Geological and Geotechnical Investigation for Preparation of DPR of

Geological and Geotechnical Investigation for
Preparation of DPR of Hydro-electric Projects
By
Prasanta Mishra
Director
GSI
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Dam Failure examples :
•Khadkawasla Dam, Maharashtra (1879 - 1961)Reason: inflow was much above the design flood
Kaddam Dam, Andhra Pradesh, IndiaReason: Underestimation of flood
Panshet Dam, Ambi, MaharashtraReason: Inadequate assessment of diversion flood
Nanaksagar Dam, Punjab, India
Reason: Inadequate assessment of diversion flood
Machhu II Dam, Gujarat
Reason: Abnormal floods and inadequate spillway capacity
which caused a loss of 1800 lives on August 1, 1979 .
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Partial Failure
•R. K. Nala dam, Little Andaman Island
•Vishnu Nala dam, Little Andaman Island
Reason: Cavernous limestone
Problems encountered in few projects :
1. Loktak Project: Casualty happened
encountering methane gas in the HRT.
because
of
2. Dul-Hasti HEP, J&K: TBM had to be buried and
excavation of HRT completed with great difficulty by DBM.
3. Nathpa-Jhakri HEP, HP: Encountered hot spring, thick
shear zones and low cover zone in HRT.
4. Tala HEP, Bhutan:
a) Stress related problem in underground Desilting
Chambers and HRT (b) Heavy seepage in HRT (c)
Encountering thick shear zones in HRT associated with
squeezing, tunnel collapse and chimney formations
resulting tunnel detour.
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(d) Choking of pilot hole of surge shaft resulting lifting of muck
from top (e) Choking of pressure shaft pilot hole formation of
cavities and flooding of construction adit.
(f) Roof collapse of underground powerhouse and rib erection (g)
Powerhouse floor heaving (h) Failure of rockbolts from
powerhouse wall (i) Detouring of TRT alignment (j) Nonavailability of suitable construction materials
5. Heavy seepage of muddy water at TBM face of Parvati II
6. Heavy seepage of water at TBM face of Tapovan Vishnugad.
7. Encountering thick fault zone in the HRT of Malana II.
8. Encountering enormously thick overburden at left bank of
Malana-II dam.
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Optimum Geotechnical Investigations
For Preparation of Bankable DPR of
Hydroelectric Projects
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A Bankable DPR
A bankable DPR is a document which can give
enough confidence to the lending agencies that each
component of the proposed project has been
adequately explored in such a way that there will be
very limited scope of deviation and nature of work
would not undergo major changes during
construction stage. The constraints and uncertainties
are well understood and suitable provisions are made
in design so that these are not met as surprises. The
main requirement of a DPR being bankable is the
assurance of the project being constructed within the
estimated cost and time schedule.
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DPR Stage Investigations
These can broadly be grouped under:
 Topographical Survey
 Hydro-meteorological investigation
 Geological and geotechnical investigation
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Regional Geomorphology and
Geology
 Physiography
 Geology
 Structure
 Tectonics
 Seismicity
on regional scale (1:50,000/1:2,50,000/available
larger scale)-for 50km radius or more
Catchment area study
 Regional geology including major thrusts/ faults/
lineaments
 Snowfed & rainfed areas, glaciers
 Glacial lakes and possibilities of breaking of such
lakes to produce Glacial Lake Outburst Flood
(GLOF)
 Impact of lake burst viz., surge of water discharge
and accompanying sediments-suitable measures
to prevent breaching
 Possibility of formation of landslide dams
Geomorphology and Geology of
Project site
 Geomorphology
 Geology
 Seismology
 Special features like presence of hotsprings,
paleochannels in and around the project site, if
any
Remote sensing and Photogeological
studies
 Photogeological /Remote Sensing map
 Lineament map
Preferably, on 1: 50,000 scale
Seismicity
 Both regional and local seismicity
 Site specific seismic parameters
 MEQ studies and active fault studies, if required
(Important for all large dams located in Seismic Zones
–IV & V)
Alternative Project Layout
 Various alternative studies of project layouts on the basis of
regional and local geology, topography regarding type and
location of dam, alignments of HRT, type of powerhouse,
etc
 The alternative locations are to be studied based on
geological mapping, geophysical explorations and drilling.
 Selection of final site - suitable type of diversion structure barrage/ weir or dam (concrete, rock fill, CFRD, arch dam,
earth dam etc), Desilting Chamber, HRT alignment,
surface or underground powerhouse
 Advantages and disadvantages of all these alternative
locations are to be discussed on techno-economic
consideration.
Detailed geology and geotechnical
investigations of project structures
 Diversion Structure-(Dam/Barrage/
Weir),Coffer Dams
1)Geomorphology, Detailed geological mapping
of project area, on 1: 1000 or 1:2000 scale and
atleast 50m above the dam height or more
depending on the site geology.
2)An outcrop geological map and an interpreted
geological map may be provided.
Subsurface Exploration
 Boreholes in river bed, each abutments/bank along dam
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axis, toe of the dam, spillway, bucket area and plunge pool
Depth of drilling - at least 20m within bedrock
At least two holes down to 2/3rd ‘H’ and one hole down to
‘H’ depth
To assess the rockmass condition, depth of grout curtain
and detect any adverse features like fault/shears etc
Additional drill holes as per site condition
Holes at coffer dams
Permeability tests
Test grouting
Geophysical methods for additional information/gap areas
Concrete Dam
 Drifts at 50m height interval on both abutments along
dam axis -to determine the stripping limits
 Cross cuts-both u/s & d/s
 Length of drifts-At least 30m or more depending on
geology
 Length of Cross cuts- width of the dam body at that
location
Barrage/Weir
 If founded in rock, same as concrete dam
 If founded on pervious foundation/riverine material,
holes to be drilled down to the hard rock level or to a
minimum depth of 30m below the deepest river bed
level
 SPT and/or geophysical investigations
 Drifts, if required
Diversion Tunnel
 Geological mapping
 Tentative distribution of different classes of rock mass
anticipated at the tunnel grade - stretch wise Rock
Mass Quality (Q) and Rock Mass Rating (RMR),
 Geology of the portal sites should be assessed along
with their slope stability measures.
 Boreholes, if required, - to assess the depth of
overburden, rockmass condition
 Stretch wise structural analysis of discontinuities
through stereographs, rose diagrams and wedge
analysis.
Water Conducting System
Intake
 Geological mapping of the intake tunnel on 1:1000 or larger
scale (Geology of the intake portal, depth of weathering, i.e
fresh rock profile/stripping limit)
 Drill hole to assess bedrock depth, nature of bedrock.
 Cut slopes of the portals
 Geology along intake tunnel, rock cover, tentative
rockmass condition /class.
 Drilling may be undertaken to ensure sufficient rock cover,
confirm anticipated shear, fault, weak zones etc. depending
upon site condition.
 Structural analysis of discontinuities through stereographs,
rose diagrams and wedge analysis
Desilting Chamber
 Geological mapping of all chambers along with
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appurtenant structures on 1:1000/2000 scale.
Underground structure - orientation on the basis of
rock type and structural data.
Drift for the entire length of the chamber with cross
cuts alongwith rockmass classification
Structural analysis of discontinuities through
stereographs, rose diagrams and wedge analysis
Boreholes to know the rock mass condition above the
cavern.
Hydrofracture test, to assess in situ stress field and
optimization of orientation.
Other tests - deformation modulus, shear parameters
etc.,
Geology of the portal of adits/ tunnel.
If required, bore holes at portal locations to assess
the bedrock depth and rockmass condition.
Head Race Tunnel (HRT)
 Geological mapping of the tunnel alignment on 1:5,000 / 10,000 scale.
 Orientation of the tunnel with respect to regional strike and weak
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zones of the rock formation, maximum, minimum vertical and
lateral rock covers, joint sets, low cover and high cover zones,
thrust/fault/shear/fold / lineament.
Drill holes and drifts (if required)
Tentative distribution of anticipated rockmass at the tunnel gradestretch wise Q and RMR value, rock category.
Groundwater table, presence of hot springs etc.
Structural analysis of discontinuities through stereographs, rose
diagrams and wedge analysis
Adits including portals - Geological mapping on 1:500/1000 scale
alongwith study of Rockmass condition, rock cover etc. Stability of
the portals
Bore holes, if required.
Power House Complex
 Geological mapping each component of the selected
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powerhouse complex is to be carried out at 1:
1000/2000 scale
Surge Shaft
For open to sky surge shaft, one drill hole including
Water Percolation Test (WPT). Sufficient lateral rock
cover.
For underground surge shaft, drifts at bottom and top
levels and Drill holes from drifts along the surge shaft,
if not feasible from the surface.
Pressure shaft/Penstock
For pressure shaft, investigation similar to tunnel
For penstock stability of penstock and foundation
condition of anchor blocks. Drill holes/ pits , if
required.
Power House Structure
 Surface Power house – Foundation condition determined
through drilling of boreholes - Minimum five boreholes (at
corners and centre) are required , Geophysical survey
required for gap area.
 Engineering / physical properties of the overburden
material/rockmass and structural analysis of the rockmass
for the cut slopes and foundation media.
 If, the foundation is proposed on overburden then nature
of overburden, liquefaction potentiality below the
foundation, bearing capacity of the foundation material
etc. are to be assessed.
 Stability of cut slope needs to be addressed.
 Underground powerhouse –
 Orientation of the cavern to be decided on the basis of
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prominent structural data and rock types.
A drift to be excavated along the entire length of
powerhouse, preferably a few metre below the crown
of the cavern to explore the rockmass condition.
3D Geological log of drift, both insitu and laboratory
rock mechanic tests to be carried out to work out the
design support system.
Hydrofracture test also to be carried out to determine
the principal horizontal stress for optimization of the
orientation of the powerhouse cavern.
Wedge analysis of the cavern to plan the support
system to be carried out.
Tail Race Tunnel (TRT)/Channel(TRC)
 Geological mapping along tunnel alignment.
 Salient features like orientation of the tunnel with
respect to discontinuity surfaces, maximum and
minimum cover over tunnel alignment, joint sets, low
cover and high cover zones, weak/shear zones likely to
be encountered, vertical and lateral rock covers in all
nala crossings etc.
 Drill holes to fix the TRT portal and to assess the
rockmass condition and holes along TRT alignment to
pick up any adverse features (weak/shear zones).
 For TRC, slope stability studies including
determination of geomechanical properties of the
overburden material is required.
 Geology and geomorphology of the reservoir
 Competency of reservoir, reservoir rim stability, seismic
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characteristics and its effects due to construction of dam,
possibilities of Reservoir Induced Seismicity, occurrence of
landslide dams.
If required, pits, trenches, drill holes, seismic surveys and
drifts
Geological mapping of the reservoir, up to 50 metres above
the FRL on 1: 5000/10,000 scale based on remote sensing
data and landsat imageries with field checks incorporating
details like geological units, critical zones, potential
reservoir leakage zones, structural discontinuities
Any mineral of economic importance, civil structures of
archaeological importance, human settlement etc, present
within the reservoir area
Potential sliding zones to be studied and explored to
decipher the effective remedial measures to contain them.
Monitoring system to record any sudden slope movement
during reservoir filling and afterwards may also be
envisaged, if required.
Construction Material
 Suitability tests for both coarse and fine aggregates
for wearing and non-wearing surface concrete.
 Availability of suitable construction material in
adequate quantity.
 Geological mapping of the selected quarry areas in 1:
1000 or less and if required, pitting, trenching or drill
holes/small drifts to establish the lateral and vertical
continuity of the source materials , vis-à-vis reserve
estimation.
Laboratory Tests
 Petrography
 Physical parameters - Bulk density, specific gravity,
grain density, water content at saturation, apparent
porosity, density and engineering parameters like
uniaxial compressive strength, modulus of
deformation, triaxial compressive strength (c and phi)
and compression and shear test.
 Required physico-mechanical and chemical test for
assessing suitability of the construction material to be
carried out and their availability in adequate quantity
to be ensured.
 To assess the corroding / abrading potential of the
suspended material carried by the water to the
turbines, composition and angularity of the
suspended materials collected systematically from
different G&D sites need to be assessed periodically
How to improve the DPR
 Quality geological Mapping
 Quality drilling
 Use of softwares for data interpretation
 Suitable time frame for preparation of DPR
 Resources of the agencies preparing DPR
Quality Geological Mapping
 Outcrop geological map
 Interpreted geological map
 Faithful recording of geological data
 Interpretation of geological data
 Presentation of map on suitable scale for appraisal of data clearly
 There is tendency to club the different lithounits into a single unit
even all units are mappable. Such as quartzite-phyllite, though
quartzite and phyllites can be mapped separately in many cases.
 All mappable units should be shown in the geological map and
sections so that the foundation condition of the dam can be
assessed properly. Again the anticipated litho units to be present
at the PH/Desilting caverns site specially at the crown portion so
that the cavern may be shifted to competent rock units
Quality drilling
 Though drilling gives point information but is one of the most
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important tool to have an assessment of sub-surface geology and
as such proper interpretation of drill hole data plays a vital role
for a bankable DPR.
It is frequently mentioned in the DPR that 2000/3000m drilling
has been done in the project, to focus sufficient work
Achievement of drilling is considered as m/week or m/month
Quality of drilling is hardly considered
Core recovery should be considered for projection of drilling
work
It is difficult to interpret the rockmass condition from low core
recovery. 80% or more core recovery should be ensured through
drilling for a realistic interpretation
There are examples that v. poor rockmass condition was
interpreted from poor core recovery which was contradictory to
surface geological condition and drifts were excavated to know
the actual rockmass condition. It was surprising that no support
was provided in the drifts owing to good rockmass condition.
Contd..
Quality drilling
 Core recovery can be increased by
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a. Using good quality drilling bit and drilling rod
b. controlling rotation and vibration of machine
c. controlling circulation of water
d. using short runs
Triple tube core barrel be used in weak zones and soft rocks
Wireline drilling may also be used
Driller’s observations should also be reflected in the DPR
such as sudden drop of drill bit due to presence of cavity,
rate of penetration, water loss, colour of return water etc.
Core should be properly numbered and maintained
Number of core pieces in a run may also be noted while
logging. This can help to assess the rockmass condition,
spacing of discontinuities, mechanical breakage etc.
Use of softwares for data interpretation
 Preparation of geological map, bedrock contour map,
interpretation of discontinuity data through stereoplots to
decipher direction and inclination of anchors/rockbolts for
effective stitching, direction and angle of grout holes etc., wedge
analysis, interpretation of core recovery-RQD- Rockmass Quality
(Q)- permeability of the rockmass etc. are being widely carried
out using softwares
 Softwares like AutoCAD , Surfer, DIPS, Unwedge, Swedge, Slide,
RockFall, RocPlane, ROCDATA, stereoplot, SlopeW are useful
 Many of the project authorities are using these softwares for
quick interpretation of the data. The presentation of data in the
DPR is also of good quality.
 The input for using the software should be realistic to have a
proper interpretation.
Contd..
 For example:
 In case of wedge analysis, the angle of internal friction of
rockmass are taken more than 50° and even 75° in many
cases. This is taken from insitu rock mechanic test in the
drift for the dam which is valid for failure towards d/s of the
dam. The angle of internal friction of the rockmass should
be different and will be influenced by the presence of
valley ward dipping and other joints
 With such high angle of internal friction of rock it is often
found that the wedges are stable but at the same time
suitable protective measures like rock bolting, anchoring
etc. are being kept in the design
 This contradiction doubts the reliability of the input data
 Therefore, purity of input data should be ensured for using
the softwares.
Stereoplots
 Stereoplot is a very effective tool for analysis of
discontinuity data and should be worked out in all the
projects.
 All the discontinuity data can be analysed at a time through
stereoplots and is not dependent on high technology
software's
 Analysis of data can be done by following procedures:
1. contouring of poles and determination of average
plane
2. determination of plunge of intersection of different
joints i.e., wedges
3. plotting of angle of internal friction and cut
slope/walls and crown of tunnel/cavern
4. The vulnerable wedges will be between the envelope of
cut slope and more than the angle of internal friction
Contd..
Identifying the vulnerable wedges and drawing of great circle
containing the line of intersection of joints
6. Deciphering the direction and inclination of
anchors/rockbolts for cut slopes/tunnel walls and crown. This
should be a point on the great circle perpendicular to the
great circle containing wedges/intersection of joints.
Gradation of joints may be done on the characteristics
(frequency, continuity, joint alteration, filling, joint surface
etc.) and thrust should be given accordingly
7. Direction of rock bolting are generally kept perpendicular to
the cut surface. It is often found that rockbolts are parallel to
the vertical/sub vertical transverse joints along the walls of
the tunnel or parallel to the longitudinal vertical joints at the
crown along the tunnels. In both the cases rock bolts become
ornamental. Therefore, calculated orientation of rockbolts
would enhance the safety of the project.
5.
Vulnerable wedges with different values of Angle of internal
friction showing variation in no. of vulnerable wedges
Stereoplots showing vulnerable envelop and direction of rockbolts
along cut slopes
Stereoplots showing vulnerable envelop and direction of rockbolts
along walls of desilting chamber
Wedge Analysis
 The software unwedge is a very good tool for
analysis of wedges.
 Purity of input data should be ensured while using
this software
 It has been found in many cases that there is no
vulnerable wedges though rockbolting has been
suggested in the design. This indicates doubt
about the reliability of data by the owner of the
DPR
Suitable time frame for preparation and Resources of
the agencies preparing DPR
 Generally, 2-3 years are being provided arbitrarily for preparation of a
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DPR of Hydroelectric projects.
Time frame for preparation of DPR should be based on complexity of
geology, accessibility of the terrain, working time period, resources of
agencies etc.
Time frame for preparation of a DPR in Jharkhand or Karnataka and
interior areas in Himachal Pradesh/Arunachal Pradesh can not be
compared
Time frame for preparation of DPR should be fixed judiciously
Similarly, adequacy of resources should also be ensured to prepare a
realistic, bankable, good quality DPR
But one of the most important factor is to carry out the investigation
under the active supervision of highly experienced personnel.
Conclusion
Criteria to become a good DPR
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A good DPR of a proposed Hydro Electric Project should
contain all the essential inputs.
A quality geological investigation is essential for a
bankable DPR.
Faithful recording of geological data is essential,
interpretation may change but geology should not
change.
Adequate sub-surface exploration be made to avoid
geological surprises, especially in the dam and
powerhouse complexes.
The description should be brief but comprehensive.
Repetition of the contents should be avoided.
As much as possible the language of a DPR should be
clear and simple so that it can be understood by all.
Due care should be taken so that navigation becomes
easy.
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Thank You all !!
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