Using Modular Trench Drains
Using Modular Trench Drains to Save Time and Money
As site designers try to utilize every square foot of space, they are including more details on how to direct storm water away from buildings and walkways. Traditionally, solutions relied on transforming the site’s topography with gentle final grade that sloped aprons and pavements to a property’s perimeter. But in recent years, municipalities have updated their storm water management directives. To comply with these restrictions, site designers are including more trench drains in their plans.
Trench drains effectively collect excess surface water from non-porous surfaces such as parking lots, sidewalks and pavements. These carefully designed structures provide a continuous path of surface drainage, directing gathered storm water to a collection basin for treatment and an outfall.
Along with the space saving benefits, trench drains enable site designers to comply with the volume of water storm events can generate. In many jurisdictions, trench drains must be properly designed to handle a 100-year rainfall by evacuating water within a specific interval. Trench drains must be architecturally pleasing, matching the overall site plan.
Hand-formed trench drains
For many years, concrete contractors had to meet these design requirement by hand-forming trench drains. This is a labor-intensive process requiring great layout skill and construction experience. Contractors must be knowledgeable in selecting the proper grate size and setting, designing the structure for its anticipated loading, and plan for all the hook-ups to adjacent drainage devices.
Hand-formed trench drains can also present service life issues. When ordinary concrete is exposed to ponded storm water containing deicing, the surface can experience spalling. Hand-formed trench drains can be difficult to construct on sites with small footprints.
And hand-formed trenches lack a finished architectural look. The trench’s grates can be uneven and cumbersome.
Modular trench drains
To address these site and operational concerns, architects often specify modular trench drains. A modular trench drain is a factory made assembly. The system includes: the channel with grate through which the storm water is directed to a catch basin or outfall. Modular trench drain assemblies come in a wide array of channel shapes and sizes, grate materials, and connection devices allowing the design professional to meet the drainage requirement on all types of projects.
Modular trench drains are factory cast with polymer concrete, which is a mixture of synthetic resin, mineral aggregates, and curing agents. This hardened material is very durable, providing long service life. The polymer concrete is also easily cut on the job site using standard powered hand tools.
Why opt for modular trench drains
Modular trench drains provide an engineered, systematic approach to the collection of surface water on complex sites that can be efficiently included into a project’s critical path. Engineers can also match trench drains to other modular drainage assemblies, such as catch basins, for a one-source solution to the site’s drainage plan. Most components are shelf-ready, allowing just-in-time inventory control. And when necessary, manufacturers can custom fabricate channel sections and connecting assemblies to match any site layout issues.
The key reason contractors prefer modular trench drains to hand-formed trench drains is the increased productivity. The modular features eliminate a large amount of form set-up as compared to hand-formed structures. Contractors can quickly install modular trench drains on all types of sub-grades. This in turns help shortens a project’s critical path, especially at the end of the schedule when time is money.
Contractors traditionally have either used one of two alignment methods to position modular trench drains. For shallow modular trench installation and on some types of replacement projects, contractors often use the “patty” method. Contractors place channels on small clumps of stiff/low slump fresh concrete while establishing the trench path and grade.
For new installations that require the channels to be positioned a foot or more above the sub-grade, contractors use the hanging method. Workers attach wooden plywood supports or a cross-fit bar to cut to each channel. These supports are positioned on the trench drain’s centerline while board suspended using threaded bar supports driven into the sub grade. Final grade is established by adjusting the nut that holds the support on the threaded rod.
Tooling up for efficiency
One manufacturer, ACO Inc, has created an installation device that refines and speeds up modular trench drain positioning as compared to the hanging method. These sacrificial chairs enable workers to quickly align the channels in all three dimensions assuring alignment and assembly stability.
Workers first layout the trench’s path, identifying where to drive in supports cut from rebar. They then slide the installation device onto the embedded rebar. Then they clamp the installation devices at each end of the channel. The workers lift the channel to the designed grade to control the elevation. Workers slip the supports onto the vertical support stake embedded into the sub grade. Once the channel is positioned to the desired elevation, workers secure the chair in its position on the rebar using a set screw.
The special installation devices secure the entire drain system restricting any movement or float when filling the area surrounding the trench drain during concrete placement. Along with better grade control, the installation tool’s clamping feature aligns the channel and helps maintain levelness between the grate seats.
ACO has developed a range of installation devices to allow concrete encasement to be installed with a single pour.
Ensuring a good placement
While these modular trench drains are easy to install, there are some practical tips that can make the installation easier and create better placement.
- Before beginning installation, determine that adjacent structural reinforcements are properly placed. In some jurisdictions, special inspections are required prior to placement.
- Start the layout from the outlet. This will help maintain the drain grade’s design.
- Choose sturdy support stakes. If so, be sure the threaded bars are straight and free from defects. Use either a 1/2” or 5/8” threaded rod with bolts instead of stakes to help establish grade. This will allow you finely raise and lower the channels to your string line.
- During set-up, be sure to drive the support stack deep into the subgrade to establish a secure attachment. This may require longer stake lengths in areas that appear to be soft. Nearby activity from skid steers and ready-mixed concrete delivery can cause the subgrade to pump and disrupt the grade and line of placement.
- When laying out the trench devices, be sure all assemblies match. Also position all fixed ends of the channels on the same side of the excavation. Some manufacturers cast directional flow arrows on the outside of the channel to help ensure proper flow.
- When filling the cavity outside the modular drain’s channel with concrete, be sure to properly consolidate the concrete leaving no voids. The best way to accomplish this, is fill the cavity in two passes. The first placement should fill in the cavity so that the top of the concrete is above the frost keys or very bottom of the channel to reduce the chance of floating. The second placement should be atop of the first placement filling the sides up to the top of grade. Use a low frequency internal vibrator to insure proper consolidation.
An effective combination of design, productivity, and aesthetics
Projects that opt for modular trench drains provide the owner with a durable, easy to maintain, and cost effective solution that maximizes site utilization. Contractors welcome the opportunity to speed the project’s critical path, especially since many installations occur in the last phase of construction.