High-pressure injection agitation method

Abstract

To set a rod movement width that effectively reduces soil clods.SOLUTION: In a high-pressure injection stirring method, a rod 100 inserted into a hole 20 drilled in the ground 10 is pulled up in stages, a cement-based solidifying material is injected from nozzles 112 and 114, formed at a tip of the rod 100 with a vertical space between them, to cut the ground 10 while rotating the rod 100, and soil 12 and the cement-based solidifying material are stirred and mixed to create a soil improvement body 150. The movement width d is set using the formula below, which represents the relationship between the vertical distance L between the nozzles 112 and 114, the movement width d for lifting the rod 100 in stages, and the cutting interval x of the ground 10. If 2L≥d≥L, x=max(d-L,L), If d<L, x=max(L-kd, d-(L-kd)), and k is the largest natural number less than or equal to L / d.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a high-pressure jet mixing method.

Background Art

[0002] Patent Document 1 discloses a technique related to a liquefaction prevention technique for preventing soft ground such as sandy ground from liquefying in an earthquake or the like. In this prior liquefaction prevention method, a rod equipped with a curing material injection device is inserted into a boring hole, and while injecting the curing material from the curing material injection device, the rod is rotated and pulled up to form a ground consolidation body. The curing material injection device is intermittently pulled up at predetermined intervals, and the curing material is injected so that the improvement area where the curing material reaches and the ground is improved joins the improvement areas in adjacent unimproved holes in the horizontal and vertical directions.

[0003] Patent Document 2 discloses a technique that enables simultaneous improvement of the efficiency of the high-pressure jet mixing method and the quality of the improved body. In this prior high-pressure jet mixing method, an injection rod equipped with nozzles on its side surface is inserted into the ground to be improved, and when the injection rod is pulled up step by step at predetermined distances while rotating it, a solidifying material liquid is injected from the nozzles at high pressure. The high-pressure jet of this solidifying material liquid cuts the soil, and at the same time, the cut soil and the solidifying material liquid are stirred and mixed to form an improved body in the ground. Then, an improved body diameter expansion process in which the injection rod is rotated at a relatively low rotational speed for a predetermined time to make the high-pressure jet reach relatively far, and an improved body high-qualityization process in which the injection rod is rotated at a relatively high rotational speed for a predetermined time to promote the stirring of the soil by the high-pressure jet are alternately executed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The high-pressure jet mixing method is said to be more likely to leave unimproved soil lumps than the mechanical mixing method that cuts and mixes the ground with mixing blades. And the soil lumps in the ground improvement body lead to quality degradation such as strength reduction.

[0006] In order to reduce the remaining amount of soil lumps, it is desirable to reduce the width of pulling up or pulling down the rod (hereinafter sometimes referred to as "moving width"). However, if the moving width of the rod is reduced, the efficiency of constructing the ground improvement body decreases. Also, the required remaining amount of soil lumps etc. varies depending on the specifications and quality of the ground improvement body to be constructed.

[0007] In view of the above facts, an object of the present invention is to provide a method for setting the moving width of a rod that efficiently reduces soil lumps in the high-pressure jet mixing method.

Means for Solving the Problems

[0008] The first aspect is a high-pressure jet mixing method in which a rod inserted into a hole drilled in the ground is pulled up or pulled down step by step, while rotating the rod, an injection material is injected from nozzles formed at the tip of the rod with a vertical interval, cutting the ground, and stirring and mixing the soil and the injection material. Using the following mathematical formula showing the relationship between the vertical interval L of the nozzles, the moving width d which is the width of pulling up or pulling down the rod step by step, and the cutting interval x of the ground, the moving width d is set. When 2L≧d≧L, x = max(d - L, L); when d < L, x = max(L - kd, d - (L - kd)), where k is the largest natural number not exceeding L / d.

[0009] In the high-pressure jet mixing method of the first aspect, the moving width is set using a formula showing the relationship between the vertical interval L of the nozzles, the moving width d of pulling up the rod step by step, and the cutting interval x of the ground.

[0010] Here, the cutting interval x of the ground does not decrease uniformly even if the movement width when raising or lowering the rod is reduced in stages. Also, the amount of soil remaining is more correlated with the cutting interval than with the movement width. Therefore, by setting the movement width d of the rod using the cutting interval x, which is determined from the vertical distance L and the movement width d, the amount of soil remaining can be reduced efficiently.

[0011] The second embodiment is the high-pressure jet agitation method described in the first embodiment, wherein when there are multiple movement widths d with the same cutting interval x, the largest movement width d is selected.

[0012] In the second embodiment of the high-pressure injection mixing method, the required cutting interval x is determined from the specifications of the ground improvement body, and the largest movement width d that results in that cutting interval x is selected, thereby efficiently reducing the amount of soil mass.

[0013] The third embodiment is the high-pressure jet agitation method described in the first embodiment, wherein the graph of the formula, with the movement width d on the horizontal axis and the cutting interval x on the vertical axis, is configured such that the movement width d is set at or near the apex of the valley.

[0014] In the third embodiment of the high-pressure injection mixing method, the movement width d of the formula is plotted on the horizontal axis and the cutting interval x on the vertical axis. Since the movement width d is set at or near the apex of the valley, the soil mass can be efficiently reduced. [Effects of the Invention]

[0015] According to the present invention, a method for setting the movement width of a rod that efficiently reduces soil clumps in a high-pressure injection mixing method can be provided. [Brief explanation of the drawing]

[0016] [Figure 1] This is a process diagram showing the construction process for creating a ground improvement body using the high-pressure injection mixing method. [Figure 2](A) is an explanatory diagram for explaining the cutting interval x when the movement width d is 25 mm, which is the same as the vertical interval L. (B) is an explanatory diagram for explaining the cutting interval x when the movement width d is 15 mm. (C) is an explanatory diagram for explaining the cutting interval x when the movement width d is 13 mm. [Figure 3] It is a configuration diagram showing the schematic configuration of the experimental device. [Figure 4] It is a table summarizing the experimental results. [Figure 5] (A) is a graph showing the relationship between the movement width d and the residual rate of soil clods in the experimental results. (B) is a graph showing the relationship between the cutting interval x and the residual rate of soil clods in the experimental results. [Figure 6] It is a graph showing the relationship between the cutting interval x and the residual rate of soil clods in the experimental results. [Figure 7] It is a graph showing the relationship between the movement width d and the cutting interval x when the vertical interval L is 25 mm.

Mode for Carrying Out the Invention

[0017] <First Embodiment> The high-pressure jet mixing method of the first embodiment of the present invention will be described.

[0018] [Configuration] First, the ground improvement device used in the high-pressure jet mixing method will be described.

[0019] As shown in FIG. 1, the ground improvement device 50 has a device body 52. A rod 100 for construction is attached to the device body 52. As shown in FIG. 2(A), a monitor 110 as an example of an injection device is attached to the tip of the rod 100.

[0020] Nozzles 112 and 114 are formed on the side surface 110A of the monitor 110, which eject a jet J containing a liquid cement-based solidifying material such as cement milk and air. One (lower) nozzle 112 is formed at the bottom of the monitor 110. The other (upper) nozzle 114 is formed at the top of the monitor 110 and is formed in the opposite direction to the one (lower) nozzle 112 in a plan view (see also Figure 3). In other words, the one (lower) nozzle 112 and the other (upper) nozzle 114 are formed with a gap in the vertical direction and are formed to eject the jet J in opposite directions. The vertical gap between the one nozzle 112 and the other nozzle 114 is called the vertical gap L.

[0021] [Overview of High-Pressure Jet Agitation Method] The outline of the construction process for the high-pressure injection agitation method of this embodiment will be described below. Note that the following construction process is an example and is not limited thereto. Also, although Figure 1 shows one (lower) nozzle 112 and the other (upper) nozzle 114 at the same height, in reality, as shown in Figures 2(A) and 3, the nozzles 112 and 114 are formed with a vertical gap between them.

[0022] • Excavation process As shown in Figure 1(A), a ground improvement device 50 is installed in the ground 10, a hole is drilled with a rod 100, and the rod 100 is inserted into the drilled hole 20.

[0023] • Pre-jet process As shown in Figures 1(B) and 1(C), the rod 100 is rotated and pulled up while a jet M containing water and air is ejected from nozzles 112 and 114 of the monitor 110 at the tip of the rod 100. Then, the soil 12 of the ground 10 is cut with the jet M up to the upper end of the ground improvement body 150 to be constructed (see Figure 1(F)) to create a pre-jetted target soil area 14 by mixing and agitating the soil.

[0024] ·Creation process As shown in Figure 1(D), the construction rod 100 is inserted to the bottom of the target soil area 14 after pre-jetting, and a jet J containing a liquid cement-based solidifying material such as cement milk and air is ejected radially outward at high pressure from nozzles 112 and 114 (see Figure 2(A)) on the side 110A of the monitor 110 attached to the tip of the rod 100, while the rod 100 (monitor 110) is rotated. Note that jet J is an example of the ejected material.

[0025] As shown in Figures 1(E) and 1(F), the high-pressure jet of jet J cuts the soil 12 within the ground 10, and the cut soil 12 is mixed with a cement-based solidifying agent. Then, by pulling up this rod 100 (monitor 110), a cylindrical ground improvement body 150 is created vertically within the ground 10.

[0026] In this embodiment, the construction process involves raising the rod 100 in stages. Specifically, the vertical position of the rod 100 (monitor 110) is fixed while the rod 100 is rotated and jet J is ejected. After one rotation, the rod 100 is raised by a predetermined width. Then, the rod 100 is rotated again while jet J is ejected. By repeating this process, a cylindrical ground improvement body 150 is constructed vertically within the ground 10. The width by which the rod 100 is raised in stages is defined as the movement width d. The method for setting this movement width d will be described later.

[0027] In the above description, the rotation of the rod 100 (monitor 110) was stopped during the lifting process, but this is not the only option; the rod 100 (monitor 110) may also be rotated during the lifting process.

[0028] Also, in the above description, the rod 100 (monitor 110) was rotated once, but it is not limited to this, and the rod 100 (monitor 110) may be rotated more than once. Further, when the cross-sectional shape of the ground improvement body is other than circular, the rod 100 (monitor 110) is rotated according to the cross-sectional shape. For example, when forming ground improvement bodies with a fan-shaped cross-section (wall-shaped cross-section of a butterfly shape) on the left and right sides of the rod, the rod 100 (monitor 110) may be rotated (oscillated) by a predetermined angle instead of being rotated once.

[0029] [Key Parts of the High-Pressure Jet Mixing Method] When pulling up step by step, the maximum vertical interval including the passing trajectories of the upper and lower nozzles 112 and 114 is defined as the cutting interval x. This cutting interval x is the maximum width at which the jet J cuts the soil 12 in the ground 10. And the equation showing the relationship among the vertical interval L between the nozzles 112 and 114, the moving width d for pulling up the rod 100 (monitor 110) step by step, and the cutting interval x is as follows. Hereinafter, the following equation will be referred to as the "cutting interval equation".

[0030] When 2L≧d≧L x = max(d - L, L) When d < L x = max(L - kd, d - (L - kd)) k is the largest natural number less than or equal to L / d

[0031] Next, the relationship among the vertical interval L, the moving width d, and the cutting interval x, that is, the above cutting interval equation will be described in detail. In the following description, the vertical interval L between the nozzles 112 and 114 is 25 mm.

[0032] As shown in Fig. 2(A), when the moving width d is the same as the vertical interval L, which is 25 mm, the cutting interval x is 25 mm. As shown in Fig. 2(B), when the moving width d is 15 mm, the cutting interval x is 10 mm. As shown in Fig. 2(C), when the moving width d is 13 mm, the cutting interval x is 12 mm.)

[0033] Thus, reducing the travel width d does not always result in a smaller cutting interval x. The formula that expresses this is the aforementioned "cutting interval formula." The upper limit of the travel width d is set to 2L. This is because if it is larger than this, there is a high possibility that areas where the jet J does not make contact or hardly makes contact will occur, i.e., unimproved areas will be created.

[0034] Figure 7 shows the cutting interval x when the vertical spacing L is 25 mm and the movement width d is increased in 1 mm increments. As the movement width d is increased in this way, the cutting interval x increases in a repeating pattern of peaks and valleys.

[0035] [Relationship between vertical spacing L, movement width d, cutting interval x, and soil mass] As mentioned above, the high-pressure jet mixing method is a construction method that, as shown in Figure 1, uses a high-pressure jet J to cut the soil 12 of the ground 10 while mixing and mixing the soil 12 with a cement-based solidifying agent to create a ground improvement body 150.

[0036] Therefore, compared to mechanical mixing methods that cut and mix the ground with mixing blades, high-pressure jet agitation is generally considered to be more prone to generating unimproved sections, i.e., residual soil clods, within the improved ground body 150.

[0037] However, reducing the cutting interval x reduces the interval at which the soil 12 is cut, which is thought to reduce the generation of residual soil clumps. Therefore, this was confirmed using an experimental apparatus.

[0038] Figure 3 is a schematic diagram of the experimental apparatus 500. In the experiment, a jet M containing water and air was injected from nozzles 112 and 114 of the monitor 110 to cut and agitate the simulated ground 510. The simulated ground 510 was made by mixing akadama soil, silica sand, water, and rapid-hardening cement, pouring it into a formwork 502, and curing it for two weeks. The vertical distance for cutting and agitation was 400 mm. Then, the soil clods 512 generated by cutting with the jet M were collected and measured. The mesh size of the wire mesh 514 used to collect the soil clods 512 was φ4.75. A PVC pipe 504 was installed at the bottom of the formwork 502 to house the part of the monitor 110 below the lower nozzle 112.

[0039] The experimental cases consisted of seven variations, Case 1 to Case 7, as shown in Figure 4. The soil mass retention rate was defined as the ratio of the weight of the collected soil mass to the weight of the cut soil, calculated from the measured volume of the cut soil mass of the simulated ground 510. In each experimental case, the rotation speed of the monitor 110 (see Figure 3) was adjusted so that the amount of water injected by the jet M ejected within the 400 mm vertical distance of cutting and stirring was equal. In other words, the larger the movement width d, the slower the rotation speed of the monitor 110 was set.

[0040] Figure 5(A) is a graph showing the relationship between the movement width d and the soil mass retention rate, excluding Case 7, and Figure 5(B) is a graph showing the relationship between the cutting interval x and the soil mass retention rate, excluding Case 7. Figure 6 is a graph showing the relationship between the cutting interval x and the soil mass retention rate, including Case 7.

[0041] As can be seen from the graph in Figure 5(A), the soil mass retention rate does not necessarily decrease even when the movement width d decreases. In contrast, as can be seen from the graphs in Figures 5(B) and 6, the soil mass retention rate decreases as the cutting interval x decreases. In other words, it was confirmed that the soil mass retention rate of soil mass 512 is correlated with the cutting interval x.

[0042] Therefore, it was confirmed that reducing the cutting interval x is important, rather than simply reducing the movement width d, in order to reduce the generation of soil clods 512.

[0043] [Method for determining the movement range] Next, we will explain an example of how to determine the movement width d.

[0044] As mentioned above, reducing the cutting interval x reduces the generation of soil clods (see Figure 6(B)). Therefore, to reduce soil clods, a smaller cutting interval x is better. On the other hand, the smaller the movement width d, the longer it takes to construct the ground improvement body 150.

[0045] Here, the target performance of the ground improvement body 150 differs from site to site. For example, the allowable soil mass (size and volume), the reach of the jet J, the material composition of the jet J, the amount of sludge produced during ground improvement, the required quality of the ground improvement body 150, and the soil type of the ground 10 to be improved differ from site to site.

[0046] Therefore, when comprehensively considering how to satisfy the target performance of the ground improvement body 150, the movement width d is set using the "cutting interval formula" from the viewpoint of efficiently reducing the soil mass.

[0047] For example, if a cutting interval x is determined that satisfies the target value for soil mass volume, and there are multiple movement widths d that result in this cutting interval x, the largest movement width d is selected. Alternatively, within the range of cutting interval x or less that satisfies the target value, the movement width d of the top (bottom) of the valley in Figure 7 or its vicinity is used.

[0048] Alternatively, for example, if there are multiple specifications that can achieve the target performance of the ground improvement body 150, the "cutting interval formula" is used to set the movement width d when selecting the specification that can efficiently reduce the soil mass.

[0049] [Example settings] Next, we will explain a specific example of setting the movement width d.

[0050] • First example Core boring was performed on 150 ground improvement bodies experimentally constructed in cohesive soil under the conditions of a vertical spacing L of 25 mm, a movement width d of 25 mm, and a cutting interval x of 25 mm. The results showed that 20% of the soil clods were 30 mm or larger, and the quality was judged to be poor. In other words, the quality was judged to be poor with a core sampling rate of 80%.

[0051] This judgment criterion is based on the Japan Building Center's ground improvement guidelines. Specifically, if core boring is performed as a quality check and soil clods are found to be mixed in 50% or more of the core area, it will be treated as an unimproved area. In the case of cohesive soil, in order for it to be recognized as an improved area, the unimproved area must be 10% or less on average over the entire length (the improved area must be 90% or more; this percentage of the improved area is called the core sampling rate).

[0052] Therefore, although the current soil clump content is 20%, we will reduce this to satisfy the aforementioned Japan Building Center's ground improvement guidelines of 10% or less. Specifically, we will reduce the soil clump retention rate from 80% (shown in Figure 6 with a cutting interval x of 25 mm) to 1 / 2 or less, i.e., to 40% or less. From the experimental results in Figure 6, it can be seen that the cutting interval x should be 12 mm or less in order to reduce it to 1 / 2 or less.

[0053] From the graph in Figure 7, created using the cutting interval formula, the movement widths d that result in a cutting interval x of 12 mm are 18.5 mm and 13 mm. Therefore, setting the movement width d to 18.5 mm is efficient and rational.

[0054] Furthermore, considering variations, it is desirable to set the cutting interval x to less than 12 mm. In that case, the travel width d to the crest of the valley in Figure 7 should be between 16 mm and 18.5 mm. Moreover, if there are no problems with construction efficiency, it is appropriate to set the travel width d to 16 mm, which is the crest of the valley.

[0055] ·Second example If the vertical spacing L is 25 mm, then from the graph in Figure 6... With a movement width d of 12 mm and a cutting interval x of 11 mm, the amount of soil remaining is 40%. With a movement width d of 16 mm and a cutting interval x of 9 mm, the amount of soil remaining is 32%. The movement width d is 7 mm, the cutting interval x is 4 mm, and the amount of soil remaining is 10%.

[0056] Based on these, the cutting interval ratio M%, which is the ratio of the cutting interval x0 of the initial planned construction specification to the cutting interval xi of the comparison construction specification, and the soil mass retention ratio N%, which represents the rate of decrease in the amount of soil mass remaining at that time, can be calculated using the following formulas. Note that the cutting interval ratio M% is an indicator of the construction speed (the larger the value, the faster the construction speed), and the soil mass retention ratio N% is an indicator of the improved soil quality (the larger the value, the better the quality).

[0057] M = [1 - (x0 - xi) / x0] × 100 N = 100 - (R² - R¹ × M)

[0058] Note that "x0" is the initially planned cutting interval, and "xi" is the cutting interval used for comparison. Furthermore, the coefficient R1 is 0.5 and the coefficient R2 is 50. These were set based on the experiment described above (Figure 6).

[0059] Then, after considering various construction specifications that satisfy quality other than the amount of soil mass, such as the diameter of the ground improvement body and the pressure of the jet, it was found that specifications A, B, and C, which differ in the lifting speed of rod 100 (L / min), the pressure of jet J (MPa), and the rotation speed of rod 100 (monitor 110) (rpm), can construct a ground improvement body 150 at almost the same cost (efficiency).

[0060] The vertical spacing L is 25 mm in all cases. The movement width d in specifications A, B, and C are as follows.

[0061] Specification A: Travel width d: 25mm Specification B: Travel width d: 17mm Specifications C: Travel width d: 13mm

[0062] The cutting intervals x for specifications A, B, and C, calculated from the aforementioned "cutting interval formula," are as follows:

[0063] Specification A: Cutting interval 25mm Specification B: Cutting interval 9mm Specification C: Cutting interval 12mm

[0064] Then, from the cutting interval reduction rate (cutting interval ratio) M% of the aforementioned cutting interval x and the soil mass reduction rate (soil mass remaining ratio) N%,

[0065] If the cutting interval ratio M in specification A is 100% and the soil mass retention ratio N is 100%, Specification B has a cutting interval ratio M of 36% and a soil mass retention ratio N of 68%. Specification C has a cutting interval ratio M of 48% and a soil mass retention ratio N of 74%.

[0066] Based on these factors, specification B, with a movement width of d17mm, is considered optimal from the perspective of improved soil quality, as it has the smallest cutting interval reduction rate M at 36% and the smallest soil mass reduction rate N at 68%.

[0067] <effect> Next, the operation of this embodiment will be described.

[0068] In the high-pressure injection agitation method of this embodiment, the movement width d is set using an equation that shows the relationship between the vertical distance L between nozzles 112 and 114, the movement width d for gradually raising the rod 100, and the cutting interval x of the ground.

[0069] As mentioned earlier, the cutting interval x of the ground 10 does not decrease uniformly even when the movement width d is reduced. Also, the amount of soil remaining is more correlated with the cutting interval x than with the movement width d. Therefore, by setting the movement width d of the rod 100 using the cutting interval formula which shows the relationship between the vertical distance L, the movement width d, and the cutting interval x, the amount of soil remaining can be reduced efficiently.

[0070] Furthermore, by determining the cutting interval x that results in a soil mass below the target amount based on the specifications of the ground improvement body 150, and selecting the largest movement width d that corresponds to that cutting interval x, the soil mass can be efficiently reduced.

[0071] Alternatively, by setting the movement width d at or near the valley apex in the graph of Figure 7, the soil mass can be efficiently reduced.

[0072] Furthermore, by using the cutting interval reduction rate M% and the soil mass reduction rate (soil mass retention ratio) N% derived from experimental results of the cutting interval formula and soil mass retention rate, it is possible to select the specifications that are optimal from the perspective of improved soil quality.

[0073] <Other> Furthermore, the present invention is not limited to the embodiments described above.

[0074] For example, in the above embodiment, the present invention was applied when the rod 100 was gradually raised to create the ground improvement body 150, but it is not limited to this. The present invention may also be applied when the rod 100 was gradually lowered to create the ground improvement 50.

[0075] Furthermore, in the above embodiment, the present invention was applied to a ground improvement body by injecting a jet J containing a cement-based solidifying agent from nozzles 112 and 114 to agitate and mix the soil and the cement-based solidifying agent, but the invention is not limited to this. The present invention may also be applied to a pre-jet target soil area 14 by ejecting a jet M containing water and air from nozzles 112 and 114.

[0076] Furthermore, for example, in the above embodiment, the vertical distance L between nozzle 112 and nozzle 114 was 25 mm, but it is not limited to this. The vertical distance L can be set according to the specifications of the monitor 110.

[0077] Furthermore, it is desirable to conduct experiments necessary to create graphs like those in Figures 5 and 6, which correspond to the vertical interval L.

[0078] Furthermore, the method for setting the movement width is not limited to the above. Setting the movement width should be used when efficiently reducing the amount of soil mass.

[0079] Furthermore, the present invention can be implemented in various forms without departing from the spirit of the invention. [Explanation of Symbols]

[0080] 10 Ground Saturday, the 12th 20 Inside the hole 50 Ground improvement equipment 100 rods 110 Monitors 110A side 112 nozzles 114 nozzles J-Jet (an example of a propellant) M Jet (an example of a propellant) L vertical spacing d Movement width x Cutting interval

Claims

1. A high-pressure injection mixing method is used, in which a rod inserted into a hole drilled in the ground is gradually raised or lowered, and while rotating the rod, a propellant is injected from nozzles formed at vertical intervals at the tip of the rod to cut the ground and mix the soil and the propellant, The vertical distance L of the nozzle and the movement width d, which is the width by which the rod is raised or lowered in stages, The cutting distance x of the ground is the maximum distance in the vertical direction, including the trajectories of the upper and lower nozzles when the rod is raised or lowered in stages, The movement width d is set using the following mathematical formula which shows the relationship: High-pressure injection agitation method. In the case of 2L ≥ d ≥ L x=max(d−L,L) If d < L x=max(L-kd, d-(L-kd)) k is the largest natural number less than or equal to L / d.

2. If there are multiple movement widths d with the same cutting interval x, the largest movement width d is selected. The high-pressure injection agitation method according to claim 1.

3. In the graph of the formula, with the horizontal axis being the movement width d and the vertical axis being the cutting interval x, the movement width d is set at or near the apex of the valley. The high-pressure injection agitation method according to claim 1.

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