Chapter VI: Conclusions

The writer in concluding this study wishes to emphasize certain principles and apparently minor details of construction, which from observation and personal experience, seem to him of vital importance.

He believes firmly in the truth contained in the following remarks by Mr. Desmond FitzGerald, of Boston, germane to this subject:

An engineer must be guided by local conditions and the resources at
his command in building reservoir embankments. His design must be
largely affected by the nature of the materials. There are certain
_general principles_, however, which must be observed and which
will be applied by an engineer of skill, judgment and experience to
whatever design he may adopt. 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; that is, where the depth to a suitable impermeable foundation is not prohibitive by reason of excessive cost.

Wherever springs of water appear, they must be carried outside the lines of the embankment by means of bed rock drains, or a system of pipes so laid and embedded as to be permanent and effective.

The drainage system must be so designed as to prevent the infiltration of water upward and into the lower half of the embankment, and at the same time insure free and speedy outlet for any seepage water passing the upper half. All drains should be placed upon bed rock or in the natural formation selected for the foundation of the superstructure. They should be constructed in such a manner as to prevent the flow of water outside the channel provided for it, and also prevent any enlargement of the channel itself. To this end, cement, mortar, broken stone, and good gravel puddle are the materials best suited for this purpose.

(2) Unite the body of the embankment to the natural foundation by means of an impervious material, durable and yet sufficiently elastic to bond the two together. 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.

(3) The continuity of surfaces should always be broken, at the same time avoiding the formation of cavities and lines of cleavage. No excavation to be refilled should have vertical sides, and long continuous horizontal planes should be intercepted by wedge-shaped offsets, enabling the dovetailing of materials together.

All loose and seamy rock or other porous material should be removed, and where the refill is not the best for the purpose, mix the good and bad ingredients thoroughly, after which deposit in very thin layers.

(4) Make the dimensions and profile of dam with a factor of safety against sliding of not less than ten. The preliminary calculations for designing such a profile have been given on p. 42.

(5) Aim at as nearly a homogeneous mass in the body of the embankment as possible, thus avoiding unequal settlement and deformation. This manner of manipulating materials will eliminate many uncertain or unknown factors, but it means rigid inspection of the work and intelligent segregation of materials, no matter what method of transporting them may be adopted. The smaller the unit loads may be, the more easily a homogeneous distribution of materials will be obtained.

(6) Select earthy materials in preference to organic soils, with a view of such combination or proportion of different materials as will readily consolidate. _Consolidation is the most important process connected with the building of an earth dam._ The judicious use of soil containing a small percentage of organic matter may be permitted, however, when there is a lack of clayey material for mixing with sandy and porous earth materials. Such a mixture, properly distributed and wetted, will consolidate well under heavy pressure and prove quite satisfactory.

(7) Consolidation being the most important process and the only safeguard against permeability and instability of form, use only the amount of water necessary to attain this. Too much or too little are equally bad and to be avoided. It is believed that only by experiment and experience is it possible to determine just the proper quantity of water to use with the different classes of materials and their varying conditions. In rolling and consolidating the bank, all portions that have a tendency to quake must be removed at once and replaced with material that will consolidate; it _must not_ be covered up, no matter how small the area.

(8) In an artificial embankment for impounding water it is impracticable to place reliance upon time for consolidation; it _must_ be effected by mechanical means. 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.

The matter of supreme importance in the construction of earth dams is that the greatest consolidation possible be specified and effected. To this end it is necessary that heavy rollers be employed, and that such materials be selected as respond best to the treatment. There are certain kinds of earth materials which no amount of wetting and rolling will compact. These must be rejected as unfit for use in any portion of an earth dam. Let the design of the structure be ever so true to correct engineering principles, it is still necessary to give untiring attention to the work of consolidation. It is therefore according to the design of a thoroughly compacted homogeneous mass, rather than to the suggested _diaphragm type_, to which modern practice should conform. This is in harmony with Nature’s own methods, and in conformity to correct principles.

(9) Avoid placing pipes or culverts through any portion of the embankment. The writer considers it bad practice ever to place the outlet pipes through a high earth dam, and fails to see any necessity for so doing.

(10) The surface of the dam, both front and rear, must be suitably protected against the deteriorating effects of the elements. This may include pitching the up-stream face, the riprap work at the toe of the inner slope, the roadway and covering of the crown, the sodding or other protection of the rear slope, and the construction of surface drains for the berms.

(11) Ample provision for automatic wasteways should be made for every dam, so that the embankment can never under any circumstances be over-topped by the impounded water. Earthquakes and seismic disturbances will produce no disastrous effects upon an earth dam. Its elasticity will resist the shock of water lashing backwards and forwards in the reservoir.

(12) Finally, provide for intelligent and honest supervision during construction, and insist upon proper care and maintenance ever afterwards.