HomeMy WebLinkAboutSlope Stability Comments Rev3 rev deis - FINAL -jkPage | Slope Stability - 1
SLOPE STABILITY CONCERNS
The DEIS has not adequately addressed slope stability concerns.
The DEIS scope calls for the DEIS to discuss “the potential of destabilization of adjacent properties and
impacts from vibration and excavation on adjacent properties, the impact of timing in between each
phase, the threat of catastrophic soil loss and erosion if there is a pause in the project schedule due to
weather or other event, and the future maintenance, type of construction, inspection schedules and
monitoring for defects of the retaining wall. The potential catastrophic failure of the retaining wall, in
whole or in part and effect on adjacent properties” (p.21).
The DEIS repeatedly concludes (pp. iv, 32, 37, 41, 79, 80, 86, 92, 290) that construction of the proposed
Evergreen retaining walls presents no slope stability concerns. It bases this conclusion on the
Geotechnical Assessment (DEIS Appendix H) prepared for the Project which states that “PWGC has been
informed that the permanent wall will be an Evergreen Wall system designed by the propriety engineer.
The wall designer has assumed a soil friction angle of 34°, cohesion of 0 psf and moist unit weight of 125
pcf.” The “proprietary engineer” is presumably Evergreen Walls, Inc. However, the Evergreen Wall
report (DEIS Appendix H) was prepared by the project engineer (Jeffrey T. Butler, P.E., P.C.) and consists
primarily of materials prepared by Evergreen Walls, Inc.
The reference to a soil friction angle of 34o is significant. In its discussion of slope stability the DEIS states
that “[p]rior to the installation of the permanent retaining wall system, the soil cut is recommended to
be sloped on 1.5:1 (Horizontal:Vertical) slope” (p.37). This is not accurate. The Geotechnical Assessment
actually recommends a “soil cut at a 1.5:1 horizontal:vertical slope or shallower to prevent slope stability
issues” (emphasis added)(DEIS Appendix H, PWGC Geotechnical Memo).
The DEIS goes on to state that the 1:1.5 slope is based on Occupational Safety and Health Administration
(OSHA) guidelines for excavation safety in Type C (granular, i.e., sandy) soils. A 1.5:1 slope is a 34° angle,
which is equivalent to the soil friction angle of the on-site soils. As indicated by PWGC, granular soils of
this composition (sand with gravel) and relative compaction (medium dense to dense) are favorable for
stable open cuts. A cut on a 34° angle taken from the base of the proposed retaining wall will not extend
horizontally onto the property of the nearest residence, 5106 Mill Road. Therefore, slope stability is not
a concern to nearby properties” (pp. 37, 86, 291).
What the DEIS fails to note is that the OSHA-required—not recommended--1.5:1 (34o angle) slope is the
“maximum allowable slope”1 and applies only to excavations less than 20 feet deep 2 (29 CFR 1926,
1 OSHA regulations define “maximum allowable slope” as “the steepest incline of an excavation face that is acceptable for the
most favorable site conditions as protection against cave-ins, and is expressed as the ratio of horizontal distance to vertical
rise (H:V).”
2 The excavation for the installation of the Evergreen retaining wall will be in excess of 30 feet deep.
Page | Slope Stability - 2
Subpart P App B - Sloping and Benching). According to OSHA regulations: “For excavations greater than
20 ft depth, the slope or bench shall be designed by a registered professional engineer.”
The project Landscape Plan (Appendix C) does include a “Typical Slope Soil Stabilization” detail prepared
by the project engineer (COMMENT FIGURE SS-1). That detail indicates how erosion control blankets
will be installed on “2:1 (H:V) Slope Max.” The 2:1 slope equates to a 27o angle. That is considerably
shallower than the 34o figure used throughout the DEIS. While this would increase slope stability, it has
other serious negative implications. The shallower slope means that the limit of excavation at the top of
the slope bordering the retaining wall will be set farther back and closer to the residence at 5106 Mill
Road.3 It also means that the limit of excavation in this area may be incorrectly depicted on project
plans. If constructed as shown on Project plans, the potential for slope instability may be considerably
greater than the DEIS indicates. It also means that a much greater volume of material will be needed to
backfill behind the completed retaining wall. It also raises the question of whether the total volume of
material to be excavated for the Project has been underestimated.
As shown on the Project’s Landscape Plan (Appendix C), the horizontal distance between the “landward
edge of the bottom the proposed wall” and the “E.O [edge of] Area of Disturbance” at the top of the
excavation area for most of its length is approximately 40 feet. The change in elevation between these
points is approximately 40 feet (note the location of the 50-foot contour which runs through the
proposed planting area at the top of the wall). That yields an approximate 1:1 slope with an
approximate angle of 45o.
The DEIS adds additional ambiguity and confusion to the issue of slope stability when it states on p.285
that “Bank slopes would not exceed 1 on 3.” This presumably means a 3:1 H:V ratio which would equate
to an 18o slope.4
The DEIS states that the “proposed Evergreen concrete retaining wall would improve slope stability as it
would correct existing slope failure due to the placement of dredge material within the Construction
Excavation Area” (p.290). As noted in other comments on soils, the placement of dredge material in the
portion of the existing SYC property where slope failure is occurring has not been demonstrated as the
cause of the slope failure in that area. To the contrary, local residents have long been aware that the
former property owner was engaged in the mining and selling of sand from this area, and comments to
this effect were submitted to the Planning Board in 2020. There is nothing precluding the Applicant from
undertaking slope stabilization measures in this area independent of the proposed Project.
3 The DEIS states that a “cut on a 34° angle taken from the base of the proposed retaining wall will not extend horizontally onto
the property of the nearest resident, 5106 Mill Road. Therefore, slope stability is not a concern to nearby properties” (p.36).
4 The reference to the 1-on-3 slope may be referring to the slope adjacent to the temporary haul road during Phase 1
excavation. A 1-on-3 slope for the haul road is shown as a detail on the Excavation Phasing Plan. However, this not clear
from the DEIS text.
Page | Slope Stability - 3
The DEIS notes that the Evergreen retaining wall will have “a safety factor of greater than 2.0, with 1.5
being the code minimum” (pp. ii, 6, 290). However, no information is included in the DEIS concerning
the safety factor of the bare slope that will be exposed for an extended period before the retaining wall
is constructed. This is of considerable concern since it is during this period that an approximately 40-
foot high unvegetated sand slope will be exposed to the elements (prolonged rainfall and rapid snow
melt can contribute to slope failure) and be susceptible to catastrophic failure. Although numerous
slope stability analysis methods are available 5, none appear to have been used by the project engineers.
Another slope stability issue has been ignored by the DEIS. The slope that will be supported by the
retaining wall is not the only steep slope that will exist during construction. During the Project’s Phase 1
excavation phase the haul road will extend into the Construction Excavation Area along the west side of
the excavation. It will be used throughout Phase 1 excavation. The Phase 1 excavation will create a
slope immediately east of the haul road that will gradually increase in height to a maximum of
approximately 30 feet in height (the bottom of the haul road will be at elev. 12, the top will be at
elev.42). The eastern edge of this portion of the haul road may be located as close as 3 feet from the
top of the temporary slope.6 The integrity of this slope could be compromised by vibration from the
passage of hundreds of fully loaded haul trucks going up, and unloaded trucks going down, into the
excavation area. A failure of this unsupported slope could compromise the portion of the haul road
inside the excavation area, rendering it unusable. Should such a failure occur when a vehicle is entering
or exiting the excavation area the conditions exist for a potential loss of human life.
Another factor relating to the stability of the temporary portion of the haul road within the Construction
Excavation Area is the fact that it will have to be continuously lengthened as the depth of excavation
continuously deepens. Given this, it seems unlikely that this portion of the haul road will be composed
of RCA. If it will be, as implied in the DEIS, a description of how this will be accomplished, without
interfering with the arrival and departure of haul trucks, is needed.
The revised7 DEIS states that “According to the project engineer, there are no slope issues for the haul
road” (pp. 18, 285). However, it is clear from the contexts in which this statement appears that it is
referring only to the portion of the haul road that will remain after the completion of construction, and
not the “temporary” portion located within the Construction Excavation Area shown on the Project’s
Excavation Phasing Plan (DEIS Appendix C).
5 Numerous technical summaries comparing the various methods are available, such as: D.P. Salunkhe (2017) An Overview on
Methods for Slope Stability Analysis, International Journal of Engineering Research & Technology 6(3):528-535)
6 The haul road cross-section detail on the Excavation Phasing Plan depicts a “3’ min” distance between the edge of the haul
road and the top of the slope (COMMENT FIGURE SS-2). It may be that this intended to apply only to the portion of the haul
road that will traverse the R-80 portion of the Project parcel, but this is not clear.
7 During its review of the adequacy of the original DEIS, the Planning Board’s consultant (NPV) expressed concern about the
grade of the haul road and its suitably for use by heavy equipment.
Page | Slope Stability - 4
The DEIS does not, as called for in the DEIS scope, discuss “the impact of timing in between each
phase, the threat of catastrophic soil loss and erosion if there is a pause in the project schedule due to
weather or other event . . .” It does not discuss the “potential catastrophic failure of the retaining
wall, in whole or in part and effect on adjacent properties.” The DEIS includes no discussion of
methods to mitigate for the possibility of slope failure. Instead, it merely concludes that there are no
slope stability concerns, and that adjacent properties will therefore not be affected.
Page | Slope Stability - 5
COMMENT FIGURE SS-1
From: DEIS Appendix C – Landscape Plan. (Modified to indicate reference to 2:1 slope)
Page | Slope Stability - 6
COMMENT FIGURE SS-2