ERDC TN-DOER-T6
September 2004
The operations manual and the accuracy of measurements is affected by human influence.
This manual operation is not conducive to automated monitoring systems like the Silent
Inspector.
As the hopper load's consistency (resistance to deformation) decreases and becomes more
"fluid" when dredging finer-grained material, the bearing strength can also decrease. When a
sounding lead is applied in these loads, it starts to sink down into the material and greater
measurement error is incurred because the solids above the lead are not accounted for. One
effort to account for the mass of solids above the sounding lead is described in the following
procedure describing hopper measurement from the USACE document
The Hopper Dredge, Its History, Development, and Operation, 1954, (also known
as "The Red Book"). The amount of settled solids in the hopper(s) is measured
by sounding after the pumping has stopped. A weighted disk attached to the end
of a light line is used for the purpose. The standard disk is 6 inches in diameter,
weighs 2 pounds 2 ounces and is assumed to rest at the top level of settled solids.
Two or more soundings are measured in each hopper and the solid content thereof
in cubic yards is read from a hopper capacity curve or from yardage tables
prepared for the dredge. Simultaneously with the soundings, the mixture above
the plane of settled solids is sampled. A special rig developed for this purpose,
having a 1-quart bottle fitted with a stopper operated by an extension rod, is
lowered to a point halfway between the top of each hopper load and the level of
the settled solids. All samples thus obtained are thoroughly mixed to produce an
average sample representing the percentage of material solids in the load. The
yardage of solids in suspension in the load is computed by multiplying the hopper
content less the settled portion by the average percentage of solids in suspension.
The total volume of solids in the load is considered to be the sum of the volumes
of settled solids and solids in suspension determined by the above procedure.
The Dutch Ministry of Transport and Public Works (Rijkswateraat) produced a sounding
technique called the "half-ball and centrifuge" method to measure volumetric "payable
quantities" (Rokosch 1989). The weight and volume (shaped like a hemisphere) of the sounding
lead was designed to stop sinking at a level where the slurry density was 1,200 kg/m3 (specific
gravity of 1.2) or greater (called the settled solids). At least four soundings were taken and
averaged together to define the 1.2 specific gravity horizon. The volume of settled solids was
then determined from the hopper capacity chart. The volume of solids above this horizon (called
the liquefied load) was calculated by retrieving slurry samples midway between the settled solids
level and the surface of the slurry in the hopper, centrifuging them for a prescribed duration, and
measuring the volume fraction of solids of the total sample. The volume of solids in the
liquefied load was calculated by multiplying this volume fraction times the total liquefied load
volume. The total volume of solids in the hopper was then calculated by adding the two solids'
volumes together (settled and liquefied).
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