JPS6024413B2 - How to measure ground pressure in a borehole using AE - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes the Kaiser effect of AE (acoustic emission) to determine the initial stress state of the ground around a measurement point from measurements in a borehole. It is related to.

The Kaiser effect is a phenomenon in which the material structure is stable and fewer AEs occur until the maximum stress the material has been subjected to in the past, but when the material is subjected to greater stress, microscopic fractures tend to occur and the frequency of AEs increases. It is used to test for structural quality deterioration during operation of metal structures such as pressure vessels and piping.

In recent years, it has been discovered that rocks also have such a Kaiser effect, and that the Kaiser effect has directionality. Focusing on these phenomena, researchers have been studying rocks in order to understand the orientation and orientation of rock formations. A very new method of estimating ground pressure by measuring the residual stress of rock specimens using the AE test method has also been proposed and implemented (Japanese Patent Laid-Open No. 53-39182).
(see issue).

However, with this method, rock specimens must be collected in a fixed orientation, requiring special equipment for sampling, slow excavation speeds, and great care, making it difficult to collect large numbers of specimens. It takes a lot of time, effort, and training to get it. In addition, if the ground is cracked, brittle, or has low strength, even if you try to collect a rock specimen, it may crack or fracture and you may not be able to obtain a satisfactory specimen, or when you pull up the sampler. The collection rate decreases due to the sample falling, and depending on the nature of the ground, it is often impossible to collect the sample. Furthermore, the samples collected from the ground may be disturbed, and there are also questions about whether the ground pressure in the current state of the ground can always be estimated with sufficient accuracy using four-dimensional samples. . The present invention was made in view of the actual state of the related technology, and its purpose is to use a borehole to determine the initial stress state of the ground around the borehole without collecting rocks. Therefore, it is an object of the present invention to provide a method that can save labor and time, can be carried out even on weak ground, and can measure ground pressure in a state closer to the current state of the ground.

Hereinafter, the present invention will be explained in detail based on the drawings.

Assuming an arbitrary surface within a rock mass, as shown in Figure 1A, stresses of equal magnitude and opposite directions act on that surface, thereby maintaining balance.

However, as shown in Figure B, when boring is carried out in the rock 1, the stress in the rock is released at the hole end 3 of the borehole 2, and the stress component perpendicular to the hole wall becomes 0. In this state, the sonde 4 is inserted into the borehole 2. The sonde 4 mainly consists of a rubber tube 5,
A plurality of AE geophones 6a, 6b, 6c with different orientations (in FIG. 3, three with different orientations on the same plane) and a pressure sensor 7 are provided. When the sonde 4 is lowered to a predetermined depth, a high-pressure fluid such as high-pressure gas or high-pressure water is supplied from the high-pressure source 8 into the rubber tube 5 to inflate the rubber tube 5 and create a uniform vertical pressure on the borehole wall 3. Add.

As this pressure is gradually increased, fluid pressure that exceeds the initial ground pressure that was previously applied to the rock in the hole wall 3 is applied, and fluid pressure is generated from within the rock. The frequency of occurrence of AEs increases rapidly. Therefore, the controller recorder 9 records the signal indicating the pressure value from the pressure sensor 7 and the signals from the AE geophones 6a, 6b, and 6c.
It detects the point where the frequency of AE occurrence increases sharply. By the way, the initial ground pressure applied to the rock mass generally has a directionality, and when viewed in a plane perpendicular to the direction of boring, the stress is the largest, as shown in Figure 4.

max and its direction, the smallest stress. There is min and its direction, and in other directions depending on that direction. With max. Indicates the stress between min. Under this stress state of the rock mass, equal pressure is applied to the borehole wall 3 in all directions, and the pressure is gradually increased. A8 is generated from the rock in the min direction due to the Kaiser effect, and as the pressure increases, the direction in which AE is generated changes, and finally the maximum stress is reached. The direction indicating max is reached. As shown in Figure 3, a,
If AE geophones 6a, 6b, and 6c that measure only AE generated from directions b and c are provided, the frequency of AE occurrence and the frequency of AE generated in each direction can be determined, for example, as shown in Fig. 5 a, b, and c, respectively. A graph showing the stress relationship can be created, and the initial stress in each direction can be determined from the inflection points (indicated by arrows).

In addition, the ground pressure state within the plane can be determined from the initial stress in these two directions. Furthermore, the three-dimensional stress distribution within the rock mass can be obtained by changing the direction of boring and attaching and measuring a geophone for measuring AE in a predetermined direction to each borehole. As described above, the present invention uses the borehole directly without collecting a sample, so it can eliminate various disadvantages associated with sample collection, that is, it can save time and labor, and it does not require a high level of skill. It has a number of excellent effects, such as being able to measure even fragile ground with many cracks, and being able to more accurately determine the ground pressure in the current state of the ground. Brief description of the drawings Figure 1A is an explanatory diagram showing the ground pressure state in the rock before excavation, Figure 1B is an explanatory diagram showing the ground pressure state in the rock after excavation, Figure 2 is an explanatory diagram showing one embodiment of the present invention, and Figure 3 Figure 4 is an explanatory diagram of the initial ground pressure at the borehole wall. Figure 5 a, b, and c are AE generation frequency and pressure from AE geophones in different directions. It is a graph showing the relationship between

1... Bedrock, 2... Borehole, 3... Hole wall, 4... Sonde, 5... Rubber tube, 6a, 6b, 6
c...AE geophone, 7...Pressure sensor.

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. A sonde, which is mainly made up of a rubber tube and equipped with multiple AE geophones in different directions, is inserted into a borehole drilled underground, and pressurized fluid is supplied into the rubber tube to cause it to expand, causing it to flow against the borehole wall. By gradually increasing the applied pressure, creating a relationship diagram of the AE occurrence frequency with respect to the change in pressure, and measuring the initial stress in the mounting direction of each AE geophone from the inflection point, A method for measuring ground pressure in a borehole using AE, which determines the initial stress state of the ground.