Cause and Solution to Roadway Deformation in Vietnam Underground Coal Mines

The deformation and support method of roadways have always been important issues in safemining and production. Vinacomin's statistics show that, by 2021, there will be 64.19 km of roadwaysthat need to be repaired (accounting for 25% of the total new roadways). Thus, the problem of maintainingroadway stability is facing difficulties in underground coal mines in Vietnam. To find out the causes ofroadway failures, a case study at roadways of the Khe Cham I and Khe Cham III coal mines, Vietnam, ispresented in this paper. Based on the results of a detailed field survey, the deformation characteristics ofroadways and the failure mode of support structures were investigated. The results show that the roadwaydeformation is severe and the main support cannot control surrounding rock mass. Also, the destructionof support structure is frequent on reused roadways, affecting production efficiency and work safety.Therefore, to reduce deformation and increase roadway stability, a new support method called “multistageanchor of rock bolt + cable bolt” has been developed and a new longwall mining system with criticalcoal pillar width has been proposed. The new findings of the research can provide references for scientificstudies, and apply them in Vietnam's underground coal mine practices.

Failure Analysis of Intersections of Large-Scale Variable Cross-Section Roadways in Deep Soft Rock and Study of Integrated Control Technology of Bolting and Grouting

In deep underground mining, achieving stable support for roadways along with long service life is critical and the complex geological environment at such depths frequently presents a major challenge. Owing to the coupling action of multiple factors such as deep high stress, adjacent faults, cross-layer design, weak lithology, broken surrounding rock, variable cross-sections, wide sections up to 9.9 m, and clusters of nearby chambers, there was severe deformation and breakdown in the No. 10 intersection of the roadway of large-scale variable cross-section at the − 760 m level in the Nanfeng working area of the Wuyang Coal Mine. As there are insufficient examples in engineering methods pertaining to the geological environment described above, the numerical calculation model was oversimplified and support theory underdeveloped; therefore, it is imperative to develop an effective support system for the stability and sustenance of deep roadways. In this study, a quantitative analysis of the geological environment of the roadway through field observations, borehole peeking, and ground stress testing is carried out to establish the FLAC 3D variable cross-section crossing roadway model. This model is combined with the strain softening constitutive (surrounding rock) and Mohr-Coulomb constitutive (other deep rock formations) models to construct a compression arch mechanical model for deep soft rock, based on the quadratic parabolic Mohr criterion. An integrated control technology of bolting and grouting that is mainly composed of a high-strength hollow grouting cable bolt equipped with modified cement grouting materials and a high-elongation cable bolt is developed by analyzing the strengthening properties of the surrounding rock before and after bolting, based on the Heok-Brown criterion. As a result of on-site practice, the following conclusions are drawn: (1) The plastic zone of the roof of the cross roadway is approximately 6 m deep in this environment, the tectonic stress is nearly 30 MPa, and the surrounding rock is severely fractured. (2) The deformation of the roadway progressively increases from small to large cross-sections, almost doubling at the largest cross-section. The plastic zone is concentrated at the top plate and shoulder and decreases progressively from the two sides to the bottom corner. The range of stress concentration at the sides of the intersection roadway close to the passageway is wider and higher. (3) The 7 m-thick reinforced compression arch constructed under the strengthening support scheme has a bearing capacity enhanced by 1.8 to 2.3 times and increase in thickness of the bearing structure by 1.76 times as compared to the original scheme. (4) The increase in the mechanical parameters c and φ of the surrounding rock after anchoring causes a significant increase in σc and σt; the pulling force of the cable bolt beneath the new grouting material is more than twice that of ordinary cement grout, and according to the test, the supporting stress field shows that the 7.24 m surrounding rock is compacted and strengthened in addition to providing a strong foundation for the bolt (cable). On-site monitoring shows that the 60-day convergence is less than 30 mm, indicating that the stability control of the roadway is successful.

Improvement of the Loading Capacity of Narrow Coal Pillars and Control Roadway Deformation in the Longwall Mining System. A Case Study at Khe Cham Coal Mine (Vietnam)

Currently, the application of coal pillars to protect an adjacent roadway is a common method in Vietnam when exploiting according to the longwall system. Therefore, the width of a coal pillar is an important issue for the stability of a roadway. In order to reduce coal loss in these coal pillars, they tend to be designed in a narrow coal pillar style but still have to ensure that the adjacent roadway can meet safe coal production conditions. The stability of roadways and coal pillars is related to many factors such as technical mechanical characteristics, physical and mechanical properties of coal, stress environment and support methods. The bearing structure of the coal pillar and the around rock a roadway is analyzed and it has been shown that enhancing roadway support and improving the carrying capacity of coal pillars can control the deformation of the surrounding rock. A study related to the stability and safety of roadways and small coal pillars in the longwall mining system has been carried out. Stabilization factors have been considered, especially the state of stress in the coal pillars and the deformation of the roadway. By applying the numerical simulation method, the stress of the coal pillar and the deformation of the adjacent roadway under different supporting solutions were analyzed and evaluated. By using this method, the rock bolt roadway support solution combined with the long cable bolt in the roadway roof and the coal pillar was selected in the safe condition of the mining process. Because cable bolt can improve the flexibility of the coal pillar such as: reducing the size of the plastic area on both sides of the pillar; enhancing coal pillar stability in the core area by providing great drag and tensile for coal pillars; contributing to improving the anchor point fixation of rock bolt. The conclusions obtained may provide a certain reference parameters to improve mining efficiency and labor safety in underground coal mines.

Analysis of Support Design in Weak Rock Drift Using a Systematic Approach

The aim of this study is to develop a systematic approach for support design of weak rock drift based on empirical, analytical, and numerical method, which is employed to estimate weak rock support demand and design support system. Detailed engineering geological investigations and rock mechanics test have been carried out in weak rock drift. The Q-system and GSI-system were used to determine the primary support design and rock mass properties, respectively. The numerical model of RS2 finite element program has been calibrated by analyzing the relation of falling height observed in the field to the frictional angles obtained from empirical method, rock mechanics test, and calculated rock mass parameters, respectively. In an attempt to check the validity of sophisticated support, support suggested by Q-system, and the combination support system proposed by analytical approach, the RS2 program was employed to analyze the depth of plastic zone and total displacement surrounding the weak rock drift. Numerical results show that the depths of plastic zone and total deformation surrounding the weak rock drift supported by the combination support system significantly descended 87% and 90% of those of sophisticated support. In particular, the rock bolt and cable bolt provide enough frictional and interlocked forces to resist weak rock falling which change the weak rock mechanicals properties and the surface holding function reinforced by the shotcrete, wire mesh, and steel strap. The factor of safety (FOS) of 8.28 of the combination support system is much more than the FOS of 1.5 for permanent drift. The combination support system with rock bolts, cable bolt, shotcrete, wire mesh, and steel straps has been applied to stabilize the weak rock drift and found to be successful to prevent further deformations surrounding the drift.

REKAYASA GEOTEKNIK UNTUK PENANGANAN SQUEEZING FAILURE DAN PENEMBUSAN AREA AMBRUKAN 1

ABSTRAKTambang emas bawah tanah UBPE Pongkor terletak di Kawasan Bayah Dome, Jawa Barat, dengan tipe endapan vein system. Sistem penambangan dilakukan dengan system cut and fill dimana upaya penembusan bijih dilakukan dengan drifting. Salah satu tambang yang memiliki kadar tinggi yaitu di area Tambang Ciurug L600 bagian Central dimana batuan di daerah tersebut memilki karakter high altered dan very weak rock strength sehingga mudah runtuh dan berpotensi untuk squeezing. Area XC 662 merupakan salah satu front yang memilki kadar Au tinggi, namun kondisi batuan yang lunak selalu menjadi permasalahannya. Dalam artikel ini akan dibahas dua cara penanganan front yang sudah dilakukan tim Geoteknik UBPE Pongkor. Kasus pertama pada XC 662 yaitu penanganan geoteknik pada batuan lemah teralterasi argilik kuat dengan tipe runtuhan squeezing failure menggunakan pendekatan Convergent Confinement. Kasus kedua pada XC 662.1 yaitu upaya penembusan pada material ambrukan (rubble material) pada heading yang sudah ditinggalkan kemudian dilakukan penembusan kembali menggunakan pendekatan teknis sederhana yang direncanakan dengan matang dan aman (Professional Judgement, Safe, and Well-Planned Method). Kasus di kedua heading ini memiliki persamaan yaitu stand-up time yang sedikit sehingga potensi delay produksi menjadi pertimbangan. Berdasarkan hasil analisis, ground support di XC 662 menggunakan kombinasi antara shotcrete setebal 15 cm dan cable bolt spasi 1,5 x 1,5 m. Maximum support pressure yang dihasilkan sebesar 2,90 MPa untuk menahan tegangan sebesar 2,16 MPa dengan nilai FS = 1,34. Ground support pada XC 662.1 menggunakan shotcrete yang diaplikasikan secara in-cycle setiap aktivitas penggalian material ambrukan, forepolling pipa spilling 6 m, batang bor, dan bantalan kayu stapling. Pemasangan pipa spilling dan batang bor dilakukan dengan spasi 20-30 cm. Bantalan kayu stapling dipasang di bagian atas forepolling untuk menghindari kontak langsung antara material ambrukan dengan pipa spilling dan batang bor. Desain ground support ini berfungsi baik untuk meningkatkan nilai kohesi pada material ambrukan dan memperpanjang stand-up time. Dari kedua kasus ini diambil kesimpulan bahwa penanganan geoteknik pada heading dengan kondisi very weak rock strength dan high altered harus dilakukan secara in-cycle dan tidak membiarkan span terlalu lama berdiri tanpa penyanggaan.Kata kunci : Cut and fill ; squeezing failure ; material ambrukan ; Convergent Confinement ; Professional JudgementABSTRACTUBPE Pongkor underground gold mine was placed at physiography of Bayah Dome, West Java, which is have vein system gold deposit. It using cut and fill mining system which using drifting mechanism for getting the ore. One of the highest grade mine location was at Ciurug L 600 Central area which have high altered and very weak rock strength characteristics so that easy to failure and have squeezing potential. XC 662 area was the other highest Au grade mine, but have the problem with the rock mass condition. This article will explain two ways about how to solve its problem which is UBPE Pongkor Geotechnical team was did it. First case at XC 662 was the engineering of squeezing failure which has very weak rock-high argillic altered by Convergent Confinement approachment. The second was placed at XC 662.1 is the engineering of rubble material support at the abandoned heading with the Professional Judgement, Safe, and Well-Planned Method approachment. This two headings have low stand-up time so that potential to making production delay. Based on analysis, the ground support of XC 662 using 15cm shotcrete and 1,5x1,5 m cable bolt so that making the maximum support pressure are 2,90 MPa to supporting 2,16 MPa pressure with FS = 1,34. The ground support of XC 662.1 using in-cycle shotcrete after mucking activities, 6m-spilling pipe forepolling, drilling rod, and wood stapling. The forepolling pipe applied with 20-30 cm spacing. Stapling wood applied above the forepolling to prevent the rubble material and spilling pipe-drilling rod contact. Its Ground support design was well-function to improved the rubble material cohesion and increase the stand-up time. The conclusion of this cases are geotechnical engineering design at very weak rock strength-high altered heading should be applied the in-cycle shotcrete and didn’t abandon the front without support for the long time.Key words : Cut and fill ; squeezing failure ; rubble material ; Convergent Confinement ; Professional Judgement

Pre-Tension Losses in Cable Bolts and their Progress in Time

Bolting and anchoring products are mostly pre-tensioned after their putting into the construction. Pre-tensioning force is a force with clearly defined size and direction and helps the construction to transfer loads. After pre-tensioning a cable bolt some pre-tension losses occur immediately after releasing the jack and other losses progress during their working time. These losses may occur due to the mechanical properties of the cable bolt and its structure itself, but also due to the properties of the surrounding rock mass.The article deals with the enumeration and description of individual losses in accordance with applicable legislation and their measurements both in the laboratory and in actual mining conditions. The comparison of calculated and measured losses and a brief overview of pre-tensionable bolting technology within OKD coal deep mines is a part of the content as well. The result of the tests is quantification of these types of pre-tension losses including proposals for their reduction.