By: Kyle Carlton, PG, Senior Geologist
Recently, one horizontal soil vapor extraction (SVE) well needed to cross nine utilities along the bore path at a target depth of 4 feet below ground surface. Preventing injuries to workers and damage to the subsurface utilities was imperative to all parties involved with the project. This case study details the design and methods used to safely drill past the utilities for installation of the horizontal SVE well and solve the site’s vapor intrusion concerns.
Background
A former gasoline station site with subsurface petroleum impacted soil and groundwater faced serious concerns of vapor intrusion to a building. The upcoming activation of an on-site air sparge remediation system was designed to volatilize dissolved petroleum compounds within the saturated zone approximately 8 feet below ground surface (bgs). Because of air sparging, soil vapors are produced, therefore vapor extraction within the vadose zone was necessary to prevent the vapor intrusion.Another complicating factor was the proximity of multiple subsurface utilities near the building. Vapors commonly collect within subsurface utilities and are capable of migration along utility pathways, threatening vapor intrusion into the building.Horizontal soil vapor extraction wells were selected and designed to extend beneath the nearby building to provide the necessary vapor recovery below the building slab. However, the utilities near the right-of-way of the street were also at risk of accumulating vapors from the air sparging. Therefore, our consultant partners, with Directional Technologies’ input from previous experience, produced a unique horizontal well design to solve the vapor accumulation concerns.
Approach
The horizontal SVE well needed to approach the utilities at a target depth of 4 feet bgs. Utilities in the area included sanitary sewer, water, gas, power, and communication lines, which traversed perpendicular to the proposed horizontal well path at depths of 3 feet to 6 feet bgs. But the question as to how to safely cross these utilities had to be addressed early in the design process.The consultant and their client specified safe utility clearance through use of an air-knife/dry-vac to uncover the utilities for visual verification of depth, diameter, and trend of the utility. Odyssey Environmental, LLC worked ahead of DTI’s directional drilling crew to uncover the utilities prior to directional drilling of the pilot bore. Like many scenarios, the marked utilities were slightly off the surface marking, and deeper than street plans depicted. One challenge was how to advance the drill bit safely past a 10-inch sewer early in the bore.To safely cross the utilities, DTI continuously and carefully tracked the location and depth of the drill bit using the walkover locating system. When possible, the crew visually verified the bit was passing safely over or under the utilities, sometimes within 5-inches of the utility.
Results
With careful planning and execution, over 200 feet of horizontal SVE wells screen was placed at the target depth to recover soil vapors and mitigate vapor intrusion in to the utility conduits and trenches. When the air sparge system is activated, the horizontal SVE wells will depressurize the subsurface above the sparging zones providing the path of least resistance of the volatized petroleum compounds to be recovered and transferred through granular activated carbon vessels in the remediation system compound. This impressive horizontal well installation was successfully and safely completed through careful planning and execution by the consultants and subcontractors.
