US1675001A - Dam - Google Patents

Description

' June 26, 1928. 1,675,001

DAM

Filed June 18, 1925 Patented June 26, 1928.

STATES HENRY W. SPOONER, OF BROOKLYN, NEW YORK.

\ DAM.

In the design of concrete or masonry dams for the impounding of water it is not always possible to compute the various opposing forces with as great a degree of accuracy as is'desirable. These forces are all governed by the general laws of gravitation, yet one of the most important'factors, namely the uplift against the bottom of the dam, is usually indeterminate. This leads to a certain amount of guesswork in the design, which in many cases is incorrect, as has been proved by the all too frequent instances of dam failures, which in many cases have resulted in great loss of life and property.

The failure of a concrete or masonry dam is always due to one or more of three causes: These are overturning, sliding down stream, or fracturing under the pressure exerted by the impounded water. The total water pressure acting against the face of the dam may be determined very closely. The opposing force of gravitation due to the weight of the structure itself may be computed with a reasonable degree of accuracy, and to this may be added a part of the inertia of the foundation by imbedding the dam into the rock. Sliding down stream can thus be prevented, but it involves considerable expensedue to the additional rock excavation and masonry required.

The danger of overturning or fracturing may be eliminated by building the dam sufiiciently heavy to overcome both the horizontal and vertical components of the water pressure, but this procedure also involves considerable extra expense. The total cost of a dam is an important consideration, and for this reason the temptation exlsts to re-- duce the factor of safety against failure to ratio far smaller than is ordinarily used in other branches of engineering design. The theoretical factor of safety may in practice he still further reduced and may even be come negative if insuflicient allowance for uplift has been made in the calculations.

Uplift is that force which operates against the bottom of the dam in a vertical direc-' tion, and is caused by seepage of water underneath the dam under the full hydrostatic head on the upstream side, or by springs arising through fissures in the foundation, or by both of these causes combined. The total force of uplift is the sum of the prod ucts of the various hydrostatic pressures in to the various areas which are not in perfect and eompiete contact with the foundation;

It is obvious that these forces and their centers of pressure are not only indeterminate, but also that they may vary in intensity and location over the entire base of the dam.

Uplift against the base of a dam tends not only to overturn it, but also to cause fracture at or near the toe, or downstream edge. It may also increase the tendency to slide downstream by reducing the friction be- Eween the base of the dam and its founda- Heretofore, the tendency for dams to overturn and the force of uplift has been counteracted solely by the force of gravity in the mass of the dam itself.

I aim to provide a dam of improved design and construction in which the tendency of uplift is reduced or entirely eliminated by the provision of means for introducing an additional force to counteract the horizontal component of the water pressure on the upstream side of the dam. I thus increase in any given dam section the factor of safety against overturning, fracturing, or sliding down stream. l

The invention, which is pointed out with particularity in the appended claims, will be apparent from the following specification when read in connection with the accompanying drawings in which- Fig. 1 is cross-sectional view of a dam embodying my invention;

Fig. 2 is a fragmentary longitudinal section on line 22 of Fig. 1:

Fig. 3 is a fragmentary View illustrating a modification. 7'

Referring in detail to the embodiments of the invention illustrated Figure 1 represents a typical cross section of a gravity dam of concrete construction foundedv in rock, part of whose length is of the bulkhead type as indicated by a; the balance of its length having an overflow or spillway section b. An inspection tunnel or gallery 6 extends throughout the length of the dam, such as is ordinarily provided in dams of fair size or importance. 7 A drainage trench d extends lengthwise under the dam at the downstream edge of the cut-off trench, and wall 6. A plurality of open grooves f extend at right angles to and connect with drainage trench d and form shallow chambers between the dam and its foundation. The total area of such chambers in a given design is dependent on the safe aiiowahle unit pressure at. the ten-raining:

lUO

stream side.

area of the base, or in other words, on that portion of the dam in actual contact "with its foundation.

The longitudinal section, Fig. 2, shows the several grooves 7 connecting into trench d A pipe g extends through the concrete between the inspection tunnel c and the drainage trench cl, its open end terminating near the bottom of the latter. Located-in-inspect ion tunnel c or in an enlarged chamber of [the same is a combined Water ejector and air pump it having a water supply pipe 7' connecting its inlet with a screen chamber 70 located near the top of the dam on itsup- A discharge pipe Z extends from the outlet nozzle th through the lower portion of the dam and terminates on the downstream side, preferably at or near the toe of the spillway section. A valveh V is provided to prevent flow of water through the pipe when thepunip is disconnected,

for making repairs. v V

The screen chamber or inlet 7c is located at a predetermined elevation such that when thehead ,water level docs-not cxceedthat eleration, the daiii will have an ample factor ill will flow out through these pipes, up

of safety tore. st through its own weight vthe combined horizontal and verti'cal components of the hydrosatic press .re. The

elevation of inlet a is such that with fan increasein the head Water level above that point the wasting, of a. comparatively small amount of water is not objectionable.

As the pipes g and'l and the interconnect,

ing pump ()l GjEJClLOi' fi oli'er a freepassage between the trench (Z and the downstreanr exterior of the dam,.any Water collecting in trench (Z or the interconnecting grooves f to the limit of their capacity under the hy rostatic head existing in trench d. Therefore the.

hydrostatic head orupliftunder thebase of the dam willbe measured by the difference in elevation between the gooves andthezhigu' ,est point of pipe Z, unless the flow of Water from trench d through pipes g and Z isvsuflicient to cause an additional friction head..

.It is therefore obvious thatthe combination'offthe vgrooves fand their communicat ing trench ct together with the pipes g and Z provide a means of limiting the hydrostatic pressure under the base of the dam.

:vl Vhen the head water level reaches or exceeds the elevation of inlet 70, Water flows down the supply pipe j and: throughthe ejector, or air. pump it, discharging downstream through pipe Z and inducing a partial vacuum in suction pipe g. This causes any water collected in trenchd-andits inter- ;communieating grooves fto flow upward through pipe 9 and to be discharged through pipeZ, due to the atmospheric pressure in the drainage trench d; I

The action of airpump 71 resulting from the flow of Water through gup y is first to remove the Water from trench (Z and int'eicomintinicating spaces 7', and then to create a partial vacuuniby removing most of the airfrom these spaces. This results in superimposing on the crest and downstream *tace of the dam an atmospheric load Which .is equalto theproduct of the negatlve pressure under the tlamtim'es the combined areas to the open spaces between the dam and its foundation V y a I he action of the pump vfirst relieves the dam from any uplift pressure due to the hydrostatic head, 'on the Water collected 7 under the dam, following which 'it builds up a. partial vacuuin'below the dam and thus siiperiinposes a corresponding atmospheric load on the outside of the dam. Thisatin'osphe'ric load opposes the horizontal. 01 mponciit of the water pressure by increasing the gene-cave weight of the lfdan i', thereby .tei'idi-ii'g to eliminate all three causes offailure, namely, overturning, fracturing, and sliding downstream. a i

The atmospheric pressure actingbn the crest exterior surface of the dam ii)- c'reas'es its effective weiglit jand corresponding'ly increases its strength throughout its height, tending thereby to reduce the likelihood of fractureat any point, 'or the danger of overthrowing any part ofthe dam above any given horizontal construction joint.

Fiirthei'more, the increase in weightha'sthe i i I t eliect of moving the pos tion of the resultant toward the upstream ed 'ewof the base, there .by reducing the pressure between the toe oi? While I have shown-and described a single pump, it will be understood any desired number may be used depending on the governing conditions.- Neither are, the pump inlet chambers required to be place'dat, the same elevation; they may be located at various levels so .as to operate in sequence and reduce the force 0t uplift inversely with an increase in head, combining, to create a vacuum afteiwtlie water level has reached the upperni'ost 'inlet and sealed the air vents formed by unsubinerged inlet pipes,

'Although I prefer to neutralize the uplift pressure by creating a -vacuum under the .dam by means of an ejector type of pump,

the same result' maybe accomplishec'lby meansof a mechanical pump oi": either the rotary or reciprocatlng types, or in fact with any pump which will handle water, and air below atmospheric pressure.

lVhile the foregoing disclosure sets forth an eflicient means for accomplishing the desired results, a less costly alternative construction but also a less efficient means could be employed as shown in 3 by connecting the trench d by means of a pipe Z or duct extending through the dam to the downstream toe and terminating in a nozzle Z of suitable construction pointing in the direction of flow of water passing over the spillway. Overflowing water submerg ing this nozzle and passing it at sufficient velocity would serve the same purpose as the pump it above described, though less effectively.

The chambers f and'trench (Z between the dam and its foundation may be constructed in any suitable manner. such as by using light metal forms, or light arches of tile or precast concrete or the equivalent placed on the foundation before pouring the lower course.

While the drawing shows the drainage trenchd in a position parallel to the axis of the dam, with the interconnecting openings f at right angles to it, I am not limited to this arrangement. Any desired arrangement of these openings may be adopted to conform to the character of the foundation and other controlling conditions peculiar to the location of the dam, and if preferred the drainage trench (Z may be placed near the center of the dam or at the downstream end.

Various modifications may be made by those skilled in the art without departing from the invention as defined in the appended claims.

hat I claim is:

l. The combination with a dam and its foundation of means for creating a partial vacuum between said dam. and foundation.

2. A dam having chambers formed in the base portion thereof and means for creating a differential pressure between said chambers and the exterior of the dam whereby an atmospheric load is superimposed on the dam.

3. A dam formed with chambers between its base and its foundation and means for utilizing the water impounded by the dam for creating a partial vacuum in said chambers.

4. A dam having chambers between its base and its foundation and means whereby the factor of safety against failure of the dam is increased with a rise in the head water level by utilizing part of the impounded water to create a partial vacuum in said vchambers.

5. A dam having at least one substantially air tight chamber between its base and its foundation and a means responsive to changes in the level of the impounded water for creating a partial vacuum in said chamber.-

6. A dam having chambers under the base thereof, apump arranged to be actuated when the water impounded by the dam reaches a predetermined level, a suction duct connecting the pump with said chambers, and. an outlet duct from the pump extending to the downstream side of the dam.

7. In combination with a dam structure having chambers under its base, a pump operated by the flow of water, a water inlet connected to said pump from the upstream side of the dam, a suction line connecting said pump with said chambers and a discharge duct leading from the pump to the downstream side of the dam.

In Witness whereof, I have hereunto signed my name.

HENRY SPOONER.

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