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HomeMy WebLinkAboutCDP 2020-0026; BEGONIA COURT RETAINING WALL; GEOTECHNICAL INSPECTION OF SLOPE FAILURE; 2020-06-01Martin R. Owen PE, GE 4596 Mount Hubbard Avenue, San Diego, CA 92117 Phone/Text: 619-813-8462 To: Ms. Marylou Powers La Costa Village HOA Email: martinowen@geotechengineer.com Subject: Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Dear Ms. Powers: RECORD COPY j)f 0:-: Initial / 2 -/1./. 2tJ ' Date Job No. 200506 June 1, 2020 In accordance with your request, I have conducted a geotechnical inspection of the subject slope failure. The purpose of the inspection was to evaluate the slope failure and provide recommendations for slope repair and restoration. My inspection was performed on May 6, 2020. SITE DESCRIPTION The subject slope is a steep (approximately 1-1/2:1 H:V) west-facing, graded fill slope, about 15 feet high, that descends from the west side of the subject residence down to a lower level and swimming pool. The residence is the westerly unit of a two-story, multi-unit townhome building constructed in 1971. The building is (mostly) supported on conventional shallow footings and has lower level slab-on-grade floors. However, you indicated that the subject residence previously experienced foundation settlement and was stabilized by underpinning with pipe piles or other deep foundations. The date and extent of the foundation repairs are not known. You also indicated you recently inspected the interior of the residence and no building damage was observed. In addition, your insurance company sent an engineer to inspect the building and no damage was reported. Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Page2 Job No. 200506 June 1,2020 Slope vegetation is moderate to good and consists of several shrubs and ivy and other ground cover. Reportedly there was a recent leak in a sprinkler line near the top of the slope. It is not known how long the pipe was leaking. The leak is believed to be factor in the slope failure. DESCRIPTION OF SLOPE FAILURE • The slope failure occurred in the upper portion of the slope below the west side of the subject residence, following recent, heavy rains. • The failure is a shallow, slump-type landslide, about 3 feet deep maximum, approximately SO feet long (parallel to slope), and extending about 2/3 of the way down the slope (see attached Figure 1). • The landslide occurred along a curved failure surface within the slope fill. The soils at the top of the slope slid downwards resulting in a 3-feet high, vertical scarp and rotated outwards accumulating as a soil pile or bulge near the bottom of the slope. • The failure is less noticeable at the north end of the slope where there has been less slope displacement at the bottom. • The failure appears to have occurred due to an increase in weight and loss of shear strength of the outer slope fill soils resulting from saturation of the slope soils. The source of water appears to be a combination of recent, heavy rainfall and water from a leaking sprinkler pipe. • The residence does not appear to been damaged by the failure. However, the scarp has exposed some PVC electrical conduits running parallel to the building. The horizontal distance from the edge of the residence to the edge of the scarp and conduits is about 5 feet. • I probed the slope soils within and adjacent to the landslide area and the soils were soft and saturated to a depth of at least 3 feet. Poor compaction of the fill soils appears to be a factor in the slope failure. Martin R. Owen PE, GE Geotechnical Engineer Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Page3 Job No. 200506 June 1, 2020 • The slope is overly steep by today's grading standards, which has also contributed to the failure. SOILS AND GEOLOGY The slope soils are fill soils consisting of gray brown, clayey sand and clayey silt. From published geologic maps the site is underlain at depth by sedimentary bedrock of the Santiago Formation. CONCLUSIONS • The primary cause of the slope failure appears to be saturation of the slope soils and loss of shear strength resulting from a combination of recent heavy rainfall and a leaking sprinkler pipe. • The steep slope and poor slope fill compaction appear to be other factors in the failure. • The slope failure is shallow and relatively minor and the subject residence does not appear to have been damaged or threatened by the failure. RECOMMENDATIONS The slope failure should be repaired and the slope restored to its original grade and configuration, as follows: • The slope can be repaired using small grading equipment or hand equipment. • The existing slope vegetation within the slope restoration area should be entirely removed . Some of the shrubs may be salvageable and can be reused. • The accumulated soils at the bottom of the slope should be removed and temporarily stockpiled on the nearby sidewalk or other area. • In addition, prior to placing fill, a 5-feet wide minimum (measured into slope) fill key, at least 2 feet deep, should be excavated at the bottom of the slope (see attached Figure 2). Martin R. Owen PE, GE Geotechnical Engineer Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Page4 Job No. 200506 June 1,2020 As grading proceeds to the north, where there has been less slope displacement, it may be possible to excavate the fill key further up the slope (instead of at the bottom). • The onsite soils may be reused as fill, provided they are free of organics and debris and rocks and cobbles over 6 inches in dimension. Any import fill soils should be approved by the undersigned or other soil testing company. • Fill soils should be placed and compacted in 6 to 8 inches thick lifts/layers starting at the bottom of the fill key and working upwards. The fill should be moistened or dried as necessary to slightly above optimum moisture conditions and compacted by mechanical means to at least 90 percent relative compaction based on ASTM D 1557. • The 5-feet wide minimum fill width should be maintained as the fill is brought up. The fill should be benched into the original slope soils in 2 feet minimum, vertical benches (see attached Figure 1). • The upper portion of the slope should be overbuilt and cut back. • Horizontal layers of geogrid fabric should be installed at 4 feet maximum vertical increments to provide extra strength. • The slope grading and compaction and placement of geogrid should be observed and tested as necessary by the undersigned or other soil testing company. OTHER OPTIONS You asked for other repair options to be considered such as constructing a retaining wall at the bottom of the slope. Constructing a retaining wall at the bottom of the slope instead of regrading the slope is not a preferred option. The retaining wall would have to be strong enough and high enough (at least 5 feet) to withstand t he soil pressures exerted by the landslide and would probably need to be supported on drilled, concrete caissons extending several feet into the ground. It would be necessary to perform a subsurface soil investigation and structurally design the wall, and to obtain permits from the City of Carlsbad. A retaining wall solution would also leave the slope in its present, failed condition. Martin R. Owen PE, GE Geotech nical E ngineer Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Pages Job No. 200506 Junel,2020 Constructing a lower (3 to 4-feet high) wall as part of a grading solution is a possible option but the only perceived benefit is that it would allow you to reconstruct the slope at a flatter angle. The wall would still have to be designed and permitted. Construction of a retaining wall at the top of the slope would provide support to the existing building foundations and conduits. However, the wall would do nothing to stabilize the landslide and further slope movement could still occur. SLOPE LANDSCAPING The slope should be re-landscaped with shrubs and deep-rooted ground cover such as ivy. Moderate irrigation will be required to establish the landscaping. FRENCH DRAIN AT BOTTOM OF SLOPE You also indicated you would like to construct a French drain at the bottom of the slope to intercept possible below-ground seepage. A French drain should be at least 3 feet deep and 12 inches wide. A 4-inch diameter, perforated SDR 35 or better PVC pipe wrapped in a filter sock should be placed at the bottom of the trench and the trench backfilled with Caltrans Class 2 Permeable Material drain rock (gravel and sand mix). The pipe should flow at a 1 percent minimum gradient to an approved outlet. LIMITATIONS Please note I have reviewed no construction plans or reports as part of t his inspection, and have performed no subsurface exploration or soil testing. The above co ncl usions and recom mendations are based on my inspection and experience with other, similar properties in San Diego County. This report provides no warranty, either expressed or implied, concerning future building or slope performance. Future damage from geotechn ical or other causes is a possibility. Martin R. Owen PE, GE Geotechnical E ngineer Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 Page6 Job No. 200506 June 1, 2020 This opportunity to be of service is appreciated. If you have any questions, please do not hesitate to call or contact me. Very truly yours, M artin R. Owen PE, GE Geotechnical Engineer Attachments: Figure 1 and 2 Martin R. Owen PE, GE Geotechnical Engineer Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 2210 Plaza Bonita 5' ~, 3' ::t FIGURE 1 SLOPE FAILURE SCHEMATIC ONLY EAST-WEST SECTION THROUGH SLOPE Scarp Original Slope / 1.5:1 (H:V) Approx. Martin R. Owen PE, GE Geotechnical Engineer Page7 Job No. 200506 June 1,2020 Soil Displacement/ Bulge from Slope / Failure 15' Approx. Geotechnical Inspection of Slope Failure 2210 Plaza Bonita Carlsbad, CA 92009 2210 Plaza Bonita FIGURE 2 SLOPE REPAIR SCHEMATIC ONLY EAST-WEST SECTION THROUGH SLOPE ~5• ~ I ' ' ' ' ~Scarp Restored Slope 1.5:1 (H:V) / Approx. ' ' ' ' Geofabric Layers ;:I at4'Max. Height Intervals 2'HighMin/ Fill Benches ➔ ..... 2' Deep Min.~ Fill Key ,~ Martin R. Owen PE, GE Geotechnical Engineer 5' Min. Page8 Job No. 200506 June 1, 2020 --I "\ 15' Approx. Lower Level ~ ,, ➔I