Specification [Title of the Invention] COMBINED STEEL WALL [Technical Field of the Invention] [0001] The present invention relates to a combined steel wall that is used for earth retaining works, coffering, shore protection, land reclamation, embankment, and the like. Priority is claimed on Japanese Patent Application No. 2012-196899, filed on September 7,2012, the content of which is incorporated herein by reference. [Related Art] [0002] A combined steel wall is a wall structure which is build by the combination of a wall body, which is built by the connection of a plurality of stee! sheet piles, and stiffening members, such as H-section steel beams or steel pipes and of which stiffness is improved. The combined steel wall can also be applied to a site, which requires the high height of a wall, or the like. Further, since the wall body is built by the fitting of adjacent steel sheet piles at a joint, it is possible to improve water cut-off performance as compared to a steel pipe sheet pile having a relatively large gap at a joint. When a steel pipe is applied as a stiffening member in the combined steel wall, there are various merits on construction. When an H-section steel beam is used as a stiffening member, there is a problem in that flanges are likely to be deformed due to the ground resistance at the time of driving of the H-section steel beam into the ground. However, since a steel pipe does not include protruding portions like the flanges of the H-section steel beam, the deformation of the steel pipe hardly occurs at the time of embedment. Further, it is also possible to embed the steel pipe in the ground while - 1 - rotating the steel pipe. Steel walls, which are disclosed in Patent Documents 1 to 3, are known as examples of a steel wall that is formed by the combination of steel pipes and steel sheet piles. [0003] In the steel wall disclosed in Patent Document 1, a machining jig for fitting a stiffening member is provided on at least one of the surface and back of a steel sheet pile and a stiffening member, such as an H-section steel beam or a steel pipe sheet pile, is installed through the machining jig. When a steel pipe sheet pile is applied as the stiffening member, a wall body is formed by fitting a joint of a steel pipe sheet pile to a machining jig mounted on a steel sheet pile for fitting the stiffening member. The transmission of a load to the steel pipe and the steel sheet pile is performed through the joint of the steel pipe sheet pile. In the steel walls disclosed in Patent Documents 2 and 3, a wall body is formed by the connection of a plurality of steel sheet piles through joints, and steel pipes come into contact with the entire wall body or some of the steel sheet piles so that the longitudinal direction of the steel pipe corresponds to the longitudinal direction of the steel sheet pile. Since the wall body is formed by the combination of the steel pipes and the steel sheet piles, it is possible to provide a steel wall that has both high water cut-off performance and high stiffness. [Prior Art Document] [Patent Document] [0004] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-299202 - 2 - [Patent Document 2] PCT International Publication No. WO2011/142047 [Patent Document 3] PCT International Publication No. WO2011/142367 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0005] In Patent Documents 1 to 3, a structure in which stiffening members, such as steel pipes, are arranged at a pitch is described as one embodiment. The structure in which the stiffening members are arranged at a pitch according to stiffness and proof stress required for a wall body can be achieved by the selection of a stiffening member, such as a steel pipe or an H-section steel beam, but there are the following problems in the setting of the pitch. (a) When the pitch of the steel pipes is excessively large, the behavior of the wall body is unstable. For this reason, there is a possibility that predetermined performance cannot be exhibited. (b) When the pitch of the steel pipes is excessively small, earth pressure cannot be appropriately shared and received by both the steel sheet piles and reinforcing members. For this reason, an uneconomical structure in which a load is concentrated on one of the steel sheet piles and the reinforcing members is formed. [0006] The stiffness and proof stress of the wall body in which the steel sheet piles are combined with the stiffening members also vary depending on the location, such as the installation positions of the stiffening members and the vicinity thereof, and the vicinity of the middle between adjacent stiffening members. However, if it is assumed that evaluation can be performed using stiffiiess obtained from the average of the stiffiiess and proof stress of the wall body, it is possible to reduce the weight of a - 3 - steel material to be used as the diameter of one steel pipe or the size of one H-section steel beam is increased and the pitch is increased. However, since an effect of stiffening the steel sheet pile wall by the stiffening members is not uniform when the pitch of the stiffening members is increased, the deformation of the steel sheet pile wall in the vicinity of the middle between adjacent stiffening members is increased. For this reason, the deformation of the wall body becomes ununiform in the extending direction of the wall (a horizontal direction). Further, when the pitch of the stiffening members is further increased excessively, a stiffening effect does not act on the steel sheet pile positioned in the vicinity of the middle between the adjacent stiffening members. That is, a portion of the steel wall in the vicinity of the stiffening member behaves as a high-stiffness wall in which the steel sheet pile wall is combined with the stiffening member, but a portion of the steel wall in the vicinity of the middle between adjacent stiffening members behaves as a steel sheet pile wall that is provided alone or a wall that is similar to the steel sheet pile wall In this case, it is not possible to average and treat the stiffness of the wall body, and, in the vicinity of the middle between adjacent stiffening members, the plastic deformation of the steel sheet pile wall occurs or excessive deformation of the steel sheet pile wall locally occurs in some cases. For this reason, a situation in which the stability of the wall body cannot be maintained is also considered. [0007] Meanwhile, when the pitch of the stiffening members is reduced, the earth pressure supported by the steel sheet pile wall is reduced. For this reason, only stiffening members resist the earth pressure. That is, although the stiffening members and the steel sheet piles are used while being combined with each other, the stifftiess or proof stress of the steel sheet pile wall cannot be utilized. When the stiffening - 4 - members and the steel sheet piles are used while being combined with each other, it can be said that a structure resisting applied earth pressure utilizing the stiffness and proof stress of both the steel sheet pile wall and the stiffening member is a reasonable structure. A pitch where an effect of stiffening the steel sheet pile wall by the stiffening members is appropriately obtained, a pitch where earth pressure can be resisted by both a stiffening member and a steel sheet pile wall, and the like are not mentioned in the above-mentioned invention. [0008] Accordingly, an object of the invention is to provide a combined steel wall capable of ensuring the safety and soundness of a wall body and having a reasonable structure that can utilize the stiffness and proof stress of both steel pipes and steel sheet piles. [Means for Solving the Problem] [0009] The invention employs the following configuration to achieve the abovementioned object. (1) According to an embodiment of the invention, there is provided a combined steel wall including: a wall body that includes a plurality of steel sheet piles connected to each other by joints and a plurality of recesses arranged at an interval in an extending direction; and a plurality of steel pipes that stand on a ground surface on a side, of which a horizontal position is low, of both sides of the wall body along a longitudinal direction of the steel sheet pile while a part of each steel pipe is received in the recess. At least a part of the wall body and the steel pipes in the longitudinal direction of the steel sheet pile are connected to each other, and a maximum interval L - 5 - (mm) as a center distance between first and second steel pipes which are adjacent to each other and between which a center distance is the maximum distance, a height H (mm) of the wall body, and a dimension D (mm) that is the sum of radii of the first and second steel pipes satisfy the following formula (A). [Formula 1] D ^ L £ ( 1 / 2 ) XH * • • (A) (2) In the combined steel wall according to (1), the wall body and the steel pipes may be connected to each other by coming into contact with each other. (3) In the combined steel wall according to (1), the wall body and the steel pipes may be connected to each other by connection members. (4) In the combined steel wall according to (3), the connection members may connect at least upper portions of the steel sheet piles and the steel pipes. (5) !n the combined steel wall according to any one of (1) to (4), the maximum interval L (mm) may be set so that the maximum interval L (mm), the height H (mm) of the wall body, yield stress cy (N/mm ) of the steel sheet pile, a section modulus Zs (mm3) of the steel sheet pile, and the maximum bending moment Mniax (N-mm) applied to the wall body satisfy the following formula (B). [Formula 2] ( 3 H - 4 L ) X ( 2 L ) * . _ . M (3 ~ 0V " " • \ B) HT z. s ~ y (6) In the combined steel wall according to any one of (1) to (5), the maximum interval L (mm) may be set so that the maximum interval L (mm), the height H (mm) of the wall body, the yield stress cy (N/mm2) of the steel sheet pile, the section modulus Zs (mm3) of the steel sheet pile, and the maximum bending moment Mmax - 6 - (N-mm) applied to the wall body satisfy the following formula (C). [Formula 3] 0.3 xay S ^ H r ^ U x i a y i , ^ ^ . . . (C) (7) In the combined steel wall according to any one of (1) to (6), the respective recesses may be formed on the wall body at regular intervals when viewed in the longitudinal direction, and the steel pipes may be disposed in every other recess. (8) In the combined steel wall according to any one of (1) to (6), the respective recesses may be formed on the wall body at regular intervals when viewed in the longitudinal direction, and the steel pipes may be disposed in every third or more recess. [Effects of the Invention] [0010] According to the above-mentioned configuration, it is possible to reduce stress generated in the steel sheet pile in the vicinity of the middle between adjacent steel pipes, to exliibit an effect of the combination of the steel sheet piles and the steel pipes, and to ensure the safety and soundness of the wall body without yielding of the steel sheet pile in the extending direction of the wall body. Further, it is possible to utilize the stiffness and proof stress of both the steel pipe and the steel sheet pile. Accordingly, it is possible to form a more reasonable structure. [Brief Description of the Drawings] [0011] FIG. 1A is a schematic plan view of an indoor model test device that is embodied for the examination of the pitch of steel pipes of a combined steel wall. FIG. IB is a schematic side cross-sectional view of the indoor model test - 7 - device shown in FIG. 1A. FIG. 2 is a view showing the summaries of a first test, a second test, and a third test. FIG. 3 is a graph showing the distribution of the strain of a steel pipe of each test in a vertical direction. FIG. 4 is a graph showing the distribution of the strain of a steel sheet pile of each test in the vertical direction. FIG 5 is a view schematically illustrating a calculation method in which it is assumed that earth pressure is applied to the steel sheet pile between the deepest portion and the same height (L) as the pitch of steel pipes. FIG. 6 is a graph illustrating the comparison between the distribution of the strain of the steel sheet pile of the first and second tests, in which the steel pipes come into contact with the steel sheet piles and are disposed on the front surface side, in the vertical direction and a calculated value that is calculated on the assumption that earth pressure is applied to the steel sheet pile between the deepest portion and the same height as the pitch of the steel pipes. FIG. 7 is a view schematically illustrating a calculation method in which it is assumed that earth pressure is applied to the steel sheet pile between the deepest portion and a height (2L) that is twice as large as the pitch of the steel pipes. FIG. 8 is a graph illustrating the comparison between the distribution of the strain of the steel sheet pile of the first, second, and third tests, in which the steel pipes are disposed on the front surface side in the vertical direction and a calculated value that is calculated on the assumption that earth pressure is applied to the steel sheet pile between the deepest portion and a height that is twice as large as the pitch of the steel pipes. - 8 - FIG. 9A is a schematic plan view showing an example of a combined steel wall according to a first embodiment of the invention, and shows a configuration in which one steel pipe is disposed per three hat-shaped steel sheet piles. FIG. 9B is a schematic plan view showing an example of the combined steel wall according to the first embodiment of the invention, and shows a configuration in which one steei pipe is disposed per two hat-shaped steel sheet piles. FIG. 9C is a schematic plan view showing an example of the combined steel wall according to the first embodiment of the invention, and shows a configuration in which steel pipes are disposed at sheet pile joints. FIG. 1OA is a schematic plan view showing a combined steel wall according to a second embodiment of the invention. FIG. 1 OB is a schematic side view of the combined steel wall shown in FIG. 10A. FIG. 11A is a schematic plan view showing a modification of the combined steel wall according to the second embodiment of the invention. FIG 1 IB is a schematic side view of the combined steei wall shown in FIG 11 A. [Embodiments of the Invention] [0012] In order to solve the above-mentioned problems, the inventors have disposed steel pipes on a side (which may be referred to as a "front surface side" in this specification), of which a ground surface is low, of both sides of a wall body in a combined steel wall including steel pipes and steel sheet piles, performed an indoor model test on a structure in which the steel pipes are arranged at a pitch, and examined the range of a pitch in which an effect of stiffening steel sheet pile walls by the steel - 9 - pipes is appropriately obtained and the range of a pitch in which earth pressure can be resisted by both the steel pipes and the steel sheet pile walls. [0013] This indoor model test is as follows: FIGS. lAand IB are schematic views of an indoor model test device. FIG. 1A is a schematic plan view of the indoor model test device, and FIG. IB is a schematic cross-sectional view taken along line I-I of FIG. 1 A. The indoor model test device has a structure in which lower ends of acrylic specimens K are fixed to the ground by an adhesive in the middle in a rigid earth tank G having a width of 1957 mm, a height of 1000 mm, and a depth of 940 mm. The acrylic specimen K forms a wall body by the combination of a wavy sheet pile Kl that is formed by the simulation of a steel sheet pile and a pipe K2 (having an outer diameter of 140 mm and a thickness of 3 mm) that is formed by the simulation of a steel pipe. Further, when the installation of a coping (connection member), which comiects the sheet pile to the pipe, at an upper portion is simulated, a connection plate K3 is attached as shown in FIG. IB. Meanwhile, the connection plate K3 is not shown in the schematic view of FIG lAfor simplification. [0014] In the indoor model test, quartz sand No. 5 (dry sand) is installed on both sides of the specimens K by an air-pluviation method. Further, quartz sand No. 5, which is present on a wall body-front surface side on which the pipes K2 are installed (the right side in FIGS. 1A and IB), is dug out to the deepest portion from this state, and the behavior of the wall body (the acrylic specimen K) is checked. Here, the ground, which is present on the side where quartz sand No. 5 is installed, is denoted by GH and the ground, which is present on the side where quartz sand No. 5 is dug out to - 10 - the deepest portion, is denoted by GL. [0015] For the examination of the contact state between the sheet pile Kl and the pipe K2 and the influence of the presence/absence of the connection plate K3, a first test, a second test, and a third test are performed while conditions are changed as summarized in FIG. 2. Meanwhile, even in the third test where the sheet pile Kl and the pipe K2 do not come into contact with each other, a part of the pipe K2 enters a recess of the sheet pile wall so that the position of the outer peripheral surface of the pipe K2 corresponds to the middle of the sheet pile wall. That is, even in any test, a part of the pipe K2 enters a recess of the sheet pile wall. In the test, a strain gauge is attached to the outer peripheral surface of the pipe K2, which is disposed at the middle portion, opposite to the side where the sheet pile wall is installed, and a strain gauge is attached to a web middle portion of the sheet pile Kl that is interposed between adjacent two pipes K2 and is most distant from both pipes K2. The strain gauges measure strain that is generated after digging. Further, a tool for measuring displacement is mounted on an upper portion of the pipe K2, which is disposed at the middle portion, and measures the displacement of the upper portion at a position that is distant from a lower end (the ground GL) by a distance of 1050 mm. [0016] In regard to each test, the distribution of vertical strain, which is generated in the pipe K2, in the depth direction is shown in FIG. 3 and the distribution of vertical strain, which is generated in the sheet pile Kl at a middle position between adjacent pipes K2, in the depth direction is shown in FIG 4. - 11 - In the graph shown in FIG. 3, a compression side is defined as a positive side in regard to strain generated in the pipe K2. Further, in the graph shown in FIG. 4, a tension side is defined as a positive side in regard to strain generated in the sheet pile Kl. Meanwhile, strain, which is calculated when it is assumed that the total earth pressure is applied to each of the sheet pile Kl and the pipe K2 as a cantilever of which a lower end is fixed, is also shown in the graph. In regard to earth pressure for the calculation of strain at this time, the same tests as the above-described tests are performed using only separate sheet piles Kl and earth pressure is calculated from the results of the tests. Furthermore, the following Table 1 shows displacement measured at a position distant from the lower end of the pipe K2, which is disposed in the middle, by a distance of 1050 mm. [0017] [Table 1] Displacement measured at a position distant from the lower end of the pipe K2, which is disposed in the middle, b}' a distance of 1050 mm FIRST TEST 4.5 mm SECOND TEST 4.6 mm THIRD TEST 4.0 mm [0018] As shown in FIG. 3, values of strain, which is generated in the vertical direction in a deep portion of the pipe K2, obtained in the first and second tests are slightly smaller than a value thereof that is obtained when total earth pressure is applied to the pipe K2, and a value thereof obtained in the third test is smaller than the values thereof obtained in the first and second tests. Further, as shown in Table 1, the - 12 - displacement of the pipe in the third test is smaller than those in the first and second tests. The amount of strain, which is shown in FIG. 4 and generated in the sheet pile Kl, varies depending on the contact condition between the sheet pile Kl and the pipe K2 and the presence/absence of the connection plate K3. However, in all tests, maximum strain is generated at the deepest portion and the strain is smaller than strain that is calculated on the assumption that total earth pressure is applied to only the sheet pile Kl. That is, since it is possible to reduce strain, which is generated in the sheet pile Kl, even in the vicinity of the middle between pipes K2 that are adjacent to each other in the extending direction of the wall body, it can be said that an effect of the combination of the sheet pile Kl and the pipe K2 can be exhibited. Further, the value of the strain of the sheet pile Kl in the third test is larger than the values thereof in the first and second tests. That is, since the sheet pile Kl shares a load, it can be said that the share of the load allocated to the pipe K2 tends to be reduced. [0019] The range of a pitch in which an effect of stiffening the steel sheet pile walls by the steel pipes is appropriately obtained and the range of a pitch in which earth pressure can be resisted by both the steel pipes and the steel sheet pile walls are calculated on the basis of these test results. [0020] First of all, a pitch in which an effect of stiffening the steel sheet pile walls by the steel pipes is obtained in the extending direction of the wall body will be examined. When the steel pipe (the pipe K2) and the steel sheet pile (the wavy sheet pile Kl) are installed so as to come into contact with each other (the first and second tests), strain is not substantially generated even at the upper portion of the steel sheet pile - 13 - between the adjacent steei pipes but strain is generated at a position substantially between the deepest portion and the same height (360 mm) as the pitch of the steel pipes (FIG. 4). The reason for this is considered that deformation is restricted by the steel pipes (the pipes K2) installed on both sides of the steel sheet pile (the wavy sheet pile Kl) but the restriction of deformation perfonned by the steel pipes is not sufficient in the steel sheet pile in the vicinity of the middle between the steel pipes of which deep portions are adjacent to each other and the behavior of the steel sheet pile and the steel pipes is locally similar to the behavior of a steel sheet pile wall provided alone. Then, strain is calculated on the assumption that earth pressure is applied to a wall body formed of a steel sheet pile between the deepest portion and the position of the same height as the pitch L of the steel pipes as shown in FIG. 5. The distribution (calculated values) of the calculated strain in the depth direction is shown in FIG. 6. Meanwhile, test values of the first and second tests shown in FIG. 4 are also shown in FIG. 6 together with the distribution of the calculated strain in the depth direction. [0021] Since calculated values substantially correspond to the test values (the first and second tests) as shown in FIG. 6, it is possible to express the vertical behavior of the steel sheet pile (wavy sheet pile Kl) at a middle position between adjacent steel pipes by calculating strain on the assumption that earth pressure is applied to a wall body formed of a steel sheet pile between the deepest portion and the position of the same height as the pitch L of the steel pipes (the pipes K2) when the steel pipes are disposed on the wall body-front surface side so that the steel pipes come into contact with the steel sheet piles in a longitudinal direction. Accordingly, when the pitch L of the steel pipes is equal to the height of the wall in this structure, stress generated in the steel sheet pile (wavy sheet pile Kl) at a middle position between adjacent steel pipes - 14 - has substantially the same behavior as that in a case in which only a steel sheet pile is applied. For this reason, an effect of stiffening the steel sheet pile by the steel pipe is not partially obtained. In other words, if the pitch L of the steel pipes is equal to or smaller than the height H of the wall (if "L