JP6466814B2 - How to improve soft ground - Google Patents

Description

  The present invention relates to a method for improving soft ground in which the time in the creation process is controlled.

  As a countermeasure against liquefaction of soft ground, sand compaction pile method (hereinafter also referred to simply as “SCP method”) is a method of compacting sand piles by press-fitting a hollow tube and supplying sand material into the ground to compact the surrounding ground. .)It has been known. When a large number of sand piles compacted by the SCP method are formed at a predetermined interval in the soft ground, the N value of the surrounding ground increases, and the resistance to liquefaction of the soft ground increases.

  In the SCP method, a design for predicting the improvement effect by the replacement rate (improvement rate) of the cast sand pile is performed based on the field data in advance. At this time, a large number of small sections partitioned by adjacent three or four sand piles to be constructed are formed on the soft ground to be constructed. That is, many sand piles are formed at equal intervals with respect to the soft ground. As described above, the sand pile interval (pile interval) is determined at the design stage. However, depending on the site, the ground displacement due to the compacted sand pile affects the existing structure, so the application of the SCP method may be abandoned. Therefore, if an SCP method is developed that can achieve the target N value even if the sand pile interval is larger than the conventionally designed sand pile interval, the ground displacement applied to the periphery can be suppressed, and the application range of the SCP method is expanded. .

JP 2003-147756 A

  Conventionally, various improvement techniques related to the SCP method such as Japanese Patent Application Laid-Open No. 2003-147756 have been proposed, but the N value obtained by the construction of the conventional design is about 30 to 40 at most, and the technology with an N value exceeding 50 is still not available. unknown.

  Accordingly, an object of the present invention is to provide a soft ground improving method capable of obtaining an efficient improving effect.

  Under such circumstances, the present inventors have conducted intensive studies and found that stress that contributes to compaction is generated in the surrounding ground if gentle compaction is performed in the creation process, such as taking time from the penetration process to the creation process. It was found that the density was easily increased and the present invention was completed.

That is, the present invention includes a penetration step of penetrating the hollow tube to a predetermined depth;
After the penetration step, the hollow tube is drawn out to an appropriate length, and a drawing step of discharging sand pile material in the hollow tube to the drawing trace and a creation step of repeating the hollow pipe back-up to the ground surface in order. In the method of placing multiple compacted sand piles in soft ground,
In placing one sand pile, the penetration process and the creation process are performed so that the time required for the creation process (T2) is more than three times the time required for the penetration process (T1). The ground improvement method is provided.

  Moreover, this invention improves the said soft ground characterized by performing a penetration process and a creation process so that the time (T2) required for a creation process may be 4 times or more of the time (T1) required for a penetration process. A method is provided.

  Further, the present invention further provides the improvement of the soft ground characterized in that the creation step is performed so that the withdrawal time (T4) is longer than the withdrawal time (T3) during one cycle of withdrawal and withdrawal. A method is provided.

  Moreover, this invention provides the improvement method of the said soft ground characterized by performing a creation process by setting back-up length as 70%-90% of drawing-out length.

  According to the present invention, a high improvement effect in which the N value of the measurement point (hereinafter also referred to as “periphery ground”) exceeds 50 was obtained. In the conventional SCP method, even if the improvement rate is as high as 20%, the N value is at most about 30 to 40, which is a remarkable improvement effect. This is because it took a longer time for the creation process than the penetration process, so the maximum excess pore water pressure in the creation process was smaller than in the penetration process, and the stress contributing to compaction was easily transmitted, leading to an increase in density. Is. For this reason, even if it constructs with a wider pile interval than before, the same improvement effect as before can be obtained. Moreover, since a pile interval can be taken large, the ground displacement given to the periphery can be suppressed and construction can be performed even in a place where conventional construction is difficult. In addition, when the construction is performed at the same pile interval as that of the prior art, a higher improvement effect can be obtained, so that it is possible to cope even under severe conditions for improvement.

It is a top view which shows an example of the improvement division in the improvement method of this invention. It is a figure explaining the penetration process and creation process in the improvement method of this invention. It is another top view which shows an example of the improvement division in the improvement method of this invention. It is a figure which shows the depth-N value of Example 1. FIG. It is a figure which shows the depth-N value of Example 2. FIG. It is a figure which shows the depth-N value of Example 3. FIG. It is the figure which put together the Example and the comparative example.

  Next, a method for improving soft ground in the embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows an attempt to drive nine sand piles 1 at an equal pitch p in a planned improvement area X formed by regular squares a, c, i, and g on a soft ground 10.

  In the method for improving soft ground according to the present invention, first, a penetration step (I) for penetrating the hollow tube 11 to a predetermined depth is performed (FIGS. 2A to 2C). Specifically, the hollow tube 11 is placed at a predetermined position (for example, the position indicated by symbol a in FIG. 1), and a certain amount of sand pile material is charged. Next, the hollow tube is rotated to penetrate a predetermined depth into the ground. Therefore, the start time of the penetration process is the start time when the hollow tube 11 is pushed into the ground as shown in FIG. 2 (a), and the end time of the penetration process is as shown in FIG. 2 (c). The time when the hollow tube 11 reaches the predetermined depth D (the lowest position). Examples of the sand pile material include sand and crushed stone.

After penetration step, an appropriate length of hollow tube 11 (in FIG. 2, reference numeral l ₁₎ withdrawal, and withdrawal for discharging Sunakui material of the hollow tube to the pull-out trace, the hollow tube 11 out return (Figure 2 In the middle, the backlash length is the code l ₂ ) in order until the ground surface is reached, and the compacted sand pile 1 is formed. The drawing process is a process (reference numeral II ₁ ) of (c) to (d) in FIG. 2, and reference numeral II _{1 in} FIG. 2 is the first extraction. 2 is a process (reference numeral II ₂ ) of (d) to (e) in FIG. 2, and reference numeral II _{2 in} FIG. ₂ is an initial return. In FIG. 2, reference numeral II ₃ is a process after two cycles, with one cycle of drawing and strike back. That is, the start of the creation process is the start of the first drawing after the intrusion process, and the end of the creation process is the same as the surface of the ground surface after the last backlash of the creation process is finished and the hollow tube is pulled out. It is time to become.

  In the present invention, in placing a single sand pile, the time required for the creation process (T2) is not less than 3 times the time required for the penetration process (T1), preferably not less than 4 times (T1). The penetration process and the creation process are performed (hereinafter also referred to as “time difference creation method 1”). As a method of (T2)> (T1) × 3, a method of extending the extraction time, a method of extending the retraction time, a method of taking a long waiting time during the creation process, and a cycle of extraction and retraction are set as one cycle. A method for increasing the number of cycles is exemplified. In addition, the time required for the creation process (T2) is longer than three times the time required for the penetration process (T1) in the placement of a single sand pile. A method in which the time (T4) is made longer than the drawing time (T3) (hereinafter also referred to as “time difference forming method 2”), and the back-up length is set to 70% to 90% of the drawing length. A method of performing a creation step by increasing the number of cycles (hereinafter also referred to as “cycle number increasing method 1”) and the number of cycles in which extraction and retraction are one cycle (hereinafter also referred to as “cycle number increasing method 2”). .). This will be described in detail below.

  In the time difference creation method 1, the time required for the creation process (T2) is 3 times or more of the time required for the penetration process (T1), preferably 4 × (T1) time or more, when placing one sand pile. X (T1) It is preferable to carry out so that it may be less than time. Even if too much time is taken, the construction time becomes longer for the improvement effect, and on the contrary, the construction cost is increased. The time (T2) required for the creation process includes a waiting time for moving to the next cycle and a waiting time for moving from drawing to strike back. This is because the stress that contributes to compaction to the surrounding ground is transmitted even during the waiting time.

  In addition to the time difference generation method 1, the time difference generation method 2 is a method in which the retraction time (T4) is longer than the extraction time (T3) during one cycle of drawing and retraction. During one cycle, it is preferable that the time to return (T4) is preferably 2 × (T3) or more, particularly 3 × (T3) or more and 8 × (T1) or less. Even if too much time is taken, the construction time becomes longer for the improvement effect, and on the contrary, the construction cost is increased. The time difference creation method 2 is the same as the time difference creation method 1 in that the time required for the creation process is lengthened, and the condition that the return time (T4) is longer than the drawing time (T3) needs to be implemented in all cycles. It is not necessary to fill in some cycles.

  The number-of-cycles increasing method 1 is a method in which the creation process is performed with the strike back length being 70% to 90% of the drawing length. Specifically, when the drawing length is 50 cm, the strike back length is set to 35 cm to 45 cm. If the retraction length and the drawing length are set as the above conditions and other conditions such as the drawing speed and the pulling speed are set as usual, the number of cycles in the creation process naturally increases. In the conventional SCP method, the strike back length is generally set to 60% of the drawing length.

  The cycle number increasing method 2 is a method in which the creation step is performed by increasing the number of cycles in which extraction and retraction are one cycle. Since the specific number of cycles varies depending on the sand pile length and backlash length, it cannot be generally stated. However, a sand pile length of 10 m is preferably 70 cycles or more, particularly 100 cycles or more and 200 cycles or less, and 16 m sand. The pile length is preferably 128 cycles or more, particularly 160 cycles or more and 320 cycles or less. The conventional SCP method is generally constructed in 80 cycles with a sand pile length of 16 m.

  According to the method of constructing the penetration process and the creation process under the above conditions, the time required for the creation process becomes longer, so that the maximum excess pore water pressure in the creation process becomes smaller than that in the penetration process. That is, in the creation process, stress contributing to compaction is easily transmitted, leading to an increase in density.

  In the soft ground improvement method of the present invention, the measurement point for confirming the improvement effect is the surrounding ground of the sand pile to be placed, for example, within 10 m from the sand pile to be measured. For example, when there are two sand piles to be placed, the measurement point 4 is a position where improvement is expected at the intermediate point or the distance from the two sand piles. In the case of three or four, it is the center of the area | region divided by connecting the sand pile, and the center vicinity. As shown in FIG. 1, the measurement point 4 in this example is the center and the vicinity of the center of the rectangular small section 31 when sand piles are placed at the placement positions a, b, d, and e. Further, when sand piles are placed at the placement positions b, e, c, and f, they are at the center and the vicinity of the center of the rectangular small section 32. Further, when sand piles are placed at the placement positions d, e, g, and h, they are at the center and the vicinity of the center of the rectangular small section 33. Further, when sand piles are placed at the placement positions e, f, h, and i, they are at the center and the vicinity of the center of the rectangular small section 34. The reason why the measurement points may be determined as appropriate is that the time required for the penetration process and the time required for the creation process in a single sand pile driving operation.

  The measurement point is not limited to the center of the area defined by the adjacent sand pile to be created or the created sand pile. For example, when placing the sand pile at the position g or h in FIG. It may be a position. Further, as shown in FIG. 3, the measurement point may be the center of a region partitioned by three adjacent sand piles to be created or created sand piles. That is, it is a point of 4b when placing a sand pile at the positions of symbols a, b and e, and a point of 4c when placing a sand pile at the positions of symbols b, e and f, This is the point 4d when the sand pile is driven at the positions of symbols f, g and j. The measurement point may be between two adjacent sand piles to be created or between a sand pile to be created and a created sand pile.

  A well-known thing can be used as a compacting sand pile production apparatus used in this invention. That is, a spiral blade may be provided on the outer peripheral surface of the hollow tube, and a drilling bit may be provided at the tip of the hollow tube. The hollow tube lifting device is not particularly limited. For example, a device using a rack and pinion, a device using a chain and a sprocket, a device using a wire rope towing, or the like, which can obtain a reaction force from the leader at the time of penetration and withdrawal. Or by vibro hammer.

  In the present invention, the N value of the surrounding ground, that is, the measurement point, is 1.1 times or more, particularly 1.2 times or more, and further 1.3 times or more than the conventional value. Further, the N value may be 50 or more at a specific position in the depth direction. In the conventional SCP method, even if the improvement rate is as high as 20%, the N value is at most about 30, and the N value 50 is remarkably high.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in detail, this is only an illustration and does not restrict | limit this invention.

Example 1
The outer peripheral surface was not provided with a spiral blade, and a hollow tube provided with a drilling bit at the tip was used. The hollow tube lifting device was based on a rack and a pinion. The improved ground was A ground as shown in FIG. In FIG. 1, the sand pile interval was 2 m, the sand pile diameter was 700 mm, and the improvement rate was 10%. Moreover, the sand pile length was 13 m. The penetration process was rotary press-fitting, and the average penetration time was 10 minutes per piece, which is a conventional standard condition. The creation process was longer than the penetration process, and the average creation time per bottle was 55 minutes (5.5 times the average penetration time). Further, the N value was measured at the position of reference numeral 4 in FIG. The results are shown in Table 1 and symbol x3 in FIG. In Table 1, sand pile numbers (1) to (7) respectively correspond to symbols a to g in FIG. In Table 1, it was expressed as “required process time / intrusion process time”. In addition, in FIG. 4, the code | symbol X1 is the N value of the ground before construction, and the code | symbol X2 is the estimated average N value after improvement. The predicted N value is a well-known design value, and it is assumed that the ground is tightened by the same amount as the volume of the supplied replenishment sand, and this is used as an empirical relationship between the relative density and the N value. The N value after press-fitting is determined, and is generally calculated from a prediction formula based on N-value data at the time of standard construction of the SCP method. From FIG. 4, the average N value after improvement was 28.2. The average N value 28.2 after this improvement was 1.18 times the predicted average N value 23.8 indicated by the symbol X2 in FIG.

(Example 2)
The procedure was the same as in Example 1 except that the distance between sand piles was set to 1.4 m, the improvement rate was changed to 10%, 20%, and the following penetration time and creation time were set. That is, the average penetration time of Example 2 was 10 minutes, and the average creation time was 73 minutes (7.3 times). The results are shown in Table 2 and symbol Y3 in FIG. In addition, in FIG. 5, the code | symbol Y1 is the N value of the ground before construction, and the code | symbol Y2 is the estimated average N value after improvement. As a result, the improved average N value was 42.7, which was 1.44 times the predicted average N value of 29.6.

(Example 3)
B ground instead of A ground, 1.4 m instead of 2.0 m between sand piles, 20% instead of 10% improvement rate, 14 m instead of sand pile length 13 m, ) To (7), except that the sand piles (1), (2) and (4) were changed to 45 minutes instead of the average creation time of 55 minutes. That is, in Example 3, the average time required for the creation process is 4.5 times longer than the average time required for the penetration process. In this example, the measurement point 4 of the sand pile is the center of the small square 31. The result of the N value at the measurement point 4 is indicated by the symbol Z3 in FIG. Z1 is the N value of the ground before construction, and Z2 is the predicted average N value after the improvement.

  The average N value in Example 3 was 40, which was 1.14 times the predicted average N value 35 indicated by the symbol Z2 in FIG. In Example 3, the N value at a depth of 8 m exceeded 50 as indicated by Z3 in FIG.

(Comparative Example 1)
The test was performed in the same direction as Example 3 except that the required time in the creation process was changed to 45 minutes instead of 45 minutes. That is, the time required for the average creation process of Comparative Example 1 is 1/3 that of Example 3, which is 2.0 times the time required for the penetration process. The results are shown in Table 3. The result of the N value at the measurement point 4 is that of the symbol Z21 in FIG. The N value of the measurement points of the three cast sand piles was 34, which was not as high as 0.97 times the predicted average N value.

(Comparative Example 2)
The same procedure as in Example 3 was performed except that the time required for the average creation process was changed to 6 minutes instead of 45 minutes. That is, the time required for the average creation process of Comparative Example 1 is 0.13 times that of Example 3. As a result, the result of the N value at the measurement point 4 is that of the symbol Z22 in FIG. From the results of FIG. 6, the average N value of Comparative Example 1 was 32, which was not as high as 0.91 times the predicted average N value.

  In addition, the result of Examples 1-3 and Comparative Examples 1 and 2 was put together in FIG. In FIG. 7, the horizontal axis is “creation time / penetration time” and the vertical axis is “ratio of average N values when the predicted average N value is 1” for the A and B grounds. From this result, it can be seen that the formation time / penetration time is 3 or more, preferably 4 or more, and a remarkable improvement effect is obtained.

  According to the present invention, since a pile interval can be taken large, ground displacement given to the periphery can be suppressed, and construction can be performed even in a place where conventional construction is difficult. In addition, when the construction is performed at the same pile interval as that of the prior art, a higher improvement effect can be obtained, so that it is possible to cope even under severe conditions for improvement.

DESCRIPTION OF SYMBOLS 1 Sand pile 4, 4a-4f Improvement target point 11 Hollow pipe X Improvement division area aj Sand pile placing planned site

Claims (4)

An intrusion process for penetrating the hollow tube to a predetermined depth;
After the penetration step, the hollow tube is drawn out to an appropriate length, and a drawing step of discharging sand pile material in the hollow tube to the drawing trace and a creation step of repeating the hollow pipe back-up to the ground surface in order. In the method of placing multiple compacted sand piles in soft ground,
In the placement of one sand pile, the penetration process and the creation process are performed so that the time required for the creation process (T2) is more than three times the time required for the penetration process (T1) .
The method for improving soft ground is characterized in that the creation step has a smaller maximum pore water pressure than the intrusion step, facilitates transmission of stress contributing to compaction, and increases density .   The method for improving soft ground according to claim 1, wherein the penetration step and the creation step are performed such that the time (T2) required for the creation step is four times or more of the time (T1) required for the penetration step.   Furthermore, the improvement process of the soft ground according to claim 1 or 2, characterized in that, during one cycle of drawing and retreating, the creation process is performed so that the retreating time (T4) is longer than the retraction time (T3). Method.   The improvement method of the soft ground according to any one of claims 1 to 3, wherein the reclaiming length is set to 70% to 90% of the drawing length.