Chainage is a linear distance measurement along a defined reference alignment (such as a road centreline, tunnel axis, pipeline route, or railway corridor), expressed in metres. It provides a standardised method for locating features, boreholes, test pits, and ground conditions along a project alignment.
Key Concepts
- Origin (Chainage 0+000): The starting point of the alignment, typically where the project begins.
- Increasing Chainage: Distances increase progressively along the alignment in the direction of design/construction.
- Format: Commonly written as Ch. 1+250 (meaning 1,250 m from the origin) or simply 1250 m.
- Equations: Where alignments change or overlap, chainage equations (e.g., Ch. 5+000 B = Ch. 4+950 A) are used to handle discontinuities.
Geotechnical Applications
| Application | Description |
|---|---|
| Borehole Location | Each borehole is assigned a chainage and offset (distance left/right of centreline) |
| Ground Profile | Subsurface conditions are plotted against chainage on long sections |
| Material Changes | Transitions between soil/rock units are referenced by chainage |
| Design Recommendations | Geotechnical recommendations are given at specific chainages |
| Construction Control | Earthworks, excavation, and support requirements are specified by chainage range |
Typical Use in Reports
A geotechnical borehole schedule might include:
| Borehole ID | Chainage (m) | Offset (m) | RL (m AHD) | Depth (m) |
|---|---|---|---|---|
| BH01 | 0+250 | -5.0 L | 12.45 | 15.0 |
| BH02 | 0+500 | 0.0 | 11.82 | 18.5 |
| BH03 | 1+050 | +3.0 R | 10.96 | 12.0 |
Where:
- Offset: Perpendicular distance from the centreline (L = left, R = right)
- RL: Reduced Level (elevation) in metres Australian Height Datum
- Chainage: Distance along the alignment from the origin