JP4931071B2 - A method for measuring the stability of submarine ground to waves. - Google Patents

Description

The present invention is a method for investigating the stability of ocean floor foundation ground against waves in offshore / coastal structures, and is related to civil engineering, coastal engineering, and foundation ground engineering.

Various structures such as breakwaters and revetments may be damaged in coastal and marine areas during stormy weather such as typhoons. Excessive water pressure generated by waves acts on the structure, which is a direct cause of damage and destruction of the structure. FIG. 1 shows a typical section of a gravity breakwater having a typical concrete caisson as a main body. Since the fluctuation of water pressure due to waves directly affects the ocean side of the breakwater, it is necessary to properly consider the resultant force as the main fluctuating external force when evaluating stability. In this general type of breakwater, in order to reduce the impact force caused by the waves, concrete wave-dissipating blocks are stacked and arranged on the open ocean side of the breakwater.

The fluctuating water pressure that directly acts on the structure is the most important factor in destabilizing the structure, but at the same time, the fluctuating water pressure acts not only on the structure but also on the seabed ground. Non-Patent Document 1 describes that there are many cases in which the seabed ground becomes unstable and the proof strength as the foundation of the ground against the structure (breakwater caisson) decreases and the function of the structure is significantly impaired.

In the field of coastal engineering, it is common to study the mechanism of breakwater damage as a “scouring phenomenon of the seabed ground” focusing on the floating or movement of the seafloor ground material according to the flow rate of seawater. However, in the disaster cases that have been clarified so far, there are cases where the instability of the seabed ground due to waves clearly reaches several meters below the seafloor. Non-Patent Document 2 and the like indicate that it is necessary to treat the ground as a three-dimensional continuum having depth and to evaluate the response of the ground to water pressure fluctuations on the surface of the seabed. Since then, geotechnical studies have been actively conducted and research results in this field have been accumulated. From a mechanical perspective, this is considered as a complex system formed by solids, porous bodies and fluids, and the mechanical properties are essentially the same. It means that it is necessary to appropriately evaluate the interaction between three different parties. The interaction between the structure and the ground (solid-porous body) is studied in the conventional geotechnical engineering field in consideration of the influence of groundwater on land, and research has been accumulated as a problem of bearing capacity of the ground for the structure.

On the other hand, the ground-wave interaction (porous body-fluid) is a relatively new problem, and the mechanism that takes this interaction into account is revealing the instability of coastal and marine structures. Research is also being conducted on analytical methods.

The interaction between the seabed and waves is mechanically the interaction between the porous body and the fluid, and modeling and formulation of the seabed ground as a porous body is necessary. Non-patent document 3 started mechanical research related to modeling and formulation of porous bodies. After that, it has become clear that the behavior of the seabed can be calculated to some extent by modeling the seabed ground as a continuum as a two-phase material of solid and fluid and applying it to the interaction with waves (non-patent literature). 4-6).

The inventors examined non-patent document 7 on the formulation of the seabed ground, analysis dimensions, and optimization of dynamic / static analysis conditions in the analysis of the interaction between the seabed ground and the waves. As a result, the effects of acceleration on both solids and fluids are negligibly small for actions with relatively long periods such as waves, so it is sufficient to formulate the ground using the up model and perform pseudo-dynamic analysis. It was shown that it was possible to analyze the behavior considering the interaction with high accuracy. In addition, when the depth of the target seabed was sufficiently small compared to the wavelength, it was shown that a sufficiently accurate response could be obtained by one-dimensional analysis in the range of several meters near the ground surface. Furthermore, what has been clarified through research by Miura et al. Is the importance of ground properties in the interaction between the ground and the ocean. From the comparison between the numerical solution and the numerical solution, the sensitivity of various ground properties was verified, and in particular, the saturation and water permeability of the ground had a great influence on the stress fluctuation in the ground.

In this regard, Non-Patent Document 8 proposes a method for investigating ground physical properties, focusing on the frequency dependence of elastic waves, as one development of elastic wave exploration.
Oka, F.M. Yashima, A .; Miura, K .; Ohmaki, S .; and Kamata. A. (1995): Settlement of Breakwater on Submarine Soil Due to Wave-Induced Liquidation, 5th International Symposium on Offshore and Polar Envelope. 2, pp. 237-242. Yamamoto, T .; (1977): Wave Induced In Stability in Seabed, Proc. Coastal Sciences, ASCE, pp. 898-913. Biot, M.M. A. (1941): General Theory of Three-Dimensional Consolidation, “Journal of Applied Physics, Vol. 12, pp. 155-164. Putnam, J.M. A. (1949): Loss of Wave Energy due to Percolation in a Permable Sea Bottom, American Geophysical Union, Vol. 30, no. 3, pp. 349-356. Madsen, O.M. S. (1978): Wave Induced Pores Pressures and Effective Stresses in a Poor Bed, Geotechnique, Vol. 28, no. 4, pp. 377-393. Yamamoto, T .; Koning, H.C. S. H. L. K. and Van Hijuum, E.A. (1978): Wave Induced Instability in Seabed, Proc. Coastal Sciences, ASCE, pp. 898-913. Miura Hitoshi, Asahara Shingo, Otsuka Natsuhiko, Ueno Katsu (2004): Ground formulation for coupled analysis of submarine ground response to waves, 49th Geotechnical Engineering Symposium, pp. 233-240 Miura, K. et al. Yoshida, N .; and Kim, Y. et al. S. (2001): Frequency Dependent Property of Waves in Saturated Sol, SOILS AND FOUNDATIONS, Vol. 41, no. 2

Non-Patent Document 1 is an investigation of actual disaster cases. This only applies a specific analysis method to a specific disaster case, and does not refer to a comprehensive method for evaluating the stability of ocean waves in the seabed.

In addition, the conventional techniques up to Non-Patent Document 2-8, which is a method that considers the interaction between the ground and the waves (porous body-fluid), are only theoretical, and are practically used to measure the stability of the seabed. Not a way.

For example, in Non-Patent Document 7, which examines Non-Patent Documents 2-6 in more detail, it is necessary to clarify the degree of saturation and water permeability of the ground. However, it is very difficult to measure them by laboratory tests or in-situ tests. For example, it will be quite difficult to obtain undisturbed soil samples while maintaining hydraulic conductivity and saturation, and there is a limit to the technology for directly measuring these properties in the original seabed. Is the current situation.

In Non-Patent Document 8, however, since this investigation method is an investigation in a considerably high frequency region, it is difficult to implement with the current measurement technique. In addition, due to the difficulty of investigating the physical properties of the ground, the stability of the foundation ground to waves during the design of coastal and harbor structures has not been fully studied and has only been studied empirically.

Therefore, an object of the present invention is to provide a method capable of practically measuring the stability of the seabed ground by using the pore water pressure in the ground, which can be easily measured, as a parameter.

In order to solve the above-mentioned problems, the present inventor has intensively studied, and as a result, has completed the following invention.

The first invention measures the pore water pressure in the seabed ground, calculates the hydraulic consolidation constant that becomes the anti-wave response parameter of the seabed ground using the dominant component of the measured pore water pressure, and calculates the hydraulic consolidation constant The one-dimensional pore water pressure coefficient is derived from the dominant component of the measured pore water pressure, the instability depth of the seabed ground is calculated from the one-dimensional pore water pressure coefficient, and the stability in the seabed ground during waves is predicted. Feature method.

  A second invention is the method according to the first invention, wherein the pore water pressure in the seabed ground is measured at three points in the ground.

Estimate the anti-sea wave stability of the submarine foundation ground of harbors and coastal structures that have not been studied so far by estimating the anti-sea wave parameters of the submarine ground, which has been difficult to estimate so far. Will be able to.

Tensor notation is used to represent the state of the fluid that fills the soil particles and the gaps between them. The displacement increment, stress increment, and strain increment of the solid phase are expressed as Δusi, Δσsij, and Δεsij, respectively, and the displacement increment, stress increment, and strain increment of the fluid phase are expressed as Δufi, Δσfij, and Δεfij, respectively. Subscripts s and f represent a solid phase and a fluid phase, respectively. The stress σsij transmitted by the soil skeleton is called effective stress. On the other hand, if the fluid phase stress has no shear resistance in the pore fluid,

である。ここで、Δｐ は間隙水圧増分、δｉｊ はＫｒｏｎｅｃｋｅｒ のデルタを表している。

It is. Here, Δp represents the pore water pressure increment, and δij represents the Kronecker delta.

The constitutive law of the solid layer is expressed from the linear elasticity theory as follows.

Furthermore, the displacement gradient

により変位と応力の関係は以下のようになる。

Therefore, the relationship between displacement and stress is as follows.

Here, Gs is a shear constant, λ s is a Lame constant, and the stiffness tensor Dijkl is as follows.

The constitutive law of the fluid phase is expressed as follows.

Here, Kf represents a volume compression coefficient, and this equation represents that the volumetric strain increment Δεfii of the fluid and the pore water pressure increment Δp are in a proportional relationship. When the pore fluid is composed of gas and pore water, it can be expressed as a fluid phase obtained by averaging the gas phase and the liquid phase by the saturation degree of the pore fluid.

The change in pore water results in a volume change, which is expressed by the following equation:

Here, n is the porosity and Ks is the volume compression coefficient of the soil particles. According to Skempton, Ks >> Kf
The term (1-n) / Ks can be ignored. Here, Bf is an average volume compression coefficient of the fluid.

Δξ is the permeation amount per unit volume and unit time (increase amount of pore water). The new vector variable Δwi represents the apparent relative displacement between the solid and liquid layers. Here, the vector variable Δwi is equal to the amount of osmotic flow per unit area passing through the porous body, and applies to Darcy flow. Δwi is given as follows.

The equilibrium conditions in the solid phase and the fluid phase as a whole are derived as follows.

Here, Δbi is a unit object force vector and becomes {0, 0, g} when gravity acts on the object. ρ
s and ρf represent the density of soil particles and pore fluid, respectively. Organizing using Equation 4,

Ρt represents the total density.

The equilibrium condition of the fluid phase is expressed as follows from Darcy's law.

Here, h is the head of water, kij is the tensor notation of Darcy's permeability, and rij is the tensor of the reciprocal of kij. Usually, in an isotropic material, the water-permeable tensor is represented as a diagonal tensor having a constant value independent of the direction.

ここで、ｋ は通常用いられているダルシーの透水係数である。
加速度の項を考慮すると、数１１は以下のように修正される。

Here, k is a commonly used Darcy permeability coefficient.
Considering the acceleration term, Equation 11 is corrected as follows.

The governing equations are formulated as simultaneous variable differential equations, and the following four types of governing equations are studied in dynamic, pseudo-dynamic and static. Dynamic analysis considers all solid phase and fluid phase acceleration terms, but pseudo dynamic analysis ignores acceleration terms and static analysis considers only solid phase displacement and fluid phase velocity terms. .
Dynamic analysis: all considered pseudo-dynamic analysis: Δus = Δw = 0
Static analysis: Δus = Δus = Δw = 0
First, the strictest formulation is expressed as follows from Equations 8, 12, and 16. Here, Δus≡Δu for simplicity.

［ｕ−ｗ−ｐ］ ｆｏｒｍｕｌａｔｉｏｎ（完全相互作用モデル）

[U-wp] formulation (complete interaction model)

This formulation is often simplified depending on the problem being handled, taking into account appropriate assumptions. The following formulation [up] formation ignores the relative acceleration of the fluid phase. That is, the acceleration of the fluid phase is handled in the same way as that of the solid phase.

［ｕ−ｐ］ ｆｏｒｍｕｌａｔｉｏｎ；Δｗ ＝ ０ （不完全相互作用モデル）

[Up] formulation; Δw = 0 (incomplete interaction model)

The relative velocity of the fluid phase is eliminated during the induction process, and the governing equation is composed of four partial differential equations in the three-dimensional condition. Since this formulation is simple and the relative acceleration of the fluid phase is extremely small during an earthquake, it is commonly used for vibration analysis during an earthquake. here,

If the influence of pore water is ignored, it is possible to ignore the osmotic flow from the governing equation, and the undrained condition is assumed. The formulation in this case is [u] formation, and the relative velocity of the fluid is ignored as well as the relative acceleration.

［ｕ］ ｆｏｒｍｕｌａｔｉｏｎ；Δｗ ＝ Δｗ ＝ ０ （弾性−非排水モデル）

[U] formulation; Δw = Δw = 0 (elasticity-undrained model)

  In the third partial differential equation of [u-wp] formulation, the relationship between Δp and Δu was obtained, and Δp was deleted from the first partial differential equation. The governing equation includes three variables under three-dimensional conditions and is composed of three partial differential equations.

If only the pore water pressure and pore water are taken into account and the displacement of the solid phase is removed from the analysis, the formulation is simplified and the solid phase can be regarded as a rigid body. This formulation called [w-p] formulation is effective for highly water permeable materials such as coarse sand and gravel.

［ｗ−ｐ］ ｆｏｒｍｕｌａｔｉｏｎ；Δｕ＝ Δｕ ＝ Δｕ＝ ０ （剛体−浸透流モデル）

[W-p] formulation; Δu = Δu = Δu = 0 (rigid body-osmotic flow model)

It was derived by setting Δu to zero in the second and third partial differential equations of [u-w-p] formation. The governing equation includes four variables under three-dimensional conditions and is composed of four partial differential equations.
The formulation and analysis conditions are summarized as shown in FIG.

The submarine ground taken up in the present invention is one in which several phases having different physical properties are accumulated in the horizontal direction. Here, the four boundary conditions of the seabed surface, layer and layer, layer and bedrock, and infinite depth are shown for one-dimensional and two-dimensional conditions, respectively.
1 and 2 illustrate the boundary conditions taken up in this study. The waveform in FIG. 1 represents the real part of the complex wave function by the phase angle θ (ωt + κx).

Since only waves are acting on the sea bottom, the boundary conditions at the sea bottom (z = 0) are expressed as follows.

一次元 ：

One dimensional :

二次元 ：
ｐ０：海底面の変動水圧振幅（Δｐ０＝ρｗｇＨ／ｃｏｓｈ（κｈ））
ρｗ：海水の密度
ｈ：水深
ω：角振動数（＝２π／Ｔ）
κ：波数（＝２π／Ｌ）
Ｈ：波高

Two dimensions :
p0: Fluctuating water pressure amplitude at the bottom of the sea (Δp0 = ρwgH / cosh (κh))
ρw: density of seawater h: water depth ω: angular frequency (= 2π / T)
κ: wave number (= 2π / L)
H: Wave height

Regarding the boundary between the deposited layers, the continuity of water pressure, stress, displacement of the solid phase, and hydrodynamic gradient must be maintained at the boundary between the two layers.

一次元 ：
二次元 ：

One dimensional :
Two dimensions :

The following describes the boundary between the sedimentary layer and the rock mass. At the boundary between the stratum and the rigid and impermeable bedrock, vertical water permeability is not allowed, ie the vertical hydraulic gradient must be zero. Also, solid phase displacement is not allowed.

一次元 ：

One dimensional :

二次元 ：

Two dimensions :

The infinite depth (z = ∞) is as follows. At an infinite depth below the sea floor, the displacement and water pressure of the solid phase must be zero.

一次元 ：

One dimensional :

二次元 ：

Two dimensions :

Here, as an example of derivation of an exact solution, a solution for a one-dimensional response of [up] formulation under a pseudo dynamic condition is derived. In the pseudo dynamic analysis, the governing equation is expressed by ignoring the acceleration term from the equation (17). Note that the object force vector increment i Δb is not considered here.

Since these simultaneous differential equations are linear, the basic form of the solution is

Substituting the equation (32) into the governing equation (31) and displaying it in matrix form, it is expressed as follows.

Since the right side is 0, the variables must be dependent on each other for this simultaneous equation to have a significant solution. This dependency means that the matrix value of the left-hand side matrix becomes zero.

The solution may be shown below.

Here, cv corresponds to a consolidation constant in Terzagi's one-dimensional consolidation theory. The general solution is written as follows from the possibility of the solution.

It can be seen that this solution contains 8 (4 × 2) unknown constants. Substituting equation (37) into the governing equation, the following four correlations are found in the unknown constant.

Four independent undetermined constants are determined by boundary conditions. Here, as an example, a solution for a ground where there is a water-impervious rock with a finite depth is obtained by boundary conditions 23 and 27 (FIG. 3). Equations 23 and 27 have 4 (2 × 2) equations and 4 unknowns, so a solution is always obtained. Solving these quaternary simultaneous equations, the unknown constant is determined as follows.

Here, D is the depth of the seabed to the bedrock.

The response of the seabed due to wave loads was calculated for three types: loose sand, regular compacted clay, and gravel.
The material and mechanical properties of the set soil are summarized in FIG. For the waves, the wave height H = 10 m, the period T = 13 sec, the water depth h = 20 m, the wavelength L = 167.6 m, and the water pressure fluctuation amplitude po at the sea bottom po = 37.9 kN / m 2. As the vertical boundary, only water pressure is acting on the ground surface (BC.1), and an impermeable rock (BC.3) is assumed at the lower end of the analysis region.
As a standard case, the depth distribution of excess pore water pressure normalized by the effective vertical stress and the amplitude of waves acting on the ground surface is [up] formulation for loose sand, normal consolidated clay and gravel. Calculated by exact solution under two-dimensional pseudo-dynamic conditions (FIGS. 4 and 5). Here, when waves act on the surface of the seabed ground, the vertical distribution of the vertical effective stress in the ground is shown corresponding to the eight phase angles. In FIG. 4, it can be seen that the vertical stress is reduced and the soil is destabilized in the process in which the water pressure on the surface of the seabed is drastically lowered. In particular, in response to loose sand, it is suggested that the effective vertical stress is in the range up to a depth of less than 4 m, and repeatedly becomes negative as the phase angle progresses, resulting in a liquefied state. The response is greatly affected by water permeability, but the vertical effective stress fluctuates with moderate loose sand. The reason is the influence of the pore water pressure coefficient B ′ value, which is a small value because the degree of saturation is low. Although the saturation level of the seabed ground is usually 99% or more, the volume compression coefficient of air is much smaller than the volume compression coefficient of water. The effect on water pressure response is significant. It can be seen that, especially in the loose sand ground, the degree of saturation in the depth direction is large because the degree of saturation is small compared to other ground, and a hydrodynamic gradient develops in the ground (Fig. 5). FIG. 6 shows the fluctuation corresponding to the phase change angle of the excess pore water pressure at the ground surface z = 0 m and the depth z = 1 m with respect to the loose sand ground. Attenuation of pressure and a phase difference of propagation are observed. As a result, an upward osmotic flow is generated in the ground around the phase of 3 / 4π to π, and the ground is unstable. On the other hand, the normal consolidated clay shows a large fluctuation near the ground surface, but it shows the behavior of undrained state deeper than that. In gravel, the pressure acting on the ground surface is difficult to attenuate in the ground due to its high water permeability, so the hydrodynamic gradient does not develop, and the vertical effective stress in the ground hardly fluctuates.

In the problem of the response of the seabed ground, the influence of acceleration is small, so in this problem, the acceleration can be ignored by [up] formulation and pseudo dynamic analysis, and in the response to a sufficiently long wavelength, the response is sufficiently reproduced by a one-dimensional response. Previous studies have shown that this can be done. Here, in [up] formulation and pseudo-dynamic analysis conditions, an equation solved by giving a boundary condition of a one-dimensional, finite depth ground is examined. The expression of the pressure response is as follows from the equations 39, 41, and 42.

here,

The pore water pressure coefficient B 'in one-dimensional deformation corresponding to the Skempton B value strongly depends on the compressibility (saturation degree) of the pore water. In addition, hv will be referred to as “hydraulic consolidation constant (reciprocal of a value obtained by correcting the consolidation coefficient Cv in consideration of the compressibility of pore water)”. This hv is determined only by the physical properties of the ground, and is a coefficient that determines the magnitude of the stress and pressure fluctuations in the seabed ground subjected to waves. Substituting these coefficients into Equation 39,

数４５における項ｅ−２ζＤ を無視すると、層の厚さＤが無限の場合に相当する以下の式が得られる（近似（１）；海底地盤の不安定化が問題になる代表的な砂の場合ｈｖ は通常１から５程度で（図１５参照）、この近似は層厚Ｄが２ｍ以上であれば誤差は数パーセント以下と考えられる）。

If the term e-2ζD in Equation 45 is ignored, the following equation corresponding to the case where the layer thickness D is infinite is obtained (approximate (1); representative sand instability of submarine ground) In this case, hv is usually about 1 to 5 (see FIG. 15), and this approximation is considered to have an error of several percent or less if the layer thickness D is 2 m or more).

Since the water pressure fluctuation acting on the seabed ground surface propagates in the depth direction while being attenuated, the fluctuation of the pore water pressure at three depths at equal intervals dp is as follows (FIG. 7).

The difference in pore water pressure between two adjacent points is

The amplitude obtained by taking the absolute value is

Therefore, hv can be calculated from the two amplitudes as follows.

If the phase difference (imaginary part of ζ) generated in the depth direction in the pore water pressure equation 46 is ignored, the amplitude can be obtained as follows (approximate (2); depending on conditions, a considerable degree of error may occur). , We will verify later.)

Therefore, B 'can be calculated for each adjacent point, and the average value is used. This calculation method is for eliminating the measurement error of d0.

Measurement of pore water pressure at three different depths in the seabed ground would be possible by inserting a rod with a pre-installed pore pressure gauge into the seabed ground, and in practice to eliminate wave irregularities. In addition, the dominant period is obtained by spectral analysis of the measured value, and the parameter calculation method described here is applied to the dominant component.

Here, as a method for determining the instability of the seabed ground, the condition here is that “the calculated vertical effective stress in the Shanghai floor ground becomes negative”, focusing on the liquefaction phenomenon that occurs repeatedly. Based on the exact solution described above, the effective vertical stress in the ground is calculated as follows.

This amplitude is

When this amplitude exceeds the vertical effective stress (()) zo t f Δσ = ρ -ρ gz in the hydrostatic pressure state of the ground, the effective stress in the ground temporarily becomes negative and leads to a liquefaction state.

Therefore, the liquefaction depth of the ground can be obtained by solving for this extreme depth (liquefaction depth) zl.

here,

Hp corresponding to this potential height is a value determined from the water pressure acting on the sea bottom, the density of the ground material, and the pore water pressure coefficient B '. FIG. 8 shows the relationship between the liquefaction depth zl and the hydraulic consolidation constant hv obtained from Equation 56 using Hp as a parameter. By using this figure, together with the measurement method of the physical constant in the ground by the “3-gauge method”, it is possible to evaluate the stability of the seabed ground against waves from the parameters estimated from the measured values. FIG. 9 is a flowchart showing a series of procedures for evaluating the anti-wave stability of the seabed ground.

In the series of submarine ground parameter estimation methods and liquefaction range estimation methods, two approximations were made in Equations 46 and 51. In the following, we will examine how the behavior of the seabed ground related to the parameter estimation method and the liquefaction range estimation method in a series of soils is affected by waves and ground parameters.

Approximation (1): This is an error due to the component ignored in Equation 46, and depends on the following terms.

Approximation (2); FIG. 10 shows the distribution in the depth direction of pore water pressure calculated under typical conditions in eight phases. Further, the blue line in the figure is an approximate envelope (Formula 59) calculated from Formula 51, and the green line represents an envelope showing the amplitude of Formula 45 (Formula 60).

here,

Therefore, the approximate envelope of the blue line calculated in Formula 51 always exists outside the envelope of the green line calculated in Formula 45. Therefore, it is expected that the B value calculated by the approximate curve underestimates the actual B value in most cases, and as a result, the liquefaction depth estimated from Equations 56 and 57 is overestimated. In order to verify the validity of these approximations, here, the B value: B ′, the hydraulic consolidation constant: hv, and the liquefaction depth by exact solution: zl input by calculation according to the following procedure, and the parameter estimation method are used. Those obtained were compared.

(1) The time calendar of excess pore water pressure at three points with different depths is calculated by exact solution.
(2) The one-dimensional pore water pressure coefficient B ′ and the hydraulic consolidation constant hv are calculated from the amplitude of the time history and the difference between the time histories.
(3) The liquefaction depth is calculated from the relationship shown in FIG. 2 and compared with the liquefaction depth obtained from the exact solution.
FIGS. 11 to 14 show a comparison of liquefaction depth when the pressure due to waves acting on the sea floor is p0, the period of waves is T, the B value of the seabed is B ′, and the hydraulic conductivity is k. ing. In these figures, the depth of the seabed to the impermeable layer: D is changed to 4m, 5m, 6m, and 100m. The variation due to D can be interpreted as variation due to approximation (1), and D is sufficiently large. Error (D = 100 m) can be interpreted as an error due to approximation (2). The standard condition is that the characteristics of waves that are basic as loose sand are pressure due to waves acting on the sea bottom: p0 = 40 (kN), wave period: T = 8 (s). In addition, the parameter estimation requires a response of pore water pressure at three depths. In this study, depths of 0.5 m, 1.0 m, and 1.5 m are targeted.

FIG. 11 shows B values when the wave pressure acting on the sea bottom is changed to p0 = 2.0 (kN), p0 = 3.0 (kN), and p0 = 4.0 (kN): B ′ The hydraulic consolidation constant: hv and the liquefaction depth by exact solution: zl are compared with those obtained by the parameter estimation method. Since the variation when the depth D of the seabed to the impermeable layer is changed is small, the error due to approximation (1) is considered to be negligible under this condition.

Further, since approximation (2) does not affect the estimation of hv, the estimated hv is very close to the input value. In the estimation of the B value, there is no change depending on p0, and the estimated B value is underestimated to be 0.26. As a result, the liquefaction depth is overestimated by about 0.5 to 0.7 (m).

FIG. 12 shows a B value when the period of waves acting on the sea bottom is changed to T = 3.0 (s), T = 8.0 (s), and T = 13.0 (s): B ′ , Hydraulic consolidation constant: hv and liquefaction depth by exact solution: zl. When the wave period T is large and the depth D to the impermeable layer of the seabed is small, the variation in the estimated value seems to be slightly large, but the fluctuation is negligible. Moreover, the fluctuation | variation by the wave period of the approximation (2) is small, and the liquefaction depth overestimates about 0.6 to 0.7 (m).

FIG. 13 shows B values when the B value of the seabed ground is changed to B ′ = 0.4, B ′ = 0.5, and B ′ = 0.6: B ′, hydraulic consolidation constant: hv and exact The liquefaction depth by solution: zl is shown in comparison. Here, the variation due to the change in D is slight, and the liquefaction depth is overestimated by about 0.6 to 0.7 m almost uniformly.

In FIG. 14, the permeability coefficient of the seabed ground is k = 1.0 * 10-5 (m / s), k = 1.0 * 10-4 (m / s), k = 1.0 * 10-3 ( m / s) and B value: B ′, hydraulic consolidation constant: hv, and liquefaction depth by exact solution: zl. In the case of k = 1.0 * 10-5 (m / s) and k = 1.0 * 10-4 (m / s), there is almost no variation in the estimated value due to the change in D, but k = 1. In the case of ground with relatively good water permeability such as 0.0 * 10-3 (m / s), the estimated value varies. In the case of such highly permeable ground, the estimated value of the liquefaction depth may be less than the exact calculation result when the depth of the seabed to the impermeable layer is small. Except when there is an impermeable layer near the ground surface in highly permeable ground, the estimated value of liquefaction depth is not significantly different from the exact calculation result, and the estimated value exceeds the exact calculation result. Yes.

At present, it is difficult to obtain undisturbed soil samples while maintaining the hydraulic conductivity and saturation, and there is a limit to the technology for measuring these samples in the original seabed. Also, due to the difficulty of investigating the physical properties of the ground, the stability of the foundation ground against waves during the design of coastal and harbor structures has not been studied. In contrast to this, the proposed “3-gauge method for investigating the physical constant of the ground” measures the pore water pressure fluctuations at three points within the seabed ground, and the stability of the seabed ground, including the “hydraulic liquefaction constant”. It is possible to easily measure the physical properties of the ground that affect the soil. In addition, this method is particularly effective for loose sandy ground where the seafloor ground has large stress fluctuations. Therefore, in addition to the “liquefaction depth estimation method”, the stability of the foundation ground of structures on the coast and offshore can be improved. It can be expected to be used for verification.

INDUSTRIAL APPLICABILITY The present invention can be used to study the stability of foundation ground against waves for the design of offshore and coastal structures.

Analysis conditions Boundary conditions for stratified ground. boundary condition Variation of depth of vertical effective stress Variation in depth of normalized excess pore water pressure Damping of pore water pressure fluctuation and fluctuation of vertical effective stress Measurement of pore water pressure Estimation of liquefaction depth of submarine ground Flow chart for evaluating the stability of ocean waves on the seabed Approximate curve of pore water pressure fluctuation Examination of wave pressure Examination of wave cycle Examination of B value Examination of hydraulic conductivity Physical and mechanical properties of typical soil used in the analysis

Claims (2)

Measure the pore water pressure in the submarine ground, calculate the hydraulic consolidation constant that is the response parameter of the seabed ground using the dominant component of the measured pore water pressure, and calculate the calculated hydraulic consolidation constant and the measured pore water pressure. Deriving the one-dimensional pore water pressure coefficient from the dominant component, calculating the instability depth of the seabed ground from the one-dimensional pore water pressure coefficient and hydraulic consolidation constant, and measuring the stability in the seabed ground during waves And how to.   The method according to claim 1, wherein the pore water pressure in the seabed ground is measured at three points in the ground.

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