Ductile Iron Pipeline Installation Technology for Water Supply Engineering
Abstract: The Yangqu Raw Water Direct Supply Project, part of the Shanxi Wanjiazhai Yellow River Diversion Project, successfully completed the installation of DN1000 ductile iron pipelines using a direct-buried underground method. Based on engineering practice, this article presents the installation technology for ductile iron pipelines, covering the construction process, installation techniques, and pipeline hydrostatic pressure testing. In recent years, ductile iron pipes have been widely adopted and rapidly developed in China. After nearly three decades of practical application, their safety and suitability have been widely recognized by the water supply industry. In medium- and low-pressure pipeline networks, generally operating at pressures below 6 MPa, ductile iron pipes offer safe and reliable operation, a low failure rate, convenient and efficient construction and maintenance, and excellent corrosion resistance. As flexible pipes manufactured by centrifugal casting, ductile iron pipes feature dense, uniform walls, spheroidal graphite, and a predominantly ferritic matrix, giving them high elongation and strength comparable to those of steel pipes. They are flexible, capable of absorbing sudden forces, and offer greater bending strength than steel pipes, making them resistant to bending and deformation during service. The installation procedure is simple, precise, and economical. Drawing on practical engineering experience, this paper presents the construction techniques for ductile iron pipelines, which may serve as a useful reference for comparable projects.
The construction process for ductile iron pipelines proceeds as follows:
trench excavation and foundation treatment → pipe socket inspection → pipe laying → socket cleaning and lubrication → rubber ring installation (including alignment, chain hoist positioning, and axis/elevation correction) → joint inspection (if rejected, return to pipe socket inspection) → trench backfilling.
During trench excavation, the cross-section shall conform to the design specifications. The clear width at the trench bottom shall be no less than 500 mm greater than the pipe's outer diameter on each side to accommodate manual operations. The natural subgrade at the trench bottom shall remain undisturbed. Where mechanical excavation is used, a 200 mm to 300 mm protective layer of soil shall be retained at the base, which shall then be excavated manually to the specified design elevation and leveled.
The trench bottom shall be maintained in a dry condition, free from inundation and frost damage. In the event of localized soft spots or disturbance, the affected area may be repaired with compacted gravel or lime-soil fill. For projects affected by groundwater, suitable drainage and dewatering systems shall be provided to meet the specified design requirements. If the trench bottom contains mixed fill or corrosive soil, it shall be entirely removed and the foundation reinforced in accordance with the design specifications.
Before pipe laying begins, the trench bottom shall be covered with a layer of gravel or medium-to-coarse sand, with the thickness determined according to the pipe diameter and design requirements. A bell hole shall be excavated at each joint to provide sufficient clearance for joint assembly, ensuring proper pipe alignment and compliance with installation requirements.
Before pipe fittings are unloaded at the construction site, they shall undergo an acceptance inspection. Prior to installation, a designated quality inspector shall conduct a thorough inspection of the fittings to identify any socket damage that may have occurred during unloading or handling. This inspection helps identify damage that could compromise the joint seal and result in leakage. The inspection items include visual quality, such as the condition of the anti-corrosion coating and cement lining, as well as structural dimensions, including socket roundness, diameter, and rubber ring dimensions. The steel ring surface of the socket shall be smooth and free of damage, burrs, and dirt. The working surface shall be flat and free of protrusions, and the lubricant shall be applied evenly.
Select appropriate lifting equipment according to the site topography and pipe burial depth. For a pipe burial depth of 4–6 m, a truck crane is a safer option when the site is firm and level. If the site terrain is undulating and pipe fittings need to be moved, a 330-class or larger crawler excavator can be used for lifting. If the pipe burial depth exceeds 6 m, a larger-capacity lifting machine should be selected based on the actual site conditions and lifting requirements. For pipe laying in trenches, a dedicated 50-class or larger crawler crane should be used. A trial lift should be performed first to verify the lifting points and confirm the safety and suitability of the lifting slings and equipment. If necessary, a lifting frame should be installed to maximize the vertical load-bearing capacity. In the Yangqu Raw Water Direct Supply Project of the Wanjiazhai Yellow River Diversion Project in Shanxi Province, the ductile iron pipes were buried at a depth of 4–6 m. A 330-class excavator was used for all lifting and installation operations, reducing the time and labor costs associated with conventional lifting operations while improving equipment utilization and construction efficiency. The pipe lifting and laying process is shown in Figure 1.
Figure 1. Excavator Lifting and Laying Pipe
To prevent pipe fittings from colliding with the trench slope and becoming contaminated with dust and sand during lifting and laying, the sockets must be thoroughly cleaned and lubricated, with a 1 m clearance maintained between adjacent joints. Before installation, clean the socket area with a stiff-bristled brush, a small air blower, or a clean cloth, paying particular attention to the groove in which the sealing ring will be installed. Ensure that the surface is free of dust, sand, stones, and other debris. Then, manually apply edible oil evenly until the surface is smooth and glossy, with no visible oil dripping or flowing.
Before installing the rubber ring, carefully inspect it to ensure that it shows no signs of aging, pores, cracks, or uneven thickness and is free from overlapping, warping, damage, and visible impurities. Furthermore, each rubber ring should have no more than one joint. Apply edible oil evenly to the rubber ring as a lubricant. For pipe diameters of DN800 or smaller, the rubber ring can be installed manually. For ductile iron pipe fittings, bend the rubber ring into a “plum blossom” shape before placing it into the socket. A detailed inspection of the pipe fittings to be installed should also be carried out to prevent damage to the socket and spigot during unloading and construction, which could result in leakage. The inspection items include:
Appearance quality: Inspect the anti-corrosion coating, cement mortar lining, and other surface conditions.
Dimensional and structural inspection: Check the socket roundness, diameter, rubber ring, and other relevant dimensions. The steel ring surface of the socket should be smooth and free from damage, burrs, and dirt. The working surface should be flat and free from any protrusions. Ensure that the lubricant is applied evenly.
Before installing the rubber ring, carefully inspect it to ensure that it shows no signs of aging, pores, cracks, or uneven thickness and is free from overlapping, warping, damage, and visible impurities. Furthermore, each rubber ring should have no more than one joint. Apply edible oil evenly to the rubber ring as a lubricant. For ductile iron pipe fittings with a nominal diameter of DN800 or smaller, bend the rubber ring into a “plum blossom” shape and place it into the sealing groove of the socket. Next, manually press the rubber ring into the sealing groove from four directions simultaneously. For ductile iron pipe fittings with a nominal diameter of DN800 or larger, bend the rubber ring into a concave shape, place it into the socket sealing groove, and then press it firmly into the groove from below. Ensure that the installed rubber ring is evenly seated in the sealing groove, without twisting, kinking, or obstruction, and that it is correctly positioned. A schematic diagram of the rubber ring installation is shown in Figure 2.
Figure 2. Schematic Diagram of Rubber Ring Installation
(1) Pipe Alignment: First, manually align the spigot of the pipe to be installed with the socket of the previously installed pipe. Then, slowly push the spigot into the socket to a depth of approximately 3 cm. Next, install the pulling straps and chain hoists at the centerline height on both sides of the pipe fitting to be installed. Pull the fitting manually and symmetrically from both sides until the spigot is fully inserted into the socket along the alignment lines. During the pulling process, continuously monitor the insertion of the spigot into the socket. If uneven insertion or significant lateral deviation is observed, immediately notify the installer to adjust the pulling speed of the chain hoists and correct the fitting's centerline alignment. If the fitting's centerline becomes misaligned or the resistance to pipe insertion is excessive, immediately stop the installation. Withdraw the pipe, inspect the position of the sealing ring and the condition of the spigot and socket, identify and rectify the problem, and then reinstall the pipe.
(2) Installation Methods for Different Pipe Diameters: Installation methods vary depending on the diameter of the ductile iron pipe. For pipes with diameters below DN150, simple tools such as pry bars can be used for installation. For pipes with diameters of DN200 and above, steel wire ropes and chain hoists are required. Fiber slings should be used for traction, and flexible protective materials should be placed at the contact points between the chain hoists and the pipes to prevent damage to the anti-corrosion coating. The Shanxi Wanjiazhai Yellow River Diversion Project's direct water supply project and the Datong Yellow River Raw Water Direct Supply Branch Line Project, part of the northern main line of the Wanjiazhai Yellow River Diversion Project in Shanxi Province, both used DN1000 ductile iron pipes. The entire 15-km pipeline was installed using manual chain hoists, with satisfactory results. A schematic diagram of the pipeline installation is shown in Figure 3.
Figure 3. Schematic Diagram of DN1000 Ductile Iron Pipe Installation
(3) Pipeline Centerline and Elevation Adjustment: After each pipe section is installed, its position should be measured and verified. The main inspection items include the pipeline elevation and centerline alignment. For any single standard-length pipe section, the vertical and horizontal deviations should not exceed ±20 mm. The socket installation gap and circumferential joint gap should be uniform, and the longitudinal clearance between the socket and spigot should not be less than 3 mm.
After installation, surveyors should recheck the pipeline centerline and elevation. The internal joints should be inspected primarily by visual examination. A uniform circumferential gap between the spigot and socket, within the specified allowable range, is considered acceptable. External joints should be inspected using dimensional measurement. A narrow steel ruler is used to check the compression of the rubber ring at four evenly spaced points around the spigot. During inspection, the insertion depth of the ruler at these four locations should be less than or equal to the distance between the rubber ring's bulb and the socket. This confirms that the rubber ring is uniformly compressed, ensuring a reliable seal at the joint and confirming that the ring has not rotated, twisted, or been damaged. A schematic diagram of the pipe joint inspection is shown in Figure 4.
Figure 4. Schematic Diagram of Pipe Joint Inspection
Before backfilling the pipeline trench, carefully inspect the installed pipeline for any signs of damage or deformation. If any defects are identified, the affected pipe shall be promptly repaired or replaced. The trench shall be thoroughly cleaned and free of standing water; backfilling in water is strictly prohibited. During trench backfilling, granular sand shall first be placed around both sides of the pipe and thoroughly compacted to ensure close contact with the pipe wall.
After the compacted backfill passes inspection, continue backfilling on both sides of the pipe. Backfill shall be placed simultaneously on both sides and compacted in successive layers. The difference in elevation between the compacted surfaces on both sides of the pipe shall not exceed 300 mm. When backfilling below the pipe springline, measures shall be taken to prevent the pipe from floating or shifting. Within 500 mm above the pipe crown and along both sides of the pipe, lightweight compaction equipment shall be used to avoid damaging or displacing the pipe. The backfilling and compaction procedures on both sides of the pipe are shown in Figure 5.
Once the backfill on both sides reaches 500 mm above the pipe crown, a hydrostatic pressure test shall be conducted on the pipeline. After the pipeline passes the pressure test, the remaining backfilling shall be completed promptly. When the backfill reaches the design elevation, the pipe deformation rate shall be measured and recorded within 12–24 hours to ensure compliance with the design requirements.
Figure 5. Backfilling and Compaction on Both Sides of the Pipeline
After the pipeline installation is completed, the length of each hydrostatic pressure test section shall be determined based on the specific project conditions and shall generally not exceed 1 km. If the water supply is insufficient or the soil conditions in the backfill area are unsuitable within a 1-km section, a test section of up to 1.5 km may be selected, provided that it has a reliable water source and suitable backfill soil conditions. In the Yangqu Raw Water Direct Supply Project of the Shanxi Wanjiazhai Yellow River Diversion Project, the pipeline has a design pressure of 1.5 MPa and uses DN1000 ductile iron pipes. The main hydrostatic pressure test sections are 1–1.5 km long. In accordance with the Code for Construction and Acceptance of Water Supply and Sewerage Pipeline Engineering (GB 50268-2008), the test pressure is 1.5 times the design pressure.
The test plan shall be developed based on the geological and topographical conditions of the test section. Before conducting the hydrostatic pressure test, the following preparations shall be completed:
· Water supply and drainage: Appropriate water supply and drainage plans shall be prepared in advance, particularly because large-diameter ductile iron pipelines require a large volume of water for testing.
· Pipeline sealing and cleaning: All openings in the test section shall be properly sealed to prevent leakage, and any debris inside the pipeline shall be removed beforehand.
· Pipeline backfilling: Except at the joints, the pipeline shall be backfilled to a thickness of at least 0.5 m on both sides and above the pipe crown to prevent pipe movement and flotation during the hydrostatic pressure test.
· Concrete strength: The hydrostatic pressure test may be conducted only after the concrete at the inflection points has reached its design strength.
After the test section is filled with water, it shall be allowed to soak for 24 hours at a pressure not exceeding the working pressure. After pressurization, monitor the pressure in the pipeline. If the pressure drop does not exceed 20% of the allowable value within 15 minutes, increase the pressure to the specified test pressure and maintain it for 30 minutes while conducting a visual inspection of the pipeline and joints. During project implementation, both the pipeline pressure drop and allowable leakage met the specified requirements, with the allowable leakage rate being no more than 3.00 L/(min·km).
The water supply pipeline constructed for the Yangqu Raw Water Direct Supply Project under the Shanxi Wanjiazhai Yellow River Diversion Project utilizes ductile iron pipes with T-type joints. The flexible yet resilient rubber-ring joints can accommodate foundation settlement and other ground movements. Ductile iron pipes are easy to install, with a simple, rapid, and efficient installation process that requires relatively low labor intensity, making them well suited to pipeline sections with challenging terrain and poor geological conditions. Following the successful installation of ductile iron pipes in the Yangqu Raw Water Direct Supply Pipeline of the Shanxi Wanjiazhai Yellow River Diversion Project, their application was further extended to the Datong Yellow River Raw Water Direct Supply Distribution Branch Line of the Northern Trunk Line of the same project. Based on engineering practice and the requirements for pipeline installation quality, the construction process was systematically summarized and refined to provide a technical reference for similar projects.
