Effect of pier bearing construction on nearby high-speed rail line bridges

2.1. Engineering overview

A new intercity railroad is located in the plain area of the Yangtze River Delta, and the construction area is characterized by large variations in stratigraphic lithology and poor engineering geological conditions. The upper part of the clayey soil, the lower part of the sand layer containing multiple layers of fine sand, shallow groundwater is pore diving, diving level depth ranging from 0 to 3.0 m, groundwater with micro-pressure, groundwater is rich, water level changes are not large. The whole line is parallel to the existing Hutong High-speed Railway, with a design speed of 350 km/h, and the total length of the line is 278.53 km. The location of the steel sheet pile cofferdam of part of the bearing platforms is less than 30 m away from the Hutong High-speed Railway line, which belongs to the neighboring existing line, and the 305 pier bearings of bridges are facing the construction of neighboring high-speed railway line. The construction site of the new pier bearing platform is shown in Fig. 1. The pier bearing platform in the area with poor soil quality, located in the reservoir and adjacent to the high-speed rail line adopts the steel sheet pile cofferdam construction.

2.2. Soil parameters

The basic physical and mechanical properties of each soil layer are derived from the proposed design parameters of the pier bearing platform in the engineering ground investigation report, with a total of five soil layers. The parameters of each soil layer are shown in Table 1.

2.3. Structural material modeling

Plaxis 3D finite element software can use plate unit, beam unit, Embedded pile unit to simulate the structure of the actual project, and interface unit to simulate the mutual contact of the structure. The cofferdam adopts Larsen SP-VIL steel sheet piles and the concrete bedding under the pier abutment is simulated by plate unit; the pier abutment supporting structure is simulated by beam unit, the inner support adopts circular steel pipe, and the bored pile is simulated by Embedded pile unit. The size of the abutment adjacent to the high-speed railway line is 9.3 m×6.6 m×2.5 m, the size of the bottom of the pier is 5.8 m×2.4 m, and the height of the pier is 14.5 m, and the bottom of the abutment is drilled piles with the diameter of 1.0 m and the length of the piles is 70 m, the nearest distance between the abutment of the pier adjacent to the high-speed railway line and the new pier abutment pit is 12.45 m, and the excavation depth of the pit of the new pier abutment is 11.35 m at most, and the weir is used as the support structure of the pier abutment, the inner support is made of circular steel pipe. The construction is driven into 24 m deep steel sheet piles, and the support structure is set up at the depth of 1, 4 and 7 m in the excavation of the foundation pit, including walings, inner supports and corner supports. The bottom sealing was done using 0.3 m thick C30 concrete and a 0.2 m thick gravel bedding. The bearing platforms and piers adjacent to the high-speed railway line are made of C30 concrete, and the main reinforcement material is HRB400 steel bar.

2.4. Finite element model

This study exclusively analyzes the impact of new pier bearing platform construction on adjacent high-speed railway line bridge sections, omitting consideration of dynamic loading from railway traffic. The neighboring high-speed rail line is in the form of bridge pier bearing platform, and the pit area of the new pier bearing platform is larger, the excavation depth is deeper, and the hydraulic conditions at the location are more complicated. Considering the spatial stress-strain effect of the new pier bearing platform construction, Plaxis 3D finite element software will be used to simulate the finite element model mesh as shown in Fig. 2. In order to ensure the accuracy of the calculation, the soil is simulated by 15-node triangular cells [13]. According to the actual situation to determine the location of the water table, the water table line is set at 0 m, the model boundary to limit the seepage, a total of 22,395 cells generated, 53,916 nodes.

Fig. 1Construction site plan of the new pier bearing platform adjacent to the high-speed rail line

Fig. 2Mesh division of the finite element model

3.1. Displacement analysis of pier bearing platforms adjacent to high-speed rail line

The magnitude of displacement difference of adjacent pier bearing platforms plays a key role in the safe and stable operation of the neighboring high-speed rail line. The displacement map of the new pier bearing platforms in its final stage and the displacement map of the pier bearing platforms of the neighboring high-speed rail line are shown in Figs. 3 and 4. As can be seen in Figs. 3 and 4, the pier bearing displacement is mainly from settlement, and the horizontal displacement is too small compared to the settlement value. As the distance to the pit increases, the displacement decreases, pier bearing platform 3 (middle bearing platform) displacement is the largest. Taking the vertical diagonal as the dividing line, the pier and bearing on the side close to the foundation pit are larger than on the other side, and the total displacement trend of each bearing is different. The pier body of pier bearing 3 is displaced towards the foundation pit, while the rest of the pier body is displaced towards the $x$-negative direction, and the bearing platform part is dominated by settlement. Take the pier body and bearing platform settlement maximum position, the rest of the pier bearing platform also choose the same position node, draw the vertical displacement curve.

Fig. 3Displacement cloud of new pier bearing (m)

Fig. 4Displacement maps of pier bearing platforms adjacent to the high-speed rail line

In order to avoid the pier bearing platform to produce too large a difference in the uneven settlement, and then the various neighboring high-speed rail line pier and bearing platform final settlement is plotted as a curve, as shown in Fig. 5.

Fig. 5Vertical displacement curves of pier bearing platforms adjacent to the high-speed rail line

a) Pier

b) Bearing platform

c) Pier bearing platform

From Fig. 5, it can be concluded that: (1) the settlement trend of pier body and bearing platform adjacent to the high-speed rail line is similar, the maximum settlement is within 6mm, and a large settlement is generated in the construction of bored piles, followed by the construction of steel sheet piles, which also has a relatively large impact on the settlement, and the closer the pit, the greater the impact. The final settlement of the platform of the existing high-speed rail line is slightly larger than that of the pier body, and at the location of pier bearing platform 3, the settlement of the pier body and the bearing platform have reached the peak. (2) At present, the limit value of total settlement of pier bearing platform of high-speed railroad in China [14] is ${∆}_1=20L^{1/2}$; the limit value of uniform total settlement of adjacent pier bearing platform is ${∆}_2=10L^{1/2}$, where $L$ is the length of the minimum span between adjacent pier bearing platforms, m. Calculation can be obtained as ${∆}_1=$100 mm, ${∆}_2=$50 mm, and the total settlement and the difference settlement of adjacent pier bearing platforms in the model are smaller than ${∆}_1$ and ${∆}_2$, which satisfy the requirements.

3.2. Impact of new pier bearing distance on neighboring high-speed rail lines

The distance from the newly constructed pier bearing to the neighboring high-speed rail line pier bearing is the key factor affecting the soil displacement of the high-speed rail line. By varying the distance between the two lines (2, 4, 8, 12, 16, 32 and 64 m), the maximum values of the displacement of the high-speed rail line pier bearings in the horizontal and vertical directions at different distances were obtained, as shown in Fig. 6.

Fig. 6Soil displacement-distance relationships for pier bearing platform adjacent to the high-speed rail line

a) Horizontal displacement

b) Vertical displacement

From Fig. 6, it can be seen that both horizontal and vertical displacements tend to decrease as the distance from the newly constructed pier bearing to the soil body where the pier bearing of the high-speed rail line is located increases, and a curve-fitting relationship for the displacement ($u_{y/z}$) of the bridge section with distance ($d)$ is derived.

The equation of the fitted curve for horizontal displacement and vertical displacement are shown in Eq. (1) and Eq. (2):

where $u_{y/z}$ is the displacement in the $y$ or $z$ direction, respectively, mm; $d$ is the distance from the new pier cap project to the pier cap of the high-speed railway line, m; $R^2$ is the correlation coefficient.

In the Technical Specification for Observation and Evaluation of Settlement and Displacement in Railway Engineering [15] and High-speed Railway Design Specification [16], the post-work settlement of ballasted track bridge pier bearing platforms should be less than 30 mm for a design speed of 250-350 km/h, and that of ballasted track should be less than 20 mm, but this is the alarm value for post-work observation, and the limit value should be controlled in a lower range in the neighboring construction stage. Numerical calculation of horizontal displacement and vertical displacement is numerically located in the same order of magnitude, so the limit value of horizontal displacement can also refer to the value of vertical displacement, and when the limit value of displacement is stipulated, the safety range of pier and bearing platform construction can be inverted through the fitted displacement curve. Given that there is a 60 % difference between the numerical calculation results for the horizontal displacement and vertical displacement quantities of the bridge section of the current high-speed railway line, let $u_y=$2 mm, $d_y=$6.31 m; let $u_y=$1 mm, $d_y=$18.28 m; and let $u_y=$0 mm, $d_y=$58.11 m. Assume $u_z=$ –5 mm, $d_z=$ 10.10 m, $u_z=$ –2.5 mm, $d_z=$ 25.83 m, and $u_z=$ 0 mm, $d_z=$ 62.02 m in separate equations. Results show that the adjacent high-speed railway bridge section is significantly impacted by distances of 6 meters or less.