International Commission on Large Dams
By Robert M. Koerner and John A. Wilkes
The 2010 International Commission on Large Dams (ICOLD) Bulletin on “Geomembrane Sealing Systems for Dams” (ICOLD Bulletin No. 135) supersedes and greatly expands upon two earlier versions published in 1981 (Bulletin 38) and 1991 (Bulletin 78).
The 265 dams incorporating geomembranes cited in this new Bulletin include 183 fill, 47 concrete, 34 roller compacted concrete, and 1 hybrid. Of the total, 118 (45%) are in Europe, 48 (18%) are in the United States, 47 (18%) are in China, and the remaining 52 (19%) are scattered in other countries and locations.
Since most of the experience gained to date is in Europe, the European Working Group on Geomembranes prepared the Bulletin with the assistance of the primary author of this article.
This article attempts to summarize the 242 pages of the English-language portion of the Bulletin and is subdivided according to the same structure. The Bulletin concludes with a brief section on quality control and another on guidance for technical content of contracts. It is published in English and French.
For more than 45 years geomembranes have been used to provide waterproofing on various types of large dams. Table 1 provides information on the 265 dams cited and detailed in this Bulletin as of 2006.Most have been installed in the dry, but some success at underwater sealing has also been achieved.
By far the largest proportion of these dams have used factory-manufactured polymeric geomembranes. When such geomembranes incorporate other types of geosynthetic materials, such as geonet drains and geotextile cushions, filters, or separators, a “geomembrane sealing system” (GSS) is achieved and will be referenced as such throughout this article. In the majority of cases the waterproofing is a GSS, although a few situations have the geomembrane by itself.
As stated, the waterproofing geomembranes used in these dams are largely polymeric, although 22 are bituminous and four are in-situ (or spray-on) types. Since in-situ types are no longer used and bituminous types are currently rare, the Bulletin focuses almost entirely on polymeric geomembranes made from various types of resins and their related formulations.
Materials, testing, durability
Most of the geomembranes cited in this Bulletin are made from thermoplastic polymers (those that can be thermally welded in the field), with European-manufactured polyvinyl chloride (PVC) the most widely used. Table 2 presents these statistics.
Recognize, however, that all polymers are actually formulations containing the designated resin (from which the name is derived), additives (mainly antioxidants), colorants (often carbon black), and some fillers. Geosynthetic literature is abundant on details of manufacturing quality control and quality assurance. The Bulletin provides a detailed comparative behavior of the various geomembranes. Table 2 shows that both exposed and covered situations are encountered and in approximately equal proportions.
The Bulleton covers testing of the manufactured geomembrane in the context of providing the desired information for various situations. Both ISO and ASTM Standards are identified. Individual tests for the following situations are include:
- quality control during manufacture
- identification testing
- performance testing
- compliance testing
Regarding the important issue of geomembrane aging and its in-service durability, a critical issue is whether the geomembrane is exposed or covered with soil, rock, or concrete. The former is more critical due to ultraviolet exposure and the usually high accompanying temperatures. This topic is addressed in the Bulletin. In this context, however, note that the oldest exposed geomembrane installation was in 1974 and the oldest covered installation was in 1960.
While not in the Bulletin, note that research at the Geosynthetic Institute on exposed geomembrane lifetime prediction is ongoing.
Loads applied to geomembrane sealing systems (GSS)
Table 3 illustrates various dam configurations and their respective GSS locations for 254 of the dams cited; the remaining 11 are special cases.As can be envisioned, mechanical loads are certainly to be anticipated. The following are addressed in the Bulletin:
- gravity load
- subgrade differential settlement
- puncture loads
- wind uplift
- reservoir waves
- ice in the reservoir
- uplift from water and air
Various physical, chemical, and biological agents can be important on a case-by-case basis, including:
- ultraviolet radiation
- water contamination ingredients
- biological activity, e.g., microorganisms
Each of these topics is discussed in the Bulletin but to a limited extent.
Geomembranes for new and rehabilitated earth and earth/rock fill dams
Table 4 indicates that both new and rehabilitated fill dams (earth and earth/rock types) are commonly waterproofed using geomembranes on, or in, their upstream slopes.This is the most widely used GSS application reported in the Bulletin.
When used on the upstream slope of fill dams, cover is provided by a geotextile cushion (typically a thick needlepunched nonwoven fabric) and then rock riprap or articulated concrete block mattresses on the surface. Most cases are of this general type. See Figure 1 for typical situations.
As seen in Table 4, a relatively high number of fill dams (47 cases) also use GSS without being covered—i.e., in a exposed condition. Confidence in geomembrane durability appears to be a factor because many of these dams are recent case histories. It is important to note that the geomembrane can be visually inspected as to its performance whenever the water level is lowered.
Internal GSS have been used in 20 large fill dams, most of them in China. The Bulletin gives numerous cost-effective placement options in comparison to conventional core walls made from clay or asphalt concrete. The internal GSS is also applicable to raising the height of existing dams.
In essentially all cases of geomembranes used with fill dams (geomembrane/geosynthetic clay liner composites were not mentioned in the Bulletin), a drainage layer is used beneath the geomembrane, creating a “system.” This drainage layer is either a thick needle punched nonwoven geotextile or a geonet composite with two geotextiles thermally bonded to it on both sides. This allows easy placement on a steep slope or even vertically.
Interface stability is important and direct shear testing is required. The inclined drainage system empties into a horizontal drainage gallery beneath the embankment and is discharged at the toe of the downstream slope. This drainage gallery is typically granular soil of adequate thickness and permeability so excess pore water pressures are not mobilized beneath the downstream embankment portion of the dam. Many configurations are illustrated in the Bulletin.
Geomembrane achorage is emphasized in a number of sections of the Bulletin.
Clearly, at the two abutments and the foundation, leakage must be minimized. Concrete beams, or “plinths,” are common. These plinths are cast with connections of various types integral with their construction. They can be continuous (with the geomembrane welded to polymer inserts) or discrete (with anchor bolts, batten strips, and associated hardware to physically fasten the geomembranes). These are critical design details.
If the strategy is for exposed geomembranes on fill dam slopes, wind and wave forces can be significant in shifting the geomembrane out of position. This will require intermediate anchorages throughout the slope. Several scenarios are presented in the Bulletin.
The seaming of the geomembrane rolls together on the slope is arguably the most important construction issue for a GSS. With thermoplastic geomembranes of the type currently used, welding of edges and roll ends is straightforward. Hot air welding can be done at any angle (even vertically) and can even be done with two parallel tracks leaving an unbonded space between. This so-called “dual channel” seam allows for air inflation and is an excellent nondestructive test of the completed weld. If flaws are detected, conventional patches or cap strips are used and retested using the vacuum box method.
Geomembranes on concrete and masonry dams
Table 5 presents statistics on the use of geomembranes in the rehabilitation of poured concrete and masonry dams. (Roller compacted concrete dams will be treated separately.)In all cases except one, the geomembranes are exposed. It is current practice to always include drainage (and cushioning) behind the geomembrane thus creating a GSS. There are no instances of using GSSs in newly designed concrete dams.
In most of the cases, a grout curtain is first constructed to minimize foundation leakage. A concrete beam (or plinth) is constructed over the grout curtain, which serves to anchor the base of the geomembrane and provide proper waterproofing. A drainage system (thick geotextile, geonet, geonet/geotextile composite, or geotextile/geonet/geotextile composite) is always used on the face of the dam to collect any seepage that bypasses the geomembrane. This drainage system leads to the dam’s existing drainage gallery where it is collected, monitored, and released downstream. Many variations are illustrated in the Bulletin. The drainage system also serves as an antipuncturing layer. Figure 2 shows such dams while being waterproofed and Figure 3 shows the final situation.
Regarding attachment to the face of the dam, experience over the years has led to linear, vertical anchorages of wide geomembrane panels as the preferred strategy. The spacing between vertical anchorages (typically 3-5m) and the type of anchorage system must leave the geomembrane panels adequately smooth, yet properly tensioned. Aesthetics is also a consideration and black geomembranes are never used, whereas any color (often gray) can be formulated providing that proper ultraviolet durability is achieved. The most common loads resisted by the exposed geomembrane panels are wind and uplift.
Geomembranes for roller-compacted concrete (RCC) dams
As seen in Table 6, approximately half of the geomembranes are exposed and the other half covered. The exposed solution has many similarities to the concrete and masonry dam rehabilitation schemes presented in the previous section (see Figure 4).The covered solution (known as the Winchester System) uses a composite geomembrane/ geotextile that is prefabricated onto a concrete panel (see Figure 5).
This panel is then used as the upstream forming system of the RCC dam with the geomembrane facing the concrete as it is being placed. The geomembranes between panels are cap-stripped and hot air welded together to form the continuous geomembrane. This configuration allows the concrete panel to act directly against the impounded reservoir, thus protecting the geomembrane against ultraviolet exposure, puncture damage, and vandalism. It has been used on 10 dams in the United States.
Special cases include waterproofing of joints on concrete dam faces for both new and rehabilitation situations. They sometimes are constructed under water. The Bulletin presents Table 7 in this regard and Figure 6 illustrates several situations.
The use of water stops (both embedded and external) in RCC dams is important in these spacial cases. The geomembrane must have excellent elasticity and flexibility, while also having the requisite durability. Good tear and puncture resistance is also required—and sometime burst resistance as well.
Quality Control (QC) and Quality Assurance (QA)
Because proper QC and QA are important, a separate chapter in the Bulletin was necessary. When dealing with geosynthetic materials, there are manufacturing and construction issues. When properly practiced, the four following interconnected quality issues lead to an acceptable project:
- MQC = manufacturing quality control at the factory
- MQA = manufacturing quality assurance at the factory
- CQC = construction quality control in the field
- CQA = construction quality assurance in the field
ISO 9000 certification for the manufacturing considerations (both MQC and MQA) and certification of both installers (for CQC) and inspectors (for CQA) are currently available for field personnel.
The Bulletin is quite detailed in giving guidance to the type and number of tests for MQA and CQA personnel to require. Some generic specifications for different types of geomembranes are currently available. Installation of the geomembrane sheets/panels as well as fastening systems are described.
Guidance on technical content of contracts
Regarding contracts, it is recommended separating the GSS waterproofing contract from the earthwork or concrete construction contract because GSS waterproofing projects require completely different work personnel and skill sets from conventional dam contractors.
Much important information about contracts is included in the Bulletin. A few points:
- A quality control plan must be submitted with the bid documents.
- It is common to specify the total acceptable leakage from the upstream face or from each compartment.
- It is typical to require a warranty on materials and a warranty on installation.
- Quality, not cost, should be the major consideration in the selection process of a contractor supplying a GSS.
Robert M. Koerner, Ph.D., P.E., NAE, emeritus professor of civil engineering at Drexel University and director of the Geosynthetic Institute in Folsom, Pa. He is a member of Geosynthetics magazine’s Editorial Advisory Committee.
John A. Wilkes, P.E., is president of Carpi USA Inc. in Roanoke, Va.
This article is adapted from a paper presented Sept. 28, 2011, at the Association of State Dam Safety Officials conference in Washington, D.C.
ICOLD Bulletin No. 135 was prepared by the European Working Group, consisting of the following members: E. Aguiar Gonzalez (Balsas de Tenerife, Spain), P. Barkek (Swiss National Committee), M. Blanco Fernandez (Laboratorio Central de Estructuras y Materials C.D.E.X., Spain), P. Brezina (Povodi Odry, Czech Republic), H. Brunold (Austrian National Committee), D. Cazzuffi (ENEL CESI, Italy), H. Girard (Cemagref, France), M. Lefranc (French National Committee), J. L. Machado do Vale (Portuguese National Committee), C. Massaro (Azienda Energetica Metropolitana Torino), J. Millmore (British National Committee), L. Schewe (German National Committee), A. Scuero (Italian National Committee), P. Sembenelli (Italian National Committee), G. Vaschetti (Italian National Committee), with the assistance of R. M. Koerner (Drexel University/GSI, USA).