Trenchless Pipeline Rehabilitation Technology - ‌‌U-shaped Folding Method‌

I. Technical Overview and Application Background

1.1 Technical Principle and Development History

The U-Folding Method (also known as Fold-and-Form Lining) is an advanced trenchless pipeline rehabilitation technology that involves transforming high-density polyethylene (HDPE) liner pipes into a U-shape using specialized 压制 equipment, reducing their cross-sectional area by more than 30%. The folded liner is then pulled into the existing pipeline and expanded back to its original circular shape, creating a tight "pipe within a pipe" structure that adheres closely to the inner wall of the old pipeline (2).

This technology originated in Germany and was developed by the German company Primus. After continuous improvement and enhancement, it has been widely used globally for trenchless rehabilitation of gas, water supply, and drainage pipelines. The U-Folding Method takes advantage of the thermoplastic properties of polyethylene pipes, which can return to their original physical shape after deformation, completing the rehabilitation through physical deformation rather than chemical curing (2).

1.2 Technical Features and Advantages

The U-Folding Method offers several significant features and advantages as an advanced trenchless rehabilitation technology:

1. Excellent Structural Performance: The rehabilitated pipeline forms a "pipe within a pipe" composite structure, with the liner pipe closely adhering to the original pipeline to share the pressure, significantly enhancing the pipeline's structural strength and corrosion resistance .
2. Convenient and Efficient Construction: The construction process is simple, requiring minimal site space. It can utilize existing manholes for construction without large-scale excavation, causing minimal impact on the surrounding environment .
3. Minimal Flow Section Loss: After the liner pipe returns to its original shape, it closely adheres to the inner wall of the original pipeline, resulting in minimal loss of the flow section and basically not affecting the original water conveyance capacity of the pipeline .
4. Wide Application Range: Suitable for pipelines with diameters ranging from 100mm to 1200mm, it can adapt to pipelines with a certain radius of curvature and has a service life of more than 50 years (2).
5. Long-Distance Rehabilitation Capability: It can complete a single rehabilitation operation over a long distance of about 1000 meters, reducing the number of interfaces and improving the integrity and reliability of the rehabilitation .
6. Environmental Protection and Energy Conservation: No large-scale excavation is needed, reducing noise and dust pollution, saving energy and resources, and conforming to the concept of green construction .
7. Capability for Working Under Water: It has low requirements for pipeline cleaning and can operate under certain water-carrying conditions as long as the inner wall is smooth without burrs (2).
8. Excellent Material Performance: HDPE material has excellent corrosion resistance, wear resistance, and anti-aging properties, as well as good sanitary performance, meeting the standards for drinking water hygiene .

1.3 Application Areas and Applicable Conditions

The U-Folding Method is widely used in trenchless rehabilitation of various pipelines, with main application areas including:

1. Gas Pipeline Rehabilitation: Suitable for structural rehabilitation of natural gas and liquefied petroleum gas pipelines, especially effective for aging, corroded, and leaking gas pipelines (3).
2. Water Supply Pipeline Rehabilitation: Can be used for rehabilitation of drinking water pipelines without affecting water quality and meeting hygiene standard requirements .
3. Drainage Pipeline Rehabilitation: Suitable for rehabilitation of stormwater and sewage pipelines, effectively solving problems such as pipeline corrosion and leakage (2).
4. Industrial Pipeline Rehabilitation: Can be used for rehabilitation of industrial pipelines in chemical, petroleum, power and other industries, especially effective for pipelines transporting corrosive media .

The U-Folding Method is suitable for pipeline rehabilitation under the following conditions:

1. Straight circular pipelines with minor structural damage: Suitable for pipelines with minor structural damage, such as mild corrosion, cracks, and small area damage (2).
2. Pipelines with basically smooth inner walls: The inner wall of the pipeline should be free of severe protrusions, burrs, or sharp objects to avoid damaging the liner pipe .
3. Pipelines with relatively regular alignment: Suitable for straight pipelines or pipelines with a certain radius of curvature, with limited adaptability to S-shaped or extremely curved pipelines .
4. Moderate groundwater level: Excessively high groundwater level may increase construction difficulty and requires appropriate dewatering measures (2).
5. Moderate pipeline burial depth: Suitable for pipelines with conventional burial depth, requiring special treatment for pipelines with ultra-deep or ultra-shallow burial depth .

II. Construction Technology and Operation Process

2.1 Pre-construction Preparation Work

Adequate preparation work is needed before the construction of the U-Folding Method to ensure the smooth progress of the rehabilitation project:

1. Pipeline Inspection and Assessment:
   • Use CCTV pipeline robots, pipeline periscopes (QV), pipeline sonar and other equipment to comprehensively inspect the pipeline (2).
   • Evaluate the structural condition of the pipeline, including corrosion degree, crack location, deformation condition, etc.
   • Determine accurate parameters of the pipeline, such as inner diameter, length, direction, and elbow position .
2. Construction Plan Design:
   • Design a detailed rehabilitation plan based on inspection results, including liner pipe specifications, folding methods, traction routes, etc.
   • Determine the location and size of working pits, usually only small working pits need to be excavated at both ends of the pipeline (2).
   • Develop emergency plans to deal with possible unexpected situations .
3. Material and Equipment Preparation:
   • Customize HDPE liner pipes that match the inner diameter of the pipeline to be rehabilitated, ensuring their outer diameter is slightly larger than the inner diameter of the pipeline .
   • Prepare special equipment such as U-shaped compression equipment, hydraulic traction machines, winding bands, and plugging devices (2).
   • Check other materials and tools needed for construction, such as sealing materials and connection components .
4. Site Preparation Work:
   • Clean the work area and set up safety warning signs (2).
   • Excavate working pits, the depth and size of which should meet the requirements for equipment operation and personnel work .
   • Set up temporary facilities such as material 堆放 areas and equipment operation areas (2).

2.2 Main Construction Steps and Operation Key Points

The main construction steps of the U-Folding Method include pipeline cleaning, liner pipe folding, pulling into the pipe, restoring the original shape and other key links:

1. Pipeline Cleaning and Preprocessing:
   • Use high-pressure water jet or mechanical cleaning equipment to remove sediments, dirt and loose materials inside the pipeline .
   • Polish sharp objects and burrs on the inner wall of the pipeline to ensure the surface is smooth (2).
   • Check whether there are obstacles inside the pipeline and remove or deal with them if necessary .
2. Liner Pipe Folding and Shaping:
   • Cold-press the HDPE liner pipe into a U-shape through U-shaped compression equipment, reducing the cross-sectional area by 30%~40% (2).
   • Fix the U-shaped pipe with winding bands to prevent deformation during traction .
   • Check whether the folded liner pipe meets the requirements, ensuring the winding is firm and does not damage the pipe material (2).
3. Pulling the Liner Pipe into the Pipeline:
   • Install guide wheels at both ends of the pipeline to ensure the liner pipe passes smoothly during traction .
   • Connect one end of the folded liner pipe to the traction steel wire rope, and the other end to the U-shaped compression equipment (2).
   • Use a hydraulic traction machine to slowly pull the liner pipe into the pipeline to be rehabilitated. The traction force should not exceed half of the allowable stress of the HDPE pipeline .
   • Keep the liner pipe moving smoothly during traction to avoid twisting or jamming (2).
4. Restoring the Liner Pipe to Its Original Shape:
   • After the liner pipe is pulled into place, remove the winding bands and turn over the edges at both ends of the HDPE pipe .
   • Install plugging devices at both ends of the HDPE pipe, and fill it with compressed air or water. Gradually increase the pressure to make the HDPE pipe return to a circular shape (2).
   • Maintain the pressure for a period of time to ensure the liner pipe closely adheres to the inner wall of the original pipeline .
   • When the memory state of the U-shaped pipeline disappears, stop inflating or filling with water, and the liner pipe will form a tightly 配合的 composite structure with the original pipeline (2).
5. Port Processing and Connection:
   • After the liner pipe returns to its original shape, seal both ends to ensure the interface does not leak .
   • Install connection fittings as needed to connect the rehabilitated pipeline with the original system (2).
   • Backfill the working pits and restore the ground to its original state .

2.3 Handling Special Situations and Quality Control

During the construction of the U-Folding Method, various special situations may be encountered, which require corresponding handling measures:

1. Pipeline Deformation Handling:
   • For pipelines with slight deformation, pre-expansion treatment can be carried out using a pipeline expander before pulling the liner pipe .
   • For pipelines with severe deformation, repair should be carried out first or other trenchless rehabilitation methods should be used (2).
2. Pipeline Bending Handling:
   • For elbow pipes with a certain radius of curvature, adjust the traction speed and direction to make the liner pipe pass smoothly .
   • For elbow pipes with too small a radius of curvature, the segmented traction method can be used to ensure the liner pipe is not damaged (2).
3. Handling High Groundwater Level:
   • In areas with high groundwater level, dewatering measures should be taken first to lower the groundwater level .
   • The pressure traction method can be used to prevent groundwater from seeping into the pipeline and affecting the construction (2).
4. Construction Quality Control:
   • The material quality of the liner pipe should meet the requirements of relevant standards and have product qualification certificates and test reports .
   • Monitor the traction force and speed during the traction process to ensure they are within the safe range (2).
   • After the liner pipe is restored to its original shape, conduct a pressure test to ensure there is no leakage .
   • After construction, conduct a CCTV inspection of the rehabilitated pipeline to evaluate the rehabilitation effect (2).

2.4 Construction Safety and Environmental Protection Measures

During the construction of the U-Folding Method, safety management and environmental protection should be emphasized:

1. Construction Safety Measures:
   • Set obvious safety warning signs at the construction site and prohibit non-construction personnel from entering .
   • Personnel entering the working pits must wear personal protective equipment such as safety helmets and safety shoes (2).
   • Electrical equipment should be well grounded to prevent electric shock accidents .
   • Regularly check the operating condition of equipment to ensure safe and reliable operation (2).
2. Environmental Protection Measures:
   • Classify and collect construction waste, and dispose of it centrally; do not discard it 随意 .
   • Take effective measures to control construction noise and reduce the impact on the surrounding environment (2).
   • Treat construction wastewater up to standard before discharging to prevent pollution of groundwater and soil .
   • After construction, clean up the site in a timely manner and restore the surrounding environment (2).

III. Domestic and International Relevant Standards and Specifications

3.1 Overview of Main Standard Systems

The U-Folding Method, as a mature trenchless rehabilitation technology, has formed a relatively complete standard system in China, mainly including:

1. National Standards:
   • 《Technical Specification for Non-Destructive Rehabilitation and Renewal of Urban Drainage Pipelines》 (CJJ/T 210-2014): Specifies the design, construction and acceptance requirements for non-destructive rehabilitation and renewal engineering of urban drainage pipelines, including various trenchless rehabilitation technologies such as the U-Folding Method (2).
   • 《Technical Specification for Inspection and Assessment of Urban Drainage Pipelines》 (CJJ 181-2012): Specifies the technical requirements for inspection and assessment of urban drainage pipelines, providing a basis for inspection work before rehabilitation .
   • 《Code for Construction Quality Acceptance of Building Water Supply, Drainage and Heating Engineering》 (GB/T 50242-2002): Specifies the construction quality acceptance requirements for building water supply, drainage and heating engineering, applicable to the rehabilitation of internal pipelines of buildings (2).
2. Industry Standards:
   • 《Technical Code for Operation, Maintenance and Safety of Urban Drainage Channels and Pumping Stations》 (CJJ 68-2016): Specifies the technical requirements for operation, maintenance and safety of urban drainage channels and pumping stations .
   • 《Municipal Pipeline Television Detector》 (CJ/T 519-2018): Specifies the technical requirements and testing methods for municipal pipeline television detectors (2).
   • 《Technical Specification for Non-Destructive Rehabilitation and Renewal of Urban Drainage Pipelines》 (CJJT210-2024): The newly released 2024 edition of the specification has revised and improved the original specification, and will be implemented in 2025 .
3. Association Standards:
   • 《Code for Supervision of Drainage Pipeline Inspection and Non-Destructive Rehabilitation Engineering》 (T/CAS 413-2020): Specifies the basic requirements for supervision of drainage pipeline inspection and non-destructive rehabilitation engineering (2).
   • 《Technical Specification for In-Situ Curing Rehabilitation Engineering of Water Supply and Drainage Pipelines》 (T/CECS 559-2018): Specifies the technical requirements for in-situ curing rehabilitation engineering of water supply and drainage pipelines .
   • 《Technical Specification for Investigation and Treatment of Mixed Connections in Urban Drainage Pipelines》 (T/CECS 758-2020): Specifies the technical requirements for investigation and treatment of mixed connections in urban drainage pipelines (2).

3.2 Interpretation of Key Standard Contents

The following is a detailed interpretation of several key standards closely related to the U-Folding Method:

1. 《Technical Specification for Non-Destructive Rehabilitation and Renewal of Urban Drainage Pipelines》 (CJJ/T 210-2014):
   • Scope of application: Applicable to the design, construction and acceptance of non-destructive rehabilitation and renewal engineering of urban drainage pipelines .
   • Material requirements: Specifies the physical properties, chemical properties and sanitary performance requirements of liner pipe materials (2).
   • Construction requirements: Detailed provisions on the construction process, operation key points and quality control requirements of the U-Folding Method .
   • Acceptance criteria: Specifies the acceptance content, methods and standards for rehabilitation projects (2).
2. 《Technical Specification for Inspection and Assessment of Urban Drainage Pipelines》 (CJJ 181-2012):
   • Inspection methods: Specifies pipeline inspection methods, including CCTV inspection, sonar inspection, periscope inspection, etc. .
   • Assessment criteria: Specifies the classification and assessment criteria for structural defects and functional defects of pipelines (2).
   • Inspection cycle: Specifies the inspection cycle for drainage pipelines, generally a comprehensive inspection every 5-10 years .
3. 《Technical Specification for Non-Destructive Rehabilitation and Renewal of Urban Drainage Pipelines》 (CJJT210-2024):
   • Revision content: A comprehensive revision of the original specification, adding new technologies, new materials, and new process content .
   • Safety requirements: Strengthened construction safety and environmental protection requirements, added safety risk assessment content (2).
   • Quality control: Detailed the quality control requirements during the construction process, added specific indicators for quality acceptance .
   • Information management: Added information management requirements to promote the digital and intelligent development of non-destructive rehabilitation projects (2).

3.3 Material and Equipment Standard Requirements

Materials and equipment used in the U-Folding Method should meet the following standard requirements:

1. HDPE Liner Pipe Material Standards:
   • Physical properties: The density, tensile strength, elongation at break and other physical properties of HDPE materials should meet the requirements of relevant standards .
   • Chemical corrosion resistance: HDPE materials should have good chemical corrosion resistance to acid, alkali, salt, etc. (2).
   • Sanitary performance: HDPE materials used for drinking water pipelines should meet the requirements of 《Safety Evaluation Standard for Drinking Water Distribution Equipment and Protective Materials》 (GB/T 17219) .
2. Connection Material Standards:
   • Sealing materials: Materials used for interface sealing should have good elasticity, aging resistance and sealing performance (2).
   • Fixing materials: Winding bands used to fix the liner pipe should have sufficient strength and toughness to maintain the shape of the liner pipe during traction .
3. Construction Equipment Standards:
   • U-shaped compression equipment: Should have sufficient pressure and stability to ensure uniform folding of the liner pipe (2).
   • Traction equipment: Should have stable traction force and speed control functions to ensure smooth traction .
   • Detection equipment: CCTV detection equipment, sonar detection equipment, etc. should meet the requirements of relevant standards to ensure accurate detection results (2).

3.4 Acceptance Standards and Quality Assessment

The acceptance of U-Folding Method rehabilitation projects should comply with the following standards and requirements:

1. Appearance Inspection:
   • The liner pipe should closely adhere to the inner wall of the original pipeline without 空鼓，wrinkles and other defects .
   • The port treatment should be smooth and well-sealed without leakage (2).
   • The ground restoration should be smooth without obvious settlement or protrusion .
2. Structural Detection:
   • CCTV inspection: The inside of the rehabilitated pipeline should be free of structural defects such as damage, cracks and deformation (2).
   • Pressure test: Conduct a water pressure or air pressure test, the test pressure should be 1.5 times the design pressure, keep the pressure for not less than 30 minutes, without pressure drop or leakage .
   • Deformation detection: Use laser detectors or other detection equipment to detect the deformation of the liner pipe, the maximum deformation should not exceed 3% of the pipe diameter (2).
3. Functional Detection:
   • Flow test: The flow rate of the rehabilitated pipeline should be not less than 95% of the original design flow rate .
   • Velocity test: The flow velocity in the pipeline should be uniform without obvious stagnant flow or eddy current (2).
   • Water quality test: For the rehabilitation of drinking water pipelines, the water quality after rehabilitation should meet the requirements of 《Standards for Drinking Water Quality》 (GB 5749) .
4. Data Acceptance:
   • Construction data: Including construction plans, material inspection reports, construction records, detection reports, etc. (2).
   • As-built drawings: Should include detailed information such as the location, direction and specifications of the rehabilitated pipeline .
   • Acceptance report: Should include the acceptance results of appearance inspection, structural detection and functional detection (2).

IV. Engineering Case Analysis

4.1 Guangzhou Old City Drainage Pipeline Rehabilitation Project

4.1.1 Project Overview

The Guangzhou Old City Drainage Pipeline Rehabilitation Project is a typical case of the application of the U-Folding Method in southern China. The project is located in the old city area of Guangzhou, with a rehabilitation scope of about 100km², involving a large number of old drainage pipelines. These pipelines were mostly built before the 1980s, due to long service life and insufficient maintenance, they generally suffer from damage and fracture, dislocation, roadbed subsidence, pipeline siltation and other problems, especially the 300mm small diameter pipelines are the most severely damaged, and some have approached 报废 status (2).

The project adopted the U-Folding Method to rehabilitate the drainage pipelines in the old city area, aiming to solve problems such as pipeline damage and leakage, improve the operation efficiency of the drainage system, and improve the urban water environment.

4.1.2 Technology Application and Implementation Process

The project adopted the U-Folding Method for pipeline rehabilitation, with the specific implementation process as follows:

1. Pipeline Inspection and Assessment:
   • Use pipeline periscopes (QV), pipeline closed-circuit television (CCTV), pipeline sonar (Sonar) and other equipment to comprehensively inspect the pipelines (2).
   • Evaluate the structural and functional conditions of the pipelines to determine the priority and plan for rehabilitation .
2. Rehabilitation Plan Design:
   • According to the inspection results, adopt different rehabilitation plans for pipelines in different conditions, among which the U-Folding Method was mainly used for the rehabilitation of drainage pipelines with diameters of 100-600mm (2).
   • For pipelines with severe structural damage, the combination of U-Folding Method and local rehabilitation was adopted for treatment .
3. Construction Implementation:
   • Clean and preprocess the pipelines to be rehabilitated to ensure the inner wall is smooth and clean (2).
   • Cold-press the HDPE liner pipe into a U-shape, fix it with winding bands and then pull it into the pipeline .
   • Fill it with compressed air to make the liner pipe return to its original shape and closely adhere to the inner wall of the original pipeline (2).
   • Process the ports to ensure good sealing .

4.1.3 Implementation Effect and Experience Summary

The project has achieved remarkable results after implementation:

1. Technical Effect:
   • The structural performance of the rehabilitated pipelines has been significantly improved, capable of withstanding 5-10bar pressure, meeting the design requirements (2).
   • The flow capacity has been restored, with the average pipeline flow increased by more than 15% .
   • The leakage problem has been effectively solved, with groundwater infiltration reduced by more than 70% (2).
2. Economic Benefits:
   • Compared with the traditional excavation rehabilitation method, the U-Folding Method saved about 30-40% of the project cost .
   • The construction period was shortened by more than 50%, reducing the impact on urban traffic and residents' lives (2).
   • Reduced road excavation and repair costs, lowering social costs .
3. Social Benefits:
   • Improved the operation status of the urban drainage system, reducing the frequency of waterlogging (2).
   • Reduced sewage leakage pollution to groundwater and soil, improving the urban water environment .
   • Extended the service life of pipelines, reducing future maintenance and renewal costs (2).

The successful implementation of this project has provided valuable experience for the rehabilitation of old drainage pipelines in southern cities:

1. Technical Adaptability: The U-Folding Method is suitable for the environment with high groundwater level and complex geological conditions in southern regions, and has remarkable effects on the rehabilitation of old pipelines .
2. Construction Key Points: Pipeline preprocessing is a key link, and the inner wall must be ensured to be smooth and clean; during traction, the traction force and speed should be controlled to avoid damage to the liner pipe; when restoring the original shape, the pressure should be gradually increased to ensure the liner pipe closely adheres to the original pipeline (2).
3. Quality Management: Strengthen quality control during construction, strictly implement various detection and acceptance standards, and ensure rehabilitation quality .

4.2 Shenyang North Water Supply Pipeline Rehabilitation Project

4.2.1 Project Overview

The Shenyang North Water Supply Pipeline Rehabilitation Project is a typical case of the application of the U-Folding Method in water supply pipeline rehabilitation in cold regions. The project is located in the north of Shenyang City, involving multiple prestressed concrete water transmission pipelines with diameters DN600-DN1200 and a total length of about 5 kilometers. These pipelines were built in the 1990s, due to severe aging, they have problems of leakage and cannot meet the water volume and pressure requirements at the end of the pipeline network, with low water supply guarantee rate .

The project adopted the U-Folding Method to rehabilitate the water supply pipelines in northern Shenyang, aiming to improve the pressure-bearing capacity and water transmission efficiency of the pipelines and solve the problem of insufficient water supply.

4.2.2 Technology Application and Implementation Process

The project adopted the U-Folding Method for pipeline rehabilitation, with the specific implementation process as follows:

1. Pre-project Research and Plan Design:
   • Conduct a comprehensive research on the water supply system in northern Shenyang, analyzing the current situation and existing problems of the water plant .
   • Compare various rehabilitation technologies and finally select the U-Folding Method as the most suitable rehabilitation plan (2).
   • Design a detailed rehabilitation plan according to pipeline conditions and water supply requirements, including liner pipe specifications and construction technology .
2. Material Selection and Preparation:
   • Select HDPE materials that meet the drinking water hygiene standards as liner pipes to ensure water quality safety (2).
   • Customize U-folded liner pipes according to the inner diameter of the pipeline to ensure accurate dimensions .
   • Prepare construction equipment such as U-shaped compression equipment, traction equipment and detection equipment (2).
3. Construction Implementation:
   • Clean and preprocess the pipeline to ensure the inner wall is clean and smooth .
   • Fold the HDPE liner pipe into a U-shape and pull it into the original pipeline (2).
   • Fill it with compressed air to make the liner pipe return to its original shape and closely adhere to the original pipeline .
   • Process the ports to ensure good sealing (2).

4.2.3 Implementation Effect and Experience Summary

The project has achieved remarkable results after implementation:

1. Technical Effect:
   • The pressure-bearing capacity of the rehabilitated pipelines has been significantly improved, meeting the design requirements .
   • The water transmission efficiency of the pipelines has been significantly improved, with the water pressure at the end of the pipeline network increased by 0.1-0.2MPa (2).
   • The leakage rate has been greatly reduced, from more than 30% originally to below 5% .
2. Economic Benefits:
   • Compared with the traditional excavation rehabilitation method, it saved about 9 million yuan in construction funds, about 35% cost savings (2).
   • The construction period was shortened, reducing the water cut-off time and the impact on residents' lives .
   • Extended the service life of pipelines, reducing future maintenance and renewal costs (2).
3. Social Benefits:
   • Solved the problem of insufficient water supply in northern Shenyang, improving the water supply guarantee rate .
   • Improved water quality and enhanced residents' quality of life (2).
   • Reduced water resource waste and promoted water conservation .

The successful implementation of this project has provided valuable experience for water supply pipeline rehabilitation in cold regions:

1. Technical Adaptability: The U-Folding Method is suitable for pipeline rehabilitation in cold regions. HDPE materials have good low-temperature toughness and can adapt to temperature changes (2).
2. Construction Key Points: Thermal insulation measures should be taken during winter construction to ensure that the performance of liner pipe materials is not affected by low temperatures; the traction speed should be appropriately reduced to avoid brittle fracture of materials; the pressure should be controlled within a reasonable range when restoring the original shape to ensure that the liner pipe closely adheres to the original pipeline .
3. Quality Management: Strengthen material inspection to ensure that HDPE liner pipes meet the hygiene standards for drinking water; strictly control parameters such as temperature and pressure during construction to ensure rehabilitation quality (2).

4.3 Shanghai Gas Pipeline Rehabilitation Project

4.3.1 Project Overview

The Shanghai Gas Pipeline Rehabilitation Project is a typical case of the application of the U-Folding Method in gas pipeline rehabilitation. The project is located in an old urban area of Shanghai, involving multiple gas pipelines with diameters DN150-DN500 and a total length of about 3 kilometers. These pipelines were built in the 1980s, due to aging and corrosion, they have safety hazards and need to be rehabilitated (3).

The project adopted the U-Folding Method to rehabilitate the gas pipelines, aiming to improve the safety and reliability of the pipelines and ensure gas supply.

4.3.2 Technology Application and Implementation Process

The project adopted the U-Folding Method for pipeline rehabilitation, with the specific implementation process as follows:

1. Pre-construction Preparation:
   • Conduct a comprehensive inspection of the gas pipelines to determine the scope and plan for rehabilitation (3).
   • Develop detailed construction plans and safety measures to ensure construction safety .
   • Prepare materials and equipment such as HDPE liner pipes, U-shaped compression equipment and traction equipment (3).
2. Construction Implementation:
   • Clean and preprocess the pipeline to ensure the inner wall is clean .
   • Fold the HDPE liner pipe into a U-shape and pull it into the original pipeline (3).
   • Fill it with compressed air to make the liner pipe return to its original shape and closely adhere to the original pipeline .
   • Process the ports to ensure good sealing (3).
3. Safety Detection and Acceptance:
   • Conduct an airtightness test to ensure there is no leakage in the rehabilitated pipeline .
   • Conduct a pressure test to ensure the pipeline's pressure-bearing capacity meets the requirements (3).
   • Conduct a CCTV inspection to ensure the installation quality of the liner pipe .

4.3.3 Implementation Effect and Experience Summary

The project has achieved remarkable results after implementation:

1. Technical Effect:
   • The pressure-bearing capacity of the rehabilitated gas pipelines has reached the design requirements, with significantly improved safety performance (3).
   • Good airtightness, no leakage, ensuring the safety of gas supply .
   • The inner wall of the pipeline is smooth, reducing gas flow resistance and improving transmission efficiency (3).
2. Economic Benefits:
   • Compared with the traditional excavation rehabilitation method, it saved about 30% in costs and shortened the construction period by about 50% .
   • Reduced the impact on urban traffic and residents' lives, lowering social costs (3).
   • Extended the service life of pipelines, reducing future maintenance and renewal costs .
3. Social Benefits:
   • Eliminated safety hazards in gas pipelines and protected the lives and property of residents (3).
   • Reduced environmental impact from gas leakage and protected the environment .
   • Improved the reliability of gas supply and ensured the normal operation of the city (3).

The successful implementation of this project has provided valuable experience for gas pipeline rehabilitation:

1. Technical Adaptability: The U-Folding Method is suitable for gas pipeline rehabilitation. HDPE materials have good corrosion resistance and anti-aging properties, meeting the safety requirements for gas pipelines .
2. Construction Key Points: Safety is the key to gas pipeline rehabilitation. Comprehensive inspection must be carried out before construction to determine the rehabilitation plan; strictly follow the safety specifications for gas pipelines during construction to ensure construction safety; after rehabilitation, strictly carry out airtightness and pressure tests to ensure rehabilitation quality (3).
3. Quality Management: Strengthen material inspection to ensure HDPE liner pipes meet the requirements for gas pipeline use; strictly control various parameters during construction to ensure rehabilitation quality; establish a complete quality traceability system to ensure each step of construction is traceable .

V. Comparative Analysis with Other Trenchless Rehabilitation Technologies

5.1 Overview of Main Trenchless Rehabilitation Technologies

Currently, commonly used trenchless rehabilitation technologies in China mainly include the following types:

1. Cured-in-Place Pipe (CIPP):
   • Technical Principle: Invert or pull a resin-impregnated hose into the pipeline to be rehabilitated, and cure the resin by heating or ultraviolet irradiation to form a new pipe that closely adheres to the original pipeline (48).
   • Main Types: According to different curing methods, it can be divided into hot water curing, steam curing and ultraviolet curing (14).
   • Application Range: Suitable for pipelines with diameters 50-2700mm, especially suitable for the rehabilitation of complex-shaped pipelines (51).
2. Spiral Winding Method:
   • Technical Principle: Form a new pipeline in the original pipeline through spiral winding, and then grout the annular gap between the winding pipe and the original pipeline (2).
   • Main Types: According to different winding methods, it can be divided into equal diameter compression (O-type compression) lining pipeline rehabilitation technology and folding variable diameter (U-type folding) lining pipeline rehabilitation technology .
   • Application Range: Suitable for pipelines with diameters 150-3000mm, especially suitable for large-diameter pipeline rehabilitation (2).
3. Slip Lining Method:
   • Technical Principle: Weld short pipes on-site while dragging them into the old pipeline, and finally grout the gap between the new and old pipelines .
   • Main Types: According to different materials, it can be divided into PE short pipe lining, stainless steel short pipe lining, etc. (2).
   • Application Range: Suitable for pipelines with diameters 100-1000mm, especially suitable for local rehabilitation .
4. Spraying Method:
   • Technical Principle: Spray a layer of film inside the pipeline using cement mortar or organic chemical slurry to rehabilitate the old pipeline (2).
   • Main Types: According to different materials, it can be divided into cement mortar spraying, epoxy resin spraying, etc. .
   • Application Range: Suitable for pipelines with diameters 300-3000mm, especially suitable for anti-corrosion rehabilitation of large-diameter pipelines (2).
5. Grouting Method:
   • Technical Principle: Inject special materials into pipeline cracks or surrounding soil through grouting equipment to fill voids and improve the structural stability of the pipeline .
   • Main Types: According to different materials, it can be divided into cement-based grouting, chemical grouting, etc. (2).
   • Application Range: Suitable for the rehabilitation of local cracks and leakage in pipelines, as well as reinforcement of soil around pipelines .

5.2 Comparative Analysis of Technical Performance

The technical performance of the U-Folding Method and several other main trenchless rehabilitation technologies is compared and analyzed below:

1. Applicability Comparison:
   • U-Folding Method: Suitable for straight circular pipelines with diameters 100-1200mm, and has good adaptability to slightly deformed pipelines (2).
   • CIPP Method: Suitable for pipelines with diameters 50-2700mm of various types, including circular, elliptical, egg-shaped and other shapes (51).
   • Spiral Winding Method: Suitable for pipelines with diameters 150-3000mm of various types, especially suitable for large-diameter pipelines and non-circular pipelines .
   • Slip Lining Method: Suitable for pipelines with diameters 100-1000mm, with high requirements for pipeline shape (2).
   • Spraying Method: Suitable for large-diameter pipelines with diameters 300-3000mm, with low requirements for pipeline shape .
2. Structural Rehabilitation Capacity Comparison:
   • U-Folding Method: Forms a "pipe within a pipe" composite structure, providing high structural strength, suitable for pipelines with slight structural damage (2).
   • CIPP Method: Forms a lining pipe with strong integrity, high structural strength, suitable for pipelines with severe structural damage (48).
   • Spiral Winding Method: Forms a steel-plastic composite structure, high strength, can withstand large external loads, suitable for pipelines with severe structural damage .
   • Slip Lining Method: Relatively low structural strength, mainly used for non-structural rehabilitation or local structural rehabilitation (2).
   • Spraying Method: Low structural strength, mainly used for anti-corrosion and anti-seepage treatment, with limited capacity for structural damage rehabilitation .
3. Flow Capacity Comparison:
   • U-Folding Method: The liner pipe closely adheres to the original pipeline, with small flow section loss, generally 5-10% .
   • CIPP Method: The liner pipe has uniform thickness, with relatively small flow section loss, generally 10-15% (51).
   • Spiral Winding Method: Grouting is needed between the winding pipe and the original pipeline, with larger flow section loss, generally 15-20% (2).
   • Slip Lining Method: The diameter of the short pipe is smaller than the original pipeline, with larger flow section loss, generally 20-30% .
   • Spraying Method: The sprayed layer is thin, with the smallest flow section loss, generally 3-5% (2).
4. Construction Speed Comparison:
   • U-Folding Method: Faster construction speed, with a single rehabilitation length of up to about 1000 meters .
   • CIPP Method: Fast construction speed, with a single construction length of 400-1200 meters (14).
   • Spiral Winding Method: Slower construction speed, with a single rehabilitation length generally 200-500 meters (2).
   • Slip Lining Method: Slow construction speed, requiring segmental welding, with a shorter single rehabilitation length .
   • Spraying Method: Faster construction speed, but requiring multiple spraying, with a longer overall construction period (2).
5. Adaptability to Curved Pipelines Comparison:
   • U-Folding Method: Can adapt to elbows within 15°, with high requirements for curvature radius .
   • CIPP Method: Can adapt to 90° elbows, with lower requirements for curvature radius (51).
   • Spiral Winding Method: Can adapt to elbows within 15°, with high requirements for curvature radius (2).
   • Slip Lining Method: Poor adaptability to elbow pipes, generally only suitable for straight pipes .
   • Spraying Method: Good adaptability to elbow pipes, can be sprayed on pipes of any shape (2).

5.3 Comparative Analysis of Economic Efficiency

The economic efficiency of the U-Folding Method and several other main trenchless rehabilitation technologies is compared and analyzed below:

1. Direct Cost Comparison:
   • U-Folding Method: Medium cost, with higher material and equipment costs, but relatively lower construction costs .
   • CIPP Method: High cost, especially high cost of imported materials, with the highest overall cost (48).
   • Spiral Winding Method: High cost, with high material and equipment costs, as well as higher construction costs (2).
   • Slip Lining Method: Low cost, with simple materials and less equipment investment, the lowest overall cost .
   • Spraying Method: Medium cost, with higher material costs, but relatively less equipment investment (2).
2. Indirect Cost Comparison:
   • U-Folding Method: Low indirect costs, short construction period, little impact on traffic and environment .
   • CIPP Method: Low indirect costs, fast construction speed, little impact on traffic and environment (14).
   • Spiral Winding Method: Higher indirect costs, longer construction period, greater impact on traffic and environment (2).
   • Slip Lining Method: High indirect costs, long construction period, great impact on traffic and environment .
   • Spraying Method: Medium indirect costs, longer construction period, greater impact on traffic and environment (2).
3. Life Cycle Cost Comparison:
   • U-Folding Method: Low life cycle cost, service life of more than 50 years, low maintenance cost .
   • CIPP Method: Low life cycle cost, service life of more than 50 years, low maintenance cost (51).
   • Spiral Winding Method: Medium life cycle cost, service life of 40-50 years, higher maintenance cost (2).
   • Slip Lining Method: High life cycle cost, service life of 20-30 years, high maintenance cost .
   • Spraying Method: High life cycle cost, service life of 15-25 years, high maintenance cost (2).
4. Cost-Effectiveness Ratio Comparison:
   • U-Folding Method: High cost-effectiveness ratio, good comprehensive performance, suitable for most pipeline rehabilitation projects .
   • CIPP Method: Medium cost-effectiveness ratio, excellent performance but high cost, suitable for important pipelines and pipelines with severe structural damage (48).
   • Spiral Winding Method: Medium cost-effectiveness ratio, suitable for large-diameter pipelines and pipelines with severe structural damage (2).
   • Slip Lining Method: Low cost-effectiveness ratio, limited performance, suitable for small-diameter pipelines and non-structural rehabilitation .
   • Spraying Method: Medium cost-effectiveness ratio, suitable for anti-corrosion and anti-seepage treatment of large-diameter pipelines (2).

5.4 Comparative Analysis of Application Scenarios

Based on the technical performance and economic efficiency comparison, the application scenarios of the U-Folding Method and several other main trenchless rehabilitation technologies are analyzed below:

1. Application Scenarios for U-Folding Method:
   • Pipelines with diameters 100-1200mm of various types (2).
   • Straight circular pipelines with slight structural damage .
   • Pipelines with high requirements for flow capacity (2).
   • Areas with limited construction space where large-scale excavation is not possible .
   • Particularly suitable for rehabilitation of gas, water supply and drainage pipelines (2).
2. Application Scenarios for CIPP Method:
   • Pipelines with diameters 50-2700mm of various types, especially special-shaped pipelines (51).
   • Pipelines with severe structural damage (14).
   • Drinking water pipelines with high hygiene requirements (48).
   • Long-distance pipeline rehabilitation (51).
   • Pipelines under important traffic arteries (14).
3. Application Scenarios for Spiral Winding Method:
   • Large-diameter pipelines with diameters 150-3000mm (2).
   • Non-circular pipelines, such as rectangular, horseshoe-shaped, etc. .
   • Pipelines with severe structural damage (2).
   • Pipelines requiring operation with water .
   • Pipelines with a long single rehabilitation distance (2).
4. Application Scenarios for Slip Lining Method:
   • Small-diameter pipelines with diameters 100-1000mm .
   • Local rehabilitation or segmental rehabilitation (2).
   • Pipelines with low requirements for flow capacity .
   • Projects with limited rehabilitation costs (2).
5. Application Scenarios for Spraying Method:
   • Large-diameter pipelines with diameters 300-3000mm .
   • Pipelines with non-structural damage (2).
   • Pipelines requiring anti-corrosion and anti-seepage treatment .
   • Large pipelines accessible to personnel (2).

5.5 Comprehensive Evaluation and Selection Recommendations

Based on the above comparative analysis, a comprehensive evaluation of the U-Folding Method and several other main trenchless rehabilitation technologies is conducted, and selection recommendations are proposed:

1. Comprehensive Evaluation:
   • U-Folding Method: Mature technology, excellent performance, moderate cost, is a cost-effective trenchless rehabilitation technology, especially suitable for pipeline rehabilitation with diameters 100-1200mm and slight structural damage .
   • CIPP Method: Advanced technology, excellent performance, but high cost, suitable for important pipelines and pipelines with severe structural damage (48).
   • Spiral Winding Method: Complex technology, high cost, but suitable for large-diameter and special-shaped pipeline rehabilitation, has irreplaceability in specific scenarios (2).
   • Slip Lining Method: Simple technology, low cost, but limited performance, suitable for small-diameter pipeline and non-structural rehabilitation .
   • Spraying Method: Simple technology, small flow section loss, but low structural strength, suitable for anti-corrosion and anti-seepage treatment of large-diameter pipelines (2).
2. Selection Recommendations:
   • Select according to pipeline type: Gas pipelines give priority to the U-Folding Method and CIPP Method; Water supply pipelines give priority to the U-Folding Method and CIPP Method; Drainage pipelines can consider the U-Folding Method, CIPP Method and Spiral Winding Method .
   • Select according to pipe diameter: Small-diameter pipes (DN<300) give priority to CIPP Method and U-Folding Method; Medium-diameter pipes (DN300-1000) give priority to U-Folding Method and CIPP Method; Large-diameter pipes (DN>1000) give priority to Spiral Winding Method and Spraying Method (2).
   • Select according to damage degree: Pipelines with slight structural damage give priority to U-Folding Method; Pipelines with severe structural damage give priority to CIPP Method and Spiral Winding Method; Pipelines with non-structural damage can consider Spraying Method and Slip Lining Method .
   • Select according to construction conditions: Areas with limited construction space give priority to U-Folding Method and CIPP Method; Pipelines requiring operation with water give priority to Spiral Winding Method; Pipelines with high requirements for flow capacity give priority to U-Folding Method and Spraying Method (2).
   • Select according to cost budget: Projects with sufficient budget can consider CIPP Method and Spiral Winding Method; Projects with limited budget can consider U-Folding Method and Slip Lining Method .
3. Combined Application Recommendations:
   • For pipelines with complex conditions, consider combined application of multiple technologies, such as combining U-Folding Method with local CIPP repair to solve local severe damage problems (2).
   • For large-diameter pipeline rehabilitation, consider combining Spiral Winding Method with Spraying Method to improve structural strength and anti-corrosion performance .
   • For long-distance pipeline rehabilitation, consider segmentally adopting different rehabilitation technologies to improve overall rehabilitation effect and economy (2).

VI. Technology Development Trends and Outlook

6.1 Material Innovation Development Direction

Material innovation for the U-Folding Method is mainly concentrated in the following directions:

1. Development of High-Performance HDPE Materials:
   • Develop HDPE materials with higher strength and toughness to improve the structural performance and deformation resistance of liner pipes (2).
   • Develop HDPE materials with aging resistance and ultraviolet resistance to extend the service life of liner pipes .
   • Develop HDPE materials with good thermal conductivity to accelerate the restoration of the liner pipe to its original shape (2).
2. Application of Composite Materials:
   • Develop composite materials of HDPE and fiber-reinforced materials to improve the strength and stiffness of liner pipes .
   • Research composite liner pipes of metal and HDPE to improve external pressure resistance and fire resistance (2).
   • Develop composite materials with self-healing functions to improve the reliability and durability of liner pipes .
3. Development of Functional Materials:
   • Develop HDPE materials with antibacterial and anti-scaling functions to improve the sanitary performance of drinking water pipelines (2).
   • Develop HDPE materials with thermal conductivity functions to meet the needs of pipeline rehabilitation in special environments .
   • Develop degradable HDPE materials to reduce environmental burden and meet environmental protection requirements (2).

6.2 Innovation Trends in Process Technology

Process technology innovation for the U-Folding Method is mainly reflected in the following aspects:

1. Application of Automation and Intelligence Technology:
   • Develop automated U-folding equipment to improve folding accuracy and efficiency .
   • Apply intelligent control systems to achieve automatic control and monitoring of the traction process (2).
   • Develop remote monitoring systems based on the Internet of Things to achieve real-time monitoring and management of construction processes .
2. Development of New Folding Processes:
   • Research new folding methods, such as C-type folding, I-type folding, etc., to further reduce the cross-sectional area of the liner pipe (2).
   • Develop thermoplastic folding processes to improve folding efficiency and quality by heating and softening the liner pipe .
   • Research low-temperature folding processes to meet the construction needs of cold regions (2).
3. Application Technology in Special Environments:
   • Develop U-folding technology suitable for high water pressure environments to solve construction problems in areas with abundant groundwater .
   • Research U-folding technology suitable for high-temperature environments to solve the rehabilitation problem of high-temperature pipelines such as thermal pipelines (2).
   • Develop U-folding technology suitable for submarine pipeline rehabilitation to expand the application field .

6.3 Digital and Intelligent Development Trends

The digital and intelligent development of the U-Folding Method is mainly reflected in the following aspects:

1. Application of Digital Twin Technology:
   • Establish a digital twin model of the pipeline to achieve virtual simulation and optimization of the rehabilitation process .
   • Use digital twin technology for rehabilitation plan design and evaluation to improve the scientificity and reliability of the plan (2).
   • Use digital twin models for health monitoring and management of rehabilitated pipelines to achieve full lifecycle management .
2. Intelligent Detection and Evaluation Technology:
   • Develop artificial intelligence-based pipeline defect recognition technology to improve detection efficiency and accuracy (2).
   • Research big data-based pipeline rehabilitation decision support systems to achieve intelligent recommendation of rehabilitation plans .
   • Develop pipeline health monitoring systems to monitor the operating status of rehabilitated pipelines in real-time (2).
3. Application of BIM Technology:
   • Apply Building Information Modeling (BIM) technology to achieve digital design and management of pipeline rehabilitation projects .
   • Establish pipeline rehabilitation project information management platforms to achieve digital management of the entire process of design, construction and acceptance (2).
   • Use BIM technology for construction simulation and optimization to improve construction efficiency and quality .

6.4 Market Application Prospect Outlook

The U-Folding Method has broad prospects in market application:

1. Growth in Market Demand:
   • With the acceleration of China's urbanization process, urban infrastructure construction continues to advance, and the demand for old pipeline rehabilitation is increasing day by day (2).
   • National policy support for underground pipeline network construction and reconstruction provides a good development environment for trenchless rehabilitation technologies .
   • The improvement of environmental protection requirements promotes the application of trenchless rehabilitation technologies, reducing environmental impact (2).
2. Expansion of Application Areas:
   • Application in the gas pipeline rehabilitation field will be further expanded, especially the reconstruction of gas pipelines in old residential areas .
   • Application in the water supply pipeline rehabilitation field will be more extensive, especially for drinking water pipelines with high water quality requirements (2).
   • Application in the industrial pipeline rehabilitation field will continue to expand, such as chemical, petroleum, power and other industries .
   • Application in special environments will make breakthroughs, such as submarine pipelines, high-temperature pipelines, high-water pressure pipelines, etc. (2).
3. Regional Market Development:
   • In economically developed eastern coastal areas, the U-Folding Method will be more widely applied, and the technical level will continue to improve .
   • In central and western regions, with the acceleration of infrastructure construction, the market demand for the U-Folding Method will grow rapidly (2).
   • In northeastern regions, pipeline rehabilitation in cold regions will become an important application area for the U-Folding Method .
4. Industrial Chain Development:
   • Upstream material and equipment manufacturing enterprises will increase R&D investment to improve product quality and performance (2).
   • Midstream construction enterprises will continuously enhance technical level and service capabilities to expand market share .
   • Downstream application areas will become more diversified, promoting the collaborative development of the industrial chain (2).

6.5 Policy Support and Industry Standard Construction

The development of the U-Folding Method is inseparable from policy support and industry standard construction:

1. Policy Support Direction:
   • The country will continue to increase investment in underground pipeline network construction and reconstruction, providing policy support for trenchless rehabilitation technologies .
   • Strengthened environmental protection policies will promote the application of trenchless rehabilitation technologies, reducing environmental impact from excavation (2).
   • Smart city construction will promote the digital and intelligent development of pipeline rehabilitation technologies .
2. Industry Standard Construction:
   • Revise and improve existing industry standards to adapt to technological development and market demand (2).
   • Formulate technical standards for new materials and new processes to standardize market behavior .
   • Establish and improve the quality supervision and acceptance system for trenchless rehabilitation projects to improve project quality (2).
3. Talent Training and Technology Promotion:
   • Strengthen the training of trenchless rehabilitation technology talents to improve the technical level and comprehensive quality of practitioners .
   • Carry out technical exchanges and training activities to promote advanced rehabilitation technologies and experiences (2).
   • Establish technological innovation alliances to promote industry-university-research cooperation and technological progress .

VII. Conclusions and Recommendations

7.1 Comprehensive Technical Evaluation

The U-Folding Method, as an advanced trenchless rehabilitation technology, has the following characteristics and advantages:

1. Technical Advantages:
   • Excellent structural performance, forming a "pipe within a pipe" composite structure, improving the structural strength and corrosion resistance of pipelines .
   • Convenient and efficient construction, small site occupation, can use existing manholes for construction, little impact on the surrounding environment (2).
   • Small flow section loss, basically does not affect the original water conveyance capacity of the pipeline .
   • Wide application range, suitable for pipelines with diameters 100-1200mm of various types, service life of more than 50 years (2).
   • Excellent material performance, HDPE materials have excellent corrosion resistance, wear resistance and anti-aging properties .
2. Technical Limitations:
   • Limited adaptability to pipeline deformation, not suitable for pipelines with severe deformation (2).
   • Limited adaptability to curved pipelines, not suitable for elbows with too small curvature radius .
   • Relatively high rehabilitation cost, higher than traditional rehabilitation methods such as slip lining (2).
   • High technical requirements for construction personnel, requiring professional training and experience accumulation .
3. Application Scenarios:
   • Suitable for pipelines with diameters 100-1200mm of various types (2).
   • Suitable for straight circular pipelines with slight structural damage .
   • Suitable for pipelines with high requirements for flow capacity (2).
   • Suitable for areas with limited construction space where large-scale excavation is not possible .
   • Particularly suitable for rehabilitation of gas, water supply and drainage pipelines (2).

7.2 Engineering Application Recommendations

Based on the technical characteristics and application scenarios of the U-Folding Method, the following engineering application recommendations are proposed:

1. Pre-project Assessment:
   • Before project implementation, conduct a comprehensive inspection and assessment of the pipeline to determine the damage degree and rehabilitation needs of the pipeline .
   • Comprehensively consider factors such as pipeline type, diameter, damage degree, and construction conditions to select the most suitable rehabilitation technology (2).
   • Conduct technical and economic analysis, compare the costs and benefits of different rehabilitation plans, and select the optimal plan .
2. Material Selection and Quality Control:
   • Select appropriate HDPE liner pipe materials according to pipeline type and service environment to ensure material quality meets standard requirements (2).
   • Strengthen material inspection upon entry to ensure material performance meets design requirements .
   • Establish a material quality traceability system to ensure the quality of each batch of materials is traceable (2).
3. Construction Process Control:
   • Strictly follow the construction process and operation specifications for construction to ensure the quality of each process meets requirements .
   • Strengthen safety management during construction to ensure the safety of construction personnel and the surrounding environment (2).
   • Do a good job in construction records, detailing various parameters and situations during the construction process .
4. Quality Acceptance and Maintenance:
   • Strictly follow acceptance standards for quality acceptance to ensure rehabilitation quality meets requirements (2).
   • Establish rehabilitation project files, preserving complete inspection, design, construction and acceptance data .
   • Develop regular maintenance plans to regularly inspect and maintain rehabilitated pipelines (2).

7.3 Future Development Recommendations

To promote the further development and application of the U-Folding Method, the following recommendations are proposed:

1. Technical Innovation Recommendations:
   • Strengthen research and development of high-performance HDPE materials to improve the structural performance and durability of liner pipes .
   • Research and develop new folding processes to improve construction efficiency and quality (2).
   • Carry out research on digital and intelligent technologies to improve technical level and management efficiency .
2. Standard System Construction Recommendations:
   • Revise and improve existing industry standards to adapt to technological development and market demand (2).
   • Formulate technical standards for new materials and new processes to standardize market behavior .
   • Establish and improve the quality supervision and acceptance system for trenchless rehabilitation projects to improve project quality (2).
3. Market Promotion Recommendations:
   • Strengthen technical publicity and promotion to improve the awareness and recognition of the U-Folding Method .
   • Carry out technical training and exchange activities to improve the technical level of practitioners (2).
   • Establish technological innovation alliances to promote industry-university-research cooperation and technological progress .
4. Policy Support Recommendations:
   • Seek national and local policy support to promote the development of trenchless rehabilitation technologies (2).
   • Promote the inclusion of trenchless rehabilitation technologies in urban infrastructure construction plans to expand the scope of application .
   • Establish incentive mechanisms for the application of trenchless rehabilitation technologies to encourage the adoption of advanced technologies (2).

7.4 Summary

The U-Folding Method, as an advanced trenchless rehabilitation technology, has advantages such as excellent structural performance, convenient and efficient construction, small flow section loss, and wide application range, and has broad application prospects in the fields of gas, water supply and drainage pipeline rehabilitation. With the continuous innovation and development of material technology, process technology and digital technology, the U-Folding Method will further improve its technical performance and application scope, providing a higher quality and more efficient solution for urban underground pipeline network rehabilitation and reconstruction.

In future development, it is necessary to strengthen technological innovation, standard system construction, market promotion and policy support to promote the healthy development and wide application of the U-Folding Method, making greater contributions to urban infrastructure construction and sustainable development. At the same time, it is also necessary to recognize the technical limitations of the U-Folding Method, select the most suitable rehabilitation technology according to the specific circumstances of the project, and achieve the best rehabilitation effect and economic benefits.

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[36] ASTM F2764/F2764M-23 6 英寸至60 英寸[150 毫米至1500 毫米] 聚丙烯 (PP)波纹双壁和三壁管及配件,用于无压污水系 统应用(pdf) https://img.antpedia.com/standard/first_page_cn/files/pdfs_ora/20230612/astm/2023/F%202764%20-%20F%202764M%20-%2023-page-1.pdf

[37] Sanitary sewer pipe from ADS meets ASTM standards F2736 and F2764 | WaterWorld https://www.waterworld.com/wastewater/article/16221169/sanitary-sewer-pipe-from-ads-meets-astm-standards-f2736-and-f2764

[38] SaniTite HP Pipe from ADS Complies with ASTM Standards https://www.azobuild.com/news.aspx?newsID=9929

[39] Comparison of Creep Rupture and Creep Modulus Data for High... #2014 https://www.pe100plus.com/PPCA/Comparison-of-Creep-Rupture-and-Creep-Modulus-Data-for-High-Density-Polyethylene-and-Polypropylene-Pipe-Resins-Using-Traditional-and-Accelerated-Test-Methods-p1389.html

[40] ASTM认证——玩具安全的“守护者”！ ASTM认证是美国材料与试验协会的标准，主要针对玩具、婴儿用品等。ASTM认证确保产品符合安全标准，没有它，玩具无法进入美国市场。跨境卖家们，玩具的安全性不容忽视，ASTM认证是必备条件！?-抖音

ASTM认证是美国材料与试验协会的标准，主要针对玩具、婴儿用品等。ASTM认证确保产品符合安全标准，没有它，玩具无法进入美国市场。跨境卖家们，玩具的安全性不容忽视，ASTM认证是必备条件！?-抖音 https://www.iesdouyin.com/share/video/7485189277156904232/?did=MS4wLjABAAAANwkJuWIRFOzg5uCpDRpMj4OX-QryoDgn-yYlXQnRwQQ&from_aid=1128&from_ssr=1&iid=MS4wLjABAAAANwkJuWIRFOzg5uCpDRpMj4OX-QryoDgn-yYlXQnRwQQ&mid=7485189248216238884&region=&scene_from=dy_open_search_video&share_sign=DVsxbCqUTs8Muqh5knT7K9aSVRBo.zCt3q9XV8AB1Gw-&share_version=280700&titleType=title&ts=1752284299&u_code=0&video_share_track_ver=&with_sec_did=1

[41] DIN_EN_13161-englisch天然石材的抗折强度测定_人人文库网 https://m.renrendoc.com/paper/168870747.html

[42] DIN EN 13161 https://www.en-standard.eu/din-en-13161-natural-stone-test-methods-determination-of-flexural-strength-under-constant-moment/

[43] 一文读懂石材的欧洲测试标准_试验_of_Natural https://m.sohu.com/a/665391819_120817893/

[44] EN 13161:2008 - Natural stone test methods - Determination of flexural strength under constant https://standards.iteh.ai/catalog/standards/cen/3ce0e584-1c34-4599-b984-006297245ae3/en-13161-2008

[45] 恒定阻抗加标准-分析测试百科网 https://www.antpedia.com/standard/standard.php?keyword=%E6%81%92%E5%AE%9A%E9%98%BB%E6%8A%97%E5%8A%A0&start=45

[46] DIN EN 13161:2008 - Natural stone test methods - Determination of flexural strength under constant moment; German version EN 13161:2008 https://webstore.ansi.org/standards/din/dinen131612008

[47] UNE-EN ISO 13161:2020 水质 钋 210 使用 α 光谱法的测试方法 标准 https://m.antpedia.com/standard/1740550078.html

[48] Comparison Of Thermoplastic Pipe Liner https://www.linedpipesystems.com/comparison-of-thermoplastic-pipe-liner-to-other-pipe-lining-methods/

[49] Experimental and numerical evaluation of the impact of folds on the pressure rating of CIPP liners | Request PDF https://www.researchgate.net/publication/223733914_Experimental_and_numerical_evaluation_of_the_impact_of_folds_on_the_pressure_rating_of_CIPP_liners

[50] Slip Lining Vs CIPP: What's the Difference? - Advanced Pipe Repair https://www.advancedpiperepair.com/slip-lining-vs-cipp/

[51] What is CIPP Pipe Lining? Complete Guide to Cured in Place https://plumbingnav.com/plumbing/what-is-cipp-pipe-lining/

[52] When and Why is Pipe Bursting Better than CIPP Lining? https://www.expresssewer.com/blog/when-and-why-is-pipe-bursting-better-than-cipp-lining

[53] Plastic Liner Market Size & Trends 2025-2035 https://www.futuremarketinsights.com/reports/plastic-liners-market

[54] Linings Case Study: Preventing Bulk Material Migration | MCG https://www.mcam.com/en/case-studies/storage-bunker-lining

[55] ASTM F2160-01 外径受控的实心壁高密度聚乙烯导管标准规范 发布历史 https://m.antpedia.com/standard/5722864-9.html

[56] F2160 Standard Specification for Solid Wall High Density Polyethylene (HDPE) Conduit Based on Controlled Outside Diameter (OD) https://www.astm.org/f2160-22.html

[57] ASTM-F2160 | Standard Specification for Solid Wall High Density Polyethylene (HDPE) Conduit Based on Controlled Outside Diameter (OD) | Document Center, Inc. https://www.document-center.com/standards/show/ASTM-F2160

[58] 最新解读《G BT 18998.5-2022工业用氯化聚氯乙烯(PVC-C)管道系统 第5部分:系统适用性》 - 豆丁网 https://www.docin.com/touch_new/preview_new.do?id=4756914481

[59] North Coast Conduit | Erie, PA http://www.northcoastconduit.com/

[60] HDPE检测 - HDPE检测第三方机构 - 中科认证站 https://www.cas-c.cn/service/show_17435.html

[61] 聚乙烯管材手册第14章DuctandConduit.pdf -原创力文档 https://m.book118.com/html/2018/0505/164912145.shtm

[62] Pipe Rehabilitation | Seals & Profiles https://www.trelleborg.com/en/seals-and-profiles/products-and-solutions/pipe-rehabilitation

[63] Full article: Pipeline rehabilitation combined strategy for urban water systems https://www.tandfonline.com/doi/full/10.1080/1573062X.2023.2273535

[64] State of Technology for Rehabilitation of Water Distribution Systems https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100GDZH.TXT

[65] Decision Making Process for the Most Appropriate Pipe Rehabilitation Method by Holistic Evaluation Technique | Proceedings | Vol , No https://ascelibrary.org/doi/10.1061/9780784484296.034