Sourcing of Reclamation Material
Sourcing of Reclamation Material
In land reclamation, fill materials usually consist of earth. There is a wide range of earth materials, from clay to rock. All these materials can be used for reclamation. However, depending upon the type of material used, transportation and filling methods vary. The quality of the land also varies depending on the material used.
All types of earth material can be used for land reclamation. The various types of earth material available include:
iii). Hill cut
v). Boulders, cobbles and gravel
3.1.1 Clay as reclamation fill
Clay is not really suitable for use in land reclamation for several reasons:
i). It is difficult to handle;
ii). Permeability is low and hence there will be drainage problems during reclamation as well as after reclamation;
iii). Few transportation methods are feasible;
iv). It has low bearing capacity; and
v). It takes a long time to settle.
However, clayey soil can be used for reclaiming below water level and can be treated with foundation soil. Several examples of reclamation with clayey soil can be found in Japan. Another way is to dump clay alternately with good permeable material to form a sandwich. With this, better quality land can be achieved with less future problems. An example
of such reclaimed land can be found in Singapore (Lee et al. 1990).
Clayey soils are usually excavated by scooping, crapping, grabbing, or cutting. Land excavation is generally carried out with excavators whereas marine excavation can be done by scooping or grabbing. Transportation by truck over land or by hopper barges and dumping the fill offshore are possible means.
Hydraulic filling is not really suitable. However, several reclamations have been carried out through hydraulic filling using clayey material. In this method, clayey soil is usually deposited as slurry and it is deemed necessary to wait until the sedimentation process is completed. Although the sedimentation process can be accelerated by adding chemical additives, this will raise the reclamation cost significantly. In addition, self-weight consolidation will occur because of the large compressibility of slurry-like soil. In order to accelerate the self-weight consolidation and draining of water, proper drainage is required.
3.1.2 Sand as reclamation fill
Sand is the best material to be used as reclamation fill for the following reasons:
i). It is easy to handle.
ii). Drainage is good and hence there is no drainage problem during and after reclamation.
iiii). Extraction and transportation are easier and hydraulic filling is feasible.
iv). The reclaimed land has higher bearing capacity.
v). There is no long-term consolidation and settlement of fill.
vi). A method is available for densifying the thick profile of sand after filling.
However, quality control of granular fill material is still necessary. The grain size distribution of imported sand has to be controlled and monitored. Although well graded sand is preferable as reclaimed fill, it is more costly to pump such sand through the discharging pipe. Wearing of the inner surface of the discharge pipe will be more significant with coarse and well graded sand. Fine sand is easier to pump hydraulically through the discharge pipe and less vulnerable to the pumping and discharging process. In addition to that, the granular material should not contain more than 10% of unsuitable material such as clay, peat, plant, or other fine materials. Unsuitable material of more than 10% will lead to difficulty in densification of granular soil. Fill material should not include a large quantity
of shell as this will lead to immediate settlement upon the static and dynamic load. Therefore, the percentage of shell content should be limited to less than 10%. The typical grain size distribution of granular fill material used for reclamation in Singapore is shown in Figure 3.1. Figure 3.2 shows the increase in immediate settlement owing to the increase in percentage of shell in the granular soil. It can be seen that more than 2% strain can occur under 200 kPa static load of sand with 20% shell.
3.1.3 Hill cut as reclamation fill
Hill cut is better than clay material. However, hill cut usually has poor drainage. Hydraulic filling is not possible, and it has to be transported either by truck or conveyer belt. Therefore, the filling operation is slow. If well compacted, it could also provide good load bearing ground. Future settlement from hill cut formation is very dependent upon the compressibility of the hill cut, but it is usually not significant. Reclaimed land using hill cut material can be found in the Marina Bay area in Singapore.
3.1.4 Rock as reclamation fill
Rock is also a good reclamation material. It will provide good load bearing foundation. However, there is a limitation to the handling of rock. Only transportation by truck or conveyor belt is possible. In general, rocks that are bigger than 2 – 3m in size are not suitable for reclamation and such large stones need to be broken down to smaller pieces. The occurrence of hollows during filling of rock will cause future settlement. Rock fills are usually difficult to densify although dynamic compaction can be applied to compact such fill. After the formation of land, it will be difficult for the construction of a basement, or raft foundation, but also for driving piles through the rock fill. An example of reclaimed land which used rock can be found in Hong Kong (Tsing Yi site) (Spaulding and Zanier 1997). Therefore, among the earth materials, granular sand is the best material for reclamation.
3.1.5 Boulders, cobbles and granular materials
Cobbles and gravel can provide good load bearing formation after filling. However, transportation by hydraulic filling becomes difficult and severe wearing of the discharge pipe will occur. Dynamic compaction has limited effect on cobbles and gravels. Vibroflotation methods need to be applied for this type of soil. Boulder clay is not difficult to handle with land equipment. Excavation with excavators is the best means of handling boulders. This type of soil requires filling lift by lift and compaction with either a vibratory roller or tamping. Hydraulic filling is not suitable, and if the material is from an offshore source, grab dredging is the most suitable method.
Earth materials are abundant throughout the earth. Thus, exploration is not a difficult task unless a particular type of earth is required. Clay fill is readily available everywhere. Hill cuts are easy to find when the topography and geomorphology of the area is available. Rocks are also easily explorable if the geology of the area is available and rock exposures are visible. Sand deposits are usually found at a river mouth or sandbar along the river and stream. It may be necessary to have a knowledge of the geology of the area. Sometimes, site investigation boreholes or a geophysical survey needs to be carried out if sand or rock sources are overlain by an overburden. However, as explained earlier, the transportation method may be limited if the borrow source is far from the land. If the intended material is granular fill, the most economical source is an offshore sand deposit. In order to explore the offshore source, geology and geomorphology of the area are deemed necessary. The preliminary exploration can take the form of a geophysical survey, such as a seismic reflection or refraction survey, and details of the exploration could be followed by the cone penetration test, or vibrocoring. From this detailed exploration, the quantity and quality of sand can be assessed. Site investigation at the borrow source is described in Chapter 4.
The method of extraction is dependent upon the type of materials and the nature of the source. The only available means of extracting land sources are excavation and blasting. Excavation is possible for clay or sand sources and hill cuts, whereas blasting is necessary for a rock source. Conventional excavators are good enough for excavating clay, sand, or hill cut. Figure 3.3 shows the excavation of a hill cut in progress.
When blasting is involved, it may be necessary to study the number of joints and the joint patterns. Suitable grid pattern of the drill hole for blasting can be arranged in order to obtain the required size of rock.
For land sources, two types of transportation are feasible. One is transportation by truck. Generally, a truck can carry a volume of 6 to 8 cubic meters per trip. Another form of transport for a land source is the conveyor belt system. A conveyor belt can transport the fill material continuously and unload at the discharge point. Rehandling is required at the discharge point. Therefore, the use of an excavator is still required at the borrow and rehandling sources. Figure 3.4 shows the transporting of
fill material using trucks, and Figure 3.5 shows the transporting of fill material using a conveyor belt system.
If trucks or a conveyor belt is used, reclamation is usually carried out by a dry method. In other words, reclamation is advanced from the coastal side towards the sea. No marine equipment is necessary. Sometimes fill material from land sources may be transported either by trucks or conveyor belts, or loaded onto barges. Thus, reclamation can be carried out from the seaward side by direct dumping. Figure 3.6 shows loading of fill material from land borrow sources on flat-bottom barge. Details of marine transportation vessels will be described in Chapter 5.
If the borrow source is offshore, extraction has to be carried out by the dredging method. For clayey soil, either a bucket or a grab dredger is used whereas cutter suction dredgers are used for extracting marine sand. Materials are usually transported with the help of barges. The various types of dredging and transportation vessels are widely discussed in Chapter 5.