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OCR for page 133
Construction of Deep Barrier Walls for Waste Containment M. Mauro, Rodio, Inc., Boston, Mass. ABSTRACT This article reviews the methods that currently are available within slurry wall technology for the construction of deep baITier walls for waste containment. The excavation of cutoff walls using self-hardening slurry is presented and indications of the achievable wall properties are given. The construction of composite walls, with the insertion of high density polyethylene (HDPE) membrane into the cutoff wall, is described. Lastly, the Hydromill technique, the latest development in slurry wall excavation, is introduced for the construction of cutoff walls with plastic concrete. INTRODUCTION The isolation of polluted areas by vertical cutoff walls is one of the current methods used to limit the contamination in the surrounding environment. ~ order to encapsulate a polluted area (Figure D, the vertical cutoff wall is generally keyed in a soil layer of Tow permeability of appropriate thickness and continuity. Several technologies currently are available to construct a vertical cutoff wall: slurry walls, jet grouting, soil mixing, or grout curtains. The selection of the appropriate technology is made considering the geology of the site, the type of contaminant to be intercepted, and the degree of water tightness to be achieved. slurry wall barrier Ha_ E~ . waste Ie~ch~te JO surface lining ., ~.. . . . . . . .. water table permeable soil . . .; FIGURE ~Containment of a polluted area

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D-116 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT If the cutoff wall is to reach a considerable depth, typically over 50 feet, slurry walls are one of the technologies most Dequently employed. Excavation equipment capable of reaching considerable depth and assuring the continuity of the cutoff walls is available. Significant progress has been made in the design of the mixes used to replace the in-situ soil. In this paper, the following three types of deep cutoff wall constructed with the slurry wall technology are discussed briefly: cutoff walls constructed with self-hardening slurry, composite cutoffwalis, and cutoff walls constructed with plastic concrete. Cutoff WaUs Constructed with Self-Hardening Slurry The construction of cutoff walls with self-hardening slurry takes place with an alternating pane! excavation sequence (Figure 21. The in-situ soil is replaced by the self-hardening slurry, a homogeneous mix of water, cement, and bentonite designed to create a very low-permeability barrier. The hydraulic conductivity of self-hardening slurry is typically in the range of 10-6 cmls and can be reduced further with the use of additives. The excavation generally is carried out with mechanical or hydraulic grabs. Mechanical grabs are operated and suspended by cables. Hydraulic grabs are opened and closed by pistons driven by a power pack mounted on the supporting crane. The hydraulic grab can be cable suspended (Figure 3) or connected to a rigid Kelly bar (Figure 4). '' 1 ) . . . . . ... .. PHASE 1: EXCAVATION' OF TIlE FIRST PANEL PHASE 2: EXCAVATION OF THE SECOND PANEL PlIASE 3: EXCAVATION OF THE THIRD PANEL BEl~h'EEN TIlE FIRST AND SECOND PANEL PROSE 4: EXCAVATION OF THE FOURTH PANEL PIl.\SE 5: EXCAVATION OF THE FIFTH PANEL BETWEEN THE SECOND AND FOURTI' PANEL FIGURE 2 Typical excavation sequence of a cutoff wall constructed with self-hardening slurry.

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D-118 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT In the case of cable-suspended grabs, great operator ability is required to maintain the verticality of the excavation. In the case of hydraulic grabs rigidly connected to a Kelly bar, the control of the verticality is in general easier because of the stiffness of the frame guiding the Kelly bar and the large weight of the Kelly bar itself. A considerable advantage of hydraulic over mechanical grabs is that the efficiency and speed of excavation are improved largely. The continuity of the cutoff wall can be assessed measuring the verticality of each pane] during the excavation and checking the overlap between adjacent panels. For the measurement of verticality, an inverted pendulum system or an inclinometer connected to the grab are commonly employed. Readings typically are taken at constant intervals dining the excavation. In case of deviation, the modification of the cutting edges of the grab or the use of reamers are usual corrective measures. A deviation from verticality of less than 1.0-2.0 percent can be achieved realistically. In order to reduce the hydraulic conductivity of the cutoff wall, the choice of the self- hardening slurry composition is very important. The following factors have to be considered: lithe choice of the cement type. In general, better performance, in terms of water tightness and chemical resistance, is obtained using blast-fu~nace or pozzolan cement instead of Portland cement (Figure 54. The bentonite content. High bentonite content may be useful in reducing the hydraulic conductivity of the self-hardening slurry and assuring a better uniformity of the cement content In the panel. Use of additives. The use of special additives, such as dispersing agents, reduces the viscosity, allowing the use of mixes with higher bentonite or cement content (Figure 51. The development of the hydrosilicate crystals during the hydration of the cement is also improved, with a consequent reduction In the hydraulic conductivity of the wall. Several cutoff walls up to 200 feet in depth have been constn~cted successfully by Rodio using the self-hardening slurry technique. -8 10- u) - c ._ .O - O 1 0- >. ._ ._ D E c' a _y) 10 -11 . ~ _, {Ordinary Portland Cement) 10: ~ 0.2 0.3 0.4 0.5 02 03 0.4 05 cementlwater ratio cementJwater ratio ~, (E}lastfurna~ e, _ ~ _ RtW ~ 5% FIGURE 5 Self-hardening slurry permeability coefficient as a function of the type of cement, cement/water and additive/water ratios (:De Paoli et al., 19931.

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APPENDIX~PAPERS PRESENTED 19-119 Composite Cutoff Walls In order to improve the resistance of the cutoff wall to chemical attacks coming from a variety of different agents, or to further reduce the hydraulic conductivity of the hairier, it is often necessary to Install a composite cutoff wall. This type of wall is constructed with the Insertion of an impervious artificial hairier, such as an HDPE membrane, In a wall excavated with self-hardening slurry (Figure 61. A practically impervious hairier can be created. .~ ,., 0 ... -,: , r .O .` .~; . i . ~ ~ . ~ ~ .' , 1 _= 1 ~:G T . ~. . ~...~._ a` me_ A..... ...... . ' . A'`~ . . .~-2 ,~~. . ..'~::~^ 2~ .~.,,~. i,.. - v ,; .~ V;2 ',.~; ~ ~ it': Air;.' . . ~ . a. . - O . o : o . o . . _ - - . . O FIGURE 6 Installation of HDPE membrane In a cutoff wall excavated with self-hardening slug. Typically, the HDPE is lowered in the trench in sheets Tom 7- to 25-feet wide. Since the density of the membrane is Tower than the density of the self-hardening slurry, a special heavy- weight guide frame is used to ensure a proper installation. The frame is withdrawn after the installation of the HDPE sheet in the wall. Continuity between the HDPE sheets is guaranteed through special joints (Figure 71. The type of joint and the method used to splice the HDPE sheets in the field must be selected carefully.

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D-120 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT - expansive strip ,,~ } ~i ;v~ . hated C~ ` ._ . ~ . `. . _. ~__ _ .~ welding spacers FIGURE 7 SET interlocking joint between HDPE sheets. _~ HOPE membrane It is very important that the self-hardening slurry maintains appropriate rehological properties long enough to allow the installation of the membrane in the cutoff wall and the interlock of the sheets at the joints. With the addition of dispersing agents, the workability time of ordinary self-hardening slurry can be substantially increased. Until a few years ago, the construction of composite barriers was limited to depths shallower than 50 feet. Recently, a composite battier with a depth of 100 feet was constructed by Rodio for the containment of a landfill close to Florence, Italy. Cutoff Wall Constructed With Plastic Concrete For the excavation of very deep cutoff walls, typically over 100-150 feet, or in special circumstances, like the presence of rock or hard layers, cutoff walls can be excavated finder bentonite slurry with the Hydromill equipment. The excavation is subsequently backfilled with plastic concrete, i.e., a mix of coarse aggregate, sand, cement, and bentonite slurry that can be designed to achieve very low permeability, usually In the range of 10-7 cm/s. The Hydromill trenching equipment (Figure 8) is equipped with two cutting wheels able to cut the soil and rock. A submerged pump located in the frame of the equipment sends the spoil- laden slurry from the bottom of the trench to the slurry treatment plant through a pipeline. At the

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APPENDIX~PAPERS PRESENTED D-121 plant, the spoils are separated Mom the slurry by means of a series of vibrating screens and cyclones, and the "clean" slurry is sent back to the top of the panel under excavation through another pipe line. The transport of the slurry from the bottom of panel, its treatment, and the return to the panel all take place in a closed loop (Figure 9~. FIGURE ~Hydromill, model "patina," for low headroom conditions. cuttings ~ ~ ~ distance up to 1,200 feet ( without booster pumps ) ll slurry from excavation ~ slurry from concreted panel $/ ':~J''~h ;7 Wit disposal area | ~I plant and I slurry tanl;s | , . . I 1 ~ l - ~ ~1 _ _ _- desanding 1' _ _ 1 _ contaminated F ' slurry L, - \~ ~ FIGURE 9 Hydromill excavation - Bentonite slurry flow chart. sluny making plant I fresh e 7 slum

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D-122 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT A primary-secondary panel sequence is usually employed to excavate the cutoff wall. During the excavation of the secondary panels, a portion of the adjacent primary panels is cut by the Hydromill. The result is the creation of a rough contact surface between the panels that provides excellent wall continuity and water tightness. The verticality of the trench is monitored constantly during the excavation with two high- resolution inclinometers, mounted on the frame of the Hydromill trencher and connected to a read- out unit located in the operator cab. The operator of the Hydromill can take corrective action at the slightest sign of deviation as transmitted by the inclinometers. If the deviation occurs perpendicular to the wall axis, the deviation can be corrected by inclining the cutters' frame with respect to the main frame of the trencher, or by moving the shield connected to each side of the main frame of the trencher. If the deviation takes place longitudinally, the corrective action generally consists of varying the relative speed of the cutters and the power delivered to them. In normal operating conditions, the Hydromill equipment is capable of achieving a vertical tolerance of 0.3-0.5 percent in both the longitudinal and perpendicular direction. The Hydromill technology has been applied by Rodio to excavate panels up to a depth of 330 feet (Bruce, et al., 19891. Rock with unconfined compressive strength of 4,000 psi has been excavated. CONCLUSIONS The selection of the most suitable method for the construction of deep barrier walls for waste containment is to be made considering the following factors: geological conditions of the site; type of soil, or rock, to be excavated; type of contaminants present in the soil; degree of water tightness to be achieved; and depth to be reached. lithe methods presented in this paper can be employed economically and effectively to construct deep barrier walls for waste containment. BIBLIOGRAPHY Bruce D. A., B. De Paoli, C. Mascardi, and E. Mongilardi. 1989. Monitoring and quality control of a 100 metre deep diaphragm wall. Paper presented at International Conference on Piling and Deep Foundations, London, England. Cavalli, N. J. 1991. Composite barrier slurry wall. Pp. 78-85 in Proceedings of the Symposium on Slurry Walls: Design' Construction, and Quality Control' Atlantic City, N.J. Sponsored by ASSAM Committee D-18 on Soil and Rock. De Paoli, B. 1984. Evolution de la technologie des patois moulees en Italie. Procedes et outillages, Symposium sur Technologie et organization de lt execution des parois moulees dans la construction d'ouvrages hydrauliques, Sofia, Bulgaria.

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APPENDIX DIAPERS PRESENTED D-123 De Paoli, B., R. Granata G. Hautman, and P. Tacconi. 1993. Confinement of Hazardous Waste by Composite Vertical Cut-Off Barriers, Colloque International sur Environment et Geotechnique organize par I'Ecale National des Ponts et Chaussees, Paris, France. Manassero M., E. Fratalocchi, E. Pasqualini, C. Spanna, and F. Verga. 1995. Containment with vertical cutoff walls. Pp. 1142-! 172 in Proceedings of the Specialty Conference Geoenvironment 2000, New OrIeans, La. Sponsored by the Geotechnical Engineering Division and the Environmental Eng~neenng Division of ASCE. Tornaghi, R. 1984. -L'experience italienne dans le domaine des parois souples d'etancheite, Symposium sur Technologie et organization de I' execution des parois moulees dans la construction d'ouvrages hydrauTiques, Sofia, Bulgaria. Zamoj ski, L. D., S. W. Perkins, and D. Reinknecht. 1995. Design and construction evaluation of a slurry wall at FER landfill Superfund site. Pp. 1192-1206 in the Proceedings of the Specialty Conference Geoenvironment 2000, New OrIeans, La. Sponsored by the Geotechnical Engineering Division and the Environmental Engineering Division of ASCE.