Laserfiche WebLink
January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 22 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />• Installation of stone columns may require predrilling to penetrate medium-dense soils, <br />fine-grained soils, or obstructions such as woody debris and boulders, to improve the <br />underlying potentially liquefiable materials. Given the near-surface elevation of the water <br />table and the presence of loose subgrade soils and woody debris, maintaining an open <br />hole will be challenging during installation. <br />• In general, installation of stone columns causes substantial vibration within the <br />subsurface soils that could potentially damage the nearby utilities and structures and <br />affect the stability of the slope. <br />• The strength of the improved soil relies heavily on the ability to densify soil between the <br />stone columns; hence, a larger area of ground improvement is required when compared to <br />other methods to achieve desired performance. <br />Based on the above-mentioned limitations and constraints, HWA does not recommend stone <br />columns as a mitigation option for this project. <br />Deep Soil Mixing: Deep soil mixing is an in-situ ground improvement technique that enhances <br />the characteristics of weak soils by mechanically mixing them with a cementitious binder. The <br />action of mixing materials such as cement, fly ash, lime, or bentonite with soil causes the <br />properties of the soil to become more like soft rock. <br />In deep soil mixing, a powerful drill advances a mixing tool as binder slurry is pumped through <br />the connecting drill steel, mixing the soil to the target depth. Additional mixing of the soil is <br />completed as the tool is withdrawn to the surface. This process constructs individual soil cement <br />columns, rows of overlapping columns, or 100% mass stabilization, all with a designed strength <br />and stiffness. Deep soil mixing is used in nearly any soil type, including organics. The ease of <br />mixing depends on the soil type, strength, water content, plasticity, stratigraphy, and texture. <br />With deep soil mixing, treatment is possible to depths up to 100 feet. Excess soil cement spoils <br />generated may range from 10 to 40% of the treated volume. Stiff soils and obstructions (i.e., <br />construction debris, buried timber) are sometimes pre-drilled ahead of the soil mixing process. <br />Limitations of using deep soil mixing as a mitigation option for this project include: <br />• Conducting deep soil mix columns in close vicinity to nearby slopes could be difficult as <br />the wet soil mix column performs as a semi-liquid zero-shear-strength material <br />immediately after mixing, which increases the potential for slope failure along the wet <br />column. <br />• Potential obstructions (such as timber and construction debris) encountered within the fill <br />could limit the construction of deep soil mix columns (e.g., pre-drilling, steel casing, etc. <br />may be required).