佢所講嘅cofferdam水下隧道建造方法已經好過時，建造成本貴同造成環境污染。我廿年前已經參與immersed tube tunnel設計。將隧道分段造好，用船拖到現場，沉到海底再接駁。成本低對環境影響亦低。 The underwater tunnel construction method he said is a very traditional method or I should say a very ancient method. A line of cofferdam to be built alongside the tunnel alignment so that the sections can be dewatered to accommodate the construction work. However this method is an extremely costly way to build a tunnel if it sits deep in the sea whilst hundreds and thousands of piles to be driven and massive steel supports are required. The wide extent of construction site, the enormous amount of soil disposal and the ground settlement led by the dewatering will create significant environmental impact. I was involving an immersed tube tunnel design 20 years ago which was a state of the art construction method at the time. The tunnel sections were cast in the dry dock and tugged to the site. The tunnel sections were sunk into the sea for connection and anchorage. The construction is much cheaper and quicker, the associated environmental impact is far less and easily controlled.
