6. PO-75 Geotechnical Report Revised 5.26.2017.pdf

Geotechnical Engineering Services Revised May 26, 2017 LaBranche East Marsh Creation Project (PO-75) St. Charles Parish, Louisiana for

US Department of Agriculture Natural Resources Conservation Service

May 26, 2017

Geotechnical Engineering Services

Revised May 26, 2017

LaBranche East Marsh Creation Project (PO-75) St. Charles Parish, Louisiana for

US Department of Agriculture

Natural Resources Conservation Service

May 26, 2017

11955 Lakeland Park Boulevard, Suite 100 Baton Rouge, Louisiana 70809 225.293.2460

May 26, 2017| Page i File No. 10883-009-03

Table of Contents

INTRODUCTION

PROJECT UNDERSTANDING

PURPOSE AND SCOPE OF SERVICES

EXISTING INFORMATION

LaBranche Wetlands Restoration Project (PO-17) Pipelines 2010 Survey Data LaBranche Shoreline Protection Project (PO-3B) PO-75 Pilot Project Survey Data (08/2013 – 08/2014)

SITE CONDITIONS

Project Location Surface Conditions Subsurface Conditions

Subsurface Marsh Stratigraphy Borrow Area Stratigraphy

CONCLUSIONS AND RECOMMENDATIONS

General Marsh Creation

Calibration of Model Using Pilot Project Results Settlement – Single Lift Settlement – Multiple Lifts Hydraulic Fill Containment

Borrow Area Borrow Area Consistency Hydraulic Dredging Fill-to-Cut Ratio

CONSTRUCTION CONSIDERATIONS

LIMITATIONS

Page ii

LIST OF FIGURES

Figure 1. Vicinity Map Figure 2. Site Location Plan with Soil Borings Figures 3A through 3C. Subsurface Profile Sections Figure 4. Pilot Project Model Calibration Figure 5. LaBranche Containment Alternatives

APPENDICES

Appendix A. Reference Information Appendix B. Not Included (Refer to November 15, 2010 Report) Appendix C. Not Included (Refer to November 15, 2010 Report) Appendix D. Design Profiles Figures D-1 through D-7. BHMC Design Parameters (Full Scale Project) Figure D-8. BHMC-PS1 and -PS2 Design Parameters Appendix E. Settlement Analyses for Marsh Fill and Foundation Soils Settlement Calculations for Marsh Fill and Foundation Soils Figures E-1a through E-7b - Marsh Fill and Foundation Soils Time Rate of Settlement Appendix F. Slope Stability Analyses for Containment Dikes Slope Stability Calculations

Figures F-1 through F-5 - Earthen Containment Dike Slope Stability Evaluations Appendix G. Alternate Containment Feature Design Appendix H. Marsh Creation Fill to Cut Ratio and Bulking Factor Calculations Settling Column and Self Weight Consolidation Test Results Appendix I. Report Limitations and Guidelines for Use

May 26, 2017| Page 1

INTRODUCTION

This report presents the results of GeoEngineers, Inc.’s (GeoEngineers) revised geotechnical engineering services for the LaBranche East Marsh Creation Project (PO-75) located in St. Charles Parish, Louisiana.

Our original services were completed under the United States Department of Agriculture - Natural Resources Conservation Service (NRCS) contract AG-7217-C-10-003, task order AG-7217-D-10-0041, dated June 30, 2010. This revision, authorized under NRCS contract AG-SPEC-C-17-0007, was commissioned to update marsh creation settlement calculations to reflect the results of the PO-75 pilot project, which constructed and monitored three small marsh areas at the north end of the PO-75 project area. Surveys of the marsh surface, water levels in and outside of the marsh areas, and settlement plates over a one year period post-construction provided site-specific settlement measurements.

The LaBranche East Marsh Creation Project is located in an area of open water between Lake Pontchartrain and Interstate 10 approximately 2 miles east of where the Bonnet Carré Spillway discharges into the lake, as shown in the Vicinity Map, Figure 1. The open marsh area will be filled with soil dredged from Lake Pontchartrain near where the Bonnet Carré spillway discharges into the lake, as also shown in Figure 1.

Relevant site features are shown in Figure 2. Figures are located at the end of the report, between the report body and appendices.

Unless otherwise noted, all elevations discussed in this report are referenced to the North American Vertical Datum of 1988 (NAVD88).

PROJECT UNDERSTANDING

Our understanding of the project is based on the June 11, 2010, NRCS Proposal No. 4 scope of services, subsequent communications, a June 18, 2010 site visit with NRCS representatives, field exploration, laboratory testing results, and analyses to provide design recommendations for the full-scale project (geotechnical report dated November 15, 2010) and pilot project (geotechnical report dated December 1, 2011). Our understanding of marsh creation construction processes at the PO-75 site was further improved through discussions with NRCS and a review of the pilot project survey results. The pilot project was completed in August 2013 to test the feasibility of the full-scale project.

We understand that this project will involve creation of approximately 729 acres of interior marsh and nourishment of approximately 202 acres of existing marsh along the perimeter of the designated marsh creation area using a hydraulic dredge to pump dredged bottom material from Lake Pontchartrain to this marsh creation area. The initial proposal indicated that non-continuous low level earthen containment dikes were planned to manage the dredge fill where there is existing marsh to build on; however, based on results presented later in this report, it appears that continuous dikes or other structures may be required.

Existing water openings/channels around the perimeter of the marsh creation area will be closed using earthen dikes, or alternatively engineered structures.

We understand that the 3-year target marsh elevation has been revised from the 1.3-foot elevation stated in Proposal No. 4 to a new design elevation of 1.0 foot which is the target elevation that was used for this report.

May 26, 2017| Page 2

PURPOSE AND SCOPE OF SERVICES

The purpose of our services was to perform geotechnical analysis as a basis for recommendations for design and construction of the proposed marsh creation. A discussion of our field and laboratory program supporting the PO-75 full-scale and pilot projects is contained in the November 15, 2010 and December 1, 2011 geotechnical reports. Field exploration and laboratory test results contained in those reports will not be repeated in this revised version of the report. As a result, Appendices B and C have not been included in this revision, but appendix identification from the November 15, 2010 report has been maintained for consistency. Our specific scope of services included the following:

1. Evaluated subsurface data and identified appropriate design profiles representing different sections along the project alignment.

2. Completed the following evaluations for each appropriate design profile:

a. Earthen Containment Dikes:

i. Stability analyses to determine stable dike side slopes and minimum berm width between the dike and borrow excavation, and

b. Marsh Creation:

i. Settlement analyses to evaluate the initial marsh elevation required to meet a 3-year target elevation of +1.0 foot, and

ii. Marsh settlement versus time over a 20-year period for a range of initial fill elevations from elevation +2.0 feet to elevation +4.5 feet.

iii. Marsh settlement versus time over a 20-year period for initial fill elevations of +3.0 and +3.5 feet with mudline elevations at -14 and -7 feet to model effects of fill in containment dike borrow excavations and channels.

3. Designed at least two different types of structures to close six breaks in the marsh creation area perimeter where no natural shoreline exists to contain the marsh fill, as shown in Figure 2.

4. Provided general construction recommendations.

5. Prepared this report of findings, including recommended earthen containment dike geometry, estimated marsh settlement, containment structure recommendations, and fill to cut ratios.

EXISTING INFORMATION

LaBranche Wetlands Restoration Project (PO-17)

The NRCS provided GeoEngineers with a copy of the 1995 as-built drawings for the adjacent LaBranche Wetlands Restoration Project (PO-17) completed by the US Army Corps of Engineers (USACE). These drawings are included in Appendix A. The limits of the wetland creation area and associated borrow area are shown in Figure 2. Based on the dredging limits shown in the as-built drawings, one of the soil borings completed for this investigation was within the previously dredged area.

Pipelines

Pipeline locations were provided by the NRCS and Air Products (Air Products responded to the One-Call notification). The approximate locations of known pipelines based on the information provided to GeoEngineers are shown in Figure 2. Copies of the reference documents showing the pipelines are included in Appendix A.

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2010 Survey Data

The NRCS contracted with T. Baker Smith, Inc. (TBS) to perform surveying for this project. TBS marked the soil boring locations and surveyed the mudline elevation at each soil boring location. This information is shown on the soil boring logs. In addition, TBS also surveyed sections at each break in the perimeter of the marsh creation area that will require some type of structure to contain the hydraulic fill. This information is included in Appendix A.

LaBranche Shoreline Protection Project (PO-3B)

GeoEngineers predecessor company (Louis J. Capozzoli & Associates, Inc.) completed a geotechnical investigation and design for shoreline protection along Lake Pontchartrain on the northern limits of the LaBranche East Marsh Creation Project, as shown in Figure 2. This investigation and report was completed in 1991 under contract with John E. Chance & Associates for the Louisiana Department of Natural Resources (LDNR). The geotechnical design report is included in Appendix A; however, GeoEngineers was not involved with the construction and does not have as-built records.

PO-75 Pilot Project Survey Data (08/2013 – 08/2014)

The NRCS provided GeoEngineers with results from TBS’s pilot project surveys and select as-built construction surveys completed by HydroTerra Technologies, LLC. TBS completed monthly surveys starting shortly after completion of pilot project dredge filling and continuing for one year after dredge filling was complete. Results from the September 10, 2014 draft report include short, tightly spaced transects adjacent to the project grade stakes, and elevation surveys each cell’s settlement plate.

Based on 2013 communications with NRCS representative Jason Kroll, we understand at least one of the settlement plates was toppled and reset around the end of construction. Based on an approximately 2.5-foot elevation survey discrepancy between the HydroTerra construction survey and the TBS Event 1 monitoring survey of the Cell 1 settlement plate, we suspect this settlement place was overturned and reset around the end of construction. Because of this discrepancy, it is difficult to discern how much of the settlement shown by this plate is attributable to the foundation and how much is within the fill in Cell 1.

SITE CONDITIONS

Project Location

The project is located at the southwestern corner of Lake Pontchartrain in St. Charles Parish, Louisiana.

The marsh creation area is an area of open water between Lake Pontchartrain and Interstate 10 approximately 2 miles east of where the Bonnet Carré Spillway discharges into the lake, as shown in Figures 1 and 2. The marsh creation borrow area is in Lake Pontchartrain near where the Bonnet Carré spillway discharges into the lake, as also shown in Figures 1 and 2.

Surface Conditions

The surrounding area is generally undeveloped but there are several significant features as listed below.

■ Interstate 10 and the Canadian National railroad exist south-southwest of the borrow and marsh creation areas. The clearance beneath the interstate and the railroad wooden trestle will limit the size of equipment that can be launched from the public launches at the Bonnet Carré spillway.

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■ As shown in Figure 2, there are several pipelines, overhead power lines, and other features in and around the marsh creation and borrow areas.

■ The strip of land separating the shallow, open-water marsh creation area from Lake Pontchartrain has a rock protection dike that GeoEngineers assumed will remain intact.

On June 18, 2010, representatives of GeoEngineers and NRCS toured the proposed project area by airboat.

Based on our observations and discussions with the NRCS representatives during the visit and observations during subsequent work, we offer the following information regarding site conditions.

Water depths in general ranged from approximately 4 to 13 feet at soil boring locations BHBA-1 through BHBA-8, and from 1.3 to 2.2 feet at soil boring locations BHMC-1 through BHMC-8. Weather, wind, and tides can substantially affect water levels. Wind can make the lake water rough and difficult to operate equipment that is sensitive to wave action.

Within the marsh creation area, duck blinds and crab traps were visible throughout the open water area.

Subsurface Conditions

The field exploration and laboratory testing programs performed by GeoEngineers for this project are described in the following paragraphs.

Subsurface Marsh Stratigraphy

A subsurface profile based on our soil borings in the marsh creation area is shown in Figures 3A. Based on review of the subsurface data, seven design profiles were developed. The soil borings used for each profile are identified on our graphical design profiles in Appendix D. Soil borings BHMC-7 and BHMC-8 were grouped together due to similarities in the soil profile in these borings.

Although undetected anomalies, (sand layers, logs, etc.) beyond the soil borings may exist, the generalized subsurface conditions can be described as follows and is also shown in Figure 3A. A 5- to 12-foot thick layer of peat was encountered in every soil boring starting at the mudline. Below the peat, we encountered between 4 and 16 feet of organic clay in every soil boring, followed by clay intermixed with silt and sand seams or layers, sometimes several feet thick. Except for soil boring BHMC-4, all soil borings terminated in this layer at about 30 feet below the mudline. In soil boring BHMC-4, starting about 50 feet below the mudline the silt and sand seams were no longer observed and a soft to medium gray clay was encountered to the soil boring termination depth about 80 feet below the mudline. Based on USACE maps (Figure 3B), it was assumed that sand and stiff overconsolidated clay (i.e. Pleistocene deposits) would be encountered just below the termination depth of soil boring BHMC-4, at elevation -85 feet.

The peat and organic clay encountered in soil borings BHMC-1 through BHMC-8 control the design for this project and are significantly different from the soils encountered for the LaBranche wetlands restoration project completed to the west of this project in 1995 (PO-17). Appendix D contains the assumed design profiles (shear strength, unit weight, and moisture content) for the seven profiles, based on the laboratory and field testing results from the 2010 full-scale project study, along with an eighth profile based on combined soil properties from pilot project cells 1 and 2 (soil borings BHMC PS1 and PS2). As presented in these profiles, typical peat moisture content in the proposed marsh creation area is between 500% and 730%, with several specimens measured at more than 800%. In organic clay beneath the peat typical moisture contents ranged from 120% to 340%.

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Based on the data contained in the USACE documentation for project PO-17 (Appendix A), the highest water content encountered in the soil samples for the completed project to the west was on the order of 200% to 300%, and the average water content was in the 100% to 200% range.

The difference in moisture content is critical to understanding the results for this project. Even though this project (PO-75) is immediately adjacent to project PO-17, the results may not be the same as project PO-17. As will be discussed later, settlement/compression and movement of the peat and organic clay complicate the construction process.

Figures 3A and 3B show detailed soil profiles through the marsh creation area. Appendix D presents the design soil properties vs. elevation based on the field and laboratory investigation.

Borrow Area Stratigraphy

The three western-most soil borings (BHBA-1 through BHBA-3) encountered predominantly clay based on soil behavior characteristics; however, grain size testing indicates that the clay material contains a significant amount of silt. The remaining soil borings (BHBA-4 through BHBA-8) predominantly encountered interbedded layers of silt, sand, silty sand, sandy silt and clay. Shells were present in many of the soil borings, both at the mudline and deeper. Figure 3C shows the soil profile through the borrow area and detailed soil boring logs are provided in Appendix B of the November 15, 2010 report. Elevations presented in this report are NAVD88; however, elevations shown in the as-built dredged borrow area in Figure 3C are referenced to the National Geodetic Vertical Datum (1929) and are approximately 2 inches higher than NAVD88 elevations at this location, which, for the purposes of this report, is negligible.

Soil boring BHBA-8 was drilled within the limits of the area dredged for project PO-17 in 1994 based on the documentation contained in Appendix A, and as shown in Figure 2. This may explain the lower mudline elevation and the presence of more organic clay than was observed in the other soil borings. The organic clay may be from recent deposits after the 1994 dredging.

The borrow area borings completed for project PO-17, presumably in the late 1980s or early 1990s cover the same general area investigated for this project as shown in Figure 2. These older soil borings (1-LB through 8-LB) indicated a more interbedded profile throughout the investigation area, consistent with soil encountered in our soil borings BHBA-4 through BHBA-8. Unless there has been additional dredging in the area, it seems unlikely that spillway openings or other events could have displaced the top 20 feet of soil, so these older borings may provide additional useful information about the soil stratigraphy in the borrow area.

CONCLUSIONS AND RECOMMENDATIONS

General

Based on pilot project results, containment features constructed from peat need to be handled carefully, but can be placed and shaped, and can provide adequate containment for dredged material for the marsh fill construction period. Furthermore, experiments completed by GeoEngineers on various soil samples show peat will recharge through capillary action if its base is exposed to water, allowing it to generally maintain its weight, shape, and strength. These findings are contrary to our initial concerns about peat drying and not being able to withstand stresses if used as containment material.

Because peat is highly organic and porous, silt and clay fill from the borrow area will weigh substantially more. The light weight and compressibility of peat result in large-magnitude settlement over a relatively

May 26, 2017| Page 6 short period of time when loaded, which should be considered in fill quantity estimates. GeoEngineers has incorporated mudline changes due to construction period settlement in our analysis.

Due to generally low shear strength and light unit weight, peat is also susceptible to being displaced laterally by dredge slurry discharging into the marsh fill area. This will result in some areas where deep holes are created and filled by the dredge slurry and other areas where mounds of peat aggregate, resulting in a heterogeneous distribution of soil types across the site. This process has too many unknowns for us to quantify and has been left out of our analysis.

Marsh Creation

Calibration of Model Using Pilot Project Results

NRCS provided monthly survey results from the first year after construction for the PO-75 Pilot Project.

GeoEngineers used the surveys to check our methods for computing marsh fill and foundation settlement with the measured results. A marsh settlement analysis was completed using the full-scale project borrow area soil slurry settlement properties assuming the borrow area material properties for the pilot project were similar to that of the full-scale project. However, we were not able to obtain a satisfactory match to the pilot project survey data using the full-scale project borrow area soil slurry properties. When pilot project borrow material test results were used in the settlement analysis, the results matched reasonably well to the pilot project survey data. This confirmed the analysis method can produce a reasonable estimate of the measured settlement in the pilot marsh creation areas. The differences between the full-scale and pilot study borrow area properties affected results. For our re-evaluation of the full-scale project, we used the full-scale borrow area properties and the pilot study calibration methodology.

Dredged material fill thickness in a contained area decreases due to primary consolidation, secondary consolidation, and desiccation. The consolidation settlement and time rate of settlement analyses for the marsh creation area were performed using the Primary consolidation, Secondary compression, and Desiccation of Dredged Fill (PSDDF) program. Soil boring and laboratory data from self-weight consolidation and settling column tests were used to determine the input parameters for the dredged fill materials.

We modeled the dredged fill and foundation settlement by breaking the construction period into several increments spaced three days apart and placing evenly divided amounts of fill in incremental lifts over the construction period (average of about 12 days per test cell, per NRCS records), using fill properties to compute stresses on the foundation and adjusting stress based on progressive densification of fill (PSDDF results) and changing buoyancy due to settlement and water level changes. The TBS survey showed an average water level drop of about 1 foot after 6 months of monitoring, which we incorporated into our analysis to calibrate settlement curves.

As mentioned in the Existing Information section, the TBS marsh platform survey method generated short transects of the marsh surface, centered around the grade stakes placed in the cells prior to construction.

This captured general conditions around each grade stake, but general involved access by foot to the survey location. Along with vertical accuracy tolerance up to 4 cm (0.13 foot) and foot traffic access through very soft material, we expect there is an appreciable error margin in the survey results. Also, the settlement plate position discrepancy between the as-built survey conducted by Hydroterra on August 13, 2013 and the first TBS survey event on August 23, 2013 adds additional uncertainty about foundation and fill contribution to settlement in Cell 1.

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GeoEngineers calibrated our model by adjusting our program inputs to provide a similar magnitude of settlement given the pilot project fill elevations, including an initial fill elevation of +2.35 feet (approximating Event 1 (August 23, 2013) survey results) to match results from Cell 2, and an initial fill elevation of +4 feet to match results measured near Cell 1 staff gauge SG 1-1. Foundation settlement measured by the settlement plates in Cells 1 and 2 was subtracted from fill surface surveys to isolate fill settlement. Using pilot project borrow area soil properties, we modeled fill placement and settlement as described in the Appendix E marsh settlement calculation approach. Because dredge fill for the pilot project was complete in an average of about 12 days per cell, we broke the construction period into 3-day increments and applied a series of thin fill layers to bring the fill elevation to the appropriate level. The resulting fill stress was used to compute settlement for the mudline during the construction period and at various times after construction up to a year, as described in our calculations approach and shown on Figure 4. Figure 4 shows elevation change with time based on our model plotted next to measured results.

It shows modeled mudline and fill settlement with time plotted next to measured settlements for the two initial fill elevations we selected for our calibration.

The pilot project demonstrated substantial settlement in the subgrade over the monitoring year, which confirms our expectation of relatively quick, large-magnitude settlement in peat foundation soils. For the lower fill elevation, measured settlement within the fill did not contribute as much to the overall elevation change as mudline settlement. For the higher fill elevation, measured settlements show the fill and foundation settling about equally for the first year after construction, though at different rates. As evident in Figure 4, we were not able to exactly model the settlement rate and quantity; however, our model estimated settlement at one year within about 6 inches of measured settlement for initial fill elevation +4.0 feet, and within about two inches for initial fill elevation +2.35 feet.

Settlement – Single Lift

The following design elevations were used to calculate settlement:

The existing marsh mudline elevation as determined by survey at each soil boring location.

Water level within a foot of the end-of-construction fill elevation for the first 6 months after construction, then water level within the fill at +1.0 foot, thereafter.

A marsh creation target elevation of +1.0 foot at three years after construction.

The NRCS communicated the assumed dredge filling period, based on the estimated volume of fill and a pump rate of 20,000 cubic yards per day, will be about 228 days. Filling activities are planned to be broken up into three distinct cells within the overall marsh creation area, resulting in an average fill time of approximately 76 days per cell. GeoEngineers assumed a construction period of 75 days to model fill placement and subgrade settlement. Existing soil beneath the fill areas will experience consolidation settlement from the additional overburden placed during dredging. It is expected that peat will compress rapidly as hydraulic fill is placed, which will add to the total fill volume required to meet the project marsh platform elevation requirement and increase the amount of settlement/consolidation within both the hydraulic fill layer and the underlying soils.

The example shown in the chart below illustrates a significant characteristic of dredged fill settlement:

initially, more than half of the hydraulic fill is above the water table; however, as time passes most of the fill is submerged. The mass of the solids within the fill remains constant, so as the fill settles/consolidates and more of the mass becomes buoyant (submerges), the stress (weight) of the fill on the underlying soils decreases relative to the time the fill was initially placed (time = 0 years). The pilot study results suggest

May 26, 2017| Page 8 that water level changes further complicate this process: for the last half of the monitoring period water levels in and out of the marsh cells generally dropped, possibly through natural processes, some form of containment breach (intentional or otherwise), or some combination that allowed increased drainage of the dredge material. Lowering the water level in the fill increases the stress on foundation soils and causes additional settlement to occur.

This is a very dynamic and complicated process with significant settlement in both the fill and soil underlying the fill. The chart below shows the projected combined settlement of the fill and subgrade soils as calculated for our November 15, 2010 report at the location of soil boring BHMC-5 for an initial fill elevation of 4.5 feet. The specific results shown are no longer relevant; however, the chart provides a valuable illustration of the processes at play.

Figures E-1a through E-7b in Appendix E summarize the predicted settlement for a range of initial fill elevations over a 20-year design life. Figures labeled E-#a show predicted marsh fill surface displacement based on the mudline measured at each soil boring for an initial fill elevation range of +2 to +4.5 feet.

Figures labeled E-#b show predicted fill surface elevation movement for mudline elevations of -7 and -14 feet and initial fill elevations of +3.0 and +3.5 feet.

The estimated elevation three years after construction, and the estimated long term elevation for initial fill elevations +3.0 and +3.5 feet are summarized in the table below for the “normal” mudlines. As shown in Figure E-4a, computed values for initial fill elevations +3.0 and +3.5 feet fall within about 0.05 feet of the values interpolated between the computed values at 20 years after construction for initial fill elevations +2.0 and +4.5 feet. After confirming negligible difference between computed values and interpolated values, GeoEngineers has opted to present only the interpolated values, except for those shown in Figure E-4a for illustration purposes. Our revised calculation method, calibrated to the pilot project results, shows that a marsh platform elevation of +1.0 foot is generally attainable in a single lift.

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Settlement – Multiple Lifts

The NRCS requested we evaluate placing a second lift 60 days after construction of the first lift is complete.

Long term settlement will eventually cause the marsh fill platform to fall below elevation +1.0 foot. Applying a second lift of dredge fill sometime after initial approval of the marsh platform elevation may help extend the time the marsh platform remains emerged; however, based on our analysis, project goals will likely be attainable in a single lift for normal mudline elevations in the area. Also, the 60 days between lifts requested by NRCS is constrained by dredge availability within the construction period. NRCS requested specific evaluations for initial fill surface elevations +3.0 and +3.5 feet. At 60 days after construction, we estimate about a foot or less of settlement at even our worst settlement cases. Because of dredge tolerances and other logistical difficulties trying to fill less than a foot hydraulically, it is not practical to suggest a second lift for normal mudline elevations.

Dressing/nourishing the new marsh platform for deeper mudlines will provide more lasting value if dredge fill is placed a year or more after initial placement is complete. However, this requires a second dredge mobilization, which we understand is not tenable within the project constraints. Because a second lift during the project timeframe provides a meager benefit, a formal two-lift construction scenario does not make sense overall. However, topping off low areas prior to demobilizing the dredge will provide additional material to offset settlement in these areas. GeoEngineers has presented a two-lift marsh settlement evaluation based on initial fill elevation +3.5 feet and initial mudlines of -7 and -14 feet in Appendix E, Figure E-7b (Locations BHMC-7and BHMC-8). Figure E-7b illustrates, for the location which generally sustains the most settlement, the gains attained by a second lift at 60 days, assuming a 30-day fill period.

Even with the additional lift, the 3-year target is not met, but there is a net gain in elevation overall.

Hydraulic Fill Containment

There are two different containment scenarios for this project as follows:

Containment to prevent overflow of fill into existing marsh areas; and

Containment to prevent flow of fill through canals, cuts, or other marsh shoreline breaks around the perimeter of the proposed marsh creation area as shown in Figure 2.

Soil Boring

Initial Fill Elevation = 3.0 ft

NAVD 88

Initial Fill Elevation = 3.5 ft

NAVD 88

3-year elevation

(ft NAVD 88)

20-year elevation

(ft NAVD 88)

3-year elevation

(ft NAVD 88)

20-year elevation

(ft NAVD 88)

BHMC-1 1.1 0.7 1.3 0.9

BHMC-2 1.1 0.6 1.3 0.8

BHMC-3 1.0 0.7 1.1 0.8

BHMC-4 1.1 0.8 1.4 1.0

BHMC-5 1.0 0.7 1.2 0.9

BHMC-6 1.0 0.6 1.2 0.8

BHMC-7&8 1.0 0.7 1.1 0.7

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In areas where there is existing marsh to build on, GeoEngineers understands the NRCS planned to construct non-continuous low level earthen containment dikes to manage the dredge fill. Based on the fill elevation required to meet the project objectives, it seems likely continuous containment will be required.

Based on pilot project results, dense emergent marsh grass can provide adequate containment if the vegetation remains dense for the entire height required to contain dredge slurry flow. The pilot project illustrated that carefully built dikes constructed of peat and organic clay can be used to hold marsh fill.

However, GeoEngineers recommends caution when directing the dredge slurry flow to avoid dike blow-out and uncontrolled release of dredge fill. Maintaining a dike of sufficient height/elevation to contain the fill will likely require regular maintenance during construction. Regular surveys and dike quality inspections will help identify problem areas where the most immediate attention is needed. The recommended configuration for a stable containment dike is shown in Figure 5. Stability calculations are included in Appendix F.

For the marsh shoreline breaks, containment generally needs to be established in two- to four-foot deep water. GeoEngineers expects it will be difficult to excavate and place the peat and organic clay materials present at this site in a manner that will be stable against water level fluctuation and wave action in these breaks. In place of constructing an earthen structure at the breaks, GeoEngineers was tasked with identifying two containment structures as alternates to earthen berms. The geometry of the shoreline breaks used for our evaluations was based on surveys by TBS.

A containment structure using wire gabion and geotextile was designed using specific material properties and fill assumptions shown in Appendix G. Different vendor materials may have different properties.

Vendors that were contacted for this evaluation have their own design support staff and may prefer to provide their own design. As long as the vendor can provide sealed engineering plans for their design, GeoEngineers believes there are other viable design options in addition to the design provided in this report.

GeoEngineers also evaluated a sheet pile cofferdam in detail, and conceptually planned sheet piling as the second alternate structure; however, using traditional construction methods and engineering evaluations, we estimate the cofferdam will require two parallel sheet pile walls with 25-foot long piles, spaced 40 or more feet apart. It was felt that this was not a reasonable containment alternative given other options that are available, and it was dropped from further consideration.

The second viable alternative is a sediment filled geotextile tube (geotube). These tubes, shown in Figure 5, can be filled using discharge from a hydraulic dredge and should be suitable for a relatively low containment elevation. The fill material in the borrow area for this site (predominantly silt & clay) is not ideal for geotubes, since it remains in suspension longer than sand and does not readily dewater; however, it may be possible to use the geotube as part of the dredge discharge system, allowing heavier silt, sand and shell to accumulate in the tube while discharging finer material as fill into the 700-acre marsh fill area.

Details of the proper geotextile tube size, configuration, and geotextile requirements will need to be evaluated during design refinement. In addition to closing open marsh gaps, geotubes may be a suitable containment alternative to small earthen dikes in the marsh. The main benefit being that the borrow area has very little peat, so a geotube placed above the water may have more strength and less shrinkage than a dike constructed of peat.

Other options, including hay bales and field fabricated structures were considered, but were not recommended do to constructability and stability limitations. Hay bales would require transporting and placing a significant number of bales, and the initial buoyant nature of the bales makes them less suitable than gabion basket. Similarly, field fabricated structures will require more labor and similar materials to prefabricated gabion baskets or geotextile tubes, which are commercially available.

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Borrow Area

Borrow Area Consistency

The proposed soil borrow area has been divided into three separate cells (Cells 1, 2, and 3) as shown in Figure 2. Based on the soil borings for this project (Appendix B) and project PO-17 (Appendix A), and laboratory testing there is no clear distinction in soil characteristics between the three cells. The soil borings indicate that there is likely to be variation in the dredged material properties within each cell. For example, taking soil borings BHBA-1, BHBA-2, BHBA-3, and BHBA-5 as representative of Cell 1, the combined silt and sand content from the mudline to elevation -20 feet were 82%, 94%, 47%, and 81% respectively; the sand content of soil boring BHBA-5 was 21 percentage points higher than any of the other soil borings.

Given the observed variability, GeoEngineers believes the settling column completed using a composite sample from all the borrow area soil borings is generally representative of the overall conditions that will be encountered. In general, the borrow area contains predominantly fine grained soil (mostly silt) from the mudline to elevation -20 feet. Based on the available information, GeoEngineers recommends using a generalized set of borrow area design parameters for all marsh creation cells.

The only additional consideration is the soil that has been deposited in the area dredged for project PO-17 shown in Figure 2. Only one soil boring was drilled within this area (BHBA-8). While not specifically indicated by soil boring BHBA-8, the sediment that has been deposited in this previously dredged area has the potential to be different from surrounding, older soils.

Hydraulic Dredging Fill-to-Cut Ratio

The fill-to-cut ratio evaluation was based on settling column and self-weight consolidation tests results, which are included in Appendix H, and design guidance in the United States Army Corps of Engineers (USACE) publication EM-1110-2-5027.

Initial Hydraulic Dredge Material

Consolidation Time (days) Fill to Cut Ratio

1 2.4

30 1.9

60 1.8

Bulking Factor – Approx. 1 day 3

The “bulking factor,” which gives a similar ratio as the USACE method for the short duration of one day, is based on guidance from the “Handbook of Dredging Engineering” by John B. Herbich (2nd Edition, pages 6.25-27).

However, dredging inefficiencies, material losses through weirs and other factors will affect dredge quantities, and should be factored into volume calculations when estimating project costs. Cut-to-fill ratios reported for construction of marsh creation projects have been between about 1.1 and 1.5 (1.1 to 1.5 cubic yards (CY) in-place borrow material cut for every 1 CY of in-place fill), depending on borrow material source and type, efficiency of retaining system, time taken to install the fill, and other factors. Larger fill areas are generally reported to have cut-to-fill ratios on the low end of this scale. GeoEngineers recommends a cut-

May 26, 2017| Page 12 to-fill ratio of 1.3, including construction settlement, to estimate fill volume quantities. Our estimate was generated as follows:

■ Based on reports of constructed projects, assume a large fill area cut-to-fill ratio of 1.1.

■ Compute average construction settlement across the site. In this case, average construction settlement for design fill elevations +3.0 to +3.5 feet is about 9 to 12 inches, respectively, or about 16% of the fill height for the average mudline elevation during our exploration.

■ Adjust the cut-to-fill by increasing the ratio by 16%. Cut-to-fill of 1.1 x 1.16 = 1.28, therefore cut-to-fill is 1.3 after adjusting for construction settlement.

CONSTRUCTION CONSIDERATIONS

Based on the site work and evaluations completed for this project, the following are offered with respect to construction.

■ Project access is limited from the Bonnet Carré Floodway launch sites to vessels that can fit under the railroad and interstate structures.

■ There are several pipelines crossing the proposed borrow area and in the near vicinity of the marsh creation area. During our 2010 investigation, GeoEngineers received a map from a pipeline owner showing a pipeline that had not previously been identified by NRCS. Figure 2 shows the pipelines GeoEngineers is aware of in the project vicinity. Because additional historic or newly laid pipelines may be present in or near the project limits, GeoEngineers recommends taking precautions when preparing to move forward with construction. We understand a pre-construction borrow area geophysical survey is typical for dredging projects. This should be sufficient to identify the potential obstructions within the project limits.

■ Water depth in the marsh creation area is shallow. Construction equipment will need to both be acceptable to the land owner and able to perform the required tasks.

■ Shoreline protection structures on Lake Pontchartrain appear to be stable and should be protected from damage during construction. The design for project PO-3B recommended a lightweight aggregate core for the rock dike to reduce settlement. The dike was constructed, but we do not have access to the as-built drawings to determine if the recommendation to use a lightweight aggregate core was followed. Any damage that may occur should be repaired consistent with the initial construction.

Further research may be required to determine the nature of the initial construction.

■ Larger soil particles will fall out of suspension more easily than smaller soil particles, resulting in mounds of high sand and shell concentration near the dredge discharge site. Construction planning that includes moving discharge locations will allow a wider concentration of higher, less settlement-susceptible material. Such strategies could include building the dredge pipe well into the marsh placement area and removing pipe sections as filling progresses, thereby allowing a more even distribution of sand and shell throughout the marsh area than occurs by keeping the dredge discharge at one end of the site.

■ Lake Pontchartrain is brackish, and is likely more saline near the mudline than at the surface.

Additional salt in the fill may induce flocculation of clay particles, which may allow for earlier acceptance in the fill than expected and earlier drainage acceleration. Careful monitoring of the fill after placement will help determine whether this is the case.

■ Peat considerations

May 26, 2017| Page 13 o Peat placed on the existing marsh above the static water level may shrink and decay significantly, and dikes constructed from this material will likely require regular maintenance to maintain the desired elevation.

o Energy diffusers or other such precautions are recommended to minimize scouring of the organic deposits during hydraulic fill placement.

o Peat is a weak, light-weight material; mud waves should be expected while placing fill.

o Peat settlement during hydraulic fill placement will result in an initial fill thickness greater than the elevation difference between the target fill elevation and the pre-construction mudline elevation.

■ Containment structures o Gabion wire and geotextile tubes must be filled with earth to make them stable. Use of peat may require periodic maintenance to maintain fill height above the water table. Use of hydraulic fill will cause settlement, but will provide a more stable structure.

o Fill in the containment structures should be maintained at or above the marsh fill elevation plus any free water on top of the fill.

o These structures are designed based on hydraulic fill retention. Extreme weather events such as storm surge from a hurricane were not considered.

o It was assumed that gabion wire and geotextile tubes will be constructed to their full design height initially, and that existing organic soils or hydraulic fill will be used to fill the structure.

The use of sand is discouraged because it will cause additional settlement and delivering it to the location may not be feasible. Given the fill materials available, it is not expected that equipment or people will be able to walk on top of the filled baskets or geotextile tubes, unless the fill has sufficiently consolidated so that it is a stable platform. As such, the containment structures need to be built and secured to their full height and fill will have to be carefully placed through the multiple layers of wire baskets. This obviously becomes more difficult with more levels of baskets.

■ Dewatering o Dewatering structures (weirs, drainage culverts) should be designed to allow retention of as much material as practical. In general, placement of such structures away from the dredge discharge point is preferred.

o Hydraulic exchange with surrounding marsh and bayous should be re-established as soon as practical. This is generally handled by strategically breaching the containment dike after the marsh fill has been allowed to set and stabilize.

LIMITATIONS

We have prepared this report for the exclusive use of the USDA-NRCS in support of design of the proposed LaBranche East Marsh Creation Project (PO-75) located in St. Charles Parish, Louisiana.

Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. No warranty or other conditions, expressed or implied, should be understood.

May 26, 2017| Page 14

Please refer to Appendix I titled “Report Limitations and Guidelines for Use” for additional information pertaining to use of this report.

FIG

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LaBranche East Marsh Creation Project (PO-75)

St. Charles Parish, Louisiana

VICINITY MAP

Figure 1

Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. can not guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.

Reference: Topographic image taken from USGS, DRG-100k Template, Quad Ponchtoula & New Orleans, Dated 9/2005

P:\10\10883009\01\CAD\vicinity map.dwg\TAB:Layout1 modified on Sep 17, 2010 - 9:28am KMCVT

MARSH

CREATION

AREA

STATE OUTLINE

SITE

BORROW

AREA

BHBA-1

BHBA-3

BHMC-2

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BHBA-4

BHBA-5

PIPELINE

(REF. 4)

BHBA-6

COMPLETED LABRANCHE WETLANDS

CREATION PROJECT (PO-17)

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BORROW AREA BASED ON AS-BUILTS

DATED FEB. 1994, FOR LABRANCHE

WETLANDS CREATION PROJECT (PO-17)

BONNE CARRE

FLOODWAY

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(REF. 3)

PROPOSED MARSH CREATION

LIMITS FOR PROJECT (PO-75)

BHMC-1

BHMC-3

BHMC-6

BHMC-5

BHMC-8

BHMC-7

BHMC-4

PROPOSED BORROW AREA

LIMITS FOR PROJECT (PO-75)

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2-LB

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9LB

11LB

CELL-1

CELL-2

CELL-3

BHBA-PS6-10

BHMC-PS3-30

BHMC-PS2-30 BHMC-PS1-30

BHBA-PS1-20 BHBA-PS2-20

BHBA-PS5-10

BHBA-PS4-10

BHBA-PS3-10

Figure 2

LaBranche East Marsh Creation Project (PO-75) St. Charles Parish, Louisiana

Site Plan

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Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files.

The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.

Data Source:

1. Aerial was taken from Google Earth Pro., Imagery dated: 4/6/2016

2. Pipeline and Dredge Limits were provided by NRCS, Dated 3/2010

3. Pipeline layout were provided by Fugro Chance Inc., Dated: 2/2/2009

Existing Pipeline

Limits of pipeline controlled area

Marsh Shoreline Breaks

Soil Borings from Projects PO-17 & PO-3B

Previous Boring Location (East Marsh Creation Project (PO-75)

BHMC-1

Previous Boring Location (Pilot Study) (East Marsh Creation Project (PO-75)

BHMC-PS1

A A'

-20

-40

-60

-80

-100 0 1000 2000 3000 4000 5000 6000 7000

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-40

-60

-80

-100

-10

-30

-50

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-90

BHMC-1BHMC-2

BHMC-4

BHMC-3 BHMC-5 BHMC-6 BHMC-7BHMC-8

WATER MUDLINE

PEAT PEAT

ORGANIC

CLAY

ORGANIC

CLAY

SILT & CLAY

W/ SILT/SAND

LAYERS

VERY SOFT TO MEDIUM CLAY

ASSUMED PLEISTOCENE (REFER TO FIGURE 3B)

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SUBSURFACE PROFILE

A - A' (BHMC SERIES)

LaBranche East Marsh Creation Project (PO-75)

St. Charles Parish, Louisiana Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. can not guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.

Reference: Mudline elevations provided by T. Baker Smith, NAVD 88, Dated 7/1/2010

SILTY CLAY

CLAY

SILT

ORGANIC CLAY

PEAT

LEGEND

BHBA-1

BHBA-3

BHMC-1

BHMC-2

BHMC-3

BHMC-6

BHMC-5

BHMC-8

BHMC-7

BHBA-2

BHBA-4

BHBA-5

BHMC-4

BHBA-6

BHBA-7

BHBA-8

Miles W E

N

Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. can not guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc.