434h_16 | Environmental Remediation | Drainage

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ch 16
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  Lecture No.16Containment Chapter 16, p.963 1.Introduction  Containment technologies are employed to minimize the risk of off-site containment.  Containment is often associated with other technologies such as in situ remediation toimplement a long-term clean-up strategy for the site.  Containment alternatives include passive controls such as slurry trench cutoff walls and coversand active strategies such as pump and treat and ground water collection. 2. Objectives  The broadest objective is to render the site safe for future use therefore the remediationscheme must minimize or eliminate the hazard to human health and the environment.  Alternatives may include removal, on-site treatment or containment.  This lesson focuses of the technical aspects of site remediation.  The site is never fully characterized and therefore there are always elements of uncertainty. 3. Passive Contaminant Control Systems  The function of passive systems is to minimize contaminant transport rates.  Consider the pathways for contamination;  Chapter 16, Containment, Page No. 2  Passive contaminant control technologies focus on controlling hydrologic pathways for contaminant migration and this approach is often called containment. 4. Surface Water Control Technologies  Surface water controls include: covers, caps, surface water diversions and erosion controlsystems. 5. Ground Water ControlA. General  Subsurface vertical barriers are employed to contain contaminants and to redirect groundwater flow.  Vertical barrier techniques include slurry trench cutoff walls, grout curtains and steel sheet piling.  Vertical barriers have not been widely employed as a method of hazardous waste siteremediation in the US.  The most common function of a vertical barrier in a remediation system is not containment butrather to inhibit the flow of clean ground water into the site and therefore vertical barriers inconjunction with ground water extraction and treatment systems.  Chapter 16, Containment, Page No. 3  The barrier may be upgradient or downgradient. The purpose of an upgradient barrier is to provide a cutoff controlling the influx of clean ground water flow from the upgradient regions.The purpose of a downgradient cut-off wall is to utilize site ground water flowing beneath thesite to actively flush contaminants from beneath the site.  Vertical barriers walls are embedded or keyed into a material of low permeability beneath thesite.  A barrier has a hydraulic conductivity of 1x10 -7 cm/sec or less.  It is usually necessary to employ ground water modeling to determine the anticipated impactof the barrier.  At present, consideration of contaminant transport across barriers is limited to advective, notdiffusive, flow.  Soil-bentonite slurry trench cut-off wall in the most common in Superfund remediation. B. Soil-Bentonite Slurry Trench Cut-off Walls  Also known as a slurry wall.  A trench is first excavated below the existing grade utilizing a liquid slurry of bentonite andwater to maintain trench stability. The liquid slurry is a construction expedient that will bereplaced by the soil-bentonite backfill.  The slurry is 95% water and 4-6% bentonite by weight which creates a viscous fluid with aspecific weight of 64-70 lb/ft 3 .  Trench collapse is controlled by the resulting hydrostatic force system where the slurrycounteracts the active soil pressures. In plain English, the slurry is always in the trench during excavation. It’s the slurry that keeps the walls from caving in, a necessary strategy as the trenches are typically narrow, several feet wide, and deep 10s of feet. Once dug, the slurry inthe trench will be replaced by the soil-bentonite backfill.  Example:  Chapter 16, Containment, Page No. 4 Given: A 100 lb bag of bentonite is mixed with 250 gallons of water. The specific weight of the water is62.4lb/ft3and the specific gravity of the benonite is 2.77.Find:1.) The % bentonite in the mixture2.) The specific weight of the mixture 1.) % bentonite Weight of water = 62.4 lb/ft3x 250gal x 1ft3/7.48 galWeight of water = 2086 lbs% bentonite = weight of bentonite / total weight = 100 lb bag / (100 + 2086 lbs) % bentonite = 4.6%2.)The specific weight of the mixture Gs =WsVs  wunits p.975Gs = specific gravity is the weight per volumeWs= weight of the solidsVs= volume of the solids, bentonite in this case  w= specific weight of water Volume of bentoniteVs= Ws/ Gs  w= 100 lb / (2.77x62.4)Vs= .58ft3Volume of water Vw= 250 gal/ 7.48 gal/ft3 Vw= 33.4 ft3 Weight of water Ww= 33.4ft3 x 62.4 lb/ft3 Ww= 2084 lbs  fluid or total= Wtotal/Vtotal = (2084lb + 100lb) / (33.4ft3+ .58ft3)    fluid or total= 64.3 lb/ft3   The factor of safety is the ratio of the resisting forces to the driving forces for clays:F = 4S u /H(  -  f  ) eq.16-1 units p. 975For sands:F =2  x  f  x tan  (  x  f  )eq.16-2 units p.975  Example: Given: A slurry trench that is to be 70’ deep and 2’ wide.  Find: Method of excavationFrom T16.1, p.976 Rotary or clamshell ** Example problem p.998, 16-7.  Backfill design. The soil-bentonite itself as opposed to the slurry should have:
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