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Challenges of Shallow Horizontal SubSlab SVE

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Ellingson-DTD recently installed a single horizontal sub-slab SVE well at an active industrial manufacturing facility in the upper Midwest. The well extended underneath the entire length of the building at an average depth of ~2ft below the floor slab. The crew installed the well in a “blind” or single-ended configuration as part of a pilot program.
The primary objective of the well includes sub slab depressurization to prevent migration of CVOC vapors in the indoor air. Following the results of the pilot program, the design team intends to calculate how many more (if any) wells will be needed to provide coverage for the entire building.

Shallow design depths present a challenge that often arises with these horizontal sub-slab SVE wells. The closer the screen is to the slab, the wider the vacuum influence will propagate in the sub-grade material. So, design engineers will always want to put the screen as close as possible to the slab. While deeper installations can theoretically exert the same influence, it requires a bigger blower, which increases operating costs.

Operationally the very shallow design depths pose issues with inadvertent returns (IR) of drilling fluid, also known as “frack-outs”. Additionally, underground utilities can often present major obstacles to the planned bore path.

HDD frack out in manufacturing facility

Inadvertent returns can be challenging when using HDD to insall very shallow horizontal subslab SVE wells

This project had its fair share of both of these common issues. With a very long screen section (350ft+), concerns over vacuum loss pushed the designed well diameter up from 3 inches to 4 inches. While that may not seem like much, there are knock on effects that led to IR issues.

The crews would normally install a 3 inch diameter well into a 6 inch diameter borehole, which represents a very standard pilot bore diameter for this size of rig. A 4 inch completion, however, needs to be reamed to 7+ inch diameter borehole, which is a little too big to complete as a single pass.

“Push reaming”, or overdrilling the initial pilot bore with a larger diameter bit, creates fluid pressure in the distal end of a single-ended borehole. This pressure drastically increases the likelihood of fluid migration. This being an active manufacturing facility, frack outs had to be carefully managed and immediately dealt with so as not to interrupt operations. Let’s just say that the shop-vacs were running non-stop once the reaming pass started.

Utilities also complicated this bore path. A sanitary sewer line, installed almost exactly at the planned screen depth, intercepted the running line perpendicularly. A small utility window cut into the slab and hand excavated to expose the pipe allowed the crew to drill within close proximity of the line, visually confirming the bit pass by without damaging anything.
That utility window later proved useful in mitigating the IR issues as well. During the reaming pass, the pit served as a kind of pressure relief valve. The frack outs diminished significantly once the reaming bit passed through the utility window.

Directional drilling provides a great method for installing horizontal SVE wells underneath occupied building space. Ideally the installation depth is ~5ft or more below the building slab. But this project proved that shallower installations are certainly possible. You just need to be ready to deal with some extra headaches.

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