NOTE SUBMITTED TO
DAM SAFETY REVIEW TEAM
CENTRAL WATER COMMISSION
SECOND VISIT (02-05-1998).
the first visit of the Dam Safety Review team to Tungabhadra Dam
from 15-19 September 1997 to the team was provided with one copy of the
"Technical Report on Tungabhadra Project" written by Shri S.C. Javali.
As has been brought out in the report, it is largely based on the specification
drawings prepared before the construction. Naturally, numerous changes
must have taken place during the execution of the project. A number of
such obvious discrepancies were noticed and it was therefore decided to
bring out the important discrepancies having a bearing on the safety aspects
of the Dam. The present note brings out some such aspects along with certain
new data observed after the first inspection as desired therein by the
team. Also included herein are certain other aspects related to the safety
of the dam which are brought to the notice of the team.
OF TUNGABHADRA DAM
dam is a medium size straight gravity one. It consists of 3 parts viz.,
Dam in masonry across the river 5,979 feet in length, of which 2,300 feet
in the middle portion acts as spillway. Out of the remaining 3,679 feet
is length of non over flow portion, 3,412 feet is in masonry and the remaining
267 feet is composite section at the left end.
Dam 500 feet long to close a small saddle in the hill adjoining the main
dam on the left side. The saddle No 1 and the Main Dam is separated by
a Pilon hill of a length 193 feet
Dam 1,527 feet long on the extreme left in saddle No 2. The saddle No 1
of earthen dam and saddle No 2 of composite dam are connected by a Hill
and Ridge of length of 1120 feet.
Thus the alignment of the dam consists of following elements starting from
Madras Right bank.
over flow section(Right side)
flow Section (Spillway)
over flow Section (left side)
over flow section(Earthen Dam in saddle No 1)
over flow section (masonry with earth back fill
in saddle No 2)
Tungabhadra Project the reference point is located on the right bank and
the longitudinal section (L.S) chainages increase from right to left bank.
The proper section of masonry i.e., the Block No 1 starts at Ch.50'. As
further to the right the hill side slope was steep, an anchor wall starting
from Ch. 28 was built filling the gap between vertical face of masonry
of the first block and the hill slope. The depth of anchorage and the material
of construction of the anchor wall is not known. However, from the day
book of Superintending Engineer, it is revealed that the face of the right
abutment was grouted with cement slurry till refusal through a series of
holes 10' centre to centre.
the left flank, composite section was used as hard rock in foundation was
not available. It was resolved that wherever there is change from hard
rock foundation to lighter soil the construction of dam may be independent
of the main dam with a copper strip joint and keyways.
The block wise
details with chainages of the main dam including location of structures,
details of foundation levels and floor levels of drainage gallery indicating
the construction materials are given in Table 1.1. Typical cross sections
of maximum over flow section and maximum non over flow section are at Fig.1.1
Tungabhadra Dam construction with stone was found eminently suitable as
good granite stone was available in the area in abundance. Further, availability
of manual labour accustomed to work with stone, was a plus point.
the specifications the dam masonry consists of granite stone thoroughly
bedded in mortar. The front face work in both spillway and non-spillway
sections consist of face stones hammer dressed on face and one line chisel
dressed on bed, top and sides for 4" from the front faces. The rear face
work in the non-overflow section consists of stones, hammer dressed on
face, sides and bed. The rear face work of spillway (over flow) section,
faces of spillway piers, cut waters and abutments are built of face stones,
one line chisel dressed on face and for 4" from face on bed, top and sides
with the courses horizontal in case of vertical walls and normal to the
face batter for walls with face batter. The hearting of both overflow as
well as non- overflow sections is of random rubble work. Lining of sluices
are of ashlar facing work, and all arch work is of rubble arching.
in the Board of Engineers appointed by the Governments of Madras and Hyderabad
for the settlement of certain disputed points in the design and construction
of Dam under the Chairmanship of Sir. M.Visweswaraya, there was a general
agreement that cement mortar shall be used for special purposes such as
buildings Sluice opening, Coping of Spillway section, Piers of spillway
shutters and other similar parts where special strength is needed. However,
after considering detailed tests results of Lime Surki Mortar, it was agreed
to construct the Spillway section in Red Cement Mortar. (Ref. correspondence
between CE, TBP, Madras, and CE, Irrigation Project, Hyderabad, through
their letter dt. 18-9-1950, 5-10-1950, 6-11-1950, and 7-11-1950). It was
also agreed to construct the non-over flow sections with Lime Surki Mortar
adopting a proportion of 1:1:1 (Lime: Surki : Sand) on both the sides
and that this proportion may be suitably changed for higher elevations.
Accordingly, proportion of 1:2:2 was adopted in the Dam in None of flow
sections above the elevation of + 1580ft (Ref: Field Laboratory Repot Compilation).
Fig 2.1 to 2.4 gives the comparative study results of compressive strength,
tensile strength and permeability of Lime Surki Mortar of proportions 1:1:1
and 1:2:2. These results are also presented in Table 2.1
Add Table And Fig
Add Fig 2.5 to 2.7
Spillway was constructed with Red Cement Mortar (RCM) in Proportion 1:4.
However, during the construction it was decided that the lower 2 or 3 courses
of masonry starting from foundation, be built with a richer RCM of 1: 2
¾ as per the decision taken by the two CEs vide their correspondence
dated 28-3-1950 and 18-4-1950. All the over flow section is provided with
impervious front to a thickness of 9' using a richer proportion of RCM
1:2 ¾. The pointing in the upstream face of the Masonry was done
with Cement mortar in 1:2. On the rear side (RCM) of 1:4 has been used
up to a level of 1558' and a mix of 1:5 thereafter. (Vide their letter dated
21-5-1950) It was also decided that as a precautionary measure the bottom
4 or 5 layers over the foundation rock in the Non of flow section be also
built of cement mortar of the same strength. Fig 2.5 to 2.7 gives the comparative
test results of the compressive strength, tensile strength and permeability
of various proportions of Red Cement Mortar.
and economy being the watch words of an Engineer, Engineering tests play
a vital role in the design of structures and choice of materials. Even
though the usage of lime for construction purposes had been in vogue in
the country from ancient times, a great debate on its use ensued between
the two agencies involved on the two banks. An earnest endeavor at studying
the properties and behavior of lime Surki mortar under different conditions
was made by Tungabhadra Project engineers and some of the results obtained
have been indicated herein. In fact, it is this detailed study that enabled
the engineers to change the proportion of Lime Surki Mortar from 1:1:1
to 1:2:2 resulting in a considerable saving of cost. Each operation, commencing
right from the manufacture of lime Surki mortar to its ultimate consumption
on works was decided upon after a thorough study of the various operations.
Grinding time of 30 minutes for example was adopted as it was found to
be sufficient from the point of view of strength as well as economy.
needle apparatus was used for determining the setting time of lime which
was screened through a 30 x 30 sieve. The normal consistency of lime was
determined akin to the procedure adopted for cement and the requisite quantity
by weight of water was added to the lime. The results show that the initial
setting time of lime is from 9 to 11 hours and final setting time from
27 to 31 hours.
comparative study was made between Lime Surki Mortar 1:1:1 and 1:2:2 with
a view of switching on to the latter proportion in case it proved better
or atleast as good as the former. Lime surki
mortar 1:1:1 was being used for the bulkhead portion of the masonry dam
and a change in the proportion of lime surki mortar from 1:1:1 to 1:2:2
meant a lot of saving in money. With this attractive object in view, lime
surki mortars of proportions 1:1:1 and 1:2:2 were ground in Pan Mixers
for a period of 30 minutes adding the requisite quantity of water for normal
consistency. Strength results (Fig 2.1 to 2.4) reveal that the proportion
of 1:2:2 if not better than 1:1:1 is in no way inferior to 1:1:1. Accordingly
the proportion of 1:2:2 was adopted in the dam above an elevation of +
tests were conducted on Red Cement Mortar (RCM) of varying proportions,
wherein, one-fifth portion of cement is replaced with Surki.
TEST ON LIME SURKI MORTAR
of the essential pre-requisites for selection of a particular mortar for
building masonry for a dam is the degree of its impermeability to water.
Permeability test on mortars used in a hydraulic structure therefore forms
a major test. Tests have been conducted on lime
surki mortars, of different proportions at various stages of curing. Tests
are being regularly conducted on mortars used on dam. A simple device has
been evolved for conducting the test.
2" in height in the form of a ring of 2 ½ diameter is placed in
a cast iron ring 4" in diameter and 2" in height. The annular space left
between the specimen and the ring is filled up with a bituminous material
such as marine glue through which water does not permeate. This specimen
is subjected to the action of water contained in a cylindrical jacket,
the water in turn being subjected to a constant pressure of 100 lbs. per
square inch, by the aid of a compressor. 4" rings are not straight away
cast as the specimen shrinks slightly thus causing a passage for the free
flow of water. This slight shrinkage cannot be closed effectively with
marine glue.Water, if permeating through the sample is collected in a graduated
jar and the quantity collected is recorded at regular intervals of half
coefficient K which is expressed as 1012 feet per second is obtained from
the formula Q = K I A t where Q is the discharge in cubic feet per hour,
I is the hydraulic gradient, A is the area of cross section of the specimen
in square feet and t is time in seconds. The graphs (Fig 2.3 and 2.4) indicate
the general behavior of the mortars.
TESTS IN MASOARY ( RCM )
holes drilled by H3 calyx in Blocks 18 to 20 were tested for permeability
on 15th September, 1951 and intake of water in these cases is as shown
TEST RESULTS IN MASONARY ( RCM )
consolidation grouting, there are two lines of holes in the non-spillway
and three lines of holes in the spillway section. But the numbers of lines
are increased depending on the actual condition of rock met with in various
reaches. The holes were drilled 20 feet into rock at 20 feet intervals,
the holes in one row being staggered with those in the adjacent row In
places like L.S.900 to 1390 feet where bed rock was found to be highly
fissured and withered it was found necessary to strengthen the foundation
considerably. Holes were drilled here at 10 feet intervals, the distances
of the lines being 2,12,22,32 and 42 feet from the axis and the depths
however remaining the same 20 feet. In addition to these, holes were also
drilled wherever joints were visible and did not cross the above pattern
consolidation grout holes on the Tungabhadra Dam were drilled with diamond
drills, Calyx shot drills and Wagon drills. Most of these low-pressure
grout holes were drilled with wagon drills using both forge sharpened steel
and earset bits. The holes drilled with forge sharpened steels are of 3
½" diameters at top and 2" at bottom. These result in the holes running
to a taper. But in case of earset bits holes of uniform diameter are obtained
from top to bottom. The different sizes of bits used are 1 ¾" and
2". These bits have tungsten carbide inserts. Drilling with forged steel
is relatively slow, while earset bits were found very useful for quick
drilling in hard epidiorite.
DATA AND OBSERVATIONS DURING GROUTING
statistical particulars regarding drilling of low-pressure grout holes
in the Tungabhadra Dam are given below.
cement grouting was generally started with ratio of 1:10 (1 of cement to
10 of water) and finally end with a richer proportion of 1:4 to 1:5. The
pressure maintained for shallow holes was generally from 40 To 70 lbs.
per square inch. For a total depth of 13,515 feet of low pressure holes
grouted the quantity of cement pumped was 1274.4 cubic feet which works
out roughly to one tenth cubic foot per running foot of hole. While grouting
operations were in progress in the dam foundations there were quite a few
holes that took in appreciable quantities of grout. They numbered about
twenty. The maximum intake of cement for a single hole in the dam was 175.4
cubic feet. This was a test hole drilled at L.S 2040 feet in epidiorite
rock near the old Raya channel course. The dip in this case was towards
the upstream side as usual and no defects could be seen in the foundation
all round on superficial examination. No surface leaks were noticed during
grouting. All the cement injected into the hole had travelled beneath the
foundation bed and also to high elevations of the Raya channel bed as revealed
when the excavation of the Raya channel was taken up later. Another such
hole was at L.S 1563 feet in pegmatite rock. The rock foundation was flat
bedded and highly micaceous. For the naked eye the exposed rock surface
appeared quite sound. When a number of holes were drilled and tested with
air, interconnections among 6 holes were detected. This hole took 67. 2
cubic feet of cement. After grouting of this hole was completed test holes
were drilled to find out the effectiveness of grouting. Core taken out
of these holes revealed cement particles below 10 feet from top.
consists of a series of holes drilled 40 feet into rock located on a line
15'-9" in rear of the axis of dam. The holes are spaced at 40 feet centers.
These were drilled from inside the drainage gallery after the dam had come
up to about 100 feet above rock level. Drilling is done through pipes extending
3 feet into the rock. Prior to the starting of masonry , holes are drilled
3 feet into rock at 40 feet centers and 3 inches metal pipes fixed in these
vertically and care is taken to see that these are maintained vertically
during the progress of masonry. These pipes extend up to the floor level
of the gallery. The drilling of these holes was done by two C.P.55 diamond
core drills operated by compressed air.
case of high pressure curtain grouting in the Tungabhadra Dam. Split spacing
method has been adopted, the final spacing of the holes being 40 feet and
pressure adopted being 150 to 200 lbs. Per square inch respectively for
the primary and secondary series of holes. In the split spacing method
a primary series of holes are drilled at some distances, say 80 to 120
feet, and grouted. A second series consisting of an equal number of intermediate
holes is then drilled and grouted at greater pressures than the first series.
A third series may also be drilled, if necessary, thereby reducing the
spacing to a quarter of that in the first series.
Tungabhadra dam, in the cement mortar sections, contraction joints have
been left at 93'-4" intervals. The longest block in surki mortar section
is 756 feet in length. However, no contraction joints have been used therein.
Such joints are provided in the non over-flow section in the blocks built
of red cement mortar at site of penstockes, sluices etc., where they are
located. In other portion of non overflow sections, which are of lime surki
mortar no contraction joints are provided. The joints are extended through
the full cross section of the dam profile including the bucket and baffle
in the case of spillway and begun from the foundation levels at the respective
sections or at any rate from not higher than 5' above foundation levels.
The faces of the contraction joints are formed smooth by casting the ends
in concrete or building the end masonry smooth with face stones single
line chisel dressed on face or precast concrete blocks.
at the upstream face of the dam is sealed by of annealed copper sealing
strips of thickness varying from 1/8" to 3/32" It is U shaped so that it
can open and close with joints. It is securely fixed in cut stone masonry
on either side and for this purpose its ends are gripped at intervals and
also provided with holes at 3' centers through which ½" rods are
passed. The U of the copper sheet is in a pocket provided in the cut stone
masonry and this is filled with asphalt A ½" steam pipe is installed
in this pocket to reliquify the asphalt if necessary.
ROCK AT TUNGABHADRA DAM AND WEAK ZONES.
foundation bed rock at Tungabhadra Dam consists mainly of two different
types, epidiorite and pegmatite. The major portion of the foundation is
hard black epidiorite while the rest is pegmatite of red and white varieties.
Even in epidiorite formation there have been a number of places where pegmatite
intrusions exist. The planes between epidiorite and pegmatite are tight
every where and can be said to be almost blended with each other. The epidiorite
rock has generally an upstream dip varying from30 degrees to 50 degrees,
while pegmatite formations are generally flat bedded. While epidiorite
rock is very compact and hard though stratified and joined in a few places,
pegmatite formations especially of the white variety are highly fissured
containing lot of quartzite, black mica and clay seams. Those clay seams
are found to exist in horizontal fissures of varying thicknesses at different
depths except in one or two places where vertical seams were also met with.
Source: K.R.Rajagopalan report
OF SOFT ROCK AT L.S 1000 FEET
zone of soft rock was met at L.S. 1000 feet at the foundation level of
R.L. 1515. This zone was only a local pocket and the rock on all sides
was quite good. The pocket was excavated to + 1479.00. The size of the
pit varied from 25 feet at top to 6 feet at bottom. It was noticed from
the cores of Calyx drill, taken up to R.L. + 1460 that the stuff obtained
below was good hard epidiorite rock. The treatment adopted for this was
similar to that adopted in between LS 1380 and 1460 feet. But instead of
one tram rail grill in that case two sets of grills were provided one at
level of + 1482.00 and the other at 1492.00. Here also mass concrete was
laid up to + 1495.00
TREATMENT BETWEEN L.S. 1380 AND 1460 FEET IN PEGMATITE ZONE.
foundation treatment as carried out for the fault in the pegmatite zone
between L.S. 1380 and 1460 feet is summerised and the various operations
followed in the order of sequence after completing drilling and grounding
operations are listed out in the following paragraphs.
a precaution, anchor rods binding the mass concrete and foundation rock
were provided at 7 feet intervals in rows and staggered in the front portion
only up to 11 feet down - stream of axis. The diameter of the bars were
taken down into the rock for 45 inches and extended into concrete above
for a length of 45 inches and hooked at ends to ensure effective bond.
anchor rods were also provided in the contact plane of epidiorite and pegmatite
rock in rear. Horizontal holes with depths extending to about 2 feet were
drilled into the epidiorite rock and the same 1 inch diameter bars were
fixed so as to extend for a length of 45 inches into the concrete and hooked
at ends. These rods were provided at 5 feet intervals in rows staggered.
pilot hole, 40 feet deep was frilled in the fissure with pipe leading to
the drainage gallery for grouting the fissure if found necessary at a future
date. This hole was in line with the other high pressure gout holes and
in addition to the usual holes for curtain grouting. The above hole was
drilled after completing the grouting of other holes.
system of grout pipes were provided, formed with two horizontal pipes 4
inches diametre each measuring 3 feet on either side of the diagonal seam
at level + 1509.00 connected with suitable length of vertical (1 inch diameter)
pipes extending up to rock surface below at 5 places on either side of the
soft seam. Each of the horizontals, in turn, was connected to the drainage
gallery by leading verticals (4 inches diameter pipes) and kept screwed
so that grouting could be resorted to when necessary from the above two
buttons were provided at L.S. 1400, 1410, 1420, 1430, 1440 and a level
of + 1509 .at the contact planes between pegmatite and epidiorite rock
intrusions in rear. One more grout button was fixed at L.S1390 at R.L.
1512.0. The pipes from these were suitably connected to a horizontal which
was taken out of the rear face of the dam at points at a convenient height
for future grouting. The diameter of the grout pipes used was 1 inch.
grout buttons were provided when the concrete level reached R.L. + 1509.00
in the above zone and then only a 1 foot concrete laid to make up the same
to + 1510.00 levels.
pipes were provided along the seam for measuring uplift pressure each connected
to the drainage gallery with suitable bends at 4 stations; the pipes were
were fixed first and after restraining the same by concrete, cap was unscrewed
and hold drilled to a minimum depth of 3 feet.
tram rail reinforcement at bottom spanning the deep transverse fissured
trench was provided. They consisted of 24 lbs. rails and were provided
at + 1502.00 level. Since the width of the trench adjoining the soft seam
was about 5 to 6 feet wide the length of each rail provided was 7 feet
so that there was a bearing of 6 inches to 1 foot on either side. Since
the rock on the right side of the scam was steep, in some places this arrangement
involved making a cavity 6 inches to 1 foot deep horizontally in pegmatite
rock to fit in the rails. Also the tram rails were spaced at 3 feet intervals.
In the cut off trench portion and just for about 4 feet adjoining it, the
steel tram rail was made to rest directly on concrete itself at the same
level of + 1502.00.
a monolithic effect of this reinforcement, all these rails were tied together
with two 1 inch diameter rods running parallel to the seam and just 2 feet
away from it, i.e. the distance between the two rods was 4 feet. Again the
1 inch diameter rods and tram rails at crossing were tied together by welding.
concrete was laid up to + 1510.00, the proportion of concrete being 1:2.38:5.04
by weight using graded jelly (metal) of granite and epidiorite in the proportions
of 1:1 by volume (3" to 1½" and 1 ½" to ¾", 35% and
¾" to 3/8": 30% by weight).
HOLES IN FOUNDATION.
L.S 110 to 1695 feet, there were many weak spots in the pegmatite. Mica
was found in different quantities and seems to be responsible for few weak
spots. These weak spots were found at L.S. 110, 1250, 1350, 1546, 1618
and 1630 feet. The holes were containing soft material of 70% gravel and
30% of clay with mica were of size 6 feet diameter at top and 8 feet deep..
The sides and bottom of these holes were found to be hard. The holes were
treated by excavating that portion to a size of 10' x 10' and depth taken
to 2 feet below the bottom of the hole and then concreted. The hole at
LS 1618 feet is about 9'x5' at top and about 7' deep. The hole at LS 1630
feet is 10'x9' at top and about 11 feet deep.
OF L.S 1408 FEET.
was a joint in the pegmatite at LS 1408 feet when foundation had been excavated
to about 1520, it was found to be soft here. So it was excavated still
further and a pit of size about 50'x50' (at top) came to be formed. It
was excavated to a level of + 1507.5(average) and it was about 10' deep.
So again a trench was excavated till a level of about + 1498 was reached.
Even in the river foundations were excavated to so deep on the deepest
level attained was only +1504
after further excavation to about R.L. 1507 and R.L. 1500 was found to
exist only in the pegmatite at 73o to the axis of the Dam and
disappeared before the pegmatite reached the epidiorite contact. The redeaming
feature was the fact that the fault did not run right across the dam foundation
but turned off at about 2/3 the width where the pegmatite hugged the epidiorite
bed rock. The fault had reached to a joint and the epidiorite on the down-stream
side, on which, part of the Dam rests did not have the fissure extending
into it and in order to ascertain, if the epidiorite formation existed
deep at the site and that the pegmatite did not intrude at the lower levels
under-neath the epidiorite bed, deep holes were drilled to a level of +
1469 close to be contact zone of the pegmatite and the epidiorite. Cores
of hard epidiorite were obtained, which never showed any indication of
holes were therefore drilled at about a foot or two away from the contact
plane on the epidiorite bed. It was found that the fissure was filled with
weathered material of pegmatite gravel and clay. The vein was 1-1/2 feet
wider and then as going lower down it dwindled to about an inch thickness
just above + 1500 level and again widened about 6" at the base of the trench
which was just below + 1500 level. At + 1500 level a cross trench was excavated
10 feet, wide at top and 6 feet wide at bottom with the fissure or fault
running North - South at 73o to the axis of the dam in the center of the
trench. The bottom trench was at + 1500 level. The filling material in
the fissure was coarse gravel 70% fine sand silt and clay 30%. The coarse
and weathered felspar and quartz were mostly angular and irregular in shape.
Probing by an iron rod revealed soft vein for a considerable depth, eight
test holes were drilled along the vein or fault in the trench. They were
all taken down to R.L. 1480, i.e.., 20 feet below the trench level. Three
of these were with the Calyx and the rest with the Wagon drill. The cores
obtained from the bores revealed highly fissured and disintegrated rock
with lot of black mica. Compressed air and water were forced through the
holes. At first, the seam or vein did not yield but continuous pressure,
forced all loose stuff to come out of the fissure or vein and large and
wide holes were formed when the weathered disintegrated and loose material
got dislodged and forced out of the vein under pressure. Wide openings
were formed adjacent to the holes and the material at lower depths appeared
to be all crushed pegmatite and clay.
above reach was inspected by the then Chief Engineer on 7-11-1950 and he
issued instructions for treatment of the above seam. He opined that the
elimination of the vein, fissure or fault in the trench for founding the
dam was out of question.
the point of view of safety, there was no necessity to have any apprehensions
that the above fault would give trouble when the reservoir would be full
and the vein would be subjected to seepage under high pressure. It was
expressed that the one main safety was the existence of the hard expidiorite
rock in the rear through which the fault did not run and as such there
was no possibility of water escaping through the vein into the rear of
the Dam and underneath it. At the worst, with the reservoir full, the water
would exert an uplift pressure on the dam along with fault line. The Chief
Engineer opined that the same could easily be counteracted by reinforcing
that width of the dam in the bottom trench and also at bout + 1510 level
with steel rails and building the dam itself in cement concrete up to +
1520 level as such a concrete fill would be the ideal fill in such cramped
and restricted foundation areas.
preparation of the foundations and concreting for the dam was instructed
to be tackled as under the following order.
holed 30 feet deep (i.e..) to R.L. 1450.00 in the wide openings formed as
a result of the washing out of the seam under compressed air and water
pressure, washing out the seam again and again under pressure through the
above deep holes. By this operation it was expected to have an almost clear
and clean fissure for a depth of 50 feet below R.L. 1500 level.
the fissure at various points and the wide openings to be filled with concrete.
a cut off trench in front for 10 feet length and 10 feet deep and about
8 feet wide upstream of the axis of the dam, drilling 40 feet holes at
5 feet intervals, three of them, the end ones, inclined towards the fault,
the centre one vertical and another three holes staggered and grouting
through the above holes and washing out.
in dowel bars in all the vertical or steeply inclined rock faces of the
trenches. The foundation pit below R.L 1520 level should be filled up with
concrete after completing the grouting operations in the cut off and the
transverse trenches and providing the minimum requisite steel tram rail
reinforcements at bottom of trenches and spanning the deep transverse trench.
The concrete should be laid up to R.L. + 1520 level and masonry work started
above that level. It was instructed not to put in drainage hole connecting
the fissure with the drainage gallery.
AT L.S 3700 FEET
the deep course of the river at L.S 3700 feet, was a "V" shaped gulley
or a rift about 140 feet wide at top and 40 feet deep. Its faces and bed
were hard epidiorate. The Drawing No 51 shows the foundation treatment
of rift. It involved large amount of excavation and heavy dewatering during
the short period of Working Season.
left flank there were no special difficulties encountered and good hard
rock exposed for founding the dam in the routine manner except the rift
portion which was specially treated because of the increased depth. However,
at the end where the masonry dam abutts the hillock, the rock dips steep
in a length of about 200 feet. Hence, instead of taking the masonry dam
foundation in the portion quite deep, a composite dam has been provided,
founding it at higher level. A good cut off appears to have been provided
for this portion as no leakage is seen.