Burr Bassell

“ One says, for instance, that a
rock is water-tight when non-absorbent of water, but that a soil is not water-tight unless it will absorb an enormous quantity of water.
This would seem to indicate that super-saturation and not pressure is necessary to increase the water-tightness of earth materials.
Again, in a recent discussion regarding the saturation and percolation of water through the lower half of a reservoir embankment, it was remarked, that
the more compact the material of which the bank is built, the steeper will be the slope of saturation. ”

“ No comments will be made upon these figures, further than to remark that the successful building of a stable dam, accomplished by the use of an excessive quantity of materials and at a cost beyond reasonable requirements, is mainly instructive as illustrating “how not to do it.” It is creditable to execute substantial works at a reasonable cost, but it reflects no credit upon any one to construct them regardless of expense. ”

“ To merely drive down iron rods feeling for so-called bed rock, or to make only a few bore-holes with an earth auger should in no instance be considered sufficient. Borings may be found necessary at considerable depths below the surface and in certain classes of material, but dug pits or shafts should always be resorted to for moderate depths and whenever practicable. Only by such means may the true character of the strata underlying the surface, and the nature and condition of the bed rock, if it be reached, become known. ”

“ The type of dam which should be built at any particular locality involves a thorough knowledge, not alone of the catchment area and reservoir basin, but also accurate information regarding the geology of the dam site itself. It would be very unwise to decide definitely upon any particular type of dam without first obtaining such information. Too frequently has this been done, causing great trouble and expense, if not resulting in a total failure of the dam.
The conditions favorable for an earth dam are usually unfavorable for a masonry structure, and vice versa. ”

“ Since the strength of an earth dam depends upon two factors, namely, its weight and frictional resistance to sliding, the effect of water upon different materials entering into an earth structure should be most carefully considered. This will therefore occupy a large place in these pages. An earth embankment founded upon rock may become saturated by water forced up into it from below through cracks and fissures, reducing its lower [Pg 13] stratum to a state of muddy sludge, on which the upper part, however sound in itself, would slide. ”

“ Mr. W. W. Follett, who is reported to have had considerable experience in building earth dams, and who has made some valuable suggestions thereupon, is emphatic in saying,
that the junction of earth and masonry forms a weak point, that either a puddle or masonry core in an earthen dam is an element of weakness rather than strength.
He also thinks the usual manner of segregating and depositing materials [Pg 45] different in density and weight, and thus subject to different amounts of settlement, as bad a form of construction as could be devised. ”

“ Dams situated upon the main drainage lines of large catchment areas are usually built of stone or concrete masonry, and designed with large sluiceways and spillways for the discharge both of silt and flood waters. It need scarcely be remarked that, as a rule, such sites are wholly unsuited to earthwork construction. It is said, however, that “every rule has at least one exception,” and this may be true of those relating to dam sites, as will appear later under the head of new types. ”

“ Puddle Trench.
There is a diversity of opinion among engineers in regard to the proper place for the puddle in dam construction. Theoretically, the inner face would be preferable to the center, for the purpose of preventing any water from penetrating the embankment. It is well known that all materials immersed in water lose weight in proportion to the volume of water they displace. If the upper half of the dam becomes saturated it must neccesarily lose both weight and stability. Its full cohesive strength can only be maintained by making it impervious in some way. ”

“ The effective head at any point of an earth dam has been defined as the difference in the elevation of the high-water surface in the reservoir and that of the intersection of the down-stream slope with the natural or restored surface of the ground below the dam. ”

“ Many persons who should know better place little importance upon the skill required for the construction of earthwork embankments, considering the work to involve no scientific problems. It is far too common belief that any ordinary laborer, who may be able to use skillfully a scraper on a country road, is fitted to superintend the construction of an earth dam. ”

“ All reservoirs formed by earth embankments must have at least two of these, namely a wasteway, which is its safety valve, and outlet pipes or outlet tunnel.
It may be stated that the proper location and construction of the outlet for a reservoir are of vital importance, since either to improper location or faulty construction may be traced most of the failures of the past. It is almost impossible to prevent water under high pressure from following along pipes and culverts when placed in an earth dam. The pipes and culverts frequently leak, and failure ensues. ”

“ Again we repeat, that consolidation is the most vitally important operation connected with the building of an earth dam. When this is satisfactorily attained it is proof that the materials are suitable and that the other necessary details have been in a large measure complied with. Light rollers are worse than useless, being a positive harm, resulting in a smoothing or “ironing process,” deceptive in appearance and detrimental in many ways. ”

“ When the depth to a suitable foundation is great, a central trench excavated with sloping sides, extending to bed rock or other impervious formation, refilled with good puddling material, properly compacted, will suffice.
When clayey earth is scarce and expensive to obtain, a small amount of clay puddle confined between walls of brick, stone or concrete masonry, and extending well into the body of the embankment and so built as to avoid settlement, will prevent excessive seepage. This form of construction is not to be carried much above the original surface of the ground. ”

“ An investigation of the location and the physical features of the dam site should include a careful and scientific examination of the materials in the vicinity, to determine their suitability for use in construction. An earth embankment cannot be built without earth, and an earth dam cannot be built with safety without the right kind of earth material. ”

“ Clay materials may be very deceptive in some of their physical properties, being hard to pick under certain conditions, and yet when exposed to air and water will rapidly disintegrate. Beds of clay, marl and very fine sand are liable to slip when saturated, becoming semi-fluid in their nature, and will run like cream. ”

“ After describing the New Croton Dam, the board of experts preface their remarks on the earth embankment by saying that
it has been abundantly proven that up to a height of 60 or 70 ft. an embankment founded on solid material and constructed of well-selected earth, properly put in place, is fully as durable and safe as a masonry wall and far less costly. ”

“ Low dams forming service reservoirs for domestic water supplies and for irrigation comprise by far the most numerous class. They are not designed to impound a large volume of water and therefore may be built across a small ravine or depression, or even upon the summit of a hill, by excavating the reservoir basin and using the material excavated to form the embankment. ”

“ The saving in cost is to be accomplished by the use of inferior materials and less care in the handling of them, or by both. It is suggested that almost any kind of material available, rock, sand or gravel, will answer every purpose where good earth is not to be found. Further, that this material may be dumped from the carts, cars or cableways, or be placed by the hydraulic-fill method. ”

“ EXPERIMENTAL DIKES AND CYLINDER EMPLOYED IN STUDIES FOR THE NORTH DIKE OF THE WACHUSETT RESERVOIR; AND (FIG. 25) CROSS-SECTION OF THE DIKE.
[Pg 50] The difference in the coefficient of friction of the same material when dry and when wet greatly modifies the form of slope. The harder and looser the particles, the straighter will be the slope line in excavation and slips. The greater the cohesion of the earth, the more curved will be the slope, assuming a parabolic curve near the top–the true form of equilibrium. ”

“ Dam Site.
The third preliminary study (that relating to the dam site itself) will be considered under three heads:
1. Location.
2. Physical features, materials, etc.
3. Foundation.
LOCATION.–The location for a dam is generally determined by the use which is to be made of it, or by the natural advantages for storage which it may possess. If it be for water power it is very frequently located upon the main stream at the point of greatest declivity. If for storage it may be, as we have seen, at the head of a river system, on one of its tributaries, or in a valley lower down. ”

“ In many parts of the arid region of the United States, the best storage-sites are situated in the upper or higher portions of the drainage systems. This is especially true of the streams on the Pacific Slope having their source in the High Sierras. As regulators of stream-flow and for power purposes such storage is peculiarly valuable, while storage for irrigation and domestic uses may be located nearer the valleys and the centers of population.
Frequently the engineer is required to build his dam where no such data are available. ”

“ Vegetable Humus is not the end of decomposition of organic matter, but an intermediate state of transformation. Decay is a process almost identical with combustion, where the products are the same, and the end is the formation of water and carbonic acid, with a residue of mineral ash. ”

“ The hydraulic laboratory of Cornell University also made tests of the permeability of several samples of materials taken from pits. All the materials examined were found to be permeable and when exposed to water to disintegrate and assume a flat slope, the surface of which was described as “slimy.”
Pipe wells were driven at different places into the dams and the line of saturation was determined by noting the elevations at which the water stood in them. In all the dams the entire bank on the water side of the core wall appeared to be completely saturated. ”

“ The form may be made to suit the configuration of the dam site. When the earthwork requires it, they may be lined with various materials to secure water-tightness. Often such dams are made composite in character, partly of earth and partly of masonry or some other material. They are also frequently accompanied by numerous accessories, such as settling-basins, aerating devices and covers, which present a diversity in form and appearance. ”

“ An abundance of water delivered by gravity under high pressure is usually regarded as one of the essential factors in hydraulic-fill dam building, but it is not essential that there be a large continuous flow. The Lake Frances Dam, recently constructed for the Bay Counties Co., of California, by J. D. Schuyler, is 75 ft. high, 1,340 ft. long on top, and contains 280,000 cu. yds. The dam was built up by materials sluiced by water that was forced by a centrifugal pump through a 12-in. ”

“ This agent is the great destroyer of the important forces, cohesion and friction. Cohesion is a force uniting particles of matter and resists their separation when the motion attempted is perpendicular to the plane of contact. Friction is a force resisting the separation of surfaces when motion is attempted which produces sliding. The hydrostatic pressure and resultant effect upon submerged surfaces need to be kept constantly in mind. When the surface is impermeable the line of pressure is normal to its plane, but when once saturated there are also horizontal and vertical lines of pressure. ”

“ The cohesive and frictional resistances of clays becoming thus very much reduced when charged with water, a too liberal use of this material is to be deprecated. The ultimate particles forming clays, viewed under the microscope, are seen to be flat and scale-like, while those of sands are more cubical and spherical. ”

“ It is in the application of these principles that the services of the professional man becomes valuable, and it is from a lack of them, that there have been so many failures.
The details and principles of construction, relating to high earth dams, may be summarized or stated in order of their application, as follows:
(1) Select a firm, dry, impermeable foundation, or make it so by excavation and drainage. All alluvial soil containing organic matter and all porous materials should be excavated and removed from the dam site when practicable ”

“ By effective size of sand grains is meant such size of grain that 10% by weight of the particles are smaller, and 90% larger than itself; or, to express it a little differently, the effective size is equal to a sphere the volume of which is greater than 1/10 that forming the weight and is less than 9/10 that forming the weight. ”

“ It is unwise to place much reliance upon an estimated flow where this is the case. The formula may be used with confidence where the layers are artificially made, and where there is no uncertainty regarding the uniform character of the material. In most natural channels there are distinct lines of flow, and under considerable hydrostatic head or pressure these lines of flow would surely enlarge. There is a wide difference between permissible and dangerously excessive percolation through an earth [Pg 58] embankment. ”