In the execution of microtunnelling and pipe jacking projects, strict control of dimensional tolerances in concrete jacking pipes is not a mere documentary formality, but the absolute boundary between drilling success and catastrophic collapse inside the tunnel. Unlike open-trench pipelines, where minor geometric deviations can be absorbed by the bedding or the joint clearance itself, a jacked pipe is subjected to brutal axial forces, often exceeding thousands of tonnes.
For quality managers and precast plant supervisors, mastering the metrology of these elements is vital. Any imperfection in the flatness of the joint faces, deviations in wall thickness, or eccentricities in the elastomeric seal will invariably translate into uncontrolled stress peaks. In this article, part of our technical series accessible from our technical blog, we deeply analyse the allowable margins of error, the phenomenology of fractures, and above all, how the quality and rigidity of the moulding equipment determine the viability of the final pipeline from minute one.
Why dimensional tolerances are critical in pipe jacking
The pipe jacking system relies on the transfer of colossal compressive force from the main thrust station (and intermediate jacking stations) through the entire pipe string to the TBM or shield. From a structural engineering perspective, this string acts as a continuous column subjected to flexural compression due to slight deviations in curved alignment and skin friction from the soil.
For the transmission of these loads to be uniform and safe, the contact surface between pipe and pipe must be flawless. If geometric requirements are not strictly met at the precast plant, load transfer ceases to be homogeneous. Stresses are no longer distributed across the entire concrete section and concentrate at specific points (areas of premature contact). It is in this scenario that poor quality control in the moulding phase triggers irrecoverable structural failures in civil works, causing unacceptable delays and cost overruns that fall directly on the precast manufacturer.
Common geometric deviations and their operational consequences
Concrete, due to its plastic nature during pouring and its shrinkage during curing, is a complex material to master within millimetric tolerances. Without a mould designed to absorb these variables, the final piece will exhibit deviations. Below, we break down the most critical errors that metrology must closely monitor in microtunnelling pipes.
Failures in parallelism and perpendicularity of pipe ends (Spalling)
The perpendicularity of the front faces (joint ends) relative to the longitudinal axis of the pipe, and the absolute parallelism between both faces, is the strictest metrological requirement. If a face presents a slight angular deviation (even of a few millimetres), upon coming into contact with the next pipe during jacking, the faces will not sit flat against each other.
The direct result of poor parallelism is the dreaded spalling. When pushed by hydraulic jacks, the force is transferred through the pressure ring (wooden packer or distribution board), but this compressible material has a limit. If the angular deviation is excessive, the pressure bursts the concrete cover at the pipe edges. To prevent this at the source, it is fundamental to apply proper structural design criteria to the manufacturing equipment, ensuring the mould bases and headers remain mathematically orthogonal under load.
Defects in wall thickness and concentricity
The concrete wall thickness not only guarantees the load-bearing capacity but also ensures the minimum cover over the steel reinforcement, protecting it from corrosion by wastewater or the groundwater environment. A deviation in concentricity means the pipe has one side thicker than the opposite.
During jacking, an eccentric pipe will cause the thrust column to tend to deviate from the planned trajectory (banana effect). Furthermore, the side with reduced thickness becomes a structural weak point against lateral soil forces and axial compression. This defect usually occurs when the inner core of the mould is not perfectly centred and anchored relative to the outer jacket, allowing oscillations during concrete densification.
Mandatory precision in joints and steel bands
A jacking pipe must not only withstand thrust; it must be absolutely watertight under high hydrostatic pressures, both from the outside (water table) and the inside (transported fluid). For this purpose, steel collars or bands are used at the ends (spigot-and-socket system) along with high-compression elastomeric seals.
Tolerances in this area are usually measured in tenths of a millimetre. The rebate that houses the steel band must be preformed in the concrete with extreme precision. If the base ring or the top ring of the mould suffer deformations or wear, the steel collar will be loose or off-centre. A poorly positioned collar will bite and shear the rubber gasket during coupling in the tunnel, causing leaks that are impossible to repair from the outside.
How does mould rigidity influence the final tolerance of the precast?
No internal quality protocol can compensate for the physical deficiencies of a poor mould. A precaster’s ability to deliver a batch of pipes compliant with regulations depends entirely on the mechanical strength and geometric indeformability of the pipe jacking moulds.
The threat of extreme vibration and hydrostatic pressure
Manufacturing these pieces requires very high-strength concrete, usually with a very low water-cement ratio, which demands massive compaction energy. The mould is equipped with high-frequency vibrators, either external or anchored to the core, subjecting the steel plate to extreme fatigue.
Simultaneously, a vertical column of fresh concrete 3 or 4 metres high generates tremendous hydrostatic pressure against the base and walls of the formwork. If the mould flexes, bulges, or yields under these forces, the resulting pipe will be deformed. The outer jacket shell, annular reinforcements, and longitudinal ribs must be calculated not only to contain the concrete but to maintain nanometric immobility during the most violent vibration process.
Custom moulds vs Modular moulds: The risk of adaptations
In the precast market, there is a tendency to try and reuse equipment for multiple purposes. While this is valid for conventional drainage pipes without major structural commitments, in pipe jacking, it represents an unacceptable technical risk.
Modular or variable-length equipment usually relies on bolts, spacers, and false joints to change its configuration. Under the brutal vibration necessary for jacking concrete, these bolted connections tend to generate undesirable clearances. Moreover, a standard mould rarely perfectly matches the specific steel collar or joint anchoring system required by each specification.
The differential advantage of Industrias Relente is that it manufactures bespoke solutions according to the client’s actual drawings, measurements, handling, and conditions. Compared to modular or standard moulds, the great technical advantage is that there is no need to adapt the piece to the mould: the formwork is manufactured directly for the exact geometry required by the project. In most highly demanding cases, this accelerates production start-up and avoids costly subsequent adjustments in the plant, allowing the operator to focus on concrete quality rather than fighting a misaligned mould. Although no industrial equipment is exempt from rigorous maintenance, starting with monolithic equipment conceived for a single purpose drastically reduces dimensional failure variables.
Control documentation and key geometric requirements for suppliers
Before launching production or auditing a problematic batch, the quality manager must establish a metrological roadmap. Likewise, when ordering new equipment, the industrialist must be clear about the parameters to demand from their metal fabrication shop. Here we present an essential technical control checklist:
- Inner and outer diameter tolerance: Check ovality and mean diameter at three different points along the vertical axis.
- Effective length tolerance: Verification of the dimension that will determine the actual advance of the TBM for each jacked pipe.
- Flatness and parallelism of joint ends: Use of precision laser levels to ensure that the deviation does not exceed the fraction of a millimetre permitted by the current regulations applicable to the project.
- Machining of base and top rings: Ensure that the tooling responsible for forming the spigot/socket joints comes from laser cutting or high-tolerance CNC machining, without heat distortion from welding.
- Wall thickness: Control of concentricity between the outer jacket and the retractable inner core, guaranteeing the uniformity of the cover mass.
Understanding these requirements is the first step before starting any project. If you wish to equip yourself properly, we invite you to review our concrete box culvert and pipe moulds to streamline technical procedures with your heavy fabrication supplier.
Frequently Asked Questions (FAQ) on quality control in jacked pipes
What tolerance is required on the ends of jacking pipes?
Although it varies depending on local regulations (such as EN 1916 or DWA-A 125 guidelines in Europe) and pipe diameter, generally, the parallelism of the joint faces must not deviate by more than 1 to 2 millimetres in small diameter pipes, and up to 3-4 mm in large interceptors. Exceeding these values compromises the uniform distribution of the main station thrust.
Why do concrete pipes break when pushed with jacks?
Breakage (usually spalling at the edges) occurs mainly due to stress concentration. If the pipe face is not perfectly perpendicular to the thrust direction, contact between pipes occurs at a single point or edge rather than across the entire annular surface. The full force of thousands of tonnes falls on a few square centimetres of concrete, instantaneously exceeding its compressive strength limit and bursting the piece.
How to avoid thickness deviations in microtunnelling pipes?
Prevention must be carried out in the formwork phase. It is necessary to use a mould where the inner core has an absolutely rigid top and bottom centring and anchoring system. In addition, the pouring of the concrete must be perimetral and symmetrical, and the vibration must be applied evenly to prevent the hydraulic pressure of the concrete from pushing the core to one side before the mixture sets.
Can these precisions be achieved by adapting a standard modular mould?
Technically it is possible but highly inefficient and risky. Modular adaptations suffer under high-frequency vibration. Bolted spacers can generate clearances or concrete burrs, and it is almost impossible to maintain perfect parallelism after several cycles of continuous demoulding. Manufacturing custom moulds according to specific geometry remains the only sustained operational guarantee in the long term.
Contact and technical advice for your project
Compliance with dimensional tolerances in highly demanding precast products is not the result of chance, but of a robust engineering strategy at the base of the production chain: the mould. If your plant is facing problems with rejected pieces on site, or is about to bid for an interceptor using a microtunnelling machine and cannot afford geometric failures, you need specialised equipment.
As experts in heavy steel fabrication solutions and custom moulds, at Industrias Relente we work from drawings to eliminate uncertainties. Our technical team will review the characteristics of your concrete, the required joint, and your compaction system to guarantee impeccable metrology piece after piece. Do not hesitate to consult our engineers to manufacture a mould adapted to the actual handling of your facility, maximising pipe safety and jacking success.