The Journal of Explosives Engineering - January/February 2015 - 11

located on the adjoining work bridge. As each pier was exposed, a sheet pile cofferdam was driven about 20 ft (6 m)
into the river bed around each of the concrete pillars. The
cofferdam sheets were about 40 ft (12.3 m) long with about
half that length exposed above water level. After the cofferdam was completed the top of the pier was about 3 ft (1
m) below the top of the sheet piles. The next step was for
the contractor to remove the gravel and river rock that was
enclosed within the cofferdam between the concrete and
steel wall. This was done with a clamshell bucket attached
to another of the cranes located on the work bridge. There
was some discussion as to how important it was to get this
packed material removed before the piers were shot. The
blasters recommended removal; the contractor did not think
it was important at this stage. A perfunctory effort was made
to remove this material from around the first pier with the
intention of seeing how the first shot went.
The specifications called for removal of the concrete and
rebar to a level that was 16 ft (5 m) or so below the river
bed. The original construction plans used to build the piers
indicated that the rebar in the concrete did not extend to the
bottom of the piers. The drill hole depth was calculated to
bottom 2 ft (0.6 m) below the bottom of the rebar in the
piers. The original piers were set on bedrock which meant
that the total height of each pier was about 40 ft (12.3 m).
The 35 ft (10.8 m) drill holes would leave 4 or 5 ft (1.5 m) of
unreinforced concrete in the bottom of each hole. By breaking

the concrete to this level it was anticipated that all the rebar
could be removed and that this would eliminate the need to
dive down and cut any rebar that was protruding from the
remaining unaffected concrete.
A volume calculation of different levels of the concrete
mass was made to determine the best distribution of the explosives charges (See figure 4). The energy level required near
the bottom of the piers, where there was more confinement
by river bed material and water, needed to be much higher
than the energy needed to break the unconfined material
nearer the top of the structure. The obvious goal was to break
all the concrete completely, but to not heave any portion of
the debris from the interior of the cofferdam. The blast designer also had to be aware that employing too much energy
carried the risk of rupturing the wall of the cofferdam, which
would negate all the other confinement efforts.
The energy level chosen used somewhat more than a
pound (0.5 kg) of explosives per cubic yard near the confined
base of the piers, grading to less than 0.5 pounds (0.23 kg)
per cubic yard nearer the top of the unconfined concrete. One
pound (0.45 kg) PETN cast boosters were chosen as the primary explosive because they exhibit very high shock impact
with minimum material heave characteristics. These properties seem to be close to optimum for breaking reinforced concrete away from the steel reinforcing bars while minimizing
the flyrock potential of the broken material. The boosters chosen for the job had two tunnels completely through the units

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January/February 2015

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The Journal of Explosives Engineering - January/February 2015

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