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Advances in HDPE barrier walls
Geotextiles and Geomembranes 14 (1996) 393-408
© 1996 Elsevier Science Limited Printed in Ireland. All rights reserved 0266-1144/96 $15.00 ELSEVIER
PII:S0266-1
144(96)00024-6
Advances in H D P E Barrier Walls
R i c h a r d W. T h o m a s a & R o b e r t M. K o e r n e r b aTRI/Environmental, Inc., 9063 Bee Caves Road, Austin, TX 78733-6201, USA bGeosynthetic Research Institute, Drexel University, Philadelphia, PA 19104, USA
ABSTRACT The cut-off and/or containment of laterally flowing liquids from landfills and impoundment reservoirs generally utilize some type of vertical wall. The most common wall is constructed using a slurry supported trench, subsequently backfllled with soil-bentonite, soil-cement, cement-bentonite or soil-cement-bentonite. Concerns have arisen as to the installation, inspection and durability of such walls. A different, or complementary, strategy uses a geomembrane by itself or in combination with any one of the standard backfill materials to provide the degree of completeness and environmental safety/security that most, if not all of these sites warrant. This paper discusses applications where geomembrane barrier walls can be used, details of their installation, idiosyncrasies of the material used (particularly the diffusive transport aspect) and concludes with selected comments regarding emerging technology. Copyright © 1996 Elsevier Science Ltd.
INTRODUCTION
Barrier or cut-off walls have been used for many years in geotechnical applications. The basic concept is to construct an in-situ vertical wall that will act as a separator between two areas. The walls are constructed using a bentonite slurry supported trench which is subsequently displaced by the permanent backfill o f a composite material like soil-bentonite (SB), soil cement (SC), cement-bentonite (CB) or soil-cement-bentonite (SCB). Concerns over the integrity of these walls come about via the nature of their construction or the type o f backfill used. Some of these concerns are as follows: 393
394
• • • • • • • • • • • • •
Richard W. Thomas, Robert M. Koerner
collapse of trench walls during and after excavation; improperly mixed slurry support material; collapse of trench walls during backfilling; sand deposits during pauses in construction; sand, gravel, boulders or other debris at the bottom of the trench; difficulty of quality control/quality assurance since none of the critical operations are visible under the slurry; discontinuities at joints and during work stoppages; backfill material drying below its initially placed high water content; long-term desiccation of the wall where it is above the water table or liquid level; freeze-thaw behavior of the wall in northern climates; chemical resistance of the wall to the liquids to be contained; permeability of the final backfilled material; diffusive transport through the final backfilled material.
One way to improve the integrity and transport properties of these walls is with the use of geomembranes. Geomembranes have been used in vertical walls since about 1980, and have been used by themselves in single or double layers, or with SB, SC, or CB material as a composite wall. The geomembranes used have almost always been high density polyethylene (HDPE). The use of other materials is possible, but H D P E is readily available and has a long history of successful use in other geotechnical applications. It is also highly resistant to water vapor transport and resistant to a variety of other chemicals. The walls are generally installed by placing individual panels that are connected together. There are several different interlocking connections available and there are different ways to seal the connections. This paper will focus on three aspects of polymer barrier walls. They are the applications where these walls can be useful, the installation techniques most commonly used to build the walls, and the properties of the geomembranes and their connections. Special emphasis will be placed on diffusive transport through HDPE, since concerns are often voiced in this regard. Some insight into future directions and summary comments will conclude the paper.
APPLICATIONS Barrier walls containing a geomembrane can be useful in any application where the flow of water or other liquids is to be controlled. The use of a geomembrane in the wall is a way to ensure continuity of an extremely low
395
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Geomembrane vertical wall system with closure cap (after SLT, 1995).
permeability material. In the light of the numerous concerns expressed earlier, it is clear that barrier walls that include geomembranes provide an added factor of safety for almost any construction application. There are a variety of different applications where these walls have been proposed or used. Some of them are illustrated in Figs 1-5. Figure I shows a typical hazardous waste containment application. The barrier wall is installed at a depth that is either well below the plume of waste or keyed into the aquitard. The wall can also be welded to a cap geomembrane by overlapping either the wall or the cap. This type of sealed system can be monitored and can include a leachate collection system. Figure 2 shows how a geomembrane barrier wall can be used to contain hazardous spills around plant sites. This figure shows how different hazardous chemicals migrate through soils. The dense nonaqueous phase liquids (DNAPL) are heavier than water, while the light nonaqueous
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396
Richard W. Thomas, Robert M. Koerner
-~low
Suitable Fig. 3. Water flow control with a vertical geomembrane barrier wall (after SLT, 1995). phase liquids (LNAPL) are lighter than water. This type of containment can be an effective way to reduce the a m o u n t of ground water contamination that would occur prior to and during remediation of a site. Figures 3 5 show how walls can be used to influence the flow of water. These applications include redirecting water flow around a construction site (Fig. 3), lowering the water table for construction (Fig. 4), or cutting off water flow through a dam, dike, or levee. These are general examples showing how these walls can be used. A recent review (Koerner, 1995) describes specific case studies that have been found in the literature (a summary of these methods is presented in Table 2 at the end of the next section). INSTALLATION METHODS Koerner and Guglielmetti (1995) identified five c o m m o n installation techniques. These are shown in Table l and will be discussed below.
i Ge°m~mbran~e ~i
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Advances in HDPE barrier walls
Aquz~r~}~'~t~'~ Fig. 5.
397
AltemateLocations/~'~"
A geomembrane cut-offwall through a dam, dike, or levee (after SLT, 1995).
Notice that the methods are different depending on how a trench is excavated, whether or not the trench is supported, and what type of backfill is used. Obviously, there are important choices to be made that are probably specific to the site and the application. The trenching machine method uses a large bucket trencher or disc cutter to excavate an unsupported trench. At the same time, the geomembrane is unrolled from a box mounted on the trenching machine or from a trailer at the ground surface. There are two major advantages to this method. It is extremely fast and there are connections made only at the ends of the rolls. Additionally, this method lends itself to welded connections, which should be the most reliable type of connection. The backfill is typically the native soil, but a sand fill with pipe leak detection can also be used. The main disadvantage of this method is that the TABLE 1 Common Installation Methods for Geomembrane Vertical Walls (after Koerner & Guglielmetti, 1995)
Method or technique
Geomembrane configuration
Trench support
continuous
none
30-60 (12-24)
1.5~,.5 (5-15)
sand or native soil
Vibrated insertion plate
panels
none
10-15 (~6)
1.5-6.0 (5-20)
native soil
Slurry supported
panels
slurry
60-90 (24-36)
no limit
SB, SC, CB, sand or native soil
Segmented trench box
panels or continuous
none
60-120 (2~48)
3.0-9.0 (10-30)
sand or native soil
panels
slurry
15-22 (f~9)
no limit
SB, SC, or CB slurry
Trenching Machine
Vibrating beam
Typical trench Typical trench width cm (in) depth m (ft)
Typical backfill
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Richard W. Thomas, Robert M. Koerner
unsupported trench depth is limited. That makes the method very site specific. The vibrated insertion plate method involves the use of geomembrane panels approximately 3 m (10 ft) wide and a steel truss 'insertion plate'. The geomembrane is fixed at its base with pins protruding from the bottom of the insertion plate. A vibratory pile hammer is used to force the entire assembly to the desired depth. The insertion plate is then removed leaving the geomembrane behind. Interlock connections are required to join the panels together. This method is fast and requires neither trench support nor special backfill. The main disadvantage is that only certain soft and/or loose soils can be penetrated to substantial depths. Also, damage to the geomembrane may be a concern with soils containing construction debris, rock, boulders or extremely hard clay strata. The slurry supported method starts by using a conventionally excavated and slurry supported trench. The geomembrane panels are inserted into the trench to their desired depth with a steel frame. They are either laid down with weights or toed into the subsoil stratum. They are also joined side-by-side with interlocking connections. The backfill is usually SB, SC, SB or SCB, but it can also be low permeability native soil. Also, a single or double lined vertical wall with a sand leak detection layer can be made by this method. The depth of the wall is virtually unlimited and the deepest walls have been constructed by this method. So far, the maximum depth has been 30m (100 ft). The segmented trench box method uses a modified trench box to support the side walls of the trench during its excavation. The geomembrane is placed between the outer segments of the box. The backfill is usually native soil, but other membranes can be used. This technique is limited by depth, so it is also site specific. The geomembrane panels are usually interlocked, but it is also possible to weld the sheets and to inspect the welds within the confines of the trench box. The last method is the vibrating beam method. This method utilizes a modified form of the slurry supported trenching method. The slurry is introduced as the vibrating beam is inserted and removed. Once the trench is slurry supported, the geomembrane panels are installed as a secondary operation. There is no depth limit and the backfill is usually a SB, SC, CB or SCB material. Thus, it is seen that these methods fall into two main categories with respect to the depth of the completed wall. A supporting slurry seems to be required for depths greater than 9 m (30 ft). Therefore, to date, there are three choices for shallow walls and two for deep walls. There are certainly other novel and unique ways to install these walls.
399
Advances in H D P E barrier walls TABLE 2
Case Studies Utilizing Geomembranes as Vertical Barriers (after Koerner & Guglielmetti, 1995) Reference
Application or containment
Installation type
Brunette & Schmelnecht (1995)
hazardous waste
Bliss & Brunette (1995)
earth dam cutoff
slurry supported
Brunette & Pierce (1994)
petroleum waste hazardous waste
Scuero, el al. (1990) Michaelaneli (1995)
Backfill
10 (35)
0.3 (0-2)
sand
15 (50)
20 (12.5)
slurry supported slurry supported
sand
4.5 (15)
0.4 (0-2)
sand
14 (45)
0.5 (0-3)
earth dam cutoff
slurry supported
CB slurry
9 (30)
0.1 (0-1)
Municipal solid waste and ash
slurry supported
CB slurry
various
various
trenching machine
native soil
3 (10)
3 (21)
Hansen & contaminated Crotty (1995) drilling waste
vibrating beamclay/cement slurry
Wall depth m Wall length km (ft) (mi)
However, it is believed that these methods illustrate the state-of-thepractice. Koerner and Guglielmetti (1995) identified six published case studies describing the application and installation of vertical geomembrane barrier walls. A summary of these is found in Table 2. It should be noted that there are many other internal documents and reports along with company brochures describing other case studies.
MATERIALS The materials that need to be discussed are the geomembranes used and the connections between geomembrane panels. An extremely impermeable sheet can be produced, but if the connections leak, the overall system will not perform well. Therefore, both the sheet and the interlocking connections need to be considered. This section will focus on the transport properties of the geomembranes and the design and performance of the connections.
Richard W. Thomas, Robert M. Koerner
400
Geomembranes The requirements of a geomembrane for use in a barrier wall include: • • • •
high stiffness for ease of installation; high resistance to a variety of chemicals, including organic solvents; ability to attach interlocking profiles on the edges of the sheets; durability in buried applications.
H D P E is a good choice for most of these requirements. It is not stiff enough to be directly driven like steel sheet, but, as just described, there are several other ways to install the sheets. When the availability and price of H D P E geomembranes are also considered, they become a natural choice for these applications. Another reason for the choice of H D P E is its ability to be shaped by profile extrusion. The interlocking connections are complicated shapes that are made by a continuous extrusion process, then cut to length and fusion welded to the geomembrane panels. By now, the issue of long term performance of HDPE in buried applications should be settled. Besides stress that can lead to slow crack growth in some cases, there are few factors that would limit the lifetime of an H D P E geomembrane in a buried application. One of these is exposure to chemicals. However, there have been hundreds of EPA 9090 compatibility tests performed on H D P E with a variety of municipal and hazardous leachates without a single failure. In fact, in the authors' opinion, the performance of H D P E when exposed to chemicals is well established. In concentrated hydrocarbons (chlorinated and aromatic are most severe), there can be a loss in tensile yield strength of up to 30%. This is due to physical plasticization which softens HDPE. This is a reversible change; when the chemicals are allowed to vent off, the strength returns. Therefore, as long as the reduced strength is above the design strength, which it should always be, this reduction in strength should not be a problem. The other mechanism that can occur during exposure to hydrocarbons is extraction of some anti-oxidants and other stabilizers. However, when buried, the need for stabilizers is minimal. One of the most important properties for barrier walls is low permeability to chemicals. It is important to distinguish the chemical permeability of geomembranes from the permeability commonly used by geotechnical engineers, i.e. from hydraulic conductivity. In the case of soils and other porous media, the transport of water (or other chemicals) occurs through soil voids, pores, cracks, and fissures. Alternatively, in chemical permeation of geomembranes, the chemical will pass
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through a nonporous membrane at a molecular level. Molecules are able to 'squeeze' through membranes by moving among and around the polymer chains. An example of this type of permeation is a helium filled balloon. Rubber balloons deflate in just a few days, while Mylar balloons last much longer. Neither balloon has holes in them; they just have different permeabilities to helium. Rubbers are characterized by being a m o r p h o u s (without ordered structure) and have high free volume, which means there is relatively large spaces between molecules. Conversely, Mylar is a semi-crystalline polyester plastic that has a high degree of order and much smaller spaces between molecules. Therefore, it is more resistant to permeation. HDPE is also a semi-crystalline plastic which makes it resistant to chemical permeation. However, it must be understood that no polymer is impermeable to chemicals. There will always be some permeation that occurs. The real question is whether or not the amount of permeation that occurs is acceptable. There are a variety of factors which affect the rate of chemical permeation through membranes. These include the chemical concentration, the temperature, and the sheet thickness. Additionally, permeation consists of two main components, diffusivity and solubility. Diffusion is the transport rate of a chemical through a barrier. Solubility is the amount of chemical that a barrier can hold. This is related to the amount of free volume in a polymer and the compatibility of the chemical with the membrane. Therefore, permeation is related to how much chemical a membrane will absorb and the speed at which the chemical will move through the barrier. Thus, the permeation rate can be affected by the concentration, temperature and liner thickness. It is fairly easy to understand why H D P E is resistant to water. The chemical composition of H D P E is similar to oil, grease, and wax, so it repels water, which means it absorbs only a small amount. HDPE is also semi-crystalline so the water molecules that are absorbed have to find their way around the crystals (molecules can only pass through amorphous regions). These features make H D P E highly resistant to water. A typical value for 2.5 m m (100 mil) H D P E is around 0.006 g/m2-day which is about 0.006 gal/acre-day. So, water transport is not really an issue with HDPE. Permeation with organic chemicals is a more complicated issue. There are only three sources for permeation data on H D P E geomembranes known to the authors. Haxo reported the solvent vapor transmission rates of a variety of neat (100%) chemicals for two thicknesses of HDPE (Haxo, 1988). Britton et al. reported weight gain and permeation results on a mixture of eight organic solvents on three thicknesses of HDPE at two temperatures (Britton et al., 1989). Both of these studies involved
Richard W. Thomas, Robert M. Koerner
402
TABLE 3 Vapor Transmission Rates of Organic Solvents through HDPE membranes (Park et al., 1995)
Chemical
Trichloroethane (d = 1-4 g/mL)
Toluene (d = 0.87 g/mE)
m-Xylene (d = 0.87 g/mL)
Methylene chloride (d = 1-34 g/mL)
Concentration
Sheet thickness
(mg/L)
(mm)
Vapor
Vapor
10 50 100 100 100
0.76 0.76 0.76 1.52 2.54
1.0 5.3 15.9 3.6 1.3
0.024 0.127 0.382 0.086 0.031
10 50 100 100 100
0.76 0.76 0.76 1.52 2.54
0.8 4.4 14.6 2-9 1.0
0.019 0-106 0-350 0.070 0.024
10 50 100 100 100
0.76 0-76 0-76 1.52 2.54
0-7 3.2 13.3 1-8 0.6
0.017 0.077 0.319 0.043 0.014
10 50 100 100 100
0-76 0-76 0-76 1-52 2.54
0-2 0-9 2-9 0.8 0.4
0.005 0.022 0.070 0.019 0.010
transmission rate transmission rate (rag/me-h) (g/m2-day)
concentrated chemicals. The third study involved diluted chemicals at several concentrations and several thicknesses of HDPE geomembranes (Park et al., 1995). Table3 shows data from the Park et al. study along with calculated transmission rates. Notice first that when the results are converted, the actual rates are small. They range from 0.04 to 0-40 g/m2-day or 0.04 to 0.43 gal/acre-day. In empirical units, this is from half an ounce to 1.7 quarts of liquid. Also notice the dramatic effects of concentration and sheet thickness on the vapor transmission rates. These are shown graphically in Figs 6 and 7. When the concentration was decreased by 10 times, the permeation rate decreased 13-19 times. This suggests that permeation rates with 100% pure chemicals vastly overestimate actual field permeation rates because leachates are typically very dilute concentrations o f chemicals. A brief survey of leachate analysis packages showed that values for organic solvents in hazardous waste rarely exceed a total of 100mg/L (Thomas,
Advances in HDPE barrier walls
403
iiiiiiiiiiii
0.4 ~'o.3
10=
&
~o.2 0.1 0
30
0
-*-Me
Fig. 6.
60 90 Concentration (mg/L)
~TCE
~- Toluene.-A- Xylene
120
]
The effect o f c o n c e n t r a t i o n o n the v a p o r transmission rate.
1995). Similarly, when the thickness was increased from 0-76 to 2.54mm, the permeation rate decreased 7-15 times. This shows that effects of permeation can be reduced by the use of thicker geomembranes. The actual effect will probably not be as great as these data suggest. In theory, the maximum permeation rate of a 2.54mm liner should be about onethird that of a 0.76 m m liner because it is 3.3 times thicker. It is believed that in this case, the permeation rates in the thicker sheets never reached their maximums, even after 3 months of exposure. However, these data do suggest that the vapor transmission rate for toluene at a concentration of 100mg/mL will not be greater than 0.13 g/me-day. This is about 1 pint of liquid per acre per day. Connections
Since the main installation techniques currently used for polymeric barrier walls involve individual panels of HDPE, the panels must be connected in
0.4 ~0.3 '10 & E 0.2
......% :
.............................................
r~ ~0.1 0 0.5
I-o- MC Fig. 7.
1
1.5 2 2.5 Sheet Thickness (mm)
-~ TCE
3
-=- Toluene -~ Xylene'']
The effect o f sheet thickness on the v a p o r transmission rate.
404
Richard W. Thomas, Robert M. Koerner
A
C Fig. 8.
D
Gasket type interlocks (after Koerner & Guglielmetti, 1995).
some way. The connections are usually a 'lock and key' type that use a hydrophilic gasket or grout as a sealant. The panels can also be heat welded together, but this is not a commonly used method. The interlocks are important because they can be the most permeable part of the wall. Additionally, the joints may become stressed during installation or if some settlement or lateral deformation occurs in the wall. Therefore, it is important to evaluate the strength and the permeation resistance of the interlocks. Four types of interlocks that use a hydrophilic gasket for liquid tightness of the installed panels are shown in Fig. 8. The gasket material is a proprietary formulation of neoprene rubber that is designed to swell on contact with water. The volume change can be up to eight times the original diameter which creates the sealing pressure. Connection A was shown to pass only negligible amounts of water under pressures up to 800kPa (Gundle Lining Systems, Inc., 1993). The performance of the connection when exposed to organic chemicals has not been evaluated to our knowledge. There are two interlocks that have been designed to be used with a sealant or grout. These are shown in Fig. 9 and are attractive because the additional thickness of grout will produce a more tortuous path for chemical migration. However, in this case, the adhesion between the grout and the HDPE needs to be studied because polyethylene is difficult to bond to. Notice that one connection can be used with both the hydrophilic seal and grout which provides double protection from chemicals. The resistance of these connections to vapor transport of water and
Advances in H D P E barrier walls
A
Fig. 9.
405
B
Grouted type interlocks (after Koerner & Guglielmetti, 1995).
chemicals is an area that requires investigation. It is likely that the materials used today provide excellent protection against chemical permeation. If not, there are other materials that can be used to seal these connections that will provide sufficient protection from chemicals.
EMERGING TECHNOLOGIES Materials H D P E is currently the material o f choice for geomembranes used as vertical barrier walls. The reasons for this include availability, cost, and ease of installation. The use o f other materials is possible, but they will have to show added value to gain inroads into this market. The types o f materials that may be useful include stiff sheets that can be driven into the ground directly which would be less expensive to install. For more critical applications like concentrated solvents, a more chemical resistant material may be useful. This could be cross-linked polyethylene which has reduced free volume by a controlled cross-linking reaction. Other highly resistant materials are certainly available, but are generally more expensive. However, it may be possible to find a thinner sheet that would perform as good or better than HDPE. Connections This is an area that will certainly produce innovations in the near future. First, full fusion welding in the trench is being investigated. This may involve an inductively heated wire or wire bundle coated in H D P L or around a H D P E core. The assembly could be inserted into a slot and the plastic melted and combined with the sheets to make a welded seam. The types of grouts and sealants can also be improved. Grouts can be made more chemically resistant and connections can be made to a c c o m m o d a t e them. In the area of seals, it should be possible to build a connection that
406
Richard W. Thomas, Robert M. Koerner
has two slots for seals. In fact, there are three already available (see Fig. 8). One seal can be the conventional hydrophilic rubber to reduce water migration and the second could be a seal designed to swell on contact with hydrocarbons. This would provide protection from both hydrocarbons and water. Another advance would be to make the connection profile on the sheet at the site or just before installation. This would reduce the cost of the materials because the profiled edge is currently made separately and welded on in a secondary operation. Installation methods
This is an area where many possible methods may appear in the future. Most likely though, the typical methods will be improved upon to make the installations better, faster, cheaper and deeper. The new excavation method known as soil sawing may be an interesting subset of the trenching method already discussed.
CONCLUSIONS To mitigate concerns expressed over vertical barrier walls consisting of SB, SC, CB or SCB backfilled materials, the use of a geomembrane is a viable alternative. Even further, the geomembrane can be used with the above mentioned backfills forming a composite barrier. Such composite barriers (geomembrane backed by a compacted clay liner) are at the heart of liners beneath hazardous and nonhazardous landfills. Now the capability exists of achieving the same degree of environmental safety and security in a vertical orientation. This paper described five techniques for geomembrane installation as vertical barrier walls. In all of them the type of backfill material in the remainder of the trench space not occupied by the geomembrane is a site specific design decision. Contrary to geomembranes placed horizontally and fusion welded together, vertical geomembranes are usually joined by a 'lock and key' type of connection. The connections are made leak proof by use of a hydrophilic gasket or grout seal. In the authors' opinion, the interlock is the essential aspect of a leakproof system over time. The concept of a double gasket, one to swell on contact with water, the other on contact with hydrocarbons, was introduced. Grouted interlocks offer the capability of injection into one channel and return via an adjacent channel. Thus, inspection monitoring can be implemented. It is perhaps this aspect of geomembrane barrier walls that distinguishes them from competitive porous material barrier walls, i.e. geomembrane walls are capable of
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407
inspection. This provides assurance that a complete barrier system has been formed. The paper also discussed at length the issue of diffusive permeability o f H D P E geomembranes in a vertical barrier application. While it is not zero, it is extremely low for water (approximately 0.006g/mZ-day or 0.006 gal/acre-day) and is even very low for aqueous solutions of organic solvents. Data from the literature in this regard show a worst case situation of toluene at a concentration o f 100mg/L will diffuse less than 1 pint of liquid per acre o f wall per day. Attenuation of such a small quantity in a low permeability soil backfill behind the geomembrane is readily achievable. Alternatively, it is even conceivable that a sand backfill (perhaps between two geomembranes as a leak detection system) could be utilized to pump seepage from behind the barrier in cases of extremely high environmental sensitivity. G e o m e m b r a n e vertical walls are currently available in a number of competitive styles and configurations. Their installation is achievable using standard equipment and techniques. It is hoped that this paper will help to stimulate the interest in and the use o f these systems.
REFERENCES Bliss, M. & Brunette, P. T. (1995). Reach 11 dikes modification: a vertical barrier wall of HDPE geomembrane. In Proceedings Geosynthetics '95, Nashville, USA, IFAI, Volume 1, pp. 147-60. Britton, L. N., Ashman, R. B., Aminabhavi, T. M. & Cassidy, P. E. (1989). Permeation and diffusion of environmental pollutants through flexible polymers. Journal of Applied Polymer Science, 38, 227-36. Brunette, P. & Schmednecht, E. (1995). Vibrating beam, curtain wall and jet grouting used to form a vertical barrier wall. In Proceedings, Polluted and Marginal Land '95, London, England. Brunette, P. & Pierce, D. (1994). Recent developments for the containment of lateral migration of petroleum hazardous waste in ground water. In Proceedings, Petro-Safe '94. Gundle Lining Systems, Inc. (1993). Laboratory tests results of gundwall locking section with hydrotite leakage potential testing, Geosyntec Consultants, project no. GL3398, Houston, TX. Hansen, P. G. & Crotty, G. R. (1995). Use of geomembranes as vertical barrier liners for containment on the north slope of Alaska, Unknown Conference Proceedings. Haxo, H. E. (1988). Lining of waste containment and other impoundment facilities, U.S. EPA 600/2-88/052, 1600 pp. Koerner, R. M. & Guglielmetti, J. L. (1995). Vertical barriers: geomembranes. In International Containment Workshop Final Report, ed. R. E. Ruiner, Baltimore, MD.
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Michalangeli, A. (1995). Italian experience with HDPE geomembranes in landfill liners. In Proceedings Geosynthetics '95, Nashville, USA. IFAI, Volume 2, pp. 569-83. Park, J. K., Sakti, J. P. & Hoopes, J. A. (1995). Effectiveness of geomembranes as barriers for organic compounds. In Proceedings Geosynthetics '95, Volume 3, IFAI, pp. 879-92. Scuero, A. M., Nuzzo, G. & Salvemini, A. (1990). A continuous barrier from bottom to crown in hydraulic earthfield structures. In Proceedings 4th International Conference on Geosynthetics, The Hague. Balkema, pp. 425-9. SLT North America, Inc. (1995). Curtain wall system--permanent subsurface barrier wall, Technical Data Package. Thomas, R. W. (1995). Unpublished results.
© 1996 Elsevier Science Limited Printed in Ireland. All rights reserved 0266-1144/96 $15.00 ELSEVIER
PII:S0266-1
144(96)00024-6
Advances in H D P E Barrier Walls
R i c h a r d W. T h o m a s a & R o b e r t M. K o e r n e r b aTRI/Environmental, Inc., 9063 Bee Caves Road, Austin, TX 78733-6201, USA bGeosynthetic Research Institute, Drexel University, Philadelphia, PA 19104, USA
ABSTRACT The cut-off and/or containment of laterally flowing liquids from landfills and impoundment reservoirs generally utilize some type of vertical wall. The most common wall is constructed using a slurry supported trench, subsequently backfllled with soil-bentonite, soil-cement, cement-bentonite or soil-cement-bentonite. Concerns have arisen as to the installation, inspection and durability of such walls. A different, or complementary, strategy uses a geomembrane by itself or in combination with any one of the standard backfill materials to provide the degree of completeness and environmental safety/security that most, if not all of these sites warrant. This paper discusses applications where geomembrane barrier walls can be used, details of their installation, idiosyncrasies of the material used (particularly the diffusive transport aspect) and concludes with selected comments regarding emerging technology. Copyright © 1996 Elsevier Science Ltd.
INTRODUCTION
Barrier or cut-off walls have been used for many years in geotechnical applications. The basic concept is to construct an in-situ vertical wall that will act as a separator between two areas. The walls are constructed using a bentonite slurry supported trench which is subsequently displaced by the permanent backfill o f a composite material like soil-bentonite (SB), soil cement (SC), cement-bentonite (CB) or soil-cement-bentonite (SCB). Concerns over the integrity of these walls come about via the nature of their construction or the type o f backfill used. Some of these concerns are as follows: 393
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• • • • • • • • • • • • •
Richard W. Thomas, Robert M. Koerner
collapse of trench walls during and after excavation; improperly mixed slurry support material; collapse of trench walls during backfilling; sand deposits during pauses in construction; sand, gravel, boulders or other debris at the bottom of the trench; difficulty of quality control/quality assurance since none of the critical operations are visible under the slurry; discontinuities at joints and during work stoppages; backfill material drying below its initially placed high water content; long-term desiccation of the wall where it is above the water table or liquid level; freeze-thaw behavior of the wall in northern climates; chemical resistance of the wall to the liquids to be contained; permeability of the final backfilled material; diffusive transport through the final backfilled material.
One way to improve the integrity and transport properties of these walls is with the use of geomembranes. Geomembranes have been used in vertical walls since about 1980, and have been used by themselves in single or double layers, or with SB, SC, or CB material as a composite wall. The geomembranes used have almost always been high density polyethylene (HDPE). The use of other materials is possible, but H D P E is readily available and has a long history of successful use in other geotechnical applications. It is also highly resistant to water vapor transport and resistant to a variety of other chemicals. The walls are generally installed by placing individual panels that are connected together. There are several different interlocking connections available and there are different ways to seal the connections. This paper will focus on three aspects of polymer barrier walls. They are the applications where these walls can be useful, the installation techniques most commonly used to build the walls, and the properties of the geomembranes and their connections. Special emphasis will be placed on diffusive transport through HDPE, since concerns are often voiced in this regard. Some insight into future directions and summary comments will conclude the paper.
APPLICATIONS Barrier walls containing a geomembrane can be useful in any application where the flow of water or other liquids is to be controlled. The use of a geomembrane in the wall is a way to ensure continuity of an extremely low
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permeability material. In the light of the numerous concerns expressed earlier, it is clear that barrier walls that include geomembranes provide an added factor of safety for almost any construction application. There are a variety of different applications where these walls have been proposed or used. Some of them are illustrated in Figs 1-5. Figure I shows a typical hazardous waste containment application. The barrier wall is installed at a depth that is either well below the plume of waste or keyed into the aquitard. The wall can also be welded to a cap geomembrane by overlapping either the wall or the cap. This type of sealed system can be monitored and can include a leachate collection system. Figure 2 shows how a geomembrane barrier wall can be used to contain hazardous spills around plant sites. This figure shows how different hazardous chemicals migrate through soils. The dense nonaqueous phase liquids (DNAPL) are heavier than water, while the light nonaqueous
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Suitable Fig. 3. Water flow control with a vertical geomembrane barrier wall (after SLT, 1995). phase liquids (LNAPL) are lighter than water. This type of containment can be an effective way to reduce the a m o u n t of ground water contamination that would occur prior to and during remediation of a site. Figures 3 5 show how walls can be used to influence the flow of water. These applications include redirecting water flow around a construction site (Fig. 3), lowering the water table for construction (Fig. 4), or cutting off water flow through a dam, dike, or levee. These are general examples showing how these walls can be used. A recent review (Koerner, 1995) describes specific case studies that have been found in the literature (a summary of these methods is presented in Table 2 at the end of the next section). INSTALLATION METHODS Koerner and Guglielmetti (1995) identified five c o m m o n installation techniques. These are shown in Table l and will be discussed below.
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Advances in HDPE barrier walls
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Notice that the methods are different depending on how a trench is excavated, whether or not the trench is supported, and what type of backfill is used. Obviously, there are important choices to be made that are probably specific to the site and the application. The trenching machine method uses a large bucket trencher or disc cutter to excavate an unsupported trench. At the same time, the geomembrane is unrolled from a box mounted on the trenching machine or from a trailer at the ground surface. There are two major advantages to this method. It is extremely fast and there are connections made only at the ends of the rolls. Additionally, this method lends itself to welded connections, which should be the most reliable type of connection. The backfill is typically the native soil, but a sand fill with pipe leak detection can also be used. The main disadvantage of this method is that the TABLE 1 Common Installation Methods for Geomembrane Vertical Walls (after Koerner & Guglielmetti, 1995)
Method or technique
Geomembrane configuration
Trench support
continuous
none
30-60 (12-24)
1.5~,.5 (5-15)
sand or native soil
Vibrated insertion plate
panels
none
10-15 (~6)
1.5-6.0 (5-20)
native soil
Slurry supported
panels
slurry
60-90 (24-36)
no limit
SB, SC, CB, sand or native soil
Segmented trench box
panels or continuous
none
60-120 (2~48)
3.0-9.0 (10-30)
sand or native soil
panels
slurry
15-22 (f~9)
no limit
SB, SC, or CB slurry
Trenching Machine
Vibrating beam
Typical trench Typical trench width cm (in) depth m (ft)
Typical backfill
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Richard W. Thomas, Robert M. Koerner
unsupported trench depth is limited. That makes the method very site specific. The vibrated insertion plate method involves the use of geomembrane panels approximately 3 m (10 ft) wide and a steel truss 'insertion plate'. The geomembrane is fixed at its base with pins protruding from the bottom of the insertion plate. A vibratory pile hammer is used to force the entire assembly to the desired depth. The insertion plate is then removed leaving the geomembrane behind. Interlock connections are required to join the panels together. This method is fast and requires neither trench support nor special backfill. The main disadvantage is that only certain soft and/or loose soils can be penetrated to substantial depths. Also, damage to the geomembrane may be a concern with soils containing construction debris, rock, boulders or extremely hard clay strata. The slurry supported method starts by using a conventionally excavated and slurry supported trench. The geomembrane panels are inserted into the trench to their desired depth with a steel frame. They are either laid down with weights or toed into the subsoil stratum. They are also joined side-by-side with interlocking connections. The backfill is usually SB, SC, SB or SCB, but it can also be low permeability native soil. Also, a single or double lined vertical wall with a sand leak detection layer can be made by this method. The depth of the wall is virtually unlimited and the deepest walls have been constructed by this method. So far, the maximum depth has been 30m (100 ft). The segmented trench box method uses a modified trench box to support the side walls of the trench during its excavation. The geomembrane is placed between the outer segments of the box. The backfill is usually native soil, but other membranes can be used. This technique is limited by depth, so it is also site specific. The geomembrane panels are usually interlocked, but it is also possible to weld the sheets and to inspect the welds within the confines of the trench box. The last method is the vibrating beam method. This method utilizes a modified form of the slurry supported trenching method. The slurry is introduced as the vibrating beam is inserted and removed. Once the trench is slurry supported, the geomembrane panels are installed as a secondary operation. There is no depth limit and the backfill is usually a SB, SC, CB or SCB material. Thus, it is seen that these methods fall into two main categories with respect to the depth of the completed wall. A supporting slurry seems to be required for depths greater than 9 m (30 ft). Therefore, to date, there are three choices for shallow walls and two for deep walls. There are certainly other novel and unique ways to install these walls.
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Advances in H D P E barrier walls TABLE 2
Case Studies Utilizing Geomembranes as Vertical Barriers (after Koerner & Guglielmetti, 1995) Reference
Application or containment
Installation type
Brunette & Schmelnecht (1995)
hazardous waste
Bliss & Brunette (1995)
earth dam cutoff
slurry supported
Brunette & Pierce (1994)
petroleum waste hazardous waste
Scuero, el al. (1990) Michaelaneli (1995)
Backfill
10 (35)
0.3 (0-2)
sand
15 (50)
20 (12.5)
slurry supported slurry supported
sand
4.5 (15)
0.4 (0-2)
sand
14 (45)
0.5 (0-3)
earth dam cutoff
slurry supported
CB slurry
9 (30)
0.1 (0-1)
Municipal solid waste and ash
slurry supported
CB slurry
various
various
trenching machine
native soil
3 (10)
3 (21)
Hansen & contaminated Crotty (1995) drilling waste
vibrating beamclay/cement slurry
Wall depth m Wall length km (ft) (mi)
However, it is believed that these methods illustrate the state-of-thepractice. Koerner and Guglielmetti (1995) identified six published case studies describing the application and installation of vertical geomembrane barrier walls. A summary of these is found in Table 2. It should be noted that there are many other internal documents and reports along with company brochures describing other case studies.
MATERIALS The materials that need to be discussed are the geomembranes used and the connections between geomembrane panels. An extremely impermeable sheet can be produced, but if the connections leak, the overall system will not perform well. Therefore, both the sheet and the interlocking connections need to be considered. This section will focus on the transport properties of the geomembranes and the design and performance of the connections.
Richard W. Thomas, Robert M. Koerner
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Geomembranes The requirements of a geomembrane for use in a barrier wall include: • • • •
high stiffness for ease of installation; high resistance to a variety of chemicals, including organic solvents; ability to attach interlocking profiles on the edges of the sheets; durability in buried applications.
H D P E is a good choice for most of these requirements. It is not stiff enough to be directly driven like steel sheet, but, as just described, there are several other ways to install the sheets. When the availability and price of H D P E geomembranes are also considered, they become a natural choice for these applications. Another reason for the choice of H D P E is its ability to be shaped by profile extrusion. The interlocking connections are complicated shapes that are made by a continuous extrusion process, then cut to length and fusion welded to the geomembrane panels. By now, the issue of long term performance of HDPE in buried applications should be settled. Besides stress that can lead to slow crack growth in some cases, there are few factors that would limit the lifetime of an H D P E geomembrane in a buried application. One of these is exposure to chemicals. However, there have been hundreds of EPA 9090 compatibility tests performed on H D P E with a variety of municipal and hazardous leachates without a single failure. In fact, in the authors' opinion, the performance of H D P E when exposed to chemicals is well established. In concentrated hydrocarbons (chlorinated and aromatic are most severe), there can be a loss in tensile yield strength of up to 30%. This is due to physical plasticization which softens HDPE. This is a reversible change; when the chemicals are allowed to vent off, the strength returns. Therefore, as long as the reduced strength is above the design strength, which it should always be, this reduction in strength should not be a problem. The other mechanism that can occur during exposure to hydrocarbons is extraction of some anti-oxidants and other stabilizers. However, when buried, the need for stabilizers is minimal. One of the most important properties for barrier walls is low permeability to chemicals. It is important to distinguish the chemical permeability of geomembranes from the permeability commonly used by geotechnical engineers, i.e. from hydraulic conductivity. In the case of soils and other porous media, the transport of water (or other chemicals) occurs through soil voids, pores, cracks, and fissures. Alternatively, in chemical permeation of geomembranes, the chemical will pass
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through a nonporous membrane at a molecular level. Molecules are able to 'squeeze' through membranes by moving among and around the polymer chains. An example of this type of permeation is a helium filled balloon. Rubber balloons deflate in just a few days, while Mylar balloons last much longer. Neither balloon has holes in them; they just have different permeabilities to helium. Rubbers are characterized by being a m o r p h o u s (without ordered structure) and have high free volume, which means there is relatively large spaces between molecules. Conversely, Mylar is a semi-crystalline polyester plastic that has a high degree of order and much smaller spaces between molecules. Therefore, it is more resistant to permeation. HDPE is also a semi-crystalline plastic which makes it resistant to chemical permeation. However, it must be understood that no polymer is impermeable to chemicals. There will always be some permeation that occurs. The real question is whether or not the amount of permeation that occurs is acceptable. There are a variety of factors which affect the rate of chemical permeation through membranes. These include the chemical concentration, the temperature, and the sheet thickness. Additionally, permeation consists of two main components, diffusivity and solubility. Diffusion is the transport rate of a chemical through a barrier. Solubility is the amount of chemical that a barrier can hold. This is related to the amount of free volume in a polymer and the compatibility of the chemical with the membrane. Therefore, permeation is related to how much chemical a membrane will absorb and the speed at which the chemical will move through the barrier. Thus, the permeation rate can be affected by the concentration, temperature and liner thickness. It is fairly easy to understand why H D P E is resistant to water. The chemical composition of H D P E is similar to oil, grease, and wax, so it repels water, which means it absorbs only a small amount. HDPE is also semi-crystalline so the water molecules that are absorbed have to find their way around the crystals (molecules can only pass through amorphous regions). These features make H D P E highly resistant to water. A typical value for 2.5 m m (100 mil) H D P E is around 0.006 g/m2-day which is about 0.006 gal/acre-day. So, water transport is not really an issue with HDPE. Permeation with organic chemicals is a more complicated issue. There are only three sources for permeation data on H D P E geomembranes known to the authors. Haxo reported the solvent vapor transmission rates of a variety of neat (100%) chemicals for two thicknesses of HDPE (Haxo, 1988). Britton et al. reported weight gain and permeation results on a mixture of eight organic solvents on three thicknesses of HDPE at two temperatures (Britton et al., 1989). Both of these studies involved
Richard W. Thomas, Robert M. Koerner
402
TABLE 3 Vapor Transmission Rates of Organic Solvents through HDPE membranes (Park et al., 1995)
Chemical
Trichloroethane (d = 1-4 g/mL)
Toluene (d = 0.87 g/mE)
m-Xylene (d = 0.87 g/mL)
Methylene chloride (d = 1-34 g/mL)
Concentration
Sheet thickness
(mg/L)
(mm)
Vapor
Vapor
10 50 100 100 100
0.76 0.76 0.76 1.52 2.54
1.0 5.3 15.9 3.6 1.3
0.024 0.127 0.382 0.086 0.031
10 50 100 100 100
0.76 0.76 0.76 1.52 2.54
0.8 4.4 14.6 2-9 1.0
0.019 0-106 0-350 0.070 0.024
10 50 100 100 100
0.76 0-76 0-76 1.52 2.54
0-7 3.2 13.3 1-8 0.6
0.017 0.077 0.319 0.043 0.014
10 50 100 100 100
0-76 0-76 0-76 1-52 2.54
0-2 0-9 2-9 0.8 0.4
0.005 0.022 0.070 0.019 0.010
transmission rate transmission rate (rag/me-h) (g/m2-day)
concentrated chemicals. The third study involved diluted chemicals at several concentrations and several thicknesses of HDPE geomembranes (Park et al., 1995). Table3 shows data from the Park et al. study along with calculated transmission rates. Notice first that when the results are converted, the actual rates are small. They range from 0.04 to 0-40 g/m2-day or 0.04 to 0.43 gal/acre-day. In empirical units, this is from half an ounce to 1.7 quarts of liquid. Also notice the dramatic effects of concentration and sheet thickness on the vapor transmission rates. These are shown graphically in Figs 6 and 7. When the concentration was decreased by 10 times, the permeation rate decreased 13-19 times. This suggests that permeation rates with 100% pure chemicals vastly overestimate actual field permeation rates because leachates are typically very dilute concentrations o f chemicals. A brief survey of leachate analysis packages showed that values for organic solvents in hazardous waste rarely exceed a total of 100mg/L (Thomas,
Advances in HDPE barrier walls
403
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60 90 Concentration (mg/L)
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1995). Similarly, when the thickness was increased from 0-76 to 2.54mm, the permeation rate decreased 7-15 times. This shows that effects of permeation can be reduced by the use of thicker geomembranes. The actual effect will probably not be as great as these data suggest. In theory, the maximum permeation rate of a 2.54mm liner should be about onethird that of a 0.76 m m liner because it is 3.3 times thicker. It is believed that in this case, the permeation rates in the thicker sheets never reached their maximums, even after 3 months of exposure. However, these data do suggest that the vapor transmission rate for toluene at a concentration of 100mg/mL will not be greater than 0.13 g/me-day. This is about 1 pint of liquid per acre per day. Connections
Since the main installation techniques currently used for polymeric barrier walls involve individual panels of HDPE, the panels must be connected in
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404
Richard W. Thomas, Robert M. Koerner
A
C Fig. 8.
D
Gasket type interlocks (after Koerner & Guglielmetti, 1995).
some way. The connections are usually a 'lock and key' type that use a hydrophilic gasket or grout as a sealant. The panels can also be heat welded together, but this is not a commonly used method. The interlocks are important because they can be the most permeable part of the wall. Additionally, the joints may become stressed during installation or if some settlement or lateral deformation occurs in the wall. Therefore, it is important to evaluate the strength and the permeation resistance of the interlocks. Four types of interlocks that use a hydrophilic gasket for liquid tightness of the installed panels are shown in Fig. 8. The gasket material is a proprietary formulation of neoprene rubber that is designed to swell on contact with water. The volume change can be up to eight times the original diameter which creates the sealing pressure. Connection A was shown to pass only negligible amounts of water under pressures up to 800kPa (Gundle Lining Systems, Inc., 1993). The performance of the connection when exposed to organic chemicals has not been evaluated to our knowledge. There are two interlocks that have been designed to be used with a sealant or grout. These are shown in Fig. 9 and are attractive because the additional thickness of grout will produce a more tortuous path for chemical migration. However, in this case, the adhesion between the grout and the HDPE needs to be studied because polyethylene is difficult to bond to. Notice that one connection can be used with both the hydrophilic seal and grout which provides double protection from chemicals. The resistance of these connections to vapor transport of water and
Advances in H D P E barrier walls
A
Fig. 9.
405
B
Grouted type interlocks (after Koerner & Guglielmetti, 1995).
chemicals is an area that requires investigation. It is likely that the materials used today provide excellent protection against chemical permeation. If not, there are other materials that can be used to seal these connections that will provide sufficient protection from chemicals.
EMERGING TECHNOLOGIES Materials H D P E is currently the material o f choice for geomembranes used as vertical barrier walls. The reasons for this include availability, cost, and ease of installation. The use o f other materials is possible, but they will have to show added value to gain inroads into this market. The types o f materials that may be useful include stiff sheets that can be driven into the ground directly which would be less expensive to install. For more critical applications like concentrated solvents, a more chemical resistant material may be useful. This could be cross-linked polyethylene which has reduced free volume by a controlled cross-linking reaction. Other highly resistant materials are certainly available, but are generally more expensive. However, it may be possible to find a thinner sheet that would perform as good or better than HDPE. Connections This is an area that will certainly produce innovations in the near future. First, full fusion welding in the trench is being investigated. This may involve an inductively heated wire or wire bundle coated in H D P L or around a H D P E core. The assembly could be inserted into a slot and the plastic melted and combined with the sheets to make a welded seam. The types of grouts and sealants can also be improved. Grouts can be made more chemically resistant and connections can be made to a c c o m m o d a t e them. In the area of seals, it should be possible to build a connection that
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Richard W. Thomas, Robert M. Koerner
has two slots for seals. In fact, there are three already available (see Fig. 8). One seal can be the conventional hydrophilic rubber to reduce water migration and the second could be a seal designed to swell on contact with hydrocarbons. This would provide protection from both hydrocarbons and water. Another advance would be to make the connection profile on the sheet at the site or just before installation. This would reduce the cost of the materials because the profiled edge is currently made separately and welded on in a secondary operation. Installation methods
This is an area where many possible methods may appear in the future. Most likely though, the typical methods will be improved upon to make the installations better, faster, cheaper and deeper. The new excavation method known as soil sawing may be an interesting subset of the trenching method already discussed.
CONCLUSIONS To mitigate concerns expressed over vertical barrier walls consisting of SB, SC, CB or SCB backfilled materials, the use of a geomembrane is a viable alternative. Even further, the geomembrane can be used with the above mentioned backfills forming a composite barrier. Such composite barriers (geomembrane backed by a compacted clay liner) are at the heart of liners beneath hazardous and nonhazardous landfills. Now the capability exists of achieving the same degree of environmental safety and security in a vertical orientation. This paper described five techniques for geomembrane installation as vertical barrier walls. In all of them the type of backfill material in the remainder of the trench space not occupied by the geomembrane is a site specific design decision. Contrary to geomembranes placed horizontally and fusion welded together, vertical geomembranes are usually joined by a 'lock and key' type of connection. The connections are made leak proof by use of a hydrophilic gasket or grout seal. In the authors' opinion, the interlock is the essential aspect of a leakproof system over time. The concept of a double gasket, one to swell on contact with water, the other on contact with hydrocarbons, was introduced. Grouted interlocks offer the capability of injection into one channel and return via an adjacent channel. Thus, inspection monitoring can be implemented. It is perhaps this aspect of geomembrane barrier walls that distinguishes them from competitive porous material barrier walls, i.e. geomembrane walls are capable of
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inspection. This provides assurance that a complete barrier system has been formed. The paper also discussed at length the issue of diffusive permeability o f H D P E geomembranes in a vertical barrier application. While it is not zero, it is extremely low for water (approximately 0.006g/mZ-day or 0.006 gal/acre-day) and is even very low for aqueous solutions of organic solvents. Data from the literature in this regard show a worst case situation of toluene at a concentration o f 100mg/L will diffuse less than 1 pint of liquid per acre o f wall per day. Attenuation of such a small quantity in a low permeability soil backfill behind the geomembrane is readily achievable. Alternatively, it is even conceivable that a sand backfill (perhaps between two geomembranes as a leak detection system) could be utilized to pump seepage from behind the barrier in cases of extremely high environmental sensitivity. G e o m e m b r a n e vertical walls are currently available in a number of competitive styles and configurations. Their installation is achievable using standard equipment and techniques. It is hoped that this paper will help to stimulate the interest in and the use o f these systems.
REFERENCES Bliss, M. & Brunette, P. T. (1995). Reach 11 dikes modification: a vertical barrier wall of HDPE geomembrane. In Proceedings Geosynthetics '95, Nashville, USA, IFAI, Volume 1, pp. 147-60. Britton, L. N., Ashman, R. B., Aminabhavi, T. M. & Cassidy, P. E. (1989). Permeation and diffusion of environmental pollutants through flexible polymers. Journal of Applied Polymer Science, 38, 227-36. Brunette, P. & Schmednecht, E. (1995). Vibrating beam, curtain wall and jet grouting used to form a vertical barrier wall. In Proceedings, Polluted and Marginal Land '95, London, England. Brunette, P. & Pierce, D. (1994). Recent developments for the containment of lateral migration of petroleum hazardous waste in ground water. In Proceedings, Petro-Safe '94. Gundle Lining Systems, Inc. (1993). Laboratory tests results of gundwall locking section with hydrotite leakage potential testing, Geosyntec Consultants, project no. GL3398, Houston, TX. Hansen, P. G. & Crotty, G. R. (1995). Use of geomembranes as vertical barrier liners for containment on the north slope of Alaska, Unknown Conference Proceedings. Haxo, H. E. (1988). Lining of waste containment and other impoundment facilities, U.S. EPA 600/2-88/052, 1600 pp. Koerner, R. M. & Guglielmetti, J. L. (1995). Vertical barriers: geomembranes. In International Containment Workshop Final Report, ed. R. E. Ruiner, Baltimore, MD.
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Michalangeli, A. (1995). Italian experience with HDPE geomembranes in landfill liners. In Proceedings Geosynthetics '95, Nashville, USA. IFAI, Volume 2, pp. 569-83. Park, J. K., Sakti, J. P. & Hoopes, J. A. (1995). Effectiveness of geomembranes as barriers for organic compounds. In Proceedings Geosynthetics '95, Volume 3, IFAI, pp. 879-92. Scuero, A. M., Nuzzo, G. & Salvemini, A. (1990). A continuous barrier from bottom to crown in hydraulic earthfield structures. In Proceedings 4th International Conference on Geosynthetics, The Hague. Balkema, pp. 425-9. SLT North America, Inc. (1995). Curtain wall system--permanent subsurface barrier wall, Technical Data Package. Thomas, R. W. (1995). Unpublished results.