The Chuo Shinkansen is a maglev line under construction in Japan that connects Tokyo, Nagoya and Osaka. Xiaobian once tracked the construction of the shaft of the project (walking into the site | What is it like to stand under an 89m deep shaft), large shield dynamics (hot spot tracking | Maglev Chuo Shinkansen First Metropolitan Area Tunnel Kita-Shinagawa Section Shield Machine Rolls Off the Line), Accident Situation (Japan Chuo Shinkansen (Maglev) Seto Tunnel Collapsed). Today, I will review the construction of one of the shafts, the emergency exit of the famous city, and come to the timeline of the water inrush accident and treatment in the construction of the shaft
Overview of the emergency exit of the famous city
In order to meet the needs of ventilation and disaster prevention in the design of the Chuo Shinkansen in Japan, an emergency exit is set up every 5km, of which the emergency exit of the famous city is located on the north side of the land of the old Famous Castle East Park. The outlet adopts a circular shaft structure, the excavation depth is about 89m, in addition to ensuring the function of the emergency exit, it is also necessary to meet the necessary dimensions of shield construction, so the inner diameter below is 27.5m, the diameter of the excavation section is about 38m, and the area is about 1100m2.
The shaft adopts the underground continuous wall as the enclosure structure, with a depth of 130 m, a total of 46 pieces, a width of about 2800 mm, and a set of milling joints. Eight of them employ a special process with bumps to prevent the base from bulging.
Accident briefing and analysis
Construction of the underground continuous wall began in March 2017 and was completed in April 2018. During the excavation, the surface of the underground continuous wall was confirmed, and there was leakage of water at the joints of the first and second stage tanks and the cracks caused by the temperature stress, but the construction of the underground continuous wall was still completed as planned.
Before the official excavation, a total of 6 pumping wells and 4 injection wells were set up in the emergency exit. Excavations began in September 2018. Since it also takes time to equip the water injection system, the setting up of the water injection well is carried out in parallel with the excavation operation. In December 2018, after the completion of the injection well setting up at a depth of about 45m, the actual pumping volume was much larger than the estimated amount of water, so additional pumping and water injection equipment were added. In late December 2018, when preparing to add pumping equipment, water inrushes were detected around the pipes of closed observation wells.
According to the situation, the reasons for the increase in water withdrawal were speculated and analyzed.
The planned increase in pumping volume is speculated about the reason
The inrush accident showed that due to the uneven and uneven formation that was not sufficiently filled, it was speculated that the groundwater flowed from the upper aquifer at the site through the bottom end of the underground continuous wall, and the infiltration pressure expanded the seepage path, eventually causing the inrush.
Action
1 Underground continuous wall bottom end grouting
Constructing a 3 m insulation wall with a length of 3 m at the bottom of the underground continuous wall, the sleeve valve tube grouting method is adopted, and the grouting liquid is a water glass-based solution (inorganic, moderately acidic) to prevent groundwater from flowing into the upper aquifer.
2 Observe the water stop grouting around the well
There were 6 pumping wells in the excavation section, and there were 3 observation wells (including the closed ones), of which 1 well had a water inrush in the outer ring of the pipeline, and the perimeter of all the pipelines was filled with water.
Processing effects
In order to form a planned reinforcement of the soil, this grouting not only requires the planned grouting amount to be completed at a single time, but also needs to be re-grouted again after the gel to improve the grouting effect. Subsequently, through the on-site pumping test, the permeability coefficient of the whole aquifer including the upper and lower aquifers of the grouting section was calculated by seepage analysis as 7.0×10-8m/s, and it was confirmed that good grouting effect was achieved.
Resume excavation
Shaft excavation resumed in late November 2019 and was successfully completed in early June 2020. Monitoring of pumping and groundwater levels is maintained during all construction phases, such as drainage, excavation and concrete pouring of the floor. After about one year of grouting, the pumped water can be stabilized at 1 to 2 L/min. The shaft was completed in September 2020 to build the floor.
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