| Building
Stabilization
By
Dr. Heng Li [bshengli@polyu.edu.hk] Tel: 2766 5879
1.
References
The
College of Estate Management, 1995, Technology - Module
4, London: The College of Estate Management
2. Learning
Objectives
"Awareness
of the methods for providing temporary and permanent support
for building works."
Temporary
Support System
Alternations, adaptations and part demolition of structures
are increasingly being undertaken as part of the upgrading process
in refurbishing, conserving and adapting existing structures
for modern usage. Often, the requirements to refurbish and renovate,
rather than demolish and reconstruct, will give rise to the
need for extensive temporary support and shoring arrangements
to support parts of the structure.
3.1
Preparatory Works
Before
any shoring or temporary support works are undertaken, the structure
of the existing buildings and those adjacent to it must be assessed.
Formation
of new openings together with the temporary shoring are likely
to create changes in loading patterns and stress distributions
in the building components. Consequently, it is important to
ensure the adequacy of the existing structure to cope with these
changes. Failure to do so may result in drastic and possibly
fatal consequences.
Similarly,
the adjacent buildings may be affected by the works and alterations
to be carried out, and a record of their conditions and assessment
of any possible consequences of the work should be carried out
before commencement.
Less than
scrupulous adjacent building owners may also attempt to attribute
existing or non-related problems to the work carried out. To
guard against this, it is a normal practice to carry out a written
and photographic condition record survey of adjacent buildings,
which can be agreed and signed by adjacent building owners prior
to the work. A similar inspection may be carried out on completion
and any damage resulting to adjacent structures may be then
be remedial.
3.2
Access and Occupancy
Assessments of the preferred method by which alternations may
be carried out and temporary support provided will also need
to give due regard to access and occupancy considerations. Temporary
screening may be required to protect occupants or contents and
may be incorporated or constructed separately from the shoring.
Temporary
roadways or walkways may also need to be incorporated to maintain
public and private access rights.
3.3
Practicality and Safety Considerations
Safety of both the building structure and persons in or around
the building is of utmost importance in all projects, including
temporary support.
Regular
inspections of the works, safety and personal protective clothing
and equipment for those within the building, and an understanding
of the structural concept of the support arrangements by the
contractor and their workforce should be provided and agreed
prior to commencement. Any necessary opening up of the structure
to ascertain its condition and exact method of assembly should
also be carried out at an early stage.
Straightforward,
simple support arrangements are normally the easiest to construct,
understand and adapt, and should be selected whenever possible.
The most practical solution is also often the safest and most
economic method.
3.4
Shoring Types and Methods
3.4.1
Deed or Needle Shoring
Dead or needle shoring, often more simply referred to as propping,
is used for supporting existing walls, floors and roofing whilst
works are carried out to form openings or remove walls at lower
level.
Steel or
timber uprights are provided to support loads from a structure,
normally in association with wedges or head and sole plates
to distribute stresses over larger areas. Needles of horizontal
timber or steel construction may be inserted through walls supported
on dead shores either sides to create working space for the
opening to be formed. The needles support the wall by corbelling
action of the masonry, and consequently its condition should
be checked beforehand.
The number
and construction of dead shores and needles will depend upon
loading and location characteristics. Adjustable steel props,
specialist scaffold sections and paired wedges are all available
commercially to simplify the installation
3.4.2
Raking Shores
Raking shores are generally used to provide lateral support
to walls that are unstable or threatened by structural or excavation
works. They are also effectively used to retain facades in place
whilst internal rebuilding takes place and to brace temporary
screens against windloadings. Consequently, whilst classed as
a temporary support, they may often remain in place for long
periods supporting unstable end walls of derelict vacated buildings
and may incorporate advertising hoardings as a disguise.
Wall plates
are used to transmit the lateral thrusts to the rakers. Ideally
these should be positioned against floors or internal wall lines
for best stress distribution. Where this cannot be achieved,
internal props or rakers may be introduced or larger plates
used to distribute the stresses.
At the base
of the rakers, sole plates are used to transmit the loads to
the ground. Often these may have housed joints, be inclined
and affixed in position by their own foundation pad.
The angle
of the raker may be determined by site constraints, but obviously
a too steep or too shallow raker may have little effect. Ideally
angles between 45 and 60 degrees should be achieved, and groups
of tow and three rakers are often effectively.
3.4.3
Flying Shores
In some situations, the close proximity of existing buildings,
or the need to maintain access, may prevent the use of raking
shores. In these situations, a flying shore may be effective.
This relies on transmitting loads to the adjacent structures,
for which obviously the owner's permission will be necessary.
The existing building to be used for support should also be
carefully inspected for its adequacy to carry additional loads.
The horizontal flyer is supported by rakers that distribute
stresses via straining cills or plates positioned against the
walls. Folding or paired wedges are then used to tighten the
framework. Flying shores are traditionally constructed in timber,
but today are commonly formed in scaffold derivatives.
3.4.4
Support Scaffolds
Increasingly, scaffold and specialist proprietary steel systems
are replacing timber for shoring techniques. Such arrangements
are also useful in providing weather screening and access to
the walls for work to be carried out.
Scaffolds
which provide lateral support to walls whilst screening the
work from the public, providing a footway beneath and access
for work to be carried out above, are increasingly used for
major refurbishment projects. Many manufacturers offer more
sophisticated scaffold derivatives for shoring, but these are
less commonly seen due to the cost implications.
Underpinning
Traditionally,
foundations for buildings were designed to provide a level
base on which to build, and to transfer the loads from the
building to the ground. Little consideration was given to
changes in the moisture levels in he ground, the effect of
trees and shrubs, or other natural causes that might adversely
affect the foundations and result in subsidence or heave as
the moisture content of the ground changes. These movements
can have considerable effect on the building structure, resulting
in cracking in walls, variations in floor levels and possibly
eventual collapse as the building components become distorted
and displaces.
Additionally,
as increases in population give rise to the need for more homes,
less suitable land has been developed, often without full consideration
of the design of the foundations. This has frequently resulted
in movement in foundations and buildings.
Natural
phenomena, such as landslide and very dry summers, also result
in foundation movements that may give rise to the need to
carry out adaptations to foundations by means of underpinning.
Underpinning
basically involves construction of additional structural foundations
below existing foundation arrangements, in order to transfer
building loads to deeper, more stable sub-strata and prevent
further movement to the building structure and components.
4.1
Identifying Progress Movement
It
is important to establish whether foundation movement is progressive
and threatening to the building, or a one-off occurrence which
can be dealt with by cosmetic repairs. Only where movement is
progressive and likely to result in serious damage to the building
should underpinning be undertaken.
It is therefore
important to establish whether movement is progressive. This
may be established in a number of ways:-
The
traditional method of fixing glass tell-tales across cracks is
now mainly discredited, as the glass may be damaged by vandalism
or misuse, and breakage does not give any indication of the amount
of movement.
4.2
Selection of Appropriate solution
Before
any underpinning, it is important to ascertain fully the cause
and extent of the problem, and select a solution designed to deal
adequately with the particular situation. Failure to do so may
result in repair works accelerating, rather than stemming, movement.
4.2.1
Crack and Level Surveys
As
part of any repair philosophy, the extent and direction of movement
must be fully ascertained. Movement is normally most pronounced
over seasonal changes, and consequently a full year of monitoring
is often prescribed. Cracks may be measured using either simple
or sophisticated devices, whilst levels surveys may be carried
out simply by use of spirit level and rule, or by use of laser
or optical levels.
Once
the extent and direction of movement has been determined, further
information and investigation will be required to ascertain the
best solution.
4.2.2
Access and Occupancy
The
availability of access and occupancy will need to be taken into
account before selecting the appropriate repair. More extensive
works, where internal walls and floors have been affected, may
require decanting of occupants whilst structural works are undertaken.
Repairs to properties that are part of terraces or attached to
adjoining buildings may also need to take account of access restrictions.
The close proximity of roads, adjoining buildings, boundaries,
trees or steep slopes will also have a bearing on the final solution
adopted.
4.2.3
Ground Conditions
There
is little point in embarking on underpinning works unless some
known strata on which adequate new foundations can be supported
exist. In order to ascertain the cause of failure and the level
at which new foundations can be formed, a detailed site investigation
and soil sampling will be required. This may involve excavation
of trail pits or boreholes, together with visual and laboratory
analysis of the soil to ascertain its behaviour and strength characteristics.
4.2.4
Structure Types
The
types of structure and its condition will also be a factor in
determination of the repair technique Masonry structures in good
condition are able to arch or corbel across unsupported sections
whilst work is carried out. In older buildings where the masonry
is in poor condition or where cracking has reduced the continuity
in masonry, it may be more difficult to achieve this. Consequently,
it may be necessary to introduce temporary support or carry out
mechanical or other repairs to the building prior to underpinning.
Framed
buildings where laods are concentrated will also need special
consideration, often involving extensive shoring and support to
enable column base to be underpinned.
4.2.5
Practicality and Safety
As
with all building operations, the practicality of carrying out
the works and the safety of the operatives, occupants and the
structure itself should be considered paramount. It is important
to remember that underpinning is being carried out because the
building is currently unsafe and the ground inadequate to support
it. These factors may often dictate the final choice of repair
technique selected.
4.3
Underpinning Methods
4.3.1
Traditional Underpinning
The
traditional technique of underpinning involves excavation in sections
beneath the existing foundations and casting a new concrete foundation.
Continuity bars of steel reinforcement are driven into each end
of the excavation to connect intervening bases into a continuous
strip on completion.
The
individual bases are cast in a staggered sequence of alternative
bays to ensure the building is fully supported at all times. Special
care and often smaller bases should be allowed for at corners
or projections such as bay windows or porches.
New
bases are normally cast to within 50 - 75mm below the underside
of the existing foundation. Following curing of the concrete,
a dry pack mix of strong cement and sand is driven into this gap
to fully support the foundations. This part of the operation may
also be carried out by pressure grouting equipment. The purpose
of this is to allow for any variations in the underside of the
existing foundations and for any shrinkage in the new concrete.
Once this work has been done, excavations may commence on the
adjacent base.
4.3.2
Scissors and Cantilever Piling
Where
adequate loadbearing strata are at lower levels making normal
excavation impractical and economically unlivable, piling solutions
may be adopted. These are also effective where high ground water
tables or loose soils prevent normal excavation methods.
Depending
on access limitations, either scissors or cantilever piling techniques
may be adopted. Such techniques may also include casting of a
reinforced concrete ring beam in the base of the wall to transfer
the load to the new piled foundations.
Piles
may be driven or bored into position. Mini-piles consisting of
100mm, 150mm or 225mm diameter metal tubes driven into the ground
into which a single reinforcing rod is placed before filling with
concrete are particularly popular. Their popularity is due to
the limited vibration that occurs when driving a pile of small
diameter, the small size of the rig that can pass through standard
doorway and operated within room height, and the general practicality
and economics of the systems. A diamond-tipped auger may be used
to pre-drill through existing floors, walls and foundations to
enable mini-piling to follow through.
Piling
techniques may also be sleeved to minimize the effect of any further
ground movements at the upper strata levels on the building.
4.3.3
Patented Systems and Grouting
A
number of systems are commercially available featuring more sophisticated
techniques of foundation stabilization. In their simpler forms,
stools or proprietary wedges are used to replace the "pinning
up", to reduce labour intensity, and to achieve better quality
results.
More
sophisticated systems involve grout injections to stabilize soils
or to fill voids beneath existing floor slabs or foundations.
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