One Story Electrical Room on Poor Ground
One Story Electrical Room on Poor Ground
(OP)
Hi, I'm working on a project involving the construction of a one story electrical room. The existing ground conditions are circa 1000mm of Made Ground (i.e. not suitable for supporting load) underlain by some clay that gets stiffer with depth. The Geotechnical report is giving me 25 kPa (so very low) capacity at the clay.
Structurally I'm looking at a 250mm RC slab designed to 7.5 kPa live load. So Dead Load circa 0.25*25 = 6.25, live load = 7.5 -> total SLS = 14 kPa so I am happy enough with that as a ground bearing slab.
I'm looking at a strip footing for the walls which is my concern. The max wall height is 4000mm (double leaf wall) with a concrete roof (no access) say 150mm thick. When I look at the line load from something like this it is very hard to get down to 25kPa with a strip footing.
I am going to get the made ground excavated out and replaced with compacted granular fill. Maybe I should be using a 45 degree spread through the granular fill (i.e. stone) in the calculation to spread it out a little more. I think I might be missing some consideration here - anyone have any thoughts?
It would seem strange to me to not be able to get a small structure like this to work on strip footings. I could thicken the whole slab to 300mm maybe as a raft? We don't have to consider frost heave heave due to the climate.
Structurally I'm looking at a 250mm RC slab designed to 7.5 kPa live load. So Dead Load circa 0.25*25 = 6.25, live load = 7.5 -> total SLS = 14 kPa so I am happy enough with that as a ground bearing slab.
I'm looking at a strip footing for the walls which is my concern. The max wall height is 4000mm (double leaf wall) with a concrete roof (no access) say 150mm thick. When I look at the line load from something like this it is very hard to get down to 25kPa with a strip footing.
I am going to get the made ground excavated out and replaced with compacted granular fill. Maybe I should be using a 45 degree spread through the granular fill (i.e. stone) in the calculation to spread it out a little more. I think I might be missing some consideration here - anyone have any thoughts?
It would seem strange to me to not be able to get a small structure like this to work on strip footings. I could thicken the whole slab to 300mm maybe as a raft? We don't have to consider frost heave heave due to the climate.
RE: One Story Electrical Room on Poor Ground
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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: One Story Electrical Room on Poor Ground
I would go with a 300mm raft without hesitation. The additional 50mm depth of concrete is very inexpensive... compared with 250mm thickness:
Cost of subgrade preparation is the same.
Cost of forming is slightly higher.
Cost of reinforcing and its' placement are slightly higher.
Cost of concrete placement is slightly higher.
Cost of concrete finishing is the same.
Cost of concrete curing is the same.
Only "substantial" additional cost is buying the extra concrete.
The RC 250mm slab plus strip footings appears to be labor intensive (excavate, backfill, compact, etc.) and when completed, is subject to differential settlement (strip footing vs. slab).
RE: One Story Electrical Room on Poor Ground
@slideruleear - thanks also. Agreed the cost implication is minimal to go to a 300mm raft style foundation. It wouldn't be an issue for this project - the contractor would probably appreciate the simplified formwork and reinforcement details. I'm purely looking for a solution that works for the bad ground conditions without resorting to mini piles or similar drastic measures.
I was also conisdering ground improvement techniques but I don't know how to quantify it or have any experience with that
RE: One Story Electrical Room on Poor Ground
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: One Story Electrical Room on Poor Ground
RE: One Story Electrical Room on Poor Ground
Well, I've had a career working in the South Carolina coastal plain with electric generating station foundations in bad ground conditions (edge of swamps)... not to mention high water table (assumed to be ground level), potential high wind loading (hurricanes) and historically high seismic (Richter 7.0+) - all to be dealt with on each project site.
The cost effective solution is simple... literally simple. That is, keep foundation design/construction uncomplicated. Complex work below grade in poor soil conditions is full of unknowns which can result in unintended consequences. For an 11 x 6 meter foundation I would excavate/backfill/compact an area of about 12 x 7 meters, perhaps to a depth of 1 meter. Since the site has poor soil, select backfill will probably have to purchased and brought in from offsite. Compaction is in layers, each a several centimeters thick. Then use a raft foundation.
Is this "cheap"? No... but it is cost effective compared to reasonable alternatives. Cutting corners or using complex designs to "save money" when working in poor soils is a guarantee for trouble... maybe not right away, but it will happen.
RE: One Story Electrical Room on Poor Ground
But assuming its correct, since your removing 1m of made ground, why done you remove 1.5m instead and backfill with light weight fill (Expanded polystyrene). That will remove approx 30kPa of load from the soil. Your net increase is likely only a couple of kPa. Should only see very small settlements then
Pu
RE: One Story Electrical Room on Poor Ground
I had one of these issues in the past - we actually recommended to excavate out the upper material and replace with Elastizel (a low density cellular concrete - unit weight about 30lbs/ft2 and satisfactory compressive strength).
RE: One Story Electrical Room on Poor Ground
@eireChch - I got the bearing capacity from another consultant - I think they are being conservative but I'm not hot on soil mechanics so don't feel too confident challenging it. I really like your idea - there is a product called Filcor used for this purpose. I think I'm going to go with this - and use the "unlocked" capacity to justify the wall pressure. am happy with that
@BigH - yes i don't think it is a strength issue either - just settlement. The preloading is a nice solution but it's a fast track project so I don't have time unfortunately. The elastizel looks like a similar solution to the Filcor - I think Filcor is more common here so I am going to proceed with that..
So I'm going to get them to excavate the made ground out, fill with the Filcor expanded polystyrene (or similar) and support an edge thickened slab off that. Feels neat to me. Thanks everyone
RE: One Story Electrical Room on Poor Ground
RE: One Story Electrical Room on Poor Ground
RE: One Story Electrical Room on Poor Ground