Attachment - Tectonic Prelim Geotech Assessment - CR 32.pdf

Denise Vesely Typewritten text

PRELIMINARY GEOTECHNICAL ASSESSMENT

ROADSIDE SLOPE DISTRESS

COUNTY ROUTE 32 (MILL STREET)

TOWN OF CORNWALL, ORANGE COUNTY, NEW YORK

Orange County, New York Department of Public Works 255 Main Street Goshen, NY 10924

Attn: Travis B. Ewald, P.E.

Deputy Commissioner of Public Works

April 5, 2022

RE: W.O. 11169.01

PRELIMINARY GEOTECHNICAL ASSESSMENT

ROADSIDE SLOPE DISTRESS

Dear Mr. Ewald;

Tectonic Engineering Consultants, Geologists & Land Surveyors, D.P.C. is pleased to submit this preliminary desktop assessment and geotechnical engineering evaluation of the stability of the slope along the west side of County Route 32 (Mill Street) in the Town of Cornwall, 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 and adjacent property.

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., PMP Vice President, Geotechnical

"G:\Newburgh\Geotechnical\11100\11169.01 OCDPW, CR 32 (Mill St) Evaluation\Report\TEC Cover Letter.docx"

PRELIMINARY GEOTECHNICAL ASSESSMENT

ROADSIDE SLOPE DISTRESS

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 AND PREDICTION

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 west side of

County Route 32 (Mill Street) in the Town of Cornwall, 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 and adjacent property. 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.

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 publicly available aerial photographs and other information related the existing slope failure along the eastern bank of Moodna Creek, 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 desk-top model of the existing slope, evaluate its relative stability, and evaluate potential causes of the ongoing failures.

• Discuss potential remedial actions to stabilize the roadway and/or the existing slope, and develop three stabilization options.

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

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

3.0 SITE AND PROJECT DESCRIPTION

The project site is located along the western side of Mill Street, approximately 700 to 1,000 feet north of the intersection of Mill Street and County Route 107 (Quaker Avenue) in Cornwall, Orange County, New York. The observed slope failure is located on a slope west of the southbound lane, which is located between the roadway and the east bank of Moodna Creek.

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Information provided by OCDPW and historic observations, indicate that the slope failure along the eastern bank of Moodna Creek has progressed eastward to a point where OCDPW has become concerned about the stability and safety of Mill Street. As Moodna Creek has eroded the bank eastward, the head scarp (the highest portion, or top of the failure surface) has been observed to progress eastward, to a point where it is now less than 30 feet from the southbound edge of the road. As of the writing of this assessment, the head scarp has begun to encroach upon the old alignment of Mill Street, as can be seen from the photos below.

Photo 1: Looking Northward Along the Head Scarp and Old Mill Street Alignment

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Photo 2: Looking Southward Along the Head Scarp and Old Mill Street Alignment

As can be seen from the photos, the failure has progressed approximately halfway across the old paved surface on the old Mill Street alignment and the guide rail for the current alignment can be seen in the upper right-hand corner of Photo 1.

As of the date that this report was completed, no topographic survey of the subject slope was performed, and therefore, publicly available topographic information was used for this study. Based on limited topographic data from the USGS and Orange County GIS (OCGIS), the slope has a total height of approximately 96 feet between the stream bank and the western edge of Mill Street, over a total length of approximately 178 feet. This results in an average slope of approximately 1.9H to 1V. However, observations show that the face of the slope varies, with the slope angle near the head scarp approaching a near-vertical condition (as can be seen in the photos). Further observations of the site conditions reveal that the slope failure is occurring within glacial till with particle sizes ranging from clay-sizes (on the order of micrometers) to boulders over 6 feet in diameter (some of which are visible in both photos). Although the majority of the slope appears to be in glacial till, aerial photos provided by

OCDPW, which will be discussed in detail below, show what appears to be a bedrock outcrop, along the bank of

Moodna Creek, near the northern end of the slope failure (See

Appendix II).

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4.0 PRELIMINARY SLOPE STABILITY ASSESSMENT

To better estimate the progression of the slope failure and potential methods to evaluate the stability of the Mill

Street roadway, Tectonic prepared an approximate model of the slope using the slope stability analysis program

SLIDE2, by Rocscience. To create the model, a cross-section of the existing slope was created. However, since topographic information for the slope was not available, only data from OCGIS

(http://ocgis.orangecountygov.com/) and observations, such as the photos above, were utilized. OCGIS topographic data shows a relatively uniform slope face from toe to crest; however, their shaded relief map, our observations of the site, and our experience with similar failures all suggest that the slope is somewhat circular in profile, with the slope angle shallower near the toe, and steeper near the head scarp. Note, the steepness near the head scarp is exacerbated by the presence of the asphalt pavement for the old route of Mill Street, which prevents some sloughing and erosion at the very top of the slope.

To develop a relatively accurate slope profile (extending approximately west to east through the point where the failed slope is closest to Mill Street), we used the OCGIS data to locate the toe of the slope and to get an estimate of the surface elevations at the toe and across Moodna Creek, as well as above the head scarp. We then used photos, like those in Section 3, to roughly estimate the slope profile within about 25 feet of the head scarp.

Finally, we used judgement and experience to model the slope face between the toe and a point 25 feet below the head scarp. Figure 1, below, shows our estimated slope profile, along with an estimated groundwater surface, and a red dashed line that represents the estimated bottom of the disturbed glacial till, which has fallen from the top of the slope and created a relatively shallower slope angle near the bottom of the slope. Note that although it appears that bedrock is visible farther downstream from the approximate location of the cross-section, we have assumed that the bedrock surface is at a uniform elevation of +140 feet, or about 10 feet below the bottom of the Creek for this model.

http://ocgis.orangecountygov.com/

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With an approximate slope profile, we next considered appropriate estimates of the soil characteristics. To do this, 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. Initially, typical strength and density values were selected, and the slope stability was analyzed to confirm or modify the values. In particular, based on observations, the existing slope (especially near the head scarp) should be marginally stable, at best.

Based on initial runs of the model, we adjusted the cohesion and friction angles of the soils to result in a marginal stability at the crest of the slope. The Materials Table presented on p used for the model.

Two items should be noted about the soil parameters. 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 with a slightly lower density than the undisturbed till.

Likewise, 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. Consequently, we have modeled the disturbed till to reflect this common change.

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The primary purpose of this study was to evaluate the existing stability of the slope, to consider how future failures could occur and look at the consequences. In doing this, we looked at the overall stability as the slope currently exists, and if the head scarp were to become flatter, through erosion or grading. We note that the AASHTO Bridge structures. The stability analysis results in Appendix I will be referenced throughout this discussion and the last six pages of that report shows output results for various analyses. These are identified as Scenarios 2 through

7.

Scenario 2 analyzed the global stability of the slope and shows that the head scarp, which again is the steepest portion of the slope, has the lowest factor of safety. This is as expected, and it should be reiterated that these analyses were performed with presumptive soil strengths, and therefore, the reported factors of safety should be considered approximate and relative, rather than exact. Also included on the output sheet for Scenario 2 are two enquiries for deeper failure surfaces that show their potential failure surfaces and relative factors of safety.

Scenario 3 considered the consequence of a relatively shallow groundwater table, as compared with the base case shown in Figure 2 and used in Scenario 2. As can be seen, although it has no effect on the potential for shallow failures, a shallow groundwater table could significantly reduce the factor of safety of the deep failure surface. Subsequently, this model indicates that it is imperative that good measurements of the groundwater depths near the slope be collected and monitored. This will be discussed further in the following section.

Scenario 4 analyzes the conditions if the disturbed till were to be eroded from the toe, either over time, or during another flood. As can be seen, the stability of the slope will diminish enough to cause a failure of the upper portion of the slope, and move the head scarp closer to the current roadway. Based on this scenario, and the progression of the head scarp described in Section 4, it is our interpretation that most of the movement is related to erosion of the toe during floods.

Scenarios 5, 6 and 7 show the changes in the global stability of the slope if the head scarp were to be removed to two different extents through excavation. Scenario 5 models the slope with minimal excavation; Scenario 6 shows the same excavation and adds a surcharge load at Mill Street; and Scenario 7 models the slope with the entire wedge containing both the old and current alignments of Mill Street removed (for example, if the roadway were to be moved a significant distance uphill from its current alignment). In total, these three scenarios show that a surcharge load on Mill Street should be included in all future analyses, that the slope cannot be fully

7 11169.01 stabilized through regrading, alone, and that some other remedial action should be taken to stabilize the toe of the slope and to prevent future erosion.

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 condition. For example, a weak layer within the glacial till, a bedrock surface that slopes downward toward the west, or similarly sloping slickensides within shallow bedrock could all decrease the overall stability of the slope. Consequently, a detailed subsurface investigation to obtain accurate measurements of the subsurface conditions at the site will be imperative to accurately analyzing the stability of the slope, and Mill Street, and to develop remedial actions that can adequately stabilize the slope in a cost-effective manner.

5.0 FAILURE PROGRESSION AND PREDICTION

Another task was to attempt to measure the progression of the head scarp toward Mill Street, and to estimate its approximate rate of progression. This was done by using several aerial photos of the site provided by OCDPW.

The aerial photos show the progression of the slope failure between 2001 and 2019. In an attempt to obtain earlier data, we reviewed historic aerial photographs on the New York State Interactive Mapping Gateway

(https://gis.ny.gov/gateway/mg/) and found one aerial photo that was taken some time between 1994 and 1998.

This provided us with approximately 21 to 25 years of imagery to approximate a rate of progression over that period.

Appendix II contains the aerial photos noted above, with a best estimate of the location of the head scarp and measurements of the shortest distance between the head scarp and the white line along the western edge of Mill

Street. 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 does not occur in the same location in each photo. Because of the approximations in locating the head scarp, the distances between it and the roadway, and the subsequent estimated rate of progression, are very approximate.

Figure 1, below, shows the approximate distance between the head scarp and the edge of the roadway, assuming that the first aerial photo was taken in 1998, which results in a slightly more-conservative linear regression of the head scarp progression rate.

https://gis.ny.gov/gateway/mg/

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As can be seen from Figure 2 (and the photos in Appendix II), there appears to have been a marked movement between the first and second photos, and then the slope appears to become approximately stable between 2004 and 2010. After 2010, the movement appears to be relatively steady. Note that the same chart was prepared with the starting point in 1994, which had a slight effect on the linear regression and slightly extends the projected dates when the head scarp will reach the roadway. The full set of tables and both charts, are included in Appendix

II.

Using linear regression, a progression rate of about 1.5 to 2 feet per year (ft/yr) was calculated, depending upon the start date; as indicated by the regression results, and by the blue dotted line and arrow in the figure. However, if we look at the movement over the past decade, a progression rate of about 2 ft/yr results, and this is shown by the red dashed line and arrow in the figure. To obtain another measure of the progression rate, we also superimposed the aerial photos from 2001 and 2016 and measured the eastward movement of the inner and outer banks in the streambed, at the bottom of the slope (See Appendix II). The inner and outer banks move some 20 to 25 feet toward the east in that time, resulting in an average rate of about 1.5 ft/yr. Based on the progression rates given above, we anticipate that the head scarp may reach a location approximately 10 feet from the white line (edge of the road) sometime in 2028. However, this estimate is based on a linear, continuous

9 11169.01 progression of the slope failure and observations of past failures have shown the authors that large flood events can cause the slope to fail quickly. This is discussed in detail in the following paragraphs.

An attempt was made to determine if the progression of the slope failure toward Mill Street could be related to flooding of Moodna Creek in other words, to discrete events, rather than continuous linear progression. To do this, we looked for hurricane, rainfall and flood data from NOAA and the USGS. We found that there is no flood gage on Moodna Creek, so we looked at the one on the Wallkill River, in Gardiner, New York, assuming that the two watersheds would have experienced similar conditions over the study period. The USGS has data for the flow rate data that reaches back to 1994 in Gardiner, but does not have readily available public information for flood stage data. However, using other data that compares flow rate to flood gage elevation, we were able to estimate that the flow rate at flood stage is approximately 17,000 cfs at Gardiner. Using this estimate, we found that the Wallkill River was at or above flood stage six (6) times between April 2005 and September 2011. It should also be noted that the flow on the Wallkill River almost reached the estimated flow rate at flood stage after

Hurricane Ida passed over the area, in September of 2021 and at several other times throughout the study period.

We also checked rainfall data from a National Weather Service rain gage in Montgomery, New York, for the rainfall amounts related to these events and for known flooding events of Moodna Creek that occurred in March of 2010 and after Hurricane Irene in August of 2011. Finally, we checked hurricane data from NOAA and found that nine

(9) tropical storms or hurricanes have passed over or near Orange County between 1994 and 2021. These data are included in Appendix III.

Returning to Figure 2, it can be seen that five events are indicated on the chart. These events were 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.

It is known that some of these events, and it is expected that all of them, led to significant erosion of the toe of the slope and subsequent failures, leading to progression of the head scarp toward Mill Street. Based on these data, it is anticipated that the head scarp largely progresses after significant flooding of Moodna Creek erodes the toe of the slope, thus removing any buttressing effect that fallen soil and boulders may provide and leading to an over-steepened slope that subsequently fails. This also suggests that the slope will remain in a pseudo-stable condition until the next flood on Moodna Creek and the resulting erosion at the toe of the slope causes another failure. However, we are unable to say anything about the degree of flooding that would be necessary

10 11169.01 to cause another slope failure. It is also likely that slower erosion of the toe and/or of the slope face, and other factors, have resulted in progressive failures, as well.

The evaluation described above leads us to conclude that the slope will continue to fail and that the head scarp will continue to progress toward Mill Street at an average rate of approximately 2 feet per year (barring extreme storm events, flooding and heavy erosion of the streambank). This will lead to the head scarp being located within approximately 10 feet from the edge of the pavement by sometime around 2028 and reaching the edge of the pavement by sometime around 2034. However, a large flood event, or several smaller flood events, could cause a significant failure, or failures, that would quicken the approach of the head scarp toward the roadway.

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 head scarp could be dangerously close to the edge of the roadway sooner than 2028.

6.0 CONCLUSIONS AND RECOMMENDATIONS

The primary purpose of this geotechnical evaluation was to identify the potential causes of the past slope failures, to estimate the rate of advancement, to provide options for protecting the current alignment of Mill Street, and to provide cost estimates for possible remedial actions. The first three 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.

In general, to conduct an accurate assessment of the slope 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 approximately square area encompassing approximately 8 to 10 acres, centered on the slope failure, be mapped. The mapping should include both sides of Moodna Creek and extend from State Route 32 to approximately 100 feet east of Mill Street.

• A subsurface investigation, including thorough laboratory testing, should be performed within the soils within which the slope is failing. At least several borings should be drilled to collect samples and data about the existing soils, groundwater, and bedrock conditions above and below the slope.

Given the number of boulders visible on the slope face and in the stream channel (some of which appear to be 6 to 8 feet in diameter), we anticipate that this investigation will be relatively difficult, 11 11169.01 and it may be necessary to utilize specialty drilling equipment, such as sonic drilling equipment. It is not anticipated that it will be feasible to perform an investigation on the slope, given the steepness of the slope; however, data from above and below the slope should be collected. It is also expected that accessing the toe of the slope with a drill rig will be very difficult, if not effectively impossible.

Consequently, we recommend that borings be drilled near the left (west) bank of the Creek, in the bend at the toe of the slope. Drilling near the crest of the slope failure will be unsafe; therefore, we recommend that borings be drilled along the old alignment of Mill Street, some 50 to 100 feet from the head scarp and along the northbound ditch of the current alignment of Mill Street.

o Most of the critical failure surfaces in the preliminary modeling are shallow and do not approach the modeled bedrock surface; however, shallow bedrock could improve the global stability of the slope, especially if it rises above the streambed east of its current position. Conversely, poor quality bedrock could compromise the global stability of the slope. Therefore, a subsurface investigation should include measurement of the bedrock surface. In areas where borings can be drilled, they should extend deep enough into bedrock to confirm the surface elevation. Due to the presence of boulders, any rock coring should extend at least 10 feet into the bedrock, to verify that it is not a boulder.

o The subsurface investigation should include alternative methods, such as seismic refraction/reflection, or soil resistivity profiling, to measure the bedrock depth along the toe of the slope. Depending upon the capabilities of the geophysics subcontractor, it may also be possible to perform a survey upon the lower portion of the slope.

o stability. It is recommended that vibrating wire piezometers be installed within several of the borings above the slope failure, to allow for accurate modeling of the groundwater table.

Vibrating wire piezometers can be quickly installed, can be less expensive than deep observation wells and can be monitored, remotely or by visiting the site and downloading stored data, over relatively long periods of time.

• To accurately design appropriate slope remediation methods, it will be necessary to calculate 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, OCDPW should also consider whether this data would be valuable for other projects.

Three conclusions can be made about the general stability of the slope and Mill Street based on our preliminary stability analyses. These are discussed in the following bullets.

• The preliminary modeling shows that stabilizing the upper portion of the slope, alone, will likely not prevent future erosion of the slope and progression of the slope failure toward Mill Street. For example, experience shows that installation of soil nails within the upper portion of the slope would immediately improve the factor of safety, but they would not significantly stop erosion of the toe. As the toe continues to erode, the soil nails would allow the upper portion of the slope to become over-steep, which could eventually result in a potential catastrophic failure of the top of the slope.

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• The modeling also shows that it is unlikely that Mill Street can be protected by construction of a wall (such as a soldier pile and lagging wall, or a secant wall) within the current right-of-way, unless designed to be well-into the bedrock materials, which are at an unknown depth. This is because progression of toe erosion and subsequent slope failures will continue to progress toward the east until the head scarp reaches the wall. This would eventually undermine a shallow wall or require that the wall be constructed to support the entire height of the slope. Given that the roadway is approximately 100 feet above Moodna Creek, it is expected that it would be prohibitively expensive to construct a wall that is tall and strong enough to support the roadway, if it were to be constructed within the right-of-way.

• Although scenario 5 through 7 show that simple grading near the crest of the slope will not adequately stabilize the slope failure, they do show that it will improve the stability. Therefore, future analyses may show that erosion prevention and stabilization at the toe, combined with other stabilization methods, such as soil nails, plantings, limited grading and erosion control, may be effective.

The modeling, and our experience with stabilizing other slopes along rivers and streams, lead to the following construction-related conclusions.

• Stabilizing the toe of the slope and preventing future erosion should be the first step in mitigating the danger of a collapse that could compromise the safety of Mill Street. This would normally be accomplished by protecting the toe of the slope with riprap. 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. Depending upon the outcome of the recommended H&H study, it may be possible to stabilize and protect the toe of the slope with standard riprap. However, based on accessibility constraints and if the flow rate is too high, it may not be possible to source, haul, or place large enough riprap to withstand the erosive forces.

• 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. 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, to prevent the riprap from being undermined. Therefore, this option would also require some dewatering, a cofferdam and/or stream diversion.

• A second alternative would be 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 like light riprap, and would need to extend to below the scour depth or to bedrock, whichever is shallower. It is possible that the wall could be a cast-in-place gravity wall, but we anticipate that construction costs would be prohibitive.

The construction of a cast-in-place wall would also require a cofferdam or stream diversions, and relatively extensive dewatering. A soldier pile and lagging wall might also be used to stabilize the toe of the slope and prevent erosion; however, the lagging would need to extend down to the scour depth, thus also requiring a cofferdam and construction dewatering. Other alternatives could be closely spaced drilled piles (drilled pipe pile or minipiles) cantilevered into the glacial till or socketed

13 11169.01 into bedrock. As with the other walls, the piles would need to stick-up above the run-up elevation.

The piles should also be backfilled with light riprap to fill the gap between the piles and toe of the slope, and to resist erosion. These would eliminate the need for dewatering and may eliminate the need for a cofferdam. Note, the riprap backfill would also be relatively conducive to reducing potential future erosion, by falling into any voids that may occur between or behind the piles below the existing ground surface. It should also be noted that some drilled pile contractors can access pile locations by using previously installed piles as an access route. This would also eliminate the need for a causeway or access road to and along the toe of the slope.

• Whichever form of toe stabilization is eventually selected, it is recommended that the stabilization begin at a point roughly 200 feet downstream of the State Route 32 bridge over Moodna Creek and be turned into the streambank, to prevent flow from getting behind the stabilization. It is also recommended that the toe stabilization extend downstream to where the flow has turned away from Mill Steet. This is approximately at the point where an apparent bedrock exposure can be seen in the aerial photos included in Appendix II. This will result in approximately 400 feet of the streambank and the toe of the slope being stabilized.

• With the toe of the slope stabilized, it appears that the middle of the slope might only need to be stabilized by preventing future erosion. It is expected that this could be accomplished with turf establishment, erosion control matting and planting of deep rooting native shrubs, such as willow and alder. 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.

It should be noted that most of the conclusions and recommendation given above show that the slope stabilization cannot reasonably be accomplished within the right-of-way of Mill Street. Consequently, the recommended stabilization will likely involve work on the neighboring private properties. It should also be noted that thorough surveying and subsurface investigation programs will also require access to neighboring properties on both sides of Moodna Creek.

Access to the toe of the slope for remediation will need to be further evaluated. The toe of the slope is the right

(east) bank of Moodna Creek, and it is approximately 200 feet west and 100 feet below Mill Street. Therefore, we do not anticipate that it can be readily accessed from Mill Street. Based on our review of the site, it appears that the best access to the toe of the slope is from State Route 32, along Moodna Creek by way of a temporary roadway or causeway within and/or adjacent to the creek. Construction of an access road will also likely require temporary removal and storage of streambed materials at the initiation of construction and stream restoration once construction of the toe protection has been installed. It was noted in Section 4 that the stream channel has moved some 20 to 25 feet toward the east since 2001. Another possible option would be to divert the stream channel back to roughly its 2001 location, thus using the native material to construct the temporary roadway, or a new stream bank along the toe of the slope. All of this will require NYSDEC involvement and careful erosion

14 11169.01 and sediment control. NYSDOT involvement may also be required, if it becomes necessary to access the site from State Route 32, and/or to temporarily install monitoring equipment on the bridge.

As described in Section 5, if the ongoing slope failure continues in a relatively progressive and approximately linear fashion, we anticipate that the head scarp will be within 10 feet from the edge of the roadway by sometime in 2029, excluding the occurrence of an extreme event, such as a major flood. A distance of 10 feet from the edge of the pavement was selected because the preliminary slope stability results shows that the points where the critical failure surfaces intersect the ground surface typically fall between 9 and 11 feet uphill of the existing head scarp and because of a progression rate of about 5 to 6 feet was measured between successive aerial photos taken between 2010 and 2019. This suggests that it can be expected that the head scarp will move about

10 feet toward the roadway, on average, with each successive failure and, once the slope failure reaches a point where the head scarp is about 10 feet from the edge of the roadway, it can be expected that the next failure would likely take a portion of the paved road and endanger the public. However, it should be reiterated that this distance is solely based upon a preliminary desktop study model, and therefore, the actual amount of loss of ground with each successive slope failure will likely be significantly different. Furthermore, there is no guarantee that failures will be evenly or widely separated in time, or that only one failure will occur with any particular flood.

The analyses discussed here in also cannot eliminate the possibility that several smaller high-water events could cause similar erosion at the toe that could contribute to oversteepening and subsequent slope failure.

OCDPW has asked for suggestions for monitoring the slope that could be used to provide a warning system for closing Mill Street, if there is a potential for a slope failure that could compromise the safety of the roadway.

Movement of failing slopes can be measured by several means. The progression of creeping slope failures is often done through the monitoring of slope indicators that are installed across the failure surface. However, this is not the case for this type of slope failure; therefore, monitoring of the existing slope face would be a better option. One method for monitoring the slope is to install prisms upon the slope face and having them monitored daily, or more frequently, using a robotic total station. This method is frequently used to measure both lateral and vertical movement of structures during construction and can be set up to monitor the slope. Although very accurate, this type of monitoring can be fairly expensive. Another option is to install tilt meters on the slope face.

They can be mounted on posts driven into the slope face and connected (either with wires or wirelessly) to a control box that has a solar power source and a wireless internet connection. As with the robotic total station, the tilt meters can be set to be monitored at different intervals ranging from minutes to hours. Furthermore, depending upon the manufacturer, the tilt meters can be powered by batteries, which would require periodic

15 11169.01 access to the meters, or by a small solar panel, which would eliminate the need to access the individual meters.

These indicate movement by measuring the tilt of the meters as the slope face moves. Furthermore, the control box can also be connected to an overall monitoring system that includes the piezometers recommended at the top of this section.

There are other methods for monitoring the slope, such as installing extensometers, using ground or aerial-based

LIDAR or radar scanning, and aerial or satellite imagery. It should be noted that any instrumentation installed on the slope face will likely require a specialty installer who can do work from ropes, given the steepness of the slope face. It should also be expected that any instrumentation that is installed on the slope face could be lost in a future slope failure. In addition to monitoring the slope, it is expected that there would be value in measuring the water level within Moodna Creek. It might be possible to correlate the water level with the amount of erosion and loss of the slope face, and measurement of the water lever might also be a valuable component of an early warning system. Consequently, it is recommended that an automated stream gauge be installed within Moodna

Creek. Furthermore, scanning instrumentation, like the robotic total station or a ground-based LIDAR scanner, will need a permanent foundation installed in a location with a clear view of the slope face. 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 6-, 4- and

2-year monitoring programs, before slope remediation can be completed, and assumes varying degrees of monitoring.

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 2018 and early 2022, available through their Pay Item Catalog (https://www.dot.ny.gov/main/business-center/engineering/specifications/pay-item-catalog). Other cost information was obtained from our various departments or other publicly available sources.

Tectonic considered best-, intermediate-, and worst-case cost scenarios for the three (3) primary phases that are expected to follow this preliminary assessment: namely investigations; monitoring and construction; and considering the span of time that these may encompass. Table 7.2.1 gives the cost estimates for three different degrees of investigation and remediation, followed by several notes on the estimates.

https://www.dot.ny.gov/main/business-center/engineering/specifications/pay-item-catalog https://www.dot.ny.gov/main/business-center/engineering/specifications/pay-item-catalog

16 11169.01

All three cost estimates assume an engineering investigation and surveying program to fully characterize the conditions at the site, in the slope and in Moodna Creek. Cost Estimate Number 1 assumes a 6-year program for monitoring the slope with a robotic total station and tilt meters, and poor soil and flood conditions leading to a need for extensive toe and slope stabilization through the installation of a toe wall and soil nails throughout the slope face. Cost Estimate Number 2 assumes monitoring of the slope over a 4-year period with a robotic total station and tilt meters, and good soil and flood conditions leading to a need for grouted riprap at the toe and stabilization of the remaining slope through installation of soil nails near the crest, combined with limited grading, plantings and erosion control measures. Cost Estimate Number 3 assumes monitoring the slope with tilt meters over a 2-year period, and excellent soil and flood conditions leading to protection of the toe using standard dry riprap and stabilization of the remainder of the slope face through limited grading, plantings and erosion control measures. These estimates incorporate a mix of engineering, monitoring and stabilization methods, and the final investigation, monitoring and remediation plan could incorporate a mix of these various methods.

Table 7.0.1 - Summary of Slope Investigation, Monitoring and Remediation Preliminary

Cost Estimates (1)

Major Line Item

Cost Estimate Number

1 2 3

Engineering and Surveying $500,000 $500,000 $500,000

Monitoring $800,000 $600,000 $200,000

Stabilize Top and Middle of Slope $2,700,000 $1,800,000 $400,000

Stabilize Toe of Slope $2,900,000 $2,300,000 $1,800,000

Estimated Construction Subtotal $6,800,000 $5,200,000 $2,900,000

Contingency, Overhead & Profit, Management, Materials, etc. $3,600,000 $2,800,000 $1,500,000

Escalation for 6, 4 and 2 years $1,900,000 $2,000,000 $1,400,000

Estimated Grand Total $12,300,000 $10,000,000 $5,800,000

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 is not directly related to stabilizing the slope and roadway. OCDPW should add some additional funds for these items, when budgeting.

2. 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. Because soil nails are included in Cost Estimate Numbers 1 and 2, the turf reinforcement typically used with them is galvanized wire and the wire should be expected to have a design life of approximately 40 years. Similarly, erosion control matting is incorporated into some of the estimates. It is assumed that plantings and turf will be well-established before these materials deteriorate, and thus, will take over surface stabilization thereafter.

17 11169.01

In addition to the notes given above, it should be clearly understood that these cost estimates have been based upon historic pricing. The extraordinary affect that the COVID-19 pandemic has had on the availability and cost of various construction materials and labor has led to extraordinary inflation. Subsequently, we have attempted to incorporate that inflation, along with a contingency of 30 percent, into our cost estimates in an attempt to compensate for this volatility. OCDPW should take this into consideration when attempting to obtain funding for monitoring, investigating and remediating the ongoing slope failure and protecting Mill Street.

8.0 RECOMMENDATIONS FOR ADDITIONAL INVESTIGATION AND ENGINEERING

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 real-time monitoring that may be used as an early warning system for the closure of Mill Street, if necessary.

• 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 at in Moodna Creek. 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 that can stabilize the slope and roadway either as a whole, or in phases, depending upon budgeting availability and importance.

• 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 and protecting the roadway. The cost estimations should also include estimations for the potential phasing noted in the bullet above.

• Prepare specifications and contract documents, and provide bidding support.

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

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

18 11169.01 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 32 (Mill Street) and the adjacent slope, in the Town of Cornwall, 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.

MAS/CBB/MCC File: G:\Newburgh\Geotechnical\11100\11169.01 OCDPW, CR 32 (Mill St) Evaluation\Report\11169.01 Mill Street Slope Stability Assessment.docx

APPENDIX I

MoodnaCr

M oo dna Cr150

JE

S S

IC

A C T

M o o d n a C r

0 22 0 24

Orange County Data Address Points

County Boundary

Municipal Boundaries

Parcel Boundaries

0 100 20050

Miles

1̄:2,257

DISCLAIMER: This map is a product of Orange County GIS Division. The data depicted here have been developed with extensive cooperation from other County departments, as well as other Federal, State and Local government agencies. Orange County make no representations as to the accuracy of the information in the mapping data, but rather, provide said information as is. Orange County expressly disclaim responsibility for damages or liability that may arise from the use of this map.

May 12, 2021

CREDITS: Orange County, Sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen, Rijkswaterstaat, GSA, CR32/Moodna Creek

Myron Polygonal Line

Myron Distance Measurement Distance:

365.41 ft

Myron Chaplin Call Out Approximate Location of Slope Cross-Section

MoodnaCreek

M oodna

C reek

Orrs MillsRd

S ta te

R o u te

M o o d n a C re e k

J e s s ic a C t

M il l S t

M il l S t

Orange County Data Address Points

County Boundary

Municipal Boundaries

Parcel Boundaries

0 100 20050

Miles

1̄:2,257

DISCLAIMER: This map is a product of Orange County GIS Division. The data depicted here have been developed with extensive cooperation from other County departments, as well as other Federal, State and Local government agencies. Orange County make no representations as to the accuracy of the information in the mapping data, but rather, provide said information as is. Orange County expressly disclaim responsibility for damages or liability that may arise from the use of this map.

May 12, 2021

CREDITS: Orange County, Sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen, Rijkswaterstaat, GSA, CR32/Moodna Creek

Myron Chaplin Line

Myron Chaplin Call Out Approximate Location of Slope Cross-Section

11169.01 Preliminary Slope Stability Assessment

OCDPW CR32…