<p><p><figure id='attachment_3507' style='max-width:887px' class='caption aligncenter'><img class="wp-image-3507 size-full" style="border: 2px solid #696969;" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic site plan for storage tank foundation support using shallow mass mixing in Vancouver Island, British Columbia, Canada." width="887" height="495" /><figcaption class='caption-text'> Site plan (Broomhead and Jasperse 1992, with permission from ASCE).</figcaption></figure></p><p><h2>Project Summary/Scope:</h2>Two concrete spill tanks for a pulp and paper mill were to be constructed on land previously reclaimed from the sea by uncontrolled dumping of sand and silt spoil. The tanks were to be set partially below ground surface so that no net load would be experienced by the soil during normal operation of the tanks.</p><p>As shown on the next page, the site consisted of pit-run gravel and sand underlain by desiccated, very stiff sand and silt fill (1.8-meters thick). The desiccated fill was underlain by loose, soft sand and silt fill deposits (3.7-meters thick). Below the loose fill, medium dense beach sand and over-consolidated silt extended down to some depth. The water table was typically at a 3-meter depth below the ground surface.</p><p>Due to site location, both static and seismic design considerations were necessary, requiring a balance of foundation costs and seismic risk. Analysis indicated that the loose layer (between about 1.5 and 3 meters) could liquefy during a seismic event. A ring of 3.6-meter diameter tangent columns was designed for each tank. The large and small tanks were supported on 71 and 45 columns, respectively. 18 additional columns were used to connect the two rings and support some ancillary tanks.</p><p>The 28-day compressive strength was specified to be 860 kPa. In-situ mix trials were performed to establish the mix design (177 kg cement /m³ soil with a water/cement ratio of 1.8 to 1) and rotation rate of the mixing tool. Column and foundation geometry was designed by considering seismic (almost empty tank) and static (full tank) conditions using force and moment equilibrium.</p><p>During mixing, the ground surface heaved due to the injection of the cement slurry. Following initial setup, heaved ground was levelled to footing grade.</p><p><figure id='attachment_3509' style='max-width:1024px' class='caption aligncenter'><img class="wp-image-3509 size-large" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic of typical soil profile under a tank with the foundation soils stabilized with shallow mass mixing." width="1024" height="356" /><figcaption class='caption-text'> Typical soil profile with tank (Broomhead and Jasperse 1992, With permission from ASCE).</figcaption></figure><h2>Alternate Technologies:</h2>Some alternative technologies were considered: piles, preload, excavate and replace, dynamic compaction columns, jet grouting, vibromortar columns, etc. Cost-benefit analysis and seismic considerations determined that shallow soil mixing was the best option.<br><h2>Performance Monitoring:</h2>Soilcrete samples were taken at various depths and locations using a discrete sampler developed by Geo-Con. Three cylinders were made from each sample and strength tested. 7-day unconfined compressive strengths of all reported samples were near 1000 kPa.</p><p>Upon first filling, the tank floors were expected to settle several inches. It was planned that pneumatic piezometers and settlement gauges would be used to control the initial filling rate to ensure that porewater pressures would not become excessive.<br><h2>Project Technical Paper:</h2>Broomhead, D., and Jasperse, B.H. (1992). “Shallow Soil Mixing – A Case History.” ASCE Geotechnical Division Specialty Conference Grouting, Ground Improvement and Geosynthetics, ASCE, New Orleans. 12p<br><h2>Date Case History Prepared:</h2>December 2014</p></p>