Project Overview
Ellingson-DTD provided horizontal directional drilling (HDD) services to design and install a multi-phase system of horizontal containment and injection wells for groundwater management beneath and adjacent to active coal ash basins at a Montana power plant. The work was conducted over several years as part of the client’s long-term groundwater containment and control program designed to manage potential migration of constituents from coal ash storage areas.
Each construction phase required precise well placement below or alongside the basins in variable geologic conditions that included coal ash, clinker, reclaimed mine workings, and native sandstone, siltstone, and coal. Wells were designed in collaboration with site engineers and hydrogeologists to achieve hydraulic capture of shallow groundwater and to maintain long-term containment under challenging drilling and site access conditions.
Phase 1 – 2020 Installations
Ellingson-DTD installed four horizontal containment wells beneath active ash basins.
• Drilling conditions: ash, reclaimed mine workings, sandstone, and coal.
• Installation methods: both blind and continuous installations, using 4-inch PVC and 6/8-inch HDPE.
• Total installed length: approximately 6,200 feet of horizontal drilling.
• Well depths: screens located 15–50 ft below entry elevation.
• Development: flushing, jetting, and pumping with fresh water and enzyme breaker.
All wells were installed using biodegradable drilling fluids to minimize environmental impact.
Phase 2 – Containment and Injection Wells
A second phase added two 8-inch HDPE containment wells and two 4-inch PVC injection wells beneath additional basins to increase hydraulic control.
• Drilling conditions: sandstone, siltstone, claystone, coal, and clinker.
• Total drilled footage: about 8,300 feet.
• Screen depths: 60–100 ft below grade.
• Development: flushing, jetting, and over-pumping with submersible pumps.
The continuous wells were fused in single lengths and pulled through each borehole, maximizing installation integrity and alignment through variable geologic strata.
Phase 3 – 2022 Expansion
The third phase extended the system to the perimeter berms of the ash basin, where three 4-inch PVC containment wells were installed to intercept groundwater migrating beyond basin boundaries.
• Total drilled footage: 4,493 feet.
• Drilling conditions: native sandstone, siltstone, claystone, coal, and clinker.
• Development methods: same proven flushing, jetting, and enzyme breaker approach.
Results and Impact
The phased installation successfully created a continuous horizontal barrier and capture system beneath and adjacent to the basins, improving control of groundwater movement and protecting downgradient receptors. Ellingson-DTD’s use of HDD technology allowed all wells to be installed with minimal surface disruption, avoiding excavation through sensitive areas and maintaining full plant operation during construction.
Technologies Used: Horizontal directional drilling (HDD), through-rod installation, continuous pullback fusion, PVC and HDPE well materials.
Total Horizontal Footage Installed: ~19,000 linear feet across three phases.
Environmental Commitment: All drilling utilized biodegradable fluids and careful well development procedures to ensure clean, high-performance systems.
