Construction techniques are extremely critical in ensuring the success of a
bioretention cell. Various construction guidelines and inspection points are
given below, as well as a general construction schedule adapted from the
Prince George's County Bioretention Manual.1
Sequence of Construction
Install sediment control devices.
1/2 day
Grade site to elevations shown on plan. If applicable, construct curb
openings and/or remove and replace existing concrete. Curb openings should
be blocked or other measures taken to prohibit drainage from entering
construction area.
1 day
Stabilize grading within Limit of Disturbance except for the bioretention
area.
1/2 day
Excavate bioretention area to proposed invert depth and scarify the
existing soil surfaces, taking care not to compact the in-situ materials.
1/2 day
Install underdrain system and observation wells, if specified.
1/2 day
Backfill bioretention area with planting soil.
1/4 day
Wet down the planting soil prior to planting vegetation to allow for
settlement.
1/4 day
Excavate or fill to achieve proper design grade, leaving space for the
upper layer of mulch that will bring the surface to final grade.
1/4 day
Plant vegetation.
1/4 day
Mulch and install erosion protection at entrance points. Remove sediment
control practices or entrance blocks with inspector authorization.
1/4 day
Active soil erosion control is the first line of defense against
contamination of the bioretention area.
All sediment control devices must be in place prior to the start of the
main construction.
Grading of any catchment area draining to the facility should be done
sparingly and stabilized immediately (within 14 days).
A bioretention cell should not be placed in service until all of the
contributing drainage area has been stabilized and approved by the
inspector.
Soil materials should not be delivered until the bioretention site has
been excavated or graded and the underdrain systems are in place. Planting
materials should not be delivered until after the soil medium has had time
to settle to the proper grade elevation.
Prior to covering the underdrain system, the inspector must observe the
underdrain itself, the connections, gravel bedding, and any filter fabric.
Manufacturer's tickets are required for the gravel, pipe and filter fabric
material.
If placing gravel over the underdrain, avoid dropping it from high levels
with a backhoe or front-end loader bucket. Spill directly over the
underdrain and spread manually.
Avoid over-compaction of the soil material by allowing time for natural
compaction and settlement. No additional manual compaction of soil
is necessary. To speed up the natural compaction process, presoaking the
placed soil may be performed.
Overfill above the proposed surface invert to accommodate natural
settlement to the proper grade. Depending upon the soil material, up
to 20% natural compaction may occur. If construction scheduling permits,
it is preferable to allow natural settlement to occur with the help of
rain events.
The mulch layer surface should approximate the final elevation as shown on
the design plans.
All plant materials should be tagged for identification in accordance with
the American Standard for Nursery Stock.
The landscaping work and materials shall be guaranteed for a minimum of 1
growing season and a maximum of 2 years from the date of installation.
The warrantee period begins when the as-built construction plan is
approved.
