Mechanical dredging

These type of dredgers are so called stationary dredgers and they are either fixed in the dredging position by spuds or by an anchor system. 

A disadvantage is that with its wire suspended grab, a grab dredger cannot reach underneath structures, such as a jetty deck on piles, which to a certain extent a backhoe dredger can do. 

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Mechanical dredging. Mechanical dredging methods use mechanical excavation equipments such as backhoes, draglines, clamshells, and bucket ladder dredges. 

The trailing suction hopper dredge (Figure 4.4) also uses a combination of mechanical dredging (cutting) and suction power to loosen the material to be dredged. 

F igure 4.10 Configurations of mechanical dredging and transport to the fill area. 

In some cases, mechanical dredging of the soft material can be a more advantageous solution since the shear strength of the relocated clay material will start from its remoulded/residual shear strength, which is higher than the shear strength of hydraulically dredged and placed slurry. 

When large deposits with soft material have to be realised, a sandwich construction technique can be implemented with alternating clay and sand layers placed upon each other (in this case the clay is mechanically dredged). An example of this technique can be found in Muthusami et al. (2004). 

For mechanical dredging, the remoulded shear strength can be used as initial shear strength. 

Estimate shear strength based on remoulded shear strength for mechanical dredging. 

Permeable treatment beds Deep well system Extraction/injection well system Extract wells/seepage basin Excavation, transport and disposal Hydraulic dredging Mechanical dredging Drum handling Slurry wall construction... 

For protection of sheet-steel piling, the anodes are normally mounted within the re-entrants of the piles to prevent mechanical damage by berthing ships, dredging, etc. (Fig. 10.30). 

The resistant nature of dormant cysts makes them likely candidates for dispersal via dredging operations, shellfish transplants, or boat ballast as well. Each of these is theoretically possible, but there is no direct evidence that demonstrates conclusively that such mechanisms have operated in the past. 

In general, it may never be possible to prove that species dispersal is facilitated by dredging, shellfish transplants,or boat traffic. Here again we are faced with mechanisms that are theoretically possible but that may be of minor practical concern relative to the introduction of cysts to new areas through advective transport of established blooms. There is little doubt that this latter mechanism has been, and will continue to be, of major importance to the geographic distribution of the toxic Gonyaulax species. 

The in vitro bioassay for dioxins with cleaned sediment extracts (DR-CALUX) proved to comply with the QA/QC criteria needed to guarantee the reliability of data in an inter- and intralaboratory study (Besselink et al., 2004). The chemical stability of dioxins makes it possible to apply destructive clean-up procedures which remove all matrix factors. Sample extraction and cleanup for other in vitro bioassays for specific mechanisms of toxicity require further development to make sure that the chemicals of interest are not lost or unwanted chemicals included in the sediment extract to be tested. Table 4 summarizes possible bioassays that could be performed in addition to chemical analyses with the dredged sediment in a licensing system. 

Depletion of species in the whole envelope by the same mechanism that adds new and s-process elements is difficult. To compound the mystery, the oxygen isotope ratios in the SC stars are consistent with the dredge-up predictions, so any depletion mechanism should not operate in these stars. 

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