Bench As-built

Bench As-built measures the benches of an open-pit wall from a mesh or point cloud of it. It finds the berm levels and the benches between them. For each bench it measures the height, the face angle actually achieved and the width of the berm at its crest, and it checks each berm against the modified Ritchie criterion. Given a design surface, it maps how far the wall stands in front of the design or behind it, and says how much of each bench is within tolerance.

Before you start

• A loaded mesh or point cloud of the wall, with z up and model units in metres.
• For the design comparison, the design as a mesh, loaded, with its triangles facing out of the rock (towards the pit). Import a design as any mesh format VRGS reads.

Running it

1. Select the scan in the Data tree.
2. Right-click and choose Attributes & Analysis → Bench As-built....
3. Check the settings and press OK.

The dialog

The line at the top names the object and what will be measured.

Benches

Field	Default	Description
Face fit radius (m)	3, or twenty point spacings if more	The radius over which the surface is fitted at each point to find its dip: about a third of a bench height.
A face dips at least (deg)	50	A point whose fitted surface dips at least this is bench face.
A berm dips at most (deg)	20	A point whose fitted surface dips at most this is berm. It must be less than the face dip.
Smallest bench height (m)	3	Two berm levels closer than this are taken as one.

Design

Field	Default	Description
Design surface	(none)	Any other mesh in the project, to compare the wall with. The design must be loaded, and its triangles must face out of the rock.
Tolerance (m)	0.5	Within this of the design counts as on design.
Reach (m)	10	Points further than this from the design get no deviation.

The dialog remembers your settings for the rest of the session.

How it works

1. Face and berm. At every point the surface is fitted over the face fit radius. A point dipping at least the face dip is face, one dipping at most the berm dip is berm, and the rest are neither.
2. Berm levels. The berm points' elevations are binned, and the peaks of that histogram are the berm levels. A peak needs at least 1% of the berm points, and at least 20 of them. Levels closer than the smallest bench height are merged. Each level sits at the mean elevation of the berm points near it. Berm points away from every level, such as a ramp, are left out.
3. Benches. A bench runs from one berm level (its toe) to the next (its crest). Its height is the difference between the two levels. Its face angle is the median dip of its face points.
4. Berm widths. At every berm point, the width is the plan distance to the crest of the face below plus the plan distance to the toe of the face above. The crest is the top 35% of that face and the toe the bottom 35%. Widths over 60 m are not measured.
5. Modified Ritchie. A berm should be at least 0.2 H + 4.5 m wide, H being the height of the bench above it, which is what it has to catch (Ryan & Pryor 2000). Each berm point's margin is its width less that.
6. Design. Each point's signed distance to the nearest triangle of the design, out to the reach. It is positive in front of the design (rock left standing, underbreak) and negative behind it (overbreak).

What it writes

Layer	Values
Bench Element	0 neither (grey), 1 berm, 2 face. Shown when the run finishes.
Bench	The bench a point belongs to, 1 the lowest. A berm point belongs to the bench whose crest it is on.
Bench Face Dip	The dip of the fitted surface at each point, 0–180°; above 90 it overhangs.
Berm Width	The berm width at each measured berm point, m.
Ritchie Margin	The berm width less the modified Ritchie width, m: negative where the berm is too narrow.
Design Deviation	Signed distance to the design, m: positive is rock left in front of the design, negative is overbreak. Only with a design surface; a run without one removes it.

The bench table

With an analysis report open, the run adds a section, Bench as-built: object, with one row per bench, from the bottom up:

Column	Meaning
Bench, Toe (m), Height (m)	The bench, the elevation of its toe, and its height.
Face angle	The median dip of its face.
Berm width (m), Berm P10 (m)	The median and 10th percentile of the widths of the berm at its crest.
Ritchie (m), Meets Ritchie	The modified Ritchie width that berm needs for the bench above, and the share of its berm points that meet it.
Mean deviation (m), Within … m	With a design: the mean signed deviation of the bench's points and the share within tolerance.

The messages panel gives a summary: the number of benches and levels, the range of face angles and berm widths, and the share of the measured berm that meets the Ritchie width. It then gives a line per bench. Without an open report it suggests opening one first.

Each run is recorded under Analysis Runs as Bench as-built, with its settings and the bench table, and can be re-run from there.

Limitations

• Level berms only. Berm levels are horizontal; a ramp's points lie between levels and are left out, so a ramped wall is measured on its level benches only.
• Design as a mesh. There is no DXF or Surpac string import; bring the design in as a mesh.
• No crest line. Backbreak is seen in Design Deviation and in the berm widths; no crest or toe line is drawn.
• Width, not retention. The berm is checked against the Ritchie width only, not against the volume of rock expected to fall onto it.

References

• Ryan, T. M., & Pryor, P. R. (2000). Designing catch benches and interramp slopes. In Hustrulid, W. A., McCarter, M. K., & Van Zyl, D. J. A. (eds.), Slope Stability in Surface Mining. SME, Littleton, Colorado, 27–38.

See also

• Slope Stability — kinematics, factors of safety and probability of failure at the bench, inter-ramp and overall scales.
• Geomechanics
• Analysis Runs