PREFABRICATED MODULAR WALLS

There are also a number of prefabricated modular wall systems in use. Such systems are generally composed of modules or bins filled with soil, and function much like gravity retaining walls. The bins may be of concrete or steel, and can be used in most cases where conventional gravity, cantilever, or other wall systems are considered. AASHTO indicates that such walls should not be used on curves less than 800 ft in radius, unless a series of chords can be substituted; or where the calculated longitudinal differential set­tlement along the face of the wall is excessive. Also, durability considerations must be addressed, particularly where acidic water or deicing spray is anticipated.

8.9 MSE BRIDGE ABUTMENT WALLS

The abutment wall is an earth-retaining wall supporting traffic surcharge load and heavy loads from the bridge superstructure. The geosynthetic-reinforced soil (GRS) earth-retaining wall is a subset of the MSE wall. The technology of GRS systems has been used extensively in transportation systems including earth-retaining walls, road­way pavement subgrades, and foundation improvement for heavy traffic loads such as bridge abutments. The increasing use and acceptance of geosynthetic soil reinforce­ment has been triggered by a number of factors, including cost savings, aesthetics, simple and fast construction techniques, and excellent performance. A comparatively new application of this technology is the use of GRS in bridge abutments and roadway approaches. When compared to conventional bridge substructures involving the use of deep foundations to support bridge abutments, the use of geosynthetic-reinforced soil systems has the potential for alleviating the “bump at the bridge” problem caused by differential settlements between the bridge abutment and the approaching roadway. It is especially effective where the GRS can be extended beyond the typical rectangular reinforcing zone of the wall and truncated gradually into a trapezoidal reinforcing zone toward the approach roadway. The FHWA published preliminary design details for bridge superstructures directly supported by MSE walls with panel facings and steel reinforcements in 1997 (Elias and Christopher, 1997), and it was included in the AASHTO 1998 Standard Specifications for Highway Bridges. A recently published FHWA report (FHWA, 2000) describes three studies on GRS bridge-supporting structures: a load test of the Turner-Fairbank pier in McLean, Virginia, in 1996; a load test of the Havana Yard piers and abutment in Denver, Colorado, in 1996-1997; and a study of a production bridge abutment constructed in Black Hawk, Colorado, in 1997. These studies have demonstrated excellent performances with negligible creep deformations of GRS bridge-supporting structures constructed with closely spaced reinforcements and well-compacted granular backfill. The maximum surcharge pressure was 4.2 kip/ft2 (200 kPa). This FHWA report concluded that the GRS abutments are clearly viable and adequate alternatives to bridge abutments supported by deep foundations or by metallic reinforced soil abutments. A complete literature overview of studies on GRS structures supporting high-surcharge loads has been presented (Abu-Hejleh et al., 2000).

The most prominent GRS abutment for bridge support in the United States is the new Founders/Meadows Parkway structure, located 20 mi south of Denver, Colorado, which carries Colorado State Highway 86 over U. S. Interstate 25 (Fig. 8.60). This is the first major bridge in the United States built on spread footings supported by a concrete block facing geosynthetic-reinforced soil system, eliminating the use of traditional deep foundations (piles and caissons) altogether. Figure 8.61 shows the bridge super­structure supported by the “front GRS wall,” which extends around a 90° curved corner in a “lower GRS wall” that supports a “concrete wing wall” and a second-tier “upper GRS wall.” Figure 8.62 shows a plan view of the completed two-span bridge and approaching roadway structures. Each span of the new bridge is 113 ft (34.5 m) long and 113 ft (34.5 m) wide, with 20 side-by-side, precast, prestressed, concrete box girders. There are three monitored cross-sections (sections 200, 400, and 800) along the faces of the “front GRS wall” and “abutment wall.” Figure 8.63 depicts a typical monitored cross-section with various wall components and drainage features in the backfill. To keep the water out of the GRS, several drainage systems were used in the trape­zoidal extended reinforcing zone, including an impervious membrane and collecting drain at the top and a drainage blanket and pipe drain near the toe of the embankment cut slope. Figure 8.63 also illustrates how the bridge superstructure loads (from bridge deck to girders) are transmitted through the girder seat to a shallow strip footing placed directly on the top of a geogrid-reinforced concrete block facing earth retaining wall. The centerline of the abutment bearing and edge of the footer are located 10 ft (3.1 m) and 4.4 ft (1.35 m), respectively, from the facing of the front GRS wall. This short reinforced-concrete abutment wall supports the bridge superstructure, including two winged walls cantilevered from the abutment. This wall, with a continuous neoprene sliding and bearing interface at the bottom, rests on the center of the spread footing.

Bridge Structure

East

FIGURE 8.62 Plan view of the Founders/Meadows structure showing the locations of monitored sections (200, 400, and 800) and construction phases (I and II). (From Research Report, Colorado Department of Transportation, Denver, Colo., with permission)

It confines the reinforced backfill soil behind the bridge abutment (see Figs. 8.61 and 8.63) and also partially supports the bridge approach slab. The old bridge was maintained in service during construction of the new bridge. A phased construction of the almost 9-m-high (29-ft), two-tier, U-shaped abutments began in July 1998, and the bridge was completed in just 12 months. This included a temporary bracing at the south side of the replaced bridge and a temporary GRS wall at the north of phase 1 of the partial new bridge. The Colorado Department of Transportation (CDOT) designed the Founders/Meadows structure in 1996, 1 year before the FHWA report of Elias and Christopher (1997) was published. It was expected that water could be kept out of the claystone bedrock formation below the base of the reinforced backfill, and that the use of an extended reinforced transition zone (Fig. 8.63) would lead to adequate overall stability for the structure and minimize settlements of the GRS wall system.

Several of the common causes for development of bridge bumps were avoided or eliminated completely in the design of the Founders/Meadows structure. The main cause of uneven settlements in typical systems is the use of different foundation types. While the approaching roadway structure is constructed on compacted backfill, deep foundations such as steel H piles typically are adopted to transfer the heavy bridge abutment loads to bedrock or competent bearing strata. At the Founders/Meadows structure, in order to minimize uneven settlements between the bridge and the approaching roadway, the approach embankment and the bridge abutment wall backfills were further integrated into an extended reinforced soil zone (Fig. 8.63). A second cause of differential settlements is erosion of the fill material behind and around the abutment wall induced by the surface water runoff collected from the bridge deck.

Several measures were implemented to prevent the peculation of surface water, as well as intercepted groundwater, from reaching the reinforced soil mass and the bedrock at the base of the fill (e. g., placement of impervious membranes with collector pipes as shown in Fig. 8.63). Finally, a third potential cause of differential settlements is sea­sonal temperature changes, which may induce expansion and contraction of the bridge superstructure with the abutment back wall pushed against the reinforced fills. A 3-in-thick (75-mm) low-density, compressible expanded polystyrene sheet spacer was installed between the reinforced backfill and the abutment back walls (see Fig. 8.63). It was expected that this system would isolate the thermally induced cyclic movements of the bridge superstructure without affecting the retained reinforced backfill.

The performance of bridge structures supported by GRS abutments had not been tested under actual service conditions to merit acceptance without reservation in highway construction. Consequently, the Founders/Meadows structure was considered experi­mental and comprehensive material testing, instrumentation, and monitoring programs were incorporated into the construction operations. Three sections of the GRS system were instrumented to provide information on the structure movements, soil stresses, geogrid strains, and moisture content during construction and even after opening the structure to traffic. The results gathered from the instrumentation program verified the suitability of CDOT and AASHTO design procedures and assumptions regarding the use of GRS walls to support bridge abutments and as a measure to alleviate the bridge bump problem. Several CDOT research reports present a summary, analysis, and assessment of all the movement results of the Founders/Meadows structure, collected at various construction stages and while the structure was in service for approximately 35 months after opening to traffic. These results include displacements of the front wall facing, settlement of the bridge footing, and differential settlements between the bridge and approaching roadway structures. These reports also provide recommendations for future design and construction of GRS abutments directly supporting bridge and approaching roadway structures. The unique features of the Founders/Meadows Bridge, and other perceived advantages of GRS walls commonly used in Colorado, as well as the excellent performance of full-scale GRS abutments and piers in monitoring programs, convinced CDOT engineers of the feasibility of GRS earth-retaining walls and abutments.