This week we introduce an application from Hong Kong, where vibro hammers are used to speed up the installation of band drains to prepare the ground for construction.

Hong Kong is a mountainous territory, the acreage available for development does not meet the needs of the island’s population. The city has severe land shortage, an issue that is addressed by extensive land reclamation. Land reclamation has been realized in areas such Sheung Wan in Central, in Shatin, and at Kowloon. It is estimated that in Hong Kong over 35% of land is reclaimed from the ocean. The land won from the sea has been used for government housing and commercial buildings.

After reclamation the soil needs to settle. There are many methods for soil settlements, one of which often used in Hong Kong is the band drain installation method. Band drains are economical, time saving and effective for large reclamation areas.

Band drains, also known as wick drains, or more accurately called strip or prefabricated vertical drains (PVDs), accelerate preconstruction soil consolidation. The drains are composed of a plastic strip with drainage channels wrapped in a nonwoven geotextile filter fabric. The geotextile filter prevents soil particles from entering the channels and clogging the drain. The installation of the drains—often, but not always vertically—into soft soils is performed using vibratory hammers or static methods. The band drain layout is typically a triangular or square pattern. The time required for consolidation depends on the permeability of the soft strata sand layers, in the strata the weight of the surcharge, and the spacing of the wicks. The drains shorten the pore water drainage path, thereby permitting the soil consolidation to occur typically in weeks instead of years.

To make Tseung Kwan O more accessible and alleviate traffic congestion a new tunnel connecting Tseung Kwan O to Kwun Tong area is being built. The project goes through heavily urbanized areas, bores through a mountain, and required the construction of a causeway to cross the sea. The soft marine sediment on the seabed needed to be removed before reclamation works could proceed. To minimize any sediment flow to the sea outside the works site during the dredging and filling works, a temporary steel cofferdam was installed to enclose the reclamation area. 24m long sheet piles were used. During installation work the site was impacted by a typhoon. A part of the cofferdam was deformed, and the piles needed to be extracted and replaced. An ICE 815C was used for this job.

To break up harder top soil layers H-beams are driven down 6-10 meters into the soil to by excavator mounted hammers to prepare for band drain installation.

For the installation of 24m long band drains the sub-contractor chose the ICE-416L. This hammer has 64t eccentric force and is powered by a 242kW power pack. The contactor used their own 24 meters mandrel design for the project, guiding the hammer and mandrel from a frame attached to the barge. The mandrel is bolted to the bottom plate of the hammers gearbox and allows two band drains to be inserted at a time. The workers will fasten an anchor at the end of the band drain, the mandrel will be inserted into the ground and then pulled out, leaving the anchor and the band drain in the soil. The stitcher is fitted with a reel that holds a roll of band drain, automating the process.  The 24m length band drains are inserted within a minute, then the hammer is extracted and moved to the next position, which means an hourly production of over 30 band drains is reached. 

This installation method was first used in Hong Kong in 2014, we reported about it then: https://www.icevibro.com/article/276.html. IF you would like to learn more about working with vibratory hammers, please get in touch with one of our specialists today!