_Provisional Supply._--It has already been stated that the Company began
operations at San Geronimo in March, 1906, by sinking a well on the
north bank of the Santa Catarina River at San Geronimo. At this point, a
little later, a small steam pumping plant, sufficient to handle about
8,000 liters per min., was installed. The lowest depth to which this
well was ultimately sunk in water-bearing strata, was 7 m., the normal
level of the water during 1906 and 1907 never falling lower than 569 m.
above datum. Tests made from time to time during 1907-08, showed that
this well was capable of supplying nearly 10,000,000 liters (264,000
gal.) of water daily.
The excellent supply yielded by this well made it desirable to adopt it
immediately as a provisional measure, pending the completion of the
larger works forming the western source of supply. To utilize the well
to its fullest extent, a reinforced concrete reservoir, of 3,000,000
liters capacity, was constructed on the south bank of the river, the top
water level being 585 m. above datum, that is, at the same elevation as
the proposed reservoir for the Estanzuela supply. The reservoir is 53.80
m. long, 21 m. wide, and has a water depth of 3.25 m. at the overflow
level. It is excavated on a steep hill slope, and has an earth
embankment on the lower side. The lining is of concrete, 20 cm. thick,
and the roof is of reinforced concrete composed of flat arches springing
from beams carried on 46 by 35-cm. reinforced columns. There are 68 of
these columns, and they are 3 m. apart longitudinally and 5 m. apart
transversely. The roof was not constructed until October and November,
1907, and prior to that time the necessity of covering the reservoir was
amply demonstrated by the growth, during hot weather, of considerable
quantities of green algae, which had to be skimmed from the surface of
the reservoir every few days.
The delivery pipe from the pumping plant was originally of riveted steel
and was asphalted. It was 30 cm. in diameter, 2 mm. in thickness, with
slip joints, and where it crossed the river it was encased in concrete.
This pipe was afterward replaced by a cast-iron pipe of the same
diameter. The supply pipe to the city was also of sheet steel, 30 cm. in
diameter. For a part of its length it was laid in the high ground of the
south bank of the river, which it crossed near the western limits of the
city, and was then connected to a 30-cm, cast-iron pipe in the
distribution system. The total length of the pipe from the reservoir to
the city distribution system was 2,850 m.
This provisional pipe continued in service from October, 1906, until
August 27th, 1909, when the river portion was completely swept away,
together with the provisional pump-house at San Geronimo, during the
great flood in the Santa Catarina River. Fortunately, the permanent
supply works were completed at the time, so that the destruction of this
pipe line, which had already served its original purpose, had no effect
on the supply of water to the city.
[Illustration: PLATE XI.--SECTION OF INFILTRATION GALLERY, SAN GERONIMO
GRAVITY SUPPLY.]
_Infiltration Gallery._--The chief feature of the San Geronimo gravity
supply is the infiltration gallery. By referring to the profile on Plate
XI it will be seen that at this place there is a considerable area of
what is undoubtedly water-bearing gravel. The main conditions were
revealed by the borings previously carried across the valley, but the
profile has been corrected to show the actual conditions as established
at a subsequent date by shafts. Practically, the water-bearing strata
are not limited merely to the sand and coarse gravels, as the clay
formation lying above and below them is full of small gravel deposits
containing considerable volumes of water. The main direction of the
underflow is toward the east, and the hydraulic gradient, which was
established from wells sunk farther west, was found to be approximately
1%, or practically the same as the average surface of the bed of the
river above the line of the infiltration gallery.
The general scheme for tapping this underflow was to drive a main
gallery at the 560-m. level on a grade of 0.05%, which was sufficiently
high to take the supply by gravity to the western reservoir, having a
top water level at 558.75 m. above datum. This elevation is sufficient
to give an excellent pressure over about 60% of the city, and a fair
pressure to reach the upper stories of the highest houses, if required,
over the whole supply district. From this gallery it was proposed to
sink shafts at frequent intervals, for a total distance of 300 m.,
carrying them below the gallery level, to tap any water-bearing gravels
there might be in the clay formation underlying the gravels and sands.
From the main gallery it was proposed to construct branch galleries up
stream on a flat gradient, so as to obtain the advantage of an increased
head due to the steep hydraulic gradient of the underflow water.
[Illustration: FIG. 12.--DIAGRAM SHOWING VARIATION IN WATER PLANE 1905
TO MARCH 1910 AT SAN GERONIMO.]
In investigations of this kind, it is of first importance to have a
continuous record of the level of the water plane, and Fig. 12 has been
plotted to show its variation at San Geronimo from the beginning of
1905 to March, 1910. From January, 1909, to March 31st, 1910, these
levels are averages of daily readings taken in 9 shafts sunk along the
proposed line of the infiltration gallery. In 1902 the water plane was
standing at 570.18 m. above datum, but from that date until 1905 the
writer has been unable to find any records. This diagram should be
examined together with the rainfall diagram, Fig. 3, and it will be
noticed that the fall in the water plane drops with the general scarcity
of the rainfall during 1907-08, and until July, 1909. The year previous
to July, 1909, is regarded, by many competent local observers to have
been the longest period of extreme drought in 30 years in Nuevo Leon,
and the evidence which the writer has been able to gather regarding
stream flow in the neighborhood of Monterrey supports this view. The
total rainfall at Monterrey for the year prior to July 1st, 1909,
amounted to 9.98 in., or 4.16 in. less than the lowest record for any
calender year since 1894, or, in other words, about 45% of the average
annual rainfall.
The lowest point to which the water plane dropped was during June and
July, 1909, when the levels stood slightly above 565.00 m., or 5 m.
above the level of the floor of the infiltration gallery. During this
period pumping tests were made in the various wells, and from these it
was quite clear that the infiltration gallery, if carried far enough to
meet them all, would yield a supply of from 25,000,000 to 40,000,000
liters daily. During the great rainfall of August, 1909, the water
levels rose very rapidly; the heavy precipitation between August 9th and
10th caused the level to rise to 568.00 m. in about 4 days, and 6 days
after the great flood of August 27th, the water level, which had
continued rising gradually, reached 571.40 m., and then fell gradually
until at the end of March, 1910, it was practically the same as it had
been from 1902 to 1905.
[Illustration: PLATE XII.--SAN GERONIMO GRAVITY SUPPLY.]
It should be noticed that the readings were taken in the shafts on the
high ground to the west of the present river bed, and were independent
of any flow there might be in the river. During times of ordinary floods
in the river, it was very noticeable that, notwithstanding the fact that
the river water might be turbid to an extreme degree, the well even in
immediate proximity to the river bed did not show the least sign of
discoloration.
_Design of Works._--Plate XII shows the general design of the gravity
scheme, which consists of a main tunnel 550 m. long and a concrete
aqueduct, 1.06 m. (42 in.) in internal diameter and 2,311 m. in length,
discharging into a low-service distributing reservoir at the extreme
western limits of the city. The tunnel and aqueduct were laid on a
gradient of 0.05%, and the latter was designed to discharge 55,000,000
liters per day (22.8 cu. ft. per sec.) if flowing to its full capacity.
_Gravitation Tunnel._--This tunnel, shown on Plate XII and Fig. 13, was
completed prior to driving the infiltration gallery into the
water-bearing gravel, so that the water encountered in the gallery could
be easily drained off by gravity, thus avoiding a heavy outlay for
pumping. The tunnel passes through various strata, the principal ones
being calcareous shale, conglomerate, and gravels. The tunneling
operations were carried on from 5 shafts, No. 1 being 23 m. deep, and
the others varying from 20 to 10 m. The shafts in loose ground were
timbered in the usual way, having clear inside dimensions of 2 m. Shaft
No. 1, which was entirely in shale, was taken out approximately to 3.35
m. in diameter, so as to permit it to be lined with concrete having a
finished internal diameter of 2.43 m.
[Illustration: FIG. 13.--GENERAL DETAILS SAN GERONIMO GRAVITY PIPE LINE.]
Fig. 13 shows the details of the tunnel, which was lined with concrete,
the bottom and sides being approximately 23 cm. (9 in.) thick. The
interior dimension is 0.91 m. at the invert level and 1.016 m. at a
height of 1.22 m., the corners between the side-walls and the floor
being slightly curved. The arch is formed of two rings of brickwork in
cement mortar, this thickness being increased in some lengths to three
rings. Where the rock was in good condition, and not likely to
disintegrate easily, the arch, for a distance of 90 m., was left
unlined. Of the total distance of 550 m., careful timbering was required
for 300 m. In lining the timbered portion of the tunnel with concrete,
all the timber was removed, except in loose ground, where the laggings
were left in position.
While the tunnel was being driven, a start was made to drive the north
end of the infiltration gallery, which was in rock for a distance of 44
m. Water appeared at about 35 m., and then the work was temporarily
suspended until the gravitation tunnel was completed and a length of the
aqueduct had been constructed far enough down stream on the north bank
of the river to permit of draining direct to the river. This point was
reached at 1,170 m. from Shaft No. 1, and there a temporary overflow
chamber was constructed.
When the tunnel was completed, the two intermediate shafts were filled
up, the remaining three being retained permanently. Shafts Nos. 2 and 3
were lined with concrete, 76 cm. (30 in.) in internal diameter, and 23
cm. thick. They were domed at the top to form circular openings to
receive cast-iron covers. Progress on this tunnel was slow, taking from
December, 1907, to November, 1908, to complete, owing chiefly to
difficulties with an incompetent contractor. The contract was
subsequently transferred to Mr. John Phillips, of Mexico City (who was
also the contractor for the aqueduct), who completed it satisfactorily.
_Continuation of the Infiltration Gallery._--When the aqueduct (to be
referred to again) was completed as far as 1,170 m. from Shaft No. 1,
the driving of the infiltration gallery, which was 2 m. high and 1-1/2
m. wide, was continued until gravel was encountered in the roof, at 44
m. from the shaft. At this point the rock dipped at an angle of 45 deg.,
and the gravels contained quantities of large boulders mixed with fine
sand; immediately after encountering the gravel, a flow of about 90
liters per sec. was met, evidently coming through from a pot-hole in the
shale. This quantity diminished in about 10 days to about one-fourth,
but gradually increased again as the driving proceeded. The operations
of driving the tunnel from 44 m. forward were begun in the dry season,
in February, 1909, and the gravel was encountered for a distance of 24
m., or up to 68 m. from the shaft. The center of this gravel bed was
about 30 m. south of the old river channel, which had been continuously
dry at the surface for several years. Up to 68 m. the work was very
difficult, owing to the upper part being of loose gravel and the lower
in very contorted shale. The timbering of the tunnel in the full gravel
section consisted of heavy square settings, 1 m. apart. At 68 m. the
clay and gravel formation was met, and the rate of progress then was
about 4 or 5 m. a week. A short branch gallery was also driven about 7
m. up stream near Shaft No. 2. The total distance the infiltration
gallery was carried from Shaft No. 1, was 100 m., when the floods of
August, 1909, caused its suspension.
During the progress of the gallery, attempts were made to sink a 3-1/2
by 2-m. shaft at a point along the line of the infiltration gallery,
about 130 m. from Shaft No. 1, but water in such abundance was
encountered that it was practically impossible to sink it in the
ordinary way more than about 6 m. deep, the quantity of water to be
dealt with amounting to about 20,000,000 liters daily. Seven shafts were
then sunk in the high ground to the north of the river, five of these
being on the line of the gallery and two 30 m. westward. They were sunk
during the dry season prior to July, 1909. These were ordinary timbered
shafts, 2 m. square between the walings, and were carried to the depths
shown on Plate XI. In Shafts Nos. 5, 6, and 7 the water was flowing with
considerable velocity, while Shaft No. 9 seemed to have penetrated a
different water plane and one which was probably independent of that
showing in any of the other shafts, in which the water was practically
at a uniform level. The evidence obtained showed that if the gallery
could be carried to Shafts Nos. 6 or 7 a great abundance of water would
be intercepted. Owing to the difficulties of sinking ordinary shafts in
the wide river channel, circular shafts were put down. These were 1.37
m. in internal diameter and 23 cm. thick, and were of concrete
reinforced with No. 10 vertical rods, 19 mm. in diameter, tied together
with No. 6 wire. These shafts were provided with steel cutting edges.
Shaft No. 2 was sunk to a depth of 1 m. below the infiltration gallery
level, No. 3 within 2 m., and No. 4 within 4 m., before August, 1909.
The shafts were sunk by digging them out and loading them at the top,
the top of the shafts being kept generally 3 m. out of the ground. Shaft
No. 3 encountered great volumes of water, and, in order to enable
sinking operations to proceed, a pumping shaft, 2-1/4 m. square, was
sunk a little west of it to draw off the water. Notwithstanding the fact
that the prolonged period of drought had lowered the general water plane
in all the shafts to 565.00 m. above datum, the difficulties of handling
the water even at that level were considerable. At the beginning of
August the work was progressing very satisfactorily, but the
extraordinary rainfall of that month caused the work to be shut down
temporarily.
_Effect of the Floods in the Santa Catarina River._--The area of the
water-shed of the Santa Catarina River above Monterrey is about 1,410
sq. km. (544 sq. miles), and its area at San Geronimo, owing to its
configuration, is practically the same. Its general character has
already been referred to. On the night of August 10th and early on the
morning of August 11th, a big flood came down the river, flowing at its
maximum about 1,130 cu. m. (40,000 cu. ft.) per sec., due to the heavy
rainfall (Fig. 4). This flood carried away all the temporary staging
around the shafts, seriously wrecking the temporary pumping station, as
well as destroying the 30-cm. cast-iron pipe, notwithstanding the fact
that it had been encased in a block of concrete 3 m. wide and 1-1/2 m.
thick right across the river; but no damage was done to the infiltration
gallery or to the shafts in the river channel. The effect of the flood
on this pipe is shown by Fig. 2, Plate XXXI.
[Illustration: PLATE XXXI, FIG. 2.--PROFILE SKETCH, LOOKING UP STREAM ON
LINE OF 24-INCH MAIN SUPPLY PIPE.]
Following this flood, which had caused the loss of 14 lives in the city,
3 miles below San Geronimo, there was practically no rain for 13 days.
Then, on August 25th the second heavy precipitation began and continued
until August 29th, the details being shown on Fig. 4.
This precipitation, therefore, fell on a water-shed which was completely
saturated, as it had already absorbed a large proportion of the 13.38
in. of rain which fell during August 10th and 11th; and at every point
along the river, prior to August 25th, springs were issuing forth, and
there had been very little evaporation during the intervening dry spell.
The writer has calculated that at Monterrey this flood reached the
enormous quantity of 6,650 cu. m. (235,000 cu. ft.) per sec., a rate
equal to 432 cu. ft. per sec. per sq. mile of water-shed.[6] The effect
of this flood was to demolish completely about 1,200 "sillar" houses
(without taking into consideration the numerous wooden houses) at
Monterrey, and to cause a fearful loss of life, variously estimated
between 3,000 and 5,000 persons; the lower figure the writer believes is
approximately correct. At San Geronimo the original pumping station was
carried away entirely, leaving practically no trace whatever.
[6] The writer, in a brief article contributed to _Engineering News_
soon after the flood (September 23d, 1909), gave this figure as 271,500,
or approximately equal to a run-off of 500 cu. ft. per sec. per sq.
mile; but, from a later and more complete study of the conditions for
many miles above Monterrey, he believes the above quantity to be
approximately correct.
Shaft No. 2 was apparently destroyed, while No. 3 was turned at an angle
of about 50 deg. down stream and filled up completely with sand. The
infiltration gallery, near Shaft No. 2, was completely blocked with fine
sand and gravel, and access could only be obtained as far as 54 m. The
profile, Plate XI, shows the change which had taken place in the river
bed. The original course of the stream was changed to the north bank, 50
m. distant, the effect of the scouring action of the flood being to
lower the general level at this point about 3.65 m., while the southern
portion of the channel was slightly raised. At present (April, 1910),
the end of the driven portion of the infiltration gallery is about 35 m.
from the center of the stream, which is still carrying about 2,270
liters (80 cu. ft.) per sec.
Immediately after the flood the flow in the gallery was 450 liters (16
cu. ft.) per sec., and this quantity has remained constant since that
time. The probable effect of the flood was to disturb the whole
subsurface above the infiltration gallery and put it in motion,
completely cleaning the gravels of their surrounding clay, which would
account for the large infiltration of water in so limited a distance.
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.
Space 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 pass a 25-mm. mesh. Where the conduit crossed the
river obliquely, immediately below the gravitation tunnel, it was
strengthened with mass 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.
[Illustration: 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.