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851 Bridger Drive, Suite 1, Bozeman, MT 59715 | phone: 406-522-8594 | fax: 406-522-9528
One 11 2.0
Water, Sewer, Storm Water Engineering Report
January 1st, 2021 Revised April 10th, 2021
Prepared for: HomeBase Montana
Prepared by: Stahly Engineering and Associates
Engineer of Record: Cordell D. Pool, PE
Quality Control Reviewer: Zach Lowe, PE
Introduction
The One 11 2.0 Project is an infill redevelopment of Lots A, B, and 8-10, Block B, of Tracy’s Third
Addition. The proposed One 11 2.0 building is an addition to the existing One 11 building, located
directly south at 111 West Lamme Street on Lot B. The five lots will be aggregated into a single
common lot (Lot A-1) for a total lot area of 53,034 sf. Lot A is currently occupied by a parking lot
serving the surrounding area, with some minimal landscaping. Lots 8-10 contain the existing
Montana Conservation Corps building, a small parking area, and a small amount of sidewalk and
landscaping.
The One 11 2.0 Building is a 6-story residential addition featuring 47 one-bedroom studio apartments
and 20 2/3 bedroom residential units. The building ground floor includes a parking garage with 14
parking spaces. The total building gross square footage is approximately 95,000 sf. An additional
surface parking lot containing 32 spaces at the southwest corner of the site in the current location of
the MCC building. The proposed building and site improvements are shown on the Civil plans
provided with the Site Plan submittal. This Engineering Report supports the design of the water and
fire service connection from the One 11 building, sewer service line, and storm water mitigation
systems.
Civil Specifications and Design Standards
The civil specifications for the project are the Montana Public Works Standard Specifications
(MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans are
developed in accordance with the City of Bozeman Design Standards.
Site Improvements
One 11 2.0 site improvements consist of streetscape improvements between the building and the
curb. Streetscape improvements are in accordance with the North Central Master Plan. The
streetscape includes a 6’ (min) concrete sidewalk, and 6’ of permeable pavers between the building
face and the curb. This design provides a flexible urban streetscape that accommodates increased
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summer pedestrian use, and winter snow storage and drainage on the paver strip. Curbed planting
beds are proposed instead of tree-grates to incorporate additional boulevard plantings. The Grand
frontage remains a 6’ curbwalk to provide better connectivity to the adjacent sidewalks.
On-site parking is located at two locations in the project. Structured parking within the building on-
grade on Level 1 will provide approximately 14 spaces. The parking is accessed through the
existing One 11 garage. The traffic flow is one-way through the building and exits onto Grand
Avenue. Accessible parking will be provided in this lot. A surface parking lot is proposed immediately
west of the existing One 11 Building. This surface lot will provide an additional 32 parking spaces.
On-street parking is available on Lamme and Beall, providing 16 on-street parking spaces for the
combined One 11 Buildings. The One 11 building previously utilized 6 of these spaces.
The proposed garage exit driveway on Grand Avenue is closer than the 40’ code requirement from
an intersection. A relaxation from this standard is requested in accordance with the BMC
38.400.090 H. criteria as explained here. The proposed garage exit is replacing two existing parking
lot accesses onto this property that are currently closer to existing intersections. The traffic flow from
the proposed exit is less than the current access since it serves far fewer parking spaces. Because
the proposed access is exit only, the turning movements will not interfere with nearby intersection
turning movements. Grand Avenue is currently stop controlled at Beall Street, reducing vehicle
speeds near the intersection. The proposed exit driveway exceeds site vision requirements. The
garage door is set back 15’ from the property line which provides adequate time to resolve
pedestrian conflicts at the sidewalk. All of these considerations result in improved safety when
compared to the existing conditions.
The proposed access for the surface parking lot, also located on Grand Avenue, is also closer than
the 40’ code requirement from an intersection, with a similar relaxation requested per
BMC38.400.090 H. An enhanced vision triangle as well as a new 18’ traffic calming circle in the
Grand Avenue and Lamme intersection are proposed to facilitate the surface parking lot access.
Access deviation letters for both deviation requests have been provided with this submittal.
Trash is stored within the building and a trash access is provided on Willson Avenue for loading.
The access is configured so there is a 50’ long straight path to the loading area without interfering
with oncoming traffic. A heated slab is proposed in front of the trash door to keep ice from
preventing loading.
Franchise Utilities
Utility service in this area of town is a little different than other downtown locations. The block
between Lamme and Beall is short and an alley was never platted through the block to provide utility
service. This situation has caused utility service providers to run utilities in Beall Street instead of in
an alley. Currently, there is a mix of overhead and underground utilities in the Beall Street right-of-
way, and very few boulevard trees. The One 11 2.0 project is working with Northwestern Energy to
have the existing overhead utilities installed underground, which will improve the streetscape along
its Beall frontage. However, the new and existing utility routing conflicts with the standard boulevard
tree location required by city standards.
The applicant would like to have the street trees and is willing to take the care required to install the
trees in close proximity to the underground utilities, such as calling for utility locates and hand
digging near utilities. The applicant will continue to work with utility providers to install or relocate
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utilities as far away from trees as practical. A new 3.5’ utility easement along the Beall Street
frontage is proposed to provide additional area for the relocated underground utilities.
New underground electrical and communication building services will be along the Grand Avenue
frontage. A 12’ easement along this street frontage is proposed to accommodate these new
underground lines. Electrical meters are located in a semi-enclosed space at the northwest corner
of the building, where they are screened from view.
The One 11 2.0 building will utilize the existing One 11 gas meter for new service. Northwestern
Energy has indicated that the existing meter has capacity to serve the One 11 2.0 building.
Water
Existing City of Bozeman water infrastructure is located in North Grand Avenue, West Beall Street,
and Willson Avenue. The mains in both North Grand Avenue and Willson Avenue are 6” cast iron
lines, while the Beall Street main is a 10” cast iron line. Additionally, water service is available for
connection via the existing One 11 building. Domestic water and fire service will be supplied by
extension of the existing services within the One 11 building, connecting through a shared fireproof
wall.
The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in
this area of town. Three fire hydrants are located within 150 feet of the building. Furthermore, as
part of the 2017 Water Facility Plan Update, a fire flow test was performed on a hydrant
approximately 400 feet away at the corner of northwest corner of the Villard Street and Tracy
Avenue intersection. One of the fire hydrants used in the test is just a block away from the site. The
results for this test (test number 44) are included with this report. The static water pressure is
approximately 137.7 psi. Two adjacent hydrants were opened simultaneously at a 2.5” diameter
nozzle, the hydrant nearest to the site flowed at 1,744 gpm while the other flowed at 1,601 gpm for a
combined flowrate of 3,345 gpm. These flows resulted in a 24.8 drop in psi at the residual test
hydrant, for a residual pressure of 112.9 psi. This indicates that reasonable urban fire flows can be
met in the area. Specifically, each of the nearby 2 hydrants could be expected to provide similar
flows, resulting in approximately 3,500 gpm of fire flow available to One 11 2.0.
Water service will be extended from the existing One 11 building into the One 11 2.0. The existing
One 11 building is currently served by a 4” service with a 3” meter. The preliminary water fixture unit
count is 1782 units, with 851 fixture units existing in One 11 and 931 proposed in One 11 2.0. The
combined fixture count results in a peak flow exceeding the capacity of the existing 3” meter. The
existing meter will need to be upsized from 3” to 4”, while the 4” service will continue to provide
adequate water supply to the increased demand.
Irrigation water supply will be provided through the domestic water service. Irrigation demands were
provided by the landscape architect. Irrigation is anticipated to occur over an approximately 100 -day
season resulting in a total annual water use of 17,667 gallons, or 0.05 ac-ft. The average daily
irrigation flow is 177 gpd.
The estimated water use One 11 2.0 is provided below in Table 1. The water use estimate provided
in this report is based on proposed building use. Residential water use is estimated using typical
values of 65 gpd/capita with an assumed 2.17 capita/unit. Retail water use is estimated at the typical
value of 30 gpd/1000sf. Irrigation water use estimates have been provided by the landscape
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architect. The average day demand is estimated to be 9,702 gpd. Applying a peaking factor of 4.5,
the anticipated peak hour demand is 30.3 gpm.
Table 1. One 11 2.0 Annual Domestic Water Use
Annual Domestic Water Use
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Unit 67 65.0 2.17 9450
Non-Residential Area (sf) Gpd/1000sf Gallons/day
Retail Space 2,500 30.0 75
Total Domestic Use 9,525
Domestic Annual Use (gallons) 3,476,753
Irrigation Annual Use (gallons) 17,667
Average Day Demand (gpd) 9,702
Peaking Factor 4.5
Peak Hour (gpm) 30.3
The North Central Master Plan anticipates water use that is lower than typical city design flow
estimates. However, there is not currently an established methodology for estimating water use in
this kind of urban, high-density residential use. High efficiency fixtures and efficient design reduces
the per capita water consumption, and this type of urban development typically does not attract
families which results in a lower water use per unit than is typical in the City. While data has not
been available for the requisite 3 years for consideration, nor are there enough comparable
developed sites, recent data from the similar Black Olive building indicate a water usage rate of
approximately 50 gallons/day/unit. It is anticipated that water use within North Central will be very
similar to that of Black Olive. Comparing of the City accepted water use estimates to measured
Black Olive meter reading data show the Black Olive water use to be approximately 35% of the City
estimates.
City provided calculations for determining Cash-in-lieu-of water rights payment has estimated the
annual water use of the One 11 2.0 building as 5.94 ac ft. The total CILWR is $35,655.
Sewer
An existing 10” Vitrified Clay sewer main is in both the North Willson Avenue and North Grand
Avenue. One new sewer service is proposed to be connected to the 10” sewer main in East Lamme
Street.
The new sewer service was sized by the plumbing fixture counts in the building and the Uniform
Plumbing Code flow rate for this number of fixture counts. The preliminary drainage fixture unit count
is a total of 916 fixture units. For this number of fixture units, the plumbing code requires an 8”
sewer service line.
Daily wastewater generation was determined utilizing the same residential unit estimates as
described in the water use estimate. Applying this value results in an average daily flow for the
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project of 6.61 gpm. Using a peaking factor of 4.5 the peak hour flow is estimated at 29.8 gpm.
These values and demands are summarized and tabulated below.
Table 2. One 11 2.0 Estimated Sewer Use and Demands
Estimated Sewer Use and Demands
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Units 67 65.0 2.17 9450
Non-Residential Area (sf) Gpd/1000sf Gallons/day
Retail Space 2,500 30.0 75
Total Domestic Use 9,525
Average Day Demand (gpm) 6.61
Peaking Factor 4.5
Peak Hour (gpm) 29.8
As mentioned in the previous section, the anticipated domestic water use for the One 11 2.0 Building
will be much lower than typical City design flow estimates, resulting in a average day demand and
peak hour demand substantially lower than presented above.
Storm Water
As mentioned previously, existing conditions on the site include substantial development and a
considerable percentage of impervious area. Stormwater analysis for the project has been divided
into two catchment areas, one for the building and one for the surface parking lot, each containing its
own stormwater mitigation system. Both drainage basins will provide stormwater mitigation via
underground infiltration chambers. Runoff from the parking garage will flow through a 1000-gallon
sand/oil separator. Due to higher flow rates the surface parking lot will include a pretreatment
manhole structure with a vortechs separator for pretreatment.
The existing site conditions within the building drainage basin consist almost entirely of a single
connected impervious area (parking lot) without any stormwater mitigation. Currently, runoff from
the site is primarily directed east or west towards the curb in North Grand Avenue and Willson
Avenue, respectively, where it is collected into City of Bozeman storm sewer. There is no existing
stormwater mitigation.
The existing conditions on the parking drainage basin include an existing structure, the MCC
building, an on-site asphalt parking lot, and some associated hardscapes and landscaping.
Currently, storm runoff leaves the site without mitigation into Grand Avenue, where it is captured by
a curb area inlet on the northwest corner of the Block.
Storm water mitigation is based on the redevelopment project low impact design (LID) requirement in
the City Design Standards to “infiltrate, evapotranspire, or capture for reuse the runoff generated
from the first 0.5” of rainfall. Additionally, to reduce impacts to the existing storm drainage
infrastructure, the stormwater mitigation will reduce peak flows from larger storm events to below
existing values. The most limiting stormwater mitigation criteria for this site is to provide mitigation of
the larger storm events; ensuring that the post developed runoff is equal to or less than existing
condition runoff values. The proposed stormwater mitigation system will capture, retain, and infiltrate
the building rooftop runoff for storms up to and exceeding 0.5” of rainfall. The infiltration system will
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capture rooftop runoff and pipe it to a subsurface chamber/gravel infiltration system within the
building footprint. All stormwater piping has been sized to handle the 25-year design event.
Permeable pavers will be installed along the sidewalk to further reduce stormwater runoff generated
from the improved sidewalk areas and to provide snow storage.
A summary of the stormwater calculations for the building and the parking lot drainage basins are
provided in Table 3 and Table 4, respectively, below. For the building drainage basin, the required
storage volume for the 0.5” event is 675 cf and the required storage volume to mitigate peak flows is
107 cf. The proposed net retention storage volume is 1,490 cf. The proposed infiltration system will
completely retain runoff from storms up to the 1.10-inch event, which represents the 99th percentile
of storm events in Bozeman. Any flows generated in storms that exceed the storage volume will be
conveyed to the City of Bozeman storm sewer via an overflow pipe that directs water to a curb inlet
near the northwest corner of the site on Grand Avenue.
For the parking lot drainage basin, the required storage volume for the 0.5” event is 540 cf and the
required storage volume to mitigate peak flows is 428 cf. The proposed net retention storage volume
is 1,1,98 cf. The proposed infiltration system will completely retain runoff from storms up to the 0.97-
inch event, which represents the 98th percentile of storm events in Bozeman. In addition to
containing the volume of the first 0.5”, the proposed storage also contains the runoff volume increase
from pre-existing to post-development in the 10, 25, 50, and 100-year 24-hour events.
Conveyance capacity of the building stormwater system is analyzed with respect to the post
development stormwater flows. Rooftop runoff is collected in a single 8” pipe to the infiltration
system. This 8” pipe connects to the stormwater system manhole at a 2% slope, allowing a full flow
capacity of 1.82 cfs with a 5.50 ft/s velocity. Overflow piping from the storm system manhole to the
curb inlet is achieved with a 10” PVC pipe at a 1.9% slope. The overflow pipe has a full flow
capacity of 3.22 cfs with a 6.23 ft/s velocity.
In addition to containing the volume of the first 0.5”, the proposed storage at both locations also
contains the runoff volume increase from pre-existing to post-development in the 10, 25, 50, and
100-year 24-hour events. Considering the similar level of impervious area in pre and post
development, the introduction of One 11 2.0’s stormwater mitigation system will greatly reduce
volumes introduced to the existing City infrastructure from the site.
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Table 3. One 11 2.0 Building Storm Water Calculations
Site Statistics
Land Classification C Existing Area (sf) Post Dev Area (sf)
Rooftops 0.9 0 17589
Pavement 0.9 17,144 0
Sidewalk 0.9 0 240
Permeable Pavers 0.3 0 0
Landscape 0.2 1,474 789
Total 18,618 18,618
Weighted Runoff Coeff. (C ) 0.84 0.87
Design Storm Information
Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres) 0.427 0.427 0.427 0.427 0.427
Drainage Area (sf) 18618 18618 18618 18618 18618
Slope (%) 2 2 2 2 2
Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0
24 Hour Precipitation Volumes (in) 0.50 1.84 2.16 2.42 2.67
Existing Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.16 1.38 1.71 1.93
Runoff Volume (cf) 655 2411 2830 3171 3499
Post Dev Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.20 1.42 1.76 1.99
Runoff Volume (cf) 675 2485 2917 3268 3605
Mitigation Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Runoff Volume Increase (cf) 20 74 86 97 107
Retention Volume (cf) 1490 1490 1490 1490 1490
Net Runoff Volume Post Mitigation (cf) 0.00 995 1427 1778 2115
% Decrease in Runoff from Existing 100.00 58.7 49.6 43.9 39.5
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Table 4. One 11 2.0 Surface Parking Lot Storm Water Calculations
Site Statistics
Land Classification C Existing Area (sf) Post Dev Area (sf)
Rooftops 0.9 2,805 0
Pavement 0.9 4,279 9,282
Sidewalk 0.9 593 0
Permeable Pavers 0.3 0 0
Landscape 0.2 3,297 1692
Total 10,974 10,974
Weighted Runoff Coeff. (C ) 0.69 0.79
Design Storm Information
Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres) 0.431 0.431 0.252 0.252 0.252
Drainage Area (sf) 18792 18792 18792 18792 18792
Slope (%) 2 2 2 2 2
Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0
24 Hour Precipitation Volumes (in) 0.50 1.84 2.16 2.42 2.67
Existing Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.16 0.81 1.00 1.13
Runoff Volume (cf) 540 1987 2333 2614 2884
Post Dev Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.22 0.84 1.05 1.18
Runoff Volume (cf) 620 2282 2679 3002 3312
Mitigation Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Runoff Volume Increase (cf) 80 295 346 388 428
Retention Volume (cf) 1198 1198 1198 1198 1198
Net Runoff Volume Post Mitigation (cf) 0.00 1084 1481 1804 2114
% Decrease in Runoff from Existing 100.00 45.4 36.5 31.0 26.7
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Storm Water Maintenance:
General Information
The proposed storm water conveyance and infiltration facilities will be operated and maintained by
the property manager.
Storm Water Facilities Maintenance Schedule
1. Site Housekeeping. (Continuously as needed)
The main cause of storm water facility damage is poor site housekeeping. Sediment tracked
onto pavement can be washed into storm water appurtenances and damage these facilities.
Trash can clog conveyance structures, potentially causing property damage.
Keep sidewalk, permeable pavers, and parking areas clean.
Pick up trash.
Restore damaged landscaping in order to prevent sediment runoff.
2. Curb, Sidewalk Chase, and Infiltration System Maintenance. (Quarterly)
All storm water conveyance structures can acquire sediment and debris buildup. If this
sediment and debris is not periodically removed, it can cause undesired ponding and
clogging. These conveyance structures need to be inspected and cleaned if required.
Inspect for sediment or debris in the structures and remove if present.
Inspect infiltration system through inspection ports for sediment accumulation.
Sediment depth less than 3” is acceptable.
Check for damage, repair as needed.
3. Curb and Infiltration System Maintenance. (Long-term)
If regular housekeeping and maintenance is not performed adequately, sediment and debris
can accumulate in the storm water conveyance structures and infiltration system and clog
them beyond repair.
If greater than 3” of sediment is present in infiltration system, hire a contractor with a
Jet-Vac chamber cleaning system to remove the sediment from the infiltration system.
If original system performance can be achieved through maintenance, hire a
contractor to repair and return conveyance structures and infiltration system to the
initial design condition found on City engineering plans.
4. System Monitoring. (Quarterly, except in winter)
The storm water facilities shall be inspected quarterly to quickly identify small issues before
expensive damage can occur. In addition to regular monitoring, the best time to inspect the
performance of storm water facilities is during runoff events.
Observe system during runoff. Look for ponding on permeable pavers or inlet
structures. This can indicate a clogged paver infiltration and/or clogged conveyance
structure.
Open infiltration system inspection ports within 24-hours of a storm event and look for
ponded water in the infiltration system. This can indicate clogged infiltration system.
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If clogged hire a contractor with a Jet-Vac chamber cleaning system to remove the
sediment from the infiltration system.
5. PERMEABLE INTERLOCKING CONCRETE PAVEMENT (PICP) Inspection & Maintenance
Guidelines. Service inspection and maintenance shall include the following activities:
Winter Maintenance:
o Ensure only joint aggregate stone (typically # 8, #89 or #9 washed chip stone)
is used for traction as needed. Sand should not be used for winter traction.
Normal Maintenance:
o Inspect surface for ponding after large rain events. If ponding is observed,
identify areas with severe sediment loading and vacuum to remove and
replace with new washed joint aggregate (typically # 8, #89, or # 9 washed
chip stone).
o Note any sediment laden run-off from adjacent areas onto permeable
pavement. If needed, correct with erosion control measures.
Annual inspection and maintenance shall include the following activities:
o Replenish paver joints with additional aggregate if level is more than ½ in.
below chamfer bottoms.
o Inspect vegetation around PICP perimeter for cover & soil stability,
repair/replant as needed.
o Inspect and repair all paver surface deformations (depressions/settlement)
exceeding 1/2 in.
o Repair paver heights offset by more than 1/4 in. above or below adjacent units
or offset by more than 1/8” lippage from paver-to-paver.
o Replace cracked paver units impairing surface structural integrity.
o Check drains and outfalls (if existing) for free flow of water. Remove any
obstructions.
o Check observation wells (if existing) to confirm reservoir is draining (based on
size of last rain event).
o Vacuum surface (typically spring), adjust vacuuming schedule per sediment
loading. Once a year sweeping is normal unless excessive silts and fines are
present in joints.
o Test surface infiltration rate using ASTM C1781. If pavement infiltration rate is
< 100 in/hr. employ remedial maintenance procedure utilizing a vacuum
sweeper/method to extract affected clogged joints/voids and replace joint/void
areas with #8, #89 or #9 washed chip aggregates and retest infiltration rate to
confirm reinstated areas exceed 100 in/hr. flow rate. Repeat remedial process
as needed to exceed the 100 in/hr. criteria.
Additional Normal Maintenance Notes:
o A dry mechanical or regenerative air-type sweeper may be used during dry
periods to remove encrusted sediment, leaves, grass clippings, etc. Vacuum
or sweeper settings may require adjustments to prevent uptake of aggregate
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from the paver voids or joints. Leaf blowers or other standard onsite manual
methods that are used for standard pavement maintenance may be employed
to remove this surface debris.
o It is not recommended to utilize a pressure washer to clean joints.
o Remove snow with standard plow/snow blowing equipment.
o Deicing salt may be used on permeable pavers (proper application and
appropriate salt type) but consult property owner or project engineer before
usage. In some regions deicing salt use is restricted. Salt use can affect water
quality and have environmental impact.