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(A) Rigid dolphin
The impact energy of vessel is absorbed mainly by fender. As such, the dolphin itself is designed as a rigid structure with a group of piles. The piles serve to transfer the reaction force from fender system to the foundation soils. The design of rigid fender is similar to other structures and the strength and stiffness of rigid dolphin should be sufficient to withstand berthing forces without causing excessive deformations.
Finger jetty is a more efficient pier structure because it could accommodate vessels at both sides of the jetty. However, there should be sufficient water depth as the berths at finger jetty is relatively close to shoreline when compared with T-shaped jetty so that it is anticipated that vessels are required to berth at shallower water. Moreover, there should be no cross current to enhance berthing at both sides of finger jetty. Also, as mooring points are often located on the jetty, leads are not ideal for larger ships.
Mooring lines are provided by vessel while the shore provides the mooring points. Wire ropes provide a more rigid mooring system than fibre ropes. When a high degree elastically is required, fibre ropes would be a better choice.
In marine environment, the cover to reinforced concrete in exposure zones is specified to be much larger than normal concrete (e.g. 75mm). Based on past experimental studies, the concrete cover is identified to be directly related to the corrosion failure of marine structures.
The presence of lateral pressure of backfilling sidesoils induces bending
moments in the opposite direction from those produced by vertical loads and bedding support reaction. Such bending moment reduces the flexure in pipe wall and as a result this causes an equivalent increase in supporting strength of the pipe.
Whenever possible box culverts should be designed for free flow at design flow rate. A box culvert with surcharged flow is similar to inverted siphon which creates maintenance problem. There is a risk of blockage by silt and debris and the inspection and maintenance of submerged culvert is difficult when compared with free flow condition. For long box culverts (more than 20m) it is even more important to design for free flow to decrease the risk of blockage which results in an inaccessible path.
Rock used for riprap should be blocky and angular, with sharp edges and flat faces. Angular stones proved to be effective to withstand external forces. Rounded stones have a high tendency to roll and inadequately protect the channel bed and bank. The ratio of length to thickness of angular stones should be less than 2.
Riprap by dumping involves the dumping of graded stone by dragline or crane in such a way that segregation would not take place. Dumped riprap is a layer of loose stone so that individual stones independently adjust to shift in or out of the riprap. The dumped riprap is very flexible and would not be damaged or weakened by minor movement of the bank caused by settlement. Moreover, local damage or soil loss can be readily repaired by placement of more rock.
Riprap is an erosion-resistant ground cover made up of large, angular and loose stones (rock, concrete or other material) with geotextile or granular layer underneath. Riprap is commonly used in drainage channel to provide a stable lining to resist erosion by channel water. It is also used in channels where infiltration is intended but the velocity of flow is too large for vegetation.
In a typical box culvert, at the interface between rockfill layer and sub-grade, a layer of geotextile filter is usually added to perform separation function. With the addition of geotextile filter layer, it avoids the intermixing of widely different soil granulations so as to reduce long-term settlement.
