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.
[Illustration: 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-pass overflow, scour-out pipes, gate-valves, and
screens, are all controlled within the valve-house.
[Illustration: 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 scrapers; 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 trimming 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 classification, 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.
_Outlet Tunnel and Valve-House._--The outlet tunnel is at the north end
of the reservoir, and was excavated in hard sillar rock. The tunnel is
lined with concrete 30 cm. thick, the finished internal dimensions being
1.52 by 0.91 m. The length of the tunnel is 22.5 m. to the point where
it enters the outlet-house. This house is divided by a wall 45 cm.
thick, which supports a 76-cm. (30-in.) penstock-valve. The supply pipe
to the city leaves this chamber in the west wall, and is also fitted
with a 76-cm. penstock-valve. The supply pipe has a copper screen of the
same design and dimensions as those in the inlet-house. A 30-cm.
(12-in.) scour-out pipe in this chamber provides for draining the
contents of the reservoir to a neighboring irrigation ditch, when
necessary.
The superstructure of the valve-house is of concrete, and at the floor
level there are bevel-geared head-stocks to raise the valves, etc.
_By-Pass and Supply Pipes._--The by-pass and supply pipes are carried
below the reservoir embankment to join the main 76-cm. (30-in.)
cast-iron distributing pipe to the city. For this short distance they
were constructed of concrete, 76 cm. in internal diameter, 10 cm. (4
in.) thick, reinforced with 6-1/2-mm. square steel longitudinal rods, 30
cm. from center to center in the circumference, and hooped with
6-1/2-mm. square steel rods spaced 30 cm. apart. The concrete forming
these pipes was a 1:1-1/2:2-1/2 mixture.
_Parapet Walls._--The parapet walls have 12 piers at each side and 8 at
each end. In these piers there are ventilating openings branching at the
top to each side of the parapet, with outlets provided with cast-iron
screens. This arrangement gives 4 sq. m. of ventilating space (exclusive
of that provided in the central tower), equally distributed at 40 points
around the walls of the reservoir.
_General Construction Scheme._--The concrete mixing plant, which
consisted of two No. 1 Smith mixers, was arranged in connection with the
bins and hoppers for the rock and sand on the high ground to the west,
and from there the material was conveyed on a framed timber gangway
carried right across the center of the reservoir, as shown by Fig. 1,
Plate XVII. From this central platform the concrete for the columns was
filled from stages placed on the top of traveling towers, 5 m. high,
which were run between two rows of columns on standard-gauge rails laid
on the floor of the reservoir. By this arrangement 24 columns could be
filled from each length of track. A main narrow track was also laid
right around the reservoir, with the necessary turn-outs.
[Illustration: PLATE XVII, FIG. 1.--FILLING PRIMARY BEAMS FROM TRAVELING
TOWER, OBISPADO RESERVOIR.]
[Illustration: PLATE XV, FIG. 1.--CONSTRUCTION OF WEST SIDE-WALL OF
OBISPADO RESERVOIR.]
[Illustration: PLATE XV, FIG. 2.--PRIMARY BEAMS AND COLUMNS, OBISPADO
RESERVOIR.]
[Illustration: PLATE XIV.--DETAILS OF FORMS FOR CONCRETE WORK, OBISPADO
RESERVOIR.]
The forms for the columns, primary and secondary beams, are shown on
Plate XIV. The side forms for the primary beams were struck in 24 hours,
so as to economize lumber; but the bottom lumber was left in position
for 28 days. To avoid much unnecessary timber, the secondary beam forms
were supported at the ends on reinforced concrete corbels cast on the
primary beams.
For placing the side-walls, a special traveling form was used, the
details of which are shown clearly on Plate XIV. At the end of each form
an expansion joint of 25 cm. was left to be filled after the roof was
placed in position. The concrete was delivered to the wall through
stove-pipe chutes, and carefully spaded by workmen in the limited space
between the forms and the embankment. The wall form was removed after 36
hours, by loosening the jacks and pulling forward the hooked tie-rods.
This form is also shown on Fig. 2, Plate XVI.
[Illustration: PLATE XVI, FIG. 2.--TRAVELING SIDE-WALL FORM, OBISPADO
RESERVOIR.]
[Illustration: PLATE XVI, FIG. 1.--PREPARING FLOOR FOR CONCRETING,
OBISPADO RESERVOIR.]
The concreting of the roof slab was carried on continuously, and, when
partly completed, the floor was laid in the shade. The bottom layer of
the floor, 13 cm. thick, was laid in continuous panels between the
columns, and brought to a fairly smooth surface. On this surface, after
keeping it wet for 10 days and then allowing it to dry thoroughly, a
layer of asphaltum, supplied by the American Asphaltum and Rubber
Company, of Chicago, was placed. The work was done by ordinary Mexican
laborers after they had received a few days' instruction from one of the
Asphaltum Company's superintendents. The surface of the lower layer was
kept perfectly clean, and then received one coat of "Pioneer" paint. The
asphaltum, heated in a boiler inside the reservoir to a temperature of
approximately 425 deg. Fahr., was then poured over the floor from buckets,
in a layer approximately 4 mm. thick. Where the floor joined the column
pedestals, and at each new panel section, a double thickness was used.
The labor cost of water-proofing, including superintendence, etc.,
amounted to 3.3 cents (Mexican) per sq. m. for painting with "Pioneer"
paint, and 5.4 cents for the asphaltum coating, or a total labor cost of
8.7 cents per sq. m. for the complete water-proofing. This cost is based
on a rate of 8.00 pesos per day for a foreman, and 1.00 peso for each
laborer. It required 50 U. S. gal. of the paint to cover 265.2 sq. m.,
and an average of about 6 lb. of asphaltum for 1 sq. m.
The upper concrete layer of the floor, 10 cm. thick, was placed so as to
break joint with the lower, and was brought to a smooth surface with
wooden floats sheathed with steel and reaching across the panels. In
this way a perfectly smooth surface was obtained without any plastering.
[Illustration: PLATE XVII, FIG. 2.--CENTRAL TOWER AND STAIRWAY, OBISPADO
RESERVOIR.]
The concrete for the beams, columns, side-walls, and floor, was a
1:2-1/2:4 mixture, crushed sand and stone being used throughout. In the
roof slab the mixture was 1:2:3.
The whole of the concrete work of the reservoir was completed in 6
months, by the Company's own administration, and the reservoir was first
put into service a few days after the great flood of August 27th, when
the Estanzuela supply main, crossing the Santa Catarina River, was
partly destroyed. Since that time frequent examinations of the
inspection pit, which is connected by a pipe to the rubble drains under
the floor, have never revealed the slightest leakage.
_Lay-Out of the Reservoir Roof and Grounds._--The Company owns about
11-1/2 hectares of land, which includes that occupied by the reservoir
and its surroundings, and as this property is in an attractive
situation, commanding fine views of the Sierra Madre Mountains, the
whole of the works have been given a pleasing architectural character,
and the grounds laid out to form a public park for the citizens of
Monterrey.
[Illustration: FIG. 14.--SKETCH PLAN OF LAY OUT AT OBISPADO RESERVOIR.]
The general plan of the scheme is shown by Fig. 14 and Fig. 2, Plate
XVIII. The roof, which has an area of 1 hectare, has been laid out with
walks and grass plots, and the surrounding embankments have been
converted into driveways. Above the reservoir a small plazuela of 1/2
hectare has been laid out with a space above it for a band-stand. The
whole of the ground has been encircled with carriage drives, on which it
is the intention to plant shade trees. The lay-out of this land also
embraced the scheme for protecting the reservoir by draining the
surface-water away to the irrigation ditches.