When a precast manufacturer who traditionally operates in open-trench projects is awarded the supply of pipes for a microtunnelling project, the first logistical and technical unknown falls upon the formwork. The immediate question is usually: what type of mould do I need for jacking pipes with a steel collar and whether it is possible to adapt the existing machinery in the plant. The short answer is no: pipe jacking requires a completely different structural approach due to the extremely high axial pressures the pipe will have to withstand underground.
The choice of machinery does not depend solely on the pipe diameter, but on the construction details dictated by the civil works. Factors such as the joint design, wall thickness, the inclusion of elastomeric sealing gaskets, and the need for special thrust sections determine the engineering behind the formwork. In this article, we analyse the different types of moulds, how the joint defines the equipment design, and what technical data is essential before requesting a quote from your metal fabrication supplier.
What is the difference between a normal pipe mould and a jacking pipe mould?
In a traditional open-trench installation, the concrete pipe is fundamentally designed to withstand vertical static loads (the weight of the ground and surface traffic). Its moulds, therefore, can allow for greater clearances and reduced wall thicknesses. However, in the pipe jacking or microtunnelling technique, the pipe acts as a horizontal structural column that is pushed by hydraulic jacks from the launch shaft.
This radical operational difference alters the formwork configuration in three key aspects:
- Thickness of the mould’s steel shell: Microtunnelling moulds require significantly thicker and stiffened steel shells to prevent any bulging or deformation during the pouring and vibration of the concrete. A millimetric deformation in the outer diameter would drastically increase friction against the ground during excavation.
- Absolute perpendicularity: The bases (bottom rings) and headers (top rings) of the mould must be machined with absolute precision. If the extreme faces of the pipe are not perfectly parallel, the thrust force of the jacks will not be distributed evenly, causing spalling or the explosive failure of the concrete. It is here that the dimensional tolerances of the pipe mark the boundary between the success and failure of the project.
- Reinforcement loads: The steel reinforcement cages of a jacking pipe are much denser and heavier. The mould must have robust reinforcement centring systems that prevent displacements during concrete compaction.
Types of pipe jacking moulds based on the structural joint
The main criterion for classifying moulds intended for thrust pipes is the morphology of the joint between pipes. Depending on whether the project specifies a fully concrete closure or a perimeter steel band, the formwork matrix changes completely.
Moulds for pipes with concrete joints (Spigot and Socket)
For shorter drives or less aggressive groundwater conditions, some projects prescribe concrete joints that integrate the housing for the O-ring seal (EPDM rubber) directly into the pipe mass. In these cases, the mould requires highly complex base and top forming rings.
The base ring usually forms the socket (the female part), while the top heading piece, which presses the fresh concrete, shapes the spigot end and creates the exact groove where the elastomeric gasket and the pressure compensating ring (usually chipboard or resin) will sit. The demoulding of these areas requires a millimetric conical design in the metal forming pieces to ensure that, upon stripping the formwork, no dragging or micro-cracks occur in the concrete, especially on the critical edges that will receive the primary thrust.
Moulds adapted for steel collar integration
In medium and long-distance microtunnelling projects, the industry standard is the use of a steel collar embedded in one end of the concrete pipe. This creates a robust, articulated, and watertight joint. But, how is the steel collar held in the concrete mould?
The mould designed for collars incorporates a machined step, generally in the base (bottom) ring. The prefabricated metal collar sits physically on this recess before lowering the inner and outer shells of the formwork. The outer metal ring embraces the collar with a very tight tolerance, ensuring it remains perfectly aligned and fixed via mechanical pressure and clamping stops.
When pouring and compacting the concrete, it envelops the anchors or studs welded to the inner face of the collar, making the metal collar and the concrete pipe become a single monolithic piece. This type of mould demands that the ring holding the collar be a high-precision machined component to prevent concrete grout leaks that could stain the outer face of the steel or create irregular burrs that would hinder assembly on the civil works site.
Special formwork in a microtunnelling project
The supply of pipes for pipe-jacked tunnels does not only consist of continuous standard pipes. Along the alignment, ground friction increases until it exceeds the capacity of the main shaft jacks, making intermediate jacking stations (IJS) essential.
Moulds for intermediate jacking stations
These elements divide the pipe string into independent sections that can be pushed sequentially. To manufacture these components, two complementary pieces are required (a primary lead pipe and a trailing pipe), which house much longer special steel shells and hydraulic cylinders inside.
The formwork for intermediate stations must have interchangeable parts or be specifically manufactured to generate the wide recess where the sliding steel shield will be housed. This forces the use of internal supplements and special stops on the formwork machinery. A precast plant undertaking a complete project will need, in addition to the standard line moulds, to acquire modified moulds for these intermediate stations, as well as for the special entry and exit pipes of the shaft.
Technical and operational criteria for choosing the demoulding system
Beyond the concrete geometry, the choice of mould must consider how the precast plant will operate in terms of rotation and cycle times. There are different opening and closing configurations for pipe jacking moulds:
- Mechanical / manual opening via wedges or bolts: These are economically accessible but require skilled operators who dedicate time to hermetically close and seal every seam of the shells. They are viable for low production runs or very unique pieces.
- Hydraulic closing and opening: These incorporate hydraulic cylinders that open and close the half-shells (outer shells) and contract the inner core. They drastically reduce demoulding time and avoid impacts or jerks from the overhead crane that could damage the green concrete.
- Quick-release conical systems: These feature a slight technical conicity that allows, alongside an optimal vibration system, the piece (or the mould) to be extracted via straight lifting almost immediately, ideal for dry or semi-dry concrete. For self-compacting concrete (SCC), the mould usually remains closed until setting.
Defining the most suitable system requires reviewing the mechanical and design criteria alongside the availability of overhead cranes and the rheological characteristics of the concrete to be used.
When custom manufacturing outweighs standardised moulds
One of the most costly mistakes in the precast industry is attempting to use or adapt generic modular formwork for jacking pipes. The geometric variability of tunnelling projects (collar thicknesses, bevel angles in the joints to allow for curved alignments, integrated bentonite injection ports) makes a standard mould inefficient.
This is where the specialisation of an experienced industrial metal fabrication manufacturer comes into play. At Industrias Relente, we know that the greatest technical and productive advantage lies in custom manufacturing according to the client’s drawings, measurements, handling, and real conditions.
Compared to modular moulds that promise flexibility but suffer from unwanted clearances, the advantage of custom work is that there is no need to adapt the final piece to the formwork using patches or supplements: the steel matrix is manufactured directly for the exact geometry demanded by the project drawings. Although there are no promises of unrealistic fixed deadlines or absolute generic guarantees, this real adaptation of the machinery to the pipe’s requirements accelerates production start-up and drastically reduces the need for adjustments, leak repairs, and rework once the project begins. Our experience in this custom concrete piece manufacturing translates into robust, stable, and highly geometrically precise heavy formwork.
Data the buyer must prepare to request a quote
If you are a purchasing manager or production director and you are going to request the design and manufacture of a mould for your next pipe jacking project, the supplier will require a much larger battery of technical data than is necessary for a conventional drainage pipe. Prepare the following information:
- Detailed civil project drawings: Final dimensions demanded for the pipe, with strict specification of the tolerances permitted by the site management.
- Steel collar drawings: Steel plate thickness, total insertion length, and concrete anchoring system (Nelson studs, claws, etc.).
- Type and dimensions of the rubber gasket: The machined recess of the mould depends on the volumetric compression rate of the specified EPDM rubber.
- Concreting technology: Indicate whether the plant will use dry, fluid, or self-compacting concrete (SCC), as this defines whether the mould requires extreme watertightness in its closures to prevent grout leaks.
- Internal logistics and lifting: Maximum load capacity of the existing overhead crane in the production facility and clear height under the hook. The mould will be structurally designed taking these limitations into account, safely locating the lifting and turning points.
Common mistakes when ordering or comparing suppliers
During the procurement phase for heavy machinery, it is common to fall into omissions that delay the start of pipe manufacturing.
The primary error is omitting the holes and recesses for bentonite injection. Jacking pipes require through-holes in the concrete (typically 3 or 4 per section) to lubricate the advance through the ground. The mould must include machined retractable cores or mandrels to form these holes cleanly. If they are not requested from the start, it will be necessary to drill the already set pipe using core bits, a slow, dirty process that can cut the reinforcement.
Another common failure is underestimating vibration. If the mould is flimsy and a high-frequency external vibration system is attached, the shell welds will suffer premature fatigue. A good custom mould incorporates perimeter reinforcements (structural ribs) calculated to dissipate the vibration into the concrete without destroying the steel.
Practical checklist before commissioning the mould design
- [ ] Has the compatibility of the mould’s outer diameter with the geometric guidelines of the tunnel boring machine or project shield been verified?
- [ ] Have the exact drawings of the bottom ring been provided to ensure the steel collar fits without clearances?
- [ ] Has it been defined if the pipe requires specific bevels to adapt to curved tunnel sections?
- [ ] Have the exact number of bases and heading rings been calculated and requested based on the curing time and demanded daily production?
- [ ] Are the insertion cores for the bentonite injection ports and the lifting sockets of the finished pipe defined?
Frequently Asked Questions (FAQ)
What types of joints do jacking pipes have?
Mainly two: embedded concrete joints (spigot and socket) with a pressure-compensating ring, or the most common joint over long distances, consisting of an external steel collar that is anchored to the end of the pipe during setting. Both feature elastomeric sealing gaskets.
Can an adapted traditional pipe mould be used to make jacking pipes?
Rarely. The millimetric tolerances in the parallelism of the faces (necessary for the axial distribution of forces) and the need to integrate and machine precise housings for steel collars mean that attempting to retrofit old moulds results in watertightness issues, geometric deviations, and pipe rejection on site.
How does the intermediate jacking station affect the moulds I must buy?
It requires supplementary moulds. You will have to manufacture special pipes (lead pipes and trailing pipes) that incorporate internal steel housings for additional hydraulic cylinders, being of different dimensions from the standard line jacking pipe.
Why is a custom mould better than a modular one for this type of pipe?
Pipe jacking is a unique engineering work. The designs of the collars, thicknesses, and radii vary in each project. A custom mould manufactured according to the real drawings avoids having to adapt the reinforcements or tolerate clearances that a standardised or modular mould inevitably introduces. This improves quality control and production speed.
Conclusion: Total adaptation to the site drawings to ensure success
Choosing the right equipment is the critical step that will dictate the profitability of precast supply in a tunnelling project. Understanding whether the alignment demands steel joints, complex compensating rings, or intermediate stations will allow you to define the formwork engineering accurately. Instead of adapting your process to standardised machines, the safest path is to invest in precision metal fabrication geared towards the final project.
At Industrias Relente, we are at your disposal to evaluate the required geometry, analyse your plant’s operations, and study your project drawings, designing the structural moulds that will allow you to manufacture the pipe with absolute reliability.