JP2020063552A - Method for sampling core from bedrock - Google Patents

Abstract

To sample in extremely precision a core diameter before expanding to be used in core deformation process.SOLUTION: A method for sampling core from a bedrock according to the present invention includes the following steps. A step is to drill a bedrock 1 to form a borehole 2. Then a step is to form an outer grove 3 extending in a depth direction and circling in the horizontal direction at a bottom portion 21 of the borehole 2. As a result, the stress relief portion 4 extended in the depth direction is generated in the bedrock 1. Then a step is to form an inner groove 5 extending in the depth direction and circling in the horizontal direction inside the outer groove 3 to extend into a position deeper than the outer groove 3. As a result, the core 6 including the non-expanded region 61 and the expanded region 63 is generated. Then a step is to sample the generated core 6 from the borehole 2.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a technique for extracting a core that can be used in a double stress release core deformation method (commonly called DCDA, hereinafter simply referred to as “core deformation method”), which is a method of calculating a stress state in rock, from rock. It is a thing.

  In Patent Document 1, Non-Patent Document 1 and Non-Patent Document 2 below, a core deformation method is described as a means for measuring the stress acting on the crust using a cylindrical core obtained by boring. Has been done.

In general, stress acts in rock due to various factors. When the core is taken from the bedrock by boring, the stress on the core is released and the shape of the core is deformed. The core diameter before stress release (so-called original diameter) is called d ₀ , the maximum value of the core diameter after stress release is called d _max , and the minimum value is called d _min . According to the core deformation method, the stress difference information depending on the direction can be calculated from the difference between the values of d _max and d _min .

However, since the deformation of the core occurs immediately after the core is separated from the bedrock, it is difficult to accurately obtain the core diameter d ₀ before releasing the stress. Here, it is generally impossible to assume that the diameter of the core bit used for boring is d ₀ . This is because the core bit diameter changes due to wear during the boring process. Moreover, even if the core bit diameter does not change, the actual core diameter d ₀ may change due to various factors such as the type of rock mass and temperature.

Since the core diameter d ₀ before the stress release cannot be accurately acquired, the conventional technique has a problem that absolute stress information cannot be calculated.

JP, 2017-025617, A

Funato Akio, Ito Takatoshi, "Core Deformation Method for Rock Mass Stress Evaluation (DCDA)", Journal of MMIJ, Vol.129, p.577-584, 2013. Funato, A., and T. Ito. "A new method of diametrical core deformation analysis for in-situ stress measurements." Int. J. Rock Mech. & Min. Sci. 91: 112-118. 2017

The present invention has been made in view of the above situation. A main object of the present invention is to provide a technique capable of accurately obtaining the core diameter (original diameter) d ₀ . Another object of the present invention is to provide a technique capable of calculating absolute stress information by the core deformation method.

  The present invention can be expressed as the inventions described in the following items.

(Item 1)
A step of forming a boring hole by excavating rock,
At the bottom of the boring hole, a step of forming a stress relief portion extended in the depth direction in the rock mass by forming an outer groove extended in the depth direction and orbited in the horizontal direction,
A core including a non-expansion region and an expansion region by forming an inner groove extending in the depth direction and circling in the horizontal direction inside the outer groove to extend to a position deeper than the outer groove. To generate
Collecting the produced core from the boring hole.

(Item 2)
A step of acquiring an unstressed core diameter used in the core deformation method by measuring an outer diameter of the non-expansion region in the core collected by the core collecting method according to Item 1;
Measuring the outer diameter of the expanded region of the core to acquire the expanded core diameter used in the core deformation method.

(Item 3)
A stress calculation method for calculating stress in the ground by a core deformation method using the core diameter acquired by the core diameter acquisition method described in Item 2.

According to the present invention, since the core including the non-expanded region and the expanded region can be collected, it is possible to accurately obtain the core diameter d ₀ before releasing the stress. Further, according to the present invention, it is possible to calculate the absolute stress information by using the obtained core diameter d ₀ .

It is explanatory drawing for demonstrating the basic concept of the core deformation method, Comprising: The figure (a) shows the state where the stress is applied to the rock mass before excavation, The figure (b) expands the core taken out from the rock mass Indicates the status. It is an explanatory view for explaining the core extraction method concerning one embodiment of the present invention, and is a figure showing the rock mass of an excavation part. FIG. 3 is a schematic diagram for explaining an example of a boring device that can be used in the core collecting method of FIG. 2.

  A core collecting method according to an embodiment of the present invention will be described with reference to the accompanying drawings. First, as a premise of the description, the basic concept of the core modification method will be described with reference to FIG.

(Basic concept of core deformation method)
For typical vertical borehole, the core diameter _{(d 0, d max, d} min) and the main stress in the rock _{(S Hmax, S hmin, S} v) relationship is as follows.

here,
E: Young's modulus of rock,
ν: Poisson's ratio of rock,
d ₀ : core diameter (original diameter) before stress release used in the core deformation method,
d _max : maximum value of core diameter after stress release,
d _min : minimum value of core diameter after stress release,
_SHmax : maximum stress in the horizontal plane,
_{Sh min} : minimum stress in the horizontal plane,
S _v : vertical stress.

In the core deformation method, the following equation (3) obtained by taking the difference between the above equations (1) and (2) is used to calculate the difference from the core diameter difference Δd (= d _max −d _min ). seeking stress _{ΔS (= S Hmax -S hmin)} .

However, it is not possible to only differential stress evaluation in this way, when the absolute value of S _Hmax and S _hmin, it is necessary to incorporate the stress information obtained in other ways. The reason why the absolute value cannot be obtained is that the original diameter d ₀ of the core before the stress is released is unknown. If d ₀ can be calculated and S _v can be calculated separately from the overpressure, for example, the absolute value of _SH max can be _calculated from equation (1), and from that value, the conventional method can be used. _Then , the absolute value of _{Sh min} can be obtained by subtracting the differential stress obtained by using the equation (3). Here, assuming that Δd _max = d _max −d ₀ and the effect of the Poisson's ratio is quadratic, the equation (1) is modified and the following equation (4) is obtained.

This relationship indicates that in order to obtain the absolute value of S _Hmax from the equation (1), it is necessary to measure d ₀ with a precision sufficiently higher than the size of the right side. For example, when S _Hmax = 10 MPa, E = 10 GPa, and d ₀ = 100 mm, the right side of the above equation is 0.1 mm. Therefore, if a 10% error is allowed in the measured value of S _Hmax , ± 0.01 mm ( = 10 μm), it is necessary to measure d ₀ . However, it is considered that such a difference in diameter easily occurs due to the easiness / hardness of rocks, the wear of bits, the temperature difference between the underground and the surface, etc. The diameter of the core obtained by excavating a rock is not d ₀ . The core deformation method itself has been conventionally known (for example, refer to Non-Patent Document 1 described above), and thus detailed description thereof will be omitted.

(Core collection method in this embodiment)
Next, the core collecting method according to this embodiment will be described mainly with reference to FIGS. 2 and 3.

(Formation of boring holes: Fig. 2 (a))
First, the boring hole 2 is formed by excavating the bedrock 1. The boring hole 2 can be formed by an ordinary boring device. The depth of the boring hole 2 is not particularly limited, but is, for example, on the order of 10 ^{1 to} 10 ³ m. It is also possible to form the boring hole 2 having a depth larger than this. As a result, the bottom portion 21 is formed at the tip portion (the deepest portion) of the boring hole 2.

(Formation of outer groove: Fig. 2 (b))
Then, the outer groove 3 extending in the depth direction and circling in the horizontal direction is formed in the bottom portion 21 of the boring hole 2. The outer groove 3 has a circular cross section. Accordingly, the columnar stress releasing portion 4 extended in the depth direction can be generated in the bedrock 1. The outer groove 3 can also be formed by a conventional boring device. By forming the outer groove 3, in the stress releasing portion 4, the stress acting on the bedrock 1 is released, so that the stress releasing portion 4 becomes substantially stressless. However, as will be described later, in the vicinity of the lower end of the stress releasing portion 4 (that is, in the vicinity of the lower end of the outer groove 3), a portion where the stress gradually changes (gradual region 62 described later) is formed.

(Formation of inner groove: Fig. 2 (c))
Then, the inner groove 5 extending in the depth direction and circling in the horizontal direction is formed inside the outer groove 3 to extend to a position deeper than the outer groove 3. The shape of the inner groove 5 is a circular cross section concentric with the outer groove 3. As a result, the cylindrical core 6 including the non-expansion region 61 and the expansion region 63 can be generated. Further, in this example, the region between the non-expansion region 61 and the expansion region 63 is the transition region 62 in which the stress state (expansion state) gradually changes.

  Here, in the present embodiment, since the core 6 is generated by continuously forming one inner groove 5, it is considered that the outer diameter (so-called core diameter) of the core 6 during excavation is substantially uniform. be able to. That is, it can be considered that the inner diameter of the core bit that generates the core 6 does not change. The length and diameter of the core 6 are not particularly limited, but are set to, for example, about 0.5 to 1 m for ease of handling.

Here, in the stress release portion 4 at the time of generating the core 6 (during excavation), the stress is already released due to the formation of the outer groove 3. Therefore, in the non-expansion region 61, which is the core 6 of the portion corresponding to the stress release portion 4, the core 6 is not expanded due to the generation thereof. Then, the obtained outer diameter of the non-expansion region 61 of the core 6 accurately represents the inner diameter of the core bit (core diameter d ₀ which is the original diameter before expansion).

  On the other hand, in the expansion region 63 of the core 6, until the core 6 is generated (that is, before excavation), the stress in the bedrock 1 is acting, that is, the so-called in-situ stress state, so the stress is released by excavation. As a result, the core expands.

(Core collection: Fig. 2 (d))
Then, the core 6 produced as described above is sampled from the boring hole 2. This sampling can be done by conventional boring equipment. For example, the core 6 can be collected by cutting the core 6 from the tip of the inner groove 5 by an appropriate method. However, the method of taking out the core 6 from the boring hole 2 is not particularly limited.

By measuring the outer diameter of the expansion region 63 of the obtained core 6, the core diameters d _max and d _min after expansion can be obtained. Further, by measuring the outer diameter of the obtained non-expansion region 61 of the core 6, an accurate core diameter d ₀ can be obtained.

Therefore, according to the present embodiment, by using the accurately measured d ₀ , d _max and d _min , it is possible to calculate the absolute information of the stress, which was not possible in the past, by using the core deformation method. There is. For example, according to this embodiment, by using a core diameter _{d 0 obtained,} it is possible to calculate the absolute value of _{S Hmax} and _{S hmin.}

  Note that, in the gradual transition region 62, since the expanded state is gradually changing, such a portion is excluded from the measurement target. The regions 61 to 63 in the core can be appropriately specified by the operator based on the depths of the outer groove 3 and the inner groove 5, or the state of change of the core diameter.

  In addition, in general, the length of the core 6 is sufficiently shorter than the distribution length (depth) of the bedrock 1, so that it can be considered that the state of the bedrock 1 is uniform in the excavation range of the core 6. it can. If the condition of the bedrock 1 changes in the excavation range of the core 6, it is difficult to apply the core deformation method.

  Further, the ratio of the depth in the stress releasing portion 4 (that is, the depth of the outer groove 3) and the diameter of the stress releasing portion 4 (that is, the diameter of the outer groove 3) is in the range of 1: 1 to 2: 1. Is preferred. When the depth in the stress releasing portion 4 becomes shallower than 1: 1, the ratio of the gradual region 62 becomes large, and the reliability of the obtained measured value may be reduced. On the other hand, when the depth in the stress releasing portion 4 is deeper than 2: 1, the stress releasing portion 4 becomes long, and thus the shape of the stress releasing portion 4 becomes difficult to hold during excavation of the inner groove 5 (that is, collapses during excavation). The possibilities increase. Therefore, by setting the ratio of the depth in the stress releasing portion 4 to the diameter of the stress releasing portion 4 in the range of 1: 1 to 2: 1, the core 6 can be collected while avoiding these problems.

(Specific examples of boring equipment)
Next, with further reference to FIG. 3, an example of the boring device used in the core collecting method of the present embodiment will be described. In this example, the inner groove 5 is generated by the boring device 7. The boring device 7 includes a boring rod 71 that is rotationally driven, a core barrel 72 attached to the tip of the boring rod 71, and a core bit 73 attached to the tip (lower end) of the core barrel 72. According to this boring device 7, it is possible to excavate the bedrock 1 by rotating the boring rod 71 to rotate the core bit 73 around the axis. Further, the core 6 separated from the bedrock 1 by an appropriate method can be stored in an inner tube (not shown) in the core barrel 72 and collected.

  As such a boring device 7, a device similar to the conventional one can be used, and thus a detailed description thereof will be omitted.

  The outer groove 3 can be formed by a similar boring machine having a large-diameter core bit.

  It should be noted that the above description of the embodiment is merely an example, and does not show an essential configuration of the present invention. The configuration of each part is not limited to the above as long as the object of the present invention can be achieved.

  For example, in the above-described embodiment, it is assumed that the rock mass 1 is excavated in the vertical direction, but the excavation direction of the rock mass is not limited to the vertical direction and may be a direction inclined from the vertical direction. It is also possible to drill horizontally. In such a case, the aforementioned depth direction is not interpreted as a depth direction with respect to the ground surface, but as a depth direction along the excavation direction (that is, a direction forward of the excavation direction).

1 Rock Mass 2 Boring Hole 21 Bottom 3 Outer Groove 4 Stress Relief 5 Inner Groove 6 Core 61 Non-Expansion Area 62 Gradual Area 63 Expansion Area 7 Boring Device 71 Boring Rod 72 Core Barrel 73 Core Bit

Claims (3)

A step of forming a boring hole by excavating rock,
At the bottom of the boring hole, a step of forming a stress relief portion extended in the depth direction in the rock mass by forming an outer groove extended in the depth direction and orbited in the horizontal direction,
A core including a non-expansion region and an expansion region by forming an inner groove extending in the depth direction and circling in the horizontal direction inside the outer groove to extend to a position deeper than the outer groove. To generate
Collecting the produced core from the boring hole. Obtaining a core diameter in a stress-free state used in the core deformation method by measuring an outer diameter of the non-expansion region in the core collected by the core collecting method according to claim 1.
Measuring the outer diameter of the expanded region of the core to acquire the expanded core diameter used in the core deformation method. A stress calculation method for calculating a stress in the ground by a core deformation method using the core diameter acquired by the core diameter acquisition method according to claim 2.

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