Trench Drains and Slotted Drains for Truck and Loading Docks

Truck docks are often below the grade of the rest of the site and therefore require a drainage system.  The standard truck dock height is 48″ below finish floor of the building.  A trench drain is the most logical solution to keep this dock from becoming full of water and simplifies the concrete construction.  Traditionally a loading dock trench drain was a 12″ wide cast in place trench drain.  The 12″ wide size was common because it was easy to construct wooden forms of this size.   With the speed and availability of prefabriacted trench drain systems, they are now the preference and are becoming more common.  In many cases the size of the trench can be reduced with a prefabricated system.  The only reason for a larger trench drain in a loading dock would be if heavy debris loads are expected.

The trench drain grates on these systems should be capable of handling vehicular loading if they will be far enough from the dock where trucks can get their wheels on them.  Often times the drains can be placed where the truck wheels will never reach the drain allowing for lighter duty systems to be used.

Design Considerations

Sizing the drain

• Truck docks range in size and therefore the drain also ranges in size.  For docks between 1 and 3 bays wide a 4″ drain is typically sufficient.  Many designers use an 8″ wide trench drain with a radius bottom due to concerns about debris.  Determine the necessary hydraulic load and go up to the next larger size.
• Flat bottom trenches tend to collect more debris because they do not have as high of a velocity flow in the bottom of the trench.  A radius bottom trench reaches scouring velocities easier and does not require as much maintenance.
• Make sure that the trench is a minimum of 6″ deep below the grate to keep debris from plugging the system.
• Debris is one of the key considerations when sizing a truck dock trench drain.  All debris that gets on the dock cannot get back out and therefore ends up in the drain system.  The channel and outlet piping can get clogged with debris.  Routine maintenance programs and oversizing of the drain and outlet piping is recommended in this application.  Use a minimum of 6″ or 8″ outlet pipe.

Choosing the channels

• Smooth interior walls are better because the flow velocity is increased and this tends to get sediment moving throught the system better.  Fiberglass trenches are ultra smooth and are a good choice for this application.
• Exterior applications will see drastic changes in temperature and should have similar thermal coefficients of expansion to that of the surrounding concrete encasement.  For more information see material properties.
• Trench drains placed where traffic will be on the grates or where fork truck traffic is possible should have a heavy duty frame to protect the top of the channels.  Frames add to the cost of the system but ensure the longevity of the drain.

Selecting the grate

• Ductile iron or cast iron grates are most common.  Iron grates are a good choice but if the drain is placed close enough to the building a lighter duty grate such as fiberglass, galvanized steel or aluminum may be a cost saving alternative.  Due to theft, we are seeing an increase in the use of fiberglass grates for this application.
• The grates should have as large of slots as possible to pass debris through the system.  Ductile iron is a good choice here as the strength to volume ratio is high meaning that you can get the desired strength with less iron.
• Consider the weight of the grates.  Light weight grates can jump out of the trench creating a hazardous situation.  Lock all light grates into place with the appropriate locking device.

Designing the layout

• On small loading docks the trench drain should slope to one end.  On larger truck docks the trench should slope to the center.
• If the pipe will change directions after the outlet it is a good idea to put a basin at the outlet of the trench drain.
• To reduce traffic on the drain place it close enough to the dock so that the truck wheels will no be on top of the drain.