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HomeMy WebLinkAboutCity of Renton Ceder River Trail Stabalization Geotechnical Evaluation ReportField Reconnaissance and Preliminary Geotechnical Evaluation Cedar River Bank Stabilization River Mile 2.5 Renton, Washington for City of Renton May 29, 2020 Field Reconnaissance and Preliminary Geotechnical Evaluation Cedar River Bank Stabilization River Mile 2.5 Renton, Washington for City of Renton May 29, 2020 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 Field Reconnaissance and Preliminary Geotechnical Evaluation Cedar River Bank Stabilization River Mile 2.5 Renton, Washington File No. 0693-087-00 May 29, 2020 Prepared for: City of Renton Community Services – Parks and Trails 1055 South Grady Way – 5th Floor Renton, Washington 98057 Attention: Cailín Hunsaker Prepared by: GeoEngineers, Inc. 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 Brett E. Larabee, PE Theo M. Malone, PE Senior Geotechnical Engineer Water Resources Engineer Lyle J. Stone, PE Associate Geotechnical Engineer Joe O. Callaghan, MS, PWS Principal BEL:TMM:LJS:JOC:leh Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record. May 29, 2020 May 29, 2020| Page i File No. 0693-087-00 Table of Contents 1.0 INTRODUCTION AND PROJECT UNDERSTANDING ........................................................................................ 1 2.0 PURPOSE AND SCOPE OF SERVICES ............................................................................................................ 1 3.0 LITERATURE REVIEW ...................................................................................................................................... 1 3.1. Geologic Setting .......................................................................................................................................... 1 3.2. Site Soil Conditions ..................................................................................................................................... 2 4.0 SITE INVESTIGATION ....................................................................................................................................... 2 4.1. Site Conditions and Slope Reconnaissance .............................................................................................. 2 4.2. River Hydraulics Observations ................................................................................................................... 3 5.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................................................... 4 5.1. Existing Slope Evaluation ........................................................................................................................... 4 5.2. Temporary Stabilization Alternatives ......................................................................................................... 4 5.2.1. General ............................................................................................................................................. 4 5.2.2. Erosion Protection ........................................................................................................................... 4 5.2.3. Slope Toe Protection and Regrading .............................................................................................. 5 5.2.4. Temporary Trail Realignment .......................................................................................................... 5 5.2.5. Discussion of Temporary Stabilization Alternatives ...................................................................... 7 5.3. Slope Monitoring ......................................................................................................................................... 7 5.4. Discussion of Permanent Repair ............................................................................................................... 8 5.4.1. Slope Reconstruction ...................................................................................................................... 8 5.4.2. Hydraulic Considerations ................................................................................................................ 9 6.0 LIMITATIONS ................................................................................................................................................... 9 LIST OF FIGURE Figure 1. Vicinity Map Figure 2. Slope Stability Results - 13 Foot Setback from Slope Crest (Center Line of Trail) Figure 3. Slope Stability Results - 20 Foot Setback from Slope Crest Figure 4. Slope Stability Results - Trail Widening APPENDICES Appendix A. Site Photographs Figures A-1 through A-8 – Site Photographs Appendix B. Report Limitations and Guidelines for Use May 29, 2020| Page 1 File No. 0693-087-00 1.0 INTRODUCTION AND PROJECT UNDERSTANDING This report presents the results of our site reconnaissance and preliminary geotechnical recommendations for the Cedar River Emergency Bank Stabilization project. The project site is located on the left bank of the Cedar River at approximately River Mile 2.5 and is about 0.5 miles east of the parking lot for the Cedar River Dog Park in Renton, Washington. A vicinity map is provided as Figure 1. Our understanding of the project is based on our discussions with City of Renton Community Services personnel and site reconnaissance completed by GeoEngineers on March 25, 2020 and May 18, 2020. We understand that during a storm event in early February of 2020, high flows in the Cedar River resulted in erosion of a section of the left riverbank. Detailed measurements of the riverbank configuration prior to the erosion event were not available; however, based on review of available Light detection and Ranging (LiDAR) and aerial imagery it appears the crest of the riverbank retreated as much as 15 to 25 feet in some locations as a results of the erosion. The crest of the riverbank is now as close as about 7 feet from the edge of the Cedar River Trail pavement and the approximately 20 feet of upper riverbank slope has eroded to a near vertical configuration. The City of Renton had closed the Cedar River Trail in the vicinity of the eroded area due to concerns that ongoing slope erosion could undermine the trail. We understand that recently temporary access was restored to the area by opening the lane of the trail furthest away from the slope crest. A temporary chain link fence was installed along the approximate center line of the trail to deter pedestrians from approaching the slope crest. The City is considering emergency repairs to temporarily stabilize the riverbank in order to protect the trail and ensure safe pedestrian access through the area. We understand that emergency repairs to the riverbank would likely be limited to areas above the ordinary high water elevation to facilitate permitting. We expect that the emergency repairs would be incorporated into a larger repair and bank stabilization project that would likely include work near and below high water levels in order to reestablish habitat and protect the riverbank from erosion in future high flow events. 2.0 PURPOSE AND SCOPE OF SERVICES The purpose of our services is to complete a site reconnaissance to document existing conditions, develop recommendations for temporary slope stabilization alternatives and provide a discussion of possible long- term repair alternatives. We anticipate that permanent stabilization alternatives could require a more detailed geotechnical site investigation and detailed hydraulic design. Our services have been completed in accordance with our agreement for this project executed on May 8, 2020. A complete list of our authorized services is provided in the agreement. 3.0 LITERATURE REVIEW 3.1. Geologic Setting The Geologic Map of the Renton Quadrangle (Mullineaux 1965) maps the soils at the site as Alluvium (Qac) and Artificial Fill (Af). Alluvial soils are described in the literature as “sand and gravel deposited by the Cedar River, and associated thin beds of silt, clay and peat…”. Artificial fill is typically associated with land May 29, 2020| Page 2 File No. 0693-087-00 modification activities including construction and regrading. In this case it appears that artificial fill is associated with construction of the former railroad grade, which was converted to the Cedar River Trail. We expect that fill at the site consists primarily of reworked native soils and possibly ballast material used to construct the railroad grade. 3.2. Site Soil Conditions Explorations were not completed as a part of this study. Our understanding of soil conditions at the site is based on observations made during our site reconnaissance and our experience working in the area. The erosion that occurred left a near vertical face, which allowed us to observe the soils that comprise the riverbank and underly the trail. A photograph of the exposed riverbank soils is provided as Photo 1 in Appendix A. Starting at the crest of the existing riverbank slope observed soils were comprised of a surficial layer of forest duff and topsoil underlain by fill and alluvial soils The forest duff layer appears to be on the order of 12 to 18 inches thick and is comprised of dark brown silty sand with decomposing organic materials. The fill, which appears to be relatively old and likely associated with modifications made for the railroad grade, consisted of silty sand with gravel and debris (bricks, wood, glass). The fill layer was generally on the order of 2 feet thick. Underlying the fill are alluvial soils. There appears to be two units within the alluvial deposits. The upper alluvial soils, which appear to be on the order of 20 feet thick were comprised of stiff sandy silt and loose to medium dense silty fine sand. These soils were observed to be standing vertical or near vertical on the slope face. We expect that these fine-grained alluvial soils were deposited by the Cedar River during lower velocity flow events. Underlying the fine-grained alluvial soils are coarse-grained alluvial soils. These soils appear to be comprised of medium dense gravel with sand, silt, and cobbles. The coarse-grained alluvial soils were observed near the water line of the river and we anticipate comprise the riverbed. 4.0 SITE INVESTIGATION 4.1. Site Conditions and Slope Reconnaissance We completed a visual reconnaissance of the site on March 25, 2020 and May 18, 2020. During our March site reconnaissance, we took detailed measurements of the eroded area and documented existing conditions. During our May site reconnaissance, we observed the site to look for changes to the slope since our original reconnaissance. A summary of features observed during our reconnaissance is provided below. Pictures from our March 25, 2020 site reconnaissance are included in Appendix A. In the project vicinity, the Cedar River Trail is a two-way pedestrian pathway surfaced with asphalt concrete. The trail is on the order of 12 feet wide with 4-foot-wide gravel shoulders. The trail is constructed on a former railroad track embankment, which appears to be on the order of 2 to 3 feet thick. The trail is oriented approximately northwest–southeast in the project area and follows a generally straight alignment. The Cedar River is located on the north side of the trail in this area. On the south side of the trail is what appears to be a drainage ditch, which has become overgrown with Himalayan blackberry (see Photo 2 in Appendix A). The south side of the trail is located near the toe of a hillside that has a relatively gradual inclination (around 2H:1V [horizontal to vertical]). This slope is densely vegetated with trees, understory shrubs, and Himalayan blackberry. May 29, 2020| Page 3 File No. 0693-087-00 The area of slope erosion is located on the north side of the trail. The eroded area has a circular shape. The length of eroded area is approximately 150 feet measured along the scarp. The proximity of the scarp to the trail varies. At the closest, the scarp is about 7 feet from the north edge of the trail pavement (see Photo 3 in Appendix A). The distance between the scarp and trail pavement increases once outside this critical location. Based on field measurements the distance between the crest of the slope and trail pavement increases to 20 feet within about 30 feet to the northwest and 40 feet to the southwest from the critical location. The observed slope erosion appears to have occurred primarily within the upper fine-grained alluvial soils. The eroded slope is currently standing at a near vertical inclination. The vertical slope height varies between about 20 and 25 feet tall. At the contact between the fine- and coarse-grained alluvial soils, the slope of the riverbank flattens forming a bench (see Photos 4 and 5 in Appendix A). The bench area is near horizontal at the contact between the fine- and coarse-grained alluvial soils and then grades gradually downwards at an approximately 2.5H:1V to 3H:1V slope to the river. We observed signs of ongoing crest retreat during our March 25th site visit as indicated by blocks of soil from the crest of the slope that had fallen and collected on the bench (see Photo 6 in Appendix A). During our site reconnaissance on May 18, 2020, we did not notice any significant changes to the slope conditions documented above. We did observe some small areas of surficial sloughing on steeper portions of the slope; however, no crest retreat was observed. 4.2. River Hydraulics Observations At the time we completed our March site reconnaissance the vertical distance between the contact of the fine- and coarse-grained alluvial soils, at the base of the eroded slope, and the water level in the river was between 6 feet and 9 feet. Photo 7 shows the water level at the time we completed our reconnaissance and Photo 8 shows the river water level around the time the erosion event occurred (Photo 8 was provided to us by the City of Renton). Downed trees, driftwood and other debris have collected on the bench area. Some of the larger trees appear to have fallen as the result of the flood event and associated erosion. Other trees have become undermined and are at risk of falling into the river or onto the slope (see Photos 9 and 10 in Appendix A). The United States Geological Survey (USGS) manages a stream gage (USGS 12119000 CEDAR RIVER AT RENTON, WA) on the Cedar River at River Mile 1.6, downstream of the site near the I-405 crossing of the Cedar River. During our March 25, 2020 reconnaissance the stream gage height was approximately 8.6 feet with a corresponding discharge of approximately 560 cubic feet per second. From visual observations of the riverbanks upstream and downstream of the erosion, it appears the ordinary high water elevation of the river is located between 4 feet and 10 feet, measured horizontally, up the bench area from the edge of water as shown in the pictures taken on March 25, 2020. Based on this estimation, about 90 feet of coarse-grained alluvial soils is exposed as measured parallel to the riverbank at the ordinary high water elevation. May 29, 2020| Page 4 File No. 0693-087-00 5.0 CONCLUSIONS AND RECOMMENDATIONS 5.1. Existing Slope Evaluation During our site investigations we did not observe signs of imminent slope movement to include tension cracks above the slope, cracking of the trail pavement, or surface settlement. We also did not observe water seepage on the face of the slope, which can indicate risk of ongoing slope instability. The ongoing crest retreat we observed during our site reconnaissance is typical of steep vertical slopes as they naturally establish a more stable configuration; however, in our opinion it is not an indication of a larger slope instability. Slope conditions observed during our May 18th site visit were generally the same as those observed during our March 25th site visit. Further, we understand that substantial changes to the slope configuration have not been observed since the original failure occurred in February of 2020. Based on our observations, it appears that the slope has eroded to a temporarily stable configuration. Ongoing erosion of this slope is likely to continue. We expect that erosion will primarily consist of small surficial slides and crest retreat. If high river levels erode the toe of the vertical portion of the slope, or hydraulic conditions significantly change, more significant movement of the slope could occur. We expect that this would most likely to occur during flood events such as what occurred in February 2020. 5.2. Temporary Stabilization Alternatives 5.2.1. General To protect the existing trail, we recommend temporary stabilization measures be implemented. At a minimum, we recommend erosion control features be installed, to the extent possible, on the vertical portion of the exposed fine-grained alluvium soils. Additional erosion protection could be achieved by placing armoring at the toe of the vertical portion of the slope. Until more significant slope reinforcement and stabilization measures are implemented, we do not recommend that the trail be reopened to public access in its current or original alignment. It appears feasible to temporarily realign a portion of the trail away from the crest of slope so that a safer set back from the scarp can be maintained. The sections below provide recommendations for temporary stabilization of the slope and set back recommendations for temporary pedestrian access through the area. As discussed further in subsequent sections, additional design and analysis will be required to provide long-term bank stabilization. 5.2.2. Erosion Protection Erosion protection features should be installed on the exposed vertical face of the eroded slope. We recommend that erosion protection features consist of jute-mats (or other erosion control blankets) secured onto the face of the slope and the planting of vegetation, where possible. We also recommend that straw wattles or other stormwater diversion features be installed at the crest of the slope to prevent surface water from running down the face of the slope. The jute-mats should be installed so the matting is in contact with the face of the slope along its entire length. The matting will need to be secured into the place at the top of the slope and along the face of the slope to ensure the matting maintains in contact with the slope. At the top of the slope, the matting could be secured using an anchor trench or stakes. The anchor point at the top of the slope should be setback at least 5 to 10 feet from the existing crest of the slope. Stakes used to secure the matting on the face of the slope should extend through areas of loose recently eroded soil and into more competent materials. Stakes might need to be on the order of 5 feet long to reach competent soils. May 29, 2020| Page 5 File No. 0693-087-00 The selected erosion control blanket should be capable of allowing vegetation to grow through its openings so the slope face can be revegetated. Plantings that will establish root systems quickly will be most beneficial. The steepness of the slope will limit where and what type of vegetation can be planted. We recommend that individual planting locations be evaluated on a case-by-case basis. 5.2.3. Slope Toe Protection and Regrading We expect that if the toe of the vertical slope is further eroded, slope movement could occur and/or slope crest retreat could accelerate. Erosion of the toe is most likely to occur during high flow events. It appears that the toe of the vertical slope is above the ordinary high water elevation, therefore. we expect that water levels will reach the toe only during flood events. If erosion were to occur upstream or downstream, adjacent to the project area, the toe of the currently eroded slope could be exposed to higher velocities, risking further erosion. Temporary protection of the toe of the vertical slope could be achieved by installing armoring such as riprap. Armoring could be placed as a temporary erosion control feature and then removed or incorporated into permanent repairs. For temporary protection, we recommend that toe armoring be at least 4 feet thick at the base and extend at least 4 feet up from the toe of the vertical slope. If riprap is used, this would likely require placing larger riprap at the toe of the slope and then using progressively smaller sized riprap to build up the armoring. The riprap at the toe should be on the order of 16 inches in the smallest dimension and riprap at the top of the section should be no smaller than about 6 inches in the smallest dimension. We do not expect that armoring placed at the toe of the slope will significantly increase the stability of the upper portions of the slope. Regrading (flattening) the upper portions of the vertical slope is likely the simplest way to increase the stability of this portion of the slope. If the slope is regraded, we recommend an inclination of 2H:1V be established. Re-grading the slope to this inclination will likely require cutting into the trail and it will be difficult to replace the material in the future. The work described above would likely require the use of long-reach excavators and other heavy construction equipment. To maintain stability of the slope, heavy equipment should not be operated within about 15 feet of the crest of the slope unless additional analyses are completed to evaluate stability of the slope considering the specific construction loads. In order to deliver riprap to the base of the slope, a small crane that can be adequately setback from the slope while still reaching the area could be necessary. Riprap might need to be hand placed once delivered to the toe of the slope. 5.2.4. Temporary Trail Realignment Even with the implementation of erosion control and toe armoring, we recommend that pedestrian access be setback from the slope crest until permanent repairs can be constructed. We completed slope stability analysis to evaluate the stability of the slope at different setback distances. Details of the analysis methodology and results are provided in the sections below. 5.2.4.1. Slope Stability Analysis Methodology Slope stability analyses were completed using the computer program SLOPE/W (GEO-SLOPE International, Ltd., 2016). SLOPE/W evaluates the stability of numerous trial shear surfaces using a vertical slice limit- equilibrium method. This method compares the ratio of forces and moments driving slope movement versus forces and moments resisting slope movement for each trial shear surface and presents the result as the factor of safety. The program then sorts the trial shear surfaces and identifies the surface with the lowest factor of safety, or the “critical” shear surface. We assumed a circular arc slip surface and used the Spencer method to calculate the forces. May 29, 2020| Page 6 File No. 0693-087-00 We evaluated the existing condition of the slope without the inclusion of erosion protection or toe armoring which, in our opinion, is conservative. The slope stability cross section we considered is based on the slope geometry at the location where the existing slope crest is closest to the trail pavement. Soil properties used in our analysis were determined by back calculating the soil strength properties required to maintain a slope factor of safety = 1.0 at the current observed geometry. In our analysis we assumed river water levels near the approximated ordinary high water elevation. We evaluated the stability of the slope considering a pedestrian setback at the center line of the trail (setback observed during our May reconnaissance) and determined the setback required to achieve a slope factor of safety equal to 1.25. There is no consistent standard for stability of trails and riverbanks. The Washington State Department of Transportation (WSDOT) Geotechnical Design Manual (GDM) recommends a minimum factor of safety of 1.25 for “…general slope stability of permanent cuts, fill and landslide repairs…”. Because this is a temporary slope condition that does not support structures or roadways, we did not evaluate seismic slope stability. 5.2.4.2. Slope Stability Results Figure 2 shows the factor of safety of a slope failure that extends to the setback location observed during our May reconnaissance (center line of trail, about 13 feet from the crest of the slope). At this setback location, the factor of safety of the slope is around 1.06. Considering a setback distance of about 20 feet from the slope crest (see Figure 3) the factor of safety of the slope is around 1.26. 5.2.4.3. Discussion and Construction Considerations In our opinion, a factor of safety of 1.06 is marginal for allowing pedestrian access through the area. While this is below a factor of safety typically used for a long-term condition, in our opinion, there is a relatively low risk that a sudden collapse of the slope extending to the trail centerline will occur without an associated flood event and/or preceding smaller slope movements that would indicate a large slope movement is eminent. In this condition we recommend that the slope be monitored for signs of instability on a weekly basis and immediately after high river flows. We recommend that the pedestrian setback is increased so the factor of safety of a slope failure impacting the pedestrian area is at least 1.25. Based on the results of our analysis, this could be achieved by increasing the pedestrian setback to at least 20 feet from the slope crest. With this higher level of stability, the slope can be monitored less frequently, on a monthly basis and immediately after high river flows. At the location where the slope crest is closest to the existing trail, it is approximately 20 feet from the crest of the slope to the far (south) edge of the trail pavement. If the pedestrian traffic is restricted within 20 feet of the slope crest, this would leave only the gravel shoulder of the trail for pedestrian traffic. We expect that this would not provide adequate access through the area. It appears feasible to temporarily widen the trail into the ditch area on the south side of the trail to maintain the recommended setback and provide adequate access. Based on field measurement we expect that an approximately 70-foot-long section of the trail would need to be widened into the shoulder. Once a 20-foot setback can be maintained at the center line of the trail, it appears feasible to begin transitioning pedestrian traffic back onto the existing trail. The hillside slope above the trail, to the south, is relatively flat and is densely vegetated. In our opinion widening the trail shoulder into the ditch area will not impact stability of the slope above the ditch. Further, based on results of additional stability results, placing fill within the ditch on the south side of the trail will not decrease the stability of the existing eroded slope (see Figure 3). May 29, 2020| Page 7 File No. 0693-087-00 If this option is pursued, we recommend that existing vegetation be removed prior to filling the ditch. We recommend that the lower approximately 18 inches of fill consist of quarry spalls conforming to WSDOT Standard Specification 9-13.1(5). The permeable quarry spalls provide void spaces to maintain water flow through the ditch. To maintain the voids and the drainage characteristics, we recommend the quarry spall fill be wrapped on all sides with a non-woven geotextile for separation per WSDOT Standard Specification 9-33.2(1) Table 3. We recommend the remainder of the fill material consist of a crushed rock product material similar to Crushed Surfacing Base Course (WSDOT Specification 9.03.9(3). A finer material can be used within the upper 4 to 6 inches as a surfacing course for the trail. Other fill materials should be compacted to a firm condition as needed to support the trail traffic. Large heavy construction equipment should not be used to complete this work and equipment should not be operated within about 15 feet of the crest of the slope unless additional analyses are completed to evaluate stability of the slope considering the construction loads. We recommend that hand-operated compaction equipment that will not induce large vibrations near the slope be used to compact materials. 5.2.5. Discussion of Temporary Stabilization Alternatives In our opinion erosion protection on the face of the vertical slope should be installed as soon as possible to prevent ongoing erosion. Widening a portion of the trail to provide a setback from the slope crest appears feasible and can likely be completed without obtaining permits related to work below the ordinary high water elevation. Further, we expect that the widened area can also be utilized, and may be necessary, during construction of permanent slope repairs. Once a permanent repair is in place it will also be possible to remove the material used to widen the trail and restore the current condition. Armoring the toe of the slope will help protect the slope during future high flow events; however, in our opinion armoring the toe is not required in order to install erosion protection on the vertical face of the slope or widen the trail. Re-grading the upper portion of the slope will help stabilize the area; however, regrading will likely require cutting into the existing trail and will make restoring the area more challenging once the permanent slope repairs are in place. We expect that placing toe armoring and re-grading the slope would be relatively difficult and could require more substantial coordination and permitting efforts then what will be needed to complete the other mitigation options. We expect that toe armoring and regrading could be part of the future long-term repairs, so there could be on opportunity to incorporate temporary armoring and regrading into future repairs; however, since the long-term repairs have not yet been developed or permitted, it is unclear how the temporary activities would impact the long-term design. 5.3. Slope Monitoring We recommend that a regular slope monitoring program be established as part of the temporary stabilization efforts. Regular visual monitoring of the slope should be completed to look for signs of ongoing slope erosion and indications of larger slope movements. We recommend the condition of the slope be observed, documented, and compared to the conditions described in this report regularly. Monitoring should occur on a weekly basis if the partial 13-foot setback is used or on a monthly basis if the full 20-foot setback is used. In either case, the slope should be observed more frequently and immediately after the river reaches flood stage, as described below. If indications of erosion or slope movement are identified, we recommend that we be notified and that the trail be closed until we can provide an evaluation and further recommendations, as appropriate. Indications of slope movement and instability include the following: May 29, 2020| Page 8 File No. 0693-087-00 ■ Erosion channels on the face of the slope or eroded soil collected at the base of the slope ■ Sloughing and retreat of the slope crest ■ Water seepage on the face of the slope ■ Tension cracks on the face of the slope or at the slope crest ■ Settlement of areas behind the slope During periods of high flow in the Cedar River more frequent observations should be made. During the February 8, 2020 flood event that caused the observed erosion, the gage height of the Cedar River as measured at USGS Stream Gage 12119000 was about 16 feet with a corresponding discharge of approximately 9,600 cubic feet per second. The USGS website indicates that flood stage for the Cedar River, as measured at this gage, is 13 feet. We recommend that weekly observations of the slope should be made if the gage height is approaching flood stage (13 feet as measured at gage 12119000). If the gage height exceeds flood stage, we recommend the trail near the eroded area be temporarily closed. After the gage height is below flood stage, we recommend the slope be inspected for the indications of slope movement and instability described above. The criteria for closing the trail should be reevaluated as regular observations are made. In our opinion, the eroded area will be at most risk of additional significant erosion when water levels are high enough to potentially erode the toe of the slope. 5.4. Discussion of Permanent Repair As discussed above, the temporary stabilization alternatives will help reduce the rate of ongoing erosion and, if implemented, reduce the risk of further toe erosion. However, these measures are temporary and, even when implemented, there is still a risk of ongoing erosion, which could result in loss of the existing trail. We understand that the City plans to construct a permanent repair within the next 2 to 4 years to reestablish the riverbank and protect the trail. Providing detailed recommendations for permanent repairs is beyond our current scope and additional site investigations, analyses, and reporting will be necessary as part of developing the permanent repair plans. Below we have provided a discussion of conceptual permanent repairs for consideration during planning. 5.4.1. Slope Reconstruction A reconstructed slope could be designed to be more resistant to erosion and protect the existing trail. Typically, permanent armored or riprap slopes are constructed at inclinations of between 2H:1V and 1.5H:1V. Steeper slope inclinations (on the order of 1H:1V to 0.5H:1V) are possible with the integration of geotextile reinforcement (reinforced soil slope). The face of the rebuilt slope would need to be appropriately armored to protect against erosion. Typically, armoring consists of large boulders and riprap, engineered woody material or other engineered hard armoring systems. Due to habitat considerations, we expect that natural armoring solutions will be preferred. If the slope is rebuilt without reinforcement, we expect that the new slope will primarily be constructed using riprap. Depending on the required inclination of the new slope, the toe of the slope may extend below or near the ordinary high water elevation. The toe of a reinforced soil slope could likely be setback further from the ordinary high water elevation. The further the toe of the new slope is setback from the river, the May 29, 2020| Page 9 File No. 0693-087-00 more excavation into the existing slope will be required in order to install the reinforcement. We expect that reinforcement will need to extend 10 to 15 feet behind the face of the slope. Reinforced slopes with faces steeper than 1.2H to 1V must have either a wrapped face or a welded wire slope face. A retaining wall could be considered to support the trail. We expect that either a soldier pile and tieback wall or a soil nail wall is feasible at this site. A retaining wall could likely be designed so that the wall remains above the ordinary high water elevation; however, erosion protection at the toe of the wall would still be recommended. Constructing a retaining wall in front of the existing slope will be challenging given access limitations and the size of the slope to be retained. We expect that a retaining wall solution will be more expensive than the slope reconstruction solutions discussed above; however, a retaining wall solution could minimize construction activities around the high water level. 5.4.2. Hydraulic Considerations Flood events on the Cedar River will continue to put erosive pressure on this location of the Cedar River Trail. The eroded slope is located on the outside of an approximately 45-degree bend in the river. Velocities are higher on the outside of bends and is, therefore, where more erosion occurs. This is part of natural river-evolution processes. The toe of a permanent slope reconstruction should be protected from erosion caused by future flood events. If the permanent slope reconstruction extends below the ordinary high water elevation, natural armoring will be needed along the toe of the reconstructed slope. Large woody material (logs with rootwads left attached) are often used to protect banks from erosive forces. They move high velocities and associated erosion away from the bank, further into the channel. They also provide added complexity and habitat to riverine environments. The logs would need to be anchored or ballasted to prevent movement during high flow events. Engineered large woody material could be incorporated into the slope repair options discussed above. 6.0 LIMITATIONS We have prepared this report for City of Renton for the Cedar River Bank Stabilization – River Mile 2.5 project in Renton, Washington. City of Renton may distribute copies of this report to owner’s authorized agents and regulatory agencies as may be required for the Project. Our services were provided to assist in the stabilization of an existing slope. Our recommendations are intended to improve the overall stability of the site and to reduce the potential for future property damage related to earth movements, drainage or erosion. Qualified engineering and construction practices can help mitigate the risks inherent in construction on slopes, although those risks cannot be eliminated completely. Favorable performance of the slope in the near term is useful information for anticipating future performance, but it cannot predict or imply a certainty of long-term performance, especially under conditions of adverse weather, seismic activity or other environmental factors such as river flow rates. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices for geotechnical engineering services in this area at the time this report was prepared. The conclusions, recommendations, and opinions presented in this report are based on our professional knowledge, judgment and experience. No warranty, express or implied, applies to the services or this report. Please refer to Appendix B titled “Report Limitations and Guidelines for Use” for additional information pertaining to use of this report. FIGURES Areal Imagery From Google Earth Pro 5/14/2020Figure 1Vicinity MapCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Project Location Not to Scale Figure 2Slope Stability Results13 Foot Setback from Slope Crest(Center Line of Trail)Cedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Notes:1. The locations of all features shown are approximate.2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.Not to ScaleCenter Line of Trail Figure 3Slope Stability Results20 Foot Setback from Slope CrestCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Notes:1. The locations of all features shown are approximate.2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.Not to ScaleSouth Edge of Trail PavementNorth Edge of Trail Pavement Figure 4Slope Stability ResultsTrail WideningCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Notes:1. The locations of all features shown are approximate.2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.Not to ScaleSouth Edge of Trail PavementNorth Edge of Trail PavementWidened Trail Area APPENDICIES APPENDIX A Site Photographs Photos Taken by GeoEngineers on March 25, 2020Figure A-1Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 1: Soil Units Observed on River Bank Slope (looking southeast)Forest Duff and Fill~ 3 feet thick at this locationFine Grained Alluvial Soils ~20 feet thick at this locationCoarse Grained Alluvial Soils extending to river bankDebris From Fill that Collected on the Riverbank Photos Taken by GeoEngineers on March 25, 2020Figure A-2Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 2: Cedar River Trail and drainage ditch on south side of trail (looking northwest) Photos Taken by GeoEngineers on March 25, 2020Figure A-3Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 3: Slope crest at “critical location” (looking southeast)Distance between slope crest and trail pavement ~7 feet Photos Taken by GeoEngineers on March 25, 2020Figure A-4Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 4: Eroded riverbank slope (looking south)Fine-grained alluvial soils standing near verticalCoarse-grained alluvial soils forming a bench area Photos Taken by GeoEngineers on March 25, 2020Figure A-5Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 5: Eroded riverbank slope (looking southeast)Fine-grained alluvial soils standing near verticalCoarse-grained alluvial soils forming a bench area Photos Taken by GeoEngineers on March 25, 2020Figure A-6Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 6 : Block of soil that collapsed from the crest of the slope(looking southeast) Photo 7 Taken by GeoEngineers on March 25, 2020; Photo 8 taken by City of Renton in February of 2020.Figure A-7Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 7: River level on March 25, 2020Photo 8: River levels around the time the erosion event occurred Photos Taken by GeoEngineers on March 25, 2020Figure A-8Site PhotographsCedar River Bank Stabilization River Mile 2.5Renton, Washington0693-087-00 Date Exported: 05/13/2020Photo 9 : Undermined tree on downstream end of eroded area (looking northwest)Photo 10: Undermined tree on upstream end of eroded area (looking southeast) APPENDIX B Report Limitations and Guidelines for Use May 29, 2020| Page B-1 File No. 0693-087-00 APPENDIX B REPORT LIMITATIONS AND GUIDELINES FOR USE 1 This appendix provides information to help you manage your risks with respect to the use of this report. Read These Provisions Closely It is important to recognize that the geoscience practices (geotechnical engineering, geology and environmental science) rely on professional judgment and opinion to a greater extent than other engineering and natural science disciplines, where more precise and/or readily observable data may exist. To help clients better understand how this difference pertains to our services, GeoEngineers includes the following explanatory “limitations” provisions in its reports. Please confer with GeoEngineers if you need to know more how these “Report Limitations and Guidelines for Use” apply to your project or site. Geotechnical Services are Performed for Specific Purposes, Persons and Projects This report has been prepared for City of Renton and for the Project specifically identified in the report. The information contained herein is not applicable to other sites or projects. GeoEngineers structures its services to meet the specific needs of its clients. No party other than the party to whom this report is addressed may rely on the product of our services unless we agree to such reliance in advance and in writing. Within the limitations of the agreed scope of services for the Project, and its schedule and budget, our services have been executed in accordance with our Agreement with City of Kent dated May 8, 2020 and generally accepted geotechnical practices in this area at the time this report was prepared. We do not authorize, and will not be responsible for, the use of this report for any purposes or projects other than those identified in the report. A Geotechnical Engineering or Geologic Report is based on a Unique Set of Project-Specific Factors This report has been prepared for Cedar River Trail Stabilization in Renton, Washington. GeoEngineers considered a number of unique, project-specific factors when establishing the scope of services for this project and report. Unless GeoEngineers specifically indicates otherwise, it is important not to rely on this report if it was: ■ Not prepared for you, ■ Not prepared for your project, ■ Not prepared for the specific site explored, or ■ Completed before important project changes were made. For example, changes that can affect the applicability of this report include those that affect: ■ The function of the proposed structure; ■ Elevation, configuration, location, orientation or weight of the proposed structure; 1 Developed based on material provided by GBA, GeoProfessional Business Association; www.geoprofessional.org. May 29, 2020| Page B-2 File No. 0693-087-00 ■ Composition of the design team; or ■ Project ownership. If changes occur after the date of this report, GeoEngineers cannot be responsible for any consequences of such changes in relation to this report unless we have been given the opportunity to review our interpretations and recommendations. Based on that review, we can provide written modifications or confirmation, as appropriate. Environmental Concerns are Not Covered Unless environmental services were specifically included in our scope of services, this report does not provide any environmental findings, conclusions, or recommendations, including but not limited to, the likelihood of encountering underground storage tanks or regulated contaminants. Subsurface Conditions Can Change This geotechnical or geologic report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by man-made events such as construction on or adjacent to the site, new information or technology that becomes available subsequent to the report date, or by natural events such as floods, earthquakes, slope instability or groundwater fluctuations. If more than a few months have passed since issuance of our report or work product, or if any of the described events may have occurred, please contact GeoEngineers before applying this report for its intended purpose so that we may evaluate whether changed conditions affect the continued reliability or applicability of our conclusions and recommendations. Geotechnical and Geologic Findings are Professional Opinions Our interpretations of subsurface conditions are based on field observations from widely spaced sampling locations at the site. Site exploration identifies the specific subsurface conditions only at those points where subsurface tests are conducted or samples are taken. GeoEngineers reviewed field and laboratory data and then applied its professional judgment to render an informed opinion about subsurface conditions at other locations. Actual subsurface conditions may differ, sometimes significantly, from the opinions presented in this report. Our report, conclusions and interpretations are not a warranty of the actual subsurface conditions. Geotechnical Engineering Report Recommendations are Not Final We have developed the following recommendations based on data gathered from subsurface investigation(s). These investigations sample just a small percentage of a site to create a snapshot of the subsurface conditions elsewhere on the site. Such sampling on its own cannot provide a complete and accurate view of subsurface conditions for the entire site. Therefore, the recommendations included in this report are preliminary and should not be considered final. GeoEngineers’ recommendations can be finalized only by observing actual subsurface conditions revealed during construction. GeoEngineers cannot assume responsibility or liability for the recommendations in this report if we do not perform construction observation. We recommend that you allow sufficient monitoring, testing and consultation during construction by GeoEngineers to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes if the conditions revealed during the work May 29, 2020| Page B-3 File No. 0693-087-00 differ from those anticipated, and to evaluate whether earthwork activities are completed in accordance with our recommendations. Retaining GeoEngineers for construction observation for this project is the most effective means of managing the risks associated with unanticipated conditions. If another party performs field observation and confirms our expectations, the other party must take full responsibility for both the observations and recommendations. Please note, however, that another party would lack our project- specific knowledge and resources. A Geotechnical Engineering or Geologic Report Could Be Subject to Misinterpretation Misinterpretation of this report by members of the design team or by contractors can result in costly problems. GeoEngineers can help reduce the risks of misinterpretation by conferring with appropriate members of the design team after submitting the report, reviewing pertinent elements of the design team’s plans and specifications, participating in pre-bid and preconstruction conferences, and providing construction observation. Do Not Redraw the Exploration Logs Geotechnical engineers and geologists prepare final boring and testing logs based upon their interpretation of field logs and laboratory data. The logs included in a geotechnical engineering or geologic report should never be redrawn for inclusion in architectural or other design drawings. Photographic or electronic reproduction is acceptable, but separating logs from the report can create a risk of misinterpretation. Give Contractors a Complete Report and Guidance To help reduce the risk of problems associated with unanticipated subsurface conditions, GeoEngineers recommends giving contractors the complete geotechnical engineering or geologic report, including these “Report Limitations and Guidelines for Use.” When providing the report, you should preface it with a clearly written letter of transmittal that: ■ Advises contractors that the report was not prepared for purposes of bid development and that its accuracy is limited; and ■ Encourages contractors to conduct additional study to obtain the specific types of information they need or prefer. Contractors are Responsible for Site Safety on Their Own Construction Projects Our geotechnical recommendations are not intended to direct the contractor’s procedures, methods, schedule or management of the work site. The contractor is solely responsible for job site safety and for managing construction operations to minimize risks to on-site personnel and adjacent properties. Biological Pollutants GeoEngineers’ Scope of Work specifically excludes the investigation, detection, prevention or assessment of the presence of Biological Pollutants. Accordingly, this report does not include any interpretations, recommendations, findings or conclusions regarding the detecting, assessing, preventing or abating of Biological Pollutants, and no conclusions or inferences should be drawn regarding Biological Pollutants as they may relate to this project. The term “Biological Pollutants” includes, but is not limited to, molds, fungi, spores, bacteria and viruses, and/or any of their byproducts. May 29, 2020| Page B-4 File No. 0693-087-00 A Client that desires these specialized services is advised to obtain them from a consultant who offers services in this specialized field. Information Provided by Others GeoEngineers has relied upon certain data or information provided or compiled by others in the performance of our services. Although we use sources that we reasonably believe to be trustworthy, GeoEngineers cannot warrant or guarantee the accuracy or completeness of information provided or compiled by others.