2 ELECTIONS, ETC. [Minutcs of Associate i&rnbm. JAMESHENRY BAILEY. FREDERICHCHARLEE LEA. ROBEIlT FORBESBENNETT. DAVIDERNEST LLOYD-DAVIES, Stud. CHARLESHERBERT BISHOP. Inst. C.E. ALFREDBROWN ERNEST BLACsBURN, ATHOLLOCKET. Stud. Inst. C.E. JOHNMACLEAN. ARTHUR CAMERON. ROBERTCREASY NASTER. ROBERTWILSON DRON. LEWISJfITCHELL. SAMUEL EDGABFEDDEN. HENRYLAWRENCE PEABSON, Stud. WILLIAM AINSLIEFOULIS. Inst. C.E. GEORGEJOHN FREUND, jun. WILLIAMHENRS PRESCOTT. HENRY DUDLEYGILL. THOMASWILLIAH RUSSELL. FREDERICE AUSTENHADOW. FREDERIC BOLTONSONNENSCHEIN. ROGERGASIKELL HETHERINGTON, M.A. DAVID MCTAGGARTSYMMERS, 3I.A. (Cantab.), Stud. Inst. C.E. (Aberdeen.) EDWARDHODGSOX. THOMASGEORGE HOWES THOMAS. GEOI~QEERNESTHOOPER,S~U~.I~~~.C.E.GILBERTTHOMSON, M.A. (Edin.) JOHNINGLIS, B.Sc. (Edin.) Stud. Inst. HORATIOEDGAR DAWSON WALKER. C.E. GERARDNORMAN WATNEY, B.A. (Can- CECILDAUBENY INMAN. tab.) JOHNWILLIAM KITCHIN. THOMAS AUBREYWATSON. HARRISJURGEN CARL ICUHL. ARTEUR HERRINGWEIR. PREDERICWINFRIED LA TROBE-BATE- EDWARDWILLIS. XAN. An Associate. ARTHURGEORQE BLOXAM. (Paper No. 3265.) “ The Burrator Works for the Water-Supply of Plymouth.” By EDWARDSANDENAN, M. Inst. C.E. TIIE waterworks of Plymouthdate from theyear 1590. In December of that year Sir Francis Drake, who appears to have been an engineer as well as a navigator,began tocut the open channel or watercoursefrom Dartmoor to Plymouth, now known as ‘I the leat,” which was destined to convey to Plymouth its supply of water during the succeeding 300 years. The leat, which was 18 miles 3 furlongs in length, appears, from the old records of the town, to have been made between December, 1590, and April, 1591. It was made in virtue of powers-conferred by an Act of Parliament of Queen Elizabeth‘sreign, dated 1585, -which authorised the Corporation of Plymouth to build a weir across the River Mewe, or Meavy, and to divert the water of that river into a trench which had a specified width of 6 feet or 7 feet Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. Pr0ceedings.J SANDEPAN ON WATER-SUPPLY OF PLYMOU'PB. 3 but no specified depth. The distance from the weir on the River Meavy to Plymouth in a straight line was not more than 103 miles, but the winding course followed by the leat, contouring the hilly country through which it passes, accounts for its much greater length. When first brought into the town the water was distributed by means of open channels through the streets, which, at a later period, were superseded by lead pipes. Later still, on theintroduction of iron pipes, thewater wassupplied under pressure from four small service-reservoirs constructedat different elevations. Thelast of theseconstructed was the Roborough reservoir,which issituated 6 miles north of thetown, at an elevation of 547 feet (Fig. 1, Plate 1). On the completion of this reservoir in 1885 the use of the portion of the leat to the south of it was abandoned in favour of a line of pipes, the northernportion, about 8 miles long, remaining in use. During the last 25 years the supply of water has frequently run short owing to drought, to leakage of water from the hat in summer, and to the effects of snow and frost in winter, and during this period many schemes for the storage of water and for the protection of the open leat have been proposed. In the year 1887 an Act of Parliament was obtained for the construction -at the foot of the gathering ground-of a reservoir having a capacity of 350 milliongallons, tobe called theHead Weir Reservoir,and land was bought for the purpose. This scheme was abandoned, however, owingto the discovery of a porous layer of decomposed granite,upwards of 100 feetthick, on thesite of the proposed embankment. In 1889 planswere depositedfor the construction of a reservoirto be called the Harter Reservoir, which was to be formed by a dam extending across the junction of two valleys, about 2 miles to the north of the first reservoir proposed. To this proposal many townspeople objected, and it was abandoned after a poll of the town had shown a majority against it. The Author, having been appointed Water Engineer to the Corporation in July, 1891, presenteda report in December of that year,recommending thebuilding of a storage-reservoir to be called the Burrator Reservoir, 2 mile below the proposed Head Weir Reservoir, and the substitution of a line of pipes for the open leat. In 1892 this report was submitted to and approved by Mr. James Mansergh, President Inst. C.E., who also assisted inpromoting the passage of theBill through Parliament, and acted throughout as Consulting Engineer. The workswere commenced on the9th August, 1893, twomonths after the Royal Assentto the Bill was received. B2 Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 4 SANDEMAN ON THE BDRRATOR W6RKS @OR [Minutes of GENERALDESCRIPTION. The watershedlies on the westernslope of Dartmoor, at an elevation varying between700 feet and1,600 feet, its most northerly point being North Hessary Tor, which is close to Princetown and 13 miles in a north-easterly direction from Plymouth. By the new scheme the area of the watershed was increased from 4,885 acres to 5,360 acres. Almost the whole of this area lies on the granite formation, only a small portion on the west being on the upper Devonian. On the hills, locally called ‘‘ tors,” the rock lies close to the surface as a rule, but in many places the valleys are filled to a considerable depth with decomposed granite, in which are large boulders. The rainfall, as recorded by the existing gauges, which with one exception were fixed in 1892, is about 58 inches to 60 inchesper annum. From the large dry-weather flowof the river it would appear that the quantityof water absorbed during wetweather is very large. Probably it is retained on various parts of the watershed in deposits of decomposed granite, which yield itin a verypure form duringdry weather.These depositshave beenproved to be upwards of 100 feetthick in several places. The Burrator Reservoir is formed by two embankments, one of masonry across the narrow gorge through which the River Meavy flows, the other of earthwork lying between two hills known as Sheepstor andBurrator. It was expected that solid granite would be found on the site fixed upon for the masonry wall, and fortunatelythe suppositionproved correct. In the case of the earth embankment, it wasknown that decomposed granite extended to a considerable depth, but it was not anticipated that it would be necessary to excavate, even in this material, to so great a depthas was afterwardsfound requisite. The peculiar geological features discovered insinking the trench, however, made the baring of the solid rock a necessity for the purpose of securing a watertight barrier. The quantity of water impounded is 657,000,000 gallons, the area covered by water being 117 acres. The length of the reservoir is 12 mile, and the greatest width is 4 mile, the top water-level being 708 feet above Ordnance datum. THEBURRATOR Dm. (Figs. 2 and 3, Plate 1.) Theheight of the masonrydam across the River Meavy is 77 feet,measured from the old river-bedto the overflow-level; whilstthe total height frombase of foundation to coping of Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] THE WATER-SUPPLY OF PLYMOUTH. 5 parapet - wall is 145 feet 6 inches. The excavatedtrench was 80 feetwide atthe level of theriver, and decreased in width on the valley sides in the same ratio as the thickness of the dam. The dammeasures 629 feet in thickness at the level of the river-bed, tapering upwards with a slope of 1 in 0.075 on the water face and 1 in 0 * 61 on the outer face. A roadway 18 feet wide is carried over the top of the dam on five segmental arches of 25 feet span. The length of the darn on top water-line is 361 feet, and the length of overflow is 125 feet. The excavation was begun in November, 1893, and was practi- cally completed by November, 1895, whenbuilding operations commenced. Fortunately, no timberingworth mentioning was required,as the ground was sufficiently hard to standwithout it.After the overlying earth and boulders on thesite had been removed, solid granite rockwas found at depthsvarying between 2 feet and 40 feet. Several wide fissures full of deposited material ran through the rock, crossing the line of the trench in such a way as to afford an admirable means of tying the mass of the masonry intothe foundation rock. The material in them, which increased in hardness with the depth,was excavated until a hard bottomwas reached. The fissures werewedge-shaped, decreasing inwidth with the depth.When the granite was exposed, it wasfound to havemany jointsand cracks, some, particularly on the sides of the valley, being quite open, others being filled in by deposit. Excavation only ceased when every crack and joint had been followed down to the solid rock. The quantity of water draining into the trench from crevices in the rock was very small. When the jointed rock was removed, it was generally found that the water came either from one side or the other of the excavated trench, and not from the base on which the dam was to be built. The rock on which the dam is founded is a hard porphyritic schorlaceous granite with large crystals of felspar, forming a perfect foundation. The rock taken from the excavations was, as a rule, cut by hand- labour with the double object (1) of obtaining stone in suitable sizes for building,and (2) of preventingthe shaking and cracking of the rock beneath by the use of blasting materials.