When it comes to the Hong Kong-Zhuhai-Macao Bridge, no one knows it. No one knows it. As the longest cross-sea bridge in the world, it would be a great honour for engineering people to have the opportunity to participate in such a century project. As an ordinary citizen, it is also a very pleasing thing to appreciate the beautiful scenery after the completion.
Such a century project is destined to be a vast and complex project. In the process of construction, there are indeed many engineering problems that have never been seen before, and there are no precedents to follow. Among them, the most famous (the most publicized) should be the " E15 tube festival back to the siltation event ."
There are quite a lot of information on the network related to the report and analysis related to this event, so I will not repeat them here. In this issue, I share with you that the Hong Kong-Zhuhai-Macao Bridge project has encountered another engineering problem:
Hong Kong port "drift" event
This incident was only mentioned slightly in the mainland newspapers and there were no in-depth and detailed reports.
As the Hong Kong port is still under construction, I am sorry that some of the project information cannot be shared with you. In the next article, I will try to stand in the perspective of a bystander and use the information that can be found on the Internet to analyze this incident in depth.
1. Overview of Hong Kong Port
The Hong Kong-Zhuhai-Macau Bridge is 55km long and contains four artificial islands: Zhuhai-Australia Port, East Artificial Island, West Artificial Island and Hong Kong Port.
The artificial island of Hong Kong Port is located next to the existing Hong Kong International Airport and covers an area of about 150 hectares.
The main role of the Hong Kong port is threefold:
(1) The west side connects Hong Kong International Airport and Hong Kong-Zhuhai-Macao Bridge Hong Kong Link;
(2) The south side of the city is connected to the MTR Tung Chung Line;
(3) The north side is connected to the Tuen Mun Link Road to the northwest of the New Territories.
It is a transportation hub connecting all over the country.
2. Exposure of drift events
In around September 2015, Hong Kong media reports revealed a “drift” phenomenon at the Hong Kong port.
Soon after, relevant media in the Mainland also quoted this news.
This is a bit sensational. Looking at the headlines of the news gives the impression that the entire artificial island is "floating" on the sea, with a purposeless drift.
In 1912, Alfred Lothar Wegener proposed the "Continental Drift Theory." He believes that the continents of today's world are splitting from a huge land mass and drifting to gradually reach their present position.
It can be said that drifting on land is not impossible. However, the drift of the continental plate has gradually occurred in hundreds of millions of years. A small artificial island, will this happen in a short time?
3. The Highways Department responded
In order to find out whether it is true or whether the media is in the eye, we need to see if the official has commented on it. On the official website of the Highways Department, I found two responses from the Highways Department.
It can be seen that the Highways Department has admitted that there is indeed a movement in the artificial island of Hong Kong port, which is about 6~7m, which occurred at the end of 2014.
Two years later, the Highways Department responded again to the incident.
Although these two publications are only a few words, they contain a lot of information:
(1) Not the entire artificial island drifts, only the seawall has undergone excessive displacement;
(2) The seawall with excessive displacement has a maximum displacement of 6~7m;
(3) The incident was discovered at the end of 2014;
(4) After the incident at the end of 2014, the seawall was reinforced.
Oh... It turned out that the sea-sea embankment of the artificial island in Hong Kong port has moved. However, 6~7m is still an astronomical figure compared to the horizontal displacement of the city's foundation pit in centimeters. How can such a large displacement occur?
4. Inference prologue: collecting data
As a qualified detective... No, engineers must collect enough information to open the fog and find the truth we need.
First, we need to know the conditions of the location.
Basically it can be divided into two major layers: Marine Deposit and Alluvium. The thickness of the silt varies from 17m to 24m, and the underlying relatively solid alluvial soil layer.
The undrained shear strength distribution of the silt layer is also relatively uniform, gradually increasing from depth to 30 kPa from the top 5 kPa. How to say it, after the bottom is not a bad mud, its undrained shear strength is close to the strength of the foundation after the drainage consolidation method.
Secondly, we look for the structural form of the seawall.
This can be said to be a traditional structural form of re-creation. First, the stone pile is crushed on the bottom of the seawall for foundation reinforcement, and then a large steel cylinder with a diameter of about 33 m is inserted in the middle of the seawall. Finally, the stone is formed into a seawall. This feeling of combining old and new is probably similar to seeing a Ming Dynasty people playing a mobile phone.
It is more intuitive to look at the 3D renderings directly.
At this point, I have already crossed out the pre-set possibility in my heart: the seawall is slipping in the silt layer and causing excessive displacement.
It can be clearly seen in the figure that both the gravel pile and the large steel cylinder have penetrated the mud and entered a relatively solid alluvial soil layer ; to take a step back, even if the gravel piles are eliminated, the seawall alone A large cylinder with a diameter of 33m is also in the middle, which is enough to resist the slippage of the seawall.
5. Inference sub-chapter: displacement comparison
Before the new exploration direction, I decided to compare the calculated and measured displacements.
In the actual construction, the backfill at the rear of the seawall should be filled with about 8m of sea level, which is equivalent to filling the backfill with a height of 16m on the seabed surface. At this time, the corresponding calculated displacement maximum value is about 0.56 m, which appears on the slope of the seawall.
According to the media survey, the displacement of the seawall is as follows.
Out of distrust of the media, I found the statistics of the Highways Department to check.
It can be seen that the figures given by the media are not groundless, which is more consistent with the statement of the Highways Department. This situation has become very interesting. By comparing the calculated and actual values, I found some of the flaws:
(1) The actual displacement is much larger than the calculated value (the difference in magnitude);
(2) Calculate the maximum displacement at the foot of the seawall, and the actual maximum displacement is at the steel cylinder;
(3) In the east, the displacement occurring in the north is the most serious;
(4) The displacement of the steel cylinder on the west side is normal (or not serious).
Please pay attention to point (4). This has not happened in any place and is based on previous information. However, this is very important. why?
Let us think about it: Under similar geological conditions and the same construction process, why is there no excessive displacement in the west compared to other directions?
Why is the calculated value so different from the actual value?
After a long period of intense thinking (10 minutes), I think both of these questions point to one point: one factor is ignored, and this factor is the answer: the key to excessive displacement of the seawall.
I went back to thinking about the results of the calculations. The seawall of this artificial island is calculated by a two-dimensional model. Using its modeling, basically every stress step and strain of the seawall and stratum has been calculated under each construction step.
Everything is there, so what is the difference between the model and the real situation?
Time and space!
That's right. Although the finite element model takes every step carefully, it does not consider the duration of each step of construction and the spatial distribution (three-dimensional effect) in the actual project.
In order to verify this idea, I need to know the construction steps of the entire artificial island at the Hong Kong port before the "drift" of the seawall, and the actual time of each step.
6. Reasoning badge: study of construction steps
The incident occurred at the end of 2014, as previously reported by the Highways Department. Let us assume that this specific time is December 2014 . Please remember this time first.
Next, let's take a look at the general construction steps of the seawall structure.
Taking December 2014 as the boundary, we use the time function of google earth to look at the actual construction from the start of the project (end of 2011) to the eve of the event.
November 2012 (25 months before the event was discovered)
At this point, it can be seen that the gravel piles in some areas have been set up, and a few steel cylinder brackets in the southeast corner of the artificial island have been installed.
January 2013 (23 months before the event was discovered)
Two months later, the steel cylinder in the northeast corner also entered the installation process.
April 2014 (eight months before the event was discovered)
Most of the steel cylinders have been installed, and reclamation began as early as the end of 2013. By April 14th, about half of the west side of the artificial island had been filled with sea level.
October 2014 (2 months before the event was discovered)
I want to know what happened two months before the seawall extension event occurred?
Please pay attention to the next article "7. Floating Mainland: Hong Kong-Zhuhai-Macao Bridge Hong Kong Port Drift Event (2)"
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