Patent Publication Number: US-8985898-B2

Title: Pneumatic anchoring system for wick drains

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application derives priority from U.S. provisional application Ser. No. 61/547, 423 filed 14 Oct. 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the installation of wick drains in the construction industry and, more particularly, to an improved pneumatic anchoring system that releases the wick drain anchor plate at depth by a sudden high pressure surge of air, and maintains a predetermined flow rate within the drainage void thereby overcoming the pore pressure at depth. 
     2. Description of the Background 
     It is often necessary in construction projects to drain and consolidate unconsolidated earthen materials such as soft wet soil, silt, sand, ore and the like (herein generally referred to as material to be consolidated or soil). Natural drainage can take years, but this can be expedited by creating closely-spaced artificial vertical drainage paths through which the water can flow. Drainage can be accomplished in a matter of weeks. The most common form of artificial drainage paths are called wick drains, and each typically comprises a central polypropylene core surrounded by a geotextile sheath. A typical wick drain is approximately 4 inches wide, ⅛ inch thick, and up to 1,000 feet in length, carried on a roll. 
     Wick drains installed with specialized equipment called “stitchers.” A stitcher is mounted on a backhoe, crane or excavator, and comprises a vertical mast housing an installation mandrel. The wick drain material, which is flexible, is placed within the mandrel. The mandrel is driven into the earth by a vibratory hammer or static method. Once at the desired depth, the mandrel alone is removed from the earth leaving the wick drain material in place. The wick drain material collects pore water from excess pore water pressure in its vicinity to the surface to stabilize the ground at that point. 
     The mandrel is forcibly driven downwardly into the earth and retracted therefrom with a drive mechanism. Typically the wick drain material is anchored in the earth by a suitable anchor plate attached to the exposed end of the wick which keeps it in place at the lead end of the mandrel. The mandrel advances into the soil pushing the anchor plate into the soil, which in turn pulls the wick drain material from its supply. When the mandrel is fully advanced the anchor plate fixes the wick drain material in the soil ahead of the mandrel, and it remains fixed during mandrel withdrawal. 
     A primary disadvantage with this form of wick drain anchoring is that the anchors sometimes fail to take hold. The mandrel creates an annulus immediately in front, and within this annulus there is nothing for the anchor plate to take hold of. Once at depth the mandrel is extracted and the advancing anchor plate is released. If the pore pressure at the depth of anchoring is in excess of the pressure inside the mandrel the anchor plate will either come out right along with the mandrel or pull off the mandrel, and the excess pressure outside the mandrel will flood the mandrel causing the wick drain to be severed. 
     Accordingly, an important object of the present invention is to provide improved apparatus and method for injecting wick drain anchors into the ground. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide an improved pneumatic mandrel that releases the wick drain anchor plate at depth by a sudden high pressure surge of air and subsequent air flow, thereby overcoming the pore pressure at depth. 
     In accordance with the foregoing objects, the present invention comprises an improved pneumatic anchoring system for installation and anchoring of wick drains comprising a construction vehicle such as an excavator or crane having an elongate support mast defined by a slot along its length, and a hollow tubular wick drain mandrel slidably carried in the mast. The mandrel is equipped with a seal plate at its upper end to prevent escape of air. An anchoring system is mounted externally on the mast, attached to the mandrel and adapted to traverse the mast downward as the mandrel penetrates the soil. The anchoring system comprises a sled in cooperative engagement with the mast and fixedly attached to the mandrel, a compressed air storage tank riding on the sled, a solenoid valve for controlling an output of compressed air from the tank, and a Venturi in fluid communication between the tank and mandrel for admitting a jet of compressed air into the mandrel. When the mandrel is at its deepest point, actuation of the solenoid releases the wick drain anchor plate by a sudden high pressure surge of air which overcomes the pore pressure at depth. The pore pressure is continually overcome as the mandrel is retracting due to the constant CFM released into the mandrel. This prevents pore pressure from entering the mandrel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments and certain modifications thereof when taken together with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a Hyundai 380 excavation vehicle with erect mast supporting a wick drain mandrel (internally), and an external pneumatic anchoring system  30  according to the present invention. 
         FIG. 2  is a plan side view of the pneumatic anchoring system  30  according to the invention. 
         FIG. 3  a photograph side view of the pneumatic anchoring system  30  as in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the mandrel  36 , mast  20  and wick drain material from above. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The present invention is a pneumatic anchoring system for wick drains. 
       FIG. 1  is a perspective photo illustrating an embodiment of the invention installed on a base carrier  10 . Either conventional excavators or cranes may be used as base carrier  10 , a HYUNDAI™ 380 excavator  10  being illustrated. Base carrier  10  supports a modified mast  20  containing an internal mandrel  36  (mandrel  36  may be internal or external). The mandrel  36  is similar to conventional mandrels in that it is a hollow tubular member adapted to enclose the wick drain material and carry it down into the soil. The mast  20  with mandrel  36  may be configured as a static-push stitcher or vibratory stitcher as known in the art, the former being suited for impact-driving of the mandrel down through the mast  20  into the soil and the latter for vibratory driving. 
     In accordance with the present invention, the mandrel  36  is attached to and carries a pneumatic anchoring system  30 , the pneumatic anchoring system  30  riding the mandrel  36  down as it is driven down through the mast  20  into the soil. When the mandrel reaches its deepest point the pneumatic anchoring system  30  is adapted to inject the mandrel with a burst of pressurized air to dislodge the wick drain anchor (overcoming pore pressure), and to maintain a set cubic feet/minute (cfm) airflow during extraction of the mandrel  36  in order to counteract the pore pressure of the hole. As described below mast  20  is modified to have a vertical track along its length to allow end-to-end transition of the pneumatic anchoring system  30 . 
       FIG. 2  is a plan side view of the pneumatic anchoring system  30 , which includes an air tank  34  having an inlet and outlet, and an air feed line  33  running from an external source of compressed air  50  (such as a compressor) to the air tank  34  inlet. The air tank  34  outlet is connected to an electric valve  32 , such as a 24 VDC Air Solenoid Valve, which controls the flow of compressed air from tank  34  through an air Venturi  35  into mandrel  36 . The Venturi  35  is ported into the mandrel  36  along a long narrow slot running lengthwise. The Venturi  35  is a pneumatic coupling with a tapering internal constriction that causes an increase in the velocity of airflow and a corresponding decrease in pressure. The Venturi  35  constriction increases the air velocity and creates a jet effect into the mandrel  36 . The wick drain mandrel  36  is a hollow tubular shaft. The upper end of the mandrel  36  is closed off by a seal plate  37  which comprises a round plate seated in a circular collar or grate, and secured thereto by a stud or other suitable fixture. Seal plate  37  minimizes air leakage in the manner of a reed valve, allowing air into the mandrel  36  as needed but preventing escape. The lower end of the mandrel  36  is open or ported for release of the wick material, and for porting the air from Venturi  35 . The pneumatic anchoring system  30  is directly attached to the mandrel  36  via the long thin Venturi  35  which passes through a slot  38  running lengthwise along a portion of the mast  20 . For better stability the air tank  34  is also preferably mounted on a sled  39  that slides along the mast  20  between opposing guide tracks  39 . This way, as the mandrel  36  drives deeper and exits the mast  20  from the bottom, the pneumatic anchoring system  30  follows it down along the mast  20 . 
     In the illustrated embodiment, vibrational or static forces are applied to the mandrel  36  to drive it into the soil through a cable  40  which is attached via a roller clamp  47  to the sled  39 . The cable may be connected to an existing excavator or some remote power plant for imparting static tension or vibration. 
     In operation, an off-axis pile driving or vibrational force imparted to cable  40  digs the mandrel  36  deeper and deeper, sled  39  sliding down along the mast  20  with the mandrel  36  between opposing guide tracks  39 . When the mandrel  36  reaches the desired depth the operator activates a cab switch inside excavator  10 . This activates the solenoid  32  to open its valve and supply a burst of pressurized air from air tank  34  through the Venturi  35  into mandrel  36 . The jet of air serves two purposes. Initially it dislodges the anchor plate attached to the wick drain material from the mandrel  36  and propels it downward into the soil at the bottom of the void. In addition, since the mandrel  36  is sealed at the top end by seal plate  37 , the void remains pressurized as a constant CFM is maintained into and out through the mandrel  36  slot, along its entire length, and this continues throughout extraction in order to counteract the pore pressure of the void. The pore pressure is continually overcome as the mandrel  36  is retracting due to the constant CFM released into the mandrel  36 . This counterpressure prevents high pore pressure material from entering the mandrel during extraction, facilitates removal of the mandrel  36 , and allows the wick drain material time to saturate and commence drainage. The wick drain is then able to reach its maximum discharge capacity (typically 1-2 gpm). 
     It should now be apparent that the above-described apparatus and method for wick drain insertion equalizes pore pressure at the depth of wick drain and avoids dislodgement of the anchor plate and/or severing of the wick drain. 
     Those skilled in the art will understand that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth in the appended claims.