Appnd_B3

Appnd_B3
Construction Specifications for
Sand Filters, Bioretention and Open Channels
Appendix
B.3
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
B.3.A Sand Filter Specifications
1.
Material Specifications for Sand Filters
The allowable materials for sand filter construction are detailed in Table B.3.1.
2.
Sand Filter Testing Specifications
Underground sand filters, facilities within sensitive groundwater aquifers, and filters designed to
serve urban hot spots are to be tested for water tightness prior to placement of filter media.
Entrances and exits should be plugged and the system completely filled with water to demonstrate
water tightness. Water tightness means no leakage for a period of 8 hours.
All overflow weirs, multiple orifices and flow distribution slots are to be field-tested to verify
adequate distribution of flows.
3.
Sand Filter Construction Specifications
Provide sufficient maintenance access (i.e., 12-foot-wide road with legally recorded easement).
Vegetated access slopes are to be a maximum of 10%; gravel slopes to 15%; paved slopes to 25%.
Absolutely no runoff is to enter the filter until all contributing drainage areas have been stabilized.
Surface of filter bed is to be level.
All underground sand filters should be clearly delineated with signs so that they may be located
when maintenance is due.
Surface sand filters may be planted with appropriate grasses; see Appendix A.
“Pocket” sand filters (and residential bioretention facilities treating areas larger than an acre) shall
be sized with a stone “window” that covers approximately 10% of the filter area. This “window”
shall be filled pea gravel (3/4 inch stone).
B.3.1
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
4.
Specifications Pertaining to Underground Sand Filters (F-2)
Provide manhole and/or grates to all underground and below grade structures. Manholes shall be
in compliance with standard specifications for each county but diameters should be 30” minimum
(to comply with OSHA confined space requirements). Aluminum and steel louvered doors are
also acceptable. Ten inch wide (minimum) manhole steps (12” o.c.) shall be cast in place or
drilled and mortared into the wall below each manhole. A 5’ minimum height clearance (from the
top of the sand layer to the bottom of the upper/surface slab) is required for all permanent
underground structures. Lift rings are to be supplied to remove/replace top slabs on pre-fabricated
structures. Manhole covers should allow for proper ventilation.
Underground sand filters should be constructed with a gate valve located just above the top of the
filter bed for dewatering in the event that clogging occurs.
Underground sand beds shall be protected from trash accumulation by a wide mesh geotextile
screen to be placed on the surface of the sand bed; screen is to be rolled up, removed, cleaned and
re-installed during maintenance operations.
B.3.2
B.3.3
MSHA Standards and Specs.
Section 902, Mix No. 3, f’c = 3500
psi, normal weight, air-entrained;
re-inforcing to meet ASTM-615-60
per pre-cast manufacturer
ASTM A-36
concrete (cast-in-place)
concrete (pre-cast)
non-rebar steel
underdrain piping
impermeable liner
(if required)
AASHTO-M-43
ASTM-D-4833 (puncture strength 125 lb.)
ASTM-D-4632 (Tensile Strength 300 lb.)
ASTM-D-4833 (thickness)
ASTM-D-412 (tensile strength
1,100 lb., elongation 200%)
ASTM-D-624 (Tear resistance - 150
lb./in)
ASTM-D-471 (water adsorption:
+8 to -2% mass)
F 758, Type PS 28 or AASHTO-M278
ash content: < 15%
pH range: 5.2 to 4.9
loose bulk density 0.12 to 0.15 g/cc
peat
leaf compost
underdrain gravel
geotextile fabric (if required)
Specification/Test Method
clean AASHTO-M-6 or ASTM-C33 concrete sand
Material
sand
n/a
n/a
4” - 6” rigid
schedule 40 PVC
or SDR35
n/a
30 mil thickness
n/a
0.375” to 0.75”
0.08” thick
equivalent opening
size of #80 sieve
n/a
Size
0.02” to 0.04”
on-site testing of poured-in-place concrete required:
28 day strength and slump test; all concrete design (cast-in-place or precast) not using previously approved State or local standards requires
design drawings sealed and approved by a professional structural
engineer licensed in the State of Maryland
SEE ABOVE NOTE
structural steel to be hot-dipped galvanized ASTM-A-123
3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” of gravel
over pipes; not necessary underneath pipes
Liner to be ultraviolet resistant. A geotextile fabric should be used to
protect the liner from puncture.
Must maintain 125 gpm per sq. ft. flow rate. Note: a 4” pea gravel
layer may be substituted for geotextiles meant to “separate” sand filter
layers.
The material must be reed-sedge hemic peat, shredded, uncompacted,
uniform, and clean.
Notes
Sand substitutions such as Diabase and Graystone #10 are not
acceptable. No calcium carbonated or dolomitic sand substitutions are
acceptable. No “rock dust” can be used for sand.
Table B.3.1 Material Specifications for Sand Filters
Appendix B.3. ...........Construction Specifications for Sand Filters, Bioretention and Open Channels
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
B.3.B Specifications for Bioretention
1.
Material Specifications
The allowable materials to be used in bioretention area are detailed in Table B.3.2.
2.
Planting Soil
The soil shall be a uniform mix, free of stones, stumps, roots or other similar objects larger than
two inches. No other materials or substances shall be mixed or dumped within the bioretention
area that may be harmful to plant growth, or prove a hindrance to the planting or maintenance
operations. The planting soil shall be free of Bermuda grass, Quackgrass, Johnson grass, or other
noxious weeds as specified under COMAR 15.08.01.05.
The planting soil shall be tested and shall meet the following criteria:
pH range
organic matter
magnesium
phosphorus (phosphate - P2O5)
potassium (potash - K2O)
soluble salts
5.2 - 7.0
1.5 - 4% (by weight)
35 lb./ac
75 lb./ac
85 lb./ac
not to exceed 500 ppm
All bioretention areas shall have a minimum of one test. Each test shall consist of both the
standard soil test for pH, phosphorus, and potassium and additional tests of organic matter, and
soluble salts. A textural analysis is required from the site stockpiled topsoil. If topsoil is
imported, then a texture analysis shall be performed for each location where the top soil was
excavated.
Since different labs calibrate their testing equipment differently, all testing results shall come from
the same testing facility.
Should the pH fall out of the acceptable range, it may be modified (higher) with lime or (lower)
with iron sulfate plus sulfur.
3.
Compaction
It is very important to minimize compaction of both the base of the bioretention area and the
required backfill. When possible, use excavation hoes to remove original soil. If bioretention
B.3.4
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
areas are excavated using a loader, the contractor should use wide track or marsh track equipment,
or light equipment with turf type tires. Use of equipment with narrow tracks or narrow tires,
rubber tires with large lugs, or high pressure tires will cause excessive compaction resulting in
reduced infiltration rates and is not acceptable. Compaction will significantly contribute to design
failure.
Compaction can be alleviated at the base of the bioretention facility by using a primary tilling
operation such as a chisel plow, ripper, or subsoiler. These tilling operations are to refracture the
soil profile through the 12 inch compaction zone. Substitute methods must be approved by the
engineer. Rototillers typically do not till deep enough to reduce the effects of compaction from
heavy equipment.
Rototill 2 to 3 inches of sand into the base of the bioretention facility before backfilling the
optional sand layer. Pump any ponded water before preparing (rototilling) base.
When backfilling the topsoil over the sand layer, first place 3 to 4 inches of topsoil over the sand,
then rototill the sand/topsoil to create a gradation zone. Backfill the remainder of the topsoil to
final grade.
When backfilling the bioretention facility, place soil in lifts 12” to 18”. Do not use heavy
equipment within the bioretention basin. Heavy equipment can be used around the perimeter of
the basin to supply soils and sand. Grade bioretention materials with light equipment such as a
compact loader or a dozer/loader with marsh tracks.
4.
Plant Material
Recommended plant material for bioretention areas can be found in Appendix A, Section A.2.3.
5.
Plant Installation
Mulch should be placed to a uniform thickness of 2” to 3”. Shredded hardwood mulch is the only
accepted mulch. Pine mulch and wood chips will float and move to the perimeter of the
bioretention area during a storm event and are not acceptable. Shredded mulch must be well aged
(6 to 12 months) for acceptance.
Root stock of the plant material shall be kept moist during transport and on-site storage. The plant
root ball should be planted so 1/8th of the ball is above final grade surface. The diameter of the
planting pit shall be at least six inches larger than the diameter of the planting ball. Set and
maintain the plant straight during the entire planting process. Thoroughly water ground bed cover
after installation.
Trees shall be braced using 2” by 2” stakes only as necessary and for the first growing season
only. Stakes are to be equally spaced on the outside of the tree ball.
B.3.5
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
Grasses and legume seed should be drilled into the soil to a depth of at least one inch. Grass and
legume plugs shall be planted following the non-grass ground cover planting specifications.
The topsoil specifications provide enough organic material to adequately supply nutrients from
natural cycling. The primary function of the bioretention structure is to improve water quality.
Adding fertilizers defeats, or at a minimum, impedes this goal. Only add fertilizer if wood chips
or mulch are used to amend the soil. Rototill urea fertilizer at a rate of 2 pounds per 1000 square
feet.
6.
Underdrains
Underdrains are to be placed on a 3’-0” wide section of filter cloth. Pipe is placed next, followed
by the gravel bedding. The ends of underdrain pipes not terminating in an observation well shall
be capped.
The main collector pipe for underdrain systems shall be constructed at a minimum slope of 0.5%.
Observation wells and/or clean-out pipes must be provided (one minimum per every 1000 square
feet of surface area).
7.
Miscellaneous
The bioretention facility may not be constructed until all contributing drainage area has been
stabilized.
B.3.6
B.3.7
sand
[1’ deep]
poured in place concrete (if
required)
underdrain gravel
underdrain piping
geotextile
shredded hardwood
pea gravel: ASTM-D-448
mulch
pea gravel diaphragm and
curtain drain
AASHTO-M-6 or ASTM-C-33
ornamental stone: washed
cobbles
Class “C” - apparent opening
size (ASTM-D-4751), grab
tensile strength (ASTM-D4632), puncture resistance
(ASTM-D-4833)
AASHTO M-43
F 758, Type PS 28 or
AASHTO M-278
MSHA Mix No. 3; f’c = 3500
psi @ 28 days, normal weight,
air-entrained; reinforcing to
meet ASTM-615-60
Specification
see Appendix A, Table A.4
sand
35 - 60%
silt
30 - 55%
clay
10 - 25%
Material
Plantings
planting soil
[2.5’ to 4’ deep]
0.02” to 0.04”
0.375” to 0.75”
4” to 6” rigid schedule
40 PVC or SDR35
n/a
n/a
pea gravel: No. 6
stone:
2” to 5”
Size
n/a
n/a
3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” of
gravel over pipes; not necessary underneath pipes
on-site testing of poured-in-place concrete required:
28 day strength and slump test; all concrete design (cast-in-place
or pre-cast) not using previously approved State or local
standards requires design drawings sealed and approved by a
professional structural engineer licensed in the State of Maryland
- design to include meeting ACI Code 350.R/89; vertical loading
[H-10 or H-20]; allowable horizontal loading (based on soil
pressures); and analysis of potential cracking
Sand substitutions such as Diabase and Graystone #10 are not
acceptable. No calcium carbonated or dolomitic sand
substitutions are acceptable. No “rock dust” can be used for
sand.
for use as necessary beneath underdrains only
aged 6 months, minimum
Notes
plantings are site-specific
USDA soil types loamy sand, sandy loam or loam
Table B.3.2 Materials Specifications for Bioretention
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
B.3.C Specifications for Open Channels and Filter Strips
1.
Material Specifications
The recommended construction materials for open channels and filter strips are detailed in Table
B.3.3.
2.
Dry Swales
Permeable soil mixture (20” to 30” deep) should meet the bioretention “planting” soil
specifications.
Check dams, if required, shall be placed as specified.
System to have 6” of freeboard, minimum above 2 year water surface elevation.
Side slopes to be 3:1 maximum; (4:1 or flatter is preferred).
No gravel or perforated pipe is to be placed under driveways.
Bottom of facility to be above the seasonally high water table per Table 2 of Appendix D.1.
Seed with flood/drought resistant grasses; see Appendix A, Section 2.4.
Longitudinal slope to be 4%, maximum.
Bottom width to be 8’ maximum to avoid braiding; larger widths may be used if proper berming is
supplied. Width to be 2’ minimum.
3.
Wet Swales
Follow above information for dry swales, with the following exceptions: the seasonally high water
table may inundate the swale; but not above the design bottom of the channel [NOTE: if the water
table is stable within the channel, the WQv storage may start at this point – see Figure 3.19]
Excavate into undisturbed soils; do not use an underdrain system.
B.3.8
Appendix B.3. Construction Specifications for Sand Filters, Bioretention and Open Channels
4.
Filter Strips
Construct pea gravel diaphragms 12” wide, minimum, and 24” deep minimum.
Pervious berms to be a sand/gravel mix [sand (35-60%), silt (30-55%), and gravel (10-25%)].
Berms to have overflow weirs with 6 inch minimum head.
Slope range to be 2% minimum to 6% maximum.
5.
Plant Selection
Recommended grass species for use in establishing permanent ground cover are provided in
Section 2.4 of Appendix A.
B.3.9
B.3.10
rip rap
geotextile
pea gravel diaphragm and curtain
drain
underdrain gravel
underdrain
filter strip sand/gravel pervious
berm
check dam (pressure treated)
check dam (natural wood)
Material
dry swale soil
dry swale sand
AASHTO M-43
F 758 Type PS 28 or
AASHTO M-278
Class “C” - apparent
opening size (ASTM-D4751), grab tensile
strength (ASTM-D4632), puncture
resistance (ASTM-D4833)
per county criteria; if
none given, use MSHA
Standards and Specs
Section 905
Specification
USCS; ML, SM, SC
ASTM C-33 fine
aggregate concrete sand
AWPA Standard C6
Black Locust, Red
Mulberry, Cedars,
Catalpa, White Oak,
Chestnut Oak, Black
Walnut
sand: per dry swale
sand
gravel; AASHTO M-43
ASTM D 448
size per county DOT
requirements based on
10-year design flows
varies (No. 6) or (1/8”
to 3/8”)
0.25” to 0.75”
4” to 6” rigid schedule
40 PVC or SDR35
n/a
sand: 0.02” to 0.04”
gravel: ½” to 1”
6” by 6” or 8” by 8”
6” to 12” diameter;
notch as necessary
Size
n/a
0.02” to 0.04”
3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” of gravel over
pipes; not necessary underneath pipes
mix with approximately 25% loam soil to support grass cover crop;
sand (35-60%), silt (30-55%), and gravel (10-25%)
see Bioretention planting soil notes for more detail.
use clean bank-run gravel
do not coat with creosote; embed at least 3’ into side slopes
do not use the following, as these species have a predisposition towards
rot: Ash, Beech, Birch, Elm, Hackberry, hemlock, Hickories, Maples,
Red and Black Oak, Pines, Poplar, Spruce, Sweetgum, Willow
Notes
soil with a higher percent organic content is preferred
Table B.3.3 Open Channel Systems and Filter Strip Materials Specifications
Appendix B.3. ...........Construction Specifications for Sand Filters, Bioretention and Open Channels
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