In the execution of underground works using the pushing or pipe jacking technique, the concrete pipe is the critical structural element that bears the entire load of the machinery. Unlike open-trench pipelines, where the pipe only supports the weight of the overlying soil, in pipe jacking, the piece is subjected to massive axial pressures that can exceed a thousand tonnes of direct hydraulic thrust. For this reason, pipe jacking mould design is not simple formwork, but a mechanical engineering feat that determines the multimillion-dollar success or failure of a civil engineering project.
If the geometry of the concrete deviates, the load distribution is no longer uniform. A pipe whose faces are not strictly parallel will concentrate hundreds of tonnes of force into a few square centimetres, causing a catastrophic structural failure underground, stopping the tunnel boring machine, and paralysing the project. Absolute control over these geometries stems exclusively from the rigidity and accuracy of the metal mould used in the precast plants.
Unforgiving pressure: The problem solved by pipe jacking mould design
When production engineers plan the manufacturing of segments or jacking pipes, the number one technical priority is to guarantee the concentric transmission of force along the entire pushed pipeline column. The jacking stations push against the last pipe introduced into the shaft, which then transfers the force to the preceding one through the joints.
For the project to proceed without incident, it is imperative to have pieces that fit together perfectly. The only way to ensure that each reinforced concrete pipe has the theoretical geometry designed is by using excellent pipe jacking moulds that do not yield to the internal pressure of fluid concrete or mechanical vibration. An inadequate mould design causes problems such as:
- Wedging: End faces not perpendicular to the longitudinal axis.
- Irregular friction: Ovalised outer diameters that rub more on one side against the excavated ground, slowing down the advance.
- Joint leaks: Deformation in the socket area that prevents the correct seating of the elastomeric seal or the steel collar (band).
Non-negotiable dimensional tolerances in manufacturing
The technical specifications of a microtunnelling project include very restrictive tolerances. To meet them, the team drafting the mould manufacturing brief must demand millimetric adjustments from the heavy steel fabrication workshop.
Perpendicularity and flatness of the end faces (joint faces)
This is, without a doubt, the most demanding tolerance. The male face (spigot) and the female face (socket) must be perfectly flat and strictly perpendicular to the main axis of the cylinder. If a pipe has a slight inclination on its contact face (even a couple of millimetres), pushing it with hydraulic cylinders will create an eccentric force and a bending moment, causing the concrete to break due to transverse tension and leading to edge spalling.
To achieve this, the design of the mould base and the top crowning ring must be machined with high precision, ensuring a hermetic and perfectly square seating relative to the outer shell and the inner core.
Diameter, eccentricity, and constant wall thickness
The thickness of the concrete wall is dictated by the clearance between the inner core and the outer shell of the mould. A concentricity error during assembly or concrete pouring will result in one side of the pipe being thicker than the other. This weakens the perimeter strength of the pipe. The design must incorporate robust centring and anchoring systems that lock both parts (inner core and outer shell), preventing any relative displacement and guaranteeing near-zero eccentricity.
Design criteria for joint areas: Spigot, socket, and steel collars
Joints are the weak points of any pipe jacking pipeline. Unlike a conventional drainage pipe, the joint of a jacking pipe usually integrates a sheet steel collar welded to the reinforcement, a distribution ring or packer (generally made of wood or compressible materials), and elastomeric sealing gaskets.
In the design of the metal mould, the area responsible for forming the socket and housing the metal collar must foresee the exact space and necessary recesses. Furthermore, since many plants precast these pipes using dry or semi-dry concrete with immediate demoulding, the forming profile of the lower spigot must have a microscopically calculated draft angle (release angle) so as not to damage the fresh concrete when extracting the pipe from the mould.
Structural rigidity against concrete vibration
To achieve the high strengths demanded by pipe jacking standards (often very high-strength concrete with a low water/cement ratio), plants use high-frequency external vibration systems or vibrating tables. This process subjects the mould to continuous centrifugal forces and mechanical fatigue.
If the mould plate is too thin, the vibration will cause elastic deformation or “buckling” in the walls of the shell, absorbing the vibrator’s energy instead of transmitting it to the concrete (generating voids and poor compaction) and, worse still, creating a pipe with an undulating surface that will radically increase friction against the ground during excavation. The mould manufacturer must specify high-thickness plates as well as longitudinal and annular reinforcing stiffeners, adapted to the client’s actual drawings and the vibration equipment available in their factory.
The technical debate: Custom moulds vs. Standard modular systems
In the precast industry, there is a temptation to acquire modular moulds adaptable to different lengths or diameters by inserting supplements. However, in the specific case of pipe jacking, this is usually a serious mistake.
Modular systems involve additional bolting, joints, cumulative clearances, and moving parts. Every clearance in a modular system translates into a millimetre of lost perpendicularity in the final pipe. Industrias Relente manufactures custom moulds based on the client’s real drawings, measurements, handling methods, and conditions. The fundamental advantage over modular or standard moulds is that there is no need to adapt the piece to the mould: it is manufactured directly for the required geometry, guaranteeing the monolithic integrity of the equipment. In many complex projects, this accelerates startup at the precast plant and avoids subsequent adjustments during production, not by promising closed deadlines or unattainable absolute guarantees, but based on a solid foundation of engineering applied to metal.
Common mistakes when ordering or comparing suppliers
When evaluating suppliers or drafting the specification for a manufacturing project, purchasing managers often fall into certain traps:
- Prioritising steel weight over geometry: Evaluating quotes solely by “kilos of plate” ignores whether the manufacturer has the necessary bending and machining processes to respect millimetric tolerances in the socket.
- Ignoring the handling and demoulding system: Failing to define how the pipe will be extracted (whether the core is retractable or truncated-conical, whether the outer shell opens in two halves by hinges or wedges) creates production bottlenecks.
- Lack of clear drawings: Requesting quotes with schematic or incomplete drawings regarding the integration of internal reinforcement and bentonite injection ports.
What information to prepare to request an exact quote
For the technical quotation of the mould to be accurate and avoid subsequent cost overruns, the engineering department must provide detailed documentation. The manufacturer not only needs to know the nominal diameter and length of the pipe but also the fine details of the production cycle.
It is essential to provide execution drawings of the pipe (including details of the end face profiles and reinforcement), the filling method (direct pouring or pumping), the type of vibration to be used (external vibrators attached to the mould, internal pokers, or a vibrating table at the base), and the existing handling and lifting equipment (overhead cranes, hoists). To delve into all the documentary requirements, you can consult our guide on the information needed to request a quote, which details the collection of this data step by step.
Relationship with other civil engineering and foundation solutions
Pipe jacking is a highly specialised phase of pipeline and underground civil engineering that requires strict quality control, similar to that demanded in major concrete works and deep drainage infrastructure.
For example, it is common for the jacking station itself to be housed in large shafts or chambers that require their own formwork systems. In this regard, dimensional accuracy technology is shared with the manufacture of other precast concrete metal moulds designed for heavy-duty supports. Similarly, the preparation of the launch and reception shafts often involves the use of heavy machinery, making it crucial to use suitable deep drilling tools to condition the rocky or unstable ground where the reaction wall will rest.
Practical checklist before validating mould drawings
Before authorising the manufacture of the matrix in the workshop, we recommend that plant engineers review the following key points:
- 1. Confirmation of tolerances: Are the ovality, perpendicularity, and straightness tolerances explicitly indicated on the supplier’s drawing?
- 2. Sealing areas: Has the housing for the rubber gasket and the steel collar been designed based on the actual compressed or theoretical cross-section?
- 3. Closing system: Are the pins, wedges, or bolts on the outer shell dimensioned so as not to yield to the hydrostatic pressure of the fluid concrete?
- 4. Anti-vibration reinforcements: Are reinforced mounting plates (tightening plates) included where the motor vibrators will be bolted?
- 5. Extraction and demoulding: Is the draft angle sufficient to remove the inner core without causing destructive friction on the green pipe?
Frequently Asked Questions (FAQ)
What tolerances should a concrete jacking pipe have?
Although it depends on the specific standard of each country (such as EN 1916 or AWWA standards) and the requirements of the particular project, the tolerance for the perpendicularity of the joint faces (end faces) is usually less than a millimetre compared to the theoretical value. Tolerances in the inner and outer diameters must also be strictly controlled to ensure a perfect fit with the steel collar and the rubber gasket without extrusions.
How to prevent pipe breakage during jacking?
To prevent cracks or breakage from pushing underground, perfect alignment of the pipeline is required, along with the use of high-quality compressible packer rings. Crucially, right from the start, the pipes must have been manufactured in a steel mould with exact geometry and non-deformable walls that prevent even the slightest load eccentricity.
What is the recommended steel plate thickness for pipe moulds?
There is no universal thickness, as it depends on the size of the pipe and the vibration equipment used. However, as a general rule, to withstand intense compaction cycles and constant external vibration without suffering dents or elastic deformation, naval or heavy fabrication plates are used with considerable transverse reinforcements. The specific choice will emerge from the structural study during the design phase.
Is a custom mould or a standard modular system safer?
For jacking pipes, a custom mould manufactured precisely to the required geometry is significantly safer. Modular moulds require adaptation pieces and assemblies which, due to their logical factory assembly tolerances, can introduce micro-clearances. During concrete curing and pipe jacking, these clearances translate into millimetric errors that end up concentrating dangerous stresses at the joints of the finished pipe.
Next steps: From theoretical design to real manufacturing
Understanding the design criteria, the metal’s behaviour under vibration, and the distribution of the concrete is the first step in guaranteeing the success of the pipe precasting phase. After compiling the detailed technical documentation, the next step should be direct consultation with experts in heavy steel fabrication and industrial moulds who understand the physics involved in their machinery.
If you are evaluating the formwork needs for your next major underground infrastructure contract, you can learn more by exploring the various technical articles available on our technical blog or, if your technical team already has preliminary outlines, contact us to request a technical study for your project. At Industrias Relente, we will analyse your drawings and working methods to materialise a high-precision mould that meets your actual on-site expectations.