Many of the single-shell tanks at Hanford are well beyond design life and are perceived as being in imminent danger of failure. Some of the tanks have already failed. Over the years, tank leaks and spills have contaminated the soils; the soil contamination raises issues about the characterization of the tanks, closure of the tanks, and the safety of the tank farms. This study investigated the application of conventional and innovative drilling techniques to the problems of soil characterization, monitoring, and barrier installation (both desiccant and physical/grout types) beneath the Hanford C tank farm.
An evaluation and ranking of twenty four types of drilling technologies included horizontal and directional drilling techniques developed by the oil and gas industry, the mining and minerals extraction industry, and others. The drilling technologies were evaluated for their ability to drill in unconsolidated soils, provide good horizontal and vertical position control, facilitate subsurface characterization and monitoring, and permit installation of subsurface barriers beneath individual tanks or beneath a tank farm. Barrier placement and soil stabilization for remediation applications presents many challenges, particularly at the Hanford Site.
For example, one facet of the study included investigation and concept development of barrier systems which could be installed beneath and around the tank farms with a minimum of excavation. The barrier concepts--existing, commercially available technologies--had to be suitable for the variety of soil conditions found beneath the Hanford tank farms. The barriers were also investigated for their ability to withstand chemical attack and seismic deformations, and for their ability to have the barrier integrity verified remotely.
The best barrier technologies and drilling systems were combined into seven integrated barrier systems which were evaluated and ranked for their suitability to the Hanford site, their ability to meet environmental constraints, the safety of personnel working on the site, and the ability of the barrier to withstand large and small tank leaks. Each of these seven systems were then evaluated in terms of relative amounts of material extracted, deployment time, and costs.
The two highest ranked barrier systems developed were a circulating air barrier and a cone grouting barrier. Both are candidates for quick deployment and application to single or multiple tank systems. Both systems are candidates for demonstration at a cold test at the Hanford site. Two other potentially applicable barrier systems were evaluated, permeation grouting from horizontal wells and fracture grouting. Both are candidates for continued development and demonstration.