HomeMy WebLinkAbout20130930-Cohen email re Sconset Beach Sheeting at Baxter Rd_201404071118294058From:Steven Cohen
To:Erika Mooney
Cc:Libby Gibson; Kara Buzanoski; Andrew Vorce; George Pucci
Subject:Sconset Beach Sheeting at Baxter Rd.
Date:Monday, September 30, 2013 1:16:24 PM
Attachments:Haley&Aldrich Sheeting Memo Sept2013.pdfBluff Angles 99 to 105.pdf
Erika,
These went to M&M on Friday. Please place in the record and otherwise distribute, as
approrpriate.
Steven
Steven L. Cohen
Reade, Gullicksen, Hanley, Gifford & Cohen, LLP
P.O. Box 2669 (6 Youngs Way)
Nantucket, MA 02584
508-228-3128 W
508-228-5630 F
SLC@readelaw.com
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MEMORANDUM
27 September 2013
File No. 26694-001
TO: Sconset Beach Preservation Fund
c/o Les Smith, Epsilon Associates
FROM: Haley & Aldrich, Inc.
Mark X. Haley
SUBJECT: Evaluation of steel sheeting adjacent to Baxter Rd.
At your request, we have reviewed the proposed option for driving steel sheet piling at the edge of Baxter
Road. The purpose of the sheet pile wall would be to provide temporary protection for the utilities located
beneath Baxter Rd. from damage due to bank erosion. Although details of the proposal were not available
to us, we have made certain assumptions. These include the following:
Depth of sheeting 45 to 50 ft. below Baxter Road grade, approximately one half
of slope height.
Sheeting would be designed to cantilever about 20 ft. (Note: normal steel
sheeting can only cantilever to about a 20 ft. height without bracing. If the sheet
pile wall was required to retain a greater height of soil, bracing would be
required.)
A ‘Z-type’ sheet would be used.
Sheet piling would be vibrated into place not top driven. This method of
installation was selected to reduce vibrations during pile driving.
That the sheeting can be driven through the dense near surface soils. (Note; the
soils in upper portion of the slope consist of dense glacial till that will be
difficult to advance a pile through.)
At first glance this proposal would appear to provide near surface soil retention adjacent to the road, but
upon further evaluation of the option, a number of issues may make this option detrimental to the overall
slope stability. These include the following:
The sheeting line will create a joint or vertical plane at the edge of road, that may
result in a shear plane, resulting in slope instability.
Disturbance of the soil on both sides of the sheeting will allow water to seep into
this zone and have the potential for weakening the soil and reducing slope
stability.
Having evaluated this slope in 2007 and again in 2012, the erosion and slope
failure occurs from loss of ground at the toe of slope. The existing medium sand
stratum at the toe of slope is highly erodible and once eroded by wave action the
Haley & Aldrich, Inc.
465 Medford St.
Suite 2200
Boston, MA 02129
Tel: 617.886.7400
Fax: 617.886.7600
HaleyAldrich.com
Sconset Beach Preservation Fund
27 September 2013
Page 2
slope becomes undermined causing slope failure of the soils above. This
proposed option does not address toe of slope instability.
The steel sheeting would probably only extend about half the height of slope, and
could thus be undermined causing a significant global slope failure.
The sheet pile line will prevent water flow towards the ocean, and water will
build up behind the sheeting thus increasing the hydrostatic pressures in the
slope, increasing the horizontal driving force and decreasing the stability of the
slope.
Installation of the sheeting will cause vibrations and potential downward
movement of the soils along the slope face.
It is our opinion; that options to consider for slope stabilization on a temporary or permanent basis should
be focused at the existing toe of slope. Protecting the soils at the toe of slope from erosion will reduce the
undermining of the slope and slope instability. Based on recent surveys, summer 2013, the slope angles
in the area of Lots 99,101 and 105 are in the range of 31 to 40 degrees except near top of slope where the
slope is much steeper in the range of 56 to 68 degrees. Based on our slope evaluations in 2007 slope
angles less than about 40 degrees are stable but become unstable when the slope angle approaches 45
degrees especially in a rain event where water is added to the soil stratigraphy.
.
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, Massachusetts
Figure 1
2013 Bank Profile, 99 Baxter Road
11 ftAngle
:
68
.2
˚
51.5 sf
An
g
l
e
:
3
9
.
6
˚
Figure 2
2013 Bank Profile, 101 Baxter Road
13 ft
An
g
l
e
:
3
9
.
2
˚
A
n
g
l
e
:
5
6
.
6
˚
56.8 sf
Figure 3
2013 Bank Profile, 105 Baxter Road
14 ft
A
n
g
l
e
:
5
6
.
4
˚
86.6 sf
Ang
l
e
:
3
1
.
6
˚
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, Massachusetts
Figure 1
2013 Bank Profile, 99 Baxter Road
11 ftAngle
:
68
.2
˚
51.5 sf
An
g
l
e
:
3
9
.
6
˚
Figure 2
2013 Bank Profile, 101 Baxter Road
13 ft
An
g
l
e
:
3
9
.
2
˚
A
n
g
l
e
:
5
6
.
6
˚
56.8 sf
Figure 3
2013 Bank Profile, 105 Baxter Road
14 ft
A
n
g
l
e
:
5
6
.
4
˚
86.6 sf
Ang
l
e
:
3
1
.
6
˚
Baxter Road and Sconset Bluff Storm Damage Prevention Project Nantucket, Massachusetts
Figure 1
2013 Bank Profile, 99 Baxter Road
11 ftAngle
:
68
.2
˚
51.5 sf
An
g
l
e
:
3
9
.
6
˚
Figure 2
2013 Bank Profile, 101 Baxter Road
13 ft
An
g
l
e
:
3
9
.
2
˚
A
n
g
l
e
:
5
6
.
6
˚
56.8 sf
Figure 3
2013 Bank Profile, 105 Baxter Road
14 ft
A
n
g
l
e
:
5
6
.
4
˚
86.6 sf
Ang
l
e
:
3
1
.
6
˚