What chamber pressure and conditioning (EPB/hydro-shield) to use in saturated and mixed soils?

The chamber pressure must balance the sum of the effective soil stress and the pore pressure at the face elevation (p≈σ’+uw), with fine adjustments on site according to earth balance, thrust/torque and auscultation. In low-medium permeability soils (silts/clays and fine-medium sands) EPB is prioritized, conditioning the detritus with foams and polymers to achieve plasticity and sealing; in high permeability soils (clean/gravel sands) and with high NF hydro-shearing/Slurry is preferred, controlling mud density/viscosity and forming a stable filter cake. Indicators of correct operation: stable pressure, balance≈1 and consistent flow rates; in case of leaks or peaks, adjust pressure/reology and perimeter sealing.

How is alignment controlled in real time: laser, gyroscope, total stations and as-built?

Q: How is alignment controlled in real time: laser, gyroscope, total stations and as-built?

Pipe jacking alignment is controlled by an on-board guidance system (laser for straight sections or gyroscope/INS for curved or limited visibility), verified with automatic total stations on a stable topographic network. The cycle is read-correction-verification-recording: the shield receives deviations from the theoretical axis, is corrected with orientation cylinders or correction rings, and externally validated with redundant observations. Horizontal/vertical deviations, slope and curvature (Rmin) are monitored with alarm thresholds. The final control is documented in the as-built (3D axis per ring/span, slopes, thrusts and pressures) and the QA/QC dossier.

At pipe jacking (microtunneling) the alignment is governed by a guiding system (laser or gyroscopic) inside the shield, external topography with automatic total stations and a continuous cycle of correction-verification-registration culminating in the as-built.

1) Operating principle (axis and slope control)

Reading: the target/target of the shield receives the laser beam or the orientation gyro/INS and calculates horizontal/vertical deviations from the theoretical axis.
Correction: the orientation cylinders apply micro-adjustments; if applicable, correction rings in the pipe line.
External verification: total station (or several) observes shield/tail prisms and contrasts with the control grid.
Registrationare recorded thrusts, torque, pressures, deflections and consolidate in the follow-up model and the as-built.

2) Laser vs. gyroscope (when to use each)

Laser (classic in straight sections)
Advantages: high accuracy, continuous reading, low cost.
Limitations: line of sight (dust, slurry mist), sensitivity to vibrations y tight curves.
Good practices: stable topographic bases, thermal compensation, daily collimation and reading quality filters.
Gyroscope/INS (ideal with curvature, long lengths or limited visibility)
Advantages: not dependent on line of sight, robust to particulates and pressure changes.
Limitations: drift long-term; requires recalibrations and periodic tethering to the external network.
Best practices: recalibration stops by forward milestones, fusion algorithms (gyro + total station).

Quick decision rule: straight and clean → laser; curved, long or difficult to see → gyro (or laser+gyro hybrid).

3) Automatic total stations (external control)

Control networkClosed polygonals and bases protected against vibration and traffic.
Observationrobotic mode with prisms in shield/tail; redundancy from two seasons when the geometry allows it.
Frequency: readings continuous or by cadences (e.g., every 30-60 s or per meter excavated).
Alarms: thresholds of Δaxis/Δslope y stop rules for inspection and correction.

4) Monitoring parameters and tolerances

Horizontal/vertical deviation (mm), tilt/swivel angle (mrad), pending (mm/m or ‰).
Tolerancesas defined in the project; as order of magnitude in microtunneling mm-cm per section, with stricter limits in gravity sewers.
Curvature (Rmin)is controlled by smoothed paths and thrust sequence; abrupt corrections are avoided.

5) As-built and QA/QC

Capture3D axis per ring/span, relevant slopes, thrusts and pressures.
Delivery: flat as-built (DWG/LandXML), lists of achieved tolerances and quality dossier.
Traceabilitydata signed by the surveyor and tunnel manager; documented recalibration milestones.

6) Typical mistakes and how to avoid them

Temperature or vibration drift → thermal compensations, anti-vibration mounts, reading filters.
Misalignment due to weak network → reinforcement of bases, redundant staking, closures, and least squares adjustment.
Loss of laser visibility → line cleaning, backup gyro, and exterior mooring checkpoints.

7) What to ask the contractor before starting

Guidance plan (laser, gyro or hybrid) with nominal accuracy and recalibration protocols.
Topographic plan (network, stations, redundancy, frequencies and alarms).
Data formatting and as-built (deliverables, coordinates, systems and versioning).
Tolerance matrix by tranche and acceptance criteria.

For more information on guidance system selection criteria and accuracy, it is useful to read the report by ITAtech about Guidance Systems for TBMs (alignment, sensors and best practices).

Next step: If you need to check your routing and tolerances, our team of Technical assistance and engineering can define the guidance and control plan according to your DN/L/Rmin and geotechnics; if you already have plans and soundings, request a quotation.