Stresses come under scrutiny
RMC presents the results of its research into why manhole covers fail
As the incidence of manhole failure increases, the real price of these failures
is becoming apparent. The costs of labour, materials and the disruption caused
can be considerable and, invariably, the same sites fail repeatedly.
As part of the research programme the process of failure was put under the
microscope. More than 2,000 manhole failures were studied with the aim of uncovering
the mechanisms at work. RMC came to believe the reasons for failure had not
been properly understood and therefore the solutions offered to-date were unable
to cure the problem.
The causes of failure were many and varied, and rarely attributable to just
one identifiable cause. The overriding conclusion was that no single aspect
of the structure can be blamed and that when considering a solution, the complete
system and the way it integrates and interacts with the surrounding road must
be taken into account.
In every case the mortar failed, with the brickwork and the road surround failing
in the vast majority of instances. Such traditional materials as bricks, cementitious
mortars, tiles, slates, etc were simply not designed to do the job that a modern
manhole structure must perform.
The factors that contribute to the failure have been identified as:
- increased traffic volumes and vehicle weights,
- the vertical and lateral stresses imposed are increased dramatically in vulnerable
locations – bends, roundabouts, traffic lights, junctions, etc,
- traditional materials are inflexible and unable to cope with these stresses,
- braking and turning forces create lateral stress and movement of the ironwork
exacerbating the inability of inflexible materials to respond,
- ironwork with a narrow or skeleton type bedding flange tends to fail earlier
than ironwork with a wide, solid flange,
- poor practices in reinstatement work,
Repeat failure is a major problem. Inadequate repairs, particularly on high
stress sites done at speed, often result in
a second and subsequent failures in a very short time.
RMC felt the cost of manhole failure had never really been considered and as
a result the true extent of the problem had not been calculated.
Applying these results to the Readyraise development programme has enabled
the creation of a system that is able to provide an effective and durable response.
Initially the Readyraise concept involved a system that could be raised and
lowered to facilitate planing and resurfacing during road maintenance. As the
more detailed consideration of manhole failure and structural requirements began
to yield tangible results, it became clear that this facility, although an important
feature, was secondary to producing a system that could withstand the stresses
that were becoming ever more apparent.
As a result, a profile of a system to meet these needs was developed:
- impact-resistant and flexible bedding – this material must have high tensile
strength be able to absorb the impact and vibrations from traffic without cracking
or crumbling, it must also allow for the natural flexing of the ironwork flange.
As a consequence rigid bedding materials with high compressive strength are
considered unsuitable as they simply transmit the vibration and impact stresses
to the structure below causing further failure,
- containment of lateral stress – the support structure must contain the ironwork
laterally and absorb the stresses from braking and turning forces, thus reducing
the stress on the carriageway surface,
- rapid installation – the system must be able to be installed rapidly. Fast-setting
of the bedding and reinstatement materials is essential to reduce disruption
and to allow the road to be reopened as soon as possible,
- ease of installation – the whole system must be capable of being installed
simply and without the potential for incorrect materials or practices being
- permanent and adjustable – the support structure should have a design life
the same or greater than the road structure and allow for future, level adjustment,
- ironwork specification – the ironwork should not only comply with BS EN 124
but should also have a wide, solid flange, be 150mm deep wherever possible and
be of a category suitable for the traffic intensity of the road.
As a result RMC produced Readyraise. Precast concrete components work in conjunction
with flexible and impact-resistant bedding material to provide a containment
structure that is durable and able to withstand the stresses imposed. The system
is simple to install, can be adjusted and has a design life of 50 years.
Thousands of Readyraise units have been installed in a wide variety of locations.
The system has been rigorously assessed, has WRc approval and has recently gained
British Board of Agrément (BBA)/Highway Authorities Product Approval
Scheme (HAPAS) approval.
RMC believes Readyraise works because its development has been based upon the
detailed consideration and assessment of the nature of the problem and a systematic
approach to providing an effective, durable and properly engineered solution.
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