ground bearing slab

[patswfc]

We have completed a design of a hall for a school.
The finished floor level was approx 1.2m above existing ground level. Our design was for a 150 thk ground bearing slab sitting on 1.2m of compacted hardcore, rolled in 150 layers to the D.O.T. spec.! The slab has mesh reinforcement in the top face only.

I did not like the idea of a ground bearing slab on such a deep layer of fill material as there could/will be longterm settlement of the hardcore (no matter how well it is compacted) which will likely lead to problems with the slab(at best fill settles evenly which will cause the slab to drop away from internal finishes or at worst fill settles unevenly which causes the slab to crack as no bottom rebar), but the senior engineer insisted on using a ground bearing slab. I tried to reason with him and showed him clauses from the NHBC that states ground bearing slabs are not suitable on fill material greater than 600mm.
He told me that the NHBC is for houses and does not apply to us and that many other buildings are built of deep fill material.

Just wondering what your thoughts on the above are and if any of you have designed ground bearing slabs on top of such deep fill.

 

[csd72]

patswfc,

This is compacted to a specification made for trucks and will only have a bunch of children running around on it.

IMHO it is not going anywhere.

csd

 

[civilperson]

What alternatives do you propose instead of the fill? Over 90% of the building sites that I have seen are graded with fills placed at the location of the buildings. A proper controlled fill is called a structural fill and is more suitable for slabs than natural earth.

 

[BigH]

If you put the hall slab on an engineered fill - such as crushed stone road-base type of material (sorry, I don't recognize the term hardcore) and compact it to, say 95% modified Proctor maximum dry density, you will not have any substantial settlement (long term) with the fill - especially, for the low live loads you have. This, of course, is subject to the foundation onto which your fill is placed will not settle to any significant degree due to the 1.2 m of fill placed above it. (That is the worrisome part) I have been involved with many jobs where floor slabs, machinery and the like were placed on much thicker engineered fills. You mention the Bldg Code commentary - what does the building code know about your specific site? Absolutely nothing. You should never, in my opinion, take the building code as "gospel" over sound engineering principals/design/judgment that are developed for a particular site condition.

 

[valleyboy]

Hello Patswfc

See also Clause 8 of BS 8103 Part 1 1995 (Structural design of low rise buildings) that states that for fill dpeths over 600mm, suspended construction should be used.

Why not use PC planks or similar? Then, the perimeter walls do not have to be designed as retaining structures.

I have recently has one of our QS's carry out a cost exercise on a similar project, and the suspended construction proved much cheaper.

Also easier than trying to carry out a large earthworks operation through a UK summer !!

VB

 

[patswfc]

Thanks for the replies.

The buildings is being constructed at the moment, so its too late to change the design, i just wanted to get others opinions on our slab design.

I know that the compacted fill shouldnt have significant longterm settlement if compacted properly but how can we be sure that it will be. This isnt the engineers responsibility its the contractors, but if a problem occured with the slab in the future, I would suspect the finger of blame to be pointed at the engineer for placing a slab on such deep fill. In my opinion the slab will more than likely be ok, but if something does happen we will get blamed for designing a slab on deep fill. We have created a situation were there could be a problem in the future.

Valleyboy, I agree that a suspeneded slab of pc planks would have been an option. This is what I wanted to use. As for the British summer, whats that?

 

[Ussuri]

Valleyboy,

Correct me if I am wrong, but is BS8103 not the code for housing and small single storey buildings, would it be applicable for a school hall?

BigH,

Hardcore is a term used to describe any material used in limited depths to make up ground. I assume from the OP post they are using a DOT sub base material (Type 1) which is a graded granular material and may comprise crushed rock, crushed slag, crushed concrete, recycled aggregate or non plastic shale. Hardcore however, may also be quarry waste, chalk, brick and tile rubble, colliery spoil, oil shale, PFA or blast furnace slag.

There is a BRE publication Digest 276 which covers hardcore, its uses, and problems such as swelling and consolidation. Its pretty brief only four pages but it offers other references.

Incidentally it also mentions the NHBC 600mm limit as differential consolidation this was found to be the most frequent structural defect in new houses.

I think this comes about due to the fact that just about anything seems to be permissable as Hardcore so it would prove difficult to get a uniform standard when the materials used have such different properties.

I have seen entire sites made up (to raise them out of the flood plain) using compacted type 1 sub base, and they seem to be ok.

 

[valleyboy]

Hello Ussuri

BS8103:1 is intended for housing and single storey structures but I think the requirements of the standard apply equally to other low-rise structures. (not sure from the OP whether this is a single storey structure or not)

I think the reason this limit exists (as you intimated in your post above regarding the limit in NHBC guidance) is that failure is common at fill depths in excess of 600mm.

 

[Lion06]

patswfc-
Doesn't your spec call for special inspections regarding compaction of fill? This is how you verify the fill is compacted to 95% modified proctor.

 

[archeng59]

all you can do is specify the properly compacted low volume change fill to be placed, require that the compaction tests be made, and trust the contractor to do his job.

 

[csd72]

I would normally call up cement stabilised sand in this situation (it is sand with 5 to 7% cement mixed in) this doesnt require compaction so it saves a lot of site costs and time.

Not sure about the availability in the UK (or the US for that matter) but we got it from one of the major cement suppliers in Australia (Boral I think).

StructuralEIT,

Special inspections is an American thing and they dont really exist as such in the UK. The DOT specification covers the type of fill and the compaction documentation required, I believe the responsibility lies with the contractor to comply.

csd