JP2002030643A - Technique for measuring deformation characteristic of ground material based on contact time - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately and easily measure the deformation characteristic of a ground material on site. SOLUTION: A lightweight, hard ball is used and caused to fall freely and the time of contact between the ball and the ground material is measured whereby the deformation characteristic of the ground material is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-situ measurement of deformation characteristics of a ground material. For more information,
The present invention relates to an in-situ measurement or estimation method for a Young's modulus and a ground modulus of a surface ground material.

[0002]

2. Description of the Related Art In embankment construction, compaction management of materials is very important. Degradation of the mixed material such as soil cement after a lapse of time progresses due to chemical action and physical action. In terms of mechanical properties, the deformation coefficient and strength of the mixed material decrease. In concrete strength tests using sampling cores, there are many cases where strength is reduced by a factor of several due to deterioration. In addition, permeability may increase with deterioration.

[0003] As for compaction control, measurement of the on-site density by the sand displacement method and measurement of the water content by the furnace drying method have been conventionally performed. However, these methods require considerable labor and time, and it cannot be said that the state of compaction of the entire embankment has been grasped, and it has been difficult to accurately reflect the compaction in the construction.

[0004] In recent years, improved (mixed) materials such as soil cement and CSG have been widely used for soil structures. There are naturally limits to managing such materials based on their density and water content. That is, in such a mixed material, the mechanical characteristics (deformation characteristics and strength characteristics), not the density and the water content ratio, are the most important factors. Therefore, how to efficiently and appropriately grasp these mechanical characteristics in the field has become a major issue.

Various methods have been developed to determine such mechanical characteristics (particularly, deformation characteristics). There are static methods such as CBR test and lateral rod loading test. However,
The static method requires working time, reaction force equipment, and is expensive.

On the other hand, many dynamic test methods have been proposed. Roughly classified, there are drop test methods and methods based on measurement of vibration response characteristics.

The drop test method includes a sphere drop test and a cone drop test. (A) Spherical drop test: This test method is based on the subgrade soil CB
It has been developed for the purpose of easily estimating the R value. A sphere (made of steel, about 9 cm in diameter, about 4 kg in mass) is dropped on the ground from a certain height, and the size of the dent (the diameter is defined as D value) of the ground formed at that time is measured. This D
It has been experimentally confirmed that the correlation between the value and the on-site CBR is extremely high, and this relationship is used. (B) Cone drop test: In this test method, a weight with a constant weight is dropped freely from a certain height, and the correlation between the values of the maximum impact acceleration and average acceleration and the physical properties of the material has been confirmed. I have.

In the method based on the measurement of vibration response characteristics, the ground and the measurement part are assumed to be a vibration system (mass, spring and dashpot), and the spring constant of the ground is obtained by measuring response characteristics such as dominant frequency and amplitude. Is the way. (B) Response acceleration method: The impact acceleration of a falling rammer on the ground surface is measured. This is to experimentally correlate the peak acceleration value of the accelerometer built into the rammer at the time of impact with an index such as density or ground reaction force coefficient. (B) Impedance head method: An impedance head is a measuring instrument for measuring a vibration frequency response function or the like of a measurement object. In this method, a weight is attached to an impedance head, dropped on the surface of soil, and an impulse input is applied to a vibration system constituted by the weight and the ground to cause vibration. This method estimates the spring constant of the ground by measuring the natural frequency of this vibration. (C) Resonance method: The vibration constant is set directly on the ground to generate vibration, and the response (mainly resonance frequency) of the vibration system between the vibration body and the ground is measured to determine the spring constant and damping ratio of the ground. This is the method to be found. (D) Wheel acceleration method: A method of measuring the degree of compaction of soil in real time by using the fact that the response characteristics of the wheel acceleration of a vibrating roller at the time of construction change with the progress of compaction. Specifically, it is a method of analyzing the spectral characteristics of an acceleration signal of an accelerometer attached to a vibrating wheel axle, and obtaining a correlation with the degree of compaction of the ground. At this time, the ground becomes hard, and the mixing ratio of harmonics generated by the repulsion of the rollers from the ground surface is an important index indicating the hardness of the ground.

Except for a part (the wheel acceleration method), these methods are considered to have the advantage that the degree of compaction can be measured more sensitively than the density measurement, and the following disadvantages. (B) The physical significance of the measured index is not as clear as density. (B) Many methods are merely correlations. When the material changes or the measuring instrument changes, the correlation characteristics also change. Therefore, it cannot be applied to improved materials in many cases, and lacks versatility. (C) The method using the vibration response characteristic has a big problem in how to determine the mass M of the ground. (D) Physical effects such as gravel content are large. (E) The depth range of the exploration is unknown, and it is almost impossible to change the depth range.

[0010] Individually, there are the following drawbacks, and at present, no satisfactory method is found at present. (B) The spherical drop method is suitable only for the ground where uniform depressions occur due to the spherical drop. (B) For the cone falling method and the response acceleration method, the response changes greatly when hitting gravel, so only homogeneous fine-grained materials can be applied. (C) In the impedance head method, the ground contact surface of the head may vary depending on the hardness and the grain size of the ground, and thus causes a large error. (D) The resonance method requires a vibrator, and the equipment becomes complicated. (E) In the wheel acceleration method, after one reciprocation of the rolling compaction, it becomes gradually insensitive. In addition, it cannot be used for deterioration investigations.

[0011]

SUMMARY OF THE INVENTION The present invention is a system for compaction management of soil materials and improved mixed materials such as soil cement and CSG, and a system for nondestructively exploring the degree of deterioration thereof.

The goals of the development of the present invention (system) are as follows. (A) This system mainly uses the mechanical properties (particularly, deformation properties) of the material as the search value. (B) This system aims to achieve both search accuracy and work efficiency. (C) This system covers from the surface to a certain depth. (D) This system has a clear theoretical background, not just a correlation. (E) This system uses a wide range of materials (fine, coarse,
Gravel, soil cement, composite materials such as CSG).

[0013]

The present invention is based on the Hertz impact theory, and determines the elastic modulus of the ground material by measuring the contact time with the ground material when a sphere (homogeneous elastic material) is dropped. Can be The harder the ground, the shorter the contact time.

According to the Hertz impact theory, the contact time between a falling sphere and an elastic body plane (for example, the ground) is determined by the following factor (see Equation 1). (A) Deformation characteristics of ground (Young's modulus and Poisson's ratio) (b) Deformation characteristics of sphere (Young's modulus and Poisson's ratio) (c) Density, diameter and drop height of sphere

When a ball having a mass M _{1 and a} radius R ₁ is dropped on the ground (fall height h), the contact time T _c between the ball and the ground
Is

(Equation 1) Becomes here

Δ = (1−μ ² ) / (Eπ) is the characteristic of the material. E and μ are the deformation coefficient and Poisson's ratio of the material, respectively.
The subscript 1 indicates a ball, and 2 indicates a ground material. g is the gravitational acceleration, which is 9.80 m / s ² in kg / m / s international unit system.

According to Equation 1, the contact time between the ball and the object (ground material) depends on the radius of the steel ball and the material (E ₁ and μ ₁ ).
It is determined by the drop height and the material of the object (E ₂ and μ ₂ ). Since the contact time of the object, the radius of the ball, the material (E ₁ and μ ₁ ) and the drop height are known,
Unknown quantity is only ₂ and the material E ₂ objects mu. Therefore, if you drop twice in the same place with different drop conditions,
Theoretically, it is possible to estimate these two unknowns, that is, the material of the object.

Analysis of Equation 2 reveals that the influence of the Poisson's ratio is small. The Poisson's ratio of the ground material during compression is usually between 0.2 (sandy) to 0.45 (clayy). Within this range, the change in δ reaches about 15%, but within the same material, the change in Poisson's ratio is small, so if a typical value of Poisson's ratio is input in advance depending on the material, It is considered that the effect of the change in Poisson's ratio is negligible.

Furthermore, by using the modified coefficient _{E 2,} it can be converted to soil factors _{K 30.}

(Equation 3) In the equation, D is the diameter of the loading plate, 0.30 m.

The range of influence can be estimated by Hertz impact theory and elasticity theory.
Naturally, the influence of the surface layer is main. As the sphere becomes larger, the range of influence becomes deeper and the applied particle size can be correspondingly larger.

Here, it should be noted that the contact time is determined by both the ground material and the characteristics and the characteristics of the ball. However, if δ of the ball is much smaller than the ground material,
That is, when the rigidity of the ball is much greater than the ground,
The contact time almost depended only on the ground material. Conversely, when the rigidity of the falling body is soft, or when the falling body is not spherical but flexible like a plate, the influence of the falling body itself increases. Therefore, it is expected that the measurement accuracy will decrease.

However, the Hertz impact theory requires that the two impacting objects be both elastic bodies. Among the ground materials, especially in the case of soft ground materials, plastic deformation occurs. While the occurrence of plastic deformation affects the contact time, it may be related to the strength of the material. In addition, the influence of the non-linearity of the ground material and the difference between the compression characteristics and the rebound characteristics can be considered. The degree of these effects, etc. will be examined through verification tests and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to measure the deformation coefficient of the ground using the method of the present invention, a falling ball exploration system was developed.

As shown in FIG. 1, the falling ball search system comprises a lightweight hard ball for dropping, a handle, an acceleration sensor,
It consists of a sensor power supply, an A / D board and a notebook computer. The acceleration sensor is built into the ball (Fig. 2).

When measuring the ground material, the ball is raised to a certain height (for example, 50 cm). Let it fall freely and record the impact process with the ground with an acceleration sensor. From the moment the ball and the ground material come into contact, the ball begins to receive upward resistance, and this upward resistance continues until the ball separates from the ground. Since the direction of the external force coincides with the direction of the measured acceleration, the time until the sign of the acceleration changes is the contact time (FIG. 3).

Further, the loading level and the strain level (the amount of settlement) can be predicted. The maximum impact force _Fm is determined as follows.

(Equation 4) Here, M and a _m is the maximum acceleration measured the mass of the ball, respectively.

[0025] The maximum amount of compression S _m can be calculated as follows.

(Equation 5) However, T _s is the time until the speed becomes zero from the moment the ball and the ground are in contact (i.e. when maximum compression).
The ball speed ν (t) is

(Equation 6) Is required. ν (t) is the time to be zero, the stopping time T _s.

The present system automatically performs all of the above analysis. That is, parameters such as the ground deformation coefficient can be obtained in real time just by dropping the ball. In addition, there is no need for reaction force equipment or shaping of measurement points, so work efficiency has been dramatically improved and real-time on-site management has become possible. Furthermore, if the ball drop method is changed (for example, by rotating with a shaft), it is possible to cope with a slope, and it can be considered that the method can be applied to deterioration inspection of a slope.

[0027]

[Embodiment] It is considered that measurement is relatively easy for hard ground because the elastic region of the material is wide. Therefore, here, a test was performed on a soft material (sand, a mixture of sand and clay) that has a large plastic effect.

In the test, compacted specimens under various conditions were prepared in a large earthen tank, and the following various tests were performed to examine the applicability of the present invention. (A) Ball drop test: Drop height 0.2, 0.3, 0.
4, 0.5m (b) Flat plate loading test: loading plate diameter φ14.5cm (c) CBR test: penetration rod φ5cm

Test quantity (number of specimens) performed in the verification test
Is shown in FIG. FIG. 5 shows the parameters of the ball, and FIG. 6 shows the Poisson's ratio of the ground material.

FIG. 7 shows an example of output on the personal computer screen.

[0031]

FIG. 8 shows a comparison between the deformation coefficient obtained by the method of the present invention and that obtained by a plate loading test (according to JIS-A-1215). Fig. 9 shows the comparison of the converted ground coefficients, and the CBR (JIS-A-12
22) is also shown in FIG.

From the correlation coefficient (r), the deformation coefficient and the ground coefficient are r = 0.778, and the correlation coefficient with the CBR is r =
0.937. All the values show high correlation, confirming the effectiveness of the present invention.

[Brief description of the drawings]

FIG. 1 is an overview of a measurement system developed according to the present invention.

FIG. 2 Fixation of an acceleration sensor.

FIG. 3 Determination of contact time.

FIG. 4 shows the test quantity of the verification test.

FIG. 5 shows parameters of a falling ball.

FIG. 6: Selection of Poisson's ratio of ground material.

FIG. 7 is an output example of a measurement screen of the measurement system.

FIG. 8 is a comparison of deformation coefficients.

FIG. 9 is a comparison of ground coefficients.

FIG. 10 Comparison with CBR results

Claims (1)

[Claims] 1. Deformation characteristics of a ground material (Young) by dropping a lightweight and hard sphere and measuring the contact time between the ball and a ground material (a natural soil material such as clay, sand and gravel, and a mixed material such as soil cement). Rate and ground coefficient).