*4.3. The Analysis of Simulation Result*

In the process of pile foundation underpinning, the displacement changes of bridge piers, underpinning beams and new piles are the key monitoring contents. The data extraction points of numerical simulation are shown in Figure 6. Because the whole construction process is simulated, the displacement changes of each extraction point in the whole construction process were analyzed. During the jacking process of pile foundation underpinning, the new underpinning pile will experience settlement due to the different engineering environments, such as insufficient foundation bearing capacity, pile bottom sediment, etc., which will induce additional stress to the upper structure to damage the beam body, which is very important in the study of the settlement law of new piles.

**Figure 6.** Data extraction points of the numerical model.

Figure 7 shows the changing trend of the new pile displacement during the pile foundation underpinning process, and it is the same as the changing trend of the displacement of the bridge pier and the underpinning beam. In the first five times of jacking, this new pile displacement has a relatively stable increase, and it starts to increase from the sixth jacking, especially the settlement of new piles near the old pile side are increased by 40% compared to the previous in the last jacking. The maximum settlement of the new pile on the side far from the old pile is 2.12 mm, and the maximum settlement value of the new pile is 3.48 mm, which has reached the warning value during monitoring but not the control value. Therefore, during the jack-up process of the pile foundation underpinning, it is necessary to focus on the settlement changes of the new piles near the old piles [19].

**Figure 7.** Displacement of the new piles calculated by numerical modelling.

Figure 8 shows the displacement change curve of the underpinning beam. It can be seen that the displacement changes of the underpinning beam on the side away from the old pile is small, and the corresponding growth rate is small, while the displacement of the underpinning beam close to the old pile side changes greatly, and the maximum displacement is 0.88 mm, but it is still within the early warning. The displacement of the underpinning beams on both sides changes smoothly during the unloading stage, and it has no sudden change when the old pile is truncated.

**Figure 8.** Displacement of the underpinning beam calculated by numerical modelling.

The difference in settlement of the new piles on both sides is caused by the difference in the lifting force. However, with the end of the jacking, this difference becomes small, indicating that the deformation of the superstructure is well controlled in the underpinning process, and the design of the jacking force in the underpinning construction is feasible.

The displacement of the bridge pier calculated by numerical modelling is given in Figure 9. It can be seen from the figure that in the first five times of the jack lifting process, the displacement increased relatively stable. However, its growth rate starts to increase from the 6th jacking to the end, and the maximum displacement is 0.68 mm. The displacement is gradually decreased by 0.43 mm when unloading, indicating that the load is well transferred. In the final jacking, the old pile is cut off, and the displacement continues to drop by 0.06 mm, indicating that the load transfer is completed in the underpinning construction, and the new pile is able to sustain the load safely. In addition, there is no sudden change in displacement during the whole simulation process, and all the displacement changes are also within the warning value. In general, the underpinning technical solution was preliminarily verified to be safe and feasible.

**Figure 9.** Displacement of the bridge pier calculated by numerical modelling.

#### **5. Field Verification of Pile Foundation Underpinning Scheme**

#### *5.1. Monitoring Content and Methods*

Pile foundation underpinning is very difficult and has a high construction risk; construction monitoring is the key to ensuring construction safety, and the setting of construction monitoring control values and early warning values is crucial. The monitored data are the deformation of the bridge pier and the underpinning beam during the underpinning process. The monitoring content and warning control value are shown in Table 4. The settlement of the bridge piers is monitored by a crystalline silicon static level instrument, and each bridge pier is installed with a static level measuring point installed on the upper part of the pier column. The inclination of the bridge pier is measured by a dual-axis inclinometer that is installed on the side of the pier parallel to the bridge axis at the top of the pier column. During the monitoring, each pier is installed with a dual-axis inclinometer. Two static level monitoring points are set at the top of the two new piles to monitor the settlement using the static level instrument. By monitoring the relative settlement of the new pile top and the pier body, the absolute settlement of the new pile top can be obtained. Thus, the settlement change of the new pile and the difference in the displacement of new piles on both sides of the main beam during the jacking process. During the underpinning process, due to the inconsistency of the jacking force on the left and right sides, the underpinning beam may be inclined to induce additional loads on the pier body and the main beam, affecting the safety of underpinning construction and bridge structure. Therefore, the monitorization of settlement of the underpinning beam is very necessary. The settlement of the underpinning beam is monitored by a static levelling instrument, and two static levelling points are set at two ends of the underpinning beam (the corresponding displacement of the new pile). The main monitored parameters include absolute settlement and differential settlement at both ends of the joist beam.


**Table 4.** Field monitoring variables and corresponding alarming and controlling values.
