«GEOTECHNOLOGY FOR REBURIAL OF ARCHEOLOGICAL SITES: APPLICATIONS AND ENGINEERING by Edward Kavazanjian, Jr. Citation: Kavazanjian, E., Jr., (2004) ...»
GEOTECHNOLOGY FOR REBURIAL OF ARCHEOLOGICAL
SITES: APPLICATIONS AND ENGINEERING
Edward Kavazanjian, Jr.
Kavazanjian, E., Jr., (2004) “The use of geosynthetics for archeological sites
reburial,” Conservation and Management of Archaeological Sites, Vol. 6, No.
3 and 4, James and James (Science Publishers), Ltd., pp. 377-394
Geotechnology applications for reburial of archeological sites include infiltration and drainage control, earth reinforcement, erosion control, and surface water management. The use of geosynthetic materials provides the most common examples of application of geotechnology on reburial projects.
Geosynthetic materials applications in reburial practice include geotextiles for separation, filtration, and protection (cushioning), geomembranes and geosynthetic clay liners for infiltration control, geonets and geocomposites for subsurface drainage, and geocells for erosion control. Earth reinforcement techniques, including mechanically stabilized earth, soil nailing, and micropiles, can provide substantial benefits on reburial projects with respect to reducing lateral earth pressure against backfilled structures, supporting excavations, reinforcing walls, and supporting foundations.
Evapotranspirative capping technology for infiltration control, including capillary break systems, can facilitate design of a robust reburial system that isolates the reburied structure from moisture and temperature fluctuations.
Optimal application of these geotechnologies requires an understanding of basic engineering principles associated with their implementation.
BIOGRAPHY Edward Kavazanjian, Jr., is a registered professional geotechnical engineer and Research Professor of Civil Engineering at the University of Southern California. He is recognized for design of waste containment systems, including applications of geosynthetic materials.
INTRODUCTIONGeotechnical engineering design and construction techniques can make substantial contributions to optimal design of an archeological reburial system, from both a technical (performance) and cost perspective.
Geotechnology developed for isolation of solid and hazardous waste from the environment, for soil stabilization and erosion control, for earthwork construction associated with transportation systems and other infrastructure development projects, and for support of excavations and foundations in sensitive urban settings can be applied to many different aspects of archeological site reburial.
The use of geosynthetic materials for separation, filtration, protection (cushioning), and infiltration control has become fairly common on reburial projects. Additional applications of geosynthetic materials such as earth reinforcement and erosion control have also been appliedin reburial practice, though somewhat less frequently than other geosynthetics applications.
However, many of these applications have been ad hoc solutions rather than engineered applications, sometimes leading to ineffective or less than optimal performance, unnecessary cost, and at times even counter-productive (damaging) field performance. Design and construction techniques developed for traditional geotechnical projects (e.g., infrastructure development, waste management) can mitigate the potential for inadequate performance and reduce unnecessary expenditures when applying geosynthetic materials to reburial projects.
Modern geotechnology can make additional contributions to reburial practice beyond those provided by application of geosynthetic materials.
Earth reinforcement techniques, including soil nailing and micro-piles, developed for construction of infrastructure projects and for protection and preservation of historic structures in urban environments, can facilitate retention of backfill, reinforce deteriorating walls, and support unstable foundations. Evapotranspirative capping technology developed for the design of waste containment systems, including capillary break systems, can be used to identify the optimal configuration for a reburial scheme with respect to isolation from temperature and moisture content fluctuations in the subsurface and to isolate a buried structure from transport and deposition of minerals dissolved in subsurface moisture in the unsaturated zone. Rational and effective application of these geotechnologies requires an understanding of their advantages and limitations as well as of the engineering principles associated with their application.
GEOSYNTHETIC MATERIALSOverview of Material Types The term geosynthetic generally refers to a man-made planar material employed for geotechnical engineering purposes. Geosynthetic materials are generally fabricated in panels, sheets, and/or rolls and are typically composed primarily of polymeric materials, though natural fibers and soils are sometimes employed when fabricating geosynthetic materials. Types of geosynthetic materials include geotextiles, geomembranes, geosynthetic clay liners, drainage cores and drainage composites, erosion control nets, geocells, and geogrids. Functions of geosynthetic materials employed in engineering practice today include separation, protection (cushioning), filtration, drainage, infiltration resistance, reinforcement, and stabilization (erosion control).
Table 1, from Bouazza (1), lists the primary functions of common geosynthetic materials.
Geotextiles include woven and non-woven fabrics and are employed for separation, protection, filtration, reinforcement, and sometimes drainage.
Geomembranes are polymeric sheets with a very high resistance to flow perpendicular to the sheet. The primary application of geomembranes is as a barrier to flow, though they have also been used for separation and protection.
A geosynthetic clay liner is composed of a thin layer of natural low permeability soil, typically bentonite (Sodium montmorillonite), either bonded to a carrier geomembrane or encased between two carrier geotextiles. The geosynthetic clay liner is also primarily used as an infiltration barrier due to its high resistance to flow perpendicular to the plane of the material.
Drainage cores are either nets composed of strands of polymeric materials or membrane-like panels and sheets with raised dimples or pedestals. When combined with a filter geotextile(s), a drainage core provides a relatively open channel for flow parallel to the plane of the core. When the drainage core and surrounding geotextile material are delivered bonded together as a single product, it is referred to as a drainage geocomposite.
Geogrids are nets or webs of high strength polymeric material used in earth reinforcement applications. Geocells are diamond-shaped cells fabricated into a sheet by linking together relatively stiff rectangular panels of polymeric material at regular intervals. Geocells provide erosion resistance by retaining soil within the cells. Geocells can also be used for earth reinforcement (e.g., to build retaining walls) or filled with concrete to form erosion resistant channel linings. Erosion control nets are open planar nets of polymeric threads and strands that hold soil in place, typically with the aid of vegetation that grows through the stands and secures the underlying soil.
Geotextiles Geotextiles are fabrics made from polymeric fibers. According to Koerner (2), over 95 percent of geotextiles are made of polypropylene or polyester, with the balance made primarily of polyethylene or nylon.
Individual fibers are sometimes twisted or spun together to form larger (thicker) strands known as yarn. The fibers or yarns are formed into geotextiles using either woven or non-woven methods.
Woven geotextiles, illustrated in Figure 1, are manufactured using traditional weaving methods and a variety of weave types. Non-woven geotextiles, illustrated in Figure 2, are manufactured by placing and orienting the fibers or yarns on a conveyor belt and bonding them by needle punching (“needle-punched”) or by heat bonding (sometimes referred to as “spunbond”). The needle-punching process consists of pushing numerous barbed needles through the fiber web, thereby mechanically interlocking the fiber into a stable configuration. In heat bonding, the fibers are heated to the point of melting and pressed together.
Geotextiles are typically provided in rolls approximately 4 m wide and from 30 to 60 m in length. Geotextiles may be seamed in the field by sewing, lystering (heat-bonding), or simple lapping. For many applications, seam strength is not important and simple lapping is sufficient. However, there are applications where the seam is required to have some strength, and sometimes seaming is useful simply to maintain the geotextile overlap until it is secured by the soil overburden. Lystering is a relatively rapid and expedient seaming process. However, lystering can weaken the geotextile, adversely affecting its performance, if too much heat is applied. Therefore, most field seams are sewn. Recently, a new generation of advanced micro-processor controlled lystering devices has been developed that significantly reduces the risk of over-heating a seam during lystering. To eliminate field seaming concerns, large geotextile panels tens of meters in dimension can be fabricated in the factory (or off-site) and delivered to the field folded in a manner that expedites deployment.
Geomembranes Geomembranes are flexible planar sheets of polymeric material.
Geomembranes may be smooth or textured, may be composed of variety of different polymers, and come in a range of thicknesses. Geomembranes are typically between 0.75 mm and 2.5 mm thick and are provided in rolls, as illustrated in Figure 3. Geomembranes are most often employed as liquid or vapour barriers due to their very low permeability. The most common types of geomembranes are high-density polyethylene (HDPE), low-density or very flexible polyethylene (VFPE), polypropylene (PP), polyvinyl chloride (PVC) and reinforced chlorosulfonated polyethylene (CSPE). However, there are a wide variety of other types of specialty geomembranes that are commercially available.
Geomembranes rolls are typically approximately 4 m wide and from 30 to 60 m in length. Roll width is generally controlled by the manufacturing process, while roll length is limited by handling considerations.
Geomembranes can also be supplied to the field in panels fabricated in the factory or offsite from geomembrane sheets the same width as geomembrane rolls. Due to its high ductility, PVC can be fabricated offsite into panels tens of meters wide. Fabricated panels are folded for handling and delivery to the site. HDPE panels, on the other hand, are rarely more than one roll width due to the limited ductility of HDPE.
Field seaming techniques for geomembrane rolls and panels include fusion welding, extrusion welding, and gluing. In fusion welding, two pieces of HDPE are joined together by heating them under pressure. The most common fusion weld is a “double track weld” that creates two parallel lines along with the geomembrane is fused together. This type of weld allows for non-destructive air pressure testing of the seam integrity along its entire length. Extrusion welding involves placing a molten bead of material (extrudate) along the seam. The extrudate bonds to both pieces of material as it cools, joining them together. In a glued seam, the adjacent pieces are joined together by an adhesive. Seams can also simply be lapped without physically joining the pieces.
Fusion welding is the preferred method of geomembrane seaming for environmental applications because it provides a high strength, redundant seam that facilitates non-destructive integrity testing. In areas where fusion testing is not possible (e.g., at corners and connections), extrusion welding is generally employed. Gluing is not generally used with HDPE or PP geomembranes due to their low ductility (which makes it difficult to glue pieces that are not perfectly flat), concerns over seam longevity, and, on environmental projects, concern over the volatile organic compounds present in most glue. Lapping is only used in non-critical situations, as it does not create a good barrier to moisture or vapor migration unless both pieces are perfectly flat and a relatively high normal stress is applied at the connection.
Geosynthetic Clay Liners A geosynthetic clay liner (GCL) is a sheet of dry, granulated sodium montmorillonite clay, commonly referred to as bentonite, which is either glued to a carrier geomembrane or secured between two carrier geomembranes by needle punching or stitching. Figure 4 illustrates these two types of GCL. Figure 5 shows a needle punch-reinforced fabric encased GCL.
The bentonite layer in a GCL is typically 6 mm thick. GCLs are the primary alternative to geomembranes for creating a hydraulic barrier with geosynthetic materials: due to the low permeability of bentonite: a 6-mm thick GCL is roughly hydraulically equivalent to 1 to 2 meters of the type of clay soil typically used in landfill liners and covers. GCLs are typically supplied in rolls of dimensions similar to geomembranes. However, rolls more than 30 m in length are difficult to handle due to their weight (a 30 m long roll of GCL weight approximately 0.75 tonnes).
GCLs are seamed by simply lapping adjacent sheets, with powdered bentonite applied to the seam for some of the needle punched products.
Laboratory testing indicates that lapping in this manner produces a seam highly resistant to fluid flow as long as sufficient normal stress) typically the equivalent of 0.5 m of soil) is placed on the seam.