Patent Publication Number: US-11047103-B2

Title: Concrete foundation structure and method for constructing same

Description:
TECHNICAL FIELD 
     The present invention relates to a concrete foundation structure using a precast concrete foundation and a method for constructing the concrete foundation structure. 
     BACKGROUND ART 
     Concrete foundations such as footing foundations, mat foundations, and independent foundations are generally used as foundations for receiving a load of a building. However, when constructing the foundations, since many steps including a step of assembling a formwork on site, a step of incorporating a rebar based on a structural design, a step of pouring concrete into the formwork, a step of hardening the concrete, a step of removing the formwork, and the like are required, there is a problem that a construction period is long. Further, since the foundations are constructed manually on site, there is a problem that an error is likely to occur in finishing accuracy. Furthermore, since it is necessary to secure a skilled technician and a special vehicle such as a concrete pump truck, there is a problem that construction cost increases. 
     As a typical technique that can solve the above problems, there is a foundation structure disclosed in PATENT LITERATURE 1. The foundation structure includes a concrete foundation plate placed on a surface of a ground and an underground anchor driven into the ground, and the concrete foundation plate and the underground anchor are connected to each other. With the foundation structure, since the concrete foundation plate can be produced in a factory, concrete pouring work and the like on site can be omitted. 
     CITATION LIST 
     Patent Literature 
     PATENT LITERATURE 1: JP-A-2011-043027 
     However, the foundation structure described in PATENT LITERATURE 1 is used for applications that receive a load of a device such as an electric water heater, and is not intended to be used for applications that receive the load of the building. Therefore, when the foundation structure is used for receiving the load of the building, there is a possibility that the concrete foundation plate sinks, or the underground anchor is pulled out by force of wind hitting the building and the concrete foundation plate moves. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to address the above problems, and an object of the present invention is to provide a concrete foundation structure capable of firmly fixing a precast concrete foundation to a ground and a method for constructing the concrete foundation structure. 
     In order to achieve the above object, a feature of a concrete foundation structure according to the present invention is that the concrete foundation structure includes: a precast concrete foundation placed on a surface of a ground; an excavation hole formed by excavating the ground below the precast concrete foundation; a plate-shaped anchor plate placed inside the excavation hole; a rod-shaped connecting member connecting the precast concrete foundation and the anchor plate; a backfill portion formed by backfilling a backfill material including a solidifying material in the excavation hole; and a filler layer formed by filling a filler including the solidifying material between the precast concrete foundation and the backfill portion, wherein the precast concrete foundation comprises: a foundation body formed to cover the backfill portion from above; and a projecting portion provided to project downward from an outer periphery of the foundation body and embedded in the ground. 
     According to this configuration, the backfill portion formed below the precast concrete foundation is solidified by using the solidifying material, and the filler layer formed between the precast concrete foundation and the backfill portion is solidified by using the solidifying material, so that the precast concrete foundation can be stably supported by the backfill portion and the filler layer. Further, since the anchor plate connected to the precast concrete foundation is embedded in the backfill portion, the precast concrete foundation and the backfill portion are integrated together, so that the precast concrete foundation can be restrained from moving by a frictional force acting between an outer surface of the backfill portion and an inner surface of the excavation hole. Furthermore, since the projecting portion of the precast concrete foundation is embedded in the ground, the precast concrete foundation can be restrained from moving by the frictional force acting between the projecting portion and the ground. Therefore, the precast concrete foundation can be firmly fixed to the ground. 
     Another feature of the concrete foundation structure according to the present invention is that the projecting portion is formed annularly on the outer periphery of the foundation body. 
     According to this configuration, since the projecting portion is formed annularly on the outer periphery of the foundation body, an upper portion of the backfill portion can be surrounded by the projecting portion, and the ground can be present all around the projecting portion. Therefore, when a horizontal external force acts on the precast concrete foundation, the precast concrete foundation can be effectively restrained from moving by the backfill portion and the ground. 
     Another feature of the concrete foundation structure according to the present invention is that the foundation body is provided with a through-hole through which the connecting member is inserted, and a portion of the connecting member projecting upward from the through-hole is provided with a movement preventing portion that contacts the foundation body and prevents the precast concrete foundation from moving upward. 
     According to this configuration, an upper end of the connecting member embedded in the backfill portion can be inserted through the through-hole provided in the foundation body, and then the movement preventing portion can be provided in the portion of the connecting member projecting upward from the through-hole. Therefore, when forming the backfill portion, the precast concrete foundation can be separated from the connecting member, and work of forming the backfill portion can be performed efficiently. 
     Another feature of the concrete foundation structure according to the present invention is that a filler of the same type as the filler is filled in the through-hole, and the filler filled in the through-hole is continuously integrated with the filler forming the filler layer. 
     According to this configuration, since a first portion of the connecting member present between the precast concrete foundation and the backfill portion and a second portion of the connecting member present inside the through-hole can be wrapped with one continuous filler, it is possible to prevent a shearing force from acting on a boundary between the first portion and the second portion, thereby improving durability of the connecting member. 
     Another feature of the concrete foundation structure according to the present invention is that the through-hole is formed to have a cross-sectional area decreasing downward. 
     According to this configuration, since the through-hole is formed to have the cross-sectional area decreasing downward, when the precast concrete foundation is to be separated from the filler layer, the filler filled in the through-hole is caught by the inner surface of the through-hole. Therefore, the precast concrete foundation is difficult to be separated from the filler layer. 
     In order to achieve the above object, a feature of a method for constructing a concrete foundation structure according to the present invention is that the method includes: a step (a) of excavating a ground to form an excavation hole; a step (b) of attaching a plate-shaped anchor plate to a lower end of a rod-shaped connecting member, and placing the anchor plate and the connecting member inside the excavation hole; a step (c) of backfilling a backfill material including a solidifying material in the excavation hole to form a backfill portion, and projecting an upper end of the connecting member upward from an upper surface of the backfill portion; a step (d) of preparing a precast concrete foundation having a foundation body provided with a through-hole through which the connecting member is inserted; a step (e) of placing the precast concrete foundation on a surface of the ground and inserting the connecting member through the through-hole; a step (f) of filling a filler including a solidifying material between the precast concrete foundation and the backfill portion; and a step (g) of attaching a movement preventing portion to a portion of the connecting member projecting upward from the through-hole, the movement preventing portion contacting the foundation body and preventing the precast concrete foundation from moving upward. 
     According to this configuration, in the step (c), since the backfill material including the solidifying material is backfilled in the excavation hole to form the backfill portion, the backfill portion can be formed firmly. Further, in the step (f), since the filler including the solidifying material is filled between the precast concrete foundation and the backfill portion, the filler layer formed of the filler can be formed firmly. Therefore, the precast concrete foundation can be stably supported by the backfill portion and the filler layer. Further, the anchor plate and the connecting member are arranged inside the excavation hole in the step (b), the backfill material is backfilled in the excavation hole to form the backfill portion in the step (c), and the movement preventing portion is attached to the portion of the connecting member projecting upward from the through-hole in the step (g), so that the precast concrete foundation and the backfill portion can be integrated together. Therefore, the precast concrete foundation can be restrained from moving by the frictional force acting between the outer surface of the backfill portion and the inner surface of the excavation hole. 
     Another feature of the method for constructing the concrete foundation structure according to the present invention is that, in the step (f), the filler is also filled in the through-hole, and the filler filled between the precast concrete foundation and the backfill portion and the filler filled in the through-hole are continuously integrated together. 
     According to this configuration, since the first portion of the connecting member present between the precast concrete foundation and the backfill portion and the second portion of the connecting member present inside the through-hole can be wrapped with one continuous filler, it is possible to prevent the shearing force from acting on the boundary between the first portion and the second portion, thereby improving the durability of the connecting member. 
     Another feature of the method for constructing the concrete foundation structure according to the present invention is that, in the step (f), the filler is filled between the precast concrete foundation and the backfill portion through the through-hole. 
     According to this configuration, since the filler is filled between the precast concrete foundation and the backfill portion through the through-hole through which the connecting member is inserted, it is not necessary to separately form a through-hole for filling the filler, and production cost of the precast concrete foundation can be kept low. 
     Another feature of the method for constructing the concrete foundation structure according to the present invention is that, in the step (c), the backfill portion is compacted. 
     According to this configuration, in the step (c), since the backfill portion is compacted, the backfill portion can be formed more firmly. 
     Another feature of the method for constructing the concrete foundation structure according to the present invention is that, in the step (e), a height adjuster having a male screw extending in a vertical direction and a female screw screwed onto the male screw is attached to a side surface of the precast concrete foundation, and the male screw is rotated to adjust a length of a portion of the male screw projecting downward from the precast concrete foundation, so that a height of the precast concrete foundation is adjusted. 
     According to this configuration, in the step (e), since the height of the precast concrete foundation is adjusted, the concrete foundation structure can be constructed with high accuracy. Further, since the height adjuster has a simple structure having a male screw and the female screw, height adjusting operation can be easily performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a structure of a concrete foundation structure according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view showing an example of using the concrete foundation structure according to the embodiment of the present invention. 
         FIG. 3  is a plan view showing the example of using the concrete foundation structure according to the embodiment of the present invention. 
         FIG. 4  is an exploded perspective view showing a part of the structure of the concrete foundation structure as viewed obliquely from above. 
         FIG. 5  is an exploded perspective view showing the part of the structure of the concrete foundation structure as viewed from obliquely below. 
         FIG. 6(A)  is a cross-sectional view showing a step of providing an excavation hole,  FIG. 6(B)  is a cross-sectional view showing a step of placing an anchor plate and a connecting member inside the excavation hole, and  FIG. 6(C)  is a cross-sectional view showing a step of backfilling a backfill material in the excavation hole. 
         FIG. 7(D)  is a cross-sectional view showing a step of placing a precast concrete foundation on a surface of a ground,  FIG. 7(E)  is a cross-sectional view showing a step of forming a filler layer, and  FIG. 7(F)  is a cross-sectional view showing a step of attaching the precast concrete foundation to the connecting member. 
         FIG. 8(A)  is a front view showing a state in which a height adjuster is attached to the precast concrete foundation, and  FIG. 8(B)  is a plan view showing the state in which the height adjuster is attached to the precast concrete foundation. 
         FIG. 9  is a cross-sectional view for describing a step of forming the filler layer in detail. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a concrete foundation structure and a method for constructing the concrete foundation structure according to an embodiment of the present invention will be described with reference to the drawings. 
     (Concrete Foundation Structure) 
       FIG. 1  is a cross-sectional view showing a structure of a concrete foundation structure  10  according to an embodiment of the present invention.  FIG. 2  is a cross-sectional view showing an example of using the concrete foundation structure  10 .  FIG. 3  is a plan view showing the example of using the concrete foundation structure  10 .  FIG. 4  is an exploded perspective view showing a part of the structure of the concrete foundation structure  10  as viewed obliquely from above.  FIG. 5  is an exploded perspective view showing the part of the structure of the concrete foundation structure  10  as viewed from obliquely below. 
     The concrete foundation structure  10  shown in  FIG. 1  is a structure that receives a load of a building  12  on a surface of a ground G. In examples of use shown in  FIGS. 2 and 3 , four concrete foundation structures  10  are constructed at a distance from each other to secure a required supporting area (for example, ⅓ or more of a floor area) determined according to the floor area of the building  12 . On an upper portion of each concrete foundation structure  10 , joists  14   a  constituting a floor structure  14  are fixed, and a load of an entrance porch  12   a  is received using each half of two concrete foundation structures  10 . 
     A space S (including soil) through which various pipes P for water and sewage, electricity, gas, and the like are inserted is formed between two adjacent concrete foundations structures  10 . The space S is a ventilation path for taking in outside air into an underfloor space and discharging moisture in the underfloor space to the outside. As shown in  FIG. 1 , the pipes P are fixed to the concrete foundation structure  10  using a pipe holding member  62 . 
     As shown in  FIG. 1 , the concrete foundation structure  10  includes a precast concrete foundation  16 , an excavation hole  18 , an anchor plate  20 , two connecting members  22 , a backfill portion  24 , and a filler layer  26 . Among them, the precast concrete foundation  16 , the anchor plate  20  and the two connecting members  22  are manufactured in a factory, and the excavation hole  18 , the backfill portion  24  and the filler layer  26  are formed on site. 
     As shown in  FIGS. 4 and 5 , the precast concrete foundation  16  is a plate-shaped or block-shaped member formed of concrete and having a rectangular shape in a plan view. The precast concrete foundation  16  includes a foundation body  28  formed to cover the backfill portion  24  ( FIG. 1 ) from above, and a projecting portion  30  provided to project downward from an outer periphery of the foundation body  28  and embedded in the ground G ( FIG. 1 ). 
     As shown in  FIG. 4 , the foundation body  28  has two recesses  32  that are open upward and are quadrangular in a plan view. The two recesses  32  are provided side by side in a length direction of the foundation body  28 , and a partition  34  is formed between the two recesses  32 . An upper surface  28   a  of the foundation body  28  in which an opening  32   a  of each recess  32  is opened is formed at a constant height and flat. The upper surface  28   a  is a support surface for receiving the load of the building  12 . Further, a plurality of female screw members  40  into which bolts  38  for fixing the joists  14   a  and the like shown in  FIG. 1  are screwed are embedded in the upper surface  28   a.    
     A through-hole  42  through which the connecting member  22  is inserted is provided in each of bottom portions  32   b  of the two recesses  32 . In addition, a plurality of female screw members  46  into which hanging metal fittings  44  ( FIGS. 8(A) and 8(B) ) such as eyebolts are screwed are respectively embedded in the bottom portions  32   b  of the two recesses  32 . 
     The through-hole  42  has both a function of inserting the connecting member  22  therethrough and a function of introducing a filler  96  ( FIG. 9 ) between the precast concrete foundation  16  and the backfill portion  24 . The through-hole  42  is formed in a tapered shape so as to have a cross-sectional area decreasing downward. 
     In the concrete foundation structure  10  shown in  FIG. 1 , the connecting member  22  is inserted into the through-hole  42 . Further, a filler  96  of the same type as the filler  96  described below that forms the filler layer  26  is filled in the through-hole  42 . The filler  96  filled in the through-hole  42  is continuously integrated with the filler  96  forming the filler layer  26 . 
     As shown in  FIG. 4 , a step portion  48  is formed at one end in a width direction (a direction perpendicular to the length direction) of the upper portion of the foundation body  28 . The upper surface  48   a  of the step portion  48  is formed flat at a lower height than the upper surface  28   a  of the foundation body  28 . The upper surface  48   a  is a support surface for supporting a rod-shaped support  50  ( FIG. 3 ) and the like that receives the load of the building  12 . A plurality of female screw members  54  into which bolts  52  ( FIG. 3 ) for fixing the support  50  and the like are screwed are embedded in the upper surface  48   a  of the step portion  48 . 
     As shown in  FIG. 5 , the projecting portion  30  is formed annularly on the outer periphery of the foundation body  28 . In the present embodiment, the projecting portion  30  is formed in a quadrangular wall shape in a bottom view so that a shape of the precast concrete foundation  16  is rectangular in a bottom view. A housing space  56  for housing an upper portion of the backfill portion  24  ( FIG. 1 ) is formed inside the projecting portion  30 . An inner surface  30   a  of the projecting portion  30  is formed to be inclined so that a transverse surface (a horizontal cross-sectional surface) of the housing space  56  gradually widens downward. A part of a rebar  74  described later is placed to extend in an inclined direction of the inner surface  30   a  of the projecting portion  30 , thereby increasing the strength of the projecting portion  30  against an external force acting in a horizontal direction. A lower surface  30   b  of the projecting portion  30  having an opening  56   a  of the housing space  56  is formed flat. 
     As shown in  FIG. 5 , a plurality of female screw members  66  into which bolts  64  for fixing the pipe holding member  62  and the like shown in  FIG. 1  are screwed are respectively embedded in four side surfaces  58   a ,  58   b ,  60   a ,  60   b  of the precast concrete foundation  16 . Further, a plurality of female screw members  72  into which bolts  70  for fixing a height adjuster  68  shown in  FIGS. 8(A) and 8(B)  are screwed are embedded in the both side surfaces  60   a  and  60   b  in a width direction of the precast concrete foundation  16 . Further, in each of the foundation body  28  and the projecting portion  30 , the rebar  74  for reinforcing them is embedded. 
     The size of the precast concrete foundation  16  is set to 2516 mm in length, 1367 mm in width, and 600 mm in height. A weight of the precast concrete foundation  16  is set to 2765 Kg. The size of the opening  56   a  of the housing space  56  is set to 2216 mm in length and 1067 mm in width. The size of the through-hole  42  is set to 60 mm in inner diameter at its upper end and 50 mm in inner diameter at its lower end. 
     As shown in  FIG. 4 , the anchor plate  20  is a plate-shaped member having a rectangular shape in a plan view, and is formed of a metal material such as a rolled steel material with a strength that is not easily bent. The anchor plate  20  is provided with two through-holes  76  through which the connecting members  22  are inserted, at intervals in the length direction. The size of the anchor plate  20  is set to 2000 mm in length, 800 mm in width, and 12 mm in thickness. 
     As shown in  FIG. 4 , the connecting member  22  is a member for connecting the precast concrete foundation  16  and the anchor plate  20 , and is formed of a metal material such as stainless steel into a rod shape having a circular cross-section. A first male screw  78   a  is provided at one end (a lower end) of the connecting member  22 , and a second male screw  78   b  is provided at the other end (an upper end) of the connecting member  22 . The size of the connecting member  22  is set to 1000 to 2000 mm in length and 20 mm in diameter. The strength of the connecting member  22  is set to 7 t in shear strength. 
     In the concrete foundation structure  10  shown in  FIG. 1 , the connecting member  22  is disposed to extend vertically inside the excavation hole  18 . Then, the first male screw  78   a  of the connecting member  22  is inserted into the through-hole  76  of the anchor plate  20 , and the anchor plate  20  is sandwiched between two female screw members  80  and  82  screwed into the first male screw  78   a . Thus, the anchor plate  20  is attached to the lower end of the connecting member  22 . 
     Further, the second male screw  78   b  of the connecting member  22  is inserted through the through-hole  42  of the precast concrete foundation  16 , and a movement preventing portion  84  is attached to a portion of the second male screw  78   b  projecting upward through the through-hole  42 . Thus, the precast concrete foundation  16  is attached to the upper end of the connecting member  22 . The movement preventing portion  84  is a member that contacts the foundation body  28  and prevents the precast concrete foundation  16  from moving upward, and has a female screw  84   a  screwed into the second male screw  78   b  of the connecting member  22  as shown in  FIG. 4 . 
     As shown in  FIG. 1 , the excavation hole  18  is formed in a rectangular shape in a plan view by excavating the ground G below the precast concrete foundation  16 . A step portion  86  is provided in an inner periphery of an upper portion of the excavation hole  18 , and a concrete layer  88  and a dry mortar layer  90  are formed in this order on an upper surface of the step portion  86 . The concrete layer  88  is a reinforcing layer for reinforcing the upper surface of the step portion  86 , and the dry mortar layer  90  is a height adjustment layer for adjusting a height of the precast concrete foundation  16 . A wall surface  92  of the ground G is formed around a region of the dry mortar layer  90  in which the precast concrete foundation  16  is placed. The size of the excavation hole  18  is set to 2100 mm in length, 900 mm in width, and 1000 to 2000 mm in depth. 
     An opening  18   a  of the excavation hole  18  is disposed inside the opening  56   a  of the housing space  56  of the precast concrete foundation  16  in a plan view, and the wall surface  92  of the ground G is disposed outside an outer surface of the projecting portion  30  of the precast concrete foundation  16  in a plan view. Therefore, the projecting portion  30  of the precast concrete foundation  16  can be placed on the step portion  86 , and the precast concrete foundation  16  can be stably supported by the ground G forming the step portion  86 . 
     As shown in  FIG. 1 , the backfill portion  24  is formed by backfilling a backfill material  94  including a cement-based solidifying material that is solidified by reaction with water and the soil into the excavation hole  18 . Further, the backfill portion  24  is firmly hardened by being compacted, and an outer surface of the backfill portion  24  is in close contact with an inner surface of the excavation hole  18 . Since the excavation hole  18  of the present embodiment is formed in a rectangular shape in a plan view, the backfill material  94  backfilled in the excavation hole  18  is solidified into a quadrangular prism shape inside the excavation hole  18 , to be integrated with the anchor plate  20  and the connecting member  22 . The upper portion of the backfill portion  24  is housed in the housing space  56  of the precast concrete foundation  16 , and an entire upper surface  24   a  of the backfill portion  24  is placed facing a lower surface  16   a  of the precast concrete foundation  16  forming the housing space  56 . 
     As shown in  FIG. 1 , the filler layer  26  is formed by filling the filler  96  including the cement-based solidifying material that is solidified by reaction with water into a space between the precast concrete foundation  16  and the backfill portion  24 . The filler  96  is sometimes called “cement milk”. As described above, in the present embodiment, since the entire upper surface  24   a  of the backfill portion  24  is placed facing the lower surface  16   a  of the precast concrete foundation  16 , the entire upper surface  24   a  of the backfill portion  24  and the lower surface  16   a  of the precast concrete foundation  16  can be connected to each other by the filler layer  26 . Therefore, the load of the building  12  acting on the precast concrete foundation  16  can be received by the entire backfill portion  24 . 
     (Method for Constructing Concrete Foundation Structure) 
       FIG. 6(A)  is a cross-sectional view showing a step of providing the excavation hole  18 ,  FIG. 6(B)  is a cross-sectional view showing a step of placing the anchor plate  20  and the connecting member  22  inside the excavation hole  18 , and  FIG. 6(C)  is a cross-sectional view showing a step of backfilling the backfill material  94  in the excavation hole  18 .  FIG. 7(D)  is a cross-sectional view showing a step of placing the precast concrete foundation  16  on the surface of the ground G,  FIG. 7(E)  is a cross-sectional view showing a step of forming the filler layer  26 , and  FIG. 7(F)  is a cross-sectional view showing a step of attaching the precast concrete foundation  16  to the connecting member  22 . When constructing the concrete foundation structure  10 , a constructor performs the following steps (a) to (g) in this order. 
     In the step (a), a position at which the concrete foundation structure  10  is constructed is determined at a construction site of the building  12  ( FIG. 3 ). Then, as shown in  FIG. 6(A) , the excavation hole  18  is formed by excavating the ground G at the determined position. At this time, the step portion  86  is formed in the inner periphery of the upper portion of the excavation hole  18 . 
     In the step (b), the anchor plate  20  shown in  FIGS. 4 and 5  is attached to the lower end of the rod-shaped connecting member  22  using the two female screw members  80  and  82 . Then, as shown in  FIG. 6(B) , the anchor plate  20  and the two connecting members  22  are placed inside the excavation hole  18 . At this time, the anchor plate  20  is placed horizontally, and the two connecting members  22  are placed vertically to the anchor plate  20 . 
     In the step (c), as shown in  FIG. 6(C) , the backfill material  94  including the solidifying material that is solidified by reaction with water and the soil is backfilled in the excavation hole  18  to form the backfill portion  24 . Further, the upper end of the connecting member  22  is projected upward from the upper surface  24   a  of the backfill portion  24 . Then, the backfill portion  24  is compacted to release air contained in the backfill portion  24 . Then, the backfill portion  24  is hardened together with the solidifying material reacting with soil moisture and being solidified. 
     In the step (d), the precast concrete foundation  16  shown in  FIGS. 4 and 5  is prepared. The precast concrete foundation  16  has the foundation body  28  and the projecting portion  30 , and the foundation body  28  is provided with the through-hole  42  through which the connecting member  22  is inserted. Since the size and weight of the precast concrete foundation  16  are determined as described above, the precast concrete foundation  16  can be carried by a vehicle, and the precast concrete foundation  16  manufactured with high precision using factory equipment can be loaded on the vehicle and brought to the site. 
     In the step (e), as shown in  FIG. 7(D) , the precast concrete foundation  16  is placed on the surface of the ground G, and the connecting member  22  is inserted through the through-hole  42 . Further, the height of the precast concrete foundation  16  is adjusted using two height adjusters  68  shown in  FIGS. 8  (A) and  8  (B). 
       FIG. 8(A)  is a front view showing a state in which the height adjuster  68  is attached to the precast concrete foundation  16 , and  FIG. 8(B)  is a plan view showing the state in which the height adjuster  68  is attached to the precast concrete foundation  16 . 
     As shown in  FIGS. 8(A) and 8(B) , the height adjuster  68  includes two male screws  98  extending vertically, support bodies  100  having female screws  100   a  into which the male screws  98  are screwed, and a base body  102  to which two support bodies  100  are fixed. A length of the base body  102  is set to be substantially the same as the length of the precast concrete foundation  16 . The two support bodies  100  are fixed to both ends in a longitudinal direction of the base body  102 . The base body  102  is provided with a plurality of through-holes  104  through which the bolts  70  screwed into the female screw members  72  ( FIGS. 4 and 5 ) of the precast concrete foundation  16  are inserted. 
     When adjusting the height of the precast concrete foundation  16 , as shown in  FIG. 8(B) , the two height adjusters  68  are attached to the both side surfaces  60   a  and  60   b  in the width direction of the precast concrete foundation  16  using the bolts  70 . Further, as shown in  FIG. 7(D) , the concrete layer  88  and the dry mortar layer  90  are formed in this order on the upper surface of the step portion  86 . Subsequently, the precast concrete foundation  16  is suspended by a wire W shown in  FIGS. 8(A) and 8(B) , and is placed on the dry mortar layer  90 , and the height of the precast concrete foundation  16  is measured. Then, by rotating the male screw  98  and changing a length of a projecting portion  98   a  of the male screw  98  projecting downward from the precast concrete foundation  16 , the height of the precast concrete foundation  16  is adjusted so that a measured value matches a design value. 
     In the present embodiment, since four male screws  98  are attached to the precast concrete foundation  16 , the height of the precast concrete foundation  16  can be accurately adjusted by appropriately rotating each of the male screws  98 . Note that the number of the male screws  98  attached to the precast concrete foundation  16  is not particularly limited, but may be three or less, or five or more. 
     In the step (f), as shown in  FIG. 7(E) , the filler  96  including the solidifying material that is solidified by reaction with water and the water is filled from the through-hole  42  into the space between the precast concrete foundation  16  and the backfill portion  24 . Then, the solidifying material of the filler  96  reacts with water to be solidified, so that the filler layer  26  is formed between the precast concrete foundation  16  and the backfill portion  24 . At this time, since a part of the filler  96  penetrates into the backfill portion  24 , the filler layer  26  is firmly integrated with the backfill portion  24 . 
       FIG. 9  is a cross-sectional view for describing a step of forming the filler layer  26  in detail. In the step (f), as shown in  FIG. 9 , a part of the filler  96  is also filled into the through-hole  42 , so that the filler  96  filled into the space between the precast concrete foundation  16  and the backfill portion  24  and the filler  96  filled into the through-hole  42  are continuously integrated together. 
     In the step (g), as shown in  FIG. 7  (F), the movement preventing portion  84  that contacts the foundation body  28  and prevents the precast concrete foundation  16  from moving upward is attached to a portion of the connecting member  22  projecting upward from the through-hole  42 . That is, the female screw  84   a  ( FIG. 4 ) of the movement preventing portion  84  is screwed onto the second male screw  78   b  of the connecting member  22 . 
     Effects of Embodiment 
     According to the present embodiment, the following effects can be obtained by the above configuration. That is, the backfill portion  24  formed below the precast concrete foundation  16  shown in  FIG. 1  is solidified using the solidifying material, and the filler layer  26  formed between the precast concrete foundation  16  and the backfill portion  24  is solidified using the solidifying material, so that the backfill portion  24  and the filler layer  26  can stably support the precast concrete foundation  16 . 
     As shown in  FIG. 1 , since the anchor plate  20  connected to the precast concrete foundation  16  is embedded in the backfill portion  24 , the precast concrete foundation  16  and the backfill portion  24  are integrated together, so that the precast concrete foundation  16  can be restrained from moving by a frictional force acting between the outer surface of the backfill portion  24  and the inner surface of the excavation hole  18 . Further, since the projecting portion  30  of the precast concrete foundation  16  is embedded in the ground G, the precast concrete foundation  16  can be restrained from moving by the frictional force acting between the projecting portion  30  and the ground G. Therefore, the precast concrete foundation  16  can be firmly fixed to the ground G. 
     In the present embodiment, since the size, weight, material, and the like of each portion are determined as described above, in the ground G having a ground supporting force of 5 t/m 2  or more, a pull-out resistance of 16.7 t can be obtained per one concrete foundation structure  10 . Therefore, in the example of using four concrete foundation structures  10  ( FIGS. 1 and 2 ), a high pull-out resistance of 66.8 t can be obtained as a whole, and it can withstand a tornado wind at a wind speed of about 80 m/s. 
     As shown in  FIG. 5 , since the projecting portion  30  is formed annularly on the outer periphery of the foundation body  28 , the upper portion of the backfill portion  24  can be surrounded by the projecting portion  30 , and the wall surface  92  ( FIG. 1 ) of the ground G can be present all around the projecting portion  30 . Therefore, when a horizontal external force acts on the precast concrete foundation  16 , the precast concrete foundation  16  can be effectively restrained from moving by the backfill portion  24  and the ground G. 
     As shown in  FIG. 1 , since the movement preventing portion  84  is attached to the portion of the connecting member  22  projecting upward from the through-hole  42 , upon forming the backfill portion  24 , the precast concrete foundation  16  can be separated from the connecting member  22 , and work of forming the backfill portion  24  can be performed efficiently. 
     As shown in  FIG. 9 , a first portion  22   a  of the connecting member  22  present between the precast concrete foundation  16  and the backfill portion  24 , and a second portion  22   b  of the connecting member  22  present inside the through-hole  42  can be wrapped with one continuous filler  96 , it is possible to prevent a shearing force from acting on a boundary between the first portion  22   a  and the second portion  22   b , thereby increasing durability of the connecting member  22 . 
     As shown in  FIG. 9 , since the through-hole  42  is formed to have a cross-sectional area decreasing downward, when the precast concrete foundation  16  is about to be separated from the filler layer  26 , the filler  96  filled inside the through-hole  42  is caught by an inner surface of the through-hole  42 . Therefore, the precast concrete foundation  16  is hard to be separated from the filler layer  26 . 
     As shown in  FIG. 9 , since the filler  96  can be filled into the space between the precast concrete foundation  16  and the backfill portion  24  from the through-hole  42  for inserting the connecting member  22 , it is not necessary to separately form a through-hole for filling the filler  96 , and production cost of the precast concrete foundation  16  can be kept low. 
     Since the height adjuster  68  shown in  FIG. 8  has a simple structure having the male screw  98  and the female screw  100   a , work of adjusting the height of the precast concrete foundation  16  can be easily performed. 
     (Modification) 
     Note that implementation of the present invention is not limited to the above embodiment and various changes can be made without departing from the object of the present invention. That is, in the above embodiment, the concrete foundation structure  10  is used for supporting the building  12 ; however, the concrete foundation structure  10  may be used for supporting street lights, signboards, greenhouses, mechanical devices, and the like. Further, the size and weight of the concrete foundation structure  10  may be appropriately changed depending on applications. 
     In the above embodiment, the precast concrete foundation  16  and the anchor plate  20  are formed in a rectangular shape in a plan view; however, they may be formed in other shapes such as a square, a circle, an ellipse, a triangle, a pentagon, and a hexagon in a plan view. 
     In the above embodiment, the projecting portion  30  is formed annularly on the outer periphery of the foundation body  28 ; however, the projecting portion  30  may be formed intermittently on the outer periphery of the foundation body  28  or may be formed in a shape in which a part of an annular shape is missing. 
     In the above embodiment, the through-hole  42  is formed in a tapered shape; however, the shape of the through-hole  42  may be any shape as long as the cross-sectional area decreases downward. For example, the through-hole  42  may be formed in a shape in which a projection is provided on the inner surface of the through-hole formed with a constant inner diameter. Further, a normal through-hole (not shown) having a constant inner diameter may be provided instead of the through-hole  42 . Even in this case, since the first portion  22   a  of the connecting member  22  present between the precast concrete foundation  16  and the backfill portion  24 , and the second portion  22   b  of the connecting member  22  present inside the through-hole (not shown) can be wrapped with one continuous filler  96 , it is possible to prevent the shearing force from acting on the boundary, thereby increasing the durability of the connecting member  22 . 
     In the above embodiment, the anchor plate  20  is fixed to the connecting member  22  with the two female screw members  80  and  82 , but a method of fixing the anchor plate  20  may be changed as appropriate. For example, the female screw member  82  placed above the anchor plate  20  may be omitted. Further, the lower end of the connecting member  22  may be bent and hooked on the anchor plate  20 . 
     In the above embodiment, the movement preventing portion  84  is formed as a member having the female screw  84   a  ( FIG. 4 ); however, a structure of the movement preventing portion  84  may be changed as appropriate. For example, when a female screw is provided at the upper end of the connecting member  22 , the movement preventing portion  84  may be formed as a member having a male screw screwed into the female screw. Even in this case, the movement preventing portion  84  is attached to the portion of the connecting member  22  projecting upward from the through-hole  42  and contacts the foundation body  28 . 
     In the above embodiment, the cement-based solidifying material is used as the solidifying material of the backfill material  94  and the filler  96 ; however, another type of solidifying material may be used. For example, a lime-based solidifying material or a composite-based solidifying material in which the cement-based solidifying material and the lime-based solidifying material are mixed may be used. However, in order to obtain stable strength over a long period of time, it is desirable to use the cement-based solidifying material. 
     In the above embodiment, the filler  96  forming the filler layer  26  is filled from the through-hole  42  through which the connecting member  22  is inserted; however, the filler  96  forming the filler layer  26  may be filled from a through-hole (not shown) provided separately from the through-hole  42 . 
     LIST OF REFERENCE SIGNS 
     G Ground 
       10  Concrete foundation structure 
       12  Building 
       14  Floor structure 
       16  Precast concrete foundation 
       18  Excavation hole 
       20  Anchor plate 
       22  Connecting member 
       24  Backfill portion 
       26  Filler layer 
       28  Foundation body 
       30  Projecting portion 
       42  Through-hole 
       68  Height adjuster 
       84  Movement preventing portion 
       94  Backfill material 
       96  Filler