Hydraulics, traffic loading, environmental factors, and grate properties are key to effective design.
Whether handling a light rainfall or a heavy downpour, properly designed surface drainage is important to protect against damage to pavement or property and to reduce safety hazards related to stormwater buildup and flooding. A common method is to use catch basin drainage, collecting stormwater at a low point and removing it through underground piping. The area surrounding each basin must have a downward grade on all four sides to ensure effective drainage and eliminate ponding. However, constructing these multi-directional grades can be difficult and costly, especially when the size of the facility requires a number of catch basins. This can also create an undulating surface when used over a large area (see Figure 1). In addition, cleaning out debris that can build up in excessive underground piping can become expensive if excavation is needed.
An alternative approach is to use trench drainage. Here, a simple, angled grade on one or both sides of the trench moves runoff into the linear drain so it can be discharged into an underground pipe system or culvert (see Figure 2). A much shallower and less costly excavation is required.
One method is a cast-in-place trench drain, where the drain is formed on-site and the frame and grate are added. Problems can include unexpected construction issues, erratic drain quality and slope, and high maintenance and replacement costs for damaged, broken areas and parts.
Another solution—the focus of this article—is a pre-cast, modular trench drain system engineered to ensure fast and efficient liquid evacuation and designed for specific site requirements. This type of system can be easily installed and maintained. Value engineering studies conducted by ACO indicate that a modular trench drain system, on average, costs 11 percent less to install and maintain than a catch basin system, and 20 percent less than a cast-in-place system.
The following five steps provide an overview of the process of planning and specifying a modular trench drain system.
Step 1: Understand hydraulic performance—Accurately calculating hydraulics ensures that the channel can properly intercept and collect the necessary volume of liquids and evacuate this quickly within a specific time period. Historically, water flow calculations have been based on assuming a steady uniform flow of water using Manning’s Theory, where liquid velocity and height remain constant along the channel. These calculations are useful when calculating size for a culvert, where liquids are carried from one area to another, or for closed-pipe flows. But, they do not take into account lateral intake of fluids along the way, the whole point of a trench drain. This system of measurement is often inaccurate and can lead to the trench drain capacity being over-estimated by more than 100 percent (trench drain designed too large), which can add significant costs to construction. Conversely, an under estimate (trench drain too small) can lead to severe flooding.
The more accurate method is to assume a steady, non-uniform flow, also called spatially varied flow. Here, liquid being carried in a trench drain is continually added to as more liquids are collected through the grates all along the trench run. Key characteristics of non-uniform flow are that liquid velocity and height change along the trench.
More than 25 years ago, ACO engineers, working with a German hydraulics institute, developed a computerized program to show what happens to fluid in a trench drain based upon full-scale laboratory tests. The analysis shows that the level of water flowing in a channel does not remain constant, but builds up along the run and then falls toward the discharge point. Consequently, if the water surface at the discharge point is lower, the velocity at that point must be greater. Engineers have confirmed that the velocity of water flowing in a trench varies continuously along a given length, being at its greatest at the discharge point and at its slowest at the upstream end of the trench. This affirms that there is an improvement in both capacity and velocity with a sloped trench drain versus a level trench of uniform depth. This also illustrates why a sloped trench drain delivers better self-cleansing dynamics than a trench drain with minimal or no built-in invert slope.
ACO offers free computer-based trench drain analyses for engineers, planners, and specifiers who need to design these systems. Using supplied project data, ACO calculates the rate of water flow and recommends the best type of trench system. First, runoff is calculated using the following data:
- length of trench run;
- size of the catchment area;
- the area’s rainfall intensity (available online);
- ground fall percentage;
- surrounding pavement material;
- position and size of the outlet pipe;
- surface roughness of the trench material; and
- angle of perpendicular approaches to the trench.
Ground fall along the trench is considered next since slope increases the velocity of the liquid within the trench drain and increases hydraulic efficiency. Slope can be introduced by using the existing pavement with natural fall, by incorporating slope along the base of the trench run, or by a combination of these. ACO also recommends using a U-shaped or V-shaped channel bottom to maximize the velocity of liquids at low volumes.
The position and size of outlet pipes are also important because ultimately the trench drain connects to an underground pipe system. There are a number of options here, but designers should ensure the outlet or subsequent pipe work is not undersized. Catch basins can be engineered into the trench drainage design to collect debris before it reaches the underground piping.
Grate hydraulics can also be a factor in design, especially in systems using restricted openings such as perforated and ADA-compliant grates. And finally, any slab depth restrictions must be noted since these will affect the type of products that can be used.
Step 2: Determine traffic loadings—Loading on the trench drain during its service life can range from light foot traffic to extremely heavy vehicles, including airplanes and heavy construction equipment. A number of U.S. standards reference grate loading and large catch basin grates, but these may not be applicable when specifying smaller trench drain grates. The only standard written specifically for trench drains is the internationally recognized DIN 19580/EN1433 that accounts for different widths of grates with various required test parameters. DIN 19580/EN1433 offers testing methods for both the complete trench drain system and individual grates, accounting for both proof loading and catastrophic failure.
Current DIN/EN Load Classifications for trench drains include:
- A: up to 3,372 pounds, for residential and light pedestrian traffic;
- B: up to 28,100 pounds, for sidewalks and small private parking lots;
- C: up to 56,200 pounds, for parking lots and general commercial use;
- D: up to 89,920 pounds, for trafficked sections of roads and highways;
- E: up to 134,800 pounds, for industrial areas, gas stations, and light commercial forklifts; and
- F: up to 202,320 pounds, for aircraft pavements, docks, heavy fork trucks, and other similar wheel loads.
When specifying a system or grate it is important to identify the type of wheel that will move over the drain. Larger and/or pneumatic tires spread the load over a larger contact area, thus exerting a lower stress (pounds per square inch). Smaller and/or solid tires concentrate the load onto a small contact area, exerting higher stress and requiring a grate or system with a higher load rating.
A key consideration is not only the weight of the vehicle but also the typical load the vehicle will carry. This combined weight is considered the static load/weight if the wheel is applied vertically onto the trench with no other movement.
However, static load weights poorly simulate real traffic situations. Once the static load has been determined, use a dynamic load model, asking the following application questions to account for the dynamic loads:
- Will vehicles or wheeled equipment travel across or along the trench?
- Will traffic brake or turn on the trench?
- What is the frequency of traffic?
- What is the speed of traffic?
- Is the trench located at the bottom of a ramp?
Moving traffic creates a dynamic load in which the movement tries to twist the trench drain out of position. Thus the trench body, grate load rating, installation, and locking mechanisms are all important factors to consider when addressing dynamic loads. The more movement (turning or braking) and the faster the traffic, the greater the dynamic load.
Step 3: Plan long-term durability—After determining the mechanical performance factors of hydraulics and loadings, specify the correct trench body material and grate for the commercial or industrial application. This decision involves all the environmental factors related to climate (freeze/thaw cycles), strength, chemical resistance, UV stability, and fire resistance. It also takes into account the pavement finish that surrounds and protects the trench system. To accommodate these factors, modular trench drainage systems are available in polymer concrete material, fiberglass, or High Density Polyethylene (HDPE).
Polymer concrete is a versatile, durable composite material manufactured by mixing mineral aggregates with polymer resin as the binding agent. It offers excellent mechanical and thermal properties, excellent corrosion resistance to many chemicals and road salts, minimal water absorption (less than 0.1 percent) to prevent freeze-thaw breakdown, and a coefficient of expansion/contraction closely tied to portland cement concrete. For increased chemical resistance, vinyl ester polymer concrete can be specified.
Lightweight HPDE drain systems are often incorrectly specified for industrial trench drain service. Although acceptable for some types of residential and light commercial drainage applications, HDPE drains have poor thermal properties. For example, in changing temperatures, an HDPE trench drain can expand (or contract) as much as 10 times that of the surrounding portland cement concrete slab, causing the HDPE trench to buckle or pull away from the concrete. Thus, polymer concrete or fiberglass is the material of choice.
Another key durability consideration is edge protection of the channel. The exposed edge of the trench holds the grate in position and is subject to the same loads as the grate. In addition to the effect of climate and the weight of vehicles, the edge may be exposed to impact from items being dropped or pulled across it, such as snow plows. Once the edge fails, the grate will move and cause major problems. Metal edges are most commonly used to withstand this abuse in heavier duty applications. Edge protection rails should be integrally cast-in or mechanically connected to the trench body.
Choosing the appropriate grate completes the application. Most grate materials are manufactured from iron, mild steel, stainless steel, or composite materials. Grates need to have better tensile properties than the trench body to withstand flexural loads. Grates can be changed or easily replaced after installation, unlike the trench drain body.
Step 4: Consider user requirements—Once hydraulics, loading, and chemical resistance requirements are met, the grate decision focuses on meeting specific user and legislative requirements, including visual and cost preferences. Grating materials such as stainless steel, ductile iron, composites, and brass offer a variety of visual effects. A grate can blend with the pavement or be used as a contrasting feature or border. Aesthetically pleasing Grate slots with perforated, slot, mesh, or decorative patterns can further enhance the appearance.
Another key grate consideration is meeting legislative requirements such as the Americans with Disabilities Act of 1990. Section 4.5.4. stipulates, “Where grates are used within walking surfaces, the open slots should be no greater than 0.5 inches (12.7 mm) wide in one direction. Where the length of the slot is greater than 0.5 inches, the opening should run perpendicular to the main direction of traffic.” This is intended to prevent wheelchair wheels and walking aids from becoming trapped or slipping on the grate surface.
Other safety considerations can include “heelsafe” grates. According to ASME: A112.6.3—2001: Section 7.12, this would be “a grate designed to resist entry of high-heeled shoes, in which the maximum grate hole size in least dimension shall be 5/16 inch (8mm).”
The final user requirement in choosing a grate is security. It is highly recommended to secure the grate to the channel to prevent movement caused by traffic, which can lead to trench and grate damage. There are a number of locking options, both boltless and bolted, that are quick and easy to install and remove.
Step 5: Installation—Even if steps one through four are correctly specified, the trench drain system may fail if incorrectly installed. Prior to starting, refer to installation specifications provided by the manufacturer. Typically, installing a modular trench drain system involves four stages, although this procedure can vary slightly from a new installation to a retrofit.
Stage One: Excavate the area to allow a minimum of 4 inches, 6 inches, or 8 inches on each side and underneath the drain system. How much depends on the load classification; some products may require a larger excavation. Ground conditions can have a major effect on the installation details and consideration of this should ensure channels do not shift during or after installation.
Stage Two: Prepare for installation by starting at the outlet and then working away from it. The trench unit should be positioned at the correct elevation so that the top of the adjacent pavement will be above the grating level 1/8 inch.
Stage Three: Pour the concrete surrounds evenly on either side of the trench following the manufacturer’s instructions for the load and pavement surface. Concrete should be low slump with a 3,000-psi minimum.
Stage Four: Install paving material to finished grade such that it is 1/8 inch above the edge of the trench drain to ensure positive drainage into the trench.
Regularly inspect and clean installed trench drains, including cover grates and locking devices, catch basins and trash buckets, and the concrete surround and adjacent paving. Work closely with the trench drain supplier on guidelines related to specific installations.
Ray Wofford is special projects executive and head of continuing education programs for ACO Polymer Products. For more information, visit www.acousa.com.