Geoscience Reference
In-Depth Information
The pump capacity is dependent on a number of variables including pipe diameter
and pump speed (stated previously), and also the length of the pressure pipe and the
pump resistance that must be overcome. What is ultimately determined using these
variables, is the required capacity of the pump to be used during tube filling.
If one assumes a sand concentration of 20% by volume (the normal sand/water
mixture ratio), then the sand quantities can be determined based on the critical pump
speed
v
cr
, pipe diameter and average sand grading, using Table 5.2.
The choice of equipment to be used must be matched with the scale of the project
and to the time available for construction of the project.
It should be noted that in the calculated sand production referred to above no
account is taken of the normal losses in the dredging process, namely:
changing barges under the barge-unloading suction dredger or sand pump;
adjustments to the cutter-suction dredger or suction dredger;
weather conditions (wind, wave, current);
changing geotextile tubes;
refilling geotextile tubes.
Experience shows that the efficiency factor, through these losses, may fall to 50%
of the work, which can increase the gross installation time to twice the net installa-
tion time.
5.2.3 Fill material
Geotextile tubes may be filled with silt or sand. In many cases the sand that is locally
available will be used. The properties of the sand (grain-size distribution and mixture
density) play a key role in the design process, such as in the choice of the geotextile.
If the sand has too many fines, then the geotextile tube will consolidate very slowly.
When a sand-water mixture is pumped into a geotextile tube, the sand will settle
and the water will flow out through the pores and the outlet ports that are used to fill
the tube with sand. The sedimentation rate determines how long it takes for the sand
to settle out of the sand-water mixture. When the velocity at which the sand-water
mixture is pumped through the tube is known, an estimate can be made of the dis-
tance required between the inlet and outlet ports. If this distance is too short, the sand
has not settled from the sand-water mixture and will flow out of the tube. In case this
distance is too long, the tube will only be partly filled.
A design formula is presented in [29] (for the filling of a hopper) for the sedimen-
tation rate of sand from a sand-water mixture:
c
4
(
1
wc
c
⋅
−
(5.2)
0
v
=
sed
1
nc
−
n
where:
sed
=
the sedimentation rate (the rate at which the sand bed rises) [m/s];
n
=
the porosity of the settled sand (fill material) [
−
];
w
0
=
fall velocity of a single grain [m/s];
c
=
the concentration of the sand in the sand-water mixture [m
3
/m
3
].
