A Study on the Mechanisms of Tension-Shear Failure and the Estimation Model for Key Parameters of Free-Face-Assisted Rock Breaking

One huge challenge TBM construction face is to improve the breaking ability of cutters for hard rocks, making the studies on new rock breaking approaches for cutters very important. Although a lot of previous tests have proved that it is feasible to reduce the cutter force by free-face-assisted rock breaking (FM), the mechanisms behind such feasibility and the estimation methods for key parameters involved, including the maximum free face distance, crushing angle, and cutting force, remain to be studied further, limiting its applications in cutterhead design and engineering construction practice. Based on the analysis of the phenomena and laws of FM tests, this paper proposes the tensile-shear failure mechanism of FM and a piecewise linear failure criterion, which could explain the reason for the reduction of cutting force. Subsequently, a series of estimation models for the parameters above are proposed, and a series of FM tests were performed. By comparing the data obtained from the tests and calculations of the estimation model and CSM model, the estimation model in this paper is verified for its feasibility and limitations, which offered some insights on these aspects in follow-up research.

Productivity Analysis of Raisebore RB-50X for Raise Slot Drilling at Extraction level GBC PT. Freeport Indonesia

PT Freeport Indonesia is the largest gold and copper mine in Indonesia, located in Mimika Regency, Papua Province. In its operation, PT. Freeport Indonesia applies two mining systems, namely open pit mining and underground mining. Underground Mine at PT. Freeport Indonesia consists of DOZ, Big Gosan, DMLZ, Kucing Liar, and GBC, with one of the methods applied by PT. Freeport Indonesia is a block caving method. The Grasberg Block Cave mine has entered the production stage since September 2018, but in its operation the GBC Mine is still developing its operational area to meet production targets (development). The development cycle (cycle development), which is the main operation carried out, namely drilling and blasting, loading & transporting (mucking & hauling), primary support, and secondary support. To support the main operations, it is necessary to have main support consisting of access and area, ventilation, water and water, electricity, pull of tests, surveys, and dewatering. In the blasting process at the drawbell, one free face is needed which functions as the direction of rock collapsing in the blasting process on the drawbell. Making free face or commonly known as slot raise using a rasiebore machine from PT. Redpath for slot raise drilling. Meanwhile, in the drilling stage, the slot raise has a problem that has resulted in delays in the drilling process for the RB-50X raisebore tool. The impact of the delay in making the slot raise resulted in not achieving the production target due to a delay in the blasting process, where the blasting process in the drawbell area functions to break the rock so that it falls and can be transported to the crusher to enter the rock sizing process.

Indonesian Millennials: Building Metrosexual Capitalist Industry through Instagram #cowokmilenial

The digital era has changed the human lifestyle of social media use. Social media has become a platform not only to show “who one is” in cyberspace but also a platform for the capitalist industry to promote consumerism. As one of the most popular social media platforms, Instagram is utilized by the capitalist industry to achieve its goals of sales and growth. This study aims to determine how posts from Instagram with the hashtag. #cowokmilenial used by the capitalist industry build a metrosexual side. The researchers employed the qualitative approach with the semiotic method on Instagram through the hashtag #cowokmilenial during 2019. The findings in this study indicate that the capitalist industry constructs the meaning of metrosexual men among Indonesian millennials through skincare products (especially facial care/Rodeos soap) which promise to help them look handsome with a clean, bright, and acne-free face.

The Effectiveness of Free Face Mask Distribution on Use of Face Masks. A Cluster Randomised Trial in Stovner District of Oslo, Norway

Face masks are recommended as a means of reducing the spread of COVID-19, but there are practically no studies of interventions to increase face mask use. Over three weeks, nine grocery stores in the Stovner District of Oslo were randomly selected each day to have distribution of free face masks outside their entrance. Free face mask distribution increased the proportion of customers wearing a mask by 6.0 percentage points (adjusted, 95% CI 3.5–8.5). Mean mask usage was 91.7% in the control group and 97.1% in the treatment group (pooled SD 5.3%). Practically all those who wore masks had both nose and mouth covered. We conclude that free distribution of face masks increased their use. Similar trials can be conducted within a short period of time.

Numerical Study on the Mechanism and Application of Artificial Free Surfaces in Bedrock Blasting of Shield Tunnels

During the pretreatment construction of blasting in shield tunnel bedrock, in order to reduce the impact of blasting vibration on the surrounding environment and improve the effect of rock blasting, the method of creating an artificial free surface is proposed. From the point of creating an artificial free surface, this paper numerically studies the function mechanism and parameter optimization of artificial free faces in shield tunnel bedrock blasting construction. The propagation characteristics of explosion stress waves at the interface between the rock and the artificial free face and the effect of the artificial free face on the shield tunnel bedrock blasting were analyzed. The results indicate that, as the explosion stress wave transmits to the artificial free face, a part of the stress wave is reflected back to the bedrock, increasing the energy in the bedrock that needs blasting and improving the blasting effect and utilization rate of the blasting energy. The reduction degree of the peak velocity of the surface particle is more than 50%, and the reduction degree of the peak velocity of the particle near the artificial free face is more than 77%. The existence of the artificial free face reflects the stress wave and superimposes with the original stress waves, increasing the effective stress in the blasting area, and the effective stress can be increased by 5 MPa or more. The peak vibration velocity of the surface particle decreases with an increasing diameter of the empty holes and the distance between the empty holes and the blasting holes. The parameter design value of the artificial free face is put forward: the diameter of the hole is 200 mm, the distance between the empty holes and the center of the blasting holes is 60 cm, and the depth of the empty hole is the same as the blasting hole.

Theoretical Analysis on Distribution Pattern of Plastic Zone in Surrounding Rock of High-Gas-Coal Roadway

The formation and expansion of the plastic zone is always accompanied by the deformation and failure of the roadway-surrounding rock. Based on elastoplastic theory, this paper considers the gas pressure parameters and uses the Mohr–Coulomb strength criterion to derive the implicit equation of the plastic zone boundary in the rock surrounding gas-coal roadways. The distribution characteristics of the plastic zone of gas-coal roadway-surrounding rock are studied, and the sensitivity to the gas pressure, cohesion, internal friction angle, and support strength of the roadway free face on the plastic zone of the surrounding rock is analyzed. The research results show that the plastic zone of the surrounding rock has four distribution patterns: circular, elliptical, rounded rectangle, and butterfly. Additionally, the lateral pressure coefficient, gas pressure, cohesion, and internal friction angle are found to jointly determine the distribution and range of the plastic zone. However, the support strength of the roadway free face does not change the distribution of the plastic zone but only affects its range. The circular and elliptical plastic zones are less sensitive to gas pressure, cohesion, and internal friction angle, whereas butterfly-shaped plastic zones are highly sensitive to these factors. The main manifestation of this sensitivity is that the four butterfly leaves degenerate rapidly with any decrease in the gas pressure or increase in the cohesion and internal friction angle. Larger butterfly leaves are prone to faster degeneration. The research results presented in this paper have important theoretical guiding significance and engineering application value for the design of high-gas-coal roadway support and gas drilling.