Excavation

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Terrain Excavation

The method used to excavate the terrain is described below.

Excavation Polygons

Excavation polygons are the tools with which material is removed from the terrain.

Two Excavation Polygons

Excavation polygons come in pairs:

1.An inner polygon, usually buried beneath the surface to be excavated. It may be at a fixed depth, or it may be draped on another surface. This polygon may also be above the excavated surface (fill operation), or partly above and partly below (mixed excavate/fill operation).

2.An outer polygon, encloses the inner polygon in plan view. It is draped on the surface to be excavated.

Drawn on Excavated Surface

Both polygons are drawn and edited on the surface to be excavated. This is convenient. But please remember that the inner polygon is actually at some other height.

Outer Encloses Inner

When set up for excavation, the outer polygon fully encloses the inner polygon. Excavation takes place within the plan view area enclosed by the outer polygon.

Inner and outer polygon placement is shown in the diagram below. This diagram also illustrates the batter angle.

Setting up Polygons for Excavation

To describe how excavation takes place, we need to think about assigning inner polygon heights (also termed polygon binding) and batter angles. These concepts also apply to Master Polygons.

Inner Polygon Heights (z-coordinates)

An excavation inner polygon may be thought of as a closed loop. This loop is made up of straight-line sections when seen in plan view. Intersections of these sections are termed polygon vertices.

The heights (z-coordinates) of the polygon may be set up in one of three possible ways:

1.In the individual vertex heights option, each polygon vertex is assigned a separate z-coordinate. Heights of intermediate points are calculated by linear interpolation.

2.In the common vertex height option, all polygon vertices are assigned the same z-coordinate. The whole polygon is then at one common height.

3.The bind to surface option drapes the polygon on to some surface. This means that the polygon's heights at each point are calculated from the surface height at that point.

Batter Angle

This is the angle of a terrain wall formed by excavation. The angle is from horizontal. So an angle of 90 degrees forms a vertical wall, zero degrees (not allowed) forms a horizontal wall. The allowed range for batter angles is 0.1-90 degrees.

Think of joining each point on an inner polygon to a "corresponding" point on the outer polygon by a straight line. The two polygons are at different heights. So this line is at some non-zero angle to the horizontal. This angle is the batter angle. It is illustrated above.

If the inner polygon is above the outer polygon, the batter angle represents a fill operation. Conversely an inner polygon below an outer polygon represents a cut operation. Remember: the outer polygon is always on the terrain surface!

Batter angles may be specified along an inner polygon or an outer polygon. In outer from inner polygon creation mode, batter angles may be set up along the inner polygon only. Conversely in inner from outer mode, batter angles are set up along the outer polygon only.

Batter angles apply to the terrain between the Inner and Outer Polygons.

Polygon Generation Using Batter Angles

Batter angles are used when a second polygon in a pair is generated using an outer from inner or inner from outer option.

Outer from Inner

Different batter angles may be defined for different sections of an existing inner polygon. The corresponding sections of the outer polygon are then created. The wall formed between the inner and outer polygon in each section after excavation will be inclined at the specified batter angles.

Inner from Outer

Different batter angles may be defined for different sections of an existing outer polygon. The corresponding sections of the inner polygon are then created. The wall formed between the inner and outer polygon after excavation will be inclined at the specified batter angles.

Connecting Generated Polygon Segments

Once batter angles are set up for a polygon, the corresponding "pair" polygon can be generated. This generation can lead to the segments of the generated polygon being disconnected.

Think of it this way:

A segment of an inner polygon has a certain batter angle. The next segment has a different batter angle. Both segments are buried at some depth below the flat surface to be excavated.

These two segments will generate two segments of the outer polygon. The two generated segments will not be connected. This is seen in the diagrams below.

Even if the two inner polygon segments have the same batter angle but are inclined to each other (such as the two sides of a rectangle), the resulting projected segments of the outer polygon will be disconnected.

To connect such disconnected segments, three options are offered:

1.Parallel projection. In this option the corresponding end (Point 1 and 2 in Figure below) of each generated segment is projected along the surface, parallel to the source segment in plan view. These projected segments cross (Point 3) to join the disconnected segments.

2.Batter angle transition. In this option using intermediate batter angles connects the two segments. These batter angles "fan out" from the junction of the two source segments. If the two disconnected segments are at the same batter angle, then the batter angle transition zone will use the same one batter angle.

3.Bezier curve. In this option, the two disconnected generated segments are connected by a Bezier curve. This curve is inscribed in the angle formed by three points: the ends of the two segments (Point 1 and Point 2 in Figure below), and the point obtained by parallel projection (Point 3). The shape of this curve is controlled by the Bezier sharpness parameter. Larger values of this parameter lead to sharper curve shapes.

The number of points (i.e. extra polygon segments) in the transition region between two disconnected generated polygon segments is controlled by the average angle step. Larger values of the step parameter lead to fewer transition segments.

How to Set Batter Angles

Here is a good way to generate batter angles for excavation:

1. Specify a single batter angle for a full polygon. For outer from inner mode this is the inner polygon. For inner from outer, it is the outer polygon. These settings are in the Excavation Settings Window.

•First draw the polygon.

•Then use the Define all by angle toolbar icon or Excavate/fill menu option to generate the corresponding outer or inner polygon.

2. Once the corresponding polygon has been generated at the single batter angle, redefine parts of the polygon to batter at different angles. This is done using the Define arc by angle toolbar icon or Excavate/fill menu option.

Batter angle values are successively specified for parts of a polygon. Excavation should be done once the right batter angles are set up.

Excavation Using Polygons

Once Excavation Polygons are set up as described above, the terrain can be excavated. In addition, various excavation constraints can be applied to shape and limit the excavation.

In plan view, the excavated surface is divided into three regions by the excavation polygons. Excavation assigns new heights to the terrain in two of these three regions. The three regions and methods of assigning new heights are:

1. Inside the inner polygon. The terrain heights inside the inner polygon are calculated in one of three ways, depending on the inner polygon binding:

•For individual vertex heights binding, heights of terrain points inside the inner polygon are calculated from the assigned vertex heights. The calculation uses triangulation of polygon vertices to divide the area inside the polygon.

•For common vertex height binding, heights of terrain points inside the inner polygon are assigned the common vertex height value.

•For bind to surface binding, heights of terrain points inside the inner polygon are calculated from the heights of the bound surface.

2. Between inner and outer polygons. Terrain heights between the inner polygon and the outer polygon are calculated from a triangulation. Each triangle is formed with two points on one polygon and one point on the other. These triangles are displayed briefly during excavation.

3. Outside the outer polygon. This one's easy! The terrain outside the outer excavation polygon is not affected by excavation.

Excavated Volume and Mass

The volume of excavated material is calculated after each excavation. This material is logged to the active general log.

Bank (in situ) material is swelled by the swell factor specified in the active general log. Material which was dumped to become part of the terrain surface (i.e. rehandle material) is not swelled if material rehandle is being logged.

Excavated mass is calculated using specific gravity values specified in the active general log.

Excavation can also be fill

Excavation can also be a fill operation (i.e. material is placed on the excavated surface). In this case, material is taken from the active general log. If more material is needed than the log contains, the log contents are allowed to become negative.

Excavation Constraints

These determine the excavation shape and limit its extent. Master Polygons provides the most important constraint.

Master Polygons

Like Excavation Polygons, Master Polygons also come in pairs. Their job is to limit the excavation. Think of it in this way:

Imagine excavating using the Master Polygons instead of Excavation Polygons. The new surface formed in this way is the final limit. It is impossible to excavate any further than this surface when master polygons are active.

Master polygons thus provide a convenient way of setting up a final excavation limit. This reduces the need to be so precise with placement of excavation polygons on each excavation near a final limit.

Most of the concepts that apply to Excavation Polygons, also apply to Master Polygons. These include inner polygon and outer polygon shapes, inner polygon binding and batter angles.

Excavate Only and Fill Only

These constraints apply to the manner in which material is taken from the excavated surface. Their names are almost self-explanatory.

When the excavate only option is enabled, no fill is ever placed on the terrain. In areas where the inner polygon is above the excavated surface, the surface is left unchanged.

When the fill only option is enabled, no material is ever removed from the terrain. In areas where the inner polygon is below the excavated surface, the surface is left unchanged.

If both excavate only and fill only are enabled, nothing happens!

Partial Excavation

This option is exercised by setting a percentage-excavated value of less than 100%. Then not all the material that can be excavated or filled at each terrain point is excavated or filled. Only the indicated fraction is processed.

The material is removed or added proportionally over the excavated region, so that the same fraction of material is processed at each point.

Log Rehandle

This option is not really a constraint. However it is included here for convenience.

Like modeling of mining machinery performance, this option makes sense and is available only for the terrain surface. It does not apply to inner surfaces.

Log Rehandle enables tracking of the volume of Rehandled material. Rehandled material is excavated material that has been dumped (or filled) on the terrain surface before excavation. Tracking takes place from the moment that the Log Rehandle option is enabled. This means that material, which was dumped on the terrain surface prior to the option being enabled, will not be counted as rehandle material if it is excavated.

After this option is enabled, each excavation separately accounts for prime and rehandle material.