Analysis method of ground improvement effect

Abstract

To provide an analysis method of ground improvement effects, which can analyze whether the effect of ground improvement is sufficiently manifested over the entire ground area to be improved, even in ground where there are local weak parts (spatial variation).SOLUTION: In an analysis method of ground improvement effects, multiple analytical models of the ground with variations in an N-value and fine particle content of the ground are created, earthquake response analysis on the multiple analytical models is performed, and after an earthquake response analysis is completed, excess pore water pressure and the amount of deformation of the ground surface are analyzed for each of the multiple analytical models, and frequency of occurrence of excess pore water pressure and the distribution of the amount of deformation of the ground surface for the multiple analytical models are calculated.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for analyzing the effect of ground improvement. [Background technology]

[0002] One of the liquefaction countermeasure methods is the Sand Compaction Pile (SCP) method. The SCP method is a widely used method that increases the liquefaction resistance, shear stiffness, and horizontal earth pressure of the ground by constructing large-diameter sand piles in loose sandy ground.

[0003] Patent Document 1 discloses a technology related to a ground improvement method. This ground improvement method is characterized in that in the ground to be improved below the paved portion, a desired thickness of unimproved portions is left so that ground subsidence after liquefaction due to an earthquake is uniform throughout the entire ground to be improved, and based on this setting, a chemical solution is injected into the ground to be improved other than the unimproved portion to improve it. In addition, it includes preparing multiple ground improvement model cases for simulation in which a desired thickness of unimproved portions is left in the ground to be improved, calculating the amount of ground subsidence and pavement deformation due to liquefaction in response to an expected earthquake motion in each of these ground improvement model cases by Monte Carlo simulation, selecting one or more optimal ground improvement model cases from the above ground improvement model cases based on the results, and injecting a chemical solution into the ground to be improved other than the unimproved portion based on this. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-62794 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, there were concerns that the ground improvement effect of the SCP method would be affected by spatial variation in the liquefaction resistance of ground improved using the SCP method, due to the heterogeneity of the physical properties of the original ground, such as the relative density and fine particle content of the ground, or the heterogeneity of the compaction effect of sand piles.

[0006] The present invention has been made in consideration of the problems inherent in the conventional techniques, and an object of the present invention is to provide a method for analyzing the effect of ground improvement, which is capable of analyzing whether the effect of ground improvement is sufficiently manifested throughout the entire area of ​​the improved ground, even in the case of ground having localized weak areas (spatial variations). [Means for solving the problem]

[0007] A method for analyzing the effect of ground improvement according to one embodiment of the present invention involves creating multiple analytical models of the ground with variations in the N-value and fine grain content of the ground, performing earthquake response analysis on the multiple analytical models, and after the earthquake response analysis is completed, analyzing the excess pore water pressure and the amount of deformation of the ground surface for each of the multiple analytical models, and calculating the frequency of occurrence of excess pore water pressure and the distribution of the amount of deformation of the ground surface for the multiple analytical models. Effect of the Invention

[0008] According to the present invention, a method for analyzing the effects of ground improvement can be provided, which can analyze whether the effects of ground improvement are sufficiently realized throughout the entire improved ground, even in ground that has localized weak areas (spatial variation). [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram for explaining a sand compaction pile method. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing an analytical model of the ground. [Diagram 3] FIG. 2 is a mesh diagram showing an analytical model of the ground. [Figure 4] FIG. 13 is a diagram showing an example of a cross section of an improved area having variation in the N-value (a numerical value representing the strength of the ground) of the original ground. [Diagram 5] FIG. 13 is a diagram showing an example of a cross section of an improved portion having variation in fine grain content Fc. [Figure 6] 1 is a table showing various parameters used in numerical analysis (Monte Carlo simulation). [Figure 7] 1 is a graph showing the waveform of an input earthquake motion used in an earthquake response analysis. [Figure 8] This is a graph showing the relationship between the number of Monte Carlo simulation trials (MCS number) and the average liquefaction rate of the improved area. [Figure 9] 1 is a graph showing the relationship between the number of Monte Carlo simulation trials (MCS number) and the average amount of ground surface subsidence at the center of the improved area. [Figure 10] 13 is a graph showing the distribution of liquefaction rate in the improved area in an analytical model of the ground. [Figure 11] 1 is a graph showing the distribution of ground surface subsidence due to dissipation of excess pore water pressure in an analytical model of the ground. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The method for analyzing the effect of ground improvement according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for the sake of explanation and may differ from the actual ratios.

[0011] [Sand compaction pile method] The Sand Compaction Pile Method (SCP Method) is a method in which sand or the like is poured into the ground 10 from the bottom end of a casing, and the casing is vibrated while being pulled out and driven back in, creating compacted sand piles 11 in the ground. This sand compaction pile method is carried out based on the "replacement rate (improvement rate)" obtained by dividing the area (volume) of the sand piles 11 by the interval at which the sand piles 11 are driven.

[0012] [Method of analyzing the effect of ground improvement] Figures 2 and 3 show the analytical model of the ground. The analytical model of the ground has a width (X direction) of 100 m and a height (Y direction) of 10 m. The number of elements (number of meshes) of the analytical model used in the finite element analysis is 2200, and the number of nodes is 2331. It is assumed that groundwater exists from the bottom of the ground to 9 m of the entire ground (height 10 m, width 100 m). The improved part (improvement range) by the SCP method is a 20 m wide range in the center of the ground (range of 40 m to 60 m in the X direction), and the number of elements in the improved part is 800 (deeper than groundwater: 720 elements, shallower than groundwater: 80 elements). The improved part (improvement range) is the area surrounded by a thick line in the mesh diagram in Figure 3.

[0013] In this embodiment, the N-value (standard penetration test value; a reference value for determining the compactness and strength of the soil) and fine particle content Fc of the original ground before improvement of the 800 elements to be improved (720 elements below groundwater, 80 elements below groundwater) were given variations. The average values ​​of the N-value and fine particle content Fc of the original ground were set to 5.8 and 35.1%, respectively, and the coefficients of variation of the N-value and fine particle content Fc of the original ground were set to 0.3 and 0.1, respectively. In addition, the N-value and fine particle content Fc of the original ground were given spatial variations (two-dimensional variations) using independent log-normal distributions.

[0014] That is, the analytical model of the ground shown in Figure 3 is defined as a two-dimensional element with a width in the horizontal direction (X direction) and a height in the vertical direction (Y direction). In this analytical model, the N value and fine particle content Fc of the ground are given variations (fluctuations) in the horizontal direction (X direction) and vertical direction (Y direction), respectively (see Figures 4 and 5). By constructing such an analytical model of the ground, it is possible to perform a numerical analysis of ground that has localized weak parts (spatial variations).

[0015] Figure 4 shows an example of a cross-section of an improved area with variation in the N-value (a numerical value that indicates the strength of the ground) of the original ground, and Figure 5 shows an example of a cross-section of an improved area with variation in the fine particle content Fc. 100 patterns of analytical models of ground with cross-sections that vary in the improved area were created and numerical analysis (Monte Carlo simulation) was performed. The improvement rates of the improved area were set to 0%, 10%, and 20%. The effective confining pressure of the ground was 98kN / m 2 and the autocorrelation distance of the ground was set to 3 m in the vertical direction (Y direction) and 15 m in the horizontal direction (X direction) (see Figure 6).

[0016] That is, for the analytical model of the ground as shown in Fig. 3, random numbers are generated for the N value and the fine fraction content Fc in the improved part for each simulation (see Fig. 4 and Fig. 5). By giving variation to the N value and the fine fraction content Fc in the improved part, a simulation (Monte Carlo simulation) for earthquake response analysis against input earthquake motion is repeatedly performed.

[0017] The fine particle content Fc of the improved ground (ground after improvement using the SCP method) was set to the same value as the fine particle content Fc of the original ground, the N-value of the improved ground was calculated using a specified design method, and the stiffness and liquefaction parameters of each element were determined.

[0018] Figure 7 shows the waveform of the input earthquake motion used in the earthquake response analysis. The earthquake motion shown in Figure 7 is assumed to be an L2 earthquake motion (Level 2 earthquake motion), with a maximum acceleration of 740 gal (see Figure 6). Additionally, the boundary conditions during the earthquake response analysis were set to viscous boundaries on the sides and bottom (see Figure 3). Furthermore, after completing the earthquake response analysis, a seepage coupled analysis was performed that took into account the dissipation of excess pore water pressure, and the amount of deformation (amount of ground surface settlement) of the ground surface in the center of the improved area due to the dissipation of excess pore water pressure after the earthquake was examined.

[0019] [Convergence of Monte Carlo simulation] In this embodiment, it was examined whether the earthquake response analysis could obtain sufficient statistics with 100 Monte Carlo simulations. Figure 8 shows the relationship between the number of trials (MCS number) of the Monte Carlo simulation and the average value of the liquefaction rate of the improved part, and Figure 9 shows the relationship between the number of trials (MCS number) of the Monte Carlo simulation and the average value of the ground surface subsidence at the center of the improved part. The liquefaction rate of the improved part and the average value of the ground surface subsidence at the center of the improved part were obtained from the liquefaction rate and the ground surface subsidence at the i-th Monte Carlo simulation. In addition, in this specification, the "liquefaction rate" is the ratio of elements in the improved ground elements given by the Monte Carlo simulation whose excess pore water pressure ratio exceeded 0.95 during an earthquake.

[0020] For both the average liquefaction rate and the average amount of ground surface settlement, the fluctuation in the average value was small at all improvement rates, and it was confirmed that both the average liquefaction rate and the average amount of ground surface settlement were sufficiently converged after 100 Monte Carlo simulations. Convergence was also confirmed after 100 runs for the standard deviation of the liquefaction rate and the amount of ground surface settlement. Therefore, it is considered that 100 trials of the Monte Carlo simulation in this embodiment is sufficient. However, it is also possible to create 100 or more patterns of cross sections (ground analytical models) with variations in the improved parts and perform numerical analysis (Monte Carlo simulation).

[0021] [Distribution of liquefaction rate and ground surface subsidence] Figure 10 shows the distribution of the liquefaction rate in the improved area, and Figure 11 shows the distribution of the amount of ground surface settlement in the center of the improved area due to the dissipation of excess pore water pressure. A prescribed goodness-of-fit test was conducted on the two distributions of liquefaction rate and ground surface settlement, and both the liquefaction rate and the amount of ground surface settlement fit the log-normal distribution at a significance level of 5% for all improvement rates.

[0022] Furthermore, the amount of ground surface subsidence also fit a normal distribution at a significance level of 5% for all improvement rates. The average value and standard deviation (degree of dispersion) were calculated to show how the liquefaction rate and amount of ground surface subsidence were distributed. The average value of the amount of ground surface subsidence for the original ground (improvement rate 0%) was 0.118m, and the standard deviation was 0.011m. In contrast, the average value of the amount of ground surface subsidence for an improvement rate of 10% was 0.075m, and the standard deviation was 0.004m. Furthermore, the average value of the amount of ground surface subsidence for an improvement rate of 20% was 0.042m, and the standard deviation was 0.002m. It was confirmed that as the improvement rate increases, the amount of ground surface subsidence and the standard deviation decrease.

[0023] [Method for evaluating the effect of ground improvement using the SCP method] The "replacement rate (improvement rate)" using the sand compaction pile method is determined based on the average value and standard deviation of the liquefaction rate and the average value and standard deviation of the amount of ground surface subsidence obtained as described above. Of course, when determining the "replacement rate (improvement rate)" using the sand compaction pile method, values ​​other than the average value and standard deviation of the liquefaction rate and the average value and standard deviation of the amount of ground surface subsidence may also be taken into consideration.

[0024] [Effects, etc.] The effects of this embodiment will be described below.

[0025] (1) The method for analyzing the effect of ground improvement in this embodiment involves creating multiple analytical models of the ground with variations in the N-value and fine grain content of the ground, performing earthquake response analysis on the multiple analytical models, and after the earthquake response analysis is completed, analyzing the excess pore water pressure and the amount of deformation of the ground surface for each of the multiple analytical models, and calculating the distribution of the frequency of occurrence of excess pore water pressure (liquefaction rate) and the amount of deformation of the ground surface (amount of ground surface subsidence) for the multiple analytical models.

[0026] In the conventional boring survey, the effect of ground improvement was evaluated at points, so if the survey point was locally weak and the improvement effect of the SCP method could not be realized as designed, the entire area of ​​the ground improved by the SCP method was judged to be poor. The method of analyzing the effect of ground improvement by numerical analysis (Monte Carlo simulation) according to this embodiment was able to evaluate whether the effect of ground improvement by the SCP method was sufficiently realized throughout the entire area of ​​the ground improved by the SCP method, even in ground with localized weak areas (spatial variation).

[0027] As described above, according to this embodiment, a method for analyzing the effect of ground improvement can be provided, which can analyze whether the effect of ground improvement is sufficiently realized throughout the entire improved ground, even in ground that has localized weak areas (spatial variation).

[0028] (2) In the method for analyzing the effect of ground improvement, the analytical model is defined as a two-dimensional element having a horizontal width and a vertical height, and in multiple analytical models, the N-value and fine particle content of the ground may be given variations in the horizontal and vertical directions, respectively.

[0029] By constructing an analytical model of the ground in this way, it is possible to perform numerical analysis on ground that has localized weak areas (spatial variations).

[0030] (3) In the method for analyzing the effect of ground improvement, more than 100 analytical models may be created.

[0031] The average value of the frequency of occurrence of excess pore water pressure (average liquefaction rate) and the average amount of deformation of the ground surface (average amount of ground surface subsidence) showed small fluctuations at all improvement rates, and it was confirmed that the average liquefaction rate and the average amount of ground surface subsidence both converged sufficiently after 100 Monte Carlo simulations. Convergence was also confirmed after 100 simulations for the frequency of occurrence of excess pore water pressure and the degree of variation in the amount of deformation of the ground surface (standard deviation of the liquefaction rate and the amount of ground surface subsidence). Therefore, it is considered that 100 trials of the Monte Carlo simulation in this embodiment is sufficient.

[0032] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0033] 10 Ground 11 Sand pile

Claims

1. We created multiple analytical models of the ground with variations in the N value and fine particle content of the ground, performing an earthquake response analysis on the plurality of analysis models; After the earthquake response analysis is completed, the excess pore water pressure and the deformation of the ground surface are analyzed for each of the plurality of analysis models; Calculating the frequency of occurrence of excess pore water pressure and the distribution of deformation of the ground surface in the plurality of analytical models; Method for analyzing the effects of ground improvement.

2. The analytical model is defined as a two-dimensional element having a width in the horizontal direction and a height in the vertical direction; In the multiple analytical models, the N value and fine particle content of the ground were varied in the horizontal and vertical directions, The method for analyzing the effect of ground improvement according to claim 1.

3. More than 100 patterns of the analysis model were created. The method for analyzing the effect of ground improvement according to claim 1 or 2.

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