JP4458465B2 - Ground investigation method and device by measuring excess pore water pressure during impact penetration - Google Patents

Description

  In addition to determining the dynamic strength (penetration resistance value) of the ground by impact penetration, the present invention estimates the liquefaction strength (liquefaction potential) from the behavior (generation and dissipation) of excess pore water pressure generated during impact penetration. The present invention relates to a ground survey method for evaluating mechanical characteristics and physical characteristics of a ground by soil analysis and a ground survey device used therefor. This technology is useful for easily and efficiently economically determining the physical and mechanical properties of the ground necessary for designing when constructing civil engineering and building structures.

  When constructing civil engineering / architectural structures, etc. on the ground made of natural or artificial objects, it is necessary to grasp the physical / mechanical characteristics of the ground in advance and to design an appropriate structure based on it. Therefore, various ground investigation methods and devices have been proposed and put into practical use.

  As a typical method of the dynamic penetration test for investigating the mechanical properties of the ground, there is a standard penetration test method (JIS A 1219) defined by Japanese Industrial Standards. This is a test method for obtaining an N value for knowing the relative values of the hardness and firmness of the soil at the original position. Specifically, a standard penetration test sampler is attached to the tip of the rod, lowered to the bottom of the drilled borehole, and the rod is hit on the ground by a free fall (fall height 76 cm) of a hammer (63.5 kg). The number of hits (N value) required to penetrate 15 to 45 cm (30 cm) from the bottom is determined. The N value obtained by this standard penetration test method has been used for many years as a design index for structures in Japan with a complex ground configuration.

  However, in the standard penetration test method, a boring machine and a boring pump are required to drill the test hole, and mud water (construction sludge) is forced to be used to stabilize the hole wall during the drilling. Investigation work such as removal of slime from the bottom of the hole and pre-working work of 15 cm is complicated, requires experience, and takes time to investigate.

  There are various sounding methods (for example, automatic ram sounding) classified as dynamic penetration testing methods as alternatives to the standard penetration testing method. This is a method in which a rod having a conical penetrating body attached to the tip is continuously struck into the ground by a free fall of a hammer and the like, and the number of hits per fixed penetration length is obtained (for example, Non-Patent Document 1) reference). However, with such a sounding method, although the dynamic strength (penetration resistance value) of the ground at the test depth can be evaluated from the number of hits, a soil sample cannot be collected as a standard at each depth, so physical properties such as soil discrimination are evaluated. I can't.

By the way, it has been reported that in the flexible land near the coast of our country, large damage was caused by the liquefaction of the soil caused by past earthquakes. The evaluation of liquefaction strength (liquefaction potential) requires indoor soil test data such as repeated undrained triaxial tests of soil using undisturbed samples collected in situ. The strength is evaluated only at a limited point. Therefore, the liquefaction strength of an area must be evaluated based on poor data. On the other hand, past surveys on earthquake damage show examples where the extent of damage changes significantly even if they are only a short distance away. Therefore, a more accurate evaluation is required to make a reliable design for liquefaction of land. However, if it is attempted to carry out conventional tests at many points for that purpose, the cost of the survey is greatly increased, so that it is very difficult to carry out. Thus, from an engineering point of view, there is a need for an investigation method that can be performed more inexpensively, simply, and with high accuracy.
"Ground evaluation by small automatic ram sounding test" Yoshiyuki Ito et al. (2002) 37th Geotechnical Engineering Conference (Osaka) pp.103-104

  The problem to be solved by the present invention is that, in the conventional dynamic penetration test method, although the dynamic strength (penetration resistance value) of the ground at the test depth can be evaluated from the number of impacts, physical characteristics such as soil discrimination at each depth are obtained. The point which cannot be evaluated, the point which cannot measure easily the liquefaction strength (liquefaction potential) in arbitrary points, etc.

The present invention incorporates a pore water pressure sensor into the penetrating body at the tip of the rod, strikes the rod, penetrates the penetrating body into the ground, determines the dynamic strength of the ground at the depth of the penetrating body from the amount of penetration, and immediately after hitting the batting Of the amount of penetration in the soil and the time of excess pore water pressure generated in the ground in contact with the intruder, and the fine particle content of the soil at the depth of the intruder from the response of the obtained penetration amount and the excess pore water pressure. This is a ground survey method based on the measurement of excess pore water pressure at the time of intrusion, characterized by evaluating soil and determining soil quality . Here, continuous penetration in the depth direction is performed , and the maximum excess pore water pressure value at the penetration depth is detected from the generation response of excess pore water pressure immediately after the impact penetration. Pressure) is used as a groundwater level index at the depth of the intrusion, and when the distribution of the groundwater level index in the depth direction is determined, the groundwater level index decreases from the surface side toward the depth direction to 1.0. It is possible to estimate the depth that converges to an almost constant value less than the groundwater level.

The present invention provides an apparatus for carrying out the ground survey method as described above, the penetrating member to be attached to the rod end, excess pore said penetrating body embedded or which are generated in the ground in contact with the penetrating body A gap water pressure sensor that detects water pressure, a displacement sensor that detects the amount of penetration of the rod, an acceleration sensor that detects impact on the rod, and an excess pore water pressure detection signal and penetration during impact penetration using the acceleration signal as a data recording trigger a data recording unit for recording the time course of the displacement detection signal, comprises a dynamic penetration apparatus for striking penetrating the rod, and strike the rod to penetrate the penetrating body to ground, penetration amount and blow during the striking penetration performing excess pore water pressure measured at the time of hitting penetration, characterized in that the response of the generation and dissipation of excess pore water pressure occurring in the soil in contact with the penetrating body such that the data recording with the intrusion It is a panel survey device.

For example, the penetrating body includes a transducer housing having a conical tip, and a cylindrical coupling portion between the transducer housing and the rod, and the transducer housing opens at a plurality of positions on a conical surface to reach a central hole. A communication hole is provided, and the opening is closed with a porous hard member to become a pressure receiving surface, a pressure transducer is installed in the center hole, and a pressure transmission medium is filled in the hole. The amount of penetration at the time of impact and the detection signal of the pore water pressure sensor are digitized by A / D conversion and digitally recorded by the data recording unit. When the rod is continuously impacted in the depth direction, it penetrates continuously in the depth direction. A configuration that records the volume and excess pore water pressure response is preferred.

  Although the present invention uses a simple measuring device that employs a continuous dynamic penetration test method, it can determine not only the dynamic strength (penetration resistance value) as the mechanical characteristics of the ground but also soil judgment and ground By simultaneously evaluating physical characteristics such as water permeability, the liquefaction strength (liquefaction potential) of the ground can be easily and economically evaluated.

  In addition, according to the present invention, since the evaluation can be automatically performed for each impact penetration, not only a significant improvement in work efficiency but also continuous measurement is possible.

  The present inventor has incorporated a pore water pressure sensor inside the penetrating body at the tip of the rod in a dynamic penetrating test method (for example, automatic ram sound etc.), and detects the excess pore water pressure and the amount of penetration in the ground around the penetrating body at the time of impact. And analyzed. As a result, it was found that the soil classification and liquefaction strength (liquefaction potential) of the ground can be evaluated from the penetration amount and the behavior of excess pore water pressure (generation and dissipation) at the time of impact penetration. For example, in the case of uniform loose sandy soil, the positive excess pore water pressure generated around the intruder increases, and in the dense case, the positive excess pore water pressure generated becomes relatively small. Moreover, the magnitude | size of the excess pore water pressure to generate | occur | produce has a favorable correlation with the dynamic strength (penetration resistance value) and liquefaction strength (liquefaction potential) of a ground. On the other hand, as the fine grain content is mixed from uniform sandy soil and the soil classification is close to that of cohesive soil, the extinction response of excess pore water pressure that occurs during dynamic intrusion becomes dull, depending on the proportion of fine grain The dissipation response changes. From these, by measuring the behavior of excess pore water pressure (occurrence and dissipation) and the amount of impact penetration that occurs during impact penetration, not only the dynamic strength (penetration resistance) of the ground at that depth, but also physical characteristics (soil properties) Discrimination and water permeability) and liquefaction strength (liquefaction potential) can be evaluated. The present invention has been completed based on the knowledge of the evaluation technique.

  In the present invention, a rod having a penetrating body incorporating a pore water pressure sensor attached to the tip is struck into the ground with a constant striking energy due to a natural drop or forced drop of a hammer. Along with this, the behavior (generation and dissipation) of excess pore water pressure generated in the ground around the intruder is detected by a pore water pressure sensor. And the behavior (generation and dissipation) of the impact penetration and excess pore water pressure is digitized by A / D conversion and digitally recorded by the data recording unit, and the dynamic characteristics of the ground at the relevant depth (dynamic Strength (penetration resistance), liquefaction strength (liquefaction potential)) and physical characteristics (soil discrimination and water permeability) are evaluated.

  FIG. 1 is an explanatory diagram showing the overall configuration of a measuring apparatus used for carrying out the method of the present invention. As the impact penetrating device main body, a dynamic penetrating test device (for example, automatic ram sound) can be used as it is. However, the rod 10 has a hollow structure because it is necessary to connect a cable for transmitting a signal from the pore water pressure gauge incorporated in the penetrating body at the tip to the ground. Here, a penetrating body 12 incorporating a pore water pressure sensor is attached to the tip portion (lower end) of the rod 10, and a hammer 16 is hit and penetrated by naturally dropping the hammer 16 from a predetermined position on the anvil 14 at the upper end portion. Yes. By the hammer 16 hitting the anvil 14, the penetrating body 12 at the tip penetrates into the ground.

In this example, a lightweight dynamic penetrating device with an improved Swedish ram sounding test device was used. The rod 10 has a diameter of 28 mm, a conical tip with a cylindrical structure, and the cylindrical penetration body 12 has a diameter of 36.6 mm, a length of 69 mm, and a vertex angle of 90 degrees. The rod 10 is mechanically driven by a 30 kg hammer 16 that falls freely from a height of 35 cm. The penetration resistance is obtained as the number of impacts (N _m ) required to drive the penetration body 12 downward by 20 cm. The N value (N _spt ) of the standard penetration test is equal to half of the number of hits (Nm).
N _spt = 1 / 2N _m (1)

  Since such a dynamic penetration system is lightweight, it can be easily brought into the field without a truck and can be installed in about 5 minutes. Dynamic penetration (raising the hammer 16) can be driven automatically by a hydraulic motor. The depth of the tip of the penetrating body 12 can be obtained by measuring the displacement amount of the rod 10 with respect to a fixed point on the ground surface by the displacement sensor 18. Here, the displacement sensor 18 is a transducer using non-contact magnetostriction.

  The excess pore water pressure value generated in the ground around the penetrating body with the impact penetration is detected by the pore water pressure sensor, and the signal is sent to the ground by the electric cable 20 inserted through the hollow rod 10. The anvil 14 is provided with an acceleration sensor 22, and the detected anvil acceleration signal is used as a trigger for starting data recording. These three types of signals (excess pore water pressure, penetration displacement, and anvil acceleration) are sent to the measuring device 24, immediately digitized by the A / D converter, and digitally recorded by the data recording unit. The recorded data is processed by a personal computer (PC) 26, and the mechanical properties (dynamic strength (penetration resistance) and liquefaction strength (liquefaction potential)) and physical properties (soil determination) of the ground at the depth are measured. Evaluate and display. Thereby, when the rod 10 is continuously hit and penetrated in the depth direction, the penetration amount and the excess pore water pressure behavior are continuously recorded in the depth direction, and the test result can be observed in real time.

  FIG. 2 shows the detailed structure of the penetrating body that performs dynamic pore water pressure measurement. A represents a longitudinal section, and B represents a bottom surface. The penetrating body 12 includes a transducer housing 30 having a conical tip (for example, a vertex angle of 90 degrees) and a cylindrical coupling portion 32 between the transducer housing 30 and the rod 10. The converter housing 30 is provided with communication holes 36 which open at a plurality of conical surfaces (three in this embodiment) and reach a common central hole 34, and the openings are porous hard members (for example, porous ceramics) 38. The pressure receiving surface is closed, and a pressure transducer (for example, a semiconductor pressure sensor) 40 is installed in the center hole 34, and the hole is filled with a pressure transmission medium, and these constitute a pore water pressure sensor. The detection signal of the pressure transducer 40 is sent to the ground by the electric cable 20. In addition, the cylindrical member shown with the code | symbol 42 is a waterproof seal material, and fulfill | performs the function which stops the muddy water which penetrates from the hollow rod 10. FIG. In this embodiment, the protective tube 44 is mounted so as to surround the lower portion of the cylindrical connecting portion 32 and the outer periphery of the converter housing 30. The protection tube 44 has a structure in which the tip (lower end) is a truncated conical surface that is continuous with the conical surface of the converter housing 30, and the inner surface step portion engages with the outer surface step portion of the converter housing 30. Accordingly, when the rod is inserted, the protective tube 44 also penetrates along with the converter housing 30. However, when the rod is pulled out, the protective tube 44 is left behind in the ground.

  FIG. 3A shows an excess pore water pressure at the time of impact penetration, and B shows an example of measurement of the penetration amount. The measured value a of A indicates a positive excess pore water pressure value (maximum excess pore water pressure value) immediately after the impact penetration, and the measured value c is an excess pore water pressure value dissipated to a certain ratio with respect to the measured value a. . The measurement value b measures the time when the excess pore water pressure has dissipated to the measurement value c. The measured value d indicates the difference between the positive excess pore water pressure value (maximum excess pore water pressure value) generated at the beginning of the impact penetration and the negative excess pore water pressure value (minimum excess pore water pressure value) generated thereafter. Thus, depending on the soil quality, a difference appears in the response of the excess pore water pressure and the penetration amount during dynamic penetration. Therefore, on the contrary, it is possible to estimate the soil quality at the depth from the response of the excess pore water pressure and the penetration amount during dynamic penetration.

  Hereinafter, the result of performing a field test at a point in the coastal area along the Tokyo Bay using the apparatus of the present invention will be described. FIG. 4 shows the soil cross-section at the test site, the standard soil penetration test N value, and the unit volume weight. The surface soil layer is an unsaturated deposit, which consists mainly of a loam containing sand and a mixed layer containing several gravel layers with a thickness of 5-10 cm. The N value of the standard penetration test observed in the deposit is 9, indicating relatively loose and uneven soil. The buried layer exists up to a depth of 2.2 m above a natural silt layer under normal coastal clay with shells. The groundwater level is 1.9m below the surface. Sand soil is 2.9m to about 6m deep. The upper part of the sand layer is coarse sand containing a small amount of fine components. On the other hand, the lower part of the sand layer consists of fine sand containing some fine components. Judging from both the N value and the particle size distribution of the standard sand penetration test, the upper layer has a small earthquake liquefaction potential. Also, the lower layer has a medium or small liquefaction potential.

  The excess pore water pressure ratio (Δu / σ′ν) and the typical time course of the penetration displacement are shown in FIG. 5A shows the response of loose and fine sand, and B shows the response of dense and fine sand. From these, it can be seen that there is a significant difference in the magnitude of the maximum excess pore water pressure. Moreover, the time course of the penetration displacement amount is also significantly different. The time course can be divided into two processes: an intrusion process and a subsequent process. Subsequent processes consist of a rebound process and re-penetration that begin immediately after penetration. In the case of dense sand, the intrusion process ends abruptly and enters the subsequent process. In the case of loose sand, there is a delay in the intrusion process. Thus, the time course of the excess pore water pressure ratio (the general pattern and duration from the moment of impact to the significant generation of pore water pressure) varies greatly depending on the soil quality. There are also significant differences in the duration of time.

  FIG. 6 shows the distribution of N values converted by a single strike against the depth. A discontinuity of the converted N value is seen between 4.0 m and 5.0 m. This discontinuity shows a significant change in relative density, indicating the presence of a mixed layer.

FIG. 7 shows a flowchart of data processing for evaluating the liquefaction strength (potential) by the method of the present invention. All the data excluding the unit volume weight are measured values obtained by the device of the present invention. Although details will be described later, the analysis processing is performed according to the processing flow shown in FIG. Incidentally, unit weight is not less error occurs even about 17~18kN / m ³ unless a special soil (if Clay system 17 kN / m ³ approximately, if sandy soil system 18 kN / m ³ Therefore, there is no particular problem even if it cannot be measured.

The distribution of the maximum excess pore water pressure Δu _max with respect to the depth is shown in FIG. They roughly agree with the effective upper pressure (σ'ν) under the groundwater level. On the other hand, those above the groundwater level are greater than the total top pressure (σν).

Since the method of the present invention is a dynamic penetration test method, the depth of the groundwater level is unknown unless pre-boring is performed. However, it is important to know the groundwater level in the liquefaction problem. From this perspective, a new index parameter (I _WL ) was introduced. that is,
I _WL = Δu _max / σν (2)
Defined by
Δu _max is the maximum excess pore water pressure, and σν is the total loading pressure. The distribution of the new index I _WL with respect to the depth is shown in FIG. The index I _WL decreases rapidly to 1.0 m and appears to be less than 1.0 at a depth of 1.9 m. The groundwater level can be estimated by this index _IWL .

When the groundwater level estimated as described above is given, the effective upper pressure (σ′ν) can be calculated. The effective upper pressure is an effective upper pressure obtained by subtracting the buoyancy of water. FIG. 10 is a plot of the maximum excess pore water pressure ratio (Δu _max / σ′ν) under the groundwater level against the depth. Surface deposits and silt layers between 1.9 m and 2.9 m show large values.

Ｉ_TIの値を計算する手順を図１１に示す。変数Ｓ_A 、Ｓ_B およびＳ_C は、過剰間隙水圧比率時間経過の積分によって累積された絶対値である。緩く細かい砂について計算された指標Ｉ_TIの大きさは、シルトのそれよりも小さい。 In order to investigate the influence of soil quality on liquefaction potential, a new index I _TI is defined by the following equation. This is a time integration of the excess pore water pressure ratio (Δu / σ′ν) from the hammer impact to the end of the penetration process (t ₁ ).

The procedure for calculating the value of I _TI is shown in FIG. The variables S _A , S _B and S _C are absolute values accumulated by integration of the excess pore water pressure ratio over time. The magnitude of the index I _TI calculated for loose fine sand is smaller than that of silt.

The distribution of I _TI with respect to depth is shown in FIG. The magnitude of the index I _TI in the silt layer was found to be greater than that in the sand layer. This trend can be explained through changes in soil properties such as permeability and dilatancy.

（４）式は、Ｉ_TIが１未満の場合にはΔｕ_max ／σ′νがそのままＩ_TIになるが、Ｉ_TIが１以上の場合には材料補正係数として、Δｕ_max ／σ′νをＩ_TIで除算する意味である。砂の場合に、Ｉ_TIが負になったり、限りなく０に近づいてしまうため、Ｉ_TIが１未満の場合にはＩ_TIで除算しないように工夫したものである。 A new index _ILP is introduced to evaluate the liquefaction strength (potential) at each step of dynamic penetration. The new index I _LP was defined as the maximum excess pore water pressure ratio (Δu / σ′ν) divided by the time integral index I _TI .

In the equation (4), when I _TI is less than 1, Δu _max / σ′ν becomes I _TI as it is, but when I _TI is 1 or more, Δu _max / σ′ν is set as a material correction coefficient. I means to divide by _TI . In the case of sand, since I _TI becomes negative or approaches 0 as much as possible, when I _TI is less than 1, it is devised not to divide by I _TI .

The distribution of the index I _LP with respect to the depth is shown in FIG. _ILP discontinuities are found in layers between 4.0 and 5.0 m below the ground surface. It can be seen that this layer is a mixed layer. Therefore, the liquefaction potential index I _LP directly indicates the degree of liquefaction rate. The threshold for this indicator, which indicates whether the soil layer can be liquefied, was 1.0 in this experiment. The depth at which this index exceeds 1.0 is located between 4.0 m and 4.2 m, which corresponds to the top of fine sand.

The following conclusions can be drawn from a series of dynamic penetration test results.
(A) The distribution of N values transformed by a single blow can be used to identify the homogeneity of the layers and find the mixed layer.
(B) The time course of excess pore water pressure during dynamic intrusion is significantly affected by N value and soil quality.
(C) The three new indicators introduced for evaluating the liquefaction rate can be used for the determination and evaluation of the following items.
I _WL : Estimation of groundwater level I _TI : Soil discrimination I _LP : Soil liquefaction strength (liquefaction potential)

  As described above, according to the method of the present invention, the interstitial water pressure sensor is incorporated into the penetrating body at the tip of the rod, the rod is hit to cause the penetrating body to penetrate into the ground, and the ground touching the penetrating body in accordance with the amount of penetration and the penetrating penetration. By measuring the generated excess pore water pressure, it is possible to obtain not only the dynamic strength of the ground at the depth but also the liquefaction strength from the obtained penetration amount and the excess pore water pressure behavior, and analyze the soil quality.

Explanatory drawing which shows an example of the test apparatus used with the method of this invention concerning this invention. Explanatory drawing which shows an example of the response of the excess pore water pressure at the time of dynamic penetration, and penetration amount. Explanatory drawing which shows an example of the chamber test result using uniform sand. Soil profile at the site where the field test was conducted. The time course figure of the typical example of excess pore water pressure ratio and penetration displacement. The distribution map of N value converted by the single blow to the depth. The flowchart of the data processing which evaluates the liquefaction intensity | strength by this invention method. Distribution diagram of maximum excess pore water pressure with respect to depth. The distribution map of index IWL with respect to depth. The figure which shows the relationship of the maximum excess pore water pressure ratio under the groundwater level with respect to depth. Explanatory drawing which shows the procedure which calculates the value of parameter | index ITI. The distribution map of index ITI with respect to depth. The distribution map of index ILP with respect to depth.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Rod 12 Penetration body 14 Anvil 16 Hammer 18 Displacement sensor 20 Electrical cable 22 Acceleration sensor 24 Measuring device 30 Converter housing 38 Porous hard member 40 Pressure transducer 44 Protective tube

Claims (5)

A pore water pressure sensor is installed in the penetrating body at the rod tip, and the rod is struck to penetrate the ground into the ground, and the dynamic strength of the ground at the depth of the penetrating body is determined from the amount of penetration, and the amount of penetration immediately after the impact is penetrated. Detecting the time course and the time course of excess pore water pressure generated in the ground in contact with the penetrating body, evaluating the fine grain content of the penetrating body depth from the response of the obtained penetration amount and the response of the excess pore water pressure, ground survey method according excess pore water pressure measured at the time of hitting intrusions and performing soil discrimination. Perform continuous impact penetration in the depth direction, detect the maximum excess pore water pressure value at the penetration depth from the response of the excess pore water pressure immediately after the impact penetration, and (maximum excess pore water pressure value) / (total top load pressure) A value is obtained and used as a groundwater level index at the penetration depth, and a distribution in the depth direction of the groundwater level index is obtained. The groundwater level index decreases from the surface side in the depth direction and is substantially constant at less than 1.0. The ground investigation method according to claim 1, wherein the depth of convergence to the value of is estimated as a groundwater level . An apparatus for carrying out the ground survey method of claim 1 or 2, wherein the penetration member to be attached to the rod end, the penetrating body embedded or is in the excess pore water pressure occurring in the soil in contact with the penetrating body Detecting pore water pressure sensor, displacement sensor detecting rod penetration amount, acceleration sensor detecting rod impact, and excess pore water pressure detection signal and penetration displacement detection at impact penetration using acceleration signal as data recording trigger It is equipped with a data recording unit that records the passage of time of the signal and a dynamic penetration device that strikes and penetrates the rod, and strikes the rod to penetrate the penetration body into the ground, and the response and impact of the penetration amount at the time of the penetration performing excess pore water pressure measured at the time of hitting penetration, characterized in that the response of the generation and dissipation of excess pore water pressure occurring in the soil in contact with the penetrating body such that the data recording with the intrusion Panel survey device.   The penetrating body includes a transducer housing having a conical tip portion, and a cylindrical coupling portion between the transducer housing and the rod. The transducer housing opens at a plurality of positions on a conical surface and communicates with the central hole. The ground survey device according to claim 3, wherein a hole is provided, the opening is closed by a porous hard member to become a pressure receiving surface, a pressure transducer is installed in the center hole, and the hole is filled with a pressure transmission medium. . The amount of penetration at the time of impact and the detection signal of the pore water pressure sensor are digitized by A / D conversion and digitally recorded by the data recording unit. When the rod is continuously impacted in the depth direction, it penetrates continuously in the depth direction. The ground investigation device according to claim 3 or 4, wherein the response of the amount and the excess pore water pressure is recorded.

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