Druid Lake Dam, Baltimore, Md.

Another very interesting and instructive example of high earth dam construction is that of the Druid Lake Reservoir embankment, Baltimore, Md.

This dam was built under the supervision of Mr. Robt. K. Martin. Construction was begun in 1864, and the dam was finished in 1870. Mr. Alfred M. Quick, present chief engineer of the water-works of the City of Baltimore has given a very lucid description of this work in Engineering News of Feb. 20, 1902.

[Fig. 26] is a cross-section of this dam, showing the method of construction so clearly as to scarcely need further description. The banks D-D on either side of the central puddle wall were carried up in 6-in. layers with horses and carts, and kept about 2 ft. higher than the puddle trench, which always contained water. The banks E-E were made of dumped material, after which the basins F-F were first filled with water and finally filled by dumping material into the water from tracks being moved in toward the center.

FIG. 26–WORKING CROSS-SECTION OF DRUID LAKE DAM.

After reaching the top of this fill, banks B-B-B were built up in layers similar to D-D. The second set of basins C-C were then filled in a manner similar to F-F. The remaining portion A-A was constructed in layers like D-D and B-B, with the addition of compacting each layer with a heavy roller.

Finally the inner face slope was carried up in 3-in. layers and thoroughly rolled, after which 2 ft. of “good puddle” was put upon the inner slope the latter was rip-rapped, the crown covered with gravel and the rear slope sodded.

Some years after completion, a driveway was built along the outer slope, as shown, which had a tendency to strengthen the dam, though not designed expressly for that purpose.

It is of interest to know that the influent, effluent and drain pipes were originally constructed through or under the embankment. These pipes were laid upon solid earth, and where they passed through the puddle wall were supported upon stone piers 6 ft. apart. As might be expected, they soon cracked badly and were finally abandoned, new ones being placed in the original ground at the south side of the lake. Mr. Quick states that so far as is known there has never been any evidence of a leak through the embankment during these 32 years of service.

New Types of Dams;
Bohio, Panama Canal.

A brief description will now be given of three different dams designed for Bohio, on the proposed Panama Canal. Mr. George S. Morison’s paper before the American Society of Civil Engineers, on “The Bohio Dam,” and the discussion thereon, especially that by Mr. F. P. Stearns, were quite fully reported in Engineering News for March 13 and May 8, 1902. In constructing the Panama Canal it will be necessary to impound the waters of the Chagres River, near Bohio, to maintain the summit level of this canal and supply water for lockage.

THE FRENCH DESIGN.–[Fig. 27] is an enlarged section of the original design of the new French Co. This design has no core wall, but at the up-stream toe a concrete wall was to be built across the river between the two lines of sheet-piling. At the down-stream toe a large amount of riprap was to be placed to prevent destruction of the dam during construction. In this case it would be necessary to construct a temporary dam above and also to use the excavation for the locks as a flood spillway. This method would involve considerable risk to the work, on account of the large volume of flood waters it might be necessary to take care of during construction.

ISTHMIAN CANAL COMMISSION.–The dam proposed by the Isthmian Canal Commission is shown by [Fig. 28]. This was designed to be an absolutely water-tight closure of the geological valley, by using a masonry core wall carried down to bed rock. The maximum depth being 129 ft., it was planned to rest the concrete wall on a series of pneumatic caissons reaching to rock. The spaces between the caissons would be closed and made water-tight. Both slopes of the earth embankment were to have horizontal benches and be revetted with loose rock.

MR. MORISON’S DESIGN.–To appreciate fully the object and aim of the third design, [Fig. 29], which may be called a new type, although similar in many respects to the North Dike of the Wachusett reservoir already illustrated and described, it should be stated that the equalized flow of the Chagres River is put at 1,000 cu. ft. per sec. Of this quantity it is estimated that 500 cu. ft. would be needed for lockage and 200 cu. ft. for evaporation. This leaves 300 cu. ft. per sec. available for seepage and other losses or to be wasted.

FIGS. 27 TO 29.–DESIGNS FOR THE BOHIO DAM,
PANAMA CANAL.

It will thus be seen that a scarcity of water is not in this instance a condition demanding an absolutely water-tight dam. The amount of seepage permissible without endangering the stability of the structure is the real point now to be discussed.

The third design, which was proposed by Mr. Morison, is shown by [Fig. 29]. The topography and configuration of this dam site is not unlike that of the San Leandro Dam, California, soon to be described, while the general design is similar, as has been remarked, to the North Dike of the Wachusett Reservoir.

This third design contemplates a compound structure, formed by two rock-fill dams situated about 2,120 ft. apart, with the intervening space filled with loose rock, earth and other available material. Immediately below the upper and higher rock-fill dam, it is proposed to place across the canyon a puddle wall 50 ft. in width, resting over two lines of sheet-piling 30 ft. apart. This piling would probably not reach farther than 50 ft. below tidewater, the solid rock floor being about 100 ft. deeper.

Mr. Morison made use of Mr. Hazen’s filtration formula for estimating the rate and quantity of seepage through the permeable strata below the dam. This formula is:

ht + 10°
V =cd²———
l60

where

This formula should be used only when the effective sizes of sands are from 0.10 to 3.0 mm. and with uniformity coefficients below 5.0[5].

Mr. Morison used the following values:

for the solution of this problem, and obtained a velocity of 0.002 ft. per sec. The bed of sand and gravel was assumed to have a sectional area of 20,000 sq. ft. for 2,500 ft. in length. This gives a seepage of 40 cu. ft. per sec.

It is believed that the above rate of 0.002 ft. per sec., equivalent to 1⅜ ins. per minute, or 7 ft. per hour, is not sufficient to move any of the material. The velocity of water percolating through sand is found to vary directly as the head and inversely as the distance.

The value of “c” in the formula is larger for sands of filters favorable for flow, and smaller for compacted materials and dams.

Mr. Morison thought it might be nearer the actual conditions to assume d = 0.50 mm.; c = 500; and l = 5,000 ft.; in which case the seepage would only amount to 2.5 ft. per sec. In this last assumption the “effective size” of sand grains is 2½ times that classed as “permeable material” at the North Dike of the Wachusett Reservoir.

Prof. Philipp Forchheimer, of Gratz, Austria, recommends the use of the formula,

h
— = a√ + b√²
l

for the percolation through soils between loam and loamy sand. Sellheim, Masoni, Smreker, Kröber and other authorities on filtration use still other formulas, to which the reader and student is referred for further research.

The writer, having had occasion in his professional practice to study quite carefully the subject of ground waters, and their percolation or flow through different classes of materials and under varying conditions, is of the opinion that rarely does the cross-section of a stream-channel, filled with sand, gravel and debris, present, even approximately, a homogeneous or uniform mass; and that there are, almost without exception, strata of material much coarser and more porous than the general average. In other words, that it is extremely difficult to arrive at a uniformity coefficient. 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 embankment. The local features, economical considerations and magnitude of the risks, all bear upon this question and must be considered for each particular case.

It is of interest to compare the estimated cost of the three designs proposed for the Bohio Dam, based upon the same unit prices, as follows:

French Engineers’ design $3,500,000
Isthmian Canal Commissioners’ design8,000,000
Mr. Morison’s design2,500,000

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.