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Trailing suction hopper dredger contains a large hopper for storage and transport of dredged materials. The dredging operation is implemented by a hydraulic dredging system including draghead, suction pip, pumps for taking up the material from seabed and putting them into the hopper.
Vertical drains are commonly installed in square and triangular patterns. The zone of influence of vertical drains (R) is a function of drain spacing (S).
For drains in square pattern: R = 0.546S
For drains in triangular pattern: R = 0.525S
Shoes are normally installed in prefabricated drains for the following reasons:
(i) It avoids the entry of soils into the mandrel by sealing it during the installation of drains.
(ii) It performs like an anchor to retain the drains at the designed depth and to stop the drains from being pulled out during the withdrawn of mandrels after driving the mandrels into ground.
For installation of band drains by marine plants, it must have sufficient water depth to accommodate the marine plants in the first place. However, due to the effect of tides and waves, the establishment of the position for installation of band drains and the subsequent installation works may be affected. In addition, the establishment cost of marine plants is higher than that of land plants.
During installation of band drains, smear zones are created in which a zone of soil surrounding the band drains are disturbed. The compressibility of surrounding soils is increased and it results in the reduction of their permeability.
Smear zones are generated during the installation of vertical drains in which the zone of soils surrounding the band drains are disturbed. Soils in the smear zones are remoulded during the installation process and the effectiveness of band drains is reduced. For instance, the compressibility of surrounding soils is increased and this brings about the reduction of their permeability. In essence, with the reduced permeability of soils around band drains, it takes longer time to complete the consolidation process.
There is a trend that sand drains are replaced by prefabricated band drains due to:
(i) The rate of installation of prefabricated band drains is high. As such, substantial cost and time could be saved by using this method of
construction.
In reclamation works, the density of filled sand varies with the method of placement. For example, bottom dumping is adopted for sand as the method of placement and this results in higher density of filled sand. For placement by pipeline discharge, the density of filled sand formed by this method is lower than that by bottom dumping. Hence a weak zone is formed for sands placed by pipeline discharge and hence these loose sand fill (relative density about 30%) may cause settlement when subjected to loading or vibrations.
A process called “ageing” occurs after the operation of vibrocompaction. It is observed that sand fill shows an increase in strength and stiffness. Upon immediate completion of vibrocompaction, the apparent increase in strength of sand fill is due to dissipation of air and water pressure. Then the ageing process shall continue owing to creep and cementation.
It is difficult to obtain undisturbed samples of sand to measure the in-situ density. The relative density of in-situ sand can be determined by correlation to other parameters in some methods. For instance, Cone Penetration Test can be employed to estimate the relative density of sands as there is well established relation between cone tip resistance and relative density.
