Drop Manholes and Manhole Construction

What Is Drop Manhole Mean?

Drop manhole is a type of manhole where the vertical pipe is permitted to flow between the main sewer and branch sewer. This type of manhole is used in steep areas or where the elevation of the inlet pipe is more than the elevation of the outlet pipe.

In drop manhole, the stop invert is at the level from the invert of the outlet. Drop manholes have a larger sloping gradient, that’s why this type of manhole construction is uneconomical.

The main sewer flow is routed outside of the manhole. The pipe which passes through the manhole barrel is called inside the drop, and that in the exterior portion is termed as an outside drop. These outside drops are for routine checking, inspection and cleaning purpose.

Manhole Construction

There are several processes and different steps available for manhole construction. Those steps are

1. Excavation
2. Bed Concrete
3. Brick Masonry
4. Plaster and Pointing.
5. Channel and Benching
6. Footrests
7. Manhole Covers and Frames

Step 1 – Excavation:

This is the first step of manhole construction. The excavation process is done as per the manhole’s dimension and depth mentioned in the plan and the drawing. You must include the excavation width for some free space for workers to work that place freely.

Step 2 – Bed Concrete:

The second part of constructing a manhole is this. Here a paste of concrete of ratio 1:4:8 is laid down the manhole bed or constructed.

Minimum bed concrete thickness should be 20 cm to 30 cm for the manhole depth of 4.25 m to above 4.25 m. Where there is a presence of loose soil, then you need to construct a special foundation.

Step 3 – Brick Masonry:

To construct a manhole regular bricks of strength 7.5N are required and mortar of 1:4 ratio used. For arched type structural manhole 1:3 ratio cement mortar is used.

The wall thickness of 5 inch is limited to 4.25 m depth of manhole, and 10inch wall is provided where the depth of manhole is more than 4.2m. The outer part of manhole should be smooth, and the masonry pipe should be specially treated and leak proof.

Step 4 – Plaster and Pointing:

This is one of the most important steps of manhole constructing. The Master of manhole should be 12 mm thick, and it requires 1:3 cement mortar paste. For archer type of structure, the plaster requires to the crown portion of the manhole, and it requires 1:2 cement mortar paste.

We don’t need to plaster outside, but in case of saturated soil, 12mm thick plaster required and inner part of the manhole should be waterproof with a waterproofing agent.

Step 5 – Channel and Benching

Benching in the manhole is a path constructed by cement mortar which converts the wastewater in a certain direction and helps the interior part of the manhole from getting damaged.

The channel and benching of the manhole are constructed by the cement concrete ratio of 1:2:4, the channel and benching must be neatly finished. The depth of the channel and benching list is given below-

Step 6 – Footrests

It is provided for easy access for cleaning and maintenance. If the depth of manhole is more than 0.8 m, then footrests are provided. It is made of either mild steel or PVC. Footrests are rooted in 200 mm deep in a 200 x 200 x 100 mm concrete block. The first footrest must be 450 mm from the manhole cover.

Step 7 – Manhole Covers and Frames

The manhole covers must be airtight and watertight. The manhole covers and frames are following types-

Types of Manholes:

The manhole types are mainly classified into two categories-

1. Based on Depth
2. Based on Construction Material

#1. Based On Depth:

Based on depth, the manholes are classified into another three categories-

• Shallow Manholes:
• Normal Manholes:
• Deep Manholes:

#a. Shallow Manholes:

The manholes with a depth of 0.75m to 0.9m is called shallow manhole. Shallow manholes are constructed where low traffic, this type of manholes are mainly rectangular type. Shallow manholes are also referred to as inspection chamber.

#b. Normal Manholes:

Those manholes whose depth are greater than 0.75 m but not more than 2 m is called normal manhole. This manholes are covered with thick cover and square in shape.

#c. Deep Manholes:

Deep manholes are that type of manhole which depth is more than 2m. This types of manholes are mainly circular in shape.

#2. Based On Construction Material:

Depending on the construction material manholes are classified into three categories, those are following-

• Precast Concrete Manhole:
• Plastic Manhole:
• Fibreglass Manholes:

#a. Precast Concrete Manhole:

This type of manholes are constructed by precast concrete, and it’s a traditional type manhole. In this type, the manhole components are directly placed in the site. This type of manhole has a huge use in all over the world.

#b. Plastic Manhole:

Plastic manholes are made of polyethene material, and it does not harm the environment. Plastic manholes do not require too much maintenance, and it’s highly resistive against corrosion.

#c. Fibreglass Manholes:

Fibreglass technology is used to construct this type of manhole. This type of manhole is lightweight and easy to install; that’s why it’s economical to use. Fibreglass manholes are durable and eco-friendly.

Purpose of Manhole

There are various purposes of manhole; the first one is for cleaning, repair and maintenance of the sewage system. Manhole allows the change of alignment and direction of sewage.

Some manholes are constructed with perforated covers; it helps to release the foul gases, so it allows the ventilation of the sewage system. Sewer lines are laid in a convenient length with support of manhole.

Manhole Components

There are mainly six types of components are present in a manhole. Those are-

• Access Shaft.
• Working Chamber.
• Base and Sidewalls.
• Bottom or Invert.
• Steps or Ladder.
• Cover and Frame.

#1. Access Shaft:

Access shaft is the upper portion of a deep manhole. It’s a vertical passage which gives access to the working chamber from the manhole cover.

The minimum size of the rectangular access shaft is 0.75 m x 0.6 m and 0.7 m dia for a circular manhole. Sometimes access shaft is covered with reinforced concrete. In circular manhole, the access shaft is made little inward slope.

#2. Working Chamber:

The lower portion of the manhole is termed the working chamber. This is provided for cleaning and inspection chamber. The minimum size of the working chamber for rectangular manhole is 1.2 m x 0.9 m. For circular type manhole, the dia of the working chamber is 1.2 m.

#3. Base and Sidewalls:

The base of the manhole is generally made of plain cement concrete, and it resists the entry of groundwater. 15 cm is the minimum thickness of concrete bed for the depth of 0.8 m manhole.

The sidewalls are made of brick masonry, stone masonry or reinforced concrete cement. The minimum thickness of the brick sidewall is 20 cm for the depth of 1.5 m and 30 cm for the depth of more than 1.5 m. Now, we use a rule to calculate the thickness of the brick wall, that is-

t = 10 + 4d

Where,

t = thickness of the wall in cm

d = depth of manhole in m

#4. Bottom or Invert:

The bottom part of the manhole is like semicircular or U-shaped and made of PCC. The side of this is perfectly vertical. The slope provided for benching is 1 in 10 to 1 in 6. The invert chamber is constructed to prevent the backflow the sewage flow.

#5. Steps or Ladder:

This is provided where the depth of manhole is more than 0.8 m. Steps are made of cast iron, and it’s horizontal centre to centre distance is 38 cm, and vertical is 30 cm. Topside is placed 45 cm from the cover of a manhole. Ladders are generally provided in deep manholes.

#6. Cover and Frame:

The cover set is provided at the top of the manhole. Generally, the covers of manholes are circular in shape, and the diameter is 56 cm, and depth is 0.9 m. Covers are made of cast iron. The dimension of this is 20 to 25 I’m height and 10 to 12 cm base. The weight of the cover and frames is between 90 kg to 270 kg.

Define Appurtenances:

In Civil Engineering appurtenances means sewer appurtenances. Sewer appurtenances are such type of structure constructed in a sewerage system in a reasonable interval. This includes manhole, clean out, lamp hole, catch basin, etc.

Appurtenances Examples:

There are several examples of appurtenances, those are-

1. Manholes:
2. Lamp Holes:
3. Cleanouts:
4. Catch Basins:
5. Inverted Siphons:
6. Storm Regulators:
7. Flushing Tank:
8. Gullies:

#1. Manholes:

Manholes are generally made of reinforces cement concrete. It’s provided in suitable interval along the sewer line for cleaning, maintenance and inspection purpose.

#2. Lamp Holes:

This is a small opening constructed along with sewer line to permit the lamp when the maintenance is running to clear the darkness.

#3. Cleanouts:

The cleanout is an inclined pipe which is started from the ground and connect the ground sewer. It is generally provided in the upper sewer instead of a manhole.

#4. Catch Basins:

Catch basins are provided to settle down the sand, grit, debris etc.

#5. Inverted Siphons:

It’s a passage which dropped below the hydraulic gradient line beneath the valley, road, railway, stream etc. It’s also known as a depressed sewer.

#6. Storm Regulators:

It’s constructed to divert the excess stormwater into another stream.

#7. Flushing Tank:

This is that type of tank which holds water, and by flushing, it cleans the dead-end of the sewer.

#8. Gullies:

These opening are constructed in the road to divert the stormwater into the groundwater sewer.

Frequently Asked Questions (FAQ) on Drop Manholes and Manhole Construction

What is a drop manhole?

A drop manhole is a type of manhole where a vertical pipe allows flow between the main sewer and branch sewer, used in steep areas or where there is a significant elevation difference between inlet and outlet pipes.

Why are drop manholes used?

Drop manholes are used in steep areas or where the elevation of the inlet pipe is significantly higher than the outlet pipe to manage the vertical drop efficiently.

What are the main components of a drop manhole?

The main components of a drop manhole include the vertical drop pipe (either inside or outside the manhole barrel), access shaft, working chamber, base and sidewalls, bottom or invert, steps or ladder, and cover and frame.

What is the purpose of a manhole?

Manholes are used for cleaning, repair, and maintenance of the sewage system. They allow for inspection, alignment changes, and ventilation of the sewage system through perforated covers.

What materials are used for manhole construction?

Manholes can be constructed from various materials including brick masonry, precast concrete, plastic, and fiberglass.

What is the process of constructing a manhole?

The process involves several steps: excavation, laying bed concrete, constructing brick masonry, plastering and pointing, creating channels and benching, installing footrests, and placing manhole covers and frames.

What are the different types of manholes based on depth?

Manholes based on depth are classified into shallow manholes (0.75m to 0.9m), normal manholes (0.75m to 2m), and deep manholes (more than 2m).

What are the different types of manholes based on construction material?

Based on construction material, manholes can be classified into precast concrete manholes, plastic manholes, and fiberglass manholes.

What are the steps involved in the excavation process for manhole construction?

Excavation involves digging according to the manhole’s dimensions and depth as per the plan and providing enough free space for workers to work comfortably.

What is the purpose of footrests in a manhole?

Footrests provide easy access for cleaning and maintenance, especially in manholes deeper than 0.8 meters.

What types of manhole covers are available?

Manhole covers are available in various grades and shapes: Light Duty (LD-2.5) – rectangular, square, circular; Medium Duty (MD-10) – rectangular, circular; Heavy Duty (HD-20) – circular, square, rectangular; Extra Heavy Duty (EHD-35) – circular, square, rectangular.

What are some common sewer appurtenances?

Common sewer appurtenances include manholes, lamp holes, cleanouts, catch basins, inverted siphons, storm regulators, flushing tanks, and gullies.

What is the function of a lamp hole in a sewer system?

A lamp hole is a small opening along the sewer line that permits light to illuminate the area during maintenance.

What is a catch basin?

A catch basin is designed to settle sand, grit, and debris to prevent them from entering the sewer system.

What are inverted siphons, and where are they used?

Inverted siphons are passages that drop below the hydraulic gradient line, used beneath valleys, roads, railways, streams, etc., and are also known as depressed sewers.