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Estimating Pipe Trench Excavation Material Storage Capacity

Estimating Pipe Trench Excavation Material Storage Capacity

About a year ago we were approached by an international company which among many other things constructs pipelines, and uses AutoCAD Civil 3D to model their projects. This involves excavating a trench, which of course produces a significant quantity of excavated material which must be stored somewhere until it can be reused as backfill after the pipe is installed. Typically, the material is stored as a berm adjacent to the excavated trench. This minimizes movement of the material on the job site, and keeps expenses to a minimum. The catch is that sometimes space for the berm is limited by right-of-way boundaries, and there are geometric constraints which must be met when constructing the berm as well. In cross section, a typical project looks something like this:

Cross section in C3D

The problem this particular customer was facing is that there are no subassemblies included with AutoCAD Civil 3D which:

  • Models a pipe trench that meets their specifications
  • Models a pipe trench which automatically reacts to the pipe diameter by resizing
  • Calculates a material area in the subassembly that is usable from another subassembly
  • Calculates the size of a berm based on the desired geometric constraints and the total area of the material used to construct it.

For this customer, not having the proper subassemblies ultimately resulted in them having to use Microsoft Excel to perform their storage capacity calculations. A section of pipeline several kilometers long could easily take several weeks to model and quantify. This process was also highly prone to mistakes. In fact, when we first started testing the solution we are about to describe, we noticed a mistake the customer had made a week prior when beginning his manual calculations that significantly altered the results he was getting, and would have resulted in a grossly erroneous bid for the project.

The Solution

The solution to this challenge involved the use of several of our applications. We used Subassembly Studio to create the custom subassembly which modeled both the trench and the berm. We then used Visual Report Designer to create a custom quantities report which also included a chart to visualize the results. Last, we used Terra Power Tools to visualize the models and materials in 3d, and to double check volumes.

The Custom Subassembly

custom subassembly in Subassembly Studio

We used several advanced techniques within subassembly studio to model the pipe trench and the berm. We also included a horizontal link which is conditionally coded based on whether we are under/over storage capacity (allows us to see hatched regions where capacity is insufficient in plan view), and a bar chart in the subassembly itself that shows the relative values of the various quantities calculated in the process of modeling this:

  • Raw excavation volume
  • Excavation volume with an expansion factor applied
  • Maximum storage capacity
  • Used storage capacity (with compaction applied)
  • Remaining storage capacity/overage

The subassembly is created as a composite subassembly. The bulk of the logic and geometry is implemented in the main subassembly, and the bar chart component is implemented as a separate subassembly which is ‘invoked’ within the main subassembly.

Implementation of the subassembly followed this general progression:

  • Create the trench geometry (including the pipe)
  • Calculate the excavation area from the trench geometry
  • Locate the storage berm extents
  • Determine the storage berm geometry based on the calculated excavation area (with factors applied)
  • Conditionally set the horizontal link coding to reflect the current state of the section (i.e. over/under capacity)
  • Invoke the bar chart component to enable the user to visualize the local results in each section view.

Subassembly Studio offers several unique features which make this sort of subassembly possible. The most critical feature is the ability to calculate the area of polygons defined from shapes. Without this one feature, none of this would be possible.

The Corridor Model

Corridor model in AutoCAD Civil 3D

With the subassembly completed, we were then able to model the geometry in AutoCAD Civil 3D as a corridor entity. The model shown above allows us to see precisely where we have sufficient storage capacity and where we have too much material, thus necessitating a mass-haul calculation. The bar chart in the subassembly allows us to see relative capacities in section view, and in the section editor:

Corridor model in plan view and Section Editor in AutoCAD Civil 3D

The entire process of modeling the corridor took no more time than a simple roadway or pipe trench might take, nor did it require any special techniques or workarounds – we simply created the corridor entity, set the baseline parameters, assigned the assembly containing our custom subassembly to a region, set targets and let it build.

Calculating Quantities

We created a custom report using Visual Report Designer to calculate the quantities from this model:

Visual Report Designer - Report Template

The report contains several notable features:

  • A bar chart to the report which shows remaining material storage capacity as a function of station.
  • Custom formatting on the bar chart shows remaining capacity in blue, and overages in red.
  • A warning icon is displayed in the section header if an overage is determined.
  • The body of the report shows a table for each station consisting of each pertinent quantity. The table shows the name, area, volume (end-area method), and ‘factored’ cumulative volume for each item.

When previewed, the report looks similar to this:

Visual Report Designer - Report Preview with Chart

At stations which have overages, the report looks like this:

Visual Report Designer - Report Preview with Warning Icons

Using Visual Report Designer instead of Quantity Takeoff (‘QTO’) in AutoCAD Civil 3D gives us several advantages. In addition to the graphics (warning icon and chart) that we included, we can add conditional formatting (note the red text in the screen capture above), and we can perform calculations within the report, as we did for the last column representing a ‘factored’ cumulative volume. We can also export the results directly to a PDF, XLS/XLSX, HTML, or any one of several other supported file formats. The results can also be printed, or embedded directly into the DWG.

We would like to note that for the customer to use this solution, it takes only a couple of minutes to create the model from existing alignment and surface geometry and ultimate get to these volume results. Compare this to the previous process which took several weeks to get to the same point.

Visualization and Composite Volumes

Using Terra Power tools, we can visualize our surfaces and materials (as solids), cut cross sections, etc. We can also calculate 3d composite volumes, which are more accurate and precise than end-area based quantities typically used for Quantity Takeoff (‘QTO’) in AutoCAD Civil 3D and in Visual Report Designer based quantity reports.

With ExistingGround turned off, the model looks similar to this:

Terra Power Tools - View Surfaces

We created a material table which results in materials being calculated for the ‘dump’ (storage berm), pipe, stripping layer, bedding, and for total excavation:

Terra Power Tools - Material Table

When we compute volumes, we get the following results:

Terra Power Tools - Volume Computation Results

The resulting materials can be viewed as solids in 3D:

Terra Power Tools - View Materials (solids) in 3D

We can also import the baseline alignment as a path and use it to cut profile and cross sections:

Terra Power Tools - View cross and profile sections with materials and surfaces shown


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