• Explosives and Blasting
• /
• v.41 no.3
• /
• pp.26-37
• /
• 2023

In order to determine the amount of explosives that can minimize the vibration generated during tunnel construction using the blasting method, it is necessary to derive the blasting vibration coefficients, K and n, by analyzing the vibration records of trial blasting in the field or under similar conditions. In this study, we aimed to develop a technique that can derive reasonable K and n when trial blasting cannot be performed. To this end, we collected full-scale trial blast data and studied how to predict the blast vibration coefficient (K, n) according to the type of explosive, center cut blasting method, rock origin and type, and rock grade using deep learning (DL). In addition, the correction value between full-scale and borehole trial blasting results was calculated to compensate for the limitations of the borehole trial blasting results and to carry out a design that aligns more closely with reality. In this study, when comparing the available explosive amount according to the borehole trial blasting result equation, the predictions from deep learning (DL) exceed 50%, and the result with the correction value is similar to other blast vibration estimation equations or about 20% more, enabling more economical design.

• Tunnel and Underground Space
• /
• v.33 no.5
• /
• pp.414-424
• /
• 2023

In this study, a total of 326 data on a variety of geometric parameters of pick cutters and the corresponding cuttable rock conditions were collected and built as a database. Statistical analysis of the database showed that there is a significant positive correlation between the parameters that define the geometry of a pick cutter, especially between the parameters related to the length of a pick cutter and the geometry of a tungsten carbide insert. The diameter of a pick cutter shaft was also strongly correlated with the geometry of the inserts. On the other hand, it was difficult to find a clear correlation between the parameters for the rock conditions defined by the four conditions and the geometric parameters of pick cutters, which may be due to the uncertainty of the rock mass and the fact that the application of a pick cutter is presented as a range rather than a numerical single value. However, the mean values of geometric parameters of pick cutters tend to increase as a rock mass becomes harder. However, the pick length parameters are found to decrease as a rock mass becomes harder, which may be a way to reduce the moments that can occur when using long pick cutters in a hard rock condition.

• Tunnel and Underground Space
• /
• v.33 no.5
• /
• pp.348-372
• /
• 2023

In relation to the high-level radioactive waste disposal project in deep fractured rock aquifer environments, it is essential to evaluate hydrogeological characteristics for evaluating the suitability of the site and operational stability. Such subsurface hydrogeological data is obtained through in-situ tests using boreholes excavated at the target site. The accuracy and reliability of the investigation results are directly related to the selection of appropriate test methods, the performance of the investigation system, standardization of the investigation procedure. In this report, we introduce the detailed procedures for the representative test method, the constant pressure injection test (CPIT), which is used to determine the key hydrogeological parameters of the subsurface fractured rock aquifer, namely hydraulic conductivity and storativity. This report further refines the standard test method suggested by the KSRM in 2022 and includes practical field application case conducted in volcanic rock aquifers where this investigation procedure has been applied.

• Tunnel and Underground Space
• /
• v.33 no.5
• /
• pp.388-398
• /
• 2023

The changes in groundwater flow due to mining development act as a contributing factor to major issues such as ground subsidence, strength reduction and collapse. For the sustainable mining development, measures for dealing with fluctuations in seasonal underground water inflow, power losses, pump damage, and unexpected increases in inflow must be put in place. In this study, the aim is to identify the causes of underground seepage through the examination of hydrological connectivity between the study area and nearby limestone mine. A tracer tes for assessing subsurface connectivity has been planned. A variety of tracers, such as dyes and ions, were applied in lab test to select the optimal tracer material, and a hydrological model of the study area was implemented through field test. Finally, the hydrological connectivity between the external stream and underground water in the mine was analyzed.

• Tunnel and Underground Space
• /
• v.33 no.5
• /
• pp.399-413
• /
• 2023

The present study conducted a numerical modeling of CO₂ injection at the Janggi Basin using the TOUGH-FLAC simulator, and examined the hydro-mechanical stability of the aquifer and the fault. Based on the site investigations and a 3D geological model of the target area, we simulated the injection of 32,850 tons of CO₂ over a 3-year period. The analysis of CO₂ plume with different values of the aquifer permeability revealed that assuming a permeability of 10^-14 m² the CO₂ plume exhibited a radial flow and reached the fault after 2 years and 9 months. Conversely, a higher permeability of 10^-13 m² resulted in predominant westward flow along the reservoir, with negligible impact on the fault. The pressure changes around the injection well remained below 0.6 MPa over the period, and the influence on the hydro-mechanical stability of the reservoir and fault was found to be insignificant.

• Tunnel and Underground Space
• /
• v.33 no.6
• /
• pp.534-546
• /
• 2023

If the disc cutter is excessively worn or damaged, it becomes incapable of rotating and efficiently cutting rockmass. Therefore, it is important to appropriately manage the replacement cycle of the disc cutter based on its degree of wear. In general, the replacement cycle is determined based on the results of manual inspection. However, the manual measurements has issues related to worker safety and may lead to inaccurate measurement results. For these reasons, some foreign countries are developing the real-time measurement system of disc cutter wear by using different sensors. The ultrasonic sensors, eddy current sensors, magnetic sensors, and others are utilized for measuring the wear amount of disc cutters. In this study, the applicability of eddy current sensors for real-time measurement of wear amount in TBM disc cutters was evaluated. The distance measurement accuracy of the eddy current sensor was assessed through laboratory tests. In particular, the accuracy of eddy-current sensor was evaluated in various environmental conditions within the cutterhead chamber. In addition, the measurement accuracy of the eddy current sensor was validated using a 17-inch disc cutter.

• Tunnel and Underground Space
• /
• v.33 no.6
• /
• pp.547-560
• /
• 2023

Recently, research on predicting ground classification using machine learning techniques, TBM excavation data, and ground data is increasing. In this study, a multi-classification prediction study for uniaxial compressive strength (UCS) was conducted by applying random forest model based on a decision tree among machine learning techniques widely used in various fields to machine data and ground data acquired at three slurry shield TBM sites. For the classification prediction, the training and test data were divided into 7:3, and a grid search including 5-fold cross-validation was used to select the optimal parameter. As a result of classification learning for UCS using a random forest, the accuracy of the multi-classification prediction model was found to be high at both 0.983 and 0.982 in the training set and the test set, respectively. However, due to the imbalance in data distribution between classes, the recall was evaluated low in class 4. It is judged that additional research is needed to increase the amount of measured data of UCS acquired in various sites.

• Tunnel and Underground Space
• /
• v.33 no.6
• /
• pp.574-593
• /
• 2023

In this study, the effectiveness and applicability of a newly designed Compact CNS shear box for conducting direct shear tests on jointed rock specimens were investigated. CNS joint shear tests were conducted on jointed rocks with Artificially generated roughness while varying the fracture surface roughness coefficient and initial normal stress conditions. In addition, displacement data were validated by Digital image correlation analysis, fracture patterns were observed, and comparative analysis was conducted with previously studied shear behavior prediction models. Furthermore, the accuracy of the displacement data was confirmed through DIC analysis, the fracture patterns were observed, and the shear properties obtained from the tests were compared with existing models that predict shear behavior. The findings exhibited a strong correlation with specific established empirical models for predicting shear behavior. Furthermore, the potential linkage between the characteristics of shear behavior and fracture patterns was deliberated. In conclusion, the CNS shear box was shown to be applicable and effective in providing data on the shear characteristics of the joint.

• Tunnel and Underground Space
• /
• v.33 no.6
• /
• pp.483-494
• /
• 2023

Measurement of the physical properties of rocks or minerals is an important factor in determining the distribution of the underground medium as well as mineral resource investigations. Resistivity and induced polarization, which are widely used in Korea, are methods for measuring electrical properties, which are representative properties of obtaining subsurface information. In order to precisely analyze the exploration data obtained from various sites, it is important to accurately measure the material properties. Electrical properties of rock is measured using two-electrode or four-electrode method. Compared to the four-electrode method, the two-electrode method is generally used because it is very easy to contact the sample and the electrode, but there is a problem in that the impedance of the electrode and the sample is measured together. In this study, the time-domain the induced polarization effects were measured using the 2-electrode method and the 4-electrode method for artificial samples mixed with graphite and cement having induced polarization characteristics, and the results were compared. Although the 4-electrode method has difficulties in installing potential electrodes, it was confirmed that it is effective in measuring electrical properties because it can reduce the problem caused by the impedance of potential electrodes compared to the 2-electrode method.

• Tunnel and Underground Space
• /
• v.33 no.6
• /
• pp.519-533
• /
• 2023

In this study, the linear cutting tests of pick cutters were carried out on a granitic rock with the average compressive strength over 100 MPa. From the tests, the correlation between the cutter acting force and the temperature measured by infrared thermal imaging camera during rock cutting was analyzed. In every experimental condition, the maximum temperature was measured at the rock surface where the chipping occurred, and the temperature generated in the rock was closely correlated with the cutter acting force. On the other hand, the temperature of a pick cutter increased up to only 36℃ above the ambient temperature, and the correlation with the cutter force was not obvious. This can be attributed to the short cutting distance under laboratory conditions and the high thermal conductivity of the tungsten carbide inserts. However, the relatively high temperature of the tungsten carbide inserts was found to be maintained. Therefore, it is recommended that a reinforcement between the insert and the head of a pick cutter or the quality improvement of silvering brazing in the production of a cutter is necessary to maintain the high cutting performance of a pick cutter.