Rogun Hydropower Project

Transcription

1 Rogun Hydropower Project Dam site Geological investigations in the Downstream Right Bank Ascencio Lara Coyne et Bellier Almaty, February 2013

2 Objectives of the presentation As the stability conditions of the downstream area of Rogun dam is of high importance for the operation and safety of the dam, with this presentation we would like to: 1)Inform on the site investigation recently finished and the new data obtained; 2)Inform on the assessment under the TEAS; 3)Inform on the PoE views on that issue. 2

3 The downstream area at the right valley side The area immediately downstream of the dam site at the right valley side, was interpreted at the Feasibility stage (1978) as the result of a large scale slope instability, considered however mostly stabilized. Though alternative geological conditions with no characters of instability could explain the atypical form of the area The TEAS report RP29 of December 2012 presents the interpretation of the site investigation works undertaken after last year s decision for the need to assess the geological conditions of this out of the ordinary 3 (atypical) vast area.

4 Investigation Works These works included: -drilling of boreholes (2 of which are related directly with this area, the 2 others being at the adjacent dam site zone), -Geophysical investigation (seismic refraction and microgravimetry), -inspection after rehabilitation of the adit 1034 which starting from the dam site was entering in this area) and -a geological mapping at a scale of 1:

5 A first PoE reflection in last September from site inspection and broader geological data consideration Part of these investigation works the member of PoE Paul Marinos had the opportunity to inspect during the site visit last September. Expert interpretation results regarding the structure and conditions of this zone, were included in the PoE report of November and presented at the riparian consultations of November 2012 in Almaty. This interpretation is repeated here after 5

6 A first PoE reflection in last September from site inspection and broader geological data consideration cont d Data from the area are in favor of complex tectonics rather than for a large scale old mass movement: Uphill valley slopes. The plateau (elevation about 1700) with small depressions Down hill valley slope. Continuity of strata is clear. The perched front of a small slide can be seen Few rock debris on the surface Topography uphill does not suggest large scale movement Geophysics in 2005 anticipated shallow overburden borehole (DZ2) revealed continuous, undisturbed, compact gypsum deposits in mudstone matrix Continuous, extended, and undisturbed bed rock exposures all along the downhill slope Only mass movements of small extent are visible on downhill slopes; for long term safety, they should be monitored and if necessary stabilized.

7 The investigations and Assessment of TEAS, Dec Regional setting The TEAS report considers the geological context of the broader area and based on the geological rules that governed its tectonic evolution proceeds to the interpretation of the lithological sequences of the area and their structural settings (thrust sheets over a basic décollement in the evaporitic-salt layer, diapirsm, inversions of initial stratigraphic sequence, dramatic changes of dip and of initial thicknesses of formations, and subsequent ductile slow rate deformations concluding to the final position of the strata). This consideration is essential and is well presented in the report. Schematic regional structural setting 7

8 The geological mapping Thus the geological reconnaissance and mapping has included, with good reason, a well extended area. A sequence that was formerly interpreted as the result of landslide is now documented that in fact is due to large scale folding that has overturned the bedding dip. The expert finds that this interpretation is well documented. Geological map of the right valley side downstream of the dam 8

9 The Geological mapping cont d The lateral eastern boundary of the atypical zone, going down to the river, with sharp changes of the disposition of beds, was previously interpreted as the landslide boundary forming a considerable offset of strata. The present investigation during the geological mapping did not find any rupture/sheared zone and considers that this offset resulted from ductile deformation, such as bending. The findings of the adit 1034, which reaches the landslide boundary do not support the presence of a tear-off of beds due to landslide The expert agrees with this interpretation. 9

10 The Geological mapping cont d Already in the PoE report of November 2012, this upstream lateral limit of the atypical area was commented: The contact gives the impression that this is not the limit of a landslide but an acme of a complex tectonic flexure during the folding of the area.analogous flexures with sharp changes of the orientation of the geologic layers can be seen in the broader area.. The upstream limit of the atypical zone. A sharp structural flexure is probably the reason for the abrupt change of strike and dip of strata and not a shear due to landslide. 10

11 The Boreholes No particular features, like loose structure and mixed rock fragments, was found under the plateau that could denote disturbance from gravity (landslide) movements. A compact gypsum is present after a surficial clayey presence on the plateau, to the end of the borehole (DZ2 depth 166m). The clayey material is attributed to residual products from dissolution of gypsum and other evaporites and infilling of the sinkholes. The clayey material of the cover zone of gypsum and evaporites The plateau of the atypical area with depressions and the borehole DZ2 The compact gypsum sequence 11 in depth

12 The plateau The sinkholes on the high plateau of the atypical area, are attributed to dissolution and not to a disturbed undulated and back tilted morphology, which could be provoked by a landslide. The expert agrees with this interpretation. The presence of gypsum under the soil cover supports it. The high plateau of the atypical area with a series of sinks and small depressions 12

13 The geophysical survey The refraction seismic investigation does not show thick loose overburden material down the slope of the plateau, as it should be the case if a landslide was the reason of formation of this atypical zone. Location of the seismic refraction profiles Summary of results on line 1 according to one of 13 the interpolation methods used

14 The geophysical survey cont d The microgravity investigation seems to confirm the presence of evaporites (gypsum, salt). The expert understands that the method does not present any exciting result regarding chaotic conditions which could have as reason a landslide. Location of the seismic refraction profiles on the plateau Residual anomaly at surface (top) 14 and extrapolated to 120m depth (bottom)

15 The reasons of the atypical area The TEAS report concludes that this atypical zone has resulted from the combination of deformation related with diapiric (salt) tectonics with subsequent dissolution of evaporites and not from a landslide. The sketch of the Consultants in TEAS report gives an interpretive approach of such a model. The expert fully endorses this conclusion. Not only because no inherent landslide features were ascertained, but also because the field data support the presence of a complex tectonic folding having provoked such tectonic deformations Interpretive cross section showing the structural setting of the right valley side 15

16 Landslide risk of such a tectonic model 1. In addition the TEAS report discusses the possibility of landslide risk at such tectonic model: There is no any evidence of a preexisting unfavorably oriented weak and persisting discontinuity (such as bedding plane) or fault that could generate a massive structural failure. 2. The case of failure through the soil or rock mass if their strength as a whole is exceeded is also discussed. The expert agrees that these cases even if strength conditions could appear low would be limited in size and certainly possible to be mitigated. They cannot be considered as a threat for the feasibility of the project. They can also be prevented by an ordinary monitoring 16

17 . Other landslide presence Small scale landslide debris are present, their surface and depth extensions are small. Such debris are not of concern for the feasibility of the project. Correctly, the report does not consider such debris to represent a large scale instability issue. Zone of ancient landslide in the central part of the downstream slope area. The arrow indicates the crown of recently initiated landslide 17

18 Monitoring Geodetic monuments were installed at the upper part of the slope below the high plateau to monitor any slope movement. The expert certainly endorses this supportive activity. Location of the geodetic monitoring 18 system

19 Conditions after the impoundment The case of rise of the pore water pressure due to the future reservoir is considered by the expert not to be an issue for the stability of the downstream atypical area, since the main part of this zone is at a much higher elevation (plateau about +1700) and the ground water level should be accordingly; higher than the reservoir top level. Only at the eastern part of the area, where the atypical zone meets the abutment of the dam such a rise could have an influence to the stability of the lower slopes of that part. Drainage measures in front of the grout curtain under the dam and its extension in the abutments, as is the usual dam practice, will keep conditions stable. In all cases the mass concerned should be by far of reduced size. 19

20 Conclusion As a conclusion the TEAS report presents an assessment of the geological setting of the right bank area, downstream of the dam which is well documented. This setting is the result of tectonic deformations and not of an old landslide. In addition in this setting the conditions are favorable for an overall stability. 20