IV. Retreating Shorelines
251
The maximum dredging depth of the backhoe dredger is about 18 m (4- 69 fi).
The boom, the stick and bucket-capacity have to be adapted to the dredging
depth, the bucket-capacity decreasing with the dredging depth. Some backhoe
dredgers have a bucket-capacity of maximum 12 m 3 (224 fP) which drops to 6 m 3
(212 fi3) at the maximum depth.
3.1.7.1.5 Barge unloading dredger
This type of dredger raises mixtures of soil [sediment] and water from barges by
suction and delivers these through a shore pipeline on areas of land that are to be
raised to higher level, filled up or reclaimed. In such cases delivery may take
place in large quantities over great distances.
The suction tube with its flattened head is lowered so as to hang lengthwise in the
barge. The pump sucks a mixture of soil with water from the barge, delivering it
under pressure through a pipeline across the bank of the dumping site. Supplywater is added by means of the waterpump which injects one or two jets of water
into the barge through tubes with conical nozzles. The so-called shore-connection
between the standpipe of the sandpump and the first part of the shore pipeline
must allow the vertical movements of the ship and any small horizontal
movement. The barges with sloping hold-sides and closed bottom, which
transport the sediment to the dredger, are elevator barges.
3.1.7.2
Production limiting factors
The main characteristics of beach nourishment sand which are of utmost
importance to evaluate dredging productions and costs are :
-
grain size : essential data to estimate pumping production ;
relative density or angle of internal friction : to estimate the production of
dense sand which can be loosened by the cutterhead or the high pressure
water jets. These characteristics have to be estimated from the results of
Standard Penetration Tests or Dutch Cone Penetration Tests ;
-
angularity of the grain and carbonate content : to ealuate the wear of the
dredging equipment which is also dependent of the grain size.
3.1.7.2.1 Trailing suction hopper dredgers
The production is determined by the maximum hopper capacity of the dredger for
the soil type to be dredged and by the cycle time. The dredging cycle of a trailing
suction hopper dredger consists of loading, turning, turning, sailing and dumping
or reclaiming.
Dividing the hopper capacity by the total cycle time gives the hourly production
in hopper m s per hour. To determine the production in situ, a bulking factor has
to be taken into account.
251
The maximum dredging depth of the backhoe dredger is about 18 m (4- 69 fi).
The boom, the stick and bucket-capacity have to be adapted to the dredging
depth, the bucket-capacity decreasing with the dredging depth. Some backhoe
dredgers have a bucket-capacity of maximum 12 m 3 (224 fP) which drops to 6 m 3
(212 fi3) at the maximum depth.
3.1.7.1.5 Barge unloading dredger
This type of dredger raises mixtures of soil [sediment] and water from barges by
suction and delivers these through a shore pipeline on areas of land that are to be
raised to higher level, filled up or reclaimed. In such cases delivery may take
place in large quantities over great distances.
The suction tube with its flattened head is lowered so as to hang lengthwise in the
barge. The pump sucks a mixture of soil with water from the barge, delivering it
under pressure through a pipeline across the bank of the dumping site. Supplywater is added by means of the waterpump which injects one or two jets of water
into the barge through tubes with conical nozzles. The so-called shore-connection
between the standpipe of the sandpump and the first part of the shore pipeline
must allow the vertical movements of the ship and any small horizontal
movement. The barges with sloping hold-sides and closed bottom, which
transport the sediment to the dredger, are elevator barges.
3.1.7.2
Production limiting factors
The main characteristics of beach nourishment sand which are of utmost
importance to evaluate dredging productions and costs are :
-
grain size : essential data to estimate pumping production ;
relative density or angle of internal friction : to estimate the production of
dense sand which can be loosened by the cutterhead or the high pressure
water jets. These characteristics have to be estimated from the results of
Standard Penetration Tests or Dutch Cone Penetration Tests ;
-
angularity of the grain and carbonate content : to ealuate the wear of the
dredging equipment which is also dependent of the grain size.
3.1.7.2.1 Trailing suction hopper dredgers
The production is determined by the maximum hopper capacity of the dredger for
the soil type to be dredged and by the cycle time. The dredging cycle of a trailing
suction hopper dredger consists of loading, turning, turning, sailing and dumping
or reclaiming.
Dividing the hopper capacity by the total cycle time gives the hourly production
in hopper m s per hour. To determine the production in situ, a bulking factor has
to be taken into account.