« PreviousContinue »
from 5° to 16°. About 18 percent of the refrigeration was lost through the supply and return headers notwithstanding the insulation.
The system was designed for a flow through each cooling coil of not less than 3 gallons per minute, but data in table 2 indicate that the average actual flow was more than 4 gallons per minute.
The refrigeration plant was rated at 825-ton capacity. One ton of refrigeration is the amount of heat required to melt 1 ton of ice in 24 hours, or a rate of 200 B. t. u.'s per minute. This plant capacity, plus the effect of the cooling tower, combined to produce an average of 1,815 tons of refrigeration during September 1934. The total amount of heat removed from the concrete was about 159 billion B. t. u.'s, of which 74 billion were removed with air-cooled water and 85 billion with refrigerated water. Data on the cooling system and its operation are given in table 2.
Temperature Control. As cooling progressed, temperatures of the concrete were obtained periodically to check the progress and to determine when cooling should be stopped.
A total of 416 resistance thermometers were embedded in the concrete throughout 2 vertical sections of the dam, 1 on either side of the cooling slot. The temperature of the concrete surrounding each cooling coil was determined before cooling was started by inserting a special resistance thermometer into the pipes some 25 to 30 feet from the face of the cooling slot. The temperature was determined again, before the connections were removed, after circulation through the coil had been discontinued for 48 hours to permit equalization of the temperature. An estimate of the concrete temperature could also be obtained by a simple calculation, using charts prepared from observed temperatures of the inlet and outlet water. Temperatures of the water were obtained directly by inserting a thermometer in samples drawn into thermos flasks through small pet cocks provided at the inlet and outlet of each coil.
Cost of Cooling. The cost of cooling Boulder Dam cannot be itemized readily because of a change in accounts and method of payment for construction and operation of the cooling plant before and after March 1, 1934. The cost to the Government of all materials, labor, and contracts relating to all work chargeable to cooling is as follows: All embedded pipes and fittings, material, and labor of installing about 590 miles of 1-inch outside diameter
Construction and operation of cooling plant, including cooling tower, refrigeration plant, pumps, and
headers 513,774. 80
Miscellaneous details involving extra work orders . . . 27, 709. 08
A total of 3,287,664 cubic yards of concrete was cooled, including 3,181,664 cubic yards in the dam and 106,000 cubic yards in the tunnel plugs. The average cost of cooling was $0,226 per cubic yard.
COOLING OTHER DAMS
Owyhee Dam. During the construction of Owyhee Dam it was determined that by the time it was proposed to grout the contraction joints, the temperature of the lower sections would still be far in excess of the mean annual air temperature. It was then proposed to remove this excess temperature before grouting by pumping river water through the embedded grout pipe system.
Cooling was started in the lower 200 feet of the dam in October 1932 but had to be discontinued for 3 months during the winter season of 1932-33 because of the accumulation of ice on the downstream face of the dam and because of freezing of the pipes. From March through July 1933 cooling was resumed, to be discontinued another 3 months because of ineffectiveness due to the rising river water temperatures and lowered concrete temperatures approaching each other. Cooling was resumed again in November 1933 and continued throughout the comparatively mild winter until March 1934 when the temperature of the con
Crete was very close to the mean annual air temperature desired for grouting. The joints' were grouted in April and May 1934.
Madden Dam. Very much the same procedure as that used in Owyhee Dam was followed in cooling the concrete in the lower part of Madden Dam (see ref. 14). River water was pumped through the grouting system during 1935 and the beginning of 1936, and the temperature of the concrete effectively reduced before the contraction joints were grouted in August and September 1936.
Grand Coulee Dam. An embedded pipe system similar to the one installed in Boulder Dam is being installed in Grand Coulee Dam now under construction. In addition to the usual transverse contraction joints, vertical longitudinal joints spaced 50 feet apart are included in the design. The combination of artificial cooling with division of the mass into small blocks is expected to lead to the same comparative freedom from cracks that was obtained in Boulder Dam. Following cooling, all contraction joints
will be grouted. Due to the necessity of providing for a flow of water over the dam during the construction period, 6 rows of shafts spaced 50 feet apart in the upstreamdownstream direction, instead of slots as at Boulder Dam, are provided for exposure of the inlet and outlet ends of the embedded coils. The cooling pipe headers are carried through the horizontal longitudinal galleries near the upsteam face of the dam at 50-foot elevations, branch into horizontal transverse galleries at the sections containing the shafts, and branch again up the 50 feet of shaft to the gallery above. Manifold connections join the inlet and outlet ends of the cooling coils with the headers in the shaft.
The cooling medium used at Grand Coulee Dam is the river water, which is suitably clear and which, for about 5 months of the year, is expected to be at a temperature less than 45° F. The two pumping plants which supply the headers are located on barges upstream from the dam. The water, after passing through the embedded coils, is wasted to the river.
Cooling was started in the west portion of Grand Coulee Dam on December 10, 1936, and has progressed satisfactorily to date.
Helch Hetchy Dam. Hetch Hetchy Dam of the water supply and power development of the city of San Francisco is being raised 85 feet and the section enlarged by an addition on the downstream face.
In order to effect the complete shrinkage of the addition before the new concrete is bonded to the old concrete and before the contraction joints are grouted, and to assure proper action of the addition under load, artificial cooling is provided (see ref. 15). Loops of 1-inch tubing, similar to the installation in Boulder Dam, are embedded in the new concrete with the inlet and outlet ends carried to the downstream face where connection is made to the headers. Cooling is accomplished in two stages: (1) by circulating reservoir water which has a temperature of about 54° F. and (2) by completing the cooling with refrigerated water. Separate headers for the two circuits are provided. The refrigerated water is pumped through a closed circuit from the plant through the coils and return. Reservoir water is pumped through the coils and the return water wasted into the river.
The work is being carried out by the Hetch Hetchy Water Supply Department of the Public Utilities Commission of San Francisco. Designs were prepared under the direction of L. T. McAfee, chief engineer and manager. Plans and specifications were reviewed by the Bureau of Reclamation.
Parker and Seminoe Dams. The plans for Parker and Seminoe Dams, now under construction, include the installation of cooling pipes. While the section of Seminoe Dam, a thin arch dam, is not so large but that natural cooling would reduce the temperatures to mean annual air temperature within a reasonable length of time, artificial cooling is provided to lower the maximum temperature, to reduce the temperature gradients from the interior to the exposed surfaces, and to lower the temperatures to the desired values so that the joints can be grouted at the most advantageous time.
The plans for cooling Parker Dam provide that the dam shall be cooled to 54° F. before the contraction joints are grouted. With the mean annual air temperature about 72° F., this would obviously be impossible without artificial cooling. Here, as in Seminoe Dam, the cooling system was so designed that cooling may be started as soon as the concrete of each lift is placed.
(1) Fredrik Vogt. Analyses of Rise in Temperature During Hardening of Concrete. 1933; obtainable from F. Bruns Bokhandel, Trondhjem, Norway.
(2) R. E. Glover. Flow of Heat in Dams. Journal of the American Concrete Institute, proceedings vol. 31, Nov.-Dec. 1934, pp. 113-124.
(3) J. L. Savage. Special Cements for Mass Concrete.
1936, U. S. Bureau of Reclamation, prepared for consideration of Second Congress of the International Commission on Large Dams, World Power Conference, pp. 105-120, obtainable from the U. S. Bureau of Reclamation, Denver, Colo., and Washington, D. C.
(4) R. F. Blanks. Boulder Dam Cement and Concrete Studies. Engineering News-Record, Nov. 22, 1934, pp. 648-651.
(5) Specifications and Plans Available for Work at Hoover Dam. New Reclamation Era, February 1931,
(6) Cooling System to Extract Heat from Concrete in Hoover Dam. Southwest Builder and Contractor, July 7, 1933, pp. 18-20.
(7) Lawrence P. Sowles. Construction of the Hoover Dam, How the Concrete is Being Cooled as it is Poured. Compressed Air Magazine, Nov. 1933, pp. 4265-4271.
(8) H. N. Royden, Refrigeration at Hoover Dam. Ice and Refrigeration, February 1934, pp. 85-88.
(9) Refrigerator to Cool Boulder Dam Concrete. Scientific American, April 1934, pp. 196-197.
(10) C. H. Vivian. Cooling the Concrete in Boulder Dam. The Military Engineer, May-June 1934, pp. 195-198.
(11) H. N. Royden and A. G. Roach. Cooling Concrete at Boulder Dam. Refrigerating Engineering, July 1934, pp. 11-13.
(12) Byram W. Steele. Cooling Boulder Dam Concrete. Engineering News-Record, Oct. 11,1934, pp. 451-455.
(13) Wesley R. Nelson. Boulder Dam Nears Completion. The Reclamation Era, March 1935, pp. 49-51.
(14) N. H. Wilson. Madden Dam Concrete Temperatures. The Military Engineer, January-February
1937, pp. 59-61.
(15) Raising O'Shaughnessy Dam 85 Feet. Western Construction News, December 1936, pp. 377-381.
DESIGN AND CONSTRUCTION OF SMALL EARTH DAMS
BY F. F. SMITH, ENGINEER, BUREAU OF RECLAMATION
INTEREST in the construction of small earth-fill, concrete, and rock masonry dams has increased throughout the Western States because of the necessity for closer control of irrigation and municipal water and for purposes of flood control and soil conservation.
Since the majority of sites chosen for the construction of impounding dams are better adapted to the earth-fill type of structure, this article will deal exclusively with earth dams. The presence of adequate foundation rock for dams of the masonry and concrete types occurs principally in the mountain areas where the stream beds are steep and afford little impounding capacity. Such sites are generally too expensive to store water for irrigation uses. They are utilized more often for municipal water supplies and for the generation of electricity. In the foothills and plains regions, abutments suitable for concrete dams are seldom found on account of the poor quality of rock that prevails or the impossibility of adapting the concrete type of structure to the profile at the dam site.
Numerous failures of small or medium height earth dams have occurred because the constructors did not recognize the fact that the same principals of engineering design and construction should be applied to small dams as are applied to the large earth dams. Large dams are usually built by the more experienced constructors with means for adequate investigation and proper design studies.
The small earth dam lends itself to the choice of the promoter with a limited amount of funds, since the costs for materials are low and labor generally cheap and plentiful. This fact is recognized by many Government agencies now constructing small earth dams for various purposes where the provision of employment is a major consideration.
There is a general misconception by those desiring the construction of small earth dams that no special skill or engineering ability is required in such construction. This misconception is based on the belief that the art is purely empirical and that engineering science provides no exact methods of computing stability and probable leakage through the dam section and foundation. Among engineers charged with the responsibility for the safety of large earth dams, it is appreciated that the outworn empirical methods have given way to thorough preconstruction investigations, careful theoretical design, and construction on known and
definite principles of soil mechanics. The proper construction of high earth-fill dams is known today to involve as much theory and to require as close an application of engineering skill in construction as does the concrete dam where the strength of the materials can be more definitely tested and determined.
Many small earth-fill dams are being constructed where failure would not cause serious flood effects. However, when such dams are built high enough to cause flows in excess of the normal high flow of the stream in event of failure, then sound engineering principles should be applied in their investigation, design, and construction. Loss of property on the lower stream and the investment in the dam, if failure occurs, will exceed many times the cost of proper engineering and the slightly greater cost of proper construction, even on small and apparently inconsequential dams.
Preliminary investigations should be conducted for all proposed dams to ascertain whether it is probable that a safe structure may be built at the selected site and to furnish adequate data for design.
The investigations should, in general, include test pits or borings along the line of the proposed cut-off trench to determine the character of the underlying materials and bedrock profile, if bedrock is present. This will indicate whether the removal of material should be extended to bedrock; or, in the absence of bedrock, the probable depth to which the cut-off should be excavated in order to shut off excessive underflow or seepage. Several pits should be excavated into the foundation material along the downstream toe and elsewhere over the foundation and abutments to determine the desirability of toe drainage and the probability of plastic movement of the material under the weight of the embankment.
The sources of all required construction materials, such as earth for the embankment, rock for riprap, and aggregates for concrete, should be located and tested to determine their suitability. Open shafts are preferable to wash borings for the examination of foundation materials provided the presence of ground water does not make these too expensive. Stratigraphic logs should be prepared for all test pits. These should show the exact locations of the various materials and the presence and locations of cobbles and boulders. Mechanical analyses and percolation tests should be made for foundation and abutment materials to determine their adequacy or the necessity for their removal.
Preliminary tests, consisting of mechanical analyses, percolation, and density at standard compaction, should be run on all available borrow pit materials to determine their suitability and to permit intelligent selection of the most desirable material from the various short-haul areas.
The investigations on important dams should include an examination by a competent geologist whose interpretations of the testing results and collaboration in deciding on the adequacy of the foundation are essential.
The adequacy of the foundation materials to provide against shearing, settlement, and excessive percolation which may cause piping at the downstream toe, is of first importance. The weak points in earth-dam construction are generally found in the foundation and at the contacts of the natural ground surface and the placed embankment. Many of the former difficulties with the construction of the embankment proper have been removed by proper laboratory analysis of the embankment material and by improved methods of compaction and mqisture control. Preliminary testing will give some idea of the amount of undesirable material to strip from the foundation area; but this operation should be carefully watched during construction to assure the removal of all material containing vegetable matter and of all material that will be rendered unstable by saturation. Test pits for further explorations should be put down during construction if any doubt exists about the presence of unstable or otherwise unsuitable material.
The character of materials in the foundation will dictate the design for the foundation cut-offs. A concrete diaphragm extending from bedrock to the crest of the dam is not recommended. Where bedrock is present, concrete walls, bonded 2 to 3 feet into rock and projecting 5 to 10 feet into the fill, will suffice. The number of walls thus constructed, will vary from one to three, depending on geologic conditions and the maximum depth of the water. Where bedrock is not within economical reach, open cut-off trenches should be used. These will vary in bottom width and depth, depending on the reservoir water depth and porosity of the material. Ordinarily, trenches about onethird of the water depth are adequate in most of the western river bed alluviums. Where earth dams are founded on bedrock with comparatively thin layers of gravel overlying, at least three-fourths of the area should be stripped to bedrock to prevent a concentrated flow of the seepage water at high velocity through the gravel. Such dams should have
a considerable length of impervious material bonded carefully to clean bedrock.
Dams founded on rather porous and deep alluvium should have a central clay core brought up through the dam from the cut-off trench; also a long percolation distance through the base with an adequate rock-fill blanket extending downstream from the toe of the earth-fill section. Where cut-off trenches are in sand and gravel, with voids filled with fine sand and silt, it is generally preferable not to unwater the trench but to puddle a good grade of watertight clay back into the trench. Unwatering tends to wash out all the fines and render the foundation more porous.
Presence of river silt, sandy clay, or fine sand demands careful consideration in foundation design. Such materials should ordinarily be entirely stripped to gravel. This is generally expensive and may greatly affect the economic feasibility of the dam. This item very often is grossly underestimated in preparing the preliminary estimates. A decision to leave such material in place can only be made after careful settlement, consolidation, and percolation tests have been conducted. If it is feasible entirely to drain the foundation material before and during the placement of embankment, about 60 percent of the final settlement can be attained by the time the embankment has been raised to a height of about 20 feet. Special attention must be given to the abutment contact and the existing overburden and the composition of the underlying rock should be carefully studied. Porous and unstable material must be removed, sometimes to considerable depth. When the cut-off trenches can be extended to rock, a concrete cut-off should be constructed, bonded into sound bedrock and the bedrock grouted along the line of the cut-off to form a continuous grout curtain. The depth of grout holes will depend upon the nature of the bedrock formation and the head of water to be brought against the abutment.
Abutments in earth material, such as cut banks, should be sloped not steeper than 1^:1, in order that effective bonding may be accomplished by the roller. All rock overhangs must be removed and the resultant slope brought to %: 1 if feasible. Seepage along rock abutments can be prevented by one or more well-founded concrete cut-off walls. Dowel bars of large diameter well bonded into the rock are often used to tie the wall to the abutment rock.
Under the latest methods of testing and earth placing control, a fair prediction can be made of the saturation slope through the embankment. Checks on these assumptions may be made by the electrical analogy and model test methods. The saturation slope should be designed so as to fall within the downstream toe with only a moderate amount of seepage.
Hydrostatic pressure cells are now being installed in all Bureau of Reclamation earth dams to indicate any approach