## 3 Experiment  
### Study Area  
Two highways in Fuoshan, Guangdong Province, namely Lungui Highway and G1501 Guangzhou belt highway, built on soft clay subgrade, are selected as test structures (Figure 2). Lungui Highway, as the significant transportation hub, connects G321 highway in the north and West Jiuhongqi Road in the south. With a total length of approximately 12 km, Lungui Highway is one of the three major  
Figure 1: Experimental flow of small baseline subset synthetic aperture radar interferometry (SBAS-InSAR) based on two models.  
south-north transportation lines in Fuoshan. Its main road was opened to traffic in October 2014, constructed under the first-class highway standard and the urban roads standard. The subgrade was designed to be 60 m in width, with soft clay being the primary rock-soil substrate along the route. G1501 Highway, located in Shunde District, Fuoshan, is the south second ring part of the Guangzhou belt highway and was opened to traffic in 2010. The standard of two-way six-lane expressway with design of 100 km/h is adopted for the whole route. The total length of the route is about 49.33 km. Connected to the Guangzhou North stretch of Beijing-Zhuhai Expressway, G1501 Highway is a key transportation line diverting large traffic load from Guangzhou city transportation and strengthening the connection of surrounding urban agglomerations. According to the design criterion of these test highways, the permissible vertical post-construction subsidence is 30 cm/yr for regular road segment, 20 cm/yr for culvert, and 10 cm/yr for bridges connections. According to the statistics of Fuoshan Transportation Bureau, the passengers flow volume of Shunde District, where the highways are located, is up to 2018.31 million people per kilometer, whereas the freights' flow using the test highways is approximately 654.64 million tons per kilometer in 2014. The huge traffic flow indicates the significant traffic situation of those two highways.  
Both highways are located in delta alluvial plain, characterized by the presence of flat and low-lying surface of liquefiable sandy soil and underlying bedrock of granites and clay sand. The main geological component of the soft clay along the routes comprises muddy clay and silty clay, with characteristics of soft-plastic behavior. Due to the geotechnical characteristics of high natural moisture content, high compressibility, and low mechanical strength, the engineering properties of soft clay are extremely poor. Especially under the action of vibration load, soft clay is prone to engineering geological problems such as transverse slip, uneven settlement, and creep, accordingly imposing serious problems on the stability of subgrade and structures. Figure 2c shows the distribution of the water system around Lungui Highway and G1501 Highway. We can see the test highways are surrounded by three main rivers--Shunde, Tanzhou and Ronggui Channel, providing adequate underground water supply to the area. Due to the fast flow rate, Ronggui and Shunde Branch Channel play an important role in the underground aquifer in this area. The water level and volume in this area are greatly affected by precipitation and tide, according to the hydrological materials of this area.