In Situ Stabilization and the Emerging World of In-Situ 3-D Environmental Construction
Stabilisation/solidification (S/S) comprises the amendment of a natural material with reagents, typically cementitious agents, to create higher strength result. It has been used for thousands of years in civil works such as road construction. In the twentieth century, it was extended to applications on contaminated sites, where the mobility and/or bioavailability of contaminants can controlled by, e.g., formation of insoluble compounds, reduction of permeability, control of pore space pH, sorption and micro-encapsulation. S/S has seen extensive application, from the Sydney Tar Ponds, to hundreds of U.S. Superfund sites.
Recent B.C. experience stabilizing tidally-influenced, saturated mixed fill containing rubble, cables and debris demonstrates that new mixing equipment and techniques are effective, even under adverse conditions. The experience also illustrates the potential of 3-D GPS-based machine control with mixing system data acquisition.
The case study includes design, pilot study and full scale in situ S/S data for a shallow (< 3 m) large-scale application in Esquimalt, British Columbia. Redevelopment of the site entailed management of a relatively large area of metals and hydrocarbon contaminated fill. Excavation, transport, and disposal of the 12,000 metric tons of contaminated fill at a licensed waste facility was initially considered; however, the cost was estimated at over $11 million, with significant uncertainty around the management of tidally-influenced seepage into the 4 to 5 metre deep excavation. In situ stabilization solved the problem at less than half the cost. The system reclaimed recyclable/reusable materials and homogenized the residue. Novel depth-specific sample collection equipment was also developed and deployed to provide depth-specific quality assurance sample collection capacity.
Based on the success of the method, a new mixing head was developed which is effective to 8 metres below grade. It is expected machine control advances in the next two to five years will allow the in-situ "printing" of honeycomb containment walls capped with mass-stabilization rafts while maintaining excellent control of both position and mix properties. Such "wall-and-cap" structures can cost-effectively contain contaminated materials while supporting large infrastructure.