Minimum height of earth cover/fill over buried Structures

[oengineer]

I am trying to determine the minimum fill height to be maintained over buried structures (i.e. Concrete pipes, Box Culverts, junction Boxes, etc.)

Is these a code or industry practice guide that states this requirement?

Suggestions/Comments are appreciated.

 

[WARose]

I'm not sure there is one. AASHTO (for example), IIRC, allows you to start spreading the load out (over something like a pipe or box) once you get to a level of fill of about 2 feet. Less than that.....and you just apply the load directly to the surface.

Beyond that, I cannot think of any

 "minimums" out there. You may want to take a look at city/county [civil] standards where you are at.

 

[phamENG]

It'll depend on what's being carried. Certain materials are placed at different elevations for either protection or layering (storm sewer over sanitary sewer to prevent a sanitary leak from entering the untreated storm flows out to a river or other unprotected watershed, for example). Best to check with the civil engineer doing the network design.

 

[bridgebuster]

I'm not aware of any code requirements either. I would say it's any owner/agency issue. For example in NYC, watermains with less than 2' of cover require steel protection plates. There's a trade group called Process Industry Practices that has this table in one of their publications.

 

[SlideRuleEra]

I am trying to determine the minimum fill height to be maintained over buried structures (i.e. Concrete pipes, Box Culverts, junction Boxes, etc.)

No, for the reasons stated by others...plus additional reasons:

1) Depends on the shape (geometry) of the structure being buried - circular pipe will have different structural properties than a "boxy" culvert.

2) Soil conditions (including location of the water table).

3) Whether the structure can be "hollow" like a pipeline (subject to flotation when empty) or not (like a culvert).

4) Material used (e.g. High Density Polyethylene -HDPE- pipe floats, even if filled with water).

5) HDPE pipe is flexible unlike concrete, steel, other plastics, or ductile iron - different backfill/cover requirements for flexible vs. rigid pipe.

6) Pipes made from virtually all materials are available in various wall thickness... different cover requirement depending on wall thickness, even for the same pipe.

http://www.SlideRuleEra.net

 

[r13]

Doesn't pipe/culvert/junction box usually have their elevation determined/controlled by other considerations other than fill depth? But I think bridgebuster nailed it.

 

[BridgeSmith]

AASHTO (for example), IIRC, allows you to start spreading the load out (over something like a pipe or box) once you get to a level of fill of about 2 feet. Less than that.....and you just apply the load directly to the surface.

In the current version of AASHTO (LRFD 8th Ed.) for less than 2' of fill on a box culvert, there are minimum 'distribution reinforcing' requirements to ensure the top slab of the culvert can sufficiently distribute the load through the concrete slab. The distribution width of the load does not consider the effect of fill.

However, the equation for the distribution length parallel to the span (Eq. 4.6.2.10.2-2) includes distribution through the fill (and the length of the tire contact patch). The commentary says that distribution may be "conservatively neglected"; i.e. the wheel load may be applied as a line load of the distribution width.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[oengineer]

I am looking at an earth fill cover of 1.85 ft.

This is over a junction box caring storm water.

 

[phamENG]

Lots of AASHTO references being thrown around - is this even under a roadway? Good references regardless, but important to take them for what they're worth in the situation. No need to bury the thing to distribute wheel loads from an 18 wheeler if it's in the middle of a field.

What's your frost depth? Probably don't want the storm water freezing in the junction box.

At less than 2ft, I'd just assume whatever loads are imposed on the surface translate directly onto the box. As long as it's strong enough to support the anticipated load, the water won't freeze and cause flooding upstream during a melt, and the local jurisdiction is otherwise satisfied, I'm not sure there's anything to worry about.

 

[oengineer]

@phamENG

This is not under a roadway.

Half of the box is under a maintenance berm and the other half is under a detention pond.

 

[r13]

If it is under road way, check out AASHTO guidance as suggested by BridgeSmith, or work out with local DOT. If it is in the public land, work out with the local engineering department. If it is in the wild, just make sure the manhole is at/above the ground level for ease of locating.

 

[oengineer]

There's a trade group called Process Industry Practices that has this table in one of their publications.

Which PIP guide contains this chart?

 

[bridgebuster]

PLSC0011 - Trenching

 

[oengineer]

PLSC0011 - Trenching

Thank you, bridgebuster. I was able to obtain this PIP guide.

The PIP Trench guide states, "Unless otherwise specified in local, state, or federal regulations, before starting excavating, an underground facility shall be probed to ensure a minimum of 0.6 m (24 inches) of cover."

Would this mean that special approval needs to be obtained in order to have a earth cover of, let's say, 18 inches above a buried structure?

 

[phamENG]

Only if the PIP Trench guide is codified in the jurisdiction where this will be built.

 

[r13]

oengineer,

As stated before, you need to get in touch with the agency that has the oversight of your project, and discuss the code/standard to be used for design as early as you can.

 

[bridgebuster]

My take is that this requirement from Section 4.4.4. applies to locations where utilities may exist. Sort of a "Call Before You Dig" requirement. I've seen (and designed) pipes and culverts with less than 24" cover. I agree with phamENG, PIP would apply if it is codified by the AHJ. However, you should check with the AHJ or facility owner to discuss codes/standards. Even if they don't have any and if something goes wrong the judge will want to know why you didn't follow an accepted industry practice.

 

[EDub24]

I've always seen 24" as a minimum but lower covers can be used if the pipe is designed for for the higher surcharge loading or there's some sort of protection for the pipe (like a concrete cap to distribute the loading).