JP2017014724A - Ground liquefaction prevention method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ground liquefaction prevention method which enables a highly reliable liquefaction prevention measure to be taken by estimating a saturation degree of the ground with higher accuracy than a conventional method.SOLUTION: A ground liquefaction prevention method comprises: a groundwater level lowering process to lower a groundwater level by pumping groundwater through a well 2; and a groundwater level restoration process to put the ground G into an unsaturated state by restoring the groundwater level in a manner that allows the groundwater W to flow into the ground G from a cut-off wall outside portion 1 or injects the pumped groundwater W back into the same. The groundwater level restoration process has: a sampling step to sample in-situ soil at an object depth; an initial saturation degree measurement step to measure an initial saturation degree of a test piece taken out from a sample obtained through the sampling step; and a saturation degree depth distribution estimation process to estimate depth distribution of the saturation degree after restoration of the groundwater level using a relation between the flow volume of groundwater and the saturation degree. The saturation degree after the restoration of the groundwater level is designed on the basis of a result of the depth direction of saturation degree estimation process.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a ground liquefaction prevention method for preventing ground liquefaction.

  As one of the measures to prevent ground liquefaction, a water well that reaches the impervious soil layer is constructed so as to surround the target ground, such as directly under the structure, and a pumping well (deep well) provided inside ), The groundwater level of the saturated sand ground is lowered, and then the groundwater level is recovered to create an unsaturated ground, thereby preventing the occurrence of liquefaction (see, for example, Patent Document 1) ).

JP 08-003975 A

  On the other hand, in the above liquefaction countermeasure construction method, since the liquefaction strength of unsaturated ground depends on the saturation, the design of the saturation after recovery of the groundwater level becomes important. However, no such design method has been proposed.

  In view of the above circumstances, an object of the present invention is to provide a ground liquefaction prevention method that accurately designs the degree of saturation of the ground after recovery of the groundwater level and enables highly reliable liquefaction countermeasures.

  In order to achieve the above object, the present invention provides the following means.

The ground liquefaction prevention method according to the present invention recovers the groundwater level by recovering the groundwater level by lowering the groundwater level by pumping water from a well and wrapping the groundwater from the outside of the water blocking wall or by injecting the pumped groundwater. In the ground liquefaction prevention method comprising a groundwater level recovery step for producing a saturated ground, the groundwater level recovery step is obtained by sampling a sample of an in-situ soil at a target depth, and the sampling step. The depth distribution of the saturation after the water level recovery is estimated from the initial saturation measurement process for measuring the initial saturation S _r0 of the specimen obtained from the sample and the relationship between the water flow rate and the saturation in the following equation (1) And a saturation depth distribution estimating step,
The saturation after water level recovery is designed based on the result of the saturation depth distribution estimation step.

ここに、Ｓ_ｒはある深度における地盤の飽和度、Ｓ_ｒ０は地下水回復直後の地盤の初期飽和度、ＤＯ（Disolved Oxigin）は地下水の溶存酸素量（ｍｇ／ｌ）、ＤＯ_ｓａｔは地下水の飽和溶存酸素量（ｍｇ／ｌ）、Ｖ_ｗは地盤の単位面積当たりの地下水通水量（ｍｌ／ｃｍ^２）、αはパラメータである。

Here, the degree of saturation of the soil in the _{S r} is the _{depth, S r0} is the initial saturation of the ground immediately after the ground water recovery, DO (Disolved Oxigin) amount of dissolved oxygen in groundwater (mg / _{l), DO sat} saturated groundwater The amount of dissolved oxygen (mg / l), _Vw is the amount of groundwater flow per unit area of the ground (ml / cm ² ), and α is a parameter.

  ADVANTAGE OF THE INVENTION According to this invention, the saturation distribution of the ground by the fall recovery of a groundwater level can be calculated | required, and saturation management can be performed accurately and suitably. This makes it possible to realize an effective and highly reliable liquefaction countermeasure.

It is a figure which shows the liquefaction prevention method by the groundwater fall and recovery which concern on one Embodiment of this invention. It is a figure which shows the relationship between the soil saturation and liquefaction strength. It is a figure which shows the water | moisture-content characteristic curve at the time of the water level fall of a test body, and recovery | restoration. It is the figure used for description of the factor which saturation increases after groundwater recovery. It is a figure which shows the relationship between groundwater flow volume and saturation. It is the figure which compared the relationship between the groundwater flow volume and saturation degree by experiment, and the relationship between the groundwater flow volume and saturation degree by a design formula. It is a figure which shows a time-dependent change of saturation. It is a figure which shows the relationship between the average particle diameter of a ground, and an effective porosity. It is the figure used by description of the method of calculating | requiring a saturation in a groundwater level recovery process. It is a figure which shows the saturation distribution in the ground at the time of assuming that a natural groundwater level lowered the water level to GL-11m with respect to the ground of GL-1m.

  Hereinafter, a ground liquefaction prevention method according to an embodiment of the present invention will be described with reference to FIGS.

  First, in the present embodiment, as shown in FIG. 1, for example, a water blocking wall 1 reaching the impermeable soil layer G1 is constructed so as to surround the target ground G, such as directly under the structure, and the inner side thereof Unsaturated ground G by pumping water from the well (deep well) 2 and lowering the groundwater level W of the saturated sand ground (groundwater level lowering process) and then refilling the groundwater level by injecting water from the well 2 (The groundwater level recovery process), and the occurrence of liquefaction is prevented by changing the degree of saturation of the ground G in this way.

  Here, according to past experimental facts, the liquefaction strength is twice that of the saturated ground (Sr = 100%) when the ground saturation Sr is 90%, and three times that of the saturated ground when the saturation Sr is 70%. (For example, FIG. 2: Yoshimi.Y, Tanaka.K and Tokimatsu.K: Liquefaction resistance of a partially saturated sand, Soils and Foundations, Vol.29, No.3, pp.157-162 , Sep. 1989).

On the other hand, the liquefaction strength τ ₁ / σ _v ′ of the saturated sand ground is calculated from the undrained repeated triaxial test and the corrected N value = N _a written in the basic structure design guidelines of the Architectural Institute of Japan. 3 is calculated. Further, N _a varies depending on the depth as in the following formulas (2) and (3). For this reason, the liquefaction strength τ ₁ / σ _v ′ of the saturated sand ground is calculated according to the depth.
N ₁ is an N value corrected by the vertical effective stress σ _v ′ at the examination depth, and ΔN _f is an increment of the corrected N value corresponding to the fine particle content FC.

  On the other hand, the external force acting on each depth (equivalent cyclic shear stress ratio τd / σv ′) is obtained by the following equation (4).

ここに、γ_ｎは等価の繰返し回数に関する補正係数でγ_ｎ＝０．１（Ｍ−１）、Ｍはマグニチュード、α_ｍａｘは地表面における設計用水平加速度（ｃｍ／ｓｅｃ^２）、ｇは重力加速度、σ_ｖは検討深度における鉛直全応力、γ_ｄは地盤が剛体でないことによる低減係数でγ_ｄ＝１−０．１５ｚ、ｚは検討深度（ｍ）である。

Here, γ _n is a correction coefficient for the equivalent number of repetitions, γ _n = 0.1 (M−1), M is a magnitude, α _max is a horizontal acceleration for design on the ground surface (cm / sec ² ), and g is a gravity force Acceleration, σ _v is the total vertical stress at the examination depth, γ _d is a reduction factor due to the fact that the ground is not a rigid body, γ _d = 1−0.15z, and z is the examination depth (m).

Next, the liquefaction safety factor FL can be calculated by Equation (5) from the obtained τ ₁ and τ _d . Moreover, in the design of normal liquefaction suppression, it designs so that FL may not go below 1 in each depth.

Further, the liquefaction strength τ _1u / σ _v ′ of the unsaturated ground can be obtained by multiplying the liquefaction strength of the saturated ground by a coefficient β corresponding to the degree of saturation S _{r as shown} in the following equation (6).

beta may be set as such on the basis of the history of the experimental result shown in FIG. 2 for example _S r = 90% at _{β = 2, S r = 70} % at beta = 3.

  Therefore, if the depth distribution of the saturation due to the recovery of groundwater drop is known, the liquefaction safety factor of unsaturated ground by depth can be calculated by Equation (3) and Equation (5).

  Next, the calculation method of the saturation distribution according to depth at the time of the fall recovery of groundwater is demonstrated.

From the past experimental facts, the soil moisture characteristic curve (= saturation degree S _r -matrix potential (suction) when the saturated soil (sand) specimen was drained and when it was reflooded from that state. ) It is known that the relationship of Pc) is different. FIG. 3 shows a conceptual diagram thereof.

When pore water is drained from the state of saturation S _r = 100% (point A), the moisture in the test body is discharged along the drainage curve in FIG. Then, when submergence (water level recovery) is performed again at point D where the degree of saturation is sufficiently lowered, the wet curve at point E is reached through the scanning curve indicated by the dotted line, and the point F is reached through the wet curve. The point F is the initial saturation S _r0 specimens when allowed to drawdown and recovery.

The initial saturation S _r0 in the ground can be calculated by the design formula shown in the following formula (7).

ここに、Ｓ_ｒはある深度における地盤の飽和度、Ｓ_ｒ０は地下水回復直後の地盤の初期飽和度、ＤＯ（Disolved Oxigin）は地下水の溶存酸素量（ｍｇ／ｌ）、ＤＯ_ｓａｔは地下水の飽和溶存酸素量（ｍｇ／ｌ）、Ｖ_ｗは地盤の単位面積当たりの地下水通水量（ｍｌ／ｃｍ^２）、αはパラメータである。

Here, the degree of saturation of the soil in the _{S r} is the _{depth, S r0} is the initial saturation of the ground immediately after the ground water recovery, DO (Disolved Oxigin) amount of dissolved oxygen in groundwater (mg / _{l), DO sat} saturated groundwater The amount of dissolved oxygen (mg / l), _Vw is the amount of groundwater flow per unit area of the ground (ml / cm ² ), and α is a parameter.

Expression (7) indicates that the saturation of the soil G2 is determined by the amount of groundwater flow per unit area and the amount of dissolved air in the groundwater W from the initial saturation _Sr0 immediately after the groundwater W is recovered. That is, the degree of saturation is expressed as a design equation that the more groundwater in contact with the bubbles, or the smaller the amount of dissolved air in the groundwater W, the easier it is to rise because the air is more soluble. It also shows that the bubbles do not move upward (ground surface) due to buoyancy or the like (see FIG. 4).

More specifically, first, in the groundwater level recovery step, the in-situ soil at a predetermined depth is sampled from the borehole in the sampling step. Then, in the initial saturation measurement step, a method for obtaining a moisture characteristic curve defined in the Geotechnical Society standard “Soil water retention test method (JSG 0151-2000)”, or a saturated specimen was drained and reflooded. The initial saturation S _r0 is measured later by a method for obtaining the saturation.

Moreover, DO of the water used for water level recovery | restoration measures the dissolved oxygen amount DO of groundwater W beforehand, when making it recover with a natural groundwater level by the wraparound of the groundwater W from the outside of the water stop wall 1. FIG. Alternatively, DO (= 1.5 mg / L) of degassed water is used as the safe side. Furthermore, the saturation after the water level recovery when it is desired to lower the S _r is the in aerated enhanced DO water may be injection into the ground G.

Further, the unit water flow rate Vw is assumed to be that the groundwater W excluding the retained water remaining in the gap rises from the gap as shown in FIG. 10, and is recovered to the natural water level as shown in the following equation (8). determined by the product of the effective porosity _{n e} amount Delta] h (cm) and sand. The effective porosity ne can be determined from the weight of water remaining in the soil after draining the saturated specimen by weight. For simplicity, ne = 0.2 to 0.35 can be set from FIG.

  Further, α is obtained from a water flow test of a sampled sample, or simply a value of about α = 0.05 based on past experimental results is used.

Then, the saturation distribution after the groundwater recovery is designed by substituting the above S _r0 , DO, α, and V _w into Equation (7).

  Here, an experiment for verifying the validity of Expression (7) will be described.

In this experiment, first, a sand specimen having a diameter of 5 cm and a height of 10 cm was prepared by an air drop method, and the gap was replaced with CO ₂ to pass deaerated water. Then, an effective restraining pressure σc = 100 kPa and a back pressure of 100 kPa are applied to saturate to a B value (saturation degree of the specimen) of 95% or more, and then the water is drained in a natural drainage state (a negative pressure of about 60 cm acts as a head differential). Thus, the specimen was made unsaturated. Next, deaerated water or tap water was injected into the specimen from the lower side with a predetermined restraint pressure and back pressure applied, and the relationship between the water flow rate and (B value) was examined.

Fig. 5 shows the unit water flow rate of the sand specimen (per unit area of the specimen) when deaerated water (DO = 1.5%) and tap water (DO = 5.5-7.0%) are passed. It shows the relationship between the water flow rate mg / cm ² ) and the degree of saturation.
From this figure, it was confirmed that the saturation increased linearly with respect to the unit water flow rate with a gradient according to DO.

FIG. 6 superimposes the result of equation (7) and the experimental data. Here, α was 0.05, DO _sat was 9.0, DO was 1.5 (deaerated water), and DO was 6.3 (tap water). The saturated dissolved oxygen amount of water at 1 atm and 15 ° C. is 9.7 mg-oxygen / l.
From this figure, it was confirmed that the change of the saturation according to the difference in DO of water to be passed can be expressed with high accuracy by the equation (7) of the design formula.

FIG. 7 shows the daily change in the B value of unsaturated specimens having different initial saturation levels.
From this figure, it was confirmed that the saturation level was kept constant for at least 80 days and the B value did not change if water did not flow through the specimen.

  The following is a specific example of saturation calculation based on groundwater recovery.

Here, the initial saturation of the ground element immediately after the recovery of the water level: S _r0 = 68%, the porosity of the sandy ground: n = 0.4, the effective porosity of the sandy ground: _ne = 0.2 (FIG. 8) Reference). And the unit water flow rate at the time of water level recovery of 5 m is 5 m / cm ² = 500 cm / cm ² = 500 ml / cm ² and 500 ml / cm ² × 0.2 = 100 ml / cm ² .

If the recovered groundwater DO is close to tap water (DO = 5.5-7.0), the unit water flow rate until the 5 m water level is recovered is 100 ml / cm ² . Then, it can be determined from FIG. 9 that the saturation at 5 m below the natural water level is about 82%. And the saturation distribution in the ground at the time of assuming that the water level was lowered to GL-11m with respect to the ground of natural groundwater level GL-1m using the method of this embodiment becomes FIG.

  Therefore, in the ground liquefaction prevention method of the present embodiment, if the saturation degree distribution of the ground due to the recovery of lowering of the groundwater level is designed by this design method, the saturation degree can be managed accurately and suitably, and the effect And liquefaction countermeasures can be realized with high accuracy and reliability.

  As mentioned above, although one Embodiment of the ground liquefaction prevention method concerning this invention was described, this invention is not limited to said one Embodiment, In the range which does not deviate from the meaning, it can change suitably.

1 Stop wall 2 Well G Ground G1 Unsaturated ground G2 Soil (soil particles)
W Groundwater

Claims (1)

A groundwater level lowering process that lowers the groundwater level by pumping from a well, and a groundwater level recovery process that restores the groundwater level by circulating the groundwater from the outside of the water barrier or by injecting the pumped groundwater to create an unsaturated ground. In the ground liquefaction prevention method comprising:
The groundwater level recovery step is a sampling step of sampling a sample of the in situ soil at a target depth;
An initial saturation measuring step of measuring an initial saturation S _r0 of a specimen obtained from the sample obtained in the sampling step;
A saturation depth distribution estimation step for estimating the depth distribution of the saturation after the water level recovery from the relationship between the water flow rate and the saturation of the following formula (1),
A method for preventing ground liquefaction, wherein the saturation after water level recovery is designed based on the result of the saturation depth distribution estimation step.

Here, the degree of saturation of the soil in the _{S r} is the _{depth, S r0} is the initial saturation of the ground immediately after the ground water recovery, DO (Disolved Oxigin) amount of dissolved oxygen in groundwater (mg / _{l), DO sat} saturated groundwater The amount of dissolved oxygen (mg / l), _Vw is the amount of groundwater flow per unit area of the ground (ml / cm ² ), and α is a parameter.

Images