An important problem arising in underground excavation is stabilizing the country rock that surrounds openings at depth. An increase in the in situ rock stresses is the essential difference between rocks at depth and near the surface. As a consequence of such an increase in the ground stress, rock burst may be of common occurrence in hard rocks, or large squeezing deformations may appear in soft and weak rocks. It has been detected in lots of mines that such phenomena begin to occur at the depth of about 600–800m below the surface level and become more significant below 1 000m. At these depths, conventional support devices may not be adapted for severe rock conditions.
Over the years, many researchers have tried to develop various ground support techniques and products for support and retention of the newly exposed faces and internal reinforcement of the soil and rock masses surrounding the excavations. One of them is a D bolt.
A D bolt is a smooth steel bar with a number of anchors along its length. The anchors, which can be spaced evenly or unevenly along its length, are firmly fixed within a borehole using either cement grout or resin, while the smooth sections of the bolt between the anchors may freely deform in response to rock dilation.
Pyon Kwang Nam, a section head at the Faculty of Mining Engineering, has carried out a numerical simulation of D bolt, a type of energy-absorbing rock bolt which is not fully encapsulated but multipoint anchored in a fully grouted borehole. Then, he considered the influence of the spacing arrangement of its anchors on its axial stress distribution and determined reasonable structural parameters.
The numerical simulation results show that for the D bolt with its whole length of 2.4m, the length of its exposed section of 0.1m, 4 anchors with the length of 0.1m and the ratio of the spacing between anchors (RSA) of 30:40:50:70, the maximum tensile stress of 3.25GPa is generated, which is about 1.13–1.31 times lower than other D bolts with different ratio of spacing, and the changing range of stress is also the smallest. Here, the ratio of 30:40:50:70 indicates a ratio of lengths of deformable sections which is determined by turns from the innermost section of rock mass around roadway to the outermost section of roadway space.
For more information, please refer to his paper “Numerical Simulation Study on Influence of a Structural Parameter of D Bolt, an Energy-Absorbing Rock Bolt, on its Stress Distribution” Advances in Civil Engineering” in “Advances in Civil Engineering” (SCI).
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