Deep Soil Mixing Pilot Study
Excavator Bucket Mixing & Single Auger Deep Soil Mixing
Southern US
Significant Project Features:
• Geotechnical ground improvement of low strength Coal Combustion Residuals (CCR) with Portland cement to facilitate an Access Pathway and Work Platform on the ash delta.
• In Situ Soil Stabilization (ISS) by means of bucket mixing to a minimum depth of 5 feet below ground surface.
• Installation of Geogrid Underlayment and Imported Stone Fill along Access Pathway and Work Platform.
• Bearing capacity Load Testing of improved CCR material upon completion of Access Pathway and Work Platform.
• Deep Soil Mixing (DSM) to a maximum depth of 96 feet below ground surface in order to stabilize fly ash material.
Project Background:
GSI installed CCR-cement mix cell elements to support the load generated by the long reach excavator advancing along the access pathway and ultimately the Casagrande B360. Once the mixing components achieved required strength, GSI would track onto previously mixed cells to continue ISS operations. GSI completed the Access Pathway and Work Platform ground improvement of low-strength fly ash materials using a Volvo EC300 long reach hydraulic excavator to mix a General Use (GU) Portland cement and water grout into the CCR.
Prior to DSM commencement, GSI performed a bearing capacity Load Test of the CCR mixed Work Platform. To ensure that the improved CCR material achieved adequate strength to support the drill rig, GSI loaded two frac tanks to simulate the expected ground pressure while continuously monitoring settlement/deflection. Upon completion of the Load Test, GSI constructed six different test zones consisting of five DSM columns each. Each zone containing different target bottom elevations (80 – 96 feet BGS) and mix designs.
Description of Work
• Construction of an access pathway and work platform by ground improvement of the ash delta.
• Load Test on the Work Platform utilizing two frac tanks to simulate the ground pressure of the drill rig.
• Installation of 25 five-foot diameter soil mixed columns across 5 zones to a maximum depth of 96 feet Below Ground Surface (BGS).
• Installation of 3 eight-foot diameter soil mixed columns to a maximum depth of 83 feet BGS.
• PQ wire-line, triple tube core barrels were utilized to collect a core from 6 test columns (1 column from each test zone).
• Maximum mix depth from ground surface of approximately 96 feet.
• Work was completed in approximately 18 weeks, including 3 weeks of setup / teardown.
Key Project Objectives
• Conform to all site safety regulations, especially safety procedures regarding work performed around and on the ash delta.
• Ensure the ash delta ground improvement exceeds the design criteria to allow the Deep Soil Mixing drill rig to traverse onto the work platform.
• Perform field constructability of ash solidification utilizing varying mix designs, depths, and column diameters.
Project Challenges
• Working on the ash delta with the ash material.
• Possibility of unforeseen challenges due to the rarity of soil mixing with CCR material.
