaranteeing it anyway", is all too often used by the
"expert" as a means of absolving his/her own responsibility.
Together with other forms of
groundwork, underpinning works are hidden from view almost immediately after their
completion, and unlike all other forms of construction are not available for inspection
for any significant period of time. Underpinning, however, unlike other forms of
groundwork tends to be carried out in much small sections and those sections tend to be
backfilled and covered over immediately. There is simply not sufficient money available
for the supervising officer to remain on site permanently, or indeed inspect on a daily
basis. The expert must therefore rely on his contractor to carry out his own supervision
work and to employ workmen, equipment and systems which inherently generate good
workmanship.
A BS5750 accreditation is, in part, a
move in the right direction, however, it does not necessarily guarantee a workable scheme.
ASUC has set out some areas which the
supervising officer should be particularly aware of. Many problems stem from failing to
realise the original cause of the subsidence problems. This often occurs more with limited
traditional schemes as they are looked upon as "low technology, cannot go wrong"
and often more complete high technology schemes are given more thought.
The three most important planks of a
successful underpinning project are firstly to establish the cause of subsidence,
secondly, to confirm that cause while carrying out work on site and thirdly, to confirm
that the work being carried out is addressing the cause of the problem. These three stages
are rarely consciously acknowledged.
On site, the more dramatic failures are
normally as a result of taking out too much support. Support removal and substitution
should be clearly identified in the contractors' proposals, and reliance should not be
placed on the initiative of the site foreman.
When involved in underpinning work the
following should be considered in addition to normal constructional factors:
Failure to identify
the cause of subsidence and placing bases to a specified (arbitrary) depth.
Base sizes inadequate
therefore cause over-stressing and settlement.
Base left exposed
before concreting (only a matter of hours can cause severe softening in highly sensitive
clays) can cause settlement problems.
Underside of
foundations not cleaned, leaving a soft layer between underpinning and underside of
existing foundation.
Dry packing is very
critical, rarely supervised and often the cause of subsequent failure.
Failure to identify
potential heave problems and therefore the provision of anti-heave precautions. Also
inadequate fixing of heave precautions.
Pinning up to loose,
damaged, or decayed footings.
The above, but include for associated
quality problems with concrete. Site concrete materials have been known to come into
contact with waste materials. Proper storage of materials and careful mixing is essential.
- Cover to reinforcement (steel is not
easy to fix or inspect).
- Arbitrary moving of bases to overcome
local problems leading to excessive spans and the over-stressing of bases and beams.
- Collapse of void formers below beam.
- Failure to isolate support stools in
anti-heave scheme
- Use of "standard" beam designs
can lead to inadequate design.
- Proper consideration of realistic
applied loads is essential to ensure a properly designed scheme both in terms of providing
adequate load-bearing capacity and preventing over-design.
Normally piling works on main
contracting sites are subject to routine inspections by a resident engineer, clerk of
works, building inspector and other supervising officers. On underpinning sites, with the
exception of the building inspector, there is no enforced supervision regime except for
that which the contractor imposes on himself/herself. Mini piling rigs can now be hired
easily and literally anybody can call themselves a piling specialist. It does not need
much imagination to realise the potential of problems that can be caused as a result of
this ignorance particularly where the supervising "expert" is not a piling
specialist.
Inadequate design or
more usually inadequate design revisions on site.
Inability to identify
cause of subsidence and therefore inadequate design.
Failure to confirm
ground conditions, cause of subsidence and ability of ground to support pile scheme
proposed.
Pile integrity
(particularly small diameter piles), will not be noticed by inexperienced crew or
supervising officer.
Weakened concrete
with water inflow (rarely noticed by inexperienced contractors).
Shallow pile depth a
common problem if crew are financially incentified and supervision is poor.
Inadequate provision
of reinforcement or slip membranes in heaving ground.
Reliance on the fact
that "a set" is "a set" is "a set" is "a set",
regardless of ground conditions. Ground conditions must be taken into consideration when
designing a driven pile - often they are ignored!
Driven piles being
used in heaving ground. Unless proper precautions are taken the will fail.
Steel casings,
finishing within concrete footing, thereby leaving unbonded section.
Failure to carry out
check sets particularly where pore water pressure is a significant factor. When using
pneumatic hammers a small drop-off in air pressure can lead to dramatic reductions
capacity. Air pressures and flows are rarely checked.
The belief, by some
contractors, that the old Cornfield formula is conservative and therefore the use of a
"lighter drop weight will do", the supervising officer is given the impression
that the weight is heavier than it actually is. Drop weight should be checked and the mass
calculated.
Small diameter mini
piles may very easily set on an obstruction or within consolidating made-up ground. This
fact is rarely checked.
Damage to the
building itself or adjacent buildings through vibration related problems.
Over use of standard
details and therefore overlooking more highly loaded areas.
Collapsing of any
void forming materials.
Failure to properly
isolate support jacks.
Failure to identify
locations of support jacks and to subsequently remove them.
The usual
steel/concrete quality problems associated in all RC work.
With a nibbed scheme
the size of the nibs can often create over-stressing in the concrete
Punching shear is
very often overlooked and can become critical.
All associated piling
problems.
Taking away too much
support - leading to severe structural damage and collapse.
Failure to confirm
the assumed cause of subsidence and to redesign if necessary.
Failure to employ
responsible operatives - regardless of supervision, fraud can occur on any site.
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Subsidence
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