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GEOTECHNICAL SUMMARY REPORT
Prepared for:
A l i t C t D t t f E i t l S i Di i i f
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Table of Contents
1. Overview............................................................................................................................ 11.1 PROJECT CORRIDOR AND STUDYAREA ............................................................................................ 1
2. Geologic Setting ............................................................................................................... 42.1 LOCAL GEOLOGY ............................................................................................................................. 4
3. Surface Conditions ........................................................................................................... 6
4. Subsurface Conditions ..................................................................................................... 75. Geotechnical Considerations and Recommendations for Future Investigation .........13
6. References .......................................................................................................................14
APPENDICES
1 Three Metropolitan Geotechnical Report
2 Crystal Drive, Route 1 and South Glebe Road Extended Geotechnical Report
3 Potomac Yard Transitway Stations B and C Geotechnical Report
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1. Overview
This report summarizes the collection and review of readily available geologic and geotechnicaldata, as well as a field site reconnaissance along the proposed Crystal City Streetcar Project.
The review performed for this task focused solely on the geologic and geotechnical aspects and
their potential impacts on the proposed streetcar design and construction. Recommendations
for continued review, additional literature review and field investigations are presented based on a
review of the literature and previous experience with design and construction on previous
projects.
1.1 PROJECT CORRIDOR AND STUDY AREA
The project corridor extends from the Potomac Avenue crossing of Four Mile Run north
approximately 2 miles to the Pentagon City Metro Station. Although alternative streetcar
alignments are still under consideration in the corridor, the streetcar will generally occupy existing
right-of-way. Construction excavation is anticipated to be relatively shallow for the track
elements, station stops and electrical improvements. The streetcar corridor will approximately
parallel U.S. Route 1 and Crystal Drive through the northern portion of Potomac Yard, Crystal
City, and Pentagon City in Arlington County, Virginia.
At the time of this report the proposed streetcar corridor and potential alignments were located
within a study area within Arlington County. The study area and base alignment are illustrated in
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Figure 1: Crystal City Streetcar Alignment
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Table 1: Crystal City Streetcar Alignment Description
Segment Name Description of Segment
Arlington Potomac Yard Potomac Avenue at the Alexandria/Arlington border, turning west on SouthGlebe Road. North on Jefferson Davis Highway merging with Crystal Drive.
Arlington Crystal City Begins at 26th Street South and runs northbound along Crystal Drive to18th StreetSouth. The southbound track runs along 18 thStreet South andthen along Crystal Drive from 18
thStreet South to 26
thStreet South, joining
the Arlington Potomac Yard segment.
Arlington Pentagon City The Arlington Pentagon City segment begins at the intersection of CrystalDrive and 18
thStreet South. It travels north on Crystal Drive before turning
west on 12th Street South, terminating at South Eads Street. The
southbound track runs along 12thStreet South from South Eads Street to
South Clark Street, turning south parallel to U.S. 1 to join the Arlington
Crystal City segment at 18thStreet South.
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2. Geologic Setting
The Crystal City Streetcar Project is geologically located completely within the Atlantic CoastPlain Physiographic Province. The project study area is located east of the boundary zone
between the Piedmont Plateau and the Coastal Plain physiographic provinces, also commonly
known as the Fall Line, or Fall Zone (Reed and Obermeier, 1982). The Fall Zone lies west of the
project study area and generally runs along Route 66 in Arlington from Rosslyn to Four Mile Run
and within Alexandria the Fall Line tends to parallel Interstate 95. The Fall Line separates the
Piedmont Plateau Province on the northwest from the Coastal Plain on the southeast.
The Piedmont Plateau Province is characterized by rolling upland topography consisting of
igneous and metamorphic rocks. The Atlantic Coastal Plain is located to the south and east of
the Fall Line and consists of a wedge of unconsolidated sentiments of gravel, sand, silt and clay.
The Piedmont bedrock began as Precambrian sediments that formed a stack of four units that
lithified into rock consisting primarily of gneiss and schist. The bedrock has been folded and
faulted and in the western portion of the Piedmont Plateau the rock has been intruded by granite
and quartz. Paleozoic metamorphic bedrock does not outcrop within the study area and can only
be seen in the far western portion of the City of Alexandria.
Two of the underlying metamorphic rock units are mapped in the Arlington-Alexandria area; the
MatherGeorge and Sykesville formations and the Annandale Group and Indian Run formation.
The Annandale-Indian Run formation consists of poorly to well foliated sedimentary mlange
consisting of medium grained quartz-plagioclase-muscovite-biotite chlorite granite matrix The
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soils are all fluvial in origin and that all of the sedimentary facies indicate that the deposits are not
deltaic, estuarine or marine in origin. The beds at the base of the layer resemble the underlying
sedimentary rock in that they contain feldspathic sand, smectitic clay and clasts of crystalline
rocks (Fleming et.al., 1994). The Potomac Group in the study area includes an upper silty and
sandy claydominated facies and a lower sand and graveldominated facies (Flemming et al.,
1994.
The Potomac formation is overlain by poorly to well bedded, lowland terrace deposits of sand, silt,
gravel and clay. Most of the preliminary streetcar alignment is located along an area mapped as
middle Pleistocene Terrace Deposits of gravel, sand, silt and clay and Holocene Alluvial deposits
and Artificial Fill. The terrace deposits are gray-brown to medium orange in color and range up to35 feet thick west of the Potomac River. The alluvium is generally poorly sorted and poorly
bedded and where streams drained is derived from weathered Saprolite and older alluvial
deposits consisting of well-rounded pebbles and cobbles as well as sand, silt and clay.
Artificial Fill is mapped along Route 1 from the Alexandria border north to near 33rd Street in
Arlington. Other areas to the north in Arlington are mapped as having Terrace deposits although
Artificial Fills can be in other areas of modern development.
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3. Surface Conditions
Surface conditions along the preliminary alignment consist primarily of 4 and 6 lane existingpaved right of way and bridge crossings. The roadways are mainly asphaltic concrete with some
new roadway being planned at Potomac Yard. Most of the streetcar corridor traverses populated
residential and commercial properties. The roadways have curb and gutter with drainage inlets
and adjacent sidewalks with grass lawn and tree landscaping in some areas.
Most of the proposed right of way will be using existing asphalt and concrete pavements and will
require removing a portion of the pavement, stabilizing and preparing the underlying fill materials
and construction of the new support section for the streetcar. The subsurface soils that are
anticipated to be encountered are described below.
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4. Subsurface Conditions
Readily available geotechnical reports were provided by The City of Alexandria and by theCounty of Arlington. These reports were reviewed to provide preliminary geotechnical data
regarding the near surface soils along the proposed alignment. No appreciable change in the
elevation of the pavement is anticipated, and no additional fill placement and subsequent
consolidation settlement are expected. Therefore, the literature review concentrated on the
characteristics of the near-surface subgrade soils. As previously mentioned, most of the
alignment is mapped as being in artificial man-made fill. The classification, thickness and the
consistency of the fill material varies along the alignment are described in the reports below:
Report of Subsurface Exploration and Geotechnical Engineering Analysis, ThreeMetropolitan Park, Arlington County, Virginia, prepared by ECS Mid-Atlantic LLC,dated September 15, 2008.
According to the boring logs, the near-surface fill soils consist primarily of a lean clay with
sand, sandy clay, silt and clay and sand. The fill contained gravel and brick fragments.
The consistency of the fill materials varied from soft to stiff and the granular fill was loose
to dense. The clayey materials generally were classified as being moist and the granular
fills were moist to wet. Groundwater level was measured to be from 25 to 35 feet in
depth.
Figure 2 on the next page shows the boring locations. Appendix 1 contains the Three
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Arlington County, Virginia Soil Survey - the most recent Arlington county Soil survey
maps the alignment as an urban land Udorthents complex, with 2 to 15 percent slopeswith more than 85 percent of the surface covered by buildings, asphalt, concrete or otherimprovements. The Udorthents consist mainly of material that has been graded, disturbedand urbanized and the materials are loamy and generally reflect the soils in the adjacentarea.
http://gis.arlingtonva.us/Maps/Standard_Maps/Environmental_Maps/Soils_Map.pdf
Arlington County Bus Rapid Transit (BRT) System, Crystal Drive, Route 1 andSouth Glebe Road Extended, Arlington, Virginia, prepared by GeoConceptsEngineering, Inc., dated April 7, 2003.
According to the boring logs, the near-surface fill soils consist primarily of a clayey sand
and lean clay, with about 17 to 76 percent fines passing the U.S. Standard No. 200 sieve.
Liquid limits and plasticity indices ranged from 27 to 37 and 9 to 16, respectively. The
natural moisture contents ranged from 7.9 to 21.0 percent. The fill soils are believed to be
grading fill associated with the sites previous use as a railroad yard. The soils beneaththe near-surface soils are lean clay believed to be river terrace deposits associated with
the nearby Potomac and its tributaries. Terrace deposits represent ancient flood plain
sediments deposited when the Potomac River and its tributaries were at a higher
elevation than they are today. Groundwater was observed at depths of about 6 to 20 feet
below the existing ground surface.
http://gis.arlingtonva.us/Maps/Standard_Maps/Environmental_Maps/Soils_Map.pdfhttp://gis.arlingtonva.us/Maps/Standard_Maps/Environmental_Maps/Soils_Map.pdfhttp://gis.arlingtonva.us/Maps/Standard_Maps/Environmental_Maps/Soils_Map.pdf -
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Report of Subsurface Exploration and Geotechnical Engineering Analysis, PotomacYard Transitway Stations B and C, Arlington, Virginia, prepared by ECS Mid-AtlanticLLC, dated January 17, 2008.
According to the boring logs, the near-surface fill soils consist of a combination of sandy
clay, clay, clayey silt, sand, and silty sand with varying amounts of gravel, asphalt, brick
fragments and organics. The soils beneath the fill soils consisted of medium stiff to stiff
clay and organic silt and clay, as well as medium dense to dense sand and medium stiff to
very stiff clay and sandy clay. Groundwater level was measured to be from 23 to 34 feet in
depth.
Figure 4 on the next page shows the boring locations.Appendix 3contains the PotomacYard Transitway Stations B and C geotechnical report, which includes the boring locationsand logs.
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5. Geotechnical Considerations andRecommendations for Future Investigation
Based on a review of the literature and the readily available subsurface information
located in the general vicinity of the proposed alignment, the preliminary geotechnical
considerations for the design and construction of the streetcar and appurtenances are: 1)
the heterogeneous nature of the near-surface fill soils, 2) and the relatively high plasticity
and moisture content of the more clayey fill soils.
The near-surface fills soils underlying the existing pavement sections are heterogeneous
in nature and potentially compressible. These soils are non-uniform and will probably
require reworking and or stabilization. It must be noted however, that the boring
information reviewed for this preliminary study were taken in areas away from the
proposed alignment pavements. It may be that the fill materials were previously stabilized
during pavement construction along the majority of the proposed alignment. Therefore, it
is recommended that a series of borings be performed along the proposed alignment to
further investigate the surface and subsurface conditions.
Some of the clayey fill soils have a moderate to high plasticity. Where these soils are
encountered below the proposed support section for the streetcar, these soils will require
undercutting and replacement with soils having a higher CBR value. The granular fill soils
th t ti t t t ill d t b d i d t d t d t t
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6. References
Arlington County, Virginia Soil Survey, (2010), - Mapped by Louis Heidel and Fred
Garst from 1996-1997, USDA Natural Resource Conservation Service Woodstock, VA
Service Center, Soils surveyed at a scale of 1:7200.
Bromberg, Francine W. (2010). The History of Potomac Yard: A Transportation Corridor
Through Time.Appendix III of North Potomac Yard Small Area Plan. City of Alexandria.
Cohen, Robert (2003). History of the Long Railroad Bridge Crossing Across thePotomac River. National Railway Historical Society, Washington D.C. Chapter.
Fleming et. al., (1994) - Geologic map of the Washington West Quandrangle, District of
Columbia, Montgomery and Prince Georges counties, Maryland, and Arlington and
Fairfax Counties, Virginia. U.S. Geological Survey Geologic Quadrangle Map GQ-1748,
scale 1:24,000.
National Railway Historical Society, Washington, D.C. Chapter. Timeline of
Washington, D.C. Railroad History. Accessed 2012-11-14 (http://www.dcnrhs.org).
Reed and Obermeier, (1982) The geology beneath Washington, D.C.; the foundations
of a nations capital, In Legget, R.F. ed. Geology under Cities; Reviews in Engineering
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a Superfund site. In 1995 the Environmental Protection Agency (EPA) approved RF&P's study
and cleanup plan, and cleanup was declared completed by 1998.
The geotechnical data search and recommendations cannot confirm or deny the validity of theEPA 1998 declaration regarding Potomac Yard. It is assumed to be a safe site for the purposes
of construction of streetcar track elements and station stops. Additional geotechnical
investigation and soil testing is recommended along the selected alignment alternative during the
preliminary engineering phase of project development.
http://en.wikipedia.org/wiki/Superfundhttp://en.wikipedia.org/wiki/United_States_Environmental_Protection_Agencyhttp://en.wikipedia.org/wiki/United_States_Environmental_Protection_Agencyhttp://en.wikipedia.org/wiki/Superfund -
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Appendix 1
Three Metropolitan Geotechnical Report
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Appendix 2
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Crystal Drive, Route 1, and South Glebe RoadExtended Geotechnical Report
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Appendix 3
Potomac Yard Transitway Stations B and C
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January 17, 2008
Mr. Ross VoorheesGreenhorne & OMara6110 Frost PlaceLaurel, Maryland 20707
ECS Job No. 14081
Reference: Report of Subsurface Exploration and Geotechnical Engineering Analysis,Potomac Yard Transitway Stations B and C, Arlington, Virginia
Dear Mr. Voorhees:
As authorized by your acceptance of ECS Mid-Atlantic, LLC (ECS) Proposal No. 28346-GPR, dated October 24, 2007, and revised November 27, 2007, we have completed thegeotechnical engineering analysis for the above-referenced project in Arlington, Virginia.O h i l i i l i i b d d illi f il b i Thi
Greenehorne & OMara
ECS Project No. 14081
January 17, 2008Page 2
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Purposes and Scope of Exploration
The purposes of this exploration were to explore the soil and groundwater conditions at thesite and to develop geotechnical engineering recommendations to guide general feasibilityplanning of the project. We accomplished these purposes by:
1. drilling borings in undeveloped areas to explore the subsurface soil andgroundwater conditions,
2.
performing laboratory tests on selected representative soil samples from theborings to evaluate pertinent preliminary engineering properties,
3. analyzing the field and laboratory data from this and earlier studies to developappropriate engineering recommendations, and
4. preparing this preliminary report.
The conclusions and recommendations contained in this report are based on four soil boringsperformed by ECS in December 2007. The boring locations for our exploration wereestablished in the field by representatives of ECS utilizing Global Positioning System (GPS)
Greenehorne & OMaraECS Project No. 14081
January 17, 2008Page 3
A field log of the soils encountered in the ECS borings was maintained by the drill crew.After recovery, each sample was removed from the sampler and visually classified.
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Representative portions of each sample were then sealed and brought to our laboratory inChantilly, Virginia for further visual examination and laboratory testing.
Laboratory Testing Program
Representative soil samples from the ECS borings were selected and tested in our laboratoryto check field classifications and to determine pertinent engineering properties. Thelaboratory testing program included visual classifications, moisture content, grain size
analysis, and Atterberg Limits tests. All data obtained from the laboratory tests are includedon the attached, respective boring logs or on separate sheets.
Each ECS soil sample was visually classified on the basis of texture and plasticity inaccordance with the Unified Soil Classification System (USCS). The group symbols for eachsoil type are indicated in parentheses following the soil descriptions on the boring logs. Abrief explanation of the USCS is included with this report. The soil engineer grouped thevarious soil types into the major zones noted on the boring logs. The stratification lines
designating the interfaces between earth materials on the boring logs and profiles areapproximate; in situ, the transitions may be gradual, rather than distinct.
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January 17, 2008Page 4
similar to natural alluvial soil deposits. Further, as the fill soils age, they may obtain thevisual characteristics of natural soils. Hence, it is difficult at times to distinguish between
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natural alluvial soils and man-made fill materials.
Subsurface Conditions
The subsurface conditions encountered within the four soil test borings were consistent with theregional geology and the findings of our geotechnical reports prepared for nearby sites. Theconditions for Borings B-1 and B-2, when compared to B-3 and B-4 varied due to the differentlocations of the proposed transit stations. Beneath a gravel layer approximately 4 to 18 inches
thick, two major soil strata were encountered within our boring termination depths. These strataare described in the subsequent text. For detailed information at specific locations, please refer tothe Boring Logs attached to this report.
Stratum I Old Fill Soils
Old fill and possible fill soils were encountered in all of the borings to depths ranging
from 8 feet to 29 feet below the existing ground surface. Deeper fill was encountered atStation C. The old fill soils consisted of a combination of Sandy CLAY and CLAY(CL), Clayey SILT (ML), SAND and SILTY SAND (SP, SM) with varying amounts
l h l b i k f d i Th ld fill id ifi d b d
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Furthermore, visual observation of the moisture content of soil samples retrieved during theauger drilling exploration can often be used in evaluating the groundwater conditions.
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Observations for groundwater were made during sampling and upon completion of drillingoperations at each boring location. Groundwater seepage was observed during drillingoperations in each of the four borings, at depths ranging between 23 and 34 feet below theexisting ground surface. These groundwater depths correspond to elevations ranging fromapproximately EL. -10 feet to EL. 1 foot. Furthermore, visual observation of the moisturecontent of soil samples retrieved during the auger drilling exploration can often be used inevaluating the groundwater conditions.
The highest groundwater observations are normally encountered in the late winter and earlyspring. Generally, variations in the location of the long-term water table may occur as aresult of changes in precipitation, evaporation, surface water runoff, and other factors notimmediately apparent at the time of this exploration. The project site may also be subject toshallower perched water conditions where surface water becomes trapped within the variablecomposition of the existing fill or within granular deposits underlain by more cohesive soils.Where perched groundwater is encountered, surficial groundwater control, such as trenchingor French Drains, may be required. For excavations which terminate above the groundwater
table, dewatering can generally be performed by sump pit and pumping operations.
G t h i l R d ti
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to depths of 12 to 25 feet below the existing ground surface and typically consist of 30-inchdiameter drilled excavations, which are backfilled in 1-foot lifts utilizing compactive effortand granular aggregate. The aggregate piers are compacted with different methods,
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g gg g gg g p p ,depending on the propriety means of the contractor/vendor. The soil reinforcement occurs asa result of the excavation of the existing soils and replacement by dense granular aggregate.In addition, some limited densification of the surrounding soils is reported to occur. Theengineering characteristics of the reinforced soil are significantly improved to allow theplacement of spread footing foundations.
The aggregate piers can be utilized under the canopy footings. Aggregate piers are normallydesigned by a design build contractor and the proposed soil improvement plan is reviewed by
the Geotechnical Engineer of Record.
To reduce and largely eliminate the risk of long term unacceptable settlement. Footingssupported by the aggregate piers could be designed for pressures of 3,000 to 5,000 psf, asdetermined by the design/build contractor of these systems. Design settlement should belimited to 1 inch of total settlement and inch of differential settlement.
Option II: Shallow Foundations with Limited Undercutting
The proposed structure may be supported on a shallow spread and/or continuous foundation
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January 17, 2008Page 7
For footings that are to bear on approved lightweight engineered fill, consistent with thesketch titled Foundation Subgrade Undercutting, a maximum allowable bearing pressure of2,000 pounds per square foot (psf) may be utilized for the design of the structure.
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, p p q (p ) y g
The allowable soil bearing pressure refers to that pressure which may be transmitted to thefoundation bearing soils. During construction, verification of the foundation subgradesshould be documented in the field by an experienced geotechnical engineer to ensure that therecommendations contained herein are followed.
Settlement of a structure is a function of the compressibility of the underlying soils, thedesign bearing pressure, column loads, fill depths, and the elevation of the footing with
respect to the original ground surface. Because of the underlying fill soils and potentiallycompressible clay (CL) and organic clay and silt (OH/OL) layers, long-term foundationsettlement values could be more than one inch utilizing the shallow foundation systemoption.
In order to reduce the possibility of foundation bearing failure and excessive settlement dueto local shear or "punching" action, we recommend that continuous footings have a minimumwidth of 2.0 feet and that isolated column footings have a minimum lateral dimension of 3
feet. In areas where individual footings are stepped down and founded at differentelevations, it is important to ensure a minimum slope of 1H:1V exists between the bottomedge of each foundation at their closest point.
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