Page images
PDF
EPUB

Engineering and Allied Questions. 59. Elephanta is one of the small islands lying off the coast of Bombay, about 7 miles from Bombay harbour. It is composed of

flows of Deccan Trap of doleritic, basaltic and Elephanta Caves andesitic composition, some of which are amygsculptures.

daloidal. The rocks are weathered to varying degrees as the island is fully exposed to moisture-laden wind from

the sea.

There are several caves containing sculptures of Hindu Gods and mythological figures hewn out of the solid rock. The caves and sculptures are said to date from the 6th century A.D. During the time when the island was under the occupation of the Portuguese (1534 to 1774 A.D.), the caves were in a very neglected state and the sculptures were also subjected to disfigurement at the hands of the garrison which occupied the island. The caves were cleaned up after the island came under British occupation, but it was not until 1872 that any effort was made to preserve the architectural treasures.

The main cave shows that dirt and debris must have accumulated to a height of about 4 to 4! ft. from the floor, the walls to that height showing deep decay due to the action of accumulated salts.

In 1935, some fragments of the sculptured figures, which had developed fractures, fell off and measures had to be devised to prevent not only the further development of cracks and fractures but also the general decay of the rock surface. Mr. M. Sanaullah, Archæological Chemist with the Government of India, made an investigation and recommended certain

to be taken. A Committee was set up by the Government with Mr. J. F. Blakiston, C.I.E., Director-General of Archæology, as Chairman, and Prof. S. S. Bhatnagar and Messrs. A Croad and S. N. Gupta as Members, and Mr. M. S. Vats as Secretary. Dr. M. S. Krishnan, Geologist of the Geological Survey of India, was deputed to advise the Committee on the geological aspects of the problem.

The Committee examined the caves and held sittings at Bombay in November, 1936. Dr. Krishnan discussed the problem of preservation with the Members of the Committee on the spot and also collected samples of the decayed rock from the floor and wall of the caves.

One set of samples was analysed for soluble salts in the Geological Survey Laboratory and duplicates were handed over to the Royal Institute of Science, Bombay. Specimens of decayel

measures

rock were also sent by the Superintendent of Archæology, Western Circle, to the Government Test House, Alipore.

Dr. Krishnan submitted a report to the Director-General of Archæology in due course, based on the results of analyses in the Geological Survey of India and the Alipore Test House. His recommendations, summarised below, have been accepted by the Committee.

The analyses of the samples collected by Dr. Krishnan revealed the presence of appreciable quantities of sulphates, chlorides and nitrates in the decayed rock. It was learnt later that the analyses made at the Royal Institute of Science showed an absence of nitrates, but this may be due to different methods of analysis.

The decay is due to the action of salts derived from wind-borne moisture from the sea, to organic matter accumulated during the years of neglect, and to decomposition of pyrites in the rock, the acid so generated acting on the minerals and producing sulphates, notably of calcium, magnesium and the alkalies. The dripping of water from cracks in the roof also plays an important part in the decay.

Dr. Krishnan has recommended the following action to be taken for the prevention of decay and the formation of cracks. The cracks in the images and sculptured parts should be cleaned free of dust and salts and cemented together with a suitable cement, the larger pieces being held together by rivets of a non-rusting alloy. The cement should be protected from atmospheric action by a coat of preservative or varnish on the surface.

For the removal of the accumulated salts on the surface, Dr. Krishnan agrees with the Archæological Chemist that the repeated application of paper pulp would be a suitable remedy where sculptured figures are concerned, and spraying by water where the walls have no figures or other ornamentation. The surface can then be dried by compressed air and preserved against future accumulation of salts by the application of a suitable varnish.

The percolation of water through the roof must also be prevented. Dr. Krishnan agrees with Mr. Croad's suggestion that the top of the roof forming the hill side should be stripped free of soil, the joints cement-grouted and the surface painted periodically with asphalt.

60. In March, 1937, at the request of the Chief Engineer, Public Works Department in the North-West Frontier Province, Dr. A. L.

Coulson visited Malakand in connection with Malakand Hydroelectric Scheme, N.-W. the hydro-electric scheme then under construcFrontier Province.

tion, to report on the nature of the geological conditions and their effect on the stability and permanence of the works.

The water of the Upper Swat canal, which is taken from the Swat river at the canal headworks at Amandarra (34° 37' : 71° 59'), is led through the Benton tunnel for about 2} miles under Malakand (34° 34' : 71° 56') to the Mazah Khwar, the bed of which is followed for a short distance before the water is again taken into a canal for distribution to a large part of the North-West Frontier Province.

The hydro-electric scheme proposes to take some 1,000 cusecs of the 1,500-1,600 cusecs of water debouching from the Benton tunnel through another tunnel, hereinafter referred to as the power tunnel, about 2,249 feet in length, and then along an open cut with a silt trap a short distance to a concrete masonry weir supporting the tops of three pipes down which the water will fall some 250 feet to drive three turbines situated on the right-hand bank of the existing Upper Swat canal. The surplus water will flow by two concrete siphons into a pit and thence over a weir to rejoin the Upper Swat canal between the turbines and the power house by means of a natural nala. The water in the power tunnel will have an approximate depth of ten feet.

The scheme will generate cheap electricity which will be supplied to the North-West Frontier Province and will be an immense boon to that Province. It was hoped to complete the scheme by September, 1937, but unforeseen difficulties have hampered the work and it was doubtful if completion by that date was possible.

The power tunnel had been driven through at the time of Dr. Coulson's visit and work was then being concentrated on the open cut and on the hillside on which the pipes were to be laid and held in position.

In 1908, Sir Thomas Holland reported geologically on the character of certain rocks in the Malakand range of hills in connection with the proposal for driving the Benton tunnel, long since completed, for the water of the Upper Swat canal. Mr. G. H. Tipper also reported on parts of the range.

mus

across,

occur.

The conditions existing in the Benton tunnel are different from those in the nearby power tunnel. In the former, the rock is practically all muscovite-granite, but no granite crops out in the vicinity of the power tunnel. Two apophyses of granite, which have found their way along the foliation planes of the schists, may be seen on the path down from Malakand to the power tunnel.

The schists are far from constant in character and a great variety of types exists. Dr. Coulson noted hornblende-schists, calc-schists, biotite-quartz-hornblende-calc-schists, muscovite-calc-schists, covite-zoisite-calc-schists, etc. The assemblage contains stringers of pegmatite, usually vein-quartz, but quartz-muscovite-pegmatites, some with “ books” of muscovite up to three-quarters of an inch

Dr. Coulson noted ample evidence of crumpling, buckling, slipping and faulting of strata. In the upper part of the Mazah Khwar, there seems to be a fault parallel to the line of the stream bed, with other cross-faults as one approaches the exit of the Benton tunnel. Through the power tunnel, also, all these phenomena were noted.

The foliation planes of the schists apparently agree with the planes of sedimentation of the original somewhat heterogeneous shale series that was metamorphosed into the present assemblage of schists.

In his detailed report, Dr. Coulson mentioned the liability of Malakand to earthquake shocks and stated that even if disastrous shocks were not felt, a slight shock might be sufficient to cause movement of strata already in a precarious state of equilibrium. He was also influenced by the fact that once the scheme was in operation, there can be no stopping for repairs to the channel along which the water is flowing. At present the headworks of the Upper Swat canal have been closed periodically to enable the Benton tunnel to be examined. But once the hydro-electric scheme is in operation, the only part of the Upper Swat canal that can be closed will be that portion below the power house.

The schists through which the power tunnel has been cut are easily permeable by water along their foliation planes. Consequently their fissility is largely increased when they are sodden with water. Dr. Coulson noted, however, that if the foliation were transverse to the line of the tunnel the roof and walls should noi. mally be fairly safe provided there was no evidence of buckling and crumpling. When, on the other hand, the foliation of the schists was parallel or nearly so, to the line of the tunnel, and the roof is consequently left unsupported for considerable distances, if water is leaking through the roof, then the condition of that cannot be regarded with equanimity, especially when there are signs of slipping or crumpling. Though it may remain in equilibrium for a long time, a slight shock may dislodge a small block of the roof, rendering probable ever larger and larger falls. Here, also, the dip of the strata is important. If the dip is to one side, the side dipping into the tunnel will have a tendency to slip into it, especially if the foliation planes are lubricated with water.

Dr. Coulson discussed the question of the safety of hillsides with varying conditions of dip, especially in reference to the open cut and the hillside from the pipe intake to the turbines below. He considered that the small amount of silt in suspension in the water during a short period each year was not likely to cause excessive erosion. He noted that as abundant water was available, small leakage losses did not matter, especially as such leakage water would normally rejoin the Upper Swat canal along the bed of the Mazah Khwar.

After describing the conditions along the length of the power tunnel in great detail, Dr. Coulson made numerous recommendations regarding lining and supporting the power tunnel and for the safety of the open channel. .

61. In view of the cost of extending to the Hazara district the grid system of the Malakand Hydro-electric Scheme, the late Mr. A. T.

Arnall, Superintending Engineer of the ElecPower for Hazara dis- tricity Circle in the Public Works Department, trict, N.-W. F. P.

North-West Frontier Province, expressed keen interest in Dr. Coulson's investigation of the coal of the Dore river (see p. 36). As Dr. Coulson discounted the economic value of the coal in question, he invited attention in his report to the hydroelectric possibilities of the perennial streams in the Hazara district. The Dore river would be rather difficult to dam at some such place as just below its junction with the Harnow river. Though the Nummulitic limestone on the right bank would offer a moderately good foundation, a fold-fault runs along the course of the stream. Damming would flood the lower part of the Harnow and would cause much expense in the provision of bridges etc., for the Abbottabad-Nathiagali road. Also the stream is very subject to spates and enormous quantities of detritus are carried down by it.

« PreviousContinue »