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, inTable 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° 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 slopewas 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.
Fig. 21.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 usingrough 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.
Fig. 2.Plate XXVIII, Fig. 2.—View of Santa Catarina River in Flood, on August 28th, 1909.
Fig. 1.Plate XXXI, Fig. 1.—Flush-Tank Carried Down by Flood of August 27th-28th, 1909.
Fig. 2.Plate XXXI, Fig. 2.—View Showing Scouring Effect Of Flood On San Geronimo Aqueduct.
Fig. 1.Plate XXXII, Fig. 1.—View of Santa Catarina River After the Flood.
Fig. 2.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° 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 to3⁄16in. 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 farapart, 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 inFig. 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.
Fig. 1.Plate XXXIII, Fig. 1.—Estanzuela Dam: Broken Concrete Basin Lining.
Fig. 2.Plate XXXIII, Fig. 2.—Estanzuela Dam: Broken Concrete Basin Lining, East Side.
Fig. 3.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, notwithstandingthe 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 consumptionto 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 371⁄2liters (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 121⁄2m. (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 forecastedaccurately; 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 than1⁄2the 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 the8-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 thegrade, 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 beneither 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 13shows 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:
Advantages of Concrete Pipe.
(a) Cement concrete pipe is usually less costly than vitrified pipe.
(b) It can be formed in any shape desired.
(c) It is not cracked by vibration, and resists impact better than vitrified pipe, for which reason it is a better material to lay near the surface of a street in which there is heavy traffic.
(d) It is not affected by ordinary sewage.
(e) The cost of repairing and maintaining is about the same as for a vitrified pipe sewer.
(f) It can be made in the city or town where it is to be installed, thus giving the locality the advantage of having some of the money paid for labor in its manufacture spent in the place where the sewers are being put in, where it is raised as a tax, etc.; also saving freight charges, etc.
(g) It can be made under the most careful local supervision and inspection, of selected material, by the engineer who is responsible for the success of the work. Vitrified pipe can seldom be made in this way.
Disadvantages of Concrete Pipe.
(a) If not carefully made and of the best of materials, it is subject to failure by disintegration, etc.
(b) It will not stand strong chemical action, and therefore the smaller sizes should not be used where they are likely to be exposed to trade wastes containing strong acids. In the larger sizes the quantity of flow mixes so quickly with the trade wastes that this danger is minimized, and it is very seldom that even the smaller sizes become affected; but vitrified pipe may be used in places where chemical action is anticipated.
(c) If not properly made, it will be attacked by steam and hot vapor, and by animal fats in the sewage; but, if properly made, it is not affected by these.
(d) Unless reinforced or made very thick, it will not stand as great a crushing load as the best vitrified stoneware pipe; but, as sewers are not intended to be used under very heavy pressure, this is not so very important. It is amply strong to withstand any internal pressure or any external crushing load to which it probably will be submitted.
(e) Under a considerable head of ground-water, it may permit water to infiltrate through its walls for a considerable time after it is laid, thereby temporarily increasing the flow, which, if the sewage is to be pumped, will increase the cost of pumping. This difficulty can be met by using a carefully selected mix of materials in making the pipe, and by makingthe joints carefully. Infiltration through concrete diminishes rapidly after the sewer is in use; it occurs in vitrified pipe, also, to some extent.
The house connection drain adopted in Monterrey, with the disconnecting trap, is very much like one which the writer has seen introduced with very bad result. These are being removed as rapidly as possible by one of his clients, a sewerage company, in the Southern States. It has been a fruitful cause of stoppages and bad smells; the ordinary method now in general use is much better. In the design shown, it would seem that there may even be some danger that the ventilation of the sewer by the stand-pipes in the streets may force the traps.
One is rather surprised to learn that the main outfall sewer is designed with a capacity of 90,000,000 liters per day, the present sewage being estimated as not more than 18,000,000 liters, and the far future being thought to require only 40,000,000 liters. Why this excessive size? Possibly the surplus water which it is to carry is to be discharged into the sewers from the water supply system direct, and is intended for irrigating the land at the disposal area, when the sewage is insufficient for this purpose. The author states that all surface water is strictly excluded.
The method of sewage disposal gives rise to several questions. It is proposed to use an extensive area for growing crops, which are to be irrigated with sewage. The paper states that the underlying strata at Monterrey contain numerous caverns, and the first question is: What will be the effect on the water supply of other towns lower down the valley? The writer recollects a serious outbreak of typhoid fever in Bluefield, W. Va., caused by the pollution of the water in similar strata finding its way through unknown underground caverns and channels to the city's water supply.
The next question is: What crops will be grown? It is a well-known fact that vegetables grown by the use of sewage as a fertilizer, are unsafe in a raw state for human consumption. This is well-known to European travelers in China and Japan, where the use of fecal matter as fertilizer renders the various water supplies (where not filtered and disinfected) and all green vegetables, unsafe, on account of typhoid germs. Moreover, crops not intended for human consumption, which are grown on lands irrigated by sewage bearing typhoid germs, etc., are unsafe for men to handle; even to store them may cause a dissemination of disease. It is evident, therefore, that the whole sewage flow should be in some manner disinfected at least, if not filtered, before it is used.
The method of sewage disposal and the use of merely settled septic sewage for irrigation seem to be open to objection. The disposal plant is not sufficiently effective to meet the present requirements of sanitaryscience; and the sludge-pit will be certain to breed a pest of flies, if it is not also an intolerable nuisance on account of foul smells. Monterrey would seem to be a proper place for the introduction of the Imhoff tank, with percolating filters, and a final settling tank, the effluent being disinfected, before entering the latter tank. The flow might then be used safely for irrigation purposes for crops not to be eaten uncooked by man. The writer does not see how the method provided can possibly fulfill the object stated, to distribute on the land an effluent which will be "innocuous and clear," or how any consequential degree of purification can be obtained in the tanks provided.
While there are described in this paper many things to find fault with, there are also many things to commend. The water supply system, with its reservoirs, etc., seems to be admirable; and the methods of construction by which the expense for forms was reduced is very interesting. The parking and ornamentation of the grounds over the reservoir roofs cannot fail to benefit the people and popularize the work.