Using servo steel support system for active control of deformation during foundation fit excavation has high superiority. To investigate the coherence of the servo support axial force, PLAXIS 3D is used to carry out numerical analysis on the coherence of the axial force applied by the construction and compare with the field test results; Field tests were conducted on the diaphragm wall joint deformation during the axial force application based on the principle of the generation of axial force coherence. The results show that the farther away from the active axial force, from which suffered get the smaller influence, and the size of the applied axial force’s effect on the support in other directions in the order of horizontal, vertical, and oblique. Moreover, the higher the application position of the active axial force of the servo support, the greater the lateral axial force loss rate generated by other supports, while the opposite in vertical axial force loss rate is true. The maximum axial force loss rate is 19%. The deformation of the diaphragm wall joint in the servo steel support zone is more significant than that in the pre-stressed steel support zone. The deformation of the joint will, in turn, affect the axial force.
1 Instruction
Urban economic growth and population expansion have led to the vigorous development of underground transportation systems. In underground space development, safety issues occupy a more critical position. The active deformation control of the foundation pit retaining structure has become the primary goal of construction safety control [1‐4]. The steel support system is widely used in deep foundation pit support systems because of its high efficiency and pre-axial force [5, 6]. Many studies have given various setting methods to determine the axial force of the support. Still, the conventional steel support will cause prestress loss due to on-site construction, environmental changes, and other factors, significantly reducing the ability to control deformation [7‐9]. The servo system is applied to the steel support in the engineering field to meet the increasingly stringent safety control requirements. Through real-time monitoring and automatic compensation of axial force, remarkable results have been achieved in the lateral deformation control of retaining structures in many engineering cases [10, 11]. However, the retaining structure is often continuous, and the supporting axial force acting on the retaining structure will be transmitted through the excellent integrity of the structure itself or the rigid connection between the structures, thus affecting each other, which is the axial force coherence. If the coherence between the axial forces is ignored, it may lead to conflict disorder in the automatic regulating system and cause safety accidents. The existing research only considers the change of axial force of uniaxial steel support and does not consider the influence of transverse axial force coherence [12, 13]. In the study of transverse correlation, the fitting formula of axial force coherence under multi-point synchronous loading is given by numerical method. Still, it is not compared with the field measurement, and the deformation at the connection of retaining structure is not considered [14]. When the transverse coherence occurs, the deformation will also happen at the joint of the retaining structure. The deformation problem at the retaining structure joint in the axial force transmission cannot be ignored.
In order to achieve the refinement of servo axial force control and reduce a series of problems caused by axial force loss, it is necessary to conduct in-depth research on the law of axial force coherence. Based on the foundation pit project of the No. 2 entrance and exit of Pudong South Road Station of Shanghai Metro, this paper uses PLAXIS 3D software to analyze the axial force coherence numerically. Compared with the field test results, the axial force coherence and the deformation of the joint of the retaining structure when the servo steel support active control system is applied are discussed, which can be used for reference in similar projects.
Advertisement
2 Engineering Background and Model
The foundation pit of Shanghai Pudong South Road Station No. 2 entrance is located in the core area of Lujiazui, adjacent to essential facilities and structures. Micro-deformation active control is needed to ensure the safety of foundation pit and surrounding environment. The outsourcing size of the foundation pit is 60.4 m × 36.7 m, and the maximum excavation depth is 18.3 m. There are five supports in the foundation pit. The first is concrete support, and the other four are servo steel support. The thickness of retaining structure is 0.8 m. The foundation reinforcement method is high-pressure jet grouting pile sticking and strip reinforcement, and the facade is shown in Fig. 1. The servo steel support elevation layout is shown in Fig. 2. The parameters used in the calculation are given in Table 1. In the hardening model of soft soil, the dilation angle ψ is 0, the unloading–reloading Poisson’s ratio νur is 0.2, the power exponent m related to the modulus stress level is 0.8, the reference stress Pref is 100 kPa, the damage ratio Rf is 0.9, and Rinter is 0.7.
A layout depicts the structure's foundation, which consists of 7 support layers. The layers are plain fill, brown yellow-gray yellow silty clay, gray muddy silty clay mixed with sandy silt, muddy clay, gray silty clay, dark green-straw yellow silty clay, and grass yellow-gray yellow sandy silt.
Fig. 1
Sectional schematic diagram of the foundation pit of the No. 2 entrance and exit of Shanghai Metro Pudong South Road Station
A layout of the structure with five supports. The support layers are concrete support of 800 by 800 and four servo steel supports of 800 by 20, with 28-section steel. The foundation has 4 clay silts with dimensions of 3400, 3500, 3600, and 3000, respectively. All units are in millimeters.
Fig. 2
Diagram of foundation pit support layout at No. 2 entrance and exit of Pudong South Road Station of Shanghai Metro
Table 1
Parameters of soil model
Geotechnical category
γ/(kN m−3)
Cref/kPa
φ/°
K0
Es/MPa
E50/MPa
Eur/MPa
② brown-gray silty clay
18.2
20
17.5
0.47
9.7
9.7
48.5
③1 gray mucky silty clay
17.5
12
19.5
0.46
9.2
9.2
46.5
③2 mixed with sandy silt
18.6
6
29
0.45
7.5
7.5
37.5
④ muddy clay
16.7
14
12
0.58
21.6
21.6
107.8
⑤ gray silty clay
18
16
17
0.54
5.8
5.8
28.8
⑥ silty clay
19.5
46
16
0.46
8.2
8.2
41.1
⑦ sandy silt
18.5
2
31.5
0.37
13.7
13.7
68.6
⑧ silt
18.8
1
33
0.34
25.3
25.3
126.6
×
×
To eliminate the external influence factors, the load of the existing station building and the south district is not considered in the numerical analysis. The simulated axial force is the actual axial force applied by the field servo steel support. The 3D model is shown in Fig. 3. The excavation simulation condition is consistent with the field test condition, which is given in Table 2.
A 3 D view of a rectangular-shaped building with 8 layers. The top four layers have several bright spots, and the top of the eighth layer has several lines connecting the dots.
Fig. 3
Schematic diagram of numerical 3D model
Table 2
Cases adopted for numerical simulation and field test
Working condition
Content
Case 1
Influence on other support axial force after excavating to the second steel support and erecting servo steel support 3–3, 3–4
Case 2
Influence on other support axial force after excavating to the third steel support and erecting servo steel support 4–3, 4–4
Case 3
Influence on other support axial force after excavating to the fourth steel support and erecting servo steel support 5–3, 5–4
Note Supports 3–1 and 3–2 were erected in case 1; Supports 4–1 and 4–2 were erected in case 2; Supports 5–1 and 5–2 were not erected in case 3
×
3 Result and Analysis
In the process of foundation pit excavation, according to the requirements of deformation control, single or multi-channel servo steel support is used to apply axial force to achieve ideal results. Since each steel support acts on the same envelope structure, the vertical coherence is bound to be generated when the axial force is applied to the single or multi-channel servo steel support, and the axial force of the steel support acting on the envelope structure will also transfer the lateral coherence through the adjacent underground continuous wall. This section will explore the influence of axial force application on the existing axial force according to various working conditions in numerical simulation and field test, and analyze the influence of prestressed steel support area and servo steel support area on the joint of retaining structure under different axial force application conditions of the same steel support.
3.1 Effect of Steel Support Axial Force Coherence
Figures 4 and 5 are the schematic diagrams of the influence of numerical simulation and field test on other steel supports under condition 1, respectively. Figure 6 is the comparison diagram of numerical simulation and field test under condition 1. Figures 7 and 8 are the schematic diagrams of the influence of numerical simulation and field test on other steel supports under condition 2, respectively. Figure 9 is the comparison diagram of numerical simulation and field test under condition 2. Figures 10 and 11 are the schematic diagrams of the influence of numerical simulation and field test on other steel supports under condition 3, respectively. Figure 12 is the comparison diagram of numerical simulation and field test under condition 3. From Figs. 4, 5, 6, 7, 8, 9, 10, 11 and 12, it can be seen that both the numerical simulation and the measured results show that the axial force of the second servo steel support (i.e.3–3, 3–4 support) has a significant influence on the axial force of the same support, and the vertical influence is small. The farther the support is apart, the smaller the influence on the axial force is. The field measurement results are consistent with the numerical simulation results. When the axial force of the third servo steel support (i.e.4–3, 4–4 support) and the fourth servo steel support (i.e.5–3, 5–4 support) is applied, it dramatically influences the vertical support axial force, and the horizontal support axial force has little influence. The farther the support is, the smaller the influence on the axial force is. The field measurement results are also relatively consistent with the numerical simulation results.
A grouped bar graph compares the servo support axial force and support number. Axial force before loading and change of axial force has high and low values of around 1900 kilonewtons and 50 kilonewtons in 2-4 and 2-6 support numbers, respectively.
Fig. 4
Diagram of axial force coherence under numerical simulation case 1
A bar graph compares the servo support axial force and support number. Axial force before loading and change of axial force have high and low values of around 1800 kilonewtons and 120 kilonewtons in 2-4 and 2-2 support numbers, respectively.
Fig. 5
Diagram of axial force coherence under field test case 1
A 2-line graph plots the axial force loss rate versus the support number. It plots two decreasing trend curves with fluctuations and data points that provide data for numerical simulation and actual measurement.
Fig. 6
Comparison between numerical simulation and field test under case 1
A grouped bar graph compares the servo support axial force and support number. Axial force before loading and change of axial force have high and low values of around 2250 kilonewtons and 10 kilonewtons in 3-4 and 2-6 support numbers, respectively.
Fig. 7
Diagram of axial force coherence under numerical simulation case 2
A grouped bar graph compares the servo support axial force and support number. Axial force before loading and change of axial force has high and low values of around 2200 kilonewtons and 50 kilonewtons in 3-4 and 2-2 support numbers, respectively.
Fig. 8
Diagram of axial force coherence under field test case 2
A 2-line graph plots the axial force loss rate versus the support number. It plots two decreasing trend curves with fluctuations. Data points provided depict higher axial force loss rate for actual measurement than numerical simulation.
Fig. 9
Comparison between numerical simulation and field test under case 2
A grouped bar graph compares the servo support axial force and support number. Axial force before loading and change of axial force has high and low values of around 3010 kilonewtons and 10 kilonewtons in 5-6 and 2-6 support numbers, respectively.
Fig. 10
Diagram of axial force coherence under numerical simulation case 3
A grouped bar graph of servo support axial force versus support number. Axial force before loading and change of axial force have high and low values of around 3010 kilonewtons and 10 kilonewtons in 5-6 and 2-1 support numbers, respectively.
Fig. 11
Diagram of axial force coherence under field test case 3
A 2-line graph plots axial force loss rate versus support number. It plots two decreasing trend curves with fluctuations. Actual measurement plots higher peaks than numerical simulation.
Fig. 12
Comparison between numerical simulation and field test under case 3
×
×
×
×
×
×
×
×
×
Advertisement
According to Figs. 6, 9, and 12, the axial force loss rate of servo steel support is further analyzed. The axial force loss rate obtained by the actual measurement is mostly higher than that of the numerical simulation. The axial force loss rate of horizontal support is the largest, followed by vertical support. The axial force loss rate of oblique support is the smallest. According to the analysis, the applied support axial force has the most significant influence on the horizontal support axial force, followed by the vertical steel support. The oblique steel support has the most minor influence.
The higher the application position of the active axial force of the servo support, the greater the lateral axial force loss rate generated by other supports, while the opposite in vertical axial force loss rate is true. Among them, the maximum lateral axial force loss rate generated by the second servo steel support is 19%, the third is 18%, and the fourth is 12%. This phenomenon is similar to the relationship between deformation and depth of diaphragm wall during excavation. The maximum vertical axial force loss rate generated by the second servo steel support is 12%, the third is 14%, and the fourth is 16%. It might be that the axial force of support close to the concrete support is less vulnerable.
3.2 Study on the Deformation of Diaphragm Wall Joints with Axial Force Coherence
The vertical coherence of the axial force of the servo steel support is mainly produced by the overall stiffness of the diaphragm wall to transfer the force. The integrity of a single diaphragm wall and the connection between the diaphragm walls together form the stiffness of the enclosure structure. Therefore, this section focuses on the change of the joints between the diaphragm walls when the axial force of the steel support is applied and studies the deformation of the joints between the diaphragm walls when the axial force of different steel supports is applied, which is helpful fine control of the axial force of the support.
In order to explore the influence of prestressed steel support and servo steel support on the displacement of underground continuous wall joints, the depth of servo steel supports joint gauge and prestressed steel support joint gauge is 11.5 m. The test conditions of the underground diaphragm wall joint meter are shown in Table 3, and the corresponding axial force is the same as that in Sect. 3.1. The horizontal deformation of the wall joint after the axial support force is applied as shown in Fig. 13.
Table 3
Cases adopted in the joint meter test of diaphragm wall
Working condition
Content
Case 1
Excavation to the third steel support, prestressed steel supports Y4-1, Y4-2 erected and applied axial force
Case 2
Excavation to the third steel support, servo steel supports 4–3, 4–4 erected and applied axial force
Case 3
Excavation to the fourth steel support, servo steel supports 5–3, 5–4 erected and applied axial force
Case 4
Excavation to the fourth steel support, prestressed steel supports Y5-1, Y5-2 erected and applied axial force
A 2-line graph plots relative deformation of the diaphragm wall joints versus date. It plots a concave-down decreasing curve and a decreasing curve with data points in 4 cases and 4 areas for servo steel support and pre-stressed steel support.
Fig. 13
Horizontal deformation of diaphragm wall joint
×
According to the working conditions, the above diagram is divided into four areas: A, B, C, and D. A pre-stressed steel support crack gauge in area A has been installed. Before it installs, pre-stressed steel support axial force of Y4-2 has been applied, and the other one has not been done. After the other one axial force of the pre-stressed steel support is applied, the crack deformation of the diaphragm wall in pre-stressed steel support area increases from 0 to 0.46 mm. Besides, the servo steel support area joint meter was not installed then.
The crack deformation of the diaphragm wall in area B gradually decreases, and the change is small. The servo steel support crack gauge is installed. Before it installs, the servo steel support axial force of 4-4 has been applied, and the other one has not been done. After the other one axial force is applied, the crack deformation of the diaphragm wall in servo steel support area increases from 0 to 0.41 mm. Area C shows that the deformation of pre-stressed steel support crack gauge does not change. After the 5-4 servo steel support axial force is applied, the deformation shown by servo steel support crack gauge increased from 0.41 to 1.04 mm. In area D, the axial force of the pre-stressed steel support is applied. The crack deformation of the diaphragm wall in pre-stressed steel support area is linearly decreasing. The relative deformation in the servo steel support area increases from 1.04 to 1.21 mm, but then converges.
There is a noticeable increase in the horizontal deformation of the joint in the servo steel support area. After the axial force of the servo steel support is applied, the horizontal deformation at the joint position of the diaphragm wall in the servo area continues to increase, and the axial force of the servo steel support will increase the joint crack of the diaphragm wall. The horizontal deformation of the wall joints in the pre-stressed steel support area is small. The horizontal deformation between the wall joints does basically not increase after the prestressed steel support is applied. The above results show that the horizontal deformation of the wall joint in the servo steel support zone is greater than that in the prestressed steel support zone. Applying the axial force of the servo steel support will affect the diaphragm wall joints. Therefore, it is necessary to combine the corresponding axial force application method to reduce the horizontal deformation of the diaphragm wall joints during the use of the servo steel support to prevent it from being too large and causing engineering hazards such as water leakage.
In addition, according to Figs. 6, 9, and 12 in Sect. 3.1, it can also be obtained that the axial force loss rate of the actual measurement is mostly higher than the axial force loss of the numerical simulation. We infer that the relative deformation between the diaphragm wall is not considered in the Plaxis 3D software. According to the data obtained from the actual construction site, there is an apparent horizontal deformation at the joint position in the servo zone. The deformation of the diaphragm wall leads to a change in the length of the steel support, resulting in a higher loss rate of the axial force than the axial force loss rate obtained by numerical simulation. That is, the deformation of the wall joints crack will in turn affect the axial force.
4 Conclusion
Based on the concept of axial force coherence of steel support in servo system, the influence of coherence on support in different directions and distances is analyzed according to the consistency between numerical simulation and field monitoring. Combined with the deformation of the joint of underground diaphragm wall measured on site, the influence of axial force coherence of steel support is analyzed, which provides theoretical for fine deformation control of foundation pit. The following conclusions can be drawn:
(1)
The axial support force can affect other support axial forces of the retaining structure at different depths and distances. The farther away from the active axial force, the smaller the influence. The axial force of the support has the most significant influence on the axial force of the horizontal steel support, followed by the vertical steel support, and the influence on the oblique steel support is the smallest.
(2)
The higher the application position of the active axial force of the servo support, the greater the lateral axial force loss rate generated by other supports, while the opposite in vertical axial force loss rate is true. The maximum lateral axial force loss rate generated by the second servo steel support is 19%. The maximum vertical axial force loss rate generated by the fourth servo steel support is 16%.
(3)
The deformation of the diaphragm wall joint in the servo steel support zone is more significant than that in the pre-stressed steel support zone. The deformation of the joint will, in turn, affect the axial force, which is an interactive relationship. The relative displacement between diaphragm walls caused by a single application of servo axial force is 1.41 mm.
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
