Project rendering

Underground space is also vividly called "an extension of the above-ground city." In recent years, along with the gradual improvement of urban surface infrastructure, the development of underground spaces such as rail transit, utility tunnels, and underground commercial complexes has become a buzzword in Wuhan's infrastructure construction.

The under-construction Optics Valley Underground Space Project has a total floor area of about 600,000 square meters, equivalent to the area of 72 standard football fields, and is the longest underground space corridor in China. What challenges are involved in building such a massive project "hidden" dozens of meters underground? What are the construction "secrets"?

Recently, the reporter visited Section II of the Optics Valley Underground Space Project, undertaken by the Second Company of China Construction Third Engineering Bureau, to explore the "cutting-edge techniques" used in its construction.‍

15,000 Pile Foundations Driven in Karst Geology

The Optics Valley Underground Space Project is located in the core area of East Optics Valley. With a total length of 5.8 kilometers, it runs north-south and covers rail transit, utility tunnels, comprehensive commercial complexes, and more. Section II, constructed by the Second Company of China Construction Third Engineering Bureau, starts at Gaoxin 2nd Road and ends at Gaoxin 5th Road, with a total length of 2.6 kilometers.

In the underground section of Gaoxin 2nd Road, several municipal pipelines for gas, heating, and tap water are buried. Among them, the 110KV Huawei dedicated line is the top priority for relocation and modification.

Project manager Chen Yaping said that if traditional relocation methods were used, the route would need to detour from Optics Valley 4th Road to Shendun 3rd Road and then to Optics Valley 6th Road, and the relocation cost would be in the hundreds of millions of yuan. To ensure nothing goes wrong, the project organized an elite technical team to build a trestle bridge on the structural roof slab specifically for this approximately 200-meter cable section. With the assistance of the power department, four circuits within the cable were laid while being powered down one by one, always ensuring that three circuits remained in normal operation... After years of careful planning and comparison, this "ultra-sensitive line" was successfully relocated in September this year.

The Optics Valley Underground Space Project involves earthwork excavation of 7 million cubic meters, enough to fill 4,000 standard swimming pools. In underground engineering, structural stability is crucial. Pile foundation construction is the prerequisite.

"Section II is further divided into sections 6 to 10. Preliminary geological surveys showed that the entire section consists of hard rock layers, with two sections basically riddled with karst caves," Chen Yaping told the reporter. If karst terrain is not properly treated, it can at best affect the integrity of pile foundations and at worst affect structural safety. To this end, advanced drilling was adopted technically, with grouting and stone filling into the karst caves in advance. "For the largest cave, grouting reached more than 100 cubic meters of cement mortar." He explained that for rock layers with particularly hard soil, impact drills were used to "apply force"... Ultimately, a total of 15,000 pile foundations were driven in this section. Upon testing, all were Class I piles of excellent quality.

After the foundation was solid, how could the construction of the two-level underground spatial structure be made stable? The project team replaced conventional steel pipes with disc-lock scaffolding as the "backbone" of the underground city. The load capacity of a single vertical pole of this component is 10 times that of traditional steel pipes, and the spacing between poles is 2 to 3 times that of traditional steel pipe fasteners, which not only ensures the stability of the frame but also reduces safety risks.‍

Self-Developed Self-Compacting Backfill Soil Reduces Cost and Increases Efficiency

With the "steel-reinforced iron bones" as support, pouring concrete injects "blood" into the building.

According to the project construction team, concrete pouring is unlike other construction work: it must be completed in one go within the initial setting time of 1 to 3 hours, otherwise cold joints are likely to occur.

In the concrete pouring for Section 9, the project needed to pour more than 4,500 cubic meters of concrete in one go. To this end, on-site personnel had to contact the mixing plant in advance, organize traffic, prepare six spare vibrators, and prepare emergency power and rainproof equipment. To ensure pouring quality, multiple temperature-measuring pipes were embedded at the center point, on the surface, and outdoors to ensure the temperature difference between inside and outside was controlled within 25°C.

During construction, the project carried out technical innovations based on actual site conditions. For example, between the foundation pit retaining piles and the structure, there is often a "fat groove" about 15 to 20 meters high and 80 centimeters wide to facilitate the erection of the structural exterior wall frame. After construction is completed, could a backfill material with better and higher cost-effectiveness be developed?

To this end, the project team worked with universities to develop a self-compacting backfill soil. This material has the function of grouting material and is lower in cost. Overall, it can reduce costs and increase efficiency by nearly 10 million yuan for the entire line. It is reported that backfill material with such performance is the first of its kind in Hubei.

Up to now, pile foundation, earthwork, main structure, and road drainage construction at the project site are proceeding in an orderly manner. Once completed, it will become the longest underground space corridor in China, helping Optics Valley become a "three-dimensional city" integrating above-ground and underground, indoor and outdoor, functional layout, and transportation and municipal utilities.‍