Defaunación

Figures 8.3 & 8.4 depict underground monitoring (see Plate 22).

The condition of water percolation/ sleepage in the underground workings were monitored closely at regular intervals. The analysis of data obtained so far indicated that the workings fall under ‘Dry and Moist’ category. There was no significant change in the ground water seepage conditions.

The support analysis on the rockmass, fill material and on timber supports of the proposed Cut and Fill stope indicated that an unsupported span of around 8–10 m can be kept during stoping. However, the maximum stand up time of unsupported excavation was worked out to be around 2 months. The analysis also showed that systematic and concurrent filling of stope would minimise the stope wall convergence.

For the purpose of monitoring, Load Cells (20 T to 50 T capacity) were installed at the stope back over timber support to measure load deformation. Physiomechanical properties of the fill material and analysis of support characteristics of fill material are vital parameters required for designing proper support system. The envisaged stope recoveries from the proposed cut and fill stoping would be 50%.

8.1.3 Geo-technical Investigations at Perlite Mine

Indian Bureau of Mines has carried out investigation for the purpose of (i) collection and testing of rock samples from the mine adits for determining the physio-mechanical properties of rock and rock mass classification (ii) carrying out numerical modelling studies of stope area for determining the maximum width of the galleries, height and size of pillar, factor of safety and stand-up time (iii) conducting blast vibration study to assess the likely effect of blasting in the neighbouring areas as per technical Circular No.7 of 1997 issued by DGMS.

The mine is located on an isolated hill amidst rock type, i.e., basalt, rhyolite, perlite and limestone and nearby a temple.

The geotechnical investigation was carried out for adit No.1 and adit No.2. The rock conditions exposed in the adit and cross cuts were mapped in detail by scanline mapping. About 14 samples were collected from cross cuts and roof of adit for assessing the physio-mechanical properties of rock. Since no borehole was drilled inside the adits or in the neighbourhood areas, it was not possible to assess various strength properties of adjacent rock. Therefore, representative block samples were collected from different locations of the adits.

Adit No.1 was advanced up to 46 m with maximum 3.4 m width and Adit No.2 was advanced up to 44 m with 2.4 m maximum width. The height of the adits varied from 2 m to 3.3 m. Both the adits were parallel and connected with cross cuts. There was seepage of water in the adit. The roof and sites of adits were strong and therefore no support was required during the development. The samples of rhyolite, perlite and basalt collected from the adits were tested in the laboratory and the test results are as below (Refer Tables 8.1 & 8.2).

Indian Bureau of Mines

121 121 121 121 121

Application of Rock Mechanics in Opencast & Underground Mines — Case Studies

1. Basalt 5 8.0 x 8.0 800 1250.00 Very strong rock

2. Rhyolite 4 8.0 x 8.0 300 468.75 Medium strong rock

3. Perlite 5 8.0 x 8.0 450 703.13 Strong rock

S. No. Rock Type No. of Samples Size (cm) Average Load at Failure (kN) Remarks As per ISRM Uniaxial Compressive Strength (kg/cm2₎ Table 8.1: Summary of Test Results

The uniaxial compressive strength of basalt, rhyolite and perlite has shown the rock as very hard and strong and that which can withstand high confining pressure. Based on geological mapping, the values of ‘Q’ and ‘RMR’ for each scanline have been determined and these are depicted in Table 8.2.

The geomechanic classification of the rocks places them under the ‘fair category’. The Q values varied from 10.00 to 21.33 and RMR values varied from 50.48 to 55.96, which indicate that the rock mass is stable and did not require any support. Design parameters for optimising pillar height, width and gallery sizes were formalised.

The load to be borne by pillar would depend on the strength of the floor and roof material. The optimum width to height (w:h) ratio of 10:1 is required for the purpose stability of pillars. The pillar strength also depends on the rock mass strength of the pillar material, shape & size of the pillar and discontinuities therein. Empirical procedure was adopted for calculation of factor of safety (FS). For

pillar of width : W_p= 4 m; height : h = 6 m; width of the opening W_o= 12 m and 14 m, depth from the

surface = 50 m; and unit weight of rock mass γ = 0.2 MN/m3_{; with Q value of 21.3; and RMR value of}

55.9; and MRMR value of 52.0; and that whose rock mass falls under ‘Fair category’, the value of the

pillar strength/uniaxial compressive strength, i.e., σs.av/σc for W_p/W_h = 4 m/6 m = 0.66 and for fair

quality rock mass, the value of average pillar strength/UCS which is σ_ls.av/ σc = 0.55 (where, σc = 70.3

MPa for perlite) and σ_ls.av = 0.55* 70.3 = 38.6 MPa and the average pillar stress σ_p for rib pillar is

σ_p = γ .z (1+ W_o/W_p). σ_p = 0.2*50 (1+12/4) = 30 MPa, the factor of safety of the pillar is given by the

formula,

FS = Av. Pillar strength/ Av. pillar stress, i.e., FS = 38.6/30 = 1.3.

Similarly, for the width of opening W_o = 14 m, considering W_p/W_h = 0.66, the factor of safety

works to be 0.9, for the perlite bearing pillars.

The stability of the pillar usually depends upon size, shape, elastic properties, rock mass quality and amount of deformation. It would also depend on the roof and floor rocks and ratio of span of yielding zone to depth below the surface. The size of the pillar should be kept wider to avoid failure. The shape of the pillar was kept with W:H ratio, i.e., 4 m:6 m. In future, if the pillar width needs to be extended at the base, the ( W/H) ratio of 4:1 would be an ideal proposition. The pillar has been designed for the factor of safety of 1.5 for the ultimate extraction of pillars.

For W:H ratio less than 4.5 :1

Pillar strength Ps = 133 Wo.50_/H0.75

= 133 x 4.00.50_/6.0 0.75

= 59.1 MPa

Scanline No. Av.RQD Jn Jr Ja Jw SRF Q RMR

1. 62.50 5 1.5 0.75 1.0 2.5 10.00 53.00 2. 56.66 4 1.5 0.75 1.0 2.5 11.33 53.48 3. 70.00 5 1.5 0.75 1.0 2.5 11.20 53.44 4. 78.75 3 1.5 0.75 1.0 2.5 21.00 55.89 5. 70.00 3 1.5 0.75 1.0 2.5 18.66 55.43 6. 59.00 9 1.5 0.75 1.0 2.5 5.25 50.48 7. 74.54 3 1.5 0.75 1.0 2.5 19.87 55.68 8. 80.00 3 1.5 0.75 1.0 2.5 21.33 55.96

Table 8.2 Rock Mass Classification of Adit No.1 and 2