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New Technologies For Subsurface Barrier Wall Construction Robert D. Mutch, Ir., P.Hg., P.E., Robert E. Ash, TV, P.E., Nashville, Tennessee, and Jeffrey R. Caputi, P.E., CHMM, ECKENFELDER INC., MaLwah, New Jersey INTRODUCTION During much of the 1980s, barrier walls of any type were regarded In some quarters as crude and antiquated. It was predicted correspondingly that remediation would be dominated bv . . . . . ~ 1 1 1 ~ ~ ~ ~ _ ~ ~ ~A~ ~ ~;~ ~ emerging treatment technologies SUCh as nloremeclatlon, air sparing, alla surIaclaIll 1lusmrlg. Notwithstanding the considerable successes of these emerging technologies, particularly bioremediation, the fact remains that a significant percentage of Superfitnd' RCRA-corrective action and other waste disposal sites present hydrogeologic, chemical, and waste matrix complexities that far exceed the capabilities of current treatment-based remedial technologies. Consequently, containment-based technologies such as subsurface barrier walls and caps are being recognized once again as irreplaceable components of practical remediation programs at many complex sites. Until quite recently, most barrier walls were constructed using traditional technologies such as soil-bentonite slurry trench, conventional sheet piles, vibrating beam technology, and in the case of shallow cut-off walls, compacted clay. Today, the remediation engineer considering a subsurface barrier wall-based cleanup is confronted with a baffling array of new technologies and permutations of these technologies. Table 1 presents a partial listing of available barrier wall technologies. TABLE 1 Subsurface Barrier Wall Technologies Compacted Clay Soil-Bentonite Slurry Trench Self-Harden~ng Slurries Plastic Concrete Slurry Trench Deep Soil Mixing let Grouting Vibrating Beam Ground Freezing Waterloo Barrier(~) Sheet Pilings Permeation Grouting Geomembrane Technologies Each of the technologies listed in Table 1 also many permutations. For instance, there are many varieties of self-hardening slurries that can be tailored to specific site conditions and design objectives. There are also a wide variety of permeation grouts and several different geomembrane technologies, as well as a variety of different materials, that can be used in vibrating beam barrier walls. Subsurface barrier walls have been constructed of compacted clay, soil-bentonite slurry trench and vibrating beam techniques for many years. These technologies are well-understood and well documented and are thus considered conventional cutoff wall technologies. Each technique has inherent advantages and disadvantages, and the cost of each is typically tied to D-23

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D-24 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT site-specific factors. (For further information on these conventional technologies, see, e.g., Mutch and Ash (1994) and Cavalli et al. (19921.) ADVANCEMENTS IN BARRIER WALL TECHNOLOGIES In the remainder of this paper, we overview the different emerging barrier wall technologies, addressing their advantages, disadvantages, limitations, documented track records, and costs. Deep Soil Mixing Barrier Walls Development of deep soil mixing (DSM) barrier walls can be traced back to the early 1960s when the Intrusion Prep akt Co. patented a process for "mixed-in-place piles" (Jaspers, 1989~. In the early 1970s, Japanese geotechnical companies developed several different types of soil-mixing methodologies and, to date, have conducted thousands of deep soil-mixing projects. Deep soil mixing involves a crane-supported set of leads that guide a series of two to four hydraulically driven mixing paddles and augers 30 to 36 inches in diameter. As the auger guides make their way through the earth, they break the soil loose and lift it to the mixing paddles, which blend the soil with a slurry that is injected through the augers. The slurry can consist of lime, bentonite, cement, or proprietary mixtures designed to solidify or stabilize the soil. A continuous barrier wall is created by sequential penetration of the augers overlapping with previous auger-treated zones. DSM can be used to create cutoff walls more than 100 feet deep. DSM offers several advantages over conventional cutoff wall methods. First, the soil does not have to be fully excavated, minimizing soil disposal costs if soils are contaminated. Since the wall is constructed In small sections, there is considerably less danger of collapse in soft soils. The technique is also capable of construction within confined areas and requires less staging and above-ground mixing areas than slurry trench techniques. A disadvantage of the technique lies ~ the fact that in-situ soils are used in the slurry soil admixture. If the soils are unsuitable or if waste materials are encountered, then additional costs and construction difficulties can result. The cost of deep soil mixing usually falls in the range of $6 to $12 per vertical square foot. Jet Grouting Cutoff Walls Jet Grouting evolved in Japan during the early 1970s from a water cutting technology originally used in American coal mines (Guaterri, 19881. Jet grouting is a general term describing construction techniques where ultra-high-pressure fluids are injected into the soil at about 800 to 1,000 feet per second. The high-speed fluid is used to cut, replace, and mix the native soil with a cementing material, typically a cement-based grout. There are three general forms of jet grouting that involve injection of a single fluid (grout), two fluids (grout/air), or three fluids (grout/air/water). Jet grouting proceeds first by drilling a vertical guide hole down to the required depth. Actual jet grouting then follows, proceeding typically from the bottom to the top of the borehole. Panels or columns can be formed by controlling the rotation of the drill rods while lifting the jet grouting device. Columns are formed when the drill rods are rotated during

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APPENDIX~PAPERS PRESENTED D-25 lifting. Panels can be created by lifting the drill rods without rotation. Subsurface cutoff walls can be created by jet grouting adjoining columns of soil sequentially. Although jet grouting has been used extensively in Japan, Italy, Germany, and South America, it has received only limited attention here in the United States, but it is expected that the technique will be used more frequently here as acceptance of the procedure grows. An advantage of jet grouting over other cutoff wall techniques is the fact that it can be used to stabilize a wide range of soils, ranging from grave! to heavy clays. Another advantage is that large-diameter columns or panels can be created, starting from relatively small-diameter boreholes. Therefore, cutoff walls can be constructed beneath buildings with limited disruption of the structure itself. let grouting also has been conducted to depths in excess of 200 feet. All three forms of jet grouting have some portions of their process covered by U.S. patents. The cost of a nominal three-foot-wide barrier wall constructed by jet grouting generally lies in the range of $15 to $30 per vertical square foot. Waterloo Sealable Sheet Piles The University of Waterloo has developed a sealable sheet pile wall that reportedly is capable of achieving bulk hydraulic conductivities of less than 10-8 centimeters per second. This product, which has patents or patents pending in several countries, is termed the Waterloo garner_. The technology involves specially fabncated sheet piles with a sealable cavity incorporated into the pile interlock. Figure 1 depicts the dimensions at the medium wall Waterloo Barrier_ constructed of 0.295-inch-thick steel. A heavier gauge, 0.375-inch-thick steel, Waterloo Barrier_ is scheduled to go into production during November of this year. The sealable cavity of the Waterloo Ba~TierTM can be sealed with clay-based, cementitious, polymer, or mechanical sealants. A footplate at the toe of the sealable cavity prevents most of the soil from entering the cavity during driving. After driving, the sealable cavities are waterjetted to remove loose soil In preparation for injection of sealant. Waterloo Barriers can be installed to depths of 70 feet and deeper if necessary by splicing piles together. Costs of the Waterloo BaIliers are on the order of $15 to $30 per vertical square foot (R. Jowett, personal communication, 19951. r SETTLE / CAVITY FIGURE 1 Medium wall Waterloo Barrier_. 22.25 in. (565 mm) SEALABLE: CAVITY me, 8.17 in. (208 mm)

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D-26 BARRIER TECHNOLOGIES FOR E~IRONMENTAL MANAGEMENT Self-Hardening Slurries Slurry trench cutoff walls can be constructed using self-hardening slurries, designed to set up in place, producing low-strength barrier walls. This type of cutoff wall is constructed in panels. A continuous wall is formed by re-excavating the end of the adjacent pane! after it has set up. Alternatively, an end-stop pipe can be placed between panels. The pipe is removed prior to full setting of the slurry, allowing the self-hardening slurry in the active panel to flow up against and form a good seal with the hardening slurry in the previously completed panel. The most commonly used self-hardening slurry consists of Portland cement and bentonitic clay (cement-bentonite). Bentonite is blended with water to produce a hydrated slurry, typically consisting of 6 percent bentonite by weight. Cement is added just prior to pumping the slurry to the trench. The cement content is typically in the range of 10 to 20 percent by weight. Upon setting, the mix resembles a stiff clay with ultimate strength in the range of 5 to 50 psi. While cement-bentonite cutoff-wall technology has been around for many years, its use in site remediation has been limited by the inability to achieve sufficiently low permeability. The permeability is typically in the range of 10-5 to 10-6 centimeters per second, whereas 10 centimeters per second commonly is specified in site-remediation applications. The relatively high permeability of cement-bentonite slurries is due, in part, to the adverse effect of the Portland cement on the swelling properties of the bentonitic clay. Alternative self-hardening slurry mixes are now available that consistently can achieve permeabilities below 1 x 10 centimeters per second. Ground, granulated blast furnace slag can be blended with Portland cement to produce slag cement. When slag cement with a slag to Portland cement ratio of 3:1 to 4:1 is combined with bentonite slurry, the permeability of the -7 ~ mixture is generally in the range of 10 to 10 centimeters per second (lefferis, 1985; Chipp, 19901. The use of slag cement also enhances chemical resistance and ultimate strength. A proprietary mix marketed by Liquid Earth Support, Tnc., called Impermix~, consists of slag cement (containing no Portland cement) and attapulgite. Attapulgite is a clay mineral with a different crystalline structure than bentonite. The combination of slag cement and attapulgite produces a mix with extremely low permeability, as well as greater resistance to chemical attack and higher ultimate strength. Permeabilities of less than 10 have been attained (Tallard, 19921. An advantage of self-hardening slurries in comparison to conventional soil-bentonite slurry trench cutoff walls is that there is no separate backfilling operation. The slurry can be prepared in a remote area and pumped to the trench. This allows for construction in limited access areas and also minimizes the time workers must spend in the exclusion zone in the case of highly contaminated sites. In addition, there is little or no slurry displaced from the trench that could require special treatment or handling. Additionally, the panel methods of construction is advantageous when working in unstable soils or near structures, since the length of open trench can be minimized. Panel lengths typically range from 10 to 30 feet. This method also allows the barrier wall to be constructed In discontinuous sections, where necessary for coordination with other site activities. The cost of self-hardening slurry walls is typically in the range of $10 to $20 per vertical square foot for a nominal two-foot wide barrier and depths less than 100 feet.

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APPEND~PAPERS PRESENTED D-27 Permeation Grouting Permeation grouting has been used extensively In the United States and abroad in the mining and geotechnical engineering fields. The most common type of grout is a mixture of Portland cement and water. Other types of grout, which have been used under certain conditions, include cement-sand-water, cement-rock flour, cement-hydrated lime, cement-calcium chloride, cement-diatomaceous earth, lumnite cement, cement-clay, or cement-bentonite (Krynine and Judd, 1957~. Asphaltic emulsions and other bituminous compounds have also been used for grouting (Krynine and Judd, 19571. More recent advancements ~nc~ucte the use of microfine cement, mineral wax, sodium silicates, and colloidal silica gel. The amenability of various soils to grouting is in large measure a function of the soil's permeability, as indicated in Table 2 (Karol, 19901. Soils with permeabilities less than 1 x 10 centimeters per second essentially are ungroutable, while soils with permeabilities greater than -1 10 centimeters per second require suspension grouts or chemical grouts containing filler materials. Grouting is also more difficult in heterogeneous soil, as the grout tends preferentially to follow pathways of least resistance through the soil. TABLE 2 1 990) Permeability (cm/see) lo-6 10-5 to 10-6 10-3 to 10-5 -l to 10-3 lo-l Approximate Relationship Between Soil Permeability and Groutability (Karol, Groutability (Ability of Soil to Receive Grout) Ungroutable Groutable with difficulty by grouts with viscosity <5 cP and ungroutable with grouts having a viscosity >5 cP Groutable with low-viscosity grouts but difficult with grouts with a viscosity greater than 10 cP , ~ Groutable with all commonly used chemical grouts Requires suspension grouts or chemical grouts containing a filler material The Department of Energy, through its Sandia National Laboratories, has been conducting a study of two grouting materials, a montan wax emulsion and a glyoxal-modified sodium silicate material. Montan wax is a fossilized plant wax with properties similar to that of natural plant waxes, such as those found In canauba palms. It is a hard, high-melting point material comprised of waxes, resins. asDhaltene-like materials with C-24 to C-32 carbon chain esters ot iong-cna~nect acids and alcohols (doss et al., 19951. Laboratory testing of soils permeated with a montan wax emulsion showed a significant reduction In soil permeability. The initial permeability of the soil tested varied from 6.5 x 10-4 to 3.6 x 10-2 centimeters per second. After permeation grouting by the montan wax emulsion, the soil permeability was reduced to between 3.7 x 10-8 and 1.6 x 10-4 centimeters per second. The glyoxal-modified sodium silicate material originally was developed by a French company, Societe Franchise Hoechst. The glyoxal-modified sodium silicate material consists of

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D-28 BOWER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT four proprietary components, the composition of which can be adjusted to modify set times. Laboratory testing of glyoxal-modified sodium silicate-grouted soil revealed final permeabilities of 6.2 x 10-6 to 5.1 x 10-5 centimeters per second. The initial soil permeability was the same as that cited above for the montan wax permeation study. It also should be noted that they were unable to grout soil with a permeability less than 5 x 10-4 centimeters per second with either the montan wax emulsion or the modified glyoxal sodium silicate material. DuPont has also developed a permeation grouting material based on a colloidal silica gel. This technology was adapted from a technology developed by Conoco for oil field applications and is termed Ludox~. The gel times of the Ludwig material cart be controlled to vary from a few hours to a few thousand hours. Its density and viscosity are similar to that of water. In laboratory experiments, it is reported that three orders-of-magnitude decrease in permeability was achieved following injection grouting by the Ludox(~) technique. Soils with initial permeability of 7.7 x 10 to S.6 x 10 centimeters per second were reduced 8 an In permeability to between 3.5 x lo and 5.4 x lo centimeters per second. However, in a larger sandbox-sized study, the permeability of the Ludox~-grouted soil was found to be 4 x 10 centimeters per second. Nonetheless, this represented a four orders-of-magnitude improvement over the permeability of the ~.ngrouted sand (Noll et al., 1993~. Ground Freezing Artificial ground freezing has been used in geotechnical construction for more than 100 years. The first application of ground freezing for construction purposes reportedly took place in Genmany in 1883 (Braun and Nash, 19851. The maximum frozen depth achieved has been 3,000 feet (Braun and Nash, 1985~. Ground freezing is accomplished by circulating a coolant through a network of closely spaced vertical or inclined pipes. The coolant can be calcium chloride brine, liquid nitrogen, or ethylene glycol. Due to relatively high maintenance costs, ground freezing generally is considered as a temporary containment measure. The Department of Energy has undertaken a pilot scale study of ground freezing at its Oak Ridge, Tennessee facility (Peters, 1994~. The test site is approximately 60 feet by 60 feet and 28 feet deep. It consists of a double ring of inclined and vertical freeze pipes to form a V- shaped bathtub ring within which is a 750-gallon steel tank. An inner, single ring of heat pipes is used to control inward growth of the freeze zone. A variety of tests are planned to evaluate the integrity of the frozen ground battier (Peters, 19941. It is reported that the cost of maintaining a frozen ground battier for approximately 70 days is comparable to the cost of constructing a conventional soil-bentonite slurry trench (Iskander, 19871. Geomembrane Cutoff Walls Geomembranes may be used alone or In combination with other technologies to create low-permeability cut-off walls. A method developed by Nick Cavalli of Hayward-Baker in the early 1980s consisted of placing a geomembrane into a previously excavated slurry trench. Vertical panels of high density polyethylene are welded to HDPE pipe. Each connection consists of panels each with a large-diameter pipe on one edge and a smaller-diameter pipe on the other. The larger-diameter pipe is slotted vertically, and the smaller-diameter pipe and membrane of the

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APPENDIX DIAPERS PRESENrTED D-29 adjacent panel are inserted into the slotted, larger-diameter pipe. The interstitial space is then grouted. A leak detection zone may be created with this technology by placing a geonet within an envelope constructed of two geomembranes (Cavalli, 19921. Gundle Lining Systems, Inc. (now GSE) also has developed a process for placement of high-density polyethylene (HDPE) as a cut-off wall. This process consists of driving vertical panels of HDPE into the soil with a steel-driving apparatus. Alternatively, the panels can be lowered into a previously excavated slurry trench. Gundle uses a jointing system that consists of an interlocking joint similar to steel sheet piling. Each half of the jointing system is welded to the vertical panels prior to installation. Each successive pane! is then driven into the soil and through the interlock of the previously placed panel. A hydrophilic gasket, which expands to several times its own volume in water, is placed within the joint to create a water-tight seal. A typical cross section of the G'ndle jointing system is shown In Figure 2 (Steve Blume, personal communication, 1996; Blume, 19951. Gunfire and Groundwater Control, Inc. have teamed to develop an alternative method for installing six foot wide, SO-mi! HDPE panels with the jointing system described above. Groundwater Control utilizes a one-pass trencher to install perforated pipe and gravel drains. The trencher has been modified to include a boot or narrow trench box that allows installation of the HDPE panels within the boot as shown in Figure 3. The trailing edge of the boot is fitted with flexible seals that move along and pass the installed pane! through the end of the boot (including the joints). This allows space for installation of a subsequent panel. The bottom key typically is ~ HYDROPH ILLIC SEAL 1~ // / HDPE JOINTING SYSTEM FUSION HDPE INTERLOCKING ~' ~ WELDED TO HDPE MEMBRANE JOINING SYSTEM PANELS FIGURE 2 Geolock panel jointing system detail. 1 4. perforated pipe Gravel surrounding pipe FIGURE 3 Groundwater control/gundle trencher panel installation.

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D-30 BARRIER TECHNOLOGIES FOR E~IRONMENTAL MANAGEMENT constructed of approximately one foot of bentonite pellets. A drain pipe and gravel also can be constructed within the same trench outside of the boot. This installation method can accommodate trench depths up to approximately 24 feet. SET Environmental, Tnc. (now GSE) has also developed a process for using HDPE as a cutoff wall. This process also includes an interlocking joint system. The SLT panel jointing system is shown in Figure 4. The joint interlock is fusion welded to vertical panels of varying width prior to a placement. The panels are then lowered into a slurry trench or specialized steel trench boxes (for shallower trenches). During this process, the interlocking joint enters the interlock of the previously placed adjoining panel. Depending on the installation, the interlock is sealed by the slurry present in the trench, by grouting, or by using several hydrophilic sealant gaskets. Slurry Systems of Gary, Indiana, recently installed two cutoff walls consisting of a OPTIONAL HYDROPHILLIC r SEA~(TYP~ HDPE INTERLOCKING JOINTING SYSTEM 1..'.'.'.-.'.'. A.. 1: HDPE JOINTING SYSTEM r FUSION WELDED TO HDPE \ MEMBRANE PANELS FIGURE 4 SLT panel joinUng system detail. combination of two technologies, including a vibrating beam wall and HDPE panels. These installations consisted of constructing a vibrating beam slurry wall using an attapulgite/cement slurry. SLT HDPE cutoff-wall panels with the SET joint interlock were then vibrated into the approximately five-inch-wide cutoff wall using a steel-driving apparatus to create a composite cutoff wall. Rodio S.p.a. of Italy has developed a composite cutoff wall involving placement of a HDPE within a self-hardening cement-bentonite slurry. The process begins with excavation of a vertical trench to the desired depth under a self-hardening slurry. HI)PE sheets varying in width from 2 to ~ meters and mounted on a steel framework then are lowered into the self-hardening slurry. The steel framework is withdrawn after installation of the HDPE. Sealing of adjacent HDPE membranes is achieved by either overlapping, a variety of socket joints, expansion strip joints, or in-situ welding. This type of composite cutoff wall was constructed around an ash

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APPENDIX~PAPERS PRESENTED D-31 landfill near Florence, Italy, In 1992. The wall attained depths of up to 30 meters below ground surface (De Paoli, 19931. Although dynamic and subject to a variety of site-specif~c factors, the costs of geomembrane barrier walls generally fall in the $10 to $30 per square foot range. Deep Barrier Walls The hydraulic effectiveness of barrier wall systems is heavily dependent upon the integrity and permeability of the aquitard into which the barrier wall is keyed (Mutch et al., 19811. Barrier walls that fail to penetrate deeply enough to key into an aquitard at all are usually only marginally effective in reducing groundwater flow (Mutch et al., 19811. The growing importance of barrier walls, together with the fact that in many geologic environments, suitable aquitards may be at depths of 150 feet or more, has spawned considerable interest and advancement in the technologies to construct deep barrier walls (defined herein as walls greater than ~ 50 feet in depth). Conventional, soil-bentonite slurry trenches generally are not used for trenches in excess of depths of 150 feet due to stability considerations. Conventional slurry trenches are constructed in a continuous manner with backfill and excavation done in the same trench as shown In Figure 5. The backfill is placed in the trench from the bohom upward until the backfill reaches grade. When backfill reaches grade, it will have an angle of repose under the slurry that normally is between seven horizontal to one vertical, to ten horizontal to one vertical. This presents two major problems. In the case of a 150-foot-deep trench, the toe of the backfill is as much as 1,500 feet from the top of the backfill. Along with this, there is often an additional 100 feet of completed trench and approximately 200 feet of trench being excavated. Therefore, as much as 1,800 linear feet of slurry trench is open at any time. At depths of 150 feet or more, the stability of the trench often becomes marginal. Second, when the excavation ends, the slurry being displaced must be recovered and disposed. A trench of this size can necessitate disposal of six to seven million gallons of contaminated slurry. ~ GROUND SURFACE SOIL-BENTONITE BACKRLL at it_ Film-';-.:-' ). ' ~ SLURRY i~. 1'~'.1. 2~' BACKFILL SLOPE FIGURE 5 Construction of a deep soil-bentonite slurry trench. -, ~ , COMPOTE WORKING

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D-32 BARRIER TECHNOLOGIESFORENVIRONMENTALMANAGEMENT Deep soil mixing, vibrating beam, and jet grouting pose a different problem. Each of these methods Involves a series of Interconnected, short sections. Verticality, therefore, becomes critical. At depths of over 150 feet, it is difficult to ensure that gaps will not occur between adjacent panels. Plastic concrete walls constructed in short panel sections of one to three clamshell bites have become the preferred type of barriers for deep walls. Alternately, a hydromill can be used. In this technique, a clam-shell or hydromill is used to excavate a short length of trench to the desired depth tinder a bentonite slurry. Plastic concrete is then tremied into the trench? displacing the slurry that is reused In subsequent excavations. Panels are interconnected either forming a joint by placing an endpipe and extracting it after backfill has been placed, or by re-excavat~ng several feet of the adjacent completed panels. Re-excavating portions of completed panels creates the problem of verticality, and therefore, continuity. The joint system, when constructed In a three-bite secondary pane! between primary panels, is more forgiving with respect to verticality. In the joint system, a series of primary panels is constructed at pre-set spacings with endpipes at both ends (Figure 61. Once the plastic concrete has set (normally 3 days), secondary ~ . ~ . ~ . ~ , .1 _ _ 1 ~ 1 _ 1 1 _ __ 1 _ _ _ ~ _ __ _ _ 1 1 ~ ~ _ _ 1 ~ 4 _ ~ panels are excavated In a three-step process. First, the ciamsnei~ or nyarom~ excavates a slot al the midpoint of the space between completed panels. Then on one side of this slot, the remaining soil between the slot and the adjacent completed panel is excavated, and the endpipe is removed. The same procedure is then repeated at the opposite side. The removal of the endpipe ensures that a good interconnection of panels is achieved. If a primary panel is out of vertical and the first bite is out of vertical, the excavating tool is guided by the space created when the endpipe is extracted and by the excavation of the first bite. Although the Lamer may not be perfectly vertical, it should be continuous at all points. ~ STOP ENDS ~ r SLURRY 7 ~7 ~ ~ is, ~ : ~ ~ it- - I'-'' ~ _~ MUSTARD ~_~ _~= ~ ID 6(C) . 6(a) SLURRY _ _ _ . . . ~ \ . ~ 1~1 , %. tiAND' .... _: - _ ~ 6(d) . ~ _ . _ =1 6(b) ~ SLURRY 6 (e) FIGURE6 An alternating panel method of plastic concrete bamer wall construciton. (a)Pr~mary panels tremie concentrated with stop ends in place. (b)Stop ends lifted. (c)Excavation of midsection of secondary panel. (c)Excavation of each end of secondary panel. (e)Tremie concreting secondary panel.

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APPENDIX~PAPERS PRESENTED D-33 Projects such as the Wolf Creek Dam (281 feet deep), and the Manicougan Dam (430 feet deep) have been constructed using this methodology although, due to extreme depths, special equipment was employed. It is believed that subsurface barriers can be built by this technique to depths of at least 500 feet. Vanel ( 1992) describes a related technique wherein a caisson beam of high-strength steel is used as an end stop. However, rather than being withdrawn prior to excavation of the adjacent panel, it is left in place to serve as a guide to a specifically designed excavating tool that slides down the beam. The beam is pulled laterally away from the concreted primary panel once the secondary panel is fully excavated. The beam can also be fitted with an additional grooved caisson into which one or more plastic or rubber water stops can be inserted. The free half of the water stops becomes concreted into the primary panel. Lateral extraction of the beam uncovers the other half, which is then sealed in the concrete of the secondary panel. CONCLUSIONS The limitations of treatment technologies to remediate many waste disposal sites fully have led to increasing usage of and reliance upon subsurface baITier walls to control contaminant migration from such sites. This more common usage has been paralleled by considerable advancements In construction technologies. Battier walls can now be constructed by many different techniques, each offering particular advantages, disadvantages, and limitations. Many of these techniques also can attain much greater depths than earlier conventional technologies. REFERENCES Blume, Steve. 1995. Unique GundWall Installation Developed Through Team Effort. Trench Topics 1~21. Braun, Bernd, and William R. Nash. 1985. Ground Freezing for Construction. Civil Engineering lanuary:54-56. Cavalli, Nicholas. 1992. Slurry Walls: Design, Construction, and Quality Control. ed. D.B. Paul, R.R. Davidson and N.J. Cavalli. Philadelphia, Penn.: ASTM. Chipp, P.N. 1990. Geotechnical Containment Measures for Pollution Control. Wetherby, West Yorkshire, England: Keller Concrete. De Paoli, B., R. Granata, G. Hautmann, and P. Tacconi. 1993. Confinement of Hazardous Waste by Composite Vertical Cutoff Walls. Paris: Environment et Geotechnique de la decontamination a la protection du soul-sol. Guatter~, Giorgio. 1988. Advances in the Construction and Design of let Grouting Methods in South America. Paper No. 5.32. Second International Conference on Case Histories in Geotechnical Engineering. St. Louis. Iskandar, Tskandar, K. 1987. Ground Freezing Controls Hazardous Waste. The Military Engineer. 516(August):455-456. raspers, Brian H. 1989. Soil Mixing. Hazmat World. November. Jefferis, S.A. 1985. Clay-Cement-Slag Grouts for Groundwater and Pollution Control. Workshop on Blast Furnace Slag Cemtnet and Concretes. King's College, London, October 14-17. Karol, R. 1990. Chemical Grouting. New York: Marcel Dekker, Tnc.

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D-34 BOWER NECROLOGIES FOR ENVIRONMENTAL MANAGEMENT K~ynine, Dimitri P., and William R. Judd. 1957. Principles of Engineering Geology and Geotechnics. New York: McGraw-Hill. Mutch, R.D., Ir., G. DiPippo, and J. Hearty. 1981. Environmental Cleanup of the Monroe Township Landfill, Proc. ASCE National Conference on Environmental Engineering. Atlanta. Noll, Mark R., Craig Bartlett, and Thea M. Dochat. 1993. In Situ Permeability Reduction and Chemical Fixation Using Colloidal Silica. Internal Paper Wilmington, Del.: DuPont Environmental Remediation Services. Peters, R. 1994. Demonstration of Ground Freezing for Radioactive/Hazardous-Waste Disposal. pp. 103-11 ~ in Proc. of the 33rd Hanford Symposium on Health and the Environment In Situ Remediation: Scientific Basis for Current and Future Technologies. Pasco, Wash. Tallard, G.R. 1992. Hazwaste Hydraulic Barriers, Update for the 90's. 45th Geotechnical Society Conference. Toronto, Ontario, Canada, October. Vanel, Paul. 1992. MalLing Diaphragm Wall Joints Water Tight with CWS System, in Slurry Walls: Design, Construction, and Qualtity Control, ed. D.B. Paul, R.R. Davidson and N.~. Cavalli. Philadelphia, Penn.: ASTM. Voss, Charlie F., C.M Einberger, and Rudy V. Matalucci. 1995. Evaluation of Two New Grouts for Constructing Subsurface Barriers. Paper presented at DOE Waste Management '95 Conference, Tucson, AZ, February.

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APPENDIX~PAPERS PRESENTED D-35 Robert D. Mutch, Jr., P.Hg., P.E., is an Executive Vice President and Corporate Director for Hydrogeology and Waste Management at ECKENFELDERINC. Mr. Mutch was formerly a Senior Vice President with Wehran Engineering in Middletown, New York. He is also an Adjunct Professor at Manhattan College in Riverdale, New York, where he teaches a graduate course in Applied Geohydrology. He has about 24 years experience in the fields of landfill design, hydrogeology and remedial engineering. His work has included the investigation and remedial design of hundreds of municipal and hazardous waste disposal sites, including dozens of Superfund sites. He has designed and, in most cases, supervised construction of 14 miles and 1,500,000 square feet of subsurface cutoff wall, 2 i/2 square miles of low permeability landfill caps, 15 miles of retrofitted leachate collection systems, and numerous groundwater extraction systems ranging in size from 50,000 gallons per day to 2,000,000 gallons per day. He has also provided consultation to the United Nations Environmental Programme (UNEP) In regard to international landfill problems. He holds B.S. and M.S. degrees in Civil Engineering from New Jersey Institute of Technology. He is certified by the American Institute of Hydrology as a professional hydrogeologist (P.Hg.) and is a licensed professional engineer in several states. Robert E. Ash, IV, P.E., is Assistant Division Director of the Waste Management Division for ECKENFELDER INC. in its Nashville, TN office. He has over 14 years of experience in the environmental consulting field, including site remediation and solid and hazardous waste management. Mr. Ash is a registered professional engineer with a B.S. degree in Civil Engineering from Rutgers College of Engineering. He is a member of the National Society of Professional Engineers and the American Society of Civil Engineers. Jeffiey R. Caputi, P.E., CHMM is a Senior Manager In the New Jersey Waste Management Division at ECKENFELDER INC. where, for the past six years, he has worked extensively on hazardous site remediation projects. His work has primarily Included feasibility studies and remedial designs for Superfund and RCRA sites, as well as sites regulated under various state programs such as the New Jersey Industrial Site Recovery Act and the Massachusetts Contingency Plan. Prior to joining ECKENFELDER INC. Mr. Caputi spent four years at Malcolm Pirnie, Inc. conducting remedial investigations, feasibility and treatability studies, remedial actions, and environmental audits. He has a Bachelor of Science degree in Environmental Engineering Technology and a Master of Science degree in Environmental Engineering from the New Jersey Institute of Technology. Mr. Caputi is a licensed professional engineer and a certified hazardous materials manager.