Tunnel As-built
Tunnel As-built measures a tunnel, drive or adit from a mesh or point cloud of it against its design profile along a centreline. For each interval of chainage it gives the overbreak and underbreak, and maps them on the scan. It also screens the stress around the opening for spalling, rates how each discontinuity set lies to the drive, gives Q support guidance for a geotechnical domain, and can make the design surface as a mesh.
Running it
- Bring in the tunnel's centreline as a polyline: drawn, or imported from the survey. It can run along the tunnel's axis or along its invert (set Centreline above invert to match).
- Select the scan — a mesh or point cloud — in the Data tree.
- Choose Attributes & Analysis → Tunnel As-built... on its menu and fill in the dialog.
| Field | Description |
|---|---|
| Centreline (polyline) | The project polyline along the tunnel. Chainage is measured along it from its first point. |
| Shape | Circular (diameter = width), D-shaped (arched roof) (vertical walls and a semicircular crown, flat invert), Horseshoe (a semicircular crown on walls that are arcs of twice its radius, flat invert) or Rectangular. Default D-shaped. |
| Width (m), Height (m) | The design opening. A D-shape must be at least half as high as it is wide; a horseshoe higher than half its width and less than 1.37 times it. Default 5 m each. |
| Centreline above invert (m) | Where the polyline runs: 0 along the invert centre (the default); half the height along the axis. |
| Interval (m) | The chainage interval of the table and the design surface's rings. Default 1 m. |
| Off the design, at most (m) | Points further from the design profile than this are not taken as the tunnel (the portal's terrain, other workings). Default 1.5 m. |
| Geotechnical domain | A geotechnical domain for the rock's UCS (from its material) and for Q support guidance; or (none: use the UCS below). |
| UCS (MPa) | The rock's strength when no domain is chosen. Default 100 MPa. |
| Make the design surface | Also add the design profile swept along the centreline as a mesh, centreline design. Ticked the first time in a session. |
The per-set drive ratings and the wedge table use the project Sets only (orientation folders do not count, unlike Slope Stability); with fewer than three Sets there is no wedge table. A run records the Sets it read, and is marked stale in Analysis Runs when one changes.
Results
Layers on the scan:
| Layer | What it holds |
|---|---|
| Chainage | m along the centreline. |
| Overbreak | m from the design profile, measured in the plane across the centreline: positive outside the design (overbreak), negative inside it (underbreak). Shown when the run ends. |
| Profile Angle | Degrees about the profile's centre: 0 the crown, 90 the right wall (looking along the drive), −90 the left, ±180 the invert. |
| Boundary Stress | MPa: the tangential stress at the excavation boundary from the active stress tensor (see below). |
| Boundary Stress / UCS | The same over the rock's UCS. Spalling starts at about 0.4. |
Points away from the tunnel have no value.
The table, added to an open report, has a row per interval:
- Points and Covered: how many points fell in it and the share of the profile they cover.
- Overbreak and Underbreak (m²): the cross-sectional area outside and inside the design, from the median deviation in each of 72 sectors round the profile times the sector's length of perimeter. Max over and Max under: the largest single deviations.
- σθ max and Spalling: the largest boundary stress and the depth of spalling it gives.
- One column per project Set: how the set lies to the drive.
Below the table are the overbreak and underbreak volumes, the stress used, Q support guidance, and, with three project Sets or more, a table of structurally controlled wedges: for each direction the drive takes, every three sets, and the roof and each wall, the largest wedge's apex height, length along the drive, volume, weight, failure mode and factor of safety, the least stable first. Wedges the rock holds in place are left out. A wedge that would run further along the drive than three times its span (two sets nearly parallel in strike make such slivers) is scaled down to that length and marked *. The messages panel gives the totals, the worst overbreak, the peak boundary stress and spalling depth, each set's rating at the start of the drive, and the Q guidance. Each run is recorded in Analysis Runs, from where it can be run again.
Method
Sections. Each point is placed at the nearest point of the centreline: its chainage, and its position across the tunnel — u to the right of the drive (horizontal), v up. Its deviation is its signed distance from the design profile in that plane.
Stress around the opening. At each interval the active stress tensor is taken at the profile's centre — at its depth, if the tensor varies with depth — and resolved in the cross-section: the major and minor in-plane stresses p and q. Around a circular opening (Kirsch), the tangential stress on the boundary at an angle θ from the direction of p is
largest, 3p − q, at right angles to p. The opening is taken as a circle of the same area, radius a. The depth of spalling in brittle rock (Martin, Kaiser & McCreath 1999) is
and none below σθ max / σci ≈ 0.41.
Sets against the drive (Bieniawski 1989, the RMR adjustment for tunnels), from each set's mean plane and the drive's direction in the interval:
| Strike to the axis | Dip | Driving with the dip | Against the dip |
|---|---|---|---|
| Perpendicular (60–90°) | 45–90° | Very favourable | Fair |
| Perpendicular (60–90°) | 20–45° | Favourable | Unfavourable |
| Parallel (0–30°) | 45–90° | Very unfavourable | Very unfavourable |
| Parallel (0–30°) | 20–45° | Fair | Fair |
| Any | 0–20° | Fair | Fair |
Strikes at 30–60° to the axis, which Bieniawski does not class, are taken as fair. The RMR adjustments are 0, −2, −5, −10 and −12 from very favourable to very unfavourable.
Wedges (Hoek & Brown 1980, ch. 9). Three joint planes through a common apex in the rock make a cone; the face of the opening cuts it in a triangle, and the block between is a tetrahedral wedge. The roof is taken as a plane across the tunnel as wide as the span, and each wall as a plane along the tunnel as high as its straight part (half the height above the springline for an arched profile, half the width for a circle). The wedge grows until its triangle is as wide across the tunnel as that face, and runs along the tunnel as far as its planes take it, up to three times the span. Each set's mean plane is used, with the set's own friction angle (30° when it has none). Its weight, at 26 kN/m³, decides the mode by the vector method:
- falls when gravity lifts it off all three planes (FoS 0);
- slides on one plane when gravity presses it onto one and the slide lifts it off the other two: FoS = tan φ × normal force / driving force;
- slides on two planes along their intersection, lifting off the third, with the normal forces on the two from the force balance;
- otherwise it is held: gravity presses it into the rock.
Q support (Barton 2002), for the chosen domain with the profile's width as the span: the equivalent dimension, the largest unsupported span 2·ESR·Q0.4, bolt length 2 + 0.15·B/ESR and the roof support pressure. The support chart's categories are not drawn.
Limitations
- The stress screen treats the opening as a circle; corners of a D-shape or rectangle concentrate stress more.
- Wedges are found on planar roof and wall faces, not on the real (or the arched design) profile, from each set's mean plane; they ignore cohesion, water and support, as a first look that UnWedge-style analysis would refine.
- Chainage is measured along the polyline in 3D; a centreline with gaps or doubled-back sections gives the wrong chainage.
References
- Barton, N. (2002). Some new Q-value correlations to assist in site characterisation and tunnel design. International Journal of Rock Mechanics and Mining Sciences, 39, 185–216.
- Bieniawski, Z. T. (1989). Engineering Rock Mass Classifications. Wiley, New York.
- Hoek, E., & Brown, E. T. (1980). Underground Excavations in Rock. Institution of Mining and Metallurgy, London.
- Kirsch, E. G. (1898). Die Theorie der Elastizität und die Bedürfnisse der Festigkeitslehre. Zeitschrift des Vereines deutscher Ingenieure, 42, 797–807.
- Martin, C. D., Kaiser, P. K., & McCreath, D. R. (1999). Hoek-Brown parameters for predicting the depth of brittle failure around tunnels. Canadian Geotechnical Journal, 36, 136–151.