Code of practice for ancillary structures in the sewerage system, Part I: Manholes
1986 Edition

According to IS 4111 Part 1 (1986), recommended manhole dimensions and spacing include: Rectangular manholes with minimum internal face sizes of 900 mm × 800 mm for depths less than 0.9 m, and 1200 mm × 900 mm for depths between 0.9 m and 2.5 m. For depths exceeding 2.5 m, circular manholes with diameters typically ranging from 900 mm to 1800 mm are suggested. Manhole covers should have a clear opening diameter not less than 560 mm for depths greater than 0.9 m, complying with IS 1726 standards. Manhole spacing is influenced by the type of mechanical cleaning equipment utilized, with no fixed interval specified in the standard.

The standard specifies that bed concrete should have a mix ratio of 1:4:8 (cement: coarse sand: graded stone aggregate, 40 mm nominal size), with thicknesses of 225 mm for manholes shallower than 2.3 m and 300 mm for those equal to or deeper than 2.3 m. Manhole walls situated in high sub-soil water conditions should be constructed using reinforced cement concrete of Grade M20 or a 1:1½:3 mix, while brick masonry walls are permissible above the water table. All walls should be plastered internally and externally with a 1:3 cement mortar, finished smoothly and treated with waterproofing compounds where necessary to ensure durability and water tightness.

Channels inside manholes must have a semi-circular bottom with a diameter matching the sewer pipe. Vertical sides should extend 50 mm above the sewer pipe crown, with smoothly rounded top edges. Branch channels require similar benching with appropriate fall at junctions. The material used should be cement concrete in a 1:2:4 mix, rendered with neat cement and finished with a steel trowel to produce a hard, impervious surface. Depths of channels and benching vary according to drain size, as detailed in the standard's tables. Benching slopes on either side of the channel are designed to confine flow, prevent sedimentation, and provide safe footing for maintenance operations.

Safety provisions include installing galvanized wrought-iron safety chains (6 or 10 mm thick) across downstream openings in manholes larger than 1 m in diameter to prevent accidental falls. Hand-rails made from galvanized pipes of nominal 38 mm diameter should be fitted around benchings, platforms, and manhole openings to protect workers. During excavation for deep manholes, measures such as timbering, staging, and dewatering must be employed following IS 3764-1966 to ensure safe working conditions. Additionally, cascades and ramps require hand-rails and chains to safeguard personnel during maintenance.

For deep manholes, the access shaft should rise to road level with the manhole cover/frame resting on top but not embedded in the RCC slab to allow easy replacement. The transition from the larger chamber to the smaller shaft is supported by a reinforced concrete slab at least 150 mm thick or by brick arches or concrete lintels. Excavation must be dimensionally accurate and safely supported with timbering and staging as per IS 3764-1966, including dewatering if necessary. Rest chambers with grated landing platforms and hinged trap doors should be provided approximately every 6 meters to facilitate safe maintenance access. Side-entrance manholes are employed where vertical access is impeded, connected through lateral passages.