JAMES D. SCHUYLER, M. AM. SOC. C. E. (by letter).--For completeness of
detail and wide range of subjects of general interest to engineers, this
paper is certainly one of the notable contributions to recent engineering
literature. It is a minute and painstaking record of the successful
accomplishment of construction work under unusual climatic conditions and
difficult circumstances, and reflects credit on the author, not only in
his capacity as an engineer, but as a faithful recorder of facts. It was
particularly fortunate that he was an eyewitness of the disastrous and
extraordinary flood which swept through Monterrey, destroying many lives
and much property, and has thus been able to give an intelligent estimate
of the maximum discharge of the river during the height of the flood wave
of August 27th-28th, 1909, when the rate of run-off per unit of area of
water-shed drained reached an amount which has seldom been equalled or
exceeded, as far as reliable records extend. It is worthy of note that
works deriving their water supply from the source of such torrential
floods should have survived with so little actual damage, and with
scarcely any interruption of service. The repair of all damages to the
system was estimated to have cost not more than $20,000.
As Mr. Conway did not assume charge of construction until May, 1907, he
was spared the responsibility of deciding on the general plan of securing
an abundant supply of pure water from sources permitting of delivery by
gravity under adequate pressure for fire protection--a responsibility
which devolved on the writer, assisted by G. S. Binckley, M. Am. Soc. C.
E., Mr. Conway's predecessor, as Chief Engineer. Not only the water-works,
but the system of sewerage and sewage disposal by broad irrigation were
subsequently carried out on the plans submitted to the State Government by
the writer in 1906, and given provisional acquiescence at that time.
There was no lack of water at hand for the supply of a city of that size,
as there are large perennial springs which flow out of the travertine of
the plain, and are used for irrigation in the valley below the city. One
of the largest of these, near the civic center, has a normal flow of
nearly 30 cu. ft. per sec.; another nearby, also within the city limits,
flows some 10 or 12 sec-ft., while both the Estanscia and Robalar springs,
but a few miles below (shown on Plate II), discharge more than 20 sec-ft.,
as nearly as memory serves. Besides this supply, the water to be developed
by sinking shafts in certain parts of the plain, as demonstrated at the
brewery and elsewhere, was apparently a reliable source of large volume.
To utilize these sources, however, would have involved condemnation of the
water-rights in the case of the springs, depriving present owners of the
use of the water, and this Governor Reyes wished to avoid. Besides, it
would have necessitated pumping the water for the city in perpetuity, an
expense which the Governor was equally anxious to save; hence a gravity
supply was made the prime requisite of the plans.
Until the concession was granted, and for a year or more afterward, it was
assumed that an adequate supply could only be obtained by the storage of
the flood-water of the Santa Catarina River in a large reservoir; and the
earlier plans of the concessionaires were based on the construction of a
high masonry storage dam at the upper end of the "narrows," where the
river turns from a western direction to a course almost due east, between
high vertical cliffs of limestone. The concession distinctly provided for
such a dam, and among the plans on file in the State Capitol is one
prepared by the late E. Sherman Gould, M. Am. Soc. C. E., for a masonry
weir across the gorge. Samuel M. Gray, M. Am. Soc. C. E., also filed a
plan and report proposing a capacious, shallow, storage reservoir near the
city, to be filled by a large flood-water canal from the Santa Catarina
Canon.
Although the writer could not have anticipated the occurrence of floods of
the magnitude of the one of August, 1909, which would surely have
destroyed any reservoir built in the Canon, he was unable to endorse the
storage plan of water development, chiefly because of the uncertainty of
the water-tightness of the reservoir in a cavernous limestone formation,
and also because of the probable impurity of water draining from such
extensive goat pastures. He, therefore, urged the development of the
underflow of the river, which was manifesting itself in the springs
referred to. Mr. Binckley secured two Keystone drilling machines and
proceeded to profile the bed-rock at Santa Catarina Canon and at San
Geronimo, the two places on the stream where the river flows between walls
of rock _in situ_. At both sites the strata were standing nearly vertical
across the channel, and, by careful sampling and testing, it was found
that in both locations there were thick strata of limestone so highly
silicious as to be insoluble, and hence free from caverns. From this
determination it was concluded that all the water which appeared in the
valley below must pass through the sections where the borings were made.
The results of this drilling, however, proved conclusively that the depth
to bed-rock at either place was too great to permit of a masonry dam being
considered as practical, and demonstrated the inadequacy of methods which
had been used in the earlier investigations when dams were regarded as
feasible.
The results have also shown that the subterranean supply at the lower
cross-section of the river, at San Geronimo, is abundant, and can probably
be increased to an indefinite degree by continuing the filtration gallery;
while at Santa Catarina the same type of development can be made for a
high-source supply, although requiring a long and expensive tunnel and
conduit.
DAVID T. PITKETHLY, ASSOC. M. AM. SOC. C. E. (by letter).--Having been
engaged on the design of sewerage systems for some years, the writer finds
this paper of peculiar interest, particularly the sewerage portion. There
are some points in the design, however, which do not appear to be clear.
The system is described as "strictly separate," and yet the sewers are
designed to run half-full, providing a capacity of 200%, the 100% basis,
or 380 liters per capita, being 90%, or 180 liters, in excess of the
calculated water supply of 200 liters per capita.
It has been the writer's practice to design sanitary sewer systems on the
basis of the water consumption, and to assume the whole daily amount to
reach the sewer in 16 hours, thus providing capacity sufficient to care
for the maximum or wash-day flow without causing the sewers to run above
the calculated hydraulic gradient, which should be placed within the pipe
so as to provide air space for ventilation under all circumstances.
The practice of calculating sanitary sewers to run half-full is a good one
when ground-water is expected in sufficient amount to fill the remaining
portion of the sewer, but when no ground-water, or roof-, or surface-water
is allowed to enter the system, or all precautions are taken to exclude
such, then the system may be designed so that the expected maximum, or
wash-day flow, will fill the sewer to the desired hydraulic gradient.
The method of ventilating the sewers does not seem practicable. The houses
are principally of one story, and yet the stand-pipes on the sewers have
openings 25 ft. 9 in. above the sidewalk. Are the ventilating or vent
pipes of the house plumbing carried to a height to balance this, or will
these chimneys draw the air from the house drains and fresh-air pipes,
breaking the seal in the so-called disconnecting traps, thus causing the
circulation of air in the house piping to be downward through the sewers
instead of upward through the fresh-air inlets and vents, as designed?
It is interesting to note that crude sewage, as well as the liquefying
(septic) tank effluent, is to be applied to land for irrigation purposes,
but the application of crude sewage without any attempt at removing the
suspended matter, or the effluent from the septic tanks where only a
partial removal occurs, seems to be bad practice.
The author states that:
"The degree of purification in the tanks was relatively unimportant; the
object to be obtained consisted chiefly in distributing on the land an
effluent which would be innocuous and clear."
How he expects to obtain such an effluent by passage through screens,
detritus tanks, and septic tanks only, is more than the writer can
understand.
The removal of suspended matter in a septic tank depends on the strength
of the sewage, the time of retention, the time elapsing between cleaning,
the presence of trade wastes, etc., and seldom exceeds 38 per cent.
The subject of septic tanks and their effect on sewage is discussed in the
"Fifth Report of the Royal Commission on Sewage Disposal" (England, 1908),
and the following extracts, relative to the application of crude sewage to
land and the effect of septic tanks on sewage, seem apropos:
"23. * * * There are also many cases in which crude sewage has
been passed over land, but the evidence shows that land treatment
of crude sewage is liable to give rise to nuisance by the
accumulation of solids on the surface of the land. Moreover, in
some cases these solids are apt to form an impervious layer,
which interferes with the aeration of the soil, and so impairs
the efficiency of the treatment."
"31. * * * At that time it was claimed that the septic tank
possessed the following, among other, advantages:
"That it solved the sludge difficulty, inasmuch as practically all
the organic solid matter was digested in the tank.
"That it destroyed any pathogenic organisms which there might be
in the sewage."
"32. As regards the first of these claims, it is now clearly
established that, in practice, all the organic solids are not
digested by septic tanks, and that the actual amount of digestion
varies to some extent with the character of the sewage, the size
of the tanks relative to the volume treated, and the frequency of
cleansing."
"At Huddersfield, Mr. Campbell estimated that about 38 per cent.
of the solids were converted into gas or digested; * * * while at
Birmingham, Messrs. Watson and O'Shaughnessy say that the figures
available indicated a digestion of not more than 10 per cent. of
the suspended matter entering the tanks."
"33. As regards the second claim, we find as a result of a very
large number of observations that the sewage issuing from the
septic tanks is, bacteriologically, almost as impure as the sewage
entering the tanks."
Messrs. Winslow and Phelps, in their interesting paper, "Investigations on
the Purification of Boston Sewage,"[8] quote a suggestion made by
Stoddart (1905):
[8] Water Supply and Irrigation Paper No. 185, p. 125.
"He finds, in a septic tank of several compartments, a
considerable deposit of sludge in the first compartment, giving
a fairly clear supernatant liquid, which in the last chamber of
all undergoes a secondary decomposition, leading to the
throwing down of an additional precipitate of offensive
sludge."
What took place in the case referred to by Stoddart corresponds to the
author's observations of the liquid leaving the tanks in a clarified
condition, but the secondary decomposition must take place in some manner,
and, when it does, a nuisance seems to be unavoidable where no provision
is made to care for it.
In view of the experience of others, some further treatment seems to be
necessary. Such treatment should include disinfection, as no method of
disposal yet devised has succeeded in reducing materially the pathogenic
germs usually to be found in sewage and tank effluents.
If the crops to be irrigated are to be eaten, uncooked, by mankind, then
disinfection at least is imperative.
GEORGE S. BINCKLEY, M. AM. SOC. C. E. (by letter).--Mr. Conway's admirable
paper is of special interest to the writer, as the entire general design
of the system, as well as the extensive hydrological studies and final
selection of the sources of water supply, was completed during 1906
through the joint labors of the writer, as Chief Engineer, and James D.
Schuyler, M. Am. Soc. C. E., as Consulting Engineer.
In this work, Mr. Schuyler and the writer had the rare privilege of
dealing from its inception with the problem of designing a complete and
somewhat extensive system of municipal water supply and drainage,
unhampered by any existing works to which the new systems would have to be
adapted. It would probably be difficult to find in the United States a
city of 85,000 inhabitants, previously totally lacking either a water
supply or sewerage system, which, under a consistent and harmonious
design, has been provided with both in the degree of completeness and
structural excellence exemplified in the works at Monterrey.
The few important changes or amplifications made in the original design,
and the manner in which its detail has been executed is naturally most
interesting to the writer, and this excellent paper should be of very
substantial value, particularly to engineers engaged on similar work in
Mexico or Spanish America.
The very novel construction method adopted by Mr. Conway in the roofing of
the South or Guadalupe Reservoir, seems to the writer rather to invite
criticism, and the fact that in the subsequent construction of the roof
over the rectangular Obispado Reservoir the customary monolithic concrete
construction was apparently reverted to after experience with the
separate-unit plan previously used, would indicate that Mr. Conway reached
the same conclusion.
The original design of the circular Guadalupe Reservoir contemplated just
about the same arrangement of columns and roof support as that actually
used, but the writer had expected that the columns would be cast in place,
and that the system of primary and secondary beams would be filled at the
same time as, and integral with, the roof slab, the reinforcement being
placed in accordance with what may be described as conventional practice.
The writer believes that the efficiency of the concrete and steel placed
in this manner would be notably higher than under the system actually
adopted, which, in effect, is pretty much the same as constructing the
supporting system of units of cut stone. If, with all the elements of
structural weakness involved in the multiplicity of mortised joints,
discontinuous reinforcement, etc., this construction is strong enough, it
would seem that an important reduction in the dimensions of the members
could have been effected by monolithic construction and continuous
reinforcement, without sacrifice of strength.
The comparison, in Table 7, of the costs of these two reservoirs, is
interesting, but very moderately illuminating, as the comparative unit
cost of the most important element in their construction--the concrete--is
not given. The total excavation cost for each reservoir is practically the
same, and the general expense, engineering, and cost of fittings and
accessories presumably so, but the total cost of the Guadalupe Reservoir
as given is $19,000 (pesos) in excess of that of the Obispado Reservoir,
while, in the latter, there were 756 cu. m. more concrete. This certainly
indicates a much higher cost of concrete per unit as laid in the South
(Guadalupe) Reservoir. An actual comparison of the cost per unit of
concrete laid under the two systems would be instructive.
The writer is interested to observe that the same system of sub-drainage
used by him in the construction of the reservoir for the provisional
supply of water from San Geronimo, has been used by the author in the
Obispado Reservoir. This arrangement of drains under the floor of the
reservoir at San Geronimo was devised as a safeguard against damage to the
lining through the accumulation of water inside the impervious bank
against its back.
It was realized that, in such a climate as that of Monterrey, perfect
water-tightness of the lining might be difficult to secure or maintain,
and, if leaks existed, a sudden draft on the contents of the reservoir
might result in serious damage through the static pressure exerted against
the lining of the sides or upward thrust against the floor. In the
writer's opinion, such a system of drains is an important element, as not
alone the fact but the quantity of leakage may be determined, and danger
of saturation of the supporting bank avoided--a matter of importance
where, as is sometimes the case, the material of such a bank is unfit to
resist the effects of saturation. The author does not state whether or not
this safeguard was omitted in the Guadalupe Reservoir. Incidentally,
however, the matter of saturation of the bank is not important in either
reservoir, as the material of which these banks are constructed is such
that settlement or failure through saturation is out of the question. It
may be remarked, however, that in fixing the angle of the sides of the
Guadalupe Reservoir at 60 deg. the writer contemplated the same system of
constructing the bank as he used in that of the San Geronimo Reservoir. In
this case, the bank was built up by spreading the material in thin layers,
wetting down, and rolling and puddling by the passage of the ox-carts used
for the transportation of the material, the wheels of the carts, and
especially the cloven hoofs of the animals, producing a most excellent
effect. The inside slope was built up in this fashion to a much lower
angle, and with a top width considerably in excess of the finished
dimensions. The excess material was then picked off to the line, and
exactly to the slope. Thus the finished slope presented a surface which
was compacted to a degree impossible to attain at or near the surface of
the bank as built, and presenting a support of the best possible character
for the concrete lining and coping.
V. SAUCEDO, ASSOC. M. AM. SOC. C. E. (by letter).--The author's
description of the water-works and sewerage of Monterrey, one of the most
extensive schemes in Mexico, will be of general interest to engineers,
especially those engaged in hydraulic and sanitary problems. The writer,
having been connected with the works for four years, knows the local
conditions well, and presents herewith some complementary data on what he
considers an important feature, the subject of floods, mentioned by the
author on different occasions, especially as certain developments in the
works show the importance of such occurrences as a factor in designing.
Abnormal rainfalls of long duration and high intensity are common in the
semi-arid region of Mexico. They come at irregular intervals, though
tending to coincide with the early fall. The floods of August, 1909, were
a repetition of similar occurrences in the past; and, though there are no
numerical records of previous cases, local traditions and historical state
documents describe them as having occurred since the foundation of the
city, at intervals of from 15 to 40 years. The graphic descriptions of the
places flooded are in accord with the character of the floods of August,
1909, and September, 1910.
The diagram, Fig. 21, is a record of the rainfall during the latter flood,
and was plotted from intermittent readings of standard gauges. It
demonstrates that the intensity increased toward the mountains on the
south, which form the tributary water-shed of the Santa Catarina River,
showing a difference of 10.54 in. between the city and the Estanzuela Dam,
which is not quite 12 miles to the southeast.
[Illustration: FIG. 21.--RAINFALL DURING FLOODS OF SEPTEMBER 14TH-16TH,
1910, IN MONTERREY.]
An estimate of the volume of discharge of the river at the time of maximum
flood is only a reasonable conjecture which (without special reference to
accuracy) aims to impress those who have not witnessed such occurrences
with the tremendous volume coming from barren steep surfaces previously
saturated.
The original computation, referred to by the author, was obtained from the
average of two different methods which gave results close to each other.
In one method the extent and nature of the water-shed were considered,
together with the maximum period of precipitation that occurred,
sufficient to gather a maximum volume of water in the river. In the other
method the volume was derived from a cross-section of the wetted perimeter
of the river at the time of maximum flow, in combination with velocity
approximations obtained by using rough floats. This gave 271,500 cu. ft.
per sec. The figure submitted by the author, 235,000 cu, ft. per sec., is
in accord with the proposed formula[9] for impervious surfaces by C. E.
Gregory, M. Am. Soc. C. E. In the first and last methods, the intensity, a
governing factor, is more or less of an assumption, and the
cross-sectional method is also unreliable, as the river-bed was greatly
disturbed, due to the high velocity of the water, which deepens the
channel to a considerable extent at times of maximum flood, the gravels
being redeposited during the period of subsidence. Such was the case
during the flood of September, 1910, when the depth of gravel above the
roof of the San Geronimo Infiltration Gallery was diminished to such an
extent that it was so inefficient as a filter for the flood as to permit
the percolation of turbid water into the underground supply.
[9] _Transactions_. Am. Soc. C. E., Vol. LVIII. p. 458.
During the floods of August, 1909, Shafts Nos. 2 and 3 were damaged beyond
repair, and sand and gravel, entering through them, blocked up the
gallery to within about 150 ft. of Shaft No. 1. The interior timbering
probably collapsed, due to cavings and disturbance in the river-bed during
the period of maximum flood, but no explorations have been possible on
account of the great quantity of water still coming through (at present
more than 650 liters per sec.). For this reason the work of driving the
gallery, as well as lining Shaft No. 1, has been suspended.
[Illustration: PLATE XXVIII, FIG. 2.--VIEW OF SANTA CATARINA RIVER IN
FLOOD, ON AUGUST 28TH, 1909.]
[Illustration: PLATE XXXI, FIG. 1.--FLUSH-TANK CARRIED DOWN BY FLOOD OF
AUGUST 27TH-28TH, 1909.]
[Illustration: PLATE XXXI, FIG. 2.--VIEW SHOWING SCOURING EFFECT OF FLOOD
ON SAN GERONIMO AQUEDUCT.]
[Illustration: PLATE XXXII, FIG. 1.--VIEW OF SANTA CATARINA RIVER AFTER
THE FLOOD.]
[Illustration: PLATE XXXII, FIG. 2.--VIEW OF SANTA CATARINA RIVER FLOWING
THROUGH LOW-LYING STREETS, 8 DAYS AFTER THE FLOOD.]
On reaching the city, the flood of August, 1909, swept away two streets
adjoining the river. These streets had been built on made ground, in what
was originally the river-bed. The sewers and water mains laid in them were
destroyed entirely, and some 460 ft. of the 24-in. cast-iron pipe, buried
under the river-bed at a depth of 8 ft., were carried away. In relaying
this portion of the main, and for protecting the remainder of it across
the river, it is now proposed to encase it in a solid rubble concrete
block, 8 ft. square, which will impart weight and stability against the
scouring effect of floods.
The South Reservoir is circular in shape, with an interior diameter of
165.68 ft. at the top, and is partly excavated in the ground and partly
completed by an embankment of vast proportions (Fig. 10). Right after the
flood of August, 1909, a wet spot appeared on the northeastern toe of the
embankment, and it was supposed for some time that it was the effect of
the saturation produced by the preceding rains, but, as it persisted for
several months, it was obvious that its origin was in the interior of the
reservoir, which was emptied when the writer took charge of the work. The
first inspection revealed a horizontal crack in the concrete lining, about
310 ft. long and extending about 153 deg. around the circumference on the
north side. Throughout its length it coincided with the line of cut and
fill. Vertical cracks, coinciding with the panel points in the lining, had
also developed, and extended from the main horizontal crack to the roof.
The circumstances originating this development can be conjectured by
considering the position of the main crack, its characteristic features,
and the conditions that preceded its formation. The coincidence of the
crack with the joint of cut and fill, points to this line as a source of
danger. An examination showed, besides, that the fracture was clean and
sharp, ranging in thickness from a hair line at the ends to 3/16 in. at
the center, and that its upper border projected over the lower one
perceptibly, a proof that horizontal motion had taken place. The vertical
cracks were a secondary effect, the consequence of the displacement
immediately after it was scoured. A fracture was discovered in the floor
of the reservoir. It started at the center and branched out into two
diverging lines in a radial direction.
The circumstance of two abnormal rainfalls, giving 35 in. in 9 days, the
precipitation being concentrated in two periods, not far apart, of 42
hours and 98 hours, respectively (Fig. 4), together with lack of provision
for shedding the water from the roof of the reservoir and from the
surrounding embankment, lead to the inference that the latter became
saturated, increasing thereby in weight and decreasing in stability,
especially in its steep inner face. A settlement and the consequent
horizontal displacement, under these conditions, was natural. The concrete
lining, only 16 in. thick at that height, was not sufficient to sustain
the resulting strain, and the main fracture developed, permitting the
stored-up water to leak into the bank. In time this seepage found its way
under the bottom of the reservoir, softening the ground and producing a
slight settlement which caused the crack in the floor. Had under-drainage
been provided, as at the Obispado Reservoir, the actual conditions would
have been noticed earlier. However, as the embankment is of vast
proportions, stable in itself to sustain with a large margin of safety the
weight of the stored-up water, there was no actual danger of failure,
except for the fact that the material forming the structure, on account of
its calcareous nature, is dissolved by water. Long exposure to this
condition would, in time, open passages in the embankment, and it is
certain that there would be cavings in its interior.
The necessary grouting has been done, and provision is being made for
water-proofing the interior of the reservoir and shedding the water from
the roof and from the embankment, thus relieving the structure of the
consequent strain.
Another place in the works where floods have had a damaging effect is the
Estanzuela intake basin, which, when the dam was completed, was filled to
the overflow level in order to test its water-tightness. As this basin,
when cleaned, was found to be slightly fissured on the north side, it was
decided to line it with concrete. As shown in Fig. 8, the lining does not
cover its entire area, but only the central portion, leaving a strip on
either side without protection. The flood of September, 1910, coming in
greater volume than the previous ones of August, 1909, in passing through
the narrow gorge at the entrance, undermined the lining in those places
where it was not founded on solid rock. Figs. 1, 2, and 3, Plate XXXIII,
show some of the damage caused by this flood. The buoyant effect of the
water and the impact of large rolling boulders caused fractures all over
the surface, and lifted the concrete lining bodily; but the dam proper,
being founded on rock bottom, did not suffer any injury. In the future, in
order to avoid the seepage of the ordinary supply, alluded to by the
author, the water will be carried to the valve-house in an open rubble
concrete channel, lined with cement mortar and built high up against the
western hillside. The remainder of the basin will be paved with large
boulders.
[Illustration: PLATE XXXIII, FIG. 1.--ESTANZUELA DAM: BROKEN CONCRETE
BASIN LINING.]
[Illustration: PLATE XXXIII, FIG. 2.--ESTANZUELA DAM: BROKEN CONCRETE
BASIN LINING, EAST SIDE.]
[Illustration: PLATE XXXIII, FIG. 3.--ESTANZUELA DAM SEPT. 26, 1910: VIEW
OF SHEARING FRACTURES OF WALL AND LINING AFTER FLOOD SEPT. 14-17, 1910.]
In conclusion, the writer wishes to emphasize the point that,
notwithstanding the severity of the test, relatively small damage was
inflicted on the extensive works carried out under the author's design and
direction. A test so severe that it caused serious damage and immense
losses in the entire region, washing away kilometers of railroad track and
destroying practically all the bridges within reach of the flood, is an
occurrence of paramount importance, and should be remembered as a leading
factor in the design of engineering works.
GEORGE T. HAMMOND, M. AM. SOC. C. E. (by letter).--In a country, such as
that described in this paper, where water is valuable, and a shortage is
at times possible, where the majority of the population is very poor, and
water and sewage discharge are both to be paid for on a basis of volume,
the question of the expected quantity of daily water supply and sewage
flow per capita is of primary importance. This question, notwithstanding
its difficulty, should be given a first place in the studies for
water-works and sewerage projects, and should never be lost sight of in
the design, which should be such that, while proper for the expected
future flow for a reasonable time, should also be proper and economical
for conditions which at present obtain and may change but slowly.
It is desirable, of course, to get as much capacity in works as one can
for the outlay, but there are instances where one can get too much for the
money, as where a larger pipe than is necessary is used for a sewer,
merely because it costs about the same as a smaller one, and as a result
the cost of maintenance is permanently increased.
The water-works were designed to supply 40,000,000 liters (10,582,000
gal.) daily, which it was assumed would be sufficient for all future
developments in Monterrey for a population of 200,000 at a per capita
consumption of 200 liters (about 53 gal.) per day. The present population
of the city is given as less than 90,000, there having been an increase of
22,000 in ten years (1891-1901), but it is evident that in the last ten
years (1901-1911) this rate of increase has not continued. Taking into
account all the data known to the writer, it does not seem that the city
will attain a population of 200,000 in a great many years, if it ever
does; but this is a matter of personal opinion, and is only stated as
such.
The present requirements of the city's population, assuming that each
person uses 200 liters (53 gal.) per day, would be, at that rate, which is
a very liberal one, only 18,000,000 liters (4,762,000 gal.) per day, or
less than half the amount which may be provided.
If the water were not to be metered and the sewage discharge paid for by
measure, it is possible that the free use of water might lead to the usual
waste with which all are fairly familiar; but the use of meters, and the
rates charged, will reduce the water consumption to a minimum. This end
will especially result from Section 5 of the Tariffs which provides that:
"Groups can be formed of two or more small houses so as to obtain a joint
service under the proportion shown in the tariff."
This provision will keep down the per capita supply, among the majority of
the people, to about 37-1/2 liters (10 gal.) per day. A similar provision
led to abuse in Santiago de Cuba, as well as in other Cuban cities, where
one householder, taking water, frequently delivers it to adjoining houses
and tenements through rubber hose. As many as ten or twelve families are
sometimes found to be supplied from one tap in this manner. Indeed, it may
be stated as a rule, having but few exceptions, that where water is paid
for by meter its use is always restricted.
The water mains and distribution system, however, are so well laid out,
and the whole design is so good, that the writer would not anticipate much
difficulty because it is on rather too liberal lines for the present or
probable future. It may, perhaps, be argued that it may cost more to keep
the mains in such a system clean; but this extra cost will scarcely be of
much moment, and will be offset by the greater lasting quality of the
larger pipes. There is another feature of the problem, however, which is
not affected favorably by a too liberal forecast of the per capita water
supply, namely, the sewerage system.
If it is assumed that, using 200 liters per capita per day, the total
water supply of the city for the present population will be 18,000,000
liters, and that this may double in fifty years, or even amount to
40,000,000 liters in that time, it would seem that a rather liberal
provision has been made for the water supply, and that this will scarcely
be exceeded by the sewage, for the latter must come from the water supply,
there being little or no ground-water and no storm-water taken into the
sewers. Designing the sewers to flow half full for all diameters less than
18 in., and seven-tenths full for all larger sizes, it would seem that
this would give ample capacity for all time to come in such a city, and
that good practice would not exceed these figures, it being more desirable
that the sewers should not be too large to work well, than that they
should be large enough in all places to meet every possible contingency.
If all the sewers of a system are too large, the condition is incurably
bad; while, if a few miles prove to be too small, on account of growth and
prosperity not anticipated by the designer, it will be easy enough to
relay such parts when this becomes necessary.
Mr. Conway states that:
"The sewers are designed on a very liberal basis, namely, on the
assumption that when flowing half full the quantity to be dealt with will
be 380 liters [100 gal.] per capita per day, with a maximum rate of flow
of 200 per cent."
If the writer understands this statement correctly, it means that the
sewers, flowing half full, will carry 380 liters per capita in 12 hours,
or are designed with 200% of the capacity required to take the assumed
flow in 24 hours.
It was assumed that each house would be occupied by 7 persons and have a
frontage of 12-1/2 m. (about 41 ft.), that is, about 700 gal. per day per
house, the maximum flow rate being 200%, or at the rate of 700 gal. per
house in 12 hours.
It is to be remembered that nearly all the houses are of one story, and
that, as a rule in tropical and sub-tropical countries, the per capita use
of water diminishes with some function of the increasing number of
inhabitants in one house. Most of the water is used in the kitchen, and
where there are 7 persons instead of 5, the quantity used by the smaller
number will generally serve the larger.
The writer is unable to understand how this quantity of sewage will be
produced, especially as the author states that, as far as the company is
concerned, it is limited to the removal and disposal of the sewage, and is
not required to provide for storm-water. He also states that:
"Apart from that fact, however, the best system for a city like Monterrey,
where rainfall for many months at a time is very scarce, is the strictly
'separate system'."
The minimum velocities in the sewers, when running full, vary between 0.91
and 1.5 m. (from 3 to 5 ft.) per sec., and will be the same flowing half
full.
From the foregoing data it will be observed that:
(1) The water supply is the only source from which sewage flow
is anticipated;
(2) The water supply is very liberally estimated at 200 liters
(53 gal.) per capita daily;
(3) For purposes of sewer design, the daily flow of sewage
expected (all of which is derived from the water supply of 200
liters per capita) is estimated at 380 liters per capita, with a
maximum rate of flow of 200% (or at the rate of 760 liters per
capita), and with this quantity the sewers are designed to flow
only half full;
(4) The gradients are such that a velocity of from 3 to 5 ft.
(0.91 to 1.5 m.) per sec. will be secured in the sewers flowing
half full with the above quantity of flow per capita.
The writer does not agree with this method of computation, as he feels
sure that it will give sewers which are too large, with grades too steep
for the best obtainable results. His experience, extending over more than
twenty years in sewer design and hydraulic work, convinces him that the
method pursued is wrong in principle.
The principles involved in sewer design are first of all hydraulic. The
quantity of flow, in the nature of things, cannot be forecasted
accurately; success depends on getting the nearest possible approximation
to average conditions. If 200 liters per capita per day is a liberal
allowance, and 40,000,000 liters per day is a liberal expectation at this
rate for double the present population, and the sewers are designed to
flow half full only, why should this again be doubled?
The design of a sewer system for a city such as Monterrey is, in fact, a
very difficult problem, especially as the quantity of sewage will be very
limited, flush-water will have to be used in considerable quantities, and
water in that part of the world is precious at all times and often scarce.
Under these circumstances, the size or shape of the pipes selected for the
lateral sewers, should have been such as would more nearly agree with the
requirements than does the 8-in. circular.
A. P. Folwell, M. Am. Soc. C. E., writing of the 8-in. circular size,
states:[10]
[10] "Sewerage," by A. P. Folwell, M. Am, Soc. C. E.
"To secure a flow in this pipe having an average depth of 4
inches would require the sewage from a population of 6,500. In
general it may be said that the ordinary depth of flow in any
sewer should not be less than 2 inches, nor should it be less
than 1/2 the radius of the invert, since if it is so there is
much more danger of deposits forming along the edges and even in
the center of the stream. It will sometimes be impossible to
meet this requirement fully, but it should be kept in mind as
extremely desirable."
Sewers of small size should be proportioned throughout the system so that
the depth of the minimum daily flow in the invert, and the velocity of
flow, will be the best possible to prevent deposits. The transporting
power of water is dependent mainly on the depth of flow, a minimum
velocity being selected rather than a minimum depth of flow. To those who
have had charge of the maintenance of sewers, as well as of their design
and construction, this principle seems so obvious that it is always a
surprise to see it disregarded by designers, who in these days seem
inclined to consider sewerage as a system of grades and sizes of pipes
installed for ideal, rather than for actual, conditions. Messrs. Staley
and Pierson have well stated the principle involved as follows:
"A stream having a depth of flow sufficient to immerse solid matter held
in suspension, to a certain extent lifts it and carries it forward. The
entire surface is also exposed to the action of the current. A stream
having an equal velocity but a less depth in proportion to the diameter of
the solid matters to be transported, evidently has less transporting
power. * * * An amount of sewage which can be properly transported by a
circular sewer of a given size, cannot be as efficiently transported by
one of larger diameter."
From some strange idea, which is apparently without foundation in logic or
based on any actual justification from experience, it has of late years
become the practice of designing engineers to make the 8-in. circular
pipe the smallest size for sewers; and it is not improbable that the
designer of the Monterrey system has merely followed this example. It has
also become the frequent practice of designers to give every length of
sewer all the grade possible, regardless of the fact, taught both by
hydraulics and experience, that the best grade is that which will give as
much depth of flow as is consistent with a scouring velocity.
Some years ago it was the standard practice, in the "strictly separate
system" of sewers, to use the 6-in. pipe as the minimum size, and, as far
as the writer has been able to discover, after giving the matter a rather
extensive investigation, the 6-in. size has given excellent results
wherever its use was proper. In places where it has not succeeded there
were excellent reasons why it should not have been selected, and these
could easily have been observed at the time the designs were made. The
best sizes for the sewers in a given system is always a matter to be
determined by local conditions; but there seems to be no reason why the
6-in. size should not be used where the flow is so slight that the 8-in.
will not work well; or where the velocity must of necessity be so great
that a flotation depth of flow cannot be maintained in the larger size. As
to likelihood of clogging and stoppage, the writer's opinion, based on the
maintenance of three rather extensive systems in different parts of the
United States, in each of which the 6-in. size comprises more than 75% of
the whole length of pipe, and of three other systems, one having 12-in.
and two having 8-in. as the minimum sizes, is that the 6-in. size, where
properly used, is less likely to become clogged than either of the others
used improperly. The cost of maintaining the 6-in. pipe lateral, under
these circumstances, is much less than that of maintaining the 8-in.
lateral.
The 6-in. pipe is not being used now as much as the 8-in., and in most
cases this is probably because the capacity of the latter is nearly double
that of the 6-in., and costs only a few cents more per foot. If there is a
sufficient population per acre, or if, within 30 or 40 years, such a
population is anticipated as will fill the 8-in. pipe half full, its use,
of course, is justified and necessary; but where it is quite evident that
this will never occur, its use is counter-indicated.
In Monterrey, where the building lots have a frontage of 41 ft., where the
houses, as a rule, are only one story high, where the water service is
metered and paid for, and the sewage flow is also paid for, there seems to
be no reason to justify the use of 8-in. pipe for the upper reaches of the
smallest sewers. The sewage flow to be anticipated will probably never be
sufficient to keep an 8-in. pipe sewer in a good clean condition at the
upper ends of the lines of sewers without excessive flushing; and the
sharper or steeper the grade on which it is placed, the worse will be the
result, because the sharper the grade, the thinner the flowing thread of
sewage will be drawn out in the invert; on the other hand, if the grades
are flat, the slight quantity of sewage flow will be spread out in a
sluggish stream, without sufficient depth, on the bottom of the 8-in.
pipe.
Where a wide surface is given to a small quantity of flowing sewage, it
stagnates slowly along the bottom of the sewer, leaving frequent deposits
to undergo decomposition and create foul air, if not to choke the sewer, a
result often produced; and where a circular sewer which is too large for
the ordinary flow is given a strong velocity by using steep grades, the
stream, though flowing rapidly, is drawn out to such a thin thread that it
will not effect the flotation of the solid masses in the sewage brought in
at house connections, and the shallow and thin stream simply flows around
such masses until a dam or obstruction forms and the sewage is backed up
sufficiently to force the obstruction farther down, to form another
obstruction in a larger pipe below. Flushing may possibly keep such a
sewer fairly clean; but, as usually practiced, it is effective only for a
few hundred feet from the flush-tank; and the quantity of flush-water
required by an 8-in. pipe is more than twice as much as that required to
keep the 6-in. pipe clean. Ventilation is better in the smaller sewer than
in the larger, as there is less air to move; but the elaborate ventilating
stacks provided at Monterrey may take care of this; and it is evidently a
place where ventilation will be needed.
The ideal size and shape of cross-section for a sewer is such as will give
the best flotation to moving solids which are being carried along by the
flow; and this means the sewer that, with the expected ordinary or average
flow, will give the best depth in the invert, when the velocity of flow is
sufficient to keep suspended solids, grit, etc., moving at a rate of from
2 to 3 ft. per sec. The size, however, is limited by practical
considerations. The circular pipes cannot well be less than 6 in. in
diameter, because the house connections cannot well be less than 4-in.
pipe, and the sewer should be larger than the house connections, for
various practical reasons; but, in order to secure flotation and a
scouring flow, the smallest pipe, or the pipe having the smallest invert
radius, that practical considerations permit, should be selected. The
grade should be such, and the collecting system so laid out, that the flow
may be conserved as far as possible, and the sewage flow should be kept of
as great a depth in the invert, or bottom of the sewer, as safety in
self-cleansing velocity will permit. This will save flush-water and
prevent stoppages, and thus reduce the cost of maintenance to a minimum.
For good sanitary practice, the sewers should be designed, first of all,
to comply with the requirements of the present, or immediately expected,
ordinary flow, with some reasonable allowance for the future. They should
be neither too large nor too small, and the grade should neither be too
great nor too little, to secure the best flotation and scouring effects
and the best flush-wave action under all circumstances.
The use of cement concrete pipe for sewers seems to be growing in favor;
nor is this surprising, in view of the many improvements made in their
design and manufacture. The excellence of the concrete pipe used in
Monterrey and its success, suggest the query: Why was it not used still
more extensively?
Table 13 shows that the cement pipe cost much less than the vitrified
tile, or "fire-clay" pipe. Thus, the 38.1 cm. (15-in.) fire-clay cost 6.14
pesos per lin. m., the 45.7 cm. (18-in.) cost 8.80 pesos, and the 50.8 cm.
(20-in.) cost 11.30 pesos. Compared with this, the concrete pipe was much
the cheaper; the 55.9 cm. (22-in.) cost 5.93 pesos, which is less than the
cost of the 38.1 cm. (15-in.) fire-clay; and the 61.0 cm. (25-in.)
concrete pipe cost 7.30 pesos, which is less than the 45.7 cm. (18-in.)
fire-clay.
The writer's experience with concrete pipe, derived mainly from a long
service in sewer design and construction in Brooklyn, N. Y., leads him to
believe that at Monterrey the whole sewer system might, with advantage,
have been built of concrete pipe, using an egg-shaped pipe with an area
slightly larger than an 8-in. circle, designed for a discharge equal to an
8-in. pipe for all the smaller sewers. The invert of such an egg-shaped
pipe would fulfill the present requirements in carrying a very small flow
with good flotation depth, better than would a 6-in. circular pipe, and
the reserve capacity of the 8-in. pipe would be secured without
interfering with good present service. Egg-shaped pipes, similar to those
used in Brooklyn, the writer believes, would have given far better
satisfaction throughout the Monterrey sewerage system than circular
fire-clay pipe, and would have cost no more, but probably less. The
egg-shaped pipe referred to is made with a flat base and a self-centering
joint, thus insuring perfect alignment, and a smoother interior surface
than can be obtained with fire-clay pipes.
Brooklyn has about 450 miles of concrete pipe sewers, of all sizes less
than 24 in., the greater part of which is egg-shaped. There are also about
250 miles of vitrified stoneware circular pipe sewers of similar sizes,
and the cost of repairs and replacing pipe, over a period of years is
about the same per mile for each kind. Incidentally, it may be stated that
the annual cost of repairs per mile on both kinds of pipe is very small,
and is only about one-fifth of the cost of repairs per mile on the brick
sewers, of which there are about 200 miles.
The principal advantages and disadvantages of cement concrete pipe sewers
may be summed up as follows: