Patent Publication Number: US-9903086-B2

Title: Friction reduction pile jacket with slip additive

Description:
TECHNICAL FIELD 
     This disclosure relates generally to pile supports for bridges and other structures, wherein the piles are driven into the earth and backfilled earthen materials and the like are placed about the pile. More specifically, this disclosure relates to an improved friction reduction pile jacket placed about the pile which reduces and remediates the downward-settling forces of the settling backfilled earthen materials from applying an adverse downward load to a pile. 
     BACKGROUND 
     In the construction of bridges and other structures in which pile supports are driven into the earth and are used as supports for the bridge, etc., it is common to form abutment walls about the piles and to backfill the space between abutment walls and the piles with backfilled earthen materials and other materials. For example, U.S. Pat. No. 3,981,038 discloses a backfilled barrier wall formed of panels mounted one upon the other and held together with elongated reinforcing straps. As the wall is constructed, the backfilled earthen material is placed in layers beside the wall about the supporting piles of the bridge abutment. These constructions are referred to in the industry as Mechanically Stabilized Earth (MSE) wall structures. 
     One of the problems encountered with bridges and other structures supported by piles with backfilled earthen materials placed about the piles is that the backfilled earthen materials placed about the pile tend to settle over a length of time, and the downward movement of the backfilled earthen materials applies an adverse downward load to the piles. The surface friction created by the settling of the backfilled earthen materials in contact with the surface facing of the piles tends to transmit a substantial amount of adverse vertical load to the pile. This load tends to adversely affect the piles, sometimes causing enough stress that the piles may bend or otherwise become deformed, affecting the integrity of the structure supported. 
     One technique for avoiding the application of the downward force from the backfill material against piles is to surround the piles with metal or plastic conduits prior to the backfilled earthen material being placed about the piles and to fill the tubes with sand, or the like. As a result, the downward settling movement of the backfilled earthen material is applied to the exterior surfaces of the conduit in lieu of the exterior surfaces of the pile. This technique generally protects the pile from the downward forces of the backfill earth. 
     While the aforementioned conduit and sand technique has been successful in avoiding adverse effects on bridge abutment piles, etc., due to the downward weight applied by the settling backfilled earthen materials, the use of the conduits and sand utilizing this technique is expensive in that the materials are relatively expensive, and the labor required to telescopically mount the conduits over the piles and then to fill the conduits with sand is expensive. 
     One technique for overcoming the aforementioned limitations is disclosed in U.S. Pat. No. 4,721,418, in which a pile jacket, formed of laminated sheet material, such as polyethylene, is placed about a pile. When the load or weight of the backfilled earthen material tends to settle, it tends to move the pile jacket downwardly with respect to the pile, with the interfacing surfaces of the pile and the pile jacket forming a slip plane. In this manner, the jacket shields the pile from the downward force exerted by the downwardly-moving backfill material during settlement. The patent discloses that a worker could coat either the outer surfaces of the pile, or the inner surfaces of the pile jacket, with a lubricant, such as grease, to reduce the friction within the slip plane. However, in practice, it has been found that grease or other lubricants attract debris during pile settlement, thereby actually increasing the friction within the slip plane. A drawback of the &#39;418 pile jacket relates to the manufacture of the sheet material for the pile jacket. Polyolefin films exhibit a high degree of tackiness in their natural state. This tackiness creates a problem during the manufacturing process. 
     It is well known that certain polymers and additives can serve to reduce both static and dynamic coefficients of friction greatly. A sheet or spray coating of PTFE (poly tetra fluoro ethylene) possesses one of the lowest coefficients of friction known. Recently, it has been shown proven that diamonds covered with a graphene sheet has the lowest measured coefficient of friction. However; these techniques are more expensive and impractical to use. History has proven the need to work within practical and economic boundaries in most applications. 
     The need persists in the art for an improved pile jacket. 
     SUMMARY 
     Disclosed is a friction reducing jacket for application to a pile including an exterior surface and a substantially uniform cross sectional shape along its length, the jacket constructed of sheet material surrounding the pile and formed in a shape conforming to the exterior surface of the pile, the sheet material comprising a thermoplastic polymer and a slip agent. The slip agent can comprise an amide. The slip agent can be a migrating slip agent. 
     Also disclosed is a method of shielding a pile from downward force exerted by backfilled earthen materials during settlement, the pile including an exterior surface and a substantially uniform cross sectional shape along its length, including the steps of providing a friction reducing jacket constructed of sheet material formed in a shape conforming to the exterior surface of the pile, the sheet material comprising a thermoplastic polyolefin and an amide slip agent; driving the pile into soil, leaving a portion of the pile above the soil exposed, thereby defining an exposed portion of the pile; wrapping the friction reducing pile jacket around the exterior surface of the exposed portion of the pile such that the interior surface of the jacket contacts the exposed exterior surface of the pile, thereby forming a jacketed section of the pile; and placing backfilled earthen materials around the jacketed section of the pile. 
     Various implementations described in the present disclosure may include additional systems, methods, features, and advantages, which may not necessarily be expressly disclosed herein but will be apparent to one of ordinary skill in the art upon examination of the following detailed description and accompanying drawings. It is intended that all such systems, methods, features, and advantages be included within the present disclosure and protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and components of the following figures are illustrated to emphasize the general principles of the present disclosure. Corresponding features and components throughout the figures may be designated by matching reference characters for the sake of consistency and clarity. 
         FIG. 1  is a side view of a bridge abutment structure, with parts broken away to show a pile, and an associated friction reduction pile jacket constructed in accordance with the present invention, surrounded by backfilled earthen materials and positioned behind a mechanically stabilized earth wall. 
         FIG. 2  is a perspective view of a pile and friction reduction pile jacket constructed in accordance with an embodiment of the present invention, both partially embedded in backfilled earthen materials. 
         FIG. 2A  is an enlarged sectional view of the section of the pile and friction reduction pile jacket called out in  FIG. 2 . 
         FIG. 3  is a horizontal cross-sectional view showing a jacketed pile embedded within backfilled earthen materials. 
         FIG. 4  is a plot of the coefficient of friction versus time after extrusion of the pile jacket material, regarding slip agent-impregnated material used in a pile jacket constructed in accordance with an embodiment of the present invention. 
         FIGS. 5A, 5B, and 5C  are a simplified illustrations of a segment of slip agent-impregnated jacket material at, respectively, day one, day five, and day nine following extrusion, showing the progressive migration of an example slip agent to the surface of the material, forming a layer on the material. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed is a friction reduction pile jacket and associated methods, systems, devices, and various apparatus. The pile jacket includes a sheet of thermoplastic, e.g., polyolefin, material comprising a slip agent, such as an amide. It would be understood by one skilled in the art that the disclosed pile jacket is described in but a few exemplary embodiments among many. No particular terminology or description should be considered limiting on the disclosure or the scope of any claims issuing therefrom. 
     One embodiment of a friction reduction pile jacket is disclosed and described in  FIG. 1 , which illustrates a friction reduction pile jacket  10  surrounding at least a portion of a pile  12  that is driven into soil  13  and that supports a structure such as a bridge abutment  14 . The pile  12  and jacket  10  are shown surrounded by backfilled earthen materials  16  and positioned behind a mechanically stabilized earth (“MSE”) wall  18 , supported by a leveling pad (footing)  20  resting at grade level  15 . The backfilled earthen materials  16  can be comprised of variable soil types, and usually will be of graded uniform granular materials. The pile  12  can be driven into the soil  13 , for example, directly behind a MSE wall  18  to be built and positioned beneath a bridge abutment concrete pile cap  22  within which the top end  24  of the pile  12  will be encapsulated. Pile  12  can be driven by means of power actuated pile drivers into the soil  13  to a depth that possess sufficient strength capable of receiving the transferred structural loads imposed through and by the structure, e.g., bridge abutment  14  in support of a bridge structure  26 . 
     The pile  12  is sized to accommodate the load transferred from the structure it supports, e.g., bridge abutment  14 . The backfilled earthen materials  16  impose a direct surcharge load upon the soil  13 . Due to this surcharge load, the soil  13  beneath is compressed, resulting in settlement of the construction materials and the backfilled earthen materials  16 . The settlement of the backfilled earthen materials  16  causes downdrag forces upon the pile  12 . This downdrag load is also sometimes referred to as negative skin friction and requires remedial efforts to mitigate its effects. The downdrag load is mitigated by placing the pile jacket  10  around the driven pile  12  during construction, in the manner discussed below. 
     Referring to  FIGS. 2 and 2A  in conjunction with  FIG. 1 , driving the pile  12  into the soil  13  leaves a portion of the pile  12  above the soil  13  exposed, defining an exposed portion of the pile  12 , shown in  FIG. 2  as having an exterior pile surface  28 , which is usually substantially smooth. The friction barrier pile jacket  10  is wrapped around the exterior pile surface  28  such that the interior surface  30  (shown in  FIG. 2A ) of the jacket  10  contacts the exterior pile surface  28 , thereby forming a jacketed section of the pile  12 . To completely cover the exposed portion of the cylindrical pile  12 , multiple scores are placed along the longitudinal direction of the pile jacket section  10  and wrapped around the exposed portion of the pile  12  and are sequentially affixed over and around each preceding section placed around the pile  12 , in conjunction with, and in corresponding lifts of, the backfilled earthen materials  16  as they are placed and compacted around the jacketed section of pile  12 . The backfilled earthen materials  16  are then placed around the pile jacket  10  (as shown in  FIG. 3 ) and compacted. Material within the backfilled earthen materials  16 , including aggregate  17 , grips the exterior surface  32  of the pile jacket  10 , forming indentations  34  in that surface  32 . Yet, the interior surface  30  of the pile jacket  10  will not be subject to the same deformation as the exterior surface  32 , owing to the spacer structure inside the pile jacket sheet, as detailed in U.S. Pat. No. 4,721,418, the disclosure of which is hereby incorporated by reference in its entirety. The grip exerted on the jacket exterior surface  32  by the backfilled earthen materials  16  results in a coefficient of sliding friction between the backfilled earthen materials  16  and the exterior surface  32  that is larger than a coefficient of sliding friction between the interior surface  30  of the pile jacket  10  and the exterior pile surface  28 , such that downward forces applied by settlement of the backfilled earthen materials  16  tend to move the pile jacket  10  downwardly about the pile  12  substantially without transmitting a major portion of the downward forces to the pile  12 . This results in a slip plane effect between the settling backfilled earthen materials  16  and the pile  12 . 
     Again referring to  FIGS. 2 and 2A , it is desirable to increase the interior contact surface area of a cylindrical pile jacket  10  that will come into contact with the exterior pile surface  28  in order to enhance the friction reduction capabilities of the pile jacket  10 . This is accomplished by utilizing multiple scores  36  formed into the interior surface  30  of pile jacket  10 . The scores are formed during the fabrication process by dull knife blades on rollers or bars placed into a jig that make indentations against the longitudinal length of one side of the sheet material comprising pile jacket  10  in order to weaken that side, thus allowing the sheet to fold and bend in a predetermined manner. The multiple scoring process provides a way for the pile jacket  10  to bend and better conform around the exterior circumference of the pile  12  for a more uniform contact surface between the pile jacket  10  and the pile  12 , thus providing an increased friction reduction component. The number of scores are determined by considering factors such as the shape of the pile to be jacketed to achieve a desired level of surface contact between the pile jacket and a pile, and the thickness, size, and type of material to be scored to fit best about a cylindrical pile. It has become apparent through application testing that the best results are achieved utilizing thinner core materials with a greater proportion of longitudinal scoring for this application. 
     The sheet of thermoplastic material can be comprised of a polyolefin. A polyolefin material comprising the pile jacket  10  is preferably polypropylene. A commercially available suitable polypropylene sheet material is from Corex Plastics Pty, Ltd. of Victoria, Australia. However, other embodiments of the present invention can use polyolefins other than polypropylene. For example, the sheet could be constructed of polyethylene instead of polypropylene. The polyolefin can be virgin or recycled materials so long as the resulting jacket has the desired coefficient of friction properties and no detrimental properties for the jacket&#39;s end use. It has been proven in application, that polypropylene performs best, because it is easy to produce, it is higher purity, has a lower melting point, greater stiffness, it has better resistance to cracking, is lighter weight, and has high resistance to chemicals including acids and organic solvents. One of ordinary skill in the art can determine an appropriate polymeric material for construction of the pile jacket. 
     The coefficient of sliding friction within the slip plane defined between the interior surface  30  of the pile jacket  10  and the exterior pile surface  28  is dramatically reduced when the pile jacket  10  is fabricated with improved friction reduction additives. It has been found that by introducing certain quantities of slip agent additives, such as amide agents, greater friction reduction can be achieved between an, e.g., extruded polypropylene pile jacket  10  comprising an amide slip agent and a driven pile  12  in order to enhance downdrag reduction, as compared to pile jackets heretofore used. 
     Polyolefin films exhibit a degree of tackiness in their natural state. This tackiness creates a problem during the manufacturing process. In order to improve the material&#39;s ability to separate and slide during manufacturing, slip additives have been introduced to enhance this capability. Slip additives are used to resist the friction of the polymer material sliding over itself and over parts of the manufacturing equipment during the manufacturing process. Slip additives can be comprised of amides. Common types of amide slip agents are oleamide and erucamide additives, although other amides can be used. Preferred slip agents include migratory slip agents. 
     Effectiveness of these slip additives is normally determined by the coefficient of friction (COF) it allows. The COF is usually measured using a Standard Method of Testing, ASTM D-1894. COF is a ratio of the force required to slide one layer of material across another relative to the gravimetric force exerted upon it. A similar method of testing, described in the Example below, has shown that the same slip additive will improve the COF results in sliding over other substrates such as steel and concrete in a much similar manner resulting in enhanced material performance. 
       FIGS. 4 and 5A-5C  illustrate the progressive decrease in the COF over time of a polyolefin sheet incorporating an amide additive. When slip additives are introduced into a thermoplastic extrusion process, they have a low compatibility with the polymer materials, allowing migration to the surface.  FIG. 5A  shows a segment  39  of a sheet of pile jacket material immediately (Day 1) after extrusion. Segment  39  includes polyolefin material  41  in which slip agent  40  is uniformly interspersed, and is shown having an upper exterior surface  42  and a lower exterior surface  44 .  FIG. 5B  shows the segment  39  at Day 5 after extrusion, by which time the amide slip agent  40  has migrated to the exterior surfaces  42 ,  44  and begins to form a partial layer while friction rapidly drops, as demonstrated by the curve  46  of the plot  38  in  FIG. 4 . As shown in  FIG. 5C , as the migration of the amide slip agent  40  reaches equilibrium, the layers of amide slip agent are mostly completely migrated, forming an upper exterior lubricating layer  43  and a lower exterior lubricating layer  45 .  FIG. 4  is a graph depicting the coefficient of friction being reduced as the amide slip agent  40  migrates over an illustrative time period of 10 days and into eventual equilibrium becoming stable. The slip agent  40  is incorporated within the polyolefin during the polymer mixing and/or extrusion processes. 
     The concentration of slip agent used can be, for example, about 1 wt % to about 2 wt % of the polymer. One skilled in the art can determine an appropriate concentration of slip agent for generating the desired end properties specified for the jacket. For example, one might want to also print company branding on the pile jacket, in which case the coefficient of friction should not be so low as to prevent printing inks from being applied and adhering to the exterior surface of a jacket. The slip agent used can, for example, be one commercially available from Croda International PLC, of East Yorkshire, United Kingdom, under the mark CRODAMIDE®. 
     EXAMPLE 
     Two types of polypropylene pile jacket materials were tested, namely: (1) a first-generation pile jacket sheet, with no slip agent additives, of the type disclosed in U.S. Pat. No. 4,721,418, and (2) a pile jacket sheet of the type disclosed herein, which includes an amide slip agent additive. Seven sheets of each type of material were cut and brought into contact with steel plates, which were previously unused, thus eliminating the effect of the plastic material rubbing off of the sheet onto the steel plate and affecting the results. For each set of data, the normal force applied included calibrated weights of 10 Kg (22.05 lb.), 20 Kg (44.09 lb.), and 30 Kg (66.14 lb.). The plates were advanced using a worm gear mechanical jacking apparatus, namely, the base of a Marshall testing machine, oriented in a horizontal position. The horizontal load applied was determined utilizing a calibrated load cell. The maximum load achieved prior to sliding was recorded for each normal force applied for each plate tested. The force required to slide the plate was divided by the normal force to arrive at a coefficient of sliding friction. The coefficients of sliding friction obtained for each trial were averaged. The following table summarizes the results: 
                                                 AVG. COEFFICIENT OF           TYPE   SLIDING FRICTION                          Conventional pile jacket sheet   0.30           Pile jacket sheet impregnated   0.26           with amide slip agent additive                        
The 0.04 difference between the two samples represents a 13.1% reduction in the coefficient of sliding friction achieved by the samples containing the amide slip agent additive.
 
     The disclosed embodiment of the present invention encompasses several variations. For instance, although the disclosed pile jacket was discussed with reference to circular piles, the same concept can be applied to piles having different shapes, such as an H-shape cross section. 
     One should note that conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more particular embodiments or that one or more particular embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. 
     It should be emphasized that the above-described embodiments are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the present disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the present disclosure. Further, the scope of the present disclosure is intended to cover any and all combinations and sub-combinations of all elements, features, and aspects discussed above. All such modifications and variations are intended to be included herein within the scope of the present disclosure, and all possible claims to individual aspects or combinations of elements or steps are intended to be supported by the present disclosure.