Patent Application
20090050025
The present disclosure is related to construction, particularly, the use construction of structures that may be exposed to water, such as roads, levees, etc.
Phosphogypsum (with a chemical formula of CaSO4.2H2O) is often produced as a by-product of phosphoric acid production at fertilizer plants. Phosphogypsum is generally considered a waste product primarily because contaminated process water is entrained within the gypsum pores (and because there has been some expressed concern about long term exposure to relatively low levels of radioactivity). On the other hand, phosphogypsum that has been leached from contaminants by rainfall infiltration or by washing with fresh water is generally a suitable construction material with excellent frictional characteristics and essentially no adverse health effects except possibly when used in confined or enclosed spaces.
Leached phosphogypsum has already been used in experimental road bases, and the feasibility of its use in road construction in lieu of limestone aggregates is being actively considered (for example, by the State of Florida DOT). Moreover, there has been discussion about the potential use of leached phosphogypsum as fill material for raising flood protection levees (e.g., in Louisiana). Because phosphogypsum is lighter than clay, has a much higher effective angle of internal friction and is much easier to handle, leached phosphogypsum could present significant benefits in raising levees constructed on soft foundation soils.
It is a known fact that phosphogypsum (CaSO4.2H2O) is quite soluble in fresh water (about 2 grams per 1000 cc). That solubility raises some concerns when phosphogypsum is used as fill material to raise levees retaining water, or if it is used as construction material in a road base on a pervious foundation because the protective asphaltic or concrete surface course is susceptible to shrinkage cracking over time. The solubility of phosphogypsum results in a loss of material over time, potential piping associated with preferential flow paths, and potential increased sulfate and calcium concentrations in the receiving waters, particularly groundwaters, unless the surface course is regularly maintained and frequently resurfaced. Therefore, a need exists to develop alternative methods to promote beneficial use of the phosphogypsum.
These and other aspects of the present disclosure will become more apparent to those skilled in the art from the following non-limiting detailed description of exemplary embodiments taken with reference to the accompanying figures.
The present disclosure relates generally to the use of a water soluble material as a construction material, particularly, the use of phosphogypsum or other similar products as a construction material.
One embodiment relates to encapsulating phosphogypsum used as construction fill in order to control damage to the structure, and deterioration or softening attributed to the solubility of phosphogypsum in flood waters (e.g., in the case of a levee) or due to infiltrating rainfall (e.g., in the case of a road base on a pervious foundation). To achieve this objective, the leached phosphogypsum would be covered or encapsulated by a relatively impervious geomembrane.
In accordance with an exemplary embodiment of the present invention a system and method are provided to use phosphogypsum as a construction material. In accordance with another embodiment of the present invention, a geomembrane is provided to ensure that the phosphogypsum remains relatively free from water.
In accordance with an example of an exemplary embodiment of the present invention, phosphogypsum is used in the construction of a road. The phosphogypsum may be used to create a base or subbase of the road. In addition, the phosphogypsum layer would be covered or encapsulated by a geomembrane. For a road base constructed above the water table, the geomembrane (with or without a geofabric) would cover the upper surface of the gypsum fill and would be placed below the asphaltic or concrete surface course to curtail rainfall infiltration into the gypsum fill.
In accordance with another example of an exemplary embodiment of the present invention, phosphogypsum may be used as a compacted fill material in the construction of a levee. In addition, a geomembrane would be used to separate the gypsum fill material from at least a portion of an external layer which is potentially in contact with water. In another embodiment of the present invention, the geomembrane may separate the fill material from at least a portion of an external layer and the foundation. In yet another embodiment of the present invention, the geomembrane may substantially encapsulate the entire phosphogypsum fill material. For a levee embankment, therefore, a textured geomembrane would be used to cover the upper surface and the upstream face of the gypsum fill and would be anchored in natural ground clays. If the levee foundation is not clayey, the geomembrane would be used to entirely wrap the base, upstream slope, and top surface of the compacted phosphogypsum fill. If erosion protection from overtopping is desired, the entire gypsum fill would be encapsulated by the geomembrane. A thin veneer of grassed clay would be used over the textured geomembrane encapsulating the phosphogypsum.
The disclosure is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements.
The following description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding of the disclosure. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the description. References to one or an embodiment in the present disclosure can be, but not necessarily are, references to the same embodiment; and, such references mean at least one.
Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.
According to one embodiment of the present invention, a body of a water soluble material can be used for a variety of purposes, including, but not limited to, roads, levees, etc.
According to exemplary embodiments of the present invention, the water soluble body is constructed using phosphogypsum leached by rainfall or washed with fresh water, designated herein as leached phosphogypsum.
In yet another embodiment of the present invention, the phosphogypsum layer is covered by another separate relatively impervious layer. Preferably, this layer greatly reduces the amount of water that may reach the phosphogypsum layer, since phosphogypsum is soluble in fresh water (with a solubility of approximately 2 grams per 1000 cc). This solubility may raise concerns about the stability of phosphogypsum in certain projects where percolating water may reach the phosphogypsum or where the percolating water through the phosphogypsum may increase sulfate concentrations in the receiving groundwater. More preferably, this outer layer may consist of an impervious geomembrane or liner. It should be noted, however, that any material now known, or hereinafter developed, may be used in the outer layer to reduce that amount of water that reaches the phosphogypsum layer.
An example of a geomembrane in accordance with exemplary embodiments of the present invention is the use of a liner such as that used in landfills. More particularly, a high-density polyethylene plastic liner may be used as the geomembrane. In accordance with aspects of exemplary embodiments of the present invention, the liner may be chemically resistant to corrosion, degradation and damage. In addition, individual sections of the liner may be seamed, joined or welded together, thus ensuring that a continuous seal is created.
Exemplary embodiments of the present invention, in which phosphogypsum is used in roadways and levees, are discussed in greater detail below.
With reference to
The phosphogypsum base 101 may lie on top of a compacted subbase 102. The compacted subbase 102 may lie on top of a natural foundation or may be part of an improved natural foundation.
With continued reference to
In yet another embodiment of the present invention, a protective geofabric layer 104 may be used to cover the phosphogypsum base 101. The woven or non-woven geofabric may comprise synthetic, felt-like fibers such as polyester, polypropylene or nylon fibers. In addition, an optional thin layer of limerock may be added on top of the geofabric to further protect the geomembrane 103 during placement of the surface course 105. Road surface layer 105 lays on top of geomembrane 103, or the geofabric layer and optional thin layer of limerock 104 if present. The road surface layer may comprise any material, either now known or hereinafter developed, that is used for pavement on roads. Examples of such surfaces include concrete, asphalt, and a combination of the two.
The phosphogypsum base 201 may lie on top of a natural foundation 202. The foundation 202 may be prepared for the addition of the phosphogypsum subbase 201. Examples of the preparation of the foundation may include, but is not limited to, leveling, compacting, and excavating.
In accordance with aspects of the present invention, a geomembrane 203 would be used to cover the phosphogypsum subbase 201. It is preferable that the geomembrane 203 be impermeable, and allow as little water as possible to reach the phosphogypsum subbase 201. An example of an exemplary geomembrane in accordance with aspects of the present invention is a liner such as that used in landfills. More particularly, a high-density polyethylene plastic liner may be used as the geomembrane. In accordance with aspects of exemplary embodiments of the present invention, the liner may be chemically resistant to corrosion, degradation and damage. It should be noted, however, that any material now known or hereinafter developed may be used as the geomembrane, as long as it assists in preventing water from reaching the phosphogypsum subbase 201. The geomembrane may be also positioned so that phosphogypsum subbase 201 does not come into contact with other layers above the phosphogypsum subbase, such as, for example, the road surface layer 205. In exemplary embodiments of the present invention, individual sections of the liner may be joined, seamed or welded together, thus ensuring that a continuous seal is created. The geomembrane 203 may also be anchored into the compacted natural foundation 202. The geomembrane 203 covers the subbase 201 from above and is anchored into the natural foundation 202. In one embodiment, the edges of the geomembrane are anchored and buried to a depth similar to that of the ditches at the sides of the road.
In accordance with aspects of an exemplary embodiment of the present invention, a conventional limerock base 204 may lie on top of the geomembrane 203.
Road surface layer 205 may lay on top of limerock base 204. The road surface layer may comprise any material, either now known or hereinafter developed, that is used for pavement on roads. Examples of such surfaces include concrete, asphalt, and a combination of the two. In addition, any method now known, or hereinafter developed, may be used to place the road surface layer 205 onto the road system 200.
Although in one embodiment of the present invention, the foundation 305 comprises a surface that is relatively flat, it should be noted that the foundation 305 does not need to comprise a flat surface and may in fact represent an existing levee which is to be raised.
Compacted phosphogypsum 301, as shown in
With continued reference to
In
In exemplary embodiments of the present invention, individual sections of the liner may be joined, seamed or welded together, thus ensuring that a continuous seal is created. The geomembrane may also comprise a textured geofabric, which may comprise synthetic, felt-like fibers in addition to a geonet of mesh-like plastic.
An external layer 303 covers the outer edge of the levee system 300. The external layer may be comprised of a compacted clay based material.
According to aspects on an exemplary embodiment of the present invention, the geomembrane 402 may comprise a liner such as that used in landfills. More particularly, a textured high-density polyethylene plastic liner may be used as the geomembrane. In accordance with aspects of exemplary embodiments of the present invention, the liner may be chemically resistant to corrosion, degradation and damage. It should be noted, however, that any material now known or hereinafter developed may be used as the geomembrane, as long as it assists in preventing water from reaching the phosphogypsum 401.
In exemplary embodiments of the present invention, individual sections of the liner may be joined, seamed or welded together, thus ensuring that a continuous seal is created. The geomembrane may also comprise a textured geofabric, which may comprise synthetic, felt-like fibers in addition to a geonet of mesh-like plastic.
The geomembrane 402 does not have to cover the entire bottom of the phosphogypsum fill 401. For example, as long as the combination of the covered bottom portion and the buried depth of the edge prevents water from seeping into the gypsum fill, the edge of the geomembrane 402 away from the body of water 410 may be buried into the foundation 405.
In addition,
According to aspects on an exemplary embodiment of the present invention, the geomembrane 502 may comprise a liner such as that used in landfills. More particularly, a textured high-density polyethylene plastic liner may be used as the geomembrane. In accordance with aspects of exemplary embodiments of the present invention, the liner may be chemically resistant to corrosion, degradation and damage. It should be noted, however, that any material now known or hereinafter developed may be used as the geomembrane, as long as it assists in preventing water from reaching the phosphogypsum 501.
In exemplary embodiments of the present invention, individual sections of the liner may be joined, seamed or welded together, thus ensuring that a continuous seal is created. The geomembrane may also comprise a textured geofabric, which may comprise synthetic, felt-like fibers in addition to a geonet of mesh-like plastic.
In the foregoing specification, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than restrictive sense, and all such modifications are intended to be included within the scope of the present invention.
