ENGINEERING APPENDIX FLORIDA KEYS COASTAL STORM RISK MANAGEMENT DRAFT INTEGRATED FEASIBILITY REPORT AND ENVIRONMENTAL IMPACT STATEMENT MONROE COUNTY, FLORIDA APPENDIX B JUNE 2020 Army Corps of Engineers ~iiiii-• Norfolk District Florida Keys Coastal Storm Risk Management Draft Integrated Feasibility Report and Environmental Impact Statement GEOTECHNICAL SUB-APPENDIX JUNE 2020 U.S. Army Corps of Engineers ,_iiiiiiiiiw Norfolk District i | P age This page intentionally left blank ii | P age TABLE OF CONTENTS 1. INTRODUCTION 1 1.1 Area Description 1 2. PROJECT DATUM 2 2.1 Horizontal Control 2 2.2 Vertical Datum 2 3. PAST AND HISTORICAL STORM EVENTS 2 4. GEOLOGY, GROUNDWATER, AND SEISMICITY 2 4.1 Regional Geology 2 4.2 Local Geology 3 4.3 Topography and Drainage 4 4.4 Ground water 4 4.5 Seismicity 5 5. EXISTING FLOOD RISK MANAGEMENT STRUCTURES 6 5.1 General 6 5.2 Existing Revetment Design 6 5.3 Historical Performance 7 6. GEOTECHNICAL ASPECTS OF STRUCTURAL STUDY MEASURES 7 6.1 General 7 6.2 Proposed Preliminary Roadway Revetment Design 8 6.3 Typical Construction Sequence 9 6.4 Expected Subsurface Conditions 9 7. CONSTRUCTABILITY 10 7.1 Construction Adjacent To Existing Structures 10 7.2 Water Impacts 10 7.3 Traffic, Tourism and Life Safety Impacts 10 8. REFERENCES 12 i | P age LISTS OF FIGURES Figure 1.1 Project Area Map.........................................................................................1 Figure 5.1 Existing Revetment Repair Design Detail .....................................................7 LIST OF TABLES Table 4.1 Largest Historic Earthquakes near Monroe County .........................................5 LIST OF EQUATIONS Equation 1 Geotextile Fabric Retention Sizing...............................................................8 ATTACHMENT: Plate I. Geologic Quadrangle Map of Monroe County………………………………...14 ii | P age 1. INTRODUCTION This report presents the results of the geotechnical engineering evaluation of the area covered by the Monroe County Coastal Storm Risk Management (CSRM) study. The purpose of this report is to document the geotechnical existing condition of the study area and to provide geotechnical information in support of the final array of alternatives in the Feasibility Study. This study was authorized by Section 4033 of the Water Resources Development Act of 2007. “The Secretary shall conduct a study to determine the feasibility of carrying out a project for hurricane and storm damage reduction and flood damage reduction in the vicinity of Monroe County, Florida.” Monroe County, Florida is the non-Federal sponsor for the project. This report utilizes existing data gathered from a number of sources, including resources shared by the Florida Department of Transportation (FDOT) and Monroe County. 1.1 AREA DESCRIPTION The Monroe County Coastal Storm Risk Management (CSRM) Study is a study conducted by the U.S. Army Corps of Engineers, Norfolk District and the Local Sponsor, Monroe County, to reduce the Florida Keys’ risk of coastal storm damages and impacts to the year 2079. The Florida Keys are located in Monroe County. The Florida Keys include over 1,700 islands that stretch approximately 120 miles from Key Largo to Key West (Figure 1.1). ~ ~ Figure 1.1 Proje ct Are a Map 1 | P age Monroe County is located on the southern tip of Florida peninsula and encompasses the Upper, Middle and Lower Keys. The county is compromised of approximately 3,737 square miles, with water making up approximately 73 percent of that area. A reefline that parallels the island on the Atlantic side is the only living coral reef formation in North America. 2. PROJECT DATUM 2.1 HORIZONTAL CONTROL The horizontal datum for this study is tied to the State Plane Coordinate System using North American Datum of 1983. Distances are in feet by horizontal measurement. Coordinates are Florida South Zone. 2.2 VERTICAL DATUM As required by ER 1110-2-8160 all elevations provided herein are referenced to the North American Vertical Datum of 1988 (NAVD 88). 3. PAST AND HISTORICAL STORM EVENTS The Florida Keys sees a number of coastal storms throughout the year and is constantly flooded by numerous regional storms over the years. These include hurricanes, tropical storms, and northeasters. In addition, intense localized storms frequently cause localized nuisance street flooding. For more information on past and historical storms see the Main Report, Engineering Appendix, or the Hydraulics and Hydrology Sub-Appendix. 4. GEOLOGY, GROUNDWATER, AND SEISMICITY 4.1 REGIONAL GEOLOGY South Florida lies within the Southern Zone of the coastal lowlands of the Coastal Plain physiographic province of the eastern United States. Most of this province originated from a combination of depositional and erosional processes associated with fluctuations in sea level during the late Pleistocene. The Florida peninsula sits atop the Florida Platform that consists of a porous plateau of karst limestone. The basement rocks of the Florida Platform include Precambrian-Cambrian igneous rocks, Ordovician-Devonian Sedimentary rocks, and Triassic- Jurassic volcanic rocks. A thick sequence of mid-Jurassic to Holocence sediments lies unconformably upon the eroded surface of the basement rocks. In response to renewed uplift and erosion in the Appalachian highlands to the north and sea-level fluctuations, siliciclast ic sediments began to encroach upon the carbonate-depositing environments of the Florida Platform. Deposition of siliciclastic-bearing carbonates and siliciclastic sediments predominated from mid-Oligocene to the Holocene over much of the platform. Numerous disconformities that 2 | P age formed in response to nondeposition and erosion resulting from sea-level fluctuations occur within the stratigraphic section. Karst landscape features are typical in the region. Limestone Formations in the region are porous, allowing the acidic water to percolate through their strata, dissolving limestone and carrying it away in solution. Over time, this persistent erosional process has created extensive underground voids and drainage systems in much of the carbonate rocks throughout the state. Typical karst landscape features include sinkholes, caves, disappearing streams and bowl shaped topography. Other typical geologic hazard in the area can include moisture sensitive expansive clay soils and erosion from flooding and wind. 4.2 LOCAL GEOLOGY The generalized geologic map units in the project region are listed below with descriptions typically summarized from geologic quadrangle maps. The Geologic Map of Monroe County is found in Plate 1. Review of existing geotechnical investigations proximate to the proposed site indicated typically shallow overburden depths consisting of light brown to gray fine sand with occasional various amounts of silt and clay. Beneath the surficial materials, limestone was encountered typically at depths less than five feet below the existing ground surface elevation. The encountered limestone was typically described light brown to gray and tan in color, containing various amounts of silt and sand. 4.2.1 KEY LARGO LIMESTONE (QK) Quaternary aged coralling limestone composed of abundant specimens of small to large reef- forming corals, with the areas between the corals filled by reefal debris and numerous whole and fragmented fossils. The formation is exposed at the surface in the Florida Keys from Soldier Key on the northeast to Newfound Harbor Keys near Big Pine Key on the southwest. The Key Largo Limestone is a white to light gray, moderately to well indurated, fossiliferous, coralline limestone composed of coral heads encased in a calcarenitic matrix. Fossils present include corals, mollusks and bryozoans. The limestone is highly porous and permeable and is part of the Biscayne Aquifer of the surficial aquifer system. 4.2.2 MIAMI LIMESTONE (QM) Quaternary aged formation that is a grainstone to packstone, composed of ooids, pellets, skeletal grains and carbonate mud with a variable, but generally minor, quartz sand component. Induration varies from poor to good and the limestone is often recrystallized. The Miami Limestone occurs on the mainland and in the southern Florida Keys from Big Pine Key to the Marquesas Keys. The formation consists of two facies, an oolitic facies and a bryozoan facies. The oolitic facies consists of white to orangish gray, poorly to moderately indurated, sandy, oolitic limestone (grainstone) with scattered concentrations of fossils. The bryozoan facies consists of white to orangish gray, poorly to well indurated, sandy, fossiliferous limestone 3 | P age (grainstone and packstone). Beds of quartz sand are also present as unindurated sediments and indurated limey sandstones. Fossils present include mollusks, bryozoans, and corals. Molds and casts of fossils are common. The highly porous and permeable Miami Limestone forms much of the Biscayne Aquifer of the surficial aquifer system. 4.2.3 HOLOCENE SEDIMENTS (QH) Occur near the present coastline at elevations generally less than 5 feet. The sediments include quartz sands, carbonate sands and muds, and organics. 4.2.4 TAMIAMI FORMATION (TT) Contains a wide range of mixed carbonate-siliciclastic lithologies and associated faunas. It occurs at or near the land surface in the southern peninsula with numerous named and unnamed members recognized within the Tamiami Formation. Its unevenness indicates that the upper part has been subjected to erosion. The Tamiami Formation has highly permeable to impermeable lithologies that form a complex aquifer. Locally, it is part