HomeMy WebLinkAboutWS Item 05Memorandum
To:
From:
Copy:
Date:
Subject:
Background•
Mayor, City Council & City Administrator
Brett Altergott, Parks & Recreation Director -VQ
Zach Johnson, City Engineer
John Hennen, Parks & Operations Manager
September 11, 2014
Kenrick Ave. Trail Slope Erosion
City of Lakeville
Parks & Recreation
The slope north of Marek's Towing is experiencing erosion issues. Since the spring the slope
has eroded twice, once following the record rainfalls in the spring after which it was
reconstructed and again following the heavy rains Father's Day weekend.
Why is the slope eroding? The primary reason for erosion is the steep slopes and granular
soils that make up the embankment. Why is the slope so steep? In late 2011, WSB was
directed by staff to design this section of the trail without affecting the snowmobile trail at
the bottom of the embankment and there is an existing box culvert within the slope that was
not extended.
In mid-June, staff directed WSB to propose the City with options and cost estimates for
solutions that reduce the likelihood of future slope erosion issues like the ones experienced in
2014. Included with this packet is a memo from WSB outlining the options considered, cost
estimates of the recommended solution and photos of the current condition of the slope.
There is approximately $44,000 remaining in the project contingency fund, any additional
costs would be funded through Park Dedication.
Don Sterna of WSB will be in attendance at the City Council Work Session to make a brief
presentation and answer any questions.
Recommendation:
Staff concurs with WSB recommendation to flatten the slopes to at least 1:2 by the
construction of a Prefabricated Modular Block Wall (PMBW), seeding, blanketing, and other
erosion measures as outlined in the WSB memo dated August 21.
WSB
& Aaurrintati, brr,
enquieering• planning- environmental• construction 701 Xenia Avenue South
Suite 300
Minneapolis, MN 55416
Tel: 763-541-4800
Fax: 763-541-1700
Technical Memorandum
To: BrettAltergott - Lakeville Parks & Recreation
From: Don Sterna, P.E. - WSB & Associates, Inc.
Date: August 21, 2014
Re: Retaining Wall Comparison
Kenrick Avenue Trail
WSB Project No. 01715-84
The purpose of this memo is to document existing conditions along the Kenrick Avenue Trail and to
present alternatives, along with a preferred recommendation to address slope stability concerns.
Summary of Existing Conditions
Currently a section of approximately 200 feet of the embankment along the Kenrick Trail (Figure 1) is
experiencing erosion. Erosion is occurring primarily as a result of the steep slopes and in-place granular
embankment material which is not able to remain in place after heavy rain events.
Post roadway construction slopes along Kenrick Avenue where approximately 1:2 and remained stable
for a number of years with no signs of erosion. In 2013, the Kenrick Trail project was designed to
maintain a 20 -foot wide snowmobile path at the toe of the slope and a 10 -foot wide trail along the east
side of Kenrick Avenue. Guardrail was placed between the roadway and trail to provide vehicular safety
and a fence was placed along the trail for pedestrian safety. In order to construct the trail, the
embankment slope needed to be steeper than the in place embankment. For a stretch of approximately
200 feet, the slope was steeped beyond the in place 1:2 slopes. The area of erosion is occurring mainly
in this 200 -foot stretch which the contractor has been unable to stabilize, even with using heavy duty
erosion control blankets and bio -rolls.
There are several factors contributing to the continued erosion. These factors include:
■ Slopes steeper than 1:2
■ Granular embankment fill
■ An increase in impervious surface
■ Heavier than normal rainfall events
The interaction plan between the granular embankment and the topsoil layer are not holding when the
topsoil becomes saturated. Once the top soil becomes saturated, the increased weight is enough that
the topsoil slides down the hill, which opens up the granular material to further erosion. The use of
geosythetic matting (erosion control blankets) over the slope does not prevent the topsoil from
becoming saturated.
Equal Opportunity Employer
vtsbeng.com
K:'.01715-8401..dmin\Docs.altagott Tech Memo 8-21-14.docx
Brett Altergott
August 21, 2014
Page 2
Alternatives Considered
There are a several wall types that could be utilized to address slope stability. These alternatives vary in
construction methods and costs. We reviewed several wall types and made an in-depth analysis of
three wall options along the corridor, along with evaluating the opportunity to extend the existing box
culvert. The option to extend the culvert was eliminated early in the process since the original culvert is
a cast in place structure. It would be technically challenging and extremely expensive to construct. This
alternative would still require some type of wall along the toe of the slope and the stream would have to
be diverted for construction.
The three wall options considered to stabilize the embankment included:
1. Prefabricated Modular Block Wall (PMBW)
2. Reinforced Soil Slope Wall (RSS)
3. Sheet pile wall
The preferred means of construction for the wall is one that requires minimal excavation and utilizes
smaller and more maneuverable equipment that can be used within tight constraints. The intent would
be to avoid impacts to the trail.
Alternatives Evaluation
Preliminary designs for the three retaining wall types listed in the previous section were developed to
determine construction costs and the associated construction limit impacts. Associated impacts include
constructability, material delivery, working limits, costs and avoid temporary or permanent easement
costs.
Evaluation criteria include:
■ Cost
o Constructability
o Right of way and easements
o Avoid impacts to existing trail
■ Construction Limit Impacts
o Overhead power line
o Material delivery
o Quick construction methods
K:101715-840`AdminlDoa 4ltergott Tech Memo 8-21-14.docx
Brett Altergott
August 21, 2014
Page 3
PMBW with Soil Reinforcement
A PMBW could be constructed without the
use of geosythetic grids up to heights of 8
feet. Heights above that require the use of
geosythetic grids. The lower height walls can
be constructed by excavating for the footing
and then placing the precast wall units.
Heights above 8 feet will require excavation IM
on some of the in place embankment to
install the grid and then the wall is back filled
with granular material. The excess material
can be placed along the base of the in place
embankment to flatten the slopes, thereby
reducing the need to haul any material off
site. This method requires less fill to flatten
the slopes outside the wall area.
Reinforced Soil Wall (RSS)
The RSS would require a significant amount of the
embankment to be excavated in order to install the
many layers of geosythetic grid necessary to would
develop the strength to support the embankment.
This alternative requires the in-place embankment
material to be removed from the grid area and be
replaced with granular borrow.
This alternative was rejected due to the challenges
associated with constructing the wall around the
existing box culvert and the amount of granular
material required. It would also take longer to
construct than other alternatives.
K. 01715-840\Admin\Dom\AItergott Tech Memo 21-14.dom
Brett Altergott
August 21, 2014
Page 4
Sheet Pile Wall
The sheet pile alternative would require driving sheeting
vertically into the ground. Approximately 12 feet would be
above ground, with another 24 feet extending below ground
to provide the needed stabilization. Constructing this wall
would require the use of a crane to drive the sheeting and
would require the relocation of existing overhead power
lines.
This alternative was rejected due to the types of construction
equipment needed, ability to fit and maneuver equipment in
the area, the conflict with the overhead power lines and the
time needed to get the sheeting material.
Recommendation
The only long-term solution to the existing problem is to flatten the slopes to at least a 1:2 and then
reseed and cover the slopes with heavy duty erosion control blankets with bio -rolls firmly staked along
the slope to assist in dissipating heavier rain fall events. After reviewing the wall options, costs,
constructability, material delivery schedules, working area, and ability to meet long-term needs, WSB
recommends the use of the Prefabricated Modular Block Wall (PMBW).
The PMBW alternative allows for the construction of flatter slopes which is needed for long-term
stabilization. This alternative keeps construction limits within existing right of way, allows for the use of
smaller equipment and can be done with minimal disruption to the existing area and trail.
Attached to this memo are the proposed plans and cross section showing the limits of the work required
to construct the preferred alternative.
Ki,01715•i46 AdminlDocsW ltergott Tech Memo 8-21-14.doc-
Y��
28* of
c
V
ev
ol
k4L.
UI I
mI1
.4
»1044)
§`
�|
|
E�eq 10
<
w
0
|
!m•
_®§/
\}kcc
. "§§
§0
;
0
\| �
m;
1;
14
N$
p
w
r
n a
F H
m ..
O N
n
F
F
°� g
•+
N
❑
p
r O�
m
u
Pi
n
a
N
M
�
p
[�
o
n
N
�J+
N
F b
p O
E
W
Fg m
y
V1
�oF�
H
o
a
H
H
x
C.ILn
cF m
Q
B 8
6OD
N
H r -I
W
P,
o
1 N
Ei
z
H
m
a
H
U
�
�
❑
'�
W �
V
F
n m
)am
z
�
r
u
U C
�• �
s
¢H¢
pl t0
Gi
•Lmll
ip&C
N
hWY
u
V
L
N
2 0
0
F
�a�y
.p]
W�
h d
pN
m
N
a1
z
7
£
3
m
U
1
u
U
v
o
a
V
v
0
41
N
Intl
m
3
x
W
a
m
=
a
F
IL
as��
� r
_t LL
r
,✓
5
as��
\ , ■ - .
.i rj � , v iM •'� A.0"
a
'a ;^'S r ':,, TR •` �''`C�., n }�sl. � .• � S� ,{ "^��'�+�+hNN �- C' q9,�� -
r
\� ' � ! _ �' 711 � �(' k. f � ► . 1 .
r
�;;�,A1:` 'Q r- c -,xs ���. ,{,.•r fir^}t,
y y LrTICv! i1
ION,
•:,' -ON1 ,
�v+r'►ti
-�� Jr dry . �� • � � �� �� �� ` � � TM, + i.,, r�.
,4 +' •. tip,
e
rc� t�
i � iM
IT
.01 Ji-
Jvat
e t
• +.,
Ohl. • r
y+ • t
It 06
40
i t
_ 'tel - � ♦ # -