Manhole cover
Contents |
[edit] Introduction
A manhole, also known as an inspection chamber, provides access to underground utilities, most commonly sewer systems. This enables operatives to undertake inspections, make modifications, and carry out cleaning and maintenance.
A manhole usually consists of a chamber or ring – a vertical circular pipe – of varying sizes and depths, which is used to access inspection points. The chamber is protected by a manhole cover (also referred to as a drain cover or the gender neutral term, maintenance hole cover) that is typically positioned away from the wheel line of traffic.
The cover acts as a removable 'plug' to protect the manhole and prevent unauthorised access. They can also be marked to identify the services (and affiliated maintenance entities) associated with them.
[edit] Manhole cover details
Evidence of manhole covers can be found as far back as ancient Rome. During this period, stone covers were used to cover limestone sewer grates.
Modern manhole covers are generally circular, but they can also be rectangular, triangular or square. Generally, covers fixed with manhole key holes, which must be free from debris.
Covers are typically made from metal or, less commonly, precast concrete, glass reinforced plastic or other composite materials. They are generally inexpensive to manufacture and are heavy enough to prevent unintended displacement (caused by traffic) or unauthorised removal (by thieves hoping to resell the scrap material for profit).
Most covers rest on a metal frame (or base) that has a smaller inset rim to fit the cover. This combination of cover and base is sometimes referred to as a casting.
[edit] Manhole cover sizes
Sewers for Adoption specifies that all manhole covers must be Kitemarked and comply with BS EN124 with Class D400 covers in areas used by road vehicles. It is recommended that all covers subject to traffic loading have 150 mm deep cover and frames fitted.
The cover and frame may rest on a cover slab, which can be made from concrete. Cover slabs must be positioned in square alignment with step irons or ladders and provide a minimum 600 x 600 sq. mm unobstructed opening. The internal face of the cover slab must be plumb with the outer edge of the step irons.
The size of manhole covers varies based on the dimensions of the opening. They commonly come in sizes from 300 mm x 300 mm to 1200 mm x 1200 mm (for square/rectangular covers) and 450 mm or 600 mm diameters for circular covers.
For 600 x 600 sq. mm openings on 1,050 mm diameter chamber rings and above, it is usual that a 600 x 750 mm cover slab is fitted, reduced to a 600 x 600 sq. mm opening by the use of an eccentric raising piece to suit the manhole cover and frame used. For 1,050 mm and 1,200 mm diameter rings less than 1.5 m deep to benching, 750 x 750 mm covers shall be fitted.
For 1,500 mm diameter rings less than 1.5 m deep to benching, 1,200 x 675 mm covers shall be fitted.
Cover slabs must not be cut to increase opening dimensions, as this will significantly weaken the cover slab.
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