|
|
||||||
|
|
|
|
|
|
|
|
SUBSIDENCE.
|
What is a structure?
What is structural movement?
Subsidence is defined as the downward movement of the bearing soil on which a building rests. There are many possible causes for a bearing soil to fail. It is possible for subsidence to occur progressively over a long period, to occur over a very short period and then stop, and other variations on this theme.
Tell Tale signs:
Cracks in brickwork, render or plaster.
In particular: Look at crack faces - how have they come apart? Are the partings fresh and clean? Is there old paint or filler in the cracks? How old are the decorations?
Movement during construction Variable ground can produce excessive differential settlement. For example, when part of a terrace of houses straddles an old riverbed, that part is likely to settle by a different amount from the rest of the terrace. Constructional 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. Similarly, 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 that may occur. Constructional 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
Response to ground movement Good 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 pre1970 unframed masonry structures permits joints to open and cracks and instability to occur more readily. In the United Kingdom, 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. Ductile structural materials such as steel and properly detailed reinforced concrete can accommodate large deformations without breaking. In contrast, a brittle material such as un-reinforced 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. If 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 cross wall structures with only small openings may have sufficient rigidity to disperse localised ground movement. If ground movement is anticipated, say from tunnelling, and then installing temporary tie-bars and bracing door and window openings may mitigate structural damage.
Survey and Assessment There 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. A process of elimination may determine the most probable causes. 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 movement monitoring.
Philosophy of Repairs Underpinning is not necessary from a purely engineering viewpoint in the following situations: Where the cause of the ground movement has ceased and is unlikely to recur, repairing the damage should be sufficient Where 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 that are more tolerant of frame distortion. When ground movement is expected to do structural damage, it may still be possible to reduce movement sufficiently to avoid underpinning, for example by: Pollarding and root-pruning trees, repairing leaking drains, modifying the superstructure or by pressure-grouting the ground.
Safety Issues Investigate services before digging Check that underpinning pits cannot flood or be gassed Strengthen superstructure before digging Check that walls above are strong enough to support themselves over pits Support sides of excavations Ensure that workers can escape from pits easily Use threaded couplers instead of dowel bars to connect reinforcement rods between sections of shallow mass concrete underpinning Ensure safe access and ventilation to pits Use a Banks man to oversee safety.
Causes of Structural Movement
Inadequate Strength From the start of the Industrial Revolution, the increasing involvement of the engineer, first with grand buildings and latterly more humble structures, ensured more adequate sizing of structural members. Exceptions include domestic buildings with timber floors overloaded by office use.
Material Decay Frost-damage to masonry, timber decay, rusting of iron and steel and sulphate-attack of cement and concrete The battle against water can largely be won by giving the building a good roof; by ensuring that driving rain is thrown clear of the building by generous drips, throatings, over-sailing copings and bonnets; and by preventing rising damp either through a damp-proof course (d.p.c.) or by ensuring that the ground is well drained.
Dimensional Instability In most structures in this country, the principal load-bearing element is the masonry. Different types of masonry move at different rates, and sometimes in opposing directions. This can give rise to differential movement and distortion. Fortunately most walls constructed before 1914 were set in lime mortar, which can accommodate considerable amounts of movement without cracking due to creep (continual strain under constant stress), whereas more modern walls require the frequent provision of movement-joints.
Subsoil and Foundation Inadequacies In good ground, corbelling continued until the First World War, latterly with a shallow strip of concrete first cast into the trench, about 500mm below ground. In poor ground, short timber piles were sometimes driven before commencing the masonry. With the advent of modern mild steel and reinforced concrete at the turn of the century, foundations became more sophisticated. Movement of shallow spread foundations is commonly caused by normal constructional settlement, mining, leaking drains, shrinkable clay, tree-roots, changes of water-table, additional loads and tunnelling. Flexible historic buildings are often better able to cope with movement than modern rigid structures, thanks to the prevalence of soft lime mortar, massive walls, timber-frames, arches, and vaulted construction. Modern structures with slender walls set in hard cement mortar with brittle plaster and no cornices, will show every crack.
Overall Instability
Alterations
and Misuse
Assessment and Conclusion Although intervention by engineers may be unnecessary for the odd symptom of distress, it is too easy to rely on the assumption that a building will last indefinitely simply because it has survived the last 200 years, while the building tiptoes to disaster. Structural movement is serious when the safety-margins of strength, stability, or integrity have been significantly eroded, or the movement is progressively leading to failure within a specified period. For a relatively modest structure such as a house, no action may be considered necessary unless the structure is likely to fail within a period of perhaps five years, but for a cathedral a much larger safety margin would be necessary, of perhaps fifty years due to its scale and the high cost involved in carrying out major works. Expectations for the duration of a repair may also vary. An engineering assessment of the seriousness of any particular symptom of structural distress is not just by calculation, but also through an understanding based on practical experience of the performance of old structures, and the intangible contribution of the non-structural fabric, such as the stiffening effect of horsehair in old plaster. The Building Research Establishment offers some guidance on the seriousness of crack-widths but this must be used circumspectly. Cracks should be examined to determine their cause, not rigidly filled in to see if they reappear, as this may restrict cyclical movement causing the problem to escalate. Careful examination can reveal the direction of movement, and whether movement is ongoing. If the probable cause of the structural movement is still unclear, or if the movement is suspected to be progressive, then movement monitoring is warranted. Monitors are aids to diagnosis and prognosis, not a substitute to understanding structures.
Cracks
can occur in brickwork and block work for many reasons, and do not
necessarily result from subsidence of bearing soils. Some surveyors
and engineers try to apply the Building Research Establishment's
guideline that cracks of 1mm or less are "insignificant", but this
is only part of the story - for example, such cracks may be in the
early stages of development, or represent a defect that only
produces minor cracking, but is significant in some other way. It
should be appreciated that cracks are symptoms.
Bulges
in external
brickwork can sometimes be "as built" (i.e. the wall was built out
of vertical from inception) but this is unusual. More usually,
brickwork bulges result from inadequate lateral restraint at
intermediate floor levels, effectively making the wall very "thin",
or cavity wall tie failure, where the cavity wall ties have rusted
through.
Roof spread
occurs when a roof
frame is not properly tied ("triangulated"), resulting in horizontal
movement, which pushes fascia boards out sideways, and often leaves
a visible gap between the soffitt board and brickwork. In extreme
cases, brickwork can be pushed out at the top and the wall develops
an outward curve. Roof spread happens mainly in "cut" roofs (made
from sawn timber) and is usually caused by poor design on the part
of the architectural designer, or site carpenter. It hardly ever
occurs in trussed roofs, which are designed and manufactured in a
factory and brought to site for final erection. Roof spread is not
covered by standard house insurance policies, therefore the expense
of remedying falls on the owner.
Slenderness
problems (the
height: wall thickness ratio) are very common. Properly, a
cavity wall should be tied ("laterally restrained") to floor
structures at each floor level. Often this happens accidentally,
where floor joists are built in to the inner skin of the cavity
wall. But where the joists run parallel, there is often no
connection between the two. Lateral restraint ties for this
situation have become a basic Building Control requirement in recent
years, but older houses have no restraint in some walls. The remedy
is to take up some floor boarding, notch the tops of the floor
joists, and install long, thin, galvanised steel ties - these are
built into the inner leaf of brickwork and nailed to the joists,
typically at 1.8m centres.
Shrinkage
is a natural phenomenon of virtually all building materials; each
with a different shrinkage rate. It is widely known that
shrinkage occurs in timber, mainly across the grain and very little
down its length, and this is well known. It can be minimised by
correct seasoning, but most wood is sensitive to changes in
moisture, and will shrink and expand as its water content falls and
rises. It is less well-known that initial shrinkage occurs in
all cementitious (concrete) products - for example; concrete
blocks, calcium silicate bricks, concrete slabs, mortars and plaster
(render) backing coats - within the first 18 months or so after
their manufacture (it tends to stop after that), which can result in
the occurrence of cracks. Such effects can be avoided by installing
"crack control" joints in the structure, and all manufacturers of
concrete products will advise on the spacing of movement joints
appropriate to the items they sell. Fortunately, shrinkage cracking
is usually not very serious, although if left unresolved it can lead
to other forms of damage. Thermal shrinkage and expansion is
a continuing natural phenomenon of building materials, and each has
a different coefficient of expansion (or contraction) - some
materials move only slightly with changes in temperature (e.g.
bricks and concrete, maybe < 3 x 10-6
per °C), others are relatively large (e.g. plastics, at around 80 x
10-6
per °C). In a cementitious product, when there is a combination of
initial shrinkage, and continuing thermal expansion/contraction
which can occur and reverse daily, cracks can get ever-wider as a
result of a phenomenon known as "ratcheting" - small
particles from the crack faces fall into the crack, and prevent it
closing when the material cools. Therefore on each re-heating, the
cracks widen slightly, starting from their new position each time.
Such movements are always small, but the forces involved are very
powerful, and quite large ultimate displacements have been recorded.
Weather effects
such as "freeze and
thaw" (rainwater enters a small fissure, freezes, expands and widens
it before melting) are common in building. Sometimes, the resulting
enlargened crack looks like a subsidence symptom. At other times the
arises of the crack are weathered by wind/rain, and not due to
continuing soils activity beneath the building.
Leaks
in drains and water supply pipes can cause localised subsidence of
bearing soils, with localised building disturbance immediately above
the failed area. If such a condition has occurred, domestic
buildings insurers might be persuaded to settle local foundation
underpinning costs on an "Escape of Water" basis, rather than under
the "Subsidence" head of claim - this would mean that the excess
would be reduced, or removed. The insured would still have to meet
the cost of remedying the cause of the leak, because this is
not insured, however insurers might be further persuaded that the
drainage or water supply pipe work has to be necessarily
disturbed to deal with foundation repairs.
Lintel
failures (or absences) result in brickwork cracks, yet are nothing
to do with foundations, or subsidence of them. Remedial works are
usually quite simple, but usually the building owner must meet the
cost, as there will be no buildings insurance cover.
Clay soil expansion / contraction
problems, known to buildings insurers collectively as
"heave", result from changes in moisture levels in clays that are
sensitive to such changes. Some clays do not move much, others a
great deal - their "sensisitivity" can be determined by testing soil
samples in a laboratory. Domestic buildings insurers deal with heave
problems in the same way as subsidence, however remedial techniques
are usually quite different. Heave solutions often require piling to
a depth where the bearing soil is stable, and the installation of
compressible/collapsible void formers to provide a gap between the
clay soil and the structure. Heave solutions can often be very
expensive.
Rises and falls in water table
can result in the collapse of soils that are sensitive to changes in
water content. Some clays are very susceptible, others are
relatively insensitive. Some sandy soils have particle sizes and
distribution that allows "collapse" (reducing their volume,
increasing their density) when wetted - alluvial river silts that
are relatively "young" in geological terms and have never been
overlain or re-immersed, can often do this. Should you have any further questions please e mail: info@ranell.org |