Unstable Rock Blocks Research Articles

Identifying unstable rock blocks by measuring micro-tremors and vibration on cliffs

Abstract. It is important to identify unstable rock blocks and take countermeasures to prevent sudden rock fall disasters. However, identifying such blocks visually is extremely difficult, so an identification method using peculiar features of unstable blocks must be developed. The method reported here uses a vibrometer, which is inexpensive and easy to operate. In order to assess the feasibility of the method, a field experiment was carried out on rock cliffs in three regions of Japan where unstable blocks are likely to exist. Vibrometers were set up on the cliffs to capture two types of vibration waves in three dimensions, i.e., micro-tremor and reactive vibration. The former type naturally exists all the time, while the latter is generated only by applying stimulation waves. At least one of the vibrometers was installed on stable baserock to compare the results with the wave patterns of unstable rock blocks. In addition to conventional items (amplitude, frequency spectrum, vibration particle trace), trace accumulation length, that is the accumulation of the trace length of a vibrating particle for ten seconds, was introduced to analyze the patterns for both types of wave. As a result, unstable rock blocks were found to generate higher amplitudes of vibration waves than stable rock blocks, and different patterns of frequency spectrum, direction of vibration particle trace, and trace accumulation length. Hence, vibrators were shown to be useful for identifying unstable rock blocks. In particular, by using trace accumulation length as an indicator, the stability of a block can be evaluated without generating stimulative waves, providing a direction for developing a cost-effective simple method for identifying unstable blocks in future.

Design of anchorage and assessment of the stability of openings in silty, sandy limestone: a case study in Turkey

Abstract This study presents a procedure for the design of anchorage and for the assessment of the stability of openings in silty, sandy limestone through investigating the geotechnical problems of the Hasankeyf historical settlement area in southeastern Turkey, of which a good portion will stay below the reservoir of the proposed Ilisu dam. Rock mass characterization was performed. The RMR and GSI methods were utilized for rock mass classification. The RocLab software was used to determine the shear strength parameters and the geomechanical properties of the rock mass according to the GSI method. The cohesion and angle of internal friction of the rock mass was determined as 257 kPa and 52°, respectively. A back analysis was applied to a structurally controlled planar failure block supporting one of the prominent historical structures in the area to determine the shear strength parameter pairs that satisfy limit equilibrium along the sliding surface of the planar block. A sensitivity analysis of the most likely shear strength pairs satisfying limit equilibrium was performed as a function of reservoir water condition (i.e., rapid drawdown, dry and wet reservoir condition) and the expected earthquake generated peak horizontal ground acceleration coefficient. The results of the back analysis led to a cohesion of 161 kPa and an angle of internal friction of 40° along the failure surface of the planar block. A limit equilibrium sensitivity analysis of the unstable rock block was performed as a function of the water condition of the reservoir, expected earthquake generated peak horizontal ground acceleration coefficient and rock anchor inclination angle. The minimum anchor force required for the stabilization of the planar block was calculated for a rapid drawdown condition as 4000 kN/m. A total of fifty six (56) rock anchors, each with a service load capacity of 1000 kN and spaced at 1 m are recommended for stability. Finite element analysis was utilized to analyze the collapse of some of the roofs of the adjacent man-made cave dwellings carved in rock due to insufficient pillar thicknesses and to determine the minimum pillar thickness required between adjacent caves in the region for stability. Nine different pillar thicknesses ranging from 0.1 to 0.93 m were analyzed. The stages included in the finite element analysis of the caves were: (a) gravity loading due to the thickness of the overburden followed by applying the far field horizontal stresses and inserting the planar block fracture; (b) excavating the caves in stressed rock assuming full face excavation and (c) applying a uniform traction load. The results of the finite element analysis led to a recommendation to apply a steel arch support to one of the adjacent caves located at Hasankeyf in case the wall thickness was less than or equal to 0.5 m.

New designs of supports for large chamber workings

1. Calculations of underground structures located in hard and soft rock masses weakened by joint systems should be carried out on the basis of physically nonlinear models taking into account not only the presence of the joints but also their structural characteristics (the characteristic distance between joints of each of these systems and their length). 2. When constructing schemes of the limit equilibrium of potentially unstable rock blocks, it is necessary to take into account the possible step character of their surfaces, which is a consequence of weakening of the rock mass by differently oriented joint systems. Movement of such blocks occurs in the direction of the slip surfaces of the step surface, determined by the dip angle of the shear-prone joint system. 3. To support the roof of chamber workings located in masses characterized by a large value of horizontal tectonic stresses andλ≫1, it is necessary to use yielding load-bearing structural members. One of the possible effective designs is the yielding reinforced-concrete arch (Fig. 5) realized in the machine hall of the Rogun hydrostation. 4. When designing the wall bolting of large chamber workings, it is expedient to take into account the load-bearing capacity of the rock-anchored arch forming around the working. The scheme of bolting proposed in the present article is one of the possible solutions.

Calculation and program of support by anchor-shotcrete for unstable rock block in the surrounding rock of a cavern

Calculation and program of support by anchor-shotcrete for unstable rock block in the surrounding rock of a cavern