Part 4 Landslides In North, Central, and South Seattle Part 4 of the report presents a general geologic and geotechnical evaluation of four additional specific study areas in Seattle not addressed in Part 3. (The original three study areas covered in Part 3 were West Seattle, Magnolia/Queen Anne, and Madrona.) The additional areas include Northwest Seattle, Northeast Seattle, Capitol Hill, and South Seattle; refer to Figure C-1, Appendix C, Volume 2 of this report. The emphasis is on evaluating factors that influence soil stability and presenting remedial measures for the types of slope instability found in the aforementioned areas of Seattle. It is to be noted that Parts 3 and 4 essentially cover the entire city. The purpose for our studies and recommendations regarding stability improvements in Northwest and Northeast Seattle, Capitol Hill, and South Seattle is to provide the City of Seattle (City) with an analysis of 17 additional areas (Stability Improvement Areas) in Seattle where landslide activity has been prevalent, and an updated landslide database with verified locations and attributes. The purpose is also to provide the City with information for prioritizing remedial efforts and to develop order-of-magnitude budgets based on the cost data from Part 2, Section 8.0 of this report. The remedial measures presented are intended to be preliminary, with final scopes of work and corresponding cost estimates based on additional engineering studies and subsurface explorations. The purpose described above has been accomplished in accordance with the following scope of services:
In general, two site visits were made to each Stability Improvement Area, as indicated above. The first site visit, actually made prior to formulating the improvement areas, was primarily to field check the database locations and make appropriate changes in the database. The second site visit was for the purpose of formulating general types of measures that could be considered by the City and/or private property owners to improve stability and reduce landslide risk. Specific sites were not evaluated. The stability improvements listed on Table 4-1 include homeowner education; existing storm drainage facilities maintenance; storm drainage facilities improvement, as may be indicated by future observations or studies; subdrainage systems; fill stabilization; and retaining wall construction. The number, length, square footage, etc., listed on the table are rough estimates presented only to formulate order-of-magnitude budgets. Upon further studies needed to prioritize improvements, such studies may conclude that the extent or type of recommended improvements may or may not be needed, or that changes and/or additions may be advisable. It should be mentioned here that some landslides have occurred outside the designated Stability Improvement Areas. These are usually isolated cases and the improvement areas were selected for locations where instability was prevalent. For landslides outside the designated areas, the stability improvement methods described in Part 2 of this report would apply, including homeowner education and drainage control. As was stated in Part 3 with respect to stability improvements, the stability measures recommended in Part 4 also do not consider the location of property lines and relate to improvements made on City property, private properties, or both. Since landslides and areas of potential instability do not obey property boundaries, improvements are sometimes necessary on both public and private land to suitably improve stability in an area. Therefore, the improvements recommended in Part 4 are those that could be made by the City to protect utilities, drainage features, streets, and other City facilities; and also those measures or actions to be taken by the City and/or adjacent property owners to improve stability of an unstable slope. In the latter case, the City and private property owners should coordinate efforts to improve stability and/or provide protection (such as catchment walls) should instability take place. It is anticipated that some improvements will be made by the City, while other improvements or protection will be the responsibility of private property owners. It should again be noted that there are always risks of damage to property and structures involving landslides for property located on or adjacent to a slope. Property owners need to accept those risks. Although the recommended improvements and homeowner education can lead to immediate or eventual improved slope stability conditions, private property owners should also obtain professional geotechnical advice to reduce current risks for their properties. The analyses and recommendations presented in Part 4 of this report must be considered only in conjunction with the Limitations Section 1.5 presented in the Preface of this report. The information presented in the next three sections 14.2, 14.3, and 14.4 is the same as that presented in Sections 10.2, 10.3, and 10.4 of Part 3. The information is repeated so that Part 4 will be complete without the need to refer back to Part 3. In the succeeding sections of Part 4, various improvement measures and other actions are presented that we recommend be considered by the City. These actions include:
Homeowner education is important so that the public is made aware of the factors that cause landslides and the steps homeowners should take to improve stability. Information should be provided to homeowners relative to prudent construction practices and obtaining professional advice for improving stability for existing homes, additions, or new construction. It is particularly important that homeowners learn that filling on a slope (especially at the top of a slope), or cutting into a slope (especially at the toe), can lead to instability and should only be undertaken with proper advice and consultation with competent geotechnical engineers or engineering geologists. Even the placement of yard waste on a slope decreases stability and, therefore, should be properly composted on flat ground or taken off-site. Homeowners should also be required to properly maintain and control their on-site drainage systems and to discharge drainage in accordance with applicable regulations, since improperly channeled water decreases slope stability, particularly when concentrated. In addition to the above, we recommend that the City continue to conduct neighborhood informational meetings to facilitate two-way discussion regarding stability matters. Valid concerns of homeowners should be taken into account in planning and implementing improvements. We also recommend that the general public be made aware of a telephone "hot line" that can be readily reached to report locations of poor drainage, landslides, or potential instability. In areas of potential landsliding, it is important that existing storm drainage facilities be maintained. In addition, storm drainage improvements could be considered when indicated by subsequent observations and studies. In this regard, the City has retained a consulting engineering firm (Black & Veatch) to evaluate surface drainage systems throughout the city. The scope of this "Needs Assessment" included visual observation of the roadway runoff where it had potential to impact landslide-prone slopes. Their studies are to be coordinated with the landslide studies presented herein, with the goal of improving stability conditions. In the succeeding sections of this report, recommendations regarding maintaining and/or improving storm drainage facilitates are subject to the evaluations and recommendations to be made by Black & Veatch. Therefore, prioritizing and budgeting relative to surface drainage improvements are beyond this current landslide study. As stated previously, the stability improvements presented in Part 4 are preliminary and for the purpose of providing the City with information they can use to prioritize remedial efforts and develop "ballpark" budgets. Further detailed studies, including subsurface explorations, should be undertaken by the City to determine final scopes and design of remedial measures, and more accurate cost estimates. Geotechnical and other consultants should be used as appropriate. Implementing stability improvements by the City would consist of preparing plans and specifications using the data presented in Part 2 of this report, and observing actual construction to verify suitable conformance with project requirements. Since landslides and potential instability cut across property boundaries, a cooperative effort between property owners is advisable in obtaining the greatest benefits of stability improvements. In addition to homeowner education, previously discussed, the City should facilitate the processing of permits submitted by private property owners so remedial work can take place expeditiously to improve stability. Variances to code requirements should be allowed where needed to improve stability for private and/or public properties. Temporary and/or permanent easements on or across City property could be granted, where allowed by ordinance, such as when needed to construct protective structures or to allow gravity flow, in lieu of pumped drainage, for suitably designed drainage facilities on private properties. Coordination between the City and private property owners may also include shared costs, such as by Challenge Grants or Local Improvement Districts (LIDs). 14.3 Actions by Private Property Owners Improvement of stability involves actions not only by the City, but actions by private property owners. Such actions by private property owners should include accepting existing conditions and the risks of slope instability. Measures should accordingly be implemented on private properties as may be needed to protect and improve stability for existing property, structures, additions, or new construction. Those measures to be taken by private property owners are the same types of improvements presented in Part 2 of this report, and professional advice should be obtained from geotechnical and other appropriate consultants regarding the improvements. Such advice should also be obtained by prospective buyers of property in slide potential areas. Stability improvements would include proper drainage of surface water, including suitable discharge of roof gutter downspouts. Surface water should not be improperly channeled to or concentrated on slopes and particularly not onto adjacent property. Other remedial measures would consist of properly designed subdrains, site grading, soil retention systems (walls, soil reinforcement, tieback anchors, etc.), drilled drains, or other measures as conditions may dictate. Of particular concern are structures located above or at the bottom of a potentially unstable slope. Private property owners should seek professional advice regarding such measures as underpinning walls and/or tieback anchors near the top, or catchment/retaining walls at the bottom of a slope. Private property owners should take advantage of the homeowner education materials prepared by the City or other entities. Cooperation with the City and with adjacent property owners is also important so that remedial measures can be coordinated to achieve the greatest benefits of stability improvement. Private property owners should also notify the City regarding areas observed with poor drainage, landsliding, or potentially unstable ground, so that drainage and stability improvements can be coordinated between City and private property owners as appropriate. 14.4 Additional Considerations The contributing factors to instability, as described for the Stability Improvements section of this report, include terms such as surface drainage, runoff, storm water runoff, surface water runoff, etc. Such drainage or runoff includes that from pavement areas as well as from soil or vegetated areas. The more pervious the soil, such as sand and/or gravel, the more that rainfall will infiltrate the ground, which reduces the amount of runoff. Conversely, for more impervious soils like silt or clay, runoff will be greater. Runoff also takes place from vegetated slopes, being greater for areas of spare vegetation than for slopes with heavy vegetation. Cuts at or near the toe of a slope, or fills on or near the top, are also contributing factors to instability. Such factors, particularly where cuts or fills took place years ago, may still have some influence on the stability of an area; however, such a factor may or may not be the predominant cause of recent or future instability. For example, a road cut area may remain stable for years, yet experience instability as the direct result of such things as a leaking or broken pipe, improper drainage from adjacent property, new filling or excavation on a slope, or other unwitting actions by owners or adjacent property owners. Each occurrence of instability requires evaluation to assess the predominant factor or factors leading to slope failure. In describing some of the Stability Improvement Areas, we noted remedial measures of landslides that had recently been completed or were taking place. However, there are probably other remedial measures being planned, in progress, or completed by the City or private property owners that are not mentioned. Furthermore, we have not mentioned specific locations where surface drainage improvements have recently been undertaken or are being planned in conjunction with the "Needs Assessment" portion of the surface drainage studies by Black & Veatch. Northwest Seattle is defined in this study as the area north of the Lake Washington Ship Canal and west of Interstate 5 (refer to Figures C-1, C-2, and C-3, Appendix C, Volume 2). From the ship canal, the ground surface rises up to the north gradually as a broad undulating plain, nearly reaching elevation 500 feet near the north city limit. It is broken by depressions such as Green Lake, Haller Lake, and Bitter Lake. It has also been incised by Pipers Creek, a west flowing, steep gradient drainage in the vicinity of Carkeek Park. The Scenic Subdivision of the Burlington Northern Santa Fe Railroad (BNSF RR) extends along the toe of the relatively steep bluff along the western margin of the study area. The stratigraphy of Northwest Seattle is typically comprised of Vashon glacial sediments overlying a relatively thick sequence of older, pre-Vashon glacial and non-glacial deposits. The contact between the Lawton glaciolacustrine clay and the overlying Esperance outwash sand (both Vashon glacial units) is mapped by Tubbs (1974) in the vicinity of the steep bluffs along Puget Sound and in the Pipers Creek drainage. Abundant groundwater seepage and springs are associated with this contact. The distribution of recorded historical landslides within the Northwest Seattle study area is generally confined to the steep slopes facing Puget Sound, above the BNSF RR. The type of instability occurring in Northwest Seattle consists of high bluff peeloff-type landslides along the upper portions of the steep bluffs above Shilshole Bay Marina, in the North Beach area, and north of Carkeek Park. Shallow colluvial-type landslides are dispersed all along the west-facing slope bordering Puget Sound and along the steep slopes of the Pipers Creek drainage (Carkeek Park). Groundwater blowout-type landslides are also confined to the bluffs adjacent to Puget Sound. Although few deep-seated landslides are recorded in the Northwest Seattle Study area, one of the largest recorded instances of instability in Seattle is located just east of Golden Gardens Park along View Avenue N.W. This section, like Part 3 of this study, presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities. It also presents measures that could be made by the City and adjacent property owners to improve the stability of an unstable slope. We present further comments regarding educating private property owners on steps they may take to improve stability. The Northwest Seattle area has been divided into six smaller Stability Improvement Areas, where landslide activity has been prevalent. As shown on Figure C-3 (Appendix C, Volume 2), the six areas are as follows: For each area, we will summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving slope stability. Also refer to Table 4-1, located following the text in Part 4 of this report. The Broadview Stability Improvement Area is located in the northwest corner of the City and north of the Carkeek Park Stability Improvement Area; refer to Figure C-3. In this area, a total of 47 landslides were recorded, interpreted from the records as high bluff peeloffs (3), groundwater blowouts (18), deep-seated landslides (2), and shallow colluvial landslides (22). Two landslides were not identified as to type. In general, most of these landslides occurred on the west-facing bluff and steep slopes located east of the BNSF RR tracks. At some locations, the landslides affected the backyard areas of residential sites located at the top of the bluffs/slopes. Toward the north, instability damaged N.W. Culbertson Drive; this area has been repaired by a crib wall. Toward the central section, some landslides occurred on the side slopes of a north-south-trending gully located uphill and east of the steep slope down to the railroad tracks. Stream-bank erosion may contribute to these instabilities. The recorded landslides in the Broadview improvement area have occurred throughout the years beginning in 1933 and extending into 1997. The subsurface soils in this area generally consist of colluvium or fill overlying glacially overridden glacial till, outwash sand, and/or lacustrine clay/silt. The sand-clay contact (Tubbs, 1974) is present in this area. The primary contributing factors to instability consist of steep topography, loose fill at the top and/or colluvium on the slope, high groundwater levels with associated seepage particularly near the sand-clay contact, and heavy rainfall (triggering cause). It is recommended that work by the City include maintaining existing storm drainage facilities and improving them when indicated to be appropriate by future observations. Homeowner education is recommended to include providing information regarding prudent construction and drainage practices, and obtaining professional advice for improving stability for existing property, additions, or new construction. Eight shallow colluvial landslides have been recorded for this Stability Improvement Area. All but two of these landslides took place during the December 1996/January 1997 storm. They occurred at random locations as shown on Figure C-3. The other two landslides took place in 1970 and 1972. The subsurface conditions in this area consist of fill and/or colluvium overlying glacially overridden sand and/or clay. The sand-clay contact (Tubbs, 1974) extends across this area as shown. The factors contributing to instability are steep topography, loose fill at the top and/or colluvium on the slope, high groundwater levels/seepage particularly near the sand-clay contact, and heavy rainfall (triggering cause). Recommended action for this area includes homeowner education and storm drainage systems maintenance and/or improvement. It is to be noted that one of the 1996/1997 landslides has been repaired by the City utilizing a mechanically stabilized earth (MSE) wall. In the Carkeek Park Stability Improvement Area, as designated on Figure C-3, 13 landslides were recorded. Twelve landslides were identified as shallow colluvial and one as a deep-seated landslide. The earliest slide was reported in January 1960. The most recent landsliding reportedly took place on or about January 14, 1998. The landslides in this area occurred primarily on or adjacent to ravine slopes. At some locations, creek erosion of the slope toe may have contributed to the instability. Construction of N.W. Carkeek Park Road, which lies near the center of this area, likely included some fills along the downhill side and cutting along the uphill side. At some locations, private property owners have placed backyard fills. At one location, a landslide was reportedly related to cutting into the toe of a roadway fill by a homeowner. Several of the landslides reported in this area were caused or exacerbated by private utility pipeline breaks, probably the latter. The subsurface soils in this area, based on geologic mapping and our experience in this area (no explorations reviewed), consist of colluvium overlying glacially overridden soils. The overridden soils consist of sand over clay, and the sand-clay contact (Tubbs, 1974) is present in the Carkeek Park ravine areas. Groundwater seepage can be expected near the sand-clay contact. The factors that contribute to instability in this area are steep topography, colluvium on the slope, and cutting or filling on the slope. The landslides were triggered by heavy rainfall that resulted in surface runoff and infiltration into the colluvium. Recommended actions in this area include storm drainage systems maintenance and/or improvement and homeowner education. Curbs and gutters along N.W. 118th Street could be considered for controlling street drainage. Homeowner education is recommended to inform property owners of the landslide risks involved with backyard fills on or near the top of a slope, and the need to properly control site drainage including downspout discharge. Side sewers on or in the slope should be checked frequently for proper functioning. In addition, the City could consider the installation of a catchment/retaining wall along the uphill side of portions of N.W. Carkeek Park Road in order to prevent landsliding onto the road. The location of the Blue Ridge Stability Improvement Area is shown on Figure C-3. In this area, 21 landslide records have been reviewed, of which 2 were interpreted as high bluff peeloffs, 11 groundwater blowouts, 3 deep-seated landslides, and 5 shallow colluvial landslides. The earliest recorded landslide was in 1933, and instability has occurred throughout the years. Several landslides were recorded in January 1997. Most of the landslides in this area took place on the steep northwest-facing slope located between uphill residences and the BNSF RR tracks at the toe of the slope. Most were natural occurrences, although some may have taken place because fill was placed behind residences (contributing factor). Two of the 21 landslides were reportedly caused by plugged catch basins that resulted in runoff onto the slope behind houses. Eleven of the landslides, which were the ones listed as groundwater blowouts, took place between 1933 and 1960, and apparently brought debris down to the railroad tracks. The subsurface soils in this area consist of colluvium overlying glacially overridden soils. Toward the south in this improvement area, where most of these slides occurred, the overridden soils consist primarily of clay. To the north, the overridden soils consist of sand over clay, and the sand-clay contact (Tubbs, 1974) is present. Seepage near this contact likely contributed to some of the landslides. The factors contributing to instability are steep topography, loose fill and/or colluvium on the slope, high groundwater levels and associated seepage near the location of the sand-clay contact, and heavy rainfall (triggering cause) that results in surface runoff and also infiltrates and reduces stability for colluvium and loose fill areas. Recommended action in this area consists of storm drainage systems maintenance and/or improvement and homeowner education. The Golden Gardens Stability Improvement Area is the area generally east and northeast of Golden Gardens Park, as shown on Figure C-3. On the west- and northwest-facing slopes, a total of 26 landslides have been recorded through the years beginning in 1930. Most of the landslides listed were of the shallow colluvial type (17), while some were listed as high bluff peeloffs (6), groundwater blowouts (1), and deep-seated landslides (2). The most recent instability recorded took place on or about March 19, 1997. The high bluff peeloff landslides occurred in the northern section of this improvement area, where a steep, northwest-facing bluff rises above the BNSF RR tracks and is present behind and northwest of residential sites fronting on N.W. Esplanade Street. (A note on two of the landslide entries listed in the database indicates that as many as 13 landslides onto railroad property took place in this area from 1949 to 1960). One shallow colluvial and the one recorded groundwater blowout also took place in this area. The rest of the shallow colluvial and the deep-seated landslides took place further south in this improvement area on the steep slopes uphill from N.W. Esplanade and downhill from View Avenue N.W. and Golden Gardens Drive N.W. The subsurface conditions in this area generally consist of fill and/or colluvium overlying glacially overridden soils. To the north in the area where the high bluff peeloffs occurred, the overridden soils (which slab off or slide) consist of glacial till or outwash sand. Toward the south, the glacial till is generally absent and the fill and/or colluvium overlies lacustrine clay/silt (north of Golden Gardens) or outwash sand (east of Golden Gardens). The sand-clay contact (Tubbs, 1974) is present to the east of Golden Gardens, as indicated on Figure C-3. With respect to groundwater, seepage can be expected in the colluvial layer, at or near the sand-clay contact, and from pervious layers within the lacustrine clay/silt stratum. In the area uphill of Golden Gardens Park, a large, deep-seated landslide destroyed three houses along the west side of View Avenue. Movement was first detected in early spring of 1974, and movement continued into July of that year, at which time 48 horizontal drains were installed for drainage of soils at or near the sand-clay contact. The drains were installed from a bench on Park Department property. Movement of the upper portion of the slope near View Avenue was stopped in August 1974 and, to our knowledge, further deep-seated movement affecting the upper slope has not occurred. However, it is our understanding that some movements on the Park Department bench and steep slope down to the railroad tracks have occurred since 1974. The primary factors that contribute to instability in this area are heavy rainfall (triggering cause), steep topography, fill at the top and/or colluvium on the slopes, high groundwater levels and associated seepage, and pipeline discharge from private properties (storm drainage). In the steep slope area located between View Avenue and Esplanade, a number of erosional gullies or slide debris chutes are present. During periods of heavy or prolonged rainfall, mud and debris flows have taken place in these chutes. At some locations, pipeline discharge has contributed to the debris flows. Recommended actions for consideration by the City in this stability improvement area include construction of catchment/retaining walls, maintenance and/or improvement of storm drainage systems, and education of homeowners. Catchment/retaining walls are recommended along the uphill side of N.W. Esplanade to protect against landslides onto the roadway. In addition, an MSE wall is recommended along the west (downhill) edge of Golden Gardens Drive N.W. to provide support for the edge of the road where signs of instability are present. Homeowner education should emphasize the suitable discharge of site drainage including downspout discharge. Homeowners located at the toe of slopes should be advised to consider constructing catchment walls to protect against debris slides from uphill land. One such catchment wall was constructed in 1998 for protection of one house along N.W. Esplanade. With respect to the area where the deep-seated landslide took place in 1974, it is recommended that a comprehensive study be made to evaluate the current stability of this area. This could include evaluating the horizontal drain system that may or may not still be operating suitably. We suspect that many of these drains may have been severed by slide movement shortly after installation in 1974. The City and/or private property owners could consider cleaning or replacing these drains as indicated. Additional stability improvement measures could also be indicated in order to improve stability. For this current study, we recommend that potential costs relative to this site be determined by assuming the cleaning of existing drains and the installation of 30 additional horizontal drains to replace non-functioning drains. In addition, deep trench subdrains on the lower bench could be assumed for cost estimating purposes; refer to Table 4-1. In the Shilshole Stability Improvement Area (refer to Figure C-3), a total of 19 landslides have been recorded. Four were interpreted to be high bluff peeloffs, six groundwater blowouts, and nine shallow colluvial landslides. The earliest recorded landslide was 1933, and landslides have occurred throughout the years. The most recent slide occurred in February 1999. Most of these landslides (14 of 19) occurred on the west-facing bluff located uphill from the BNSF railroad tracks. Debris from these landslides sometimes reached the railroad tracks, and at least two debris flows came down onto Seaview Avenue N.W. and/or the parking area for the Shilshole Bay Marina. The other five landslides occurred on residential sites located in the southern portion of this improvement area. The subsurface conditions in this area generally consist of fill and/or colluvium overlying glacially overridden soils. The overridden soils include glacial till and sand over clay. The sand-clay contact (Tubbs, 1974) is mapped only in about the northern third of this improvement area. The factors that contribute to instability in this area are steep topography, high groundwater levels/seepage, and improper fills at the top or on slopes. The triggering mechanism is generally heavy rainfall. Where existing residences are located at the top of the slope, surface runoff from the top and/or storm water discharge (downspouts) into slope soils could contribute to instability unless suitably controlled. Recommended action consists primarily of homeowner education. Maintaining and/or improving storm drainage would also be appropriate. Northeast Seattle is defined in this study as the area north of the Lake Washington Ship Canal and east of Interstate 5 (refer to Figures C-4 and C-5, Appendix C, Volume 2). From the ship canal, the ground surface rises up to the north relatively gradually as a broad undulating plain, nearly reaching elevation 450 feet near the Maple Leaf area. The northern two-thirds of the study area is incised by Thornton Creek and its tributaries and the southern third by Ravenna Creek. Steep slopes predominate along the eastern portion of this study area adjacent to Lake Washington. The Burke Gilman Trail (formerly railroad tracks) is located at the toe of the steep bluff along the shore of Lake Washington and extends south from the northern city limit to the ship canal. The stratigraphy of Northeast Seattle is comprised of Vashon Glacial sediments overlying a relatively thick sequence of older, pre-Vashon glacial and non-glacial deposits. The contact between the Esperance outwash sand and the underlying Lawton glaciolacustrine clay (both Vashon glacial units) is mapped by Tubbs (1974) along the steep slopes above the Burke Gilman Trail and within the Thornton Creek drainage basin. The locations of recorded landslides within the northeast Seattle study area are generally confined to the steep slopes facing Lake Washington with the exception of the instability recorded along Thornton Creek and its tributaries. The type of instability occurring in this study area primarily consists of shallow colluvial-type failures. Several deep-seated failures are recorded in the vicinity of the Inverness area. Groundwater blow-out-type landslides are documented along the shore of Lake Washington in the northeast portion of the study area where the sand-clay contact extends along the steep slope just west of the Burke Gilman Trail. The instability recorded in Northeast Seattle has primarily occurred after 1940 and the majority of the older recorded events are confined to the Laurelhurst neighborhood. Several landslides are recorded in the Inverness neighborhood between 1950 and 1970; these are primarily related to grading and excavations during development. This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities in the Northeast Seattle area. Furthermore, this section includes measures that could be made by the City and adjacent property owners to improve the stability of an entire landslide or unstable slope. We further present comments regarding educating private property owners on steps they may take to improve stability. The Northeast Seattle area has been divided into three smaller Stability Improvement Areas where landslide activity has been prevalent, in order to describe various improvements and homeowner education suggestions. As shown on Figure C-5 (Appendix C, Volume 2), the three areas are as follows: For each area, we will summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving stability. Also refer to Table 4-1. The Burke Gilman Stability Improvement Area is located in the northeast corner of the City, as indicated on Figure C-5. In this area, a total of 39 landslides were recorded. Most of them (35) were shallow colluvial landslides. Two were high bluff peeloffs, one groundwater blowout, and one deep-seated landslide. The landslides generally occurred on the east-facing slope that is present in this area. The earliest recorded landslide took place in 1955, and instability was reported throughout the years including January 1999. Fourteen of the landslides were recorded for the bluff area located between the Burke Gilman trail on the east and private properties uphill to the west. Many other non-recorded landslides have occurred here as well. Many of these brought debris down onto the trail. Ten of the recorded 14 landslides took place in early January 1997, one in March 1972, one in May 1983 (due to sprinkler left running), and two in February 1996. The landslide database indicates that at least one of these landslides resulted in some damage to a residence at the top of the slope. The other landslides in this improvement area occurred on the uphill and downhill sides of various streets (38th to 42nd Avenues N.E.), and on the east-facing slope located behind and to the east of a number of residences on these streets. The subsurface soils in this area generally consist of colluvium overlying glacially overridden sand over clay. The sand-clay contact (Tubbs, 1974) is generally located just east (upslope) of the Burke Gilman trail. At some locations, fill located behind (east of) residences was involved in instability. Groundwater seepage can be expected at the sand-clay contact. The factors that contribute to instability in this area are steep topography, colluvium on the slope, high groundwater levels and associated seepage at the sand-clay contact, and heavy rainfall (triggering cause). At some locations, fill at the top or on a slope contributed to landslide potential. As indicated previously, one reported landslide was due to a sprinkler that was left running. Recommended actions in this area include storm drainage systems maintenance and/or improvement and homeowner education. Surface drainage along 40th Avenue N.E. (13700-block) could be evaluated and improved as needed. Homeowner education is recommended to inform property owners of the landslide risk involved with a steep slope when located behind a residence, particularly with backyard fills on or near the top of a slope. Site drainage including downspout discharge also needs to be properly controlled. In addition, the City could consider the installation of a continuous catchment/retaining wall along the uphill side of 40th/41st Avenue N.E. between N.E. 142nd and N.E. 144th Streets. This is to prevent landslide debris from coming onto the road. To improve stability for the downhill edge of this road, an MSE wall could be constructed. The Inverness Stability Improvement Area is located uphill of Sand Point Way and is shown on Figure C-5. In this area, 22 landslides are recorded, consisting of 5 deep-seated and 17 shallow colluvial landslides. The earliest recorded landslide occurred in February 1955. Instability has taken place throughout the years. The most recent instability was noted in January 1999. In general, instability in this area has occurred on ravine slopes often where filling has occurred in conjunction with residential development. The subsurface soils in this area generally consist of fill and/or colluvium overlying glacially overridden sand over clay. Most of the landslides occurred near the sand-clay contact (Tubbs, 1974) mapped for this area. Seepage at the contact is likely. The primary contributing factors to instability consist of steep topography, loose fill at the top and/or colluvium on the slope, high groundwater levels and associated seepage near the sand-clay contact, and heavy rainfall (triggering cause). Recommended actions in this area consist of storm drainage systems maintenance and/or improvement and homeowner education. Homeowner education is appropriate, particularly involving the instability risks regarding fills on or near the top of slopes. It is recommended that homeowners also be provided with information regarding prudent drainage practices including downspout water discharge. Twenty landslides have been recorded for the Laurelhurst Stability Improvement Area; refer to Figure C-5 for location. All 20 recorded landslides have been listed as shallow colluvial events. Beginning in December 1933, instability has reoccurred in this area at about 10-year intervals. The last recorded event was January 1997. Most of the landslides in this improvement area consist of instability on the southeast-facing slope west of Lake Washington and rising above N.E. Laurelcrest Lane. Two recorded landslides occurred further north and on the downhill side of residences located on 55th Avenue N.E. The subsurface soils in this area consist of colluvium overlying glacially overridden soils. The sand-clay contact (Tubbs, 1974) is not shown in this area. Based on geologic mapping and our experience in this area (no explorations reviewed), the overridden soils consist of till, sand, and/or clay. The factors that contribute to instability consist of steep topography, colluvium on the slope, and heavy rainfall (triggering cause). Recommended actions in this area include storm drainage systems maintenance and/or improvement and homeowner education. It is recommended that homeowner education include informing uphill property owners of the risks involved with fills on or near the top of a slope, and the need to properly control and maintain site drainage including downspout discharge. Homeowner education could also include information regarding construction of catchment walls at the toe of slopes to retain landslide debris and protect residences, garages, and the private drive that extends south of N.E. Laurelcrest Lane. The City could also consider the installation of a catchment/retaining wall along the uphill side of portions of N.E. Laurelcrest Lane in order to prevent landsliding onto the road. The Capitol Hill area is defined in this study as the peninsular area east of Lake Union, south of Portage Bay and the Montlake Cut, west of the Arboretum (see Figure C-1), and north of East Roy Street (refer to Figures C-6 and C-7, Appendix C, Volume 2). Capitol Hill proper, is a north-south-trending ridge that rises gently to the south with relatively steep west-facing slopes along Interstate 5 on the west side and a highly incised drainage (Interlaken Park) to the east. Interstate 5 extends north-south just west of the longitudinal axis of Capitol Hill and State Route 520 extends east-west, south of the Montlake Cut, to its western terminus at Interstate 5. The stratigraphy of Capitol Hill is comprised of a thin veneer of Vashon Glacial sediments overlying a relatively thick sequence of pre-Vashon glacial and non-glacial deposits. The sand-clay contact, representing the interface between the Lawton glaciolacustrine clay and the overlying Esperance outwash sand (both Vashon glacial units) is mapped by Tubbs (1974) in the vicinity of the steep slopes east of Interstate 5 and west of the southern margin of the University of Washington Arboretum. The contact is conspicuously absent around the northern margin of Capitol Hill. Based on exploratory borings for the Sound Transit project, it is believed that a pre-Vashon, east-west trending fluvial (outwash) channel extends through Capitol Hill just south of the I-5/SR 520 interchange. Our opinion that these natural cut-and-fill deposits extend in an east-west-direction is corroborated by the City of Seattle shaded relief map of the area (refer to Figure A-1, Appendix A, Volume 2). This map shows erosional ravines and landslide bowls on the east (Interlaken) and west (Lakeview Drive) hillsides of Capitol Hill. In our opinion, these features are indicative of cohesionless soils, such as outwash sand, and thus explain the absence of glaciolacustrine clay in portions of the Interlaken area. The distribution of recorded landslides within the Capitol Hill study area is generally confined to three areas: the steep slope just east of Interstate 5 (I-5) along Lakeview Boulevard East, upslope of Portage Bay Place East on the east side of Capitol Hill, and in the highly incised area of Interlaken Park. Shallow colluvial and deep-seated-type landslides predominate in the Capitol Hill area. The absence of groundwater blowouts, in our opinion, may be a result of the lack of detailed information in the City files as well as the absence of glaciolacustrine clay, especially in the Interlaken area. Because the Capitol Hill area is among one of the older neighborhoods in Seattle, the record of landsliding dates back to the early 1900s. This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities. Measures are also presented that could be made by the City and adjacent property owners to improve stability of an unstable slope. We present further comments regarding educating private property owners on steps they may take to improve stability. The Capitol Hill area has been divided into three smaller Stability Improvement Areas, where landslide activity has been prevalent. As shown on Figure C-7 (Appendix C, Volume 2), the three areas are as follows: For each area, we will summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving slope stability. Also refer to Table 4-1, located following the text in Part 4 of this report. Fourteen landslides have been recorded for the North Capitol Hill Stability Improvement Area; refer to Figure C-7 for location. One landslide was noted as deep-seated, 8 as shallow colluvial, and 5 were unidentified as to landslide type. The earliest slide was reported in 1923, and instability has occurred through the years. Five landslides were listed for January and March 1997. Seven of the landslides in this area took place on the steep slope located behind (east of) buildings on Fuhrman Avenue E., extending downhill to Portage Bay Place N.E. Four of these landslides occurred in 1997, which apparently resulted in the construction of a soldier pile and wood-lagged retaining wall along a portion of the slope toe on the west side of the lower road. Another landslide took place behind a building on Fuhrman Avenue in August 1986, reportedly caused by excessive landscaping watering. The instability uphill of Portage Bay Place can and has caused debris to block the roadway. The other seven landslides in this improvement area took place at the locations shown on Figure C-7. Four of them involved fill placed by private property owners. The subsurface soils in this North Capitol Hill area generally consist of fill and/or colluvium overlying glacially overridden glacial till, outwash sand, and/or lacustrine clay/silt. The sand-clay contact (Tubbs, 1974) is not mapped in this area. The primary contributing factors to landsliding consist of improper fills, steep topography, colluvium on the slope, improper irrigation (one instance), and heavy rainfall (triggering cause). It is recommended that actions by the City include maintaining existing storm drainage facilities and improving them when indicated by future observations. Homeowner education is also recommended to stress the risks involved with improper filling. Information regarding prudent construction and drainage practices should also be made available to private property owners. The City could also consider the installation of a catchment/retaining wall along currently unsupported portions of the toe of slope west of Portage Bay Place, including adding more debris catchment height to the recently constructed retaining wall. The Interlaken Stability Improvement Area is generally a northeast-facing slope located as shown on Figure C-7. Numerous landslides have taken place through the years in this area. Forty-two landslides have been recorded, interpreted as one groundwater blowout, 10 deep-seated events, 22 shallow colluvial landslides, and 9 unidentified as to type. The earliest recorded landslide was in February 1927. A number of landslides occurred in 1997, 1998, and 1999. Most of the landslides in this improvement area occurred upslope of E. Interlaken Boulevard and Interlaken Drive E. A number of landslides also took place downslope of E. Boston Terrace, and others occurred upslope of Delmar Drive E. and 14th Avenue E.; refer to Figure C-7. Sixteen of the 42 landslides in this area reportedly involved fill material of which ten were fills at residential sites. A number of the landslides in this area have received remedial measures consisting of retaining structures, grading, and/or subsurface drainage. The subsurface soils in this Interlaken area generally consist of fill and/or colluvium overlying glacially overridden sand, silt, and clay. The colluvium consists of intermixed sand, silt, and clay. High groundwater levels occur in the colluvium during the wet-weather times of the year. The sand-clay contact (Tubbs, 1974) is not mapped in this area. The primary contributing factors to landsliding consist of improper fills, steep topography, high groundwater levels/seepage in colluvium on the slope, and heavy rainfall (triggering cause). It is recommended that actions by the City include maintaining and/or improving storm drainage systems in this area. Homeowner education is also recommended to stress the risks involved with improper filling and storm drainage practices. The City could also consider construction of the improvements described in the following paragraphs. A trench subdrain may be appropriate to improve stability for a portion of the slope uphill from Interlaken Drive. Such a subdrain would be parallel to slope contours in the area where previous landslides have occurred, and should be extended through the colluvium and into the glacially overridden soils. An MSE wall could be considered at two locations: 1) along the downhill shoulder of Interlaken Drive (near the south end of the improvement area), and 2) along the downhill side of 20th Avenue E. near its transition to Interlaken Place E. The purpose for the MSE walls is to strengthen the roadways. The construction of retaining/catchment walls could be considered for support and/or debris catchment along the uphill sides of Interlaken Drive (where landslides have previously occurred), Delmar Drive E. (north of 14th Avenue E.), and 14th Avenue E. (between Delmar Drive and Boyer Avenue E.) to protect the street. We also recommend consideration of fill stabilization (roadway replacement) for portions of Interlaken Drive and 20th Avenue. Sixteen landslides have been reported for the West Capitol Hill Improvement Area, located as shown on Figure C-7. Of these 16 landslides, 2 were noted as deep-seated, 11 as shallow colluvial, and 3 not identified as to landslide type. The earliest recorded landslide date was 1916. Four landslides occurred in the 1930s, one in 1961, one in 1974, two in 1986, and seven in 1997. In general, the landslides in this area have taken place on the steep, west-facing slope located between 10th Avenue E. and Lakeview Boulevard E. A number of the landslides resulted in debris blocking Lakeview Boulevard. One landslide in 1997 occurred on the downhill side of Lakeview Boulevard. A few of the landslides involved fill material. The subsurface soils in this area generally consist of fill and/or colluvium overlying glacially overridden till, sand, or clay. The sand-clay contact (Tubbs, 1974) is mapped downslope to the west of this improvement area. The primary contributing factors to instability consist of steep topography, fill and/or colluvium on the slopes, and heavy rainfall (triggering cause). High groundwater levels/seepage is also a contributing factor, particularly in the vicinity of Lakeview Boulevard. Recommended actions in this area consist of storm drainage systems maintenance and/or improvement and homeowner education, particularly involving prudent drainage and construction practices. The risks of instability involving fills on or near the top of a slope could also be emphasized. The City could also consider the installation of retaining/catchment walls along the east side of Lakeview Boulevard, as protection against landslide debris blocking the roadway. South Seattle is defined in this study as the area south of Interstate 90, north of the City limit, west of Lake Washington and east of 21st Avenue S.W. (refer to Figures C-8 and C-9, Appendix C, Volume 2) The area is characterized by the broad, north-south-trending, floodplain of the Duwamish River that is bounded on the west by the steep, east-facing slope of Puget Ridge and on the east by Beacon Hill. Other significant slopes in this study area are located along the west shore of Lake Washington near the Rainier Beach and Mount Baker neighborhoods. Interstate 5 extends in a north-south direction along the toe of the west-facing slope of Beacon Hill. The stratigraphy of South Seattle is comprised of Vashon glacial sediments overlying a sequence of pre-Vashon glacial and non-glacial deposits. Older, Tertiary bedrock crops out sporadically throughout the South Seattle study area, with notable exposures along the east side of Interstate 5. The contact between the Lawton glaciolacustrine clay and the overlying Esperance outwash sand (both Vashon glacial units) is mapped upslope of West Marginal Way S.W., and around the northern tip and along the west side of Beacon Hill. Abundant groundwater seepage and springs are associated with this contact as well as other contacts between relatively permeable glacial units (older glacial outwash deposits) and relatively impermeable soils of older, pre-Vashon glacial deposits and Tertiary bedrock. The distribution of recorded historical landslides within the South Seattle study area is generally confined to the steep slopes on both sides of Beacon Hill, the east-facing slope along West Marginal Way S.W., and the steep, northeast-facing slope in the Rainier Beach neighborhood. Deep-seated and shallow colluvial-type landslides predominate along the margins of the Duwamish floodplain, while nearly all of the documented landslides in the Rainier Beach area are classified as shallow colluvial. Groundwater blowout-type landslides are documented along the west-facing slope adjacent to Interstate 5 and south of Highland Parkway S.W. in the vicinity of the contact between the underlying Lawton glaciolacustrine silt and clay and the Esperance glacial outwash sand and gravel. There are no high bluff peeloff-type landslides documented within the South Seattle study area. The timing of landslides within the South Seattle study area is strongly influenced by the construction of public works projects. For example, the majority of the instability along the west side of Beacon Hill was recorded prior to the 1960s before the construction of I-5. The construction of Interstate 5 effectively increased stability, with cylinder piles and retaining walls, for large portions of the chronic landslide areas along the west-facing slope. Conversely, the grading of Rainier Avenue S., in the Rainier Beach neighborhood, along the toe of the steep, northeast-facing slope may have oversteepened the slope and exacerbated instability of the colluvium-covered till slope. This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities. Measures are also presented that could be made by the City and adjacent property owners to improve stability of an unstable slope. We present further comments regarding educating private property owners on steps they may take to improve stability. The South Seattle study area has been divided into five smaller Stability Improvement Areas, where landslide activity has been prevalent. As shown on Figure C-9 (Appendix C, Volume 2), the five areas are as follows: For each area, we will summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving slope stability. Also refer to Table 4-1, located following the text in Part 4 of this report. Fourteen landslides were listed in the database for the Mount Baker Stability Improvement Area, categorized as three deep-seated events, eight shallow colluvial landslides, and three unidentified as to landslide type. The earliest landslide was recorded for 1922. Thereafter, two took place in the 1930s, five in the 1960s, and three each in 1986 and 1997. The landslides in this area generally have occurred on the east-facing slope that extends downhill to Lake Washington Boulevard S. The seven southernmost landslides involved Park Department property located between Lake Washington Boulevard and private properties located on the next street (Lakewood and Cascadia Avenues S.) uphill to the west. In three or four of these landslides, debris from private properties came down onto Park Department land, reaching Lake Washington Boulevard in two reported events. In the other landslides, instability apparently occurred on Park Department property to the east of private property. One residence was reported to be threatened by a 1986 landslide. In connection with two of the seven landslides, property owners claimed that sewer backup, leakage, and/or surface drainage led to the instability. Three shallow colluvial landslides occurred immediately uphill from Mount Claire Drive S., in 1961, 1963, and 1997. These apparently involved Park Department property (Mount Claire Park). In the 1997 event, landslide debris from Park Department property came down and crossed Mount Claire Drive. Further to the west, an unidentified type of landslide occurred on private property, involving failure of a rubble wall and fill material. To the north, three reported deep-seated landslides took place in 1922, 1933, and 1936. Stability in this latter area was improved by drainage facilities installed as a Works Progress Administration (WPA) project in 1935 and 1936. The subsurface soils in this area generally consist of fill or colluvium overlying glacially overridden till, sand, and/or clay. The sand-clay contact (Tubbs, 1974) is not mapped in this area. The factors contributing to instability in this area are relatively steep topography, fill and/or colluvium on the slope, and heavy rainfall (triggering cause). In several instances, as previously mentioned, sewer backup, leakage, and/or surface drainage contributed to the instability. Recommended actions in this area include storm drainage systems maintenance and/or improvement and homeowner education. In addition, the City could consider installing a retaining/catchment wall along portions (two sections) of the west side of Mount Claire Drive, to protect the roadway and private property from potential landslide debris. In the 25th Avenue S. Stability Improvement Area (refer to Figure C-9), a total of ten landslides have been recorded. They are listed as four deep-seated events and six shallow colluvial landslides. With the exception of one recorded landslide in March 1997, the others occurred in 1974 and before. The earliest listed events (two landslides) took place in December 1933. The landslides here have occurred at approximately 20-year intervals. In general, this area straddles 25th Avenue S. and slopes downward toward Rainier Avenue S. Eight of ten landslides reportedly involved fill material, presumably placed in conjunction with street grading or residential construction. Two of the deep-seated landslides took place on the downslope side of 25th Avenue S. just south of S. McClellan Street. One of these was initiated by an excavation made for a building that was constructed on the west side of Rainier Avenue S. The subsurface conditions in this area consist of fill and/or colluvium overlying glacially overridden clay. The sand-clay contact (Tubbs, 1974) is not mapped in this area. The predominant factors contributing to instability are the soil conditions on this sloping area (fill and/or colluvium overlying glacially overridden clay), undercutting or filling on the slope, and heavy rainfall (triggering cause). Other possible contributing factors of instability are steep topography at some locations and high groundwater levels/seepage in the colluvium. Recommended actions in this area include homeowner education and storm drainage systems maintenance and/or improvement. To improve subsurface drainage, a curb could be installed along the east side of Cheasty Boulevard S. between S. Hinds Street and S. Winthrop Street with the curb extending along Winthrop to 27th Avenue S. The City could also consider construction of an MSE wall along a portion of Cheasty Boulevard (east side) to increase support for the downhill side of the roadway. In the West Beacon Hill Stability Improvement Area, as designated on Figure C-9, 38 landslides are listed in the database. These consisted of groundwater blowouts (6), deep-seated landslides (13), shallow colluvial landslides (16), and 3 not identified as to landslide type. The earliest landslide was recorded in 1921, and 21 of the 38 instabilities occurred prior to 1960. Others occurred in the 1960s (7), one in 1972, two in 1986, one each in 1987 and 1990, three in 1997, and two were noted to have taken place in 1999. The landslides in this improvement area took place on the west-facing slope downhill from 15th Avenue S., and uphill from the Interstate 5 (I-5) alignment (I-5 constructed in this area in the 1960s). As previously noted (Section 18.1), construction of I-5 effectively increased stability in this location. A number of landslides occurred in areas of residential fills. The subsurface soils in this area consist of fill and/or colluvium overlying glacially overridden till, sand, or clay. The sand-clay contact (Tubbs, 1974) runs in a north-south direction through this area, where groundwater seepage can be expected into near-surface soils. The factors contributing to instability in this area consist of steep topography, fill and/or colluvium on the slope, high groundwater levels with associated seepage near the sand-clay contact, and heavy rainfall (triggering cause). Recommended actions consist of storm drainage systems maintenance and/or improvement and homeowner education. Homeowner education could stress prudent drainage and construction practices, and filling should not take place unless suitably supported using competent geotechnical advice. There are a number of locations where steep slopes exist adjacent to residential properties or roadways. Yard waste and filling at the top or over the slope should not take place. The City could also consider the construction of a new retaining/catchment wall on the east of 13th Avenue S. between Bayview and S. Lander Streets to protect the roadway. Figure C-9 shows the location of the Duwamish Stability Improvement Area, located west of the Duwamish Waterway. Twenty-four landslides occurred in this area throughout the years beginning with a 1922 event. Three occurred in 1997 and two were observed to have occurred in early 1999. Most of these landslides (19) were recorded as shallow colluvial events, while the others were groundwater blowout (1), deep-seated (3), and unidentified as to type (1). Many of the landslides in this area brought debris down onto W. Marginal Way S.W. Two early landslides (one dated 1922; the other 1923) were reportedly related to the grading of W. Marginal Way. Another landslide, dated 1926, was reportedly related to the grading for 9th Avenue S.W. (located near Highland Park Way S.W.). Along the west side of W. Marginal Way and some distance to the north of Highland Park Way, an ivy-covered toe wall (appears to be wood), approximately 500 feet in length, is present. This improvement area is generally an east-facing slope, except near the north and south portions of this improvement area. At the north end, a portion of this area slopes down to the north. In the south where Highland Park Way follows a ravine uphill to the west and south, landslides have occurred on slopes facing east, north, and west. Colluvium overlying glacially overridden clay is generally present in this area. At two locations, the glacially overridden soils were listed as sand or glacial till, and at two other locations, fill material was involved in the instability. The sand-clay contact (Tubbs, 1974) runs in a north-south direction through this area as shown on Figure C-9. Groundwater seepage can be expected near the sand-clay contact. The factors that contribute to instability in this area are steep topography, colluvium on the slope (mostly overlying glacially overridden clay), cutting and filling on the slope, and high groundwater levels/seepage. The landslides were triggered by heavy rainfall that results in surface runoff and infiltration into slope soils. It is recommended that actions by the City to improve stability include maintaining existing storm drainage facilities and improving them when indicated by future observations in this area. Homeowner education could stress prudent construction practices. The City could also consider construction of a retaining/catchment along a portion of W. Marginal Way on the west side of the roadway to protect the roadway; its location would be further north than the existing toe wall mentioned previously. The Rainier Beach Stability Improvement Area is located in the southeast corner of the City as indicated on Figure C-9. In this area, 27 landslides are shown. The earliest landslides were recorded in 1914 (two events), 1918, and 1924. No further instability was reported until 1951, and then landsliding occurred in the 1950s (3), 1960s (4), 1970s (3), 1980s (8), and 1990s (5). The most recent instability was observed in 1999. Most of the landslides in this area were shallow colluvial instability (21). The others were a groundwater blowout (1), deep-seated landslides (4), and one that was not identified as to type. The landslides in this area generally occurred on the northeast-facing hillside that slopes down to Lake Washington. A number of the slides took place immediately upslope from Rainier Avenue S., bringing landslide debris down onto the sidewalk or roadway. At present, portions of the sidewalk along the south side of Rainier Avenue are permanently closed. One landslide that occurred in 1914 was reportedly related to the grading work (cutting at slope toe) for Rainier Avenue. Uphill from Rainier Avenue, a number of landslides occurred in fill material placed on private properties. The subsurface conditions in this area consist of fill and/or colluvium overlying glacially overridden soils. For the most part, based on the database information, the overridden soils consist of clay. (Geologic maps indicate that glacial till may also be present in this area.) The sand-clay contact (Tubbs, 1974) is not mapped in this area. Recent (1999) field visits have noted that groundwater seepage is present in this area, particularly in the bowl-shaped areas extending uphill (west) from Rainier Avenue. The factors that contribute to instability in this area are steep topography, fill and/or colluvium on the slope, high groundwater levels/seepage, cutting or filling, and heavy rainfall (triggering cause). Recommended actions include storm drainage systems maintenance and/or improvement and homeowner education. In addition, the City could consider the construction of a retaining/catchment wall along the west side of Rainier Avenue to prevent debris from accumulating on the sidewalk or roadway. |
We completed field verifying the balance (those not
field checked for Part 3) of the reported landslides within
the City. During this effort and an additional field visit,
we evaluated the alternatives for stability improvements
in areas of concentrated historical landslide activity based
on the conditions observed (slide type, groundwater and
surface water conditions, soil stratigraphy, etc.).Department of Planning and Development (DPD)