Sweeping across the tidal flats, the Xiamen Xiang'an International Airport in Fujian is taking shape. Not far away, the foundation pit for Airport West Station, the transfer station between Xiamen Metro Lines 3 and 4, has been backfilled, with 1,728 pressure-dispersed, retrievable anchors having "completed their mission." These steel "skeletons," with a total length of nearly 28,000 meters, once supported a solid space within the soft foundation formed by land reclamation.

The Airport West Station project for Xiamen Metro Lines 3/4, undertaken by The Sixth Engineering (Xiamen) Co., Ltd. of CCCC Third Harbor Engineering Co., Ltd., required sharing a massive foundation pit of nearly 100,000 square meters with the adjacent open-cut sections. The station's width of 96 meters far exceeds the standard 20-meter width of conventional metro stations. "Traditional internal bracing systems create 'compartmentalized' stress distribution in wide foundation pits, akin to cutting tofu into blocks, which can actually increase the risk of deformation," explained Project Manager Lin Sixin.

The strata formed by land reclamation consist of under-consolidated loose silt-sand layers and original marine silt layers, prone to thixotropy and rheology. In anchor support, the anchors act like "rubber bands" pulling backward against the foundation pit, preventing it from collapsing inward. In October 2022, through 18 preliminary comparative tests, the project team found that ordinary tension-type anchors were prone to breaking due to localized stress reaching its limit. In contrast, pressure-dispersed anchors, with their structure of 4 to 7 unbonded steel strands per bundle (each strand itself multi-wire), distribute stress evenly. A single bundle could bear a load of 804 kilonewtons, sufficient to withstand the greater tension required to counter the "active" soil of the foundation pit.

"Just selecting the right type wasn't enough. To make the anchors 'stand firm' in the soft silt, we had to precisely control every parameter," said Chief Project Engineer Ma Zhuang. The test log on his desk recorded the construction method optimized by the team over more than two months: adopting a full casing follow-up process; during grouting, first injecting neat cement slurry at normal pressure, followed by secondary high-pressure grouting after the grout strength reached 5 Megapascals, increasing the grip strength of the anchor section by 20%; and employing a multi-cycle tensioning method for compensation tensioning. This process reduced the construction cycle for a single anchor from 14 days to 9 days, and the overall project duration is expected to be shortened by nearly 30%.

However, in March 2023, the foundation pit displacement monitoring system suddenly issued a warning. The team traced the problem to the fixing of the upper end of the anchors. The lower end of the anchor needs to be buried deep in the soil and fixed with grout, while the upper end must be securely locked at the pit edge using wedges. Only when both ends are taut can a stable rear pulling force be formed. "Manual fixing inevitably leads to angular deviations, like pulling a rubber band with a crooked hand – the force deflects sideways, the anchor can't be taut, and the pit naturally deforms more easily," Ma Zhuang explained.

How to realign this "misaligned force"? The project team found inspiration in the uniform force transmission characteristic of spherical hinge bearings used in bridges. They installed a socketed force transmission plate with 360-degree rotation capability at the upper end of the anchor. This "universal joint" automatically corrects angular deviations, ensuring the pulling force is always transmitted along a straight line. Simultaneously, the team equipped each anchor with an "intelligent wristband" that collects 100 sets of data per second to monitor stress, creating a dual safeguard of "passive correction" and "active monitoring."

After solving the challenge of "standing firm," the issue of "leaving clean" also needed consideration. The Airport West Station later needs to receive tunnel boring machines, and the surrounding area is planned for comprehensive underground development. Residual anchors would become "underground obstacles." For this reason, the team specifically designed a "releasable and retrievable" structure. At the bottom of each anchor bundle, composed of 4 to 7 unbonded steel strands, interlocking wedges were installed. During retrieval, simply striking one strand to make it move backward causes the wedges to release, allowing the strands to be pulled out one by one.

In May of this year, with the backfilling of the pit around Airport West Station completed, all 1,728 anchors were successfully retrieved, saving over 900,000 RMB in material costs. This technology has also obtained a national utility model patent, becoming a "golden key" for supporting foundation pits in soft ground.

Today, on the backfilled foundation pit of Airport West Station, construction vehicles come and go. The planning for underground connection passages linking the station with surrounding commercial complexes and transport hubs is steadily progressing, adding confidence to the seamless integration between Xiamen Xiang'an International Airport and the urban transport network.

(Content provided by: Ma Zhuang, Liu Ruixin, Shen Danyuan)