Site applications

Sitework is a very broad category and can include drive entrances, parking areas, landscape areas, walkways, and numerous other situations.  Typical challenges have to do with pedestrian traffic and vehicular traffic in the same areas, transverse and longitudinal loading on the grates, debris, aesthetics, cost, limited depths due to underground utilities, etc.  Almost any system can be used in a site work application depending on where it is located and the exact parameters of it's use.

Sizing the drain
  • The channel size should be based on your hydraulic load, debris load, storage capacity, and outlet capacity.  Common trench drain sizes start at 4" wide and go all the way up to 24" wide.  These sizes will be able to handle 99% of all site drainage applications.  The downloads area of the web site has information on the different size systems and their flow capacities .

Choosing the channel
  • Due to the ease of installation and quality, most designers and contractors prefer a prefabricated trench drain.
  • Make sure the trench drain that is selected has a frame that is designed to handle the loading that it will encounter.  For vehicular loading a metal frame with proper anchoring into the surrounding concrete should be selected.  Remember that dynamic loads from turning and breaking can exceed static loads.  Be careful of systems that do not have anchors or have very small anchors as these systems can fail over time.  The frame should transfer the load received to the surrounding concrete without bearing on the prefabricated trench walls.  Sufficient bearing area is required to transfer these loads.  Proper frame design ensures that the trench body will not crack and fail over time.
  • Exterior applications will see severe temperatures or changes in temperature and should have similar thermal coefficients of expansion to that of the surrounding concrete encasement. One of the most prominent failure mechanisms for trench drains that receive light duty traffic only is due to differing thermal coefficients of thermal expansion.  When the channel grows at a different rate than the surrounding concrete the channel material is stressed and eventually will fail.  This is almost always a factor outdoors.  For more information see material properties.
  • Make sure that the outlet properly matches the design flow capacity.  Do not accept systems that do not offer the proper outlet size connections.  The system is only as good as it's weakest link.  If the design requires a 24" outlet pipe make sure the product you select has the appropriate size outlet pipe.
  • Outdoors channels should be restricted to fiberglass, polymer-concrete, concrete, or metal trench drain bodies.  Plastic bodies should be restricted to indoor use or short runs where thermal expansion will not pose a problem.
  • Sealing channel joints is typically not necessary. Some areas will require the joints to be sealed due to freeze thaw cycles but most states rely on proper vibration of the concrete to prevent water from getting in the joints. However, if you do want the joints sealed you must specify such and stress to the contractor that this be done properly. Most exterior applications can simply use a urethane joint sealant.

Selecting the grate material

  • Grates for sitework applications should be Cast or Ductile iron in roadways.  In occasional use entrances a galvanized steel grate is acceptable.  Pedestrian or landscape areas can be almost any material, even plastic, as long as it is able to handle the ultraviolet light without degradation.
  • If a grating must receive both vehicular traffic and pedestrian traffic look for grates that are both heel proof and rated for vehicular.  After these considerations are met the next most important thing is the amount of open area in the grate.  It will want to plug up with smaller holes so the more open area you have the more likely the system is to function properly over the life of the facility.
  • When there is a choice between Cast iron and ductile iron choose ductile iron.  Cast iron grates are more susceptible to failure over time and the ductile iron grates cost little more than cast iron.  The other huge advantage of choosing ductile iron is that the strength to weight ration is much greater.  What this means is that for the same strength grate you can have more open area in the top of the grate.  This is important with the amount of debris that is often present in site work.  The more open area and the larger the geometry of the openings the less likely the grate is to plug and not work properly.  You should be mindful of pedestrians where applicable.
  • Make sure that the openings on the grate are as large as possible to ensure that clogging is not a factor. The smaller the openings the more frequent cleaning of the grates will be necessary. The larger the openings the more likely the debris is to flush through the system.
  • Be sure the grates are properly anchored. The best is to use a system that locks all four corners into the concrete.

Designing the layout

  • The layout can be simple or quite complex depending on the job specific requirements.
  • To minimize the depth of the systems consider putting the pipe outlets in the center of long trench drain runs.
  • It is best to try and keep the trench drains out of direct pedestrian traffic when there will be large amounts of debris. Sometimes this is not possible so look for systems with maximum open area while still meeting the traffic requirements.
  • The depth of the system that you are going to use can conflict with piping and other buried utilities.
  • Outlets should be placed to minimize piping, but try to avoid making the outlets too far apart.  The further a trench runs in one direction the deeper it gets and the more expensive.  Where possible limit the outlets to under 150 linear feet to keep costs reasonable.
  • For radius trench drains it is important that you select a supplier before you complete the design.  Few companies offer this type of design and the cost will be significantly more.  If this cost is not figured properly in the design and bidding phase it can be an issue at a later date.