[Footnote: Paper, with slight abbreviation, read by Mr. David Gravell,
Assoc. M. Inst. C.E., before the Society of Civil and Mechanical
Engineers. The paper brings together in a convenient form the sections
and salient facts concerning many dams. It was illustrated by numerous
diagrams, from which our engravings have been prepared.--_The Engineer._]
By DAVID GRAVELL.
The construction of dams, in some form or other, may probably rank among
the very earliest of engineering works. Works of this character are not
infrequently referred to in the accounts of the earliest historians; but
it is to be feared that they are not always perfectly trustworthy. The
subscribers to the Mudie of the period had to be considered, and their
taste for the marvelous was probably not much inferior to that of our own
day. When, therefore, Herodotus describes the reservoir of Moeris as
formed for the control of the river floods of Nile-nourished Egypt, and
of another constructed by Nebuchadnezzar at Sippara, of 140 miles in
circumference, we must make allowances. But there is no question as to
the existence in the East at the present day, and especially in India and
Ceylon, of the remains of what may correctly be termed stupendous works;
and the date of the construction of which, as regards India, is in many
cases prehistoric. In Spain also the Moors, whose occupation of the
peninsula terminated in the thirteenth century, have left reservoir dams
of great magnitude, situated mostly in the south-eastern provinces of
Murcia and Alicante, and many of which are still serviceable.
In India and Ceylon the greater number of the ancient dams or bunds are
now in ruins, and this can occasion but little surprise, considering the
meteorological condition of these countries. In Ceylon, for instance, the
whole rainfall of the year occurs within a period of six to eight weeks,
and often amounts to as much as 12 in. in the twenty-four hours, and has
been known, comparatively recently, to reach nearly 19 in., the latter an
amount only 2 in. or 3 in. less than the average rainfall of Lincolnshire
for the whole year. In London it is only 25 in. and in the wettest
district in Great Britain, viz., Cumberland, averages not more than 70
in. per annum.
The rainfall in Bombay is from 80 in. to 100 in. per annum, and
throughout India may be taken as from 50 in. to 130 in., varying, as is
the general rule, in direct ratio with the altitude, and limited to a few
weeks in the year. Notwithstanding this, there still exist in the Madras
Presidency a not inconsiderable number of ancient bunds which serve their
intended purpose at the present day as well as ever. Slight mistakes did
occasionally occur, as they ever will till no more dams are wanted, as is
proved by the remains of some works in Ceylon, where the failure was
evidently due to error, possibly due to the instruments being out of
adjustment, as their base is at a higher level than the bed of the stream
at the point where water from the latter was to be diverted to afford the
supply.
Among the most remarkable of these ancient works is the Horra-Bera tank,
the bund of which is between three and four miles in length and from 50
to 70 ft. in height, and although now in ruins would formerly impound a
reservoir lake of from eight to ten miles long and three to four miles
broad. There is also the Kala-Weva tank, with a bund of twelve miles in
length, which would, if perfect, create a lake of forty miles in
circumference. Both of these ruined works are situated in Ceylon. The
third embankment of a similar character is that of the Cummum tank,
situated in the Madras Presidency, and which, though ranking among the
earliest works of Hindoo history, is still in such a condition as to
fulfill its original intention. The area of the reservoir is about
fifteen square miles, the dam about 102 ft. high, with a breadth at the
crest of 76 ft., and of the section shown in the diagram.
The by-wash is cut in the solid rock altogether clear of the dam; the
outlet culverts, however, are carried under the bank. We will now
consider generally the methods employed in determining the site,
dimensions, and methods of construction of reservoir dams adapted to the
varying circumstances and requirements of modern times, with a few
references to some of the more important works constructed or in
progress, which it will be endeavored to make as concise and burdened
with as few enumerations of dimensions as possible.
The amount of the supply of water required, and the purposes to which it
is to be applied, whether for household, manufacturing, or irrigation
uses, are among the first considerations affecting the choice of the site
of the reservoir, and is governed by the amount of rainfall available,
after deducting for evaporation and absorption, and the nature of the
surface soil and vegetation. The next important point is to determine the
position of the dam, having regard to the suitability of the ground for
affording a good foundation and the impoundment of the requisite body of
water with the least outlay on embankment works.
It has been suggested that the floods of the valley of the Thames might
be controlled by a system of storage reservoirs, and notice was
especially drawn to this in consequence of the heavy floods of the winter
of 1875. From evidence given before the Royal Commission on Water Supply,
previous to that date it was stated that a rainfall of 1 in. over the
Thames basin above Kingston would give, omitting evaporation and
absorption, a volume of 53,375,000,000 gallons. To prevent floods, a
rainfall of at least 3 in. would have to be provided against, which would
mean the construction of reservoirs of a storage capacity of say
160,000,000,000 gallons. Mr. Bailey Denton, in his evidence before that
commission, estimated that reservoirs to store less than one tenth that
quantity would cost £1,360,000, and therefore a 3 in. storage as above
would require an outlay of, say, £15,000,000 sterling; and it will be
seen that 3 in. is by no means too great a rainfall to allow for, as in
July of 1875, according to Mr. Symons, at Cirencester, 3.11 in. fell
within twenty-four hours. Supposing serious attention were to be given to
such a scheme, there would, without doubt, be very great difficulty in
finding suitable situations, from an engineering and land owner's point
of view, for the requisite dams and reservoir areas.
In Great Britain and many European countries rain gauges have been
established at a greater or less number of stations for many years past,
and data thereby afforded for estimating approximately the rainfall of
any given district or catchment basin. The term "watershed" is one which
it appears to me is frequently misapplied; as I understand it, watershed
is equivalent to what in America is termed the "divide," and means the
boundary of the catchment area or basin of any given stream, although I
believe it is frequently made use of as meaning the catchment area
itself. When saying that the rain gauges already established in most of
the older civilized countries afford data for an approximate estimate
only, it is meant that an increase in the number of points at which
observations are made is necessary, previous to the design of a reservoir
dam on the catchment area above, the waters of which are proposed to be
impounded, and should be continuous for a series of five or six years,
and these must be compared with the observations made with the old
established rain gauges of the adjacent district, say for a period of
twenty years previously, and modified accordingly. This is absolutely
necessary before an accurate estimate of the average and maximum and
minimum rainfall can be arrived at, as the rainfall of each square mile
of gathering ground may vary the amount being affected by the altitude
and the aspect as regards the rainy quarter.
But this information will be of but little service to the engineer
without an investigation of the loss due to evaporation and absorption,
varying with the season of the year and the more or less degree of
saturation of the soil; the amount of absorption depending upon the
character of the ground, dip of strata, etc., the hydrographic area
being, as a rule, by no means equal to the topographic area of a given
basin. From this cursory view of the preliminary investigations necessary
can be realized what difficulties must attend the design of dams for
reservoirs in newly settled or uncivilized countries, where there are no
data of this nature to go on, and where if maps exist they are probably
of the roughest description and uncontoured; so that before any project
can be even discussed seriously special surveys have to be made, the
results of which may only go to prove the unsuitability of the site under
consideration as regards area, etc. The loss due to evaporation,
according to Mr. Hawksley, in this country amounts to a mean of about 15
in.; this and the absorption must vary with the geological conditions,
and therefore to arrive at a satisfactory conclusion regarding the amount
of rainfall actually available for storage, careful gaugings have to be
made of the stream affected, and these should extend over a lengthened
period, and be compounded with the rainfall. A certain loss of water, in
times of excessive floods, must, in designing a dam, be ever expected,
and under favorable conditions may be estimated at 10 per cent. of the
total amount impounded.
As regards the choice of position for the dam of a reservoir, supposing
that it is intended to impound the water by throwing an obstruction
across a valley, it may be premised that to impound the largest quantity
of water with the minimum outlay, the most favorable conditions are
present where a more or less broad valley flanked by steep hills suddenly
narrows at its lower end, forming a gorge which can be obstructed by a
comparatively short dam. The accompanying condition is that the nature of
the soil, i.e., the character, strata, and lie of the rock, clay, etc.,
as the case may be, is favorable to assuring a good foundation. In Great
Britain, as a rule, dams for reservoirs have been constructed of
earthwork with a puddle core, deemed by the majority of English engineers
as more suitable for this purpose than masonry.
Earthwork, in some instances combined with masonry, was also a form usual
in the ancient works of the East, already referred to; but it would
appear from the experience of recent years that masonry dams are likely
to become as common as those of earthwork, especially in districts
favorable to the construction of the former, where the natural ground is
of a rocky character, and good stone easily obtained.
As to the stability of structures of masonry for this purpose, as
compared with earthwork, experience would seem to leave the question an
open one. Either method is liable to failure, and there certainly are as
many cases on record of the destruction of masonry dams as there are of
those constructed of earthwork, as instanced in Algeria within the past
few years. As regards masonry dams, the question of success does not seem
so much to depend upon their design, as far as the mere determination of
the suitable profile or cross section is concerned, as that has been very
exhaustively investigated, and fairly agreed upon, from a mathematical
point of view, but to be principally due to the correctness of the
estimate of the floods to be dealt with, and a sufficient provision of
by-wash allowed for the most extreme cases; and, lastly, perhaps the most
important of all, the securing a thoroughly good foundation, and a
careful execution of the work throughout.
These remarks equally apply to earthwork dams, as regards sufficient
provision of by-wash, careful execution of work, and security of
foundation, but their area of cross section, supposing them to be
water-tight, on account of the flatness of their slopes and consequent
breadth of base, is, of course, far in excess of that merely required for
stability; but in these latter, the method adopted for the water supply
discharge is of the very greatest importance, and will be again referred
to.
Before commencing the excavation for the foundations of a dam, it is most
essential that the character of the soil or rock should be examined
carefully, by sinking a succession of small shafts, not mere borings,
along the site, so that the depth to which the trench will have to be
carried, and the amount of ground water likely to be encountered, can be
reliably ascertained, as this portion of the work cannot be otherwise
estimated, and as it may bear a very large proportion of the total
expense of construction, and in certain cases may demonstrate that the
site is altogether unsuitable for the proposed purpose.
The depth to which puddle trenches have been carried, for the purpose of
penetrating water-bearing strata, and reaching impenetrable ground, in
some cases, has been as much as 160 ft. below the natural surface of the
ground, and the expense of timbering, pumping, and excavation in such an
instance can be easily imagined. This may be realized by referring to
Fig. 4, giving a cross-section of the Yarrow dam, in which the bottom of
the trench is there only 85 ft. below the ground surface. In the Dale
Dyke dam, Fig. 2, the bottom of the trench was about 50 ft. below the
ground surface.
There is one other point which should be mentioned in connection with the
form of the base of the puddle trench--that instead of cutting the bottom
of the trench at the sides of the valley in steps, it should be merely
sloped, so that the puddle, in setting, tends to slide down each inclined
plane toward the bottom of valley, thereby becoming further compressed;
whereas, should the natural ground be cut in steps, the puddle in setting
tends to bulge at the side of each riser, as it may be termed, and so
cause fissures. It will be noticed that the slopes of these earthwork
dams vary from 7 to 1 to 2 to 1.
The depths to which some puddle trenches are carried has been objected to
by some engineers, and among them Sir Robert Rawlinson, as excessive and
unnecessary, and, in the opinion of the latter, the same end might be
obtained by going down to a depth say of 30 ft. only, and putting in a
thick bed of concrete, and also carrying up the concrete at the back of
the puddle trench, with a well for collecting water, and a pipe leading
the same off through the back of the dam to the down stream side. An
arrangement of this kind is shown in the Yarrow dam, Fig. 4.
The thickness of the puddle wall varies considerably in the different
examples given in the diagrams before you, a fair average being the Row
bank of the Paisley Water Works, Fig. 6; and although in instances of
dams made early in the century, such as the Glencorse dam--Fig. 5--of the
Edinburgh Water Works, the puddle was of very considerable thickness, and
it would appear rightly so. This practice does not seem to have been
followed in many cases, as, for instance, again referring to the Dale
Dyke dam, Fig. 2, where the thickness of the top was only 4 ft., with a
batter of 1 in 16 downward, giving a thickness of 16 ft. at the base. For
a dam 95 ft. in height this is very light, compared with that of the
Vehui dam at Bombay, of which the engineer was Mr. Conybeare--Fig.
7--where the puddle wall is 10 ft. wide at the top, with a batter
downward of 1 in 8, the Bann reservoir--Fig. 8--of Mr. Bateman's design,
where the puddle is 8 ft. broad at the top, and other instances. The same
dimension was adopted for the puddle wall of the Harelaw reservoir, at
Paisley, by Mr. Alexander Leslie, an engineer of considerable experience
in dam construction.
There appears to be a question as to what the composition of puddle
should be, some advocating a considerable admixture of gravel with clay.
There is no doubt that clay intended for puddle should be exposed to the
weather for as long previous to use as possible, and subject to the
action of the air at any rate, of sunshine if there be any, or of frost.
When deposited in the trench, it should be spread in layers of not more
than 6 in. in thickness, cut transversely in both directions, thoroughly
watered, and worked by stamping.
The position of the puddle wall is, as a rule, in the center of the bank
and vertical; but laying a thickness of puddle upon the inner or up
stream slope, say 3 ft. thick, protected by a layer of gravel and
pitching, has been advocated as preventing any portion of the dam from
becoming saturated. There are, however, evident objections to this
method, as the puddle being comparatively unprotected would be more
liable to damage by vermin, such as water rats, etc.; and in case of the
earthwork dam at the back settling, as would certainly be the case,
unless its construction extended over a very lengthened period, the
puddle would be almost certain to become fissured and leaky; in addition,
the comparative amounts of puddle used in this manner, as compared with
the vertical wall, would be so much increased. With the puddle wall in
the position usually adopted, unequal settlement of the bank on either
side is less liable to affect the puddle, being vertical.
It would be interesting to refer to the embankment of the Bann, or Lough
Island Reavy reservoir, Fig. 8, designed by Mr. Bateman, now nearly fifty
years ago, where a layer of peat was adopted both on the slope, 15 in.
thick, and in front or on the up stream side of the puddle wall, 3 ft.
thick. The object was, that should the puddle become fissured and leaky,
the draught so created would carry with it particles of peat, which would
choke up the cracks and so reduce the leakage that the alluvial matter
would gradually settle over it and close it up. On the same diagram will
be noticed curved lines, which are intended to delineate the way in which
the earthwork of the embankment was made up. The layers were 3 ft. in
thickness, laid in the curved layers as indicated.
It is a moot question whether, in making an earthwork embankment,
dependence, as far as stanchness is concerned, should be placed upon the
puddle wall alone or upon the embankments on either side, and especially
upon the up-stream side in addition. Supposing the former idea prevails,
then it can be of little moment as to how or of what material the bank on
either side is made up--whether of earth or stone--placed in thin layers
or tipped in banks of 3 ft. or 4 ft. high; but the opinion of the
majority of engineers seems to be in favor of making the banks act not
merely as buttresses to the puddle wall, and throwing the whole onus, as
it may be termed, of stanchness upon that, but also sharing the
responsibility and lessening the chances of rupture thereby. But to
insure this, the material must be of the very best description for the
purpose. Stones, if allowed at all--and in the author's opinion they
should not be--should be small, few, and far between. Let those that are
sifted out be thrown into the tail of the down stream slope. They will do
no harm there, but the layers of earth must not approach 3 ft. in
thickness nor 1 ft.--the maximum should be six in., and this applies also
to the puddle. Let the soil be brought on by say one-horse carts, spread
in six inch layers, and well watered. The traffic of the carts will
consolidate it, and in places where carts cannot traverse it should be
punned. In the Parvy reservoir dam a roller was employed for this
purpose. It comprised a small lorry body holding about a yard and a half
of stone, with two axles, on each of which was keyed a row of five or six
wheels.
At the Oued Meurad dam, in Algeria, 95 ft. high, constructed about 23
years ago, the earthwork layers were deposited normal to the outer slope,
and as the bank was carried up the water was admitted and allowed to rise
to near the temporary crest, and as soon as the bank had settled, the
earthwork continued another grade, and the same process repeated.
It was the practice until comparatively recently to make the discharge
outlet by laying pipes in a trench under the dam, generally at the lowest
point in the valley, or constructing a culvert in the same position and
carrying the pipes through this, and in the earlier works the valves or
sluices regulating the outflow were placed at the tail of the down stream
bank, the pipes under the bank being consequently at all times subject to
the pressure of the full head of the water in the reservoir. An instance
of the first mentioned method is afforded by the Dale Dyke reservoir,
Fig. 2, where two lines of pipes of 18 in. diameter were laid in a trench
excavated in the rock and resting upon a bed of puddle 12 in. in
thickness, and surrounded by puddle; the pipes were of cast iron, of the
spigot and faucet type, probably yarned and leaded at the joints as
usual, and the sluice valves were situated at the outer end of the pipes.
As the failure of this embankment was, as we all know, productive of such
terrible consequences, it may be of interest to enter a little more fully
into the details of its construction. It was situated at Bradfield, six
or seven miles from Sheffield, and at several hundred feet higher level.
Its construction was commenced in 1858, the puddle trench was probably
taken down to a depth of 40 ft. to 50 ft., a considerable amount of water
being encountered. This trench was 15 ft. to 20 ft. broad at the top, and
of course had to be crossed by the before mentioned line of pipes; and
although the trench was filled with puddle, and the gullet cut in the
rock already mentioned for carrying the pipes under the site of the dam
was "padded" with a layer of 12 in. of puddle, we can imagine that the
effect of the weight of the puddle wall and bank upon this line of pipes
would be very different at the point where they crossed the puddle trench
to what it would be where they were laid in the rock gullet and partially
protected from pressure by the sides of the latter. At the trench
crossing there would be a bed of puddle 50 ft. in thickness beneath the
pipe, in the gullet a bed of 1 ft. in thickness. So much as regards the
laying of the pipes.
The embankment had scarcely been completed when, on March 11, 1864, a
storm of rain came on and nearly filled it up to the by-wash, when the
bank began slowly to subside. The engineer was on the crest at the very
time, and remained until the water was running over his boots; he then
rushed down the other slope and was snatched out of the way as the bank
burst, and the whole body of water, about 250,000,000 gallons, rushed out
through the trench, carrying with it in the course of about twenty
minutes 92,000 cubic yards, or say one fourth of the total mass of
earthwork, causing the death of 250 human beings, not to mention cattle,
and destruction of factories, dwellings, and bridges, denuding the rock
of its surface soil, and, as it were, obliterating all the landmarks in
its course. The greatest depth of the bank from ground level to crest was
95 ft., the top width 12 ft., and the slopes, both on the up stream and
down stream sides, 2½ to 1, and the area of the reservoir 78 acres.
Mr.--now Sir Robert--Rawlinson, together with Mr. Beadmore, were called
in to make a report, to lay before Parliament, upon this disaster; and
having made a careful examination of the ruins, and taken evidence, they
were of opinion that the mode of laying the pipes, and in such an
unprotected way, was faulty, and that subsidence of the pipes probably
occurred at the crossing of the puddle trench. A fissure in the puddle
was created, affording a creep for the water, which, once set up, would
rapidly increase the breach by scour; and this event was favored by the
manner in which the bank had been constructed and the unsuitability of
the material used, which, in the words of one engineer, had more the
appearance of a quarry tip than of a bank intended to store water. This
opinion of the cause of failure was, however, not adopted universally by
engineers, the line of pipes when examined being found to be, although
disjointed, fairly in line; and there having occurred a land slip in the
immediate neighborhood, it was suggested that the rupture might be caused
by a slip also having taken place here, especially as the substratum was
of flagstone rock tilted at a considerable angle. The formation was
millstone grit. This catastrophe induced an examination to be made of
other storage reservoir dams in the same district, and a report on the
subject was presented to Parliament by Sir Robert Rawlinson.
[Illustration: TYPICAL MASONRY AND EARTHWORK DAMS OF THE WORLD.]
The dam of Stubden reservoir, of the Bradford water supply, also on the
millstone grit, was constructed about 1859, and caused considerable
anxiety for a length of time, as leakage occurred in the culvert carrying
the pipes, under the embankment at a point a short distance on the down
stream side of the puddle trench. This was repaired to some extent by
lining with cast iron plates; and an entirely independent outlet was made
by driving a curved tunnel into the hill side clear of the ends of the
dam and lining it with cast iron plates. In this tunnel was then laid the
main of 2 ft. diameter, and as the original culvert again became leaky,
the water had to be lowered, the old masonry pulled out, and the space
filled in with puddle.
The Leeming compensation reservoir of the same water supply, with a dam
of 50 ft. in height, and culvert outlet, had to be treated somewhat in
the same manner, as, although the reservoir had never been filled with
water, in 1875, when it was examined previous to filling, it was found
that the culvert was cracked in all directions; and it was deemed best to
fill it up with Portland cement concrete, and drive a tunnel outlet
through the hill side, as described in the case of the Stubden reservoir.
The Leeshaw dam, which was being constructed at that time upon the same
lines, viz., with culvert outlet under the dam, was, at the advice of Sir
Robert Rawlinson, altered to a side tunnel outlet clear of the dam.
Some years previous to the failure of the Dale Dyke reservoir there
occurred, in 1852, a failure of a similar character--though, as far as
the author is aware, unattended by such disastrous results--at the
Bilberry reservoir at Holmfirth, near Huddersfield, which had never been
filled previous to the day of its failure, and arose from the dam having
sunk, and being allowed to remain at a level actually below that of the
by-wash; so that when the storm occurred, the dam was topped and
destroyed. An after examination proved that the bank was badly
constructed and the foundation imperfect.
Besides the above instances, there have been numerous failures within
recent times of earthwork dams in Spain, the United States, Algeria, and
elsewhere, such as that which occurred at Estrecho de Rientes, near
Lorca, in Murcia, where a dam 150 ft. high, the construction of which for
irrigation purposes was commenced in 1755 and completed in 1789, was
filled for the first time in February, 1802, and two months later gave
way, destroying part of the town of Lorca and devastating a large tract
of the most fertile country, and causing the death of 600 people. The
immediate cause of failure in this case the author has been unable to
ascertain. In Algeria the Sig and Tlelat dams were destroyed in 1865; and
in the United States of America, at Williamsburg, Hampshire Co.,
Massachusetts, in 1874, an earthwork dam gave way, by which 159 lives
were lost and much damage done to property. In another case, viz., that
of the Worcester dam, in the United States of America--impounding a
volume of 663,330,000 gallons, and 41 ft. high, 50 ft. broad at the
crest, and formed with a center wall of masonry, with earthwork on each
side--which gave way in 1875, four years after its completion; here, as
in almost all other instances of failure, the leakage commenced at a
point where the pipes traverse the dam. In this case they were carried in
a masonry culvert, and the leak started at about 20 ft. on the up stream
side of the central wall. The opinion of Mr. McAlpine as to the cause of
failure, which agrees with that of the most eminent of our own water
engineers, was to the effect that "earthen dams rarely fail from any
fault in the artificial earthwork, and seldom from any defect in the
natural soil. The latter may leak, but not so as to endanger the dam. In
nine tenths of the cases, the dam is breached along the line of the water
outlet passages."
The method of forming the discharge outlet by the construction of a
masonry culvert in the open has no doubt many advantages over that of
tunnel driving through the hill side clear of the dam, permitting as it
does of an easy inspection and control of the work as it proceeds; but a
slight leakage in the instance of a side tunnel probably means nothing
more than the waste of so much water, whereas in the case of the culvert
traversing the site of the bank, the same amount or less imperils the
stability of the bank, and in ninety-nine cases out of a hundred would,
if not attended to, sooner or later be the cause of its destruction. I
think the majority will therefore agree that the method of discharge
outlets under the site of embankments should not be tolerated where it is
possible to make an outlet in the flank of the hill, to one side, and
altogether clear of the dam.
At Fig. 9 is a diagram of the Roundwood dam of the Vartry Water Works,
supplying Dublin, which is a fair specimen of the class of earthwork dam
with the outlet pipes carried in a culvert under the embankment, and
which, perhaps, is one of the most favorable specimens of this method of
construction, as the inlet valves are on the up stream of the dam, and
consequently when necessary the water can be cut off from the length of
pipes traversing the dam. A short description will be given. This dam is
66 ft. high at the deepest point and 28 ft. wide at the crest, having to
carry a public road. The slope on the inner face is 3 to 1, and on the
outer 2½ to 1. The by-wash is 6 ft. below the crest, which is about the
average difference. The storage capacity of the reservoir is
2,400,000,000 gallons, or sufficient for 200 days' supply to the city.
The puddle wall is 6 ft. wide at the top and 18 ft. at ground level, the
bottom of the puddle trench about 40 ft. below the surface of the ground.
The culvert was formed by cutting a gullet 14 ft. wide with nearly
vertical sides through the rock, and covering it with a semicircular arch
4 ft. in thickness. Through this tunnel are laid a 33 in. and 48 in.
main; the former for the water supply, and the latter for scouring or for
emptying the reservoir on an emergency. There is a plugging of brickwork
in cement under the center of the dam in the line of the puddle wall, and
then stop walls built at the end of the plugging, projecting 25 ft.
beyond the sides of the culvert and 8 ft. above, the space between them
being filled up with cement concrete tied into the rock, and on this the
puddle wall rests. This bank, like almost all others pierced by outlet
pipes or culverts, was not destined to be perfect. In 1867, four years
after the completion, spurts of water showed themselves in the culvert
in front of the puddle wall, which began to settle, and the water had to
be drawn off to admit of repairs. Diagram No. 10 shows a structure of a
different character to any of these already described. This character of
work is adopted on the North Poudre Irrigation Canal, in N.E. Colorado.
Timber is there plentiful, and a dam of this character can be rapidly
constructed, although probably not very durable, owing to liability to
decay of timber. That represented is about 25 ft. high.
The author has now concluded the consideration of earthwork dams, and
proposes making a few remarks upon those of masonry or concrete, with
reference to some of the most important, as shown on the diagrams. Their
stability, unlike those of earthwork, may be considerably increased where
the contour and nature of the ground is favorable by being curved in
plan, convex toward the water, and with a suitable radius. They are
especially suitable for blocking narrow rocky valleys, and as such
situations must, from the character of the ground, be liable to sudden
and high floods, great care is necessary to make sufficient provision for
overflow.
When of masonry, the stones should be bonded, not merely as they would be
in an ordinary vertical wall, where the direction of the stress is
perpendicular, but each course should be knit in with that above and
below it in a somewhat similar manner to what is termed "random" work.
And lastly, if hydraulic mortar be used, a sufficient time should elapse
after construction before being subjected to strain, or in other words,
before water is allowed to rise in the reservoir. For this latter reason,
and also the liability to damage by sudden floods during the progress of
the works, dams of Portland cement concrete, on account of their quick
consolidation, possess advantages over those of hydraulic masonry apart
from the necessity in the latter instance of constant supervision to
prevent "scamping" by leaving chinks and spaces vacant, especially where
large masses of stone or Cyclopean rubble are used.
Again, should the dam be drowned by flood during its erection, no harm
would accrue were it composed of Portland cement concrete, whereas should
it be of hydraulic mortar masonry, the wall would probably be destroyed
or, at all events, considerably injured by the mortar being washed out of
the joints. Portland cement, however, is only suitable for situations
where the foundation is absolutely firm, as, should there be the
slightest settlement, fissures would certainly be produced.
As regards foundations, the dam of the Puentes reservoir in Spain is
somewhat remarkable--see Fig. 12. Its height is 164 ft., and the profile
or cross section is of precisely the same character as that of the
Alicante dam, the latter being 135 ft. in height, 65 ft. wide at the
crest, and 65 ft. at the base, and erected about 300 years ago. At the
Puentes dam the flanks of the valley were reliable, but, as must be
frequently the case in such situations, the bed of the valley was
composed to a great depth of gravel, _debris_, and shaky strata. The
difficulty was overcome by throwing an arch, or arches, across the
valley, the abutments being formed by the solid rock on each side, and
building the dam upon this arching and filling in below the latter down
to a sufficient depth with walling.
Bearing in mind the sudden and great floods to which dams constructed in
such situations must be subjected, and, if the valley be very narrow, the
probability that sufficient space at the side for a by-wash will be
difficult to obtain, it would seem reasonable that in the calculation for
their section allowance should be made for the possible condition of the
whole length of the dam being converted into a weir, over which the
waters may flow without risk of injury to the dam, to a depth of, say, at
least twice that ever probable.
The topping of dams by floods is not uncommon, and if the extra strain
thus induced has not been allowed for, their destruction is nearly
certain, as instanced in more than one case in Algeria, where, although
the average rainfall is only 15 in. yearly, a depth of 6¼ in., or more
than one-third of the annual total, has been known to fall in twenty-four
hours.
The Habra dam--see Fig. No. 13--completed in 1871, was destroyed by a
sudden flood of this kind in December, 1881. This reservoir, with a
storage capacity of 6,600,000,000 gallons, was intended for the
irrigation of a cultivated bordering on the Mediterranean and the storage
of floods. The height of the dam was 116.7 ft. and was provided with a
by-wash of 394 ft. in length, and outlets for irrigation formed by four
cast iron pipes of 31½ in. diameter through the dam. It was composed of
rubble set in hydraulic mortar, the latter composed of two parts of sand
to one of hydraulic lime.
For getting rid of the large deposits of sand to which all reservoirs in
that country are liable, two scouring outlets were provided of the same
description as those in the old Moorish dams. The profile was calculated
from Delocre's formula, and was correct in this respect, supposing the
by-wash to have been sufficient. But as it was otherwise, and the flood
swept over the crest to the depth of about 3 ft., the enormous extra
strain thus induced overthrew the dam and caused the destruction of
several villages and the death of 209 persons. It must be mentioned that
when the reservoir was filling, the water percolated through the masonry,
giving the face wall the appearance of a huge filter, which at the time
was attributed to the porous nature of the sandstone used in
construction, but which more probably was due to the washing of the green
mortar out of the joints.
At the Hamiz dam, also in Algeria, the water was admitted in 1884, but it
showed immediately signs of weakness, so that the water had to be run out
and an immense retaining wall erected to strengthen the main dam. Algeria
seems to have been singularly unfortunate as regards the success of works
of this description. Water was admitted to the Cheurfas reservoir in
January, 1885, and it at once began to make its way through permeable
ground at one end of the dam. The flushing sluice in the deepest part of
the dam had become jammed, so that the pressure could not be relieved,
and in February 30 ft. length of the dam was carried away, causing a
flood in the river below. At some distance down stream was the Sig
reservoir. The flood rushing down, topped this dam by 18 ft. and
overthrew it also.
Allusion has been made to provision for scouring out sand and deposit,
especially in the dams of Algeria and of Spain. The amount of sand, etc.,
brought down by the floods is something enormous, and the question of the
best means of getting rid of it has occupied much attention. In the old
Moorish reservoirs the flushing gallery, piercing the lower part of the
dam, was closed by iron doors on the down stream face and blocked with
timber at the upper end. When required to be flushed out, laborers passed
through the gallery and broke down the timber barrier, the silt forming a
wall sufficiently thick to resist the pressure of the water for the time
being, and allow of the retreat of the Forlorn Hope--if the latter had
luck--before giving way.
One method adopted in Algeria, which has the advantage of permitting the
sediment to be utilized together with the irrigation, this sediment being
very fertilizing, is to pump air down through hose extending to the
bottom of the reservoir, the pumps being actuated by steam power or
turbine, and the sediment thus stirred up and run off with the water
through the irrigation pipes. As an example of one of the early types of
masonry dams in France, reference may be made to Fig. 13, on which is
shown an elevation and cross section of the Lampy dam, forming a large
reservoir for feeding the Languedoc canal.
I will now refer to some of the most notable masonry dams in existence,
commencing with France, where perhaps the finest is that known as the
Furens, in connection with the St. Etienne Water Works, constructed
between the years 1859-66, and designed by the engineers Graiff and
Grandchamps. It is curved in plan, struck with a radius of 828 ft. from a
center on the down stream side, and founded upon compact granite, the
footings being carried down to a depth of 3 ft. 3 in. below the surface
of the rock. It is of rubble masonry, in hydraulic mortar, carried up in
courses of 5 ft. in depth.
The height is 170 ft. on the up stream side and 184 ft. high on the lower
side, with a breadth of 9 ft. 8 in. at the crest and 110 ft. at the base,
and the cross section is so designed that the pressure is nearly constant
in all parts, and nowhere exceeds 93 lb. to the square inch--13,392 lb.
to the square foot. The contents is equal to 52,000 cubic yards of
masonry, and the cost of erection was £36,080. The capacity of the
reservoir is equal to 352,000,000 gallons.
The reservoir discharges into two tunnels (see Fig. 11), driven one above
the other through a hill into an adjacent valley. The lower tunnel
contains three cast iron pipes, with a masonry stopping of 36 ft. long.
Two of these pipes are 16 in. diameter, with regulating valves, and
discharge into a well, from whence the water can be directed for the town
supply or into the river. The third pipe, of 8½ in. diameter, is always
open, and serves to remove any deposit in the reservoir, and to furnish a
constant supply for the use of manufacturers.
The author drew attention to the difference in the section of the Furens
dam, Fig. 11, as compared with that of Alicante, and of Puentes, which is
similar to the latter. These two last illustrate the ancient Moorish
type, and the former that of the present day. The Gileppe dam at
Verviers, in Belgium, Fig. 14, although quite recently erected, viz.,
between the years 1869 and 1875, differs very much from the Furens type,
in so far as it is of very much larger sectional area in proportion to
its height, but this is accounted for by the desire of the engineer, M.
Bodson, to overcome the opposition to its construction, and meet the
objections and combat the fears of those whose interests--and those
serious ones, no doubt--would be affected in the event of its rupture,
the body of water stored being 2,701,687,000 gallons, or about eight
times as much as the capacity of the Furens reservoir.
In addition to this, there was another reason, which was quite sufficient
in itself to account for the extra substantiality of the dam. This
reservoir is for supplying water to the cloth factories of Verviers, on
the Belgian-German frontier. It is curved in plan to a radius of 1,640
ft., with a length of 771 ft., and the additional strength of the
structure due to so flat a curve is probably slight.
It is built of rubble masonry, with ashlar facework, laid in hydraulic
mortar. The total amount of masonry is 325,000 cubic yards. There are two
weirs, at a level of 6 ft. below the crest, each 82 ft. wide. The total
height, including the foundations, which are carried down from 3 ft. to 5
ft. into the rock, is 154 ft., and the breadth of the crest, which
carries a road, is 49 ft. 3 in., and at the base 216 ft. The outlet pipes
are carried through tunnels, which are driven on the curve into the hill
side a considerable distance clear of each end of the dam.
Another very important structure is the Villar dam, Fig. 15, in
connection with the water supply of Madrid, and situated on the river
Lozoya. The storage capacity of this reservoir is very considerable,
viz., 4,400,000,000, or nearly thirteen times as great as that of Furens.
The height of the dam is 162 ft., with a breadth of 14 ft. 9 in. at the
crest. It is built on the curve to a radius of 440 ft., and the length of
the dam measured along the crest is 546 ft., of which 197 ft. is by-wash,
thus describing nearly one-fifth of a circle, and consequently well
designed to resist pressure. The dam is built of rubble masonry in
hydraulic mortar, and cost £80,556.
The Stony Creek lower reservoir dam of the Geelong water supply, Fig. 16,
colony of Victoria, is interesting as being constructed of concrete, in
the proportion of 1 to 8½. Its erection occupied eighteen months, and
cost about £18,000. It is curved in plan to a radius of 300 ft., and the
greatest depth or head of water is 52 ft. 4 in. The width at the crest is
only 2 ft. 8 in., although surmounted by a heavy coping of bluestone 3
ft. 3 in. broad and 1 ft. 9 in. deep. There being no facility for making
a by-wash at the side, the center of the dam is dished to form a weir 30
ft. long. There are both outlet and scour pipes, and valves of 2 ft.
diameter, and the capacity of the reservoir is 143,145,834 gallons.
The Paramatta dam, in New South Wales, built of masonry in hydraulic
mortar, is another instance of a dam built on the curve, and which has
resisted a flood of water 4 ft. in depth over the crest; and in the case
of a dam of about 40 ft. high across the river Wyre, in connection with
the Lancaster Water Works, made of cement concrete in proportion of 4 to
1, there has, according to Mr. Mansergh, frequently been a depth of 5 ft.
of flow over it. This dam is built to a radius of 80 ft. only, and as it
measures 100 ft. along the crest, must include about the fifth of a
circle.
There now remain only two other examples of masonry dams, the first being
that in connection with the Liverpool water supply, and known as the
Vyrnwy dam, Fig. 17, this being thrown across a stream of that name in
North Wales. It is now under construction, and when completed will
impound an area of 1,115 acres.
The dam will be 1,255 ft. long, and formed of Cyclopean rubble set in
cement mortar, and the interstices or spaces between the large masses of
stone, which are rough hewn and not squared, are filled with cement
concrete. The proportion of the cement mortar is 2½ to 1. These masses of
stone weigh from two to eight tons each, and it is expected that the wall
will be of a most solid description, as great care is being taken to fill
up all spaces. The face next to the water is cemented. The area of the
cross section shown on the diagram, which is at one of the deepest
points, is 8,972 square feet, and the height from foundation to flood
level is 129 ft., the breadth at the base being 117 ft. 9 in.
The existing dam of the New York water supply, Fig. 18, known as the
Croton reservoir, is shown on the diagram. Its capacity is 364,000,000
gallons and the area 279 acres. The height is 78 ft. and width at crest 8
ft. 6 in., and is built of masonry in hydraulic mortar. The face walls
are of stone laid in courses of 14 in. to 26 in., and are vertical on the
up stream side, and with a batter of 1 in 2½ on the down. The hearting is
of concrete for a depth of 45 ft. from the top, and the remaining depth
is in Cyclopean rubble.
At Fig. 19 is shown the section of the Quaker Bridge dam, which when
completed will be the largest structure of the kind in existence. It is
situated on the Croton River, which is a tributary of the Hudson, about
four miles below the present Croton dam. The length will be 1,300 ft. and
the height 170 ft. above the river bed, or 277 ft. above the foundation.
The water by-wash is 7 ft. below the crest, and the dam is 26 ft. broad
at the crest and 216 ft. at the base. The capacity of the reservoir will
be 32,000,000,000 gallons, or nearly a hundred times as great as that of
Furens. The geological formation at the site is sienitic gneiss. The cost
of the dam is estimated at £500,000.
[Illustration: TYPICAL MASONRY AND EARTHWORK DAMS OF THE WORLD.]
The accompanying table gives the pressures to which various dams are
subjected, and it may be noted with regard to the weight of water,
generally assumed as 62.4 lb. per cubic foot, that it will, in some
districts, in time of flood, carry so much matter in suspension as to be
increased to as much as 75 lb. weight, or an addition of 20 per cent.,
which, it may be easily imagined, will affect the conditions of stability
very seriously.