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Marine Works

Should silt curtain be designed to touch seabed?

Silt curtains are impermeable vertical barriers extending from the seawater surface to their designed depths. The curtains are held in a vertical position by the carrier float on their top and a curtain weight at their bottom. A tension cable is designed at the carrier float to resist stresses incurred by currents. Moreover, the silt curtains are anchored to the seabed to hold them in the designed configuration.

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Why are observed settlements in reclamation normally larger than calculated?

Settlement in reclamation area occurs as a result of primary consolidation (i.e. by dissipation of excess pore water pressure) and secondary compression which involves creep of soils. Creep of soils occurs by viscous squeezing out of absorbed water in double layers of clay particles and rearrangement of clay particles under loading.

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In reclamation involving large volumes of fill and tight programme, shall engineers use marine fill or mud extracted from land borrow area as filling material?

There are two advantages of adopting marine fill over mud extracted from land borrow area:

(i) In some land borrow areas, it involves breaking up of rock to suitable sizes for reclamation and the production rate is not high. With modern equipment for dredging and placing marine fill, the filling rate is much higher.

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In reclamation by filling sand, what is the effect of filling operations below mean sea level?

Filling below mean sea level usually has a low density. The settling sand in standing water would form a loose skeleton leading to a low density. However, as the sand level is rising, the increased load causes reallocation of sand grains in lower layers.

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What is the importance of geotextiles and sand in reclamation works?

For geotextiles used in reclamation, they serve mainly the following two purposes:

(i) they separate reclamation fill from marine mud;
(ii) they may act as reinforcement to enhance the stability of reclamation.

However, the reinforcement function is still under heated debate because its performance as reinforcement depends on several factors like the directional strength of woven geotextiles and damage effect by installation of vertical band drains.
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In case mud waves occur during reclamation, what are the possible solutions to rectify the situation?

(i) Option 1 – Complete Removal of All Disturbed Mud
To remove all disturbed mud once mud waves occur is the fastest way to treat the problem. After that, filling material is used for replacing the disturbed mud. However, this option is a rather expensive option because it involves dredging of all disturbed mud and replacement of large amount of fill.

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What is the purpose of formation of bunds in reclamation?

Reclamation works normally proceed behind the seawall to protect against typhoon attack. In case where soft marine mud is encountered during reclamation, bunds may be formed on planned alignment of road and drainage works and locations of early development to displace mud to other less important areas should mudwaves indeed occur.

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In dredged reclamation, what are the considerations in selecting between trailer suction hopper dredgers and grab dredgers?

Trailer suction hopper dredgers are vessels which remove material off the seabed through hydraulic suction by using pumps. During the dredging operation, a mixture of soil and water is transported through suction pipe to storage hoppers. Significant turbulence inside the hoppers keeps the dredged mixture in suspension and this should be minimized to enhance the material to settle swiftly prior to the process of overflowing.

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What are different approaches for reclamation in deep water region and shallow water region?

To illustrate the different approaches adopted for reclamation in deep water and shallow water region, the following example is used:

In deepwater region, consider the seabed level is –8.5mPD. After laying of geotextiles and 1.5m thick sand blanket, the top level of sand blanket is about –7mPD. Split barges are deployed for dumping public fill to –2.5mPD. Afterwards, end dipping of public fill by trucks will be carried out up to +2.5mPD which is the designed reclamation level. Between level –2.5mPD and +2.5mPD, it is too shallow for split barges to enter the water, thus the method of end dipping is used instead.

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What are the pros and cons of using timber fenders, plastic fenders and rubber fenders?

Timber fenders:
They are low in strength and are subject to rotting and marine borer attack. Moreover, they have low energy absorption capacity and the berthing reaction depends on the point of contact. The contact pressure between fender and vessels are high. They are considered to be environmentally unfriendly because they consume tropical hardwoods in their production.

Plastic fenders:
Their strength is similar to that of timber fenders but they have relatively high abrasive resistance. They are resistant to chemical and biological attack. Their energy absorption capacities are moderate and the berthing reactions are also dependent on the point of contact. The reaction is lower when compared with timber fenders for a given energy absorption. They are considered to be environmental friendly because they are manufactured from recycled material.

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