Stresses come under scrutiny

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

  employed,

• 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.