Intelligent Drilling Depth Monitoring System for Geotechnical Site Investigation

School of Civil Engineering, Shaoxing University, Shaoxing 312000, Zhejiang, China Zhejiang Provincial Key Laboratory of Rock Mechanics and Geological Hazards, Shaoxing University of Arts and Sciences, Shaoxing 312000, Zhejiang, China State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology (Beijing), Beijing 10083, China Geological Survey Engineering Department, Shaoxing Huahui Engineering Design Group Co. Ltd., Shaoxing 312000, Zhejiang, China


Introduction
Conducting geological surveys before project construction can help to gain more information about geological conditions. is information can better help us understand the impact of unfavorable geological conditions, which make e ective treatment measures to ensure the development of the project. e drilling and coring method is an e ective method used to obtain geological information, which is very extensive in engineering applications [1][2][3][4]. e key to the method is to get the accurate drillhole depth [5][6][7]. At present, the drillhole depth is obtained by calculating the number of drill pipes under the supervision of the operator. e drillhole depth obtained by this traditional method of counting the number of drill pipes is greatly a ected by some subjective factors [8,9]. Hence, this traditional method of calculating the number of drill pipes needs to be further optimized [10].
With the development of technology, some researchers have proposed intelligent methods to monitor drillhole depth [11][12][13][14]. Among them, the use of waveforms (e.g., sound waves, low-voltage pulses, and electromagnetic waves) to monitor the drillhole depth is a common intelligent monitoring method [15][16][17][18]. However, using waveforms to monitor the drillhole depth has some disadvantages. e waveform will re ect during the propagation, resulting in the gradual attenuation of the energy during the propagation. In this case, the information received will be inaccurate. For energy attenuation, some scholars use the secondary curve or index curve to make digital compensation, and some even calibrate the drill rod on site, but this increases the workload [19,20]. In this case, the use of wired sensor monitoring can greatly reduce the problems encountered in the signal transmission process [21][22][23]. e wired sensor monitoring method is less affected by the surroundings, which can reflect the real on-site monitoring situation in real time. Some scholars applied wired sensor monitoring to logging while drilling (LWD) [24][25][26]. e LWD is mainly used in drilling, but few monitoring fields are related to the drillhole depth of drilling core in engineering investigation. Yue et al. [27] conducted a more in-depth study on the whole process of monitoring the drilling with wired sensors. On this basis, the authors of [28][29][30] proposed a system for drilling process monitoring (DPM), which can obtain the mechanical strength of the underground rock and soil. It also provides a reference for us to build a drillhole depth monitoring system with wired sensors.
is study proposes a drillhole depth monitoring system with wired sensors. e monitoring system is easy to install and suitable for hydraulic drilling rigs that are widely used in China. According to the oil pressure data and laser displacement data collected by the monitoring system, the drillhole depth calculation formula for the soil layer geology is proposed. e calculated drillhole depth is close to the drillhole depth obtained by calculating the number of drill pipes. Additionally, the monitoring system is also successfully applied in the rock layer geology.

Traditional Manual Calculation Method.
e engineering core drilling rigs commonly used in inland China are hydraulic, and the drillhole depth of onsite is generally calculated by counting the number of drill pipes. e main calculation formula is as follows: where L is the total length of drill pipes, N is the number of drill pipes, and H is the length of a single drill pipe. is is greatly affected by the subjective factors because the drilling construction is mostly paid for by footage and different people have different standards when calculating the drillhole depth. On the other hand, in order to obtain the benefits, there is a phenomenon of falsely reporting the number of drill rods. If a special person is arranged to supervise, it will waste human resources and increase management costs. Accordingly, this study proposes a drillhole depth monitoring system to better serve the hydraulic core drilling rig commonly used in public works.

Monitoring Equipment.
e hardware of the monitoring system is divided into four modules in accordance with their function, including the data acquisition module, the transmission module, the power supply module, and the terminal display module. e data acquisition module includes the laser displacement sensor and the oil pressure sensor (Figure 1). e transmission module transmits the obtained data to the display terminal in a wired manner. e power supply module provides electricity to other modules to ensure normal operation. e terminal display module can display the changes of monitoring data in real time and save them. e software of the monitoring system includes four major functional modules, as shown in Figure 2. e parameter setting module can set the serial port, save path, etc. e functions of the real-time monitoring module include real-time data acquisition, statistics, and output functions. e database management module includes data backup and viewing of historical data. e system setting module can set the background color, grid color, and coordinate axis color of the graph curve.

Installation of Equipment.
e drilling rig widely used in China is the GXY-1C hydraulic drilling rig, which includes hydraulic jacks, drill pipes, drill bits, racks, diesel hydraulic presses, and others, as shown in Figure 3. e main power source of the drill is a hydraulic jack, which is supplemented by the rotation and propulsion force of the drill bit. When the drilling rig is working, hydraulic oil flows through the tubing in the following manner: it flows in four different directions to power the rig's different operations after flowing out of the main pipe. e oil pressure and laser displacement sensors are installed in a reasonable position in accordance with the working principle of the on-site drilling rig. e oil pressure sensor is mounted on one of the pipes which provides downward power to the power head. According to the designed installation equipment, the laser displacement sensor is installed under the drilling rig power head.
e schematic diagram of the installation of the sensing device is shown in Figure 4.

Project Overview.
e designed drillhole depth monitoring system is applied in the engineering investigation of a school laboratory expansion (Shaoxing, Zhejiang). Four holes are drilled on the site. e drillhole depth of the soil core holes (denoted as ZK1-ZK4) is about 30 m. e soil core obtained onsite is mainly clay ( Figure 5), and the installation of the site sensing equipment is shown in Figure 6.

Result Analysis.
Under the geological condition that the drilling stratum is soil, the working stages of the drilling rig during the process can be roughly divided into four stages (a, b, c, and d). Stage a (installing the casing). It indicates that the drilling rig reaches the specified hole location and prepares to install the casing. First of all, a core rod with a height of 0.8 m and a diameter of 108 mm is used to drill down until about 3 m. en, a casing with a length of 3-4 m is placed, which is for subsequent convenience to connect or remove the thin rod to ensure the fluency of the drilling work. e drilling depth of this section is recorded as Ha. Stage b (drilling into the core sample). is stage indicates that the power head moves up and down and the core rod begins to fill the core sample. is process takes about 3 min from start to finish. During this process, the oil pressure and laser displacement change considerably, and the drillhole depth can be monitored by calculating the effective displacement. Stage c (taking out the drill pipe).
is stage indicates that the mandrel is filled and the process of extracting core samples by oil pressure. e process can be divided into the process of raising the core rod, lifting the drilling rig, moving the power head back, removing the thin rod, raising the core rod, passing the core sample out of the core rod, putting back the core rod, and connecting the thin rod. Stage d (hole washing). ere are two types of hole washing, one is that of the downward hole washing by the active drill pipe with self-weight plus drilling speed. e other is that of the hole washing by way of the power head going up and down. e time points of each stage were recorded and are shown in Figure 7 along with the monitored oil pressure and laser displacement. In Figure 7(a), the whole drilling process includes the four stages (a, b, c, and d) of the abovementioned core soil sample, but it is mainly a cycle of (b, c, and d) three stages. e cycle of the three stages expresses the complete operation from putting in the core rod to drilling into the core sample and then to extracting the core sample.
e displacement of taking out the drill pipe in stage c is invalid for the calculation of the drillhole depth. For stage d, the displacements produced by the two washing methods correspond to the second d and the third d in Figure 7(a). During the up and down displacement of the power head, no core sample enters the mandrel, and the displacement in this process is invalid. Moreover, the oil pressure in the two cleaning methods is extremely small, and the oil pressure changes in a similar pattern. e stage b can be well distinguished from the three stages, and the drillhole depth is mainly calculated from it. e oil pressure and laser displacement data of the part surrounded by the dotted line in Figure 7(a) are analyzed, as shown in Figure 7(b). In Figure 7(b)), when the laser displacement starts to fall from the maximum value, the power head applies pressure through the jack. e oil pressure rapidly and abruptly changes from approximately 0.02 MPa to 1.8 MPa or even 3.5 MPa. Subsequently, when the power head is lowered to the lowest point, it needs to be lifted. e oil pressure value rapidly drops to approximately 0 MPa when the displacement increases (i.e., the power head lifts upward), showing the same pattern as rock drilling in the following. is process expresses the complete process of the drilling power head being drilled down once and then lifted up again. We define the interval of the power head dropping from the highest point to the lowest point as i. e effective drilling into the core sample displacement can be well distinguished and counted. Let the oil pressure be M, the rate of change per second is ΔM, when M ≥ 1.5 and ΔM ≥ 1, there are m drilled core sample

Shock and Vibration
e drillhole depth of the four measuring points is calculated by monitoring the system and is recorded in Table 1. It is close to the drillhole depth obtained by calculating the number of drill pipes. e relative errors of the two are 0.94%, 0.83%, 0.84%, and 0.74%, indicating that the proposed method is feasible in the calculation of drillhole depth. e results show that it can detect the drillhole depth conveniently and quickly, which is better than the traditional manual calculation method and has great importance for reducing the falsification of the drillhole depth.

Discussion
Under the condition of the rock layer, three holes are drilled on the site. e obtained core is mainly tuff (Figure 9) and the depth of the rock core holes (denoted as ZK5-ZK7) is about 5 m. e drilling stages in the rock layer are different from that of the soil layer, which are divided in the following categories. Stage a (idling drilling). is stage indicates that the drilling rig imitates normal drilling, and the core rod is drilled in the hole with the highest oil pressure and the fastest drilling rate. e drillhole depth of this section is recorded as Ha, but this section is generated by empty drilling and is invalid displacement. Stage b (drilling into the core sample).
is stage indicates that the power head moves up and down and the core rod begins to fill the core sample. In this process, the oil pressure is kept at a certain value, and the core rod bit keeps wearing the rock to achieve the effect of core entry, thereby increasing the displacement slowly. Stage c (unclogging hole). is stage indicates that the drilling rig performs high-frequency drilling, and the laser displacement under the drill head and the size of oil pressure keep changing. is process is to unclog holes and prevent sticking for better subsequent drilling. Stage d (taking out the drill pipe). is stage refers to the process of further compaction of the core rod. After the compaction is complete, the mandrel is pulled out and then connected to the water pump pipe, and the core sample is extruded by water pressure.
As shown in Figure 10(a), the main drilling process is the cycle of stages b and c. e displacement in stage c is generated by the simultaneous up-down movement of the power head and the active drill pipe. During the up and down displacement of the power head, no core sample enters the mandrel, and the displacement in this process is invalid. erefore, the drillhole depth is still reflected by stage b. Selecting a stage b (part of the dotted line surround) in Figure 10(a) for analysis, as shown in Figure 10(b). In Figure 10(b), it can be seen from the selection of the ith interval that this process expresses the complete process of the drilling head being drilled and lifted. It is characterized by keeping a certain oil pressure unchanged, and the core rod bit is constantly wearing the rock to achieve the effect of coring. e effective drilling into the core sample displacement can be well distinguished. Hence, the proposed calculation formula for the drillhole depth could also be used in the rock layer geology.
e key is to identify the stage b. According to Figure 10(a), the conditions for identifying stage b are (1) M ≥ 1.5, (2) ΔM ≤ 3% and the duration is greater than 10 s. e drillhole depth is then calculated by formulas (2) and (3). e calculation result is close to the drillhole depth calculated by the traditional manual calculation method ( Figure 11). erefore, the applicability of proposed monitoring methods in the rock layer geology is validated.

Conclusion
A drillhole depth monitoring system while drilling is designed based on summarizing the existing engineering geological exploration drilling technology, and the test results are analyzed. e conclusions are summarized as follows: (1) Combined with the working characteristics of drilling coring rigs commonly used at this stage, an intelligent drillhole depth monitoring system is built based on the drilling parameter. e system consists of hardware and software, and it could be installed conveniently on-site drilling rigs. (2) Different working stages of the rig could be matched with the oil pressure and laser displacement data collected by the monitoring system. It is concluded that the effective drillhole depth is reflected by stages a and b. rough the data analysis from stage a and b, the calculation formula of the drillhole depth suitable for soil layer geology is proposed. Moreover, the calculated drillhole depth is close to the actual situation. (3) rough the application of monitoring system in the rock layer geology, the effective drillhole depth is mainly concentrated in stage b. e results show that the drillhole depth calculated by the proposed formula agrees well with the actual situation, which validates the feasibility of the proposed method and system.
In summary, this paper proposes an intelligent drillhole depth monitoring system for engineering investigation. It is noted that this system is used in common hydraulic core drilling rigs, its applicability to other types of drilling rigs will be further verified. In addition, the proposed system has not been used in the weak intercalated layer, but it would be studied in the future.

Data Availability
e data used to support the findings of the study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Shock and Vibration 7