Patent Publication Number: US-2021190043-A1

Title: Wind turbine foundation and method of constructing a wind turbine foundation

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATION(S) 
     This application is a divisional of and claims priority to U.S. Nonprovisional patent application Ser. No. 16/880,259 entitled “Wind Turbine Foundation and Method of Constructing a Wind Turbine Foundation” which was filed on May 21, 2020 which is a divisional of and claims priority to pending U.S. Nonprovisional patent application Ser. No. 16/591,720 entitled “Wind Turbine Foundation and Method of Constructing a Wind Turbine Foundation” filed on Oct. 3, 2019 which claims priority to Provisional Patent Application No. 62/741,184 entitled “Wind Turbine Foundation” filed on Oct. 4, 2018 and pending U.S. Provisional Patent Application No. 62/874,029 entitled “WK Wind Turbine Foundation” filed on Jul. 15, 2019, all of which are incorporated herein by reference in their respective entireties. 
    
    
     FIELD 
     This disclosure relates to the field of construction related to wind turbines or other tower-like structures. More particularly, this disclosure relates to a foundation for a wind turbine. 
     BACKGROUND 
     The wind energy generation market has experienced tremendous growth over the past decade with wind energy currently recognized as the lowest cost source of renewable energy generation. The key driver of this growth has been the advancements made in wind turbine technologies, with wind turbines growing in capacity, size and height every year. The advancements in wind turbine technologies has placed increasing strains on the other classical approaches to wind project design and construction, and as a result several of the classical, brute force approaches to wind project design and construction are reaching their limits of effectiveness and cost efficiency. Change in wind project design and construction is required to complement the turbine technology changes being experienced in the industry. 
     In 2018 US wind energy generation capacity grew by over 8%, and the installed capacity of wind generation is anticipated to exceed that of hydro by the end of 2020. This growth has been driven by the reductions in cost of the wind generation technologies. The key driver of these reductions in cost have been the advancements in wind turbine technologies. Wind turbines have grown consistently in the past few years with turbine size, weight and tower heights increasing significantly every year. In 2018 the largest turbines installed in North America had capacities on the order of 3.6 MW, with tower heights of 110 m. In 2020 wind turbine installations will include 4.8 MW turbines with tower heights exceeding 140 m. 
     While turbine sizes are growing every year, turbine logistics remain constrained with road, rail and truck transport limiting tower base dimensions. These increases in turbine size combined with limit on growth of the tower base dimensions has resulted in significant growth in the load demands being placed on the wind turbine foundations. In contrast to the technology improvements seen on the turbines, wind turbine foundation technologies have not advanced significantly over the past 20 years. Today&#39;s predominant wind turbine foundations are the traditional concrete raft foundation, with minor variations being applicable for unique ground conditions (shallow bedrock situations, etc.). While the concrete raft foundation was a good solution for the turbines installed in 2016, with capacities of 2.6 MW and tower heights of 70 m, they are now approaching their limits of applicability. Increasing the size and strength of the concrete raft foundation is not a simple matter, with rebar and anchor bolt cage densities reaching the limits of constructability, the complexities of very large concrete pours creating significant logistics issues and quality risks. What is needed therefore are new approaches to wind turbine foundations to meet the needs of the continuing advancements in wind turbine technologies. 
     SUMMARY 
     The above and other needs are met by a wind turbine foundation comprising a core member which may include, for example, a metal base can or a metal spool. In some embodiments wherein the core member comprises a metal base can, the metal base can further comprises a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the base can, a second outer flange extending out from the main body along a lower section of the base can, and a tower flange including a plurality of apertures for attaching a wind turbine tower to the base can; and a plurality of metal radial girders connected to and radiating out from the base can wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange. Preferably, the wind turbine foundation of claim  1  wherein the wind turbine foundation is located in an excavated hole in the ground, wherein the hole in the ground is created by removing soil, and wherein at least some of the removed soil is laid over at least a portion of the plurality of metal girders. The wind turbine foundation preferably further includes an underlying slab and a layer of rebar located above the underlying slab. The wind turbine foundation preferably further includes a base layer of concrete poured along the underlying slab and the layer of rebar. 
     In some embodiments, the plurality of radial girders includes an upper girder flange and a lower girder flange wherein each upper girder flange is connected to the first outer flange and each lower girder flange is connected to the second outer flange. 
     In some embodiments, the wind turbine foundation includes an inner shell of concrete lining an inside surface of the base can. In similar embodiments, the wind turbine foundation may further include concrete substantially filling the base can. 
     In some embodiments, the wind turbine foundation includes a reinforced concrete base slab supporting the metal base can and the plurality of radial girders, wherein the excavation under the slab is tapered so that a bottom side of the slab filling the excavation is tapered and bulges along a middle portion of the base slab. 
     In some embodiments, the wind turbine foundation includes a reinforced concrete base slab wherein the excavation under the slab is in a stepped configuration so that a bottom side of the slab filling the excavation is in a stepped configuration. 
     In some embodiments, the wind turbine foundation includes a plurality of first transverse girders wherein individual members of the plurality of first transverse girders are located between and connected to pairs of the plurality of radial girders. The wind turbine foundation may further include a plurality of second transverse girders wherein individual members of the plurality of second transverse girders are located between and connected to pairs of the plurality of girders at distal ends of the radial girders. 
     In some embodiments, the wind turbine foundation includes a perimeter grade beam of concrete and a mid-grade beam of concrete beneath a reinforced concrete base slab. 
     In some embodiments, at least a first portion of the upper girder flanges are substantially parallel with a portion of the lower girder flanges. In some embodiments, the first portion of the upper girder flanges comprises most of the upper girder flanges. 
     In some embodiments the plurality of radial girders comprises a plurality of truss girders. 
     In some embodiments the wind turbine foundation includes a plurality of piles supporting the plurality of radial girders at distal ends of the plurality of radial girders. 
     In some embodiments the wind turbine foundation further includes a core column inside the base can and a plurality of stiffener plates connected to and radiating out from the core column wherein distal edges of the stiffener plates are connected to an interior surface of the base can. The wind turbine foundation may further include a first plurality of rock anchors connected to the plurality of radial girders wherein there is at least one rock anchor per radial girder extending into bedrock. The wind turbine foundation may further include a plurality of transverse girders wherein individual members of the plurality of transverse girders are located between and connected to pairs of the plurality of radial girders. The wind turbine foundation may further include a second plurality of rock anchors connected to the plurality of transverse girders wherein there is at least one rock anchor per transverse girder extending into bedrock. In some embodiments the base can further comprises a plurality of vertical flanges wherein individual vertical flanges of the plurality of vertical flanges are connected to individual radial girders of the plurality of radial girders. 
     In some embodiments the wind turbine foundation includes a plurality of vertically oriented beams connected to an interior surface of the base can to stiffen the base can. 
     In another aspect, a wind turbine foundation is disclosed comprising a metal spool; a plurality of metal radial girders connected to and radiating out from the metal spool; and a ring girder connected above the plurality of radial girders wherein the ring girder further comprises a tower flange including a plurality of apertures for attaching a wind turbine tower to the ring girder. The ring girder may further include a composite ring girder comprising a plurality of ring girder sections forming the composite ring girder wherein ring girder sections are individually connected to the plurality of radial girders with one ring girder section per radial girder. The spool may further include a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the spool, and a second outer flange extending out from the main body along a lower section of the spool wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange. The spool may further include a plurality of vertical flanges wherein individual vertical flanges of the plurality of vertical flanges are connected to individual radial girders of the plurality of radial girders. The spool may further include a plurality of pairs of vertical flanges located in an area between the first outer flange and the second outer flange wherein individual pairs of vertical flanges of the plurality of pairs of vertical flanges are connected to individual radial girders of the plurality of radial girders. 
     In another aspect, a method of erecting a wind turbine foundation is disclosed, the method comprising the steps of excavating a foundation area in the ground by removing excavated soil from the ground; pouring a mud slab in the excavated foundation area to create a level work surface; placing a metal core member in the excavated foundation area; and attaching a plurality of metal radial girders to the core member. The core member may include, for example, a metal base can or a metal spool. The core member preferably includes a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the core member, and a second outer flange extending out from the main body along a lower section of the core member wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange. 
     The summary provided herein is intended to provide examples of particular disclosed embodiments and is not intended to cover all potential embodiments or combinations of embodiments. Therefore, this summary is not intended to limit the scope of the invention disclosure in any way, a function which is reserved for the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, aspects, and advantages of the present disclosure will become better understood by reference to the following detailed description, appended claims, and accompanying figures, wherein elements are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  shows a perspective view of an embodiment of a base can used in the construction of wind turbine foundations described herein; 
         FIG. 2  shows a cutaway partial side view of a portion of a wind turbine foundation showing a girder attached to the right side of the base can shown in  FIG. 1  and, for illustrative purposes to better show features of the base can, with no girder shown attached to the left side of the base can; 
         FIG. 2A  shows a close-up view of a first highlighted section of the wind turbine foundation shown in  FIG. 2 ; 
         FIG. 2B  shows a close-up view of a second highlighted section the wind turbine foundation shown in  FIG. 2   
         FIG. 2C  shows a partial plan view of the wind turbine foundation shown in  FIG. 2 . 
         FIG. 3A  shows a plan view of the wind turbine foundation shown in  FIG. 2  including a plurality of girders attached to a base can like the one shown in  FIG. 1 ; 
         FIG. 3B  shows a full side view of the wind turbine foundation shown in  FIG. 3A ; 
         FIG. 4A  shows a plan view of a wind turbine foundation including a solid concrete center inside of a base can like the one shown in  FIG. 1 ; 
         FIG. 4B  shows a side view of the wind turbine foundation shown in  FIG. 4A ; 
         FIG. 5A  shows a plan view of a wind turbine foundation including no concrete in the center of a base can like the one shown in  FIG. 1 ; 
         FIG. 5B  shows a side view of the wind turbine foundation shown in  FIG. 5A ; 
         FIG. 6  shows a side view of a wind turbine foundation including a tapered mud slab and base layer; 
         FIG. 7  shows a side view of a wind turbine foundation including a tapered stepped mud slab and base layer; 
         FIG. 8A  shows a plan view of a wind turbine foundation including a plurality of transverse girders between a plurality of girders; 
         FIG. 8B  shows a side view of the wind turbine foundation shown in  FIG. 8A ; 
         FIG. 9A  shows a plan view of a wind turbine foundation including a base slab (or base layer) including an inner beam and an outer beam; 
         FIG. 9B  shows a side view of the wind turbine foundation shown in  FIG. 9A ; 
         FIG. 10A  shows a plan view of a wind turbine foundation including girders with a first tapered profile; 
         FIG. 10B  shows a side view of the wind turbine foundation shown in  FIG. 10A ; 
         FIG. 11A  shows a plan view of a wind turbine foundation including girders with a second tapered profile; 
         FIG. 11B  shows a side view of the wind turbine foundation shown in  FIG. 11A ; 
         FIG. 12  shows a side view of a wind turbine foundation including a plurality of girder trusses; 
         FIG. 13A  shows a plan view of a wind turbine foundation comprising a base can, a plurality of girders connected to the base can, and a plurality of piles supporting the plurality of girders with one pile per girder; 
         FIG. 13B  shows a side view of the wind turbine foundation shown in  FIG. 13A  wherein the plurality of piles includes a plurality of screw piles; 
         FIG. 13C  shows a side view of the wind turbine foundation shown in  FIG. 13A  wherein the plurality of piles includes a plurality of concrete bell piles; 
         FIG. 14A  shows a plan view of a wind turbine foundation including a base can including radial stiffeners inside the base can connected to a central core member and the inside of the base can; 
         FIG. 14B  shows a side view of the wind turbine foundation shown in  FIG. 14A ; 
         FIG. 14C  shows a partial plan view of the wind turbine foundation shown in  FIG. 14A  and  FIG. 14B ; 
         FIG. 14D  shows a partial side view of the wind turbine foundation shown in  FIGS. 14A-14C  including a girder attached to the right side of the base can and, for illustrative purposes to better show features of the base can, with no girder shown attached to the left side of the base can; 
         FIG. 14E  shows a plan view of the base can used in the wind turbine foundation shown in  FIGS. 14A-14D ; 
         FIG. 14F  shows a segmented partial side view of the wind turbine foundation shown in  FIGS. 14A-14D  wherein the image is cut and truncated both horizontally and vertically to show the top and bottom corners of one side of the wind turbine foundation; 
         FIG. 14G  shows a partial view looking down cut from a line shown in  FIG. 14F ; 
         FIG. 15A  shows a plan view of a wind turbine foundation including a base can including vertical beams connected to the inside of the base can at locations adjacent to where girders are connected to the base can; 
         FIG. 15B  shows a side view of the wind turbine foundation shown in  FIG. 15A ; 
         FIG. 16A  shows a wind turbine foundation including a plurality of girders connected to a base can, transverse girders between and connected to pairs of girders, and rock anchors connected to distal ends of the plurality of girders and along the transverse girders; 
         FIG. 16B  shows a side view of the wind turbine foundation shown in  FIG. 16A ; 
         FIG. 17A  shows a plan view of a wind turbine foundation including a spool and a plurality of girders connected to the spool; 
         FIG. 17B  shows a side view the wind turbine foundation shown in  FIG. 17A  with a first tower piece added; 
         FIG. 17C  shows a close-up partial plan view of the spool and the plurality of girders attached thereto from the wind turbine foundation shown in  FIG. 17A ; 
         FIG. 17D  shows a cut-away side partial view of the spool and girders from the wind turbine foundation shown in  FIG. 17A ; 
         FIG. 17E  shows a cut-away partial view looking down from the center of the spool as viewed from line “ FIG. 17E ” shown in  FIG. 17D ; 
         FIG. 17F  shows a cut-away partial view looking down a girder toward the spool as shown from the view of line “ 17 F” in  FIG. 17C  wherein a ring beam has been added to the apparatus from  17 C and is bolted to the plurality of girders; 
         FIG. 17G  shows a partial side view of a girder connected to a ring girder which is connected to a first tower piece of the wind turbine foundation shown in  FIG. 17B ; 
         FIG. 17H  shows a cut-away partial view looking down a girder toward the spool wherein a curved ring girder section has been added by welding to the girder shown in  FIG. 17H ; 
         FIG. 17I  shows a partial side view of a girder welded to a ring beam which is attached to a first tower piece of the wind turbine foundation; and 
         FIG. 17J  shows a close-up partial plan view of the spool, the plurality of girders attached thereto from the wind turbine foundation shown in  FIG. 17A  and further shows a plurality of curved ring girder subsections connected to the girders and forming a composite ring girder. 
     
    
    
     The figures are provided to illustrate concepts of the invention disclosure and are not intended to embody all potential embodiments of the invention. Therefore, the figures are not intended to limit the scope of the invention disclosure in any way, a function which is reserved for the appended claims. 
     DETAILED DESCRIPTION 
     An example of a wind turbine foundation  100  and its components is shown in  FIGS. 1, 2, 2A, 2B, 2C, 2D, 3A and 3B .  FIG. 1  shows a base can  102 —a central component of the wind turbine foundation  100  shown more completely in the plan view of  FIG. 3A  and a side view of  FIG. 3B . The base can  102 , which comprises a metal shell, is referred to as a “can” because of its preferred cylindrical shape which looks like a traditional can as well as its preferred composition (i.e., including mostly or completely metal or metal alloy, hereinafter collectively referred to as “metal”). A rounded cylindrically-shaped base can is preferred but other shapes would work including a polygonal base can with multiple faces. A plurality of radial girders  104  are connected to the base can  102 . The base can  102  preferably includes a first outer flange  106 A and a second outer flange  106 B. The plurality of girders  104  are preferably connected to the base can  102  by bolting the girders  104  to the first outer flange  106 A and the second outer flange  106 B. Although bolting is specifically described in this example, other devices and/or methods of attachment may be used such as, for example, welding. 
     The plurality of girders  104  preferably includes twelve substantially similar girders of the same size and shape. In other examples, the plurality of girders  104  can include more than twelve or fewer than twelve girders. The girders  104  and other similar objects described herein are preferably made of steel but other metals or metal alloys could be used instead of or in addition to steel. The girders  104  are preferably made using traditional steel plate girder design used in bridge girders and existing steel bridge design codes and associated manufacturing methods. Each of the girders  104  is preferably tapered as shown and preferably has a length ranging from about 8 meters (m) to about 14 m and a height at the highest point ranging from about 2.5 m to about 5 m. 
       FIG. 2  shows a closer view of the base can  102  connected to a first girder  104 A. A first close-up view of the connection between the first outer flange  106 A and the first girder  104 A is highlighted and shown in  FIG. 2A . A second close-up view of the connection between the second outer flange  106 B and the first girder  104 A is also highlighted and shown in  FIG. 2B . A plan view of the connections between the base can  102  and the plurality of girders  104  is shown in  FIG. 2C . The base can  102  also preferably includes a tower flange  108  for attaching a first tower piece  110  to the base can  102 . A close-up view of a preferred connection between the tower flange  108  and the first tower piece  110  is highlighted and shown in  FIG. 2A  showing a preferred embodiment using bolts on the inside of the base can  102  and the first tower piece  110  but not on the outside. Other embodiments may include a two-sided tower flange for using both internal and external bolting to hold the base can  102  to the first tower piece  110 . Use of only internal bolts and only an inward facing tower flange is preferred because the mechanical connection is protected from the elements, thereby reducing corrosion or other deterioration of the connection between the base can  102  and the first tower piece  110 . The base can  102  preferably has a diameter that substantially matches the diameter of the first tower piece  110 . The wall thickness of the base can is preferably at least as thick as the wall thickness of the first tower piece  110 . 
     The wind turbine foundation  100  preferably includes a mud slab  112  on which the base can  102  rests. The mud slab preferably comprises concrete with a level top surface and is preferably about 100 millimeters (mm) to 150 mm thick in some embodiments. Rebar  114  and a base slab layer  116  (preferably made of concrete) is preferably located above the mud slab  112  inside and outside of the base can  102 . The base slab layer  116  (or “base slab” or “base layer”) is designed at a nominal thickness that is much less than the mass concrete of a traditional raft foundation, thus avoiding prevalent heat-of-hydration and associated cracking and performance concerns. The thickness of the base slab layer  116  is selected such that the required strength is achieved with a nominal, lower reinforcement ratio suitable to handle punching shear at the edges of the girders  104 . In some embodiments, the thickness of the base layer  116  preferably ranges from about 300 mm to about 600 mm. 
     The girders  104  preferably include downward facing studs  118  (e.g., Nelson™ studs) that are enmeshed with the rebar  114  and the base slab layer  116  and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. Each of the plurality of girders  104  preferably includes upper girder flanges  120 A and lower girder flanges  120 B as shown, for example, in  FIG. 2 . Preferably, the upper girder flanges  120 A are bolted to the first outer flange  106 A of the base can  102  and the lower girder flanges  120 B are bolted to the second outer flange  106 B of the base can  102 . Each of the plurality of girders  104  also preferably includes a solid girder web  122  and a plurality of stiffener plates  124 . Crushed gravel  126  is preferably placed directly adjacent to an upper section  128 A of the base can  102  at surface level, covering backfill  130  which is preferably placed along and/or above the girders  104  and base slab layer  116 . The backfill  130  will principally be the excavated in situ materials excluding topsoil. Only in instances of saturated soils or unusual soil composition would imported material be required. Backfill will be placed in standard 200 mm to 300 mm lifts compacted to about 95% standard proctor maximum dry density or better to achieve a dense soil ballast over the entire foundation. The top of the base slab layer  116  is screeded to the second outer flange  106 B for convenience and to provide assurance of complete contact between the concrete and the underside of the second outer flange  106 B. The lower girder flanges  120 B preferably include a plurality of “bleed holes” used to observe concrete flow under the girders  104  for this purpose. Corrosion protection will vary based on soil types but typically includes full epoxy coating of all steel components and galvanized bolts, as well as a site-specific designed impressed current grounding and monitoring system. 
     The radial girders  104  are proportioned at the base can  102  connection based on strength or stiffness. The girder  104  geometry is tapered towards the outside perimeter to maintain a relatively constant section capacity to resistance demand ratio. The can-ends of the girders  104  (the ends of the girders  104  closest to the base can  102 ) have a short and preferably substantially horizontal sections of the top flange to facilitate the bolted connection to the can. This type of connection is selected because the first outer flange  106 A (or “bolting ring”) on the base can  102  also facilitates circumferential load distribution and ring stiffness acting as Tee Ring Beams. The connection is preferably designed as a “slip-critical” connection because shifting of the joint could lead to incremental tower misalignment. The structural design of the girders  104  preferably follows typical practice for traditional plate girders for bridges. In fact, in preferred embodiments, the girders  104  and base slab layer of concrete  116  act as a Composite Radial Inverted Bridge Section (CRIBS). The ends of the radial girders  104  are preferably fitted with a support leg  111  including levelling bolt positioned over a steel plate on the mud slab  112  to facilitate level installation prior to concreting. 
     The embodiment of the wind turbine foundation  100  shown in  FIG. 2  through  FIG. 3B  includes an inner layer ring of concrete  132  located inside the base can  102 . The ring of concrete  132  may further include rebar  134  included therein. Granular fill  136  (preferably compacted to at least or about 98% standard proctor maximum dry density) may be added inside the ring of concrete  132 . An example of the average size of granular fill that can be used in some embodiments is 40 mm. Additionally or alternatively, gravel could be added inside the ring of concrete  132  for added weight and to discourage water retention. Another layer of rebar  138  and concrete  140  may be added above the ring of concrete  132  and the granular fill  136 . In a different embodiment shown in  FIGS. 4A and 4B , a wind turbine foundation  141  includes a full concrete core  142  located inside the base can  102 . In the embodiments shown in  FIG. 2  through  FIG. 4B , inward facing studs  144  (e.g., Nelson™ studs) along the base can  102  are preferably included extending inside the base can  102  enmeshed with concrete. Corrugated Steel Pipe (csp)  146  is used to act as sacrificial steel form for the concrete. 
       FIGS. 5A and 5B  show an embodiment of a wind turbine foundation  150  including the base can  102  and the plurality of girders  104  but not including a concrete ring or concrete core inside the base can  102 . In certain embodiments, it may be preferably to minimize the use of concrete inside the base can  102 . 
       FIG. 6  shows an embodiment of a wind turbine foundation  152  wherein ground excavation  154  for the overall apparatus  152  is tapered. The wind turbine foundation  152  preferably includes a tapered mud slab  156  having a thickness preferably ranging from about 150 mm to about 300 mm. Above the mud slab  156  is a tapered base slab  158  which is preferably made of concrete and preferably reinforced with rebar. The base slab  158  is thickest beneath a base can  102  which is attached to a plurality of girders  104  in similar fashion to the wind turbine foundation  100  described above with reference to  FIG. 2  through  FIG. 3B . The thickness of the base slab  158  beneath the base can  102  preferably ranges from about 500 mm to about 1500 mm. The girders  104  include downward facing studs  118  which are enmeshed with the base slab  158 . Although tapered along a peripheral section  160 , a first portion of the base slab  162  is preferably substantially flat beneath the base can  102 . 
       FIG. 7  shows an embodiment of a wind turbine foundation  164  wherein ground excavation  166  for the overall apparatus  164  is in a tapered stepped pattern. The wind turbine foundation  164  preferably includes a tapered stepped mud slab  168  having a thickness preferably ranging from about 150 mm to about 300 mm. Above the mud slab  168  is a tapered stepped base slab  170  which is preferably made of concrete and preferably reinforced with rebar. The base slab  170  is thickest beneath a base can  102  which is attached to a plurality of girders  104  in similar fashion to the wind turbine foundation  100  described above with reference to  FIG. 2  through  FIG. 3B . The thickness of the base slab  170  beneath the base can  102  preferably ranges from about 500 mm to about 1500 mm. The girders  104  include downward facing studs  118  which are enmeshed with the base slab  170 . In a preferred embodiment, the tapered step pattern includes three steps from the periphery of the tapered stepped base slab to its center as shown in  FIG. 7 . 
       FIG. 8A  and  FIG. 8B  show views of a wind turbine foundation  200  including a mud slab  202 , a base can  102  on or otherwise above the mud slab  202 , a base slab  204  preferably made of concrete, and a plurality of girders  104  connected to the base can  102 . A peripheral section  206  of the base slab  204  preferably extends deeper into the ground than a central section  208  of the base slab  204 . Additional features include a plurality of inner transverse girders  210  connecting midsections  212  of adjacent radial girders  104  together and a plurality of outer transverse  214  connecting outer sections  216  of adjacent radial girders  104  together. The inner transverse girders  210  and outer transverse girders  214  are preferably steel I-beams which are preferably bolted or welded to adjacent girders  104  providing further structural support to the base can  102  and girders  104 . Use of the transverse girders in some circumstances could allow for a thinner mud slab or an alternative type of slab such as, for example, corrugated or ribbed steel panels or composite rigid panels. Granular backfill  218  preferably covers the girders  104  and the base slab  204 . 
       FIG. 9A  and  FIG. 9B  show views of a wind turbine foundation  300  including a mud slab  302 , a base can  102  on or otherwise above the mud slab  302 , a base slab  304  preferably made of concrete, and a plurality of girders  104  connected to the base can  102 . The base slab  304  preferably includes an inner beam  306  and an outer beam  308  which both extend deeper into the ground than the surrounding portions of the base slab  304 . The inner beam  306  is preferably beneath midsections  310  of the girders  104  and the outer beam is preferably beneath outer sections  312  of the girders  104 . 
       FIG. 10A  and  FIG. 10B  show views of a wind turbine foundation  400  that includes a mud slab  202 , a base can  102  on or otherwise above the mud slab  202 , a base slab  204  preferably made of concrete reinforced with rebar, and a plurality of radial girders  406  connected to the base can  102 . The plurality of girders  406  are like the plurality of radial girders  104  described above except for profile shape. The girders  406  are preferably tapered as shown and the length of each of the girders  406  preferably ranges from about 8 m to about 15 m and the height of each of the girders  406  at the highest point ranges from about 2.5 m to about 5 m. In the example shown in  FIG. 10A  and  FIG. 10B , rectangular sections  408  of the plurality of girders  406  extend out substantially horizontally from about 20% to about 50% the length of each of the girders  406  before angling downward along tapered sections  410 . In another example, a wind turbine foundation  412  shown in  FIG. 11A  and  FIG. 11B  includes a plurality of radial girders  414  wherein rectangular sections  416  of the plurality of girders  414  extend out substantially horizontally from about 50% to about 80% the length of each of the girders  414  before angling downward along tapered sections  418  of the girders  414 . The girders  414  are preferably tapered as shown and the length of each of the girders  414  preferably ranges from about 4 m to about 12 m and the height of each of the girders  414  at the highest point ranges from about 2 m to about 5 m. 
       FIG. 12  shows a side view of a wind turbine foundation  500  that includes a mud slab  202 , a base can  102  on or otherwise above the mud slab  202 , a base slab  204  preferably made of concrete reinforced with rebar, and a plurality of radial girders  502  connected to the base can  102 . The plurality of girders  502  are like the plurality of girders  104  described above; however, the plurality of girders  502  shown in  FIG. 12  include girder trusses (open web girders) which allows for the plurality of girders  502  to be lighter than the formerly described plurality of girders  104  but maintain substantially the same level of strength. Each of the girders  502  is preferably tapered and preferably has a length ranging from about 8 m to about 15 m and a height at the highest point ranging from about 2.5 m to about 5 m. 
       FIG. 13A  shows a plan view of a wind turbine foundation  600  including a leveling slab  602 , a base can  102 , and a plurality of radial girders  604  connected to the base can  102 . The plurality of girders  604  are supported at distal ends  606  by a plurality of piles  608  extending into the ground. Each of the girders  604  is preferably tapered and preferably has a length ranging from about 5 m to about 15 m and a height at the highest point ranging from about 2 m to about 5 m.  FIG. 13B  shows an example in which the plurality of piles  608  include helical piles  610 .  FIG. 13C  shows an example in which the plurality of piles include concrete bell piles  612 . In cases where there are soft soils near the surface and stiffer soils or bedrock at depth using piles  608  to provide support will sometimes be advantageous as opposed to making the overall foundation much larger in diameter. Types of piles  608  that can be used include, without limitation, pipe piles, H piles, helical screw piles and concrete bell piles depending on the soil properties, groundwater depth and depth to the firm soils or bedrock. The piles  608  may be used alone with just the radial girders (buried or not buried) such that no concrete base slab is used. However, the piles may also be used in combination with a concrete base slab and buried as usual depending on the soil characteristics, groundwater depth and load requirements. In the case of expanding clay soil in the upper soil strata, piles  608  may be used in combination with a compressible foam panel or similar void form placed under the mud slab, base slab or girders to prevent the soil expansion from imposing uplift forces on the foundation. 
     The base can generally requires increased shear stiffness relative to the towers above to provide overall rotational stiffness. In some embodiments, this is achieved by a combination of inner radial stiffeners  702  connected (preferably by welding) to the inside of a base can  704  as required by site conditions and turbine manufacturer requirements. For additional strength and support, concrete can be added in the base can  704  between the radial stiffeners  702 . An example of a wind turbine foundation  700  including these features is shown in  FIGS. 14A-14G .  FIG. 14A  shows a plan view of the wind turbine foundation  700  including the base can  704 , inner radial stiffeners  702  connected to a central core member  708  (e.g., a steel pipe) along proximal edges and connected to the inside surface of the base can  704  along distal edges. The radial stiffeners  702  preferably include steel stiffener plates which can be connected to the base can  704  by, for example, welding or using bolts. 
       FIG. 14B  shows a side view,  FIG. 14C  shows a closer partial plan view, and  FIG. 14D  shows a closer partial side view of the wind turbine foundation  700 .  FIG. 14E  shows a plan view of the base can  704  by itself.  FIG. 14F  shows a close-up side view of the wind turbine foundation  700  cut along a line revealing what is shown in  FIG. 14G . In these various figures, different features are shown including a first outer flange  710 A near the top of the base can  704  and a second outer flange  710 B near the bottom of the base can  704 . A plurality of radial girders  712  are connected to the base can  704 . Each of the girders  712  includes upper girder flanges  714 A, lower girder flanges  714 B, and girder webs  716 . Each of the girders  712  is preferably tapered as shown and preferably has a length ranging from about 8 m to about 15 m and a height at the highest point ranging from about 2.5 m to about 5 m The base can  704  further includes a plurality of vertical flanges  718  which extend between the first outer flange  710 A and the second outer flange  710 B. The vertical flanges  718  are preferably situated directly adjacent to the girder webs  716  and first vertical plates  720 A and second vertical plates  720 B are preferably situated on either side, overlapping the vertical flanges  718  and the girder webs  716  such that, for example, bolts can be used to connect the vertical flanges  718 , girder webs  716 , first vertical plates  720 A and second vertical plates  720 B together. A close-up view of this is shown in  FIG. 14G . In addition to this connection, the first outer flange  710 A is preferably connected to the upper girder flanges  714 A using, for example, bolts tightened through first upper horizontal plates  722 A and second upper horizontal plates  722 B as shown in  FIG. 14F . Similarly, the second outer flange  710 B is preferably connected to the lower girder flanges  714 B using, for example, bolts tightened through first lower horizontal plates  724 A and second lower horizontal plates  724 B as shown in  FIG. 14F . 
     The base can  704  further includes a tower flange  726  which preferably extends inward and outward (like a “T”), preferably with at least two rows of apertures  728  through which bolts can be inserted to attach a first tower piece  730  to the wind turbine foundation  700 . The base can  704  preferably includes upper stiffener plates  732  which preferably extend from the first outer flange  710 A to or near the tower flange  726  and alternating partial stiffener plates  733  which alternate between inner radial stiffeners  702 . The upper stiffener plates  732  are preferably dispersed in line with girder webs  716  as well as spaces in between where girder webs  716  are angled toward the base can  704  as shown, for example, in  FIG. 14E . The base can  704  and girders  712  are preferably placed on support legs  734  including leveling bolts for leveling the base can  704  and girders  712  above rebar  736  on a mud slab  738 . After leveling is completed, a base layer  740  of concrete can be poured. The girders preferably include downward facing studs  742  (e.g., Nelson™ studs) that are enmeshed with the rebar  736  and the base layer base layer  740  and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. 
       FIG. 15A  and  FIG. 15B  show an embodiment of a wind turbine foundation  800  including a base can  802  and a plurality of girders  712  connected to the base can  802 . Inside the base can  802 , metal beams  804  (e.g., H beams) are connected (preferably by welding or field bolted) to an inside surface  806  of the base can  802  at locations adjacent to where girders  712  are connected to the base can  802 . The base can  802  further includes a first outer flange  808 A near the top of the base can  802  and a second outer flange  808 B near the bottom of the base can  802 . Each of the girders  712  includes upper girder flanges  714 A, lower girder flanges  714 B, and girder webs  716 . The base can  802  further includes a plurality of vertical flanges  810  which extend between the first outer flange  808 A and the second outer flange  808 B. The vertical flanges  810  are preferably situated directly adjacent to the girder webs  716  and first vertical plates  720 A and second vertical plates  720 B are preferably situated on either side, overlapping the vertical flanges  810  and the girder webs  716  such that, for example, bolts can be used to connect the vertical flanges  810 , girder webs  716 , first vertical plates  720 A and second vertical plates  720 B together. A close-up view of this type of connection in a previous related embodiment is shown in  FIG. 14G . In addition to this connection, the first outer flange  808 A is preferably connected to the upper girder flanges  714 A using, for example, bolts tightened through first upper horizontal plates  722 A and second upper horizontal plates  722 B. Similarly, the second outer flange  808 B is preferably connected to the lower girder flanges  714 B using, for example, bolts tightened through first lower horizontal plates  724 A and second lower horizontal plates  724 B. An example of these types of connections is shown in a previous embodiment shown in  FIG. 14F . 
     The base can  802  further includes a tower flange  812  which, in this embodiment, extends inward and outward (like a “T”), preferably with at least two rows of apertures through which bolts can be inserted to attach a first tower piece  730  to the wind turbine foundation  800 . The base can  802  and girders  712  are preferably placed on support legs  734  including leveling bolts for leveling the base can  802  and girders  712  above rebar  736  on a mud slab  738 . After leveling is completed, a base layer  740  of concrete can be poured. The girders preferably include downward facing studs  742  (e.g., Nelson™ studs) that are enmeshed with the rebar  736  and the base layer  740  and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. 
       FIG. 16A  and  FIG. 16B  show a different embodiment including a wind turbine foundation  900  which would typically be used when suitable bedrock is close or at the surface at a location where a wind turbine is to be built. Depending on the rock characteristics and the degree of rock weathering or quality, the foundation  900  may be either placed on the surface without backfill or excavated and backfilled as per previously described foundation installations with a concrete base slab or not. Given the stronger rock qualities for bearing and support, the diameter of the overall foundation  900  would typically be smaller and the forces would project out to the ends of a plurality of radial girders  902 . As such, in preferred embodiments, the girders  902  would not be tapered like those of the pure gravity base versions described above. The foundation  900  preferably includes the base can  704  including radial stiffeners  702 , central core member  708 , first outer flange  710 A, second outer flange  710 B, and vertical flanges  718 . The girders  902  are preferably connected to the base can in the same manner as the connection between the girders  712  and the base can  704  shown in  FIGS. 14A-14G . 
     The girders  902  include rock anchors  904  at distal ends  906  of the girders  904  wherein the anchors  904  penetrate into surrounding bedrock. The rock anchors  904  will be drilled in place to a depth suitable to meet the uplift force requirements according to the rock mechanics and bonding design, and some consolidation grouting of the surrounding rock also may be required. Typically, a double corrosion protected grouted bar anchor will be used in this application with post tensioning. However, a multi-strand cable anchor or multiple bar anchor with some canting could also be deployed. Rock anchor heads  908  at the top of the rock anchors  904  preferably would be designed to be accessible to check their post tensioning from time to time and the anchor heads  908  preferably will be corrosion protected with removable caps and grease or a similar system. The wind turbine foundation  900  also preferably includes a plurality of transverse girders  910  preferably connected between at the ends  906  of the girders  902 . Preferably, one or more rock anchors  904  are also connected to the transverse girders between the radial girders  902 . 
     In another aspect, an embodiment of a wind turbine foundation  1000  and associated parts is shown in  FIGS. 17A-17G . Instead of a wide base can with a diameter substantially the same a bottom tower piece, the wind turbine foundation  1000  has a narrower spool  1002  which preferably includes a metal cylindrical pipe including a top horizontal flange  1004 A, a bottom horizontal flange  1004 B, and a plurality of vertical flanges  1006 . The top horizontal flange  1004 A and the bottom horizontal flange  1004 B preferably extend out from the spool  1002  from about 2 m to about 6 m. The spool height preferably ranges from about 2.5 m to about 5 m. A plurality of girders  1008  are connected to the spool  1002  preferably using bolts along the top horizontal flange  1004 A, a bottom horizontal flange  1004 B, and a plurality of vertical flanges  1006 . The girders  1008  preferably have a length ranging from about 9 m to about 18 m and a maximum height ranging from about 2.5 m to about 5 m. The tower is mounted directly above the top of the girders themselves. The girders include upper girder flanges  1010 A, lower girder flanges  1010 B, and girder webs  1012 . The upper girder flanges  1010 A are connected to the top horizontal flange  1004 A, the lower girder flanges  1010 B are connected to the bottom horizontal flange  1004 B, and the girder webs  1012  are connected to the vertical flanges  1006 . As one example, the vertical flanges  1006  are preferably situated in pairs defining a plurality of slits  1014  wherein each pair includes a slit between each of the vertical flanges making up that particular pair of vertical flanges  1006 . Portions of the girder webs  1012  along proximal ends  1016  of the girders are slid into the slits  1014  and the girder webs  1012  are connected to the pairs of vertical flanges  1006  preferably using bolts. The upper girder flanges  1010 A along proximal ends  1016  of the girders  1008  are preferably tapered so that the girders  1008  can be connected to the spool  1002  in radial fashion as shown, for example, in  FIG. 17C . 
     The girders  1008  preferably include curved flanges  1018  which are preferably an extension of the upper girder flanges  1010 A at a location along the girders  1008  above which a first tower piece  1019  would rest. The curved flanges  1018  together form a circle as shown, for example, in  FIG. 17C . In one embodiment, a ring girder  1020  is placed above and connected to the curved flanges  1018  as shown, for example, in  FIGS. 17B and 17F-171 . In this embodiment, the ring girder  1020 —preferably a short cylinder of metal including an upper ring girder flange  1021  and a lower ring girder flange  1022 —is bolted to the curved flanges  1018  along the lower ring girder flange  1022 . The first tower piece  1019  is connected to the ring girder  1019  along the upper ring girder flange  1021 . In an alternative embodiment shown in  FIGS. 17H-171 , curved ring girder subsections  1023  are welded directly to the upper girder flanges  1010 A instead of using the curved flanges  1018 . In this alternate embodiment, there is preferably one ring girder subsection welded to each girder (one ring girder subsection per girder). When all girders  1008  are assembled in place (i.e., connected to the spool  1002 ), the curved ring girder subsections  1023  form a composite ring girder  1024  similar to the ring girder  1020 . The curved ring girder subsections  1023  include upper ring girder subsection flanges  1025  for connection with the first tower piece  1019 .  FIG. 17J  shows a plan view of the plurality of girders  1008  connected to the spool  1002  and including curved ring girder subsections  1023  connected to the radial girders  1008  to form the composite ring girder  1024  including the upper ring girder subsection flanges  1025  for connecting a tower piece to the composite ring girder  1024 . 
     In these examples, the girders  1008  include tapered stiffener plates  1026 . The stiffener plates  1026  are wide and are added to support the curved flange  1018  (if present), the ring girder  1020 , and/or the composite ring girder  1024  and distribute the tower forces to the full girder  1008  height. 
     During installation, the spool  1002  and girders  1008  are supported by support legs  1027  including leveling bolts. The support legs  1027  rest on a mud slab  1028 . A base layer  1030 , preferably of reinforced concrete, is laid above the mud slab  1028 , beneath the spool  1002  and girders  1008 . The girders  1008  preferably include downward facing studs  1032  (e.g., Nelson™ studs) that are enmeshed with the base layer  1030  and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. The wind turbine foundation  1000  provides a stiffer direct connection between the tower piece  1019  and the radial girders  1008  with the center girder connection done in a lower stress location reducing bolting and plate thicknesses as well as lessening fatigue issues. This configuration also provides a more direct load flow along each radial girder  1008  set from the compression side to the tension side of the foundation  1000 . 
     An example of a construction sequence for certain embodiments described herein is as follows: 
     1. Excavate foundation area (e.g., ˜3 m×20 m diameter), verify in situ ground conditions and improve as per normal foundation preparation.
 
2. Pour concrete mud slab to protect the exposed ground and create a level work surface.
 
3. Install base slab reinforcing over entire area noting the pattern for foundation orientation.
 
4. Install base can or spool on support legs which include leveling bolts.
 
5. Install all radial girders by bolting to the base can or spool. Final levelling of the tower flange is conducted by adjustment of the levelling bolts on the girder ends and base can or spool perimeter. The perimeter levelling bolts are used to perform final levelling adjustments to the tower flange. The levelling bolts on the base can or spool are raised during this process and re-lowered once the final levelling is complete.
 
6. Pour the base slab concrete and screed to the top of the second outer flange (girder base flange) ensuring full concrete contact to underside of flange by watching the air bleed holes in the flanges.
 
7. Install electrical conduits and grounding cables.
 
8. Pour concrete fill in base can (if applicable) and trowel finish top surface.
 
9. Backfill foundation with excavated soil stockpiled adjacent to the area.
 
10. Grade area for drainage, install gravel surfacing and install precast stairs foundation.
 
     In one specific nonlimiting example, the wind turbine foundation  100  is preferably housed in a hole dug in the ground with a preferred height of from about 2 meters to about 4 meters and diameter of from about 15 meters to about 25 meters for use with a 3.5 megawatt (MW) wind turbine. Although specific preferred dimensions are provided herein for an example of a foundation for use with a 3.5 MW wind turbine, it should be understood that the technology described herein can be scaled with different dimensions to accommodate different sized wind turbines. Digging the hole is a first step (A1) in building the wind turbine foundation  100 . An additional step (B1) includes pouring an underlying mud slab  112  in the hole wherein, in this specific example, the underlying mud slab  112  is preferably from about 50 mm to about 200 mm thick. The base can  102  and girders  104  are preferably situated on the mud slab  112  that, in this specific example, is preferably round with a diameter of from about 15 meters to about 25 meters and most preferably about 20 meters. In this specific example, the girders  104  are preferably about 2 meters tall along the tallest edge of the girders  104  where the girders  104  attach to the base can  102 , however other sizes are contemplated for different embodiments. 
     An additional step (C1) in making the wind turbine foundation  100  includes placing the base can  102  in the hole on the underlying mud slab  112 . The base can  102  is preferably placed at the approximate center of the underlying mud slab  112 . In this specific example, the base can  102  is preferably from about 2 meters to about 6 meters high and most preferably about 3.5 meters high. In this specific example, the base can  102  is preferably about 4 meters to about 6 meters in diameter and most preferably about 5 meters in diameter. The base can  102  preferably includes a collector port  172  through which electrical and potentially other connections can be made to a wind turbine resting on the wind turbine foundation  100 . 
     An additional step (D1) includes placing rebar  114  in the hole along the underlying mud slab  112 . The rebar  114  preferably has a diameter of about 20 mm, a linear mass density of about 2.4 Kg/m and a cross-sectional area of about 300 mm 2  but other rebar sizes may be used. In this specific example, the total weight of rebar used per wind turbine foundation should be from approximately 10,000 Kg to about 16,000 Kg and most preferably less than about 13,000 Kg. 
     Another step (E1) includes attaching the girders  104  to the base can  102  preferably using bolts. In a following step (F1), a base layer of concrete  116  is poured beneath the girders  104  and beneath the base can  102 . In this specific example, preferably from about 100 m 3  to about 120 m 3  of concrete is used to form the base layer  116 . 
     A next step (G1) includes placing a mass of material (e.g., backfill  128  from the excavation to dig the hole) above the base layer  116  and preferably up to the collector port  172 . Another step (H1) includes installing a collector conduit bundle  174  preferably through a culvert. 
     Another step (I1) includes adding additional backfill to the hole and inside the base can  102 . Preferably, the backfill is added to a consistent depth across the hole with a slight slope of from about 2% to about 5% away from an upper section  128 A of the base can  102  that remains exposed. In another step (J1), additional rebar is added inside the base can  102 . A following step (K1) includes pouring concrete into the base can  102  to form a base can slab  176  wherein, in this specific example, from about 1 m 3  to about 3 m 3  of concrete is used. Another step (L1) includes attaching the first tower piece  110  to the tower flange  108  along the upper section  128 A of the base can  102 . The first outer flange  106 A extends out from a main body  178  of the base can  102  and is also located along the upper section  128 A of the base can  102 . The second outer flange extends out from the main body  178  of the base can  102  and is located along a lower section  128 B of the base can  102 . As an example, the first outer flange  106 A and the second outer flange  106 B may be welded to the main body  178  or may be formed as a part of the base can  102 . 
     In an embodiment where the base can is replaced by a spool, the placement of the spool, the installation of rebar, the attachment of the girders to the spool, the pouring of the concrete base layer, the placing of the mass of material including installation of a collector conduit and additional backfilling of the spool follow similar steps as outlined above with the base can being replaced by the spool. 
     The previously described embodiments of the present disclosure have many advantages. As described in the Background section, current methods of making foundations for wind turbines of the 3.5 MW size typically use about 400 m 3  of concrete, 83,000 Kg of steel, require a 5 week build cycle, and, depending on the geographic location, can only be built for certain months out of the year. For example, in many parts of Canada, construction can only be best carried out for about 8 months out of the year. Some of the embodiments described herein relating to 3.5 MW turbines typically use about 140 m 3  of concrete and 70,000 Kg of steel. Some of these embodiments described herein have a three week build cycle, and can be built all twelve months of the year regardless of geography since many of the components including the girders can be made offsite during colder or otherwise inclement months. Current methods for making foundations for 3.5 MW wind turbines require on average about 80 truckloads of material. Some of the embodiments described herein relating to 3.5 MW turbines require approximately 20 truckloads of material since much of the ballast used is backfill from the initial excavation (which, therefore, does not need to be hauled away). 
     The various embodiments preferably use pre-fabricated structural steel components for efficient load transfer and distribution as part of the foundation. Such embodiments maximize use of natural in-situ materials (e.g., excavated soil) to provide stability. The embodiments described herein do not use a pre-tensioned anchor bolt cage embedded in concrete for transferring load from the tower to the foundation. Eliminating the anchor bolt cage eliminates a major construction step and makes rebar placement easier. A bolted flange connection eliminates the entire anchor cage typically consisting of about 180 4 m long×40 mm bolts and associated steel anchor rings. Embodiments described herein have a design type that is a raft foundation, like a traditional concrete raft foundation, however instead of concrete providing part of the bending and shear resistance and most of the ballast, the embodiments described herein use radial girders connected to a base can or spool for primary load transfer and use mostly backfill as ballast over the thin concrete base slab. The loads transferred to the girders are distributed into the proximal parts of the concrete slab. The slab is held in place by bearing on the subgrade below and the weight of the backfill on top of it. Similar to traditional concrete raft foundations, in medium to low strength soil conditions, the design is typically governed by rotational stiffness, depending on the turbine manufacturer requirement for stiffness. In stronger soils and on bedrock, the foundation size tends to be governed by overturning stability and sometimes bearing capacity. 
     New 4+MW wind turbines forces and diameters are causing design limits to be reached for traditional concrete raft foundations so an alternative to such foundations is becoming more necessary. High shear, rebar spacing issues and high-strength concrete are now common. Site conditions are dictating multiple traditional foundation solutions that increase cost and logistical challenges. For example, high groundwater and shallow weak bedrock are often found. One foundation type—a universal solution—is better than two or three different foundation types on one site from an economies-of-scale and simplicity-of-construction perspective. Pre-fabricated foundations offer year-round construction opportunity which decreases build time and reduces constraints. Shop manufactured components can be built and shipped any time of the year. In embodiments described herein, the tower to foundation joint is a bolted steel flange connection instead of a grouted base and anchor bolt connection. As turbine sizes increase, grouted connections are now reaching their maximum capacity. Bolted steel flanges offer much higher capacity that are in concert with the other tower connections above and have much better fatigue performance. 
     The foregoing description of preferred embodiments of the present disclosure has been presented for purposes of illustration and description. The described preferred embodiments are not intended to be exhaustive or to limit the scope of the disclosure to the precise form(s) disclosed. Different features of some embodiments can be substituted for other features of other embodiments to arrive at different embodiments of the concepts described herein. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the disclosure and its practical application, and to thereby enable one of ordinary skill in the art to utilize the concepts revealed in the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the disclosure as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.