ROLE OF ENGINEERING GEOLOGY IN VARIOUS CIVIL … · ROLE OF ENGINEERING GEOLOGY IN VARIOUS CIVIL STRUCTURES - RIVER LINKING n-, S.R. Kulkarni Bharati Vidyapeetlt University, College

ROLE OF ENGINEERING GEOLOGY IN VARIOUS CIVILSTRUCTURES - RIVER LINKING

n-, S.R. KulkarniBharati Vidyapeetlt University,

College of EngineerPune-411043.

115 J, Sadashiv Peth,Laxmi Keshav Society.

PUNE-411030Tel No.:- +91-20-24477464

Er. Chandramohan HangekarSuperintending

Dam safety Organisation,Na hik-422004

([email protected])

ABSTRACT

The overall success of the river linking is based on the engineering safety of the dams.channels and structures in the link. As geological conditions that would lead to failure oftunnels, dams. aqueducts and erosion of channels are of common occurrence a geologicalstudy of the linking channels is essential. While comparing the cost of a tunnel with that of aopen channel, the cost of protection works required for the tunnels and channels must also betaken into account. Dams required to divert the channels as well as the foundations of theaqueducts will also have to be properly investigated in engineering geological perspective.Sufficient time will usually be available to carry out the necessary geological investigationsand to design and to execute appropriate protective works. But it is important to takecognizance of the problem immediately as it manifests itself and to proceed scientifically totackle it.

PREFACE:

- Aqueducts In River Linking Project

While carrying water through different regions,thousands of cross drainage works will have to be 'constructed. Very high and long aqueducts will haveto be designed before construction. Propergeological investigation of the foundations of allsuch structures is very important. The success of thewhole: project is depending on the engineeringsafety of channels, structures and dams in the link.

774

- .''- ..\:~::-

'-'"- . ',' -.', .

Is V2= input voltage at no load and loading conditions respectively;

V = volume of liquid in tank (L3);v = characteristic tangential velocity, proportional to ND (U T);X = power per unit volume parameter;. = kinematic viscosity of water (L2iT);I = dissipation rate of turbulent kinetic energy.

773

DAMS IN THE RIVER LINKING PROJECT

, -

Dams are key structures in river linking schemes. Insuch projects besides the existing old dams, new darnsor di version barrages will have to be planned. Thepresent statistics of large dams in Maharashtra is shownin the table-l.Besides being a source of wealth, darnscan also be a source of accidents, albeit it few innumber: an average of less than 1% of dams hassuffered accidents over a long period of time. Yet theresulting damage and loss of life mean that all suchaccidents are unacceptable.

Table-I Number of Large dams in Maharashtra as on 1-1-2006

Sr.No Region Total of Dams under Total. completed dams construction1 2 3 4 51 Pune 316 4l 3572 Konkan 171 24 195'"I NMR 324 31 354.)

4 Aurangabad 425 47 4725 Nagpur 147 32 l796 Amaravati 269 19 288

Total 1652 193 1845

A dam prevents the flow of water on surface, but if water is to be stored, it has also to be seenthat there is not a flow below the surface either. This means that the foundation rocks must bewatertight, and if they are not naturally so, suitable steps have to be taken to prevent loss of waterthrough them. Also, to avoid the disastrous effects of dam failure safety and stability of a damhave to be assured. These will depend among other things on the strength and soundness offoundation rocks, which in turn will depend on the nature and structure of these rocks. Ananalysis of dam failures of the past has shown that failure to recognize or to treat properly defectsin foundation rocks was responsible for a substantial number of them. The safety, stability andeffectiveness of a dam therefore will depend largely on the geological conditions at thefoundation and these must be known with accuracy and in sufficient detail before the work on adam is undertaken. Detailed geological investigations have therefore to be carried out forobtaining the necessary information about rocks at the dam site and over the reservoir area

Today, dam building calls for scientific and technical knowledge in such varied disciplines as:• Geology, seismology and geotechnical engineering• Hydrology• Soil and rock mechanics and building materials• Performance monitoring and instrumentation• Operational research and data processing, and many others

775

Each of these disciplines makes 'use of less specialized tools provided by mathematics(continuum mechanics, material strength), physics and chemistry. In terms of safety, it is in fact"rare" or "extreme" events that are of interest. that is. accidents. The usual approach is thereforeto specify the design flood and design earthquake at the design stage.

Regarding foundations, it is not only the overall behavior of a foundation that is of interest.Equally if not more important are any discontinuities and their possible persistence as channelsfor seepage, and the structural stability of the foundation under the various forces exerted bygravity, uplift pressure, hydrodynamic loads, and so on. This of coarse applies equally well toembankment dam materials (earth and rockfill) and all soil foundations.

TUNNELS IN RIVER LINKING PROJECT

Tunnel is a very essential component now-a-days in a dam project. The modernised methods andexperience in tunneling techniques-enables an engineer to opt 'for the tunnels. Howeverengineering geology plays very important role for a successful tunnel. Proper knowledge of thestrata and investigation by proper geologist is therefore very important. Collapse of a bottlenecktunnel may prove a total project economically unsafe.

GEOLOGY OF MAHARASHTRAAbout 85% of Maharashtra is covered by igneous volcanic rocks- Deccan trap basalts. In rest ofthe portions older rocks belonging to Archaen, Dharwar, Cuddapah, Vindhyan and Gondwanaseries of Indian Geology occur. The Deccan Traps mainly consist of basalts, but as there isconsiderable variation in the characteristics of basalts, and as rocks. derived from them bymodifications taking place in the volcanic process also occur. It is proposed to describe heresome case histories of dams, at which we have carried out geological work, in an attempt depictthe varying geological conditions in the Deccan Trap rocks that effect tunneling operations.

THE DECCAN TRAP ROCKS

As the suitability or otherwise of geological conditions for tunneling will depend on thecharacteristic of rocks met with along the alignment, an acquaintance with the various rock typesoccurring commonly in the Deccan Traps and their engineering behaviors is necessary.

Compact and Amygdaloidal Basalts

In the Deccan Trap basalts two main types occur: The compact nonvascular basalts without gascavities filled with secondary minerals such as zeolites. and chlorophaeite which give themspotted appearance. Both compact and amygdaloidal basalts often contain small slender laths asphenocrysts giving polyphyritic varieties.

Chlorophaeitic Basalts

Chlorophaeitic basalts in which a major portion of groundmass glass has been converted intochlorophaeite are common. When large amounts of chlorophaeite are present the rocks becomedark black.

776

Hydrothermal Alteration

Magmatic gases that produce cavities sometimes chemically alter the basalts, and this action iscalled hydrothermal alteration. The normal grey or bluish colour of basalts is turned into shadesof green. pink. red. purple or brown by hydrothermal alteration. Most commonly hydrothermalalteration brings about only such colour changes in basalts without affecting their physicalproperties but more intense hydrothermal alteration at times weakens rocks.

Tachylytes

Basalts in which practically no crystallization has taken place and which consist mostly of basaltglass are called tachylytes. They are very fine grained because of the low degree of crystallizationand pitch black when fresh. Black tachylytes on hydrothermal alteration acquire a vivid redcolour and red tachylytes are more common than the black variety. Both black and red tachylytesdisintegrate on exposure to atmosphere and fall to powder. The red powdery material commonlypassing under the name of red bole is nothing but red tachylyte which has disintegrated onexposure to atmosphere. These rocks are always troublesome when exposed in a tail channel or ifoccurring in a dam.

Volcanic Breccias

When rock fragments blown up by the explosion with which volcanic eruption often starts areconsolidated into hard rocks, volcanic breccias are formed. The explosion fragments may be heldtogether by being embedded in the basalt lava that follows or by the deposition of zeolitesbetween them. The lava matrix is most commonly normal basalt, but may consist ofhydrothermally altered basalt or chlorophaeitic basalt or red or black tachylyte. Such breccias areeasily erodable if occurring in a tail channel. Dam foundation through these rocks is veryproblematic.

ENGINEERING BEHAVIOVR

Divisional planes such as joints, bedding planes etc. are of great importance from the point ofview of dam as they are the main source of trouble during dam foundation. In the Deccan Trapbasalts joints are the only divisional planes present and their presence or absence determine thesuitability of basalts for dam foundation. Of the two main types of basalts. the compact basaltsare well jointed and therefore not suitable for tunneling. As joints provide passage ways forwater, compact basalts are likely to be water bearing. Also the fragmentation brought about bythe joints may make them unstable for excavation. Tunnels in compact basalts may thereforeprove troublesome because of the inflow of water through joints and roof falls and overbreaks.

Amygdaloidal basalts on the other hand are always unjointed and are quite impervious whenfresh. Also because of the absence of divisional planes they are stable in all kinds of cuts andexcavations. The massive impervious amygdaloidal basalts are therefore a very suitable mediumfor dam foundation.

Compact basalts are troublesome because as a rule joints in them are closely spaced. However insome compact basalt flows joints are broadly spaced, are not interconnected or are inconsistent,and tunnels in such compact basalt flows will be trouble free. Similarly top and bottom portions

777

of thick compact basalt flows become arnygdaloidal and therefore unjointed. Dams in top andbottom portions of compact basalt flow will therefore not be troublesome.

Basalts that have suffered only moderate hydrothermal alteration have the same engineeringbehaviour as the original rock, but with more intense alteration basalts are weakened and maycause di fficulty due to collapse of loose material.

Chlorophaeitic basalts and black and red tachylytes are sound rocks as long as they are notexposed to atmosphere. However. on exposure to atmosphere, they deteriorate and fall to powder.This may lead to falls from the crown.

Fractures

In addition to engineering characteristics of rocks, some other field structures also effect theoperations of dam foundation. Chief among them are fractures along which no movement hastaken place, occurring at a number of places in western parts of Deccan Trap outcrop. Somefractures are seen as vertical or steeply inclined tight clean cracks with no gap between the twosides. More commonly they are seen as vertical or steeply inclined sheet jointing. The sheetjointing is result of basalt brought about by water seeping along the fracture, and the weatheredzone may be from a few centimeters to 1.5 m in width.

The most important feature of the fracture is that they show ample geological evidence to provethat no movement has taken place among them. This shows they are not faults, but are cracksalong which no movement has taken place.

Behaviour of fracture in dams has been variable. A large number have not given any trouble, butothers have troublesome in various degrees.that they are not faults, but are cracks along which nomovement has taken place. Behaviour of fracture crossing dams has been variable. A largenumber have not given any trouble, but others have troublesome in various degrees.

CASE HISTORIES:-

At the Doodhganga dam site in Kolhapur District the Kaladgi quartzites on which the dam hasbeen founded have intercalations of shale and there is a deep pool downstream of the damalignment. The danger of slipping along the shale bands had therefore to be guarded against.

The profuse leakage taking place through the joints in the compact basalt at the foundations in thegorge portion of the Chas Kaman Dam (Maharashtra) vividly brings out the magnitude of theleakage that can take place through joints. The heavily jointed compact basalt flow on which thedam is founded in the gorge portion was left untreated and copious leakage took place even whenreservoir was only partially filled. This dam also illustrates the consequences of failure torecognize and to treat properly the defects in the foundation rocks revealed by drilling. The largenumber of short pieces of core of the exploratory drill holes had clearly indicated that thecompact basalt flow occurring at the foundations was heavily jointed. However, the rock was nottreated to prevent leakage through the joints; nor was an impervious cut off upto sufficient depthwas provided below the earth dam.

778

~..

As against this similar jointed compact basalt flows occurring at the foundations of other damssuch as Bhama Askhed, Nira Gunjavni, Nira Devghar, Urmodi, Temghar etc wereeffectively grouted during construction to prevent leakage.

Cavernous marbles have been encountered at the Kamtikhedi Weir Site of the Pench project(Maharashtra) and limestones at the sites at Koppili Dam in Assam and Navagam Dam inGujrat.

Bhakra Nangal Dam: - Engineering geological knowledge played a key role in the successfulcompletion of major engineering projects in India. Bhakra Nangal Dam is_ very good Indianexample in which, geological.considerations at the time of construction were taken care off,which played a vital role in its stability and durability. Bhakra Dam is a straight high gravity damfounded on the soft rocks and medium hard rocks belonging to siwalik supergroup, with faultzones cutting across the foundations and abutment in different attitudes (Krishnaswami, 1982).The preventive measures carried out comprised excavation of the heel clay stones andexcavations of the spillway apron, which was tied down to the sandstone member overlying thedownstream claystone band to prevent erosion. Beside these special treatments, the cross shearzones on the abutments were treated by providing concrete tunnel plugs and the entire foundationarea of the Dam was grouted. Grouting and drainage curtains were further provided fromfoundation galleries in the dam and through drainage and grouting, tunnels were provided in theabutments. No major problems have been faced during the post construction period of the dam,except for cracks in the upstream (Reddy, 1995).

Case study of Nandgavan dam-Masonry waste weir of about length 90m out of Total length of 190mwas washed away due to heavy flood. On left side some of the bucketportion of waste weir wall was also heavily damaged. Heavyscouring and undermining due to water is observed. The foundationbelow waste weir wall was also seen exposed. The basalt withcolumnar jointing is seen underneath the foundation of damagedportion of waste weir.

At Kolkewadi Dam of the 3rd stage of the Koyna Hydro ElectricProject foundations could be kept 6 meters higher by accepting 6meters of compact basalt as sufficient cover for the weakdecomposed basal part of the basalt flow upto 0.25 meters inthickness. Foundations of the Vaitarna Dam of the Vaitama HydroElectric Project could be raised through 13 meters in gorge portionby resting the dam on good volcanic breccia overlying weak jointed

basalt, instead of removing both to take the foundations below the weak jointed basalt.

CASES IN OTHER COUNTRIES:

The St. Francis Dam in California, which was built without any geological investigationswhatever, failed because the argillaceous cement of the coglomerate on which it was founded onthe right bank softened and the rock gave away.

779

The history of dam construction repeats with cases of dams where limestones have made trouble.The most important case is that of the Hales Gar Dam on the Tennessee which was built withoutsufficient geological exploration and without sufficient corrective treatment. A [he cavities in thelimestones at the foundation were not properly treated serious leakage took place and it to k tenyears and ten million dollars to stop the leakage through the limestones at the foundation.

Teton Dam: - The panel noted that the design did not provide for downstream defence againstcracking or leakage, and did not ensure sealing of the upper part of rock under the grout cap. Thegrout curtain was not constructed in three rows, and the reliance on the singie curtain was judgedto be "unduly optimistic". The dam -and foundation were not instrumented sufficiently to warn ofchanging conditions.

Austin Dam (Texas,USA) is a classical example of dam failure due to ignorance of geologicalfactors at the time of its construction in 1893. The foundation of the dam was cretaceouslimestone, shale and clay, which were highly jointed and faulted.

EROSION OF RIVER CHANNELS:An important item of investigation while investigating river links is the geology of the riverchannel carrying the actual water. In the interests of economy there is usually an attempt to locatethe spillway of a dam on either flank or in a side saddle. Where such a side spillway is providedthe flood waters leaving the spillway are led to the river by means of a specially excavated tailchannel. But the success of such a layout largely depends on the capacity of rocks along the tailchannel to resist erosion by the floods waters which have great erosive power. Because, in suchcases the river is diverted along a new course at steep gradients, and unless rocks along the tailchannel are capable of resisting erosion, rapids and falls are bound to develop. These willgradually recede towards the spillway which may in the long run be endangered because of theerosion of the foundation rocks.

CASE STUD[ES OF SP[LL CHANNEL EROS[ON [N MAHARASHTRAPanshet Dam: It is about 35 km from Pune. It is an earth dam with spillway to the right.Hyrdrothermally altered block jointed and deeply weathered compact basalt and volcanic brecciawith red lava matrix occur along the spill channel. lt is an ideal condition for serious erosion.Therefore very elaborate protection works were carried out. Three fall structure and guide wallwere constructed and all around the bed concrete apron protection wall adopted. Even after theseenergy dissipation arrangements, deep gullies have been formed.

Warasgaon Dam: It is a major masonry dam with LO.7 T.M.C storage. It is in the adjoiningvalley of panshet dam and geology is same. But as the spillway is centrally located and as thespill water is flowing on the established channel of the river, after suitable treatment, very littleerosion has taken place.

,Qn

c=>:~:....

Ghod Dam:

It is an earth dam about 95 KmN-W of Pune. The spillway islocated to the right. Bed gradientof tail channel is I in 50. Thehard rocks exposed in the tailchannel are columner jointedcompact basalt and volcanic. breccia which is soft and altered.This is an ideal condition forwaterfall. About 6-m deep waterfalls were created in just fouryears. A number of retainingwalls were constructed to resisterosion. But majority of them

collapsed. Spill channel protection has proved to be very expensive.

Gangapur Dam: It is the first major earth darn in the state about t3km west of Nashik. Thereoccur three basic dykes and sheet jointed weathered amygdaloidal basalt Tremendous tailchannel erosion has taken place. Guide walls. guide protection and small fall structures areconstructed.

Radhanagri Dam ~ It is about 6.3Km west of Kolhapur , which is a major city in southernMaharashtra. In first 150m the f2.1! is 22m. The rocks in the tilil channel are block jointed compactbasalt underlain by hrdrothermally altered volcanic breccia Deep gullies are formed.

Malangaon Minor Irrigation Tank; It is very SmtI\I M.£ tank in Dhule District. In two seasonstremendous spill channel erosion has taken place. 6m*8m*4m slabs of compact basalt areremoved and are lying up side down in the channel.

Karvand Dam( Ohule District), Manyad dam, Thulshi dam (Kolhapur Oisrtict etc.) aresome major dams in Maharashtra where serious spill channel erosion has taken place due tosimilar bed geological conditions.

Dams where original proposal of side spillway was changed to central spillway:

At number of places where tail channel erosion is indicated either the proposal for side spillwayhas to abandoned as was done at Katepurna dam, Kolkewadi dam (Koyna stay In), Warasgondam, Mula Dam, or adequate protection has to be provided for the spill channel. Spill channelprotection will usually prove to be very expensive. Therefore. while comparing the cost of a sidespillway with that of a Gorge spillway, the cost of protection works required for the spill channelof the side spillway must also be taken into account.

781

CASE STUDIES IN OrUSSA

Alikuan Dam: Spillway is constructed to the left flank. Tremendous tail channel erosion hastaken place. No rock is exposed in the spill channel. Water is allowed to flow on thicko erburdcn. which is mainly sandy soil. Very deep gullies are eventually formed. No rock is meteven upto about 20m of depth.

Full protection to the tai I channel will have to be given. A number of fall structures will have tobe constructed. Protection to the bed needs to be given. By doing this, i.e. constructing fallstructures, water cushioning, cascading, energy of the spilling will have to be dissipated.

Moreover, protective measures will have to be taken even where the tail channel meets the mainriver as that is the point from where the erosion starts backwards and if undermining takes placebelow any fall structure, which washes away resulting in continued headward erosion.

Jharanai Dam: Spillway is located to the right flank. In the left abutment hill closely jointedgneisses are exposed. After about 200 m from the spillway tail channel erosion is observed.About 5 to 6 m deep gullies are developed. There is thick overburden which has been easilyeroded. In the bottom portions of the tail channel are closely jointed and highly weatheredgneisses are exposed. They have also been easily eroded. Two fall structures are constructed. Butas they have been founded on over burden, erosion below their foundations has taken place. Thesidewall has collapsed. In the tail channel area four trial pits are taken and upto about 6m whereonly overburden is available.

Ganianala Dam: Spillway is situated to the left. At this site also tail erosion has taken place. Asa protective measure, two fall structures have been constructed. First structure is at about 30mdownstream.

Another fall structure is constructed at 50m downstream. Beyond this and upto about 300m thereare no rock exposures. There is deep weathering and thick overburden. Deep gullies are formed.Between 300m and 350m there are outcrops of gneisses. Beyond 350m and upto 800m againthere are no rock exposures. Deep weathering and thick soil overburden is present.

Ghodahado Dam: Spillway is located to the right saddle. About 54m downstream a low levelbridge is constructed for crossing the tail channel. Foundation of the spillway is reported to be onrock. The spillway is about 72m wide. In the vertical cliff to the right there are exposures ofaugen gneiss. It is very coarse drained. Joints are broadly spaced and they are few in numbers. Inthe top portions it is in weathered condition. In the left cut charnokite is exposed. It is closelyjointed. It shows spheroidal weathering.

Just downstream of the spillway there are exposures of augen gneiss. The junction between augengneiss and chamokite is not clearly exposed. There is a deeply weathered zone. Just downstreamof the bridge, tail channel erosion is seen. Two deep gullies are formed. A gully to the right isabout 11m deep and the gully to the left is about 8m deep. Beyond ch. 90m downstream on boththe banks there are exposures of chamokite. Charnokites are closely jointed. Spheroidalweathering has been developed. In the tail channel bed deep weathering is seen. Thick

782

overburden is present. In this overburden due to spilling waters deep gullies have been formed.Due to these falls headward erosion has started. Protecti ve measures have already been started.

Bhanjanagar Dam (Russel Konda Lake) : In the tail channel deep erosion has taken place. Fallstructure has been constructed. Some protective measures are being given. But even after that atabout 150m to 200m downstream deep gullies are formed. Thick overburden is present. About180m downstream in the tail channel bed a number of pot holes have been formed.

Five drill holes have been taken in the tail channel. In B.H. No.1 hard rock is met with at a depthof 2.4m in B.H. No.2 is at 18m, in B.H. No.3 hard rock is at 20.30m, in B.H. No.4. Hard rock isat 21.25m and in B.H. No.5 hard rock is at 23.15m.

Though it has been reported that hard rock is encountered, the core recovery is varying between2% and 8% only. This needs further verification and confirmation.

Remedial Proposals

As discussed above, in case of all the dams, in the tai I channel there are rarely rock exposures. Inthe case of Jharanai dam weathered rock is met at about 8m depth. But at Alikuan dam or GaniaNala even upto 18 to 20m depth rock is not available. At a number of places deep gullies, rapidsand falls are formed. They are gradually receding towards the spillway. Therefore verysystematic elaborate protective measures such as construction of series of fall structures, guidewalls, energy dissipation arrangements and bed protection wi II have to be adopted. Anextenuating feature of tail channel erosion is that it does not usually pose an immediate threat, asits harmful effects take some time to reach dangerous proportions. It thus gives time to study thephenomenon and devise preventive measures. Hence, if a tail channel is being eroded there is nonecessity to get unduly alarmed and do something desperate in panic. Sufficient time will usually

r be available to carry out the necessary geological investigations and to design and to executeappropriate protective works. But it is important to take cognizance of the problem immediatelyas it manifests itself and to proceed scientifically to tackle it.

IMPORTANCE OF PRELIMINARY GEOLOGICAL INVESTIGATIONS.

The importance of preliminary geological investigations for tunnels is well brought out by casesof tunnel in which difficult experience could have been avoided if proper preliminary geologicalwork helped to avoid difficulties.The Bhivpuri Tunnel of the Tata Electric companies has been excavated to carry the water of theThokarwadi dam to the Bhivpuri generating station. The Tunnel passes through volcanic brecciaswhich are excellent for tunneling. But at the crown level a layer of blank tachylyte about 1mthick from which roof falls took place as the tachylyte deteriorated on exposure. These falls couldhave been avoided by lowering the crown a couple of meters if the occurrence of the tachylytelayer had been established by preliminary geological investigations.Drilling along the alignment chose for the tunnel in km.3 of the Bhatsa Left Bank Canal showedthat tunneling would be difficult due to roof falls and leakage in the closely joined basaltsoccurring along the alignment. This alignment was therefore abandoned and an alternativealignment along which drilling established the occurrence of fresh amygdaloidal basalt was

783

adopted. As anticipated tunneling was completed without difficulty, proving the importance ofpreliminary geological investigationsThe Bassein Creek tunnel is another case where serious difficulties could be avoided becausepi iper geological investigations had been carried out. This runnel has been excavated near Thaneto can'y the water of the Upper vaitarna Dam across the Creek for Bombay water supply.Originally it was proposed to excavate the tunnel 50 m. below river bed. However drillingshowed up to LOOmbelow the river bed jointed compact basalts. The entire horizontal portion ofthe tunnel in the arnygdaloidal basalts was excavated without trouble. But the upper portions ofthe inclines on the two banks leading down to tunnel level had to pass through compact basaltsuntil they reached LOOmbelow river bed. In these upper portions of the inclines there was profuseleakage of water through the joined compact basalts and large quantities of water had to bepumped out. At the Kalher end the inflow was so large that it proved impractical to pump out somuch water. Work at this end had therefore to be abandoned and excavation had to be carried outonly from the KoLshet end. The difficulties experienced in the portions of the inclines passingthrough compact basalts showed that it would have been very difficult to excavate the tunnel SOmbelow the river bed and proved the correctness of the decision to lower the tunnel. It is importantto note that it was preliminary geological investigations that had revealed that conditions wouldbe difficult at 50m depth and would be favorable at l50m depth and hence the tunnel waslowered. It was thus only because preliminary explorations had been carried out that a difficultalignment was discarded and a favourable one chosen. If preliminary geological investigationshad to face very difficult conditions and this difficult situation was avoided by preliminarygeological investigations.

TUNNELS IN AMYGDALOIDAL BASALTS:

The suitability of amygdaloidal basalt for tunneling because of absence of divisional planes iswell illustrated by a number of tunnels, chief among them being the tunnels of the Bor Ghat onthe Mumbai Pune line of Central Railway. Out of the twenty five tunnels excavated inamygdaloidal basalt in the Bhor Ghats 19 have been left unlined. Even 6 tunnels that needed fullof partial lining have needed it because of extraneous factors such as fractures or hydrothermalalternation and not because of any inherent defects in the arnygdaloidal basalt.

.;. The ability of the arnygdaloidal basalt to standunsupported is very well demonstrated by theunsupported slab of amygdaloidal basalt jettingout L3m from the Pune portal of Tunnel NO.l4.The very thin wall separating Tunnel No.l7from Tunnel NO.l7 A which varies in thicknessfrom 4.5 to 6 m. also testifies for the stability ofamygdaloidal basalts. Bla ting of TunnelNo. L7 A which was excavated later by the side ofTunnel No. L7 could be carried out so close toTunnel No. L7 without damaging it only becauseof the stability of amygdaloidal basalts. The very

thin outer wall of Tunnel No.26C which is only about 2 m thick also shows how stableamygdaloidal basalts are. Another proof of stability of amygdaloidal basalts is the very thin coverof some tunnels, which is only about L.5 to 2 m. In case of tunnel number 26C it is still thinner

784

throughout its length of I.SOm, the minimum being only 80cm. In spite of such thin cover thetunnel has been left unlined except for 28.Sm from the Pune portal and 30m from Mumbai portalwhich have been lined to cover a total of seven fractures. All tunnels in amygdaloidal basalts.even including those with the thin cover. do not show any percolation even in the monsoonthough they receive a rainfall of 4000mm per year.

~: ..

A number of other tunnels have also been excavated without trouble in amygdaloidal basalts. TheHead Race Tunnel of Koyna Hydro Project Stage 3 lies entirely in amygdaloidal basalts and hasbeen excavated correctly to design section without trouble. The approach tunnel of theK.H.E.P.Stage 4 lies for the most part in amygdaloidal basalts and was excavated withoutdifficulty. It cuts across 3 fractures only one of which shows some seepage. The PanchdharaTunnel on the project road from Koynanagar to the Stage 4 area also lies in amygdaloidal basaltfor the most part. The tunnel was excavated without trouble, but some small boulder falls tookplace after completion because of deterioration of some chlorophaeitic basalts which occurred atthe crown.

The Malshej Ghat Tunnel on the Murbad-Balshej Road, Stage Highway No.2 is another tunnelexcavated without difficulty in amygdaloidal basalts ans left unlined. It cuts across 3 featureswhich show some seepage in the monsoon.

The tunnel excavated for the Dimbhe Left Bank Canal in Pune District also lies in amygdaloidalbasalts. This being a free flow tunnel sides have been lined to improve discharge and crown hasbeen left unlined. It cuts across a tight fracture.

The Mankhurd-Belapur line of Central Railway passes through a tunnel near Vashi in NewMumbai. The tunnel penetrates a pile of thin amygdaloidal basalt flows some of which havebecome red and brown due to hydrothermal alteration. As some of these which had been softenedby more intense tiydrotherrnal alteration fell from the roof it was feared that large scale roof fallsmay take place and lining was proposed. A detailed geological examination however showedthat but for a few thin softened flows, most of the crown was stable. As the rock above thesoftened thin flow was sound and stable there was no fear-of continued roof falls. It was thereforerecommended that the thin soft portions should be knocked off and the tunnel should be leftunlined.

The Kasheli under Creek Tunnel near Thane, 800 m. long and 4m in diameter, was excavated bythe Municipal Corporation of Greater Bombay below the Bassein Creek to carry the water of theBhatsa Dam across the creek for Mumbai water supply. The tunnel was excavated without anymishap 130 m below the surface through amygdaloidal baslts which were bone dry even thoughthe tunnel lay below a perennial river. Only some small quantities of water percolated into thetunnel through a dyke and two fractures.

TUNNELS IN COMPACT BASALTS

Compact basalts are unsuitable for tunneling because they contain JOIl1ts which render themunstable in excavations and through which water may be discharged into tunnel. Tunnels injointed compact basalts therefore prove troublesome as can be seen from the experience oftunnels described below.

785

"

The Panchdhara Tunnel described above passes through jointed compact basalt for some length atthe Koynanagar portal and rock falls took place in this stretch because of the collapse ofunsupported joint rocks.

It is seen from the exposure of closely jointed rock around the Mumbai portal of Tunnel No.26 inBor Ghat of Central Railway that this tunnel has pa sed through closely jointed compact basalt. Ithas therefore been fully lined except some patches on sites. Sirni larly 19 m of tunnel nO.23C thatpass through closely jointed compact basalt have been lined.

Copious leakage of water took place in the inclines of the Bassien Creek Tunnel, to be describedmore fully later in this account, through the joints of the closely jointed compact basalts in whichthe lay, making tunneling impossible on one side.

Perhaps the worst example of trouble experienced in tunneling through compact basalts isprovided by the K.H.W.P. Stage 3 Tail Race Tunnel, which passes through a closely jointedcolumnar compact basalt over considerable length. Constant roof falls, heavy overbreaks andcopious leakage of water through closely spaced joints and tunneling is very difficult andexpensive. Difficulties were aggravated by other defects in rocks also. The tunnel encountered anumber of vertical veins of intensely hydrothermally altered basalf and serious roof falls tookplace because of the softness of the material. The tunnel also had the misfortune of passingthrough a minor slip with a small displacement. This displacement led to the formation of widezone filled with fragmented and highly decomposed rock. On removal of support this zonecollapsed into the tunnel bringing down with it the closely jointed basalt on both sides. As aresult the entire cover about 6m thick collapsed and the tunnel was delighted over a length ofabout 20m.

KHATA V TUNNEL: Detailed engineering geological mapping of Khatav, Man and Kanhertunnels Near Satara and Karad has been carried out. The case history of Khatav tunnel isdiscussed in detail. Similar geological conditions exist for remaining two tunnels.

The tunnel is passing through the Deccan trop rocks. The rocks of the project area are compactbasalt and volcanic breccia with red tachylytic basaltic lava matrix. The lava flows are thick andextensive. Some fractures cutting across the lava flows are also encountered. At various locationsheavy leakages were observed. As discussed above compact basalts are always jointed. Along thejoint planes leakage takes place. Nearly at all the places in the tunnel leakage along the junctionbetween above lying compact basalt and underlying volcanic breccia was observed. The tunnelpasses through flows of Deccan Trap basalt. The rock exposed in the tunnel is compact basalt andvolcanic breccia Compact basalt is jointed. Joints are at various angles like vertical, horizontal,columnar, block etc. At places sheet jointing is also seen. Majority of the joint surfaces arestained white and brown indicating circulations of water. Between ch. 1200 m and 1350 mcontinuous leakage was observed. In addition to this zone at number of places heavy leakage andseepage was also observed. Compact basalt is jointed and at number of places joints are oblique,intersecting and deeping into the excavation. This has led to overbreak. Volcanic breccia with redtachylytic basalt lava matrix is soft and it becomes softer in contact with water. On exposure redtachytytic basalt matrix has disintegrated. All these properties of volcanic breccia with redtachylytic basalts have led to over break. At number of places there are big zeolite cavities.Permanent supports have been given. The over breaks and treatment given described above aredue to the rock types. there structures like jointing, fracture and soft nature with cavities and

786

disintegration of red tachylytic matrix of volcanic breccia. Due to these geological featuresvarious protective measures, such as rock bolting and permanent supports are adopted.

Trouble free Tunnels in Compact Basalts

Jointing proves to be troublesome in tunneling only if it is closely spaced and regular. If jointingis irregular and inconsistent, it is not troublesome as it is shown by Tunnel Numbers 3, 4 and 5 inthe Bor Ghat of Central Railway. These tunnels pass through a compact basalt flow, which iswell jointed, but the jointing is irregular and inconsistent.

This irregular jointing has not made the rock unstable and the tunnels are standing unlined. Theyare also quite dryas the basalt flow has become impervious as the joints are not consistent andinterconnected.

Another favorable feature that makes compact basalts less unfavorable for tunneling is that thetop portions of compact basalt flows become amygdaloidal and therefore unjointed. Tunnels aresuccessfully excavated through such unjointed top portion of compact basalt flows and leftunlined include, among others.

1. Old Tunnel No.24 which is not now in the Bor Ghat section as it was abandoned whenTunnel No.24, 25 and 26 were constructed is now used for the Mumbai-Pune Road, NH-4.

2. Some portions of Tunnel No.23 C in Bor Ghat.3. Tunnel No.25 in Bor Ghat. The crown of this tunnel could not be left unlined as the

uppermost portion of the flow in which the crown is situated are in an advanced stage ofdecomposition due to hydrothermal alteration. Hence the crown is lined and only the sightshave been left unlined.

4. Construction Adit from the railway track between Tunnel Number 25 and Tunnel No.26 tothe alignment of Tunnel No.25 C.

5. Three tunnels for the Dimbhe Right Bank Canal in Pune District.6. Construction Adit of the tail race tunnel of the Bhatsa dam base powerhouse.

Hydrothermal Alteration

Difficulties in tunneling due to hydrothermal alteration are comparatively rare. Lining of crownof Tunnel No.25 in Bor Ghat and roof falls in the Mankhurd- Belapur Tunnel and the Tail Race

.Tunnel of stage 3 of Koyna Hydro Electric Project due to hydrothermally astered rock havealready been described. A case of more serious difficulties due to hydrothermal alteration isTunnel No.23 in Bor Ghat. At the Mumbai end all rocks are in excellent condition and thererorethe first 86 m. from the MumbaiSomewhere midway in the tunnel a zone of intense hydrothermal alteration starts in which rockhave been completely decomposed and reduced to soft incoherent material. As the tunnel lies inthe soft material the Pune end must have given considerable trouble during construction and thelast l60m at the Pune end have been lined. When the tunnel was widened in 1950 the Murnbaiend could be easily widened. The lined portion however could not be widened as it would havemeant removing the lining, exposing the soft unstable material which would have provedtroublesome. Hence after widening the unlined portion the tunnel was diverted on the each sideand one track was laid in the diverted portion and in the open cut east of the tunnel.

787

FracturesFractures along which no movement has taken place have been met with in a number of tunnels,but in a large majority of cases they have not proved troublesome and case of fracturesnecessitating elaborate treatment are few. In the Bor Ghat tunnels 7 fractures in Tunnel No.26Chave been lined while 52 have been left unlined. There is only one case of serious difficulties dueto fractures. Tunnel No. 15 requiring lining as the rocks of the crown were weathered due tofractures.

The 270m long katraj Tunnel 17 km from Pune on the Pune Bangalore Road, NH4 should nothave required lining at all as it passes through the unjointed top portions of a compact basaltflow. However it has been excavated along a fracture which is seen at the crown at both portalsand weathered rock along the fracture and leakage through it has necessitated lining.

Groups of closely spaced fractures have caused serious trouble almost throughout the length ofthe Tail Race Tunnel of the Bhatsa Dam Base power House. Roof collapses and heavy leakageshave made necessary corrective measures such as guniting and concrete lining appropriate to theconditions or rocks in weathered zones of fractures.

CONCLUSION

Geological conditions in the Deccan Traps are not on the whole unfavorable for dam foundations,deep cuts for canals, tunneling etc. However favorable conditions can not be taken for grantedbecause of the possible occurrence of unfavorable features such as jointed rock, fractures,tachylytic basalts volcanic breccias, hydrothermal alteration etc. and it is essential to carry outproper geological studies to ascertain beforehand what the geological conditions are along thealignment. The long term economics depends on the engineering safety of the manmadeconstructions. Durability and maintenance free service of the dams, canals, structures likeaqueduct etc. is only possible if engineering safety of them is assured. As every structure isrelated to rock beneath, proper geological investigations are of utmost importance.

788