Basic site investigations for lightweight structures 2

[ChrisMcLean]

I work for a large conservatory company in the UK and am responsible for ascertaining ground conditions and, in conjunction with a structural engineer (useful fella) designing appropriate foundations.

As those of you how know the UK systems will appreciate, most of our structures are exempt from building regulations and so we do not get a great deal of guidance from buildong control on foundations. We have in the last couple of years radically changed our procedures to reduce the frankly staggering rate of base failures we were experiencing. We guarantee our bases and units for ten years and so have to think at least mid-term to ensure everything stays roughly where we leave it!

At the moment, we rely a lot on the existing house foundations for guidance, as well as SI and foundation design data from local building control or NHBC sources. This is fine for newer houses, but for older houses yields no information.

What I am looking to do is to improve our SI gathering - currently it largely consists of a builder sticking his head in a hole and reporting what he observes, which is useful but hardly scientific. We utilise strip, pad & beam, raft and pile type foundations (the latter normally only in proximity to large, thirsty trees). I have been looking into basic plasticity testing (as 75% or so of the UK appears to be clay soils), which a local lab will do fairly cheaply for me and some kind of test to ascertain the bearing strength of the ground.

I would like to either make use of hand shear-vane testing (which will give information down to ~1.5m) or possibly plate bearing tests (which I realise will only give information on the top metre or so). What I would like to ask is what useful information I could reasonably expect to obtain from vane testing and how easy it would be to apply the results of the testing to deciding what foundation to use. I am capable of working out the loads based on the unit size and roof material type.

Help would be appreciated!

 

[BigH]

Just a thought - you seem to want to shy away from doing borings. Is this so? Too expensive which is why you want vanes? If you "know" the area to a degree (as to whether you have crustal clay over soft clay - or heaven forbids with peat deposits) - I might like to suggest digging a test pit about 1m deep and taking three or four split spoon samples "down the same hole". The SPTs' N of the latter samples might be affected, but you can get continuous samples, I would say to about 3m depth (I've done this using hand methods of dropping the 140 pounder hammer the 30 inches. (check out previous threads for discussions on suitability of using split spoon sample for rough estimate of Su for firm clays.) For strip footings of, say 600mm wide, this should be deep enough - but . . . this assumes you have good local knowledge of the strata to be encountered.
An initial thought to get the ball rolling!!

 

[ChrisMcLean]

Fundamentally, you are right. The issue is cost with boreholes - works out at about 500GBP per site which for typical 10000GBP order values doesn't stack up. Vane testing is something we could do ourselves relatively quickly and cheaply but obviously I need to satsify myself that I can get what I need from these tests. I am a postgraduate chemist by trade but I am the best we have lol!

We do have problems with peat in some areas, particularly East Anglia (Cambridgeshire primarily) which normally means we end up using a piled footing. As yet, I have not found anyone happy to design a raft to acount for stratas of high organic content material - which is fair enough as I wouldn't myself. We do not like attaching rafts with movement joints as the range of movement is limited and it commits us to inspecting the unit every six months or so to check the weather sealing etc of the unit.

Sometimes we have good knowledge of ground conditions - other times very poor, particularly when the site has been subject to remediation works. Most new housing developments in the UK (where the bulk of our work is) appear to be 'brownfield' redevelopments, frequently of contaminated former industrial land.

 

[Mccoy]

Chris,
in Italy we have the same problem in areas where exploratory borings and field tests are not requested by building regulations. The additional overhead always drives the contractor nuts! Basically what we do is
1- assess what's the type of buildings, its loads and tolerances to settlements
2-assess the most likely stratigraphy and soil strenght, based on local knowledge, inference from close-by buildings, shallow excavations
3-assess potential legal consequences in lack of proper subsoil investigations and decide wheter to turn the job out (extreme but not unheard-of condition)

From the price range you outline you should be dealing with low-rise residential buildings, with usually contained pressures on the foundation soil. What we do in that case is to have a backhoe dig exploratory pits, test the soil by pocket penetrometers, vane tests (soil must be clayey or with undrained behaviour!), taking samples as BigH suggests, we do it by hand (ideally isolating a soil cube to be cut at the base, most frequently just taking big lumps of soil). The problem with this is that you don't get information on the deeper strata, a big potential problem in peat areas and other tricky grounds. some guys even fall back on a reduced-scale version of the dinamic penetrometer, hand operated, with internal combustion engine, cheaper than the standard version but yielding only qualitative data. If I were your employer and the soil is mainly soft with no hard layers or boulders I'd contemplate the purchase of a static penetrometer, rigged on a frame to be car-towed. If you build a lot, the initial investment would be soon recovered, with the added advantage of safe, quantitative foundation design and avoidance of reimbursment expenses, deriving from the 10-year guarantee.

 

[ChrisMcLean]

This appears to support my primary hypothesis, i.e. shear vane testing offers a relatively easy route to improving SI data collection.

Where we encounter peat soils, which as I have mentioned tends to be in the East of England (Cambridgeshire fens primarily) we have no option but to use the house foundations as a guideline - this of course has benefits in terms of avoiding issues with differential settlement. One of the advatanges we have is that, as you correctly point out, our structures are small single story types with very low foundation loadings (generallly, less than 30% of the wallframe comprises cavity brickwork and the roof structure is always glass/polycarbonate units in a reinforced uPVC frame structure.

Most of our sites do appear to comprise clay subsoil types - certainly, most of the problem ones do in any case! This obviously lends itself to shear vane and lab based plasticity tests, and the pocket penetrometer tests would look to be useful.

Anyone else have any other great ideas regarding easy site investigations?

 

[Grouser]

ChrisMcLean

Does the British Ministry of Agriculture have soil maps identifying native near-surface soil types? This may help to identify soil types and to catagorize properties as you acquire data.

Test pits or continuous sampling are a good idea especially in locations where fill soils are probable.

If you do a lot of these structures, you could purchase or lease a ground penetrating radar unit that would give you a "picture" of the subsurface without digging and replacement. This should show buried peat layers easily and should help to quantify the subsequent subsurface investigation (mentioned above) portion of the site surveilance.

Good Luck.

 

[adamu2]

Chris,

Interesting to view your thread. It actually prompted me to join the forum!

I trust that you are aware of the British Standard (BS5930) for the description of soils and are familiar with estimation of shear strength, relative density etc. It may be prudent to consider some degree of soils engineering training to your field staff, in order for them to relay onto you and your engineer some of the basic soil properties. I however assume that you do not subcontract the installation out to another company.

Although you only have a light structure I can think of many types of ground where I would want some degree of "comfort", particularly as you are guaranteeing the structural integrity of the "extensions". Proximity to trees, contamination issues, slope stability, ground gases, organic materials, soft layered materials would need to be examined and to possibly some depth beneath and around the proposed structure.

Incidentally do you guarantee against contamination, landslips etc????

In the first instance it is always worth considering contacting the local building inspector and environmental health officer to determine whether there are ground of contamination issues.

I suppose the bare minimum should be an inspection of the foundation excvations, with a proper field description, supplemented by shear vane tests (pocket penetrometers in my experience are not representative enough). For more "difficult" sites it may be necessary for a simple desk study, consultations, chemical and geotechnical testing and a more extensive probehole survey. There are some good small contractors with drilling kit that can be mobilised very economically. Alternatively hand held soil augering kits are available.

If you need any further help, just ask.

 

[PSlem]

Has anyone used dynamic probing in lieu of SPT? A search showed the speed to be great and only a small rig and computer monitoring required.

 

[ChrisMcLean]

Generally, we build in line with what would be expected for house type structures. This allows us the comfort you mention, despite the fact that the weight of our structure is almost negligible for a glass to ground unit in comparison with the weight of a conventional brick-built extension.

The only time we really use the weight of the structure in foundation design considerations is where we utilise raft type foundations which are obviously justified on the basis of loadings.

BS5390 is not one I have been using previously - primarily we use BS8004 (1986) and the NHBC guidelines for building in proximity to trees (which are crippling!). We generally find that between us, the local council (where planning is required, unfortunately increasingly commonly) and use of handy reference sites like homecheck we normally pick up issues with gas contamination and use a foundation scheme taken directly from BRE212 (Buildings on gas contaminated land).

Our guarantees do have limits - if we fail to pick up contamination and the customer is not able to provide details at the point of survey or installationand problems later arise I doubt we would be held liable - thus far, it hasn't been tested I'm glad to say! - though it is likely we would probably at least attempt remedial measures. We do normally consult in a limited form with the local building control and the NHBC (increasingly responsible for new developments) and occasionally with the environmental health bods if we identify or suspect a problem. Landslip is obviously a different matter. If the house foundation was designed with this in mind (for example a piled foundation or similar, piled retaining walls etc.) and we missed it we would probably end up liable. In theory, the research carried out by us at the point of survey should mean that we do pick up known problems. If we could prove that the foundations we provided were in fact 'fit for purpose' (the latest legal buzz term from our company solicitors) then the odds are that we would invite the customer to take the issue up with their insurer, as we have done recently over some exceptional hailstone damage to polycarbonate roofs in Eastbourne.

I concur that the biggest improvement could be gained from increased training to both our surveying and our installation staff; however, the biggest problem I have at the moment is persudaing my bosses that rather than being a £75 million p.a. conservatory company we are a £30 million p.a. building company and a £45 million p.a. conservatory company. Unitl the culture changes away from what is effectively a double glazing ethos it will be difficult to move much further on!

It's interesting to note that according to several loss adjusters I have dealt with in the last few years we are the only company who routinely deal with piling for conservatory foundations - most other companies turn down rebuilds requiring piling.

 

[dirtsqueezer]

Just a note about the bearing test your local lab was going to perform along with the atterbergs. This is the equipment my lab uses:

http://www.triggstechnologies.com/

We mostly work in residential and light building structures. Just to emphasize, this is the hard way. 40lb weight with up to 50 blows per hole! Gotta love internships. Anyway, a little more information to round out the discussion.