TKR-SHP Geological Investigation

8/15/2019 TKR-SHP Geological Investigation

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Small Hydro Power (SHP)

Geological Investigations

A Lecture

By

Tarun Kumar Raghuvanshi PhD

Associate Professor

School of Earth SciencesAddis Ababa University,Ethiopia



Short Term Training Course on “ DPR Preparation and Evaluation for SHP Projects” GEOLOGICAL INVESTIGATIONS

Identification of a Geologically suitable SHP site is a prerequisitefor the safe functioning and long lasting performance of any SHPproject.

General selection criteria for a suitable SHP site are;

topography, geology, hydrology, power requirement,

accessibility, socio-economic constraints, environmental protection policies, availability of funds, climatic conditions etc.

Geology is the most important factor for the safety and safe performance of a project

It can significantly affect;

site selection, design, construction and performance of the SHP project.

Small Hydro PowerGeological

Investigations

Tarun K.Raghuvanshi PhD AHEC, IIT Roorkee, August 24’ 2015



Ideally, no site is suitable from geological point of view,

There may always be some problems associated to geological setupof the area.

Identification of possible adverse and unfavorable conditions in the project area,during the investigation stage may helps in adopting proper remedial measures

This is to ensure that the project do not pose any problems during the construction orperformance stage.





Geological considerations

Safe performance is broadly dictated by Geological conditions

Geological setup must be thoroughly Assessed.

Geology –type of Rocks and soilsengineering characteristics of soil and rock mass.

Rock mass -type,strength andslope stability characteristics.

Soil mass -genetic origin,

engineering behavior andinstability potential.

Purpose of Geological investigations is to Assessengineering behavior of Rocks and Soils.





Rock Mass

Type of Rocks

Various types of Rock mass may be encountered at differentSHP sites.

Genetically, rocks may be classified as;

igneous, sedimentary and metamorphic rocks

Granite

Sandstone Gneiss

Igneous Sedimentary Metamorphic





Rock Mass

Type of Rocks Igneous rocks

These are formed by the solidification of magma eitherunderneath the surface or above it.

Igneous rocks have silicate mineralcompositions and interlockingtextures.

For the fresh igneous rock themineralogy and texture areresponsible to provide highstrength characteristics.

However, strength of these rocks

mass may be influenced by thespecific rock type, degree ofweathering and intensity of joints.

Some common type of igneous rocksare; Basalt, Granite, Gabbro, Tuffetc.

Basalt Granite

Gabbro Tuff





Rock Mass

Type of Rocks Sedimentary rocks

Derived from pre-existing rocks, erosion, transportation and deposition

by wind, water, glacier, etc.

The loose sediments undergo compaction and form sedimentary rocks.

Sedimentary rocks present many challenges to the engineering geologist.

Primary sedimentary structures such as bedding planes creates discontinuities in addition to secondary structures like joints and faults.

These primary and secondarystructures reduce rock strength.





Rock Mass

Type of Rocks Sedimentary rocks

Some common type of sedimentary rocks are;

sandstone, limestone, shale, dolomite etc.

Sandstone Limestone Breccia

Conglomerate Dolomite Shale





Rock Mass

Type of Rocks Metamorphic rocks

These rocks are formed by the alteration of pre-existing rocksby the action of temperature or pressure or both.

Metamorphism causes textural, structural and mineralogical changes in the originalrock this result in the modification of its physical properties.

Some common type of metamorphic rocks are;

Quartzite, Marble, Gneiss, Schist, phylites etc.

Gneiss Marble Slate





Rock Mass

Weathering of Rock mass

Weathering plays a significant role in altering the engineeringproperties of rock mass

Weathering may influence all types of rocks, however the degree of weathering maydepend upon;

rock type,

the type of weathering process, the climate, the environment to which the rock

is subjected and the time.

Weathering may be divided into two;

Physical disintegration, Chemical decomposition

Highly weathered Limestone – Central Ethiopia





Rock Mass

Discontinuities in rock mass

The rock mass contains discontinuity planes in the form of joints, fault planes, bedding plane and foliations.

Three sets of overstressed

discontinuities in Quartzite

Discontinuities due to Cooling

of volcanic rocks

Discontinuities due to

Sedimentary beddings

These discontinuity planes are responsible to give different physical properties torock mass.

These discontinuity planes are responsible to control the engineering properties of

the rock mass





Fresh Jointed Quartzite

Rock along road cut

Arunachal Pradesh





Rock Mass

Discontinuities in rock mass

While making an selection of various components of SHP we needto assess the general characteristics of these discontinuityplanes.

Discontinuities Characteristics

The important factors of discontinuity planes

which influence the properties of the rock mass are;

Orientation, Spacing, Continuity, Surface Characteristics, Separation of discontinuity Surface and Thickness and nature of filling material





Rock Mass

Discontinuities inrock mass

DiscontinuitiesCharacteristics



GEOLOGICAL INVESTIGATIONS



Rock Mass

Orientation

Discontinuities in slates day light the slope at lessthan the slope angle (Tons Valley 14 km Milestone

Kwanu- Minas Road, Uttrakhand, India)

Plunge of line of intersection of twowedge forming planes day light the slope

at less than the slope angle



Sh i i C “ d l f ” GEOLOGICAL INVESTIGATIONS



Rock Mass

Engineering behavior of rock mass

The engineering behavior of rock mass in terms of its bearingcapacity or for slope stability will mainly be controlled by;

the rock type, degree of weathering and the characteristic of discontinuities present in it.

While making a selection of site for SHP we need to look that what type of rocksare present at different SHP component locations.

Some rocks are generally weak like;

shales, clay stone, mud stone, tuff

whereas there are rocks which are generally competent like;

granite, basalt, quartzite, sandstone, limestone etc.

The strength of rocks may highly be influenced by degree of weathering and thecharacteristic of discontinuities.



Sh t T T i i C “DPR P i d E l i f SHP P j ” GEOLOGICAL INVESTIGATIONS



Rock Mass

Engineering behavior of rock mass

During the initial site selection we may look for degree ofweathering of rock mass and the general characteristic ofdiscontinuities.

If the orientation of beddings istowards the valley it is obvious thatsuch rock mass may pose problem of

instability if any engineeringcomponent is placed over it.

Orientation ofBedding towards

valley



Sh t T T i i C “DPR P ti d E l ti f SHP P j t ” GEOLOGICAL INVESTIGATIONS



Soil Mass

Different type of soil mass may be present at various SHP siteunder investigation.

Soil mass may vary in its engineering behavioralcharacteristic.

This may be due to its general composition in terms of;

particle size and shape present in it, mineralogical composition,

geometry of slope formed by the soil mass, erosional features present on slope, vegetation cover etc.



Short Term Training Course on “DPR P ti d E l ti f SHP P j t ” GEOLOGICAL INVESTIGATIONS



Soil Mass

Residual soils

These soils generally are formed by erosional and weatheringprocesses and are well compacted.

Such soils perform well for engineeringpurposes

However, a detailed analysis would be

required to know its engineering behaviorfor intended use.

During initial identification of site suchsoil mass may be preferred over theother type of soils.





Short Term Training Course on “DPR Preparation and Evaluation for SHP Projects” GEOLOGICAL INVESTIGATIONS



particle size varying from silt toboulders

Pose considerable problems if usedfor engineering purpose

Soil Mass

Alluvial Soils

contain granular material with varied particle size.

Short Term Training Course on DPR Preparation and Evaluation for SHP Projects GEOLOGICAL INVESTIGATIONS





Soil Mass

Recent Landslide deposits

Such soils may be loose and can easily be eroded by water.

Thus, these are not suitable for any engineering use.






Engineering behavior of soil mass

While making a site selection we need to see which type of soilmass is present at the proposed component of SHP project.

Well compacted residual soils may be preferred.

Compacted alluvial soils may also be considered, - may requiremore detailed investigation at later stages.

Colluvial and recent landslide deposits must be avoided.






Diversion and intake site

The rocks exposed at diversion weir and intake site must becompetent and relatively fresh so that these rocks provide

stable foundation to engineering structures.

The rock mass on valley slopes must bestable.

Any manifestation of instability such as;

orientation of beds or joints dippingtowards valley,

slope distress – bulged rockmass, presence of scarp face on the slope, toe erosion of slope by stream,

rock fall or toppling etc.must be observed.

All such conditions are not desirable forthe safety of structure.







manifestation of instability

Scarp Face Beds Dipping

into valley

Toe erosion orcutting

Toppling andSliding Rock Fall

Rock Fall







Diversion weir and intake structure may also be founded on soilmass.

It may require

thorough investigation for theengineering behavior of such soils,

adequate design and proper protection works.

All efforts must be made to selectdiversion weir and intake structure atlocations where

competent and relatively fresh andstable rock mass is exposed.






Desilting tank and Forebay tank

Desilting and forebay tank site must be located over

flat, gentle and stable slope.

The rock exposed must berelatively fresh and competent to provide stable foundation.

It may also be founded overthick well consolidated residual or old alluvial deposits,

However, more detailed investigation for engineering behavior of such soils may berequired at later stages.

In no case the desilting or forebay tank must be placed on Colluvial or unstable soil

mass.








Penstock Alignment

Penstock must be aligned along steepest slope section .

The saddle pillars must be founded over stable competent rockor soil mass

The stability of slope is theprime concern for the penstockalignment.

The beddings or other jointspresent in the rock mass mustnot favor the possibilities for theinstability, particularly for thelocations where saddle pillars areplanned along the alignment.

The penstock imposes high staticand dynamic loads through thesaddle pillars therefore the soilor rock mass must provide a safebearing strength to the pillars.

g p j





Powerhouse Site

From the geological point of view Powerhouse must be locatedover competent rock or stable soil mass.

The rock or soil mass mustpossess good bearing capacity toaccommodate high static anddynamic stresses imposed bythe operational turbines placedwithin the power-house.

The slope adjacent to streambank must be stable and thepowerhouse must be placedwell above the maximum flood

line.

g p j





Geological Investigation

The core of geological Investigation of SHP site is;

to understand various geologic principles and processes and

its possible impact over various components of SHP project.

Attempts are made

to assess the engineering behavioral characteristics of the rocks and soils for theintended engineering uses.

The rock or soil mass present at various SHPcomponent sites is assessed for;

Its bearing strength, stability,

settlement potential, alterations during performance stage and its possible impact on various components.





As a part of engineering geological investigation attempts are made;

to study the possible impact of rock and soil properties over the civil engineeringstructure and of the civil engineering structure over the foundation material.

The design of various components of SHP project will mainly be governed by thegeneral findings of the engineering geological Investigation.







Stages of Site Investigation

There are three distinct stages of site investigation:

Preliminary investigation that supports a feasibility stage of the project,

Detailed investigation which provides information necessary for project designand construction planning and

Implementation studies which permit changes in expected conditions that areincorporated in design modifications and evaluation of completed work, thussupporting the actual constructionof the project.






Preliminary Investigation

Preliminary investigation supports

the project feasibility

Address geologic factors for:

the relative suitability of alternative sites or project designs, the extent of detailed investigation required for construction and

geologic information required for basic cost estimates.








Detailed Site Investigation

Detailed site investigation is the intensive exploration of the proposed site.

The results obtained from the site explorations are incorporated into the finaldesign of the project.

Site exploration basically involves two important objectives.

To know surface and subsurface conditions.

Behavior characteristic of earth materials.






Geological Mapping

a detailed topographical survey, generally on 1:500 or 1:1000 scale, depending uponthe size of the project, is carried out.

The topographical survey is carried out for the general alignment, covering all theSHP components.

geological mapping is carried out on 1:1000 or 1:500 scale .

all lithological units, joint patterns, faults shear zones etc. are marked.

The various soil types present are also mapped.

Further, samples of rocks and soils are collected for further reference and

laboratory tests.








Inventory mapping for slope instability

Most of the run-off-the river (ROR) SHP projects are identified and developed inmountainous regions.

SHP projects are selected where the stream gradient is high.

The water conductor has to be planned through moderate to steep slopes.

Slope instability problems will be high in moderate to steep slopes.

This does not mean that all moderate to steep slopes will show instability problems

as, the instability of slope will be a collective effect of other causative andtriggering factors.






Inventory mapping for slope instability

Under inventory mapping various past landslides are identified and marked over themap.

Potential slopes which show manifestation for instabilities are marked over themap.

Manifestation for instabilities;

presence of scarp face on the steep slopes, unfavorable orientation of bedding or joints, removal of toe support, presence of evidences of slope distress, such as, development of cracks, bulging

of slope face and other such features and

presence of zones of excessive seepage.







Inventory mapping forslope instability

TYPE OF FAILURES






Investigations at Powerhouse site

Investigations carried out at powerhouse site are basically focused to know theengineering geological behavior of the foundation material.

Powerhouse must be founded over competent rock or soil mass as the electro-mechanical equipment placed in the powerhouse will impose high static and dynamicloads on the foundation material.








Investigations at Powerhouse site

The bank slope stability around the powerhouse location must be resistant to toeerosion by the stream water.

For this it is important that thepowerhouse location must be abovethe maximum flood level and

The rock or soil mass must beresistant to toe erosion.

Stream Bank instability triggered due to Toe erosion






Investigations at Penstock Alignment

The main factor which is important for penstock alignment is the stability conditionof the slope.

Data pertaining to slope stability condition must be collected.

Firstly, the potentially unstable zones are identified, based

on their field manifestations of instability, such as;

Presence of scarp faces on the steep slope section. Removal of toe support by natural or manmade activities. Presence of evidences of slope distress, such as,

development of cracks, bulging of slope face andother such features.

Presence of zones of excessive seepage.







Slope instabilityproblems along penstockalignment may damagethe powerhouse buildingalso.








Other important engineering geologicalaspect which has to be considered along thepenstock alignment is to assess thefoundation conditions at the pillar locations.

The rock or the soil mass present at thepillar locations must be competent enough tobear the high static and dynamic loads.

Thus, test pits must be dug to the requiredfoundation levels to examine the rock or soil

mass.






Investigations at Forebay and Desilting Tank Location

The rock or the soil mass present at the forebay and desilting tank location must becompetent enough to bear the load of the structure.

The adjoining slopes must be stable and

The rock or the soil mass present at the forebay and desilting tank should be

examined for its engineering behavioral characteristic for the intended use.

In case of rock mass;

the rock type, degree of weathering and discontinuity characteristics must be assesses.







Investigations at Headrace Channel

The headrace channel alignment in most of the cases will pass through the slopeswhich may be of varied soil or rock type.

The slope stability condition along the headrace alignment is of prime concern fromengineering geological point of view.





Investigations at Headrace Channel

The collection of field data is generally done following the principle of `whole topart’.

Through this principle where the entire alignment is surveyed initially and thepotentially unstable zones are identified.

based on their field manifestations of instability,

Once the critical slope sections are identified based on above mentioned instabilitymanifestations further detailed slope stability studies are carried out.

Later, based on the stability studies proper remedial measures are worked out.






Investigations at Intake and diversion weir site

Efforts must be made to locate these structures on competent rock mass.

Though, with proper design and adequate safety measures these structures can alsobe founded in soil mass.

As a part of engineering geological investigation rock mass must be investigated for

its quality and stability along the stream bank slopes and within the stream section.

Depth to fresh rock should be assessed at diversion weir and intake structure.







The exposed rock mass at diversion weir and intake structure site must bethoroughly mapped for;

its type, weathering grade and discontinuity characteristics.







Test pits can also be made to know the physical condition of rock mass particularly incase of weak rock mass.

The depth to overburden material should be assessed in the stream section.

Ideally the foundation for diversion weir must be laid down on fresh rock,

but where the thickness of overburden material is more the foundation must be laiddown at depth and proper protection for undercutting by stream water must bemade.







Diversion weir and intake structures can also be founded over soil mass,

however thorough investigation of soil mass must be carried out to know theengineering behavioral characteristic.

In case of soils the primary concern is to protect the foundation from undercutting

and internal erosion of the foundation by the stream water. Thus, detailed preventive and curative measures have to be employed to protect the

soil foundation.




THANK YOU!!

??



?Questions

Tarun K. Raghuvanshi Ph.D

School of Earthsciences

Questions ?

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TKR-SHP Geological Investigation

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