TECHNO-ECONOMIC ASSESSMENT STUDY OSHPC …siteresources.worldbank.org/INTECCU8/Resources/563344... · TECHNO-ECONOMIC ASSESSMENT STUDY FOR ROGUN HYDROELECTRIC CONSTRUCTION PROJECT

OSHPC BARKI TOJIK TECHNO-ECONOMIC ASSESSMENT STUDY FOR ROGUN HYDROELECTRIC CONSTRUCTION PROJECT

Geology assessment February 2013

TECHNO-ECONOMIC ASSESSMENT STUDY FOR ROGUN HYDROELECTRIC CONSTRUCTION PROJECT

• Previous investigations provided abundant data for many geological issues

• Additional investigations were deemed necessary for the following issues: – Slope stability conditions in the Right Bank downstream of the dam site

– The salt wedge and Ionaksh fault

– Permeability, principally of the Right Abutment of the dam

– Ground water in the dam foundation

Geological investigations


Geological investigations

• Surface geological mapping at 1:5 000 scale

• Drilling with core recovery and permeability testing (4 boreholes, 608 m)

• Refraction seismic profiles: 3 lines, with cumulated length of 2380 m

• Microgravimetry measurements: 684 stations on 25 profiles

• Installation of 30 stations for monitoring of potential slope movements

• Inspection of existing galleries, purposely rehabilitated

• Springs: discharges measurements, chemical analyses

• Piezometer drilling and measurements (19 piezometers installed and followed-up)

• Pumping-dissolution test of Ionaksh fault zone.


Regional geology

Regional structures and tectonic units • Tajik Depression

– Rogun HPP site lies in this unit

– Mesozoic-Tertiary sedimentary cover

– Paleozoic basement

– N boundary = Vakhsh crustal fault

• Tien-Shan – Paleozoic rocks

– S boundary = Gissar crustal fault

• Pre-Gissar ‘Downfold’ – Narrow strip between crustal

faults


• Crustal faults – >500 km long, 50 km deep

– Guissar-Kokshal fault, S boundary of Tien Shan

– Illiak-Vakhsh fault, N boundary of the Tajik Depression, with evaporite domes at surface

• Regional faults in the TD – ~100 km long, rooted in

Jurassic salt layer

– Ionaksh fault

– Gulizindan fault

• Local faults – <5 km long

– Fault no.35, in dam foundation

Principal faults

Regional geology


Regional geology

Tectonic units and regional faults

View upstream

View downstream


Regional geology

• Background map: recent tectonic deformation

• Knot of faults

• Sharp bend of regional faults to NW: Gissar, Vakhsh

• Sharp bend of the Vakhsh River course to the S – In the reservoir: runs parallel to the

Vakhsh Fault

– Dam site: cuts across the regional geological structures including Ionaksh fault

– Downstream: again parallel to the regional structures

Particularities of the Rogun HPP site


• 3 units (old to recent):

Marine sedimentary layers (RB) Continental sedimentary layers (LB)

Ionaksh fault

• Upper Jurassic evaporite: halite, gypsum, anhydrite, 400 m thick; 20 m reddish mudstone cap

• Lower Cretaceous continental series: alternant sandstone, siltstone, mudstone, 1100 m thick

• Marine sedimentary series: siltstone, shales, gypsum, dolomite, limestone; often fossiliferous; 550 m thick

Site geology – Lithology


Top of salt wedge 952-970 masl, close to the GWL

Thickness increasing 15 m / 100 m depth

Base of the salt layer at Rogun HPP site ~2000 m below the river level

The salt wedge is watertight, but the residual cap is locally very permeable

Extrusion rate of salt wedge is estimated at 2.5 cm/year

Site geology – Salt dome


Site geology - Morphology

River valley across the main geological structures

Favourable topography

Narrow valley, steep slopes

Atypical flat area RB above 1600 masl

Downstream view

Upstream view


Regional thrust faults: • IONAKSH, GULIZINDAN

Local faults • Antithetic: Fault no.35,

Fault no.70

• Other faults: Fault no.28, Fault no.367

Site geology - Faults



• IONAKSH fault, GULIZINDAN fault • Rooted in the Jurassic salt layer, salt wedges extruded in the hanging wall • Oriented NE-SW, steep dip SE, SE bloc thrusts over NW bloc • Fault zone acts as hydraulic barrier in the upstream – downstream

direction, except for the residual cap above the salt wedges • IONAKSH fault – medium-high permeability measured in BH-DZ1 in the

atypical zone, at elevation 1450 (150 m above the FSL)

Regional thrust faults within the Tajik Depression unit



Local faults Antithetic faults: dip opposite to the main thrusts Fault no.35, Fault no.70

The main discontinuity system (S4) has similar attitude

Faults and discontinuities cross the dam foundation and the tunnels

The PH cavern is tackled by the zone of influence of F no.35 and is crossed by S4 discontinuities

Other faults Fault no.28, near parallel with bedding, steep dip, in the area of the outlets and

downstream pool

Fault no.367, near perpendicular to the bedding strike, steep dip, upstream, cut by Ionaksh fault. Tackles the upstream cofferdam.



Vakhsh F. Ionaksh F.

Gulizindan F.

1 mm/y 1.8 mm/y

1 mm/y

0.7 mm/y F35

Uplift rates on the main faults

Name and nature of faulting Average rate of recent vertical movements

(mm/year)

Vakhsh (SE wall) 1.0

Yonaksh Walls offset 1.8 Tectonic lens 2.8

Gulizindan 1.0

Fault no.35 Walls offset 0.7

Tectonic lens 2.3

Extrusion rate of tectonic lenses is 2-3 times higher than wall displacement Extrusion rate of salt wedge is estimated at 2.5 cm/year Monitoring results were variable from one station to another The dam is located in the tectonic bloc between Ionaksh and Gulizindan faults Present kinematic model considers pure uplift of the tectonic bloc


Site geology – Joint pattern

Most conspicuous discontinuity systems: bedding planes and S4 joints, antithetic to regional thrusts

Frequent low angle discontinuities with decimetric offset, uncertain kinematic role

S4 S4

Right Bank Left Bank


Site geology – Faults and joint pattern

TEAS conclusions

Most of the deformation is accommodated by displacements along the major faults which were studied by monitoring

Part of the deformation is diffuse, takes place as displacement along minor discontinuities

The nature of the deformation on minor discontinuities is not known ( creeping / seismic rupture ?)

The creeping rate on major faults is in the order of 1 – 2 mm/year. The rate of the minor deformation could be one or two orders of magnitude lower: 0.1 to 0.01 mm/year

This diffuse deformation is difficult to predict

The kinematic model should be refined


Slope stability – General considerations

Rock falls

Frequent occurrence, mainly a safety threat

Recommended: cleaning, rock bolts, wire meshes and monitoring

Landslides involving Quaternary deposits

Well identified since the early stages

Standard mitigation measures are recommended to prevent their development (drainage, reshaping the slopes, retention structures)

Landslides controlled by structural discontinuities (following slides)

Downstream Right Bank

Left Bank


Atypical zone initially interpreted as ancient landslide, now stabilized

TEAS observations: large scale folding overturning the sequence and small scale, tight, highly plastic folding recalling the bubble wrap folds in the Left Bank

Slope stability – Downstream Right Bank


Interpretive cross section of the Right Bank

• TEAS conclusions privilege tectonic deformation instead of landslide

• Consequences of this interpretation: • Low risk of massive landslide, provided that pore water pressure is controlled by drainage

• The NE boundary of the atypical zone is not a tear-off landslide boundary

Slope stability – Downstream Right Bank

Recommendations 1. Boreholes to investigate the piezometric level

and the permeability

2. Design of adequate drainage to control the pore water pressure

3. Exploratory gallery for investigation and monitoring of the NE boundary of the atypical zone


Slope stability – Left Bank

The risk is related with gravitational sliding along S4 discontinuities

S4 discontinuities daylight on the slopes above the Diversion Tunnels inlets and water intake

Damages to reinforced concrete already reported at the inlet of the DT

Risk expected to increase during impounding

Recommendations

Identification of persistent daylighting discontinuities

Carry out stability calculations integrating pore water pressure variation during impounding and fluctuation of reservoir level


Rock mass ClassThickness or

upper boundary

Modulus of deformability

(GPa)

Permeability (Lugeon Units)

Class I 7 to 40 1.2 - 2.5 20Class II 15-25 to 40-50 3.5 - 4.5 1 - 3Class III Top at 20 - 80 > 4 < 1Class IV Top at 60 - 140 > 4 << 1

The main distinctive characteristics of the rock mass classes (TD, 1978)

TEAS conclusions

Permeability data does not include the anisotropy mudstone/sandstone

High convergence and creeping related with siltstones in PH cavern highlighted the overestimation of rigidity and behaviour of the weak layers

Engineering geology – rock mass zoning


Engineering geology – Dam foundation

The core is judiciously founded in the mudstones of the Lower Obigarm formation – low permeability, relatively lower fracturing degree at the foundation level.

All unstable rock masses and slope deposits will have to be removed

At this stage, the designed depth of the grout curtain seems appropriate

The design of the lateral extent in the Right Bank has to be based on additional investigations of the GWL and of the permeability


Engineering geology – PH and TH caverns

The caverns are located principally in sandstone, subordinately in siltstone, deep away from the surface

Monitoring of convergence, geophysical investigations, in situ testing and joint survey showed:

Unexpected weakness of the siltstone rock mass

Impact on stability of S4 discontinuities

The proximity of the Fault no.35 calls for the accurate location of its zone of influence, in order to conceive the appropriate support measures


Engineering geology – Tunnels

Reported damages to the reinforced concrete lining occurred in different geological settings: Weak zone near Ionaksh fault (inlet of the Diversion Tunnels)

S4 discontinuities (near the inlet of the DT)

Tectonic lens near intersection of Fault no.35 and discontinuity no.111

Major lithological contact between sandstones and siltstones

TEAS conclusion The causes of the damages have to be clearly determined

The lining could be sheared by deformation on minor faults, difficult to predict by the simplified kinematic model involving only major faults

Monitoring of the deformation should be resumed and extended


Reservoir geology

The reservoir is roughly aligned on the Vakhsh and Gissar fault systems

Particularity: evaporite masses extruded along the Vakhsh fault

Sinkholes in Passimurakho valley

Sinkholes at the foot of the Vakhsh Ridge


Reservoir geology - landslides

Particular mechanism: slope adapting to dissolution of salt at the foot of the slope

Currently developing in the left bank of Passimurakho valley (photo)

TEAS conclusions • Field observations indicate slow development

• The risk of sudden occurrence of massive sliding seems low

• However, impounding and reservoir level fluctuations are rapid changes of natural conditions. The impact of these changes has to be assessed.


Reservoir geology - leakage

The shortest potential by-pass would be the Gulizindan Fault

Geological and geophysical investigations showed that this fault is watertight

At Technical Design stage, a 1 km long grouting gallery driven parallel to the fault, upstream along the Obishur valley was envisaged. This preventing measure is supported by TEAS and it should be implemented.


Assessment in progress

Issues related with the salt dome

Geotechnical assessment

Construction materials

Documents

TECHNO-ECONOMIC ASSESSMENT STUDY OSHPC …siteresources.worldbank.org/INTECCU8/Resources/563344... · TECHNO-ECONOMIC ASSESSMENT STUDY FOR ROGUN HYDROELECTRIC CONSTRUCTION PROJECT