What is a Grade Level Manhole?
Installing a manhole flush with grade is essential for ensuring avoidance of safety hazards such as creating a tripping hazard or an unlevel surface that might catch a snowplow. But, does this mean a manhole cover should be installed in a completely flat surface?
At times in construction, you may run into a problem where installation of a manhole cover was completed without considering grade level pitch or slope. When a concrete slab is poured perfectly flat or without any slope, this can cause problems with collected water. This can be especially problematic if water is collecting around an installed manhole cover like a bird bath.
What is Grading?
In construction, grading is the process of reshaping land. This determines the grade level of a finished space and can include leveling the ground surface, raising or lowering the ground level, or adding or removing slopes. The main purpose of modifying the grade is to create proper drainage or to prepare land to bear weight.
What does “Grade Level” mean?
Grade level or ground level is the average elevation of the finished ground level at the center of a space. Essentially, grade level is the level of the ground under your feet.
How does grade level affect manholes?
Think about how your car bumps around when driving in a parking lot. Although you may generally say a parking lot looks flat, the entire surface is likely sloped at different locations. The grade of the asphalt might be slightly sloped to move water away from or toward one direction of the property or toward or away from manhole covers or drains.
How do I decide what grade is required for my manhole cover installation?
Recessed or Sunken Surface Level
You may choose to recess the slab of concrete or asphalt around your manhole frame if your goal is to invite water flow toward the drain or cover. Setting the top edge of the manhole frame slightly below grade and sloped or dipped toward the manhole in this way will help the surface shed water, drain water away from one area, and allow it to flow into another area, especially if there is no ground absorption. Generally, when the surface level is pitched toward a drainage system, the water flows toward a catch basin or storm drain system below.
Elevated or Raised Surface Level
You may choose to elevate the slab of concrete or asphalt so that it is pitched away from your manhole frame if your goal is to deter water flow away from the manhole cover. Setting the top edge of the manhole frame slightly above grade or sloped away from the manhole cover in this way will encourage water flow down and away from the manhole cover and out of your utility space beneath. Diverting rainwater, runoff, and debris away from your manhole cover will decrease the degradation of the cover over time.
The scope of your project will determine how you should install your manhole frame in the ground. If you have additional questions about determining the slope required for your installation, give us a call today!
