Since the beginning of this year, Jinan Rail Transit Group has continuously tackled difficulties and overcome obstacles, taking the city's "Project Deepening Year" as the overarching guide to keep accelerating project progress. Recently, the shield-tunnel section of the longest running tunnel on the entire Jinan Rail Transit Line 4 (from Jinan West Station to Dayang Station) achieved double-line breakthrough, playing an important role in promoting the full-line opening and operation of Line 4. It is reported that this section is constructed by China Railway 21st Bureau Group, and is one of the most complex shield-tunnel sections among Jinan's metro projects currently under construction. The shield machine needs to continuously pass beneath three high-speed railway lines—the Beijing-Shanghai High-Speed Railway, the Jinan-Zhengzhou High-Speed Railway, and the Beijing-Jinan Connecting Line—as well as Jinan Rail Transit Line 1 and the main road of Jingshi Road. A single shield-tunnel section continuously passing beneath four operating railway lines represents a complex construction condition that is the first of its kind nationwide.

The double-line shield-tunnel section has a total length of 4,582 meters, the longest on the entire line; it continuously and densely passes beneath four railway lines, the most on the entire line. According to reports, this section has a high groundwater level and complex geological conditions. How to precisely control settlement and deformation through technical and management measures was a major challenge facing the construction team. Moreover, this section is located in the core area of the "Jixi Grand Reservoir," where underground water content is abundant. When densely passing beneath railway lines meets water-rich strata, this undoubtedly adds another layer of challenge to the project construction.

First in the country to continuously pass beneath 3 high-speed railway lines

Plus water-rich challenges to achieve "zero settlement"

Every hurdle is difficult yet overcome; every step is hard yet taken. In January last year, shield tunneling began in the "Ji-Da" section. The first obstacles facing the shield machine were the three "roadblocks"—the Beijing-Shanghai High-Speed Railway, the Jinan-Zhengzhou High-Speed Railway, and the Beijing-Jinan Connecting Line. As important components of the national high-speed railway network, these three lines see a train pass on average every 4 minutes. Facing such important railway lines, there are extremely stringent settlement standards for passing beneath them, measured in millimeters. The horizontal distance between the edge of the shield tunnel and the high-speed railway bridge piers is only 9.4 meters, making the difficulty imaginable. Li Huan, project manager of China Railway 21st Bureau Group for Jinan Rail Transit Line 4, said that from the early stage of the project they set the goal of ensuring "zero settlement." To ensure tunneling safety, the project established a dedicated team to carefully study engineering data, conduct comprehensive surveys and statistics of above-ground and underground conditions within the project construction scope, carry out diagnostic analysis and demonstration, select the optimal shield machine model, formulate the optimal construction plans for "railway-related," "rail-transit-related," and "road-related" works, strictly enforce node verification procedures, and maximize risk pre-control.

Jinan Metro Line 4 "Ji-Da" section continuously passes beneath 3 high-speed railway lines

A person in charge of Phase I construction of Line 4 from Jinan Rail Transit Group Construction Investment Co., Ltd. introduced that this section is located in the "Jixi Grand Reservoir" area, where groundwater is extremely abundant and the strata are silty clay layers with strong vertical permeability. Therefore, during shield tunneling, mud-water mixtures can easily leak in all directions, and if not attended to, this can lead to over-excavation of soil. During shield machine tunneling, the soil output per ring is usually about 68 cubic meters, and once this value is exceeded, ground settlement may occur.

After comprehensive assessment, the shield adopted the traditional and stable earth pressure balance shield mode for fully enclosed construction. As the foremost excavation face advanced, pressure balance was maintained throughout by means of a certain air pressure and added foam filler, reducing the possibility of collapse during tunneling. Simultaneously with tunneling, prefabricated segments were installed as each ring was jacked forward, and grouting was carried out between the excavation face and the shield segments. "It can be understood this way: shield construction is not simply digging a tunnel, but building an underground structure simultaneously with tunneling and ensuring one-time formation, which can greatly reduce stress loss in the strata."

To address the problem of water-rich ground, the construction team of China Railway 21st Bureau Group carried out synchronous grouting reinforcement inside the tunnel and injected quick-setting materials to shorten the slurry setting time, fully ensuring waterproofing safety. In addition, water-swellable sealing strips were added at the segment joints to block water seepage through gaps and ensure no risk remained. It was precisely through such meticulous and thorough organization and construction that zero settlement was achieved for each of the continuously crossed high-speed railway lines.

Building a metro beneath a metro

Lines 1 and 4 "face off" underground

During construction, the shield machine encountered a "face-off" with the operating Jinan Rail Transit Line 1. Unlike the lateral crossing beneath high-speed railway lines, passing beneath Line 1 was a true "head-on confrontation," with the two lines forming a cross-shaped crossing underground.

Dayang Station on Jinan Rail Transit Line 4 is an interchange station between Line 1 and Line 4. This means that it was necessary both to complete the underpass beneath the Line 1 tunnel and to ensure the corresponding relationship with the station. As is well known, the closer a train is to a station, the closer it is to the ground, so when Line 4 passes beneath Line 1, the closest distance to the bottom of the Line 1 tunnel is only 3.01 meters. This was indeed a great challenge for the shield, requiring the shield team to strictly control every parameter.

What metro underpasses and high-speed railway underpasses have in common is that both face water-rich strata. Beneath the Line 1 tunnel are water-rich silty clay layers and cobble layers. Compared with passing beneath high-speed railways, the difficulty was no less, if not greater. Li Huan, project manager of China Railway 21st Bureau Group for Jinan Rail Transit Line 4, introduced that to prevent potential water seepage risks during tunneling, China Railway 21st Bureau Group scientifically selected shield machines equipped with four rows of tail brushes to enhance the sealing effect of the shield tail and prevent water seepage at the tail; added water-swellable sealing strips at segment joints to block water seepage through gaps; adopted a double-seal design for the portal at shield launching and a fully wrapped sleeve design for the portal at reception to improve portal airtightness and prevent water leakage; during advancement, installed a water-stop ring every 6 meters in the completed tunnel to block water coming from behind the shield machine; and modified the synchronous grouting pipelines to inject quick-setting materials simultaneously during grouting in the advancement process, fully ensuring waterproofing safety.

The longest shield-tunnel section on the entire Line 4

Double-line breakthrough completed in 1 year and 5 months

The "Ji-Da section" began shield tunneling in January last year and finally, after 1 year and 5 months, completed the breakthrough of the longest shield-tunnel section on Jinan Metro Line 4. The smooth breakthrough of this section marks the completion, in Jinan rail transit construction, of the country's ** shield-tunnel section in which a single tunnel passes beneath four railway lines. On June 29, the shield machine was successfully received at Dayang Station in Huaiyin District, marking the successful conclusion of shield work in the section. Next comes comprehensive settlement monitoring and the construction of ancillary structures such as the station.

The person in charge of shield tunneling at Jinan Rail Transit Group introduced that for subsequent settlement monitoring, Jinan Rail Transit Group Construction Investment Company installed four total stations on the high-speed railway bridge piers and in the metro tunnel for round-the-clock real-time monitoring, and arranged dedicated personnel for inspections. The person in charge stated that after metro construction, whether settlement has occurred in existing buildings requires monitoring for about 100 consecutive days. If during this period the settlement caused by construction is clearly within the safe range, then the possibility of subsequent settlement is very small and can be ignored.

The double-line breakthrough of the "Ji-Da" section marks that China Railway 21st Bureau Group has overcome the most complex challenge in the construction of Jinan Metro Line 4, which will greatly promote the full-line opening and operation of Line 4. It is understood that this is also the second Jinan rail transit project in which China Railway 21st Bureau Group has participated. As early as the construction of Line 2, China Railway 21st Bureau Group undertook the construction of Laotun Station and Baliqiao Station, which also accumulated construction experience for its work on Line 4, and it has continuously innovated in construction methods, technical training, safety management, and other aspects, embarking on a path of innovation. At present, among the three stations on Jinan Metro Line 4 undertaken by China Railway 21st Bureau Group, the fastest-progressing Jinan West Station has completed 90% of its construction tasks, Dayang Station has completed 80%, and Lashanhe West Station has completed 20%.

As of now, among the 33 stations on the entire Line 4, 27 stations have achieved capping of the main structure, and 6 stations are in the main structure construction stage; there are a total of 35 sections across the line, of which 26 sections have achieved double-line tunnel breakthrough, and 12 shield machines are currently tunneling. Track laying has commenced on site in 14 stations and 18 sections, with a cumulative total of 32 kilometers of track laid. A relevant person in charge of Jinan Rail Transit Group Construction Investment Company stated that in the future, Jinan Rail Transit Group will always adhere to "building metro for the people and building a metro that satisfies the people" as the guide for its work, continue to accelerate the pace of construction, and help Line 4 open to traffic as soon as possible.