Precast catch basin types: Type 1 vs. Type 2

By November 6, 2018 No Comments
Catch Basins from Columbia Precast Products

Ancient civilizations created catch basins to hold seasonal rainwater, ensuring crops could be irrigated until harvest. The purpose of precast catch basins has evolved to fit today’s concrete and asphalt landscapes. These hard surfaces can’t absorb stormwater and some areas would quickly become large, debris-filled lakes without the capabilities of a precast catch basin.

Importance of a Precast Catch Basin

Catch basins are concrete grated entryways to storm drainage systems. You’ll find them in parking lots, on street curbs or in maintenance yards – just about any location where rainwater travels from a hard surface into the system. Unlike manholes or grated inlets, catch basins include a sump, or hollow, at the bottom.

The basin grate filters large pieces of debris, while its sump collects sediment, oils, grease and smaller material like leaves and litter found in stormwater runoff. Debris is cleared by hand or a Vactor catch basin cleaning truck. The precast catch basin helps prevent downstream sewer pipes from clogging and reduces the amount of solid material discharged into lakes, streams, and rivers.

Determining Which Type of Precast Catch Basin to Use

Catch basins have been designed in several sizes and structures to meet the wide variety of needs for managing runoff. The availability of precast catch basins makes it easy to customize dimensions to meet specific site conditions. Answers to these questions will help you select the catch basin for your project:

  • What are the stormwater management goals (in terms of water volume) for the site?
  • Which catch basin type is most appropriate for the site’s geography?
  • Will the installation and operational requirements of the catch basin be able to be met?

Type 1 Catch Basin

A Type 1 precast catch basin is typically rectangular or circular with a maximum knockout size of 20 inches. With a recommended 1 ½ inch gap between the basin wall and the drainage pipe, the maximum drainage pipe to use is 17 inches. Since pipes with skew angles will have larger diameters, take this into account when estimating the knockout size required.

Usually, contracts allow several options of pipe material and pipe alternatives. Be sure to check that the Type 1 catch basin can accommodate all approved alternates.

The maximum depth for Type 1 catch basins is 5 feet, as measured from the lowest pipe invert to the finished road.

Catch basins must be watertight according to Washington state requirements. To comply, you may need to grout the catch basin sides or apply mortar to the joint between the catch basin wall and drainage pipe.

Type 2 Catch Basin

Type 2 catch basins are designed for larger pipes and deeper inverts. These catch basins are available in several diameters including 48-inch, 54-inch, 60-inch, 72-inch, 84-inch, 96-inch, 120-inch, and 144-inch.

To facilitate the installation of drainage piping into the Type 2 basin, a gap of 2 to 2 ½ inches between the knockout wall and the outside of the pipe is recommended. After the pipe is installed, concrete grout or joint mortar is used to fill the gap.

Maximum depth for Type 2 catch basins is based on the reach of Vactor trucks, typically 15 feet, as measured from the road surface to the catch basin bottom.

When dealing with a Type 2 catch basin, OSHA safety requirements call for a 28-inch maximum adjustment section from the roadway to the first step of the catch basin barrel. If the full 28-inch depth isn’t necessary, the catch basin can be placed deeper than 12 feet, but not more than 15 feet.

A Type 2 catch basin has two foundational structures:

  • A precast base section with riser walls between 2 and 5 feet high. Smaller base sections may not contain the knockouts. Additional risers are used to reach full height.
  • A precast base slab fit with an o-ring gasket providing a watertight seal. Typically, a 2 riser is added for a sump, followed by a 4-to-5-foot riser with the knockouts. Additional risers may be used to attain more height.

To learn more about the type of precast catch basins needed for your project, contact our team of experts at Columbia Precast Products.