I used the caisson technique to construct a base for a 60 foot
self-supporting tilt-over telescopic mast.
Couldn't dig a deep hole here due to high water table in estuarine sands,
quite firm but any excavation below 2 foot would need well-pointing to
prevent collapse.
I purchased an 8 foot length of steel spiral welded pipe, 30 inch internal
diameter and 1/4 inch wall thickness, from a nearby pipe works. The casing,
manufactured for a large water supply main, was concrete lined for corrosion
protection, and had been rejected due to failing a test. Broke out the
concrete (my ears are still ringing) and ground a bevelled cutting edge on
one end. The pipe was then stood on end and two stacks of timber sleepers
(8" x 6" x 6' long) placed on each side of the steel casing. Two heavy
steel beams (10"x10" H piles) were installed on top of the timber stacks
straddling the casing and about 8 ton of deadweight (called kentledge and
comprised of concrete crib wall blocks) stacked on the steel beams. A fifty
ton portable hydraulic jack was positioned between the steel beams and the
casing and the casing was slowly jacked in stages into the ground against
the weight of the beams and kentledge. The ground inside the casing was
progressively removed scooping with a gallon tin attached to a light steel
pipe (all this inside a 30" dia casing), and layers of timber were
progressively removed from beneath the steel beams to lower the kentledge
and maintain only a small gap between the beams and the casing where the
hydraulic jack was placed. Initially the ground cut like cheese but lower
down I struck tree roots which took some time to chisel through and which
caused the cylinder to take a 1 in 15 tilt (of negligible effect). Jacking
against the roots lifted the beams and kentledge off the timber stacks.
Eventually jacked the casing entirely into ground, cleanly excavated, and
with undisturbed ground at the foot of the casing but, of course, containing
a 6 foot depth of water. The casing could not be pumped dry, to have
attempted this would likely have caused the sand to heave upwards in the
bottom of the casing, and the ground could have become "quick" and likely to
displace the casing and potentially affect my house a few feet away.
I stood the base section of the 30 foot lower mast section (6"x6" steel
rolled hollow section) in the 8 foot deep hole (needed a small mobile crane
for this), centred and plumbed the mast and filled the casing with coarse
concrete aggregate. The mast was fully supported by the aggregate.
Next step - to concrete up the mast by injecting cement grout through a 3/4"
NB
pipe to the bottom of the casing. Hired a grout pump and mixer from the
local bridge piling contractors and injected a thick grout (cement only, no
sand) to displace the water upwards from the bottom of the casing,
eventually the grout permeated upwards through the aggregate to the surface.
After several days for grout to harden I cut out the top 6" or so of the
grout to remove the weak laitance and expose a roughened concrete surface.
Welded several steel bars between the steel casing and the steel mast for
earthing. Have still to cap the casing and form a concrete mowing strip
around the mast to finish off.
This was cheaper than forming up and constructing a reinforced concrete
spread footing, and the only heavy equipment used was the small mobile crane
for 45 minutes. Everything else done by manpower and a crowbar in leisurely
stages. I haven't finished the top mast sections.
Mike ZL1BNB
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