Reinforced electric duct bank

[cuky2000]

Went is an electric duct bank is recommended to be reinforced.
We understand that 3 ft below grade from the top of a concrete encased duct bank, heavy load in the surface will be distributed around the earth that minimize the impact on the duct bank assembly.
Any suggestion?

 

[7anoter4]

It is a very interesting subject of which I was not worried about since the duct bank resistance calculation was the structural engineer job. As I understood they consider the duct bank as a beam supported on two manholes windows and "free field "loaded by prospective burden-the above earth and the heaviest track considered. From the following articles I understood it is about the stress transfer.
See-for instance:
Pressure Reduction on Wide Culvert

http://surface.syr.edu/cgi/viewcontent.cgi?article=1548&context=etd

and
EXPERIMENTAL AND NUMERICAL ANALYSIS OF A PIPE ARCH CULVERT SUBJECTED TO EXCEPTIONAL LIVE LOAD

https://etd.ohiolink.edu/rws_etd/document/get/ohiou1172090797/inline

The transfer of stresses in a multi-component support system is known as arching. In the case where the buried structure is more rigid than the adjacent soil, arching will be defined as negative arching, where the structure will be exposed to greater pressures relative to the calculated “free-field” pressures from the sum of overburden pressures alone.

 

[cranky108]

The duct bank I have seen installed prior to the completion of the road above, used fiberglass to reinforce. But not being a civil engineer, I could only look at it with untrained eyes.

 

[bacon4life]

Seismic loading on unreinforced duct banks may cause the whole bank to sheer all conduits in a single large crack.

We had a case where a sewer contractor excavated a wide trench under a concrete duct bank without providing structural support. Eventually the duct bank cracked and required replacing all of the cables in the duct bank. I don't know the reinforcing details of the duct bank, but clearly the contractor overestimated the strength.