How Do You Build a Manhole? [PDF]

 How Do You Build a Manhole? [PDF]

A manhole, also known as an inspection chamber, is an underground unit built to provide access to utilities such as a sewer system, drainage system, and so on. As a result, underground utilities are inspected, modified, cleaned, and maintained using a manhole.

This article will go over the construction specifications for manholes.

1. Excavation

1. The manhole excavation must adhere to the dimensions and levels specified in the plan or drawings.
2. The excavation width must include the necessary working space for the mason to work freely below ground.

2. Bed Concrete

1. A 1:4:8 concrete bed will be laid over which the manhole will be built.
2. The minimum thickness of bed concrete for manholes up to 4.25m and above 4.25m shall be 20cm and 30cm, respectively.
3. Special foundations must be built to support the manhole in the case of loose soil.

3. Brick Masonry

1. The standard quality of bricks used in manhole construction shall be class 7.5N with a cement mortar ratio of 1:4.
2. The cement mortar ratio for arched and circular manholes shall be 1:3. (1 cement: 3 fine sand).
3. The walls of the manhole must be built with one brick thickness up to a depth of 4.25m.
4. Manholes less than 4.25m deep in ordinary soil must be built with one and a half bricks, and if the manhole is less than 9.75m deep, the walls must be two bricks thick.
5. The external face of the brick masonry should be finished smooth.
6. The masonry and pipe joint must be specially treated to ensure that it is completely leak proof.
7. The excess shaft in the circular manholes must be corbelled inwardly at the top in order to fit the cover frame.

4. Plaster and Pointing

1. The inside of the manhole's brick masonry must be plastered with a 12mm thick 1:3 cement mortar.
2. Plastering up to the crown level with cement mortar 1:2 is required for circular or arched manholes.
3. In most cases, the outside of the manhole is not plastered, but in the case of saturated soil, the outside of the manhole must be plastered with a 12 mm thick cement plaster 1:3.
4. A waterproofing agent must be applied to the inside plastered surface.

5. Channel and Benching

Benching in manholes refers to the path made of cement mortar to direct waste water in a specific direction and prevent waste splashing, which damages the manhole.

1. The channels and benching in a manhole must be built with cement concrete in the 1:2:4 ratio.
2. The channel and benching should be finished smoothly and neatly.
3. The channel and benching depths must be as specified in Table 1 below-

Table 1: Depth of Channel and Benching

Sizes of drain in mm	Top of channel at the centre above bed concrete in Cm	Depth of benching at side walls above bed concrete in cm
100	15	20
150	20	30
200	25	35
250	30	40
300	35	45
350	40	50
400	45	55
450	50	60

6. Footrests

The footrests in the manholes are provided for ease of access during cleaning and maintenance.

1. If the depth of the manhole is greater than 0.8m, footrests are provided.
2. PVC or mild steel footrests are required.
3. Any type of footrest, PVC or mild steel, must be embedded 200mm deep in the 200x200x100 mm 1:3:6 concrete block.
4. The footrests must be staggered laterally and vertically 40cm apart.
5. The footrest must extend 100mm beyond the inner surface of the manhole walls to provide protection.
6. The first footrest must be installed 450mm below the manhole cover.
7. To prevent corrosion, the mild iron footrest should be painted with coal tar.

7. Manhole Covers and Frames

1. The manhole cover must be both gas and watertight.
2. The manhole covers and frames must be of the four grades, types, and locations listed below.

Table 2: Manhole grade, Designation, Shape, and Application.

Grades	Grade Designation	Type / shape of cover	Application
Light Duty	LD – 2.5	Rectangular, Square, Circular	Residential and institutional complexes/areas with pedestrian but occasional LMV traffic.
Medium Duty	MD – 10	Rectangular, Circular	Service lanes/roads, car parking areas etc.
Heavy Duty	HD – 20	Circular, Square, Rectangular, (Scrapper Manhole)	Institutional/commercial areas/carriageways with heavy-duty vehicular traffic like buses, trucks, etc.
Extra Heavy Duty	EHD – 35	Circular, Square, Rectangular, (Scrapper Manhole)	Carriageway in commercial, industrial/port areas/near warehouses/godowns having wheel loads up to 11.5 tonnes

3. Following the completion of the manhole construction, the covers must be sealed with thick grease.

FAQs

1. What is a Sewer Manhole?

A manhole, also known as an inspection chamber, is a unit built underground to provide access to utilities such as a sewer system, drainage system, and so on. As a result, underground utilities are inspected, modified, cleaned, and maintained using a manhole.

2. What is benching or channeling in manhole?

Benching in manholes refers to the path made of cement mortar to direct waste water in a specific direction and prevent waste splashing, which damages the manhole.

3. What is the type of brick used in the construction of manholes?

The standard quality of bricks used in manhole construction shall be class 7.5N with a cement mortar ratio of 1:4. The cement mortar ratio for arched and circular manholes shall be 1:3. (1 cement: 3 fine sand).

4. What is the thickness of the walls in a manhole?

Manholes up to a depth of 4.25m shall have walls constructed of one brick thickness. Manholes less than 4.25m deep must be built with one and a half bricks, and if the manhole is less than 9.75 m deep, the walls must be two bricks thick.