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HomeMy WebLinkAboutAGENDA REPORT 2017 0607 CCSA REG ITEM 09B ITEM 9.B. C/TY OF MOORPARK,CALIFORNIA MOORPARK CITY COUNCIL City Council Meeting AGENDA REPORT w too - 7-620I7 TO: Honorable City Council FROM: Steven Kueny, City Manager 114 BY: Jessica Sandifer, Program Manag DATE: May 30, 2017 (CC Meeting of 6/7/17) SUBJECT: Consider Studies to Develop Strategies for Future Development of City-owned Properties in the Downtown Area BACKGROUND Recent and on-going geotechnical studies in the downtown area have identified the potential for seismic related geohazards including liquefaction, seismic dry settlement, and hydroconsolidation. Several projects have been impacted by these soil conditions, including the Area Housing Authority's Walnut Street apartments project and the Apricot Lane Farms project. In addition, preliminary geotechnical results indicate that the future civic center site also has poor soil condition. Prior to the State's action to eliminate redevelopment, this is the type of situation where a redevelopment agency could have provided funding to mitigate these impacts. With these recent developments regarding soil conditions within the downtown area, staff recommends that a strategy be developed that will address development on City- owned parcels in the downtown area. DISCUSSION Seismic safety in Southern California is important. Properly constructing a building to withstand the ground shaking of an earthquake and its attendant problems (liquefaction, hydro consolidation and seismic dry settlement) is a critical public health and safety issue and is also necessary for property owners to protect their investment in the event of an earthquake. The recent studies conducted in the downtown area have identified the potential for liquefaction to occur between depths of 15 feet to about 60 feet below ground level. The estimates of liquefaction and seismic dry settlement for the future civic center site compared to what is required to achieve compliance with the California Building Code (CBC) are provided below to illustrate the severity of the soil conditions: • Liquefaction — 12 to 15 inches • Dry seismic settlement— 12 to 17 inches • Lateral displacement—400 to 500 inches • To achieve CBC compliance—2 inches of total settlement and 1 inch of differential settlement 129 Honorable City Council June 7, 2017 Page 2 In order to meet compliance with the code, the soil has to be re-worked in order to support a building and guard against these geologic hazards in the event of an earthquake. Traditionally, when you construct a building foundation you want the soil beneath the foundation to achieve a specified level of compaction. When those soils can't achieve that compaction level, you typically excavate the soil to a prescribed depth, and replace the soil with fill material that is stable and can be compacted to the required level ("overexcavation"). In the case of the High Street soil studies, the work is showing that overexcavation would be required to depths of up to 60 feet, making traditional grading methods very expensive. In instances where you have this type of soil there are other non-traditional methods to treat the soil and achieve a "buildable" area such as vibro replacement, wet (or deep) soil mixing, and dynamic compaction. Vibro Replacement is the construction of dense aggregate (stone) columns, which provide a surface to build the foundation on. The Area Housing Authority project is utilizing this method for their project. Wet (or deep) soil mixing consists of using a mixing tool to add a cementitious binder slurry to the native soil to improve the stability of the soil. Dynamic compaction consists of repeatedly dropping a heavy metal plate on the ground. The vibrations created by dropping the plate densify the soil. While not as high as overexcavating 60 feet, these other methods come at a higher cost than traditional grading, which impacts project costs and timelines. The Area Housing Authority foundation work added an additional $500,000 to the project and it is estimated that the soil work for the civic center site, will add an additional $750,000 per building foundation area. Robert Anderson, from RJR Engineering will be present to address questions about soil conditions. In order to mitigate these impacts to future projects, staff needs to explore several areas of concern. First, a geotechnical firm needs to be hired to determine to what extent liquefaction issues affect City-owned parcels in the downtown area. Staff is in the process of obtaining quotes from geotechnical firms to perform these studies. Once the studies are completed, staff will need to consult with a structural or civil engineer, or possibly both, to determine potential costs for future projects to mitigate the soil issues identified by the geotechnical firm, and provide a "buildable" site. The City's appraiser has also indicated that these issues will have an effect on the value of the land. Staff needs to further explore with the appraiser the extent it devalues the land and how to have the property appraised to reflect these conditions. Lastly, staff will consult with the City Attorney on what ways the City can attempt to mitigate these effects through the Disposition and Development process and to address how they may affect the prevailing wage requirements. Staff feels it is important to have answers to these questions so that a strategy can be developed for future developers of City-owned properties in the downtown area. FISCAL IMPACT The costs to hire a geotechnical firm, structural or civil engineer, and to work with the City Attorney are estimated to cost $60,000. Staff will return to City Council with a request for funding once firmer price quotes have been obtained. 130 Honorable City Council June 7, 2017 Page 3 STAFF RECOMMENDATION Authorize City Manager to continue the process to prepare studies on City-owned properties in the downtown area including geotechnical and structural evaluations in order to prepare a strategy for future disposition of the properties. 131