JP3052224B2 - Measuring method of allowable bearing capacity of ground - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring an allowable bearing capacity of a ground, and more particularly to a method for measuring a ground surface using a Rayleigh wave.

[0002]

2. Description of the Related Art Among the methods for investigating the soil ground, there are two methods, which are referred to as a physical exploration method, a borehole exploration method and a surface exploration method. The surface exploration method is a logging method that measures the physical quantity of the ground on the ground surface without performing boring and analyzes the measurement results to grasp the soil layer composition, and is widely used for rock ground, It is said that the measurement accuracy is low on soil ground and that there is not much experience. on the other hand,
The borehole exploration method is a well-logging method that measures the physical quantity of the ground in situ using a borehole, and is widely used to measure the permissible bearing capacity of soil because it can be measured continuously. Have been.

[0003] Among the borehole exploration methods, a method widely used is a velocity logging method for measuring the velocity of an elastic wave, a P wave and an S wave.
This is a PS logging method that measures waves. When the speed logging method is subdivided, there are a speed logging method based on falling or blasting of a weight, and a speed logging method using a sound wave pulse. The PS logging method includes a weight falling method and a plate knocking method. In all of these cases, the geophone is placed in the borehole. In the velocity logging method, the oscillation source is located in the hole together with the geophone, and in the PS logging method, the oscillation source is located on the ground surface. .

[0004]

However, in any of the above-described methods, a large amount of cost is required to form a boring hole, and there are also troublesome problems such as maintenance of the wall surface in the hole. . In addition, in the velocity logging, the measurable depth is shallow, and the installation space on the surface of the ground for the measured depth is, for example, 50 m for a depth of 10 m.
There is a problem that implementation is difficult depending on the situation in consideration of the effect of vibration on the surrounding environment. Further, there is a problem that the generated P-wave and S-wave are greatly attenuated, and it is not easy to generate vibrations sufficient for measurement.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and an improved surface exploration method which does not require a boring hole can solve the above-mentioned drawbacks of the inside hole exploration method. It is an object of the present invention to provide a method for measuring the allowable bearing capacity of the ground, which can obtain the same measurement result as the method.

[0006]

The inventor of the present application has developed and proposed various methods for measuring the propagation speed of surface waves (Rayleigh waves) generated on the ground. These include Tokuhei 2-13
No. 249, Tokuhei 3-50993, Tokuhei 4-118
No. 276. According to the development results of these methods, Rayleigh waves can be generated very easily on the surface of the earth, the wave energy is large, the attenuation is small, and the area to be measured can be small. all right.
Therefore, the inventor of the present application has conceived of measuring the allowable bearing capacity of the ground based on the propagation speed of the Rayleigh wave applied to the ground, and has developed the following measuring method as a result of earnest research.

The method for measuring the allowable bearing strength of the ground according to the present invention measures the Rayleigh wave velocity Vr on the ground to be measured, and calculates the S wave velocity Vs using the Rayleigh wave velocity Vr and a predetermined coefficient k.

Vr = k · Vs, and the uniaxial compressive strength q of the ground based on the S-wave velocity Vs
u

(Equation 6) And the adhesive strength C of the ground below the bottom of the foundation

[Mathematical formula-see original document] Obtained as C = qu / 2, and using the adhesive strength C, the allowable ground force qa of the ground
To

Qa = (α · C · Nc + β · γl · B · Nr) / 3 (where, α and β are basic shape factors, Nc,
Nr is the bearing capacity coefficient, γl is the unit volume weight of the ground under the base bottom, and B is the minimum width of the base bottom.

Further, in the method for measuring the allowable bearing strength of the ground according to the present invention, the measurement may be performed with the predetermined coefficient k being 0.8 to 0.99.

Further, in the method for measuring the allowable bearing capacity of the ground according to the present invention, the predetermined coefficient k may be set to 0.9.

【Example】

FIG. 1 is a conceptual diagram of a Rayleigh wave velocity measuring system using an exciter for carrying out the present invention. In FIG. 1, 1 is an arithmetic unit, 2 is a seismograph, 3 is an exciter signal, 4 is an exciter, 5 is a first exciter, and 6 is a second exciter. In this system, a vibration signal is generated and amplified by a vibration signal device 3, and a vibration device 4 is vibrated in a vertical direction to generate a Rayleigh wave, which is one of surface waves, on the ground surface. First
The time passing between the geophone 5 and the second geophone 6 is defined as T, and the average speed Vr bar of the Rayleigh wave is determined by the time T and the distance S between the first and second geophones 5 and 6.

## EQU9 ## Determined as Vr bar = S / T. Further, the speed of the Rayleigh wave can be similarly obtained by varying the excitation frequency f, and the dispersion characteristic of the Rayleigh wave in the depth direction can be obtained. Assuming that the measurement depth is D and the wavelength of the Rayleigh wave is L,

D = L / 2 = Vr bar / 2 · f That is, most of the Rayleigh waves travel in a region of approximately one wavelength depth in the semi-infinite elastic body, and the average property in this region is considered to be approximately equal to the property at a half wavelength depth. This is because you can do it.

FIG. 2 is a conceptual diagram of a system for performing S-wave logging measurement using a borehole used for data collection for studying the present invention. In FIG. 2, 11 is a recorder, 12
Is a power supply, 13 is a ground vibration receiver, 14 is a three-component vibration receiver, 1
5 is a pressurizing device, and 16 is a slap plate. This system receives an S-wave generated by a beating plate 16 which is an S-wave exciter by a ground vibration receiver 13 and a three-component vibration receiver 14 in a boring hole 17, and records the waveforms thereof with a recorder 11.
And the S-wave velocity Vs is determined by the arrival time T and the distance S of the three-component geophone 14 in the borehole 17 from the ground.

Vs = S / T The S-wave structure is measured while gradually lowering the three-component geophone 14 every 1 m, for example, while changing the distance S.

The following method is used to determine the allowable ground force qa of the ground using the S-wave velocity Vs measured by the system shown in FIG. That is, the uniaxial compressive strength qu of the ground to be measured is

(Equation 12) And the adhesive strength C of the ground below the bottom of the foundation

## EQU13 ## C = qu / 2 is obtained, and the allowable supporting force q of the ground is obtained by using this adhesive force C.
a

## EQU14 ## qa = (α · C · Nc + β · γl · B · Nr) / 3 (where α and β are the shape factors of the foundation, Nc and Nr are the bearing force factors, and γl is the ground under the foundation bottom surface. Unit weight, B
Is the minimum width of the bottom of the foundation). As shown in Table 1 below, the shape factor of the foundation is

[Table 1] The bearing force coefficients Nc and Nr are obtained as functions of the internal friction coefficient φ as shown in Table 2 below.

[Table 2]

The Rayleigh wave velocity Vr and the S wave velocity Vs
If we can identify some proportional relationship between

By setting Vr = k · Vs, it can be seen that the allowable ground force qa of the ground can be obtained only by measuring the Rayleigh wave velocity Vr without measuring the S-wave velocity Vs.

That is, the present inventor has found that the velocity V of the Rayleigh wave
Assuming that r is proportional to the velocity Vs of the S wave and that the proportionality constant k varies depending on the Poisson's ratio of the ground, the velocity Vr of the Rayleigh wave is measured by the system of FIG. The S-wave velocity Vs was measured by the plate tapping method described above, and their relationship was examined.

The first ground of the ground (lowland of alluvial soil) subjected to this investigation had a Poisson's ratio of 0.48 for the upper layer and 0.45 for the lower layer. The number of measurement points is five, and the S-wave logging is performed using the system of FIG. 2 using a borehole up to a depth of 20 m to determine the S-wave velocity for each layer. At the same time, the Rayleigh-wave logging is performed by the system of FIG. In the same way, the Rayleigh wave velocity of each layer was obtained. The number of samples was 32, and the measurement results are shown in FIG.

The graph of FIG. 4 is obtained from the result of FIG. 3, and when the Poisson's ratio of the ground is 0.45 to 0.48,

Vr ≒ 0.885 Vs-3.362 was obtained.

Similarly, the measurement results at the first to sixth different points on the second ground are shown in FIGS.
6 is shown. The first to fourth measurement points are alluvial lowlands, and the fifth and sixth measurement points are alluvial plateaus. At the first point, from the data in Figs.

Vr ≒ 0.9Vs At the second point, from the data in FIGS.

Vr ≒ 0.83Vs At the third point, the data shown in FIGS.

Vr４0.99Vs At the fourth point, the data shown in FIGS.

At the fifth point, Vr で は 0.93Vs is obtained from the data shown in FIGS.

At the sixth point, Vr６0.8Vs is obtained from the data shown in FIGS.

Vr ≒ 0.96 Vs was obtained. That is, the Poisson's ratio of the ground is 0.45 to 0.4.
48, the most common The fastest Rayleigh wave velocity on alluvial ground is 0.9
9 Vs and the slowest speed was 0.8 Vs.

By substituting these results into the above relational expression between Vs and qu,

## EQU23 ## In general, higher-order waves in which the speed of Rayleigh waves increases are mixed with the ground in a multilayered structure, the ground becomes softer as it goes to the lower layer (in this case, the fundamental wave disappears), and it is compared with the lower layer. Occurs when there is a hard layer. The generation of higher-order waves in the S-wave logging also occurs when the ground has a multilayer structure, when pure S waves are not generated, or when the power is weak. Therefore, when adopting the constant k, it is desirable to know the structure of the ground and adopt a numerical value suitable for it. However, according to many measurement results performed by the present inventors, it is considered that k is fixed at 0.9. There is no practical problem in doing so.

Next, the criterion for determining the ground strength based on the Rayleigh wave will be described. First, the basic criteria are: Is there a possibility of differential settlement? (1) Is the thickness of the embankment constant? (2) Is the flow of the stratum under the embankment parallel? (Within 5 m depth) (3) Is there any local soft layer (homogeneous)? (5m depth
Within) 2. Is the velocity of the formation below the bottom of the foundation 85 m / sec or more and the layer thickness is 2 B or more (is the long-term allowable bearing capacity 3 t / m ² or more)? 3. Whether the speed of the formation below the bottom of the foundation is 105 m / sec or more and the layer thickness is 4 B or more (long-term allowable bearing capacity is 5 t / m ²
Is there more?) For the determination of 1 above, for example, curves showing the relationship between the depth D and the average Rayleigh wave velocity Vr bar shown in FIGS. FIG. 17 is a D-Vr bar curve when the flow of the stratum is parallel, and FIG. 18 is a curve when the flow of the stratum is inclined and uneven settlement exists.

Next, the building foundation determination standard is as shown in FIG. In the figure, X is a zone that can be said to be relatively safe foundation work, Y
Is a zone that can be said to be safe if careful foundation work is performed.
Is a zone where uneven settlement is a concern. It should be noted that whether or not the rolling force can improve the supporting force in the Y zone requires the judgment of an expert. The figure B is the width of the foundation for 5t / m @ 2, also B 'represents the width of the foundation for 3t / m ^2, respectively 400 cm, a 600 cm.

[0021]

As described above, the method for measuring the allowable bearing capacity of the ground according to the present invention is as described above. Therefore, it is possible to measure the allowable bearing capacity at the same level as in the borehole exploration method without the need for a boring hole. Has the effect of becoming

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a Rayleigh wave velocity measuring system using an exciter for carrying out the present invention.

FIG. 2 is a conceptual diagram of a system for performing S-wave logging measurement using a borehole used for data collection for studying the present invention.

FIG. 3 is a diagram showing measurement data of a first survey ground measured using the system of FIGS. 1 and 2;

FIG. 4 is a graph showing the results of regression analysis of Vr and Vs obtained from the results of FIG.

FIG. 5 is a diagram showing measurement data at a first point on a second survey ground measured using the system of FIGS. 1 and 2;

FIG. 6 is a graph in which Vr / Vs values obtained from the results of FIG. 5 are plotted.

FIG. 7 is a diagram showing measurement data at a second point on a second survey ground measured using the system of FIGS. 1 and 2;

FIG. 8 is a graph in which Vr / Vs values obtained from the results of FIG. 7 are plotted.

FIG. 9 is a diagram showing measurement data of a third point on the second survey ground measured using the system of FIGS. 1 and 2;

FIG. 10 is a graph in which Vr / Vs values obtained from the results of FIG. 9 are plotted.

FIG. 11 shows a second example measured using the system shown in FIGS.
It is a figure which shows the measurement data of the 4th point of an investigation ground.

FIG. 12 is a graph in which Vr / Vs values obtained from the results of FIG. 11 are plotted.

FIG. 13 shows a second example measured using the system shown in FIGS.
It is a figure which shows the measurement data of the 5th point of an investigation ground.

FIG. 14 is a graph in which Vr / Vs values obtained from the results of FIG. 13 are plotted.

FIG. 15 shows a second example measured using the system shown in FIGS.
It is a figure which shows the measurement data of the 6th spot of an investigation ground.

FIG. 16 is a graph in which Vr / Vs values obtained from the results of FIG. 15 are plotted.

FIG. 17 is a curve showing the relationship between the depth D used for the basic determination of the ground and the average Rayleigh wave velocity Vr bar.

FIG. 18 is a curve showing the relationship between the depth D used for the basic determination of the ground and the average Rayleigh wave velocity Vr bar.

FIG. 19 is a flowchart showing a building foundation determination criterion.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 arithmetic unit 2 seismometer 3 vibration signal 4 vibration generator 5 first vibration receiver 6 second vibration receiver 11 recorder 12 power supply 13 ground vibration receiver 14 three-component vibration receiver 15 pressurizing device 16 plate beater 17 boring Hole

Claims (3)

(57) [Claims] 1. A Rayleigh wave velocity Vr on a measurement target ground is measured, and an S wave velocity Vs is expressed by the following equation using the Rayleigh wave velocity Vr and a predetermined coefficient k. The uniaxial compressive strength q of the ground according to the speed Vs
u is given by Then, the adhesive strength C of the ground below the bottom of the foundation is obtained as C = qu / 2, and the allowable supporting strength qa of the ground is obtained using the adhesive strength C.
Qa = (α · C · Nc + β · γl · B · Nr) / 3 (where α and β are basic shape factors, Nc,
Nr is a bearing capacity coefficient, γl is a unit volume weight of the ground below the foundation bottom, and B is a minimum width of the foundation bottom. 2. The method according to claim 1, wherein the predetermined coefficient k is 0.8 to 0.9.
9. The method according to claim 1, wherein the allowable bearing strength of the ground is measured. 3. The method according to claim 1, wherein said predetermined coefficient k is 0.9.