Clarification on Backfill Materials Used in 33kV Cable Trench Design

[NomadicOwl]

Dear Engineering Community,

I am seeking insights into the rationale behind the selection of specific backfill materials in our existing 33kV redundant power cable trench design.Photo attached . The current configuration utilizes the following materials:

Cement-Based Sand (CBS): Encasing the power cables.

Fluidized Thermal Backfill (FTB): Positioned below the 33kV power cables and surrounding the control cables.

Engineering Backfill: A 500mm wide layer separating the redundant 33kV cables.

Dune Sand: Encasing the earth wire within the engineering backfill.


As we plan to install new 33kV cables parallel to the existing setup, also featuring redundancy, I aim to determine whether all these specific backfill materials are necessary for the new trenches or if there are opportunities for optimization without compromising the system's integrity.

Specific Questions:

1. Cement-Based Sand (CBS):

Can the use of CBS be optimized or replaced with alternative materials that provide similar thermal and mechanical protection?



2. Engineering Backfill:

Is the 500mm width between redundant cables essential, or can it be reduced without affecting performance?



3. Fluidized Thermal Backfill (FTB):

Given its role in thermal management, is there potential to optimize the use of FTB, or is its application critical as specified?



4. Dune Sand:

Are there alternative materials to dune sand for surrounding the earth wire that offer comparable benefits?

5. Can I route control cables and FO cables in the same trench as 33kV power cables?



I would appreciate insights into industry best practices, relevant standards, or personal experiences related to optimizing these backfill materials for high-voltage cable installations.

Thank you for your assistance.

 

[waross]

Cement-Based Sand (CBS): Encasing the power cables.

Research suggestion;
I am not sure what is meant by "Cement Based Sand" but it may be productive to look up "Ufer Grounds" and consider surrounding the bare earthing conductor with CBS.

 

[bacon4life]

I am confused why they have FTB below the power cables and CBS surrounding/above the cables. I haven't ever used CBS, but I thought FTB had better thermal properties.

It is not obvious to me why you need 250 mm of non-native material below the cables. I can see a possible need for a much thinner (100 mm) layer of material to provide a smoother bed for the the cables. I am more used to having cables in conduit, so perhaps the thinker layer is applicable to directly embedded cables.

The 500 mm distance between cables is barely enough. A couple consideration:
1) the typical width of a backhoe is more than 500 mm. The spatial separate should be large enough to ensure a bad equipment operator doesn't damage both trenches at the same time.
2) the trenches should be far enough apart such that there can be a 1:1 slope during excavation. This implies that a separation of 1200 mm might be better.
3) the trenches are probably close enough to thermally influence each other.

I am assuming the dune sand is likely designed to make it easy to excavate the ground cable for making future connections. However, it does seem very far from the conductors. It may be worth checking the that the conductor sheilds are adequately sized for fault currents.