In recent years, Tianjin has seen rapid advancements in its urban infrastructure development, with the city’s scale steadily expanding and wastewater discharge steadily rising. At the same time, efforts to protect rainwater resources, implement stormwater-sewage separation systems, and enhance the city’s flood-control capabilities have driven a significant annual increase in the scale of Tianjin’s newly constructed underground drainage networks. Meanwhile, as urban traffic continues to grow—leading to longer and wider roads—the length and diameter of rainwater and sewage pipes beneath road surfaces have also been expanded, ensuring the city can meet its evolving needs.
　　1. Project Overview
　　1.1 Overview: Geological Conditions, including the geological characteristics of the soil layers where construction pipelines will be laid in this area.
　　The design pipeline is located beneath the road surface, at a depth of approximately 7 meters, where the soil consists of silty clay, silt, and muddy silty soil. (Below 4.9 meters, the color turns brownish-gray with occasional shell fragments.) Groundwater conditions: The local groundwater table is relatively high, with groundwater visible just about 1 meter below the ground surface.
　　1.3 Landform and Engineering Characteristics
　　1.3.1 Dagukou South Road is a key urban thoroughfare; regardless of the construction process, the existing road must remain fully functional for traffic flow. The jacking distance for pipe-jacking work must meet the requirements of the well locations—otherwise, no excavation on the road will be permitted.
　　1.3.2 The construction site is located in a bustling area, with numerous overhead and underground pipelines, utilities, and obstacles both on and around the roads. These challenges significantly complicate construction efforts while requiring extra precautions to safeguard the safety of various underground pipes and cables.
　　1.3.3 The construction site is large in scale but confined in area. At the same time, the contractor is required to minimize disturbance to residents and reduce the footprint of the project in the surrounding area. Notably, the pipeline routes are situated primarily alongside government offices, businesses, and residential buildings. Due to the close proximity of these structures, special precautions will be taken during construction to ensure the safety of the nearby residential buildings.
　　1.3.4 Construction pipelines are being carried out concurrently with other works, such as bridge construction. The key challenge lies in carefully coordinating these overlapping operations, meticulously organizing three-dimensional intersecting work sequences, while prioritizing civilized construction practices and environmental protection efforts—ultimately ensuring the project is completed on schedule.
　　2. Construction
　　2.1 Construction Preparation
　　2.1.1 Selection of Construction Techniques: Considering the numerous existing pipelines running along Dagulu South Road, a d2600mm diameter was chosen for the sewage interception design, and all pipes will be installed using the pipe jacking method. Can pipe jacking effectively meet the project requirements? To ensure success, it’s essential to select the most suitable pipe-jacking method based on varying soil conditions and specific construction site constraints. Currently, our company offers three primary pipe-jacking techniques: manual excavation, earth-pressure balanced (EPB), and slurry-balanced pipe jacking—all of which have their own unique advantages. In this particular project, the pipeline will be buried deep beneath the soil, and groundwater activity is expected to be significant. If manual excavation were used, the first priority would be to lower the groundwater level. However, forced dewatering could lead to settlement of the road surface and underlying foundations, potentially causing structural damage to nearby buildings and triggering a host of related issues. Therefore, manual excavation is not recommended for this project. Moreover, since the pipeline installation must occur after the road construction is complete, adopting the earth-pressure balanced pipe-jacking method is the optimal choice. This approach minimizes the need to break up large sections of the road while effectively preventing any risk of surface settlement.
　　2.1.2 Process Determination: The pipe jacking method with a diameter of up to 2,600 mm is currently the largest-diameter pipe jacking project in our city. Construction in southern China has successfully implemented this method using multi-cutterhead machines, ensuring that the road surface remains stable and minimizing risks to nearby buildings.
　　This construction involves a d2600mm drainage pipe with an outer wall diameter of Ф3120mm. If a multi-cutterhead machine is chosen for the project, the machine’s head design is relatively simple and cost-effective, requiring less equipment investment compared to other methods. The multi-cutterhead configuration features four distinct zones within the circular head interface, each equipped with smaller-diameter cutting discs. These discs are individually driven by motors, rotating independently to excavate the surrounding soil. Together, the four cutterheads cover approximately 50% of the entire cross-sectional area, while the remaining portion is efficiently compacted and pushed into the spiral soil-ejection mechanism. This lightweight head design makes it particularly well-suited for working in soft soils, such as those exhibiting fluid or plastic characteristics.
　　The soil layers beneath the Dagu South Road are highly variable, and geological data indicate that the N-values of the soil are relatively high, suggesting a fairly hard soil condition. Additionally, the area is dotted with numerous existing ground-level structures, underground utilities, and buried objects. Based on our experience, the optimal choice for tunneling machinery would be a machine equipped with a large cutterhead capable of excavating large cross-sections of soil—making this method both practical and effective. The reason is that a large cutterhead ensures 100% cutting efficiency, making it perfectly suited to handle the diverse soil types commonly found in this region. In fact, we’ve successfully deployed a machine with a diameter of 2200 mm in the past, and our experience confirms that this type of cutterhead performs exceptionally well in dealing with clay, silt, and even loose sandy soils characteristic of our city.
　　2.1.3 Mechanical Excavation Performance: For this D2600mm earth-pressure balanced pipe jacking machine, a large cutterhead model has been selected for cutting operations. Prior to construction, the cutterhead torque required for the D2600mm tunneling equipment must be determined based on local geological data and other relevant factors specific to this city. Given the increased pipe diameter and corresponding enlargement of the cutterhead cross-section, it’s essential to calculate the torque generated by soil-cutting resistance when the cutterhead is operating in clay. This information will serve as the basis for the manufacturer’s machining specifications. The next step is to determine the optimal torque level that ensures the cutterhead delivers efficient cutting performance. Assuming an optimized cutterhead rotational speed of 2.0 rpm, the cutting force can be calculated using the following formula: \[ T = \int_{r_1}^{r_2} 10khr \, dr \]
　　T – Cutterhead torque, kN·m;
　　k——the cutting resistance per unit area experienced by the tool during the machining process, measured at 1 kg/cm²;
　　h——the thickness of soil cut by the tool per revolution is 0.3 m;
　　r—cutter head cutting radius: 1.56 m;
　　By calculating, the torque of the d2600mm cutterhead is 365.8 kN·m, equivalent to (36.58 t·m).
　　The electrical power required to meet the above cutterhead torque can be calculated using the following formula:
　　N = {Constant × 1.026 × (Cutter Torque T × Cutter Speed r)} / Efficiency η = kW
　　N – Effective power (kW) of the cutterhead torque used for pipe jacking
　　r —— cutter head speed: 2.0 r/min η —— mechanical efficiency: 0.85
　　After calculation, the input power required for d2600mm is: 1.026 × 36.58 × 2.0 / 0.85 = 88.308 kW
　　Mechanical design of the large cutterhead tunneling machine
　　The sewage interception design calls for a pipe diameter of 2,600 mm, and construction will be carried out using a soil-pressure balanced tunneling machine to jacking the pipes into place. Depending on the existing soil conditions, an appropriate large cutterhead model will be selected, ensuring flexible operation, advanced technology, and smooth soil removal. A dry soil-disposal method is employed, minimizing environmental impact and pollution from waste soil, while also simplifying the process of transporting and handling the excavated material.
　　2.2 Construction Design
　　2.2.1 Working Pit and Receiving Pit Design
　　Since, in practice, setting the controlled earth pressure cannot be a constant value, the additional pressure ΔP varies empirically within a range of 20 kPa, with P控 set at 110 kPa.
　　After surveying and setting out the layout, excavate the pile trenches and construct working pits. Drive steel piles into the ground: Use a crawler-type vibratory pile driver to install steel piles along the inner perimeter of each pit, from #1 to #10. A 16-ton crane will assist in the construction process. When driving the piles, ensure that they remain perfectly vertical by carefully controlling their alignment with the help of the vibratory hammer, gradually sinking the entire length of the steel pile to the required depth. After driving the steel piles into the working pits, maintain a back-to-back spacing of 0.3 meters between adjacent piles, while keeping the remaining gaps at 0.8 meters. Each pile will be 15 meters long. On either side of the dragon gate opening, position one steel pile, ensuring they are placed 0.2 meters away from the outer edge of the pipe. To achieve precise control during installation, use two 15 cm x 15 cm square timbers as horizontal control beams, and employ 10 cm x 10 cm angle steel sections as vertical support frames.
　　2.2.2 Installing Cement Mixing Piles
　　Cement mixing piles are installed along the edge of the working pit to prevent collapse and water seepage, forming a waterproof curtain around the pit. First, boundary lines are marked outside the pit, followed by excavating the pile trenches. The drilling rig is then positioned, and the cement mixing piles are driven into place, creating a curtain with a pile diameter of 0.7 meters, an overlap of 0.20 meters, and a pile length of 14 meters, featuring a 15% cement content. For critical areas, such as the rear support structure, the cement content is increased to 20%. To ensure precise cement usage per meter length, a metering pump is employed during the grouting process. The piles are formed through uniform mixing, with continuous overlapping throughout the project. To accelerate the setting of the cement slurry, 3% gypsum powder is added as an admixture. Additionally, the water-cement ratio is carefully controlled to maintain optimal cement strength. A dual-axis, rack-mounted piling machine is used to guarantee the verticality of the piles and ensure seamless overlap between adjacent piles.
　　2.2.3 Determining the Controlling Earth Pressure and Jacking Thrust
　　(1) Earth pressure calculations must be based on the soil conditions of the construction site:
　　The soil bulk density γt = 19.3 kN/m³, soil cohesion C = 14 kPa, and soil internal friction angle Ø = 24°. The depth to the center of the pipe is h = 6.5 m.
　　Determine the controlled earth pressure as the product of the at-rest earth pressure and an empirical coefficient, calculated using the formula: P_control = P0 × K0
　　P₀ – Rankine's at-rest earth pressure (kPa), which is the product of the soil unit weight γₜ and the depth h from the pipe centerline.
　　K0—Coefficient of at-rest earth pressure, with an empirical value of 0.75 (this value typically ranges from 0.4 to 0.9 in sandy soils). Calculated controlling earth pressure P_control = 90 kPa.
　　Since, in practice, setting the controlled earth pressure cannot be a constant value, the additional pressure ΔP varies empirically within a range of 20 kPa, with P控 set at 110 kPa.
　　(2) Calculation of jacking thrust:
　　In earth-pressure balanced pipe jacking construction, the total design jacking force \( F_{\text{total}} \) is primarily composed of two components: the face resistance \( F_0 \) encountered by the cutterhead during pipe advancement, and the cumulative resistance \( F \) acting along the length of the pipe being pushed forward. Thus, \( F_{\text{total}} = F_0 + F \). For a pipe diameter of φ2600 mm, the face resistance \( F_0 \) under normal jacking conditions is approximately 70 tons. This initial jacking force—equivalent to the face resistance—is made up of the soil pressure and groundwater pressure acting against the front of the tunneling machine, as well as any additional resistive forces. The total jacking force is calculated using the empirical formula \( F = K_f \pi D L \), where: - \( K_f \) is a coefficient accounting for various factors like soil type and machine efficiency, - \( D \) is the pipe diameter, - \( L \) is the length of the pipe segment being advanced.
　　F—Jacking Thrust (t)
　　K—Safety factor (taken as 1.2)
　　D—Outer diameter of the pipe: 3.12 m
　　L—Jacking length 140 (m)
　　f—Grouting friction coefficient (friction coefficient in sandy soil is taken as 0.6).
　　The design jacking force for the 140-meter jacking distance is F_total = 1058 tons.
　　2.3 Installation of Construction Equipment
　　2.3.1 Main Jacking Equipment Installation
　　Each main jacking pit is equipped with 6 hydraulic jacks (320-ton capacity), and the jack supports are arranged in three layers to securely hold the jacks in place. On the ground, a hydraulic power unit is set up, featuring two high-pressure oil pumps, each delivering a flow rate of 2 × 25 L/min at a pressure of 31.5 MPa. High-pressure oil lines connect these pumps directly to the jacks. A crane is used to lift and install the jack supports, followed by the precise positioning of the jacks themselves. Finally, flat shims are employed to fine-tune the jacks’ alignment, ensuring they are perfectly level and stable—while simultaneously adjusting all six jacks so their combined force acts precisely through the centerline of the pipe.
　　2.3.2 Installation of the Stabilizing Rail and Back Iron
　　The rails are made from a custom-designed heavy-duty composite rail and are welded to the embedded steel plates in the foundation. The spacing between the rails should ensure that the outer bottom of the pipe remains at least 5 cm above the top surface of the transverse channel steel. During rail installation, it’s crucial to maintain precise control over elevation and centerline alignment errors. For the backrest, a pre-fabricated composite backrest is used, with its plane positioned perpendicular to the rail. Finally, concrete (C20 grade, 10–15 cm thick) is poured between the backrest and the pit wall to achieve vertical leveling.
　　2.4 Jacking Construction
　　2.4.1 Cutterhead Installation: Before the cutterhead enters the tunnel entrance, all steel piles at the working pit opening must be removed. After clearing the jet-grouted piles, use a jack to push the cutterhead along the guide rails into the soil at the tunnel entrance. Ensure the cutterhead’s front face is tightly pressed against the surrounding soil inside the tunnel, allowing the soil chamber pressure gauge to reach the preset value. Once this pressure stabilizes for a specific duration and then begins to drop slightly, the reading at that point represents the actual static earth pressure—this value will serve as a reference for controlling the soil pressure during excavation. Additionally, to prevent the cutterhead from sinking unexpectedly due to ground settlement, install four sets of M36 bolts (arranged in a well-like pattern) between the first tunnel segment and the cutterhead itself. These bolts should be securely welded to the embedded steel plates for added stability.
　　Before starting the jacking process, it’s crucial to accurately measure the elevation and centerline displacement of the first pipe section to ensure precise equipment installation. Any issues—such as jack synchronization problems, significant differences in jacking force among jacks, or insufficient installation accuracy—can lead to misalignment of the resultant force line, ultimately compromising the overall quality of the pipe segment being jacked forward. For each pipe section to be jacked, make sure the rubber gasket is properly fitted and evenly coated with petroleum jelly. Then, place the O-shaped steel shoe underneath and slowly activate the jack to firmly connect the current pipe to the preceding one, ensuring a secure and stable joint.
　　Under standard conditions, control the entry into the ground to ensure a smooth start for the entire pipe section's jacking process. When the TBM cutter head first enters the soil, use four hydraulic jacks (320 tons) and maintain a slow jacking speed of 2 cm/min.
　　The earth-pressure balanced cutterhead machine excavates soil using a screw conveyor. The large cutterhead at the front of the machine slowly advances, cutting into the soil ahead. The excavated material is then conveyed through the screw conveyor and collected in the muck bucket. A lightweight 15 kg/m rail system is installed inside the tunnel to transport the muck buckets along the track, which are later lifted by a crane to the surface for loading onto trucks and final transportation away from the site.
　　When the cutterhead has already advanced about 10 meters into the soil, grouting should be initiated to reduce the resistance of the soil against the pipeline. The grouting pressure should be maintained at around 0.3 MPa, and the grout volume should be controlled at approximately 1.5 times the theoretically calculated value. During grouting, the procedure "advance first, then inject; inject continuously as you advance" must be strictly followed.
　　2.4.2 Measurement
　　Install the laser theodolite: Position the laser theodolite between the two sets of jacks, 1 meter away from the back wall. Ensure the theodolite is aligned along the centerline of the pipeline. When installing, first mount the leveling instrument bracket, which should be constructed using Φ40mm steel pipes and 3mm-thick steel plates. The front control point for the instrument will be established by transferring the pipeline’s center point—from the ground-based theodolite—onto the theodolite bracket inside the excavation pit. Before starting the pipe jacking process, set up settlement monitoring points based on the pipeline’s location and the surrounding structures. Finally, record all measurement results meticulously.
　　The measurement guidance system utilizes a J-2 laser theodolite, which projects a laser beam onto a receiver target positioned at the center of the cutterhead to precisely locate the machine head. Inside the machine compartment, an inclinometer is also installed to monitor the longitudinal and horizontal alignment as well as the radial deflection of the tunneling machine’s cutterhead. Correction adjustments are made based on the observed trends in pipe movement, with corrective values calculated using data from the instruments. These proactive corrections ensure that the jacking process meets the highest quality standards.
　　2.4.3 Operation During Jacking
　　The jacking operation personnel within the jurisdiction are qualified and possess considerable experience. During the jacking process, they regularly plot random jacking curves, randomly measure and verify alignment deviation errors, and adjust the jacking speed as needed to facilitate corrective actions.
　　During the jacking process, the controlled earth pressure closely matches the theoretically calculated values. Typically, we maintain the earth pressure between active and passive pressures, keeping it around 100 kPa in practical applications. The cutterhead must apply an earth pressure lower than the passive pressure ahead of the cutter disc but higher than the active pressure directly in front of the disc. Throughout operation, carefully monitor changes in earth pressure while continuously observing the cutterhead motor current. If the forward earth pressure exceeds the set value—indicated by an increase in the cutterhead motor current—the system triggers an immediate shutdown. For the D2600mm machine equipped with four cutterhead motors, each is designed to operate at 90% of its rated current (22 kW / 37 A). Should the forward cutting torque surpass the design limit, the electrical circuit breaker will instantly trip, halting both the cutterhead’s excavation and the main jacking cylinder’s propulsion. To mitigate this situation, reducing the jacking speed can help ease the forward torque and decrease the cutterhead load, allowing the breaker to reset automatically. Once the conditions return to normal, the rear hydraulic station resumes the jacking process.
　　When the pipe jacking back wall shifts or becomes uneven, it can lead to excessive deviation in the pipeline axis, causing the line of action of the jacking force to shift and resulting in pipeline misalignment. If the pipeline deviation becomes too severe, it may cause bending of the pipeline, leakage at the joints, or even damage to the pipe sections that cannot be repaired.