Chapter 6
Reclamation Methods

6.1 DRY METHOD
6.2 HYDRAULIC RECLAMATION METHOD
6.3 REHANDLING FROM A REHANDLING PIT
6.4 HYDRAULIC FILLING
6.5 SAND SPREADING
6.6 PUMPING INSIDE THE BUNDS

There are several methods of land reclamation, depending upon the type of fill material, foundation soil, topography of the seabed, the availability of equipment, and allowable fine material for reclamation.

6.1 DRY METHOD

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 (Figure 3.4 and 3.5).

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.

6.2 HYDRAULIC RECLAMATION METHOD

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.

6.2.1 Direct dumping

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. Figure 6.2 shows dumping of fill material by a bottom-opening barge.

6.3 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. Figure 6.3 shows dredging and loading of fill material at the borrow source, and Figure 6.4 shows transportation of fill materials by a deep draft cutter suction hopper dredger and dumped at the rehandling pit for storage. Figure 6.5 shows reclamation by hydraulic filling from the rehandling pit using a stationary cutter suction dredger. The production rate of such reclamation is dependent upon the stationary cutter suction dredgers and the number of barges used for transportation. Filling up to 2 million m³ per month is possible with this method of reclamation.

6.4 HYDRAULIC FILLING

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.

To carry out direct hydraulic filling from a trailer suction hopper dredger, sufficient draft of the seabed is needed near the reclamation area. Now as big as 33,000 m³ trailer suction hopper dredgers are available, and either dredging or unloading can be carried out within two hours. The sailing time is dependent upon the distance between the borrow source and the reclamation area. If the source is close to the reclamation area, many trips per day are possible. In such a situation, as much as 4.0 million m³ per month of production is possible with the trailer suction hopper dredger. Figure 6.6 shows a dredging operation with a cutter suction hopper dredger at the borrow source, and Figure 6.7 shows a hopper dredger sailing with a full load of fill material. Figure 6.8 shows a dredger pumping sand through a discharge pipe. Figure 6.9 shows the leveling of dumped sand fill with the help of a bulldozer. If the sand source is less than 5 km from the reclamation area neither a rehandling pit nor a cutter suction dredger is feasible. Direct pumping from the sand source to the reclamation area is possible. Pumping through a discharge pipe is possible up to 10 km. Some intermediate booster pumps may be added to pump over such a long distance. Long distance reclamation using intermediate booster pumps is shown in Figure 6.10. If the seabed is deeper or the location of the reclamation is far

away from the dredger location, rainbow pumping is implemented, as shown in Figure 6.11. Rainbow pumping is normally suitable for underwater filling. Hydraulic filling is not suitable when the seabed is too shallow or the seabed soil is too soft. In that case, a sand spreading method is applied.

6.5 SAND SPREADING

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. A sand spreader was used in the Changi East reclamation project, shown in Figure 6.12. Figure 6.13 shows sand spreading in progress. The details on land reclamation on slurry-like soil using sand spreading method can be found in Bo et al. (1998d). In the Pulau Tekong project in Singapore, TOA-Jan de Nul JV has fabricated a fully automatic purpose-built spraying pontoon which can discharge the sand at 11,000m³ per hour, in layers of 50 cm thickness (Figure 6.14). Since sand spreading is not stationary and moving from one end to another is required, moving the spreader is made it possible with a winch system and a heavy duty bulldozer. Sand deposits using a sand spreading method usually results in a loose profile (Figure 6.15).

6.6 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. Figure 6.16 shows reclamation carried out in Singapore after the formation of a bund. Figure 6.17 shows the discharge outlet provided for reclamation with a containment bund.