Numerical Analysis Study on the Influence of Shield Cutting Pile on the Settlement of Pile–Soil Composite Foundation

Abstract

Based on the shield cutting pile–soil composite foundation project of Zhengzhou Metro Line 5, numerical analysis was employed to replicate the in situ experimental conditions on site. The study focused on investigating the impact of shield cutting piles on the settlement of pile–soil composite foundation and analyzing the sensitivity of shield construction parameters, face pressure (P), and grouting pressure (Q). Comparing the numerical analysis results of land surface subsidence with field in situ test monitoring results reveals a consistent pattern, validating the rationality of the numerical modeling method presented in this paper. The results demonstrate that the vertical displacement of the formation caused by shield cutting pile construction can be divided into settlement and uplift areas. The region with significant formation displacement settlement spans within 0.9D (5.4 m above the tunnel vault), whereas the area with pronounced formation displacement uplift ranges within 0.58D (3.5 m below the tunnel arch bottom). The influence of face pressure (P) on the bearing characteristics of the composite foundation primarily occurs during the prepile cutting stage, whereas the influence of grouting pressure (Q) on the bearing characteristics of the composite foundation mainly manifests after the shield tail separates from the residual pile of the composite foundation.

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.

1. Introduction

The shield tunneling method offers numerous advantages, such as minimal environmental impact, high mechanization, and low construction costs, making it a widely adopted approach for urban railway construction globally. As urban underground space development and utilization continue to expand across various countries, there is an increasing demand for improved construction environments during metro shield tunneling. In recent years, metro construction projects have frequently involved tunnels being constructed adjacent to existing underground structures (such as buildings), and in some cases, even coming into contact with the foundations of upper buildings, presenting complex working conditions. As shield equipment and construction technology become more advanced, it has now become feasible to utilize shield machines to directly cut piles through existing buildings.

Currently, only a limited number of scholars have conducted research on shield pile cutting. Professor Yuan Da-jun’s team [1, 2, 3, 4] has undertaken extensive research on cutting large-diameter reinforced concrete piles using shield technology. Their work encompasses innovative research on pile cutting theory, new tool development, cutterhead and tool configuration methods, and the first-ever shield pile cutting test conducted both domestically and internationally. Wang et al. [5, 6] have selected cutters based on the characteristics of soft soil areas and large-diameter pile foundations. They have combined this with the Advant Edge FEM finite-element software to design the angle of the shell cutter and study tool arrangement. In addition, Liu et al. [7] have carried out numerical simulation analysis of existing bridge pile foundations encroaching on Beijing Metro Line 12 using the discrete element method–continuum mechanical coupling method. They have proposed a theoretical calculation model for shield cutterhead thrust that considers the pile cutting effect. Peng et al. [8] have comprehensively analyzed the cutter plate force when shield cutting piles from a mechanical standpoint, simplifying the pile–soil composite foundation to an equivalent composite soil body, and deriving a formula for calculating the cutter plate load when shield cutting composite foundation group piles. Furthermore, Xu et al. [9] have conducted indoor tests of shield cutter disc cutting reinforced concrete pile foundations; statistically analyzing the effect of cutting pile foundations, reinforcement damage patterns, vibration characteristics of the cutter disc, and the form of tool damage; and revealing the principle of hobbing and tearing cutter cutting pile foundations. In addition, Xu et al. [10] have conducted indoor cutting tests to investigate the interaction law of shield cutter and concrete, verifying that cutting is essentially a cyclic process involving repeated collision, extrusion, cutting, and spalling between the cutter and the concrete specimen. Moreover, Li et al. [11] have conducted hob and tear cutter cutting pile foundation tests to analyze rebar fracture patterns, force characteristics, cutting parameters, tool damage, and cutter vibration characteristics. Liu et al. [12] have developed an analytical model for single-tool cutting concrete and obtained the single-tool penetration force, which can be used to predict the thrust force when shield cutting piles. Wang et al. [13], relying on actual engineering cases, have investigated the tool wear during shield cutting of concrete piles with diameters ranging from 800 to 1,000 mm. They found that during the cutting process of reinforced concrete piles, steel reinforcement usually breaks under tensile stress.

Most of the aforementioned related research focuses on the enhancement and advancement of shield cutting pile foundations and cutting tools. However, there is a scarcity of cases detailing shield cutting of pile–soil composite foundations as well as limited research on the settlement law of composite foundations and strata resulting from shield cutting effects. Therefore, there is an urgent need to investigate the impact of shield pile cutting construction on pile–soil composite foundations. This paper addresses this need by examining the effect of shield pile cutting construction on the pile–soil composite foundation beneath the masonry structure of the Zhengzhou Metro Line 5. It conducts field tests of shield structures directly cutting single-pile composite foundations in cement soil and analyzes the settlement patterns of the composite foundation and surrounding strata during the shield cutting process. This is achieved through the restoration of field test conditions using numerical analysis methods. In addition, the paper explores the sensitivity of shield construction parameters such as palisade pressure (P) and grouting pressure (Q). The research findings hold significant practical value and can serve as a foundational reference for similar projects in the future.

2. Engineering Background and Geological Conditions

The section from Zhengzhou Metro Line 5 Children’s Hospital Station to Zhacheng Station utilizes Ø6410 earth pressure balance shield construction. The tunnel has an outer diameter of 6.2 m, an inner diameter of 5.5 m, a ring width of 1.5 m, and a wall thickness of 0.35 m. During construction, the shield cutting group pile intersects with the 1# building of Zhenghe Community, which features a 7-storey masonry structure. This section utilizes a cement–soil group pile composite foundation, with a pile diameter of 0.5 m, a length of 11.5 m, and a spacing of 0.95 m between piles. The shield machine cuts piles to lengths ranging from 2.6 to 3.7 m, resulting in approximately 224 shield cutting piles on the left line and about 114 shield cutting piles on the right line. Figure 1 illustrates the relative position of the masonry structure and the tunnel.

[figure(s) omitted; refer to PDF]

The tunnel passes through the following main geological conditions: clay silt ②₂₂, silty clay ②₄₁, and clay silt ②₅₁. The depth of the tunnel top ranges from 12.1 to 13.5 m. The engineering geological conditions are depicted in Figure 2. A field test of shield cutting cement–soil monopile composite foundation was conducted in an open temporary parking lot located 30 m away from the #1 building of Zhenghe Community. During this test, the surface settlement was monitored throughout the shield construction process, with the layout of surface settlement measurement points illustrated in Figure 3.

[figure(s) omitted; refer to PDF]

3. Numerical Modeling Analysis

3.1. Calculate Models and Parameters

The numerical model of the shield cutting composite foundation adopts the Moore Coulomb model [14, 15, 16] as the soil constitutive model, and the Midas GTS NX finite-element solid element unit is used to simulate the soil mass. Based on the engineering geological survey data shown in Figures 4(a) and 4(c), the soil layers consist of plain fill soil ①₁, clayey silt ②_31 A, clayey silt ②₃₂, silty clay ②₃₃, clayey silt ②₂₂, silty clay ②₄₁, clayey silt ②₅₁, fine sand ②_52A, fine sand ②₅₂, and silty clay ③₂₃. The physical and mechanical parameters of the soil are provided in Table 1.

Table 1

Physical and mechanical parameters of each soil layer.

Layer no.	Soil name	H (m)	w (%)	e	γ (kN m3)	Es (MPa)	μ	c (kPa)	φ (°)	fak (kPa)
①1	Qml	2.7	18.0	0.95	20	4.5	0.33	20	20	110
②31A	Clayey silt	2.6	19.8	0.707	18.9	9.2	0.29	16.7	20.7	140
②32	Clayey silt	2.9	20.8	0.676	19.4	8.5	0.30	16.9	20.6	130
②33	Silty clay	1.0	25.1	0.73	19.6	5.7	0.36	25.5	14.3	120
②22	Clayey silt	1.5	23.2	0.668	19.9	7.5	0.32	16.0	19.2	130
②41	Silty clay	3.0	27.8	0.85	19.4	6.1	0.38	24.9	14.4	120
②51	Clayey silt	2.5	21.1	0.625	20.2	8.7	0.33	16.2	19.3	140
②52A	Fine sand	5.9	2.1	0.7	19.4	20.0	0.27	1.0	30.0	200
②52	Fine sand	5.8	2.0	0.7	19.5	22.0	0.27	1.0	32.0	220
③23	Silty clay	6.0	19.5	0.594	20.3	9.8	0.30	29.7	15.3	230

Notes. φ is the drained type friction angle; μ is the Poisson’s ratio; γ is the saturated unit weight; e is the void ratio; w is the water content; c is the effective cohesion force; H is the layer thickness; E_s is the oedometric compression modulus; and f_ak is the characteristic value of bearing capacity.

[figure(s) omitted; refer to PDF]

To simulate the load plate and bedding, Midas solid units are used, whereas the piles are simulated using Midas beam units. The shield shell, lined pipe sheet, short-term grouting layer, and postconsolidation grouting layer are simulated using Midas shell units. The material parameters for the bedding layer and hydraulic soil pile are selected based on references [17, 18], whereas the material parameters for the shield shell are taken from the research of Komiya et al. [19].

The modified inertia method [20, 21] is employed, introducing the concept of bending stiffness efficiency η to account for the stiffness reduction effect caused by pipe joints on the overall lining pipe ring. In this project, a bending stiffness efficiency η value of 0.75 is used to reduce the stiffness of the staggered-seam assembled pipe ring, based on the experimental study by Huang et al. [22]. The modulus of elasticity for the pipe sheet ring is assumed to be 25.9 GPa, and the Poisson’s ratio is set at 0.2.

As illustrated in Figure 5, the equivalent homogeneous ring method [23, 24] is applied to simulate the shield-tail grouting layer. A series of influencing factors such as shield-tail gap (0.03 m), shield-shell thickness (0.08 m), shield-tail grouting filling degree, and shield-perturbed disturbance degree are simulated and analyzed using a series of equivalent 0.11 m thick homogeneous linear elastic shell units. The structure parameters and grouting layer parameters are provided in detail in Tables 2 and 3.

Table 2

Structure parameters.

Structure name	γ (kN m3)	μ	E (MPa)	r (m)	t (m)	L (m)
Load plate	25.0	0.2	34.5	—	0.02	0.95
Cushion layer	20.0	0.3	0.05	—	0.02	0.95
Soil–cement pile	20.0	0.3	0.2	0.5	—	15.6
Shield shell	97.0	0.2	200	3.23	0.045	9.0
Lining segment ring	25.0	0.2	25.9	3.1	0.35	1.5

Notes. L is the structure length; t is the structure thickness; μ is the Poisson’s ratio; E is the oedometric compression modulus; γ is the unit weight; and r is the structure diameter.

Table 3

Grouting layer parameters.

Structure name	γ (kN m3)	μ	E (MPa)	Distance from shield tail (m)
Short-term grouting layer 1	19.5	0.25	1	1.5 (1 ring out of shield tail)
Short-term grouting layer 2	19.5	0.25	10	3.0 (2 rings out of shield tail)
Short-term grouting layer 3	19.5	0.25	20	4.5 (3 rings out of shield tail)
Grouting layer after setting	19.5	0.25	40	＞4.5 (4 or more rings out of shield tail)

Notes. μ is the Poisson’s ratio; E is the oedometric compression modulus; and γ is the unit weight.

[figure(s) omitted; refer to PDF]

The load transfer method [25, 26] is employed to simulate the load transfer relationship between the pile and the soil. The transfer function chosen is the widely used linear elastic-perfectly plastic transfer function proposed by Japanese scholar Satoru [27].

The boundary condition settings of the model are depicted in Figure 4(b). Constraints in the X direction are applied to the left and right faces of the model, whereas constraints in the Y direction are applied to the front and back of the model. Constraints in the X, Y, and Z directions are simultaneously imposed on the base surface of the model. The top face of the model is set as a free boundary with no constraints in any direction. The positions of the numerical models corresponding to the monitored cross-sections of surface subsidence are shown in Figure 4(d).

3.2. Shield Excavation Analysis Step

The numerical model analysis steps for the shield cutting cement–soil monopile composite foundation are shown in Figures 6(a) and 6(b) provides the simulation diagram for shield excavation into the Nth ring. The specific excavation steps are as follows:

[figure(s) omitted; refer to PDF]

First, the excavated soil unit ① in front of the shield machine is deactivated and the shield shell unit ⑤ is activated to achieve the forward thrust of the shield machine. At this stage, the face pressure ② and jack thrust ③ required to excavate the Nth ring are applied.

Second, the n-6th ring lining segment unit ⑥ at the tail of the shield is activated and the grouting pressure ④ is applied within the width of the n-6th ring at the tail of the shield. The short-term grouting layer 1 (E = 1 MPa) of the Nth ring is also activated.

Finally, following the above excavation step cycle, steps n + 1, n + 2, and n + 3 are carried out sequentially to simulate shield boring, segment assembly, and shield-tail synchronous grouting. By varying the material properties of the grouting layer according to the different distances between the grouting layer and the shield tail, the condensation hardening process of the grouting slurry over time is simulated, approximating the “time effect” of the condensation hardening of shield-tail synchronous grouting grout.

3.3. Analysis of Land Surface Subsidence Calculation Results

Six typical working conditions, namely S7, S11, S12, S18, S22, and S31, were selected to analyze the surface settlement caused by the construction of the shield cutting cement–soil monopile composite foundation. The details of the selected typical operating conditions are presented in Table 4.

Table 4

Typical operating conditions.

Stage name	Construction analysis step	Number of excavation rings	Driving distance of cutter head	Shield construction conditions
Ⅰ stage	S7–S11	646–650 rings	−8.25 to −0.75 m	Before shield cutting pile.
Ⅱ stage	S12	651 ring	0 m	Shield cutting pile.
Ⅲ stage	S13–S17	652–656 rings	+0.75 to +8.25 m	Shield tail detached from residual pile composite foundation.
Ⅳ stage	S18	657 ring	+9.75 m	Shield body passes through residual pile composite foundation.
Ⅴ stage	S19–S21	658–660 rings	+9.75 to +14.25 m	Grouting layer condensation.
Ⅵ stage	S22–S31	661 –670 rings	+14.25 to +29.25 m	Shield is away from residual pile composite foundation.

Notes. “−” means that the cutter head of shield machine is not excavated to soil–cement monopile composite foundation; “0” means that the cutter head of shield machine is just excavated to contact soil–cement monopile compound foundation pile body; and “+” means that the cutter head of shield machine has cut the pile through soil–cement monopile composite foundation.

Figure 7 depicts the comparison curve between the simulated and measured values of the surface settlement trough under typical working conditions. The figure illustrates that, as the shield excavation construction progresses, the surface settlement trough gradually deepens, with a width of approximately 3.33D. In the presence of the existing cement–soil monopile composite foundation, the lateral influence range of shield tunnel construction extends to about 1.67D from the tunnel axis. The maximum settlement value of the cement–soil monopile composite foundation is situated at the foundation’s center point, measuring −33.3 mm.

[figure(s) omitted; refer to PDF]

Through comparative analysis of the construction steps S18 (shield machine cutterhead pitch composite foundation pile body + 6.75 m/shield tail detached pile composite foundation) and S22 (shield machine cutterhead pitch composite foundation pile body + 15.75 m/shield tail detached from the residual pile composite foundation 1.125D), it was observed that the field test surface settlement measured data and numerical simulation results follow a similar pattern, both exhibiting a normal distribution. Furthermore, the maximum settlement amount of the cement–soil monopile composite foundation is approximately consistent. Considering slight deviations in the settlement values due to measurement instrument errors and uncontrollable factors in the construction environment, it is deemed feasible to employ finite-element numerical simulation for studying and analyzing the influence of shield cutting pile penetration on the bearing characteristics of the cement–soil monopile composite foundation.

4. Vertical Displacement (Z) Cloud Map

Figures 8(a), 8(b), 8(c), 8(d), 8(e), and 8(f) depict the vertical (Z) displacement cloud diagrams for S7, S11, S12, S18, S22, and S31 under typical working conditions, respectively, as outlined in Table 4. From these figures, it can be observed that the vertical displacement of the ground layer during the construction of shield cutting piles is distinctly divided into two zones: a settlement zone and an uplift area. The settlement zone is located above the tunnel vault, where the amount of settlement increases progressively from the surface down to the tunnel vault. The area experiencing significant displacement and settlement around the tunnel vault extends up to 0.9D (5.4 m above the tunnel vault).

[figure(s) omitted; refer to PDF]

Conversely, the uplift area is situated below the tunnel arch bottom, where the degree of uplift decreases gradually from the tunnel arch bottom to the deeper ground layers. The region experiencing considerable uplift at the tunnel arch bottom extends up to 0.58D (3.5 m below the tunnel arch bottom). The maximum surface settlement occurs at the center point of the composite foundation, which is attributed to the combined effect of the load loading on the composite foundation and the disturbance caused by the shield cutting pile.

5. Shield Boring Parameter Sensitivity Analysis

5.1. Palm Face Pressure P

Figure 9 illustrates the change curve of the surface settlement trough at different pressures (P) on the tunnel face. The figure shows that when the pressure on the tunnel face changes, the final morphology of the lateral surface settlement trough caused by the excavation of a shield cutting through a cement–soil monopile composite foundation remains similar, exhibiting a Gaussian distribution. The width of the settlement trough is consistent across different pressures. However, as the pressure on the tunnel face increases, the curve of the settlement trough gradually shifts upward and narrows within the width range of −3.0 to +3.0 m (centered on the axis of the shield machine, within a radius of 0.5D). This indicates that the settlement of the shield cutting composite foundation within a certain range (centered on the axis of the shield machine, within a radius of 0.5D) can be effectively reduced by controlling and adjusting the tunnel face pressure during shield construction.

[figure(s) omitted; refer to PDF]

By comparing the maximum values of the final surface settlement under Working Conditions 1–4 (see Table 5), it is evident that the surface settlement decreases with the increase in tunnel face pressure. However, when $P=0.20$ MPa, the maximum value of the final surface settlement is not significantly different from that when $P=0.25$ MPa. This suggests that beyond a certain point, increasing the tunnel face pressure does not proportionally inhibit the settlement of the composite foundation. Specifically, when the tunnel face pressure reaches 0.20 MPa, further increasing the pressure has a minimal effect on reducing the settlement of the composite foundation.

Table 5

Maximum surface settlement at different palm surface pressure P.

Working conditions	Palm face pressure value P (MPa)	Surface settlement maximum (mm)
Condition 1	0.10	−30.6385
Condition 2	0.15	−28.561
Condition 3	0.20	−27.647
Condition 4	0.25	−27.897

5.2. Grouting Pressure Q

Figure 10 illustrates the change curve of the surface settlement trough at different grouting pressures (Q). By comparing the curves of the final settlement trough under four different working conditions, we can observe that within a certain range, increasing the grouting pressure effectively suppresses the settlement of the composite foundation. The suppression effect is centered on the axis of the shield machine and extends within a radius of 0.5D. However, when the grouting pressure exceeds 0.35 MPa, the surface settlement trough shifts slightly downward, indicating a slight increase in settlement. Preliminary analysis suggests that when the grouting pressure exceeds 0.35 MPa, the soil above the tunnel vault splits, reducing its strength and leading to the collapse of soil within a certain range above the vault. This, in turn, causes an increase in surface settlement. Consequently, beyond 0.35 MPa, further increasing the grouting pressure results in diminished control over the settlement of the composite foundation, exhibiting the principle of “less is more.” The maximum values of surface settlement at different grouting pressures (Q) are presented in Table 6.

Table 6

Maximum surface settlement at different grouting pressure Q.

Working conditions	Grouting pressure value Q (MPa)	Surface settlement maximum (mm)
Condition 1	0.20	−32.686
Condition 2	0.25	−30.854
Condition 3	0.35	−28.358
Condition 4	0.45	−29.547

[figure(s) omitted; refer to PDF]

5.3. Variation of PileSoil Stress Ratio

Figure 11 shows the change curve of the pile–soil stress ratio at different face pressures. Analyzing the change rule of the pile-soil stress ratio at different face pressures through the six typical stages outlined in Table 4, it can be observed that: during the four stages of before shield cutting the pile (Stage I), shield cutting the pile (Stage II), shield passing through the residual pile of the composite foundation (Stage III), and the shield tail detaching from the residual pile of the composite foundation (Stage IV), the pile–soil stress ratio generally increases with higher face pressure. However, during the grouting layer condensation stage (Stage V) and the shield away from the residual pile composite foundation stage (Stage VI), the change rule of the pile–soil stress ratio obtained from Working Conditions 1 to 4 is consistent and tends to be uniform. The analysis indicates that within the significant range of longitudinal disturbance (rings 646–657) caused by shield construction, the face pressure is used to balance the sum of the water and soil pressure in front and the additional stress generated by the composite foundation, playing a crucial role in maintaining the stability of the stratum ahead of the face. Nonetheless, as the shield machine moves away from this significant range of longitudinal disturbance, the impact of face pressure on the bearing properties of the composite foundation diminishes, causing the pile–soil stress ratio to ultimately converge to a similar change pattern.

[figure(s) omitted; refer to PDF]

Figure 12 depicts the variation curve of the pile–soil stress ratio under different grouting pressures. Analyzing the change law of the pile–soil stress ratio under different grouting pressures based on the six typical stages outlined in Table 4, it is evident that the change law of the pile–soil stress ratio from Working Conditions 1 to 4 is essentially the same and tends to be consistent across three stages of shield pile cutting (Stage I), shield through residual pile composite foundation (Stage II), and shield through residual pile composite foundation (Stage III). However, during the three stages of shield tail detachment from the residual pile composite foundation (Stage IV), grouting layer coagulation (Stage V), and shield away from the residual pile composite foundation (Stage VI), a phenomenon emerges where the higher the grouting pressure, the greater the pile-soil stress ratio. This indicates that the impact of grouting pressure on the bearing properties of the composite foundation becomes more pronounced after the shield tail detaches from the residual pile composite foundation. The rationale behind this phenomenon is that when the shield tail detaches from the residual pile composite foundation, the synchronous grouting pressure of the shield tail significantly inhibits the settlement of the residual piles and strata in the upper part of the tunnel vault. This gradual increase in load borne by the pile body and the corresponding decrease in load borne by the soil between the piles ultimately leads to the incremental rise of the pile–soil stress ratio.

[figure(s) omitted; refer to PDF]

6. Conclusions

In this paper, Midas GTS NX finiteelement software is employed to simulate the excavation process of a shield cutting through a composite foundation. This study investigates the settlement behavior of the composite foundation and ground caused by the cutting pile and analyzes the response of surface settlement and the pile–soil stress ratio of the composite foundation when the face pressure (P) and grouting pressure (Q) vary. The main conclusions are as follows:

(1) A comparative analysis of the measured surface settlement data and numerical simulation results during the shield cutting process reveals that both datasets follow Gaussian distributions with high consistency, verifying the reasonableness and effectiveness of the numerical modeling method used in this paper.

(2) The vertical displacement of the stratum during the cutting pile process can be divided into a settlement area and an uplift area. The settlement area is located above the tunnel vault (within 5.4 m above the tunnel vault), whereas the uplift area is situated below the tunnel arch bottom (within 3.5 m below the tunnel arch bottom).

(3) The influence of face pressure on the bearing properties of the composite foundation is primarily significant before pile cutting. Conversely, the grouting pressure’s influence on the composite foundation’s bearing properties is mainly significant after the shield tail detaches from the residual pile composite foundation. This study finds that when the face pressure is 0.2 MPa and the grouting pressure is 0.35 MPa, the impact of the shield cutting pile on the composite foundation’s bearing properties and settlement is minimized.

Acknowledgments

The research described in this paper was financially supported by A new round of construction project of key academic discipline in Henan Province (Teaching and Research (2023) No. 414 issued by Education Department of Henan Province), Key Research Projects of Higher Education Institutions in Henan Province (No. 24A560023), and Zhengzhou University of Technology High-level Talent Research Project (No. 24GC02).

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