Soil reclamation (Land reclamation)
Soil reclamation is the process of improving lands to make them suitable for a more intensive use. Reclamation efforts may be concerned with the improvement of rainfall-deficient areas by irrigation, the removal of detrimental constituents from salty or alkali lands, the diking and draining of tidal marshes, the smoothing and revegetation of strip-mine spoil areas, and similar activities.
The dry method is suitable for filling material from land sources, especially rock, hillcut and clay fill. Filling or transporting clay fill material into the sea would create viscous slurry which would take much longer to become usable land. As explained earlier, the dry method usually uses a truck or conveyor belt to transport fill material to extend the land towards the sea.
Generally, the dry method works well for foreshore locations with underlying competent seabed soil. If the seabed soil is weak, a mud wave will be created in front of the fill because of displacement. In that case, a greater quantity of fill material would be required. In addition, the dry method usually results in a loose profile of fill especially when granular soil is used as fill material.
A comparison of the density profile of granular fill carried out by hydraulic filling and land filling is shown in Figure 6.1. It can be seen that the density profile of landfill is much lower than hydraulic fill. Therefore, landfill generally requires densification of granular soil.
A wet method of reclamation is implemented when fill material is obtained from an offshore borrow source. However, this method is only suitable for granular fill, which has good drainage characteristics. As explained earlier, the method of filling is selected based on the availability of equipment, type of seabed soil, topography of seabed, and the production rate required.
A direct dumping method is used when the seabed is deep or the underlying seabed soil is soft. A bottom-opening barge usually carries fill material from the borrow source and either sails with a self-propeller or pushed by the powerful tugboat to the designated location. At the location, fill material is dumped by opening the bottom of the barge. Sufficient draft and clearance is required for this method. Generally, a seabed of 6 – 8 meters depth is suitable for bottom dumping. This method is used not only for granular material but also for stiff clay and soft clay. However, dumping of soft clay is not appropriate for deeper seabed conditions since soft clay can be dispersed, and the environment can be affected. Bottom-opening barges usually have a capacity of a few thousand cubic meters and the production rate of reclamation using bottom-opening barges is largely dependent upon the number of barges used and the distance between the borrow sources and the reclaimed area. The dumping location is generally controlled by a global positioning system. However, bottom dumping alone cannot complete the reclamation because it can only operate up to 2 – 3 meters depth below sea level. The next level of fill has to be raised by hydraulic filling or other means.
Rehandling from a rehandling pit
Sometimes, if cutter suction hopper trailers are not available or direct dumping is not feasible, a rehandling method is used. The rehandling method involves transporting sand by barges and dumping the fill material temporarily in the pit for storage. The pit should have a storage capacity of a few million cubic meters. Rehandling pit locations are generally selected at natural depressions on a firm seabed or created by dredging. To create a rehandling pit, one needs to consider the stability of the pit slope. Such an operation would require two stationary cutter suction dredgers, one at the borrow source and another at the rehandling pit. In that case, sand barges are required to transport sand to the rehandling pit. Alternatively, one cutter suction hopper dredger dredges the sand at the borrow source and transports it to the rehandling pit, while another stationary cutter suction dredger will operate at the rehandling pit to fill the reclamation area.
The hydraulic filling method is suitable for granular fill. Generally, this method is used when filling is carried out from an offshore source, either from a rehandling pit, as explained earlier, or from a trailer suction hopper dredger. In the case of pumping from a cutter suction hopper dredger, the fill material is dredged from the borrow source with its own trailer suction dredger which is moved adjacent to the reclamation area and then pumped through the discharge pipe. Bulldozers are used to grade and spread the fill material around the discharge pipe. The discharge pipe is usually set slightly above the required finished level. Pumping is usually done with a mixture of fill material and water. The ratio of fill material to water is adjusted according to the grain size of the fill material. A large ratio of material to water would lead to wearing of the inner walls of the sand transportation pipe. On the other hand, a smaller ratio of material to water will reduce the production rate. After a certain amount of land has formed, the pipes are extended accordingly. Usually, the diameter of the sand transportation pipes is about 800 – 1000 m and 10 meters in length. Normally, wearing occurs at the bottom of the pipe, therefore, frequent rotation of the pipe after usage is necessary. Pipes that have to run above water can be floated with floaters attached to the pipes.
Sand spreading is implemented when a shallow seabed is encountered or when the seabed soil is too soft. When sand spreading is carried out, a rehandling pit is generally necessary. The spreader is mounted on a small floating barge. The end of the discharge pipe is usually closed and several perforations are provided along the last two to three sections of the discharge pipes. Sand is discharged through the perforations with water.
Pumping inside the bunds
Reclamation can start from the coastal line and advance towards the sea. However, this type of reclamation may lead to great loss of fill material because of wave and current action. Therefore, sometimes reclamation is carried out within a protected area after a bund has been formed around the proposed reclamation area. In this way, losses caused by wave and current action can be minimized. However, this type of reclamation requires an outlet for the overflow of water and fine material, otherwise mud can be trapped at or near the corner of the bund.