Soil Mixing Case Studies

To determine the best method for the remediation of volatile organics, benzene and vinyl chloride, Remedial Construction Services, L.P. (RECON) performed five unique in-situ pilot studies. These in-situ studies were competed using RECON ’s Single Auger Mixing (SAM) drill rig to provide a homogeneous mixture of the selected oxidizing agents and soil. To entrap the volatile organics, a gas collection hood was set up over the excavation and fixed to the drill rig.

Single Auger Mixing

SAM uses a specifically designated, hollow stem mixing tool, consisting of a configuration of fixed rotating blades – 6 feet in diameter. Injection points are strategically located along the base of the tool to insert the slurry into the soil. As the tool descends into the ground, at the prescribed revolutions per minute, slurry is introduced at a predetermined rate. This progress continues until the final depth is reached; then, the auger is repeatedly raised and lowered to ensure a homogenous mix of the entire area or column.

Pilot Tests

Perimeter boundaries were established and demarked for each individual test section. The top 5 feet of soil was removed from each area, because of the swelling potential during mixing operations. The columns were advanced 25 feet below ground surface. Recon Services used a varying percentage of sodium persulfate, hydrated lime, Portland cement, and sodium percarbonate during the 5 tests. Wet grab samples were collected from the top, middle and bottom of each column for testing. Area 4 was the “hottest” mix; the soil began to boil upon completion. All off-gasses produced by this procedure were contained by the fabricated hood, removed and contained by the Drager pump with a 0.5 to 1.0 ppm tube. The Site Safety Officer thoroughly tested the area using a photoionization detector and a benzene monitor.

The project consisted primarily of treating approximately 23,000 cubic yards of contaminated soil by in-situ solidification / stabilization methods. The site was contaminated with chromium, lead, PCB, toluene, benzene, and several other organic compounds as a result of improper waste oil storage and incineration operations.

The work included a pre-bid treatability study, and a post-award trial on an area approximately 25-feet by 25-feet by 10-feet to demonstrate the treatment system, including the chemical formulation, effectively treated site soils. Additional work included abandoning existing monitoring wells and surfacing treated soil with compacted gravel.

The equipment utilized was a Manitowoc 3900 with a crane mounted drill attachment with 450,000 foot pounds of torque and an 8-foot diameter, three bladed insitu soil mixing tool. A Moyno Pump was used for pumping the grout like material through the Kelly bar and auger head, blending it into the soil.

Requirements

The TCLP, Permeability, and the USC performance requirements of the project were:

In this case, the situation that needed to be addressed was that the land was excessively wet and oil contaminated land needed for re-developement.

The Problem

For industrial land owners, the process of closing old industrial sites including the mitigation of free phase petroleum hydrocarbons in the soil and ground water that resulted from prior facility operations poses unique challenges.

In meeting these challenges, BP Amoco looked to Environmental Resources Management, Inc. (ERM) to design and manage the closure of one of its sites, the former Amoco Baltimore Asphalt Terminal, located in Baltimore, Maryland. Originally constructed in 1922 as a bulk transport terminal for gasoline and kerosene, the facility eventually operated solely as an asphalt refinery before operations ceased in 1982, after which the facility served as an asphalt storage terminal.

The closure plan prepared by ERM was approved by the Maryland Department of Environment (MDE) Oil Control Progam (OCP). The primary objective of the plan was to remove, to the best extent practicable, free product from soil and ground water to achieve the site closure and increase property value for future development. The closure plan required the excavation of approximately 15,000 cubic yards of soil containing free product. The excavated soil was to be managed on site and placed within a clay-lined berm formerly used for storage tanks. The cleanup criterion required one part per million (ppm) or less of leachable total petroleum hydrocarbons (TPH) diesel range organics (DRO). To perform this work, ERM turned to RECON of Houston, Texas, a leading contractor who specializes in soil stabilization and remediation.

The Solution

ERM and RECON determined that soil stabilization with pebbled quicklime was the most effective method to achieve the criterion for clean-up of free product, and also pr o vide optimum dr ying and stabilization for the soil. This soil modification practice is a common and highly proven method with identical uses in road construction and site development where wet, plastic soil is improved beneath pavement subgrades or building slabs.

The process relies on the use of quicklime (calcium oxide, CaO), which reacts with the existing soil moisture to cause lime hydration to help dry the soil. During hydration, the soil.s high pH levels cause the calcium ions from the hydrated lime to attack and separate the clay par ticles by decreasing the levels of water. Subsequently, this reduces the amount of water the soil can then absorb. Clay particles are agglomerated and their texture rapidly changes to a granular form with larger particle properties and less surface area, resulting in greatly reduced soil plasticity. The new modified condition significantly reduces soil leaching capacity. Stabilized soil is also easier for contractors to maneuver than wet soil, and improves compaction properties.

The multi-weather attributes of lime stabilization allows for application in the colder winter climate by which quicklime can still react. For this operation the lower temperatures provided additional advantages in keeping the viscosity of the free product high, and minimizing odors. As summarized by Karl Peckhaus of RECON, Quicklime was used to solidify the saturated soil during the winter months. Its long-lasting reaction time and drying ability virtually eliminated petroleum hydrocarbon leaching all together.. With the importance of environmental dust control for site work contractors, Peckhaus was also quick to point out. During the soil stabilization phase, RECON maintained a virtually dust-free environment by using 1/4. by 1/2. quicklime product.

The Results

Samples of the stabilized soil were collected throughout the project to document and verify the stabilization results. In all cases, the soil passed a .paint filter test. by which it proved to be dry enough (lowered moisture content) so that free liquids were no longer present (or at negligible levels) in the soil. TPH-DRO results were below one part per million (ppm), which achieved MDE.s cleanup criterion. In fact, all leachable results were non-detect except for one, for which the laboratory reported a leachable result of 0.90 ppm.

Verification monitoring results since show that free product is not being detected in the remaining monitoring wells indicating that the corrective measures, as approved by MDE, have removed significant levels of free product. Structurally, the stabilized soil provided an all-weather working platform capable of supporting typical construction traffic. According to Leonard Rafalko of ERM, .The use of quick¬lime as the stabilizing agent was instrumental in allowing the project to meet regulatory criteria, and BP Amoco.s schedule for site closure.. Since the work was completed, MDE.s State Superfund, Oil Control, and Voluntary Cleanup programs have issued a No Further Action or No Further Requirements Determination for the site.

When soils in one area of the former Halby Chemical Company site, in New Castle, Delaware, were found to contain high concentrations of Carbon Disulfide (CS2), Terra Constructors, an acquisition of Remedial Construction Services, L.P. (RECON), was awarded a contract to conduct a remediation test program using insitu chemical oxidation.

In this process, as oxygen is brought into contact with the CS2, a chemical reaction takes place that breaks down the contaminant into carbon dioxide and sulphate salts, both harmless compounds. RECON/ Terra delivered the oxygen to the target soils in the form of an oxidant slurry, using the Single Auger Mixing (SAM) process.

Single Auger Mixing

In this technique, a specially designed, hollow-stem mixing tool with carbide cutting teeth is advanced and rotated into the soil by means of a crane-mounted turntable. Concurrently, the reagent is pumped through ports in the mixing tool and blended with the in situ soil as the mixing tool is lowered and raised through the target zone. A grid treatment pattern designed to achieve overlapping “columns” ensures uniform mass treatment.

For this project, a 60-ton crane supported the drilling platform, which was capable of producing as much as 100,000 foot-pounds of torque. Torque was transferred to the 6-foot diameter mixing tool, and “down pressure” was provided, by a specially fabricated kelly bar.

Test Program

Three areas, approximately 6 feet wide, 10 feet long and 12 feet deep, were designated for the test program. The oxidant, an adduct of soda ash and peroxide, was blended with water in a high shear mixer to produce a pumpable suspension. Heat generated during the oxidation process resulted in the formation of gases that were drawn through a thermal oxidation unit for treatment.

Soils with median CS2 concentrations of 5,000 mg/kg, ranging up to 160,000 mg/kg, were successfully treated to below the clean-up goals of 1,010-mg/kg average and 2,020-mg/kg maximum.

Production Contract

The success of the test program led to RECON/Terra being retained to treat approximately 11,000 cubic yards of contaminated soil. A solidified cap was formed by adding cement into the top four feet of soil with an excavator.