Experimental Study on Bending Moment of Double-Row Steel Pipe Piles in Foundation Excavation

Double-row steel pipe piles have been widely used in retaining and protection of foundation excavation because of the advantages of high bearing capacity, high ﬂexural rigidity, fast construction speed, and so on. This study presents a ﬁeld test to assess the feasibility of strain gauges in monitoring the strain of double-row steel pipe piles during foundation excavation. Two steel pipe piles were instrumented with strain gauges and then installed into the drilling holes. The installation method of strain gauges is introduced ﬁrst. Then, the bending moment of the test piles during the foundation excavation was analyzed. The ﬁeld test results indicate that the survival rate of strain gauges was 100%, and the monitoring method used in the test was feasible to measure the bending moment of double-row steel pipe piles. Moreover, with the increase in foundation excavation depth, the bending moment of the test piles all increased, and the bending moment of the inner pile was obviously higher than that of the outer pile. The bending moment distribution of the whole support system accords with the conventional pile-anchor mode. The test results can provide reference and basis for the design and construction of double-row steel pipe piles.


Introduction
With the rapid development of high-rise buildings and underground structures, the scale of underground projects was increasing, and the design and construction of deep pit support was becoming more and more important [1][2][3]. How to ensure the stability of the deep pit, control the deformation of the pit support structure, and reduce the impact on the surrounding environment has become a research hotspot [4][5][6][7]. e double-row steel pipe pile is a kind of spatial composite support structure, which is composed of two rows of reinforced steel pipe piles and rigid connected top beams [8]. e bearing capacity of this support structure is good, and the deformation of the foundation pit can be controlled well [9,10]. Moreover, the double-row steel pipe pile has a good applicability for foundation pit support engineering [11]. In order to improve the bearing capacity, optimize the design, and guide the construction of this support structure, many scholars have studied the mechanical characteristics of the steel pipe pile.
Liu and Fu [12] established a finite element model of the planar bar system of double-row piles considering the pilesoil effect and studied the key parameters and soil pressure distribution of the finite element model of double-row piles. Bai et al. [13] established a calculation model of double-row pile support structure in the deep foundation pit considering the spatial effect and analyzed the influence of the top beam on the bearing capacity of double-row pile structure. Huang et al. [14] divided the support structure of double-row piles into two parts and established the flexural differential equation of the piles. Shi and Gong. [15] used finite element software to simulate the excavation process of the deep foundation pit supported by double-row piles and studied the mechanical characteristics of double-row piles. Wang et al. [16] used MIDAS/GTS simulation software to study the stress and deformation state of double-row piles. Most of the previous studies have investigated the mechanical characteristics of double-row piles through theoretical calculation [12,17] and numerical simulation [18][19][20], and some scholars studied the stress characteristics of double-row piles through the laboratory model test [21,22]. However, due to the complexity of the construction site, theoretical calculation and numerical simulation values cannot fully reflect the real stress characteristics of the double-row piles, and the measured values from field tests are more realistic [23,24].
Studies on the mechanical characteristics of double-row steel pipe piles in foundation excavation through the field test are rarely reported, and few relevant research studies can be found, especially in the field of long-term monitoring of double-row steel pipe piles. In this paper, the strain gauges were installed on the surface of double-row steel pipe piles to monitor the strain of piles in foundation excavation and studied the feasibility of the installation method of gauges used in the test. Moreover, the bending moment of doublerow steel pipe piles during the foundation excavation was analyzed.

Site Condition.
e site is a typical soil-rock combination of top soil and bottom rock, and the soil layer distribution from top to bottom is plain fill, powdered clay, coarse sand, strong sand, strong clay, weathered granite, strongly weathered dunite porphyry, massive fractured rocks, and slightly weathered granite, respectively. e stable elevation of the ground water level in the field is 15.85∼20.33 m, the buried depth of the stable water level is 2.20∼4.30 m, and the underground water level in the test site is 15.99 m. e physical and mechanical parameters of each layer are shown in Table 1.

Support Structure.
is project is a transfer station of a subway, and the plan of the foundation pit is shown in Figure 1. e construction method is an open-cut method and smooth practice. e foundation pit adopts the microsteel pipe pile advance support system, which  has three steps from top to bottom. e first-order support structure used double-row steel pipe piles with the external diameter of 168 mm, wall thickness of 8 mm, and pile length of 12.3 m. e horizontal spacing of steel pipe piles was 1 m, and the steel pipe piles were embedded in the rock strata 1.5 m. e top of the doublerow steel piles was connected by a reinforced concrete crown beam with a width of 1000 mm and a height of 400 mm. e second-order and third-order support structures adopted single-row steel pipe piles with the length of 11.8 m and 16.6 m, respectively, and the diameter and wall thickness of the piles are the same as those of the double-row steel pipe piles. In order to improve the stability of the support structure, the prestressed anchor cable combined support was adopted in the soil layer, numbered AC1∼AC6, and the bolt combined support with HRB400 steel was adopted in the rock layer, numbered BL1∼BL9.

Test Program.
Eleven pairs of strain gauges were installed on the surface of one steel tube pile, which were marked as A1∼A11 and A1'∼A11', respectively, from the top to the pile tip. e strain gauges were used to monitor the strain state of the pile at different positions under different working conditions. e profile A-A and sensors installation location are shown in Figure 2. e installation steps of strain gauge are as follows: (1) Mark the installation positions of strain gauges with a pen, and drill a hole with a diameter of 20 mm next to each installation position for threading ( Figure 3(a)). (2) e sandpaper was used to polish the installation position. After wiping the polishing place with alcohol, 502 glue was used to paste the strain gauge on the surface of the steel pipe pile. en, the ohmmeter was used to test the survival rate of the strain gauges ( Figure 3(b)). (3) In order to improve the survival rate of the strain gauges during the test, the strain gauges were coated with 704 adhesive and epoxy resin, respectively ( Figure 3(c)). (4) e strain gauge line was connected with the data line, and the data line was inserted into the pile through the hole next to the installation position of the gauge. In order to protect the data line during pile driving, insert the data line into the plastic hose and fix the data line on the pile top (Figure 3(d)).
e mechanical drilling method was adopted, and the diameter of the drilling is 200 mm, as shown in Figure 4(a).  Advances in Civil Engineering e test steel pipe piles were put into the drilling holes, and the test piles were slowly pressed to the designed elevation ( Figure 4(b)). After that, the grouting pipe was inserted into the bottom of the micro-steel pipe pile, and the cement paste was injected into the piles. e water-cement ratio of the cement paste was 0.5, and the grouting pressure is 0.5 MPa. e DH3816N dynamic strain acquisition instrument was used to monitor the strain change of the strain gauges on the pile body under different working conditions. e bending moment value of the pile body can be calculated through the measured strain value of the gauges. e monitoring date and working conditions are shown in Table 2. e monitoring date is the number of days from the first data collection.

Bending Moment of Piles during the First-Order
Excavation. Bending moment of the test piles during the first-order excavation is shown in Figures 5(a) and 5(b). With the excavation of the foundation pit, the bending moment of test pile 1 increased gradually, and there are many points of contraflexure, which was consistent with the results of Liu and Fu [12] and Bai et al. [13]. e bending moment at the pile top increased with the excavation depth. At the beginning of excavation, the bending moment of the lower section of the pile was small. When the foundation pit was excavated to 10.75 m, the bending moment at the pile top reached the maximum of 15 kN·m. is indicates that in the process of foundation pit excavation, the upper section of the steel pipe pile played the support role first and bore a large load, according to the literature [8,10]. At beginning of excavation, the deep section of the pile can be regarded as the fixed end due to its large buried depth and embedment in the soil and rock, which has borne a small load [23].
It can be obviously seen that both positive and negative bending moments exist on the pile, which was consistent with the results of Shi and Gong. [15] e bending moment at 4.30∼6.45 m of the pile was negative, indicating that the soil surface of the pile was under tension, and the extreme value of negative bending moment at measuring section A5-A5' was −13.01 kN·m. e bending moment at 6.45∼8.60 m of the pile was positive, indicating the foundation pit surface of the pile was under tension, and the extreme value of positive bending moment at measuring section A7-A7' was 11.44 kN·m.  Advances in Civil Engineering e installation of the anchor cable had influence on the bending moment of piles. After AC2 installation, the bending moment of the 4-4' measuring section decreased from 6.82 kN·m to 2.73 kN·m. After the installation of the third anchor cable tension, the pile bending moment at the 6-6' measuring section decreased from 7.73 kN·m to 1.80 kN·m. When the foundation pit was excavated to 10.75 m, AC4 and AC5 were not installed in time, and the bending moment at the bottom of the pile increased significantly. is is because the steel pipe piles can be regarded as cantilever piles when anchor cables were not installed, and the lateral earth pressure was mainly balanced by the passive earth pressure at the bottom of the pile. e installation of anchor cables was equivalent to adding spring support to the steel pipe pile, which restricted the deformation of the pile and reduced the bending moment of the pile [19].
As shown in Figure 5(b), the bending moment variation of pile 1 and pile 2 was similar. e bending moment increased with the increase in excavation depth; the point of contraflexure appeared at the anchor cable installation position of 2.15 m, 4.3 m, 6.45 m, and 10.75 m, respectively. Compared with the bending moment of test pile 1, the bending moment of test pile 2 was smaller, and the maximum negative bending moment of test pile 2 was less than 10 kN·m, which is 70% of test pile 1. Moreover, the bending moment at 4.30∼6.45 m of test pile 2 was positive, and the bending moment at 6.45∼8.60 m of test pile 2 was negative. e bending moment value of test pile 2 is positive, which was opposite to the bending moment value of test pile 1. is is because top beam 1 and the top of test pile 1 and test pile 2 were rigidly connected; top beam 1 and the pile cannot rotate each other, so this structure can resist the bending moment together [20]. e bending moment of test pile 1 was obviously higher than that of test pile 2 due to the action of soil pressure between test piles 1 and 2, according to the literature [9].

Bending Moment of Piles during the Second-Order
Excavation. Bending moment of the test piles during the first-order excavation is shown in Figures 6(a) and 6(b).  Compared with the bending moment of test pile 1 during the first-order excavation ( Figure 5(a)), with the increase in excavation depth and the longer exposure time of the foundation pit, the bending moment of test pile 1 gradually increases. e bending moment of the pile increases obviously in the rock layer (7.0∼8.5 m), 7.55 m is the position with the maximum positive bending moment of the pile during the second-order excavation, and the maximum bending moment was 17.98 kN·m.
Compared with the bending moment of test pile 1 during the first-order excavation, the bending moment at the pile top was significantly reduced, and the value was between −4.72∼−10 kN·m. e maximum negative bending moment of the pile was moved from 5.4 m to 9.7 m. is indicates that the flexural rigidity of top beam 1 was high, and the composite structure of the top beam and double-row steel pipe piles was similar to rigid support [10]. Moreover, the structure can adjust the bending moment of the piles with  the change of the support condition of the lower end of the steel pipe pile so as to adapt to the complex and changeable load action, according to the literature [11,19]. As shown in Figures 6(a) and 6(b), the bending moment of test pile 2 was still significantly smaller than that of test pile 1. e maximum negative bending moment of test pile 2 was reduced from −7.94 kN·m to −2.78 kN·m, which was similar to the changing trend of the bending moment of test pile 1 and consistent with the results of Zhou et al. [24] e bending moment at anchor cable position was always small, without the phenomenon of sudden increase. is is because BL1∼BL4 were installed in time during the construction of the second-order excavation. e second-order support structure was the single-row steel tube pile, and the designed bearing capacity of BL1∼BL4 was 210 kN∼250 kN, which was higher than that of AC1∼AC5 (145 kN∼202 kN). It can be seen that the timely installation of bolts and the increase in bearing capacity of bolts can effectively restrain the deformation of steel pipe piles and improve the bearing capacity of support structure [10]. Figure 7(a), with the continued excavation of the foundation pit, the bending moment of the pile increased rapidly. e maximum negative bending moment of measuring section A5-A5' reached −26.6 kN·m, and the maximum positive bending moment of measuring section A5-A5' reached 19.01 kN·m. e positive and negative bending moments of test pile 2 also appear in the soil-rock-bound layer, which was similar to the bending moment distribution of test pile 1. is indicates that the pile in the soil-rock layer bears a large lateral earth pressure, and the support strength of the soil-rock layer of the pile should be strengthened in the structural design of the foundation pit. After the installation of top beam 3, the bending moment of test piles 1 and 2 was lower, indicating that the top beam can effectively limit the displacement of the steel tubular piles and improve the stability of the support structure, which was consistent with the results of Liu et al. [21] and Zhou et al. [24].

Bending Moment of Piles during the ird-Order Excavation. As shown in
As shown in Figure 7(b), the distribution and variation of the bending moment of test pile 1 were similar to those of test pile 2; with the continued excavation of the foundation pit, the bending moment of the pile increased rapidly. However, the bending moment of test pile 2 was less than that of test pile 1, which was similar to the bending moment during the second-order and third-order excavation.

Conclusions
e objective of this paper is to validate the feasibility of using strain gauges for monitoring the strain of double-row steel pipe piles during the foundation excavation. e installation and protection method of strain gauges was proposed. e mechanical properties of the double-row steel pipe pile were investigated. Main conclusions could be drawn as follows: (1) e combined support form of double-row steel pipe piles combined with prestressed anchor cables was suitable for deep foundation pit support. As the main support structure, double-row steel pipe piles play an important support role in the process of foundation pit excavation.  Advances in Civil Engineering 7 requirements and can carry out long-term monitoring of the strain state of steel pipe piles at different working conditions. (3) With the increase in foundation pit excavation depth, the bending moment of the pile increased. e bending moment distribution of the whole support system accords with the conventional pile-anchor mode.
In the future research, soil pressure sensors can be installed on the side of steel pipe piles to monitor the variation law of the lateral earth pressure during pit excavation. e displacement of the steel pipe pile can be monitored in the process of pit excavation, and the internal force change and displacement can be compared and analyzed.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.