Footer near manhole 2

[PrimeCivil]

During excavation for construction of a new warehouse building, a deep sanitary manhole was discovered underneath existing asphalt pavement. The manhole is in conflict with a footing for the new building.

I'm looking at relocating the building but don't have much space to play with due to some high voltage electric lines on the opposite side of the building. The warehouse is a PEB which has already been purchased so I cannot adjust the structure of the building or column locations. The site is being built up about 5' with aggregate. It is an area with extremely sandy soils, pretty much all sand and rock, no clay at all (cohesionless). Depth to bedrock is unknown but is likely less than 10' down and there is no surcharge. The footer will be above the invert of the manhole (~15' down). I doubt I will be able to get the building more than 5' from the manhole. I'm concerned about soil pressure on the sides of the manhole and subsequent failure of the manhole wall. I have considered some kind of manhole reinforcement, either mechanical, or by thickening the walls, but think it may be unnecessary.

Does anyone have any experience with this sort of situation or suggestions?

 

[hokie66]

Footing, not footer. But this sounds like it requires a deeper type of footing, perhaps a bored pier.

It is not just the manhole location you should be concerned about. Any footings which would otherwise place loading on the sewer itself should also be founded at or below the invert of the pipe.

 

[PrimeCivil]

Agreed, fortunately there is no conflict with the sewer.

 

[geodad]

Need to look into calculating the lateral earth pressures acting on the manhole from the footing load. This will give you a better idea of how the building loads will effect the manhole. As hokie66 mentioned, you will probably need to lower the footing elevation near the manhole to reduce the lateral loading.

 

[darthsoilsguy2]

get a geotech egr to give you a recommendation. Hopefully you already had one anyway. The structure and the utility are an issue, but the old utility trenches in old yard areas are a dice roll when it comes to whether they were compacted to specified so its a safe bet to assume the entire trenchline is uncontrolled fill. a suggestion you will want to consider is over-excavating the perimeter on this edge and placing lean concrete into the trench on suitable bearing soils to lower the bearing pressure point at the perimeter below the influence zone on the piping to build the footing at original planned elevation. that work is so deep that lowering the actual footing would likely have a lot of cost compared to improving the soils. That approach could have lots of unintended consequences if done so without studying the site, drainage, foundation design, and the soils so again get a geotech. Another aspect that you should be considering is what is the plan if that old utility needs to be replaced. is there land to reroute the alignment with 2 new structures in a way that it could be abandoned in place without a deep dig next to your bldg?