Defining Underground Storage Chambers

Underground storage chamber systems are one of the most common ways to provide on-site stormwater detention or retention when surface space is limited. CivilGEO software allows the user to specify a manufactured chamber product, lay out a complete chamber field within an excavation footprint, and automatically compute the resulting storage volume.

This article describes underground storage chambers and how to interact with the Underground Storage Chamber panel of the Storage Area Data dialog box. Refer to this article in our knowledge base for more information about the Storage Area Data dialog box.

What Is an Underground Storage Chamber?

An underground storage chamber system is a subsurface stormwater storage system made up of a series of high-density polyethylene or polypropylene arch-shaped chambers installed within an open-graded stone bed. These systems are sometimes referred to as chamber fields, plastic chamber systems, or arch chamber systems.

Stormwater runoff is directed into the chamber system through inlets, pipes, and header rows. As runoff enters the system, water fills the open space inside the chambers and also occupies the void spaces between the surrounding stone. Together, the chamber volume and the stone void volume provide the total available stormwater storage capacity.

Once the system begins to fill, the stored stormwater can either infiltrate into the surrounding soil, slowly discharge through a controlled outlet, or use a combination of both methods, depending on the site design and local drainage requirements.

Underground storage chambers are commonly installed beneath parking lots, driveways, athletic fields, landscaped areas, and other usable surface spaces. This allows the land above the system to remain functional while providing the required stormwater detention, retention, or infiltration capacity below ground.

These systems are typically designed to manage the increase in runoff caused by site development. They are commonly sized to control the difference between pre-development and post-development runoff for a selected design storm, or to meet local low-impact development (LID), detention, retention, or infiltration requirements.

Typical Underground Storage Chamber System Components

• Chambers: Manufactured arch units that provide most of the open storage volume.
• Stone embedment (base, sides, and cover): Open-graded angular stone surrounding the chambers. The void space within this stone contributes additional storage.
• End caps and headers: Closed-end pieces and manifold pipes that distribute flow across chamber rows.
• Geotextile and impermeable liners: Separate the stone from native soil and, in retention systems, trap and contain water until it exfiltrates.
• Inlet, outlet, and overflow structures: Manage flow throughout the system, including controlled discharge for detention designs.

Defining Specifications for Underground Storage Chambers

Follow the steps below to define the specifications for the underground storage chambers:

1. In the Storage Area Data dialog box, select Underground Storage Chamber from the Storage Area Specifications dropdown combo box.
   

2. The corresponding panel with options for defining the underground storage chamber will be displayed.
   

The following subpanels are available in the Underground Storage Chamber data panel:

• Chamber Details
• Chamber System Design
• Summary

Chamber Details

In the Chamber Details subpanel, the Underground Storage Chamber Details section displays the information about the selected storage chamber manufacturer and associated model.

The following parameters are available:

• Storage chamber manufacturer
  This read-only field displays the selected storage chamber manufacturer name.
• Storage chamber model
  This read-only field displays the selected storage chamber model. Clicking the [Select] button temporarily removes the Storage Area Data dialog box and displays the Select Underground Storage Chamber lookup dialog box. This dialog box allows the user to select the storage chamber model. After selecting the storage chamber model, the Storage Area Data dialog box will be redisplayed. The selected storage chamber model will be displayed in the Chamber Details subpanel. Refer to the Select Underground Storage Chamber section (explained below) to learn more about this dialog box.
• Storage chamber model Image
  The image available in this section displays the preview of the selected storage chamber model.
• Description Box
  The description box displays the selected storage chamber model specifications provided by the manufacturer. The following specifications are displayed:
• 1. Manufacturer’s name
  2. Storage chamber model
  3. Chamber height
  4. Chamber width
  5. Chamber length
  6. Chamber volume

Select Underground Storage Chamber

The following panels are available in the Select Underground Storage Chamber dialog box:

• All Storage Chambers
• Recently Selected
• Favorites
  

All Storage Chambers

In the All Storage Chambers panel, the Underground Storage Chamber Selection section allows the user to select the storage chamber manufacturer and associated model to be used for the selected storage area.

The following parameters are available:

• Storage chamber manufacturer
  This dropdown combo box allows the user to select the storage chamber manufacturer. The software supports the following storage chamber manufacturers:

• 1. ADS StormTech
  2. Cultec
  3. NDS StormChamber
  4. Prinsco
  5. Triton

Clicking the […] button displays the Select Storage Chamber Manufacturer lookup dialog box that allows the user to select additional storage chamber manufacturers. The selected storage chamber manufacturers will then be listed in the Storage chamber manufacturer dropdown combo box.

• Storage chamber model
  This read-only field displays the currently selected storage chamber model. Note that only one storage chamber model can be selected at a time.
• Search Box
  The search box allows the user to quickly find a specific underground storage chamber model from the available list. The user can click the [Clear] button to remove any text entered in the search field.
  

Recently Selected

In the Recently Selected panel, the software lists the most recently used storage chamber models for each manufacturer.

Favorites

In the Favorites panel, the user can quickly access frequently used storage chamber models. This panel displays storage chamber models that have been marked as favorites. To add a storage chamber model to the Favorites panel, select the model from the All Storage Chambers panel and click the Star icon.

Once the storage chamber model has been selected, click the [OK] button. The selected storage chamber model, along with associated information such as manufacturer name, model name, dimensions, and image, will be displayed in the Chamber Details subpanel.

Chamber System Design

In the Chamber System Design subpanel, the Chamber System Specifications and Layout section allows the user to define the specifications and layout of the storage chamber system.

The following tab panels are available in the Chamber System Design subpanel:

• Chamber System Design
• Chamber System Layout

Chamber System Design

In the Chamber System Design tab panel, the Define Chamber System Layout Specifications section allows the user to define the parameters for the chamber system layout.

The following parameters are available:

• Terrain surface
  This dropdown combo box allows the user to select the terrain elevation surface available in the project.
• Space between chamber rows
  This entry field defines the gap between storage chamber rows so that the stone fill can be inserted.
• Storage area excavation wall to chambers
  This entry field defines the buffer distance provided from the storage area boundary. Note that the storage chambers can only be placed inside this buffer region.
  
• Chamber row header distance (optional)
  This entry field defines the width of the header manifold, which will be provided to each chamber row.
  
• Stone cover
  This entry field defines the stone cover to be placed above the storage chamber crown.

• Stone base
  This entry field defines the stone base to be placed under the storage chamber.
• Stone base invert elevation
  This entry field defines the invert elevation of the stone base.
• Stone void space
  This spin control entry field defines the stone void space, represented as a percentage. By default, the software uses a value of 40. However, the user can enter a different value ranging from 10 to 90.

Once all required data have been defined, click the [Generate] button. The software generates the storage chamber system layout and immediately takes the user to the Chamber System Layout tab panel. Additionally, the software updates the storage area with the maximum number of storage chambers, which makes it possible to maximize storage volume.

Chamber System Layout

The Chamber System Layout tab panel displays a preview of the storage area created on the Map View. Initially, the preview shows the current Map View extents, but the user can zoom and pan this view using the mouse cursor.

The following parameters are available:

• Project base map provider
  This dropdown combo box allows the user to select the project base map from several high-quality base maps.
• Use grayscale base map
  This checkbox option allows the user to change the base map to grayscale instead of color.

Editing the Storage Chamber Layout

Clicking the [Edit Layout] button temporarily removes the Storage Area Data dialog box and displays the Edit Layout dialog box. This dialog box allows the user to adjust the layout of the storage chamber more precisely. In this dialog box, the user can define the storage area boundary and manually add or remove storage chambers. After making the necessary changes, click the [OK] button to save the layout. The Storage Area Data dialog box will be redisplayed, and the Chamber System Layout preview tab will be updated with the changes made. To abort the process, click the [Cancel] button.

The Edit Layout dialog box provides the following toolbar commands for editing the storage chamber system layout:

Expand table

Toolbar Command Icon	Toolbar Command	Description
	Move & Rotate Storage Chamber Layout Grid	This command moves and rotates the layout grid along with the storage chambers.
	Move & Rotate Storage Chamber Boundary	This command moves and rotates the boundary of the storage chamber.
	Edit Storage Chamber Boundary	This command edits the vertices of the storage chamber boundary.
	Move Storage Chambers	This command moves selected storage chambers within the storage chamber layout grid.
	Add Storage Chamber	This command adds a new storage chamber to the storage chamber layout grid.
	Add Chamber Header	This command adds a storage chamber header to the ends of the storage chambers.
	Remove Chamber Header	This command removes the storage chamber/header from the storage chamber layout grid.
	Flip Chamber Header	This command flips the storage chamber header from top to bottom or bottom to top within the chamber guidelines.
	Measure Distance	This command measures the distance on the storage chamber layout view by drawing a temporary polyline.
	Toggle display of Base Map	This command shows or hides the base map layer in the Map View.

In addition, the user can zoom and pan the storage chamber layout grid in Map View. Refer to this article in our knowledge base to learn more about the other tools available in this dialog box.

Summary

The Summary subpanel provides a volume summary for the storage chamber system, including the depth-area-volume plot and corresponding data in a tabular format.

The following tab panels are available in the Summary subpanel:

• Volume Summary
• Depth-Area Volume Plot
• Data Table

Volume Summary

In the Volume Summary tab panel, the Storage Chamber System Volume Summary section displays the volume summary for the storage chamber system. Note that this section is populated only when the user has generated the storage chamber system layout from the Chamber System Design subpanel.

The following parameters are available:

• Total storage chambers
  This read-only field displays the total storage chambers included in the storage chamber system.
• System footprint area (including buffer)
  This read-only field displays the footprint area of the storage chamber system, which is equal to the area of the buffer polygon.
  
• Total excavation volume (including buffer & cover)
  This read-only field displays the volume of the earthwork to be excavated for the storage chamber system, including exterior buffer and ground cover.
• Total ground cover volume
  This read-only field displays the volume of excavation required to provide ground cover to the storage chamber system.
• Total storage chamber volume (without stone fill)
  This read-only field displays the total storage volume of the storage chambers.
• Total stone fill required (includes void space)
  This read-only field displays the storage volume of the stone fill (including void space).
• Stone void volume
  This read-only field displays the storage volume provided by the voids of the stone fill.
• Total system storage volume (chambers & stone void space)
  This read-only field displays the total storage volume defined by the storage chamber system. This volume includes the storage volume of the storage chambers and the stone fill voids.

Clicking the [Calc] button computes the volume summary for the storage chamber system. Note that the [Calc] button is enabled only when changes are made in the chamber system layout using the Edit Layout dialog box. Otherwise, this button is disabled (i.e., grayed out).

Clicking the [Chamber Report] button displays the Underground Storage Chamber Report dialog box, which allows the user to generate a report of the storage chamber system in Microsoft Word or PDF format. Refer to this article in our knowledge base to learn more about the Underground Storage Chamber Report dialog box.

Depth-Area-Volume Plot

In the Depth-Area-Volume Plot tab panel, the Storage Chamber System Depth-Area-Volume Plot section displays the depth-area-volume plot of the storage chamber system based on the computed volume summary from the Volume Summary tab panel.

Data Table

In the Data Table tab panel, the Storage Chamber System Depth-Area-Volume Data Table section displays the depth vs. area and volume data in a tabular format based on the computed volume summary from the Volume Summary tab panel.

The following read-only columns are available in the Storage Chamber System Depth-Area-Volume Data Table section:

• Depth
  This read-only column displays the cumulative depth value of the storage chamber system.
• Void Space Area
  This read-only column displays the cumulative value of the void space area of the storage chamber system at the corresponding depth.
• Void Space Volume
  This read-only column displays the cumulative value of the void space volume of the storage chamber system at the corresponding depth.

Practical Design Tips

• Mind the cover: Most chamber manufacturers specify a minimum and maximum cover depth based on traffic loading. Confirm that the design satisfies the applicable load case before finalizing the stone cover value.
• Match the void ratio to the stone gradation: The default 40% void space is typical for AASHTO #3 or #57 angular stone. Make sure to confirm with the geotechnical specification on your project, as some designs require a different value.
• Plan for inspection ports and isolator rows: Many municipalities require inspection ports and at least one isolator row for sediment capture. These are typically detailed outside this dialog box, but should be accommodated in the boundary footprint.
• Set the invert before placing chambers: The Stone base invert elevation drives the system's vertical position relative to the terrain surface. Establish the controlling invert (often the outlet pipe invert) before generating the layout.
• Use the Edit Layout tools sparingly: The auto-generated layout is usually close to optimal. Use manual edits to handle utility conflicts, easements, or locally required offsets, then re-run [Calc] to refresh the volume summary.
• Document with the chamber report: Generate the Underground Storage Chamber Report as part of the project deliverables. It gives reviewers a clean record of chamber selection, layout, and computed storage volume.

Conclusion

Underground storage chamber systems provide an efficient way to manage stormwater where surface space is limited. By combining the storage volume inside the chambers with the void space in the surrounding stone, these systems can deliver reliable detention, retention, or infiltration capacity while keeping the land above available for other uses. When properly selected, laid out, and checked against site conditions, these systems can be a practical and effective part of modern stormwater design.