Download Engineering Geological Conditions and Tunneling in Deccan traps

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ENGINEERING GEOLOGICAL CONDITIONS AND
TTJNNELLING IN DECCAN TRAPS
B.
v. D. u. College
Dr. S. R. Kulkarni
Consulting Engineering Geologist,
1151, Sadashiv Peth, Near Peru Gate,
Laxmi Keshav Society, Pune-411030
Of Engineering
Pune-411043
Tel. No. :- (020)-24477464
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INTRODUCTION
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Tunnels are commonly used in Deccan Trap areas of Maharashtra for roads, railways, irrigation canals, water
supply projects, hydroelectric projects etc. The Deccan Traps mainly consist of basalts, but as there is considerable
variation in the characteristics of basalts, and as rocks derived from them by modifications taking place in the
volcanic process also occur, the tunnels penetrate rocks whose characteristics important from the point of view of
tunneling vary within wide limits. It is proposed to describe here some tunnels, at which we have carried out geological
work, in an attempt depict the varying geological conditions in the Deccan Trap rocks that effect tunneling operations.
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THE,DECCAN
TRAP ROCKS
As the suitability or otherwise of geological conditions for tunneling will depend on the characteristic of rocks
met with along the alignment, an acquaintance with the various rock types occurring commonly in the Deccan Trapsand their engineering behaviors is necessary.
Compact and Amygdaloidal
Basalts
In the Deccan Trap basalts two main types occur: The compact non-vascular basalts without gas cavities filled
with secondary minerals such as zeolites and chlorophaeite which give them spotted appearance. Both compact and
arnygdalcidal basalts often contain small slender laths as phenocrysts giving polyphyritic varieties.
Chlorophaeitlc
Basalts
Chlorophaeitic basalts in which a major portion of groundmass glass has been converted into chlorophaeite are
common. When large arnounts.of.chlorophaeite.are present the rocks.become.dark black.
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Hydrothermal
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Alteration
Magmatic gases that produce cavities sometimes chemically alter the basalts, and this action is called hydrothermal
alteration. The normal grey or bluish color of basalts is turned into shades of green, pink. red, purple or brown by
hydrothermal alteration. Most commonly hydrothermal alteration brings about only such color changes in basalts
without affecting their physical properties but more intense hydrothermal alteration at times weakens rocks.
Tachylytes
Basalts in which practically no crystallization has taken place and which consist mostly of basalt glass are called
tachylytes. They are very fine grained because of the low degree of crystallization and pitch black when fresh. Black
tachylyte on hydrothermal alteration acquire a vivid red colour and red tachylytes are more common than the black
variety. Both black and red tachylytes disintegrate on exposure to atmosphere and fall to powder. The red powdery
material commonly passing under the name of red bole is nothing but red tachylyte which has disintegrated on
exposure to atmosphere. These rocks are always troublesome when exposed in a tail channel or if occurring in a
. tunnel.
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Volcanic Breccias
When rock fragments blown up by the explosion with which volcanic eruption often starts are consolidated into
hard rocks, volcanic breccias are formed. The explosion fragments may be held together by being embedded in the
basalt lava that follows or by the deposition of zeolites between them. The lava matrix is most commonly normal
basalt, but may consist of hydrothermally altered basalt or chlorophaeitic basalt or red or black tachylyte. Such
breccias are easily erodible if occurring in a tail channel. Tunneling through these rocks is very problematic.
Engineering Behaviour
Divisional planes such as joints, bedding planes etc. are of great importance from the point of view of tunneling
as they are the main source of trouble during tunneling. In the Deccan Trap basalts joints ate the only divisional
planes present and their presence or absence determine the suitability of basalts fortunneling. Of the two main types
of basalts, the compact basalts are well-jointed and-therefore not suitable for tunneling. As joints provide passage
ways for water, compact basalts are likely to be water bearing. Also the fragmentation brought about by the joints
may make them unstable for excavation. Thnnels in compact basalts may therefore prove troublesome because of the
inflow of water through joints and roof falls and over-breaks.
Amygdaloidal basalts on the other hand are always unjointed and are quite impervious when fresh. Also because
of the absence of divisional planes they are stable in all kinds of cuts, and excavations. The massive impervious:
amygdaIoidal basalts are therefore a very suitable medium for tunneling.
Compact basalts are troublesome because as a rule joints in th~J!l ~e _c-'~~ly ~Qac_ed...:l!o~ever in some 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 of thick compact basalt flows become amygdaloidal
and therefore unjointed. Tunnels in top and bottom portions of compact basalt flow will therefore not be troublesome.
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Basalts that have suffered only moderate hydrothermal alteration have the same engineering behaviour as the
original rock, but with more intense alteration basalts are weakened and may cause difficulty due to collapse of loose
material.
Chlorophaeitic basalts and black and red tachylytes are sound rocks as long as they are not exposed to atmosphere.
They however deteriorate on exposure to atmosphere 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 the operations of tunneling.
Chief among' them are fractures along which no movement has taken place, occurring at a number of places in
western parts of Deccan Trap outcrop. Some fractures are seen as vertical or steeply inclined tight clean cracks with
no gap between the two sides. More commonly they are seen as vertical or steeply inclined sheet jointing. The sheet
jointing is result of basalt brought about by water seeping along the fracture, and the weathered zone 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 prove that no movement
has taken place among them. This shows that they are not faults, but are cracks along which no movement has taken
place.
Behaviour of fracture in tunnels has been variable. A large number have not given any trouble, but, others have
troublesome in various degrees.
Tunnels in the Deccan Trap area have passed through all these different kind of rocks and have been trouble free
or have had to face difficulties of various types depending on the rocks penetrated. Some of them which illustrate
how tunneling operations have been influenced by geological conditions along the alignment as described below.
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TUNNELS IN AMYGDALOIDAL BASALTS
The suitability of amygdaloidal basalts for tunneling because. of absence of divisional planes is well illustrated by
a number of tunnels, chief among them being the nlnnels of the Bor Ghat on the Mumbai-Pune line of Central
Railway. Out of the 25 tunnels excavated in amygdaloidal basalts in the Bor Ghats 19 have been left unlined. Even
the 6 tunnels that needed full or partial lining have needed it because of extraneous factors such as fractures or
hydrothermal alteration and not because of any inherent defects in the amygdaloidal basalts.
The ability of the amygdaloidal basalts to stand unsupported is very well demonstrated by the unsupported slab of
amygdaIoidaI basalt jutting-out 13 m from the Pune portal of Tunnel No.14. The very thin wall separating Tunnel
No.17 from Tunnel No.17 A which varies in thickness from 4.5 to 6 m also testifies for the stability of amygdaloidal
basalts. Blasting of Tunnel No.17 A which was excavated later by the side of Tunnel No.17 could be carried out so
close to Thnnel No.17 without damaging it only because of the stability of amygdaloidal basalts. The very thin outer
wall ofThnnel No.26C which is only about 2 m thick also shows how stable amygdaloidal basalts are. Another proof
of.stability of amygdaloidal basalts is the very thin cover of some tunnels, which is only about 1.5 to 2 m. In case of
tunnel number 26C it is still thinner throughout its length of 150m, the minimum being only 8Ocm. In spite of such
thin cover the tunnel has been left unlined except for 28.5m from the Pune portal and 30m from Mumbai portal
which have been lined to cover a total of seven fractures. All tunnels in amygdaloidal basalts, even including those
r: ; "\ with the thin cover, do not show any percolation even in the monsoon though they receive a rainfall of 4000mm per
year.
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A number of other tunnels have also been excavated without trouble in amygdaloidal basalts. The Head Race
Thnnel of Koyna Hydro Electric Project Stage 3 lies entirely in amygdaloidal basalts and has been excavated correctly
to design section without trouble. The approach tunnel of the K.H.E.P Stage 4 lies for the most part in amygdaloidal
basalts and was excavated without difficulty. It cuts across 3 fractures only one of which shows some seepage. The
Panchdhara Tunnel on the project road from Koyananagar to the Stage 4 area also lies in amygdaloidal basalt for the
most part. The tunnel was excavated without trouble, but some small boulder falls took place after completion
because of deterioration of some chlorophaeitic basalts which occurred at the crown.
The Malshej Ghat Tunnel on the Murbad-Malshej Road, Stage Highway No.2, is another tunnel excavated without
difficulty in amygdaloidal basalts and left unlined. It cuts across 3 fractures which show some seepage in the monsoon .
. The tunnel excavated for the Dimbhe left Bank Canal in Pune District also lies in amygdaloidal basalts. This
being a free flow tunnel sides have been lined to improve discharge and crown has been left unlined. It cuts across a
tight fracture.
The Mankhurd- Belapur line of Central Railway passes through a tunnel near Vashi in New Mumbai. The tunnel
penetrates a pile of thin amygdaloidal basalt flows some of which have become red and brown due to hydrothermal
alteration. As some of these which had been softened by more intense hydrothermal alteration fell from the roof it
was feared that large scale roof falls may take place and lining was proposed. A detailed geological examination
however showed that but for a few thin softened flows, most of the crown was stable. As the rock above the softened
thin flow was sound and stable there was no fear of continued roof falls. It was therefore recommended that the thin
soft portions should be knocked off and the tunnel should be left unlined.
The Kasheli Under Creek Tunnel near Thane, 800m. long and 4m. in diameter, was excavated by the Municipal
Corporation of Greater Bombay below the Bassein Creek to carry the water of the Bhatsa Dam across the creek for
Mumbai water supply. The tunnel was excavated without any mishap 130m. below the surface through amygdaloidal
basalts which were bone dry even though the tunnel lay below a perennial river. Only some small quantities of water
percolated into the tunnel through a dyke and two fractures.
TUNNELS IN COMPACT BASALTS
Compact basalts are unsuitable for tunneling because they contain joints which render them unstable in excavations
and through which water maybe discharged into tunnel. Tunnels in jointed compacrbasalts therefore prove troublesome
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as can be seen from the experience of tunnels described below.
TIle Panchdhara Thnnel described above passes through a jointed compact basalt for some length at the Koynanagar
portal and rock falls took place in this stretch because of the collapse of unsupported joint rocks.
It is seen from the exposure of closely jointed rock around the Mumbai portal of Tunnel No. 26 in Bor Ghat of
Central Railway that this tunnel has passed through closely jointed compact basalt. It has therefore been fully lined
except some patches on sites. Similarly 19m. of tunnel no. 23C that pass through closely jointed compact basalt have
been lined.
Copious leakage-of water took place in the inclines of the Bassien Creek Tunnel, to be described more fully later
in this account, through the joints of the closely jointed compact basalts in which they lay, making tunneling impossible
on one side.
Perhaps the worst example of trouble experienced in tunneling through compact basalts is provided by the K.H.E.P.
Stage 3 Tail Race Tunnel, which passes through a closely jointed columnar compact basalt over considerable length.
Constant 'roof falls, heavy overbreaks and copious leakage of water through closely spaced joints and tunneling is
very difficult and expensive. Difficulties were aggravated by other defects in rocks also. The tunnel encountered a
. number of vertical veins of intensely hydrothermally altered basalt and serious roof falls took place because of the'
softness of the material. The tunnel also had the misfortune of passing through a minor slip with a small displacement.
This displacement led to the formation of wide zone filled with fragmented and highly decomposed rock. On removal
of support this zone collapsed into the tunnel bringing down with it the closely jointed basalt on both sides. As a
result the entire cover about 6m. thick collapsed and the tunnel was delighted over a length of about 20m.
KilATAV TUNNEL
Detailed engineering geological mapping ofKhatav, Man and Kanher tunnels Near Satara and Karad has been
carried out. The case history of Khatav tunnel is discussed in detail. Similar geological conditions exist for remaining
two tunnels.
The tunnel is passing through the Deccan trap rocks. The rocks of the project area are compact basalt and volcanic
breccia with red tachylytic basaltic lava matrix. The lava flows are thick and extensive. Some fractures cutting
across the lava flows are also encountered. At various locations heavy leakages were observed. As discussed above
compact basalts are always jointed. Along the joint planes leakage takes places.Nearly at all the places in the tunnel
leakage along the junction between above lying compact basalt and underlying volcanic breccia was observed. The
tunnel passes through flows of Deccan Trap basalt. The rock exposed in the tunnel are compact basalt and volcanic
breccia. Compact basalt is jointed. Joints are at various angle like vertical, horizontal, columnar, block etc. At places
sheet jointing is also seen. Majority of the joint surfaces are stained white and brown indicating circulations of water. '.
Between ch.1200 m and 1350 m continuous leakage was observed. In addition to this zone at number of places heavy
. leakage and seepage was also observed. Compact basalt are jointed and at number of places joints are oblique,
intersecting and deepening into the excavation. This has led to over-break. Volcanic breccia with red tachylytic
basalt lava matrix is soft and it becomes more soft in contact with water. On exposure red tachylytic basalt matrix has
disintegrated. All these properties of volcanic breccia with red tachylytic basalts has led to over-break. At number of
places there are big zeolite cavities. Permanent supports have been given. The overbreaks and treatment given
described above are due to the rock types, there structures like jointing, fracture and soft nature with cavities and
disintegration of red tachylytic matrix of volcanic breccia. Due to these geological features various protective measures,
such as rock bolting and permanent supports are adopted. Trouble free Thnnels in Compact Basalts.
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Jointing proves to be troublesome in tunneling only if it is closely spaced and regular. If jointing is irregular and
inconsistent, it is not troublesome as it is shown by Tunnel Numbers 3, 4 and 5 in the Bor Ghat of Central Railway.
These tunnels pass through a compact basalt flow, which is well jointed, but the jointing is irregular and inconsistent.
This irregular jointing has not made the rock unstable and the tunnels are standing.unlined. They are also quite dry
as the basalt flow has become impervious as the joints are not consistent and interconnected.
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Another favorable feature that makes compact basalts less unfavorable for tunneling is that the top portions of
compact basalt flows become .amygdaloidal and therefore unjointed. Tunnels successfully excavated through such
.unjointed top portion of compact basalt flows and left unlined include, among others:
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Old Thnnel No. 24 which is not now in the Bor Ghat section as it was abandoned when Tunnel No.24, 25 and 26
were constructed, and is now used for the Mumbai-Pune Road, NH-4.
2. Some portions of Tunnel No. 23C in Bor Ghat.
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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 anadvanced stage of decomposition due to hydrothermal alteration.
Hence the crown is lined and only the sights have been left unlined.
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4. Construction Adit from the railway track between Tunnel Number 25 and Tunnel No. 26 to the alignment of
Tunnel No. 25C.
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5. Three tunnels for the Dimbhe Right Bank Canal in Pune District.
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Construction Adit of the tail race tunnel of the Bhatsa dam base powerhouse.
HYDROTHERMAL ALTERNATION
Difficulties in tunneling due to hydrothermal alteration are comparatively rare. Lining of crown of Tunnel No. 25
in Bor Ghat and roof falls in the Mankhurd - Belapur Tunnel and the Tail Race Tunnel of Stage 30f Koyna Hydro
Electric Project due to hydrothermally altered rock: have already been described. A case of more serious difficulties .
due to hydrothermal alteration in Tunnel No. 23 in Bor Ghat. At the Murnbai end all rocks are in excellent condition
and therefore the first 86m. from the Mumbai, somewhere midway in the tunnel a zone of intense hydrothermal
alteration starts in which rock have been completely decomposed and reduced to soft incoherent material. As the
tunnel lies in the soft material the Pune end must have given considerable trouble during construction and the last
160m. at the Pune end have been lined; When the tunnel was widened in 1950 the Mumbai end could be easily
widened. The lined portion however could not be widened as it would have meant removing the lining, exposing the
soft unstable material which would have proved troublesome. Hence after widening the unlined portion the tunnel
was diverted on the east side and one track was laid in the diverted portion and in the open cut east of the tunnel.
.FRACTURES
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Fractures 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 fractures necessitating elaborate treatment are few.
Inthe Bor Ghat tunnels 7 fractures in Tunnel No. 26C have been lined while 52 have been left unlined. There is only
one case of serious difficulties due to fractures. Tunnel No. 15 requiring lining as the rocks of the crown were
weathered due to fractures.
The 270m. long Katraj Tunnel 17km. From Pune on the Pune Bangalore Road, NH4 should not have required
lining at all as it passes through the unjointed top portions of a compact basalt flow. However it has been excavated
along a fracture which is seen at the crown at both portals and weathered rock along the fracture and leakage through
it have necessitated lining.
Groups of closely spaced fractures have caused serious trouble almost throughout the length of the Tail Tace
Tunnel of the Bhatsa Dam Base Power House. Roof collapses and heavy leakages have made necessary corrective
measures such as gunniting and concrete lining appropriate to the conditions or rocks in weathered zones of fractures.
The 3.8km.long Shivdi-Futka Tank Tunnel with a diamater of3.8m has been bored by a Tunnel Boring Machine
6Om. below the surface for Mumbai water supply. It passes through volcanic breccias and has encountered 69
fractures. Moderate to heavy leakage is taking place through a majority of fractures. By the removal of soft weathered
material gaps upto 2m. wide have been formed and minor roof collapses have taken place. The entire tunnel will be
lined with concrete.
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IMPORTANCE OF PRELIMINARY GEOLOGICAL INVESTIGATIONS
The importance of preliminary geological investigations for tunnels is well brought out by cases of tunnels in
which difficult experience could have been avoided if proper preliminary geological work had been done, and also
cases in which preliminary geological work helped to avoid difficulties.
At Katraj Tunnel referred to above, though geological conditions were otherwise favourable, trouble was caused
as the tunnel was excavated along a fracture A proper understanding of the geological conditions before excavation
would have made it possible to avoid the trouble due to the fracture by shifting the alignment about 10m. to the east
of the fracture.
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, The Bhivpuri Tunnel of the Tata Electric Companies has been excavated to carry the water of the Thokarwadi
Dam to the Bhivpuri Generating Station. The tunnel passes through volcanic breccias which are excellent for tunneling.
But at the crown level occurs a layer of black tachylyte about 1m. thick from which roof falls took place as the
tachylyte deteriorated on exposure. These falls could have been avoided by lowering the crown a couple of meters if
the occurrence of the tachylyte layer had been established by preliminary geological investigations.
Drilling along the alignment chose for the tunnel in km. 3 of the Bhatsa Left Bank Canal showed that tunneling
would be difficult due to roof falls and leakage in,the closely joined basalts occurring along the alignment. Th' _
alignment was therefore abandoned and an alternative alignment along which drilling established the occurrence 0
fresh amygdaloidal basalt was adopted. As anticipated tunneling was completed without difficulty, proving the
importance of preliminary geological investigations.
The Bassein Creek tunnel is another case where serious difficulties could be avoided because proper geological
investigations had been carried out. This tunnel has been excavated near Thane to carry the water of the Upper
Vaitarna Dam across the Creek for Bombay water supply. Originally it was proposed to excavate the tunnel 50m,
below river bed. However drilling showed that upto 100m. below the river bed jointed compact basalts occurred and
amygdaloidal basalts' below. It was therefore decided to take the tunnel 130m. below river bed so that it would pass
through amygdaloidal basalts. The entire horizontal portion of the tunnel in the amygdaloidal basalts was excavated
without trouble. But the upper portions of the inclines on the twobanks leading down to tunnel level had to pass
through compact basalts until they reached 100m. below river bed. In these upper portions of the inclines there was
profuse leakage of water through the joined compact basalts and large quantities of water had to be pumped out. At
the Kalher end the inflow was so large that it proved impractical to pump out so much water. Work at this end had
therefore to be abandoned and excavation had to-be carried out only from the Kolshet end. The difficulties experienced
in the portions of the inclines passing through compact basalts showed that it would have been very difficult to '
excavate the tunnel 50m. below the river bed and proved the correctness of the decision to lower the tunnel. It is
important to note that it 'was preliminary geological investigations that had revealed that conditions would be difficult!
at 50m. depth and would be favourable at 150m. depth and hence the tunnel was lowered. It was thus only because
preliminary geological explorations had been carried out that a difficult alignment was discarded and a favourable
one chosen. If preliminary geological investigations had to face very difficult conditions and this difficult situation
was avoided by preliminary geological investigations.
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,CONCLUSION
Geological conditions in the Deccan Traps are not on the whole unfavourable for tunnelling. However favourable
conditions cannot be taken for granted because 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 out
proper geological studies to ascertain beforehand what the geological conditions are along the alignment.
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