Langan Geotech Report.pdf

GEOTECHNICAL ENGINEERING REPORT

for

PRESERVATION OF MERIDIAN HILL PARK

Washington, DC

Prepared For:

Mills + Schnoering Architects, LLC

200 Forrestal Road, Suite 3A

Princeton, New Jersey

On Behalf of:

United States Department of the Interior

National Park Service

Denver Service Center – Contracting Services

12795 W. Alameda PKWY

Denver, Colorado 80225

Prepared By:

Langan Engineering and Environmental Services, Inc.

1300 Wilson Boulevard, Suite 450

Arlington, Virginia 22209

Kyle J Lawson

Ronald Manney, P.E.

Washington D.C. Professional Engineer License No. PE907655

1 December 2017

270060901

Geotechnical Engineering Report

Preservation of Meridian Hill Park

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Langan Project No.: 270060901

Page i of i

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 PROJECT DESCRIPTION

3.0 REVIEW OF AVAILABLE INFORMATION

4.0 SUBSURFACE INVESTIGATION

5.0 SUBSURFACE CONDITIONS

6.0 DESIGN RECOMMENDATIONS

7.0 CONSTRUCTION RECOMMENDATIONS

8.0 CLOSURE/LIMITATIONS

FIGURES

Figure 1 Site Location Map

Figure 2 Investigation Location Plan

Figure 3 Geologic Map

Figure 4 FEMA Flood Map

APPENDICES

Appendix A Concrete Coring Photographs

Appendix B Lightweight DCP Methodology and Test Results

Appendix C Medium Weight DCP Methodology and Test Results

Appendix D Hand Auger Logs

Appendix E Laboratory Testing

Appendix F Historic Boring Logs By Others

Appendix G Historic Photographs

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1.0 INTRODUCTION

This report presents the results of our geotechnical engineering study performed for the Preservation of

Meridian Hill Park project located in Northwest Washington D.C. The purposes of this study were to:

1) Research and review available site information;

2) Obtain shallow subsurface information by performing an investigation consisting of hand augers and dynamic cone penetrometer (DCP) testing;

3) Provide recommendations for geotechnical aspects of the proposed project;

All elevations given in this report are referenced to a project specific datum given in the December 2015

Phase 5 Schematic Design Drawings survey provided by Mills + Schnoering Architects, LLC (M+S). No environmental testing or analysis was completed as part of our study.

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2.0 PROJECT DESCRIPTION

2.1 Existing Conditions

Meridian Hill Park is located in northwest Washington D.C. and is owned and operated by the National

Park Service (NPS). The park is bordered by Euclid Street to the north, W Street NW to the south, 15th

Street NW to the east, and 16th Street NW to the west. The adjacent properties consist of multi-story residential and mixed-use structures; see Inset 1.

Inset 1

Meridian Hill Park was designed and constructed between 1912 and 1936 and includes various memorials/statues, pedestrian walkways, and landscaped areas. Existing structures and features within the park include the following:

Upper Park – The northern portion of the park consists of an open grassed mall with paved pedestrian pathways, benches, and trees. The Lodge House, a one story structure that houses public bathrooms, is located on the east side of the Upper Park. The Serenity statue is located on the west side of the Upper Park.

Great Terrace – This area is elevated approximately 20 feet above the south portion of the site and consists of an approximate 60 by 320 foot (19,000 square foot) area with decorative concrete pavers, landscaped planters, a Joan of Arc statue at the center, and two fountains on the east and west sides. The elevation difference is supported by an approximate 19-foot-tall retaining wall on the south side of the Terrace referred to as the Great Wall. Access to the Great

Terrace from the south side of the wall is provided by two stairways referred to as the Great

Stairs. Storage spaces and out-of-commission public restrooms are located beneath the Great

Stairs.

Cascades – This portion of the park is located south of the Great Terrace and north of the Lower

Plaza and contains an approximate 230-foot-long, 13 basin cascading fountain at the center. The cascading fountain drops approximately 30 vertical feet to meet the grade at the Lower Plaza.

Euclid Street

W St NW th S t N

W th S t N

W

Upper Park

Great Terrace

Cascades

Lower Plaza

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On either side of the cascading fountain are the East and West Ascents, which consist of a series of stairs and ramps connecting the bottom of the Great Stairs with the Lower Plaza. The

Dante statue is located along the East Ascent.

Lower Plaza – This Lower Plaza is located at the south portion of the park and contains an approximate 8,000 square foot reflecting pool at the central portion of the plaza and the James

Buchanan Memorial at the east portion of the plaza. The plaza consists of decorative pavers and landscaped areas. Two park entrances via stairs from W Street NW are located on the south portion of the plaza, and one entrance via stairs from 16th Street NW is located on the west side of the plaza.

Site grades vary significantly across the site. The Great Terrace is located at a high point above the retaining wall at approximate el 198. Grades to the north of the Great Terrace slope gently downward to the north to a low point at approximate el 190 at the north edge of the park. Immediately south of the

Great Terrace the grades drop approximately 20 feet to el 179 at the base of the retaining wall. Grades slope steeply downward to the south and west to approximate el 133 at the south edge of the park in the Lower Plaza.

2.2 Project Understanding

According to the 30 December 2015 project program prepared by Mills + Schnoering, we understand that this phase of the restoration will include investigations to understand and evaluate the following:

Rehabilitation of the Great Terrace to repair differential settlement of pavers that has occurred over the years, Restoration of the Great Stair and underlying storage spaces to correct water penetration;

Restoration of the Lower Plaza, including the repair of heaving pavement/pavers and repair/replacement of the existing drainage system, Construction of three walkway ramps at the north portion of the Lower Plaza. Construction of the walkways will include lowering grades along the west perimeter of the park along 16th

Street NW and the construction of a retaining wall along the north side of the proposed accessible walkway; and

Restoration of the Buchanan Memorial, which has deteriorated over the years due to vegetative growth, graffiti, and displacement of stone units.

As a part of our geotechnical scope of services, we evaluated the near-surface subsurface conditions at the following areas:

The Great Terrace

The Buchanan Memorial

The area of the proposed walkway ramps

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3.0 REVIEW OF AVAILABLE INFORMATION

We reviewed available historic tax maps, Sanborn maps, and aerial photographs as well as regional geologic and flood maps for the site vicinity. We also reviewed pertinent historic geotechnical boring logs provided by NPS. Information obtained from these documents is summarized in the following sections.

3.1 Historic DC Tax Maps

We reviewed the 1861 District of Columbia Tax Map entitled Topographical Map of the District of

Columbia Surveyed in the Years 1856, ’57, ’58, & ’59 published by D. McClelland, Blanchard & Mohun.

This map indicates the site area as being undeveloped, vegetated land. Small structures border the north, east, and west sides of the site.

3.2 Historic Sanborn Maps

We reviewed available historic Sanborn maps dated 1903, 1928, 1959, 1977, 1984, 1988, 1990, 1991, 1992, 1994 and 1998. These maps are described below.

1903: This map depicts the site prior to becoming Meridian Hill Park. The site’s east and west borders are 15th and 16th Street NW. Two streets oriented east to west (Prospect and

Morris Street NW) trisect the site into three sections. The southern section of the site is vacant; the middle section has a 2-story house at the south portion. The northern section of the site contains various small 1 to 2-story structures adjacent to Morris and

15th Street NW. The lots adjacent to the site are mostly vacant.

1928: The map depicts the site as Meridian Hill Park. The small structures and Prospect and

Morris Streets NW that were depicted on the 1903 map are no longer depicted at the site. The adjacent lots bordering the east and west sides of the site are generally more developed. Various 2 to 3-story structures are depicted to the east of the site. Various 3 to 7-story structures have been constructed to the west of the site. Vacant land is depicted on the southwest portion of the site.

1959: This map depicts the site similar to the 1928 map. Additional structures are depicted to the east and west of the site including apartment complexes. A majority of the land to the southwest of the site is vacant.

1977: The map depicts the site similar to the 1959 map. A 3-story building is depicted southwest of the site and previously developed land is now depicted as vacant to the northeast of the site.

1984: This map depicts the site similar to the 1977 map. Faircliff Plaza is depicted within vacant lot described in the previous map.

1988-1998: These maps depict the site conditions similar to those shown on the 1984 map.

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3.3 Historic Aerial Photographs

We reviewed available aerial photographs for the site dated 1951, 1963, 1964, 1979, 1988, 1994, 2002, 2003, 2006, 2007, 2008, 2009, 2011, 2012, and 2013. These photographs are described below.

1951: This photograph depicts Meridian Hill Park in a similar condition to as it appears today.

Adjacent site conditions are depicted in similar conditions to as they appear today;

however, partially wooded plots of land are depicted along 16th Street NW and at the southwest corner of the intersection of 15th and W Streets NW.

1963: This photograph depicts a parking lot at the southwest corner the intersection of 15th and W Streets NW, replacing the partially wooded lot depicted in the 1951 photograph.

1979: This photograph depicts a housing development having been constructed adjacent to the southwest corner of the site along 16th Street NW and a multi-story structure constructed at the southwest corner the intersection of 15th and W Streets NW.

1988 - 2003: These photographs depict the adjacent site conditions as similar to those shown in the

1979 photograph.

2006: This photograph depicts a multi-story U-shaped building has been constructed at the intersection of Belmont Avenue and 15th Street NW.

2007-2013: These photographs depict the adjacent site conditions as similar to those shown in the

2006 photograph and represent the adjacent site conditions as they appear today.

3.4 Regional Geologic Map

We reviewed the regional geologic map entitled “Geological Map of the Washington West Quadrangle, District of Columbia, Montgomery and Prince George’s Counties, Maryland, and Arlington and Fairfax

Counties, Virginia,” dated 1995; see Figure 3. Based on this map, the site is underlain by highly weathered sedimentary surficial deposits including Potomac Group sands generally characterized as quartz-rich gravel, sand, silt and clay.

We also reviewed the United States Geologic Survey’s Map Showing Configuration of Bedrock Surface in the District of Columbia Region, dated 1948-1949. Based on the map, top of rock is noted at approximate el 90 to 100 MSL in the Cascades and Lower Plaza area, or about 25 to 40 feet below grade. Rock is noted as approximate el 100 to 115 MSL in the Upper Park and Great Terrace area, or about 75 to 90 feet below grade.

3.5 FEMA Flood Map

According to the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map Number

1100010016C dated 27 September 2010, the site is located in an area outside of the 1.0% annual chance (100 year) and 0.2% annual chance (500-year) floodplains; see Figure 4.

3.6 Historic Borings By Others

Previous geotechnical borings were performed at the Great Terrace by GeoConcepts Engineering, Inc.

in 2010 and by Froehling & Robertson, Inc. in 1981. These previous geotechnical borings indicate the general subsurface conditions consist of a layer of fill overlying successive strata of clay/silt and clayey sand. Subsurface information from these logs is incorporated into Section 5 of this report.

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3.7 Review of Historic Photographs

We reviewed pertinent historic photographs of the construction of Meridian Hill Park provided to us by

NPS; see Appendix G for select pertinent photographs. Date stamps were not given on a majority of the historic photographs; therefore, an accurate construction timeline cannot be determined. Based on our review, pertinent photographs taken during and after-construction of the Great Terrace and the

Great Wall indicate the following:

Photograph MEHI110 depicts an excavation extending from the bottom of the Great Wall to the

Great Terrace final grade. In plan view, the excavation extends approximately 25 feet from the back of the retaining wall to approximately 1.5 feet south of the Joan of Arc statue.

Photograph MeridianHill86 depicts the Joan of Arc statue placed directly adjacent to the edge of the excavation for the retaining wall.

Photographs MEHI22 and MEHI23 depict the Great Terrace without visible differential settlement between the Joan of Arc statue and the front of the Great Wall.

Photographs MEHI33 and MEHI34 depicts a water line installed within a trench between the

Joan of Arc statue and the Great Wall. Loose fill is depicted covering the water line and compaction equipment is not depicted in the photographs.

Photographs MEHI195 and MEHI199 depict settlement of the pavers on the Great Terrace immediately north of the Great Wall and in the same general location it is observed today.

Photograph MEHI194 depicts settlement of the plaza near the west fountain on the Great

Terrace in the same general location it is observed today.

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4.0 SUBSURFACE INVESTIGATION

Our subsurface investigation consisted of concrete coring, lightweight and medium weight dynamic cone penetrometer (DCP) testing, and hand augering. The surface elevation at each DCP test location was inferred from the December 2015 survey provided by M+S. The sections below describe our subsurface investigation.

4.1 Concrete Coring

The concrete pavers on the Great Terrace were cored at five locations on 30 October 2017. The core locations were laid-out by Langan’s field engineers and coordinated with M+S and NPS; see Figure 2.

Concrete coring was performed by Eastern Testing and Inspection Corporation (ETI) using a 6-inch-diameter concrete core bit. Concrete core sample thicknesses were measured and recorded by our field engineers. Select photographs of each concrete core are given in Appendix A.

4.2 Lightweight Dynamic Cone Penetrometer Testing

Seven lightweight dynamic cone penetrometer (DCP) tests identified as LDCP-1 through LDCP-7 were performed on the 30 and 31 October 2017 by Langan using a Kessler K-100 DCP apparatus. The tests were performed within each concrete core hole or at the ground surface; see Figure 2. The lightweight

DCP tests were performed by driving a 5/8-inch diameter steel drive rod fitted with a 60 degree angle cone tip into the existing subsurface using a 17.6 pound hammer dropped from a height of 22.6 inches.

At each test location, the number of hammer blows to advance the cone for a two-inch-increment was recorded. Each lightweight DCP test was performed in accordance with ASTM STP-399 and was advanced to depths of 40 to 50 inches below the top of the existing concrete pavers or ground surface.

The lightweight DCP data reduction methodology and test logs are given in Appendix B.

4.3 Medium Weight Dynamic Cone Penetrometer Testing

Four medium weight DCP tests identified as MDCP-1 through MDCP-4 were performed by Langan on

31 October 2017 using a Wildcat DCP, developed by Triggs Technologies, Inc. All medium weight DCP tests were performed at existing grade, or between approximate elevation 141.5 to 145. The medium weight DCP tests were conducted by driving a 1-inch-diameter steel rod fitted with a 1.4-inch-diameter cone tip into the subsurface using a 35-pound hammer dropped from a height of 15 inches.

The cone was advanced to depths ranging from approximately 4.8 to 7 feet below existing grade and the number of hammer blows to advance the cone for each 4 inch increment was recorded. The medium weight DCP test results were analyzed using software developed by Triggs Technologies, Inc.

The medium weight DCP data reduction methodology and test logs are given in Appendix C.

4.4 Hand Augering

Following DCP testing, a 3-inch-diameter hand auger was used to obtain samples of the underlying soil strata at test locations LDCP-1 through LDCP-7, MDCP-1, and MDCP-2. The hand auger and sampling depths ranged from 3.3 to 6 feet below existing grade. Our field engineers logged each hand auger excavation and classified encountered soils. Hand auger logs are given in Appendix D.

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4.5 Laboratory Testing

During our study, soil samples were visually examined in the field, and classifications were confirmed by re-examination in our Arlington, Virginia office. Select samples were sent to a specialty testing laboratory where the following tests were performed:

Grain Size Distribution

Atterberg Limits

Natural Water Content

The laboratory testing results are discussed together with the soil strata descriptions in the following sections. The laboratory test results are given in Appendix E.

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5.0 SUBSURFACE CONDITIONS

The subsurface conditions have been separated into two areas, the Great Terrace and the Lower Plaza.

The following sections describe the subsurface conditions encountered at each area.

5.1 Great Terrace Subsurface Conditions

The subsurface conditions at the Great Terrace generally consist of surficial concrete pavers overlying successive strata of fill, clay/silt, and sand.

The subsurface information presented herein includes both historic boring information and information obtained during our near-surface investigation at the Great Terrace.

5.1.1 Surficial Materials

Great Terrace Concrete Pavers

The existing concrete pavers on the Great Terrace were observed to be between 4.75 to 8.5 inches thick at the core locations. Approximate concrete paver thicknesses at locations LDCP-1 through

LDCP-5 are given in Table 1.

CONCRETE PAVER CORES

Core Location Measured Concrete

Thickness (inches)

LDCP-1 6.0

LDCP-2 5.5

LDCP-3 8.5

LDCP-4 5.25

LDCP-5 4.75

TABLE 1

Topsoil

A 3-inch-thick layer of topsoil was reportedly encountered at the surface in borings GC-1 and GC-2.

5.1.2 Stratum A – Fill

An approximate 0.6 to 18-foot-layer of fill was encountered in all hand augers and was reportedly encountered in historic borings GC-1 and GC-2 beneath the surficial materials, and was assumed to be encountered based on blow counts and soil descriptions in historic borings B-6, B-7, and B-10. The fill is generally characterized as brown silt with varying amounts of clay, fine to coarse sand, and fine to coarse gravel, and trace amounts of roots, cinders, mica, asphalt, brick, and concrete. Stratum A extended to a depth of approximately 1 to 18 feet below grade, or to approximate el 180 to 197.

The fill encountered in the historic borings was primarily coarse-grained and was reported to be very loose to medium dense as evidenced by SPT N-values ranging from 3 to 27 blows/foot.

The fill encountered in the DCPs/hand augers was primarily fine-grained and observed to be very soft to firm as evidenced by DCP index values ranging from 0.16 to 2.25 inches/blow. These DCP index values correspond to CBR values ranging from 3 to 59 percent.

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Results from specialty laboratory testing performed on a sample from Stratum A resulted in a moisture content of 13 percent, a liquid limit of 46, a plastic limit of 19, a plasticity index of 27, and 55 percent fines.

5.1.3 Stratum B.1 – Clay/Silt

A 2.5 to 26-foot-thick stratum of clay/silt was reportedly encountered beneath the fill in historic borings

GC-1, GC-2, and B-6, and was encountered beneath the fill at LDCP-2. Stratum B.1 extended up to depths of approximately 34 feet below existing grade in the borings, or about el 164. This stratum generally consisted of brown, red, orange, or to gray clay or silt with varying amounts of fine sand and fine gravel.

An interbedded 5-foot-thick layer of clayey sand was reportedly encountered in historic boring GC-1 at a depth of 18.5 feet below existing grade, or at approximate el 179.5.

Stratum B.1 soils encountered in the historic borings were reported to be soft to very stiff as evidenced by SPT N-values ranging from 3 to 25 blows/foot (average of 12 blows/foot).

Stratum B.1 soils encountered in LDCP-2 were observed to be very soft to firm as evidenced by DCP index values ranging from 0.38 to 0.95 inches/blow (average of 0.72 inches/blow). These DCP index values correspond to CBR values ranging from 8 to 23 percent (average of 12 percent).

Results from specialty laboratory testing performed on samples from Stratum B.1 resulted in moisture contents ranging from 13 to 17 percent, liquid limits ranging from 47 to 49, plastic limits ranging from 19 to 22, plasticity indices ranging from 27 to 28, and 58 to 83 percent fines indicating low to medium plasticity clays and silts.

5.1.4 Stratum B.2 – Sand

Stratum B.2 was reportedly encountered beneath Stratum B.1 in historic borings GC-1, GC-2, and B-6, and beneath the fill in historic boring B-10. This stratum was encountered beneath the fill in LDCP-4 and

LDCP-5. This stratum was generally characterized as light brown fine to coarse sand with varying amounts of silt and clay, and trace amounts of fine gravel and mica.

An interbedded 6.5-foot-thick layer of clayey silt was reportedly encountered in historic boring B-10 at a depth of 22.5 feet below existing grade, or at approximate el 175.5.

Stratum B.2 soils encountered in the historic borings were reported to be loose to dense as evidenced by SPT N-values ranging from 5 to 50 blows/foot (average of 18 blows/foot).

Stratum B.2 soils encountered in the DCPs were observed to be very loose to dense as evidenced by

DCP index values ranging from 0.04 to 1.5 inches/blow (average of 0.73 inches/blow). These DCP index values correspond to CBR values ranging from 5 to 99 percent (average of 17 percent).

5.2 Lower Plaza Subsurface Conditions

The subsurface conditions at the Lower Plaza generally consist of topsoil overlying silty sand. Fill was encountered beneath the topsoil in one hand auger at the Lower Plaza.

The subsurface information presented herein includes information obtained during our near-surface investigation at the Lower Plaza.

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5.3.1 Topsoil

An approximate 1-foot-thick layer of topsoil was encountered at the ground surface in all hand augers performed at the Lower Plaza. The topsoil was generally characterized as brown, dark brown, and black silt with varying amounts of fine to coarse sand and clay, and trace amounts of fine gravel and roots.

5.3.2 Stratum A – Fill

An approximate 1.3-foot-thick layer of fill was encountered in LDCP-6 below the topsoil. The fill was characterized as brown fine to coarse sand with trace amounts of silt, fine gravel, cinder, brick, and roots. This stratum extended to a depth of approximately 2.2 feet below existing grade, or to approximate el 143.8.

The fill was observed to be very loose as evidenced by DCP index values ranging from 1.00 to

2.00 inches/blow. These DCP index values correspond to CBR values ranging from 3.6 to 7.8 percent.

5.3.3. Stratum B.2 – Silty Sand

Stratum B.2 was encountered beneath the topsoil in LDCP-7 and MDCP-1 through MDCP-4, and beneath the fill in LDCP-6 at depths of approximately 1 to 2.2 feet below existing grade, or approximate el 140 to 144. The medium weight DCPs encountered refusal at depths ranging from 4.8 to 6.8 feet below existing grade, or approximate el 134.4 to 139. This stratum was generally characterized as brown, light brown, orange-brown, and tan silty fine to coarse sand with varying amounts of clay, fine gravel, and trace amounts of mica.

Stratum B.2 was observed to be very loose to dense as evidenced by DCP index values ranging from

0.04 to 1.5 inches/blow (average of 0.45 inches/blow). These DCP index values correspond to CBR values ranging from 8 to 109 percent (average of 30 percent).

Medium weight DCP blow counts in Stratum B.2 generally ranged from 4 blows/4 inches to refusal of the medium weight DCP (greater than 50 blows/4 inches) with an average of 23 blows/4 inches. Cone resistances ranged from 440 to 4,060 psi (average of 1,455 psi). The test results indicate that this stratum is generally loose to very dense with correlated SPT N-values of 5 to 50 blows/foot (average of

19 blows/foot).

Results from specialty laboratory testing performed on samples from Stratum B.2 resulted in moisture contents ranging from 10 to 16 percent, liquid limits ranging from 38 to 66, a plastic limit of 22, plasticity indices ranging from 16 to 44, and 22 to 45 percent fines indicating sands with low to medium plasticity silt.

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6.0 GEOTECHNICAL DISCUSSIONS AND RECOMMENDATIONS

Our geotechnical discussion and recommendations are divided into three sections addressing the Great

Terrace, the proposed Lower Plaza walkways, and the James Buchanan Memorial.

The following sections briefly discuss our findings and recommendations based on our site visit observations, review of available information, and subsurface investigations.

6.1 Great Terrace

The subsurface conditions at the Great Terrace generally consist of surficial concrete pavers overlying successive strata of fill, clay/silt, and sand.

Settlement of the Great Terrace pavers was observed within an approximate 25-foot-wide area between the Great Wall and the Joan of Arc statue. Historic photographs taken during the construction of the

Great Wall depict an excavation extending from the base of the below-grade wall and sloped at a steep angle to the base of the Joan of Arc statue. The plan-view limit of the excavated area shown in the historic photograph is located directly within the area of observed settlement during our site visit; see

Inset 2.

Although the historic photographs are undated, several photographs of the Great Terrace taken soon after construction do not show visible settlement of the pavers behind the Great Wall. However, presumably later photographs indicate that settlement began to occur after construction.

Based on the historic borings and DCP-1 through DCP-5, and our review of historic photographs, we opine that the settlement occurring at the Great Terrace is likely due to under-compacted soil placed behind the Great Wall.

To correct the paver settlement and minimize future settlement, we recommend a two-step approach:

1) compaction grouting be completed to stabilize the fill soils beneath the Great Terrace and

2) over-excavate the upper 12 inches of soil supporting the pavers and replace with subbase material and a layer of tri-axial geogrid.

The remediation should also be supplemented by providing adequate drainage for landscaped areas, planters, walkways, and other areas along the surface of the Great Terrace to prevent water from collecting behind the Great Wall. Detailed recommendations are given in the following sections.

Inset 2: Historic and recent photographs depicting the construction of the Great Wall.

North Below-Grade Wall

Behind Great Wall

Joan of Arc Statue

Excavated Area

(~25 feet wide ±)

Northern Below-Grade

Wall of the Great Wall

Joan of Arc Statue

Excavated Area

(~25 feet wide ±)

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6.1.1 Compaction Grouting

Compaction grouting is recommended to improve the loose fill soils under the Great Terrace to mitigate future settlement. Compaction grouting is a process by which a hollow steel mandrel is drilled or inserted to a predetermined treatment depth and low-flow grout is pressure-injected into subsurface soils in sequential lifts that create a column of concrete and densify the surrounding soils. See Inset 4 for a diagram of compaction grouting.

We preliminarily recommend that compaction grouting be performed from a minimum depth of 20 feet below grade and extend up to 3 feet below existing grade.

Spacing of the grout columns and injection-pressure thresholds should be evaluated by a qualified specialty ground improvement contractor to provide a stiffened soil mass that is less susceptible to settlement.

Care should be taken during compaction grouting operations so that the existing Great Wall below-grade walls, foundations, utilities and other structures to remain are not affected by excessive soil pressure. Real-time monitoring methods using laser tell-tails, or other approved methods, should be implemented so that ground heave or movement of existing structures can be monitored. Lower pressures may be required adjacent to these features. Allowable displacement thresholds of the Great Wall below-grade walls and the Great

Terrace should be established by the structural engineer. Grouting operations should immediately stop if heave or lateral displacements are detected by the real-time monitoring sensors.

6.1.2 Great Terrace Paver Subgrade

After grouting operations have been completed and prior to reconstruction of the concrete pavers, the paver subgrade should be over-excavated a minimum of 12 inches and the over-excavation subgrade proofrolled/compacted with at least six passes using a sheepsfoot vibratory roller having a minimum static drum weight of 1.5 tons. Smaller equipment can be used if needed in areas of limited access. Any areas of subgrade or fill exhibiting excessive rutting or pumping during proofrolling and compaction should be removed and replaced with structural fill. The Contractor should be responsible for maintaining all subgrades in their as-approved condition until the concrete pavers are replaced. Even with proper preparation and stone placement, the slab subgrade can deteriorate over time. Therefore, we recommend that the concrete pavers be replaced as soon as possible after the subgrade is prepared.

We recommend that the 12 inch over-excavation beneath the pavers be filled with a well-graded subbase such as Maryland State Highway Administration GAB. The existing subbase is also acceptable for reuse. We recommend that the subbase be placed in 6-inch-thick lifts and compacted to 95% of the materials maximum dry density in accordance with ASTM D-1557. A layer of tri-axial geogrid should be installed at mid-depth of the subbase to help minimize future artificial settlement between the pavers.

Inset 4: Compaction Grouting

Schematic (Reference: Hayward

Baker Geotechnical Construction)

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6.2 Lower Plaza Walkway

Existing fill and natural sand soils are expected at the proposed Lower Plaza walkway subgrades.

Subgrade preparation procedures are given in the Sections 7.1 and 7.2 of this report and should be followed to prepare walkway subgrades.

The DCP test results indicate correlated California Bearing Ratio (CBR) values in the vicinity of the walkways range from approximately 4 to 20. Due to the variability of these soils, we recommend a CBR value of 5 be used for walkway design. We anticipate a concrete pavement consisting of 4 inches of concrete over 6 inches of subbase is appropriate for the new walkways.

Walkway subgrade preparation work should be inspected by a qualified geotechnical engineer. Should isolated areas exhibit unsuitable conditions, the isolated areas should be over-excavated to a depth as determined by the geotechnical engineer and immediately replaced with approved compacted fill or crushed stone.

6.3 James Buchanan Memorial

During our site visit, select stone block units at the monument were observed to be slightly offset, joints between stone units were open or white caulking was placed within the joint, and vegetative root jacking was observed at select block units. A portion of the offset blocks were observed to contain soil infill or were open creating a space for water to enter.

Subsurface conditions in the vicinity of the James Buchanan memorial generally consist of a 1-foot-thick layer of topsoil overlying a loose to dense silty sand with densities generally increasing with depth.

Based on the information gathered during our investigation, these soils are geotechnically suitable to support the monument. Observations of the stone units around the base of the monument did not show apparent signs of excessive differential settlement between blocks. Sand subsurface soils are not generally susceptible to long-term settlement; therefore, we anticipate that a majority of the stone units were offset or dislodged due to a combination of mechanisms such as freeze/thaw of water within the joints or possible water infiltration between joints degrading the subsurface soils.

We recommend that site grades and pavers surrounding the monument be pitched away from the

James Buchanan memorial and that proper drainage utilities be installed to convey stormwater away from the memorial. Stone paver joints along the ground surface should be cleaned of soil and resealed.

At this point, we do not anticipate that foundation remediation is necessary. However, if elected, long-term monitoring of the monument can be implemented to establish any possible settlement trends.

This monitoring program should consist of crack gauges and survey targets. If this option is selected, we recommend the following:

Survey Targets - Survey targets should be used to monitor for vertical and horizontal movement of the monument. A baseline survey should be performed, then subsequent monitoring surveys should be performed once every other month for a year, with measurements taken to the nearest 0.005 feet. The survey data should be overseen and reviewed by a surveyor licensed in the District of Columbia. Establishment of structure movement points along the monument should be coordinated with NPS.

Crack Gauges - Crack gauges should be installed to monitor crack or stone block joint changes and monitored once every other month for one year.

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Langan Project No.: 270060901

The data collected will give the project team an understanding of typical seasonal and long-term movements (if any) the monument is subject to throughout the duration of a year.

Washington, DC

Langan Project No.: 270060901

7.0 CONSTRUCTION RECOMMENDATIONS

Our geotechnical recommendations for site preparation, subgrade preparation, and protection of adjacent structures are given in the following sections.

7.1 Site Preparation

Prior to commencement of grading or fill placement, any miscellaneous trash, debris, or other unsuitable materials should be removed from the site. All debris should be properly disposed off-site in accordance with applicable District of Columbia regulations.

At this time, we recommend the following regarding site preparation:

Existing utilities designated for abandonment should be removed or abandoned in-place by complete filling with grout. Utilities designated for abandonment should be completely removed.

Topsoil should be stripped from the new walkway areas plus a minimum of 2 feet beyond. The topsoil should be stockpiled and protected from erosion. The reuse of topsoil should be evaluated by a Landscape Architect.

The concrete pavers in remediation zones should be removed, stockpiled, and protected for reuse at the site. The gravel subbase beneath the pavers should be stockpiled separately to avoid mixing with the site soils and saved for reuse. Care should be taken during removal of the concrete pavers adjacent to monuments, statues, and other structures.

All site preparation work should be performed in accordance with any environmental regulations and requirements established for the site as well as all District Department of the Environment

(DOEE) regulations inclusive of sediment and erosion control.

All work should be performed so as not to adversely impact the existing park structures, roadways/pathways, slopes, utilities, or landscaping designated to remain. Protection of these elements should be provided as necessary during the course of all construction activities at the site.

7.2 Subgrade Preparation

After performing the aforementioned site preparation work (removal of concrete pavers/over-excavation, or removal of topsoil) the exposed soil subgrade should be conditioned to within 3 percent of optimum moisture content and compacted to at least 95 percent of the material’s dry density is accordance with

ASTM D1557. Subgrades should then be proofrolled with at least six passes of a 1.5 ton double-drum vibratory trench roller (Wacker Neuson RTSC3 or equivalent). All proofroll activities should be inspected by a qualified geotechnical engineer. Additional proofrolling coverages should be performed in any areas deemed necessary based on observations made by the engineer. Soft areas identified during proofrolling should be undercut and replaced with structural fill or subbase meeting the gradations given in Section 7.3. Any low areas should be leveled using similar material and compacted.

7.3 Engineered Fill

7.3.1 Fill Designation

Structural fill should be used beneath slabs, walkways, pavements, and utility pads. We recommend that structural fill have no more than 35 percent fines (silt and clay) and a plasticity index not more than

Washington, DC

Langan Project No.: 270060901

12. Structural fill should not contain man-made debris (unless it is processed recycled aggregate) or organics. The maximum particle size should not exceed 3 inches.

We recommend that subbase meet the requirements of Maryland State Highway Administration graded aggregate base (GAB).

Non-structural fill should be used in all other areas of the site and should not have particles exceeding

6 inches below 2 feet from final grade and 4 inches within 2 feet below final grade.

7.3.2 Imported Fill

Imported fill should meet the requirements previously described in Section 7.3.1. Suitable fill should be free of organics and other deleterious materials. Any approved imported fill should be “certified clean fill” free of all hazardous substances and meeting all DOEE and Federal regulations. The contractor should provide documentation of compliance prior to delivery of any fill to the site to the Geotechnical

Engineer. Grain size distribution, maximum dry density, and the optimum water content determinations should be made on representative samples of the backfill and fill materials proposed by the Contractor.

From a geotechnical perspective, recycled concrete material free of reinforcing steel and other deleterious materials can be imported and used as compacted fill at the site subject to approval by

DOEE and NPS. Mixing of recycled concrete with on-site soils for re-use as fill should be evaluated by the Geotechnical Engineer during construction. The use of recycled concrete materials should be performed in accordance with any established environmental requirements for the site.

7.3.3 Fill Placement and Compaction

All fill in structural areas (beneath structures, pavements, walkways, and pads) should be placed in uniform lifts and compacted to at least 95 percent of the material’s maximum dry density as determined by the Modified Proctor Compaction Test (ASTM D1557). Fill placed in exterior landscaped areas should be compacted to at least 88 percent of the material’s maximum dry density as determined by ASTM

D1557.

Fill placed in structural areas are recommended to be placed in maximum 8-inch-thick loose lifts and compacted using a walk behind trench roller with a minimum static drum weight of 1.5 tons. Smaller compaction equipment (jumping jack compactor) and a maximum loose lift size of 6 inches may be necessary in areas of limited maneuverability or within 5 feet of structures. Fill placed in landscaped areas should be placed in maximum 12-inch-thick lifts.

During periods of wet weather, the contractor should make provisions to dry portions of the excavated and stockpiled material such as by discing/air drying prior to compaction to an acceptable moisture content as determined by the project geotechnical engineer.

No fill material should be placed on areas where free water is standing, on a subgrade covered with snow, on frozen subgrade areas, or on surfaces which have not been approved by the inspecting geotechnical engineer.

7.4 Protection of Adjacent Structures

All construction work should be performed so as not to adversely impact or cause loss of support to adjacent structures, monuments, slopes, asphalt walking paths, and utilities that are to remain. Site preparation and subsequent construction will need to be performed so as not to adversely impact these structures.

Washington, DC

Langan Project No.: 270060901

Prior to beginning repair work, we recommend that pre-construction documentation of the Great Wall, below grade walls below the Great Terrace, the Great Terrace, and Lower Plaza be performed using still pictures and video. An engineer should observe these structures signs of existing damage.

Washington, DC

Langan Project No.: 270060901

8.0 CLOSURE/LIMITATIONS

This report presents our recommendations regarding the geotechnical aspects of the Preservation of

Meridian Hill Park project in northwest Washington DC. These evaluations, conclusions, and recommendations are based on our site subsurface investigations, project documents, and review of available information. Recommendations given are contingent upon one another and no recommendation should be followed independent of the others. Langan Engineering and Environmental

Services, Inc. cannot assume responsibility for the use of this report to generate recommendations for other than the specific site and structures addressed in this report.

FIGURES

Figure 1 – Site Location Map

Figure 2 – Investigation Location Plan

Figure 3 – Regional Geology Map

Figure 4 – FEMA Flood Map

Filename: \\langan.com\data\ARL\data9\270060901\Project Data\CAD\01\2D-DesignFiles\Geotechnical FIgures\Site Location Plan 2.dwg Date: 11/21/2017 Time: 09:04 User: bsperry Style Table: Langan.stb Layout: ANSIA-BP

Arlington, VA 22209

T: 571.366.6800 F: 571.366.6801 www.langan.com

L a n g a n

SOURCE: NEARMAP AERIAL ACCESSED 07 NOVEMBER 2017.

12/01/2017

Filename: \\langan.com\data\ARL\data9\270060901\Project Data\CAD\01\2D-DesignFiles\Geotechnical FIgures\Boring Location Map.dwg Date: 11/22/2017 Time: 13:46 User: bsperry Style Table: Langan.stb Layout: Investigation Location Plan

Arlington, VA 22209

T: 571.366.6800 F: 571.366.6801 www.langan.com

L a n g a n

Filename: \\langan.com\data\ARL\data9\270060901\Project Data\CAD\01\2D-DesignFiles\Geotechnical FIgures\Geologic Map.dwg Date: 11/21/2017 Time: 08:47 User: bsperry Style Table: Langan.stb Layout: ANSIA-BP

Arlington, VA 22209

T: 571.366.6800 F: 571.366.6801 www.langan.com

L a n g a n

LEGEND:

T1 AND T2: TERRACE DEPOSITS: GRAVEL, SAND, SILT, AND CLAY

Kps: POTOMAC GROUP SANDS AND GRAVELS

Q5: CHICAMUXEN FORMATION GRAVEL, SAND, SILT AND CLAY

af: ARTIFICIAL FILL

SOURCE: GEOLOGIC MAP OF WASHINGTON WEST QUADRANGLE, DISTRICT OF COLUMBIA, MONTGOMERY AND PRINCE

GEORGES COUNTIES, MARYLAND, AND ARLINGTON AND FAIRFAX COUNTIES, VIRGINIA

UNITED STATES GEOLOGICAL SURVEY, 1989

Filename: \\langan.com\data\ARL\data9\270060901\Project Data\CAD\01\2D-DesignFiles\Geotechnical FIgures\FEMA Flood Map.dwg Date: 11/21/2017 Time: 08:48 User: bsperry Style Table: Langan.stb Layout: ANSIA-BP

Arlington, VA 22209

T: 571.366.6800 F: 571.366.6801 www.langan.com

L a n g a n

SOURCE: FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA) FLOOD INSURANCE RATE MAP 1100010016C DATED

27 SEPTEMBER 2010

LEGEND: Z

EZONE X: AREAS OUTSIDE OF THE 500 YEAR (0.2 PERCENT ANNUAL) FLOODPLAIN.

Appendix A

Concrete Coring Photographs

Meridian Hill Park

Langan Project No.: 270060901

Concrete Core Photographs

Photograph 1: Concrete slab at core LDCP-1

Photograph 2: Cored slab at core LDCP-1

Photograph 3: Concrete slab at core LDCP-2

Photograph 4: Concrete slab at core LDCP-3.

Photograph 5: Cored hole at core LDCP-3.

Photograph 6: Cored hole at core LDCP-3.

Photograph 7: Cored concrete slab at LDCP-4.

Photograph 8: Cored concrete slab at LDCP-5.

Photograph 9: Cored hole at LDCP-5.

Appendix B

Lightweight

DCP Methodology and Test Results

Lightweight DCP Testing

Lightweight Dynamic Cone Penetration (DCP) Testing Methodology

Seven lightweight dynamic cone penetration (DCP) tests, identified as LDCP-1 through LDCP-7, were completed between 30 and 31 October by a field engineer from our office using the

Kessler K-100 Standard DCP. LDCP-1 through LDCP-5 were completed on the Great Terrace and LDCP-6 and LDCP-7 were completed on the Lower Plaza; see Figure 1. LDCP-1 through

LDCP-5 were performed within the concrete core holes on the Great Terrace and LDCP-6 and

LDCP-7 were performed at existing grade. Each lightweight DCP test was conducted by driving a 5/8-inch diameter steel drive rod fitted with a 60 degree angle cone tip into the subsurface using a hammer dropped from a height of 22.6 inches. All the light weight DCP tests used a

17.6 pound hammer.

Each lightweight DCP test was advanced to depths of 40.0 to 50.0 inches below the top of the existing concrete pavers or the existing ground surface and the number of hammer blows to advance the cone for each about two-inch-increment was recorded. Each lightweight DCP test was performed in accordance with ASTM STP-399.

Photograph 1: Langan Field Engineer Performing a LDCP Test.

DCP Data Reduction Methods

The recorded number of hammer blows for each approximate 2-inch-increment was used to calculate the California Bearing Ratio (CBR) value of the in-situ soils. Specifically, the measured

DCP Index (inches/blow) at each two-inch increment was calculated and correlated to a CBR value using the US Army Corps of Engineers Waterways Experiment Station equation (1992).

The general equation, applicable to a majority of the encountered site soils, is given as:

Lightweight DCP Testing

For encountered site soils classified as clay the METHOD ST6 (1984) equation was used to calculate the CBR value. The equation is given as:

DCPI and CBR vs. depth plots are given in this Appendix. DCPI value ranges and associated soil consistency are given in Tables A1 and A2.

DCPI, CBR, AND SOIL CONSISTENCY RELATIONSHIP

(For Coarse Grained Soil)

DCPI Value Range

(inches/blow)

Corresponding CBR

Value Range Soil Consistency

0.4 to 2 3.5 to 22 Very Loose

0.13 to 0.4 22 to 74 Loose

0.12 to 0.13 74 to 100 Medium Dense

TABLE A11

DCPI, CBR, AND SOIL CONSISTANCY RELATIONSHIP

(For Fine Grained Soil)

DCPI Value Range

(inches/blow)

Corresponding CBR

Value Range Soil Consistency

0.67 to 2 3.5 to 12 Very Soft

0.5 to 0.67 12 to 17 Soft

0.5 to 0.16 17 to 58 Firm

0.12 to 0.16 58 to 100 Stiff

TABLE A21

1 Tables A1 and A2 are adapted from ASTM Special Technical Publication #399 entitled Dynamic Cone for

Shallow In-Situ Penetration Testing by Sowers and Hedges, Fig. 3.

DCP-1 TEST DATA

Project: Date:

Location: Soil Type(s):

No. of Accumulative Type of

Blows Penetration Hammer

(mm)

152.4 1

3 203.2 1

4 254 1

4 304.8 1

3 355.6 1

3 406.4 1

2 457.2 1

3 508 1

4 558.8 1

4 609.6 1

2 660.4 1

3 711.2 1

2 787.4 1

2 825.5 1

1 863.6 1

2 914.4 1

2 965.2 1

1 1022.35 1

2 1070.61 1

Meridian Hill

Washington DC.

10/30/2017

See Hand Auger Logs

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

D E

P T

H in

CBR

D E

P T

H m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

Hammer

0 14 28 42 56 69 83

0 2000 4000 6000 8000 10000 12000

D E

P T

H

BEARING CAPACITY, psi

D E

P T

H in

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of Concrete Airport Pavement, Portland

Cement Association, page 8, 1955)

L

DCP-2 TEST DATA

Location: Soil Type(s):

No. of Accumulative Type of

Blows Penetration Hammer

(mm)

139.7 1

3 190.5 1

4 241.3 1

4 294.64 1

3 345.44 1

3 403.86 1

2 447.04 1

3 502.92 1

4 546.1 1

4 599.44 1

2 645.16 1

3 718.82 1

2 779.78 1

2 845.82 1

1 883.92 1

2 952.5 1

2 1023.62 1

1 1051.56 1

2 1112.52 1

2 1168.4 1

2 1229.36 1

Meridian Hill

Washington DC.

10/30/2017

See Hand Auger Logs

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

D E

P T

H in

CBR

D E

P T

H

Both hammers used

Soil Type

CH

CL

All other soils

Hammer

0 14 28 42 56 69 83

0 2000 4000 6000 8000 10000 12000

D E

P T

H

E

P T

H of CBR and Bearing values (Design of Concrete Airport Pavement, Portland

DCP-3 TEST DATA

Location: Soil Type(s):

No. of Accumulative Type of

Blows Penetration Hammer

(mm)

330.2 1

6 383.54 1

6 431.8 1

4 482.6 1

6 533.4 1

5 594.36 1

4 655.32 1

3 695.96 1

4 736.6 1

4 792.48 1

4 845.82 1

3 891.54 1

4 955.04 1

2 993.14 1

3 1046.48 1

3 1104.9 1

2 1153.16 1

2 1203.96 1

2 1249.68 1

2 1300.48 1

2 1356.36 1

2 1407.16 1

2 1455.42 1

20 1490.98 1

Meridian Hill

Washington DC.

10/30/2017

See Hand Auger Logs

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

D E

P T

H in

CBR

D E

P T

H

Both hammers used

Soil Type

CH

CL

All other soils

Hammer

0 14 28 42 56 69 83

0 2000 4000 6000 8000 10000 12000

D E

P T

H

E

P T

H of CBR and Bearing values (Design of Concrete Airport Pavement, Portland

Cement Association, page 8, 1955)

DCP Refusal at 4.9 ft.

DCP-4 TEST DATA

Location: Soil Type(s):

No. of Accumulative Type of

Blows Penetration Hammer

(mm)

133.35 1

2 194.31 1

2 260.35 1

2 290.83 1

5 339.09 1

5 392.43 1

4 450.85 1

2 488.95 1

3 549.91 1

2 590.55 1

3 646.43 1

3 697.23 1

4 750.57 1

3 796.29 1

3 849.63 1

3 905.51 1

3 963.93 1

2 1009.65 1

2 1057.91 1

3 1118.87 1

2 1159.51 1

3 1220.47 1

2 1261.11 1

Meridian Hill

Washington DC.

10/30/2017

See Hand Auger Logs

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

D E

P T

H in

CBR

D E

P T

H

Both hammers used

Soil Type

CH

CL

All other soils

Hammer

0 14 28 42 56 69 83

0 2000 4000 6000 8000 10000 12000

D E

P T

H

E

P T

H of CBR and Bearing values (Design of Concrete Airport Pavement, Portland

DCP-5 TEST DATA

Location: Soil Type(s):

No. of Accumulative Type of

Blows Penetration Hammer

(mm)

120.65 1

2 166.37 1

4 224.79 1

4 278.13 1

3 323.85 1

11 369.57 1

11 422.91 1

8 476.25 1

8 527.05 1

6 582.93 1

6 628.65 1

6 679.45 1

4 735.33 1

4 781.05 1

4 834.39 1

3 890.27 1

4 930.91 1

4 986.79 1

20 1007.11 1

Meridian Hill

Washington DC.

10/30/2017

See Hand Auger Logs

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

D E

P T

H in

CBR

D E

P T

H

Both hammers used

Soil Type

CH

CL

All other soils

Hammer

0…