Containment dykes

Figure 20.8 Marine engineering applications of geotextile tubes, (a) Revetments — exposed and submerged, (b) offshore breakwaters, (c) protection dykes, (d) containment dykes, (e) training walls and (f) groins.

Geotextile containers are used for a range of marine engineering applications which require placement of the units at water depth. These include offshore breakwaters (Fig. 20.10(a)), containment dykes (Fig. 20.10(b)), artificial reefs (Fig. 20.10(c)) and slope buttressing (Fig. 20.10(d)). Some of these applications are discussed in further detail later in this chapter. 

Basal filters beneath breakwaters and containment dykes... 

The presence of breakwaters and containment dykes in marine environments can result in water turbulence at the toes and along the base of these structures, especially if they are constructed of rock bll. This can cause scour beneath the toes, undermining the structure and resulting in instability. To prevent scour and undermining of the breakwater or containment dyke, a geotextile biter is placed across the base of the structure, as shown in Fig. 20.16. Where breakwaters and containment dykes are located on rock or on overconsolidated clay foundations, base scour does not arise however, when the foundation consists of sand, and in particular loose sand, base scour can become a major problem. 

To enable the efficient installation of the basal geotextile biter on the seabed before construction of the breakwater or containment dyke, commonly the geotextile is brst formed into large panels onshore and then combined with a semirigid lattice of timber or bamboo. This combined structure is known as a fascine matttess and is the modern version of an old technique in which fascines (brushwood) were interwoven into a sheet to provide foundation protection for old dyke mounds. The fascine mattress is... 

Breakwaters and containment dykes must maintain their shape and height over their required design life. If the foundation cannot support the breakwater or containment dyke loading, this can lead to deformations, settlements and ultimately instability of the structure. The preferred location for the construction of breakwaters and containment dykes is on good, stable foundations. However, in certain circumstances this may be impossible and foundation improvement techniques may have to be used when the foundation alone cannot support loading of the overlying structure. 

Figure 20.19 Basal reinforcement technique applied to breakwaters and containment dykes on soft foundations.

One technique to improve the stability of breakwaters and containment dykes con-stmcted on soft foundations is to use geotextile reinforcement across the base of the stmcture (Fig. 20.19). This basal reinforcement technique has been used successfully for many years in the onshore construction of embankments on soft foundation soils, and is discussed in detail in chapter Geotextiles used in reinforcing foundations. Because of its cost—benefits, this technique has also been used in marine engineering structures over the past 20 years. 

Figure 20.20 Design limit modes for basal reinforced breakwaters and containment dykes on soft foundations, (a) HydrauUc stabiUty, (b) rotational stabiUty, (c) strain in reinforcement and (d) settlement.

Ameratunga et al. (2006) describe the design and construction of a basal-reinforced rubble seawall (containment dyke) over soft marine clay for the expansion of the Port of Brisbane, Australia. Fig. 20.21 shows a typical cross section of the basal-reinforced rabble seawall. The seawall was constructed to contain the reclamation fill for the port expansion. The thickness of the soft foundation varied from 8 to 30 m in some locations it had an undrained shear strength as low as 5 kPa at the foundation surface which was approximately 3.5 m below the average water depth. The rabble seawall varied in height to 7.5 m, and an analysis showed that the soft foundation could not support this... 

Containment dykes are constmcted for land reclamation and artificial islands to prevent the loss of fill by erosion during construction. Typical containment dyke layouts are shown in Fig. 20.24. The containment dyke delineates the extent of the reclamation land area boundary and consists of erosion-resistant materials. The fill inside the containment dyke may be installed by hydraulic means or by dry placement. 

Nonwoven geotextiles are commonly used as filters on the inside of mbble fill containment dykes. The reason for this is that they exhibit relatively high elongation and thus can readily deform around mbble surface undulations without damage. Although woven geotextiles have also been used for this applicafion, normally the mbble fill surface has to be specially prepared to ensure no undulations are present. 

Figure 20.24 Different containment dyke configurations using geotextiles, (a) Rubble containment dyke and (b) geotextile tube containment dyke.

Figure 20 5 Rubble containment dyke with geotextile filter for land reclamation. Courtesy C. Lawson.

Alternative containment dyke structures have been constructed in which mbble fill has proved to be expensive or unsuitable. For example, geotextile mbe containment dyke structures (shown in Fig. 20.24(b)) have become common in which geotextile tube units substitute for the rubble fill. Here, the geotextile tubes can be filled hydraulically with locally available sand fill to construct the containment dyke. Later, the outside of the structure can be covered with rock armour for permanent protection. Fig. 20.26 shows an example of a geotextile mbe containment dyke with rock armour being placed over the exterior for permanent protection. 

Lawson (2008) describes the construction of an artificial island using a geotextile mbe containment dyke for the overhead viaduct approaches to be constructed wholly above water for the Grand Incheon Bridge in Korea (Fig. 20.27). The foundation conditions consisted of soft marine clay, which made alternative containment solutions. 

Figure 20.26 Geotextile tube containment dyke for land reclamation. Courtesy TenCate Geosynthetics.

Alternative spoil containment dyke structures have been used where hydrauhc conditions permit and where cost savings are important. For example. Fig. 20.28(b) shows the use of geotextile tubes as a dredged spoil containment dyke. The advantage of this solution is that the dredged spoil itself may be used as the fill inside the geotextile mbes, which increases the effective spoil capacity of the containment facility. This... 

Offshore wetlands developments are usually located in sheltered marine locations and serve as habitats for local and migrating birds. Fig. 20.29(a) shows a cross section of a typical offshore wetlands development area in which dredged spoil has been used as the hydraulic fill to construct the wetlands habitat. The wetlands development is contained within geotextile tube containment dykes which also can be filled with the dredged spoil material, which thus makes full use of the dredged spoil. If the... 

Figure 20.29 Offshore wetlands development, (a) Cross section through typical offshore wetlands development area and (b) construction of combined spoil containment and wetlands development area utilising geotextile tube containment dykes.

Fig. 20.29(b) shows the constmction of an offshore wetlands habitat area using spoil excavated from a nearby mnnel construction. The spoil, which consisted of fine, uniform sand, was also used to fill the geotextile tube containment dyke surrounding the wetlands development area. For permanent protection, a rock armour layer was placed over the outer surface of the geotextile mbe containment dyke. 

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