Sewer and Stormwater Manhole constructionFebruary 9, 2014
Section 10 of NZS 4452 sets out alternative acceptable solutions for the construction of manholes.
Note: These requirements, previously used by most territorial authorities, may now exceed the performance criteria of NZBC El and G13.
Shape: Manholes may be circular, square or rectangular in plan. Manholes that are circular in plan should have a minimum internal diameter of 1050 mm. Non-circular manholes on sewers where depth from ground surface to invert exceeds 1.5 m shall have minimum dimensions of 900 mm in either direction.
Size: The internal dimension of a circular manhole lid frame should be not less than 500 mm. The dimensions stated in this clause allow the use of television cameras for inspection. Shallow inspection chambers less than 900 mm in depth may be of lesser dimensions than stated above, but consideration shall be given to the possible need to insert inspection and cleaning equipment.
Specially designed computer-controlled machinery is available to locate, inspect and test whole underground pipe systems, with no need for excavation.
Lightweight covers may be fitted when it is desirable for a blocked sewer to Pop the manhole lid and discharge safely rather than build up and cause a nuisance elsewhere.
The base of precast manholes should extend beyond the walls to prevent the manhole floating (clause 10.4).
Means of access: All manholes over 1 m deep shall be provided with step irons, recessed in the case of small manholes. When the depth exceeds 2 m, galvanized steel ladders are recommended. Step irons and ladder rungs projecting not less than 125 mm should be evenly spaced at nominal 300 mm vertical intervals. The lowest from or rung shall be not more than 450 mm above the benching and the highest not more than 600 mm below the top surface of the manhole cover.
In deep manholes, removable steel grillage type platforms shall be placed at 6 in vertical intervals. Steel ladders connecting platforms shall be placed on opposite walls, or staggered alternately right and left on the same wall. The least dimension in access openings of platforms shall be not less than 500 mm.
Step irons should be formed from mild steel rod of 20 mm minimum diameter and all steel work shall be hot-dipped galvanized at the rate of 0.7 kg/m2. Alternatively, the use of marine quality (type 316 or similar) stainless steel or high strength synthetic (plastic) coatings should be considered in situations likely to be exceptionally corrosive.
Euro Plumbing’s Drain layers have been designing and installing manholes successfully for some time now, if you need help or any recommendations for design or build give us a ring and we will gladly help.