Stability and Load Transfer in Soft Soil Foundations Using Geosynthetic-Reinforced Pile-Supported Embankments for an International Motor-Racing Circuit

Soft soil is characterized by high void ratios, significant compressibility, and low strength, which cause inadequate bearing capacity, excessive settlement, and considerable lateral displacement and ultimately can jeopardize the stability and longevity of the constructed facilities. To address these limitations various soil improvement techniques have been developed such as pile-supported embankments (PEs) which is unknown for it cost-effectiveness and high construction efficiency. However, still the performance of PEs is highly dependent on pile spacing, diameter, and the type of pile cap used and to enhance the load-transfer efficiency of PEs to control deformations often has high engineering costs. Therefore, finding a balance between project costs and foundation performance is critical. To achieve this balance, geosynthetic reinforcement is commonly integrated with Pes which creates geosynthetic-reinforced pile-supported embankments (GRPEs). In a GRPE, a load transfer platform (LTP) is installed above the pile caps to improve the long-term performance of the foundation and the LTP consist of granular materials such as gravel and one or more layers of geosynthetics that facilitate better load transfer to piles, and reduce foundation settlements. Recently, motor-racing circuit in Wuhan, China was constructed and the soft soil subgrade posed significant challenges due to excessive settlement. To address this challenge, new paper published in KSCE Journal of Civil Engineering and led by Professor Xianwei Zhang from the of Rock and Soil Mechanics (IRSM) at the Chinese Academy of Sciences (CAS) and conducted by PhD candidate Gang Wang, Dr. Xinyu Liu, Zhixiong Chang & Zhihai Liu, the researchers evaluated the performance of geogrid-reinforced embankments and pile-supported embankments and provided a reliable reference for the design and construction of similar projects.

In their experiments, the authors performed two large-scale field tests on the main track of the Wuhan International Circuit. The first test involved a PE without geogrid reinforcement, and the second test included a triaxial geogrid reinforcement to create a GRPE. They systematically monitored earth pressures, settlements, lateral displacements, and pore water pressures to understand the impact of geosynthetic reinforcement. They also monitored the earth pressures on pile caps (Ep) and the soil surface (Es) over approximately 100 days and found in both test sites that the values of Es initially increased with the embankment height, peaking at around 12 to 15 days before stabilizing. In contrast, the values of Ep continued to grow throughout the monitoring period. The geogrid reinforcement in test site 2 resulted in smaller fluctuations in Es and higher values of Ep compared to test site 1 which indicated that the geogrid reinforcement transferred more load to the piles and enhanced the load-transfer efficiency. Moreover, the researchers measured total settlements on the pile caps and the soil surface beneath the embankment at both test sites and observed the settlements increased rapidly in the initial stage of embankment construction and then gradually stabilized. However, in the unreinforced site the differential settlements between the piles and the surrounding soil were more pronounced with the surrounding soil have higher settlement rates. According to the authors, the layered settlement gauges provided valuable data on the subsoil’s layered settlements and the results indicated that the uppermost 13-m-thick soft-soil layer contributed the most to the total settlements in both test sites and that the reduction in settlements was possibly due to the load-transfer mechanism of the GRPE where the geosynthetic layers acted as a tensioned membrane and distributed the load more effectively and prevented excessive subsoil compression. Furthermore, the authors monitored the lateral soil displacements both outside and beneath the embankment and found in the unreinforced site, the lateral displacements outside the embankment were significant with the maximum displacements occurs near the embankment toe. The geogrid reinforcement in test site 2 reduced these lateral displacements by 32.2% to 44.0%. They also measured beneath the embankment, lateral displacements using inclinometers and showed the inclusion of geogrid reinforcement reduced the influence depth of lateral displacement from approximately 20 meters to 15 meters below the ground surface. The maximum lateral displacement recorded was 1.39 mm in the unreinforced site and 1.15 mm in the reinforced site which means that the geosynthetics minimized horizontal displacement beneath the embankment. Additionally, the geogrid reinforcement reduced the lateral displacement of the shallow soft-soil layers and contributed to the overall stability of the embankment. The team measured as well the pore water pressures using piezometers installed at various depths within the subsoil and reported that during embankment construction, high excess pore water pressures were generated in the shallow soil layers due to the rapid placement of fill and slow water dissipation. The peak values of excess pore water pressure reached 80 kPa in the unreinforced site and 40 kPa in the reinforced site and that the geogrid reinforcement in test site 2 significantly reduced the excess pore water pressures and enhanced the load transfer from the soft soil to the adjacent piles. Indeed, the maximum excess pore water pressure was minimized by approximately 50% in the reinforced site compared to the unreinforced site which demonstrates the effectiveness of geosynthetics in mitigating pore pressure buildup. In summary, Professor Xianwei Zhang and colleagues demonstrated the effectiveness of GRPEs in addressing the geotechnical challenges associated with constructing on soft soils. The practical implications of innovative engineering solution are far-reaching for infrastructure projects in areas with soft soil conditions. The authors’ findings provided a strong empirical basis for the adoption of geosynthetic reinforcement in pile-supported embankments and offered cost-effective and efficient solution to the limitation of   soft soils which can lead to more stable, durable, and reliable infrastructure.