Advances in Civil Engineering
Volume 2019 (2019), Article ID 5476354, 16 pages
https://doi.org/10.1155/2019/5476354

Research Article

Investigation of the Axial Force Compensation and Deformation Control Effect of Servo Steel Struts in a Deep Foundation Pit Excavation in Soft Clay

Honggui Di,¹ Huiji Guo,¹ Shunhua Zhou,¹ Jinming Chen,² and Lu Wen¹

¹Shanghai Key Laboratory of Rail Infrastructure Durability and System Safety, Tongji University, Shanghai 201804, China
²Ningbo Rail Transportation Group Co. Ltd., Ningbo 315012, China

Correspondence should be addressed to Huiji Guo; guohuiji@tongji.edu.cn

Received 3 August 2019; Accepted 8 October 2019; Published 13 November 2019

Academic Editor: Khalid Abdel-Rahman

Copyright © 2019 Honggui Di et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This study presents a comparative analysis of the deformation control effect of the hydraulic servo steel struts and ordinary steel struts of a foundation pit based on the measured axial force of the steel struts, lateral wall deflection, and ground surface settlement due to pit excavation. The results indicate that ordinary steel struts installed via axial preloading exhibit a disadvantageous axial force loss with a maximum value equal to 86.7% of the axial preloading force. When compared with ordinary steel struts, the hydraulic servo steel strut exhibits a superior supporting effect. The hydraulic servo steel strut adjusts the axial force in real time based on the deformation of the retaining structure and the axial force of the struts. Thus, the ratio of maximum lateral deflection to the excavation depth of a deep foundation pit in soft soil is less than 0.3%. Concrete struts undergo unsupported exposure during the excavation process, leading to sharply increasing deformation of the retaining structure. Therefore, regarding a foundation pit with strict requirements for deformation control, the use of hydraulic servo steel struts rather than concrete struts is recommended.

1. Introduction

Metro stations are typically built in urban areas with a large population density and numerous buildings. To minimize the impact of foundation pit excavation on the surrounding environment, foundation pit excavation should meet the strict requirements of deformation control [1]. However, with rapid advancements in urban construction, metro stations and their surrounding environment are becoming increasingly complex, and the urgent problem of control and reduction of excavation deformation in foundation pits and its effect on the surrounding environment should be addressed [2].

To understand the deformation mechanism of foundation pit excavation, Clough and O’Rourke [3] investigated the relationship between lateral wall deflection, pit uplift, and strut stiffness. The relationship between deformation of the retaining structure and excavation depth under various geological conditions and construction conditions has also been examined by Long [4], Leung and Charles [5], and Liu et al. [6]. Sun et al. [7] and Xu et al. [8] investigated the relationship between deformation in foundation pit excavation and the geometric shape of the foundation pit. The effect of foundation pit excavation on the surrounding environment was examined by Tan and Wei [9] and Shi et al. [10].

Based on the deformation law of foundation pit excavation, extant studies propose various deformation control methods for foundation pit excavation from design method and construction control method perspectives. Wong et al. [11] summarized the supporting effect of different foundation pit retaining structures. Luo et al. [12, 13] proposed a reliability-based design method for the retaining structure of the foundation pit that considers spatial variability of the soil. Tan et al. [14] proposed that rapid excavation, timely support, timely pouring of floors, and sectional construction are critical for deformation control of the foundation pit. Tan and Wang [15] and Li et al. [16] proposed that excavation by sections effectively controls the deflections of the diaphragm wall.

Recently, engineers began improving deformation control techniques for foundation pits from a support system standpoint. Yao et al. [17] illustrated that an increase in the stiffness and preaxial force of steel struts reduces diaphragm wall deflections to a certain extent. Tanner Blackburn and Richard [18] stated that the axial force of steel struts is related to the mode and speed of excavation and the installation of the adjacent strut. Chen [19] noted that the use of concrete support effectively restrained the development of deformation in the retaining structure of the foundation pit. However, all traditional support methods exhibit shortcomings [20]. For example, the concrete strut is cast-in-place, and there is unsupported exposure during the excavation process. Typically, there are high losses in axial force in the ordinary steel struts.

To compensate for the flaws in traditional support methods that exhibit axial force losses in ordinary steel struts, a hydraulic automatic servo control system (with a working mechanism that compensates for axial force loss in real time) is adopted to control the axial force of the steel struts and deformation during pit excavation [21]. However, given its high cost, only a few engineering cases and studies have focused on introducing the engineering application of hydraulic servo steel struts [22]. A focus problem in the field of pit engineering involves whether the hydraulic servo steel strut delivers a superior supporting effect to that from traditional support methods.

In this study, a new hydraulic servo steel strut system with a working mechanism that simultaneously controls axial force and displacement was designed and adopted to control deformation of a foundation pit excavated in soft soils. A comparative analysis of the deformation control effect of hydraulic servo steel struts and ordinary steel struts in pit excavation is presented based on measured data. Section 2 introduces the background of the project while Sections 3–5 discuss the measured axial force of the steel strut, lateral wall deflection, and ground surface settlement caused by the pit excavation. Section 6 presents the discussion, and the conclusions are presented in Section 7.

2. Project Description

2.1. Project Overview

The Haiyan Road station is a transfer station of Ningbo Metro Line No. 1 and Line No. 5. The existing Haiyan Road station of Line No. 1 lies along the east-west direction and is an underground two-story island station, as shown in Figure 1. The Haiyan Road station of Line No. 5 lies along the north-south direction and is an underground three-story island station. The existing Haiyan Road station of Line No. 1 has been in service since 2014, and a transfer point reserved for Line No. 5 is in its design stage. The Haiyan Road station of Line No. 5 consists of a north foundation pit (Pit A) and a south foundation pit (Pit B) with lengths of 125 m and 33 m, respectively. The width of the standard section of Pit A and Pit B is 22.1 m, and the excavation depths vary from 24.7 m to 24.8 m.

Figure 1: Plan view of the foundation pit of the Haiyan Road station of Line No. 5.

A group of foundation pits (a pit complex) is under construction on the northwest side of the Haiyan Road station of Line No. 5. The pit complex is 19 m deep and approximately 11–13 m from the edge of Pit A, and its excavation is complete. The Hongtai Building Group (located on the southwest side of the station of Line No. 5) is approximately 10 m away from the edge of Pit B. The main building of the Hongtai Building Group is a 24-story building with a 21.5 m–27.5 m bored pile foundation. The Muqi River flows on the east side of the station of Line No. 5, approximately 60 m away from the edge of the foundation pit of Line No. 5. The surrounding environment is complex, thereby leading to higher requirements for deformation control on the project.

Furthermore, the soft clay layer in the area is characterized by high natural water content, a high natural pore ratio, high compressibility, low strength, and low permeability, as shown in Figure 2. Consequently, the foundation pit of the station of Line No. 5 exhibited large deformation during excavation.

Figure 2: Physical and mechanical index of the soil layer. —moisture content; —liquid limit; —plastic limit; γ—soil gravity; Es0.1-0.2—compression modulus; —effective stress cohesion; —effective stress friction angle; e—porosity; —vertical permeability coefficient; Kh—horizontal permeability coefficient.

To reduce the impact of foundation pit excavation of Line No. 5 to the foundation pit group of Ningbo Center, the Hongtai Building Group, and the current Line No. 1 station, hydraulic servo steel struts were adopted in this project. Although the diaphragm wall near corners featured much smaller displacements than those near the middle span of the excavations [23], the hydraulic servo steel struts were adopted in the section near the existing station of Line No. 1 to preserve normal operations at the existing stations.

2.2. Pit Supporting Structure

The primary foundation pit of the Line No. 5 station was constructed via the open-cut method. The retaining structures consisted of diaphragm walls and an internal support system. The thickness of the diaphragm wall was 1000 mm (or 1200 mm), and its depth was approximately 57.8 m. The internal support system consisted of concrete struts, ordinary steel struts, and hydraulic servo steel struts.

There were seven struts along the depth of the foundation pit, with the first and fifth struts being concrete struts, while the other five were steel struts. The dimensions of the first concrete strut were 0.8 m wide × 1.0 m high, and for the fifth concrete strut, they were 1.0 m wide × 1.0 m high. The diameter of the steel strut was 609 mm. Figure 3 presents the plan view of the supporting system. The average distance between the concrete struts in Pit A and Pit B was 8.5 m and 7.5 m, respectively, and the average distance between the steel struts in Pit A and Pit B was 3.0 m and 2.2 m, respectively.

(a)

(b)

(c)

Figure 3: Plan view of the foundation pit: (a) layout of 1st and 5th struts; (b) layout of 2nd strut; (c) layout of 3rd, 4th, 6th, and 7th struts.

Part 1 of Pit A (see Figure 3) is some distance from the existing station and is the ordinary steel struts section. Thus, all the steel struts in the cross section are ordinary steel struts, as shown in Figure 4(a). Part 3 of Pit B and Part 2 of Pit A (see Figure 3) are close to the existing station and are hydraulic servo steel strut sections. Hence, hydraulic servo steel struts are used for the third, fourth, sixth, and seventh steel struts in the cross section, as shown in Figure 4(b).

(a)

(b)

Figure 4: Cross section of the foundation pit in the standard section: (a) part 1; (b) parts 2 and 3.

The bottom of the pit was reinforced with cement mixing piles (3 m wide × 2.5 m deep) placed at intervals of 3 m. The cement mixing pile was also used to reinforce the soil within 3 m below the fifth concrete strut.

The hydraulic servo steel strut system consists of a hydraulic module and automatic control system module. The hydraulic module is composed of a hydraulic pump station and a hydraulic jack while the automatic control system module is composed of automatic control hardware and computer software. The field arrangement of the hydraulic servo steel strut is presented in Figure 5.

(a)

(b)

Figure 5: Installed hydraulic servo steel strut system: (a) hydraulic servo struts; (b) control system.

The hydraulic servo steel strut system adopts the working mechanism of simultaneous control of axial force and displacement, as shown in Figure 6. The control of axial force element of the working mechanism entails that when the measured axial force of the strut is less than a set value, the axial force is actively restored to the set axial force of the strut. This reduces the increasing deformation of the retaining structure caused by loss of stress in the steel struts. Concurrently, the adopted control of displacement working mechanism optimizes the set axial force and actively updates the set axial force in the automatic control system module. The working mechanism of control of displacement implies that when the installation of a hydraulic servo steel strut is completed, the displacement of the servo steel strut jack is set as the displacement datum point, and subsequently, the displacement is controlled from −1 mm to 2 mm relative to the displacement datum point. The positive value indicates that the servo steel strut jack shrinks and the diaphragm wall moves to the inside of the pit. The negative value indicates that the servo steel strut jack extends and the diaphragm wall moves out of the pit, effectively decreasing the deformation of foundation pit excavation caused by the improper initial set axial force.

Figure 6: Working mechanism of the hydraulic servo steel strut system.

The specific working mode of control of displacement is as follows: when the measured displacement of the servo steel strut jack exceeds the maximum displacement constraint of 2 mm (this implies that deformation of the diaphragm wall to the inside of the foundation pit is high and the set axial force is low), the set axial force is actively increased. When the measured displacement of the servo steel strut jack is lower than the minimum displacement constraint of −1 mm (this implies that deformation of the diaphragm wall to the outside of foundation pit is high and the set axial force is high), the set axial force is actively reduced. The working mechanism adjusts the hydraulic servo steel strut and dynamically compensates for the axial force in real time, and thus the foundation pit excavation deformation meets the high control requirements.

2.3. Construction Operation and Instrumentation

To clarify the excavation process of the foundation pit, Table 1 lists the working conditions of the standard section. The overall excavation direction of the foundation pit ranged from north to south and from top to bottom. The soil above the fifth concrete strut of Pit A was excavated downwards layer by layer, and the soil beneath it was excavated via the slope. Pit B was excavated downwards layer by layer. During the excavation process, an inclinometer, settlement tester, and axial force tester were installed around the foundation pits to monitor deformation in the retaining structures and soil and the axial force of the struts. As shown in Figure 7, there are 20 inclined monitoring points (CX01–CX20) for monitoring the lateral deflection of diaphragm walls, 20 settlement monitoring groups (D1–D20) for monitoring the ground surface settlement, and 7 axial force monitoring groups (T1–T7) for monitoring the axial force of the struts.

Table 1: Construction operation of the foundation pit.

Stage
Construction operation
Date

1
Preparation of Pit A, installation of 1st concrete strut
2017/04/26–2017/05/22

Preparation of Pit B, installation of 1st concrete strut
2018/12/01–2019/12/24

2
Excavation of Pit A to 5.0 m, installation of 2nd steel strut
2017/05/22–2017/06/18

Excavation of Pit B to 5.0 m, installation of 2nd steel strut
2018/12/24–2019/01/05

3
Excavation of Pit A to 8.0 m, installation of 3rd steel strut
2017/06/18–2017/07/05

Excavation of Pit B to 8.0 m, installation of 3rd steel strut
2019/01/05–2019/01/13

4
Excavation of Pit A to 11.5 m, installation of 4th steel strut
2017/07/08–2017/07/16

Excavation of Pit B to 11.5 m, installation of 4th steel strut
2019/01/13–2019/01/18

5
Excavation of Pit A to 15.0 m, installation of 5th concrete strut
2017/07/16–2017/08/15

Excavation of Pit B to 15.0 m, installation of 5th concrete strut
2019/01/18–2019/02/23

6
Excavation of Pit A to 18.5 m, installation of 6th steel strut
2017/07/26–2017/10/02

Excavation of Pit B to 18.5 m, installation of 6th steel strut
2019/02/23–2019/03/11

7
Excavation of Pit A to 21.5 m, installation of 7th steel strut
2017/08/08–2017/10/15

Excavation of Pit B to 21.5 m, installation of 7th steel strut
2019/03/11–2019/04/01

8
Excavation of Pit A to 24.5 m, pouring of bottom slab
2017/09/01–2017/10/30

Excavation of Pit B to 24.5 m, pouring of bottom slab
2019/04/01–2019/04/20

Figure 7: Layout of the monitoring points.

3. Lateral Movement of the Diaphragm Walls

Lateral diaphragm wall deflection was obtained from 20 inclinometers (see Figure 7). However, it was a complex task, considering the engineering environment on the west side of the foundation pit (see Figure 1). Thus, the measured deformation data on the east side of the foundation pit were selected for analysis to reduce the effect of external factors. Figure 8 shows the variation in the measured cumulative lateral deflection of the diaphragm walls with respect to depth during construction. The cumulative lateral deflection of the diaphragm walls increased with increase in the excavation depth, and the maximum cumulative lateral deflection generally occurred near the position of excavation depth. These observations agree well with the study performed on foundation pits in Zhejiang soft clay by Ding et al. [24]. Furthermore, between completion of the installation of the fourth steel strut and completion of the installation of the fifth concrete strut, the cumulative lateral deflection of the diaphragm walls changed significantly, and its value reached 19.7–34.1% of the final deflection. This is because the foundation pit exhibits a space-time effect, and the concrete strut requires a certain time to garner strength. Thus, there is a period of unsupported exposure, which significantly increases the lateral deflection of the diaphragm walls.

Figure 8: Curve of the measured cumulative lateral deflection of the diaphragm walls with respect to depth.

To compare the supporting effect of the hydraulic servo steel strut with that of an ordinary steel strut, the measuring points CX11, CX13, and CX17 representing Part 1, Part 2, and Part 3, respectively, were selected for further analysis. Figure 9 shows the variation in maximum lateral deflection with respect to time at different observation points. It was observed that the maximum lateral deflection of CX11 in Part 1 always exceeded that of CX13 in Part 2. Furthermore, the maximum lateral deflection of CX11 and CX13 increased with increase in excavation depth. Conversely, with respect to the maximum lateral deflection development curve of CX17, there is an evident turning point, which occurred at the completion of the fourth hydraulic servo steel strut installation. The maximum lateral deflection of CX17 tends to be stable following the completion of the installation of the fourth hydraulic servo steel strut. The phenomenon is a preliminary indication that the supporting effect of the hydraulic servo steel strut exceeds that of the ordinary steel strut.

Figure 9: Curve of the maximum lateral deflection of the diaphragm wall with respect to time.

The relationship between the cumulative maximum lateral wall deflection () and the corresponding excavation depth () is plotted in Figure 10. The ratio of CX13 (0.29%) and CX17 (0.11%), which meets the control standard of level I foundation pit (), was smaller than that of CX11 (0.35%). Similarly, observation points CX03 and CX19 were selected to represent End Well 1 and End Well 2, respectively. The ratio of CX19 (0.13%) close to Part 2 was smaller than that of CX03 (0.18%) close to Part 2, illustrating that the hydraulic servo steel struts provide a more stable support environment.

(a)

(b)

Figure 10: Ratio of the maximum lateral deflection of the diaphragm wall to excavation depth (): (a) Pit A; (b) Pit B.

Furthermore, with respect to the ratio of Pit A, there is an evident turning point with increase in excavation depth, as shown in Figure 10(a). The turning point occurred between the completion of the installation of the fourth steel strut and completion of the installation of the fifth concrete strut. However, with respect to Pit B, there is no evident turning point, as shown in Figure 10(b). Potential reasons include the following: (1) The size of Pit B is smaller than that of Pit A, and thus the exposure period of the unsupported concrete strut in Pit B is shorter than that of the unsupported concrete strut in Pit A. Consequently, a large lateral deflection is absent during the exposure period of the unsupported fifth concrete strut, resulting in the absence of a turning point of in Pit B. (2) The supporting effect of Part 3 in Pit B exceeds that of the combination of Part 1 and Part 2 in Pit A. To verify this standpoint, the change in the rate of maximum lateral wall deflection with respect to time for points CX11, CX13, and CX17, representing Part 1, Part 2, and Part 3, respectively, was analyzed.

As shown in Figure 11, the average value of the maximum lateral deflection rate of point CX13 (0.47 mm/day) and point CX17 (0.19 mm/day) is lower than that of point CX11 (0.53 mm/day). The distribution of the maximum lateral deflection rate of points CX13 and CX17 in the hydraulic servo steel strut section was more concentrated than that of point CX11 in the ordinary steel struts section. This implies that the hydraulic servo steel struts restrain the development of deflection in diaphragm walls more effectively.

Figure 11: Change in the rate of maximum lateral wall deflection with respect to time.

It should be noted that there are several negative points in Figure 11. When compared with the excavation of a pit foundation that is supported by only concrete struts and ordinary steel struts [25], almost all the deflection rates were positive. This is potentially due to the high axial force of the hydraulic servo steel struts, which causes the diaphragm wall to move to the exterior of the foundation pit. When the diaphragm wall moves to the exterior of the foundation pit, the hydraulic servo steel struts maintain the stability of the support system via elongation of its hydraulic jack. However, ordinary steel struts can exhibit support dislocation, which can lead to loss of axial force. This implies that the adjacent ordinary steel struts could function erroneously if the axial force of the hydraulic servo steel struts is not set correctly. Therefore, when the hydraulic servo steel struts are utilized together with ordinary steel struts, they may not work well together.

To further examine the control effect of the deflection of the hydraulic servo steel strut section, the final cumulative lateral deflection of the diaphragm walls on the west side and east side of the foundation pit is plotted in Figure 12. The final cumulative lateral deflection of the hydraulic servo steel strut section was lower than that of the ordinary steel struts section. Considering the space-time effect of the foundation pit, the deformation data on the enclosure structure of the two end wells were compared. From that, we can see the final cumulative lateral deflection of point CX19, near the hydraulic servo steel strut section in Part 3, was significantly lower than that of point CX03 near the ordinary steel strut section in Part 1. This further illustrates that the supporting effect of the hydraulic servo steel struts exceeds that of ordinary steel struts.

Figure 12: Final cumulative lateral deflection of the diaphragm walls around the foundation pit.

Figure 12 also shows that the final cumulative lateral deflection of the east side of Pit A (End Well 1, Part 1, and Part 2) exceeds that of the west side, although the final cumulative lateral deflection of the east side of Pit B (Part 3 and End Well 2) is lower than that of the west side. This is because a foundation pit group (Ningbo Center) was under construction near the west edge of Pit A, which led to unloading on the ground near the west side of the Pit A. Conversely, on the west side of the Pit B, there is a construction surcharge (Hongtai Building Group), resulting in an overload on the ground near the west side of Pit B (see Figure 1), which led to the differences between Pit A and Pit B. The observed results are consistent with the results of previous research [25, 26], which reported that different existing structures around the foundation pit have different effects on the deformation of the foundation pit. Furthermore, it should be noted that CX13 develops much greater lateral wall displacement than CX17, although both were retained by hydraulic servo struts. The reasons may be as follows: (1) On the right side of CX13, it is close to the standard section of Pit A, while on the right side of CX17, it is close to the existing station, and the stiffness of the existing station is greater than the standard section of Pit A. (2) Pit B, where CX17 is located, was smaller than Pit A, where CX13 is located, and its construction process was fast, thus leading to smaller deformation. (3) Hydraulic servo steel struts in Pit B work better than the mixed support system of hydraulic servo steel struts and ordinary steel struts in Pit A.

4. Ground Surface Settlement

The ground surface settlement values of the D11, D13, and D17 settlement monitoring groups were selected for analysis purposes to compare the effect of the hydraulic servo steel strut and ordinary steel strut on the control of surrounding environment deformation. Figure 13 shows settlement monitoring points , , and , which are 5 m, 10 m, and 15 m, respectively, away from the edge of the foundation pit. The ground surface settlement increased gradually with increase in the excavation depth, and the maximum value occurred 10 m along the edge of the foundation pit.

Figure 13: Curve of the measured ground surface settlement with respect to time.

To study the relationship between the maximum ground surface settlement () and the corresponding excavation depth (), the ratio of points D11-2, D13-2, and D17-2, which represent Part 1, Part 2, and Part 3, respectively, is selected for analysis, as shown in Figure 14. The ratio of D13-2 (0.38%) and D17-2 (0.17%) is lower than that of D11-2 (0.52%). Furthermore, there is an evident turning point with increase in excavation depth for Pit A and no turning point for Pit B. The reason for the phenomenon is potentially the same as that for the ratio of the maximum lateral deflection to excavation depth ().

Figure 14: Ratio of the maximum surface settlement to the excavation depth ().

Figure 15 presents further details on the final ground surface settlement at 10 m along the edge of the foundation pit. The final settlement of the hydraulic servo steel strut section (Part 2 and Part 3) was significantly smaller than that of the ordinary steel struts section (Part 1), and this indicates that the hydraulic servo steel struts control the surface settlement more effectively and reduce the effect of foundation pit excavation on the buildings in the surroundings.

Figure 15: Final surface settlement at 10 m along the edge of the foundation pit.

5. Axial Force of Struts

Subsequently, the axial force variation in the ordinary steel strut and hydraulic servo steel strut was analyzed. Figure 16 shows the variation in the axial force of the third steel strut, T4-3 (third steel strut of the T4 axial force group), and the adjacent hydraulic servo steel strut, SF1-3. The changes in the axial force of steel struts were characterized by three stages: increasing stage (stage I), slowly decreasing stage (stage II), and suddenly increasing stage (stage III).

Figure 16: Changes in the trend of the axial force of the steel struts.

The increase in stage I occurred prior to the installation of the fifth concrete strut. The reason for the increase in axial force in stage I is that increments in the excavation depth continuously increased the difference in soil pressure between the inside and outside of the foundation pit. Thus, the retaining structures exhibited a tendency to deform into the pit in the horizontal direction, thus resulting in increments in the axial force. Furthermore, the change of the axial force near the strut is not obvious after the installation of the next strut was completed. This is because the excavation of foundation pit and the installation of strut are carried out synchronously. When the strut is installed, the soil pressure on the sidewall also increases. Thus, despite the new strut installation, there will be no significant downward trend in the strut axial force.

A slow decrease in stage II occurred after the installation of the fifth concrete strut. The reasons for that can potentially include three parts: (1) the concrete strut exhibits greater stiffness and stability and bears most of the soil pressure; (2) with the installation of underpass struts, the soil pressure behind the wall is shared with the underpass struts, and thus the axial force of the struts decreases slowly; (3) the ordinary steel strut exhibits the disadvantage of stress loss, and thus the axial force of the ordinary steel strut exhibits a slowly decreasing trend.

A sudden increase in stage III occurred after the removal of the sixth and seventh steel struts. The removal of the sixth and seventh steel struts caused the remaining steel struts to share more soil pressure from behind the wall, and this resulted in an increase in axial force. Therefore, the monitoring frequency should be strengthened, and the internal structure should be installed quickly after the removal of the struts.

Furthermore, as shown in Figure 16, the axial force of the ordinary steel struts was generally lower than the axial preloading force, and this indicates that there is axial force loss in the ordinary steel struts. Table 2 lists the axial force of some ordinary steel struts in the foundation pit. Axial force loss is a common phenomenon in ordinary steel struts, and the maximum loss corresponds to 84.54% of the axial preloading force. In addition to the loss of axial force being caused by defects in the ordinary steel strut itself, it is also caused by the unreasonable setting of the set axial force of the hydraulic servo steel strut. The diaphragm wall moves to the exterior of the foundation pit if the set axial force of the hydraulic servo steel strut is excessively high. Subsequently, the axial force of the hydraulic servo steel strut is maintained above the set axial force due to its control of axial force working mechanism. However, the ordinary steel struts can exhibit support dislocation, and this leads to axial force loss in ordinary steel struts and an increase in the force on the adjacent struts. This is potentially the reason why the measured axial force of the hydraulic servo steel struts exceeded the set axial force, although the measured axial force of ordinary steel strut was lower than the set axial force.

Table 2: Axial force loss in ordinary steel struts.

Strut number
Measured axial force (kN)
Axial preloading force (kN)
Percentage of axial force loss

T1-2
327.34
930
64.80

T2-2
143.77
930
84.54

T3-2
339.21
930
63.53

T4-2
257.02
930
72.36

T1-3
320.14
1200
73.32

T2-3
588.27
1200
50.98

T3-3
668.16
1200
44.32

T4-3
412.67
1200
65.61

T1-4
647.61
810
20.05

T2-4
402.88
810
50.26

T3-4
653.89
810
19.27

T4-4
397.32
810
50.95

Furthermore, as shown in Figure 16, low amplitude fluctuation of steel strut axial force is observed. Based on measured data on a foundation pit, Chen et al. [27] reported that the axial force of an ordinary steel strut varies with temperature, and the value was 37.08 kN/°C. To prove that low amplitude fluctuation was caused by temperature, the axial force and temperature of the hydraulic servo steel strut during the period of the axial force fluctuation (August 2 to August 22) are selected for analysis, as shown in Figure 17. It is evident in Figure 17 that the change in axial force is consistent with the change in temperature. Furthermore, Table 3 presents the axial force of some of the hydraulic servo steel struts in this study. The results indicate that the rate of change in the hydraulic servo steel struts with temperature corresponds to 12.6–31.5 kN/°C.

Figure 17: Changes in the trend of the axial force and temperature of the hydraulic servo steel struts.

Table 3: Statistics of the axial force of hydraulic servo strut.

Number of servo steel struts
Set axial force (kN)
Minimum temperature (°C)
Maximum temperature (°C)
Minimum axial force (kN)
Maximum axial force (kN)
Change in rate (kN/°C)

SF1-3
1300
30.75
45.44
2131.5
2563.9
29.4

SF2-3
1320
30.75
45.75
1999.4
2291.3
19.5

SF3-3
1120
30.75
45.62
1360.2
1753.1
26.4

SF4-3
1160
30.75
45.44
1596.8
2007.4
28.0

SF5-3
1080
30.75
45.62
1328.7
1696.4
24.7

SF6-3
760
30.75
46.19
878.2
1200.0
20.8

SF7-3
1040
30.75
45.44
1174.8
1638.2
31.5

SF8-3
1020
30.75
45.75
1038.3
1285.3
16.5

SF9-3
696
30.75
45.62
687.0
908.4
14.9

To verify the supporting effect of the hydraulic servo steel struts under temperature variation, the axial force and displacement variation in the hydraulic jack are plotted in Figure 18. Although the axial force fluctuation of the hydraulic servo steel struts was high, the fluctuation value of the hydraulic jack was always between 1.4 mm and 3.4 mm. Essentially, although the axial force of the hydraulic servo steel struts varies with temperature, the hydraulic servo steel strut adjusts in real time based on the hydraulic jack displacement variation, and this prevents deformation of the retaining structure caused by change in axial force with respect to temperature fluctuation.

Figure 18: Trend of the axial force and displacement variation in the hydraulic servo steel struts.

The relationship between the lateral deformation and ground settlement with the axial forces of hydraulic servo steel strut is important for this topic. Considering the long interval between the two monitoring of ground surface settlement, we select the deformation of lateral wall for analysis. To investigate the relationship between lateral deformation and the axial forces of hydraulic servo steel strut, Figure 19 shows the curve of the lateral deformation at the elevation of the servo support in the section where the SF1-3 support is located with respect to time. From Figure 19, we can see that deformation to the outside of the foundation pit occurred at the position when the servo strut was installed. The reason for this phenomenon is that the set axial force of the servo struts was large, which makes the retaining structure deform outside the pit. Then, deformation of the servo strut position tends to be stable, which indicates that the hydraulic servo steel strut plays a beneficial role in controlling the deformation of the foundation pit. Furthermore, the retaining structure at the elevation of the 6th and 7th struts experienced large deformation before the concrete strut was put into use. However, when the 5th concrete strut was put into use, the deformation tended to be stable. This phenomenon proves yet again that the exposure period without support caused by the long maintenance period of the concrete strut leads to a sharp increase in the deformation of the enclosure structure. However, the concrete strut has good overall performance and can effectively restrain the development of sidewall deformation after it has been put into use.

Figure 19: Curve of the lateral deformation at the elevation of the servo support with respect to time.

6. Discussion

Compared to an ordinary steel strut, a hydraulic servo steel strut is better at controlling deformation in a foundation pit. However, a few problems still persist in its application:

(1)A definite design method for the set axial force of the hydraulic servo steel strut is absent, and it is necessary to investigate how to optimize the design of a hydraulic servo steel strut based on its working mechanism.(2)In the application process of a hydraulic servo steel strut, the actual axial force typically exceeds the design axial force, and this leads to an increase in the internal force of the diaphragm wall. Future studies should investigate whether the reinforcement setting of the diaphragm wall should be adjusted accordingly in the design stage.(3)Presently, the working mechanism of the hydraulic servo steel strut is based on a single strut, but the deformation effect of the foundation pit is affected by the entire support system. Consequently, it is of great significance to study the influence of the axial force adjustment of the hydraulic servo steel strut on the adjacent support and to propose a new servo adjustment method based on the support system.

7. Conclusions

The following conclusions were obtained in this study:

(1)When compared with an ordinary steel strut installed by preloading the axial force, the hydraulic servo steel strut (which adopts the working mechanism of simultaneous control of axial force and displacement) exhibits a superior supporting effect. The hydraulic servo steel strut adjusts the axial force in real time based on deformation of the retaining structure and the supporting axial force. Thus, the ratio of maximum lateral deflection to excavation depth of the deep foundation pit in a soft soil area is less than 0.3%.(2)The ordinary steel strut (installed via axial preloading) exhibits the disadvantage of axial force loss due to several factors. In this study, the maximum axial force loss of the ordinary steel strut reached 86.7% of the designed axial force. Therefore, in foundation pit engineering with strict deformation control, the use of hydraulic servo steel struts, which overcome the axial force loss, is recommended.(3)A concrete strut is typically used in a deep foundation pit and is cast-in-place. Thus, there is unsupported exposure during the excavation process, leading to a sharp increase in deformation of the retaining structure of the foundation pit. Thus, the construction organization design should be optimized to the maximum possible extent to reduce the unsupported exposure period of the foundation pit. The use of a hydraulic servo steel strut rather than a concrete strut can accelerate construction and preclude an unsupported exposure period.

Data Availability

Some or all data used during the study are available from the corresponding author by request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The study on which this paper is based was supported by the National Natural Science Foundation of China through the grant no. 51808405.

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