B08_Section_J_Attachment_3_Geotechnical_Exploration.pdf

Prepared for:

Mr. Josiah M. Berg, PE

HDR

15450 New Barn Road, Suite 304 Miami Lakes, Florida 33014

Prepared by:

Mr. Uday Bhate, PE Bhate Geosciences Corporation 5217 5th Avenue South Birmingham, Alabama 35212

Geotechnical Exploration and Remediation Recommendations

May 2022

Fort Jefferson Counterscarp Repairs

Dry Tortugas National Park

BHATE Project Number: 121287

DRAFT

SUBJECT TO REVISION

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations

Bhate Geosciences Corporation Geotechnical, Materials, Environmental Engineers

May 7, 2022

Mr. Josiah M. Berg. P.E.

Ports Section Manager HDR Engineering, Inc.

15450 New Barn Road, Suite 304 Miami Lakes, Florida 33014

Subject: Geotechnical Exploration & Remediation Recommendations Fort Jefferson Dry Tortugas BHATE Project Number: 121287

Dear Mr. Berg:

Bhate Geosciences Corporation (BHATE) has completed a Counterscarp Condition Survey and Limited Geotechnical Exploration along the counterscarp at Fort Jefferson, located on Garden Key, Dry Tortugas National Park, Florida. Our services were performed in general accordance with BHATE proposal number 1881-21R dated October 8, 2021.

The purpose of our condition survey and limited geotechnical exploration was to determine general conditions of the counterscarp wall, foundation, and to the extent practicable, underlying bearing materials. Limitations to the condition survey and geotechnical exploration should be acknowledged. The condition survey documented readily apparent and observable conditions, and no coral, vegetation, or debris was disturbed or moved during the survey. Consequently, some existing features or conditions may have been obscured. The geotechnical exploration was conducted at a limited number of discrete points along the counterscarp, and material composition and condition varied along the alignment of the counterscarp. Consequently, features or conditions present between core locations were likely not encountered.

We appreciate the opportunity to be of service to you and look forward to assisting you as the project progresses. If you have any questions or need any additional information, please call us.

Respectfully submitted, BHATE GEOSCIENCES CORPORATION

Marc L. Langlois, P.G. (AL) Senior Consultant

Kevin A. Smith, P.G. (AL) Senior Engineering Geologist

Uday Bhate, P.E.

Senior Principal

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations

TABLE OF CONTENTS

EXECUTIVE SUMMARY

1.0 BACKGROUND

1.1 COUNTERSCARP CONSTRUCTION

2.0 SCOPE OF SERVICES

3.0 COLLINS ENGINEERS, INC. UNDERWATER INSPECTION REPORT

4.0 COUNTERSCARP CONDITION SURVEY

4.1 COUNTERSCARP CONDITION SURVEY

4.2 FRONTS 2 AND 3

4.2.1 Fronts 2 and 3 - Ocean Side

4.2.2 Fronts 2 and 3 - Moat Side

4.3 FRONTS 4 AND 5

4.3.1 Fronts 4 and 5 - Ocean Side

4.3.2 Fronts 4 and 5 - Moat Side

5.0 COMPARISON TO PREVIOUS CONDITION SURVEY

6.0 LIMITED GEOTECHNICAL EXPLORATION

6.1 FIELD EXPLORATION PROCEDURE

6.2 FINDINGS

6.3 DAMAGE FROM HURRICANE IRMA

7.0 CONCLUSIONS

8.0 REMEDIATION CONSIDERATIONS

8.1 DIRECTED ALTERNATIVE

8.2 CHEMICAL GROUTING

8.3 CEMENTITIOUS GROUTING

8.4 USE OF GEOTUBES

8.5 REPAIR OF BRICK FAÇADE

8.6 OCEAN-SIDE ARMORING

9.0 ENVIRONMENTAL CONSIDERATIONS

10.0 CLOSING

APPENDIX A

Figure 1 – Condition Inventory Map Figure 2 – Condition Photographs Figure 3 – Condition Photographs Figure 4 – Condition Photographs Figure 5 – Condition Photographs Figure 6 – Condition Photographs Figure 7 – Condition Photographs Representative Photographs of Counterscarp Conditions

APPENDIX B

Coring Location Plan Logs of Boring with Core Photographs

APPENDIX C

Avanti AV-330 Safeguard Technical Data Sheet Avanti AV-330 Safeguard Safety Data Sheet Avanti AV-330 Safeguard Water Quality Certificate

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

EXECUTIVE SUMMARY

Project

Fort Jefferson Counterscarp Condition Survey, Geotechnical Exploration & Remediation Recommendations

Scope of Services

• Document conditions of the counterscarp below the waterline o Field Observation Dates: February 1, 2022 through February 3, 2022 o General Weather Conditions: Overcast to clear, windy o Field Personnel: Marc L. Langlois, P.G. (AL)

Mike Ellison

• Geotechnical exploration to evaluate the condition and composition of the counterscarp wall, foundation, and underlying bearing materials.

o Field Observation Dates: March 22, 2022 through March 29, 2022 o General Weather Conditions: Mostly clear to clear, warm o Field Personnel: Kevin A. Smith, P.G. (AL)

Chris McGowan Joseph Midkiff

Summary of Findings – Condition Survey:

• We reviewed the previous (2019) underwater inspection report prepared by Collins Engineers as a baseline to conduct our condition survey and also to compare current change in conditions since the earlier inspection.

• Three major breaches extending through the counterscarp from the moat to the Gulf of Mexico were identified. Repair of these breaches should be considered Critical Repairs.

Locations are shown on Figure 1.

• The previously identified scour depression near the diesel fuel lines appears to have increased in size since 2019. This is based on comparison of Collins report and our current observations and measurements. This portion of the counterscarp is undermined and unsupported. The Collins report documented the scour depression as 15 feet in diameter.

At the time of our survey, the scour hole had increased to approximately 30 feet in diameter.

• There is loss of materials due to scour along the majority of the ocean side brick/foundation interface, resulting in one to two wythes of brick being unsupported. At multiple locations the subsurface mortar between the bricks has deteriorated, leaving gaps between bricks.

• A ledge on the moat side of the foundation, approximately 1’ wide and 6’ below the top of the counterscarp, was observed to extend along the majority of the length of the counterscarp. At some locations, remnant crenulations from the forms used during construction of the ledge were visible. We could not confirm the construction timeframe of the extension of cementitious material, but the extension may have been associated

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 with original construction of the counterscarp footing or a historical repair. However, our review of historical documents did not reveal use of corrugated formwork.

• Steel sheet piling is present on the ocean side of the counterscarp from approximately Stations 12+95 to 16+50, and Stations 17+20 to 21+00, but was not visible between Stations 16+50 and 17+20. Sheet piling was likely installed after original construction, as a means of stabilization. Based on the information provided to us, the construction sequence / timeline of sheet pile installation could not be determined. The steel sheet piling between Stations 22+80 and 21+00 appeared newer than the other sheet pile.

• We could not determine when the sheet piling was installed, but the condition of the sheet piling corresponds to the conditions described in the 2019 Collins report.

• Corals, sea fans, and other sea life has heavily colonized the ocean side of the counterscarp which could impact the repair methods on the ocean side face of the counterscarp.

Summary of Findings – Geotechnical Exploration:

• Our borings indicated that the counterscarp wall/foundation height (top of wall to bottom of coral concrete footing) ranged from 7’-4” to 11’-9”.

• Coral concrete used during original construction is highly degraded and crumbly.

Cementitious materials have been mostly to completely eroded out of some coral concrete sections.

• The coral concrete foundation is currently supported on unconfined unconsolidated sand which is susceptible to on-going scour and erosion.

• Remediation and stabilization of the counterscarp are needed for long-term stability and continued use of the counterscarp structure.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

1.0 BACKGROUND

Fort Jefferson is located on Garden Key in Dry Tortugas National Park, in the Florida Keys. The fort is constructed of brick and coral concrete, and is protected by a counterscarp of similar construction. Due to its age and the results of wear from the ocean and storms, the counterscarp has deteriorated and requires remediation / stabilization to permit long-term continued use.

1.1 COUNTERSCARP CONSTRUCTION

Materials referenced as part of our research pertaining to the Fort Jefferson Counterscarp include the following.

1. Underwater Inspection Report Fort Jefferson Counterscarp. Dry Tortugas National Park, FL - prepared by Collins Engineers, Inc. and dated March 2019.

2. Dry Tortugas National Park Geologic Resources Inventory Report, Revised April 2014 – Natural Resource Report NPS/NRSS/GRD/NRR-2014/809 – prepared by Rebecca Port, National Park Service Geologic Resources Division.

3. Fort Jefferson Counterscarp Repairs Survey – prepared by Atkins Engineers – Undated (survey conducted in March 2019).

4. National Park Services Cultural Landscapes Inventory – dated 2007

5. Historic Structure Report, Historical Data Section, Fort Jefferson 1846-1898, Fort Jefferson National Monument – prepared by Edwin C. Bearss and dated July 1983

6. NOAA Nautical Chart 11438 – dated November 2012

7. National Park Service, Dry Tortugas National Park – Fort Jefferson Counterscarp Alternatives Analysis – prepared by HDR and dated January 2020

8. NPS DRTO Draft Set for 4-5 Workshop – prepared by HDR and dated March 31, 2021

Based on our review of historical documentation the counterscarp was constructed using wooden sheet piles to place a foundation utilizing coral concrete. Once the foundation was laid, brick wythes were utilized to form the walls of the counterscarp. Once the wythes were erected on each side of the counterscarp the space between the wythes was filled with coral concrete. Once placement of the concrete was complete the counterscarp was capped with a layer of brick. The counterscarp structure is likely over 150 years old.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

2.0 SCOPE OF SERVICES

HDR Engineering, Inc. contracted with BHATE to perform the following scope of services related to proposed counterscarp repairs:

• Conduct an existing conditions survey of the counterscarp to assess the overall condition of the counterscarp from water surface to the bottom of the moat and/or sea floor.

• Advance exploratory cores into and below the counterscarp to evaluate the composition of the coral concrete and underlying bearing material.

• Develop a remediation plan to stabilize the counterscarp and foundation coral concrete and underlying bearing materials.

Details of our observations, geotechnical exploration, findings and conceptual recommendations for stabilization of the counterscarp are discussed in the following sections.

3.0 COLLINS ENGINEERS, INC. UNDERWATER INSPECTION REPORT

Prior to performing our condition survey and geotechnical exploration, we reviewed the Underwater Inspection Report Fort Jefferson Counterscarp Dry Tortugas National Park, FL, prepared by Collins Engineers, Inc., dated March, 2019. The stated purpose of the Collins report was to:

• Determine the condition of counterscarp components below the water surface

• Determine shoreline condition in close proximity to the counterscarp

• Provide photographic and video documentation of the counterscarp and significant defects

• Obtain material samples along the alignment of the counterscarp for testing.

The Collins report was used as a baseline for conditions observed during our fieldwork. In some cases, the conditions observed during our exploration were similar to those reported by Collins.

In other cases, continued erosion since submission of the Collins report has resulted in further degradation of the structure and scouring of bearing materials.

4.0 COUNTERSCARP CONDITION SURVEY

The counterscarp condition survey was conducted from February 1, 2022 through February 3, 2022.

• The condition survey was conducted by utilizing an underwater video camera to record the condition of the counterscarp from the water surface to the moat and/or ocean floor.

• Transects were done in approximately 10-foot sections. A PVC bar marked with in 1-foot sections was used to provide a photographic reference scale.

• The condition survey resulted in 735 individual photographs. The text of this report only documents typical conditions from each section of the counterscarp, along with significant

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 damage/defects identified. The bulk of the photographs are provided on an accompanying thumb drive.

4.1 COUNTERSCARP CONDITION SURVEY

To simplify references to areas of observed distress, the counterscarp condition survey is divided into four sections, based on documented Fronts. Fronts 2 and 3 generally face Southwest and West, and Fronts 4 and 5 generally face Northwest and Northeast. The following descriptions are divided as follows:

• Fronts 2 and 3 – Ocean Side

• Fronts 2 and 3 – Moat Side

• Fronts 4 and 5 – Ocean Side

• Fronts 4 and 5 – Moat Side

Representative figures and photographs of observed conditions are presented in Appendix A.

Illustration 1: Approximate Limits of General Condition Survey Adapted from overall site plan provided by HDR.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

A survey of conditions along the ocean side of the counterscarp was performed between the southwest sand embankment at approximately Station 4+60 and the northeast sand embankment at approximately Station 22+40. As anticipated, the ocean side of the counterscarp generally exhibited more deterioration and had a greater density of marine life than the moat side.

4.2 FRONTS 2 AND 3

4.2.1 Fronts 2 and 3 - Ocean Side

A large portion of the vertical face of the counterscarp base was exposed along Fronts 2 and 3.

The exposed portion of the counterscarp in that area has been subjected to scour and wave action, and continued exposure will result in continued erosion and general deterioration of the counterscarp foundation over time. Though some degradation was observed, the visible portions of the foundation generally appeared to be intact at the time of the survey. It should be noted that extensive growth of marine life prevented observation of portions of the foundation.

A high percentage of mortar joints were deteriorated, which weaken the integrity of the brickwork. However, growth of coral over large portions of the brickwork appears to have partially recemented the joints.

Significant scour was observed between the brickwork and the foundation of the counterscarp along virtually the entire length Fronts 2 and 3. The scour gap ranged from approximately 8 to 12 inches high, and extended 4 to 8 inches into the counterscarp. Our observations generally coincide with conditions reported in the 2019 Collins report. The referenced scour condition has resulted in loss of support of one to two wythes of brick along Fronts 2 and 3. The scour gap should be repaired in a manner that restores support to the overlying brick, and should be considered a Needed Repair1.

A major section of collapsed counterscarp was observed approximately between Stations 11+20 and 11+80. This breach allows wave motion through the gap in the counterscarp, and is beginning to undermine the ends of the intact portions of the counterscarp. Repair of the breach/collapse should be considered a Critical Repair1.

Because of the above-referenced collapse and a sluice gate located at approximately Station 12+85, a 105’ long segment of counterscarp is isolated between approximately Stations 11+80 and 12+85. Evidence of steel sheet piling was not observed along the isolated segment. The scour gap between the brickwork and foundation continued along this portion of the counterscarp. The scour gap ranged from approximately 6 to 8 inches high and extended 6 to 8 inches into the counterscarp. A vertical crack was present near the sluice gate. The crack was observed over the full cross section of the counterscarp, and widened downward from approximately 1/8” near the top of the counterscarp to 1.5” near the foundation ledge.

Additional armoring in this area is recommended to prevent additional future degradation.

1 For purposes of this report, Critical Repair is defined as a repair that should be performed within 6 months of submission of this report to reduce the risk of further degradation or future collapse. Needed Repair is defined as a repair that should be performed immediately after completion of Critical Repairs. Recommended Repair is defined as a repair that should be performed after completion of Needed Repairs.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

Marine Life Observations

Marine life is more prevalent on the ocean side than on the moat side of the counterscarp at this location. Corals and sea fans have colonized both the exposed foundation and brickwork, covering portions of the brickwork. Some of the corals have colonized the gap that is present between the brick work and the foundation. Repairs to the counterscarp would almost certainly require disturbance of the marine life on the ocean side of this portion of the counterscarp.

4.2.2 Fronts 2 and 3 - Moat Side

Counterscarp conditions along Fronts 2 and 3 from approximately Station 4+50 to 11+20 generally appeared to be intact, and similar to conditions described in the Collins report. Some localized areas of brickwork exhibited deteriorated mortar joints. While not an immediate repair need, re-pointing of the mortar will be needed to maintain the integrity of the brick wall. Based on our observations, a total of approximately 150 linear feet of the brickwork exhibited significant deterioration.

A breach through the lower portion of counterscarp, documented in the Collins report, was observed approximately between Stations 6+45 and 6+60. The approximate 15’ lateral dimension of the breach appeared to be consistent with the Collins report. However, due to continued scouring of sand below the foundation, the vertical dimension of the breach has increased from 5’ (Collins report) to 8’. Stabilization and repair of this scour should be considered a Critical Repair.

Water flow through the breach has resulted in a significant scour depression in the moat. The Collins report documented the scour depression as 15 feet in diameter. At the time of our survey, the scour hole had increased to approximately 30 feet in diameter.

A significant ebb and flow is present at this location from wave action through the void under the counterscarp. It is anticipated that the condition of the counterscarp and the size of the scour depression will continue to worsen. Stabilization and repair of this scour should be considered a Critical Repair. It appeared that two diesel fuel lines documented in the Collins report as being present at this location have been removed or relocated, as the PVC sheaths were empty.

A ledge on the moat side of the foundation was observed approximately 6’ below the top of the counterscarp. The ledge appeared to be comprised of cementitious material and was approximately one foot wide. Based on portions of the ledge in other areas having crenulations corresponding to corrugated formwork, ledges were likely related to a subsequent repair.

However, our review of historical documents did not reveal use of corrugated formwork.

The ledge exhibited minor deterioration, damage, or undermining along approximately 80% of its length along Fronts 2 and 3. Given the relatively calm water inside the moat, surface repairs of the ledge should be considered a Recommended Repair performed after Needed Repairs are made.

A major section of collapsed counterscarp was observed between approximately Stations 11+20 and 11+80. This breach allows wave motion through the gap in the counterscarp, and is beginning to undermine the ends of the intact portions of the counterscarp. A resulting scour depression is also present in the moat side of the counterscarp. The undermining of the

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 counterscarp and the scour depression will worsen over time. Repair of the breach in the counterscarp should be considered a Critical Repair.

As discussed in the previous section, the above-referenced collapse and a sluice gate located at approximately Station 12+85, have isolated a 105’ long segment of counterscarp approximately between Stations 11+80 and 12+85. The moat side of this isolated segment of counterscarp generally appeared to be intact, similar to the condition described in the Collins report. Mortar joints in the brickwork exhibited minor deterioration, and a vertical crack was present near the sluice gate. The crack widened downward from approximately 1/8” near the top of the counterscarp to 1.5” near the foundation ledge. Repair of the brickwork will be needed to maintain the integrity of the brick wall and should be considered a Needed Repair.

The counterscarp foundation was undermined at the collapsed end of the counterscarp, approximately Station 11+80. Due to the wave action/current through the collapsed area, this condition will continue to deteriorate. Repair of the breach/collapse should be considered a Critical Repair.

Marine Life Observations

Sea grass is present on the majority of the moat floor along Fronts 2 and 3, and localized areas of marine growth are present on the brick. The majority of the ledge was covered with marine life growing on it, though coverage was not continuous. Depending on conservation requirements pertaining to the marine life, repairs to the counterscarp could require specialized techniques to minimize disturbance to the marine life.

4.3 FRONTS 4 AND 5

4.3.1 Fronts 4 and 5 - Ocean Side

The counterscarp on the ocean side of Fronts 4 and 5 exhibits more deterioration than that observed on the moat side. Steel sheet piling was observed from the ocean floor to just below the brick/foundation interface along the majority of the ocean side of the counterscarp. The sheet piling was installed approximately 3-feet oceanward from the ocean-side face of the counterscarp, and the space between the sheet pile and the counterscarp has been filled with cementitious material. This has resulted in a 3’ wide ledge that extends outward from the counterscarp. The piling is present approximately between Stations 12+95 and 16+50. A deviation in ledge width was observed between Stations 16+50 and 17+20, where no evidence of sheet piling was observed and the ledge width was reduced from 3’ to approximately 1’. The deviated feature appeared to be constructed using similar cementitious material. Presence of similar sheet piling and 3’ ledge width resumed from Station 17+20 to Station 21+00. Sheet piling that appeared to be from a later construction episode was present from Station 21+00 to Station 22+80. The sheet piling and cementitious materials were likely installed in an effort to stabilize or repair deterioration of the counterscarp foundation, similar to the conditions observed along Fronts 2 and 3.

When the sheet piling was installed, and the ledge created, installation did not extend above the contact between brickwork and foundation. At the time of our condition survey, the ledge was approximately two feet below water surface. Consequently, the brickwork/foundation contact has remained exposed to wave action. A scour gap was observed along the entire length of the brickwork/foundation contact. In many areas the brickwork is damaged or missing above the contact. The scour gap ranged from 6 to 8 inches high, and extended 6 to 12 inches into the

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 counterscarp. The scour gap requires repair to stabilize the brick and prevent further loss of material from the face of the counterscarp and should be considered a Needed Repair.

Sheet piling along Fronts 4 and 5 generally appeared to be intact. Minor pitting and corrosion were present on the sheet piling, as noted in the Collins report, but overall structural integrity of the sheet piling did not appear to be compromised.

Marine Life Observations

Marine life is more prevalent along the ocean side of this portion of the counterscarp than in other locations. The ledge has been heavily colonized by corals and sea fans. In some areas the coral has colonized the entire width of the ledge. The sheet piling has also been colonized by marine life, though not as extensively as the ledge. Repairs to this section of the counterscarp would require significant disturbance of the marine life.

4.3.2 Fronts 4 and 5 - Moat Side

Counterscarp condition along the moat side of Fronts 4 and 5, between Stations 12+85 and 21+75 generally appeared to be intact and conditions were similar to those described in the Collins report. The brickwork exhibited minor deterioration of mortar joints that were present at and above the cementitious ledge that is present along the floor of the moat/counterscarp contact. Re-pointing of the mortar will be needed to maintain the integrity of the brick wall and should be considered a Recommended Repair.

A ledge at the base of the counterscarp was observed from the end of the counterscarp sluice gate to Station 14+05, and approximately between Stations 14+95 and 21+30. The ledge was not observed between Stations 14+05 and 14+95. The ledge appeared to be comprised of cementitious material and was approximately one foot wide. the vertically exposed ledge varied based on the depth to the floor of the moat. Though not observed beyond Station 21+30, the ledge may continue beneath the accumulated sand on the moat floor.

The ledge exhibited minor pitting and corrosion along the majority of the segment between Stations 12+85 and 21+75. However, the ledge generally appeared to be to be intact and immediate repairs are not critical. Given the relatively calm water inside the moat, repair of the ledge should be considered a Recommended Repair.

Crenulations on the vertical face of the ledge were observed approximately between Stations 15+05 and 16+45, and between Stations 17+05 and 18+55, indicating that corrugated formwork or similar material had been used in that area. This condition may be related to a historical repair. However, our review of historical documents did not reveal use of corrugated formwork.

A cast iron pipe was observed at approximately Station 16+80. The cast iron pipe extended through the cross section of the counterscarp. The pipe and immediately surrounding counterscarp appeared to be intact.

A “blind” void that did not extend all the way to the Gulf of Mexico was present approximately between Stations 18+25 and 18+35. The void was approximately 12 inches high and extended into the counterscarp approximately one to two feet.

An approximately 5’ long breach was observed approximately between Stations 18+03 and 18+08. The breach was approximately 2 feet high, and extended through the entire cross

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 section of the counterscarp. Without repair, this condition will continue to deteriorate. Repair of this breach should be considered a Critical Repair.

The Collins report documented as “Area of Concern D” a breach at the north corner of the counterscarp, approximately Station 18+50. The Collins report indicated the breach was approximately 15 feet long, 3 feet high, and extended through the entire cross section of the counterscarp. Collins identified this condition as “a significant structural concern”. Our condition survey identified severe degradation, generally consistent with the Collins description, on the ocean side of the counterscarp in this area.

An approximately 13’ long breach was observed approximately between Stations 21+10 and 21+23. The height of the breach ranged from approximately 4 to 8 inches, and extended through the entire cross section of the counterscarp. At the time of our survey, the void was occupied by a school of fish. Without repair, this condition will continue to deteriorate. Repair of this breach should be considered a Critical Repair.

Marine Life Observations

Sea grass is present to a lesser extent on the floor of the moat side along Fronts 4 and 5.

Localized areas of marine growth are present on the brick. The majority of the ledge had some form of marine life growing on it, even if it was not continuous. Depending on conservation requirements pertaining to the marine life, repairs to the counterscarp could require specialized techniques to avoid impacting the marine life.

5.0 COMPARISON TO PREVIOUS CONDITION SURVEY

Prior to conducting our Condition Survey, we reviewed the Underwater Inspection Report, Fort Jefferson Counterscarp, Dry Tortugas National Park, FL, prepared by Collins Engineers, Inc. and dated March 2019. During our condition survey we found similar conditions as identified in the Collins Engineering Report.

• The scour and associated void under the counterscarp listed as “Area of Concern A” in the Collins report is still present and the scour depression in the moat has increased since 2019.

• The collapsed portion of the counterscarp listed as “Area of Concern B” in the Collins report is still present and has not been repaired.

• The collapsed walkway listed as “Area of Concern C” in the Collins report is still missing and has not been repaired/replaced.

• Our survey of the area identified in the Collins report as “Area of Concern D” at the north corner of the counterscarp did not reveal a 15’ by 3’ void extending through the cross section of the counterscarp. We did observe a smaller void west of the north corner.

• The Collins report listed a culvert/pipe as being located 30 west of the north corner of the counterscarp. The pipe we observed was approximately 150’ from the north corner of the counterscarp. The pipe is also depicted as approximately 150’ to the west of the north corner on the ATKINS Topographic Survey (2019).

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

• The two voids identified in the Collins report as “Area of Concern E” were not located during our reconnaissance. The Collins report stated they were present at the mudline.

It is possible they were covered with sand or sea life and were obscured from view at the time of our observations.

• We identified a breach, approximately 15’ long, on the moat side of the counterscarp, near the northern beach area.

• The current condition of steel sheet piling appeared to be in similar condition to that documented by Collins, including the gap at the brick interface.

Findings from BHATE’s current condition survey are included in our Photo log, including captions describing conditions depicted in the photo.

6.0 LIMITED GEOTECHNICAL EXPLORATION

From March 23, 2022 through March 29, 2022, BHATE performed a limited geotechnical exploration at selected locations along the alignment of the Fort Jefferson counterscarp. Prior to exploration, proposed core locations were provided to, and subsequently approved by, the NPS compliance team.

Planned geotechnical exploration consisted of coring through the counterscarp wall and foundation, and into the underlying subgrade material, at twenty-one (21) core locations along the alignment of the counterscarp. However, based on field conditions observed during layout of core locations, it was determined that two (2) core locations would be omitted. Hole #1 was omitted because it fell within the narrower portion of the moat wall, within the beach area. Hole #9 was omitted because it was within the large breach on Front 3. Water level in the breach was too deep to safely perform exploration, and containment of the core cuttings would not have been possible.

6.1 FIELD EXPLORATION PROCEDURE

Geotechnical exploration was conducted by coring through the concrete footpath surface of the counterscarp, through the coral concrete foundation, and into underlying bearing material. Coring through the coral concrete wall and foundation permitted viewing and field evaluation of the concrete matrix and cementing materials, and coring into the underlying bearing material provided a general indication of the support capability of the subgrade.

Core locations were generally located along the approximate centerline of the footpath along the top of the counterscarp. However, field conditions such as degraded or missing surface concrete necessitated offsets of 3 to 6 inches in some locations. The bridge column probe location was drilled adjacent to the second column from the north, on the east side of the bridge.

Coring was performed with a portable, gas-powered core rig manufactured by Shaw Tool, and utilized a carbide-tipped core barrel. The coring process recovered samples measuring approximately 37 mm (1.4”) in diameter, and created an approximately 41 mm (1.6”) diameter core hole.

The coring operation was performed in the following sequence.

• Coring commenced from the surface of the footpath portion of the counterscarp.

• Due to the relatively poor quality of the concrete, core runs were generally limited to 1’ length.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

• Core runs were repeated to a typical maximum depth of approximately 15’ below top of wall.

Due to bearing material conditions below the foundation, some core locations were terminated at shallower depths.

• Cores were described in the field for each core location, and included descriptions of penetrated materials for which no sample was recovered. Core logs and photos are included in the appendix of this report.

• Recovered core samples were retained in wax-coated cardboard boxes. The majority of the cores were left with NPS representatives onsite. Selected, representative core segments were returned to our office in Birmingham, Alabama. Though no laboratory testing of cores was included in our proposed scope of services, the representative cores are available if the need for testing or further inspection arises during the design phase. If no testing or inspection is required, the samples can be returned to NPS at the end of the design phase.

• All completed core holes were backfilled with fast-setting mortar/hydraulic cement. Where possible, the upper segment of “modern” concrete was returned to the top of the core hole in an effort to minimize signs of exploration.

6.2 FINDINGS

Detailed Logs of Boring and photographs are included in Appendix B. The following sections summarize our findings.

• Counterscarp concrete penetrated during coring generally consisted of cementitious materials and varying amounts of coral fragments ranging from sand size to cobble size.

• Several areas of immediate concern were observed around the perimeter of the counterscarp, and included a collapsed section of counterscarp along Front 3, severe scouring under portions of Fronts 2, 3 and 4, and sections of collapsed brick façade along portions of Fronts 3, 4 and 5.

• Based on visual examination of recovered cores, the coral concrete used in original construction of the foundation and wall is highly degraded. At several core locations, most to all of the cementitious materials had eroded away, with only coral and/or sand present in the recovered samples. The samples of highly degraded coral concrete obtained during our exploration support the Collins laboratory findings. The Collins report indicated coral concrete compressive strength of 570 psi.

• Based on visual examination in the field of recovered core samples, interpretation of drill performance during coring, and general observations of the counterscarp, large segments of the counterscarp wall and foundation are highly eroded, and the loose subgrade sands have experienced significant scouring. Our observations validate the Collins assessment of overall counterscarp conditions being “poor to fair”.

• Based on the distribution of core locations along the counterscarp alignment, sixteen (16) core locations initially penetrated a “modern” concrete surface repair. We understand this repair was performed at some time in the 1960s and/or 1980s. Where present, the “modern” concrete ranged from approximately 5.5” to 10.75” thick.

• At all core locations, the counterscarp (below historic brick or modern concrete) was constructed of coral concrete. In most cases the concrete matrix was tan, but in some

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287 cases, the concrete matrix was gray near the base. Based on conversation with Park staff, this may be the result of variations in the concrete mixes used during original construction.

• Below the coral concrete counterscarp, coring penetrated loose sand with varying amounts of gravel size coral fragments, which extended to termination depth. Planned termination depth was 15’ below counterscarp surface. However, some core locations were terminated at depths shallower than 15’ because of rapid inflow of loose sand into the core holes during coring.

• Due to degraded conditions near the base of the wall at five (5) locations (core locations

C-2, C-3, C-6, C-11, and C-18), precise vertical wall height was difficult to determine. This was particularly true at C-18. Consequently, the reported coral concrete height at C-18 is an approximation based on drill performance during coring.

The following Table 1 summarizes total counterscarp height and final coring termination depth at each of the core locations. Counterscarp height was measured from the top of “modern” concrete, where present, to the base of the counterscarp foundation.

Hole Number Total Counterscarp Height Coring Termination Depth C-1 Omitted due to field conditions C-2 8’-10” 15’ C-3 9’-0” 13’ C-4 8’-4” 15’ C-5 10’-1” 15’ C-6 10-3” 15’ C-7 10’-3” 15’ C-8 8’-11” 11’ C-9 Omitted due to field conditions C-10 9’-8” 15’ C-11 11’-9” 15’ C-12 7’-7” 13’ C-13 8’-4” 11’ C-14 7’-4” 11’-6” C-15 7’-9” 12’-3” C-16 8’-11” 11’ C-17 7’-8” 17’ C-18 8’ to 9’ estimate 10’ C-19 9’-10” 15’ C-20 10’-2” 15’ C-21 Probed within moat to 15’7” below top of bridge (5’-7” into moat floor) Table 1: Summary of Counterscarp Height and Coring Termination Depth

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

6.3 DAMAGE FROM HURRICANE IRMA

Several sections of brick façade, on the order of 4’ x 20’, located along Fronts 4 and 5 have broken away from the concrete core of the counterscarp, and are currently lying on the ocean floor near the base of the structure. A corresponding section of modern concrete surface pavement on Front 4 has also been washed into the moat. According to Park Maintenance and Mechanical Supervisor, Mr. Tree Gottshall, a significant portion of the façade collapse in those areas occurred during Hurricane Irma in September, 2017.

Erosion of the coral concrete exposed as a result of impacts from Hurricane Irma is similar to other exposed segments of coral concrete.

7.0 CONCLUSIONS

1. Exploration confirmed that the coral concrete used in original construction of the counterscarp wall and foundation was highly porous and highly degraded. At several core locations, cementitious material had been mostly or totally eroded from the retained sample. The concrete was likely degraded by many years of continuous wave and storm action.

2. Based on our findings, it appears the entire counterscarp foundation is supported on loose sand mixed with coral fragments. All core holes were terminated in the sand layer because at many core locations, the sand collapsed into the hole during coring. The depth of sand penetrated below the bottom of the footing ranged from approximately 2’ to 9’.

Degradation of the counterscarp, including collapsed sections, is likely due to a combination of highly-porous, weakly cemented coral concrete, and shifting sands below the structure.

3. Remediation and stabilization of both the counterscarp and the underlying loose-sand support material will be necessary for continued long-term functionality of the structure.

4. Illustration 2 identifies critical and recommended repair areas of the coral concrete counterscarp. It should be noted that conditions between core locations have potential for significant variation over relatively short lateral distances. Consequently, the indicated boundaries of repair areas should be considered approximate.

5. Areas shaded red indicate existing collapse, breaches, and/or an advanced degree of degradation, and represent areas in critical need of stabilization and repair. Critical Repair areas should be repaired as the first priority.

6. Areas shaded blue indicate Needed Repair areas that require remediation, but degradation is not as severe as in areas shaded red. Needed Repair areas include areas currently exhibiting moderate to significant ocean-side scouring and areas where core samples indicated moderate to significant erosion of cementitious materials from the coral concrete. These areas should be repaired after repair of Critical Repair areas.

7. Areas shaded yellow indicate Recommended Repair areas that currently exhibit minor to moderate degradation on the ocean side of the wall, and core samples contained relatively in-tact core segments and had high core recovery. Recommended Repair areas should be repaired after repair of Needed Repair areas.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations

Illustration 2: Critical Repair, Needed Repair, and Recommended Repair Areas

Adapted from overall site plan provided by HDR.

8.0 REMEDIATION CONSIDERATIONS

BHATE’S findings indicate that remediation should be undertaken promptly to reduce the potential for continued degradation. Without remediation and stabilization, water migration through and under the structure will continue to contribute to degradation of the counterscarp, due to removal of cementitious materials within the counterscarp and mobility of the underlying sand.

Remediation of both highly porous concrete and loose granular foundation materials can be accomplished by targeted, methodical placement of chemical or cementitious grout.

8.1 DIRECTED ALTERNATIVE

HDR originally presented to NPS eight (8) alternative action options for remediation of the counterscarp. Proposed alternative actions ranged from no action to complete in-kind replacement of the structure. We understand that after review of the remediation alternatives with NPS, a “hybrid” directed alternative was selected by which minimal ocean-side stabilization with rip rap would be performed in areas not significantly affected by coral growth. Additionally, counterscarp foundation stabilization would be performed as required.

Critical repair area.

Severe degradation of coral concrete.

Immediate repair needed.

Needed repair area.

Significant degradation of coral concrete.

Repair needed.

Recommended repair area. Minor to moderate degradation of coral concrete.

Repair needed.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations

Based on our current geotechnical findings, the following recommendations are provided in support of the directed alternative.

8.2 CHEMICAL GROUTING

Remediation/repair could be accomplished along the majority of the counterscarp wall, foundation, and underlying sand by injection of chemical grout. The low viscosity of chemical grout at the time of injection would allow permeation into the porous concrete.

To achieve adequate lateral and vertical grout coverage within the counterscarp or sand zones, multiple injection points would be required, and multiple vertical zones would be targeted within each injection point. Because tight control of injected volume and pressure per injection point would be required, the injection pattern would likely include two rows of points along the alignment of the counterscarp, one row being inset approximately 1.5’ to 2’ from the ocean-side edge of concrete and the second row inset a similar distance from the moat-side edge of concrete. Spacing along the length of the counterscarp would likely need to be on the order of 3’ to 4’. The resulting grid of injection points would permit a qualified specialty geotechnical grouting contractor to execute a well-controlled, accurately documented remediation program.

Chemical grout remediation and repair work should be performed by a qualified specialty geotechnical grouting contractor. The grouting contractor would develop means and methods for executing the repair program, including:

• Develop required project documents, including drawings to identify injection pattern, staging and wash-out areas, grout curtains and other containment devices, etc.

• Determine equipment and tooling to be used. Chemical grout injection equipment is generally compact and highly portable.

• Select and submit for approval the grout materials to be used.

• Develop project schedule.

Remediation by chemical grouting involves injection of a catalyzed, hydrophilic (strong affinity for water) or hydrophobic (little or no affinity for water), polyurethane, acrylate, or similar resin into the area to be remediated. A polyurethane resin would be activated by interaction with water, and would expand to create a closed-cell foam within the concrete pores or within the loose sand mass. An acrylate grout would be used to fill the pores within the concrete and permeate the sand mass, and cure to a semi-rigid foam or gel.

Reaction time, or the time required to cure from a low-viscosity fluid to a solid or gel, could be controlled to several seconds to 1 minute (or longer), depending on the specific formulation utilized. Also depending on the particular formulation utilized, the hardened resin/foam could be significantly harder and more durable than the existing counterscarp materials and supporting sand in their current state.

If remediation by chemical grouting is performed, a containment curtain or formwork located immediately adjacent to the ocean- and moat-sides of the counterscarp would be required.

Chemical grout injection would not be appropriate for remediation of large breaches or collapsed areas. Remediation of large, open areas such as breaches and collapsed areas would require sheet piling or temporary formwork and placement of low mobility cementitious grout or concrete.

Fort Jefferson Counterscarp Repairs Dry Tortugas National Park Geotechnical Exploration & Remediation Recommendations BHATE Project Number: 121287

8.3 CEMENTITIOUS GROUTING

As an alternative to chemical grouting, remediation/repair could be accomplished along the entire counterscarp wall, foundation, and underlying sand by injection of a combination of Low- Mobility Grout (LMG) within the loose-sand support material, followed by injection of High- Mobility Grout (HMG) within the counterscarp structure.

Injection of cementitious grout would involve similar spacing of injection points and project execution as chemical grouting. However, the LMG phase would require slightly larger equipment, tooling and retaining formwork than HMG or chemical grout injection. Set times for cementitious grout would be substantially longer than chemical grout, and would be measured in many minutes to hours. Increased set time of cementitious grout would likely add significant time to project duration, compared to chemical grouting.

Cementitious grout remediation and repair work should be performed by a qualified specialty geotechnical grouting contractor. The grouting contractor would develop means and methods for executing the repair program, similar to execution of a chemical grouting program.

LMG injection would utilize regular Portland cement/sand/bentonite grout, and would be performed with the objective of strategically displacing and/or densifying the loose sand underlying the counterscarp. Undermined “voids” under the counterscarp could be filled with LMG from the surface, via the injection point, to reestablish contact between the structure and the foundation materials. Installation of temporary formwork would be required in many scour and breach areas. Relocation of coral or vegetation may be required in areas requiring installation of formwork.

HMG injection would utilize micro-fine or ultra-fine cementitious grout, and would be performed with the objective of filling the open pore space within the counterscarp structure. The fine-grain grout particles would be well-suited for limited migration through the structure. Viscosity modifiers and anti-washout admixtures are also available to enhance placement and durability properties of the grout.

Remediation of the counterscarp and bearing materials by injection of cementitious grout could significantly increase the strength of the structure, depending on the formulation of the grout utilized, and may provide sufficient improvement to support the future addition of limited additional wall height in order to provide additional protection of the fort from adverse effects…