Making land
History of Reclamation in HK, abridged:
When the British forced the Chinese to cede HK to us in the 19th Century, we gained a strategic harbour surrounded by a fairly useless chunk of mountainous terrain (the only naturally flat land in HK, apart from Happy Valley, is in the northern New Territories which we didn't get our hands on until 1898). Unfortunately, for what was to come, flat land was pretty much a necessity. So we shovelled a lot of rubble, building debris, rubbish, even fire debris, off the coastline until there was more coastline. This was the first stage, and formed the tram route, among other things. It still manages to cause Engineers nightmares when we come to build basements. Unfortunately, the top layers of soil under the sea are 'Marine Deposits' - soft black clay with the consistency of toothpaste and which takes years to consolidate and gain strength as the excess water eventually drains out. Moreover, if you tip stuff in like that, it causes 'mud waves'... which are extra thick parts which will consolidate even more later.
After this was realised, the next generation of reclamation started. Dredgers would remove all the soft marine sediments down to decent founding material, and then dump nice graded fill (generally) on top of that, which settled predictably and became stable. That formed the bulk of reclamation in the 20th Century. Probably the last major reclamation to adopt this method was the 2% added to HK's total land area during the construction of the airport and related infrastructure in the 1990s, but don't quote me on that one.
However, in recent years we have a growing environmental awareness both in the engineering community and our regulatory authorities and clients, but also among the general public. We are all intimately familiar with the favoured swimming routes of Chinese White Dolphins in the commercially important waters off western Lantau; and we are aware that these marine deposits are actually pretty nasty materials and if you dig them out you have to dump them somewhere - and simply, there is nowhere we are allowed to dump this vast quantity of material amounting to thousands upon thousands of tonnes. So the latest tendency is towards constructing 'Drained Reclamations' - where you install vertical drains to encourage the excess water out of the marine deposits and therefore they can gain strength in 1-2 years instead of 10-20 years. During which time you might expect them to settle 1m for every 5m thickness of deposits. At which point, the design even for basic stabilityb gets tedious.
This is where I come in, and why I am typing this in the office on what is supposedly my Saturday off. Bummer.
When the British forced the Chinese to cede HK to us in the 19th Century, we gained a strategic harbour surrounded by a fairly useless chunk of mountainous terrain (the only naturally flat land in HK, apart from Happy Valley, is in the northern New Territories which we didn't get our hands on until 1898). Unfortunately, for what was to come, flat land was pretty much a necessity. So we shovelled a lot of rubble, building debris, rubbish, even fire debris, off the coastline until there was more coastline. This was the first stage, and formed the tram route, among other things. It still manages to cause Engineers nightmares when we come to build basements. Unfortunately, the top layers of soil under the sea are 'Marine Deposits' - soft black clay with the consistency of toothpaste and which takes years to consolidate and gain strength as the excess water eventually drains out. Moreover, if you tip stuff in like that, it causes 'mud waves'... which are extra thick parts which will consolidate even more later.
After this was realised, the next generation of reclamation started. Dredgers would remove all the soft marine sediments down to decent founding material, and then dump nice graded fill (generally) on top of that, which settled predictably and became stable. That formed the bulk of reclamation in the 20th Century. Probably the last major reclamation to adopt this method was the 2% added to HK's total land area during the construction of the airport and related infrastructure in the 1990s, but don't quote me on that one.
However, in recent years we have a growing environmental awareness both in the engineering community and our regulatory authorities and clients, but also among the general public. We are all intimately familiar with the favoured swimming routes of Chinese White Dolphins in the commercially important waters off western Lantau; and we are aware that these marine deposits are actually pretty nasty materials and if you dig them out you have to dump them somewhere - and simply, there is nowhere we are allowed to dump this vast quantity of material amounting to thousands upon thousands of tonnes. So the latest tendency is towards constructing 'Drained Reclamations' - where you install vertical drains to encourage the excess water out of the marine deposits and therefore they can gain strength in 1-2 years instead of 10-20 years. During which time you might expect them to settle 1m for every 5m thickness of deposits. At which point, the design even for basic stabilityb gets tedious.
This is where I come in, and why I am typing this in the office on what is supposedly my Saturday off. Bummer.