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.

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. Again, there may be local conditions requiring some entirely different type.

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.

In a general way, the location of high earth dams is governed by the configuration of the ground forming the storage basin. It may not be possible, however, to decide upon the best available site without careful preliminary surveys and examinations of the geological formation.

All earth dams must be provided with a wasteway, ample to discharge the maximum flood tributary to the reservoir. Whatever type of wasteway be adopted, no reliance should ever be put upon the outlet pipes for this purpose. The outlet should only figure as a factor of safety for the wasteway, insuring, as it were, the accuracy of the estimated flood discharge. The safety of the dam demands that ample provision be made for a volume of water in excess of normal flood discharge. This most necessary adjunct of earth dams may be an open channel, cut through the rim of the reservoir basin, discharging into a side ravine which enters the main drainage way some distance below the dam. It may be necessary and possible to pierce the rim by means of a tunnel where its length would not prohibit such a design. Lastly, there may be no other alternative than the construction of an overfall spillway, at one or both ends of the embankment. This last method is the least desirable of any and should be resorted to only when the others are impracticable; even then, the volume of water, local topography, geology, and constructive materials at hand must be favorable to such a design. If they are not favorable it may be asked, “what then?” Simply do not attempt to build an earth dam at this site.

PHYSICAL FEATURES, MATERIALS, ETC.–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.

Test pits judiciously distributed and situated at different elevations will indicate whether there is a sufficient amount of suitable material within a reasonable distance of the dam. The type of earth dam best suited for any particular locality, and its estimated cost, are thus seen to depend upon the data and information obtained by these preliminary studies. Economical construction requires the use of improved machinery and modern methods of handling materials, but far more important even than these are the details of construction.

FOUNDATION.–We may now assume that our preliminary studies relating to the location and physical features of the dam site are satisfactory. We must next investigate the foundation upon which the dam is to be built. This investigation is sometimes wholly neglected or else done in such a way as to be practically useless. 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. If a satisfactory stratum of impermeable material be found it is necessary also to learn both its thickness and extent. It may prove to be only a “pocket” of limited volume, or if found to extend entirely across the depression lengthwise of the dam site it may “pinch out” on lines transversely above or below. Shafts and borings made in the reservoir basin and below the dam site will determine its extent in this direction, knowledge of which is very important.

[Fig. 1], showing a longitudinal section of the site of the Yarrow Dam of the Liverpool Water-Works, England, illustrates the necessity of such investigation. A bore hole at station 2 + 00 met a large boulder which at first was erroneously thought to be bed rock. The hole at station 3 + 50 met a stratum of clay which proved to be only a pocket.

The relative elevation of the different strata and of the bed rock formation, referred to one common datum, should always be determined. These elevations will indicate both the dip and strike of the rock formation and are necessary for estimating the quantities of material to be excavated and removed, including estimates of cost. They furnish information of value in determining the rate of percolation or filtration through the different classes of material and the amount of probable seepage, as will appear later. The cost of excavating, draining and preparing the floor or foundation for a dam is often very great, amounting to 20 or 30% of the total cost.

[Fig. 2] is a transverse section of the Yarrow Dam. This particular dam has been selected as fairly representative of English practice and of typical design. It is one of the most widely known earth dams in existence.

FIG. 1.–LONGITUDINAL SECTION OF YARROW DAM SITE.

FIG. 2.–CROSS-SECTION OF YARROW DAM.

At the Yarrow dam site it was necessary to go 97 ft. below the original surface to obtain a satisfactory formation or one that was impermeable. A central trench was excavated to bed rock, parallel to the axis of the dam, and filled with clay puddle to form a water-tight connection with the rock, and prevent the water in the reservoir from passing through the porous materials under the body of the embankment. This interesting dam will be more fully described later, when the different types of earth dams are discussed.

CHAPTER III.
Outline Study of Soils.
Puddle.

The following study of soils is merely suggestive and is here given to emphasize the importance of the subject, at the risk of being considered a digression. Soil formations are made in one of three ways:

1. By decomposition of exposed rocks.

2. By transportation or sedimentation of fine and coarse materials worn from rocks.

3. By transformation into humus of decayed organic matter.

The transforming agencies by which soils succeed rocks in geological progression have been classified as follows:

Heat and its counter agent frost are the most powerful forces in nature, their sensible physical effects being the expansion and contraction of matter.

Water has two modes of action, physical and chemical. 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. 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 stratum to a state of muddy sludge, on which the upper part, however sound in itself, would slide. The best preliminary step to take in such a case is to intersect the whole site with wide, dry, stone drains, their depths varying according to the nature of the ground or rock.

Air contains two ingredients ever active in the process of decomposition, carbonic acid and oxygen.

Organic Life accomplishes its decomposing effect both by physical and chemical means. The effect of organic matter upon the mineral ingredients of the soil may be stated as follows:

1. By their hydroscopic properties they keep the soil moist.

2. Their decomposition yields carbonic acid gas.

3. The acids produced disintegrate the mineral constituents, reducing insoluble matter to soluble plant food.

4. Nitric acid results in nitrates, which are the most valuable form of nutritive nitrogen, while ammonia and the other salts that are formed are themselves direct food for plants.

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 conditions essential to organic decomposition are also those most favorable to combustion or oxidation, being (1) access of air, (2) presence of moisture, and (3) application of heat.

Now the coöperation of these chemical and physical forces, which are ever active, is called “weathering.” Slate rock, for instance, weathers to clay, being impregnated with particles of mica, quartz, chlorite and hornblend. Shales also weather to clay, resulting often in a type of earth which is little more than silicate of aluminum with iron oxide and sand.

In the vicinity of the Tabeaud Dam, recently built under the personal supervision of the author, the construction of which will be described later, there is to be found a species of potash mica, which in decomposing yields a yellow clay (being ochre-colored from the presence of iron), mixed with particles of undecomposed mica. This material is subject to expansion, and by reason of its lack of grit and its unctuous character it was rejected or used very sparingly. Analysis of this material gave, Silica, 54.1 to 59.5%; potash, 1.5 to 2.3%; soda, 2.7 to 3.7%.

Soil analysis may be either mechanical or chemical. For purposes of earthwork, we are most interested in the former, having to deal with the physical properties of matter. Chemical analysis, however, will often afford information of great value regarding certain materials entering into the construction of earth dams. The most important physical properties are:

There are two distinct methods of mechanical analysis: (1) Granulating with sieves, having round holes. (2) Elutriating with water, the process being known as silt analysis.

It would require a large volume to present the subject of soil analysis in any way commensurate with its importance. Experiments bearing upon the subjects of imbibition, permeability, capillarity, absorption and evaporation, of different earth materials, are equally interesting and important.[1]

The permeability of soils will be discussed incidentally in connection with certain infiltration experiments to be given later.