Types of Coastal Protection
Types of Coastal Protection
Coastal protection is deemed necessary to protect the completed profile of the reclaimed land. Coastal protection prevents erosion caused by wave, current, tide and flooding by the open sea. On the other hand, coastal protection also protects leaching of reclaimed material from the reclaimed land, which could occur because of groundwater flow. Sometimes coastal protection is provided to divert the pattern of the current, which is affected by the newly reclaimed land.
Coastal protection is usually constructed after the formation of the reclaimed land. Although coastal protection prevents erosion by waves and current, the structure also serves an additional purpose: it can retain the soil behind the structure and also serve as a berth or jetty since the structure is usually a vertical wall. These structures are usually constructed before reclamation. A selection is made between using a shore protection rock bund and a retaining structure, depending upon the area available and its future usage.
Rip-rap is a single-layer shore protection structure, which protects the reclaimed land from erosion, wave, current, and tide actions, and leakage of material. It is usually constructed in a less dynamic environment with a shallow seabed. Rip-rap usually has a single stable slope of 1:3 to 1:7 and some graded stones are generally provided between the armor stones and the sand fill. Nowadays the thickness and layers of graded stones have been reduced and a geofabric layer is provided instead. The size of the armor stones is selected based on the expected force of the waves and currents. The typical profile of a rip-rap is shown in Figure 8.1, while Figure 8.2 shows a photograph of a completed rip-rap.
A retaining rock bund is usually provided where the seabed is deep and has more dynamic waves and currents. A more systematic layering of graded stones is required for a retaining rock bund. Larger armor stones are also required for protection against the greater dynamic forces expected in the open sea. Table 8.1 shows various sizes of graded stones and armor stones used for a retaining rock bund. Rocks used for shore protection works are generally granite or sandstone. Granite is preferred to sandstone. To control the quality of stones, some specifications are deemed necessary. Table 8.2 shows the specification of rocks for shore protection works used in the Changi East reclamation projects. Since a retaining rock bund is usually constructed for a deep seabed, sometimes several berms are required to stabilize the structure. Details on stability analysis of shore protection works will be discussed in Chapter 9.
Sometimes a sandkey is necessary to improve the stability of the shore
|Table 8.1 Type and size of stones.|
|A||1 500 to 10 000 kg|
|B||1 100 to 1 500 kg|
|C||2.2 to 1 100 kg|
|Crusher Run||3 kg|
|Table 8.2 Specifications of granite rocks used for shore protection works at Changi East reclamation project.|
|Weight loss after (5) cycles of tests by NaSO4 (%)||<12|
|Water Absorption (%)||<2|
|No fracture shall occur when dropped onto a steel plate from a height of (meters)||1.5|
|Table 8.3 For geofabric propex 4516 & 4557 or equivalent.|
|No.||Fabric Property||Propex 4516||Propex 4557|
|(a)||Thickness (ASTM D4632-91) (ASTM D4632-91)||4.90mm (min)||3.50mm (min)|
|(b)||Mean grab tensile strength:|
test in both directions (ASTM D4632-91)
|1560 N (min) in each direction||1200 N (min) in each direction|
|(c)||Mean grab extension at maximum load (ASTM D4632-91)||30% (min)|
|(d)||Mean trapezoidal tear strength (ASTM D4632-91)||580 N (min)||500 N (min)|
|(e)||Puncture resistance (ASTM D4833-88)||980 N (min)||850 N (min)|
|(f)||Drop test (400 kg rock dropped from 1.5m height onto the designed stone layer laid on top of the geofabric)||No puncturing of geofabric||No puncturing of geofabric|
|(g)||U V resistance||Geofabric shall retain 80% of minimum grab tensile strength after one year exposure to sunlight in Singapore||Geofabric shall retain 80% of minimum grab tensile strength after one year exposure to sunlight in Singapore|
|(h)||Equivalent opening size (ASTM D4751-87)||200 microns (max)||200 microns (max)|
|(i)||Permitivity||0.7 sec-1||1.1 sec-1|
|(j)||Weight (g/m2)||540 (min)||400 (min)|
protection structure as well as to minimize the settlement of the rock bund. Even when a sandkey is provided, the shore protection structure can still settle especially at the crest. Thus, such shore protection should be constructed with sufficient overheight during the construction stage.
The stability of the sandkey excavation should also be analyzed before carrying out the dredging works. As in rip-rap construction, geofabric is used for filter and separation. To control the quality of the geofabric, a standard specification for geotextile is necessary. Table 8.3 shows the specification of geofabric for shore protection works at Changi East in Singapore. Figure 8.3 shows the typical design of a shore protection structure usually constructed at a reclamation site. Sometimes man-made materials such as gybon and tetrapod are used for shore protection structures, as shown in Figures 8.4 and 8.5. Figure 8.6 shows various types of man-made shore protection materials. Figure 8.7 shows a rock bund under construction. When the offshore location has turbulent waves and current, a solid structure rock bund is constructed with armor stones, as seen in Figure 8.8.
A breakwater is usually constructed to break the waves which are directed towards the reclamation. Such structures are long arms protruding from the reclaimed land to protect the land from strong waves and currents. The structure is usually constructed with armor stones. The whole structure has either a rock or sand core with a shell of armor stones depending upon the force of the waves and currents. The length of the shore protection is generally determined based on the hydraulic model. Figures 8.9a and 8.9b show various types of breakwaters. Figures 8.9c and 8.9d show a breakwater under construction. Figure 8.9e shows a completed breakwater.
Headlands are an alternative for breaking the waves and currents. Headlands are normally constructed perpendicular to the wave direction. Such headlands are provided when beaches are required to be formed at the edge of the reclaimed land. When a headland is provided, tabular shaped beaches are naturally formed in the process of coastal action (Figure 8.10). When headlands are required, the shore protection structure is constructed only to a certain level, usually under water. Headlands are constructed at the crest of the lower bund and beaches with gentle slopes are formed behind the headlands.
Headlands are formed with rock. Figures 8.11a and 8.11b show a cross-sectional profile of a lower bund and headland together with a plan view of the headland and the reclamation area. A slope stability analysis is also required in the design of the lower bund and headlands.
Vertical walls are constructed when there is a constraint in area, such as a limited navigation channel or a deep seabed. When reclamation is carried out for a seaport and jetty, vertical walls are deemed necessary since sufficient draft is required for ship berthing. Several types of vertical walls are described in the following sections.
8.5.1 Cantilever, counterfort and gravity walls
Cantilever walls are suitable for shallow seabed conditions. These walls are usually placed before the filling at the periphery area. For cantilever walls, sufficient weep holes are required in order to maintain the groundwater level behind the wall to be the same as the sea level in front. Insufficient weep holes would result in poor drainage from the groundwater flow and the wall will have to carry unnecessary additional water pressure. In order to improve the drainage, vertical drainage is usually provided behind the wall. Vertical drainage is formed with geotextile at the drainage core. Figure 8.12 shows a typical design of a cantilever wall, a counterfort wall and a gravity retaining wall usually used in reclamation projects. Figure 8.13 shows high groundwater level behind the wall, which is not affected by tidal fluctuation due to insufficient weep holes.
8.5.2 Sheet pile wall
A sheet pile wall is an alternative type of retaining wall generally used for deep and soft seabed conditions. For a soft seabed condition, sufficient penetration depth is required for sheet pile installation. The sheet piles are usually supported by raker pipe piles at reasonable intervals. Raker piles give support from the passive side and these piles are usually strengthened again by toe pins. On the active side the piles are usually pulled back by internal anchors. A typical design of a retaining wall, with raker piles, toe pin, and anchor are shown in Figure 8.14. Figure 8.15 shows the installation of a ground anchor with a raker pile. Figure 8.16 shows sheet piles and raker piles after installation. Figure 8.17 shows a completed sheet pile wall at one of the reclamation projects. Some sheet pile walls are tied back some distance with raker piles (Figure 8.18a). Some sheet pile walls are also reinforced with a rock bund in front (Figure 8.18b).
Retaining walls constructed with sheet piles are necessary as protection from corrosion especially when the structure is constructed in a marine environment. Several coats of paint are necessary to protect them from the corrosive action. On top of the coating, catholic protection is usually applied to counteract the corrosion action.
A caisson is an alternative vertical wall structure. This type of structure is usually used in reclamation for port and harbor construction. Caissons are either circular or square in shape. Inside the caisson are several sub-divided cells and these hollow cells are filled with granular material after the caisson is positioned at predetermined locations.
Whenever the foundation is not sufficiently strong, either a sand key, a sand blanket, a rock key, or a rock blanket is provided below the caisson. Figure 8.19 shows a typical design of a caisson on natural seabed sand. Figure 8.20 shows the transportation of a caisson to the construction site, and Figure 8.21 shows a retaining wall constructed with a caisson.
8.5.4 Box gabion
At some locations where the underlying formation is firm, simple box gabions are used in the retaining structure, as shown in Figure 8.22. Figure 8.23 shows the retaining structure constructed with box gabions. Figure 8.24 shows a photograph of a retaining structure constructed with box gabions.
A quay wall is usually constructed for a port facility. This type of wall is either of masonry or a rock structure. A berthing facility can be constructed in front of a rock structure using pile foundation. A typical example of such structure is shown in Figure 8.25.
There are some retaining structures that are constructed with a combination of methods in order to strengthen the foundation or in order to achieve a stable retaining structure. Figure 8.26 shows an embankment built on a sand pile whereas Figure 8.27 shows a caisson constructed on a sand pile. Some retaining structures are constructed after the soft foundation soil has been improved, as seen in Figure 8.28. Some quay wall structures can be constructed with sand piles behind the wall, which can carry vertical and horizontal loads (Figure 8.29). There are several combinations of structures to form a wharf or berthing facility depending on the nature of the foundation soil.