B08_Attachment_6_Design_Analysis_95pct.pdf

US Army Corps of Engineers Norfolk District Norfolk, Virginia 23510

DESIGN ANALYSIS

FOR

COLONIAL NATIONAL HISTORICAL

PARKWAY

YORK RIVER SHORELINE

STABILIZATION PHASE IIB

YORKTOWN, VIRGINIA

NOVEMBER 2020

95% SUBMITTAL

TABLE OF CONTENTS

SECTION 1 - GEOTECHNICAL REPORT

SECTION 2 - STRUCTURAL CALCULATIONS

SECTION 3 - STORMWATER MANAGEMENT

PLAN AND EROSION & SEDIMENT CONTROL PLAN

SECTION 1

Geotechnical Report 95% Submittal

Submitted by:

Geo-Environmental Section

U.S. Army Corps of Engineers Norfolk District

October 2020

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Table of Contents

1. INTRODUCTION

1.1. PROJECT DESCRIPTION

1.2. PROJECT DATUMS AND STATIONING

2. GEOLOGY, GROUNDWATER, AND SEISMICITY

2.1. REGIONAL GEOLOGY

2.2. REGIONAL GROUNDWATER

2.3. LOCAL GEOLOGY

2.4. SEISMICITY

3. SUBSURFACE EXPLORATIONS

3.1. 2012 USACE EXPLORATIONS

3.1.1. STANDARD PENETRATION TEST (SPT)

3.1.2. CONE PENETRATION TEST (CPT)

3.2. 2019 USACE EXPLORATIONS

3.2.1. STANDARD PENETRATION TEST (SPT)

3.2.2. CONE PENETRATION TEST (CPT)

3.2.3. LABORATORY TESTING

3.3. VIRGINIA DIVISION OF GEOLOGY AND MINERAL RESOURCES (VDGMR) EXPLORATIONS

4. SUBSURFACE STRATIGRAPHY

4.1. GROUNDWATER

5. SLOPE STABILIZATION DESIGN

5.1. ROCK REVETMENT

5.2. BLUFF AREAS

5.2.1. STABILIZATION METHOD

5.2.2. SLOPE STABILITY

5.2.3. SLOPE STABILIZATION RECOMMENDATIONS

5.2.4. SHEETPILE/TIEBACK DESIGN

5.2.5. SHEETPILE WALL SLOPE STABILITY

5.3. LOWLAND AREAS

6. CONSTRUCTION RECOMMENDATIONS

6.1. FUNDING CONSTRAINTS

7. REFERENCES

List of Figures Figure 5‐1. Typical Vegetated Wall Slope Stabilization With Geotextile Bags (and Rock Revetment) Detail Figure 5‐2. “Hybrid” Solution Detail

List of Plates Plate 1. Colonial Parkway Map Plate 2. Project Area – Reaches 1 and 2 Plate 3. Geologic Map Plate 4. NRCS Soil Map – Reach 1 Plate 5. NRCS Soil Map – Reach2 Plate 6. Exploration Locations – Reach 1 (Map 1 of 2) Plate 7. Exploration Locations – Reach 1 (Map 2 of 2) Plate 8. Exploration Locations – Reach 2 (Map 1 of 2) Plate 9. Exploration Locations – Reach 2 (Map 2 of 2) Plate 10. Stick Log Profile – Reach 1 (Profile 1 of 2) Plate 11. Stick Log Profile – Reach 1 (Profile 2 of 2) Plate 12. Stick Log Profile – Reach 2 (Profile 1 of 2) Plate 13. Stick Log Profile – Reach 2 (Profile 2 of 2)

List of Tables Table 1‐1. Project Stationing Table 3‐1. 2012 SPT Exploration Information Table 3‐2. 2012 CPT Exploration Information Table 3‐3. 2019 SPT Exploration Information Table 3‐4. 2019 CPT Exploration Information Table 3‐5. 2019 Laboratory Testing Program Table 3‐6. VDGMR Exploration Information Table 5‐1. Shear Strength Parameters, Reach 2 Table 5‐2. SLOPE/W Results, Sheetpile Wall

List of Enclosures Enclosure 1. Site Photographs Enclosure 2. Soil Boring Logs Enclosure 3. Geotechnical Laboratory Test Data Enclosure 4. Historic Topographic Maps Enclosure 5. Tall Slope Protection Material Questionaires Enclosure 6. SLOPE/W Output Enclosure 7. Sheet Pile Tieback Calculations

York River Shoreline Stabilization Project

YORK RIVER SHORELINE STABILIZATION PROJECT

COLONIAL NATIONAL PARKWAY

GEOTECHNICAL REPORT

1. INTRODUCTION

The purpose of this report is to present the results of a subsurface exploration and (in future submittals) to provide geotechnical recommendations for use in preparation of the final design of the York River Shoreline Stabilization project along the Colonial Parkway, Colonial National Historic Park, Virginia.

The report summarizes the results of the subsurface exploration and laboratory testing performed by the Norfolk District U.S. Army Corps of Engineers (Corps) and others. The report presents our understanding of the project, reviews the exploration procedures, and presents our observations, evaluations, and recommendations. The report does not address the archeological or environmental concerns related to this project; therefore appropriate references should be used to specifically address those concerns.

1.1. PROJECT DESCRIPTION

The proposed project is located along the south/southeast shoreline of the York River that parallels the Colonial Parkway in Yorktown, Virginia. Colonial Parkway is within the Colonial National Historic Park, Virginia. The parkway is owned by the National Park Service (NPS) and connects the cities of Yorktown at the east, through Williamsburg, to Jamestown on the west. A general map of the parkway is on Plate 1; the K‐xx markers are kilometer markers along the parkway. The project site is between Williamsburg and Yorktown. The project is divided into four reaches. Reach 1 extends from Felgates Creek on the west to Indian Field Creek on the east, a distance of about 1.7 miles. Reach 2 extends from the mouth of Indian Field Creek on the west to the west pier of the Yorktown Naval Weapons Station on the east, a distance of about 1.4 miles. Reach 3 extends from the east pier at the Yorktown Naval Weapons Station on the west to Yorktown Creek on the east, a distance of about 1.3 miles. Reach 4 begins at the picnic area just downriver of Yorktown Beach on the west and extends to the U.S. Coast Guard Station on the east, a distance of about 1.4 miles. This report only covers Reaches 1 and 2; it is anticipated that Reaches 3 and 4 will be covered under a future project. Plate 2 is a map of the Reach 1 and 2 area.

Most of the banks along this portion of the York River are eroding. Rock revetment and some local breakwaters were placed over much of the project area in the early 1980’s but the revetment has been overtopped, causing erosion behind the revetment. The height of the erosion scarp varies from 2‐3 feet at low‐lying areas (mostly near the creeks) to a maximum of 30 feet in the bluffs between the creeks. Photos of the erosion, some of which were taken from a boat in the York River, are in Enclosure 1. At one location near the K‐5 marker in Reach 2, the

NPS installed an anchored high performance turf reinforcement mat (HPTRM) on the slope (Photo 7). That repair attempt has failed and the slope is highly unstable at that location. We believe the repair failed because the slope was too steep for an anchored HPTRM and the installation was likely faulty. The erosion scarp is steep (mostly less than 2H:1V; less than 1H:1V at the anchored HPTRM location) and is only 10‐15 feet away from the Parkway in some places.

Larger coastal storms will continue to overtop the lower existing revetments and erode the base of the scarp face. In addition to river erosion at low elevation, sheet flow of rainwater off the Parkway is causing erosion in the upper reaches of the bluffs. Localized upland erosion has also happened at storm drain pipe outlets. As part of this project, the existing revetments will be raised and existing breakwaters will be rehabilitated to protect the shoreline. In lowland areas where there is no existing rock revetment and the erosion scarp is short, it is anticipated that natural and nature‐based features (NNBFs) such as plantings, erosion control matting, oyster reefs, and living shorelines will be used for stabilization. In the tall bluff areas it is anticipated that proprietary block‐type or terraced walls with plantings will be used for stabilization. If no block‐type wall suitable for the site conditions can be found, a steel sheetpile wall may be used.

The NPS originally funded the Norfolk District to design slope stabilization for all 4 reaches in 2012‐2014. That effort resulted in a geotechnical report and a set of 50% plans being prepared before the funding ran out. The earlier work only included improvements to stop the wavewash erosion at the base of the slope. The current project also includes stabilization of the upper slopes and storm drainage improvements along the Parkway to alleviate the sheet flow erosion of the upper slopes. This report uses and adds to the 2014 geotechnical report.

1.2. PROJECT DATUMS AND STATIONING

The horizontal datum is Virginia South State Plane, based on the North American Datum of 1983 (NAD83). The vertical datum is Mean Low Water (MLW). Conversion factors between MLW and other commonly‐used vertical datums are given on Sheet 3 of the plans.

The main project stationing is along the north/northeast pavement edge of Colonial Parkway. Project stationing increases to the east/southeast. Table 1‐1 shows the approximate stations of major existing and proposed features. Project features are discussed in detail in Section 5 of this Geotechnical Report.

Table 1‐1. Project Stationing

Station Feature

0+00 Start of Reach 1, west abutment Colonial Parkway bridge over Felgates Creek

0+23 Start new rock sills

3+00 to 6+00 Ringfield Pull‐Out, Contractor Staging Area

8+60 End new rock sill

12+20 Start vegetated wall slope stabilization

12+35 Start revetment rehabilitation

16+00 to 19+00 Cheetham Annex Pull‐Out

72+20 End rehabilitation existing rock revetment

72+50 End vegetated wall slope stabilization

Station Feature

84+50 End of Reach 1, west abutment Colonial Parkway Bridge over Indian Field Creek

86+00 Start of Reach 2, east abutment Colonial Parkway Bridge over Indian Field Creek

91+55 Start rock sill rehabilitation

94+00 to 97+25 Powhatan’s Village Pull‐Out

95+50 to 96+75 Short break in rock sill

103+75 to 104+40 Short break in rock sill

113+20 End existing rock sill rehabilitation, start new rock sills

118+80 End new rock sills, start existing rock sill rehabilitation

121+75 End existing rock sill rehabilitation

122+75 Start of breakwater area

128+10 Start vegetated wall slope stabilization

128+30 End vegetated wall slope stabilization, start “hybrid” solution

129+50 End breakwater area

131+00 Start revetment rehabilitation

131+35 End “hybrid” solution, start vegetated wall slope stabilization

134+00 to 137+50 York River Pull‐Out

141+90 End vegetated wall slope stabilization

142+00 End revetment rehabilitation

155+20 Navy Property Fence, End of Reach 2

2. GEOLOGY, GROUNDWATER, AND SEISMICITY

2.1. REGIONAL GEOLOGY

York County is located within the Atlantic Coastal Plain physiographic province. The province consists of deformed and consolidated pre‐Jurassic basement rocks that are overlain unconformably by wedges of Mesozoic and Cenozoic marine and fluvial sediments that thicken and dip gently eastward. The basement rocks are similar to those exposed in the Piedmont physiographic province, which is located west of the Atlantic Coastal Plain Province. The contact between the Atlantic Coastal Plain and the Piedmont physiographic provinces is called the Fall Line.

The Coastal Plain is an area of low elevation and low relief characterized by narrow well drained ridges, broad poorly drained flats, and marine coastal areas. Coastal areas consist of marshes, beaches, and dunes. The formation and character of the coastal areas have been shaped not only by sea level rise and fall but also by torrential winds associated with hurricanes.

The sedimentary deposits have been characterized by the continued deposition of clays, silts, sands, gravels, and peat bogs. The late Pleistocene‐Holocene geology of the Virginia Coastal Plan has mostly been characterized by marine transgression onto the land, filling what is today know as Chesapeake Bay.

The Chesapeake Bay and the numerous tidally influenced rivers that flow into the bay characterize the drainage of the region. The most recent formation of the Chesapeake Bay began some time after approximately 15,000 years ago, at the end of the Wisconsin glacial advance. At this time, the ancestral Susquehanna River drained the region; however, as glaciers began to melt and sea level rose, ocean waters began to flow into the Susquehanna valley, eventually creating the estuary that exists today.

The area around the southern Chesapeake Bay, including the City of Hampton, is undergoing subsidence. The Virginia Institute of Marine Science (VIMS) estimates general land subsidence in the Chesapeake Bay area is approximately ‐0.0131 feet per year, which is 1.3 feet over the next 100 years. The subsidence is attributed to the following three factors:

Ground water extraction: When ground water is pumped from permeable aquifer soils (sand and gravel), water is extracted from the space between individual soil particles.

When the aquifer soils have lenses or beds of clay or silt within or adjacent to them, the lowered water pressure in the sand and gravel causes the slow drainage of water from the clay and silt beds. Drainage of water from the clay and silt beds causes those beds to consolidate. This consolidation causes the overlying land surface to subside. This action is believed to be the primary cause of local subsidence.

Glacial isostatic adjustment: During the most recent Ice Age, the Laurentide ice sheet extended from the Arctic south to central Pennsylvania. The weight of the ice sheet caused the underlying land to subside. In response, the land immediately in front of the ice sheet, including the greater Norfolk area, was forced upwards like a seesaw, creating what is called a glacial forebulge. When the ice sheet melted and it’s weight was removed, the land formerly underneath the ice sheet rebounded upwards while the land formerly in the forebulge area subsided, continuing the seesaw action. This action is still occurring.

Chesapeake Bay meteor strike: In the mid 1980’s scientists discovered evidence of a meteor strike in the southern Chesapeake Bay, northeast of the City of Norfolk. The strike caused the settlement of sediments in the region around the strike site. The contribution of this mechanism to regional subsidence is in question.

2.2. REGIONAL GROUNDWATER

Ground water in the Virginia Coastal Plain is contained in a series of aquifers (coarse‐grained permeable units) and confining units (fine grained impermeable units) within the coastal plain sediments. Due to the complex sedimentary history none of the aquifers or confining units extends over the entire Virginia Coastal Plain. The margins of the aquifers and confining units create a complex overlapping patchwork configuration. Major discontinuities among aquifers and confining units exist along the margin of the Chesapeake Bay impact crater. Ground water is recharged primarily by precipitation infiltration. Most of the ground water within unconfined aquifers flows short distances before discharging into streams but a small amount percolates downward through the confining layers to the deeper aquifers. Due to withdrawals for municipal and industrial uses, ground water levels in Virginia Coastal Plain aquifers have dropped; the drop is up to 200 feet near the largest withdrawal centers. Ground water flow direction has also generally changed from a seaward to a landward direction, creating the potential for increased salt water intrusion.

2.3. LOCAL GEOLOGY

Plate 3 is the Geologic Map of Virginia for the area around Reaches 1 and 2 of the project. The surficial geology primarily consists of the Quaternary‐age Shirley Formation (Qsh). The Shirley formation is described by the Virginia Division of Mineral Resources as light to dark gray, bluish‐ gray, and brown sand, gravel, silt, clay, and peat. The Shirley Formation was deposited by riverine terraces and bay‐floor plains.

Plates 4 and 5 are the Natural Resource Conservation Service (NRCS) surficial soil map for the area around Reaches 1 and 2 of the project. The primary soil types along the shoreline are as follows:

3: Axis Very Fine Sandy Loam. Parent material marine deposits. Very fine to fine sandy loam. Very poorly drained

4: Beaches. Parent material marine deposits. Sand. Excessively drained

13: Dragston Fine Sandy Loam. Parent material marine deposits. Fine sandy loam and loamy fine sand. Somewhat poorly drained

15F: Emporia Complex, 25 to 50 percent slopes. Fine sandy loam to loam to clay loam.

Parent material marine deposits. Well drained

35: Udorthents, loamy. Udorthents soils are soils that have been disturbed by human activity (cutting/filling).

Note that the NRCS defines loam as soil composed of 7‐27% clay, 28‐50% silt, and less than 52% sand particles.

2.4. SEISMICITY

The seismic hazard in the vicinity of Yorktown is low. Earthquakes occur along fault planes when the stresses acting on the planes exceed the frictional resistance between the planes. Many of Virginia’s faults are “blind faults,” hidden underground without surface evidence to indicate their location. The Virginia Department of Mines, Minerals, and Energy has identified three seismic zones in the state of Virginia. These zones are areas of increased earthquake activity compared with the surrounding area.

The city of Norfolk is not located within any of these zones. The closest seismic zone to Norfolk is the Central Virginia Seismic Zone (CVSZ). The CVSZ is an east‐west trending zone extending from Richmond (95 miles northwest of Norfolk) on the east to Charlottesville on the west. The Central Virginia Seismic Zone sits in the middle of the North American tectonic plate, which extends from California on the west to the mid‐Atlantic ridge on the east. Quakes generated in the middle of a tectonic plate tend to be weaker than quakes generated along plate edges.

The largest earthquake in Virginia since the development of seismographs occurred on 23 August 2011.

The earthquake had a magnitude of 5.8 and the epicenter was located near the town of Mineral in Louisa County, about 95 miles northwest of Yorktown and within the CVSZ. While the earthquake was felt in Yorktown, no damage was reported.

3. SUBSURFACE EXPLORATIONS

This section describes the geotechnical subsurface explorations that were conducted for the slope stabilization design of Reaches 1 and 2. Locations of explorations are shown on Plates 6 through 9. Stick log profiles are shown on Plates 10‐13. Detailed soil borings are in Enclosure 2.

3.1. 2012 USACE EXPLORATIONS

Field explorations conducted by USACE in 2012 are listed in this section. For more detailed information, see the June 2013 geotechnical report.

3.1.1. STANDARD PENETRATION TEST (SPT)

Fourteen SPT borings were conducted in Reaches 1 and 2. Table 3‐1 gives general information about the borings.

Table 3‐1. 2012 SPT Exploration Information

Exploration Number

Depth (feet)

Elevation (feet)

Northing Easting

12DH‐041 10 +3.0 3629031.26 12039956.11

12DH‐042 8 +3.0 3629129.92 12040398.21

12DH‐043 10 +3.0 3628937.56 12040647.03

12DH‐036 40 +29.5 3628055.74 12043240.85

12DH‐037 40 +29.0 3627679.0 12045519.20

12DH‐044 10 +6.5 3627557.45 12047625.50

12DH‐045 10 +7.5 3627528.10 12047947.73

12DH‐046 10 +7.3 3627373.71 12048477.43

12DH‐047 10 +7.0 3627072.10 12048989.89

12DH‐048 10 +4.5 3626547.43 12049433.20

12DH‐049 10 +6.0 3625979.44 12049629.35

12DH‐050 10 +7.5 3625441.51 12050062.89

12DH‐051 10 +5.0 3624844.25 12050647.15

12DH‐038 40 +19.0 3623467.60 12052200.50

3.1.2. CONE PENETRATION TEST (CPT)

Four CPT probes were conducted in Reaches 1 and 2. Table 3‐2 gives general information about the CPTs.

Table 3‐2. 2012 CPT Exploration Information

CPT Number Depth (feet)

Elevation (feet)

Northing Easting

CPT‐01 40 +28.0 3628235.09 12042330.34

CPT‐02 40 +30.2 3627789.66 12044669.53

CPT‐03 40 +28.5 3627639.50 12045877.50

CPT‐11 41 +23.5 3624341.00 12051009.00

3.2. 2019 USACE EXPLORATIONS

3.2.1. STANDARD PENETRATION TEST (SPT)

Eight soil borings (2F‐19‐1 through 2F‐19‐8) were conducted to complete the design of the shoreline stabilization of Reaches 1 and 2. The borings were 40‐80 feet deep. The SPTs were conducted between 6 and 12 March 2019 using a truck‐mounted CME 550 SPT rig and crew from the Savannah District of the Corps. The borings were drilled using 4‐1/4 inch diameter hollow stem augers in general accordance with ASTM D 1586. The borings were sampled continuously to 16 feet depth and then every 5 feet to the bottom of the hole (BOH). Soil samples were obtained with a standard 1.4” I.D., 2” O.D., and 30‐inch long split‐spoon sampler driven with a 140‐pound automatic hammer falling 30 inches. The number of blows required to drive the sampler for 6 inch penetration increment was recorded. The recovered soil samples were visually classified in the field in accordance with ASTM D 2488 by a geotechnical engineer from the Norfolk District. A portion of the recovered soil from the split spoons was placed in plastic jars and returned to the Norfolk District Office for review and laboratory testing assignment.

The groundwater level was recorded during drilling of the SPT borings and at the end of drilling.

When the completion of a hole coincided with lunch, the end of drilling groundwater level was recorded after the lunch break prior to pulling the augers and backfilling. When a hole completion coincided with the end of a work day, the augers were left in the hole overnight and the groundwater level was recorded first thing the next morning before the augers were pulled and the holes backfilled. For safety reasons, other SPT holes were not left open overnight and the augers were pulled and the holes backfilled at drilling completion.

The SPT holes were filled with drill cuttings. Excess drill cuttings were spread around the ground near the drill site locations.

Table 3‐3 gives general information about the borings.

Table 3‐3. 2019 SPT Exploration Information

Depth (feet)

Elevation (feet)

Northing Easting

2F‐19‐1 40 +5.6 3628927.413879 12040393.444519

2F‐19‐2 80 +22 3628580.262695 12041524.484497

2F‐19‐3 80 +27.1 3628160.644714 12042703.669678

2F‐19‐4 80 +30 3627833.209106 12044488.506531

2F‐19‐5 80 +28.2 3627565.003906 12046873.155273

2F‐19‐6 40 +9 3627456.970886 12048075.281921

2F‐19‐7 80 +27.6 3624077.136 12051341.067

2F‐19‐8 60 +20.1 3623518.495728 12052141.187683

3.2.2. CONE PENETRATION TEST (CPT)

Nine CPTs, designated CPT‐19‐1 through CPT‐19‐8, were conducted for the design of the Reaches 1 and 2 stabilization. The CPTs were conducted between 13 and 16 June 2019 in general accordance with ASTM D 5778 using a Savannah District Geoprobe rig and crew from a Norfolk District barge in the York River. The intent of the CPTs was to provide a 3‐dimensional soil profile, with one over‐water CPT for every 2 or 3 deep land‐based SPTs. Due to the time involved in moving and setting up the barge at each location and the shallow draft needed to get close to shore, the faster CPT method was used over water in place of the SPT. The CPT is a probe with transducers attached that is pushed into the ground at a constant rate as measurements of tip resistance, sleeve friction, and porewater pressure are taken at 0.07‐foot increments. Due to issues with the GPS system on the barge, some of the CPTs were not conducted in the planned location. Some of the CPTs encountered refusal before reaching total depth. When this happened, the barge was moved several feet and another attempt was made. We believe the presence of extensive shell fragments in the Yorktown Formation caused the refusal, which has been experienced on other projects in the area. The CPT holes were left to collapse. The field test data was reduced by a geologist with the Savannah District using established correlations. Table 3‐4 gives general information about the CPT probes.

Table 3‐4. 2019 CPT Exploration Information

Elevation (feet)

Northing Easting

CPT‐19‐1 42.1 +0.6 3629141.440125 12040246.774292

CPT‐19‐2 35.4 ‐1 3628248.623474 12042820.386108

CPT‐19‐2b 60.2 ‐1.6 3628350 12042792

CPT‐19‐3c 13.4 ‐1.5 3627717.373474 12046813.441711

CPT‐19‐4 41.9 ‐1.7 3627359.73468 12048657.191711

CPT‐19‐5 42.1 ‐0.8 3626554.179077 12049650.247314

CPT‐19‐6b 20.9 ‐1.6 3625349.317871 12050480.108276

Elevation (feet)

Northing Easting

CPT‐19‐7b 42.4 ‐0.4 3624196.540283 12051403.719482

CPT‐19‐8 47.1 ‐0.3 3622807.651306 12052973.163879

3.2.3. LABORATORY TESTING

Laboratory testing of selected samples was performed by the Savannah District Materials Testing Regional Center of Expertise lab in Marietta, Georgia. Table 3‐5 summarizes the testing program. Test results are in Enclosure 3. Test results have been incorporated into the boring logs in Enclosure 2.

Table 3‐5. 2019 Laboratory Testing Program

Test Reference Quantity

Gradation ASTM D422 40

Atterberg limits ASTM D 4318 40

Water Content ASTM D2216 40

3.3. VIRGINIA DIVISION OF GEOLOGY AND MINERAL RESOURCES (VDGMR) EXPLORATIONS

Three boring logs in the Reaches 1 and 2 area were obtained from the Virginia Division of Geology and Mineral Resources. These borings were drilled as part of a research program to locate a fault associated with the Chesapeake Bay impact crater. Drilling method, drill rig type, and sampling methods are not known. Table 3‐6 gives general information about these explorations.

Table 3‐6. VDGMR Exploration Information

Depth (feet) Elevation (feet)

Northing Easting

2015317‐1 50 21.6 3623516.177 12052107.271

2015317‐2 40 6.4 3628871.986 12040506.096

2015338‐1 25 11.9 3628756.191 12040901.348

4. SUBSURFACE STRATIGRAPHY

The subsurface conditions discussed in this section and those shown on the boring logs represent interpretations of the boring data and are for information only. The lines designating strata breaks on the boring logs and those indicated below represent approximate boundaries between soil types, as the transition may be gradual or may occur between samples.

The generalized subsurface stratigraphy below is our interpretation of the soil stratigraphy and subsurface conditions at the project site based on the subsurface explorations. The interpretation necessarily assumes uniform subsurface conditions across the site, which is probably, but not necessarily, correct. The general descriptions below should not be considered as a substitute for the boring logs.

It is uncertain whether the bluff areas in Reaches 1 and 2 are “natural” bluffs or whether those areas were filled for the construction of Colonial Parkway. The (retired) geologist who provided the VDGMR boring logs believes the entire area was once lowlands and the bluffs were created by deposition of dredged material from the York River (Berquist 2019). NPS personnel reported they found a photo taken in the Reach 1 area, believed to be from the mid‐1800’s, that showed uplands very similar to what it looks like today (Hardin 2019). Three USGS quadrangle maps (1906, 1952, and 2019) were obtained and examined. The maps are in Enclosure 4. The maps show that some fill was placed in the lowland area in Reach 1 on the east side of the mouth of Felgates Creek between 1906 and 1952. A larger amount of fill was placed in the lowland area on both the east and west sides of the mouth of Indian Field Creek in Reaches 1 and 2, and an unnamed creek in Reach 2 east of Indian Field Creek was mostly filled in between 1906 and 1952. This filling was likely done to reduce the spans of the bridges over Felgates and Indian Field Creeks and eliminate the need for a bridge over the unnamed creek for construction of Colonial Parkway. The bluff area in Reach 1 appears to be relatively unchanged between 1906 and 2019. It is not obvious whether the bluff area in Reach 2 changed between 1906 and 2019.

On the stick log profiles (Plates 10‐13) it is not possible to divide the layers into distinct stratums (Stratum 1, Stratum 2 etc.) that are distinguishable over all or even most of the area.

Therefore general soil types (sand, silty sand etc.) will be described. The statistics given for SPT and soil sample data (blow counts, percent fines, liquid limit etc) were obtained using both the 2012 and the 2019 borings. The 2012 CPT excel data files were not found on the Geo‐ Environmental server, so CPT tip resistance statistics were obtained from only the 2019 probes.

The predominant soil types within the project area are silty sand (SM), poorly graded sand (SP), and clayey sand (SC). Uncorrected SPT N‐values vary from less than 1 to 22 blows per foot (bpf) with an average of 7 bpf. The moisture content ranged from 6.4 to 36.5 with an average value of 21.9. Frequently although not always lower N‐values correspond to depths where groundwater was first encountered during drilling. The percent fines (percent passing the No.

200 sieve) varies from 4 to 47 percent with an average value of 18 percent. For tested soils with plastic fines, the Liquid Limit (LL) varies from 18 to 45 with an average value of 29 and the

Plasticity Index (PI) varies between 2 and 23 with an average value of 12. CPT tip resistance varies between 0.5 and 280 tons/ft2 .

Lean Clay and sandy lean clay (CL) occur in borings primarily in Reach 1. Uncorrected SPT N‐ values vary from 0 (weight of rod) to 18 bpf with an average value of 5 bpf. Note those statistics do not include one samples with what appeared to be an anomalous high blow count (26). Shell fragments in that samples could have artificially raised the blow count. Laboratory testing was only done on two samples of this soil so no statistics were calculated for index properties.

Fat Clay (CH) soils are found in locations over the entire project area. These are primarily the lowland areas near Felgates and Indian Field Creeks and the in bluff area of Reach 2. Fat clay in the lowland areas of Reach 1 occurs mostly below elevation ‐10 feet. Fat clay in the lowland area of Reach 2 near Indian Field Creek occurs below elevation ‐3 feet. Fat clay in the upland area of Reach 2 occurs between approximate elevations +4 and +17 feet. Uncorrected SPT N‐ values vary from 0 (weight of rod) to 13 bpf with an average value of 4 bpf. The moisture content ranged from 31.9 to 61.8 with an average value of 44.5. The percent fines (percent passing the No. 200 sieve) varies from 81 to 90 percent with an average value of 87 percent.

The Liquid Limit (LL) varies from 56 to 93 with an average value of 72 and the Plasticity Index (PI) varies between 37 and 63 with an average value of 51. CPT tip resistance for all clays varies between 0 and 65.8046 tons/ft2 with an average value of 7.7104 tons/ft2.

4.1. GROUNDWATER

Groundwater was encountered during drilling in most of the soil borings at elevations between +9.5 and ‐2 feet. The explorations were conducted in different years and at different times of the year. Groundwater levels fluctuate over time due to rainfall, nearby river stage, nearby irrigation, nearby well pumping, and other factors. The field measurements reported herein do not reveal the actual year‐round groundwater conditions.

5. SLOPE STABILIZATION DESIGN

5.1. ROCK REVETMENT

The existing rock revetment along the shore will be raised and some of the existing rock breakwaters will be rehabilitated. New rock sills will be constructed at some of the lowland areas. The rock revetment will be designed by the project Hydraulic engineer. Most of the rock revetment will have a crest elevation of +9 feet. This elevation was selected because it was the water level recorded in the York River during Hurricane Isabel in 2003. That water level has only been reached twice since 1933.

Only very large, infrequent storms will overtop the revetment.

The 2014 geotechnical report said the rock revetments and sills are expected to settle 1 inch during construction and 1‐2 inches long term after construction completion, but no documentation of how those values were determined has been found in the project records. In general, sands settle instantaneously, while clays consolidate slowly over time.

5.2. BLUFF AREAS

To prevent continued erosion from reaching the Parkway, the bluff slopes above the raised riprap will need to be stabilized. Given the poor performance of the anchored HPTRM near the K‐5 marker in Reach 2, erosion control matting is not considered to be a viable option. The NPS prefers a stabilization method that allows for planting of the slope after installation to preserve the scenic aspect of the Parkway. While the slope itself will need to be cleared and grubbed prior to installation, trees located between the edge of the Parkway pavement and the top of slope hinge point will be preserved to the extent possible.

5.2.1. STABILIZATION METHOD

After an internet search of slope stabilization treatments, the following products were selected for evaluation for potential use on this project:

Envirolok: Non-woven geotextile bags filled with soil, stacked, attached together with metal plates or stakes or binding straps, can be vegetated after installation

Evergreen Wall: Precast concrete crib wall system; blocks stacked, filled with soil, and planted (used at an NPS site in Hopewell, Virginia)

Keysystem II: Segmented block vertical retaining wall with geogrid reinforcement Rivel: Metal formwork facing with geogrid reinforcement and retaining bars, face can be vegetated Strataweb: Geocells placed on slope, held by short metal anchors, anchor trench at top of slope, face can be vegetated

Product brochures were reviewed by the Norfolk District Project Delivery Team (PDT) members and NPS personnel. Different product brochures contain different information, and none of the brochures addressed some concerns identified for this project (impact of storm drainage pipes through the protection, brackish water in the York River, potential mower impact damage). To evaluate the products using a standard set of information, a material questionnaire was developed and the product manufacturers/distributors provided the requested information. The product brochures were attached to the completed questionnaires and distributed to the team. The questionnaires are in Enclosure 5.

After extensive discussion among PDT and NPS personnel, the Envirolok system will be used over most of the bluff area. Because there is more than one supplier of similar systems, the generic name of “vegetated wall slope stabilization with geotextile bags” has been used on the plans and in the specifications. A typical vegetated wall slope stabilization with geotextile bags detail is shown in Figure 5‐1. It is anticipated the bags will be placed directly against the existing slope, with a small amount of fill placed behind the bags in localized areas to “smooth out” the slope where needed. The exception is the very steep area near the K‐5 marker in Reach 2, location of the failed HPTRM installation. The existing slope is too steep and too uneven for a standard vegetated wall slope stabilization with geotechnical bags installation. The Parkway is about 35 feet from the top of slope hingepoint and the existing slope is about 23 vertical feet high and steeper than 1H:1V, Also the slope cannot be cut back. Fill will be needed to create a shallower slope.

A full‐height steel sheetpile wall with tiebacks was considered, but NPS personnel did not want a vertical slope so close to the parkway for safety reasons. After discussion with a vegetated wall slope stabilization with geotextile bags distributor, a “hybrid” solution was developed where the lower half of the slope will be stabilized with a steel sheetpile wall with tiebacks. The upper half of the slope will be stabilized with the vegetated wall slope stabilization with geotextile bags. The slope between the top of the sheetpile wall and the base of the vegetated wall slope stabilization will be 10H:1V with a small drainage ditch and vegetated with grass. Figure 5‐2 is a detail of the “hybrid” solution.

Figure 5‐1. Typical Vegetated Wall Slope Stabilization With Geotextile Bags (and Rock Revetment) Detail

Figure 5‐2. “Hybrid” Solution Detail

Placing new fill directly against an existing slope without benching into the existing slope is generally not recommended because the contact between the new fill and the existing slope becomes a weak zone that is subject to slope instability, especially when wet. At this location, the existing slope is so steep that benching is not recommended for worker safety reasons. The sheetpile wall will provide stability for the fill. Additionally, the tieback for the sheetpile wall will extend through the fill into the existing slope. Sheetpile and tieback design is discussed in a later paragraph.

5.2.2. SLOPE STABILITY

Several sections were analyzed for slope stability within Reach 1 during the earlier work (using the computer program SLIDE) and are reported on and discussed in the 2014 geotechnical report. No stability analyses were conducted within Reach 2. For this study, a typical existing‐conditions section within Reach 2 was analyzed to determine the maximum slope where stabilization will be required.

Stability analysis was performed using the SLOPE/W computer program, part of the GEOSTUDIO 2019 program suite. The analysis consisted of running a circular search routine using the Spencer procedure which assumes all side forces have the same inclination and all requirements for static equilibrium are satisfied. To prevent the reporting of small, shallow surficial slip surfaces as the critical surface, the minimum slip surface depth was set at 10 feet and the failure surface entry and exit points were set such that the smallest failure surface would encompass most of the slope.

Since the existing slope has been standing for some time, the end‐of‐construction condition is not applicable and long‐term shear strength parameters were used for the fat clay layer. SPT N‐values and CPT tip resistances (qt) were used to determine the friction angles for the silty and clayey sand layers, using the “one‐third rule”. That rule states that strength parameters be selected such that one‐third of the data is lower than the selected strength and two‐thirds of the data is higher than the selected strength. The 33rd percentile of the N and qt values were used to determine the shear strengths. For the fat clay, the effective friction angle was determined from correlation with the plasticity index. For the existing riprap, typical values for that material were used. Adopted shear strength values are given in Table 5‐1 and the SLOPE/W output is in Enclosure 6.

Table 5‐1. Shear Strength Parameters, Reach 2

Material Unit Weight (pounds per cubic foot)

’ (degrees) C’ (pounds per square foot)

Unsaturated Loose Silty\Clayey Sand

115 28 0

Saturated Loose Silty\Clayey Sand

120 28 0

Medium Silty/Clayey Sand

120 33 0

Fat Clay 105 25 50

Riprap 135 37 0

The first slope analyzed was 2 horizontal to 1 vertical (2H:1V). Two analyses were conducted; one with the water level at the toe of the slope, which corresponds with high tide, and one with the water level 4 feet above the toe, which corresponds with a small storm event. The factor of safety are 1.3 and 1.298 respectively. EM 1110‐2‐1902, known as “the stability manual”, does not list recommended minimum factors of safety for riverbank slopes; that is typically set by the designer. For this project, a factor of safety of 1.3 is considered the minimum acceptable.

5.2.3. SLOPE STABILIZATION RECOMMENDATIONS

It is recommended that all bluff areas where the existing slope is steeper than 2H:1V be stabilized using vegetated wall slope stabilization with geotechnical bags, with the “hybrid” solution for the near‐vertical slope near the K5 marker in Reach 2. Based on survey data, the bluff stabilization will extend between approximate stations 12+20 and 72+50 in Reach 1 and between approximate stations 128+10 and 141+90 in Reach 2. There are a few short‐in‐length areas within those station ranges where the existing slope is 2H:1V or greater. However, “checkerboarding” of the bluff stabilization is not recommended due to the potential of concentrated erosion in the short areas between stabilization. The “hybrid” stabilization in Reach 2 will extend between approximate stations 128+30 and 131+35.

Over most of the bluff area it is anticipated that the vegetated wall slope stabilization with geotechnical bags will be placed against the existing slope with minor “smoothing” of the slope after clearing. It is also anticipated that riprap or a concrete or metal “flume channel” may be needed for protection at the outlets of storm drain pipes.

5.2.4. SHEETPILE/TIEBACK DESIGN

Design of the sheetpile wall was a joint effort with the Structural Design section. A representative cross section was provided to the structural engineer. The program CWALSHT was used to design the sheetpile. The sheetpile design from the structural engineer is as follows:

Top of Wall Elevation: 16.5 FT Anchor Elevation: 12.5 FT Anchor Angle: 0 degrees (horizontal) Wall Bottom Elevation: -18.5 FT Pile Length: 35 FT Pile Penetration: 21 FT Anchor Force: 9,037.4 lbs Anchor Spacing: 12.5 FT

Drilled and grouted anchor bars cannot be installed horizontally because the grout will tend to run out of the drilled hole. Helical anchors were selected for use because they can be installed horizontally.

Helical anchors consist of a central shaft with helical steel plates welded to the shaft and are installed by rotating (screwing) the anchor into the ground using standard drilling equipment. The helical plates transfer the load from the shaft to the surrounding soil. Solid square shafts are generally used for tension applications. The shaft and the helical plates vary in diameter and an anchor may have more than one plate depending on the load requirement. The first plate must be at least 3 plate diameters beyond the assumed failure plane. Each following plate is at least 3 diameters of the previous plate behind the previous plate to develop the maximum resistance.

Although a performance specification will be used for the tiebacks, a preliminary design was done in‐ house to aid in the preparation of the Government cost estimate. The individual plate bearing capacity method as presented in Missouri University of Science and Technology (MST) Design and Installation of Torque Anchors for Tiebacks and Foundations (undated) was used to estimate the number and diameter of the required helical plates as well as the minimum length of the anchor. Calculation sheets are in

Enclosure 7. Per EM 1110‐2‐2504, Design of Sheet Pile Walls, a factor of safety of 2.5 was used for steel anchor rods. The anchor spacing had to be reduced to 6 feet 3 inches to obtain an allowable anchor load for an anchor with four 14‐inch plates.

5.2.5. SHEETPILE WALL SLOPE STABILITY

The global stability of the sheetpile wall was evaluated using the SLOPE/W computer program. Two failure surface sizes were analyzed; a large circle cutting through the wall and a large circle underneath the wall. The initial run included the sheetpile wall modeled as a pile with the shear strength supplied by the structural engineer (115,000 lb) and the helical anchors modelled as an anchor with a “typical” shear strength obtained from manufacturers literature (90,000 lb). Every trial failure circle did not converge and no factors of safety were calculated. Reinforcements (which include both piles and anchors) are difficult for slope stability computer programs to handle because of the high structural forces. Removing the helical anchors produced the same result (no factors of safety calculated). The sheetpile wall shear strength was reduced to a very small value (10 lb) and the program calculated factors of safety with adequate convergence. Results of the stability analysis are given in Table 5‐2. All factors of safety are above 1.9.

Table 5‐2. SLOPE/W Results, Sheetpile Wall

Failure Surface Size Condition Factor of Safety

Large Circle Cutting Through Wall

End of Construction 2.275

Long Term 1.905

Large Circle Underneath Wall End of Construction 2.395

Long Term 2.281

5.3. LOWLAND AREAS

Stabilization of the short slopes in the lowland areas is anticipated to consist primarily of natural and nature‐based features (NNBFs) such as plantings, erosion control matting, oyster reefs, and living shorelines. Rock breakwaters will also be used. Design of these features will be performed by the project Hydraulic and Civil engineers.

6. CONSTRUCTION RECOMMENDATIONS

The Contractor will be warned in the specifications (Clearing and Grubbing and Earthwork) that the existing slope is unstable, particularly in the area of the “hybrid” slope stabilization. The Contractor will have to take care and cover this topic in the Activity Hazard Analysis that he will prepare as part of his Safety Plan.

Due to the relatively loose soils and the steep (<2H:1V) slopes to be stabilized, the Contractor shall clear but not grub the existing slope where the sheet pile wall and the vegetated wall slope stabilization with geotextile bags are proposed. At the location of the failed anchored HPTRM installation, the anchors will remain in place and the Contractor will cut and remove the mat between the anchor plates.

Clearing will be limited to 100‐foot long sections ahead of the vegetated wall slope stabilization with geotextile bags installation to minimize the amount of time the steep slope sections are exposed to the elements (rain, wind) between clearing and stabilization.

The preferred installation method for the bottom row of bags is to excavate into the existing ground.

Excavation is not allowed in identified cultural resources areas (shown as “Environmentally sensitive areas” on the plans). In those locations the bottom row of bags will be placed directly on the existing ground. The subgrade underneath the bottom row of bags, whether excavated or not, will be compacted with a minimum of 3 passes of a hand‐operated compactor. The logistics of working at the base of a slope near the water will preclude the use of vehicular compactors. The bottom row of bags will be placed with the long dimension perpendicular to the riverbank. Subsequent layers of bags will be placed with the long dimension parallel to the riverbank in an offset pattern similar to brickwork. The top row of bags will be placed with the long dimension perpendicular to the riverbank.

A vegetated wall slope stabilization with geotextile bags supplier recommended the installed stabilization system be sprayed with water so it starts out wet and then hydroseeding directly onto the bags. There is no potable water on site and the Contractor will have to supply water.

To reduce vibration levels at the near‐vertical slope, a vibratory or hydraulic‐push pile driver will be specified for the installation of the sheet pile wall. Impact hammers will not be allowed.

Backfill material behind the sheetpile wall and the vegetated wall slope stabilization with geotextile bags will be specified as VDOT aggregate 21B. The fill will have to be laid in shallow lifts (6 inches) and compacted with a minimum of 2 passes of a hand‐operated compactor because there is no access for trucks at the location of the hybrid option.

6.1. FUNDING CONSTRAINTS

NPS funded USACE Norfolk District with preparing plans and specifications for a complete design of Reaches 1 and 2. However, the NPS has limited funding for construction of this project. The Government estimate for the project features described in this report and shown on the project plans is higher than the available funding. The contracting mechanism will be a base contract with at least 2 bid options. NPS prioritized the work based on the Government estimate, available funding, and site conditions. The base contract will consist of rock revetment rehabilitation and vegetated wall slope stabilization with geotextile bags for 4,000 lineal feet within Reach 1 (approximate stations 32+20 to 72+20) as well as the “hybrid” option in Reach 2. Bid option 1 will consist of approximately 2,000 lineal feet of rock revetment rehabilitation and vegetated wall slope stabilization with geotextile bags

(approximate stations 12+20 to 32+20). Bid option 2 will be the breakwaters in Reach 2. Portions of the project not constructed now may be constructed at a later date if funding becomes available.

7. REFERENCES

Berquist, Craig, (retired) Virginia Division of Geology and Mineral Resources, personal communication, 12 March 2019.

Duncan, J.M. et al, Shear Strength Correlations for Geotechnical Engineering, Virginia Polytechnic Institute and State University Center for Geotechnical Practice and Research, August 1989.

Hardin, Lily, National Park Service, personal communication, 2019.

McFarland, E.R. and Bruce, T.S., The Virginia Coastal Plain Hydrogeologic Framework, U.S.

Geological Survey Professional Paper 1731, 2006

Missouri University of Science and Technology, Final Report, Design and Installation of Torque Anchors for Tiebacks and Foundations, undated.

Natural Resource Conservation Service Web Soil Survey, https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

Oaks, Robert Q. and Nicholas K. Coch, Post‐Miocene Stratigraphy and Morphology, Southeastern Virginia, Bulletin 82, Virginia Division of Mineral Resources, 1973

U.S. Army Corps of Engineers, Headquarters, EM 1110‐2‐1902, Slope Stability, 31 October 2003.

U.S. Army Corps of Engineers, Headquarters, EM 1110‐2‐2504, Design of Sheet Pile Walls, 31 March 1994.

U.S. Army Corps of Engineers, Norfolk District, York River Shoreline Stabilization – Colonial Parkway, Yorktown, Virginia, Geotechnical Report, June 2013

Virginia Department of Mines, Minerals, and Energy Interactive Geologic Map, https://www.dmme.virginia.gov/webmaps/dgmr/

PLATES

Yorktown

Jamestown

York River Williamsburg

Ballard Creek

Felgates Creek Indian Field Creek

Yorktown Naval Weapons Station k-0 k-18 k-17 k-24 k-25 k-26 k-27k-28k-30k-31k-32k-33k-34 k-35 k-36 k-15 k-11k-21 k-29 k-01k-02 k-04 k-07k-08k-10 k-13 k-12 k-14 k-16 k-19 k-20 k-22 k-23

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

³ 0 9,200 18,400 27,600 36,8004,600

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Legend K_markers

Colonial National Historic Parkway - York River

Plate 1. Colonial Parkway Map

York River Felgates Creek

Indian Field Creek

Yorktown Naval Weapons Station k-06 k-05 k-04 k-07 k-08k-09

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

³ 0 1,800 3,600 5,400 7,200900

Feet

Legend K_markers

Plate 2. Project Area - Reaches 1 and 2

Developed By: Geospatial Services Section, USACE Norfolk District via CorpsMap:

https://corpsmap.usace.army.mil/nao Email: geospatial@usace.army.mil

600m4002000

Date Printed: 10.21.2019 Map Scale: 1:27084.0

Geologic Map of Virginia

Plate 3. Geologic Map

Soil Map—James City and York Counties and the City of Williamsburg, Virginia

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

10/21/2019

37° 16' 53'' N

5'

5' ' W

37° 16' 53'' N

3'

2' ' W

37° 15' 39'' N

5'

5' ' W

37° 15' 39'' N

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N

Map projection: Web Mercator Corner…