Part 6 (1/2)
The water has always been limpid and pure, but its hardness remains the same as it was prior to the flood.
With the copious supply of water from this source, due of course to abnormal conditions and not likely to be permanent, the operations of tunneling have been suspended temporarily; but it is proposed to continue the driving of the gallery, from a new shaft west of No. 3.
The water encountered will be drained off by pumping until the main water-bearing gravels, in the neighborhood of Shaft No. 5, are reached.
It is also proposed to reconstruct the 30-cm. high-level pipe line, from San Geronimo along the high road on the north bank of the river, so that by pumping water can be delivered to the city system from Shafts Nos. 5, 6, and 7, in the event of a shortness of supply from the Estanzuela River.
_Shaft No. 1._--Shaft No. 1 is designed to connect the infiltration gallery with the gravitation tunnel. This shaft has an inner diameter of 2.43 m. (8 ft.) and is fitted with a special gate-valve. In the bottom of the door of this valve there is a smaller valve, 30 cm. in diameter, so that, when the infiltration gallery is closed for cleaning out the sump, the smaller door, which is operated through the same spindle by a bevel-geared head-stock at the top of the shaft, can be opened first.
s.p.a.ce is also left for screens if these should be found necessary.
Access to this shaft is gained by a reinforced concrete stairway in nine stages. The superstructure is to be supported on reinforced concrete column foundations carried to the firm rock, owing to the loose condition of the strata at the top of the shaft.
_Aqueduct._--The construction of the concrete conduit was begun in April, 1908. Fig. 13 shows the general types. Type _A_ was adopted in gravel and conglomerate formation, and Type _B_ where the excavation was in ”sillar,” the soft nature of this rock permitting it to be excavated exactly to the required external diameter of the concrete lining.
The concrete which was without steel reinforcement was a 1:2-1/2:3-1/2 mixture, the sand being from the crusher and the aggregate from the river bed, screened to pa.s.s a 25-mm. mesh. Where the conduit crossed the river obliquely, immediately below the gravitation tunnel, it was strengthened with ma.s.s boulder concrete of Type _C_. During the great flood this heavy section withstood its effects without damage of any kind, but beyond this point, where the conduit had been laid in compact cemented gravels, the scouring action of the flood on the north bank lowered the level of the gravels from 2 to 3 m.; the only damage, however, was the scouring away of the gravels at the south side of the conduit. To prevent such an occurrence in the future, the conduit at that point was strengthened with additional concrete for a distance of 195 m., as shown on Fig. 13. The extra concrete, amounting to 733 cu.
m., was a 1:3:5 mixture, in which was embedded 20% of heavy boulders.
The top of this special length now forms a weir for the present river flow. Where the conduit enters the bluff on the north side of the river, at 1,200 m., there is an overflow chamber which has a sluice-gate 76 cm.
wide, arranged so that the conduit can overflow at the present time when running 76 cm. deep. To deflect the flow in the conduit, a wrought-iron plate, provided with a balance weight, is dropped into a groove on the lower side. The outlet is a 61 cm. concrete tube, having its invert above ordinary flood level, and arranged to be closed by a gate.
At 1,963 m. the conduit is carried over an arroyo on a segmental arch of 8 m. clear span, as shown on Fig. 13. There are 5 ventilating columns and 5 manholes on the aqueduct.
[Ill.u.s.tration: PLATE X, FIG. 2.--SETTING FORMS FOR SAN GERONIMO CULVERT.]
The aqueduct terminates in the Obispado distributing reservoir valve-house, at a level of 558.50 m. The work in connection with this aqueduct was completed by December, 1908.
DISTRIBUTING RESERVOIR AT OBISPADO.
The main distributing reservoir for the San Geronimo gravity supply is immediately below the historic Obispado (Bishop's Palace), at the western limits of the city. The general arrangement and lay-out is shown on Plate XIII.
[Ill.u.s.tration: PLATE XIII.--GENERAL PLAN AND SECTIONS, OBISPADO RESERVOIR.]
_Valve-House._--The invert of the conduit from San Geronimo, where it enters the valve-house, is 558.50 m. above datum. The valve-house, which is built in the center of the reservoir, is shown on Fig. 2, Plate XVIII.
One of its special features is the provision of the main overflow at this point instead of within the reservoir proper. The inlet, main supply tunnel, independent by-pa.s.s overflow, scour-out pipes, gate-valves, and screens, are all controlled within the valve-house.
[Ill.u.s.tration: PLATE XVIII, FIG. 2.--VIEW OF ROOF OF OBISPADO RESERVOIR, LOOKING NORTH.]
_Reservoir._--The reservoir is rectangular, 126 by 81 m. (413.28 by 265.68 ft.) at the top, and has a water depth of 4 m. (13.1 ft.). In the design it was necessary to limit it to the lowest economical depth, so as to increase the static pressure over the low-pressure district as much as possible.
_Excavation and Embankment._--The excavation, except for a depth of about 1 m. which was in black soil, was chiefly in a disintegrated ”sillar” stratum of a heavy clayey nature, the greater part of which could be handled conveniently with plows and sc.r.a.pers; the actual foundation on the eastern half required blasting for the final depths.
The total excavation amounted to 56,479 cu. m., of which 7,255 cu. m.
were placed in the embankment, the remainder being deposited in the immediate neighborhood of the reservoir. The final tr.i.m.m.i.n.g of the banks, which were left 30 cm. full, was not undertaken until the lining was begun. The work was done under contract with Mr. J. S. Nickerson, of Monterrey. The excavation had only one cla.s.sification, and the contract prices were 0.50 peso per cu. m. for material carried to spoil banks, and 1.00 peso for material placed in the embankment. The excavation was begun in December, 1907, and completed in April, 1908. The work was then left standing until the end of 1908 to allow the banks to consolidate thoroughly prior to lining, which was begun on January 4th, 1909.
_Concrete Lining and Roof._--Plate XIII shows the general plan and sections, the main feature being the simple division of the reservoir into 24 rows of columns longitudinally and 15 rows transversely, making a total of 360 columns, less the four left out at the central tower. All the columns are 5 m. apart both ways. The roof was designed for a live load of 100 lb. and a dead load of 150 lb., the same as at the South Reservoir. With the exception of the floor, all the concrete work was reinforced with twisted steel lug bars. The foundation load on the columns for the eastern half of the reservoir is 0.9 ton per sq. ft.; that on the columns for the western half, where the foundation is of very hard sillar and conglomerate, is 1.95 tons per sq. ft.
_Under-drainage of the Floor._--To provide for proper drainage in case of seepage, the floor was underdrained with rubble drains, 30 cm. wide and 23 cm. deep, filled with large round gravel carted from the bed of the Santa Catarina River. The total length of these underdrains is 1,160 m. In order to facilitate the detection of any seepage, they were conducted to a permanent inspection pit outside of the reservoir.
_Main Distributing Conduit._--The main distributing conduit is laid along the inside of the reservoir, at the inlet end, and carried on elliptical arches of 2 m. span to a height of 71 cm. above the finished floor level. This conduit is 76 cm. high and 45.7 cm. wide, and it branches in two directions from the inlet tunnel to each side of the reservoir, its total length being 69 m. In order to prevent any stagnation and to give a continuous circulation, the water is delivered at eight points, in the length of the distributing pipe, through square openings with semicircular tops, the areas of the openings increasing toward the ends. These inlets are placed so that the current will not strike the roof columns.