JP4071988B2 - Ground survey method using S-wave amplitude associated with impact penetration - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ground investigation method for estimating the penetration resistance value from the maximum amplitude of S wave vibration generated at the time of penetration and evaluating the mechanical characteristics of the ground. This technique is useful for obtaining a penetration resistance value corresponding to the N value of the ground, which is a design guideline for constructing a structure, etc. simply and efficiently economically.
[0002]
[Prior art]
When constructing a structure or the like on a ground made of natural or artificial objects, it is necessary to grasp the mechanical characteristics of the ground in advance and to perform an appropriate structure design based on that. For this purpose, various ground survey methods have been proposed and put into practical use.
[0003]
As a typical method for investigating the strength 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, it is lowered to the drilled borehole bottom, and the rod is hit on the ground by a free fall (drop height 76 cm) of a hammer (63.5 kg). The number of strikes (N value) required to penetrate 30 cm between 15 and 45 cm from the bottom of the hole 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.
[0004]
However, a boring machine and a boring pump are required for drilling the test hole, and mud water (construction sludge) is forced to be used to stabilize the hole wall during drilling. The survey work such as the work and the 15 cm pre-striking work is complicated, requires experience, and takes time to investigate. Such a standard penetration test is essentially inconsistent in impact energy due to the shape of the hammer and knocking head, the deflection of the rod, the condition of the drilling hole, the way of working, and other conditions, and the variation in measured values Easy to grow. Moreover, since the measurement interval is wide, it is difficult to obtain detailed ground information in the depth direction. Therefore, recently, with the increase of design load (especially seismic external force) and the introduction of the limit state design method (reliability design), various errors in which the N value has existed until now have become a big problem.
[0005]
One of the sounding methods that replaces the standard penetration test method is a dynamic penetration test method (such as automatic ram sounding). This is a method in which a rod with a cone attached to the tip is driven continuously into the ground by a free fall (falling height 50 cm) of a hammer (63.5 kg), and the number of hits (Nd 'value) per 20 cm penetration length is obtained. is there. As the rod penetrates, frictional resistance against the soil is generated on the rod peripheral surface. Therefore, the torque Mv acting on the rod is measured for each predetermined penetration amount, and the following correction formula Nd = Nd′−a · Mv (however, a is a coefficient)
To obtain the penetration resistance value Nd at the tip. The Nd value obtained in this way is evaluated and used as Nd≈N.
[0006]
[Problems to be solved by the invention]
In a dynamic penetration test method that does not require boring, if a strata with a high penetration resistance such as gravel is present, it will take time to hit and may not be able to penetrate. Furthermore, the influence of the friction between the rod connected to the tip cone and the surrounding ground affects the penetration resistance value Nd of the tip. That is, since the energy hit on the surface of the rod is not completely transmitted to the cone at the tip due to loss due to friction between the rod and the surroundings, the obtained penetration resistance value is overestimated. For this reason, as described above, in the dynamic penetration test, the maximum torque is measured by rotating the rod for each penetration test, and this is corrected as the frictional force between the rod and the surrounding ground. The procedure for determining the resistance value must be taken. Such a procedure impairs workability and affects data reliability.
[0007]
An object of the present invention is to provide a method capable of correctly evaluating the ground strength while having a simple measurement system with little variation in measurement data, good workability.
[0008]
[Means for Solving the Problems]
The inventors of the present invention installed a seismometer on the ground surface, conducted a hit penetration test, detected and analyzed the elastic wave generated by the hit penetration, and found that the maximum amplitude of the S wave (shear wave) and the N value were between We found that there is a correlation. For example, the amplitude of the S wave increases in the case of sandy soil (sand or gravel) having a large N value on the ground (measuring soil), and the amplitude of the S wave decreases in the case of viscous soil with a small N value. . Since the S wave amplitude depends on the magnitude of the striking energy, it is a condition that the striking energy is stable. However, the penetration resistance can be directly evaluated by measuring the maximum amplitude of the S wave. The present invention has been completed based on such knowledge.
[0009]
The present invention hits and penetrates a rod with a penetrating body attached to the tip, and detects an elastic wave generated therewith with an S wave sensor installed on the ground surface, and determines the penetrating body depth and the maximum amplitude of S wave vibration, The S wave maximum amplitude is corrected for attenuation according to the elastic wave propagation distance from the penetrating body to the S wave sensor, and corresponds to the S wave maximum amplitude and N value by the impact penetration test at at least one point in the investigation site. Obtaining a relational expression with the penetration resistance value, and estimating the N value at the penetration depth by using the relational expression based on the maximum S- wave amplitude obtained at the time of impact penetration at another point of the investigation site. This is the survey method.
[0010]
In the present invention, the detection signal of the S wave sensor is digitized by A / D conversion and digitally recorded by the data recording unit, and the digitized S wave amplitude is automatically processed by a computer to automatically process the maximum amplitude of the S wave. The penetration resistance value at each penetration depth is analyzed. In this method, by continuously hitting in the depth direction, the penetration resistance value of the ground can be obtained and displayed continuously in the depth direction. The S wave sensor to be used is preferably a speedometer type seismometer capable of detecting two components, a vertical component and a horizontal component.
[0011]
The installation position of the S-wave sensor is arbitrary, but it is preferable that the S-wave sensor is located at a distance 0.5 to 1.5 times the survey plan depth from the impact penetration point, and the level is about the same as the survey plan depth. It is best to set it at a distance. This is because if it is too close to the impact penetration point, work noise is likely to enter, and if it is too far, it is difficult to detect the direct wave. Therefore, it is better to install the S-wave sensor at a more distant position as the survey plan depth becomes deeper. It is preferable to change. One S-wave sensor may be used, but it goes without saying that a plurality of S-wave sensors may be arranged at intervals.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
A rod with a penetrating body at the tip is struck into the ground with a constant striking energy by the natural or forced drop of a hammer. On the other hand, an S wave sensor is installed on the surface of the ground at an arbitrary distance (preferably a distance similar to the penetration depth) from the impact penetration point so that the S wave amplitude can be observed. The S wave vibration generated with the impact penetration is detected by the S wave sensor. The S wave detection signal is digitized by A / D conversion, digitally recorded by the data recording unit, and the digitized S wave amplitude is processed by a personal computer to automatically obtain the maximum S wave amplitude, and each intruder Estimate the penetration resistance value at the depth and evaluate the mechanical properties of the ground.
[0013]
According to the method of the present invention, it is possible to continuously hit in the depth direction and obtain the ground penetration resistance value continuously in the depth direction. For hitting penetration, a free fall of a hammer may be used as in the prior art, but a penetrating device (for example, a hydraulic hammer, a diesel hammer, etc.) having a great hitting ability can also be used. As a result, the work efficiency of the impact intrusion is increased, and it is possible to apply it to a ground having a high penetration resistance such as a gravel layer or a ground with a high degree of consolidation and a large depth.
[0014]
The observed elastic wave waveform is recorded and stored in the data recording unit. In addition, it is preferable to record in the header various information related to the relative position information, gain information, and other data recording conditions between the penetrating body and the S wave sensor at this time because subsequent analysis and result display are facilitated. .
[0015]
The S wave sensor is preferably installed on the ground surface because of its good workability, but it can also be installed in an existing geological survey boring hole or the like. Elastic waves are generated when the penetrating body at the tip breaks the ground and penetrates into the ground by hammering. This elastic wave propagates through the surrounding ground and reaches the S wave sensor. Of course, if there is a stratum boundary, reflected waves, refracted waves, and the like reach the S wave sensor, but the maximum amplitude when the reaching wave is assumed to be a direct wave contributes to the penetration resistance value of the ground. The relative position between the penetrating body at the tip and the S wave sensor changes for each impact, and the attenuation varies depending on the propagation distance of the elastic wave. Therefore, it is necessary to correct the propagation distance attenuation for the amplitude. That is, since the propagation distance from the penetration body depth to the S wave sensor can be calculated, the attenuation of the amplitude may be compensated according to the propagation distance. This correction can be automatically performed by a personal computer. The method for observing the S-wave amplitude of the present invention can be sufficiently applied if the depth is about 50 m or less.
[0016]
If the ground having a known penetration resistance value is previously correlated with the amplitude of the elastic wave, the penetration resistance value can be directly determined from the amplitude of the elastic wave in subsequent measurements. These series of calculation algorithms can be easily incorporated in a computer, and can be automatically analyzed from reading the maximum amplitude to calculating the penetration resistance value.
[0017]
Since the method of the present invention does not require correction work of the rod and the surrounding friction as in the conventional dynamic penetration test, the investigation can be continuously performed in the depth direction, and hence the penetration resistance value in the depth direction. The distribution of can also be obtained continuously. Thereby, compared with the conventional method, dense penetration resistance value distribution can be calculated | required in the depth direction, and a highly accurate ground evaluation can be performed.
[0018]
【Example】
FIG. 1 is an explanatory diagram showing an example of a measurement system used for carrying out the method of the present invention. As the impact penetrating device itself, a conventionally used device such as a standard penetrating test or auto-match ram sounding can be used as it is. Of course, any other impact penetrating device may be used, but it is important that the impact energy is stable. In FIG. 1, an example using an auto-matchock ram sounding device having a configuration in which a cone 12 is attached to a tip (lower end) of a rod 10 and a hammer 16 is freely dropped from a predetermined position to a knocking head 14 at an upper end to allow a hammer to penetrate. Show.
[0019]
An S wave sensor 22 is installed on the ground surface separated from the impact penetrating device 20 by an arbitrary distance L. The installation position is optimal at a point that is approximately the same distance as the penetration depth D, but the depth changes due to the impact penetration, so L is a point that is separated by about (0.5 to 1.5) D. It's okay. However, it is necessary to be able to correctly grasp the relative position between the cone and the S wave sensor, and the penetration depth D is also measured for each impact penetration. The S wave sensor 22 may be a seismometer that detects two components, a vertical component and a horizontal component, and here, a speedometer type sensor is used. The output signal of the S wave sensor 22 is guided to the measuring device 24, where it is amplified, A / D converted, and recorded in the data recording unit. The digitized S-wave amplitude is processed by a personal computer (PC) 26, the maximum amplitude of the S-wave is automatically obtained, and the penetration resistance value at each hitting point (penetration depth) is analyzed. Displayed.
[0020]
When the hammer 16 strikes the knocking head 14, the cone 12 at the tip breaks the ground and penetrates into the ground. At that time, an elastic wave is generated. This elastic wave propagates through the surrounding ground and reaches the S wave sensor 22 installed on the ground surface. If there is a stratum boundary as well as a direct wave directly from the penetrator position to the S wave sensor, there are also reflected waves and refracted waves. Since the relative position between the tip cone 12 and the S-wave sensor 22 changes with each hit, the amount of attenuation differs depending on the propagation distance of the elastic wave, so the distance between the cone 12 and the S-wave sensor 22. Is used to correct the distance attenuation of the maximum amplitude of the S wave to reach. In this method, if the penetration depth is shallower than about 50 m, it is possible to estimate the N value from the maximum amplitude regardless of the transmission path of the S wave.
[0021]
FIG. 2 shows an example of an elastic wave observation result when using a standard penetration tester and using the impact as a vibration source. A shows the N value by the standard penetration test, B shows the vibration waveform detected by the seismometer installed 20 m away from the standard penetration test machine installation position, and C shows the N value and amplitude obtained by the standard penetration test. The comparison with the S wave maximum amplitude after the distance attenuation correction is shown. From this result, it can be confirmed that there is a good correspondence tendency between the N value and the amplitude of the elastic wave. That is, the amplitude of the S wave is large in the case of sand or gravel with a large N value of the measurement target soil, and the amplitude of the S wave is small in the case of clayey soil with a small N value of the ground.
[0022]
Next, FIG. 3 shows the relationship between the dynamic penetration resistance Nd value and the S wave amplitude in the dynamic penetration test (automatic ram sounding). This is a ram sounding test and the amount of penetration of the rod and the S wave generated at that time are measured for each impact. Then, the Nd value (penetration resistance value at the tip end corrected for the influence of the rod peripheral surface friction due to the torque) converted to the number of hits per 20 cm is obtained from the penetration amount per hit, and the S wave due to the hit is obtained. The maximum amplitude was corrected for distance attenuation. A is the case below the groundwater level (unsaturated), and B is the case below the groundwater level (saturated). In FIG. 3, the data of the gravel layer near the depth of 11 to 17 m is excluded.
[0023]
From FIG. 3, it can be seen that the relationship between the Nd value and the S wave amplitude differs between unsaturated soil and saturated soil, but there is almost no difference due to soil quality. This is because rapid destruction of soil due to impact penetration depends on undrained shear strength regardless of soil permeability. Therefore, it is important to understand the groundwater level that is the boundary. FIG. 4 shows a result of estimating the Nd value from the S wave amplitude using the relational expression between the S wave amplitude and the Nd value obtained from FIG. 3 and comparing it with the Nd value and the N value as the Ns value. As can be seen from FIG. 4, the Ns value, the Nd value, and the N value are generally well matched. At a depth of 11 to 17 m corresponding to the gravel layer, the Nd value of the dynamic penetration test result and the N value of the standard penetration test result are obtained. It can be seen that indicates an excessive value.
[0024]
The actual investigation procedure is shown in FIG. First, an application test of the impact penetrating device to be used is performed as necessary. The relationship between the respective penetration resistance values of the hitting penetration device to be used and the standard penetration test is clarified in the ground whose geology and N value are known by the conventional standard penetration test. This is done at one or more locations. If the impact penetrating device to be used is a device that has already been correlated with the N value, such as auto-match ram sounding, this application test need not be performed. This application test does not necessarily have to be performed at the survey site, and may be performed at any test site.
[0025]
Next, a preliminary survey is conducted at the survey site. This preliminary investigation is an investigation for obtaining the relationship between the impact penetrating device to be used and the maximum amplitude of the elastic wave (S wave) generated by the impact penetrating, and is performed at least at one location. In this case, if each relationship is already clear, as in auto-match crum sounding, it is only necessary to confirm that the relationship is applicable. That is, when the geology is known in advance at the survey site (when the standard penetration test is performed), the preliminary test is performed in the vicinity to achieve the purpose of the application test and the preliminary survey. it can. When the standard penetration test is not performed, S-wave observation may be performed during the standard penetration test. As a result, it is possible to collect soil samples simultaneously with the sampler.
[0026]
And this survey is conducted. In this investigation, the above-mentioned impact penetrating device is used, and the maximum amplitude is obtained by continuously measuring the elastic wave (S wave) at each penetration depth. It is not necessary to measure the torque associated with the impact penetration. After the penetration test, measure the water level in the penetration hole and check the groundwater level. This work is repeated for necessary parts in the survey site.
[0027]
Finally, organize the results. The penetration resistance value obtained by the S wave amplitude is shown for each depth (penetration curve), and the geology is assumed from the distribution state of the penetration resistance value. Since this method can be carried out continuously in the depth direction, the distribution of penetration resistance values in the depth direction can be obtained continuously. Thereby, compared with the conventional method, dense penetration resistance value distribution can be calculated | required and it becomes possible to perform an accurate ground evaluation (estimate division and N value estimation).
[0028]
【The invention's effect】
As described above, the present invention is a method of measuring the maximum amplitude of elastic waves together with the depth of the penetration body by installing an S wave sensor at an arbitrary distance away from the impact penetrating device. Investigate economically. Further, since the amplitude information of the elastic wave is used, the influence of data variation due to the friction between the rod and the surroundings is small, and the strength of the ground can be correctly evaluated regardless of the soil quality.
[0029]
The method according to the present invention is not limited to the natural fall of the hammer, so that it is possible to use a penetrating device having a large striking penetration capability, such as a hydraulic hammer, and it can be applied to the ground having a large penetration resistance and a large depth. In addition, since correction work for the rod and the surrounding friction is not required, the work efficiency can be greatly improved, and continuous dense measurement is possible.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an example of a test apparatus used in the method of the present invention.
FIG. 2 is an explanatory diagram showing a comparison relationship between an N value and a S-wave maximum amplitude in a standard penetration test.
FIG. 3 is an explanatory diagram showing a relationship between an Nd value and an S wave amplitude in a dynamic penetration test.
FIG. 4 is an Ns value estimated from a converted Nd value and an S wave amplitude, and a depth distribution diagram of the N value.
FIG. 5 is an explanatory diagram showing an investigation procedure.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Rod 12 Cone 14 Knocking head 16 Hammer 20 Impact penetrating device 22 S wave sensor 24 Measuring instrument 26 Personal computer

Claims (5)

A rod fitted with a penetrating member to the tip struck penetrate the ground, is detected by the S-wave sensor disposed an elastic wave to the surface to be generated with it, determine the maximum amplitude of the penetration body depth and S-wave vibration, S-wave maximum amplitude Is to perform attenuation correction according to the elastic wave propagation distance from the penetrating body to the S wave sensor, and the penetration resistance value corresponding to the S wave maximum amplitude and the N value is determined by the impact penetration test at at least one point in the investigation site. A ground survey method characterized in that the N value at the penetration depth is estimated by using the relational equation based on the maximum S- wave amplitude obtained at the time of impact penetration at another point of the survey site . The detection signal of the S wave sensor is digitized by A / D conversion, digitally recorded by a data recording unit, and the digitized S wave amplitude is processed by a computer to automatically obtain the maximum S wave amplitude. The ground investigation method according to claim 1, wherein the penetration resistance value at the penetration depth is analyzed. The ground investigation method according to claim 2, wherein the impact intrusion is continuously performed in the depth direction, and the penetration resistance value of the ground is continuously obtained in the depth direction and displayed. The ground survey method according to any one of claims 1 to 3, wherein the S wave sensor is a seismometer that detects two components of a vertical component and a horizontal component. The ground survey method according to claim 4, wherein the horizontal distance from the impact penetration point of the installation point of the S wave sensor is set to a range of 0.5 to 1.5 times the planned depth of survey.

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