Mill River Levee System - Geotechnical Data Memo_12-22-2020 - Changes TrackedM E M O R A N D U M
To:David Veleta (Northampton, DPW)Kris Baker (Northampton, DPW)
From:Matthew A. Taylor, P.E. (GZA)
Christopher Baker, P.E. (GZA)
Chris Tsinidis, P.E. (GZA)
Date:December 22, 2020
File No.:01.0174343.00
Re:Geotechnical Data Memorandum Engineering Services for Levee CertificationMill River Levee SystemNorthampton, MA
In accordance with our agreement (WF7-19-2019) datedJuly 19, 2019, GZA GeoEnvironmental is pleased to submit this memorandum to the City of Northampton Department of Public Works (DPW),
summarizing our geotechnical subsurface exploration data collection efforts for the Mill River Levee System. A separate memorandum summarizing the geotechnical exploration data collection
efforts for the Connecticut River Levee System is provided under separate cover. This memo was prepared in accordance with Task 3 ofthe referenced contract and is subject to the Limitations
provided in Appendix A.Unless noted otherwise, elevations reported herein are in referenceto the North American Vertical Datum of 1988 (NAVD88).
GZA’s subsurface exploration program was performed to gather the necessary data to fill previously identified subsurface information data gaps within the project record. The program
was also intended to obtain additional information to support updated seepage analyses, slope stability analyses, wall stability analyses, and settlement analyses for the System. The
results of those analyses, which will be submitted under separate cover, will be used to support the levee system certification and eventual Federal Emergency Management Agency (FEMA)
accreditation in under Regulation 44 CFR 65.10.
BACKGROUND
MILL RIVER LEVEE SYSTEM
In response to significant flooding events in the 1920’s and 1930’s, the United States Army Corps of Engineers (USACE) designed and constructed flood control works along the Mill River
to protect the City of Northampton (City), Hampshire County, Massachusetts. The Work was authorized under the Flood Control Act approved on June 22, 1936. Construction along the Mill
River was conducted over a series of construction contracts initiated in December 1938 and completed in June 1942. In 1944, the City accepted responsibility to operate and maintain the
Mill River Levee System (System) in accordance with the Flood Control Regulations. Refer to Figure 1 for the location of the System. Refer to Appendix B for relevant Mill River Levee
System Record Drawings obtained from the 1945 Operations and Maintenance Manual.
The System protects the western part of the City against high water from the Mill River. The Mill River Levee System was constructed in conjunction with a diversion channel and is approximately
2,400 feet long and has a maximum height of approximately 25feet.The Smith College Dike is the most upstream component of the System; its upstream end is located about 70feet downstream
of the Paradise Pond Dam. The Smith College Dike has a maximum height of about 16-feet and extends southward along the east bank of the Mill River about 1,100feet to West Street from
Station (Sta.) M 0+71 to Sta. M 11+57. At West Street, a 50-foot wide and 14.5-foot tall stop log closure structure is provided from Sta. M 11+57 to Sta. M 12+04, which is located between
the Smith College Dike and a downstream concrete floodwall (Floodwall).The Floodwall, of “T” type cantilever construction, has a maximum height of about 21feet and extends approximately
450feet from about Sta. M 12+00 to Sta. M 16+52 tying in a downstream dike (Diversion Dike). The Floodwall has a 20-foot wide opening provided for the former New York, New Haven, and
Hartford Railroad, which has been converted into a paved “Rail Trail”.
The Diversion Dike has a maximum height of about 25feet and extends approximately 900-feet from Sta. M 16+50 to high ground at Hebert Avenue at Sta. M 23+50. This Diversion Dike protects
the east overbank area and also diverts the Mill River to a diversion canal (Diversion Canal).The Diversion Canal was constructed to divert the Mill River from a populated area in the
southwestern section of the City, through the former Hulbert’s Pond, which is sparsely populated, where it discharges to the Connecticut River Oxbow. The Diversion Canal is approximately
11,042feet long from its upstream end located at the New York, New Haven and Hartford Railroad, adjacent to the Diversion Dike at Sta. C -1+40, to its downstream end situated at Connecticut
River Oxbow at Sta. C 109+02. A hydraulic grade structure (Drop Structure) was built near South Street Bridge to lower the water surface elevation in the Diversion Canal to accommodate
the fluctuating tailwater formed by backwater from the Connecticut River. The System also includes a small bridge constructed on Old Springfield Road near the point where the Diversion
Canal discharges into the Connecticut River Oxbow.
The Bridle Path Bridge Closure consists of the filling in of former bridge/river-bed opening through the Bridle Path (existing embankment pre-dating the System). The opening through
the Bridle Path was located approximately 900feet south of the Drop Structure on the Diversion Canal. An existing plank and girder bridge was removed and the stone abutments were leveled
to El. 117 (Mean Sea Level Datum). The 90-foot-long opening in the Bridle Path embankment was filled with soil described as “random material.” The Bridle Path Overflow Section consists
of a 946-foot-long excavated section of the Bridle Path embankment located perpendicular to the Canal at about Sta. C 85+00. The Overflow Section was partially excavated to create an
“overflow section” for the Diversion Canal. The crest of the original Bridle Path embankment in this area was originally at about El. 112 (Mean Sea Level Datum), and was cut to El.
106 (Mean Sea Level Datum) as part of the Project. The Overflow Section was designed to add discharge capacity to pass the Diversion Canal designed flow of 20,000 cubic feet per second.Mean
Sea Level Datum can be converted to NAVD88 by adding 0.31 feet.
SCOPE OF workThe Scope of Work for the Mill River Levee System Subsurface Exploration Program included the following:
Review the available existing subsurface information.
Prepare the Drilling Work Planfor review and approval by the USACE.
Complete the Subsurface Exploration Program which consisted of eight (8) test borings (locations designated MR-1 through MR-8 on Figure 2) to evaluate soil and groundwater conditions.
Perform gradation analysesand organic content testing on nine (9)soil samples collected from the test borings to confirm field classifications and determine engineering parameters to
be used in the proposed geotechnical engineering analysis.
Prepare this data geotechnical data memorandumto summarize our findings.
SUBSURFACE EXPLORATION Programs
PREVIOUS TEST BORINGS
Thirteen (13) borings were completed by the USACE is support of the original design and subsequent floodwall improvements for the Smith College Dike, Flood Wall, and Diversion Dike of
the Mill River Levee System. Below are various excerpts from the Mill River Levee System subsurface exploration program that were summarized in a Pre-Inspection Packet (PIP) prepared
by GZA/Watermark for the 2010 Periodic Inspection of the System regarding planning and design phase subsurface explorations and subsurface conditions assessments:
Smith College Dike, Flood Wall, and Diversion Dike
Subsurface Explorations:
The design phase subsurface explorations for the Smith College Dike, Flood Wall, and Diversion Dike consisted of nine test borings (BH-17 to BH-21 and BH-39 to BH-43). In support of
the Flood Wall Improvements completed by the USACE in 1957, four additional test borings (FA-1, FD-1, FA-2, and FA-3) were drilled on the landside of the Flood Wall near West Street.
The test borings consisted of foundation drive sample borings (FD) and foundation hand auger borings (FA).
Refer to the Select Record Drawings in Appendix B for previous test boring locations and a subsurface profile with the previous boring data.
As noted in the USACE 2010 Periodic Inspection Report, the majority of the previous test borings were performed prior to the construction of the levee/dike and floodwalls. As such, the
previous explorations did not sample the materials used to construct the levee/dike. In addition, the previous test borings revealed the presence of a discontinuous “artificial fill”
layer along portions of the levee/dike and flood wall alignment.
2020 GZA Test Borings
The 2020 GZA Test Borings were drilled through the dikes/levees and adjacent to the floodwall approximately at 300 to 500-foot intervals between the upstream end of the Smith College
Dike and the downstream end of the Diversion Dike. The objectives of the 2020 GZA Test Borings were:
To obtain information about the embankment and foundation soils to support the geotechnical engineering analyses necessary to support the FEMA’s levee certification/accreditation effort
for the System; and
To determine if the “artificial fill” layer was removed or left in-place below the System.
As a first step,
GZA prepared and submitted a drilling work plan entitled “Revised Drilling Work Planfor Subsurface Exploration Program – FEMA Certification Efforts for the Connecticut River and Mill
River Levee Systems – Northampton, MA”, dated November 20, 2020 to the USACE for review and approval. TheWork Plan was prepared in accordance with the USACE’s ER 1110-1-1807, Engineering
and Design, “Drilling in Earth Embankment Dams and Levees”, dated December 2014. The final Drilling Work Plan was approved by the USACE on 11/21/2019. The Work Plan has been provided
to the City as separate document and should referred to as needed for additional information.
GZA engaged New England Boring Contractors, Inc. (NEBC) of both Glastonbury, Connecticut and Hermon, Maine to perform nine (9)test borings (MR-1, MR-1A through MR-8). The 2020 test borings
were performed between the dates of January 15 and 22, 2020from the top of the levee/dike, except for MR-4 and MR-5, which were performed at grade on the landside of the floodwall.
The 2020 test borings were performed using an ATV mounted drilling rig and a truck mounted drill rig. The borings were advanced using drive-and-wash, cased drilling techniques to depths
ranging from 24 to 44feet below ground surface (bgs). Split spoon samples were obtained continuously through the levee fill material at each boring location, and at 5-foot intervals
thereafter. Split spoon samples were collected in general accordance with ASTM D1586, the Standard Penetration Test (SPT). Upon completion, each borehole was backfilledwith a cement-bentonite
grout mixusing a tremie grouting technique.
A GZA engineer coordinated with the drilling subcontractor, observed the borings on a full-time basis, classified the soil samples, prepared the daily field reports attached as Appendix
C and prepared the boring logs attached as Appendix D. The as-drilled boring locations were located by GZA using the Trimble R1 GPS unit with submeter accuracy. The boring location
plans are attached as Figure 2. Ground surface elevations were estimated from a plan entitled “PLAN OF LAND IN NORTHAMPTON, MA” prepared by Northeast Survey Consultants, Inc. and dated
July9, 2015.
GEOTECHNICAL LABORATORY TESTING
Nine(9) soil samples collected from the 2020 test borings were selected by GZA for analysis and submitted to Thielsch Laboratories in Cranston, Rhode Island, for testing. The geotechnical
laboratory testing program consisted of gradation analyses and organic content test. The purpose of the testing was to evaluate/confirm field classifications and to support assignment
of engineering parameters for use in subsequent geotechnical engineering analyses. Geotechnical laboratory test results are attached as Appendix E.
Inferred SUBSURFACE CONDITIONS
Soil AND RoCK
Based on the as-built record drawings, the Mill River Levee System construction consisted of five distinct levee cross sections as shown in Plate XIII of Appendix B as well as one section
of flood wall. From Station 0+00 to the West Street Closure Structure at Station 11+60, the cross sections indicate typical levee/dike embankment construction consisting of Impervious
Blanket Fill on the riverside of the levee/dike followed by Random Fill for the remainder of the embankment. Following the section of flood wall after the West Street Closure Structure,
from Station 16+50 to Station 23+50, the cross sections indicate typical levee/dike embankment construction consists of Random Fill on both the riverside and landside with a Selected
Impervious Fill core. The cross sections also indicate an approximately 1-foot-thick layer of riprap along the riverside of the entire levee/dike.
All of the 2020 test borings were performed on the top of the levee, with the exception of the floodwall borings (MR-4 and MR-5). The levee/dike boring locations werealternatedbetween
the riverside edge and the landside edge of the top of the levee/dike to capture subsurface data from both the pervious (landside) and impervious fill (riverside) layers of the levee/dike
construction.The foundation soils generally consist of a discontinuous layer of Existing Fill (i.e. the “artificial fill” identified in the previous borings that existed prior to the
levee/dike construction) underlain by a naturally deposited Gravel and Sand layer and/or Varved Claydeposit. Weathered Bedrock was encountered below the naturally deposited soils at
four of the nine2020 test boring locations.
A general description of each strata/deposit encountered in the 2020 explorations is presented below, in order of increasing depth. Refer to the test boring logs in Appendix D for the
conditions encountered in each exploration location. A summary of the subsurface conditions is also provided in Table 1.The foundation soil subsurface conditions encountered in the 2020
borings were generally consistent with the conditions indicated in the previous test boring logs and as shown on the record drawings.
Topsoil Fill – An approximate3- to 6-inch thick layer of Topsoil Fill was encountered at the ground surface on the top of the levee/dikeat borings MR-1, MR-2, MR-3, MR-6, MR-7, and
MR-8. The Topsoil Fill predominately consisted of dark brown, fine to coarse SAND, with up to 35 percent Silt, up to 20 percent roots and other organic material, and up to 20 percent
gravel.
Gravel Base Course –An approximate 0.6- to 1.8-foot thick layer of Gravel Base Course was encountered beneath a thin layer of asphalt at borings MR-4 and MR-5 (landside of the floodwall),
and below the Topsoil Fillat borings MR-1, MR-2, MR-3, MR-6, MR-7, and MR-8 on top of the levee/dike. The Gravel Base Course predominately consisted of brown, fine to coarseSAND, with
up to 45 percent gravel, and up to 45 percent Silt/Clayey Silt. Additionally, 50 percent of the samples contained up to 10 percent organics.The SPT N-values within the Clayey Gravel
Base Courseranged between 6 to 36blows per foot (bpf), indicating a loose to dense relative density.However, the typical density of the Gravel Base Course was observed via the SPT to
be Medium Dense.
Impervious Blanket Fill – An Impervious Blanket Fill layer was encountered in test boring MR-3, below the Gravel Base Course layer on top of the levee/dike and was observed to be approximately
3 feet thick. The Impervious Blanket Fill generally consisted of brown, SAND and GRAVEL, with up to 20 percent Silty Clay. The SPT N-value within the fill was 19bpf, indicating a medium
dense relative density.
Random Fill – A Random Fill layer was encountered in test borings MR-1, MR-2, and MR-3 beneath the GravelBase Course layer on the levee/dike. The Random Fill thickness ranged between
approximately 11 feet to 24 feet. The Random Fill generally consisted of brown, fine to medium SAND, with up to35 percent Silt, up to 10 percent clay, and up to 10 percent roots. The
SPT N-values within the Random Fill ranged from 7bpf to 57bpf, indicating variable relative density ranging from loose to very dense. However, the typical density of the Fill was observed
via the SPT to be “dense”. It is noted that N-values greater than 50 bpf are anticipated to indicate the presence of cobbles or boulders, and do not necessarily represent the relative
density of the Random Fill.
Selected Impervious Fill – ASelected Impervious Fill layer was encountered in test borings MR-6, MR-7, and MR-8 beneath the Gravel Base Course layer on the levee/dike. The Selected Impervious
Fill thickness ranged between approximately 17 feet to 20 feet. The Selected Impervious Fill generally consisted of gray, SILT & CLAY, with up to 35 percent fine to coarse SAND, and
up to 10 percent roots. At test boringsMR-7 and MR-8, thebottom of the Selected Impervious Fill layer at the interface with the naturally deposited VarvedClay appeared to also include
up to 10 percent wood fibers and cinders.The presence of the wood fibers and cinders may be indicative of the original river-bed material prior to construction of the levee/dike. The
SPT N-values within the Selected Impervious Fill ranged from WOH (weight of hammer) to 36bpf, indicating variable relative density ranging from very soft to hard. However, the typical
density/consistency of the Select ImperviousFill was observed via the SPT to be “medium stiff”.
Existing Fill – An Existing Fill layer was encountered in test borings MR-1 beneath the Random Fill Layer, MR-4, and MR-5 beneath the Sand Base Course layer. It appears the Existing
Fill layer was in-place prior to the construction of the levee, hence the “existing” descriptor. The “Existing Fill” also appears to be of similar consistency and location as the “Artificial
Fill” shown on the Record Drawings for the System. The Existing Fill thickness ranged between approximately 6.5 feet to 11 feet and generally consisted of brown, fine to coarseSAND,
with up to45 percent Gravel, andup to 45 percent Silt. Test boring MR-1 contained up to 45 percent red-brick fragments, and test borings MR-4 and MR-5 contained up to 20 percent coal
slag and cinders. The SPT N-values within the Fill ranged from 2bpf to 72bpf, indicating a widely variable relative density ranging from loose to very dense. However, the typical density
of the Existing Fill was observed via the SPT to range betweenLoose and Medium Dense. It should be noted that the blow counts atN-values greater than 50 bpfwere only noted in test Boring
MR-1 and are anticipated to have been caused bythe presence of the red bricks. Therefore, these high blow counts may not necessarily be indicative of the actual relative density of
the Existing Fill layer.
Gravel and Sand – A naturally deposited Gravel and Sand layer was encountered in test borings MR-1 through MR-6, below the constructed levee/dike (i.e. Existing Fill, Random Fill, or
Selected Impervious Fill layers). The Gravel and Sand layer thickness ranged between approximately 3.5 feet to 11 feet. The Gravel and Sand layer generally consisted of gray, GRAVEL,
up to 50 percent fine to coarse SAND (occasionally over 50 percent fine to coarse SAND), and up to 10 percent Silt. The SPT N-values within the Gravel and Sandlayer ranged from 1bpf
to 49bpf, indicating variable relative density ranging from very loose to dense. However, the typical density of the Gravel and Sand layer was observed via the SPT to be Medium Dense.
Varved Clay – A naturally deposited Varved Clay deposit was encountered in test borings MR-3,MR-4,MR-5, MR-7, and MR-8 below the naturally deposited Gravel and Sand layer. The Varved
Clay deposit generally consisted of a grey, SILTY CLAY or CLAY & SILT, with alternating 1/16-inch to 1/4-inch thick lenses of a dark grey Clayey Silt, or reddish-brown fine to medium
sand. The SPT N-values within the Varved Clay ranged from WOR (Weight of Rod) to 6bpf, indicating a variable relative density, ranging from very soft to mediumstiff. However, the typicalrelative
density/consistency of the Varved Clay deposit was observed via the SPT to be Very Soft.
Weathered Bedrock – Weathered Bedrockwas encountered at test borings MR-1A, MR-2, MR-4, MR-5, and MR-7beneath the naturally deposited soils (i.e. Gravel and Sandlayer or Varved Clay
deposit)at depths of approximately 31.5 to 39feet bgs. The Weathered Bedrock generally consisted of reddish-brown SANDSTONE that has been weathered into a soil-like matrix consisting
of fine to coarse SAND, withup to 20 percent Gravel and up to 20 percentSilt. The SPT N-values were greater than 100bpf, indicating a Very Dense relative density. The test borings were
not advanced through the weathered zone. Therefore, the depth to sound bedrock was not determined at the noted test boring locations.
Groundwater
Water level observationswere made during drilling at test borings MR-1, MR-3, MR-4, MR-6, and MR-7. The water levels ranged between approximately 0.3 feet bgs to approximately 25 feet
bgs, whichcorrelate to a range between about El. 120 feet and El. 137.7 feet.
However,it must be recognized that these water level readings were obtained inside the casing during drilling. Water was added to each borehole as part of the drive-and-wash drilling
methods used and water was added to the drill casing at the end of each day to mitigate the potential of soil “blow-in” into the casing, as stipulated in part of the Drilling Work Plan.Therefore,
the water levels obtained during drilling are not representative ofthe actual stabilized groundwater conditions at the sitenor the location of the phreatic surface within the levee/dike.
The water levels reported on the boring logs are intended to provide qualitative permeability information for the various soil layers/deposits where the bottom of the casing was located
at time of each reading.
Independent of the 2020 test boring program, an existing, previously installed, groundwater monitoring well (well number ID unknown) was found on the landside of the floodwall near boring
MR-5. Refer to Figure 2 for the approximate location. On January 21, 2020, GZA measured the water level inside the well to be about 10.2 feet bgs, correlating to approximately El 114.5
feet. The depth to the bottom of the well was recorded to be 14.5 feet bgs. No other information about the existing well was available.
It should be noted that fluctuations in groundwater levels may occur due to seasonal variations in rainfall and temperature, site features, and other factors different from those existing
at the time of the explorations and measurements.
CLOSING
We trust that the information contained in this memorandum meets the City’s needs at this time. Please feel free to contact us if you have any questions or comments regarding the content
of this memorandum.
Attachments:Table 1 – Summary of Subsurface ConditionsFigure 1 – Locus PlanFigure 2 – Exploration Location Plan
Appendix A – Limitations
Appendix B – Record Drawings and Previous Test Boring Data
Appendix C – Daily Field Reports
Appendix D – 2020 Test Boring Logs
Appendix E – Geotechnical Laboratory Test Results
J:\170,000-179,999\174343\174343-00.CLB\Memos\Geotechnical Investigation\Mill River\Mill River Levee System - Geotechnical Data Memo_12-22-2020.docx
TABLE
FIGURES
APPENDIX A
LIMITATIONS
APPENDIX B
RECORD DRAWINGS AND PREVIOUS TEST BORING DATA
APPENDIX C
DAILY FIELD REPORTS
APPENDIX D
2020 TEST BORING LOGS
APPENDIX E
GEOTECHNICAL LABORATORY TEST RESULTS