“Parking difficulties” plague many large cities. Recently, beneath two century-old buildings in Nanjing, an 8-story underground mechanical parking garage with a depth of 28 meters has been constructed. Currently, the garage is in the stage of basement equipment installation and decoration construction, and is scheduled to be officially put into use in May this year.
　　Entering the construction site, there is no flying dust or dirt; the deep pits common on construction sites have been replaced by several small openings near the buildings. Walking underground, a deep, bottomless garage lies quietly, while the old buildings above remain completely undamaged. Wang Zhihua, general manager of the Fifth Engineering Company of Shanghai Construction Group Second Construction Co., Ltd., the construction unit, introduced that this garage can park 252 cars, greatly alleviating local parking pressure. This garage model will be promoted in residential and commercial areas of Shanghai in the future.
Two Buildings Moved a Total of 4 Times
　　The two buildings under construction this time have considerable “background”; every red brick and tile bears the marks of history. Among them, the building at No. 57 Beijing West Road was originally built in 1937, while the building at No. 32 Tianmu Road was built in 1946. Since both historic buildings are located in downtown Nanjing, where parking demand is high but space is insufficient, they had to turn to underground space.
　　According to reports, the on-site work mainly uses two methods: “translation” and “top-down construction.” First, jacks lift the buildings to a height of about 1 meter to begin translation. The building at No. 57 Beijing West Road, originally on the north side of the site, was moved to the south side, and top-down construction began on the north side. After the underground floor on the north side was completed, both buildings No. 32 and No. 57 were moved together to the north side, and construction began on the south side. Finally, the building at No. 32 Tianmu Road, originally on the south side, was “returned” to its original position. The two buildings underwent a total of 4 translations.
　　How can the historical charm of the old buildings be preserved during construction? Yu Xiaodong, project manager of the construction unit, said that construction adheres to three principles: first, the principle of integrity, protecting not only individual buildings but also the surrounding environment compatible with them; second, the principle of continuity, not completely erasing the historical context but acknowledging traces left from different historical periods; finally, reversibility, reducing the use of irreversible materials and processes to reserve space for future renovations.
　　In specific operations, the team first reinforced the exterior walls with steel bars, used concrete as a base, and wrapped the internal wooden structure with steel wire mesh to form a “large cover.” At the same time, the team found old bricks of the same age as the buildings, crushed them into mortar for repair and filling.
　　Top-Down Construction Like Minimally Invasive Surgery
　　“Compared to surgery, traditional construction sites are like major operations, while top-down construction is minimally invasive surgery,” said Long Libo, chief engineer of Shanghai Construction Group Second Construction Co., Ltd. In underground engineering construction, the conventional bottom-up method is generally adopted, which first excavates a deep pit, then repairs layer by layer from bottom to top, with temporary supports erected for each layer. Top-down construction is the opposite: only a protective slab needs to be laid flat on the ground, and construction proceeds from top to bottom. People passing by the site will only see a few “small openings” on the ground for workers and materials to enter and exit, and will not realize that a massive project is underway underground. Moreover, each completed floor can provide space for storing materials for the next floor's construction, so materials do not need to be stacked on the ground, ensuring that the buildings above can continue to operate.
　　Top-down construction places higher demands on construction techniques. For example, the one-column-one-pile technique used in the project requires each steel pipe to be positioned precisely. Because top-down construction is underground, it avoids flying dust and noise, is unaffected by weather, and greatly improves efficiency. In terms of cost, because top-down construction reduces support and maintenance costs, the garage cost is only 60% of that of a similar-scale garage built with conventional bottom-up construction nearby. In terms of schedule, because the main structure replaces temporary supports, the project team completed structural construction in only 9 months, saving at least 3 months compared with traditional techniques.
　　“Most importantly, top-down construction has great potential in urban renewal,” Long Libo said. “Top-down construction causes less damage to the on-site environment and provides better control of foundation pit deformation. The deformation of a 28-meter-deep foundation pit was only 3.61 millimeters, one-sixth of the design value. For fragile old buildings, top-down construction can better protect the original building structure from damage.
　　Better Promote Intensive Land Use
　　Shanghai faces the same parking difficulties. In August 2017, statistics from the Municipal Transport Commission showed that the city had 1.181 million parking spaces for various types of passenger cars, with a shortage of 849,000 residential parking spaces and 158,000 non-residential parking spaces. Not only is there a huge gap in residential parking facilities, but parking facilities around schools, hospitals, and other units are also insufficient, and nighttime parking demand in commercial districts is difficult to meet.
　　In Shanghai, where land is extremely precious, top-down construction for deep underground garages has broad application prospects. “I hope that in the future, garages can be added beneath old buildings such as those along the Bund, to better promote intensive land use and enhance economic value,” Long Libo said. Future underground garages will also undergo more renovations. The team plans to install intelligent robots in the garage; people only need to set the pickup time on their mobile phones, and the robot will deliver the car to the entrance on time, reducing queuing time in the garage.
　　In addition, the development of deep underground garages will also promote the development of industrial clusters. When using top-down construction to build garages, a large amount of compact and lightweight equipment needs to be introduced, which will bring new development opportunities to the domestic equipment manufacturing industry. At the same time, underground logistics and utility tunnels will also become key areas of underground space development.