Geological and Engineering Details

NOTE SUBMITTED TO DAM SAFETY REVIEW TEAM 

OF 

CENTRAL WATER COMMISSION SECOND VISIT (02-05-1998)

INTRODUCTION

During 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.

GENERAL ALIGNMENT OF TUNGABHADRA DAM

      The dam is a medium size straight gravity one. It consists of 3 parts viz.,

•  Main 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.

• Earthen 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

• Composite 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.

MAIN DAM

1)	Non over flow section(Right Side)	1,985 ft
2)	Over flow Section (Spillway)	2,300 ft
3)	Non over flow Section(Left Side)
	a)   Masonary Section	1,427 ft
	b)   Composite Section	267 ft
TOTAL		5,712 ft

 

EARTH DAM

Non over flow section(Earthen Dam in saddle No 1)                        500 ft

COMPOSITE DAM

Non over flow section (masonry with earth back fill                      1,527 ft
in saddle No 2)

                                                                                                                            ———-

Grand Total                                                                                                     8,006

                                                                                                                            ———– 

RIGHT ABUTMENT DETAILS 

In 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.

LEFT FLANK

On 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 and 1.2

2. MASONRY AND MORTARS
   

In 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.

As per 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.

Initially 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

 The 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.

 

3. ENGINEERING TESTS
   

Efficiency 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.

The Vicat 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.

A special 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 + 1580 ft.

Similar tests were conducted on Red Cement Mortar (RCM) of varying proportions, wherein, one-fifth portion of cement is replaced with Surki.

PREMEABILITY TEST ON LIME SURKI MORTAR 

One 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.

A specimen 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 an hour.

The permeability 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.

PREMEABILITY TESTS IN MASOARY ( RCM )

6 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 below.

PREMEABILITY TEST RESULTS IN MASONARY ( RCM )

 

Sl.No	Borehole No	LS	Masonry level	Permeability  test results Gal/Minute Litres/sec
1	864 M	2650	1577.00	0.0116                0.0009
2	885 M	2660	1577.00	0.0082               0.0006
3	916 M	2727	1577.00	0.0227               0.0017
4	923 M	2737	1577.00	0.0262               0.0020
5	936  M	2860	1556.00	0.0262              0.0020
6	943 M	2850	1556.00	0.113                0.00867

4. DRILLING AND GROUTING

Consolidation Grouting

For 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 drill holes.

These 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.

STATISTICAL DATA AND OBSERVATIONS DURING GROUTING

The statistical particulars regarding drilling of low-pressure grout holes in the Tungabhadra Dam are given below.

 

With wagon drills With calyx drills	11, 681 feet 18,354 feet
Total	————– 13,515 Feet. ————–

The 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.

HIGH PRESSURE GROUTING

This 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.

In the 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.

5. CONTRACTION JOINTS

In the 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.

The joint 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.

1. FOUNDATION ROCK AT TUNGABHADRA DAM AND WEAK ZONES.
   

The 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 

2. ZONE OF SOFT ROCK AT L.S 1000 FEET

A 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

 

3. FOUNDATION TREATMENT BETWEEN L.S. 1380 AND 1460 FEET IN PEGMATITE ZONE
   

The 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.

4. LARGE HOLES IN FOUNDATION.
   

Between 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.

 

5. FISSURE OF L.S 1408 FEET.
   

There 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

The fissure after further excavation to about R.L. 1507 and R.L. 1500 was found to exist only in the pegmatite at 73^o 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 pegmatite intrusion.

The 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.

The 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.

From 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.

The preparation of the foundations and concreting for the dam was instructed to be tackled as under the following order.

(a)	Drilling 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.
(b)	Grouting the fissure at various points and the wide openings to be filled with concrete.
(c)	Excavating 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.
(d)	Putting 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.

6.GULLEY AT L.S  3700 FEET

In 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.

On the 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.