## 2 Materials and Methods  
### Description of Engineering Project and Geology Condition  
In this study, the reconstruction and expansion project of the Qinzhou-Beihai section of the Lanzhou-Haikou (i.e., Lanhai) expressway, as the engineering project, is located in Qinzhou City and Beihai City, Guangxi. This project is divided into the Qinzhou section mainline of Lanhai Expressway and the Beihai branch line, which are important parts of the national expressway network. Qinshan section mainline (k2126+277-k2156+500) is routed from west to east, and the total length of the Qinshan section mainline is designed as 30.223 km. The Beihai branch line (lk540+000-lk550+000) is routed from north to south, and the total length of the Beihai branch line is designed as 10 km. Most of the pile foundations in the bridge design of this engineering project are end-bearing piles, in which the pile foundation is not deep and the pile column is not high. Therefore, the prefabricated construction of the bridge substructure could be easily realized. According to the field investigation and engineering geological conditions, the test site was selected as an open space on the downstream line of the Tieshangang service area. Figure 1a shows the schematic diagram of site selection and site conditions. The site is located in the northeast corner of the Tieshangang service area, which is a turf green land at present. Several oil tanks were stacked during the survey, which has little impact on the operation of the service area and has no conflict with the parking plan of the reconstructed service area, and, thus, this site can be considered used for the construction of a novel prefabricated bridge substructure \"pile-column integration\" [37]. In addition, the depth of the on-site geological survey is shallow and does not reach the moderately weathered or slightly weathered rock stratum, and the core samples of rock and soil are shown in Figure 1b. The basic parameters and analysis of soil behavior are presented in Figure 1c.  
The original bridge adopted 3 \(\times\) 20 m prestressed concrete simply supported bridge deck continuous hollow beam, the substructure abtument was column abtument and pile foundation, and the pier was column pier and expanded foundation. The original bridge deck was 28 m in width and 64.04 m in length. The abtument pile foundation at the lower part of the original bridge was designed as a rock socketed pile, embedded with weakly weathered quartz sandstone. The pier expanded the foundation, and the foundation entered strongly weathered quartz sandstone. All the superstructure of the original bridge will be removed, and a new prestressed concrete simply supported, and then a continuous box girder will be replaced. The expansion will be carried out with the same span and the same number of holes as the original bridge. After the reconstruction and expansion, the whole line will be uniformly expanded to eight lanes, that is, the reconstruction and expansion bridge is expanded from 28 m of the original bridge width to 44.5 m, 7 m on the left and 9.5 m on the right. The substructure of the new bridge could use the original bridge as a whole.  
Figure 1: Application project for “Pile–Column Integration”: (**a**) schematic diagram of site selection and site conditions; (**b**) core samples of rock and soil; and (**c**) the basic parameters of soil.  
pier column and foundation, and a new capping beam (abutment cap) would be built. The expansion part adopts a column pier and pile foundation with a column diameter of 1.3 m and pile diameter of 1.5 m according to the original pier and abutment structure layout and geological conditions and considering the feasibility of construction. The abutments on both banks shall be on both sides of the foundation of the original pile foundation column abutment, the conical slope shall be removed, and a 2 cm construction joint should be reserved at the connection part of the abutment cap between the existing old bridge and the widened bridge.