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BACKGROUND: I'm burying a 3 foot x 4 foot drainage box (2 foot tall) in my driveway to hold/distribute excess gutter flow (1" of rain produces ~21cuft). I've constructed the walls with brick, laid on their edge so that the water can drain out through the 10 holes in each. I need to construct a cover that, when itself covered with ~3" of crush&run will support a potentially heavy vehicle driving over it. To further support the cover, I'm laying 1.25" iron pipe across the top (counter sunk into the brick), at 1 foot in from each side, so that the top is supported on one foot centers (horizontally: earth, then 3" gap to brick wall, then 12" gap to pipe, then 12" gap to pipe, then 12" gap to brick wall, then 3" gap to earth). I could pour a 4" slab over the hole, but then it would be nearly impossible to lift, should the need arise. QUESTION: If I fill 5' long 1.25" pvc pipe with quikrete and a rebar, and then covered the top with them (placed tightly), then covered the top with the 3" of crush&run, would that be strong enough to support a heavy truck (UPS, Dump truck, etc.)? (This way, if I needed access, I could just dig up a few of the pipe.)

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  • $\begingroup$ i guess you are too far into this project, but laying a bunch of perforated pvc (perhaps connecting some much smaller dry wells) would give you the volume without the safety issue. If you are determined to follow the path you are on, i'd suggest you can brick up internal walls to reduce the span without losing much volume. $\endgroup$ – agentp Sep 20 '16 at 12:35
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A sketch would be helpful. But if my understanding of what you want to do is correct, you want to cover a 3 by 4 feet sump pit by light material which you could remove in case it is needed.
The pvc pipe filled with quickrete or even high strength concrete or any other mix of epoxy and cement is going to fail because it basically hardly can sopprt its own weight let alone dead and live load surcharge.

your brick walls are going to fails as well.
you need to contact the building department of your city and ask what are the codes and if they have a basic hand out which details what you need to do.

Usually they expect you to make a foundation on competent soil at the depth they specify, or or concrete pad and build the wall with adequate reinforcement and specified to take the traffic load which runs up to 1000 or more lbs/square foot.

The cover can be made of steel plate with stiffener beams under it.
The entire cover and its attachment to the walls and the pit have to be engineered.
Otherwise you are setting a trap for somebody to fall into and get seriously hurt!
Alternatively if there is enough space to build the pit outside of traffic pattern you can use lighter live loads, 100 lbs/sqr.ft.

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