Introduction

Currently, both domestic and international scholars have conducted extensive research on the impact of circular and non-circular shield tunneling construction on existing underground pipelines and tunnels. Among these studies, Zhang et al.1 developed a two-stage method for calculating pipeline vertical deformation based on the dual-parameter Pasternak foundation model. Deng et al.2 derived a deformation formula for existing pipelines induced by curved-section circular shield tunneling, combining the mirror-source method with Mindlin’s solution. Zhou et al.3 established a two-stage deformation calculation method for existing tunnels overlapped or undercrossed by circular shield tunnels. Zhang et al.4 analyzed longitudinal deformation of existing tunnels using the finite difference method. Fu et al.5 created a calculation model for quasi-rectangular shield tunnels crossing above existing tunnels, based on the mirror method and Mindlin’s solution. Zhang et al.6 obtained analytical solutions for pipeline deformation adjacent to quasi-rectangular tunnels using a two-stage approach combining the source method and Winkler beam model. Wang et al.7 proposed a pipeline deformation calculation method for orthogonal shield tunnel undercrossing based on small-hole expansion theory. Guan et al.8 developed a settlement prediction method for existing tunnels affected by newly excavated tunnels at various intersection angles using a modified Vlasov model.

Regarding model experiments, scholars have conducted systematic studies on both circular and quasi-rectangular shield tunnels. Fang et al.9 investigated the relationship between soil deformation, existing structures, and shield tunneling parameters during circular tunnel undercrossing beneath a metro station. Wei et al.10 examined surface and subsurface soil deformation caused by soil loss at different new-old tunnel intersection locations through laboratory model tests. Wei et al.11 analyzed pipeline settlement patterns and surface deformation during quasi-rectangular shield tunneling in sandy soil. Ran et al.12 proposed a BP neural network-based method for monitoring deformation of existing metro structures during shield construction. Zhao et al.13 conducted laboratory tests to evaluate effects of new quasi-rectangular tunnels on existing tunnel pressure, arch strain, and crown displacement. Wu et al.14 performed sandy soil model tests to study vertical pipeline settlement caused by multi-pipeline vertical undercrossing. Liu et al.15 analyzed settlement and deformation patterns in existing tunnels and surrounding soil during shield undercrossing. Shi et al.16 investigated ground settlement and pipeline deformation under small-scale water leakage conditions through indoor modeling.

In terms of field experiments, Ding et al.17 examined the behavior of synchronous grouting in quasi-rectangular shield tunneling through large-scale visual field excavation tests. Ma et al.18 conducted in-situ field tests to monitor the distribution of earth pressure during the construction of large-section quasi-rectangular shield tunnels in water-rich sandy strata, and assessed the adaptability of existing earth pressure calculation methods.

In numerical analysis, Cui et al.19 developed a 3D numerical model for shield tunneling through existing pipelines, examining effects of Poisson’s ratio, tunnel diameter, and pipeline presence on deformation. Jiao et al.20 created a 3D finite element model for dual-line shield tunneling through existing pipelines, analyzing impacts of soil elastic modulus, tunnel curvature radius, and pipeline burial depth. Notably, most existing studies focus on soft soil layers, with limited research on water-rich sandy strata.

This paper takes the large-section quasi-rectangular shield tunnel project of Zhengzhou Metro Line 8 in water-rich sandy layers as its engineering background. Through field tunneling tests and comprehensive monitoring, it systematically investigates the deformation effects of large-section quasi-rectangular shield tunneling on existing underground pipelines and power tunnels. By analyzing surface settlement characteristics, pipeline deformation distribution, and power tunnel responses during construction, this study provides data support and scientific basis for optimizing quasi-rectangular shield tunneling parameters and ensuring safety of existing underground infrastructure.

Project overview

Tunnel and surrounding environment overview

The left tunnel section of the large-section quasi-rectangular shield tunnel for Zhengzhou Metro Line 8 has a length of 87.369 m, comprising 72 rings, with a maximum longitudinal slope of −26‰. The right tunnel section measures 51.621 m in length, consisting of 43 rings, with a maximum longitudinal slope of + 28‰. The tunnel cover depth ranges from 17.26 m to 19.0 m. The shield machine dimensions are 11.830 m in width, 7.267 m in height, and 4.342 m in length. The segment thickness is 0.45 m, with a segment width of 1.2 m. The segment assembly lining structure forms a single-tube, double-track rectangular tunnel, constructed using staggered-joint assembly. M27 and M33 circumferential bolts are used for segment connections, with concrete strength of C50 and waterproofing grade P12.

This tunnel section is located within the Zhengzhou East Railway Station high-speed rail station area, where the surrounding environment - characterized by water-rich sandy strata - is sensitive and complex. The alignment passes beneath Dongzhan North Street and Dongzhan South Street, as well as existing operational Metro Line 5 tunnels, underground gas pipelines, water supply pipelines, stormwater pipelines, and power tunnels, all of which constitute critical infrastructure. The complex construction environment demands high-precision control throughout the rectangular shield tunneling process. The surrounding environment of the rectangular tunnel is illustrated in Fig. 1.

Fig. 1
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Satellite map of the surrounding environment of quasi- rectangular tunnel. (Satellite imagery provided by Google Maps, Annotations by the corresponding author).

Engineering geological overview

The quasi-rectangular shield tunnel test section is divided into southern and northern segments. The southern segment extends from DK46 + 478.706 to DK46 + 566.179, passing primarily through ②41 silty sand and ②51 fine sand strata. Its horizontal alignment comprises a transition curve followed by a straight section. The vertical alignment features an initial slope of −26‰ transitioning into a variable slope section (−26‰ to −2‰).

The northern segment spans from DK46 + 781.379 to DK46 + 833.000, traversing ②41 silty sand, ②51 fine sand, and ③51 A clayey silt strata. This segment maintains a straight horizontal alignment, while its vertical alignment consists of a variable slope Sect. (28‰ to 2‰).

Geological longitudinal profiles for both segments are presented in Fig. 2. Table 1 summarizes the physical and mechanical parameters of each soil layer.

Fig. 2
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Geological longitudinal section map of the test section. (a) The northern segment. (b) The southern segment.

Table 1 Physical and mechanical parameters of each soil layer.
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Monitoring plan

To monitor deformation of underground pipelines and power tunnels caused by uneven surface settlement induced by quasi-rectangular shield tunnel construction, surface settlement monitoring sections DM-5 and DM-35 are arranged during southern and northern shield segment construction, respectively. Surface settlement monitoring points are positioned at distances of 3 m, 3 m, 5 m, and L m (with the ‘L’ layout adjusted according to actual site conditions) from the quasi-rectangular tunnel centerline. Monitoring sections are numbered DM-N−1 to DM-N-n, where: DM = surface settlement monitoring section code, N = shield ring number, n = number of monitoring points per section.

Monitoring points for underground pipelines and power tunnels are positioned directly above these structures. These points are located at 3 m, 3 m, 5 m, and L m (with ‘L’ layout adjusted per actual site conditions) from the rectangular tunnel centerline. Monitoring sections are numbered RQ/GS/YS/DL-N-1 to RQ/GS/YS/DL-N-n, where: RQ = gas pipeline monitoring code, GS = water supply pipeline monitoring code, YS = stormwater pipeline monitoring code, DL = power tunnel monitoring code, N = shield ring number, n = number of monitoring points per section.

The plan view of the quasi-rectangular shield tunnel section and monitoring point layout are shown in Figs. 3 and 4. Table 2 summarizes underground pipelines and power tunnels in southern and northern sections.

Fig. 3
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The plan view of the rectangular shield tunnel section.

Fig. 4
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The layout plan of the monitoring points.

Table 2 The situation of underground pipelines and power tunnel in the Southern and Northern segment sections.
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Measured results and analysis

Deformation of underground pipelines

Figure 5 illustrates surface settlement and deformation curves of existing underground pipelines (gas, water, and stormwater pipelines) induced by the entire quasi-rectangular shield tunneling process in northern and southern sections. The shaded region represents the corresponding surface extent of the quasi-rectangular shield. Key observations include:

(1) The deformation patterns of underground pipelines and surface settlement exhibit general similarity, though deformation curves vary by pipeline type. Gas pipelines demonstrate “U”-shaped deformation due to their steel/PVC construction with high circumferential stiffness and ductility, enabling gradual bending under soil pressure. Water pipelines (brittle cast iron/concrete) exhibit “V”-shaped deformation from stress concentrations at joints. Stormwater pipelines (composite materials) show intermediate behavior due to anisotropic stiffness distribution.

(2) Surface settlement and pipeline deformation are significantly influenced by rectangular shield operating conditions. During active tunneling, both surface and pipelines experience initial uplift, peaking at the tunnel centerline. As the shield advances, deformation transitions from uplift to settlement, reaching maximum values after shield tail passage until disturbed soil consolidation completes.

(3) Pipeline material strength significantly affects deformation: PE pipes > cast iron pipes > concrete pipes. As material strength increases, the settlement trough width decreases, though material type has minimal impact on trough position.

(4) Three influence zones were identified around Zhengzhou’s water-rich sand layer quasi-rectangular EPBM tunnel. Pipeline displacement (SP) and surface settlement (SG) correlations follow R = SP/SG: Zone A (R > 1): Pipeline settlement exceeds surface settlement by 1–5 mm, Zone B (45°intermediate zone): SP ≈ SG, Zone C (R < 1): Pipeline settlement < surface settlement. These findings validate that existing pipeline deformation from rectangular shield construction can be accurately predicted using the Peck settlement curve.

Fig. 5
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Surface settlement and underground pipe settlement curves for south and north sections. (a) North section RQ-20 gas pipeline. (b) South section RQ-27 gas pipeline. (c) North section GS-25 water supply pipeline. (d) South section GS-21 water supply pipeline. (e) South section YS-15 rainwater pipeline.

Power tunnel deformation

Figure 6 illustrates the displacement curves of the DL-18 power tunnel in the southern section. From Fig. 6(a), it is evident that the vertical deformation of the power tunnel closely mirrors the ground surface settlement pattern, with the settlement curve exhibiting a “V”-shaped profile. As shield tunneling progresses, the ground surface initially experiences uplift while the power tunnel undergoes minor settlement. Subsequently, both the surface and the tunnel transition to settlement deformation, reaching maximum values after shield tail passage until disturbed soil consolidation stabilizes.

Figures 6(b) and 6(c) respectively show the horizontal displacement curves of the power tunnel in X and Y directions. The X-axis represents the north-south direction (along tunnel excavation), with negative values indicating southward movement; the Y-axis represents the east-west direction (cross-sectional), with negative values indicating eastward movement. In the X direction, points to the right of the tunnel centerline initially move northward before shifting southward, culminating in symmetrical lateral displacement. In the Y direction, all points exhibit eastward movement, with left-side points showing greater displacement than right-side points. Ultimately, maximum displacement reaches 1.5 mm in the X direction and − 8.3 mm in the Y direction. This demonstrates that in water-rich sand layers, quasi-rectangular shield tunneling induces significantly smaller horizontal displacement effects on existing power tunnels compared to vertical displacement.

Fig. 6
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Displacement curve of DL-18 power tunnel in the southern section. (a) Z-direction vertical displacement curve. (b) X-direction horizontal displacement curve. (c) Y- direction horizontal displacement curve.

Deformation control technology and measures

Deformation control for large-section quasi-rectangular EPBM tunnel construction in water-rich sand layers centers on “dynamic regulation and multi-dimensional collaboration,” integrating refined construction techniques with intelligent monitoring systems to achieve “active ground deformation control” rather than “passive repair of existing infrastructure.” The primary control measures include:

(1) Friction Reduction via Lubricants: High-molecular-weight materials such as polyacrylamide are employed to reduce shield-ground friction. Dosage must be strictly controlled to prevent groundwater contamination. Foam conditioners enhance soil fluidity while reducing cutterhead-soil adhesion.

(2) Shield Machine Design Optimization: The cutterhead opening ratio is appropriately increased to minimize contact pressure with the soil formation. A hybrid tool configuration combining roller cutters and disc cutters reduces cutterhead torque requirements.

(3) Construction Parameter Optimization: Soil chamber pressure is dynamically adjusted to maintain excavation face stability. The shield machine operates at uniform advance speed with controlled cutterhead torque to mitigate shear failure risks. Cement-water glass grouts fill shield tail voids, with compensatory grouting prioritized in the final 2–3 rings - particularly beneath existing utilities. Grouting pressure is maintained at ≤ 0.5 MPa to prevent soil fracturing.

(4) Pressure Relief Hole Installation: Strategically positioned pressure relief holes between existing pipelines, power tunnels, and the quasi-rectangular tunnel section alleviate construction-induced pore water pressure, thereby reducing additional stress concentrations.

Conclusions

(1) While deformation patterns of underground pipelines and power tunnels generally exhibit similarities, functional differences result in distinct deformation curves. Gas pipelines demonstrate “U”-shaped deformation profiles, water supply pipelines and power tunnels show “V”-shaped patterns, and stormwater pipelines exhibit intermediate behavior between these two extremes.

(2) Pipeline/power tunnel material strength significantly influences deformation characteristics. Deformation severity follows the order: polyethylene (PE) pipelines > cast iron pipelines > concrete pipelines/power tunnels. As material strength increases, the width of the settlement trough progressively narrows, though material variation has negligible impact on the trough’s positional center.

(3) In water-rich sand strata, large-section quasi-rectangular EPBM construction emphasizes “active ground deformation control” for existing infrastructure. Key control measures include: friction reduction through lubricant application, optimized shield machine design, refined construction parameter adjustment, and strategic installation of pressure relief holes.