Structure Backfill [SIG]

The Structure Backfill object calculates the net volume of fill material needed after foundation construction. It computes the gross backfill volume and automatically subtracts the volumes of structural components (footing, columns, abutments) that occupy space within the backfill zone.

General

Foundation: Select the footing that defines the backfill boundary. The backfill zone extends from the footing bottom to the finished grade. This is a required input.

Excavation Type: Determines the shape of backfill boundary walls. Should match the excavation slope.

• Vertical (0): Straight vertical walls from bottom to top

• Sloped (1): Angled walls matching the excavation slope

Slope Ratio (H/V): The horizontal to vertical ratio for sloped backfill walls. Only shown when Excavation Type is set to Sloped. Should match the excavation slope ratio to ensure consistent geometry.

Boundary Offset Beyond Footing: Distance beyond the footing edge in all directions. Should match the excavation boundary offset. Default is 12 inches. Must be non-negative.

Finished Grade

Grade Source: Method to define the finished ground surface after construction.

• Elevation (0): Manual input of a single elevation value for flat finished grade

• From DTM (1): Use a Digital Terrain Model for variable finished grade

Finished Grade Elevation: Manual elevation of the finished ground surface. Only shown when Grade Source is set to Elevation. Represents the final ground surface after construction.

Finished Grade DTM: Select a DTM object representing the finished grade. Only shown when Grade Source is set to From DTM. The DTM should represent the final graded surface, which may differ from the original ground.

The finished grade may be different from the original ground surface used in Structure Excavation. For example, the finished grade might be higher or lower than the original ground to accommodate drainage requirements.

Display

Show Backfill: Toggle to show or hide the backfill volume in the 3D view.

Opacity: Transparency of the 3D visualization. Range from 0 (fully transparent) to 1 (fully opaque). Default is 0.7.

Quantities

All quantity fields are read-only computed values.

Gross Backfill Volume: Total volume of the backfill zone before subtracting structural components. Calculated using the same methods as excavation volume:

• DTM mode: Computed prism volume from terrainGrid function

• Elevation mode (Vertical): Average Depth × Bottom Area

• Elevation mode (Sloped): Frustum formula

Structure Volume (Subtracted): Combined volume of the footing and substructure elements within the backfill zone. This is the sum of Footing Volume and Substructure Embedded Volume.

Net Backfill Volume: Actual fill material needed after construction. Calculated as:

Net Backfill = Gross Volume - Structure Volume

Top Surface Area: Area of the finished grade surface within the backfill boundary.

Bottom Surface Area: Area at the bottom of the backfill zone (at footing bottom elevation).

Average Fill Depth: Average depth of backfill from footing bottom to finished grade.

Footing Volume: Volume of the footing within the backfill zone. Retrieved from the footing's Volume property.

Substructure Embedded Volume: Volume of columns or abutments embedded within the backfill zone. Calculated differently based on substructure type:

• Pier Columns: Only the embedded portion within the backfill depth is counted. Uses the ratio: min(1, Backfill Depth / Column Height) × Column Volume

• Cantilever Abutments: Full abutment volume is used since the entire stem is typically within the backfill zone

Piles are NOT subtracted from backfill volume since they extend below the footing, outside the backfill zone.

Supported Foundation Types

Structure Backfill works with:

• Custom Footing (OBPFoundation): Footing with point list or section-based geometry

• Rectangular Footing (OBPFoundationRectangular): Standard rectangular footing

The backfill boundary is automatically derived from the footing polygon, offset outward by the specified boundary offset distance.

Supported Substructure Types

Structure Backfill automatically detects and subtracts volumes for connected substructure elements:

• Pier Columns (substructure_type = 2): Embedded portion based on column height ratio

• Cantilever Abutments (substructure_type = 1): Full abutment volume

These are determined by the substructure reference array from the connected foundation.

Workflow Tips

1. Create Excavation First: Set up the Structure Excavation object with the desired boundary offset and slope ratio.

2. Match Parameters for Backfill: When creating Structure Backfill, use the same boundary offset and slope ratio as the excavation to ensure consistent geometry.

3. DTM Usage:

   • For excavation: Use a DTM representing the existing ground surface

   • For backfill: Use a DTM representing the finished grade (may differ from existing)

4. Verify Quantities: The net backfill volume should always be less than the excavation volume, accounting for space occupied by structures.