Trench method

The trench method is applicable only when the water table is relatively shallow, less than 10 to 15 ft below the ground surface. For a deeper water table, the cost of the trench method becomes more expensive than other methods such as pump systems. Another limitation of the trench method is the soil structure. The soil above the water table has to be firm and well aggregated to allow for the trench to be self-supporting. Otherwise, embankment enforcement or screening would be needed. A third limitation is that continuous pumping and skimming is required to maintain a flow gradient towards the trench. Otherwise, the free product will move back and reenter the soil. 

The pumping well method is more suitable for a water table that is too deep for the trench method. Pumps draw water, forming a cone of depression in the water table to control the movement of floating gasoline. The gasoline is then pumped out. The pumps can be either single- or a dual-pump... 

The remedial technologies83-85,90-93 described in previous sections for gasoline release are applicable, for the most part, for remediation of DNAPLs. For example, the pumping or trench method for free products, vacuum extraction, biodegradation, pumping and treatment, soil flushing, and soil excavation and treatment are suitable for cleanup of various phases of DNAPLs. Again, because of... 

Trench and area methods, along with combinations of both, are used in the operation of landfills. Both methods operate on the principle of a cell, which in landfills comprises the compacted waste and soil covering for each day. The trench method is good in areas where there is relatively little waste, low groundwater, and the soil is over 6 ft (1.8 m) deep. The area method is usually used to dispose of large amounts of solid waste. 

In the trench method, a channel with a typical depth of 15 ft (4.6 m) is dug, and the excavated soil is later used as a cover over the waste. Grading in the trench method must accommodate the drain-off of rainwater. Another consideration is the type of subsurface soil that exists under the topsoil. Clay is a good source of soil because it is nonporous. Weather and the amount of time the landfill will be in use are additional considerations. 

Figure 14.2 Safe trenching methods without the use of timber...

Traditional open trenching methods for instalUng pipe can be expensive, particularly in congested urban areas. Construction involves digging aronnd existing utilities to get to the required depth, which slows down the operation. Also, not only must the trench be backfilled, often sidewalks, pavanent, brick paving, sod, or other surfaces must be replaced. In addition, open cut operations often cause interruption of traffic and disraption of near by commercial activities. Excavation requirements... 

Plume Containment. WeUs can be placed at a contaminated site to prevent the contamination from spreading further or migrating offsite. In the past, containment efforts often reHed on physical methods such as bentonite slurry trenches, grout curtains, sheet pilings, weU points, and fixative injections. Containment by judiciously placed weUs generally costs less, takes less time to install, and is more flexible because pumping rates and locations can be varied. 

These two methods, which are used as an emergency action, can also be utilized for the cleaning of plumes. Containment methods can often be extended to plume treatment by using the trench or well pumping to recover the free product. 

Wet methods Use wet methods to minimize contamination with dusts. Clean vessels chemically vs. sandblasting. Use water sprays for cleaning. Clean areas frequently. Use water sprays to shield trenches or pump seals. 

Major disadvantages include the slow rate of recovery, exposure of large surface areas of flammable fluids (in open trenches), minimal containment, and odor or air quality concerns. Additionally, this recovery method does not address the treatment of dissolved hydrocarbon constituents in groundwater. 

A method frequently used to improve performance of trenches is to incorporate flow enhancement. Typically, this involves the use of a pump to lower the fluid level in the trench, and thus increase the hydraulic gradient. An additional benefit is that the continued flow toward the pump also tends to collect the LNAPL in a smaller area, where it is easier to recover. Water recovered from the trench may be treated for off-site disposal or reinjected upgradient to enhance the flow further. Increased... 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

A key benefit of accurate CMP models that needs emphasis is the capability to optimize layout design before polishing. Post-CMP ILD thickness variation is a serious concern from both functionality and reliability concerns. An effective method of minimizing this effect is the use of dummy fill patterns that lead to a more equitable pattern density distribution across the chip. Evaluation of such schemes before actual product implementation has become a major use of CMP modeling [53]. Dummy fill is also being investigated for front-end processes where shallow trench isolation CMP suffers from substantial pattern dependencies. 

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. 

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