Hydraulic and earth engineering – Earth treatment or control – Shoring – bracing – or cave-in prevention
Reexamination Certificate
1997-04-28
2001-05-29
Graysay, Tamara (Department: 3672)
Hydraulic and earth engineering
Earth treatment or control
Shoring, bracing, or cave-in prevention
C405S262000
Reexamination Certificate
active
06238144
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to retaining wall systems and more specifically to a retaining wall system which includes fie rod assemblies for attaching the wall facing elements with a reduced height, compared to the overall wall height, to the confined fill layers of a separate stabilized earth (MSE) structure.
BACKGROUND OF THE INVENTION
Various methods have been used in the past to construct precast walls for retaining earth, soil, sand or other fill generally referred to as soil. As typical precast wall system is disclosed in U.S. Pat. No. 4,914,876 assigned to the Keystone Retaining Wall Systems Inc. by Paul J. Forsberg. The Keystone Patent illustrates a typical modular block wall system wherein the wall face is comprised of concrete masonry units connected to the geosynthetic wall reinforcement layers. The geosynthetic tensile inclusion members for this type of retaining wall structure are typical referred to as “geogrids”.
A disadvantage of such a system is that a considerable amount of hand labor is required to install the numerous small block facing units. This limits the amount of wall structure that can be completed in any work shift. In addition, if the wall is placed on weak foundation soils, the manifestation of wall settlement is cracking or more significant crushing or crumbling of the facing units. If the settlement is excessive, the geogrid material can be sheared at the concrete masonry unit horizontal joints which can result in wall failures.
Numerous other types of concrete block mechanically stabilized earth (MSE) wall systems are available. All of these products, such as the Keystone wall type previously described, mandates precise grading and compacting of the wall backfill to correspond to increments of the vertical height of the block facing units so that the tensile inclusion material used to mechanically reinforce the retained wall backfill material will be at the horizontal joint elevation of the concrete masonry units. Although the material costs for these types of wall systems are low, due to the high labor costs of various stages of the wall construction for the these systems the resultant installed price of walls constructed with these products can be substantially higher than the material costs.
Another broad range of mechanically stabilized walls include walls that use precast concrete panels for the wall facing elements such as walls that utilize components provided by the Reinforced Earth Company of Arlington VA. U.S. Pat. No. 4,961,673 issued to Pagano et al. along with U.S. Pat. Nos. 3,421,326; 3,686,873; 4,0425,965 and 4,116,010 to Vidal describe such a wall system. Wall systems such as the Reinforced Earth products and those of the VSL Company, U.S. Pat. No. 4,725,170 by Edgar Davis, require the use of metal reinforcing strips or steel grids to be used as soil inclusion members in the wall backfill and to be connected to the precast wall panels to hold the panels in place and to provide stability for the wall backfill.
All of these types of wall systems require that the facing panels be placed on a continuous cast in place leveling pod. The elevation of the foundation pad for these systems corresponds to the base elevation of the MSE wall structure. The base of all retaining walls, either cast- in-place or MSE, is typically required to be depressed with respect to the final: grade in the front if the wall for geotechnical stability or for frost protection. Heretofore all wall systems currently in use have a bottom of wall facing elevation that corresponds to the bottom or base elevation of the MSE reinforcement elevation. In addition the facing elements of these systems are required to be installed simultaneously with the placement of the wall backfill and soil reinforcement.
A disadvantage of MSE walls that use metal soil reinforcement is that the metal soil tensile inclusion members used are subject to corrosion since the metal is in direct contact with the wall backfill. Numerous catastrophic failures have resulted from the effects of unchecked corrosion on the metal tensile inclusion members for these types of wall systems. Although the metal strips or steel grids can be galvanized to reduce the effects of the oxidation process this technique is not effective for all soil types due to the diverse mineral content present in some soils. Other methods such as epoxy coating for the metal soil reinforcing members have been used to further resist the deleterious effects of potential chemical reactions of the minerals present in the soil in contact with the soil reinforcement. A disadvantage of the epoxy coating is that the coating is easily scratched during the construction process which result in the exposure of the steel or metal soil reinforcement to the corrosive effects of the minerals present in the backfill. Also, epoxy coatings increase the costs of these systems.
Since the wall facing components in all precast panel or concrete masonry unit wall systems currently in use are installed simultaneously with the wall backfill, another disadvantage of these systems, besides the need for close backfill placement tolerances, is the fact that a portion of the soil mass adjacent to the wall facing units does exert a horizontal force on the face.
Typical wall facing units for existing MSE systems in current use may range in size from 8″×16″ for block systems to 25 to 50 sq. ft. for precast panel wall systems. The concrete masonry block systems, due to the high unit weight and relatively small size of each block, do not require bracing or interlocking to hold the face units in a vertical position as the wall backfill is placed. Since the blocks are heavy (exceeding 100 pounds for some applications) the placement of the blocks is physically demanding which adds to the placement cost of the facing units. For currently available MSE wall systems that use panels for facing units the panels are large in size compared to the block facing units and the panels (typically between 25 to 80 sq. ft. in area) are held in place during backfilling operations by interlocking with the previously placed or adjacent panels. For some systems the facing units are “wedged” or leaned by other methods so that the effect of the interaction of the backfill pressure and the metal soil reinforcement will, in theory, force the panels into a plumb or vertical position. Panel placement for these systems require skilled experienced workers to erect the units so that the resultant structure will be vertical and not leaning either in or out of a vertical plane.
Full height panels have been used on MSE walls where the MSE layers are connected to the wall face. Temporary erection braces are required for these system to hold the panels in place as the backfill is placed behind the wall. This requires additional working right-of-way in front of the wall and restricts site access. Since the soil reinforcement material, whether geosynthetic or metal, is not designed for concentrated high loads at the connections of the soil reinforcement material it is critical that all panel connections should, in theory, have quantifiable uniform loads. This condition is extremely difficult, if not impossible, to achieve in the field. This is one of the primary reasons why few full height MSE panel walls have been built with precast face units. An indeterminacy situation exists for the load determination at the numerous connections of the soil reinforcement material to the panels for these types of walls since typically the number of soil reinforcement connections to the wall facing exceeds the number of equations available to solve for the individual connection loads.
There is a portion of retained soil loading on the wall face in full height and all MSE panel systems currently in use and any vertical settlement (relative motion) between the tensile inclusion soil reinforcement layers and the panel face can induce excessive shear loads on the soil reinforcement material at the connection point to the panel. Typically there is no adequate provi
Graysay Tamara
Lagman Frederick L.
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