Excavation Methods from eti

Biopolymer Trenching One of the most cost effective and versatile PRB construction methods has been biopolymer (BP) trenching. Installation of a treatment zone of granular iron using BP is similar to constructing a conventional impermeable slurry wall. As the trench is excavated, BP is added as liquid shoring to provide stability the trench walls. The BP used is typically guar gum based. Excavation continues through the BP without the need for dewatering. Granular iron (or iron-sand mixture) is placed through the BP by tremie. Recirculation wells are spaced along the length of the trench. After placement of the granular iron is complete, an enzyme is circulated through these wells to initiate the BP breakdown process and allow groundwater to flow through the granular iron PRB. Over 27 full-scale iron PRBs have been constructed with the BP method. The maximum depth for a granular iron PRB completed to date is 70 ft (21 m) bgs.
Continuous Trenching Continuous trenching machines allow simultaneous excavation and backfilling without an open trench. Excavation is performed by a cutting chain immediately in front of a trench-box (boot) that extends the width and depth of the PRB. Both the cutting chain and boot are attached to the trenching machine. As the trencher moves forward, granular iron or an iron/sand mixture is added to the boot to create a continuous treatment zone. Trenchers are available to install treatment zones from 1 ft to 3 ft (0.3 to 0.9 m) in width to depths of 35 ft (11 m). Several passes of the trench can be made to install thicker walls. Continuous trenching was first used to install a PRB in 1996 at a site in North Carolina. About 450 tons of iron was placed in a trench 150 ft (46m) long and 24 ft (7 m) deep in about 6 hr. Since then continuous trenchers have been used at over 15 sites.
Unsupported Excavation The PRB can be constructed without any sidewall support if the trench will stay open for about 4 hours without significant caving of the side walls. The PRB is excavated to the depth, width and length required. The granular iron or iron-sand mixture is placed directly into the trench. The iron of iron-sand mixture can be placed though shallow depths of water. This method is typically limited to depths of less than 20 feet (6 m). This is the simplest and least expensive construction method.
	Supported Excavation Trench boxes or hydraulic shores can be used to provide support to the trench side walls. These methods can be used where only minor side wall support is required. Trench boxes can be either standard trench boxes used for laying pipe or custom fabricated for PRBs. The trench is excavated and the trench box is pulled along close to the excavation face to support the trench. The iron or iron-sand mixture is placed in the back portion of the trench box. Hydraulic shores typically consist of two hydraulic shores fastened to bearing plates. These shores are placed across the trench immediately behind the excavation. Large metal "road plates" can be placed along the trench wall to provide additional support. The iron or iron-sand mixture is placed in the trench with the supports in place, and then the supports (and road plates) are removed. This method is typically limited to depths of less than 20 feet (6 m).
Cofferdam (Sheet Piling) Sheet pile is driven around the perimeter of the PRB and the soil within the sheet pile is excavated. Typically internal bracing is required with depth. The sheet pile maintains the dimensions of the treatment zone during excavation and backfilling. After backfilling is complete, the sheet piling is removed and groundwater is allowed to flow through the treatment zone. This traditional installation technique has been installed at over a dozen sites, including sites in California, Kansas, Colorado, Missouri, New Jersey, New York and Australia.
Injection Methods
Vertical Hydrofracturing Vertical Hydrofracturing enables placement of PRBs far deeper than possible by conventional construction methods. Continuous PRB treatment walls as deep as 300 ft (91 m) and up to 9 in (23 cm) thick can be injected into subsurface using Vertical Hydrofracturing. This installation method is minimally invasive, requiring only the drilling of 6 in (15 cm) boreholes every 15 feet (4.6 m) on the planned placement line of the PRB. Specialized tooling is inserted into the borehole to the required depth and oriented to control the direction and fracture pathway for what will become the PRB. The vertical interval for fracturing and injection is isolated in the borehole by packers. Iron flings of medium sand size are mixed with HPG biodegradable slurry. Immediately before injection a special breaker enzyme is included in the slurry mixture, which is then cross-linked to form a highly viscous gel containing 16 lbs (7.3 kg) of iron filings per gallon. This highly viscous iron filings carrier is then injected under low pressure (25 psi) through the down-hole tooling to propagate the fracture and form the PRB wall. The gel carrier follows the fracture pathway causing the soil to separate, creating the iron treatment zone. The enzyme breaks the gel within an hour or two, reducing it to water and harmless sugars, leaving a clean wall of iron filings. The wall is built from the bottom up by coalescing injections from each borehole to form a continuous PRB (i.e. a continuous vertical wall of iron filings). Vertical Hydrofracturing has been used at 8 sites in New Jersey, Iowa and Virginia, Texas and California. Vertical Hydrofracturing PRB systems have been installed up to a depth of 100 ft (30 m) below surface and the longest is 1,160 ft (354 m) in length.
Jetting Installation of an iron PRB using jetting uses high pressures of about 5,000 to 6,000 psi to jet a finer grained iron into the natural aquifer formation. The jetting tool is advanced into the formation to the desired depth. The iron is suspended in biodegradable slurry and is injected from nozzles as the tool is withdrawn. If the tool is rotated a columnar iron zone is created. The diameter of injection will depend on several factors, but distances of 2 to 7 ft (0.6 to 2.1 m) to are expected. If the tool is not rotated, and has only one or two opposing nozzles, a thin diaphragm treatment wall can be created. Diaphragm walls may be 2 to 3 inches (5 to 8 cm) of 100 percent iron thickness near the point of injection, but may be of a mix of iron and aquifer material further away. Pilot demonstrations have occured at Travis AFB (1999), Warren AFB (2002) and Memphis Army Depot (2006), and a full-scale application was completed in North Carolina in August 1999 (diaphragm wall).
Pneumatic Fracturing and Injection The injection method is completed in two steps, pneumatic fracturing and pneumatic injection, which are completed sequentially in one step. Pneumatic fracturing is used to create and /or enhance subsurface fractures with controlled bursts of high-pressure gas at pressures exceeding the natural in situ geostatic pressures and at flow volumes exceeding the natural permeability of the subsurface. Typically nitrogen gas is used. Fracturing allows greater volumes of iron to be distributed in the subsurface and provides better access to hydraulically isolated zones in the plume. Granular iron is then injected into the fractures using the gas as a carrier. Pneumatic fracturing and injection has been applied in many types of geologic media including sands, silts, silty clays and highly weathered fractured bedrock and up to depths of 160 feet (50 m). This technique has been used to emplace iron at over 15 sites.
Direct Push Injection This method typically involves hydraulic fracturing, which is the injection of a slurry solution at a pressure that exceeds the combined lithostatic pressure and cohesive strength of the formation. Rods are pushed into the subsurface to the required depth, and then retracted to expose an injection nozzle. The granular iron is suspended in a biopolymer slurry and pumped into the formation at a rate that exceeds the ability of the formation to accept the fluid. As a result, the pressure continues to rise until fractures are created. The injection pressure drops dramatically at the onset of fracturing. After fracturing occurs, slurry injection is continued to keep the fractures open. Enzymes are added to the biopolymer slurry during injection to degrade the slurry.
Soil Mixing Soil mixing rigs use one or more large diameter augers to thoroughly mix iron and soil. The iron is initially mixed with biopolymer and pumped to the mixing augers while they are advanced slowly through the soil. Over time the biopolymer breaks down allowing the groundwater to flow through the treatment zone. Alternatively the iron can be placed in a smaller diameter borehole and mixed into the adjacent aquifer material using the soil mixing material.