Underwater ground improvement method and underwater ground improvement structure

Abstract

To provide a water bottom soil improvement method and a water bottom soil improvement structure capable of using without removing swelling soil generated in accordance with creation of a sand pile and suppressing increase of a swelling height and a swelling range to reduce influence on peripheral ground or the like.SOLUTION: A water bottom ground improvement method according to the present invention is a method for improving a water bottom ground 5 containing cohesive soil, and includes a sand pile creation step of creating a sand pile in the ground of the water bottom ground 5, and a modification step of stirring and mixing a modification material 23 for hardening the cohesive soil with raised soil 3 generated in the sand pile creation step at an original position to modify the raised soil 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and structure for improving waterbed soil for placing underwater structures on the waterbed. [Background technology]

[0002] When constructing structures such as caissons or submerged breakwaters on soft ground at the bottom of a body of water that mainly contains clay, ground improvement is required to ensure the stability of the structures. A well-known method for improving soft ground is the sand compaction pile method, in which a large number of firmly compacted sand piles are placed underground.

[0003] The sand compaction pile method involves using a vibratory hammer to drive a casing pipe into the ground, supplying sand or other materials through the pipe, and then compacting the material to create a sand pile (see Non-Patent Document 1 and Figures 2 and 3 of this specification). In the sand compaction pile method, sand piles are forcibly driven into the ground, causing the surrounding clayey soil to pile up above the ground surface. The clayey soil that piles up as a result of the construction of these sand piles is called mounded soil (see Non-Patent Document 2).

[0004] As shown in Figure 20, mounded soil 3 occurs on the ground surface 5a on top of the sand pile construction area 7 (the area where many sand piles are constructed at a specified pitch) and in the surrounding area. However, since the mounded soil 3 is soft clayey soil, it is not suitable for placing structures on top of it. Furthermore, if a large amount of soft mounded soil 3 occurs, it may have an impact on the surrounding ground and surrounding structures.

[0005] Therefore, in the past, it was common to dredge and remove the piled soil 3 after constructing the sand piles, but this required a huge amount of disposal costs to dispose of the dredged soil as industrial waste. Furthermore, in recent years, it has become difficult to secure soil disposal sites, so there has been a social demand for a way to utilize the piled-up soil without disposing of it.

[0006] Therefore, Patent Document 1 proposes a method of utilizing the mounded soil without removing it by constructing sand piles in the mounded soil. Furthermore, Patent Document 2 proposes a method of recovering the piled up soil, mixing it with a modifying material and coarse grain material at sea, and effectively utilizing it as a filling material for constructing sand piles. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 9-316864 [Patent Document 2] Japanese Patent Application Laid-Open No. 3608539 [Non-patent literature]

[0008] [Non-Patent Document 1] Japan Geotechnical Society: Sand Compaction Pile Method with Push-Back Construction, Design and Construction Manual, March 30, 2009, p. 33, Figure 2.15 [Non-patent document 2] Tsutomu Fukute, Yoshiaki Higuchi, Masahiko Furuichi, Hideo Tsuboi: Prediction of the shape of the seabed uplift caused by large-scale installation of sand compaction piles, 33rd Soil Engineering Symposium, pp.23-28, 1988 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, the method of Patent Document 1 does not require the disposal of mounded soil because it does not require dredging and removal. However, by constructing sand piles in the mounded soil, the height and range of the mound from the ground surface before improvement further increases, which has the problem of having a greater impact on the surrounding ground and surrounding structures.

[0010] Furthermore, although the method of Patent Document 2 minimizes the impact on the surrounding ground and surrounding structures, it uses the soft mounded soil as a fill material for constructing sand piles, which means that it must be mixed with a large amount of modifiers and coarse-grained materials, which limits the amount of mounded soil that can be effectively used.

[0011] Furthermore, the method of Patent Document 2 requires dredging of the raised soil and mixing of modifying materials and coarse-grained materials in parallel with driving the sand piles, which means that sand compaction pile ships, dredgers, premix ships, etc. are required at the same time, which creates the problem of large-scale construction work.

[0012] The present invention has been made to solve the above-mentioned problems, and aims to provide a waterbed ground improvement method and waterbed ground improvement structure that can utilize the mounded soil that occurs when constructing sand piles without removing it, and that suppresses an increase in the height and range of the mound, thereby minimizing the impact on the surrounding ground, etc. [Means for solving the problem]

[0013] (1) The waterbed ground improvement method according to the present invention is a method for improving waterbed ground containing clayey soil, a sand pile construction step of constructing sand piles in the ground of the bottom of the water; and a modifying process in which the mounded soil generated in the sand pile construction process is modified by stirring and mixing in situ a modifying material that hardens the clayey soil.

[0014] (2) In addition, in the above-described (1), the sand pile construction step constructs the sand pile so that the upper end of the sand pile is formed at a position deeper than the surface layer of the waterbed ground, The modifying step involves stirring and mixing the modifier with the clayey soil in the surface layer in situ to also modify the clayey soil in the surface layer.

[0015] (3) In addition, in the above (1) or (2), the modification step uses a modifier stirring and mixing device having a casing capable of taking in the soil to be modified inside, a stirring and mixing means capable of moving up and down inside the casing, and a modifier supply path for supplying the modifier toward the stirring and mixing means, With the casing pressed into the soil to be modified, the modifier is supplied toward the stirring and mixing means, and the stirring and mixing means is operated so that the soil to be modified and the modifier taken into the casing are stirred and mixed, thereby modifying the soil to be modified inside the casing from bottom to top.

[0016] (4) Furthermore, in the above (3), the sand pile construction process constructs the sand pile so that the distance d from the ground surface of the bottom ground to the upper end of the sand pile satisfies the following formula (1). -18.677a s 2 +1.041D-0.393≦d≦-0.442a s 2 +1.025D-0.115(where 0≦d) (1) where: D: Layer thickness of the soil to be improved (m) a s : Replacement rate (the ratio of the area of ​​the sand pile when the plane area of ​​the waterbed is 1) d: Distance from the ground surface before construction to the top of the sand pile (m)

[0017] (5) The waterbed ground improvement structure according to the present invention is an improvement structure for waterbed ground containing clayey soil, a sand pile construction section in which sand piles are constructed underground in the waterbed; A modified soil portion is provided on the sand pile construction portion and is made of modified soil obtained by modifying the clayey soil. The modified soil is obtained by mixing the mounded soil that was created during the construction of the sand piles with a modifier that hardens the clayey soil.

[0018] (6) In addition, in the above (5), the upper end of the sand pile construction portion is formed at a position deeper than the surface layer of the bottom ground before the sand pile is constructed, The modified soil further includes soil modified by mixing the modifier into the clayey soil in the surface layer.

[0019] (7) Furthermore, in the above (5) or (6), the distance d from the ground surface before the construction of the sand pile to the top end of the sand pile satisfies the following formula (1). -18.677a s 2 +1.041D-0.393≦d≦-0.442a s 2 +1.025D-0.115(where 0≦d) (1) where: D: Layer thickness of improved soil (m) a s : Replacement rate (the ratio of the area of ​​the sand pile when the plane area of ​​the waterbed is 1) d: Distance from the ground surface before construction to the top of the sand pile (m)

[0020] (8) In addition, in the above (5) or (6), the modifying agent is a material that reacts with the clayey soil and hardens, The modified soil portion has strength due to the modifier being mixed into the clayey soil.

[0021] (9) In addition, in the above (8), the material is made from converter-based iron and steel slag. [Effects of the Invention]

[0022] The present invention solidifies mounded soil by stirring and mixing in situ a modifier that hardens clayey soil with the mounded soil, so that the mounded soil can be used without being removed by dredging. Furthermore, since the volume of the modifier mixed into the mounded soil is significantly smaller than the volume of the sand pile, the amount of mounded soil generated can be reduced compared to the method of constructing sand piles in mounded soil as in Patent Document 1, and the impact on the surrounding ground, etc. can also be reduced. Furthermore, since the mixing of the modifier into the raised soil can be carried out after the construction of the sand piles is completed, it does not require large-scale construction like that in Patent Document 2, and construction costs can be reduced. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is an explanatory diagram of a submerged ground improvement structure according to an embodiment of the present invention. [Figure 2] FIG. 1 is an explanatory diagram (part 1) of another aspect of the submerged ground improvement structure according to the embodiment. [Figure 3] FIG. 2 is an explanatory diagram (part 2) of another aspect of the waterbed ground improvement structure according to the embodiment. [Figure 4] FIG. 3 is an explanatory diagram (part 3) of another aspect of the waterbed ground improvement structure according to the embodiment. [Figure 5] FIG. 4 is an explanatory diagram (part 4) of another aspect of the submerged ground improvement structure according to the embodiment. [Figure 6] FIG. 5 is an explanatory diagram of another aspect of the submerged ground improvement structure according to the embodiment (part 5). [Figure 7] FIG. 1 is an explanatory diagram (part 1) of a sand pile construction process in a waterbed ground improvement method according to an embodiment. [Figure 8] FIG. 2 is an explanatory diagram (part 2) of the sand pile construction process in the waterbed ground improvement method according to the embodiment. [Figure 9] FIG. 2 is an explanatory diagram of the improvement process in the waterbed ground improvement method according to the embodiment. [Figure 10] FIG. 2 is a diagram illustrating each variable in equation (2). [Figure 11] In equation (4), the variables D and as are fixed values, and the values ​​that the variable d can take are graphed when the other variables are randomly substituted. [Figure 12]12 is a graph showing how the upper limit value d99max and the lower limit value d99min shown in FIG. 11 change depending on the values ​​of the variables D and as. [Figure 13] FIG. 1 is an explanatory diagram of a conventional example according to the first embodiment. [Figure 14] 14 is a graph comparing the height of the raised portion of the conventional example of FIG. 13 with the invention examples of FIGS. 1 and 4. [Figure 15] 1 is a cross-sectional view showing a ground to which the underwater ground improvement structure according to the present invention is applied and a structure provided on the ground (Example 2). [Figure 16] 1 is a cross-sectional view showing a ground to which the underwater ground improvement structure according to the present invention is applied and a structure provided on the ground (Example 3). [Figure 17] 1 is a cross-sectional view showing a ground to which the underwater ground improvement structure according to the present invention is applied and a structure provided on the ground (Example 4). [Figure 18] 1 is a cross-sectional view showing the ground to which the submerged ground improvement structure of the present invention is applied and a structure provided on the ground (Example 5). [Figure 19] 1 is a cross-sectional view showing the ground to which the underwater ground improvement structure of the present invention is applied and a structure installed on the ground (Example 6). [Figure 20] FIG. 1 is a diagram illustrating the mounded soil that occurs when ground improvement is performed using the sand compaction pile method. DETAILED DESCRIPTION OF THE INVENTION

[0024] A submerged ground improvement structure 1 according to one embodiment of the present invention will be described with reference to FIG. FIG. 1(a) is a plan view of the submarine ground improvement structure 1, and FIG. 1(b) and FIG. 1(c) are cross-sectional views taken along the lines AA and BB, respectively, of FIG. 1(a). In addition, although the mounded soil 3 is illustrated in the shape of a quadrangular pyramid in FIG. 1, this is a schematic illustration of the mounded soil 3 and is not limited to this in reality.

[0025] The submerged ground improvement structure 1 of this embodiment is intended to improve the submerged ground 5 containing clay soil so as to obtain sufficient stability for constructing a structure on the submerged ground 5, and as shown in Figure 1, has a sand pile construction section 7 and an improved soil section 9 provided on top of the sand pile construction section 7.

[0026] The waterbed ground 5 is a soft ground that mainly contains clay, and the clay content is, for example, 50% or more. Here, soft ground refers to ground with an N-value of 4 or less obtained by a standard penetration test (Japanese Industrial Standards JIS A1219 "Standard penetration test method for soil"). In addition, clayey soil refers to soil that falls into the fine-grained soil category (containing more than 50% clay or silt with a particle size of less than 0.075 mm) in the geotechnical engineering standard "Engineering Classification Method for Geotechnical Materials" (JGS0051-2000).

[0027] The sand pile construction section 7 is formed by constructing a large number of sand piles (not shown) made by compacting sand at a predetermined pitch underground in a predetermined area (the area indicated by diagonal lines in Figure 1) below the waterbed ground 5. In the sand pile construction section 7, the soft clayey soil has been replaced with sand piles, which increases the stability of the water bottom ground 5 and provides a predetermined bearing capacity against the superimposed load.

[0028] The modified soil section 9 is formed from modified soil obtained by modifying the clayey soil of the waterbed ground 5. Specifically, the modified soil section 9 is formed from modified soil obtained by modifying the mounded soil 3 that was generated when constructing the sand piles in the sand pile construction section 7.

[0029] As shown in the figure, the mounded soil 3 occurs on the ground surface 5a above the sand pile construction section 7 and in the surrounding area, but at least the part of this that occurs above the sand pile construction section 7 (the part shown in gray in the figure) has been reinforced by mixing in a modifier that hardens the clayey soil.

[0030] The modifier is not particularly limited to any particular material as long as it can modify the soft mounded soil 3 to a predetermined strength, but examples that can be used include steel slag, cement, cement-based solidification material, quicklime, and lime-based solidification material.

[0031] A specific example of the modifier is a material that hardens over time through a hydration reaction with clay, such as a calcia modifier. Calcia modifier is a material that uses converter steelmaking slag, a type of steel slag, as a raw material, with its components controlled and particle size adjusted. By using calcia modifier as a modifier, steelmaking slag can be effectively utilized.

[0032] By mixing the above-mentioned modifying material, the weak mounded soil 3 is modified to become modified soil with a predetermined strength, and the modified soil forms the modified soil portion 9. In this way, by modifying the mounded soil 3 that has formed on the sand pile construction section 7 to form modified soil section 9, it becomes possible to build structures on the modified soil section 9. Therefore, there is no need to dredge and remove the mounded soil 3, and problems related to the disposal of the mounded soil 3 do not arise.

[0033] Furthermore, as will be explained in more detail in Example 1 below, the submerged ground improvement structure 1 of this embodiment reduces the height and range of mounding compared to the method of Patent Document 1 in which sand piles are constructed in mounded soil 3, and therefore has less impact on the surrounding ground and surrounding structures.

[0034] Next, other aspects of the submerged ground improvement structure 1 according to the present embodiment are shown in Figs. 2 to 6. In each figure, (a) is a plan view, (b) is a view corresponding to the AA cross section of FIG. 1(a), and (c) is a view corresponding to the BB cross section of FIG. 1(a).

[0035] The improved soil portion 9 of the submerged ground improvement structure 1 may be formed by improving not only the portion directly above the sand pile construction portion 7 but also the entire mound soil 3, as shown in FIG. As shown in FIG. 3, the modified soil portion 9 may be formed by modifying not only the mound soil 3 but also the clayey soil in the surface layer of the bottom ground 5. In either case, at least the mounded soil 3 above the sand pile construction section 7, i.e., the part on which the structure will be placed, will be modified to form a modified soil section 9, and the mounded soil 3 that has formed around the sand pile construction section 7 and the clayey soil in the surface layer of the waterbed ground 5 should be modified as needed and at the appropriate time.

[0036] In addition, while it was common in the past to construct sand piles down to the ground surface 5a of the waterbed ground 5, in this embodiment, the clayey soil above the sand pile construction section 7 is modified to form a modified soil section 9, so it is not necessarily necessary to construct the sand piles down to the ground surface 5a. Therefore, as shown in FIG. 4, an embodiment may be adopted in which the upper end of the sand pile construction portion 7 is formed at a position deeper than the surface layer of the waterbed ground 5 before the sand pile is constructed.

[0037] In this case, the modified soil portion 9 is formed from modified soil obtained by modifying the clayey soil in the surface layer of the water bottom ground 5 and modified soil obtained by modifying the mound soil 3. In the embodiment of FIG. 4, the length of the sand piles can be made shorter than in the past, thereby reducing the cost of constructing the sand piles. Furthermore, since the amount of clayey soil replaced by sand piles is reduced, the amount of mounded soil 3 is also reduced, the height and range of the mound are reduced, and the impact on the surrounding ground, etc. can be suppressed.

[0038] Also, in this embodiment, as in the example of Figure 2, the modified soil section 9 can be formed by modifying not only the mounded soil 3 that has formed directly above the sand pile construction section 7, but also the mounded soil 3 that has formed on the surrounding ground surface 5a (see Figure 5). As in the example of FIG. 3, the modified soil portion 9 can also be formed by modifying the clayey soil in the surface layer of the waterbed ground 5 around the sand pile construction portion 7 (see FIG. 6).

[0039] Next, a method for improving the waterbed soil for constructing the waterbed soil improvement structure 1 as described above will be described. The submerged ground improvement method of this embodiment includes a sand pile construction process for constructing sand piles underground in the submerged ground 5, and a modification process for modifying the mounded soil 3 generated in the sand pile construction process by stirring and mixing a modifier in situ with the mounded soil 3.

[0040] The sand pile construction process can be carried out using the conventional sand compaction pile method, and an example of the procedure is shown in Figure 7 and will be explained in detail below. In the sand pile construction process, first, a vibro hammer 11 is used to drive a casing pipe 13 into the waterbed 5 until the tip of the casing pipe 13 reaches the bearing layer. Next, sand or the like is supplied from the upper end of the casing pipe 13 to fill the inside of the casing pipe 13 with sand, and then the casing pipe 13 is pulled out to create a sand pile in the waterbed ground 5 with a diameter equivalent to the pipe diameter (approximately 0.8 m).

[0041] Thereafter, the vibrohammer 11 is again applied while the casing pipe 13 is inserted into the sand pile, compacting the sand pile and expanding its diameter to about 2 m. By repeating this process and gradually pulling up the casing pipe 13, the sand pile 15 is formed from the bottom up, and once the sand pile 15 has been formed to a predetermined height, the casing pipe 13 is pulled out to complete the construction of the sand pile 15.

[0042] Here, as shown in Figures 1 to 3, if the upper end position of the sand pile construction portion 7 is to be at the same height as the ground surface 5a of the waterbed 5, the sand pile 15 may be constructed until it reaches the ground surface 5a. Furthermore, as shown in Figures 4 to 6, if the upper end position of the sand pile construction section 7 is to be located deeper than the ground surface 5a of the waterbed 5, it is advisable to stop construction of the sand pile 15 at a predetermined depth before reaching the ground surface 5a.

[0043] Then, by constructing a large number of sand piles 15 at a predetermined pitch in a predetermined area of ​​the waterbed ground 5, as shown in Figure 8 (citing Figure 2.12(b) of Non-Patent Document 1), a sand pile construction section 7 can be formed underground in the waterbed ground 5. In this sand pile construction process, the clayey soil that forms the waterbed ground 5 is replaced by sand pile 15, and as shown in Figure 8, an amount of clayey soil equivalent to the volume of sand pile 15 is pushed up onto the sand pile construction section 7 and the surrounding ground surface 5a, resulting in mounded soil 3.

[0044] After the sand pile formation portion 7 is formed in the sand pile formation step, a reforming step is carried out to reform the mounded soil 3. In the modification step, the modification material is stirred and mixed in the mounded soil 3 in situ to modify the mounded soil 3, thereby forming a modified soil portion 9.

[0045] The method for stirring and mixing the modifier in situ with the mounded soil 3 is not particularly limited, but it is preferable to use, for example, the method described in Japanese Patent No. 6936456. The modification process of this embodiment is based on the method described in the above document, and is specifically as follows.

[0046] The construction procedure for the modification process is shown in Fig. 9. In the modification process, as shown in Figure 9, a modifier stirring and mixing device 27 is used, which has a casing 19 that can take in the soil 17 to be modified inside, a stirring and mixing means 21 that can move up and down inside the casing 19, and a modifier supply path 25 for supplying modifier 23 toward the stirring and mixing means 21. The modifier stirring and mixing device 27 is provided on a work boat (not shown) that can move on the sea, and can be raised and lowered along a leader erected on the work boat.

[0047] The soil to be improved 17 in FIG. 9 is clay soil to be mixed with the improving material, specifically the clay soil in the mounded soil 5 and the surface layer of the water bottom ground 5 in the gray areas in FIGS. That is, when the sand pile construction portion 7 is constructed down to the ground surface 5a of the water bottom ground 5 as shown in FIG. 1, the gray part of the mounded soil 3 in FIG. 1 becomes the soil to be improved 17 in FIG. Furthermore, if the sand pile construction section 7 is not constructed down to the ground surface 5a of the waterbed ground 5 as shown in Figure 4, the gray part of the raised soil 3 in Figure 4 becomes the upper to middle part of the soil 17 to be modified in Figure 9, and the gray part of the clayey soil in the surface part of the waterbed ground 5 in Figure 4 becomes the middle to lower part of the soil 17 to be modified in Figure 9.

[0048] In the modification process, first, a casing 19 is pressed into the soil 17 to be modified. During the process of pressing the casing 19, the soil 17 to be modified is taken into the casing 19. When the pressing of the casing 19 reaches the target depth (the depth at which the bottom end of the casing 19 reaches the top end of the sand pile construction section 7), the pressing of the casing 19 is stopped (Figures 9(a) and 9(b)). In addition, when modifying the mounded soil 3 in areas other than directly above the sand pile construction section 7 or the clayey soil in the surface layer of the waterbed ground 5, the casing 19 can be pressed in to an appropriate depth as needed.

[0049] Next, the stirring and mixing means 21 and the modifier supply passage 25 are lowered to the lower end position within the casing 19 . The stirring and mixing means 21 is, for example, a rotating stirring and mixing blade, and in addition to the role of stirring and mixing the modifier into the clayey soil, it also plays a role of loosening the clayey soil inside the casing 19. By rotating the stirring and mixing means 21 and lowering it inside the casing 19, the clayey soil (the soil to be modified) inside the casing 19 is loosened from top to bottom (Figure 9(c)).

[0050] Next, the modifier 23 is introduced into the casing 19 through the modifier supply path 25. The introduced modifier 23 is discharged from an outlet located directly above the stirring and mixing means 21 and supplied to the stirring and mixing means 21. The amount of modifier 23 introduced here is, for example, the amount required to modify the lower layer of the soil 17 to be modified. The amount of modifier 23 to be added is not limited to the above amount, and the entire amount required to modify the soil 17 from the lower layer to the upper layer may be added all at once (FIG. 9(d)).

[0051] After the modifier 23 is added, the stirring and mixing means 21 is rotated to stir and mix the soil 17 to be modified and the modifier 23 in the casing 19. At this time, while rotating the stirring and mixing means 21, it is simultaneously raised and lowered in short strokes, moving the stirring and mixing position of the stirring and mixing means 21 from bottom to top. By stirring and mixing the modifying material 23 into the soil to be modified 17 in the casing 19 in the above manner, modified soil 29 is formed by mixing the soil to be modified 17 and the modifying material 23 (Figure 9(e)).

[0052] When the position of the stirring and mixing means 21 in the casing 19 reaches the top of the lower layer of the soil 17 to be modified, the modifier 23 is again introduced into the casing 19 through the modifier supply path 25. The amount of modifier 23 introduced here is the amount required to modify the middle layer of the soil 17 to be modified, for example. In this embodiment, the modifying agent supply passage 25 is provided so as to follow the up and down movement of the stirring and mixing means 21, and the relative distance between the stirring and mixing means 21 and the outlet of the modifying agent supply passage 25 is always kept at a constant close distance (FIG. 9(f)).

[0053] Then, similarly to FIG. 9(e), the stirring and mixing means 21 is rotationally driven to stir and mix the soil to be modified 17 and the modifier 23 in the casing 19 (FIG. 9(g)).

[0054] When the position of the stirring and mixing means 21 in the casing 19 reaches the top of the middle layer of the soil 17 to be modified, the modifier 23 is again introduced into the casing 19 through the modifier supply path 25. The amount of modifier 23 introduced here is, for example, the amount required to modify the upper layer of the soil 17 to be modified (Figure 9(h)). Then, similarly to Figures 9(e) and 9(g), the stirring and mixing means 21 is rotationally driven to stir and mix the soil to be modified 17 and the modifier 23 in the casing 19 (Figure 9(i)).

[0055] When the position of the stirring and mixing means 21 in the casing 19 reaches the upper end of the soil to be improved 17, the stirring and mixing means 21 is stopped (FIG. 9(j)). Thereafter, the casing 19 is pulled out from the soil 17 to be improved, and the top of the improved soil 29 is leveled (FIG. 9(k)). Through the above steps, the formation of modified soil 29 at one location is completed.

[0056] Thereafter, the position is changed as necessary and the same process as above is repeated to modify the modification target soil 17 over the entire target area to form modified soil 29, thereby forming the modified soil portion 9. According to the above-described method, the mounded soil 3 can be modified in situ to form the modified soil portion 9, which eliminates the need to dredge the mounded soil 3 and is therefore efficient. Furthermore, by adjusting the injection depth of the casing 19, construction can be performed so that the mixing range of the modifier 23 and the construction range of the sand pile 15 do not overlap.

[0057] As mentioned above, the depth to which the casing 19 is pressed during the modification process must be set to a depth at which the lower end of the casing 19 reaches the upper end of the sand pile construction section 7, i.e., a depth equivalent to the layer thickness of the soil 17 to be modified. The thickness of the soil to be modified 17 varies depending on the total volume of the sand piles 15 constructed in the sand pile construction section 7, so a method for estimating the thickness of the soil to be modified 17 will be described below.

[0058] First, in the aforementioned Non-Patent Document 1, the following formula (2) is proposed as a formula for estimating the height of the heap that occurs when constructing sand piles in a clayey soil ground.

[0059]

number

[0060] The above formula (2) is a formula for estimating the height H of the mound, assuming that the shape of the mounded soil 3 is a truncated quadrangular pyramid as shown in Figure 10, based on the construction results of the sand compaction pile method to date. The variables in formula (2) are as shown in Figure 10. The rise rate μ is calculated by the substitution rate a s and the height of the sand pile l, it can be expressed as the following equation (3). μ=0.718a s +2.117 / l+0.056 (3)

[0061] Since all of the clayey soil in the area above the sand pile construction section 7 is subject to improvement, the layer thickness D of the soil to be improved is H+d. Therefore, the layer thickness D of the soil to be improved can be estimated using the following equation (4).

[0062]

number

[0063] The layer thickness D of the soil to be improved can be predicted using the above formula (4), and the injection depth of the casing 19 can be set based on this prediction. However, in actual design, there may be restrictions on the layer thickness D of the soil to be improved due to construction constraints.

[0064] For example, in the case of the construction method shown in Figure 9, the maximum depth to which the casing 19 can be injected is approximately 5 m due to restrictions on the weight that can be hoisted by the work vessel and the length of the casing 19. Therefore, if the layer thickness D of the soil to be modified exceeds 5 m, it is not preferable because the modifying material cannot be sufficiently mixed into the lower layer of the soil to be modified, which may reduce stability. Thus, depending on the construction method of the reforming process, there may be restrictions such as limiting the layer thickness D of the soil to be reformed to 5 m or less.

[0065] As mentioned above, the layer thickness D of the soil to be modified varies depending on the total volume of the sand piles constructed in the sand pile construction process, so the inventors carefully considered how to construct the sand pile construction process section 7 so that the layer thickness D of the soil to be modified does not exceed the construction limit.

[0066] First, since D = H + d, it is necessary to adjust H or d so that D does not exceed the construction limit. Here, the height of the rise H varies depending on the total volume of the sand piles in the sand pile construction section 7. Therefore, the replacement rate a can be reduced by reducing the diameter of the sand piles or by reducing the number of piles. s The protrusion height H can be reduced by reducing the substitution rate a s If the value is made smaller, it will affect the stability of the sand pile construction portion 7, which is not preferable. Therefore, the substitution rate a s It is preferable to adjust the distance d from the ground surface 5a before constructing the sand pile to the top end of the sand pile while setting the value necessary for stabilizing the sand pile construction portion 7.

[0067] Therefore, the layer thickness D of the soil to be improved and the replacement rate a s The possible values ​​of distance d were statistically investigated when the height l of the sand pile, the longitudinal length A of the sand pile improvement range, and the transverse length B of the sand pile improvement range were independently and randomly varied with a fixed value. Here, the ranges of variation for l, A, and B were set to 5m≦l≦40m, 10m≦A≦200m, and 10m≦B≦200m, taking into account a typical construction range. Note that the influence angle θ from the bottom end of the sand pile is often calculated as 60°, so it was fixed at θ = 60°.

[0068] An example of the study results is shown in Figure 11. Figures 11(a) to 11(c) show the relationship between the layer thickness D of the soil to be improved and the replacement rate a s The distance d that satisfies equation (4) was calculated when the other variables were randomly changed within the above ranges with a fixed value, and the results were compiled in a histogram (5000 samples). Note that the layer thickness D of the soil to be improved and the replacement rate a s The values ​​are as shown in each graph.

[0069] From the results of Figures 11(a) to 11(c), it was found that the frequency of each value calculated as the distance d roughly follows a t-distribution. Therefore, the upper limit value d in the 99% confidence interval (the range that falls within with a 99% probability) based on the t-distribution was calculated. 99max and the lower limit d 99min This range is determined by the layer thickness D of the soil to be improved and the replacement rate a s is a fixed value, the range of values ​​that the distance d can take.

[0070] Next, the layer thickness D of the soil to be improved and the replacement rate a s The upper limit d 99max and the lower limit d 99min The results are shown in Figure 12. Figure 12(a) shows the substitution rate a s The upper limit value d when the value of the layer thickness D of the soil to be improved is changed while the value of 99max and lower limit d 99min 10 is a graph showing the change in In addition, Fig. 12(b) shows the relationship between the thickness D of the soil to be improved and the replacement rate a s The upper limit d when the value of 99max and lower limit d 99min 10 is a graph showing the change in

[0071] As shown in Fig. 12(a) and Fig. 12(b), the upper limit value d 99max and the lower limit d 99min changes linearly with the value of the layer thickness D of the soil to be improved, and the replacement rate a s It was found that the value of varies quadratically. Therefore, the layer thickness D of the soil to be improved and the replacement rate a s When multiple regression analysis was performed using the square of d as an explanatory variable, the upper limit d 99max and the lower limit d 99min It was found that these can be approximated by the following formulas (5) and (6), respectively. Note that the adjusted coefficient of determination for both formulas is 0.999 or higher. d 99max =-0.442a s 2 +1.025D-0.115 (5) d 99min=-18.677a s 2 +1.041D-0.393 (6)

[0072] Therefore, the appropriate range of the distance d when there is a construction restriction on the layer thickness D of the soil to be improved can be calculated using the following formula (1). -18.677a s 2 +1.041D-0.393≦d≦-0.442a s 2 +1.025D-0.115(where 0≦d) (1) where: D: Layer thickness of the soil to be improved D (m) a s : Replacement rate (the ratio of the area of ​​the sand pile when the plane area of ​​the waterbed is 1) d: Distance from the ground surface before construction to the top of the sand pile (m)

[0073] From the above considerations, in the design of the submerged ground improvement structure 1, the replacement rate a is set based on the stability required for the sand pile construction part 7. s It is advisable to determine the thickness D of the modified soil section 9 (corresponding to the thickness D of the soil to be modified) based on the construction restrictions in the modification process, and then determine the distance d based on this so as to satisfy the above formula (1).

[0074] Then, in the sand pile construction process, by constructing the sand pile so that its upper end is formed at a position a distance d deeper than the ground surface 5a of the waterbed ground 5, it is possible to ensure that the sum of the mound height H of the mound soil 3 and the distance d, i.e., the layer thickness D of the soil to be modified, does not exceed the construction limit in the modification process. As a result, in the modification process, the soil to be modified from the surface of the mounded soil 3 to the top end of the sand pile construction portion 7 can be reliably modified, and a stable waterbed ground improvement structure 1 can be obtained.

[0075] As described above, in this embodiment, the modifier 23 is stirred and mixed in situ with the mounded soil 3 that occurs during the construction of sand piles, and solidified, so that the mounded soil 3 can be used without having to be dredged away.

[0076] Furthermore, compared to the conventional method of improving the mounded soil 3 by constructing sand piles in the mounded soil 3, the height and range of the mound can be reduced, so the impact on the surrounding ground and surrounding structures is also small.

[0077] Furthermore, since the modification process is carried out after the sand pile construction process is completed, the number of work boats can be reduced and the scale of construction can be scaled down compared to the conventional method of using the raised soil 3 as a filling material for the sand piles.

[0078] Furthermore, if there is a limit to the layer thickness D of the soil to be improved due to construction reasons, the distance d can be set using the above-mentioned equation (1), thereby ensuring that the soil to be improved is improved, and a stable submerged ground improvement structure 1 can be obtained. [Example]

[0079] The effect of reducing the swelling height in the waterbed ground improvement method and waterbed ground improvement structure 1 of the present invention will be specifically described below with reference to examples shown in Figs. FIG. 13 is a diagram showing a conventional waterbed ground improvement structure 31 in which sand piles are also constructed in the mounded soil 3 to ensure the strength of the mounded soil 3.

[0080] In the improved structure of Fig. 13, sand piles are also constructed in the mounded soil 3, which increases the mound height. Based on past construction results, it is known that the mounding ratio μ' when sand piles are also constructed in the mounded soil 3 can be estimated by multiplying the mounding ratio μ calculated by equation (3) by 1.24 (see Non-Patent Document 2).

[0081] Figure 14 is a graph comparing the rise height H (calculated assuming the rise rate μ´ = 1.24μ) in the above-mentioned conventional example with the rise height H in the submerged ground improvement structure 1 of the present invention shown in Figure 1 or Figure 4 when the height l of the sand pile, the longitudinal length A of the improved range of the sand pile, and the transverse length B of the improved range of the sand pile (see Figure 10) are independently and randomly changed. In both the conventional example and the invention example, the substitution rate a s was fixed at 0.5 and the influence angle θ was fixed at 60°. In addition, the layer thickness D of the modified soil portion 9 in the invention example was fixed at 5 m.

[0082] The conventional example in Figure 13 and the invention examples in Figures 1 and 4 are both technologies that utilize mounded soil without dredging it away, but as shown in Figure 14, according to the present invention, the mound height H can be reduced to less than 80% of the conventional height. [Example]

[0083] Figure 15 shows an example in which the submerged breakwater 1 according to the present invention is applied to a submerged breakwater required for shallow areas, tidal flats, seaweed beds, etc. In Figure 15, 33 denotes a submerged breakwater, 35 denotes sand capping, and 37 denotes a filling material. By applying the present invention to improve the water bottom ground 5 as shown in FIG. 15, the stability of the submerged breakwater 33 can be ensured and the mounded soil 3 can be used without dredging. Furthermore, the modified soil portion 9 is a solidified body, and unlike sand piles, it exhibits strength even in areas with low overburden pressure, which also has the effect of suppressing the occurrence of shallow slides that pass through the toe of the submerged breakwater on the offshore side. [Example]

[0084] Figure 16 shows an example in which the underwater ground improvement structure 1 according to the present invention is applied to a caisson-type breakwater. In a caisson-type breakwater, the water depth is shallowed by the foundation rubble 41 constructed below the caisson 39, but by positioning the upper end of the sand pile construction section 7 deeper than the ground surface 5a before improvement as shown in Figure 16, the height of the rise can be suppressed, and the impact on the water depth can be reduced. [Example]

[0085] Figure 17 shows an example in which the underwater ground improvement structure 1 according to the present invention is applied to a caisson-type breakwater with wave-dissipating blocks 43. By extending the construction range of the improved soil section 9 further offshore than the sand pile construction section 7 as shown in Figure 17, the stability of the caisson 39 can be ensured and the settlement of the wave-dissipating blocks 43 installed on the offshore side of the caisson 39 can be suppressed. [Example]

[0086] Figure 18 shows an example in which the submarine ground improvement structure 1 according to the present invention is applied to a caisson-type breakwater with a shoring work 45. By extending the construction range of the improved soil section 9 further inland than the sand pile construction section 7 as shown in Figure 18, the stability of the caisson 39 is ensured, and the installation of the shoring work 45 can prevent sliding on the land side of the caisson. [Example]

[0087] Figure 19 shows an example in which the submarine ground improvement structure 1 according to the present invention is applied to a gravity-type quay wall. By extending the construction range of the improved soil section 9 further offshore than the sand pile construction section 7, as shown in Figure 19, the stability of the caisson 39 is ensured and shallow sliding that occurs on the offshore side of the caisson due to eccentric inclined loads caused by the horizontal earth pressure of the backfill stones 47 and backfill soil 49 can be suppressed. Furthermore, the modified soil portion 9 is a solidified body and exhibits strength even in areas where the overburden pressure is small, so it has a large effect of suppressing shallow slides. [Explanation of symbols]

[0088] 1 Underwater soil improvement structure 3. Raised soil 5 Underwater ground 5a Ground surface 7 Sand pile construction department 9. Modified soil section 11 Vibro Hammer 13 Casing pipe 15 Sand pile 17 Soil to be improved 19 Casing 21 Stirring and mixing means 23 Modifier 25 Modifier supply channel 27 Modifier stirring and mixing device 29 Modified soil 31 Underwater ground improvement structure (conventional example) 33 Submerged Embankment 35 Sand covering 37 Filling material 39 Caisson 41 Foundation rubble 43 Wave-dissipating blocks 45 Belly work 47 Backfill Stone 49 Backfill soil

Claims

1. A method for improving a waterbed containing clayey soil, comprising: a sand pile construction step of constructing sand piles in the ground of the bottom of the water; This method for improving the ground at the bottom of the water includes a modification process in which the mounded soil generated in the sand pile construction process is modified by stirring and mixing in situ a modifier that hardens the clayey soil.

2. The sand pile construction step constructs the sand pile so that the upper end of the sand pile is formed at a position deeper than the surface layer of the waterbed ground, 2. The method for improving a submerged ground according to claim 1, wherein the modifying step involves stirring and mixing the modifying material in situ with the clayey soil in the surface layer to also modify the clayey soil in the surface layer.

3. The modification process uses a modifier stirring and mixing device having a casing capable of taking in the soil to be modified inside, a stirring and mixing means capable of moving up and down inside the casing, and a modifier supply path for supplying the modifier toward the stirring and mixing means, A method for improving submerged ground as described in claim 1 or 2, wherein the modifier is supplied toward the stirring and mixing means while the casing is pressed into the soil to be modified, and the stirring and mixing means is operated so that the soil to be modified and the modifier taken into the casing are stirred and mixed, thereby modifying the soil to be modified in the casing from bottom to top.

4. The method for improving the waterbed ground according to claim 3, wherein the sand pile construction step constructs the sand pile so that the distance d from the ground surface of the waterbed ground to the upper end of the sand pile satisfies the following formula (1): -18.677a s 2 +1.041D-0.393≦d≦-0.442a s 2 +1.025D-0.115 (where 0≦d) ... (1) where: D: Layer thickness of the soil to be improved (m) a s : Replacement rate (the ratio of the area of ​​the sand pile when the plane area of ​​the waterbed is 1) d: Distance from the ground surface before construction to the top of the sand pile (m)

5. An improvement structure for a waterbed containing clayey soil, a sand pile construction section in which sand piles are constructed underground in the waterbed; A modified soil portion is provided on the sand pile construction portion and is made of modified soil obtained by modifying the clayey soil. The modified soil is a submerged ground improvement structure in which the soil is modified by mixing a modifier that hardens the clayey soil with the mounded soil that was created during the construction of the sand piles.

6. The upper end of the sand pile construction portion is formed at a position deeper than the surface layer of the waterbed ground before the sand pile is constructed, The waterbed ground improvement structure according to claim 5, wherein the modified soil further includes clayey soil in the surface layer that has been modified by mixing the modifying material into the clayey soil.

7. The waterbed ground improvement structure according to claim 5 or 6, wherein the distance d from the ground surface before constructing the sand pile to the upper end of the sand pile satisfies the following formula (1): -18.677a s 2 +1.041D-0.393≦d≦-0.442a s 2 +1.025D-0.115 (where 0≦d) ... (1) where: D: Layer thickness of improved soil (m) a s : Replacement rate (the ratio of the area of ​​the sand pile when the plane area of ​​the waterbed is 1) d: Distance from the ground surface before construction to the top of the sand pile (m)

8. The modifying material is a material that reacts with the clayey soil and hardens, 7. The waterbed ground improvement structure according to claim 5, wherein the modified soil portion has strength due to the modifier being mixed into the clayey soil.

9. 9. The waterbed ground improvement structure according to claim 8, wherein the material is made from converter-based steel slag.

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