total friction force along the length of the pipeline becomes too large, 
the flow is stopped. However, flow can be started again by increasing 
the driving force--that is by pushing the concrete by a higher pressure. 
This is in marked contrast to blockage due to arching which, as described 
above, is not overcome by additional driving force, and in fact gets 
worse. 
For the proposed application the frictional resistance will be used 
to advantage for limiting terminal flow velocity. The concrete falling 
in the long vertical pipe is driven primarily by gravity. Additional 
force is provided by the pump pressure which will be maintained at a 
positive value. The force due to pump pressure, added to the force due 
to gravity, can be varied and will be used as a control method. If 
velocity becomes too great, the pump pressure can be reduced (but still 
positive); if velocity becomes too low, the pump pressure can be increased. 
Prevention of concrete setting (hardening) in the pipeline is an 
important operational consideration but is not considered to be a serious 
technical problem. This type of blockage is avoided by limiting the 
length of time between mixing the concrete and placing it. Set retarder 
admixtures are available for delaying the initial set for up to several 
hours, if needed. If the operational delay occurs after the concrete is 
in the pipe, the pipeline should be pumped out before initial set of the 
concrete occurs, and the job re-started when fresh concrete is again 
available. 
Blockage due to build-up of mortar on the pipe wall is also controlled 
by operational procedures; it can be prevented by cleaning with a "go- 
devil" periodically, say once per hour. Build-up may be more of a 
problem with grouts, slurries and mortars than with concrete in plug 
flow. However, its possibility should be planned for and expendable 
cleaning devices provided. 
Concrete Deposited Underwater 
For the proposed application the major characteristics of interest 
of the concrete placed at the seafloor are: (1) the flow behavior of 
fresh concrete underwater, particularly the mounding behavior of the 
concrete placed without forms, (2) the capability of the concrete to 
form a continuous coherent consolidated mass without layers of water, 
laitance, etc., and (3) the capability of concrete to mature into a good 
quality structural grade material in terms of strength, density and 
durability. 
The underwater flow behavior includes characteristics such as the 
distance of lateral flow, the steepness of advancing front, and the 
residual surface slope of the concrete after it has stopped flowing. 
For concrete placed without forms it is desired that the concrete be 
able to form a mound of concrete with surface slopes of about 1 vertical 
to 2 horizontal or steeper. For concrete placed in forms and with 
reinforcing steel the concrete should flow into the forms with a minimum 
of trapped water in corners, and develop adequate bond to the reinforcing 
Seeewe 
20 
