Attachment - Tectonic Prelim Geotech Assessment - CR 74.pdf

Orange County, New York Department of Public Works

PO Box 509, 2455-2459 Route 17 M Goshen, NY 10924

W.O. 13053.01

May 9, 2025 October 16, 2025 - REV 1

1279 Route 300, 2nd Floor Newburgh, NY 12550

(P) 845.567.6656

(F) 845.567.6248

Tectonic Engineering Consultants, Geologists & Land Surveyors, D.P.C.

Submitted To: Submitted By:

PRELIMINARY GEOTECHNICAL ASSESSMENT

ROADSIDE SLOPE DISTRESS

COUNTY ROUTE 74 (FORGE HILL ROAD)

TOWN OF NEW WINDSOR, ORANGE COUNTY, NEW YORK

Orange County, New York Department of Public Works PO Box 509, 2455-2459 Route 17 M Goshen, NY 10924

Attn: Mr. Ryan McGuire - Deputy Commissioner of DPW, Division of Engineering Via email (RMcGuire@orangecountygov.com)

May 9, 2025

Revised October 16, 2025

RE: W.O. 13053.01 Rev. 1

PRELIMINARY GEOTECHNICAL ASSESSMENT

ROADSIDE SLOPE DISTRESS

Dear Mr. McGuire;

Tectonic Engineering Consultants, Geologists & Land Surveyors, D.P.C. is pleased to submit this Town of New Windsor, Orange County, New York. It is noted that this evaluation and report constitute the first step in the overall program that is anticipated to include a field investigation, monitoring, and ultimately remediating the existing conditions of the slope. This report presents descriptions of the scope of the assessment and evaluation; the geotechnical findings; conclusions; recommendations for supplemental investigations; and recommendations for monitoring and stabilizing the existing conditions, as well as providing a preliminary cost assessment of the potential stabilization options, as of the date of this study.

We appreciate this opportunity to assist you with this project. If you have any questions, please do not hesitate to contact the undersigned.

Sincerely, TECTONIC ENGINEERING CONSULTANTS, GEOLOGISTS & LAND SURVEYORS, D.P.C.

Christopher Burke, Ph.D., P.E. (NY, NJ, PA, FL), P.M.P. Myron C Chaplin, P.E.

Senior Vice President, Geotechnical Senior Engineer

"G:\Newburgh\Geotechnical\13000\13053.01 Evaluation of CR 74 Bank Erosion\Report\TEC Cover Letter.docx"

PRELIMINARY GEOTECHNICAL ASSESSMENT

SLOPE EVALUATION

TABLE OF CONTENTS

SECTION ITEM PAGE

1.0 INTRODUCTION

2.0 SCOPE OF WORK

3.0 SITE AND PROJECT DESCRIPTION

4.0 PRELIMINARY SLOPE STABILITY ASSESSMENT

5.0 FAILURE PROGRESSION

6.0 CONCLUSIONS AND RECOMMENDATIONS

7.0 PRELIMINARY ROUGH ORDER OF MAGNITUDE COST ESTIMATES

8.0 RECOMMENDATIONS FOR ADDITIONAL INVESTIGATION AND ENGINEERING

9.0 LIMITATIONS

APPENDIX I SLOPE STABILITY ANALYSIS INFORMATION AND FIGURES

APPENDIX II HEAD SCARP PROGRESSION INFORMATION

APPENDIX III HURRICANE, RAINFALL AND FLOOD DATA

APPENDIX IV PRELIMINARY COST ESTIMATES

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1.0 INTRODUCTION

Tectonic Engineering Consultants, Geologists & Land Surveyors, D.P.C. (Tectonic) has performed a preliminary desktop assessment and geotechnical engineering evaluation of the stability of the slope along the south side of

County Route 74 (Forge Hill Road) in the Town of New Windsor, Orange County, New York. It is noted that this evaluation and report constitute the first step in the overall program that is anticipated to include a field investigation, monitoring, and ultimately remediation of the slope. This report presents descriptions of the scope of the assessment and evaluation; the geotechnical findings; conclusions; recommendations for supplemental investigations; and recommendations for monitoring and stabilizing the existing conditions, as well as providing a preliminary cost assessment of the potential stabilization options, as of the date of this study. This report has been revised from the original to include Figure 1 and associated text.

2.0 SCOPE OF WORK

The geotechnical engineering services described below were performed for Orange County Department of Public

Works, herein referred to as OCDPW.

• Review a previous desktop evaluation of a similar slope located between Moodna Creek and County Route 32 (Mill Street), approximately 1.5 miles upstream of this site and detailed in a report by Tectonic, entitled " Preliminary Geotechnical Assessment, Roadside Slope Distress, County Route 32 (Mill Street), Town of Cornwall, Orange County, New York

• Review publicly available aerial photographs and other information related to the existing slope along the northern bank of Moodna Creek in the project area to assess the rate of progress of the failure and estimate future progress.

• Use publicly available topographic information, site observations and our knowledge of the local soils to prepare a preliminary desktop model of the existing slope, evaluate its relative stability, and evaluate potential causes of the ongoing distress.

• Discuss potential remedial actions to stabilize the existing slope and develop three (3) stabilization options.

• Perform preliminary cost assessments of three (3) potential stabilization options, as of the date of this study.

• Prepare this engineering report presenting the results of the preliminary geotechnical assessment, preliminary geotechnical recommendations for remedial options to stabilize the subject slope, and provide preliminary estimates for stabilizing the existing conditions.

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3.0 SITE AND PROJECT DESCRIPTION

The project site is located along the southern side of Forge Hill Road, approximately 1,300 to 1,600 feet east of the intersection of Forge Hill Road and County Route 94 (Blooming Grove Turnpike) in New Windsor, Orange

County, New York. The observed slope failure is located south of the eastbound lane, which is located between the roadway and the northern bank of Moodna Creek. The site is located at a point where Moodna Creek makes an approximately 90 degree bend from flowing northward, to flowing eastward, immediately downstream from the confluence with Silver Stream. The site of the failing slope is also located on the property of the Knox

Headquarters State Historic Site.

Information provided by OCDPW and historic observations, indicate that the slope failure along the northern bank of Moodna Creek has progressed northward to a point where OCDPW has become concerned about the stability and safety of Forge Hill Road. As Moodna Creek has eroded the streambank, the slope has progressively failed causing the head scarp (the highest portion, or top of the failure surface) to progress northward (toward County

Route 74), to a point where by 2022 it had reached approximately 21 feet from the eastbound edge of the road

(see Photo 1).

Photo 1: Aerial Photo of Slope and Forge Hill Road Showing the Distance Between the Head Scarp and

Pavement as of 2022 (Source Orange County GIS)

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Photo 2: Oblique View Looking Northward Across Moodna Creek Toward Forge Hill Road (May 2023)

As of the date that this report was completed, a project-specific topographic survey of the subject slope was not available, and therefore, publicly available topographic information was used for this study. Topographic data presented on the USGS National Map Viewer website (see in Figure 1 on the following page), which is based on

LIDAR mapping that was performed between January of 2014 and April of 2022, indicates that the slope has a total height of approximately 133 feet between the stream bank and the crest at Forge Hill Road, over a total length of approximately 144 feet. This results in an average slope of approximately 1.1H to 1V (An approximate section of this profile is also included at the beginning of Appendix I). However, observations show that the face of the slope varies, with the steepest section located several feet above the toe of the slope and a less-steep

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Figure 1 2025 USGS Topographic Map (https://apps.nationalmap.gov/viewer/) of Site Showing Spot

Elevations at the Crest and Toe of Slope (Contour interval: 2.5m {~8.2 ft})

Further observations of the site conditions reveal that the slope consists of glacial till soil with particle sizes ranging from clay-sizes (on the order of micrometers) to boulders (some of which are visible at the bottom of the slope in both photos, in the stream at the toe of the slope). Although the slope appears to completely consist of glacial till, some aerial photos show what appears to be bedrock outcrops within Moodna Creek, immediately east of the eastern end of the slope. Another feature that is visible in the two photos presented above is well defined erosion gullies on the face of the slope. A particular one, near the western (left) edge of the slope appears to be particularly deep and, in 2022, had extended to less than 30 feet from the County Route 74 pavement edge.

It should be noted that the latest publicly available data regarding the slope (aerial photos and topography) dates to 2022. Consequently, although there may have been further deterioration of the slope in the time since, the lack of available data prevents us from presenting it herein.

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4.0 PRELIMINARY SLOPE STABILITY ASSESSMENT

To better estimate the progression of the slope failure and potential methods to stabilize the slope, Tectonic prepared a preliminary approximate model of the slope using the slope stability analysis program SLIDE2, by

Rocscience. To create the model, the above-noted USGS website was used and to establish a cross-section through the slope approximately at the eastern end of the guiderail above the slope. The publicly available topographic data, as noted above, is based on various lidar surveys performed between 2014 and April of 2022, and shows a relatively constant slope between the crest and a point approximately 60 feet above the streambed, where the slope steepens, and then shows another flatter section that begins about 30 feet above the streambed.

It is our interpretation that these changes in slope angle reflect the remains of an over-steepened toe section, talus It should also be noted that this model may not represent changes that may have occurred between 2022 and the date of this report (2025).

Figure 2 shows the slope profile established from data as described above. The green line in the profile represents the ground surface as of 2022, while the gray dashed line represents a best-estimate 2011 ground surface profile across the slope. Along with the two surface profiles are a red dashed line that represents the estimated bottom of the disturbed glacial till, which has fallen or eroded from the slope and created a relatively shallower slope angle near the bottom of the slope; an estimated line representing a bedrock surface; and an estimated groundwater surface. Note that we have assumed that the bedrock surface is at a nearly uniform elevation of +53 feet, which matches the ground surface elevation at the apparently exposed bedrock downstream of the site.

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Figure 2 CR-74 (Forge Hill Road) Slope at Moodna Creek Cross-Sections

The 2011 ground surface profile was selected because aerial images show that the slope was relatively stable, with only some evidence of a few eroded gullies, between 1994 and 2011, when the toe was eroded and became significantly steeper (see Photo 3). Later photos also show that the upper portion of the slope, which was tree or brush covered until 2011, began to significantly erode. We did not have any topographic information for the slope in 2011, so the profile is a best-estimate based on the conditions visible in the aerial photos. This configuration results in the steepest portion of the slope having had a gradient of approximately 0.4H:1V.

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

El ev at io n

Ft

Length (Ft)

Ground Surface Groundwater Bottom of Disturbed Till

Bedrock 2011 Ground Surface

Moodna Creek

Forge Hill Road

0.4H:1V Portion of 2011 Slope

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Photo 3 Aerial Photo of Slope, Taken on October 7, 2011

To establish appropriate estimates of the soil characteristics, we reviewed available data that Tectonic has collected for glacial till in and around Orange County to assign reasonable strength and density values for the undisturbed and disturbed glacial till. Initial strength and density values were also selected based on the results of the Mill Street Slope desktop study, and then the soil parameters were adjusted until the very steep toe section of the 2011 cross-section had a factor of safety of 1 to calibrate the model. This was done based on the assumption that immediately following the oversteepening of the slope, it would have failed to a point where it was marginally stable (only had a factor of safety of approximately 1). Figure 3 shows the 2011 slope profile, with the critical failure surface and the resulting soil parameters. Forge Hill Road, OCDPW County Route 74

Erosion Evaluation provides details of the preliminary slope stability analyses for optimizing the soil parameters and evaluating the existing conditions of the slope. Figure 3 is also included as the first output plot included at the end of the report included in Appendix I.

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Figure 3 2011 Slope Profile with Design Soil Parameters

Two items should be noted about the soil parameters presented in Figure 3. First, glacial till especially basal till, which forms below a glacier tends to be very dense and as the soil is disturbed during a slope failure, it tends to break apart and loosen. Therefore, the disturbed till has been modeled using a slightly lower density than the undisturbed till. Second, undisturbed glacial till typically exhibits a higher strength than disturbed till.

This may be because of the density and cohesion from clays within the soil, or because of some cementation.

Therefore, the disturbed till has been modeled with a lower cohesion.

The primary purpose of this preliminary study was to evaluate the existing stability of the slope (as of 2022 due to available data), to consider how future failures could occur, and to look at the consequences. We therefore investigated the overall stability of the slope as it currently exists (as of 2022), and then investigated the effects of further deterioration, whether through a slope stability failure or erosion, and how that would affect the safety and stability of the roadway. We note that the AASHTO Bridge Design Manual recommends a minimum resistance factor of 0.75 (FS of 1.33) for slopes that do not contain structures, such as this one.

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The 2022 Slope Configuration -section using the latest USGS LIDAR-based topography, as noted above, and the approximate location of the potential failure surface with the lowest factor of safety against global failure. It should be reiterated that these preliminary analyses were performed with presumptive soil strengths, and therefore, the reported factors of safety should be considered approximate and relative, rather than exact. This preliminary analysis suggests that the current slope has a factor of safety against global failure of approximately 1.2. This is below the required 1.33 factor of safety that is required by AASHTO. However, because the critical failure surface is relatively shallow it does suggest that if a global failure were to occur, it might not immediately affect the roadway. Consequently, it might not be necessary to perform real-time monitoring of the slope, like at Mill Street, but the slope should be periodically monitored for evidence of tension cracks, displacement, or erosion approaching the roadway.

The desktop evaluation of the existing condition suggests that regrading to flatten the upper portion of the slope stability. The Graded Slope Configuration considered these changes. Regrading of the slope and installation of riprap at the toe can improve the factor of safety and might improve it enough to exceed the AASHTO recommended minimum factor of safety (see the second-to-last output plot in Appendix I). The Graded Slope

Configuration (under earthquake loading) also shows that a regraded slope, with enough riprap or other toe stabilization might be adequate (see the final output plot in Appendix I). It should be noted, however, that if the groundwater depths within the slope are significantly higher than anticipated, it is possible that the slope could be less stable. This will be discussed further in the following section.

It has become apparent from our analyses that the leading cause of the deterioration appears to be erosion.

Specifically, the aerial photos show a tree-covered slope (with some evidence of small erosional gullies) between the first available aerial photo in 1994 and the spring of 2011. They then show that significant erosion of the streambank occurred sometime between March 26, 2011, and October 7, 2011. With the latter aerial photo having been taken only 38 days after Hurricane Irene, causing significant flooding and damage along the length of Moodna Creek. A comparison of these two photos is included in the first three pages of Appendix II and shows the approximate locations of the left (northern) shoreline of Moodna Creek on the two dates, as well as the estimated maximum extent of shoreline erosion during Hurricane Irene on the second photo. The third image is the result of overlaying the two previous images and includes several measurements of the northward progression of the shoreline between the two dates.

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The aerial photos taken after 2011 show a progression of erosion between the crest of the eroded toe (shown in

Photo 3) and the crest of the slope along Forge Hill Road. This shows that erosion of the over-steepened lower portion of the slope has progressed uphill toward the roadway. These photos and the preliminary stability analyses performed for this desktop study both suggest that the primary stability issue for the subject slope may be related to erosion, and that erosion stabilization, both along the toe of the slope to prevent undercutting of the toe, and on the rest of the slope to prevent further progression toward the roadway might be sufficient to secure the slope and roadway.

Finally, it should be noted that this desktop evaluation assumes uniform soil and bedrock conditions throughout the slope. Experience generally shows that this is not normally the case. For example, a weak layer within the glacial till, a bedrock surface that slopes downward toward the south, or a significantly shallower groundwater table than was modeled, with seepage out onto the slope, could all decrease the overall stability of the slope.

Also, analyses were based on a relatively rough cross-section from USGS data and the current slope configuration could be significantly different. Conversely, a shallower bedrock surface could improve the stability of the slope.

Consequently, a detailed topographic survey (with 1-foot contours) and a subsurface investigation should be performed to obtain accurate measurements of the surface and subsurface conditions at the site to analyze the slope and develop remedial actions that can adequately stabilize the slope in a cost-effective manner. It is noted, however, that it will be very challenging to obtain detailed subsurface information at the toe or on the slope itself;

therefore, most of the subsurface investigation will likely need to be completed from Forge Hill Road.

5.0 FAILURE PROGRESSION

We were able to make some very rough measurements from the available aerial photos and therefore perform a preliminary approximate estimation of the rate of progression of the head scarp toward Forge Hill Road. This analysis was completed by using several aerial photos of the site collected from public sources, like Google

Earth®, Orange County GIS, and the Discover GIS Data NY (https://orthos.dhses.ny.gov/).

The series of photos presented in Appendix II were used to estimate the progression of the head scarp toward the roadway. Note that it was difficult to estimate the location of the head scarp in some of the early photographs, because of poor resolution, tree limbs and shadows, and that the shortest measured distance occasionally occurs at other locations in some photos. Because of the approximations in locating the head scarp, the distances between it and the roadway, and the subsequent estimated rate of progression, should be considered approximate and are only provided to establish trends, not specific measurements or predictions. It is also noted

11 13053.01 that the accuracy of the scales for the photos was found to be inconsistent. Consequently, we measured the distance between the two white lines near the eastern end of the guiderail and assumed that the roadway width had not significantly changed over the study period. However, as can be seen from the distances between two large boulders visible in the early photos, there appears to be some remaining discrepancies in the scales.

Figure 4 shows the approximate distance between the head scarp and the centerline of the roadway, between

1994 and 2022, as well as a linear trend-line between those two dates.

Figure 4 Distance between Head Scarp and the Center of Road (Yellow Line) (1994 to 2022)

Based on the figure, it appears that the distance between the roadway centerline and the head scarp increased between 1994 and 1995. This is expected to be an artifact of the poor resolution of the early photos, unless the roadway alignment was moved during that timeframe. The trends show the distance to the crest of the slope

(head scarps of eroded gullies) and the centerline of the roadway tended to be constant in the late 1990s, and for approximately a decade after Hurricane Irene. These periods are then followed by decreases in the distance that both appear to be related to erosion of the upper portion of the slope. The full set of tables and charts are included in Appendix II.

y = -0.241x + 528.16 R² = 0.6841

1990 1995 2000 2005 2010 2015 2020 2025

Distance Tropical Storm Beryl Hurricane Floyd

2010 Spring Flood Hurricane Irene Heavy 2011 Fall Rains

Linear (Distance)

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Using the linear regression shown in Figure 4, the progression rate may be as slow as 3 inches per year and the nearest eroded gullies may reach a point approximately 15 feet from the pavement edge by .

However, as noted above and as is evident from Figure 4, the progression of erosion in not linear, but rather shows relatively long periods of no progression, followed by spans of relatively steady progression. If we consider the rate of progression between 2001 and 2011, the progression rate increases to approximately 8 inches per fear and the nearest eroded gullies may reach within 15 feet of the pavement edge .

It should be noted that even this more aggressive estimate assumes a constant rate of progression and ignores revegetation of the slope, which would lengthen the timespan, or another significant flooding event within the next decade, which would accelerate erosion of the upper portion of the slope and shorten the time until the erosion approaches the roadway.

We reviewed hurricane, rainfall and flood data from NOAA and the USGS specifically, flood data on the Wallkill

River, in Gardiner, New York, and rainfall data at Montgomery, New York (see Appendix III). Using that data, we estimated that the Moodna Creek may have experienced significant flooding six (6) times between April 2005 and September 2011, and further review for this study found that there was likely flooding after Hurricane Ida in

August 2021. We also reviewed the major flooding event that impacted southern Orange County, and Rockland, Putnam and Westchester Counties on July 9 and 10, 2023. However, it appears to have not had the same effect on the Moodna Creek.

Five relatively significant events are indicated on Figure 4: Tropical Storm Beryl (~August 19, 1994), Hurricane

Floyd (~September 19, 1999), a spring flood (~March 13, 2010), Hurricane Irene (~August 25, 2011) and two heavy rain events that occurred between September 5 and 23, 2011 (one of which appears to be related to the remnants of Tropical Storm Lee). Because of the poor resolution of the early aerial photos, it is impossible to clearly evaluate whether flooding before 2001 eroded the streambank at the bottom of the subject slope.

However, comparing two aerial photos taken in March and October of 2011 clearly shows significant erosion, with the streambank along the length of the subject slope having moved between approximately 20 and 50 feet northward during that period. Subsequent aerial photos show deposition (mainly through mass wasting of the slope) along the streambank at the toe of the slope.

It is anticipated that occasional flood-induced erosion of the streambank at the toe of the slope leads to oversteepening of the lower portion of the slope, which then promotes erosion of the upper portion of the slope, with progression of erosional gullies toward the roadway. It is likely that erosion of the upper portion of the slope

13 13053.01 will continue to progress toward the road until the slope reaches a relatively uniform grade and plant growth returns to the slope and stabilizes it from further erosion. After that point, the slope would likely remain relatively stable until an event would start the process again.

The degree of flooding that would be necessary to cause another event that would result in a significant loss of soil at the toe of the slope is beyond the scope of this assignment. However, given that extreme storm and flooding events appear to have increased in frequency as the climate has changed over the past few decades, this suggests that the crest of the eroding slope could be close to the edge of the roadway sooner than was identified above ( ).

6.0 CONCLUSIONS AND RECOMMENDATIONS

The primary purpose of this preliminary geotechnical desktop evaluation was to identify the potential causes of the observed deterioration of the subject slope, to estimate the rate of advancement of erosion on the slope, to provide options for protecting the current alignment of Forge Hill Road, and to provide rough order-of-magnitude cost estimates for possible remedial actions. The first four of these items will be discussed herein, while the last will be discussed in the following section. This report will end with recommendations for additional investigation and analyses.

The following conclusions can be made about the general stability of the slope and Forge Hill Road based on our preliminary stability analyses:

• The analyses performed for this desktop study suggest that the primary causes of instability of the subject slope are related to two forms of erosion:

o The first consists of undercutting of the toe of the slope during heavy flooding events. This over-steepens the lower portion of the slope that then accelerates erosion of the upper portion of the slope.

o The second is the erosion of the upper portion of the slope particularly in the form of eroded gullies. This erosion progresses uphill from the over-steepened lower portion of the slope toward the crest, and roadway, working to produce a uniform overall slope angle.

• The preliminary modeling shows that global slope stability may not have a significant effect on the stability of the subject slope and Forge Hill Road, except in that over-steepening of the lower portion of the slope can lead to localized slope failures that work toward decreasing the angle of the lower portion of the slope. There is no evidence at this stage that structural slope stabilization is necessary.

However, this will be required to be confirmed in the next phase of work based on field work.

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• We believe that long-term stabilization cannot reasonably be accomplished within the right-of-way of Forge Hill Road. Furthermore, stabilizing either the upper portion of the slope or the lower portion (streambank) alone will not be sufficient to protect the roadway over the long-term. Stabilization of the streambank will not prevent the ongoing erosion of the upper portion of the slope. Conversely, preventing the erosion of the upper portion of the slope through the installation of various forms of erosion control will only protect the roadway until the next flooding event occurs and starts the cycle of oversteepening and erosion over again. Consequently, it is our opinion that both the streambank needs to be protected and stabilized, and the upper portion of the slope should be protected from further erosion through both structural and biological methods.

• Graded Slope Configuration scenarios suggest that relatively minor grading of the slope (mainly cutting several feet off the face to flatten the slope) will improve its global stability to a point that might meet the general requirements set forth by AASHTO. This may require shortening the distance between the edge of the roadway and the crest of the slope by a few feet and adding erosion prevention material (see below) at the toe of the slope. However, a future subsurface investigation, laboratory testing and analyses may show that the final extent of improvement to the slope may require more or less improvement of the slope.

The following construction-related conclusions are provided based on the preliminary modeling, and our experience with stabilizing other slopes along watercourses:

• Consideration should be given to stabilizing the toe of the slope and preventing future erosion as the first step in mitigating the danger of a collapse that could compromise the safety of Forge Hill Road.

This would normally be accomplished by protecting the toe of the slope with riprap and flow diversion or calming devices. However, riprap (if it can be used) must be designed to withstand the high flow rates, and the run-up of the water on the outside of the bend, that occurs during flooding. A hydrology and hydraulics study of the lower Moodna Creek will be necessary to develop a design flow velocity at the site before it can be determined that the toe of the slope can be protected with standard heavy riprap, or if a more resilient form of erosion protection will be necessary.

• If it is found that normal riprap will not be sufficient to prevent the toe of the slope from eroding, because the size of the needed riprap would be prohibitive, or uneconomical to obtain and place, then two other options might be feasible.

o One alternative is to use grouted riprap, which would allow for smaller riprap to be used, while still resisting the erosive forces of the creek by filling the voids between the riprap with a cement-sand grout. This option would require that the grouted riprap be keyed into the streambed at the toe below the scour depth, or extend down to bedrock, whichever is reached first, to prevent the riprap from being undermined. Therefore, this option would also require some dewatering, a cofferdam and/or stream diversion.

o A second alternative is to install a wall along the toe of the slope. The wall would need to extend above the maximum water elevation on the outside of the bend (the run-up elevation), during a design flood, and/or be backfilled with a non-erodible material (such as light riprap), and would need to extend to below the scour depth or to bedrock, whichever is shallower. We anticipate that a cast-in-place gravity wall, a modular block wall (provided that the block will not fail during flooding), a soldier pile and lagging wall, or closely spaced drilled piles cantilevered into the

15 13053.01 glacial till or socketed into bedrock could be viable alternatives that can resist the high flow velocities and impacts from large boulders that could be suspended within floodwaters.

• Whichever form of toe stabilization is eventually selected, it is recommended that the stabilization begin at the old confluence with Silver Stream and extend approximately 600 feet downstream from that point along the toe of the slope. This configuration will begin the wall approximately 100 feet upstream, and end it about 100 feet downstream, of the currently observed eroded slope.

• With the toe of the slope stabilized, it is possible that the middle and top of the slope might only need to be graded to a relatively smooth condition that can be stabilized with erosion control materials. It is expected that the slope will likely need to be graded using specialty excavators designed to work on relatively steep slopes s of equipment that is commonly used by typical earthwork contractors, and therefore, it is likely that a contractor who specializes in working on steep slopes will need to be hired to conduct this work, either as the general contractor or as a sub-contractor. Once the slope is graded, erosion control could be accomplished with turf establishment, installation of Geocell matting, or erosion control matting, and planting of deep rooting native shrubs, such as willow and alder and/or trees. However, if later analyses show that the slope will need reinforcement, it may be necessary to include soil nails, or similar reinforcement methods into the design.

Access to the toe of the slope for remediation and stabilization will need to be further evaluated; however, it appears that the streambank can be accessed at a point approximately 300 feet below the downstream extent of the subject slope, at a small drive that extends from the New Windsor Public Water Supply treatment plant, at

181 Forge Hill Road to the streambank. It is anticipated that a temporary roadway or causeway within and/or adjacent to the creek can be constructed. Construction of an access road may also require temporary removal and storage of streambed materials at the initiation of construction and stream restoration once the toe protection has been installed. It is noted the stream channel in this area has moved some 20 to 70 feet toward the north

(been widened) since the spring of 2011. This work will require NYSDEC involvement, and careful erosion and sediment control will be require to protect both the stream habitat and the Town of New Windsor s water supply.

Given the likely cost of preventing future erosion of the slope and based on our experience with evaluating the remediation of the Mill Street Slope, it is anticipated that it will take several years to analyze the slope condition, obtain funding to repair the slope, and to perform the construction. Consequently, it is recommended that the ongoing erosion of the slope be periodically monitored. The monitoring may include yearly aerial photography of the slope that can be scaled to allow for accurate measurements to be made, as well as physical surveying and/or aerial-based LIDAR or radar scanning of the slope. The LIDAR scanning should consist of an initial scan of the slope to obtain a baseline topography, with follow up scans after significantly flooding events, or at most every year.

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It is recommended that an automated stream gauge be installed within Moodna Creek to begin monitoring the water level, flow rates, and to identify flooding conditions. The cost of the monitoring will greatly depend upon the equipment used, the amount of labor needed to install it, and the duration of the monitoring. The rough order of magnitude estimates included in the following section assume five (5) years of monitoring before slope remediation can be completed.

7.0 PRELIMINARY ROUGH ORDER OF MAGNITUDE COST ESTIMATES

Tectonic has been asked to provide preliminary rough order of magnitude cost estimates for three (3) remedial options related to stabilizing the slope failure. These cost estimates are limited and are based on our experience with similar projects. The cost estimates primarily use data for NYSDOT projects that were bid between 2024 and early 2025, available through their Pay Item Catalog (https://www.dot.ny.gov/main/business-center/engineering/specifications/pay-item-catalog), with some earlier bid prices when recent ones were not available. Other cost information was obtained from our various departments, GeoStructures, Inc., or other publicly available sources.

Tectonic considered cost scenarios for investigations and monitoring of the slope, as well as best, intermediate, and worst-case scenarios for the construction. A Schematic Slope Remediation Plan, presented in Appendix IV, shows the approximate areas of the slope where various forms of work will be performed, shows an access roadway, and generally represents the lowest cost remediation option (described below as Cost Estimate Number

3). Table 7.1 gives the cost estimates for investigation, monitoring, and three different degrees of remediation, followed by several notes on the estimates.

All three cost estimates assume an engineering investigation and surveying program to fully characterize the conditions at the site and in Moodna Creek. In general, all cost estimates assume that the span from investigation to construction will be approximately 5 years (2025 to 2030) and includes a small program of scanning the subject slope once per year to monitor erosion. These estimates incorporate a mix of engineering and stabilization methods. The final investigation, monitoring and remediation plan could incorporate a mix of these various methods.

• Cost Estimate Number 1 assumes that the geotechnical investigation finds that the slope is globally unstable and will require reinforcement, and assumes that the flood flow in Moodna Creek will be high enough that some form of structural erosion control, such as a wall, will be needed to prevent future erosion of the toe of the slope.

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• Cost Estimate Number 2 assumes the engineering costs will be approximately ¾ that of Cost Estimate Number 1, and good soil and flood conditions leading to a need for grouted riprap at the toe and stream barbs in the Creek, as well as grading and erosion control on the upper portion of the slope using Geocells, and grass, shrub and deep-rooting tree plantings.

• Cost Estimate Number 3 assumes that the engineering costs will be approximately ½ that of Cost Estimate 1, and excellent soil and flood conditions lead to protection of the toe using standard dry riprap and stream barbs, as well as grading and erosion control on the upper portion of the slope using Geocells, and grass, shrub and deep-rooting tree plantings.

Table 7.1 - Summary of Slope Investigation, Monitoring and Remediation Preliminary Cost Estimates (1)

Major Line Item Cost Estimate Number

1 2 3

Engineering Investigation, Analyses and Design $600,000 (2)

$400,000 $300,000

Site Preparation (clearing, slope grading, temp causeway) $2,100,000 $2,100,000 $2,100,000

Stabilize Toe of Slope and Streambed $6,100,000 $2,600,000 $1,500,000

Stabilize Top of Slope $1,500,000 $0.00 $0.00

Slope & Site Finishing and Final Stabilization $2,700,000 $2,700,000 $2,700,000

Contingency, Overhead & Profit, Management, Materials, etc. $7,200,000 $4,400,000 $3,700,000

Escalation for 5 years (2025 - 2030) $2,800,000 $1,700,000 $1,500,000

Estimated Construction Grand Total $23,000,000 $13,900,000 $11,800,000

Notes:

1. These are partial cost estimates that do not include repair of the pavement, traffic control, and other ancillary parts of the construction that are not directly related to stabilizing the slope and roadway. OCDPW should add some additional funds for these items, when budgeting.

2. The cost estimates presented in the table have been rounded to the nearest $100,000. See the individual cost estimates in Appendix IV for more details about what is included in these estimates, and to see other notes and caveats.

3. Cost Estimates 2 and 3 assume that the slope will be found to be stable enough that no mechanical stabilization will be required. They assume that the slope conditions will only require placement of aggressive erosion control features, as described above.

4. These estimates are completely contingent on obtaining permission from New York State Parks, Recreation and Historic Preservation and the Town of New Windsor to access the site.

The recent introduction of increased tariffs on various construction materials, such as steel, has led to volatility in the costs of some construction materials. Subsequently, we have added a contingency of 30 percent into our cost estimates in an attempt to compensate for this volatility.

18 13053.01

8.0 RECOMMENDATIONS FOR ADDITIONAL INVESTIGATION AND ENGINEERING

In general, to conduct an accurate assessment of the stability and to design potential remedial actions, a field investigation program is required. The types and extent of investigation are detailed in the following bullets.

• An accurate topographic survey of the slope is required. Slight differences in the slope angle greatly affect the global factor of safety. Furthermore, given the anticipated extent of the work required to stabilize the slope, a thorough survey will be needed for construction and the survey should include a wide perimeter around the slope failure, so that all potential areas that may be utilized or modified during construction can be mapped. We recommend that an area encompassing approximately 8 to 10 acres, extending from at least 50 feet beyond the northern shoulder of Forge Hill Road, across the slope failure, to 50 feet beyond the right (southern) bank of Moodna Creek. The survey should extend at least 200 feet beyond the currently visible eastern and western extents of erosion on the slope, and should also include the drive extending from the New Windsor public water supply treatment plant to the right (northern) bank of Moodna Creek.

• A subsurface investigation, including thorough laboratory testing, should be carried out within the Forge Hill Road right-of-way, near the top of the slope. Several borings should be drilled to collect samples and data about the existing soils, groundwater, and bedrock conditions above the slope.

Unlike at Mill Street, we do not see a large number of boulders exposed on the face of this slope, so it might be possible to perform the borings using conventional drilling equipment. However, if a large number of boulders are encountered, it may become necessary to utilize sonic drilling equipment. Drilling near the crest of the slope may be unsafe; however, we anticipate that borings can be drilled in the shoulders of Forge Hill Road. At a minimum, two borings should be drilled on either side of Forge Hill Road, as far apart from each other as practicable, near the eastern end of the guiderail. At least two more should be drilled near the eastern and western ends of the eroding slope. The investigating geotechnical engineer may also conclude that additional borings should be performed at other locations.

o Most of the critical failure surfaces in the preliminary modeling are shallow and do not approach the modeled bedrock surface; however, some of the failure surfaces exit the slope near the toe and shallow bedrock could improve the global stability of the slope. Conversely, poor quality bedrock could compromise the global stability of the slope. Therefore, the subsurface investigation should include measurement of the bedrock depth and it is recommended that each boring extend 10 feet into bedrock, to avoid the potential for mistaking a boulder for bedrock. Consequently, given the height of the slope (approximately 133 feet) it is expected that the borings will be on the order of 140 to 150 feet deep, depending on the depth and slope of the bedrock surface.

o It is not anticipated that it will be feasible to perform borings on the slope, given its steepness, at the toe, or within Moodna Creek. Consequently, we recommend that a geophysical investigation be performed along the toe of the slope to measure the depth to bedrock.

Geophysical testing could be seismic refraction/reflection, or soil resistivity profiling, and should extend along the entire 600-foot length of the bottom of the slope.

o stability. It is recommended that temporary groundwater observation wells be installed within several of the borings above the slope failure. These may be either standard PVC wells and/or

19 13053.01 vibrating wire piezometers either grouted into the borings upon completion of drilling, or installed within the PVC wells. If standard wells are used, they can be periodically measured throughout the investigation and design phases of the project, while if piezometers are installed, they can be monitored remotely or by visiting the site and downloading stored data over relatively long periods of time. The piezometers would provide continuous groundwater measurements, while manual monitoring would be periodic.

• To accurately design appropriate slope remediation methods, it will be necessary to accurately estimate the water level and the speed of the flow during a design flood in Moodna Creek. It is our understanding that NYSDEC has performed a hydraulic and hydrologic (H&H) study of Moodna Creek above Blooming Grove, but one has not been performed for the lower reach of the Creek. Therefore, an H&H study of the lower reach of Moodna Creek should also be undertaken. Given the number of county roads that cross the lower reach of Moodna Creek and the fact that the Mill Street slope lies only 1.5 miles upstream from this site, OCDPW should also consider whether this data would be valuable for other projects.

It is understood that this first phase of the project was limited to a desktop study, without the benefit of site-specific investigations, which are required. Tectonic envisions several phases to this project, including the following:

• Develop and implement a monitoring program to provide periodic documentation of the rate of erosion on the slope

• Conduct a topographic survey of the existing slope failure and surrounding area for accurate slope stability analyses and for a basis for developing various construction plans.

• Develop and conduct subsurface, and hydraulic & hydrologic investigations that will characterize the soil and bedrock conditions in the slope, and the flood condition in Moodna Creek. Perform an environmental assessment of potential constraints that could affect the remediation of the slope and the types of work that can be performed. These investigations would then be used to better-evaluate the stability of the existing slope and to develop detailed remedial options.

• Develop a remediation plan that can stabilize the slope. This may include submittals of options for the County to review and preparation of preliminary drawings.

• Prepare detailed cost estimations for the entire project that includes each of the potentially viable stabilization options. These should include all aspects of stabilizing the slope.

• Prepare specifications and contract documents, obtain permits from the NYSDEC and/or other regulatory agencies, and provide bidding support.

• Perform construction materials testing during installation of the slope stabilization.

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9.0 LIMITATIONS

Our professional services have been performed using that degree of care and skill ordinarily exercised under similar circumstances by reputable geotechnical engineers and geologists practicing in this or similar situations.

The interpretation of the field data is based on good judgment and experience. However, no matter how qualified the geotechnical engineer or detailed the investigation, subsurface conditions cannot always be predicted beyond the points of actual sampling and testing. No other warranty, expressed or implied, is made as to the professional advice included in this report.

The recommendations contained in this report are intended for design purposes only. Contractors and others involved in the construction of this project are advised to make an independent assessment of the soil and groundwater conditions for the purpose of establishing quantities, schedules and construction techniques.

This report has been prepared for the exclusive use of Orange County Department of Public Works for the specific application to the stabilization and protection of County Route 74 (Forge Hill Road) and the adjacent slope, in the

Town of New Windsor, Orange County, New York, as described herein. In the event that any changes in the design or location of the proposed structure are planned, Tectonic Engineering Consultants, Geologists & Land

Surveyors, D.P.C. (Tectonic) shall not consider the conclusions and recommendations contained in this report valid unless reviewed and verified in writing. It is further recommended that Tectonic be retained to provide construction monitoring and inspection services to ensure proper implementation of the recommendations contained herein, which would otherwise limit our professional liability.

CBB/MCC File: G:\Newburgh\Geotechnical\13000\13053.01 Evaluation of CR 74 Bank Erosion\Report\13053 Forge Hill Road Slope Stability Assessment.docx

APPENDIX I

Source: https://apps.nationalmap.gov/viewer/

Forge Hill Road

OCDPW County Route 74 Erosion Evaluation

Tectonic Engineering Consultants, Geologists & Land Surveyors DCP

Date Created: 4/15/2025, 12:24:58 PM

Software Version: 9.038

Table of Contents Project Summary

Currently Open Scenarios

General Settings

Analysis Options

Group 1 (2011)

Group 2 (2022)

Group 3 (Graded)

Groundwater Analysis

All Open Scenarios

Surface Options

All Open Scenarios

Seismic Loading

Group 3 (Graded) - Scenario 2 (earthquake)

All other Scenarios

Materials

Materials In Use

Global Minimums

Group 1 (2011)

Method: spencer

Method: gle/morgenstern-price

Group 2 (2022)

Method: spencer

Method: gle/morgenstern-price

Group 3 (Graded) - Master Scenario

Method: spencer

Method: gle/morgenstern-price

Group 3 (Graded) - Scenario 2 (earthquake)

Method: spencer

Method: gle/morgenstern-price

Entity Information

Group 1 (2011)

Shared Entities

Scenario-based Entities

Group 2 (2022)

Shared Entities

Scenario-based Entities

Group 3 (Graded)

Shared Entities

Scenario-based Entities

2/16

Tuesday, April 15, 2025Forge Hill Road

Slide2 Analysis Information

Forge Hill Road

Project Summary

File Name: Forge Hill Road.slmd

Slide2 Modeler Version: 9.038

Project Title: OCDPW County Route 74 Erosion Evaluation

Analysis: Desk-Top Stability Analyses

Author: Myron Chaplin, PE

Company:

Tectonic Engineering Consultants, Geologists & Land Surveyors DCP

Date Created: 4/15/2025, 12:24:58 PM

Currently Open Scenarios

Group Name Scenario Name Global Minimum Compute Time

Group 1 (2011)

Master Scenario

Spencer: 1.002030 Gle/morgenstern-price:

1.002690

00h:00m:04.998s

Group 2 (2022)

Master Scenario

Spencer: 1.216130 Gle/morgenstern-price:

1.214480

00h:00m:04.454s

Group 3 (Graded)

Master Scenario

Spencer:…