Underground manholes and cable ducts

Below is what is called an underground manhole for electrical cables. This one is still under construction for 33kV electrical incoming authority supply cables.

Picture 01 – A 33kV underground manhole

A 33kV Electrical Manhole Under Construction

This post is another anchor post. I will upload pictures for underground electrical manholes and underground electrical cable ducts here.

If I come across more than a few of related pictures on this subject, I will probably send a separate article, but one or two of the pictures from the new post will also be added here together with a link to the new post.

Manholes for telecommunication cables will also be located here, as well as the ductworks for the underground telecommunication cables.

Even though the signals and the power carried by the two types of cable are totally different, the nature of their construction and installation works of the manhole and the duct-bank are generally very similar.

The skills and knowledge to handle the trade contractors of both works are also the same. So one will benefit the other and vise versa.

Picture 02 – An 11kV Manhole Being Delivered To Site

An 11kV Manhole Being Delivered To Site

This manhole was still on the truck and it was for the 11 kV underground incoming cables to an office complex.

An onboard crane was preparing to loading the unit from the truck.


Picture 03 – Manhole Pit Being Prepared

Manhole Pit Being Prepared

A few days after the precast electrical manhole was delivered to the construction site, the pit for the manhole was dug out and prepared.

Beginners please take note here. Do not dig out the pit for the manhole first while waiting for the manhole to arrive.

Things happens. Maybe it’s a heavy rain instead of the manhole that arrives. That is possible.

The manhole may not arrive the next day as promised by the supplier. In fact, the manhole may only arrive two weeks after the pit was excavated and prepared.

A pit exposed too long may collect rain-water and the base of the pit would be unsuitable to received the manhole. It will need to be rectified first … this will mean extra man-hours and extra machine time … extra money need to be spent.

Picture 04 – 11kV Underground Manhole Being Lowered Into Pit

11kV Underground Manhole Being Lowered Into Pit

Here the manhole was being lowered and guided into the base of the pit. The orientation of the manhole will need to align with the cable ducts that will connect to it.

Otherwise the ducts may not properly slide through the precast openings in the manhole walls.

When this happens, the wall will need to be hacked and modified on-site to suit the connecting ducts. Again, extra time and money.

Picture 05 – A Heavy-Duty Manhole Cover Being Put Onto Position

A Heavy-Duty Manhole Cover Being Put Onto Position

Now the heavy duty manhole covers being put into place. Note that there may be several pieces of the covers as can be seen in the following picture.



Picture 06 – A Completed 11kV Underground Manhole

A Completed 11kV Underground Manhole

Completed. Job done.





Update 20160720:
Picture 07 – 11kv manhole and underground duct crossing roadside drain

11kV manhole and duct crossing roadside drain

The above picture and the one below shows an underground manhole for 11KV incoming supply cables.

In the above picture, you can see the precast openings for connecting G.I. pipes to the manhole. The are two rows of six round openings each around 150 mm diameter.

Each duct would slide into an opening to make a nice connection. That’s why it is best to align the manhole and ducts properly.

Often it is just not possible to nicely slide the straight rigid ducts into the precast openings due to the layout of the underground cable route and the location available to place the manhole.

In cases like that the precast openings (the manhole wall) have to be hacked off and an overall opening for all the ducts wound be made so they could enter the manhole in an angle.

The new large opening would later be sealed off after all the ducts have been properly positioned.

Picture 08 – G.I. ducts connected to the manhole wall at 90-degree angle

GI duct connection alignment to manhole

In this picture you can also see that the incoming ducts had to cross a roadside drain before entering the manhole.

Beginners should take note here that we cannot simply cross roadside drains like this.

Civil engineers responsible for the drainage system should be consulted first and approval should be obtained in writing before directly crossing a drainage system.

Of course, in theory the design engineers for each engineering discipline should have coordinated the cable routes and the drainage systems during the design stages.

And the construction supervision people only need to ensure that the construction of the road and drainage, and the installation of underground cable ducts and manholes follow exactly as per the construction drawings that have been issued to site.

Life, however, seldom run perfectly.

And every now and then, we the electrical just have no other choice than crossing through the middle of a drainage system as in the case here.

Update 20160727:
Picture 009 – Telecom cable ducts connecting to a JC9C manhole at an angle

Telecom cable ducts connecting to a JC9C manhole at an angle

If Picture 08 above shows rigid underground G.I. ducts connecting to the 11kV manhole by nicely sliding into the factory-made 150 mm diameter openings, the new picture above shows otherwise.

They are black PVC pipes for the incoming telecommunication authority cables.

Due to the site layout, the cable ducts was designed to enter the telecommunication manhole here at an angle which is not a 90-degree to the opening wall.

Therefore, the manhole wall was hacked off a bit, just to make an opening for the 4-ducts-in-a-row.

The manhole came from factory with a two layer duct arrangement (2 row x 2 ducts per row) as you can see in the next picture below.

As the ducts at this site is laid in a 1-row x 4-ducts arrangement, some modification was still necessary to the manhole wall, even if the ducts came at a 90-degree angle.

Picture 010 – The telecommunication JC9C precast manhole being lowered in the the manhole pit

A JC9C precast telecommunication manhole

Beginners please observe also the number of precast openings are not the same at different sides of the manhole walls.

I will go into a bit more details when I send a separate post for this work in the future.

Picture 011 – Black PVC conduits installed underground for telecommunication incoming cables to an office building

Underground black PVC conduits in sand bedding and cover

I put this picture here just to show the compacted sand bedding below the 4-in diameter black PVC conduits, and more sand over them.

The compacted sand below is called the “bedding”, usually compacted to 2- or 3-inch thick.

After the conduits are laid, more sand is poured over them and compacted to 3- or 4-inch thick above the top of the conduits.

The compacted sand above the conduits is called the “cover”.

That’s all for now. See you all again soon.

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