3. Retaining Walls Issue Paper
CITY OF
CHANHASSEN
7700 Market Boulevard
PO Box 147
Chanhassen, MN 55317
Administration
Phone: 952.227.1100
Fax: 952.227.1110
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Fax: 952.227.1190
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Phone: 952.227.1400
Fax: 952.227.1404
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Natural Resources
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1591 Park Road
Phone: 952.227.1300
Fax: 952.227.1310
Senior Center
Phone: 952.227.1125
Fax: 952.227.1110
Web Site
www.ci.chanhassen.mn.us
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MEMORANDUM
TO:
Planning Commission
FROM:
Bob Generous, Senior Planner
DA TE:
February 21,2005
SUBJ:
Potential Subdivision and Zoning Ordinance Amendments
Retaining Walls
ISSUE
Retaining walls appear to be becoming a larger component of subdivisions and
development design. However, the current city code provides little direction,
guidance or regulation.
Staff has prepared the following discussion items to stimulate investigation into
the issue and determine if we should proceed with the preparation of an ordinance
amendment. The goal is to have standards for high quality design and
maintenance assurances. Following is a discussion of the issues.
DISCUSSION
Existing Chanhassen Ordinance:
Sec. 20-1025. Retaining Walls
Retaining walls exceeding four feet in height, including stage walls which
cumulatively exceed four feet in height, must be constructed in accordance with
plans prepared by a registered engineer or landscape architect and shall be
constructed of a durable material. A permit is required for the construction of
such retaining walls.
Other Communities
Staff researched zoning ordinances of numerous Twin Cities area municipalities
including Eden Prairie, Minnetonka, Shakopee, Woodbury and those communities
comparable to Chanhassen as part of the "Key Financial Strategies List". It was
found that the majority of these city codes generally contain very similar or less
restricti ve ordinances than that of Chanhassen. The City of Woodbury did
provide standards for the fencing specification: "black, brown or dark green vinyl
clad chain link fences or wrought iron fences or better shall be used on top of
retaining walls for safety reasons".
The City of Chanhassen · A growing community with clean lakes, quality schools, a charming downtown, thriving businesses, winding trails, and beautiful parks. A great place to live, work, and play.
Planning Commission
Retaining Walls Issue Paper
February 21, 2006
Page 2 of 3
Design: While the city does not want to prescribe all retaining wall designs, we want to
encourage that suitable standards are being met. Normally, the manufacturers, for retaining
walls less than four feet tall, and the design engineer, for retaining walls over four feet in height,
must determine the need for backfill area depth, backfill material, compaction, drainage piping,
foundation requirements, geo-fabric, etc.
For retaining walls as part of subdivisions and site plans, staff is recommending that we require
the design comply with the Minnesota Department of Transportation (MnDOT) standards.
MnDOT has performed the research and analysis for retaining wall design.
Drainage: Drainage is an important issue when designing retaining walls. Retaining walls
should be designed to reduce or eliminate the volume of runoff flowing over the wall in order to
reduce the potential of erosion at the bottom of the slope. Should the city require retaining walls
greater than four feet high to comply with MnDOT standards, a drainage system consisting of
porous backfill material behind the wall and drain pipes will be installed.
Landscaping: Landscaping between staged retaining walls should be low or no maintenance
because of potential difficulty in accessing the area.
Location: City Code currently prohibits retaining walls in drainage and utility easements.
However, staff would also recommend that retaining walls over six (6) feet in height located
within 10 feet of any public way (sidewalk, street, trail, alley, etc.) be required to have a fence or
other barrier, such as a berm or landscaping, to impede access to the retaining wall. Staff
recommends that fences or berms be installed no closer than 18 inches from a sidewalk or trail to
allow adequate room for bicycle handlebars and to provide a recovery zone adjacent to the
sidewalk or trail.
Maintenance: Staff is proposing that retaining walls over four feet high constructed as part of a
subdivision or that cross property lines, be maintained by the development, either through a
homeowners association, condo association, property manager or cash security and not the
individual property owners on whose lots the retaining walls are located.
Materials: As part of the 2004 Zoning Ordinance amendment, staff was initially recommending
the following materials - brick, concrete or natural stone with a caveat that artificial material may
be approved if appropriate.
Rather than specify materials to be used, staff is recommending that we prohibit the use of
certain materials: smooth face concrete (poured in place); however, stamped or patterned
concrete face may be acceptable, masonry (mortared), railroad ties, and timber. These materials
are less durable, tend to absorb moisture and are less aesthetically pleasing than patterned walls,
segmental block or rock retaining walls.
When required, black, brown or dark green vinyl clad chain link or wrought iron fences or better
shall be used on top of retaining walls for safety reasons (wood privacy fences?). Retaining
walls shall be constructed of a permanent material and finish and shall be of a compatible
material with the principal structure.
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Planning Commission
Retaining Walls Issue Paper
February 21,2006
Page 3 of 3
Safety: Safety issues include location, height, fencing and barriers and design. Staff is
recommending that retaining walls over six (6) feet in height located within 10 feet of any public
way (sidewalk, street, trail, alley, etc.) be required to have a fence or other barrier, such as a
berm or landscaping, to impede access to the retaining wall.
Separation: At one time, the city required separation between walls to equal the height of the
wall, unless the wall was designed by an engineer. Currently there are no separation
requirements. Staff proposes that we require a separation of 1 Y2 times the wall height; otherwise,
the retaining walls must be engineered.
RECOMMENDA TION
Staff is requesting Planning Commission discuss the issue of retaining walls and provide
direction regarding the preparation of regulations for retaining walls.
ATTACHMENTS
Pictures
Anchor Wall System Glossary
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Page 1 of 3
Backfill
The material used to fill the excavated area
behind the wall. This can sometimes be the
original excavated soil. We recommend using
3/4" (19.1mm) minus aggregate with no fines
as your backfill material.
Base
The area in which you will lay your first course
of block. We suggest using 3/4" (19.1mm)
minus aggregate with fines as your base
material.
To prepare your base complete the following
steps: Lay 6" (152.4mm) of compactable
aggregate (sand and gravel) in the bottom of
the trench and compact. We recommend using
3/4" (19.1mm) minus aggregate with fines as
your base material and the use of a hand
tamper to compact.
Base Course
The very first row of blocks installed on the
prepared base.
Always bury the first course: After you've
spent time compacting the base, bury and level
the first course. This will help assure a long-
lasting, beautiful wall.
Compaction
Compressing or densifying the soil material
used for the base and backfill is called
compacting. For best results, use a hand
tamper to compact.
Course
A course is the horizontal layers of blocks used
to build a wall.
Drainage Aggregate
Drainage aggregate is important for wall
stability because it allows water to drain down
past the wall instead of being absorbed in the
soil and building up pressure behind the wall.
We recommend aggregate be placed directly
behind the blocks and acceptable native
(original) soil being used for the remaining
void.
Grade
This refers to the ground level.
Gravity Wall
There are two types of retaining walls, gravity
and reinforced. Gravity walls are typically 2-3
feet (.6m-.9m) high and recommended for do-
it-yourselfers. A gravity walls basically stands
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Page 2 of 3
on its own, with the help of some basic steps,
due to the weight of the blocks themselves and
the compacted soil behind the wall. See the
chart below to find out how high to build your
gravity wall.
Highland Stone: 3 1/2 feet (1.1m) or 8 courses
of block, which includes a buried base course.
Diamond: 3 1/2 feet (1.1m) or 8 courses of
block, which includes a buried base course.
Meadow Stone: 20 inches (S08mm), or 6
courses of block, which includes a buried base
course.
Windsor: 20 inches (S08mm), or 6 courses of
block, which includes a buried base course.
Border Stone: 12 inches (.3m), or four
courses, which includes a buried base course.
Sahara: 20 inches (S08mm), or 6 courses of
block, which includes a buried base course.
Running Bond
This refers to a staggered vertical alignment
used to create a consistent pattern and add to
your wall stability. The wall in fig. A is on bond.
This is how you want your wall to look. The
wall in fig. B is off bond.
Partial Unit
A block that has been split to keep a course of
block on bond. To split a block, use a hammer
and chisel to score the block on all sides.
Pound the chisel along the score line until the
block splits. If the block does not split easily,
you may need to use a circular cut-off saw with
a masonry blade. Read and understand the
operating manual before using the saw. Always
wear eye protection when splitting blocks.
Partial blocks should always be adhered with a
concrete adhesive.
Terraced wall
Walls placed in a multi-level arrangement.
Split a block
Use a hammer and chisel to score the block on
all sides. Pound the chisel along the score line
until the block splits. If the block does not split
easily, you may need to use a circular cut-off
saw with a masonry blade. Read and
understand the operating manual before using
the saw. Always wear eye protection when
splitting blocks. Partial blocks should always be
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adhered with a concrete adhesive.
2:1 Ratio Rule
If the lower wall is "H", the height of the upper
wall should be less than or equal to H. And, the
upper wall must be built at a distance away
from the lower wall of at least 2 H. For
example, if the lower wall is 2 feet high (.6m),
the upper wall must be no higher than 2 feet
(.6m), and built at least 4 feet (1.2m) behind
it.
More questions? You can contact us at: Anchor
Wall Systems 5959 Baker Road, Suite 390,
Minnetonka, MN 55345 1-877-295-5415
http://www.anchorwal1.com/Contractors/Contractors_Installation_ Glossary .asp
1/ 18/2006