Adjusting a catch basin refers to the task of raising or lowering a casting or precast top from its initial roughly placed elevation to line up in harmony with the adjacent grade, pavement, curb, etc. This task is often looked at separately by a contractor because it is typically done after final surrounding grade elevations are achieved, after the drainage structure below was placed. While the final grade may be known during the initial catch basin and casting installation, it is almost impossible to gauge exactly where the final surrounding elevations or grades will end up before they are actually finished.

The action of adjusting a catch basin can consist of any of the following based on the the installed elevation of the manhole and the finish elevation of the adjacent pavement or grade:

-installing one or several grade rings

-removing one or several grade rings

-installing brick/block and mortar

-removing brick/block and mortar

-shimming with brick and mortar

-shimming with stone and mortar.

4.1.jpg
Adding precast grade rings to a round catch basin

The intent is for the elevation of the top of the catch basin casting to be as close as possible to the final grades when it is first installed.

The extent and scope of the catch basin adjustment depends solely on the scope of the work surrounding it. A common industry rule of thumb is that a catch basin can only be adjusted with precast grade rings, brick. Mortar, etc up to 12″ before an actual precast catch basin riser piece or brick/mortar wall is required. The idea behind this requirement is that anything more than 12″ of grade rings or brick and mortar is ultimately unstable and will be more apt to infiltration and exfiltration of water than a monolithic piece of precast concrete. If an actual precast riser piece is required, or more than 12″ of adjustment is required, this scope of work is often considered a catch basin ‘reconstruction’ rather than an ‘adjustment’.

After the catch basin casting is essentially ‘shimmed’ into its final vertical and horizontal location to match the final grades, it is often required that mortar be mixed and plastered on the inside of the structure from the bottom of the casting to the top of the catch basin. This limits infiltration and exfiltration of drainage water and ground water and inhibits corrosion and damage over time.

Lastly, depending on whether the catch basin is within an area with vehicle traffic, a ‘concrete collar’ may be required.  This is typically the last task done when installing a catch basin and is often lumped into the overall task of adjusting a catch basin. A concrete collar is a cast in place concrete section formed and poured around the casting giving additional strength and reinforcement to the catch basin from vehicle traffic. The concrete collar spreads traffic loads off the structure and also helps resist the entire assembly from shifting or moving with vehicle traffic or freeze thaw conditions.

PRO-RING-RECTANGULAR_1
Shimming catch basin casting before pouring concrete collar

The following are different conditions in which a catch basin will commonly need to be adjusted:

New Catch Basin Installed in ‘non-paved’ area

Concrete collar not typically necessary. Catch basin must be adjusted so adjacent grade will send drainage water to it smoothly. Pipe penetrations if not booted are commonly grouted/mortared in annular space to limit infiltration/exfiltration which can lead to perimeter dirt settling. Also the area inside the structure from the bottom of the casting frame to the top of the structure itself is typically mortared smooth as well.

New Catch Basin Installed in ‘paved’ area

Most commonly catch basins within pavement occur in parking lots and are sqaure shaped with a square poured concrete collar, if a collar is poured at all. Collar will typically be poured to a depth 2-12″ below the bottom of the casting. Pipe penetrations if not booted are commonly grouted/mortared in the annular space to limit infiltration/exfiltration which can lead to perimeter soil settling.

Existing Catch Basin Curb Inlet in newly paved road

Most developed roads currently have curb inlet catch basins which collect storm drainage water from the curb and convey it to the main. The scope of an adjustment for such basins in a newly paved road commonly involves minor vertical elevation adjustments and a concrete collar. Also the area inside the structure from the bottom of the casting frame to the top of the structure itself is typically mortared smooth as well.

(Different phases of an existing curb inlet adjustment)