How to Construct a Manhole: A Step-by-Step Guide

Manholes, also known as inspection chambers, are essential underground units that provide access to sewer systems, drainage systems, and other utilities. Constructing a manhole requires careful planning and execution to ensure its functionality and durability.

In this article, we will guide you through the process of constructing a manhole, covering excavation, bed concrete, brick masonry, plastering, channeling and benching, footrests, and manhole covers and frames. Let’s dive in!

Excavation:

1. Begin by excavating the designated area according to the dimensions and levels specified in the plan or drawings. This includes allowing sufficient working space for masons to operate comfortably underground.

Bed Concrete:

1. Lay a bed concrete layer with a mix ratio of 1:4:8. The minimum thickness of the bed concrete should be 20cm for manholes up to 4.25m deep, and 30cm for manholes deeper than 4.25m.
2. In the case of loose soil, special foundations should be constructed to support the manhole.

Brick Masonry:

1. Use standard quality bricks of class 7.5N with cement mortar of 1:4 ratio for the construction of manholes.
2. For arched or circular manholes, adjust the cement mortar ratio to 1:3 (1 cement: 3 fine sand).
3. Build the walls of the manhole with one brick thickness for depths up to 4.25m.
4. If the manhole is deeper than 4.25m in ordinary soil, use one and a half brick thickness. For manholes below 9.75m, construct the walls with two brick thickness.
5. Ensure a smooth finishing of the external face of the brick masonry.
6. Pay special attention to treating the joint between the masonry and pipe to ensure a leak-proof connection.
7. Corbel the excess shaft in circular manholes inwardly at the top to reduce its size for fitting the cover frame.

Plaster and Pointing:

1. Plaster the inside of the brick masonry with a 12mm thick layer of 1:3 cement mortar.
2. For circular or arched manholes, plaster the entire surface up to the crown level using cement mortar of 1:2 ratio.
3. While the outer side of the manhole typically does not require plastering, in cases of saturated soil, plaster the external surface with a 12mm thick layer of cement plaster using a 1:3 mix.
4. Apply a waterproofing agent to the inside plastered surface to ensure water resistance.

Channeling and Benching:

1. Construct the channels and benching within the manhole using cement concrete of mix ratio 1:2:4.
2. Ensure a smooth and neat finish for the channel and benching to facilitate proper water flow and prevent damage to the manhole.

Footrests:

1. Install footrests if the manhole’s depth exceeds 0.8m.
2. Use PVC or mild steel footrests embedded 200mm deep in a 200x200x100 mm concrete block with a mix ratio of 1:3:6.
3. Place the footrests staggered laterally and 40cm apart vertically.
4. Ensure that the footrests extend 100mm beyond the inner surface of the manhole walls.
5. The first footrest should be positioned 450mm below the manhole cover.
6. Apply a coat of coal tar to mild iron footrests to protect against corrosion.

Manhole Covers and Frames:

1. Install gas-tight and watertight manhole covers.
2. After completing the construction of the manhole, seal the covers using a thick grease.

Common Questions:

1. What is a Sewer Manhole?A sewer manhole, also known as an inspection chamber, is an underground unit constructed to provide access to sewer systems and other utilities. It allows for inspections, modifications, cleaning, and maintenance of underground utilities.
2. What is benching or channeling in a manhole? Benching refers to the path made of cement mortar within a manhole, which directs wastewater flow in a specific direction and prevents splashing that could damage the manhole.
3. What type of brick is used in manhole construction?
   Standard quality bricks of class 7.5N with a cement mortar ratio of 1:4 are typically used in manhole construction. For arched or circular manholes, a cement mortar ratio of 1:3 (1 cement: 3 fine sand) is recommended.
4. What is the thickness of manhole walls?For manholes up to 4.25m in depth, one brick thickness is sufficient for constructing the walls. Below 4.25m, one and a half brick thickness is used, and for manholes deeper than 9.75m, two brick thickness is recommended to ensure structural integrity.

Constructing a manhole requires careful attention to detail and adherence to the recommended procedures. From excavation and bed concrete to brick masonry and channeling, each step plays a crucial role in ensuring the functionality and durability of the manhole. By following the guidelines provided in this article, you can construct a well-designed manhole that facilitates efficient access, inspection, and maintenance of underground utilities.