6980 Sierra Center Parkway, Suite 90 Reno, NV 89511

Transcription

1 6980 Sierra Center Parkway, Suite 90 Reno, NV January 8, 2018 Project No: 2004 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION 7650 Town Square Way Reno, NV RE: Rockery Wall Preliminary Failure Investigation Report 2320 Trail Ridge Court Reno, Nevada Dear Mr. Carter: This letter transmits the results of the rockery wall preliminary failure investigation for 2320 Trail Ridge Court. This preliminary investigation report summarizes readily available soils, grading, and observational information obtained during the repair investigation phase of this project. Furthermore, this report provides recommendations on acquiring additional information to supplement this investigation. 1.0 BACKGROUND The rockery walls are located on a slope behind the residence at 2320 Trail Ridge Court. The rockery wall configuration is two tiered (upper and lower wall) with the walls separated by an approximate 15 foot wide terrace having a 3H:1V slope gradient (refer to Pictures #1 & #2). The slope fronts a golf course fairway. The rockery walls failed in January 2017 after a period of considerable precipitation. The upper rockery wall completely failed, being displaced both vertically and horizontally with an estimated displacement distance of about 4 to 5 feet (refer to Picture #3). The lower rockery wall completely failed at an isolated location below the upper wall with the individual rock particles being displaced and falling onto a golf cart path located adjacent to the wall. In the remaining sections of the lower rockery wall, the individual rocks have noticeably been displaced as indicated by an outward individual rock displacement and enlarged gaps between the rocks. Typically, rockery walls are constructed with a 1H:4V to 1H:6V wall batter and the wall now appears to be vertical or outward leaning at some locations. The slope is currently being repaired. The design repair strategy is to abandon the existing tiered rockery wall design and replace with a rock buttress placed adjacent to the lower wall. A 2H:1V fill slope will be constructed above the rock buttress to the top of the slope. It is understood that the walls were designed and constructed by Parsons Brothers. It is further understood that they hired Harlan Fricke Consulting to provide structural calculations for the rockery walls. A copy of rockery wall structural calculations were obtained for Somersett Village 5-C. To provide a basis discussion of the rockery wall design, it is assumed that these calculations are similar design calculations that were completed for the rockery walls at the subject location. Acquiring the actual design calculations for the rockery walls at this location is recommended. Office fax

2 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 2 of 7 Picture 1: Showing Panoramic View of Two Tiered Retaining Wall Picture 2: Looking upslope from the base of the lower rockery wall at golf cart path

3 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 3 of 7 Picture 3: Top of slope showing upper rockery wall vertical and horizontal displacements

4 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 4 of EXISTING INFORMATION In the preparation of this report, the following documents were reviewed: Construction Materials Engineers, Inc. (2017). Somersett - Slope and Rockery Wall Repair, Recommendations Investigation, 2320 Trail Ridge Court, Reno, Nevada. CME Project No: 2004, August 3, Harlan Fricke Consulting (2004), Specifications and Stability Calculations For Dry Stacked Rock Walls Somersett Village 5-C, job No: KANE GeoTech, Inc. (2017). Somersett Trail Ridge Slope Failure, Slope Stability Modeling Results, Reno, Nevada. KGT Project No. KGT17-16, July 25, Manhard Consulting LTD, Topographic Map of Slope Area Lots 10,11, and 12, dated 4/19/2017. Odyssey Engineers (2003), Grading Plan Somersett. Odyssey Engineers (2004), Plot Plan for 2320 Trail Ridge Court Somersett. Summit Engineering Corporation (2003). Geotechnical Investigation, Units 3C at Somersett, Reno, Nevada. Report to Somersett, L.L.C., File No , February 19, Wood Rogers (2013). Byde Residence, 2320 Trail Ridge Court. Letter from Mischelle J. Smith to Mr. Tom Byde, Wood Rogers Project No , September 11, 2013,. Wood Rogers (2013). Johnson Residence, 2310 Trail Ridge Court. Letter from Mischelle J. Smith to Mr. AbeJohnson, Wood Rogers Project No , September 11, 2013, Amended September 17, GEOLOGIC AND GENERAL SOIL PROFILE DESCRIPTIONS Based on the referenced Geotechnical Investigations (CME, 2017 and Wood Rogers, 2013), the slope area appears to have been constructed with fill soils. CME s investigation, 2017, drilled a boring to a depth of 31.5 feet in the backyard of the lot where the upper wall failed. This field exploration encountered two distinct soil profiles in this fill horizon, as follows: The uppermost soil profile encountered to a depth of about 6 feet consisted of two prominent soil types: sandy lean clay (CL) to a depth of about 4 feet and a clayey sand with gravel (SC) encountered from about 4 to 6 feet in depth. The lowermost soil horizon encountered below 6 feet to the depth of exploration classified as a sandy fat clay (CH). Based on SPT blow counts, the material had a relatively uniform stiff consistency. Below this fill soil horizon, the general subdivision area is underlain by several different geologic units (Summit, 2003): Side stream and Wash Alluvium (Q a), Pediment gravel deposits of the Verdi Basin (Q p4), and claystone, siltstone, and sandstone members of the Tertiary Hunter Creek Formation (T h).

5 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 5 of Soil Moisture and Groundwater Conditions Subsurface soils were encountered in a moist to very moist soil condition (CME, 2017). Based on the measured soil moisture contents, the moisture content of the fill soils progressively increased with depth. The existing moisture contents are likely higher than when the fill soils were originally placed and indicate that seasonal and landscape moisture has migrated into soils profile with depth. Ground water was not encountered during exploration and is expected to lie at a depth well below that which would affect construction. 4.0 DISCUSSION It is understood that the lower rockery wall is located on the golf course jurisdiction and the upper wall is located within the Somersett Homeowners (HOA) jurisdiction. An opinion of the cause of the rockery wall failures is desired to determine the responsible party for repair costs. The two possible wall failure modes that will be considered with this preliminary investigation report are as follows: The upper wall failed first and caused the lower wall to fail; The lower wall failed first causing the upper wall to fail. Based on an existing grading plan (Odyssey Engineering, 2003, presented in Appendix A), both walls were originally designed with heights of 10 feet. These walls were separated by an approximate 15 foot wide bench sloped at a 3H:1V gradient. The grading plan also shows a 2H:1V slope in front of the lower wall with a height ranging from 7 to 9 feet. The golf cart path is not shown on the plans. Plans showing the golf cart path were not available. Based on a topographic map of the slope area, completed after the rockery wall failures (Manhard, 2017, presented in Appendix B) and field measurements, the lower rockery wall was not constructed as presented on the grading plan. Based on field measurements and a review of the existing slope topographic map (Manhard, 2017), the lower rockery wall has an exposed height of over 10 feet with the high-point of this wall having an exposed height of 14 feet, or about 4 feet higher than the design height. The wall height gradually decreases from this high point in both the northern and southern direction. The high point of the wall is where the lower wall completely failed and is located directly below the failed upper wall. Additionally, the finished grade in front of the lower wall is slightly sloped to the cart path, but does not have the same slope height as originally designed. Based on this grading plan, it appears that the wall was constructed higher than designed to offset the reduced slope height at the base of the wall. Based on a review of the referenced rockery wall design calculations (Harlan Fricke 2004, presented in Appendix C), the maximum exposed wall height that was designed is 10 feet. Based on our experience in the Somersett area, the 10 foot wall height is typically the maximum rockery wall height that was constructed at Somersett. The rockery walls are designed as gravity walls with the weight of the wall providing resistance to the lateral soil pressure. As the walls increase in height, the base of wall has a corresponding increase in width as additional wall weight is required to resist the backfill soil lateral weight. As presented on the referenced design calculations, an 8 foot exposed wall height has a base width of 5 feet, while a retaining wall with a 10 foot exposed height has base width of 6 feet. Based on these designed wall bases, the proportioned base width for a 14 foot exposed wall height wall would be about 8 to 9 feet, although verification by a

6 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 6 of 7 structural engineer would be required. The constructed/design base width of the 14 foot high wall is currently unknown. Kane GeoTech, Inc. (2017) completed a slope stability modeling investigation on the subject slope. The purpose of this investigation was to provide design recommendations for slope repair. Their investigation recommended the construction of a rock buttress at the base of slope to provide a long-term solution to the slope stability. 4.1 Observations and Mechanisms for Wall Movement Based on the current available information and observations, can a wall failure mechanism be defined to determine if either the upper or lower rockery wall movement contributed to the overall wall failures? To provide a basis for a potential answer, the following observations are listed below: The upper rockery wall extends through several lots on either side of the subject lot, but failed in the area above the lower wall failure; It is assumed for this discussion that the upper rockery wall backfill soils are fairly similar (CME, 2017 and Wood Rogers, 2013) within the adjacent lots to the subject wall failure lot and, consequently, likely experienced similar lateral loads. The upper rockery walls in the other lots did not fail and, consequently, the failure of the rockery wall at the subject lot due to an increased backfill soil lateral load pressure is not likely. Based on this conclusion, another mechanism likely caused the upper wall to fail; If the upper wall failed by either a sliding or overturning failure mechanism such as caused by an increased soil lateral pressure, individual rock particles from the retaining wall would likely be scattered on the slope face, which was not observed. Instead, the observed failure shows that the rockery wall, being relatively in-tact, displaced entirely down slope. This down slope movement would likely be caused by an overall slope instability and not a sliding or overturning failure mechanism; The upper rockery wall foundation is supported on a 3H:1V fill slope directly above the lower wall. If the slope moves in a downward direction below this wall, as likely a result of the lower wall failure, this would cause a loss of bearing support for the upper wall; The two-tiered wall failure topography (Manhard, 2017) indicates that the maximum upper wall displacement is above where lower wall completely failed. This observed failure geometry provides a tenable explanation for the upper wall failure in regards to the lower wall movement; A head scarp is located at the top of the slope (Kane, 2017) and is a feature typically associated with a rotational type slope failure. Typically, this slope failure would be caused by a loss of support toward the base of the slope. Based on the existing information and observations, it is our opinion that the best scenario for the two-tiered wall failure is that the lower wall failed initially causing a loss of bearing support for the upper wall and subsequent wall displacement. The walls have been constructed and in service for many years. It is unknown the exact date of wall construction, but it is assumed that the walls have functioned adequately for many years. So what was the

7 Mr. Tracy Carter SOMERSETT OWNERS ASSOCIATION January 8, 2017 Page 7 of 7 mechanism that initiated the lower wall instability and eventual failure? It is our premise that the lower wall failed for these potential reasons: The January storm caused an increase in the soil lateral pressure exerted on the wall by increasing the backfill soil unit weight and reducing the shear strength of the backfill soils due to saturated conditions; Because of the constructed height of the lower retaining wall, the factor of safety for the potential movement mechanism consisting of sliding and overturning may have been reduced. The retaining wall backfill soil pressure for a 14 foot high wall is considerable higher than a 10 foot high retaining wall. Based on the referenced retaining wall design calculations, the 10 foot high retaining wall factor of safety for sliding is 1.5 and for overturning is 3.0. If the retaining wall was not designed to resist a 14 foot high wall soil pressure, the factor of safety values would be considerable reduced such that a small increase in the soil lateral pressure may have initiated wall movement. 4.2 Recommended Additional Investigations To verify the structural integrity of the lower wall, it is recommended that research be conducted to determine if the lower wall was engineered for the as-built wall heights. A structural engineer should review any calculations completed on this wall. The permit on the wall construction should also be reviewed to determine the designed wall heights. Additionally, design grading plans showing the golf cart path and revised grades should also be obtained. The following appendices are included and complete this letter. Appendix A: Odyssey Engineers (2003), Grading Plan and Plot Plan for 2320 Trail Ridge Court Somersett. Appendix B: Manhard Consulting LTD, Topographic Map of Slope Area Lots 10,11, and 12, dated 4/19/2017. Appendix C: Harlan Fricke Consulting (2004), Specifications and Stability Calculations For Dry Stacked Rock Walls Somersett Village 5-C, job No: If you have any questions or desire additional information, please let us know. Sincerely, CONSTRUCTION MATERIALS ENGINEERS, INC. Randal A. Reynolds, PE Senior Geotechnical Engineer rreynolds@cmenv.com Direct: Mobile: v:\active\2004\trail ridge rockery wall failures docx

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