Laserfiche WebLink
January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 21 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />implementing ground improvement to reduce the impact of slope instability and lateral loading <br />on new structures is a feasible mitigation option. <br />4.4.1 Ground Improvement Methods <br />Ground improvement methods can be utilized to mitigate the impacts of lateral spreading events. <br />The ground improvement methods evaluated for this project include stone columns, deep soil <br />mixing, and jet-grout columns. <br />It is important to note that the above-described ground improvement methods were evaluated to <br />mitigate lateral loading due to slope instability and lateral spreading on the proposed Effluent <br />Pump Station. Consequently, the proposed ground improvement will mitigate impacts of slope <br />instability and lateral loading on structures located east of the proposed Effluent Pump Station <br />(i.e., ground improvement will prevent lateral movement of liquified soil and the non-liquefiable <br />crust towards the west). However, it is HWA’s opinion that implementing ground improvement <br />to prevent lateral loading on the existing clarifier structures (to be used as secondary storage) on <br />the southwest side of the property is cost prohibitive and not feasible. HWA recommends that the <br />design team assume that the existing clarifier (to be repurposed as secondary storage) on the <br />southwest side of the property may experience substantial lateral displacement during a large <br />seismic event and could be rendered non-functional. <br />It is important to note that the proposed ground improvement will only mitigate the impact of <br />slope instability and lateral loading during a large seismic event within the area of <br />implementation. <br />Stone Columns: Stone columns, otherwise known as Vibro-Compaction or Vibro-Replacement, <br />are one of the most cost-efficient and popular ground improvement methods to mitigate soils <br />susceptible to liquefaction. Stone columns could be used to improve the subsurface soil <br />conditions to reduce the potential for lateral displacement. Stone columns are installed using a <br />vibratory probe that penetrates through the soil under its own weight from the ground surface to <br />a predetermined depth, or the top of a dense soil layer. Once at the desired depth, crushed rock is <br />introduced at the bottom of the hole through an opening at the base of the probe. The probe is <br />then raised and lowered while the crushed rock falls out the end of the probe. Each time the <br />probe is raised and lowered it forces more crushed rock into the substrata and builds a column of <br />rock within the subsurface soils. Stone columns are generally installed with a large crane, in a <br />triangular grid, with each column measuring between 30 to 36 inches in diameter. The stone <br />column installation laterally densifies granular soils around the columns and creates vertical <br />load-bearing columns of crushed rock. The crushed rock columns are highly permeable, aiding <br />drainage of porewater pressures developed by ground shaking. <br />Limitations of using stone columns as a mitigation option for this project include: