| Hydro power caverns


The prevailing host rock type is listed in the accompanying tables instead of combinations of formations. Average values represent preliminary evaluations of the past, and are by no means a prediction of cavern host rock formations in already distinguished regions.


4. Typical rock pillars between caverns The evaluation of Figure 3 shows the close correlation


of intact rock pillar of cavern groups with the main power cavern height. A maximum value of 50m rock pillar width seems to cover all risks of unwanted structural failure modes of cavern groups even for large cavern height of ~85m. Furthermore, the cavern width seems to not be a valid criterion for the required pillar. Considerable influences on the required rock pillar,


like height, rock type and rock mass compressive strength have been discussed in previous Yearbooks. It may be concluded from the figures presented in this edition that an intact rock pillar height of 50m is a suitable asymptotic value regardless of cavern height and unfavourable rock quality, particularly within distinctly bedded and inclined rock layers.


5. Geotechnical risks at hydro caverns Geotechnical risks during cavern construction are


comparable with those associated with tunnel construction, but with significantly higher impact with regard to delays and costs due to the almost impossible task of support reinforcement at higher excavation levels. In previous case histories, ‘adverse site conditions’


are seen as part of a realized risk within all possible construction risks.


Even in ‘best case’ scenarios, studies show that the number of risks increase by 40-50% during construction. As a summary of general or project- specific investigations, the percentage observations of considerable, serious, and severe geotechnical risk impacts may be 55-65%, 15-20%, and 20-25%, respectively.


An ideal distribution for four impact grades, including disastrous risks, would be 54%, 27%, 13%, and 6% for a first attempt. A 67% serious risk and 33% severe risk occurred due


to late realisation under time pressure. Likely risk is 25%, occasional risk is 55%, and unlikely risk is 20%. A list of adverse site conditions at approximately 50


hydro caverns was first compiled and published by Hoek & Brown in 1980[10] Fernandez in 1983[7]


, and again by Hendron & , and were taken as starter values


for the addition of projects post-1980. The observation of adverse site conditions were


taken as risks for comparison with earlier editions of the hydro cavern tables introduction. As expected, three separate rock type groups show different contributions of risk groups, and the conclusions reached in the 2015 edition of the Yearbook are repeated here, i.e.: ● Sedimentary rocks are sensitive to adverse change in strong support measures, visible by the number of caverns with post-tensioned anchors and parameter variations for poor rock quality. This observation is similar in slightly metamorphic rocks.


● Bedded rocks are sensitive to structurally controlled cavern contour failure, clearly visible from this modelling item.


● Magmatic rocks are highly sensitive to the influence of lateral in situ stresses. This is also found f


www.waterpowermagazine.com | Yearbook 2022 | 295


The seven most frequent geotechnical risks are:


Layered/intercalated rocks Structure as blocks/wedges Disturbed or clay-filled zones Valley effect, varied stresses Water inflow, permeability Very poor rock or rock burst Large deformation, squeeze


20% 20% 20% 15% 10% 7.5% 7.5%


Typical work performance risks are: Delayed field investigations


Delayed support installation Monitoring system ineffective Poor applied work sequence Poor support performance Poor shotcrete/grout quality Grout-take massively higher


~20% ~25% ~20% ~10% ~10% 7.5% 7.5%


Figure 1. Total capacity: evaluation of 64 conventional and 43 pumped storage plants in China


14 12 10 8 6 4 2 0


-200


300 800 1300 1800 2300 2800 3300 3800 4300 4800 Conventional PSP


Poly. (Conventional) Poly. (PSP)


Figure 2. Cavern host rock formations: evaluation of 49, 21, and 46 conventional, pumped storage, and plants referred by Hudson & Feng[12]


20 16 12 8 4 0


Observations


Calcar. Sediments


Effusive Intrusive


01 234 5 Conventional


Pump storage Hudson & Feng


Figure 3. Cavern group pillars related to power cavern size: evaluation of 40 conventional and 10 pumped storage plants


50 40 30 20 10


y = 0.297x +19.4


y = 0.22x +12 30 40 Trafo 50 Pillar 60 Linear (Trafo) 70 80 Linear (Pillar) 90


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241  |  Page 242  |  Page 243  |  Page 244  |  Page 245  |  Page 246  |  Page 247  |  Page 248  |  Page 249  |  Page 250  |  Page 251  |  Page 252  |  Page 253  |  Page 254  |  Page 255  |  Page 256  |  Page 257  |  Page 258  |  Page 259  |  Page 260  |  Page 261  |  Page 262  |  Page 263  |  Page 264  |  Page 265  |  Page 266  |  Page 267  |  Page 268  |  Page 269  |  Page 270  |  Page 271  |  Page 272  |  Page 273  |  Page 274  |  Page 275  |  Page 276  |  Page 277  |  Page 278  |  Page 279  |  Page 280  |  Page 281  |  Page 282  |  Page 283  |  Page 284  |  Page 285  |  Page 286  |  Page 287  |  Page 288  |  Page 289  |  Page 290  |  Page 291  |  Page 292  |  Page 293  |  Page 294  |  Page 295  |  Page 296  |  Page 297  |  Page 298  |  Page 299  |  Page 300  |  Page 301  |  Page 302  |  Page 303  |  Page 304  |  Page 305  |  Page 306  |  Page 307  |  Page 308  |  Page 309  |  Page 310  |  Page 311  |  Page 312  |  Page 313  |  Page 314  |  Page 315  |  Page 316  |  Page 317  |  Page 318  |  Page 319  |  Page 320  |  Page 321  |  Page 322  |  Page 323  |  Page 324  |  Page 325  |  Page 326  |  Page 327  |  Page 328  |  Page 329  |  Page 330  |  Page 331  |  Page 332  |  Page 333  |  Page 334  |  Page 335  |  Page 336  |  Page 337  |  Page 338  |  Page 339  |  Page 340  |  Page 341  |  Page 342  |  Page 343  |  Page 344  |  Page 345  |  Page 346  |  Page 347  |  Page 348  |  Page 349  |  Page 350  |  Page 351  |  Page 352  |  Page 353  |  Page 354  |  Page 355  |  Page 356  |  Page 357  |  Page 358  |  Page 359  |  Page 360  |  Page 361  |  Page 362  |  Page 363  |  Page 364  |  Page 365  |  Page 366  |  Page 367  |  Page 368  |  Page 369  |  Page 370  |  Page 371  |  Page 372  |  Page 373  |  Page 374  |  Page 375  |  Page 376  |  Page 377  |  Page 378  |  Page 379  |  Page 380  |  Page 381  |  Page 382  |  Page 383  |  Page 384  |  Page 385  |  Page 386  |  Page 387  |  Page 388  |  Page 389  |  Page 390  |  Page 391  |  Page 392  |  Page 393  |  Page 394  |  Page 395  |  Page 396  |  Page 397  |  Page 398  |  Page 399  |  Page 400  |  Page 401  |  Page 402  |  Page 403  |  Page 404  |  Page 405  |  Page 406  |  Page 407  |  Page 408  |  Page 409  |  Page 410  |  Page 411  |  Page 412  |  Page 413  |  Page 414  |  Page 415  |  Page 416  |  Page 417  |  Page 418  |  Page 419  |  Page 420  |  Page 421  |  Page 422  |  Page 423  |  Page 424