JP2008019561A - Construction method of countermeasure against liquefaction under breakwater caused by creation of underground wall - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a construction method of countermeasures against liquefaction under a breakwater which enables construction to be inexpensively performed in a short period of time, and can effectively prevent the occurrence of a liquefaction phenomenon. <P>SOLUTION: A rubble-mound 11 is created on a sandy soil layer 10 on the bottom of the sea. The breakwater is constituted by installing a concrete dam body 12 on the rubble-mound 11. Under the breakwater, a pair of impermeable or semipermeable underground walls A are created in the sandy soil layer 10 under the slightly outside sections of both the front and back edges of the dam body 12 along both edges of them. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In the present invention, the ground under the breakwater built on the sandy ground of the seabed is designed to make the breakwater sunk or collapse due to liquefaction when an earthquake occurs and to prevent the breakwater function from being impaired. It relates to a liquefaction countermeasure method under a breakwater by creating a middle wall.

  In general, breakwaters such as jetties and offshore dikes are constructed on the ground so as to secure a supporting force that does not cause sliding and overturning against external forces such as waves.

  Conventionally, the sandy ground under and supporting the breakwater is a replacement sandy soil in which the original ground made of clay soil is replaced with sand, or the ground is against a wave or other external force. In some cases, it is sandy soil having a supporting force that does not cause sliding or falling.

  However, even in sandy ground that has sufficient support for external forces such as waves, it is assumed that the support will be lost due to the liquefaction phenomenon at the time of the earthquake and the breakwater will sink. When a liquefaction phenomenon occurs due to an earthquake, a volume change occurs in the sandy soil layer, and the dam body is expected to sink, tilt, or in some cases collapse.

  If the levee body sinks or tilts more than a certain level, the wave blocking function is impaired, the amount of overtopping to the inside of the port increases, and the calmness within the port cannot be ensured. In addition, when a tsunami comes after an earthquake, the amount of overtopping of the tsunami increases, and there is a problem that the function to stop flooding damage to the land on the back is impaired.

  Conventionally, as a countermeasure for liquefaction when building a breakwater on such sandy ground, there is a sand compaction pile method (for example, Patent Documents 1 and 2). As shown in FIG. 12, before construction of the breakwater, an improved ground 2 is created in the sandy soil layer 1 below so that liquefaction can be prevented by a ground improvement method such as creation of a sand compaction pile. This is a method in which a rubble mound 3 is created on top of which a concrete dam body 4 made of concrete caisson or cast-in-place concrete is installed.

  Further, as a liquefaction countermeasure for the existing breakwater, there is a raising method as shown in FIG. This is to raise the height just above the levee body 4 from the static water surface to the required height, and build the raised portion 5 on the dam body 4 with cast-in-place concrete to ensure stability against sliding and falling of the dam body. In order to do so, it is widened as necessary to ensure weight.

Furthermore, in order to ensure stability against the sliding and falling of the levee body as well as the widening of the levee body, a required amount of stone of about 1 to 70 kg / piece is put inside the breakwater port to create the backfilling stone portion 6.
JP-A-8-27759 JP 7-207653 A

  In the conventional construction of the new breakwater described above, the method of applying ground improvement by creating a sand compaction pile in the sandy soil layer under the breakwater increases the construction cost, because the ground improvement range is wide, There is a problem that the construction period becomes longer.

  Moreover, the raising method for the existing breakwater cannot prevent liquefaction of the sandy soil layer itself, and when the liquefaction phenomenon occurs, the levee body undergoes considerable subsidence, and the breakwater normal is not straight. There is a risk that the function as a breakwater cannot be fully achieved.

  In addition, as the expected height of the tsunami increases, it is necessary to increase the dam body width by a considerable amount in order to prevent sliding and falling over the wave force. Similarly, in order to ensure stability against sliding, the amount of lining stones inside the port must be increased, and when raising the height, the cost increases accordingly, and the sand under the breakwater There is a problem that the weight applied to the soil layer increases, which may cause settlement.

  In view of the conventional problems as described above, the present invention can be applied in a short period of time at low cost regardless of whether it is applied to any existing or new breakwater, and the settlement of the levee body due to liquefaction phenomenon It was made for the purpose of providing a liquefaction countermeasure method under a breakwater that can effectively prevent the dam.

  The feature of the invention described in claim 1 for solving the conventional problems as described above and achieving the intended purpose is that a rubble mound is formed on a sandy soil layer on the seabed, and concrete is formed on the rubble mound. A liquefaction countermeasure method under a breakwater constructed by installing a dam body, and a pair of impervious water in the sandy soil layer slightly below the front and rear edges of the levee body The purpose is to create underground walls with low permeability or poor permeability.

  According to a second aspect of the present invention, in addition to the configuration of the first aspect, the breakwater is a newly installed breakwater, and the underground wall is formed in the sandy soil layer that becomes a breakwater before the rubble mound is formed. Is to build.

  According to a third aspect of the present invention, in addition to the configuration of the first aspect, the breakwater is an existing breakwater, and the underground wall is constructed in a sandy soil layer under the existing breakwater. It is in.

  According to a fourth aspect of the present invention, in addition to the constitution of the first, second or third aspect, the pair of impermeable or hardly permeable underground walls are cemented solidified in the sandy soil layer. It is to create by mixing materials.

  According to a fifth aspect of the present invention, in addition to the configuration of the first, second, or third aspect, the pair of impermeable or hardly permeable underground walls hits a sheet pile in the sandy soil layer. It is to create by setting up.

  In the present invention, a rubble mound is formed on a sandy soil layer and a concrete dam body is installed on the rubble mound. By creating a pair of impervious or hardly permeable underground walls by mixing cement-based solidified material or placing a sheet pile in the sandy soil layer slightly outside The liquefaction phenomenon occurs in the sandy soil layer directly below, but the volume change of the sandy soil layer between the underground walls is suppressed by the effect of suppressing shear deformation by the underground wall, and as a result, the settlement of the levee body is suppressed. The

  In addition, in the breakwater liquefaction countermeasure of the present invention, since the ground wall is not formed on the entire ground below the breakwater, but a part of the ground wall is created, it is constructed as a liquefaction countermeasure for the newly built breakwater. Even if it does, it can construct in a short time at low cost compared with the construction method by the creation of the conventional sand compaction pile.

  Furthermore, even when constructing an existing breakwater, it is not intended to secure the required height even when subsidence occurs by raising the bulk as in the conventional raising method, but the subsidence itself due to liquefaction is not achieved. Since it is what prevents, there exists an effect that safety | security is higher.

  Next, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a cross section of a breakwater and a ground below the breakwater countermeasure according to the present invention. In the figure, reference numeral 10 indicates a sandy soil layer in the case where the original ground under the breakwater is sandy soil, and 11 is formed by throwing foundation rubble weighing about 10-200 kg / piece onto it. The rubble mound 12 is a dam body made of concrete caisson or cast-in-place concrete placed on the rubble mound 11.

  The upper surface of the rubble mound 11 is covered with a covering stone 13 having a weight of about 1 t / piece except for the bottom of the dam body 12, and the solid stone 14 is stacked before and after the root portion of the dam body 12. ing.

  The countermeasure against liquefaction in the present invention is that the underground walls A and A are improved by the ground improvement continuously in the direction along the edge of the dam body 12 in the sandy soil layer 10 slightly outside the dam body 12. It has been created.

  The underground walls A and A are hardened in comparison with the surrounding sandy soil layer by mixing the sandy soil layer 10 with a cement-type fluidized solidifying material such as cement slurry to improve the ground. It is a highly impermeable or hardly permeable wall.

  In addition to this, a water glass-based, special silica-based or polymer-based ground improvement chemical solution may be formed by penetrating into the ground and solidifying.

  The greater the thickness of the underground wall, the greater the effect of suppressing shear deformation and the more the subsidence of the dam body during an earthquake is suppressed. However, considering the necessary subsidence suppression effect, economic efficiency and workability, 1 About 0.0-3.0 m is preferable.

  For the formation of the underground wall A, a device similar to a ground improvement device for injecting and mixing a solidified material to a soft ground with a water bottom and hardening it can be used. As shown in FIG. 2, this apparatus includes a rotating shaft 22 integrally provided with an auger 21 on the outer periphery in a parallel arrangement, and a plurality of stirring blades 23 project from the outer periphery of the lower end of each rotating shaft. The agitating blade 23 is provided with a solidifying material discharge nozzle (not shown) so that the solidifying material can be discharged from the nozzle through the rotating shaft 22 and the rotating shaft 22 rotates the rotating shaft 22 at the upper end. A ground improvement machine 20 having 24 is provided.

  As shown in FIG. 5A, the ground improvement machine 20 is installed so as to be vertically movable along a leader 26 standing on the deep mixing treatment ship 25, and the ground improvement machine 20 is moved from the deep mixing treatment ship 25 to the bottom of the water. So that it can be moved up and down.

  Next, an embodiment when a breakwater is newly installed on the sandy soil layer 10 will be described.

  FIG. 3 shows the construction flow. In the laying sand work shown in the construction flow, the laying sand 30 is laid on the upper surface of the sandy soil layer 10 where the underground walls A and A are to be created. This laying sand 30 is laid to prevent the improving material such as cement slurry mixed in the ground to form the underground wall A from flowing into the water, and has a thickness of about 1.0 m. . As shown in Fig. 4, the laying sand construction is conventionally used, such as using an open-bottomed earth-moving ship 31 and putting it directly by opening the bottom or using a self-propelled transport ship with a grab and throwing it from the ship with a grab bucket. It is carried out by the existing sand method.

  Next, the underground wall construction work by ground improvement will be carried out. This underground wall construction work is carried out using the ground improvement machine 20 described above, and as shown in FIG. 5 (a), a deep mixing treatment vessel 25 is used to turn the ground improvement machine 20 into a sandy soil layer. The columnar body 20a in which the solidified material is discharged from the nozzle of the stirring blade 23 at the same time as it is rotated while being pulled up and then rotated while being pulled up, thereby mixing the solidified material into the sandy soil of the sandy soil layer 10. Create.

  In this way, the solidifying material is injected into the sandy soil layer 10 to repeat the operation of forming the columnar body 20a with its side portions integrated with each other as shown in FIG. The underground wall A is created. In addition, when the N value of sandy soil is large and the improvement work requires difficulty, an auxiliary construction method such as loosening the ground by prior drilling using a work ship is also used.

  Thus, after extending a pair of underground wall A and A sequentially in a horizontal direction in the mutually parallel arrangement | positioning, the installation sand removal work which removes the installation sand 30 as needed is constructed. This laying sand removal work is performed using, for example, a grab ship 32 as shown in FIG.

  It is judged that it is inappropriate to use the material with non-homogeneous physical properties as it is as the foundation for the sand removal work because the improvement of the sand by the solidified material is insufficient and the physical properties are inhomogeneous. If you do. In addition, when it is judged that the material of the ground sand and the uplifted soil near the surface layer is homogeneous and suitable for use as a foundation, it will not be removed.

  Next, as shown in FIG. 1, a rubble mound 11 is formed by a method similar to the conventional method, and a bank body 12 is installed thereon.

  Next, the Example with respect to the sandy soil layer under the existing breakwater is demonstrated.

  When the above-described underground wall A is created, first, the ground improvement machine insertion portion for allowing the ground improvement machine 20 to be inserted into the sandy soil layer 10 under the rubble mound 11 as shown in FIG. 40 is formed. This ground improvement machine insertion part 40 removes a part of foundation rubble temporarily, and forms a window hole in the rubble mound front and back, and it is good also as operating a working machine by a diver and removing rubble. As shown in FIG. 8, it may be formed by cutting into a cylindrical shape using a cutting machine comprising a core cutter 43 in which a stone cutting blade 42 is fixed to the tip of a cylindrical rotating cylinder 41.

  In this way, the ground improvement machine insertion part 40 is formed to have an insertion width of one to several times of the ground improvement machine 20, and the ground improvement machine 20 is passed through the ground improvement machine insertion part 40 as shown in FIG. After inserting while rotating to the bottom of the sandy soil layer 10, the solidifying material is discharged from the nozzle of the stirring blade 23 at the same time as it is pulled up, thereby mixing the solidifying material into the sandy soil of the sandy soil layer 10. The columnar body 20a is formed.

  After the columnar body 20a is formed in this way, as shown in FIG. 7 (c), the basic rubble is returned to the ground improvement machine insertion section 40 to restore the rubble mound to its original state. By repeating this process, the column states 20a, 20a,...

  The formed ground improvement machine insertion portion 40 is filled with backfill sand, and the solidification material mixed by the ground improvement machine 20 is solidified into the backfill sand following the solidification material mixing into the sandy soil layer 10. The material may be mixed, and in this case, the ground improvement machine insertion portion 40 is filled with the underground wall A as shown in FIG.

  In the above-described embodiment, the raw ground is sandy ground, and the countermeasure for liquefaction under the breakwater built on the sandy soil layer 10 has been described. However, as shown in FIG. Is a soft ground 50 having a small bearing capacity (N value) made of cohesive soil, and is constructed on a sandy soil layer 10a composed of so-called replacement sand in which a part of the original ground is replaced with sand. As a countermeasure against liquefaction under the breakwater, it may be constructed as shown in FIG. The same parts as those in the above-described embodiment are denoted by the same reference numerals, and redundant description is omitted.

  In each of the above-described embodiments, a pair of underground walls are formed by mixing cement-based solidifying material in the sandy soil of the sandy soil layer. The underground wall which is impermeable or hardly permeable may be formed by joining and sequentially placing. When placing this sheet pile, if it is to be constructed under an existing breakwater, the rubble mound at the placement position is removed to form the sheet pile placement portion as described above. When removing rubble, it may be removed one by one or by cutting using a cutting machine. Moreover, the sheet pile to use can use steel sheet piles, steel pipe sheet piles, etc. other than concrete sheet piles.

Confirmation of effect Next, confirmation of the effect of the liquefaction countermeasure method according to the present invention by an earthquake response analysis by the finite element method will be described. In this analysis, the structure for the analysis was modeled, the material constants for the modeling were set, and the seismic response was calculated by selecting the input ground motion (maximum amplitude, waveform). As analysis results, horizontal and vertical maximums and residual displacements are calculated.
a. Outline of numerical analysis Outline of numerical analysis: Fig. 11
Target structure: Breakwater (width B = 15.0m)
Sandy soil layer thickness under breakwater: H = 20m Input seismic motion: Hachinohe basement incident waveform calculated from records obtained at Hachinohe Port during the 1968 Tokachi-oki earthquake.
Maximum acceleration: 400 gal
Underground wall thickness: 2 types of L = 4.5m, L = 1.5m b. Study cases Three cases as shown in Table 1 were used.

ｃ．解析結果
堤体の残留時の最大沈下量の算出結果を表２に示した。

c. Analysis results Table 2 shows the calculation results of the maximum subsidence when the bank remains.

  As shown in the above results, the method of the present invention is about 0.29 and 0.36 (the amount of settlement when measures are taken / the amount of settlement when no measures are taken) compared to the case where measures are not taken. The amount of settlement was suppressed. From this result, it was found that if the underground wall width is 1.5 m or more, the effect is sufficiently obtained.

It is a longitudinal cross-sectional view which shows an example of the existing breakwater which applied the countermeasure against liquefaction by this invention, and the ground under it. It is a front view which shows an example of the ground improvement machine used for the liquefaction countermeasure by this invention. It is a flowchart in the case of constructing the method of the present invention before building a new breakwater. It is sectional drawing which shows a laying sand work same as the above. (A) is sectional drawing which shows the continuous underground wall construction construction same as the above, (b) is a cross-sectional view in the middle of underground wall construction. It is sectional drawing which shows a laying sand removal work same as the above. (A) to (c) explain the construction process when the present invention construction method is carried out using the ground improvement machine of FIG. 2 when the present invention is carried out on a sandy soil layer under an existing breakwater. It is a schematic sectional drawing. It is sectional drawing which shows the other example of the formation method of the ground improvement machine insertion part in Fig.7 (a) in this invention construction method. It is sectional drawing of the state which built the underground wall continuously in the ground improvement machine insertion part in this invention construction method. The present invention shows an example in which the method of the present invention is carried out under a breakwater built on a sandy soil layer made of replacement sand in which a part of soft ground is replaced with sand, (a) a cross section before the liquefaction countermeasure (B) It is sectional drawing of the state after a liquefaction countermeasure. It is a schematic diagram of the numerical analysis used for confirmation of the effect of the present invention. It is a longitudinal cross-sectional view which shows an example of the conventional liquefaction countermeasure under a breakwater. It is sectional drawing which shows an example of the liquefaction countermeasure with respect to the existing existing breakwater.

Explanation of symbols

A Underground wall 10 Sandy soil layer 11 Rubble mound 12 Deck body 13 Cover stone 14 Negotite 20 Ground improvement machine 20a Columnar body 21 Auger 22 Rotating shaft 23 Stirring blade 24 Rotating drive 25 Depth mixing treatment ship 26 Leader 30 Laying Sand 31 Self-propelled carrier ship with grab 32 Grab ship 40 Ground improvement machine insertion part 41 Rotating cylinder 42 Stone cutting blade 43 Core cutter 50 Soft raw ground

Claims (5)

A liquefaction countermeasure method under a breakwater constructed by creating a rubble mound on a sandy soil layer on the seabed and installing a concrete dam body on the rubble mound,
According to the underground wall construction characterized by creating a pair of impervious or hardly permeable underground walls in the sandy soil layer slightly below the outer edge along both front and rear edges of the levee body A liquefaction countermeasure method under the breakwater.   2. The countermeasure for liquefaction under a breakwater by underground wall creation according to claim 1, wherein the breakwater is a newly installed breakwater, and the underground wall is constructed in the sandy soil layer that becomes the breakwater before the rubble mound is created. Construction method.   The liquefaction countermeasure method under a breakwater by underground wall creation according to claim 1, wherein the breakwater is an existing breakwater, and the underground wall is constructed in a sandy soil layer under the existing breakwater.   The breakwater by underground wall construction according to claim 1, 2 or 3, wherein the pair of impermeable or hardly permeable underground walls are formed by mixing a cement-based solidified material in the sandy soil layer. The liquefaction countermeasure method below.   The pair of impervious or hardly permeable underground walls is formed by placing a sheet pile in the sandy soil layer. Liquefaction countermeasure method.

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