Benching or strip footing on a grade?

[Mjft82]

I've recently did a design for box culvert with an open base that had to protect and allow access to pipes under a road. The pipes travel at a grade of 1 in 8 down a hill. I've designed using a strip footing which has a retaining wall at the end with a key. I found the box culverts were stable with a friction factor of 0.3 which is concervative for concrete on concrete or on the rock with has an allowable gbp of 400kpa. The fill varies across the road from 300mm to 3m My question is at what grade would people change the design from a strip footing down a slope to benching at various levels? I was thinking that 1:8 is the limit based on safety factor and friction.

 

[oldestguy]

I would think the controlling factor would be how will this be formed up. Working on an incline, it would seem that paralleling the forms with the main body of the box with the footings would be easiest to build. That should control.

 

[Mjft82]

Thanks for the reply. That is the method I went for. The ground was cleaned and blinding poured. The strip footing were made oversized in depth. The box culverts I should have added were precast. The backfill of the footing was stabilized sand inside the culvert due to difficulty compacting and the outside was road fill. Then locked in with a retaining wall at the end of the strip footing with a large key.

I went for construct ability of a single long strip footing rather than having benches every 2.4m or 4.8m along the slope.

It was just something I've never seen much off, only seen in mine portal entrances at this grade or storm water drainage which has a shallower angle due to water control.