Design patterns for roadways can be broadly classified into four different categories such as triangular or restricted gutter flow, sheet flow, applications related to curb return, and designs for retrofitting or designing. In general, a triangular gutter flow application is found within a gutter in the roadway’s curbed section or adjacent to a barrier wall. This pattern is also suitable while placing trench drains in a valley. Sheet flow designs are implemented mostly in access ramps on roadways with limited access, entrances to a driveway, and across the streets that don’t have a crown. The most common example of a retrofitting or repairing design is a trench drain with a slope for a system where the inlet was not placed at a low point. Whereas, a curb return design is frequently applied to ensure that a wheelchair ramp remains dry. This design helps overcome the problems while placing the inlet structure in close vicinity to the radius of intersection.
A robust roadway design must ensure that every single rating related to AASHTO M309 and AASHTO H-20 is met. Moreover, the design should not lead to any restrictions in flow within the cross section of the trench to arrest any debris. All grates used should be made of ductile iron. Being susceptible to degradation by UV rays and heat, all types of plastic components should be avoided.
While designing roadways, there are four primary design considerations for you.
- The quantity of the runoff to be captured by the drain determines the capacity of your trench design. The Rational Method is the most common alternative to determine this, following the equation Q=CIA
- The triangular gutter flow equations should be applied for valley flow or constrained flow.
- In case of a sheet flow, if there is a steep slope, sizing up the drain is important in order to catch the water. Otherwise, the channel’s hydraulic requirements should be taken into account.
- The process of drain size calculation should also add the site slope. This will help increase the flow rate of the trench
- The minimum calculated size in cubic feet/second should be used for exterior applications to handle storm water. Keeping freeboard for large storms is not advisable because it not only increases the cost, but also results in debris build up because of low flow rate.
- Your outlet capacity must not limit the drain’s flow.
- Roadway applications are subject to several external factors such as heavy traffic, cyclic loading, UV light, and different climatic cycles. Therefore, the channel should be made of materials that are capable of expanding and contracting at the same rate compared to the concrete. Also, the material should have adequate strength and thickness to handle cyclic loading and remain unaffected by UV light.
- Use of radius bottom trench channels is a good alternative for the accumulation of debris.
- Use properly anchored frames or rails to support the grades against excessive dynamic load.
- In case you are looking to seal the channel joints, use urethane sealants for fibreglass channels and polymer concrete. Similarly, follow a process of continuous heat welding for holding your HDPE channels together.
- If the system is not temperature controlled, opt for a material with same thermal expansion coefficient compared to the concrete encasement surrounding it.
Grate material selection:
- Use of ductile iron is recommended because cast iron grates are more expensive and tend to fail over a period of time. With its higher weight ratio, ductile iron also offers more open area for the same strength of the grate.
- In order to minimize clogging, the openings on your grate should be made as large as you can. In case of smaller grate openings, the system will require frequent cleaning.
- Make sure you have properly anchored the grates. Utilize an anchoring system that locks the corners within the concrete. Toggles can be used in gutters, curbs, and areas with occasional traffic. While using a removable grate, the anchoring should be done with stainless or galvanized steel anchors.
- Most of the trench drains have a relatively flat longitudinal slope, making them rather lengthy. As a result, depth becomes an important attribute of a trench drain. It is always better to design systems that have multiple outlets or are capable handing longer runs.
- Be mindful of the fact that the depth of your system can cause obstruction to the buried utilities including the piping.
- Place your outlets in a way that minimizes the piping. However, the distance between the outlets should not be greater than the standard run length. Use sidewall extensions or neutral channels if this happens by any chance.