Permeable trench

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

Figure 10a. Passive gas control using a permeable trench (U.S. EPA, 1985).

Several recovery scenarios were considered for remediation. Initially, construction of a narrow, permeable trench parallel to the canal appeared to be an appropriate interception system. The construction technique considered was use of a specially designed deep trenching unit. This type of trench would have included a tile drain leading to a single two-pump recovery well. However, a review of the subsurface site plans and interviews with long-term employees determined that an unknown number of buried pipes traverse the area intended for the trench construction. Disruption of refining operations and safety considerations resulted in rejection of this option. 

High-permeability passive perimeter gas control systems entail the installation of highly permeable (relative to the surrounding soil) trenches or wells between the hazardous waste site and the area to be protected (Figure 16.6). The permeable material offers conditions more conductive to gas flow than the surrounding soil, and provides paths of flow to the points of release. High-permeability systems usually take the form of trenches or wells excavated outside the site, then backfilled with a highly permeable medium such as coarse crushed stone. 

The soil used in the experiment was a low plasticity sandy material with a PI of about 11%. The variations in hydraulic conductivity probably reflected zones of material that contained more sand in some places and more clay in others. Tests have been performed on a couple of liners in the field where liquid flowing into the soil liners has been dyed and traced by cutting a cross section or trench through the liner. The result seems to indicate that dyed liquid finds a defect in the top lift, moves down and spreads along a more permeable zone between lifts finds another defect, moves downward, spreads finds another defect and so forth. 

All passive systems rely on the natural hydraulic gradient to transport LNAPL to the recovery location. Under most circumstances, the flow of LNAPL into this type of system is very slow. At open surface recovery sites (trenches and ponds) constructed in low-permeability soils, the LNAPL migrates in so slowly that free volatile product often evaporates before it accumulates sufficiently to be collected. High-permeability soils typically are subject to a low hydraulic gradient, which limits the rate of flow into the system. Conditions that are more favorable to passive recovery, shown schematically in Figure 7.1, include ... 

Variations of linear interceptors include closed trenches that are backfilled with highly permeable material or contain a conduit pipe (classic French drain). When set slightly below the yearly low water table elevation, a French drain can serve as an effective interception device (Figure 7.6). 

During the design phase, all of the data derived from the hydraulic characterization are evaluated for use in the selection of recovery pumping equipment and for the determination of the most appropriate subsurface fixtures (whether wells, trenches, or drains, etc.). A variety of generic scenarios may be appropriate to optimize product recovery. If the product thickness is sufficient, the viscosity low, and the formation permeable, a simple pure-product skimming unit may be the best choice. Other combinations of permeability, geology, and product quality will require more active systems, such as one-pump total fluid, or two-pump recovery wells. 

The adsorbent can be used in several ways for field applications. In one method, the material can be placed into a trench installed in the path of a contaminant plume. The material wiU form an in situ permeable barrier, removing contaminants as they pass through the Humasorb. Another method involves injecting or angering the adsorbent into the soil to accomplish the same task. The technology can also be applied as part of an ex situ remediation system. 

In a 1997 estimate provided by the vendor, the cost of permeable barrier installed during one year that operates for approximately 20 years would be about 682,000. This estimate was prepared for the Denver Creek site, in the Coeur d Alene district of Idaho. The apatite was assumed to cover a depth of about 5 yd, and the emplacement trench was assumed to be approximately 30 yd across and 8 yd thick. The flow rate through the site was estimated to be 2 ft /sec. For this estimate, design costs were determined to be 48,000 feasibility study costs 110,000 health and safety costs were estimated to be 112,000 the cost of the apatite was placed at 210,000 and monitoring well emplacement and operation, including samples... 

Field and laboratory experience continually emphasize the need for three-row grout patterns in linear cutoff grouting. In terms of resistance to extrusion and reduction of permeability, very thin cut-off walls would suffice. (A 6- to 12-in.-thick grouted curtain in sands and silts will support unbalanced hydrostatic heads of several hundred pounds per square inch. This is reflected in the fact that slurry trenches are generally very thin also.)... 

Regardless of its source, oil released into the subsurface soil moves along the path of least resistance and downwards, under the influence of gravity, as shown in Figure 31. Oil often migrates towards excavated areas such as pipeline trenches, filled-in areas around building foundations, utility corridors, and roadbeds. Such areas are often filled with material that is more permeable or less compacted than the material removed during the excavation. 

Interceptor trenches are ditches or trenches dug down-gradient from the spill, or in the direction in which the spill is flowing, to catch the flow of oil. They are placed just below the depth of the groundwater so that oil flowing on top of the groundwater will flow into the trench. Both water and oil are removed from the trench to ensure that flow will continue. Interceptor trenches are effective if the groundwater is very close to the surface and the soil above the groundwater is permeable. 

Electrolytic reactive barriers (also known as e barriers) consist of closely spaced permeable electrodes installed in a trench perpendicular to the direction of ground-water flow intercepts a groundwater contaminant plume, similar to PRBs. Rgure... 

Troubles from seepage usually can be controlled by exclusion or drainage techniques. Cut-off trenches, carried into bedrock, may be constructed across cols occupied by permeable deposits. Grouting may be effective where localized fissuring is the cause of leakage. 

The effect of a grout curtain is to form a wall of low permeability within the ground below a dam. Holes are drilled and grouted, from the base of the cut-off or heel trench downwards. Where joints are vertical, it is advisable to drill groutholes at a rake of 10-15°, since these cut across the joints at different levels, whereas vertical holes may miss them. 

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