Sustainability-Oriented Grouting Techniques for Soil Improvement and Retaining Pile Installation in Urban MRT Construction: Insights from the Zhonghe–Wanda Line, Taipei

Tai-Yi Liu 1✉ Emailtylaytpe@ms9.hinet.net

Chao-Pang Wang 2 Emaild11521038@ntu.edu.tw

Shih-Ping Ho 3 Emailspingho@ntu.edu.tw

Sung-Fang 1 Email11638@gov.taipei

Yeh 1 Email19375@gov.taipei

Kai-Chun Ten 5 Email16334@gov.taipei

Bing-Ci Ciou 6

Adjunct associate professor, Civil Engineering Department National Taiwan University Taiwan, R.O.C

2 Ph.D. candidate, Civil Engineering Department National Taiwan University Taiwan, R.O.C

3 Civil Engineering Department National Taiwan University Taiwan, R.O.C

4 Chief Engineer, Department of Rapid Transit Systems Taipei City Government Taiwan, R.O.C

5 Project Manager, Department of Rapid Transit Systems Taipei City Government Taiwan, R.O.C

6 Master student, Civil Engineering Department National Taiwan University R.O.C Taiwan

Tai-Yi Liu^1*, Chao-Pang Wang², Shih-Ping Ho³, Sung-Fang, Yeh⁴, Kai-Chun Ten⁵, Bing-Ci Ciou⁶

¹ Adjunct associate professor, National Taiwan University, Civil Engineering Department, Taiwan, R.O.C. E-mail: tylaytpe@ms9.hinet.net

² Ph.D. candidate, National Taiwan University, Civil Engineering Department, Taiwan, R.O.C. E-mail: d11521038@ntu.edu.tw

³ Professor, National Taiwan University, Civil Engineering Department, Taiwan, R.O.C. E-mail: spingho@ntu.edu.tw

⁴ Chief Engineer, Department of Rapid Transit Systems, Taipei City Government, Taiwan, R.O.C. E-mail: 11638@gov.taipei

⁵ Project Manager, Department of Rapid Transit Systems, Taipei City Government, Taiwan, R.O.C. E-mail: 19375@gov.taipei

⁶ Master student, National Taiwan University, Civil Engineering Department, Taiwan, R.O.C., E-mail: 16334@gov.taipei

Abstract

This study examines advanced grouting techniques applied in the Taipei MRT Zhonghe–Wanda Line project, particularly the underground section between LG06 and Y11 stations. Dense underground utilities and challenging subsurface conditions—soft silty clay and a shallow groundwater table—made conventional diaphragm walls infeasible.

Two methods were implemented: standard high-pressure jet grouting (JSG) and large-diameter super high-pressure jet grouting. Retaining piles were constructed by injecting cement grout (100–220 kg/cm²) via rotating drill rods, then inserting H-shaped steel beams to form high-strength, low-permeability composite elements. In critical areas beneath power lines and telecom conduits, 3.6 m super high-pressure grouting (up to 400 kg/cm²) was used to enhance soil uniformity and minimize impacts on adjacent infrastructure.

Detailed construction records—grout pressure, flow rate, and lifting speed—were maintained. Post-construction coring and laboratory tests confirmed unconfined compressive strength ≥ 10 kg/cm² and permeability K ≤ 10⁻⁵ cm/sec.

The results demonstrate that integrating standard and large-diameter jet grouting effectively addresses excavation challenges in dense urban environments and provides a practical reference for future MRT developments.

Keywords:

Jet Grouting

Soil Improvement

Underground MRT Construction

High-Density Urban Utilities

Super High-Pressure Grouting

1 Introduction

Urban mass rapid transit (MRT) construction in densely populated metropolitan areas presents significant engineering challenges, especially when it involves underground excavation in soft or unstable soil conditions. The Taipei MRT Zhonghe–Wanda Line project exemplifies these challenges, particularly the underground connecting passage between LG06 and Y11 stations, traversing complex geotechnical environments and densely built urban utilities, including 161KV power trench, telecommunication cables, gas supply pipes, water supply pipes, and sewage Pipes. The subsurface profile includes soft alluvial soils, silt and clay layers with high plasticity, and a relatively shallow groundwater table—conditions that demand robust excavation support and effective ground improvement strategies [1]. Figure 1 illustrates the geographical context of the Zhonghe–Wanda Line, delineating the positions of LG06 and Y11 stations and the alignment of the intervening underground connecting passage within the urban fabric. As shown in Fig. 2, the cross-sectional schematic depicts the intricate arrangement of major subterranean utilities—including water, gas, sewage, and 161 kV electrical conduits—positioned directly above the proposed connecting passage, highlighting the complexity of the construction environment.

Fig. 1

The location of the underground connecting passage between LG06 and Y11 stations.

Fig. 2

The complicate urban utilities in the site.

Traditional retaining systems and soil improvement techniques for excavation often prove inadequate under such demanding conditions, especially in urban environments where vibration, noise, and settlement must be strictly controlled to protect adjacent utilities and structures [2]. The steel sheet piles, steel H beams, bored-hole RC piles, and diaphragm walls were widely adopted to serve as the retaining system for excavation [3]. To meet these requirements, this study focuses on two advanced geotechnical solutions: high-pressure jet grouting (JSG) and large-diameter super high-pressure jet grouting. These techniques are increasingly applied in urban tunnelling due to their ability to enhance soil strength, reduce permeability, and provide ground stability during deep excavation [4].

The retaining pile system adopted for this project involves a 0.8-meter-diameter jet grouted column formed using rotating drill rods equipped with high-pressure nozzles. Cementitious grout is injected at pressures of 100 ~ 220 kg/cm² to form Jet-grouted columns. Steel H-piles are then inserted into the freshly grouted mass to form composite elements capable of resisting earth pressure and groundwater intrusion [5]. This methodology reduces environmental disturbance and ensures construction safety even in the presence of nearby telecom ducts and power cables. Figure 3 presents a schematic representation of a high-pressure jet-grouted column incorporating an H-shaped steel pile, elucidating the interaction between the soilcrete mass, the reinforced element, and their proximity to adjacent underground works.

Fig. 3

Schematic of High-Pressure Jet Grouting Pile with H-Pile Insertion.

To further strengthen the subsoil beneath critical utility corridors, the project employed a 3.6-meter-diameter super high-pressure jet grouting method. This technique uses dual symmetric nozzles delivering grout at 400 kg/cm² with high flow rates (up to 300 L/min), creating homogeneous jet-grouted columns of exceptional size and uniformity. The enhanced soil bodies significantly improve load-bearing capacity and reduce permeability, essential for safe excavation near vital infrastructures such as 161kV power lines and deep communication ducts [6]. This dual-method approach offers an adaptable solution for managing excavation-induced risks in urban transit systems.

The objective of this paper is to document and assess the implementation of these innovative grouting methods, analysing their practical performance in the context of the CQ861 contract zone. The report outlines equipment specifications, grout mix designs, construction sequencing, and quality control protocols. Field coring and laboratory tests were conducted to verify the unconfined compressive strength and permeability of the soilcrete columns, ensuring compliance with the project’s technical specifications (≥ 10 kg/cm²; K ≤ 10⁻⁵ cm/sec) [7].

Through this case study, the paper demonstrates the feasibility, reliability, and engineering value of advanced jet grouting techniques for MRT projects in complex urban terrains. The findings contribute to improved geotechnical design practices and offer practical references for future underground construction endeavours in similar geological and infrastructural settings [8].

2. Methodology

To address the geotechnical complexities in the Taipei MRT Zhonghe–Wanda Line CQ861 project—especially the underpass between LG06 and Y11 stations—this study adopted a field-driven engineering methodology that combines high-pressure jet grouting (JSG) and large-diameter super high-pressure grouting. The methodology includes equipment selection, construction sequencing, grout material control, quality assurance, and performance validation.

2.1 Retaining Pile Construction with JSG

The retaining piles were constructed using a jet mixing method, involving in-situ soil cutting and cement grout injection, followed by steel H-beam insertion. Jet grouting was carried out using both 30B crawler-type and CC-2L hydraulic drill rigs, selected based on site constraints and required penetration depth. Drill rods equipped with high-pressure nozzles rotated while injecting grout at 100–220 kg/cm² to form cylindrical silcrete columns approximately 0.8 m in diameter.

After reaching the target depth of 18–19 meters, the rods were withdrawn at a controlled rate (e.g., 13 sec/5 cm), and H428*407*20*35 steel piles were placed into the fresh grout to form composite piles with high structural integrity and permeability. Figure 4 comprises a series of site photographs documenting the installation of H-Pile within a Jet-Grouted Column. The cement grout was mixed at 505 kg of cement and 840 L of water per cubic meter, targeting a specific gravity of 1.345 ± 5% [8].

Fig. 4

Some site photos of H-Pile within Jet-Grouted Column.

2.2 Large-Diameter Super High-Pressure Grouting for Soil Improvement

In areas beneath critical infrastructure, such as 161kV power lines and major telecom conduits, traditional JSG was inadequate. A super high-pressure, large-diameter grouting method (3.6 m) was thus employed. This system used double-tube drilling rods with dual 2.6 mm nozzles that delivered cementitious slurry at 400 ± 40 kg/cm². The technique involved initial waterjet cutting at 300 kg/cm² to fracture and loosen the soil, followed by grout injection in alternating cycles. Figure 5 illustrates the sequential procedure for constructing a 3.6-meter super high-pressure jet-grouted column, encompassing double-tube drilling, waterjet pre-cutting, and subsequent grout injection required to generate a uniform large-diameter soilcrete body. Each column was formed incrementally from the bottom up, using lifting speeds of 51 sec/2.5 cm and rotation speeds of 2 ~ 3 rpm.

Fig. 5

The Process Diagram of Super High-Pressure 3.6 m Grouting with Waterjet Pre-cutting.

The process ensured uniform mixing of in-situ soils with the grout, forming continuous cylindrical bodies with superior load-bearing and waterproofing performance [9]. The grouting sequence followed a leapfrog (jump pile) pattern to avoid interaction between adjacent piles and maintain column continuity.

2.3 Quality Control and Verification

Comprehensive quality control protocols were applied during and after construction. Real-time monitoring systems recorded injection pressures, flow rates, grout volumes, and drill rod movements. Using NX casing, the central and peripheral coring locations were employed for post-construction verification of column integrity, strength uniformity, and material homogeneity, as shown in Fig. 6.

Fig. 6

Coring and sample locations for grout column strength verification.

The unconfined compressive strength of the silcrete was tested in accordance with CNS 12384 A3282, targeting ≥ 10 kg/cm². Permeability was assessed through falling head tests, with the allowable coefficient set to ≤ 10⁻⁵ cm/sec, in line with project specifications [10]. The Recovery Quality Designation (RQD) of the core samples was required to exceed 90% for column acceptance. Figure 7 presents photographic records of on-site inspections, demonstrating both the grouting operations and the subsequent evaluation of extracted core samples for Recovery Quality Designation (RQD) compliance.

Fig. 7

The site inspections of the grouting and the RQD verification after the core sampling.

(Inspected by the corresponding author)

This integrated methodology ensured both structural performance and environmental compatibility in an urban MRT context, while offering a replicable model for future underground construction projects under similar geotechnical constraints. Figure 8 illustrates the spatial configuration of the jet-grouted H-piles and the adjacent super high-pressure grouting zones, emphasizing column overlap, geometric continuity, and their collective role in stabilizing the connecting passage.

Fig. 8

The plan layout of the JSG H-piles and the Super High-Pressure Jet Grouping.

3. Problems Faced and Solutions

The construction of the Taipei MRT Zhonghe–Wanda Line between LG06 and Y11 stations encountered numerous technical and logistical challenges due to complex urban geologic conditions and highly congested utility corridors. This section outlines the major problems encountered during the retaining pile and soil improvement phases, and the engineered solutions implemented to ensure construction quality and safety.

3.1 Congested Underground Utilities

One of the most critical problems was the high density of underground utilities, including 161kV power lines, major telecommunications ducts, water mains, and gas pipelines. Trial excavations revealed that some of these could not be relocated due to service continuity requirements and spatial limitations. As shown in Fig. 9, trial excavation photographs reveal the density and complexity of underground utilities at the conflict zone, including multiple intersecting ducts and pipelines that impose significant constraints on excavation strategy. A traditional bored pile or diaphragm wall posed a high risk of damaging these installations through vibration or displacement.

Fig. 9

Trial Excavation Revealing high-density utility conflict zone.

(Inspected by the corresponding author)

Solution:

A “jump pile” sequencing strategy was adopted in combination with small-diameter high-pressure jet grouting, which reduced construction-induced ground movement. For areas under sensitive facilities, large-diameter (3.6 m) super high-pressure jet grouting was implemented using a waterjet pre-cutting method to reduce lateral stress propagation. The use of symmetrical double-tube systems also minimized eccentric disturbance [11]. Real-time monitoring of displacement and settlement was employed to guide adjustments in pile layout and injection pressure during execution [12].

3.2 Variable and Weak Soil Profiles

The subsurface stratigraphy consisted of loose fill, soft silty clays, and interbedded sand seams. The strength and permeability characteristics varied significantly even within short lateral distances. These inconsistencies made it difficult to achieve uniform soil improvement, leading to potential seepage risks and excavation instability.

Solution:

The project team adjusted grouting parameters dynamically based on in-situ conditions. For example, in sandier zones, cement content was increased by 10–15% to enhance the binding effect. In cohesive layers, pre-cutting with waterjets was extended to increase grout penetration. This adaptive approach ensured that the resulting silcrete columns met the required unconfined compressive strength (≥ 10 kg/cm²) and permeability (K ≤ 10⁻⁵ cm/sec), as confirmed by post-construction coring and testing [13].

3.3 Limited Construction Access and Urban Constraints

The site was located at a major traffic intersection, with commercial buildings, residential towers, and public infrastructure on all sides. There was limited working space, strict noise and vibration restrictions, and constrained access for machinery mobilization and material delivery.

Solution:

Compact drilling equipment and modular grout batching systems were selected to reduce the construction footprint. Night-time deliveries and modular staging areas were arranged to reduce daytime traffic disruption. A detailed Environmental Protection Plan (EPP) was executed, incorporating acoustic barriers, sedimentation tanks, and vibration control protocols, in compliance with city construction regulations [14].

3.4 Groundwater Control and Seepage Risk

The shallow groundwater table (~ 2–3 m below the surface) posed a significant challenge during excavation. The risk of uplift pressure and seepage through untreated zones had to be minimized to ensure bottom stability of the cut.

Solution:

To mitigate the challenges posed by the shallow groundwater table (~ 2–3 m below ground level), a combination of large-diameter super high-pressure jet grouting columns was employed to enhance both seepage resistance and uplift stability. These grouting columns formed an impervious and reinforced subgrade, effectively minimizing seepage through untreated zones and counteracting potential uplift pressure. The effectiveness of this measure was verified through piezometric monitoring, which indicated stable hydraulic conditions and negligible groundwater intrusion during excavation [15].

3.5 H-Pile Removal Difficulty

Following the completion of jet grouting and excavation support, the removal of embedded steel H-piles posed significant engineering challenges. These piles, firmly bonded to the surrounding soilcrete, extended deep into stratified ground layers with varying stiffness. Extraction was further complicated by limited working space, stringent noise and vibration restrictions, and the presence of critical aboveground and underground infrastructure.

In particular, the overhead steel girder of the Yellow Line MRT restricted the vertical clearance at the site to just 6 meters, severely limiting the allowable hoisting height for pile removal. This constraint rendered conventional full-length extraction methods infeasible, especially for standard H-piles exceeding 6 meters in length.

Solution:

To overcome these challenges, a segmented removal method was adopted. Each H-pile was cut into sections shorter than 4 meters to accommodate the constrained headroom beneath the Yellow Line MRT girder. High-torque vibratory drivers (e.g., PC400) were combined with waterjet perimeter cutting, which effectively weakened the bond between the steel and surrounding soilcrete. In shallower layers (GL: 0 to -2 m), waterjet cutting was omitted to avoid disturbing adjacent facilities. A leapfrog (jump extraction) sequence was also implemented to reduce ground reaction and facilitate safe removal. This modified approach ensured efficient pile recovery under severe spatial and structural constraints.

Table 1 lists the summary of the Problems and Solutions in CQ861 Project.

Table 1

The summary of the Problems and Solutions in CQ861 Project.
Problem	Description	Engineered Solution
Congested Underground Utilities	High-density utility networks (161kV lines, telecom ducts, etc.) restricted conventional excavation.	Used “jump pile” sequencing with small-diameter JSG; applied 3.6 m super high-pressure grouting in sensitive zones; real-time displacement monitoring guided adjustments.
Variable and Weak Soil Profiles	Soft silty clays and sand seams caused uneven grout behavior and seepage risks.	Adjusted grout mix in situ; increased cement in sandy layers and extended waterjet pre-cutting in cohesive zones.
Limited Construction Access	Narrow work zones with strict noise and vibration limits hampered logistics.	Used compact rigs and modular systems; arranged night-time deliveries; implemented EPP with acoustic/vibration controls.
Groundwater and Seepage Risk	Shallow water table (~ 2–3 m) created uplift and seepage hazards.	Installed large-diameter super high-pressure grout columns as a bottom seal; confirmed performance via piezometric monitoring.
H-Pile Removal under Height Limitation	MRT Yellow Line steel girder limited headroom to 6 m, restricting full pile extraction.	Cut H-piles into < 4 m segments; used vibratory drivers with waterjet pre-cutting and jump extraction sequence.

4. Findings

Following the implementation of high-pressure and super high-pressure jet grouting techniques in the Taipei MRT Zhonghe–Wanda Line CQ861 project, comprehensive field testing and performance assessments were conducted. This section presents the key findings related to structural performance, ground improvement effectiveness, permeability control, and environmental impact mitigation.

4.1 Compressive Strength and Structural Integrity

Core samples retrieved from both standard jet grouting and super high-pressure columns exhibited excellent strength development. Laboratory testing on unconfined compressive strength (UCS) samples, prepared following CNS 12384 A3282, showed that most silcrete specimens exceeded the project requirement of 10 kg/cm². The average UCS values were:

(1)

12.6 kg/cm² for 0.8 m standard JSG columns.

(2)

15.2 kg/cm² for 3.6 m super high-pressure columns.

Edge and center samples indicated consistent distribution, with a coefficient of variation (COV) below 8%, demonstrating good homogeneity (Please see Fig. 6). The presence of H-piles in jet-mixed columns further enhanced load distribution and bending resistance, improving pile-wall performance during excavation stages [16].

4.2 Permeability and Seepage Resistance

Falling-head permeability tests on core samples and in-situ packer tests confirmed that permeability coefficients consistently fell below the target of 1×10⁻⁵ cm/sec. Most results ranged between 2.3×10⁻⁶ and 7.1×10⁻⁶ cm/sec. These values are indicative of near-impermeable behavior, especially critical in zones with shallow groundwater and flood-prone conditions [17].

Seepage monitoring using piezometers installed at the tunnel invert level showed negligible fluctuations during excavation, validating the efficacy of the impermeable grout curtain system. This minimized uplift pressure and water ingress, ensuring safe and dry excavation conditions throughout construction.

4.3 Geometric Accuracy and Column Continuity

Non-destructive testing using borehole camera inspection and ground-penetrating radar (GPR) revealed good geometric conformity of the grout columns to design specifications. The deviation in column diameter was typically within ± 5% of target values. In areas where the double-tube super high-pressure method was employed, column overlap achieved over 95% continuity, with no significant voids or soft zones detected.

The column grid installed beneath the connecting tunnel formed an effective load-transfer platform and groundwater barrier. The excavation progression of the underground connecting passage, documenting exposed soil strata, temporary structural supports, and field personnel operating within the confined subsurface workspace, as shown in Fig. 10.

Fig. 10

Field excavation photos of the underground connecting passage.

(Inspected by the corresponding author)

4.4 Environmental and Construction Efficiency Outcomes

The selected methods proved advantageous in reducing environmental impact. Ground vibration monitoring during drilling and injection phases showed peak particle velocities (PPV) below 2 mm/s at sensitive locations, well under regulatory limits. Noise levels remained under 75 dBA due to the use of low-noise batch mixing units and acoustically shielded drilling rigs [18].

In terms of productivity, the average installation rate reached:

(1)

12–15 m/day per rig for standard JSG piles.

(2)

5–7 m/day per rig for large-diameter super high-pressure columns.

Despite the slower rate for larger columns, the method’s effectiveness near critical infrastructure justified its use. No utility damage or safety incidents were recorded during execution, underscoring the reliability of the applied technologies.

These findings demonstrate that the combined approach of JSG and super high-pressure grouting not only met but exceeded the performance criteria of the CQ861 project [19], offering a validated model for future underground construction in constrained urban settings.

5. Conclusion and Recommendations

This study presented the successful application of high-pressure jet grouting and large-diameter super high-pressure jet grouting techniques in the Taipei MRT Zhonghe–Wanda Line CQ861 contract, particularly in the geotechnically complex and urban-sensitive zone between LG06 and Y11 stations. The engineering challenges encountered—such as weak and heterogeneous soils, shallow groundwater, and dense utility networks—required advanced and adaptable ground improvement solutions. Through carefully designed grouting methodologies, quality control protocols, and field testing, the project achieved its technical and safety goals without compromising the surrounding urban environment.

The use of standard high-pressure jet grouting in retaining pile construction enabled effective integration of structural elements (H-piles) into strengthened soilcrete bodies, resulting in a system that offered both load-bearing and impermeability functions. Meanwhile, the deployment of large-diameter super high-pressure jet grouting beneath sensitive facilities and the connecting tunnel provided enhanced strength and seepage resistance, even in variable soil conditions. The leapfrog sequencing and double-tube nozzle systems ensured uniform grout distribution while minimizing disturbance to nearby utilities and structures.

Post-construction evaluations confirmed that both the unconfined compressive strength and permeability performance of the grouted bodies met or exceeded project specifications. Furthermore, geometric precision, column continuity, and environmental compliance were all verified through rigorous monitoring and inspection. These results demonstrate that the combined use of high-pressure and super high-pressure grouting can effectively address the multifaceted requirements of urban underground infrastructure development.

Based on the experience and outcomes of this project, several recommendations are offered for similar future applications:

Early Integration of Utility Mapping and Risk Assessment:

Accurate utility surveys and proactive coordination with utility owners are essential. Risk zones should be identified early, and flexible grouting solutions should be planned accordingly.

Adaptive Grouting Strategy:

Grouting parameters should be dynamically adjusted based on soil variability encountered during execution. Real-time monitoring tools and soil classification logs are critical for guiding in-field decisions.

Use of Super High-Pressure Grouting in Critical Areas:

For sections beneath sensitive infrastructure or where conventional methods pose a risk, super high-pressure jet grouting should be considered. Though slower in installation, its benefits in strength, uniformity, and safety outweigh the longer execution time.

Comprehensive Quality Assurance:

Field sampling, lab testing, and non-destructive evaluation should be consistently applied to verify performance and minimize construction uncertainty.

Environmental Mitigation Planning:

Projects in urban environments must implement stringent vibration, noise, and groundwater management plans. Compact equipment, shielded systems, and staged work schedules can greatly reduce the project’s environmental footprint.

In conclusion, this case study reinforces the effectiveness, adaptability, and reliability of advanced grouting methods for MRT projects in challenging urban contexts. It provides a valuable reference for engineers, planners, and contractors tasked with delivering safe, high-quality underground infrastructure amid complex site constraints.

Data Availability

The datasets generated and/or analyzed during the current study include publicly accessible technical specifications and standards, while project-specific construction data are available from the corresponding author, T.-Y. Liu, upon reasonable request, subject to confidentiality restrictions of the Taipei MRT project.

Author Contributions Statement

T.-Y. Liu conceived the study, supervised the research, and led manuscript writing.

C.-P. Wang contributed to methodology and data processing.

S.-P. Ho provided academic guidance and technical review.

S.-F. Yeh supported project information verification.

K.-C. Ten assisted with field investigations and construction documentation.

B.-C. Ciou prepared figures and contributed to data compilation.

Acknowledgement

The authors sincerely acknowledge the Department of Rapid Transit Systems, Taipei City Government, for their continuous guidance, coordination, and provision of essential project information throughout the execution of this work. The authors also extend their appreciation to New Asia Construction and Development Corporation for offering technical support, site access, and valuable professional insights during field investigations and construction activities. The dedicated efforts of the onsite engineers, project teams, and technical staff in assisting with data collection, documentation, and quality verification significantly strengthened the rigor and completeness of this study. Their collective contributions are gratefully recognized.

Disclosure

Statement

The authors declare that they have no known financial, commercial, legal, or professional conflicts of interest that could have influenced the research, analyses, or conclusions presented in this manuscript. All work was conducted independently, and no external organization or individual provided support that could be perceived as creating a competing interest. The affiliations of the authors, including roles related to the project, did not influence the objectivity or integrity of the study.

Author Contribution

T.-Y. Liu conceived the study, supervised the research, and led manuscript writing.C.-P. Wang contributed to methodology and data processing.S.-P. Ho provided academic guidance and technical review.S.-F. Yeh supported project information verification.K.-C. Ten assisted with field investigations and construction documentation.B.-C. Ciou prepared figures and contributed to data compilation.

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