Dredging in Ports and Harbor Construction - Report Example

Abstract This report looks at application of dredging in ports and harbor construction. The methods discussed dredging: pontoon-mounted grabbing; pontoon-mounted (or shoreline) dragline; pontoon-mounted pumping; self-propelled trailing suction dredging; blasting and bucket dredging. The advantages and advantages associated by these methods have also been discussed Key word: Dredging, ports, harbours, technique Introduction Dredging is undertaken in ports and harbours where there the access channels, harbor docks and swinging basins are created or the existing ones are deepened, widened are enlarged (Jan De Nul Group, 2012). Enlargement of ports and harbours are of economic importance since the expanding world trade makes inevitable for ports and harbours to increase their capacity. This is addressed through deepening harbor basins and channels which are already in place or the facilities are just created altogether. The increase in need o transport raw materials such as coal, iron ore and oil and gases there is need for construction of specialized harbours (ABP Research, 1999). Making an accurate estimate of volumes that are to be dredged is important in assessment of dredging works. Having accurate estimates of volume of materials to be handled is important in choosing the appropriate dredging plant, in estimating production, time of execution and the cost of project. The materials that are to be dealt with during dredging need to be characterized. Characterization involve evaluation of physical, biological and chemical properties of the sediment so as to determine the appropriate dredging methods to be used, beneficial use that the materials can be put to and the disposal options available and likely environmental impact (Erftemeijer, 2006). Dredging methods Dredging may be suitable the following materials: cohesionless materials such as sand or granular materials which are loosely cemented together; clayey materials which are highly cohesive; silt mad originating from the inside of port areas and rocks of wide rage of hardness (USACE, 1983). The dredging techniques which may be put to use depending on deposit type are: pontoon-mounted grabbing; pontoon-mounted (or shoreline) dragline; pontoon-mounted pumping; self-propelled trailing suction dredging; Pontoon-mounted grabbing A pontoon –mounted grabbing may be a grab dredge which has self-propelled vessels with on board hoppers with one or more cranes being mounted on the deck. Another type is the pontoon dredger which has a barge-mounted crane which is propelled by a tug aided by hopper barges. The operations in a grab dredger involve lowering, closing and raising of a single grab by use a wire rope. Grab dredger advantages are: it is flexible in applications in maintenance activities in all marine sediments where a variety of grabs may be feasible may be used with desired efficiency to a depth of 35m beyond which there is impediment of free-fall effects of grab that lowers efficiency of grab it is very suitable when dredging is being undertaken along quays and in harbor basin corners. On the other hand the this technique has a shortcoming causing obstruction in see traffic when stationary in navigation channels in addition to leaving irregular bottom topography Dragline In this method an open steel bucket is dragged on the sea bed up to the point it will fill up. The bucket is then lifted up after being filled up and the load dumped in a barge or on the shore. The use of this method is predominant in North America (PIANC., 1977). The method has a shortcoming of much of the dredged materials being spilled resulting to high level of turbidity. Most of functions performed by dragline have been taken over by backhoes because of the ability to perform with much higher efficiency. Pontoon-mounted pumping This technique involves a large submersible pump being suspended from a pontoon and dunked into the ocean bed. The slurry which consists of about 11% of sand on weight basis is pumped through floating pipes. (Erftemeijer, 2006). Where slurries have extremely sand particles which are extremely fine , the discharge pipe velocity would be expected to be beyond 1.5m/s while where coarser sand are involved the speed should range between 3 and 4m/s (ABP Research,1999). The ability of handling larger outputs is advantage using pontoon pumping. The pumps used may also have cutter blades to take care of sea grass and winds (Erftemeijer, 2006). This method is incorporated with air lift is very suitable in dredging areas rich in archaeological remains. This method is associated with the following shortcomings the operation are interfered with where rough seas are involved as the pump is made to loose contact with the bed there is a narrow range of sediments that can be dredged efficiently the result of use of this method is an irregular sea bed obstruction is caused where the equipment is stationary in navigation channel Trailing suction This method find application where large volumes of sediment which are cohesionless. This dredger consists of a hopper vessel which has a trailing arm that is suspended at the side and is dragged at the sea bed. The vessel moves forward at a speed of about 6knots with a computerized hydraulic system being used to ensure swell and tidal variations is compensated for automatically. Powerful pumps are used to draw the mixture of water and sand passing them through a chain of decanters with solid deposition into internal hoppers which have a carrying capacity ranging from 2000 to beyond 25000m3 with the decanted seawater which is relatively clear being dumped overboard (Vellinga, 1984). If no intermediated pumps are used the maximum possible depth of dredging is around 25m while incorporating one or two intermediate pumps there can be increase in the depth to between 80 and 120m. There is full automation of the process with dredging running 24-hour non-stop (Vellinga, 1984). The clean dredged material may be dumped offshore while unclean material is pumped onshore through pipes which are laid ashore so as to be used in reclamation The advantages of this method are: there is little interference with operations of ports; exhibits versatility by being able to handle both cohesive and cohesionless sediments. The other advantage is that it is possible for dredged load being pumped ashore as reclamation and it also exhibit versatility in size depending on the size of the project. On the other hand the disadvantage of this method is t lack of precision in the final dredged depth thus calling for over dredging and the cost of mobilization may also be high. Blasting In situation where the areas being dealt with have thick layers of rock or there are isolated rocks out crops then blasting may be the best option. Blasting involves drilling holes into sea bed which are closely spaced; the holes are filled with explosives then fired. The drilling of the holes done from above water surface or directly at sea bed. The hydraulic power for operating the machines is supplied a power park located at the shore and it is connected either through a floating or a submerged line. Blasting involve engagement of specialist subcontractors. This method is regarded as being a quick method that can be used where the rocks are of small amount very hard rock (Vellinga, 1984). The current practice involves use of rock-cutter suction dredgers or bucket dredgers in the excavation of large quantities of hard rock (Jan De Nul Group, 2012). The disadvantages associated with blasting are: for removal of blast materials grabbing must accompany blasting it causes obstruction unless done at sea bed This results to indiscriminate loss of sea life May involve considerably high mobilization cost Bucket dredging The bucket dredger also referred to as ladder dredger is compost of endless bucket chain that scraps the sea bed as the dredger traverses the area being dredged by user of a series of anchor ropes. This method has highly been replaced with cutter-suction dredgers, but some are still in operation up to date. The maximum depth of dredging is about 25 metres with a possibility of 34metres where the largest dredgers of this type are considered with the buckets used have a capacity of 50 litres to 1000litres (ABP Research, 1999). Advantages associated with this method are The method is applicable in dredging a level bottom topography They are applicable in situation involving narrow and restricted areas There is versatility in dealing with a range of sediments The side loaded usually will be filled high solids-to-water ratio The disadvantages of the method are It will result into obstruction when stationary in navigation channel This method is interfered with in situations where the seas are rough Bucket dredging is associated with high noise level and thus they may not be allowed to work at night when the area is in urban setting. Conclusion From this paper it has been seen that dredging involves several techniques which can be chosen depending on various factors including the amount of work to be undertaken, the type of material to be dealt with; the need for environment conservation and location of the area as far as human habitation is concerned. Reference ABP Research, (1999). Good practice guidelines for ports and harbours operating within or near UK European marine sites. English Nature, UK Marine SACs Project, 20 pp. Adger, W.N. et al. (2005). Social-ecological resilience to coastal disasters. Science 309: 1036–1039. Erftemeijer, P.L.A. and R.R. Lewis III, 2006. Environmental impacts of dredging on seagrasses: a review. Marine Pollution Bulletin 52: 1553-1572. Jan De Nul Group (2012). Dredging and marine works. Australia PIANC. (1977). Final Report of the International Commission for the Study of the Environmental Effects of Dredging and Disposal of Dredged Materials. Annex to Bulletin No. 27. Brussels, Belgium. USACE, 1983. Dredging and dredged material disposal. Engineering and Design. U.S. Army Corps of Engineers, Engineer Manual EM 1110-2-5025.. Vellinga T. (1984). Dredging and disposal of contaminated dredged material. Dredging and Port Construction, Vol. XI, No 12. Read More