PUMPING
CONCRETE UNDER WATER
Underwater pumping is not difficult, if you follow a few
basic rules. If you try and shortcut these guidelines you
will have trouble that could have been prevented.
Whether your pour is displacing seep water from a caisson
or a large load bearing structure for a bridge the most
important part of the job is communicating. Everyone involved
must know the ‘who what when and where’ of the
pour; to not plan the pour, then pour the plan is to fail.
Any time you are on a construction site it is dangerous.
When your site includes a body of water, you have added
one more dimension of risk. Always check with the appropriate
regulating authority as to the requirements of items such
as respirators, medical personnel, decompression chamber
availability, back up divers, safety boats, etc. If it is
a well planned job with all of the safety gear and procedures
in place an accident may still happen; that is when your
pre planning will pay off. Please do not short cut this
part of your preparation.
All successful pump jobs begin with a good ‘prime’
of the placing system. Since we are pumping under water
this prime will take on some additional requirements to
be successful, and stay within specification.
1. All placing line, which is below the surface, or susceptible
to the water’s pressure, must be grouted, “primed”
in a manner by which no water is in contact with the priming
material prior to the prime reaching the discharge end of
the pumping system.
This rule is very important for several reasons. The first
reason is that if the grout becomes too diluted it will
loose its ability to lubricate the interior of the placing
system. This will cause the system to become plugged; causing
extra work and wasted time. The second reason for this rule
is contamination of the water. The cement in the grout will
be suspended in the body of water. Many times this will
put a contractor and the local water authority at odds,
which is not a good place to be. The next reason is visibility.
If the divers can not see the work progress, or note any
special embedded items or details, the slowdown in production
could be the difference between a profitable day and a looser.
The correct procedure for grouting this water-filled line
is to introduce two (2) sponges of appropriate diameter
for the system used, to the pipe line, just above the level
of the water. When the pumping begins, the material should
be pumped at a slow enough rate so not as to force any grouting
material past the sponges. When done correctly, the water
will be displaced by the priming material without any contamination
or co-mingling of the two. After the priming process is
complete, the two sponges will float to the top of the water,
letting you know that you have been successful.
As with all pumping, the last hopper full of grouting material
should be mixed with the first of the concrete, creating
a gradual transition from one material to the other.
We are now pumping concrete under water.
2. While pumping concrete under water do not allow the
discharge end of the placing system to be raised above the
level of the concrete being placed.
We are probably placing concrete in this application because
of its strength. In order to maintain the integrity of what
ever it is that we are pumping we must keep the water out
of our concrete. This may sound a bit strange, in that we
are after all, pouring under water. Yes, it will get wet
on the outside. What we are protecting, and trying to further
is the strength of the body of concrete that is not on the
‘outside’. The discharge of the concrete, once
started, is only done inside the existing concrete. By keeping
the discharge end of the placing system IN the concrete;
we are keeping the water out. It is imperative that this
be done at all times. This is what makes the difference
between a structurally sound, correct pour and a pour that
has been contaminated, and the strength washed away by the
allowing of dilution. Once the absolute usurpation of the
allowable water – cement ratio has taken place; the
structure is no longer sound.
The height of the concrete within the pour, and the height
of the discharge end of the pumping system must both be
known quantities at all times during the pour. This may
be monitored by divers, cameras, story poles or what ever
means you have provided for in your pour planning. Do not
fail in this very important aspect of your pour.
3. Do not ever allow any pumping boom to become overloaded
at any time.
If you are using a boom pump or placing boom; land based
or barge mounted, pay close attention to the potential overload
conditions. When doing your calculations always anticipate
that the placing system, including any system hanging off
of the end of the boom to be FULL OF CONCRETE. With most
underwater pumping, a crane will be required to raise and
lower the extra system. Any concrete pumping boom that is
attached to additional system must be attached by a flexible
hose of sufficient length as to insure that it does not
assume the load. The easiest visual assurance of this is
to SEE a droop in the hose between the placing boom and
the additional system. The concrete pump operator, and the
crane operator must know, in advance, what the sequence
of events will be. This pre planning must include a plan
in the case of clutch failure of the crane. All personnel
in the area need to be made aware of that, and any other
emergency actions, that may be taken.
With proper planning, good safety program, and reliable
communications an under water pour can be as easy as a day
at the beach.
Written By Bob Sanderson
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