Overcoming the World’s Problem -Undersea Weathered Slots
11 min readAccompanied by a muffled bang,a gust of strong shock waves came from the deep tunnel, and the construction vehicle’s windows rattled… The drilling and blasting method was adopted in the excavation of Xiang’ an Tunnel. Commonly known as “drilling and blasting”, this method involves blast working. Similarly, the blast working also requires the use of specific technology.
“At all times, we had to keep in mind that we were constructing under the sea, and the consequences that one action could have.”Zeng Chao said.
The drilling and blasting method has a drawback: the vibration generated by the blasting will cause disturbances to the surrounding rock of the tunnel. When surrounding rock is affected by the blasting disturbance,a loose ring of surrounding rock of a certain thickness could form around the tunnel, and then the anti-permeability and the supporting capacity of the surrounding rock would greatly decrease. Xiangan Tunnel in particular had to go through several fault fracture zones, if the blasting programs and blasting parameters were unreasonable, after one blast, they would not besure whether water or rock would come out. If the water burst into the tunnel, the consequences would be disastrous.
For example, when the Japanese constructed the Seikan Tunnel, this kind of accident happened.When large amounts of seawater burst into the tunnel, without even considering the casualties, water shutoff is a problem. They had to send the underwater divers to investigate the flooded tunnel. In order to save the tunnel, the Japanese dug a tunnel next to the main tunnel, went around to the accident spot, and blocked the gushing point using certain technical methods. In this process, the drainage alone took over 150 days.
Therefore, the quality of blast working scheme is particularly important. During the Xiang’ an Tunnel construction, they did a lot of work on the controlled blasting: adopting smooth blasting technology and the pre-splitting blasting technology to reduce the thickness of the loose ring of surrounding rock, and adopting the vibration reduction blasting technology to protect the stability of the surrounding rock structure. Meanwhile, blasting vibration monitoring was conducted during the excavation. The information was promptly fed back to adjust the blasting parameters, reduce the effect of blasting vibration, thus ensuring the safety of tunnel construction.
The amount of the explosive material also matters. Meanwhile, millisecond-order detonation technology was introduced to the Xiang’ an Tunnel construction. This method could detonate the total explosive material in sequence at millisecond intervals.A large number of experimental studies have shown that, under the same conditions including the same amount of total explosive material, the vibration intensity of millisecond-order detonation was 1/3 to 2/3 lower than volleyed detonation.
In order not to affect the protected animals in the surrounding waters, the constructors also strictly detected the explosion noise underwater. For example, each time before the blasting, the construction workers would drive a boat out to sea to drive away the dolphins in the surrounding waters via special sonar equipment. This instrument can send out the sound that the white dolphinsdislike, so they would get away from the surrounding areas temporarily, and avoid being hurt by the explosion noise.
Compared to the heavily weathered stratum and the permeable sand stratum, the biggest difficulty of Xiangan Tunnel was the weathered slot(or weathered sack), which has even been called the key to Xiangan Tunnels successful completion. It was undoubtedly the “stumbling block “.
February 7,2007 was one of the most memorable days for all constructors -the Xiang’ an Tunnel A1 Section dealt with a global problem: weathered slot. Huang Lingqiang said,”When we were facing this global problem, we had no project examples or successful experience for reference.We had to use independent innovation and scientific research, probing while drawing conclusions.We had to advance step by step.”
As mentioned above, the weathered slot was an extremely complex phenomenon in the seabed geological structure. The slots were mainly formed by the long years of rock weathering. They inserted vertically into the stratum, and connected with the seawater. In simple terms, the weathered slot was an unstable and permeable interwoven layer full of mud and gravel, connecting with the seawater. When the construction was carried out in such a complex geological body, withunexpected characteristics, the seawater would squeeze into the narrow construction site of the tunnel. The whole project could be abandoned or devastating fatal accidents could also be caused.
“In the early stages of surveying, we thought that the difficulty of the construction of the weathered slots would be very high, but in reality we found that it was even more difficult than we imagined.”said Sun Zhenchuan, Deputy Manager of A1 Section Project of China Railway Tunnel Company Limited, which was the first construction unit in the weathered slot. This broad northwestern tough guy was stationed in the front line for almost 24 hours a day. He acted as the pioneer of the construction, doing, in his words, the “most delicate work of his life”.
As the construction encountered difficult situations, there were too many uncertainties and any unforeseen problem could occur. The youth pioneer team of Xiang’ an Tunnel took the initiative to strengthen their sense of duty during weathered slot construction, to ensure that there were members on the site at any time, in order to detect and solve the problems.
For example, one of the team members was a native of Xiamen. He lived in the nice downtown area and he could go home every day. But since construction began, he lived and ate on the site, stationed in the harsh environment of the site for a long time, and only went back home once a week. Another team member from Sichuan spent almost all of his time on the construction site. He hadn’t been to the Xiamen downtown in the two or three years since arriving at the site.
Thus, in the staff quarters on the construction site of Xiang’ an Tunnel, you could often see the scene of off-duty workers sleeping with their clean clothes on after they ate and bathed. The workers said with a smile,”This will save us the time to put on the clothes in case there is an emergency situation after falling asleep. So we can rush to the scene straight away.”
Such a team can overcome the most dangerous difficulties.
Wang Xuebin remembered this scene: in the borehole construction of undersea F1 weathered slot, in order to identify the situation at the front, they drilled a borehole on the tunnel face.”When it reached about 20 m, with a sudden bang,a high pressure water column shot straight out from the orifice. It was seawater.”The scene has remained fresh in Wang Xuebin’s memory.
The high pressure water was like a silver ribbon in the black tunnel. The flow exceeded the expected amount, and it was difficult to swiftly drain the water. The water pressure was too high, so
the water had to be stopped immediately. The workers shut down the orifice valve according to the construction procedures, and then the water column stopped.
“But they immediately found a new problem: there was water seepage on the tunnel face!”Wang Xuebin said. This phenomenon showed that seawater was penetrating into the tunnel face through boreholes and cracks. Without timely reinforcement, the tunnel face might not be able to withstand high water pressure and might collapse. In that way, the tunnel would be flooded in a very short time, and the construction staff could not escape from there.”
It appeared that the water could not be simply “held back”. They had to drain out the water while blocking it. After analysis, the construction side decided to partially open the orifice, in order to relieve the pressure. They set up the emergency power pumps to pump the water out of the tunnel, and immediately reinforced the tunnel face. After tens of minutes, this emergency program worked, and the dangerous situation was controlled.
The constructors thought a lot after the accident. The general idea of the construction in the undersea weathered slot was determined: in order to go through the weathered slot safely, they had to conduct pre-grouting and rock reinforcement in this section. They had to adopt the “fast support, early ring formation”method until they could complete the weathered slot section. The excavation should be carried out by humans or small machinery. The weak blasting should be carried out if necessary, to avoid large disturbances to the surrounding rock. In the excavation, the advanced pipe-shed support and grouting should be conducted in strict accordance with the design requirements.
Wu Shishu, Safety Director of the Xiamen Xiangan Tunnel, said,”Slow work yields fine products.”
The tunnel construction “fears the soft but not the hard”. In order to make the tunnel hard in the weathered slot, the constructors decided to adopt the “full face curtain grouting technology”. In short, this technology was for injecting the quick-drying cement into the mud, so the mud and gravel would harden into cement blocks with the hardness comparable to the rock. When the hardness reached the standard, they could carry out the excavation.
On February 12,2007, the grouting work began. The cement slurry used in the grouting was different from the common cement. It was a composite material, with fast solidification, and had aplugging effect. The cost of it was 7-8 times of that of the ordinary cement. This kind of cement slurry was injected into the weathered slot full of mud and gravel. Generally, within a few weeks, they would gradually solidify, and block the seawater.
From February 12 to October 27, the grouting depth only reached 55 m in 8 months. Then, with some apprehension, the constructors slowly excavated the weathered slot in the first section.”When the results presented themselves, the crowd was in silence. Everyone choked.”Wang Xuebin said,”The texture of the tunnel face in the front seemed like pork belly. We could clearly see that the cement had solidified in the fractured stratum. It was solid as the rock.”
Data collection was crucial: after inspecting the excavation situation, in the grouting segment, the 0.0-5.5 m part was relatively dry, the stratum contained more slurry, and the grouting effectwas better. When the excavation reached 5.5 m,a small amount of water dripping could be found in the left part of the arch on the tunnel face, and a wet mark appeared in the central part, which meant it had reached the water-containing interface. When the excavation reached 11 m, the excavation face could basically self-stabilize, but larger seepage appeared in the left upper part of the tunnel face.
“At that time, the seepage flow reached 0.6 m3/h. In order to ensure the safety of excavation, they sealed the tunnel face, and conducted small duct supplement grouting. Then they continued to excavate, and finally reached 18 m.”Zeng Chao said.
18 meters in 8 months was the speed of the excavation in the first section. The speed was just like digging with bare hands. But the payoff was great.
First, the construction technology was successful. When they drilled into the weathered slot for the first time, the outflow speed was 50 m3/h, and it was finally reduced to 0.6 m3/h. This had reduced the risk of excavation one hundredfold. Through technical testing, the feasibility of the full-face advance pre-grouting technology had been proven.
Second, the construction speed could be improved. Initially, the construction adopted a small drilling rig with working platform+grouting sets method. However, due to the large quantity of boreholes, the high workload of drilling and grouting, the cross-operation, the mutual interference, the complex geological conditions, and the equipment loss, it required the serialization,diversification, automation and efficiency of the drilling and grouting machinery and equipment.
In order to ensure safety and quality,a large amount of money was spent on Xiang’ an Tunnel.This time, they spent six million Yuan to purchase a “Casa”drilling-injection machine. However, it made the process more efficient, it actually reduced the cost. After being optimized, the length of the curtain grouting can reach 45 meters. The number of the grouting holes was only 150, and the grouting time was only one month. It ensured the construction safety, improved the construction efficiency, and saved millions of Yuan of investment.
During the construction, all parties had worked hard, giving their pound of flesh. The Xiang’ an Tunnel construction started in the hottest season. In the fight against the sea, with seawater over head, saline mud under feet, facing temperatures of about 40 degrees Celsius, the constructors kept working hard on the working face. The heat from the high-performance concrete evaporated the seawater from the tunnel into gas and salt and the constructors’ own body heat made them sweat somuch that the sweat would drip down their bodies, across their skin and down their legs into theirboots. During each shift, the constructors had to pour out a total amount of over 1,000 ml of sweat from their boots three to four times.
Li Zhijun, Director of the Engineering Department of Xiamen Xiangan Tunnel Project Department of China Railway First Bureau Group, came to Xiamen in August 2005. At that time the Project Department had not been fully established. He lived in a hotel in Xindian with a college for over a month, and then he lived in the board room of the Project Department on the left side of Xiangan Tunnel. Although he was always longing for Gulangyu Island, he barely had the opportunity to visit it in the five years.
Huang Mingqi, the Director of Engineering Department and Science Research Department of Xiamen Xiangan Tunnel Project Department of China Railway 12th Bureau Group, said, the weathered slot between the No.2 and No.3 tunnel faces of the left channel that the Group tookcharge of in Xiamen Xiang’ an Tunnel reached 123 meters.”The longer the weathered slot was, the higher the construction pressure was. In the excavation of more than a year,I didn’t sleep well for one day.I was always awakened in the middle of the night.”
With the wisdom and sweat of the constructors,a deep chasm turned into a thoroughfare: the A1Section of the left channel of the F1’s first section started the grouting on February 12,2007 and ended on October 27, taking eight months. After adopting the new scheme, the grouting depth of the second section reached 48 meters, reducing the time by one month.
The experiences and wisdom during the construction process better solved the key technical problems of China’s first undersea tunnel construction, provided reliable technical support for the success of Xiangan Tunnel’s construction, achieved good results, and effectively promoted the progress and development of domestic tunnel construction technologies. It had become an extremely valuable asset.
After the weathered slot was defeated, the tunnel construction had won half the battle. On June 13,2009, after overcoming four undersea weathered slots, the right channel of Xiangan Tunnel opened. In October and November of the same year, the service channel and the left channel opened.
The final blasting was on November 5，2009.The breakthrough point that day was at about 2，300 meters into the tunnel，which was the deepest point of the tunnel，70 meters below sea level. In order to successfully blast the upper part of the last working face，the workers prepared 24 kg of explosives.At 16：00，with a loud muffled noise，the rocks scattered，smoke and dust filled the air，and the blast wave blew.One notch was blasted on the working face.There was light coming through it.
The workers from both sides rushed with banners，shook hands and cheered with excitement.In the past few years，many of them were doing the same thing every day，but they had never seeneach other.At this moment，the working face finally disappeared.With these strange and familiar faces in their eyes，all sorts of feelings welled up in their minds.
