Abstract:
A system and method removes contaminated sediments during mechanical dredging operations, while maintaining a predetermined low moisture content of the contaminated sediments. The system uses a specially designed positive pressure controlled enclosure or enclosed excavator that houses a mechanical bucket and prevents ambient water from contacting the sediment during the excavation process.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/812,996, filed on Mar. 20, 2001, now U.S. Pat. No. 6,432,303; which claims the benefit of U.S. Provisional Application No. 60/190,809, filed on Mar. 21, 2000 and of U.S. Provisional Application No. 60/197,551, filed on Apr. 17, 2000. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This application relates to a process for removing contaminated sediments from the bottom of lakes, reservoirs, rivers, streams, and other water bodies, while at the same time minimizing the moisture content present in these extracted sediments. 
     BACKGROUND OF THE INVENTION 
     Current methods of dredging can be divided into two general categories. They include mechanical dredging and hydraulic dredging. The fundamental difference between these categories is in the form in which the sediments are removed. Mechanical dredges remove the sediments directly with clamshell-type buckets. The operation consists of lowering the bucket with a crane to the bottom of the waterway, scooping or extracting the sediment, and bringing the sediment to the surface for disposal (typically in a dredge barge). Hydraulic dredges, sometimes referred to as vacuum dredges, are designed to vacuum up bottom sediments. Unless the sediments are very loose, vacuum dredges require cutter heads or alternative means to dislodge the dredge material so that the sediment can be vacuumed into the dredge head. Mechanical dredging operations typically yield much lower liquid to solid ratios (30 to 70 percent by weight) compared to hydraulic dredges (less than 1 to 10 percent by weight). 
     Due to the very high cost of contaminated sediment removal and disposal, which is typically priced on the basis of $ per unit volume or $ per unit weight (e.g., $ per cubic yard or $ per ton), the presence of moisture in the sediment will unavoidably raise the price of a dredging cleanup. The higher the moisture content, the higher the cost of disposal. 
     As previously noted, mechanical dredges tend to produce less moisture per weight of mud (lower liquids to solids ratio), but the sediment is exposed to the water column during the excavation process, which tends to supersaturate the excavated sediment. Due to the very high moisture content associated with hydraulic or vacuum dredging operations, the muds collected from such operations typically employ costly dewatering processes to try to reduce the quantity of moisture in the mud, thereby reducing the overall disposal cost. 
     The invention being disclosed herein also relates to the development of a system equipped to collect sediment from the marine environment with minimal moisture content, thereby reducing the cost of disposal over that of conventional dredging methods. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of this invention to provide for the removal of sediments from the subsurface of a water body with low moisture, closely approximating the in-situ moisture content of the mud sample. 
     It is further an object of the present invention to improve over the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     In this embodiment, the system provides for housing a mechanical bucket in a specially designed enclosure into which positive pressure, equivalent to the hydrostatic pressure present at the corresponding submergence depth, is introduced into the enclosure to prevent water from entering the enclosure and providing the means to submerge, excavate and retrieve the sediment in a dry environment. As a result, such sediments can be recovered at a moisture content that approximates that of the in-situ condition of the sediment. 
     The operation of this system includes the following steps: 
     a) enclosing a dredge bucket within a sealed housing having an openable door; 
     b) providing a source of positive air pressure to said housing; 
     c) maintaining said positive air pressure at an amount equal to a predetermined hydrostatic pressure imposed by a surrounding water column surrounding said housing, thereby preventing the introduction of water from said surrounding water column into said housing; 
     d) submersing said housing having said bucket down to the in situ contaminated sediments to be dredged; 
     f) extending said bucket into said in situ contaminated sediments and collecting the contaminated sediments having said predetermined low moisture content into said excavator bucket; 
     g) retracting said bucket under positive pressure within said housing; 
     h) closing said openable door attached to said housing; 
     i) lifting of said positively pressurized housing with said bucket with to the surface with said dredged contaminated sediments having said low moisture content; and, 
     j) disposing of said contaminated sediments with said low moisture content to a disposal site. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention can best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic of the enclosed excavator, which consists of a mechanical dredge or bucket situated within a pressure-controlled enclosure; 
     FIG. 2 is a schematic of the enclosure&#39;s unique single-hinged door shown in a fully opened and fully closed position; 
     FIG. 3 is a schematic showing the pneumatic, pumping, hydraulic, and mechanical controls associated with the enclosure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, the process is affected by a clamshell-type bucket  10  housed within a pressure-controlled enclosure  20 . Both the enclosure and dredge bucket contained within the enclosure can be lowered with a lifting shackle  30 , and connected to a dredge bucket shaft  31  by means of a clevis and pin  33 . The dredge bucket shaft  31  guided into the top of the enclosure by a stuffing box  32  to prevent leakage into the shaft or loss of pressure inside the enclosure. The dredge bucket is opened and closed by means of a hydraulic piston  34 . 
     As shown in FIG. 2, in this sealed sediment excavator embodiment, the pressure-controlled enclosure  20  is outfitted with a rotating single-hinged door depicted in FIG. 2 in both the fully opened  21  and fully closed positions  22 . This door would typically be in the open position during the submersion cycle of the operation, and in the closed position following the excavation process or recovery cycle. The door would be closed as soon as the enclosure  20  is clear of the bottom. It remains closed as the enclosure is lifted from the bottom and reopened prior to the sediment load being discharged to a top surface dredge barge or other container designed to collect and dispose of the dredged material. 
     The single-hinged door can be opened and closed with a door control piston  23 , which is attached to piston support columns  24 . 
     As shown in FIG. 3, depicting the sealed sediment extractor embodiment, the rotating single-hinged door  21  is outfitted with a water pump line  25  and a submersible pump  26  or surface supplied pump (not shown) that can evacuate water that drains from the excavated sediment during the recovery cycle of the operation. A pneumatic pressure line  27  is incorporated into the design to control the air pressure within the enclosure. Access is provided for the hydraulic bucket control lines  13 , which are used to open and close the dredge bucket and the hydraulic door control lines  28 , which are used to drive the door control pistons. The hydraulic lines used to drive the pump and the hydraulic lines for the dredge bucket are not shown on the drawings, but are also be included as part of the system. 
     Operation of the system to remove solids, where the primary objective is to minimize moisture control of extracted sediments, can be effected with the aforementioned system where such a system is maintained in a pressurized mode equivalent or greater than that of the hydrostatic pressure imposed by the surrounding water column. 
     This positive air pressure is introduced into controlled enclosure  20  by pneumatic pressure line  27  connected to a source of air pressure continuously and sequentially applying a positive pressure equal to a predetermined external hydrostatic pressure imposed by a surrounding water column surrounding controlled enclosure  20 . This positively applied pressure is applied from the source of positively pressurized air during a first mode of deployment (submergence) of lowering the excavator bucket  10  within controlled enclosure  20  to the sediment sample at the bottom of the surrounding water column. The positively applied air pressure is also applied during a second mode (excavation) of the excavator bucket  10  excavating aquatic sediments having the predetermined low moisture content and also during a third mode (retrieval) of removal of the excavator bucket  10  containing the dredged aquatic sediments having the low moisture content. 
     Although the aforementioned particular embodiments are shown and described herein, it is understood that various other modifications may be made without departing from the scope of the invention, as noted in the appended claims.