Design and Construction of Mixed Ground Working Platform for a CCR Surface Impoundment-Paper

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The paper describes the design and construction of a temporary access pathway and working platform for a deep soil mixing (DSM) pilot study on a coal combustion residual (CCR) surface impoundment. The saturated, soft CCR deposits—about 70–80 feet thick—created significant risks for equipment access, including bearing failure, instability, and vibration-induced liquefaction.<br /><br />Two-dimensional limit-equilibrium and finite-element analyses assessed global stability, bearing capacity, punching shear, and equipment loading. The analyses showed that the proposed 10-foot layer of in-situ mixed CCR could be reduced to 5 feet while meeting safety requirements, substantially lowering cost and schedule demands. The design supported an excavator and a 290-kip DSM rig, with required minimum mixed-ground strengths of 12 psi and 28 psi, respectively.<br /><br />A long-reach excavator mixed grout into the CCR in cells, advancing over previously treated areas after sufficient curing and acceptance of quality-control results. Cement type and grout proportions were adjusted during construction based on observed conditions. A static load test using water-filled tanks, with pressures comparable to the DSM rig, confirmed that the completed platform could support the anticipated loads.<br /><br />Instrumentation monitored settlement, vibration, and pore pressure. Settlement during rig mobilization was less than 0.1 inch, and excess pore pressures dissipated quickly; no signs of liquefaction were observed. Field pocket-penetrometer and proving-ring tests provided timely strength estimates to guide equipment movement.<br /><br />The case study demonstrates that engineering analysis, monitoring, and construction quality control can make temporary platforms safer and more economical in difficult ground. It also shows that mixed-ground thickness can be optimized without compromising performance.

Keywords

deep soil mixing

temporary access pathway

working platform

coal combustion residual

surface impoundment

ground stability analysis

bearing capacity

liquefaction risk

instrumentation monitoring

quality control