06_Geotech_TB_Geotechnical_Memo_1.pdf

DRAFT

Geotechnical Report

Rehabilitation of Seawalls and

Shoreline NPS National Mall and Memorial Parks Washington, DC

Submitted to:

HDR Engineering, Inc.

1670 Broadway, Suite 3400

Denver, CO 80202

Submitted by:

GEI Consultants, Inc.

1615 L Street, N.W. Suite 630

Washington, DC 20006

202.828.9510

November 11, 2022

GEI Project 2201647

Paul Landers, P.E.

Senior Project Manager

Giovanni Bonita, PhD, PG, PE

Senior Vice President

ABOZORGI

Text Box Note: This memorandum is not final.

JV provided GEI with comments, and GEI is currently revising this memorandum. The comments mainly include, among other comments:

- "North TB" and "NTB" will be revised to "TB West" and "TB-W", respectively, everywhere within the document.

- Additional geotechnical parameters will be included to sec 6.1

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Table of Contents

1. Introduction 1

1.1 Project Description 1

1.2 Scope of Work 1

1.3 Limitations on Use of this Report 2

2. Historical Information 3

2.1 Background on North Plaza Settlement Mechanism 3

2.2 Historical Geotechnical Explorations 4

3. Subsurface Exploration and Testing 5

3.1 Geotechnical Exploration Program 5

3.2 Soil Borings 5

3.2.1 Split Spoon Sampling 6

3.2.2 Tube Sampling 6

3.2.3 Rock Coring 7

3.3 Soil Classification 7

3.4 Borehole Gas Measurement 7

3.5 Water Level 8

3.6 Boring Logs 8

3.7 In-situ Testing 8

3.8 Elevation Datum and Coordinate System 9

3.9 Geotechnical Laboratory Testing 9

3.9.1 Index Testing 10

3.9.2 Strength Testing 10

3.9.3 Compressibility Testing 10

3.9.4 Rock Testing 11

4. Geologic Conditions 12

4.1 Regional Geology 12

4.2 Local Geology 12

4.2.1 Bedrock Geology 13

4.3 Local Soils 13

4.3.1 Fill 13

4.3.2 Alluvium 14

4.3.3 Decomposed Rock 14

4.3.4 Bedrock 15

5. Geotechnical Instrumentation 16

5.1 Inclinometer and Vibrating Wire Piezometers 16

5.1.1 VWPZ Calibration and Initial Zero Reading 16

5.1.2 VWPZ and INC Installation 16

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5.1.3 VWPZ Data Collection 17

5.1.4 VWPZ Data Interpretation 17

5.1.5 INC Initial Baseline Reading 17

5.1.6 INC Data Collection 18

5.1.7 INC Data Interpretation 18

5.2 Multiple Position Borehole Extensometer 18

5.2.1 MPBX Calibration and Initial Zero Reading 18

5.2.2 MPBX Installation 19

5.2.3 MPBX Data Collection 19

5.2.4 MPBX Data Interpretation 19

5.2.5 MPBX Data Limitations 19

5.3 Surface Protection 20

6. Geotechnical Design Parameters 21

6.1 General 21

6.2 Site Class 22

6.3 Liquefaction Potential 22

7. General Design and Construction Considerations 23

7.1 Design Considerations 23

7.2 Construction Considerations 23

8. References 24

Figures

Figure 1 – Site Vicinity Map

Figure 2 – Wall Location Map

Figure 3 – Northern Tidal Basin Boring Location Plan

Figure 4 – Eastern Tidal Basin Boring Location Plan

Figure 5 – Northern Tidal Basin Section

Figure 6 – Eastern Tidal Basin Section

Figure 7 – Summary of LEL Readings

Figure 8 – Summary of VWPZ Readings

Appendices

A. Historical Boring Logs From Others

B. Boring Logs

C. Laboratory Test Results

D. In-Situ Testing Results

E. Instrumentation Data

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B:\Working\NATIONAL PARK SERVICE\2201647 National Capital Area Seawall\08_GDR\NPS Seawalls-Geotech Report.docx

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1. Introduction

1.1 Project Description

The Rehabilitation of Seawalls and Shoreline Project (Project) for the National Park Service

(NPS) consists of the rehabilitation and reconstruction of seawalls in the West Potomac Park

(WPP) and the Tidal Basin to prevent water from the Potomac River and the Tidal Basin from encroaching on the land areas over the seawalls The location of the general project area is shown in Figure 1. This report is focused specifically on the seawalls within the Tidal Basin.

For purposes of this Project, the Tidal Basin seawalls have been divided into two sections, North

Tidal Basin and East Tidal Basin as shown in Figure 2. The North Tidal Basin consists of the seawalls that start at the Ohio Drive Bridge and extend approximately 850 feet to the north near the location of the Japanese Pagoda. The East Tidal Basin consists of approximately 1,250 feet of the seawalls starting from the Inlet Bridge to the western edge of the North Plaza in front of the Jefferson Memorial. The WPP seawall, which is not included as part of this study, starts from the northwest side of the Ohio Drive Bridge and runs to the north along the Potomac River for approximately 4,650 feet to the Memorial Bridge.

The purpose of this Geotechnical Report is to present data collected by GEI Consultants, Inc.

(GEI) during a geotechnical exploration program in the Tidal Basin area. The data will be used for use in the planning and design of components associated with the overall Project. Data obtained from previous geotechnical explorations and laboratory tests performed by others in the vicinity of the Project are also briefly described. Recommended soil strength and compressibility parameters are also provided.

1.2 Scope of Work

GEI was contracted by HDR/M&N JV to perform geotechnical fieldwork and testing. The purpose of this work is to supplement existing geotechnical data and reports provided by the

NPS. The scope consisted of drilling soil borings and performing in-situ testing along the alignment of the Tidal Basin walls. A total of four (4) soil borings with rock coring were performed and were utilized for the installation of geotechnical instrumentation. In addition, eleven (11) Cone Penetration Tests (CPTs) and seven (7) Dilatometer Tests (DMTs) were performed.

Geotechnical instrumentation included three (3) Inclinometers (INC) and one (1) multi-point borehole extensometer (MPBX). Each INC contained five (5) multi-level vibrating wire piezometers (VWPZ) attached to the outside of the inclinometer casing.

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Geotechnical laboratory testing consisting of classification, strength, and compressibility testing was performed on disturbed and undisturbed samples collected in the borings.

1.3 Limitations on Use of this Report

This report has been prepared for the exclusive use of the NPS and its consultants and contractors for the planning, design, and construction of the Project. The data presented in this report are based on subsurface conditions encountered at the time of the subsurface exploration.

NPS is not responsible for the interpretation by others of the data contained herein.

The data presented in this report is based solely on the results of the explorations performed for this Project. Observations and data not collected during these exploration programs (i.e., Historical Data) is provided for reference and information only.

The services described herein have been performed in a manner consistent with the level of care and skill ordinarily exercised by members of the profession currently practicing under similar conditions in the area. No warranty, express or implied, is made.

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2. Historical Information

2.1 Background on North Plaza Settlement Mechanism

Data collected during explorations as part of the emergency repairs to the North Plaza Seawall from 2007 to 2009 revealed several important facts about the settlement processes that have occurred in the Tidal Basin. Large ground surface settlements were originally expected by designers of the Jefferson Memorial from the excavation and fill operations that were required for its construction. A portion of the existing fill to the East of the Memorial was excavated and used to form the western half of the North Plaza that extended the previous shoreline west of the

Memorial into the Tidal Basin.

The North Plaza was initially constructed on grade, while the North Plaza Seawall was constructed on timber piles due to scarcity of steel at the time of construction. The timber pile length was estimated to be between 60 and 70 feet in length and did not reach the bedrock. By the late 1960s, up to three feet of settlement of the North Plaza had occurred. The North Plaza was then reconstructed on a system of concrete piles extending to rock. Monitoring data was collected on the North Plaza Seawall periodically since the end of construction of the Memorial, which revealed that the seawall also underwent settlement, but had tapered off by the 1970s.

The North Plaza Seawall, still founded on its original timber piles, started undergoing settlement again in the late 1990s to early 2000s. At the time, the reasons for the re-initiation of movement were not known since no additional loading had been imparted on the seawall. This prompted the

NPS to commission a study of the causes for the “settlement/heave” of the seawall and North

Plaza since the available survey data suggested that the entire North Plaza was heaving, even though it was now supported on piles. It soon became obvious that the survey monuments were settling while the North Plaza was stable. The seawall, however, was settling at a relatively fast rate requiring prompt action. Both the seawall and North Plaza were also experiencing lateral movement.

Measurements performed in 2007 to 2008 west of the North Plaza and into the area that is part of the current scope revealed that the Tidal basin seawalls seemed to be experiencing a similar phenomenon, with significant settlement as well as lateral movement toward the Tidal Basin.

Data collected from instrumentation installed for the North Plaza study revealed that the piezometric gradient at the site did not follow hydrostatic conditions, nor did it show excess pore pressures above hydrostatic, which are characteristic of typical surcharge-induced settlement mechanisms. Instead, the data revealed that the piezometric head within the soft soils followed a hydrostatic regime near the surface and dropped significantly below hydrostatic near the bedrock interface.

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The mechanism for settlement for the North Plaza was interpreted to be compression of the soft soils progressing upward from the bedrock interface as pore pressures decreased to adjust to a different piezometric condition (Gomez, 2011). Because the soils compressed below the tip of the North Plaza Seawall piles, it was concluded that the seawall would continue to settle until a new equilibrium was reached in the future. The causes for the drop in piezometric head in bedrock were not studied at the time, but NPS indicated that design of the North Plaza Seawall reconstruction should assume these conditions would continue.

2.2 Historical Geotechnical Explorations

Schnabel Engineering Inc. (Schnabel) performed subsurface investigation near the Jefferson

Memorial in 2006. Fill material, soft clay and silt, medium-stiff clay and silt were encountered from the ground surface to the bedrock, respectively. Fill material was observed from ground surface up to about 30 feet in some locations. The top of bedrock encountered during the geotechnical exploration was approximately between Elevation (El.) -88 ft and El. -93 ft. Boring logs prepared by Schnabel in 2006 are presented in Appendix A.

The upcoming Potomac River Tunnel project is planned to run from the Joint Base Anacostia-

Bolling and then north up the Potomac River to a drop shaft located in West Potomac Park. This tunnel will run parallel to the seawalls along East and West Potomac Parks. The borings drilled for this project are not directly next to the Tidal Basin and were not available at the time of this report. However, stick logs and a profile provided by the NPS reveal that the bedrock rises in elevation moving upstream along the Potomac. These profiles indicate that the bedrock is at about El. -95 ft. near the Ohio Drive Bridge and around El. -40 ft. near Memorial Bridge at the northern end of the WPP seawalls. This profile also reveals that the Potomac Group soils are mostly eroded away and replaced with Alluvial soils in this area.

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3. Subsurface Exploration and Testing

3.1 Geotechnical Exploration Program

The purpose of the geotechnical exploration program was to collect information related to the geologic, hydrogeologic, soil, and groundwater conditions along the proposed alignments for use in the planning and design of the Tidal Basin seawalls.

The geotechnical explorations included soil borings (hollow-stem auger); rock coring; Standard

Penetration Tests (SPTs); Dilatometer Tests (DMTs); Cone Penetration Tests (CPTs);

installation and monitoring of Vibrating Wire Piezometers (VWPZs), Inclinometers (INC); a multi-point borehole extensometer (MPBX); and the collection of soil samples for geotechnical laboratory testing.

The geotechnical exploration program included performing of eleven (11) CPTs, seven (7)

DMTs, four (4) SPT borings, vane shear testing, installation and monitoring of three (3) inclinometers each with five (5) multi-level VWPZs, installation and monitoring of one (1)

MPBX, and field and laboratory testing. Boring locations for North and East Tidal Basin are shown in Figures 3 and 4. The geotechnical exploration program included laboratory testing on soil from both split spoon samples and tube samples.

3.2 Soil Borings

Four (4) SPT soils borings were drilled at select locations along the north and east Tidal Basin walls. Drilling was performed by E2CR, Inc. of Baltimore, Maryland under contract with the

HDR/M&N JV. The drilling and field-testing activities were observed by GEI Consultants, Inc.

of Washington, DC.

The borings were advanced using truck-mounted, and track-mounted conventional drill rigs.

Borings were advanced using 3-¼” minimum ID hollow stem augers (HSA) following the

ASTM D6151procedures until refusal was encountered at the top of bedrock. All borings were extended to the top of bedrock except Boring ETB-B-02 which was terminated at a depth of 55 feet due to the high level of methane gas. HSAs support the sides of the borehole while leaving the center open for sampling and additional drilling. This method is subject to “blow in” or

“plugging” of soil at the bottom of the auger flights which can compromise soil sample integrity and the accuracy of the N-value measurements. To prevent this, an auger pilot bit (also referred to as a plug) was attached to the end of the drill rods to fill the void within the HSA and block soil cuttings from plugging the inside of the augers.

Pocket penetrometer and pocket Torvane tests were performed on SPT samples in the field to provide an estimate of undrained strength for fine-grained cohesive soil. Pocket penetrometer

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After the completion of drilling, Inclinometers with five (5) multi-level VWPZs were installed in three borings (ETB-B-01, NTB-B-01, and NTB-B-02). One MPBX was installed in boring ETB-

B-02 (BTX) after reaching the final depth.

All soil cuttings brought to the surface by the auger flights and were collected and placed in steel drums for disposal off-site. All borings were tremie-grouted with a bentonite-grout mix upon completion. For the instrumentation holes, grouting encompassed the instruments sealing the hole from the migration of water vertically through the borehole column.

3.2.1 Split Spoon Sampling

Representative soil samples were obtained by means of the split-barrel sampling procedure in accordance with ASTM D1586. A 2-inch outside diameter, split-barrel sampler is driven into the soil a distance of 18 to 24 inches by a 140-pound automatic hammer falling 30 inches. The number of blows required to drive the sampler through the second and third six-inch interval is termed the Standard Penetration Test (SPT) N-value. N-values can be used as a qualitative indication of the in-place relative density of cohesionless soils and the consistency of cohesive soils. This indication is qualitative, since many factors can significantly affect the standard penetration resistance value and prevent a direct correlation among drill crews, drill rigs, drilling procedures and hammer-rod-sampler assemblies.

3.2.2 Tube Sampling

Thin-walled tube samples were obtained using the direct push method. Conventional direct push sampling was performed in general accordance with ASTM D1587 in all borings. In general, two

“undisturbed” (Shelby tube) samples were collected from fine-grained (clayey) soil strata in each boring.

Tube samples are commonly referred to as “undisturbed samples.” However, samples collected using these methods can be subject to some level of disturbance. The field personnel observed the collection of tube samples and handled and transported the tube samples with care during the exploration program. The tubes were stored in a climate-controlled room and were properly oriented while awaiting shipment to the geotechnical laboratory.

Sampling tubes were new, 3-inch-O.D., thin walled, in good condition, and made of stainless steel. Tubes advanced were 30 inches long. The cutting edge of each tube was sharp, free of dents or nicks, and had a slightly smaller I.D. than the I.D. of the rest of the tube.

Samples extruded from the tubes were used for laboratory strength and deformation testing. The locations of the undisturbed samples are denoted by “U-#” on the boring logs.

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3.2.3 Rock Coring

Ten (10) feet of rock coring was performed in two (2) of the borings (ETB-B-01 and NTB-B-

01). Boring ETB-B-02 (BTX) was drilled for the sole purpose of installing the MPBX. This hole was blind drilled using HSA to the top of bedrock and then five (5) feet of rock coring was performed. Rock coring was performed through the HSA using an NX-sized single core barrel.

3.3 Soil Classification

Soil samples were logged by a GEI field engineer during the exploration activities. Soil samples obtained from the borings were observed and classified in the field in general accordance with the ASTM Visual-Manual Procedure (ASTM D2488). In addition, information on color, moisture, miscellaneous material, any unusual characteristics or observations, and possible stratum designations were also noted.

Percentages of gravel, sand, and fines are reported using descriptor terms in general accordance with Section 16.1.4 of ASTM D2488. The terminology and the percentage ranges are provided below.

• Trace – Particles present but estimated to be less than or equal to 5%

• Few – Greater than 5% but less than 15%

• Little – Greater than or equal to 15% but less than 30%

• Some – Greater than or equal to 30% but less than 50%

• Mostly – Greater than or equal to 50%

3.4 Borehole Gas Measurement

GEI field personnel recorded gas meter readings to screen for potentially hazardous atmospheric conditions for the field crews, and for potential gassy soil conditions that construction workers could encounter during drilling. The borehole was monitored for hydrogen sulfide (H2S) gas, carbon monoxide (CO), the percentage of available oxygen (O2), VOCs, and potentially combustible gases reported as a percentage of the Lower Explosive Limit (LEL). Gas monitoring was performed during drilling using a multi-gas meter. The LEL sensor was calibrated to methane gas. During the drilling a plastic tube connected to the multi-gas meter was placed just below the top of the casing or HSA and the meter recorded the gas measurements. When the

LEL level was higher than the allowable range, drilling was stopped until the gas vented out and

LEL level was within the allowable range.

Information on the LEL readings throughout the subsurface exploration is provided in the boring logs as well as a summary of the readings in Figure 7. H2S, CO, and VOC readings are reported in parts per million (ppm), and O2 and LEL readings are reported in percent (%). The alarms on the meters were set at current worker levels to assist them in stopping work if there were unsafe gas levels recorded within the breathing zone.

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The methane gas venting activities created delays when drilling borings ETB-B-01, ETB-B-02, and NTB-B-02. Boring ETB-B-02 had to be terminated at El. -53 feet due to the high LEL levels that would not dissipate to acceptable levels after venting for seven (7) days.

3.5 Water Level

Water levels were measured in the borings during the subsurface explorations. The water level encountered during the drilling is provided in the logs. The water level is being monitored using the installed VWPZs. The VWPZ data is presented in Appendix E and shows the recorded water level collected to date by each of the five (5) VWPZs at each boring location. A summary plot of the water readings versus depth is presented in Figure 8.

It must be noted that fluctuations in the level of the groundwater may occur due to variations in season, rainfall, temperature, and other factors not evident at the time observations were made and reported herein. The monitoring wells will remain operational during the entirety of the project.

3.6 Boring Logs

The boring log is a written record of the observed subsurface conditions, drill rig response, and other observations and measurements made during drilling and sample collection and field testing. A boring log was prepared by the field engineer or geologist during the drilling activities.

It includes a description of each sample collected and the soil conditions and soil strata observed at each boring. These boring logs are provided in Appendix B

The type and depth of soil samples, sampler penetration and recovery, and SPT N-Values are included on the boring logs. Other information generally included on the boring logs include field test results, general remarks about the drilling methods and progress, observed groundwater levels, gas readings, ground surface elevations, coordinates, and drill rig and tooling information.

Soil descriptions shown on the logs are based on the field observations of the soil contained in the sampler. There are instances where a sample or core was not recovered, or recovery was poor. When this occurred, a description was typically inferred from the material contained in the sampler (if any), drill action (e.g., ease or difficulty of drilling, rate of advancement, etc.), and the cuttings observed at the ground surface. These descriptions were noted on the logs and should be considered as only general indicators of subsurface conditions at those depths.

3.7 In-situ Testing

In-situ testing was performed by In-Situ Testing, LLC and Stable Ground In-situ (SGI) under contract with the HDR/M&N JV.

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Eleven (11) CPTs were performed to identify strata and develop profiles. Dissipation tests were also performed at specific depths within several of the CPTs. Soil behavior type, shear strength parameters, and soil deformation parameters are estimated using the CPTs data.

Seven (7) DMTs were also performed to collect measurement of the Constrained Modulus values of the soil. Seismic DMT were also performed to determine the shear wave velocity and small strain modulus (Gs) of the soils.

All in-situ tests were advanced until refusal of the probe occurred. Refusal was encountered at elevations ranging from El. -60 ft to El -90 ft. Based on nearby borings, this appears to closely match the top of bedrock elevation.

Upon completion of the testing, the rods were pulled out of the ground. In several locations in the East Tidal Basin area we did observe gas venting from the test hole once the friction reducer was removed. This venting was observed as bubbling of the water in the hole. All in-situ test holes were either backfilled with a tremie-placed bentonite-grout mix or with bentonite chips upon completion

E2CR performed two vane shear tests in boring ETB-B-01 at depths of 57 ft and 67 ft corresponding with El. -52.1 ft and El. -62.1 ft, respectively. Undrained shear strength was estimated using a vane with 50 mm width and 100 mm long.

Results for the CPTs, DMTs, and vane shear tests are presented in Appendix D.

3.8 Elevation Datum and Coordinate System

Elevations are measured in feet and are referenced to the North American Vertical Datum of

1988 (NAVD88). Boring coordinates are in feet and are referenced to Virginia State Plane

Coordinate System, Virginia North Zone defined by the North American Datum of 1983

(NAD83).

3.9 Geotechnical Laboratory Testing

The objective of the geotechnical laboratory testing program was to measure the physical, mechanical, and chemical properties of soil and rock samples obtained during the field explorations. The laboratory testing program included index, strength, and consolidation tests.

The laboratory test results, combined with the field test data collected, help to provide a basis for characterizing the observed subsurface conditions and for developing engineering parameters for use in the design and construction phases of the Project.

Table 1 below provides a summary of the type and number of tests performed.

The laboratory testing report for the samples tested by Soil and Land Use Technology, Inc.

(SaLUT), and Baltimore Materials Testing (BMT) are provided in Appendix C. This report

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Table 1 – Summary of Lab Testing Performed

Description of Test Test Standard Number Performed

Moisture Content ASTM 2216 71

Grain Size – Mechanical Sieve ASTM D422 18

Grain Size – with Hydrometer ASTM D422 7

Atterberg Limits ASTM D4318 22

Specific Gravity ASTM D854 4

Organic Content ASTM D2974 10

Direct Shear ASTM D3080 6

UU Triaxial ASTM D2850 4

Incremental 1-D Consolidation ASTM D2435 5

Unconfined Compression on Rock ASTM D7012 5

3.9.1 Index Testing

Index testing was performed on select soil samples collected from both split spoon sampling and

Shelby tubes. These tests included natural moisture content, particle size distribution, Atterberg limits, specific gravity and organic content. A summary table with the results from these tests along with the lab reports are included in Appendix C.

The specific gravity of the soils across the profile and site were found to be close to 2.6.

Organic content tests were performed on samples that were visually identified as containing organics in the boring logs. We also tested samples within the depth zone where high LEL levels were measured. In general, the organic contents were found to range between 2 and 12%.

3.9.2 Strength Testing

A total of three (3) samples had direct shear testing performed on both undisturbed and remolded fine-grained alluvial soil samples. The tests indicate that the remolded samples had a slightly higher friction angle but lower cohesion. The undisturbed samples were found to have a peak friction angle between 28 and 34 degrees with cohesion values between 500 and 620 psf. The tests on the remolded samples provide fully softened strengths since the structure of the soil is fully broken down during the remolding process. The fully softened friction angles were found to range between 35 and 39 degrees with cohesion values between 290 and 540 psf.

3.9.3 Compressibility Testing

A total of five (5) undisturbed samples had consolidation testing performed using one-dimensional incremental loading. These tests were all performed on alluvial samples with depths

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ft. The moist unit weight of the soils were measured as part of this testing and the values were found to range between 95 and 112 pcf. Initial void ratio values were found to range from 1.0 to

2.1. Compression index values were estimated from the consolidation curve and were found to range between 0.28 and 0.6. The measured compression index values match closely with published correlations with the natural water content. The preconsolidation pressure was also estimated from the curves and compared to an estimated in-situ effective stress to calculate the over-consolidation ratio (OCR). In general, it was found that the shallower samples appear to have an OCR value greater than, but that values appears to be closer to 1.0 within the middle soft fine-grained alluvial soils. This trend is consistent with the values calculated in the CPT test results.

3.9.4 Rock Testing

Unconfined compression tests were performed on intact rock core samples. The results of these tests found the unconfined compression strength of the tested rock samples to be between 5,700 psi and 9,900 psi. The cores were found to shear both vertically and diagonally.

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4. Geologic Conditions

4.1 Regional Geology

The Washington, DC area is located within the Coastal Plain and Piedmont Physiographic

Provinces. The boundary between these two provinces, known as the Fall Line, runs southwesterly from the District of Columbia-Montgomery County boundary near Silver Spring across the

Potomac River north of Roosevelt Island. Downtown Washington, DC is predominantly located within the Coastal Plain. The Coastal Plain typically contains Pleistocene terrace deposits and recent river alluvium at the lower levels, rising into exposed Cretaceous sediments on higher ground. The Piedmont Province extends from the Hudson River near Nyack, NY to a point just

North of Montgomery, Alabama. It is predominantly a rolling upland developed on intensely folded and faulted metamorphic and igneous rocks. Moving southeast from the fall line, the bedrock is overlain by increasing thicknesses of sedimentary deposits, the oldest and lowest dating from the Cretaceous period (Potomac Group Soils), followed by Quaternary deposits and recent alluvium and artificial fill. East of the shoreline is the Atlantic Continental Shelf Province, the submerged continuation of the Coastal Plain, which extends eastward for at least another 75 miles where the sediments reach a maximum thickness of about 40,000 feet. Local relief is on the order of 50 ft, with occasional greater relief near deeply cut stream valleys. Dissection is often greatest near the Fall Line. The metamorphic rocks in the Washington area include the Wissahickon

Formation, the Sykesville Formation, and the Laurel Formation. The igneous rocks are more recent intrusions into the older metamorphic rocks. A thin mantle of soil covers much of the Piedmont.

4.2 Local Geology

The geologic history of the project area is characterized by successive periods of sedimentary deposition and erosion over millions of years. These processes resulted in the current configuration of the bedrock surface and the physical character and configuration of the overlying wedge of overburden soils.

The subsurface materials in this area of Washington, DC typically consist of the following layers in order with depth; recent and older fills, Late-Pleistocene and Holocene Alluvium soils, Late-

Pleistocene Terrace deposits, Early-Cretaceous Potomac Group soils, overlying crystalline bedrock. In this specific project area, the majority of the Terrace and Pleistocene soils have been eroded away by high velocity flow of the Potomac River. As the river flow decreased over time the area was filled with river alluvial deposits. Documented filling operations were performed as early the late 1800s and early 1900s to reclaim this area from the river.

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4.2.1 Bedrock Geology

The underlying bedrock is part of the Mid-Atlantic Piedmont province, which occupies the area between the Blue Ridge Mountains and the Atlantic Coastal Plain. The bedrock in the area formed about 450 to 550 million years ago in a volcanic arc associated with the Taconic subduction zone. The oldest bedrock was probably deposited in or near a submarine trench before being metamorphosed, folded, and intruded by several phases of plutonic rocks.

The bedrock in the Project area is generally early Paleozoic crystalline schist and gneiss bedrock of the Chopawamsic Formation. The top of bedrock encountered during the geotechnical exploration was encountered between Elevations (El.) -60 ft and El. -90 ft.

4.3 Local Soils

General descriptions of the main soil strata in the project area are provided below, based on the available geotechnical data, the surficial geology maps, and the observations from the completed explorations. From the top down (i.e. youngest to oldest deposits) the major soil strata are:

• Fill

• Alluvial Deposits

• Decomposed Rock

• Bedrock

Subsurface profiles have been prepared for the North and East Tidal Basin walls and are presented in Figures 5 and 6, respectively. These profiles show our interpretation of soil strata breaks. These layer breaks are based on data collected at the discrete test locations and have been interpolated between these points and should therefore be considered very approximate and used with caution for design purposes.

4.3.1 Fill

In recent history, many areas in the area have been filled or re-graded to increase the amount of available land for development. Fill was placed using hydraulic placement and mechanical dumping over the past 120 plus years. Older Fill materials are assumed to have been generated from dredging of the river alluvial materials. Given this, the contact between man placed Fill and the underlying alluvium can be difficult to identify.

Based on an available report associated with the nearby Long Bridge Project, existing man placed Fill soils extend across the alignment of the bridge from the ground surface to depths of up to 28.5 feet. The recent boring logs for the seawalls project identified Fill material extending to depths up to 13 feet. The existing Fill soils typically consist of sands, gravelly sands, clayey and silty sands, and sandy clays with varying amounts of organics and debris. The existing Fill soils are of variable consistencies and densities.

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The results of the field investigation in the Tidal Basin area found the fill to range in thickness from 8 to about 13 feet. The fill materials were identified to consist of clay and silt with varying amounts of sand and gravel, and sand with varying amounts of silt, clay and gravel. SPT N-values in this layer were found to range from weight of hammer (WOH) to 13 blows per foot

(bpf).

4.3.2 Alluvium

Quaternary-age Alluvium was deposited within the current Potomac River valley and its tributaries incised into the pre-existing Coastal Plain sediments. The Alluvium generally consists of interbedded deposits of clay, silt, sand, and gravel with varying amounts of organic materials, including peat and wood fragments. The Alluvium was deposited by rivers and tributary streams within channels, bars, flood plains, terraces, estuaries, and alluvial fans. It is generally characterized by cross-bedding, rapid pinching and thickening of beds, and lateral and vertical particle-sized grading.

Fine-grained soils are deposited during low energy river flows while coarse-grained soils are deposited during high velocity river flow events. Seasonal and storm-based river flows will result in interbedded layering of these coarse and fine soils.

The results of the field investigation in the Tidal Basin area found that the alluvial soils can be broken down into three layers, the upper loose sandy soils, the middle very soft fine-grained soils, and the lower stiffer fine-grained soils.

The upper sandy soils were generally found to extend to elevations between El. -28 ft and El. -36 ft and consist of narrowly graded sand, and silty and clayey sand, and sandy clays. The upper alluvial materials were found to have SPT N-values ranging from WOH to 13 bpf with an average N-value of around 2 bpf.

The middle fine-grained soils were found below the sandy soils in all borings and extend down to elevations between El. -52 ft and -72 ft and consist of low to high plasticity clays and elastic and non-elastic silts with varying amounts of sand. The middle fine-grained soils were found to have SPT N-values ranging from WOH to 13 bpf with an average N-value of around 2 bpf.

The lower stiffer alluvial soils were found in all borings and typically range in thickness between

10 and 20 feet and generally consist of low to high plasticity clay with varying amounts of sand as well as silty to clayey sand. The lower fine-grained soils were found to have SPT N-values in this layer were found to range from 3 to 15 bpf with an average N-value of around 8 bpf.

4.3.3 Decomposed Rock

The decomposed rock layer was identified based on high SPT N-values, high tip resistance in the

CPTs, and refusal of the DMTs. The decomposed rock was found in most of the borings between

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4.3.4 Bedrock

The bedrock was encountered between El. -60 ft and El. -95 ft with the shallowest location being on the northern end of the North Tidal Basin near the Japanese Pagoda. The bedrock was identified as a coarse-grained metamorphic Schist. The percentage recovered in the rock coring was found to range from 80% to 100% with an average of 93%. The Rock Quality Designation

(RQD) is a measure to indicate the quality of the rock based on a summation of the solid intact cores recovered that are greater than 4 inches in length. The RQD values measured were found to range between 25% and 100% with an average of 67% with the lower values typically found at the top of the bedrock indicating higher weathering at the surface. The values of the recovery and the RQD indicate that the bedrock in this area is of high quality.

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5. Geotechnical Instrumentation

Inclinometers (INC), vibrating-wire piezometers (VWPZ), and a multiple position borehole extensometer (MPBX) were installed in boreholes to measure lateral and vertical soil deformations and groundwater levels. INCs and five (5) multi-level VWPZs were installed and fully-grouted in borings ETB-B-01, NTB-B-01, and NTB-B-02 with approximate tip elevations of El. -105 ft, El. -92.5 ft, and El. -91 ft, respectively. A five anchor MPBX was installed in

Boring ETB-B-02 (BTX) with an approximate tip elevation of El. -94 feet. The location of the extensometer was changed after having a high methane gas level in its original planned location of ETB-B-02.

5.1 Inclinometer and Vibrating Wire Piezometers

Five (5) vibrating wire piezometers (VWPZ) were installed and co-located with an inclinometer

(INC) that were all fully-grouted in each boring. The VWPZs were factory calibrated and checked in the field prior to installation in general accordance with the manufacturer’s instructions before they were installed.

5.1.1 VWPZ Calibration and Initial Zero Reading

Each VWPZ was provided with an individual calibration report from the manufacturer. Field calibrations were therefore not performed. A device-specific linear gage factor (G) (psi/digit) and a thermal factor (K) (psi/°C) was provided with the manufacturer’s calibration.

An initial zero reading was established for each device prior to installation. This was achieved by saturating the filter stone to remove air bubbles from the sensing chamber. The device was submerged in a bucket of water until the temperature of the device stabilized with the surrounding water temperature. The instrument was plugged into the readout device and was lifted just above the surface of the water after the temperature had stabilized. A set of readings were taken at this position and the average was considered the initial zero reading. This reading was represented as a digit (R0) and its corresponding temperature (T0).

The initial zero reading was used as a baseline value to compare with the pressure reported by the instrument while submerged.

5.1.2 VWPZ and INC Installation

The INC casing was placed on the ground and the VWPZs were fastened to the casing at the appropriate locations. A service loop was added to the cable near the sensor location to provide slack and reduce stress on the cable connection to the VWPZ. The casing and the attached instruments were then assembled while being lowered into the borehole to the specified depth

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5.1.3 VWPZ Data Collection

Data from the VWPZ was collected in the field using a data logger (Worldsensing LS-G6-VW).

The VWPZ data was recorded once per hour. Data collected from the data logger was sent to a central gateway using RF radios where it is packaged and sent using a cellular modem to the monitoring server.

5.1.4 VWPZ Data Interpretation

Similar to the initial zero reading, each data point collected from a VWPZ was represented by a digit (R1) and a temperature (T1). The water pressure was calculated by multiplying the difference between the measurement and the Initial Zero Reading by its calibrated gage factor

(G) and thermal factor (K) and summing the two results.

The water level was computed by first translating the calculated pressure into a height of water above the VWPZ. This was done by dividing the calculated pressure (force/area) by the unit weight of water (force/volume). The resulting distance was then added to the known elevation of the VWPZ to calculate the corresponding water level.

From the data collected to date, it has been observed that all of the water pressures measured are directly influenced by the Potomac River level and exhibit tidal fluctuations. We have also observed that the water pressure in the upper alluvial soils matches closely with the hydrostatic pressure. The water pressure measured in the lower alluvial soils was found to decrease below hydrostatic progressively more at deeper depths down to the bedrock. Plots of the VWPZ data to date are included in Appendix E. A summary of the current water pressures versus elevation for each borehole is presented in Figure 8.

5.1.5 INC Initial Baseline Reading

Each INC was read two times using a Digitlt AT Inclinometer system (DGSI) a minimum of one week after grouting. Readings were taken where the A0 was set as the grooves closest to perpendicular with the water. The probe was lowered to the bottom elevation in the A0 direction then 2-foot intervals were recorded using the cable gate as reference, after completing a reading in the A0 direction, the readings were repeated in the A180 direction. The sum of the two readings were checked to ensure repeatability. The average of these two readings is used as the baseline reading in which subsequent readings are compared.

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5.1.6 INC Data Collection

Data from the INC was collected in the field using a Digitilt AT (DGSI) in the same method as in the baseline once every two weeks. The data collected from INC was sent to the monitoring server for data reduction.

5.1.7 INC Data Interpretation

Each data point collected at two-foot intervals is reported in inches. The displacement was calculated by multiplying the difference between the measurement and the Initial Reading added by the displacement from an initial bottom reading which is assumed to be fixed.

The data currently collected with the inclinometers is showing lateral deflections in the readings to date. These movements appear to be deep seated down near where the bottom of the soft fine-grained alluvial soils. This type of movement is consistent which what was observed at the

Jefferson Memorial North Plaza. However, the measured movements are showing some fluctuations back and forth and therefore, it is necessary to observe longer term readings to determine trends. Plots of the INC data to date are included in Appendix E.

5.2 Multiple Position Borehole Extensometer

A multiple position borehole extensometer (MPBX) consisting of five rebar anchors and stainless-steel extension rods encased in protective PVC sleeves were installed and fully-grouted in the boring at separate elevations. After fully grouting the anchors, the transducer head was installed and baselined. MPBX transducers were factory calibrated and checked in the field prior to installation in general accordance with the manufacturer’s instructions before they were installed onto the extension rods.

5.2.1 MPBX Calibration and Initial Zero Reading

Each MPBX transducer was provided with an individual calibration report from the manufacturer. Field calibrations were therefore not performed. A device-specific linear gage factor (G) (in/digit) was provided with the manufacturer’s calibration.

The MPBX transducers were threaded onto the extension rods, then the transducer head was bolted onto the tube mount. Each transducer was locked off at an approximate half stroke range to allow for 2 inches of settlement and 2 inches of heave.

An initial zero reading was established for each transducer as an average of initial readings taken after the transducers were locked in place. This reading was represented as a digit (R0).

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5.2.2 MPBX Installation

The MPBX stainless steel rods, and protective PVC sleeves were laid out on the ground. The rods and PVC were connected to reflect the appropriate elevations of El. -19 ft, El. -39 ft, El. -59 ft, El. -79 ft, and El. -99 ft. The rods were attached with internal threads while the PVC was attached with PVC couplers with telescoping members. The rebar anchors were attached to the bottom end of the rods and the top end of the rods were attached to the tube mount using sacrificial rod extensions and compression fittings. A rope loop was added to the tube mount to suspend the MPBX at the correct depth in the road box to accommodate the transducer head. The

MPBX anchors and the attached rods were then lowered into the borehole to the specified depth and the borehole was tremie grouted with cement-bentonite grout.

5.2.3 MPBX Data Collection

Data from the MPBX was collected in the field using a data logger (Worldsensing LS-G6-VW).

The MPBX data was recorded once per hour. Data collected from the data logger was sent to a central gateway using RF radios where it packaged and sent using a cellular modem to the monitoring server.

5.2.4 MPBX Data Interpretation

Similar to the initial zero reading, each data point collected from a VWPZ was represented by a digit (R1). The displacement of the transducer head is calculated by assuming the bottom anchor in rock is fixed. This is calculated by multiplying the difference between the measurement and the initial zero reading by its calibrated gage factor (G) and summing the two results.

The displacement of the remaining anchors is computed by first calculating the displacement of the anchor relative to the transducer head and adding that to the displacement of the ground surface.

The data collected to date is showing an ongoing trend of settlement of several of the points with the magnitude of the settlement decreasing with depth. Plots of the MPBX data to date are included in Appendix E.

5.2.5 MPBX Data Limitations

Anchor 1 set at 20 feet was not able to be read due to damage to the anchor during installation.

Therefore, data for Anchor 1 is omitted from the data presented. The transducer for anchor 5 set at 100 feet was found to have fully compressed on September 28, 2022, and was subsequently reset on November 9, 2022. From the time anchor 5 was fully compressed until it was reset, ground surface displacement was calculated assuming the anchor 4 set at depth of 80 feet was fixed.

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In addition, the transducer for anchor 2 set at a depth of 80 feet was also fully compressed on

October 20, 2022, and was reset on November 9, 2022. As a result, no readings were collected during this time.

5.3 Surface Protection

All instrumentation were installed in borings that were finished with a flush-mount cover consisting of a 12"-diameter roadbox set in an approximately 6-inch-thick concrete collar flush with the existing ground surface.

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6. Geotechnical Design Parameters

6.1 General

We have reviewed all of the data collected and have prepared a summary of recommended soil parameters for each layer that can be used for the design of the new seawalls and an assessment of settlement and global stability. The recommendation soil parameters are based on lab test results and in-situ test results collected for the project. Results were also checked based on publish typical values and from correlations with index properties of the soils. Soil parameters are broken down by Strength Properties in Table 2 and Compressibility Parameters in Table 3.

Table 2 – Recommended Design Strength Parameters

Stratum Bottom El.

(ft)

N

(bpf) γm

(pcf) φ

(deg) su (psf)1

UCS (psi)2

Em

(tsf)

Fill -8 to -10 5 110 30 0 100

Sandy Alluvium -28 to -36 2 105 28 0 60

Fine-Grained

Alluvium -52 to -72 2 100 0 600 50

Lower Fine-

Grained Alluvium -58 to -85 8 115 0 1000 150

Decomposed

Bedrock -60 to -95 50 130 38 0 1200

Bedrock NA Refusal 170 N/A 5700 10000

1 – Undrained Shear…