HomeMy WebLinkAboutRS_1716.00_Geotechnical Report Pinnacles on Lake Washington_03-16-2022GEOTECHNICAL EVALUATION OF HAZARD
TREE MANAGEMENT PLAN
Pinnacles on Lake Washington
1400 Lake Washington Boulevard North
Renton, Washington
Project No. 1716.00
March 16, 2022
Prepared for:
Essex Property Trust, Inc.
Prepared by:
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
19019 36th Avenue West, Suite E
Lynnwood, WA 98036
ZGA
Pinnacles
Development
Zipper Geo Associates, LLC
Geotechnical and Environmental Consulting
19019 36th Avenue West, Suite E Lynnwood, WA 98036 (425) 582-9928
Project Number 1716.00
March 16, 2022
Essex Property Trust, Inc.
1400 Lake Washington Boulevard North
Renton, Washington 98056
plynch@essex.com
Attention: Mr. Peter Lynch
Subject: Geotechnical Evaluation of Hazard Tree Management Plan
Pinnacles on Lake Washington
1400 Lake Washington Boulevard North
Renton, Washington 98056
Dear Mr. Lynch,
Zipper Geo Associates, LLC (ZGA) has completed a geotechnical evaluation of the Hazard Tree
Management Plans prepared by Savatree Consulting Group for the Pinnacles on Lake
Washington development located at 1400 Lake Washington Boulevard North in Renton,
Washington. This report presents the findings of our document review and site reconnaissance,
and our geotechnical recommendations for the project. Our services were completed as
supplemental services in general accordance with our Short Form Agreement (Proposal No.
P16303) dated October 21, 2016. We appreciate the opportunity to be of service to you on this
project. If you have any questions concerning this report, or if we may be of further service, please
contact us.
Sincerely,
Zipper Geo Associates LLC
3/16/22
3/16/22
James P. Georgis, L.E.G. Thomas A. Jones, P.E.
Principal Senior Consultant
Distribution: Addressee via email (1)
TABLE OF CONTENTS
Table of Contents
INTRODUCTION ............................................................................................................. 1
SITE DESCRIPTION ....................................................................................................... 1
PROJECT UNDERSTANDING ....................................................................................... 1
GEOLOGIC SETTING .................................................................................................... 2
SITE RECONNAISSANCE ............................................................................................. 3
CONCLUSIONS AND RECOMMENDATIONS ............................................................... 3
General .................................................................................................................... 3
Trees 47, 48, and 51 ................................................................................................ 4
Trees 49, 50, and 52 ................................................................................................ 4
Geologic Hazard ECA Considerations .................................................................. 5
Planting Recommendations .................................................................................. 5
Temporary Erosion Control Considerations ........................................................ 6
Retention of Woody Material ................................................................................. 6
General Steep Slope Considerations .................................................................... 7
CLOSURE ....................................................................................................................... 7
FIGURES
Figure 1 – Site Map
APPENDICES
Appendix A – March 15, 2022 Arborist Report, prepared by Savatree Consulting Group
GEOTECHNICAL EVALUATION OF HAZARD TREE MANAGEMENT PLAN
PINNACLES ON LAKE WASHINGTON
1400 LAKE WASHINGTON BOULEVARD NORTH
RENTON, WASHINGTON
Project No. 1716.00
March 16, 2022
INTRODUCTION
This report documents our geotechnical evaluation of the Hazard Tree Management Plan
prepared by Savatree Consulting Group for the Pinnacles on Lake Washington development
located at 1400 Lake Washington Boulevard North in Renton, Washington. The project
description, site conditions, and our geotechnical conclusions and recommendations are
presented in the text of this report. The arborist report prepared by Savatree Consulting Group is
presented in Appendix A.
SITE DESCRIPTION
Pinnacles on Lake Washington (Pinnacles) is a multi-building, multi-family residential
development located at 1400 Lake Washington Boulevard North in Renton, Washington. The
Pinnacles development consists of two distinct areas. The upper portion of the development
includes Buildings A through E and is accessed by a private drive off of Lake Washington
Boulevard North. The upper portion of the development is constructed on a relatively level bench
located on a moderately steep to steep west-facing slope. In the central portion of the
development (Building C), the slope extends down from the I-405 freeway to an off-site multi-
family residential building. In the northern portion of the development (Buildings D and E), the
slope extends down from I-405 to the lower portion of the Pinnacles development (Buildings F
through H). Buildings F through H are accessed via North 20th Street. The total relief between
the upper Pinnacles development bench and the bottom of the steep west-facing slope is on the
order of 100 feet. Portions of the steep west-facing slope include near vertical bluffs, some of
which are up to 40 feet tall. The approximate site location, the location of the Pinnacles buildings,
and approximate hazard tree locations are shown on the enclosed Site Map, Figure 1.
PROJECT UNDERSTANDING
The site is mapped by the City of Renton as containing Erosion Hazard, Steep Slope Hazard, and
Landslide Hazard Environmental Critical Areas (ECAs). We understand that the City requires that
a geotechnical evaluation of the Hazard Tree Management Plan be completed to assess the risk
to regulated Geologic Hazard ECAs and their associated buffers, as defined in Part 4-3-050-G-5
of the City of Renton Municipal Code (RMC) that may result from the Hazard Tree Management
Plan and provide recommendations to mitigate identified risks. Our understanding of the project
is primarily based on a review of the following hazardous tree evaluation report and discussions
with Essex Property Trust.
• Hazard Trees, 1400 Lake Washington Blvd (Pinnacles on Lake Washington Apts),
prepared by Savatree Consulting Group, dated March 15, 2022.
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The Savatree Consulting Group report identifies a total of 6 hazard trees comprising two distinct
site conditions, as summarized below.
• Trees located at the edge of a near-vertical bluff that are a direct hazard to property and
people: The report identifies three Douglas Fir hazard trees (Trees 47, 48, and 51) as
being severely undermined by the site bluff with about half of their root system either
missing or undercut. The report indicates the likelihood of tree failure as Probable, the
likelihood of impacting targets as High, and the consequences of failure as severe, with
an overall rating of High Risk. The report recommends that all three trees be cut to short
stumps with their root systems left in place and wood debris left in place on the site slopes.
The report does not indicate whether the three Douglas Fir trees are expected to survive
the recommended cutting. ZGA contacted Mr. Scott Selby with Savatree Consulting Group
on March 16, 2022 to inquire on the survivability of the Douglas Fir trees. Mr. Selby
indicated that the trees are not expected to survive.
• Trees located on a steep slope below the near-vertical bluff that are a direct hazard to
property and people: The report identifies three Black Cottonwood hazard trees (Trees 49,
50, and 52) on the steep slope below the bluff. The report indicates the likelihood of tree
failure as Probable, the likelihood of impacting targets as High, and the consequences of
failure as severe, with an overall rating of High Risk. The report recommends that all three
black Cottonwood trees be cut to a short snag height of about 10 feet, that the wood be
left in place on the slopes, and that the limbs and smaller debris be removed and chipped.
The report does not indicate whether the three Black Cottonwood trees are expected to
survive the recommended cutting. ZGA contacted Mr. Scott Selby with Savatree
Consulting Group on March 16, 2022 to inquire on the survivability of the Black
Cottonwood trees. Mr. Selby indicated that Black Cottonwood trees sprout profusely after
cutting and expected the trees and their root mass to survive the cutting.
A copy of the referenced hazard tree report is enclosed with this geotechnical evaluation. The
approximate locations of Trees 47 through 52 are presented on the Site Map, Figure 1, and in the
Savatree Consulting Group report. Photographs of the trees are presented in Savatree Consulting
Group report.
GEOLOGIC SETTING
We assessed the geologic setting of the site and surrounding vicinity by reviewing The Preliminary
Geologic Map of Seattle and Vicinity, Washington, U.S. Geological Survey Map I-354, 1962. The
publication maps the bench on which the upper portion of the Pinnacles development is located
as being mantled by Quaternary age Vashon till (Qt). The Vashon till is generally described as a
compact, unsorted mixture of sand, silt, clay, and gravel, and is commonly referred to as
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“hardpan”. The lower portion of the steep slope and near vertical bluffs are mapped as older
Quaternary clay till (Qcg). The older clay till is described as sand and gravel lenses in an older
clay till. Both deposits have been glacially over-consolidated and are typically very dense in their
undisturbed, unweathered condition.
SITE RECONNAISSANCE
We completed a reconnaissance of the identified hazard tree locations and immediate vicinity on
March 10, 2022. The purpose of the reconnaissance was to identify surficial features of site
conditions such as the presence or absence of groundwater seepage and surface erosion,
indications of past slope instability, indications of past land use, the type and extent of existing
vegetation, the location of trees identified for removal/pruning, and other site features that may
be affected by the hazard tree management plan. The enclosed Site Map, Figure 1, shows the
approximate location of the identified hazard trees. Our primary reconnaissance observations are
present in the Conclusions and Recommendations section of this report.
CONCLUSIONS AND RECOMMENDATIONS
General
The identified hazard trees appear to be located within Erosion Hazard, Steep Slope Hazard, and
Landslide Hazard Environmental Critical Area (ECAs), or their associated buffers. As such, the
Hazard Tree Management Plan must address the effect of the tree management plan on slope
stability.
In general, there are two primary mechanisms which help trees stabilize slopes; 1) mechanical
reinforcement of the soil through roots, and 2) hydrologic effects (reduction of soil water content
through interception and evaporation of precipitation and transpiration). Although the hydraulic
effect of deciduous trees is greatly reduced in the winter (when the potential for slope instability is
generally the highest) due to the reduction in interception (no leaves) and transpiration (trees are
dormant). This winter reduction in the stabilizing effect of trees is much less pronounced in
evergreen trees.
It should be noted that large trees located on or near the crest of steep slopes or bluffs can contribute
to slope instability through two primary mechanisms; 1) large trees exposed to wind can transmit
that load through the root mass to the soil, thereby contributing to slope instability, and 2) the weight
of the tree on the slope or at the crest of a bluff can contribute to slope instability.
There is some debate in the scientific community regarding the relative contributions of these
different factors to slope stability, but the general consensus appears to be that under most
conditions the presence of well-rooted, healthy trees (in particular the bio-reinforcement they
provide) results in a net improvement to slope stability.
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Based on the results of our site reconnaissance, document review, and geotechnical analysis, it is
our opinion that the proposed hazard tree maintenance plan is feasible from a geotechnical
perspective. Provided that the conditions and recommendations contained within this report and
the provisions of the Renton Municipal Code (RMC) are satisfied, it is our opinion that the
proposed Hazard Tree Maintenance Plan will not increase the potential for soil movement and
the risk of damage to existing structures, the site, and to adjacent properties from soil instability
will not be increased. However, it should ne noted that the erosion and sloughing occurring on
the steep slope will continue whether the subject trees are present or not.
The following sections provide our conclusions and recommendations regarding the Hazard Tree
Maintenance Plan at specific locations, and a discussion regarding City of Renton regulated ECAs.
Trees 47, 48, and 51
These Douglas Fir trees are located west to southwest of Building D and above the carport and
parking lot of the lower portion of the multi-family residential development. All three trees are
located at the edge or within about 2 feet of the edge of a 15 to 25 foot tall near-vertical bluff. The
bluff generally consists very dense, poorly sorted silty sand to sandy silt with variable gravel and
cobble content interpreted as glacial till. The near vertical bluff undermines a significant portion
of the root mass of all three trees.
It is our opinion that the near-vertical glacial till bluff which has undermined these trees is
marginally stable and the weight and wind loading effect of these trees could trigger a landslide
resulting in down-slope deposition of soil and wood debris (which could potentially impact the
carport area) and upslope regression of the bluff. In our opinion, cutting these trees to short
stumps and leaving the root systems intact as generally recommended in the arborist report would
retain the soil reinforcement of the root system in the short-term, while reducing the loading
conditions promoting slope instability and result in a net short-term increase in slope stability.
However, decomposition of the root systems over time could lessen the slope stabilization
contribution of an intact root system. We therefore recommend that replacement trees be planted
at a 2 to 1 ratio to mitigate this risk, as discussed in more detail below. We recommend that cut-
wood be prevented from cascading down the near vertical bluff to the extent feasible, as this
condition could contribute to erosion and instability.
Trees 49, 50, and 52
These Black Cottonwood trees are located west to southwest of Building D and the near-vertical
glacial till bluff and above the carport and parking lot of the lower portion of the multi-family
residential development. These located on a 30- to 45-degree slope mantled by very loose to
loose soil derived from a recent regression (failure) of the up-slope near-vertical glacial till bluff.
In our opinion, cutting these trees back and allowing them to re-sprout thereby reducing their size
and weight as recommended in the arborist reports would retain the soil reinforcement of the live
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root system while reducing the loading conditions that contribute to slope instability. As such, a
reduction in slope stability is not anticipated provided the trees survive the pruning.
Geologic Hazard ECA Considerations
The site is mapped by the City of Renton as containing Erosion Hazard, Steep Slope Hazard, and
Landslide Hazard Environmental Critical Areas (ECAs). ZGA completed a geotechnical
evaluation of the Hazard Tree Maintenance Plan to assess the risk to regulated Geologic Hazard
ECAs and their associated buffers, as defined in Part 4-3-050-G-5 of the City of Renton Municipal
Code (RMC) that may result from the planned maintenance and provide recommendations to
mitigate identified risks.
Part 4-3-050-C-3 of the RMC indicates that the removal/pruning of dangerous trees which have
been approved by the City and certified dangerous by a licensed landscape architect or certified
arborist is exempt from RMC ECA restrictions and may be permitted by the City. The following
sections present our evaluation of Erosion Hazard, Steep Slope Hazard, and Landslide Hazard
ECA’s relative to the Hazard Tree Removal/Pruning Plan.
Erosion Hazard Areas: The City of Renton maps the planned hazard tree maintenance areas as
Erosion Hazard ECA’s. Based on our document review and site reconnaissance, it is our opinion
that the planned hazard tree maintenance areas meet the RMC Erosion Hazard Area criteria.
Steep Slope Hazard Areas: The City of Renton maps the planned hazard tree maintenance areas
as Steep Slope Hazard Areas with slope inclinations ranging from 25 to 40 percent slopes and 40
to 90 percent slopes. Based on our site reconnaissance, it is our opinion that the planned hazard
tree maintenance areas meet the RMC Steep Slope Hazard Area criteria.
Landslide Hazard Areas: The City of Renton maps the planned hazard tree maintenance areas as
Moderate Severity Landslide Hazard Areas. Based on our document review and site
reconnaissance observations, it is our opinion that the planned hazard tree maintenance areas
meet the RMC Landslide Hazard Area criteria.
Risk Evaluation Summary: Provided that the conditions and recommendations contained within
this report and the provisions of the Renton Municipal Code (RMC) are satisfied, it is our opinion
that the proposed Hazard Tree Maintenance Plan will not increase the potential for soil movement
and the risk of damage to existing structures, the site, and to adjacent properties from soil
instability will not be increased.
Planting Recommendations
The three Douglas Fir trees are not expected to survive the planned cutting. Research indicates
that the soil reinforcing benefit of the root mass may only last three to nine years after a tree has
died. We therefore recommend that that new trees be planted at replace the Douglas Fir trees
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with a 2 to 1 replacement ratio. We recommend that the replacement trees and or woody shrubs
be planted at least 10 feet behind (east of) the bluff edge. We recommend that the type of
replacement trees and/or shrubs planted be determined by the project arborist based on the site
conditions at the planting locations. We recommend that the replacement plantings be selected
to optimize root mass spread and penetration while minimizing the weight and sail area of the
mature plants.
Based on our discussions with the project arborist, the Black Cottonwood trees and their root
mass are expected to survive the planned cutting. However, it is possible that a tree may not re-
sprout and may not survive. Based on our evaluation of the site conditions, it is our opinion that
the decay of the root systems associated with cottonwood trees could result in a localized
reduction in the stability of the slope. Therefore, we recommend that the project arborist inspect
the cottonwood trees an appropriate time interval after cutting to verify that the trees are re-
sprouting as intended and are recovering from the pruning process. If a tree is determined to be
dead or dying, we recommend that at least two new trees and/or woody shrubs be planted in its
place. We recommend that the type of replacement trees and/or shrubs planted be determined
by the project arborist based on the site conditions at the planting locations. We recommend that
the replacement plantings be selected to optimize root mass spread and penetration while
minimizing the weight and sail area of the mature plants.
Temporary Erosion Control Considerations
We anticipate that the tree cutting process will likely result in disturbance of the ground surface
and a temporary increase in the potential for erosion. We therefore recommend that the tree
removal service utilize means and methods intended to minimize ground disturbance.
The steep slope located between the near vertical glacial till bluff and the lower parking lot is
mantled by about 6 to 18 inches of loose soil derived from a recent regression (failure) of the near
vertical glacial till bluff. These soils are near their angle of repose and very susceptible to erosion.
We recommend that these exposed soils by hydroseeded shortly after the hazard tree
maintenance to reduce the potential for erosion and promote revegetation. We recommend that
the hydroseed be applied in accordance with the manufacturer’s directions using a native seed
mix appropriate for the site conditions.
Retention of Woody Material
It appears geotechnically feasible to retain much of the woody debris generated from the hazard
tree maintenance as habitat enhancement within wooded portions of the site. However, we
recommend that woody material be placed and oriented in a manner that will prevent the material
from rolling down slopes. We further recommend that smaller limbs, branches, and foliage be
removed as necessary to prevent the debris from smothering the existing ground cover.
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General Steep Slope Considerations
Owners of properties containing or adjacent to steep slopes should recognize that soil loss or
downslope displacement from steep slopes is a naturally occurring mass wasting process, and
that some soil loss from steep slopes should be expected over time, particularly in response to
episodic severe storm events. The conclusions and recommendations presented in this report are
intended to allow for the cutting/removal of hazard trees while maintaining the existing stability of
the site slopes. The findings presented in this report should not be interpreted as indicating that
the site steep slopes will be stable under all possible future conditions.
CLOSURE
This report has been prepared for the exclusive use of Essex property Trust, Inc., and their agents,
for specific application to this project and has been prepared in accordance with generally
accepted geotechnical engineering practices. No warranties, express or implied, are intended or
made. Site safety is the responsibility of others. In the event that changes in the nature, design,
or location of the tree removal/pruning as outlined in this report are planned, the conclusions and
recommendations contained in this report shall not be considered valid unless Zipper Geo
Associates, LLC reviews the changes and either verifies or modifies the conclusions of this report
in writing.
BLDG EBLDG CBLDG DBLDG FBLDG GBLDG HTREES 47 THROUGH 52.
SEE ARBORISTS REPORT
FOR INDIVIDUAL TREE
LOCATIONS
REFERENCE: GOOGLE EARTH 2016. NOT TO SCALE
FIGURE
Job No.
Zipper Geo Associates, LLC
19019 36th Ave. W.,Suite E
Lynnwood, WA SHT. of 11
Site Map
1716.00Date: March 2022
1
PINNACLES ON LAKE WASHINGTON
1400 Lake Washington Boulevard North
Renton, Washington
APPENDIX A
SAVATREE CONSULTING GROUP REPORT
Date: March 15, 2022
To: Peter Lynch, Service Manager – Essex Property Trust
Subject: Hazard Trees
Location: 1400 Lake Washington Blvd (Pinnacle on Lake Washington Apts)
From: Scott Selby, Registered Consulting Arborist #749, Board Certified Master Arborist #PN-
1775B, ISA Tree Risk Assessment Qualified
Peter:
Overview
At your request, I visited the Pinnacle on Lake Washington site March 14th to evaluate trees of concern. A recent
landslide resulted in significant undermining of three Douglas fir (Pseudotsuga menziesii) trees and resulted in burying
the root crowns of two large black cottonwood (Populus trichocarpa) trees and exposing the root crown of one other
large black cottonwood. A very broad swath of soil was released as well as a large intact block of soil.
You had a geotech visit the site to assess the condition of the slope and trees undermined. You indicated the geotech
recommended removal of the fir trees due to the precarious placement at the top of the slope and the potential for
failure of the trees to impact targets (structures or parked cars) at the base of the slope.
During my site visit, I noted the three cottonwood trees mid slope downhill from the fir trees and located in the middle
of the slide area. The demonstrated instability of the slope and the potential for the cottonwood trees to impact the
same targets in the event of failure represents an imminent risk.
Risk Assessment
Tree risk assessment involves qualitative analysis of several factors including the Likelihood of Failure, Likelihood of
Impacting Target, and Consequences of Failure (likelihood of injury or damage). The two tables that follow are from the
International Society of Arboriculture Tree Risk Assessment Form and are the current standard for assessing tree risk.
Matrix 1. Likelihood Matrix Likelihood of
Failure
Likelihood of Impacting Target
Very low Low Medium High
Imminent Unlikely
Somewhat
Likely Likely Very Likely
Probable Unlikely Unlikely
Somewhat
likely Likely
Possible Unlikely Unlikely Unlikely
Somewhat
Likely
Improbable Unlikely Unlikely Unlikely Unlikely
Matrix 2. Risk Rating Matrix
Likelihood of
Failure & Impact
Consequences of Failure
Negligible Minor Significant Severe
Very Likely Low Moderate High Extreme
Likely Low Moderate High High
Somewhat Likely Low Low Moderate Moderate
Unlikely Low Low Low Low
I rate all six trees as follows: the Likelihood of Failure as Probable, the Likelihood of Impacting Target as High and the
Consequences of Failure as Severe = HIGH Risk Rating.
Recommendation
Due to the potential for all six trees to impact targets in the event of failure and the imminent nature of the situation
due to failure of the slope, I recommend all six trees be removed without delay.
Tree Removal Plan
Due to the sever undermining of the fir trees as a result of slope failure, these trees are unsafe to climb and dismantle.
Safety dictates the trees be felled into the adjacent slope with the resulting debris left in place to help stabilize the bank.
Activity along the slide area should be kept to a minimum to avoid destabilizing the slope further. The trees will be cut to
short stumps with the roots left in place.
The cottonwood trees appear safe to climb and will be cut to a short snag height of approximately 10 feet. The removal
of these trees will generate a significant amount of debris which could represent a fire hazard if left in place.
Consequently, the wood will be left in place to help hold the slope, but the limbs will be removed and chipped.
Please let me know if you have any questions about this report.
Scott Selby, Account Manager
SavATree
206.849.4718
sselby@savatree.com
47
slide area
4950
52
Targets
49
50
49
5052