One Story Electrical Room on Poor Ground

[interstructeng]

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.

 

[dik]

If you can tolerate movement and maybe a little cracking, then a stiffened slab foundation... with a 5" or 6" slab and a 12x16 or 18 deep edge stiffner. I've used them in pretty poor soil. It's best to use a stiffened slab than a solid one to minimise the added dead load. Also, the building footprint should be relatively small. If you cannot tolerate movement, they you might be looking at a perimeter grade beam and friction piles or something of that ilk... The building envelope should be flexible, and not rigid.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

I could thicken the whole slab to 300mm maybe as a raft?

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).

 

[interstructeng]

@dik thank you for the reply. Reducing the dead load is definitely something for me to consider - I suppose I'm not too concerned from the slab loads as they are so spread out. But the wall loading is coming down in a more concentrated way. Some settlement wouldn't be a huge deal - I might get them to cast it a bit higher than existing ground level to allow for some settlement (and to keep out storm water). The building envelope is pretty much set as masonry walls and a concrete roof due to fire requirements and contractor. If the whole structure settles downwards as one then I suppose it wouldn't cause much cracking. Or if there is some way of constructing a masonry wall in a more flexible way (mortar mix maybe)...

@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

 

[dik]

I would not be using masonry... best to use wrinkletin. How large is the enclosure?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[interstructeng]

It's around 11 x 6 metres. Needs to be properly fire rated (4 hrs) hence the blockwork

 

[SlideRuleEra]

I'm purely looking for a solution that works for the bad ground conditions without resorting to mini piles or similar drastic measures.

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.

 

[EireChch]

I would re assess your bearing capacity in clay. In reality your 25kPa undrained shear strength is probably very conservative.

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

 

[BigH]

I doubt that bearing is the issue but potential settlement. It would be nice to see a soil log for the site . . . I like the idea of removing a metre of the soil and backfilling with engineered fill. Given that the one metre is "fill", the engineered fill will probably weigh more than the material removed so will add to the applied pressure - why, then, not put on another metre of fill - perhaps the material that you dug out and put on as a small preload - monitor, remove and then use the sog as SRE has recommended.

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).

 

[interstructeng]

@slideruleera agreed - I'm trying to keep in as uncomplicated as possible. I'm used to soils with allowable bearing pressures of 150 kPa +. For a shed this size I wouldn't have been looking for a geotechnical report normally - but I have one so need to come up with something that will satisfy an engineering check. thanks for the suggestions

@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

 

[BigH]

Be careful about expanded polystyrene - It is used a lot but it does need to be covered as it is flammable. Perhaps Filcor is different or has handled that particular issue.

 

[interstructeng]

@BigH very good point, I'll follow up on that. Also if I am to use it I want to protect from any potential contamination later - I see they have a wrap product for this

 

[SlideRuleEra]

interstructeng - Poor soil is often accompanied by wet conditions. If the water table will ever be "high" (above bottom elevation of Filcor) the slab may need to be designed to resist uplift from polystyrene flotation. Depends on how much of the Filcor will be below the water table.