137. Describe tunnel alignment setting‑out methods.

Introduction

Setting out the precise alignment of a tunnel is a critical surveying task that ensures the underground excavation follows the designed horizontal and vertical path. Accurate alignment prevents costly deviations, maintains structural integrity, and guarantees that the tunnel meets its intended connection points, gradients, and clearances. Describe tunnel alignment setting‑out methods.

Key Considerations

• Control Network: A robust network of surface benchmarks and underground reference points.

• Datum and Coordinates: Consistent datum for elevation and a common coordinate system for horizontal positioning.

• Instrumentation: Optical instruments, gyroscopic tools, or inertial guidance systems depending on method and tunnel length.

• Redundancy: Multiple checks and overlapping methods to guard against single-point failures.

Surface Control and Underground Transfer

1. Establish Surface Control Points

   • Lay out a series of accurately surveyed benchmarks along the tunnel’s centreline on surface.

   • Record precise coordinates (Easting, Northing, Elevation) for each.

2. Access Shaft or Portal Reference Transfer

   • Use a plumb line down a vertical shaft or an optical tunnel to transfer surface control into the underground environment.

   • Install permanent underground monuments at shaft base or portal for repeated reference.

Optical String Line Method

Useful for short drives and pilot tunnels:

1. String Line Setup

   • Stretch a high-tension wire between two fixed underground monuments marking the centreline.

   • Adjust sag and tension so the wire is straight and level.

2. Sight Through Optical Alignment

   • Mount an optical level or theodolite co-linear with the string line.

   • Ensure the crosshair coincides with the string at regular stations.

3. Marking Excavation Points

   • From the string line, measure perpendicular offsets and establish points on the tunnel face.

   • Lay out guide marks for drill-and-blast or mechanical excavation.

Gyrotheodolite Method

Ideal for longer tunnels or those lacking line-of-sight:

1. Gyroscope Orientation

   • Spin up the gyrotheodolite at a stable underground bench point.

   • Determine true north based on Earth’s rotation, independent of magnetic interference.

2. Traverse between Stations

   • Set the theodolite on successive underground stations.

   • Record bearings to transfer the initial azimuth along the tunnel axis with minimal cumulative error.

3. Compute Alignment

   • Use recorded bearings and distances to calculate coordinate changes.

   • Adjust heading and grade continuously as excavation proceeds.

Inertial and Laser Guidance Systems

For mechanized tunneling (TBMs):

• Inertial Measurement Units (IMUs) track the machine’s orientation in 3D, guiding the cutter head automatically.

• Laser Targeting: A laser transmitter at the face projects beams toward receivers on the TBM shield; software computes position and issues correction commands.

• Data Logging: Continuous recording of alignment parameters aids in post-drive verification and reporting.

Shaft-to-Shaft and Pilot Drive Connections

When two headings must meet precisely:

1. Surface Tie-In

   • Calculate expected meeting point coordinates for both drives.

   • Ensure control points on each side share a common reference.

2. Mid-Drive Checks

   • Periodically verify heading and grade by sighting back to initial monuments or using gyro reference.

   • Compare projected coordinates to design to detect deviation early.

3. Final Breakthrough

   • Conduct high-precision checks (e.g., refined optical sightings or rapid gyro bursts) in the last few meters.

   • Communicate adjustments in real time to the excavation team.

Best Practices and Tips

• Instrument Calibration: Check and adjust levels, theodolites, and gyros before each shift.

• Environmental Controls: Minimize temperature and vibration fluctuations around instruments.

• Redundant Measurements: Always back up critical readings with an independent method.

• Documentation: Maintain clear field logs of bearings, distances, offsets, and any corrections applied.

• Team Coordination: Keep surveyors, machine operators, and site engineers in constant communication to implement alignment tweaks swiftly.