Philosophy of Underpinning
Clay Victorian heating pipes under the Horniman Museum dried out
the clay subsoil, causing shrinkage to a depth of 5.5m. Construction
of a new basement eliminated the problem and gave the museum additional floorspace
According to the London Weather
Centre, the summer of 1996 had 23 per cent below average rainfall, 21
per cent above average sunshine and 10 per cent above average temperatures.
As many buildings in the UK are built on clays that shrink when their
moisture content reduces, these warm, dry weather conditions have led
to an increase in building movement; more cracked buildings are being
reported and insurance claims for subsidence are on the increase.
the usual remedy, underpinning, is far from being a universal panacea.
Not only is it expensive, but it is now widely recognised that underpinning
can also be traumatic for both the buildings and their occupants.
Where cracks are caused by subsidence (not all are), underpinning
should be specified only with great caution.
article looks at subsidence in low-rise and medium-rise buildings, which
usually have shallow foundations or cellars up to about 3m deep. Subsidence
and settlement are taken as interchangeable terms, meaning the sinking
of ground on which a structure is founded. Ground can also rise, in
which case the movement is known as 'heave'. Depending on the cause
of subsidence or heave, horizontal stretching or squeezing of the
ground can accompany the vertical movement.
movement is, or is likely to become excessive, so that the use or
safety of the building is compromised, then underpinning may be a
solution. It is usually achieved by digging underneath shallow footings
and pouring concrete to extend the reach of the foundations down to
a stable stratum. Other forms of underpinning include piles (column-like
foundations). In some circumstances, pressure grouting (the use of
a fluid cement to consolidate the ground itself) provides an alternative
While some early medieval timber buildings had their main posts dug
into the ground, almost all surviving buildings have timber sill beams
resting on low masonry plinths. Masonry buildings of the period had
walls which sat straight into the ground with no attempt being made
to spread the load over a broad foundation. These buildings largely
succeeded because their builders followed tried and tested construction
techniques and because they could be more selective in choosing suitable
ground on which to build.
later buildings, masonry walls were sometimes supported on one or
two wider courses in a series of steps (a form of 'corbelling') to
provide a better distribution of the load on the soil. Where the ground
was reliable, this practice continued until the First World War, sometimes
on a shallow strip of concrete cast into the trench about 500mm below
ground. In poor ground, short timber piles were driven before starting
the masonry, or cellars were constructed to reach stable ground below
the advent of modern mild steel and reinforced concrete at the turn
of the century, the design of foundations became more sophisticated.
These and the forms seen today fall into three broad categories (and
spread footings: where surface soils have sufficient load
bearing capacity and stability, concrete pads, strips, or rafts,
are used to spread the forces from the superstructure over the
ground, just deep enough to avoid unstable topsoil, frost penetration
and seasonal moisture variations
where the surface soils are so weak that even a raft foundation
under the entire footprint of the building is insufficient, piles
are used to reach deeper and stronger strata
foundations: as an alternative to piles, a basement box of
reinforced concrete can be constructed which, in effect, floats
in the ground. The weight of the ground removed to form the basement
compensates for the weight of the new building. To a certain extent,
old brick cellars, vaults and crypts behave similarly.
MOVEMENT DURING CONSTRUCTION
During construction, buildings settle as the ground adjusts to the
new weight imposed upon it. Where built on rocks, gravel or sands,
constructional settlement is substantially complete by the end of
construction. For clays, silts and peat however, settlement takes
many years. Once constructional settlement is complete it will not
recur, unless the status quo alters. Constructional settlement is
not usually detrimental provided the structure settles uniformly or
is robust enough to accommodate differential settlement.
ground can produce excessive differential settlement. For example,
when part of a terrace of houses straddles an old river bed, that
part is likely to settle by a different amount from the rest of the
settlement may also occur when existing structures are substantially
extended or underpinned, as the stress in the ground is increased
at a greater depth than before.
there is a risk of differential settlement occurring between a building
which has been disturbed and neighbouring parts which have not, such
as adjoining buildings which may have finished their constructional
settlement years ago. The settlement can be difficult to control due
to the constraints of the existing fabric. If structural damage does
occur, then it should be monitored and repaired at the end of the
settling-in period. Provision should always be made within the project
costs to pay for the monitoring and repair of any distress which may
settlement does not always stop. Old buildings with overloaded footings
on soft clay, including some Georgian and Victorian houses, have never
quite achieved equilibrium and are still sinking slightly today, due
to the nature of clay.
MOVEMENT AFTER CONSTRUCTION
which substantially disturbs the balance between the ground and the
structure can promote new settlement. Tunnelling, mining and deep
excavations, or altering loads - by building on old foundations, for
example - can promote new movement in all types of ground. Ground-specific
causes include frostheave, ground vibrations, changing water-tables,
leaking drains, droughts and trees (see table 'Causes of Ground Movement',
RESPONSE TO GROUND MOVEMENT
building's response to ground movement depends upon the continuity,
ductility, and stiffness of its structure.
structural continuity (or 'tensile connection') can be provided by
timber, steel and reinforced concrete frames which enable buildings
to flex without coming apart at the seams. However, the lack of structural
continuity or 'togetherness' of most pre-1970 unframed masonry structures
permits joints to open and cracks and instability to occur more readily.
After 1970, the Building Regulations and British Standards were amended
to provide continuity in the wake of the progressive collapse of Ronan
Point in 1968.
structural materials such as steel and properly detailed reinforced
concrete can accommodate large deformations without breaking. In contrast,
a brittle material such as unreinforced masonry set in cement mortar
can only deform within its elastic limit. Historic unframed masonry
structures can accommodate large distortions without cracking due
to the 'creep' of the lime mortar, if movement is not too fast. (Creep
is the continuing deformation or 'strain' of a material under constant
stress). Modern cement mortars do not creep.
a structure is sufficiently stiff it may be able to ride out the ground
movement, moving or tilting as a whole, and heavily braced frames
and compact crosswall structures with only small openings may have
sufficient rigidity to disperse localised ground movement.
ground movement is anticipated, say from tunnelling, then structural
damage may be mitigated by installing temporary tie-bars and bracing
door and window openings.
SURVEY AND ASSESSMENT
Not all distortions and cracks in buildings are necessarily due to
ground movement. Symptoms of distress can also be caused by inadequate
strength of materials, inadequate structural togetherness, material
decay, dimensional instability (caused by thermal and moisture movement),
overall instability, alterations, misuse and accidental loads (Editor's note: see this author's article, 'Structural Movement: Is it Really a Problem?' in
The Building Conservation Directory 1996).
are no foolproof rules for distinguishing between the causes of movement
in buildings, and correct assessment can only be made with experience
and by following good surveying practice. It is essential to be thorough:
examine every part of the structure and every possible cause of failure;
consult geological maps; record all individual symptoms; and keep
an open mind. The most probable causes may be determined by a process
of elimination. If symptoms are consistent with ground movement as
well as other causes, further investigations must be made to distinguish
between them, including trial-pits, boreholes, drain testing, and
PHILOSOPHY OF REPAIRS
is a messy, noisy and traumatic operation for buildings and their
occupants alike. Unless sophisticated and expensive jacking systems
are incorporated, the underpinning will almost inevitably promote
some additional subsidence as the works settle in. If a structure
is partially underpinned, for example one house in a terrace, then
future damage may recur as the rest of the non-underpinned structure
continues settling. For these reasons, underpinning should be avoided
if at all possible.
is not necessary from a purely engineering viewpoint in the following
the cause of the ground movement has ceased and is unlikely to
recur, repairing the damage should be sufficient
the rate and total magnitude of anticipated ground movement is
unlikely to significantly threaten the structural strength, stability
or integrity of a building during its required lifespan, periodic
repairs and redecoration should suffice. Doors and windows may
have to be eased from time to time or changed for other types
which are more tolerant of frame distortion.
ground movement is expected to do structural damage, it may still
be possible to reduce movement sufficiently to avoid underpinning,
for example by:
and root-pruning trees
In certain cases, such as when a building is to be sold, an owner
may be compelled to underpin in order to attract a purchaser even
though it may be unnecessary in engineering terms.
1 Typical causes and effects
- Yielding retaining walls
- Tunnelling and mining
- Settlement of soft ground in swallow-holes and fissures
heave (caused by the expansion of moisture as it freezes)
in aquifer pumping
types of underpinning involve digging holes under buildings in confined
spaces. The existing structure is expected to defy gravity and temporarily
arch over the excavation. Collapses can occur. The risks must be identified
and managed in accordance with CDM legislation.
services before digging
that underpinning pits cannot flood or be gassed
superstructure before digging
that walls above are strong enough to support themselves over
sides of excavations
that workers can escape from pits easily
threaded couplers instead of dowel bars to connect reinforcement
rods between sections of shallow mass concrete underpinning
safe access and ventilation to pits
a Banksman to oversee safety.
- C Richardson, 'The AJ Guide to Structural Surveys', The Architect's
to Subsidence of Low Rise Buildings, The Institution of Structural
in Clay Soils', BRE Digest 412, Feb 1996
article is reproduced from The Building Conservation Directory, 1997
CLIVE RICHARDSON BSc, CEng, FICE, FIStructE, ACIArb is a
Chartered Engineer and Associate Director of JAMES Consulting Engineers.
He is also Engineer to the Dean and Chapter of Westminster Abbey,
and Visiting Lecturer in building conservation at The Architectural
Association School of Architecture, London.
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