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Article Excerpt
Conventional treatments for streambank stabilization such as riprap revetments and concrete retaining walls have been criticized for their environmental effects (Simpson, 1982) and poor aesthetics (Riley, 1998). Alternative approaches relying on the stabilizing properties of vegetation, alternatively known as soil bioengineering (Schiechtl, 1996), biotechnical engineering (Gray and Sotir, 1996) and phytostabilization (Berti and Cunningham, 2000), have been known for many years (Gray and Leiser, 1989; Schiechtl, 1980). The role played by vegetation in enhancing soil stability is well recognized, and comprehensive reviews may be found in several publications (e.g., Greenway, 1987; Gray and Leiser, 1989; Coppin and Richards, 1990; Gray and Sotir, 1996). Many studies have analyzed roots as soil reinforcement. These include laboratory shear tests of soils with roots (Waldron, 1977; Goldsmith, 2002), in situ shear tests on soil blocks with roots (Nilaweera, 1994), and laboratory plate load tests of mine waste tailings with roots (Silva et al., 1998a; Silva et al., 1998b). These studies showed increases in shear strength due to soil-root interaction. Mechanically, vegetation increases the strength and competence of the soil by root reinforcement. A root-permeated soil behaves as a composite material in which fibers of relatively high tensile strength are embedded in a matrix of lower tensile strength.
Despite this knowledge of the bioengineering techniques and soil-reinforcing properties of vegetation, the above-mentioned, non-vegetative conventional treatments still dominate (Li and Eddleman, 2002). While bioengineering seems to be gaining popularity where perceived risks of failure are low (such as in rural settings or small streams), the approach remains uncommon near critical infrastructure such as highways, landfills, or remediation sites.
Soil bioengineering projects have been rarely documented in peer-reviewed literature. One reason for this is that stream restoration projects are generally poorly monitored and documented. Bernhardt et al. (2005) report that documented monitoring is performed on roughly 10 percent of projects, with a smaller subset reporting on these results in the literature.
"Green" or ecological engineering approaches in general, and vegetative-based stabilization treatments in particular, are attracting attention because their perceived environmental and social benefits, including lower cost (Watson et al., 1999), water quality improvement, fish and wildlife habitat, aesthetics, recreation potential, acceptance by the public, and easier permitting by regulatory agencies (Barrett, 1999).
Vegetative-based treatments also may perform as well as or better than hard structures, especially over the long term. Li and Eddleman (2002) cite numerous failures of hard-armor stabilization treatments like riprap or concrete. Bioengineering treatments, on the other hand, make use of the self-maintaining properties of ecological systems (Mitsch and Wilson, 1996), which can help a streambank repair itself after a stressful event like a large flood. Watson et al. (1999) document a project in which willow posts were used to stabilize eroding streambanks using an "experimental bioengineering technique." They found the treatment "to be successful in bank stabilization for the period of monitoring, in comparison with more traditional riprap stabilization methods."
Modern practitioners of bioengineering recognize that vegetation can be integrated with conventional geotechnical engineering materials to take advantage of the strengths of each type of treatment while minimizing its weaknesses (Henderson, 1986; Elias et al., 2001; Di Pietro and Brunet, 2002). Materials such as geotextiles and rock can provide stability in high-stress settings such as very steep slopes and the toe of streambanks. These materials can also provide initial protection against erosion while newly planted vegetation develops its full root system. Initial protection is important because the period in which vegetation has not fully developed has been identified as vulnerable to failure (Simon and Steinemann, 2000). Vegetation, on the other hand, complements geotextiles and rock by masking their visual unattractiveness and, when planned and designed correctly, can provide wildlife habitat and water quality improvement. Furthermore, as the vegetation does develop, roots help stabilize the geotechnical materials.
In the project described herein, the objective was to integrate bioengineering with geotechnical methods and materials to provide an aesthetically acceptable and economically feasible restoration and stabilization of an eroding streambank along a sanitary landfill. The article describes pre-project conditions, design of stabilization treatments (including a new leachate collection system), construction phase issues, and post-construction performance.
Methods and Materials
Pre-project conditions. The Center Hill Landfill is located in the City of Cincinnati, Ohio (39[degrees]12'N, 84[degrees]28'W; ~1.7 million metro population). Available geologic mapping (Osborne, 1974) shows that the site is underlain by a combination of recent alluvial silt, sand and gravel and Pleistocene aged fluvial gravel, sand, silt and clay. These materials may comprise dissected terraces, abandoned river channels, glacial lake deposits and one or more till units. Within the project area, dense clays derived from lake deposits form the lower half of the streambank profile, with a mixture of alluvial gravels and waste materials forming the upper half.
The landfill was owned and operated by the City for disposal of residential, commercial and industrial waste from 1955 until its closure in 1977. A compacted soil cap 0.6 to 1.2 m (2 to 4 ft) thick was added after closure. Future development plans call for industrial/commercial land use on site, and the adjacent property immediately upstream on the Mill Creek has already been developed in this manner.
The landfill is located along Mill Creek on the outside bend of a slight meander (radius of curvature ~150 m or 500 ft; Figure 1). Two gravel-filled trenches were installed along the streambank to intercept leachate from the landfill. The trenches were about 7.5 m (25 ft) deep. extending to the interface with the relatively impermeable lacustrine clay layer. A pump removed leachate from the bottom of the trenches into a sanitary...
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