## 1 Introduction  
When constructing roads and seawalls on soft clay foundations, due to the higher strength of the roadbed fill material compared to the soft clay, the constructed roadbed-foundation system is characterized as an upper-hard, lower-soft system or a soft-sandwiched system. In this system, traffic vibration loads will have a significant impact on the soft clay layer beneath the roadbed. Under the long-term action of traffic loads, the soft clay will experience reconsolidation (relative to the initial consolidation under the static loads of the roadbed), leading to substantial post-construction settlement. The following example from a seawall in situ observation project undertaken by our research group illustrates this situation. The project is the Nanyangtu reclamation project in Linhai City, Taizhou City, Zhejiang Province, China. The total length of the seawall is 7410 m, and the total reclamation area is about 10,000 mu (approximately 6667 hectares). The seawall is not designed for traffic. The soft clay foundation under the seawall is treated with a combination of plastic drainage boards and geotextile reinforcement. A typical cross-section of the seawall is shown in Figure 1. Construction of the seawall began in September 2008 and was completed in March 2012. The seawall was constructed in stages and sections. Settlement observations show that the settlement pattern of the seawall (2009-2014) is highly consistent with the classical static consolidation theory: The primary consolidation settlement is fast during loading, and the secondary consolidation settlement is slow and tends to stabilize after a period of time after loading is completed. The total settlement of different measuring points in the main control section of the seawall is about 1500-2200 mm [1]. After the completion of the project, a third party verified the elevation of the seawall in June 2015, and the elevation met the design requirements. Subsequently, with the development of the land within the enclosure (factory construction), the top of the seawall was used as a single-lane road for construction, as shown in Figure 2. The third party verified the elevation of the seawall again in March 2020 and found that the entire seawall had experienced significant subsequent settlement, with the maximum subsequent settlement in the main control section reaching 900 mm [2]. The completion acceptance report organized by the Zhejiang Provincial Water Resources Department in November 2020 showed that the top of the seawall is lower than the 50-year flood prevention standard of the design but can meet the 20-year flood prevention standard. In addition, for road construction projects on soft clay foundations, design calculations typically employ the quasi-static method to apply traffic loads to the roadbed. The super-loading pre-compression foundation treatment method is frequently used to reduce the post-construction settlement of the roadbed. This treatment results in the foundation soil becoming overconsolidated. Classical static consolidation theory suggests that after super-loading pre-compression reinforcement, the soft clay foundation has eliminated most of the primary consolidation settlement and a significant portion of the secondary consolidation settlement. Consequently, the post-construction settlement of the roadbed  
Figure 1: Typical cross section of the seawall. Figure 2: Truck traffic on the seawall crest. should be significantly reduced, satisfying the requirements for both differential and total settlement. However, engineering practice demonstrates that even after reinforcing the roadbed foundation with the super-loading pre-compression method designed according to traditional soil mechanics theory, the roadbed can still experience significant post-construction settlement under the action of traffic loads. This settlement often persists for a long time.