Are there any rules of thumb or rough guides for how much static load a pile say 100mm dia and 1m (or varying) depths will support in clay soil?
I'm mentally planning the workshop and I would like to timber frame the floor, raised off the ground and supported on 4-6 piles (hence the question).
The main reason is I will be *very* close to drains (shared sewer) and hand dug piles will present the (albeit small) load below the depth of the drain - and more importantly - when the water co want to dig it all up and fix them, my workshop will not fall over.
Also, piles are not going to be much more work than a concrete or brick foundation wall.
Don't know. But saw sonotubes mentioned the other day on a beekeep forum (Alternative use) - am sure I saw some figures on one of the sites about them that gave dimensions vs load.
Piles can be driven or bored: on larger projects the ground will determine which is used - if it's a loose sandy soil overlaying gravel then boring won't work as the sides will fall in, so driving to a set is the norm. In clay IME boring is the norm.
Re the original OP's question, in my BCO days the rule of thumb (literally!) for firm clay (defined as thumb press will leave an impression but not sink in) was 1 ton/ft2, which for a 100mm bore would allow 86kg - make the bore a more practical (in terms of being able to get concrete down the bore) 150mm, and this increases to 193kg.
Sounds like I would need wider piles (as Phil L suggested for practical purposes) and maybe more.
I presume that what really matters here is the surface area at the load face - ie the bottom of the hole? Concrete is far far stronger in compression than the bearing load of a pile.
Would it make any difference to put 6" of concrete in the bottom of the hole with a small pile formed into that (using 100mm drain pipe) to save on quite a lot of concrete? Backfilling with earth.
eg
SSSSSSSSSSSSSSSSSSSSSS D D D D D D D D D D D D D D D D D D FFFFF FFFFF
D=100mm drain pipe pile F = wider footing, eg 1sq ft.
Assume a few bits of rebar are dropped in.
More practical for hole digging, practical in terms of concrete.
Yes, for short piles like this it is end bearing only [for the mega deep piles that hold up tower blocks, under-reaming - tapering out the bottom to give a large area - is common, and friction on the pile sides also increases the capacity].
You're not going to use that much concrete, but you're right: even weak concrete will be way stronger than the ground. In my old UK home I built a deck on mini-piles like this and lost quite a bit of clean rubble in them.
It might not.. quite a lot of the resistance is from friction at the sides of the pile. (Well it is on the ones that don't reach rock.) Some large buildings are built on sand by doing this and there wouldn't be any difference in the weight bearing capacity at the bottom than at the top if it weren't for the friction on the sides.
For anyone who has the time to read it, this is the report into the pile boring machine which drilled into a Network Rail tunnel, leaving auger sections across the track.
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"Summary During the morning of Friday 8 March 2013, a train driver reported that flood water was flowing from the roof of a railway tunnel north of Old Street station near central London. The driver of an out-of-service passenger train was asked to examine the tunnel at low speed and check for damage. The driver stopped short of the water flow and reported that two large drills (augers) had come through the tunnel wall and were fouling the line ahead of his train.
The augers were being used for boring piles from a construction site about 13 metres above the top of the tunnel. The operators of the piling rig involved were unaware that they were working above an operational railway tunnel. Its position was not shown on the site plan, or on any map available to either the developer or the local planning authority. As a consequence, Network Rail was not consulted during the planning application stage and was unaware of the construction activity.
The RAIB has determined that approximately half of the piles required for the new development would have intersected with the tunnel had they had been constructed. It has identified two learning points from this incident which are relevant to the construction industry: clients and design teams should be aware of the importance of information shown on land ownership records; and those carrying out investigations for proposed developments should be aware that not all railway tunnels are shown on Ordnance Survey mapping.
The RAIB has also made five recommendations: three are addressed to railway infrastructure managers, and relate to: the provision of information to organisations undertaking property-related searches; the provision of information on the location of railway tunnels and associated subterranean structures; and the identification of development work by third parties. One recommendation is made to the British Standards Institution relating to the enhancement of a British Standard, and one recommendation is addressed to the Department for Communities and Local Government relating to a recommendation made by the RAIB in
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