Keywords

1 Introduction

The shear strength of rock mass is an important parameter in the design of large-scale rock mass engineering, which is directly related to the above-ground high-rise civil construction and the excavation of the bank slope of the surface high dam [1]. At the same time, with the use of underground space, especially the rapid mining of coal, roadway and stope surrounding rock stability control, rock burst, coal and gas outburst and other disasters become more prominent. However, it is difficult to accurately grasp the strength parameters of rock mass because the stress environment of rock mass is complex and contains randomly developed spatial discrete structural planes. The laboratory test is limited by size, mining disturbance and rock separation, and the test conditions are different from the actual environment. The traditional large-scale field test has a long period, complex operation, a lot of manpower and material resources, and there are constraints of soil and rock mass properties. In view of the limitations of the existing methods, it has become one of the important research topics in the field of geotechnical engineering to find a fast, simple and reliable means for rock mass in-situ testing [1, 2].

In the 1960s, Handy et al. from the University of Iowa proposed an in-situ test method using Borehole shear test (BST) to measure rock and soil shear strength parameters, and developed related equipment. After more than 40 years of development, borehole shear test has been applied in situ to test the shear strength index of fine grained sedimentary soils such as loess, hard clay, glacial moraine, Marine soft clay and residual soil [3]. Borehole shear test has been used in slope stability analysis and pile bearing capacity design abroad. In 2009, the Survey and Design Institute of Machinery Industry introduced a set of soil borehole shear instrument [4, 5] from the United States to test the shear strength indexes of typical Malan loess and leishite loess in Xi’an area. The China Institute of Water Resources and Hydropower Research has introduced a set of rock borehole shear instrument from the United States, and applied it to the hollow cylinder of concrete in the laboratory and the rock foundation of Xiangjiaba Hydropower Station.

At present, the equipment used in the borehole shear test in China is mainly focused on the BST borehole shear meter, Iowa borehole shear meter, and Phicometre geotechnical dual-purpose borehole meter in France [6]. The shear modulus test is mostly focused on soil, and the rock mass test is less. Aiming at the shortcomings of the existing test, developed in situ rock mechanics properties was designed to measure system, the system is compared with the United States and France similar equipment, in addition has the advantages of simple structure, light weight small size, can quickly measure the shear strength of rock mass advantage for many times, magnetostrictive sensor installation, can be in situ detection intrusion rock mass displacement size, a data acquisition module, Test data can be collected and stored in real time [7]. In addition, the end of the rock shear instrument is designed with a conversion joint, which can be used in coal mine when combined with the power head drill. The shear modulus of coal and rock is tested at 360 in situ, which expands the range of rock detection.

2 Shear Principle

Rock borehole shear instrument similar to the principle of direct shear test indoor and field, rock borehole shear apparatus [8] is in a certain depth in the borehole drilled in the two parallel wedge pressed into the drill hole wall, the shear apparatus between two parallel cutting tool to form a thin layer of rock, and then through the tyra shear apparatus tail, variable diameter joint implementation into dentate convex surface and hole direct shear failure of surrounding rocks. The experiment can be regarded as a forced direct shear failure test along this thin rock slice. In the rock borehole shear test, if the area of the rock slice embedded between dentate bulges is A, the normal force acting on the rock slice and the lifting force of the connecting rod are P and T, then the normal stress and shear stress acting on the rock slice are respectively.

$$ \sigma = \frac{P}{A} $$
(1)
$$ \tau = \frac{T}{{2{\text{A}}}} $$
(2)

The in-situ rock mass mechanical property measurement system is composed of rock mass intrusion module, shear strength measurement module, and data acquisition and processing module. It can detect different types of rock mass, generate test reports of mechanical properties, and analyze and process test data in the later stage. In the rock intrusion module, the manual hydraulic pump compressively invades the rock mass by the tool of the rock shear apparatus, and the magnetostrictive sensor measures the intrusion displacement. Shear strength measurement module by drilling or hollow plunger jack to provide axial tension, by shear instrument to shear rock axial; The data acquisition and processing module is mainly responsible for the acquisition of data in the shear instrument, and the realization of data reception, display, graphics, storage, analysis and other functions. The overall block diagram of module design is shown in Fig. 1.

Fig. 1.
figure 1

Overall module design block diagram.

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3 The Design of System Hardware

3.1 Pre Drilling In-Situ Rock Mass Shear Instrument

The predrilling in situ rock mass shear instrumentv [9] is designed as a lumen cylindrical structure, equipped with an elastic plate and a wedge tool, with an outer diameter of 75 mm (with a wedge tool and a metal positioning sleeve), and connected with a sensor adapter and a drill pipe adapter at the upper end. The sensor adapter is provided with a tubing interface, which is used for pressurizing oil injection or pressure relief of the pressure chamber, so that the elastic sheet and wedge cutter can be stretched out and the rock mass can be tested.

The structure of the shear instrument is mainly composed of shear device, pressure device, displacement device, diameter converter and other parts, as shown in the Fig. 2. The pressure device mainly converts the hydraulic pressure in the tube provided by the manual hydraulic pump into the axial forward force of the slider, and the displacement device uses the axial movement of the slider to make the tool on the elastic shear sheet move radially into the rock mass. The diameter changer head is mainly used to facilitate the shear meter from the drilling position, extraction, and subsequent rock shear axial tension, hollow plunger jack to provide shear force, shear meter through the jack hollow, using casing or caliper fixed upward pull; When the drill provides shear force, the reducer is connected to the drill pipe and shear in the direction of power head tension.

Fig. 2.
figure 2

Pre drilling in-situ rock mass shear instrument.

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3.2 Parameter Collection and Display

The digital display table is used for accurate display of displacement value, power supply of sensor and transmission of displacement signal. In order to improve the display accuracy, the filtering algorithm and discount calculation are applied to accurately process the displacement signal. The display meter is directly connected to the 220 V AC power supply to provide power for the magnetostrictive sensor in the shear meter. The sensor displacement signal is accurately processed by the display table and transferred to the computer serial port to enter the data acquisition and management system (Fig. 3).

Fig. 3.
figure 3

Parameter collector.

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The instrument is equipped with filtering algorithm, first smooth filtering and then inertial filtering. Smooth filtering is a common preprocessing method in signal analysis, which has good suppression effect on periodic interference and high smoothness.

The displacement obtained through the measurement process may have errors due to various reasons such as sensors, transmitters, leads or instruments. The correction function provided by instruments can effectively reduce the errors and improve the measurement and control accuracy of the system. In order to further improve the accuracy of the collected displacement data, the display table was modified by conversion.

3.3 Design of Auxiliary Accessories

Auxiliary accessories include manual hydraulic pump, anti-vibration pressure gauge, leveling base, hydraulic pipeline, data transmission line, etc. Depending on the mode of shear force provision, a drill, drill pipe connected to the shear meter, or a plunger jack and hollow steel pipe are used for axial shear.

4 System Software Platform Design

The design idea of rock shear data acquisition and processing system software is to build a software system with strong expansibility, high reusability, alarm and flexible control. Running the Windows operating platform, the software system based on intrusion displacement parameters, formed by the sensor is converted signal synchronization of real-time data acquisition and processing, the system of data acquisition and processing, curve fitting, report output and multifarious work can be done in an instant, can not only intuitive and clearly display and print processing results, Moreover, the fitting curve characterizing the change law of physical quantity can be drawn, so that the whole software system integrates the advanced nature and practicability.

Specific design, in order to improve the operability and convenience of the system. The computer acquisition interface adopts the display mode of combining data and curve, which is intuitive and clear. In the center of the screen, the displacement curve is displayed in real time with time as the abscissa and displacement as the ordinate. The displacement data on the right side of the curve is in millimeters and accurate to thousandths. In order to improve the data scalability, the interface can display a maximum of 16 channels of curves and data values simultaneously. Above the data report, print, save picture button, real-time record generated data; The lower part of the screen is equipped with various function keys, and there are operation hints, you can check the reference at any time; The function key includes the automatic curve refresh time, which can be adjusted according to the difference of intrusion time of different lithologies. Set up/down displacement alarm values to detect rock intrusion outliers in time to improve the accuracy of testing (Fig. 4).

Fig. 4.
figure 4

System software platform.

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5 Conclusion and Prospect

Through the test of the predrilling in situ rock shear system, the rock intrusion and data acquisition performance of the system is significantly improved compared with similar foreign equipment, which is reflected in the following points:

  1. 1)

    The system has the advantages of simple structure, light weight and small volume, and little disturbance to rock mass. It has the advantages of in situ testing and can quickly measure the shear strength of rock mass for many times.

  2. 2)

    The selected magnetostrictive sensor has the characteristics of high accuracy, good stability, high sensitivity and fast response time. Compared with similar equipment, it can accurately measure the intrusive rock mass displacement and improve the shear effect.

  3. 3)

    The software system has the characteristics of strong expansibility, high reuse and flexible control. It has the function of real-time data recording and overcomes the shortcoming of large error of handwriting by eye observation.