Collapse of Littlefield Dam Due To Inadequate Design and Faulty Construction Methods Investigating Engineer Finds

BY: J. A. Fraps

ARIZONA HIGHWAYS

VOLUME V. SEPTEMBER, 1929 NUMBER 9

By J. A. FRAPS Department Engineer on Dam Supervision and Control INVESTIGATION of the collapse Aug-ust 5 of the Littlefield dam, under construction on the Virgin River, six miles above the town of Littlefield in the extreme northwestern corner of Ari-zona, showed that the failure was due to inadequate design and to faulty con-struction methods.

The site of the dam was known locally as "The Narrows." The name was derived from the fact that the river flows through a narrow gorge at this point. A box canyon, with high, pre-cipitous sides, extends for several miles above the dam site, while below the river flows through an open, rolling country.

The purpose of the Littlefield dam primarily was diversion. The water was to be diverted at the elevation of spill-way crest, but a small amount of stor-age was to have been secured by the installation of eight-foot automatic spill way gates. Actual storage capacity thus to be obtained has not been ascertained, as no reservoir surveys have been made. This was not the first attempt to construct a dam at the "Narrows," for in former years a dam 60 feet in height was built by blasting rock from the can-yon sides into the river below. Insuffi-cient spillway capacity was said to be the cause of the failure of this early structure, although the belief is ex-pressed that the mode of construction may have been an additional factor. Af-ter the failure of this dam it was found that the stream bed was 20 feet higher than originally, due probably to the de-posit of immense boulders in the bottom of the river.

With the changes caused by former construction, the existing dam site is about 100 feet wide at stream bed, and 300 feet wide 125 feet above. Depth to bed rock was not known by any avail-able parties connected with the work, al-though it was said that wash borings were made after the failure of the 60-foot dam.

In order to convey a general idea of the Littlefield Dam, a plan, elevation and section, drawn roughly to scale from a description by the construction fore-man, is presented in Fig. 1. It will be noted that the dam was to have been constructed to elevation 2037, or 137 feet above stream bed. Crest width was to have been six feet, and side slopes approximately 1.4 horizontal to 1 verti-cal. Dotted lines on Fig. 1 indicate the status of construction on August 5, 1929, and water line at the time of fail-ure is also indicated.

The structure was of the rock fill type, but without any foundation prep-arations, cut-off wall, core wall, or im-pervious upstream face. Material for the fill was secured from the spillway excavation on the north canyon side, and the fill carried across at spillway floor level, very much in the fashion of con-structing a railroad or highway em-bankment. The material was permitted to take its natural slope, and no effort was made to place any of the rock It can be readily seen, that by starting at one side and working toward the other, large rocks would roll to the bottom of the canyon, and the interstices between them would provide a devious though continuous path for the passage of wa-ter. It was expected by the owners that the spaces between the rocks would be filled with silt, and thus a watertight structure would result, but this view was too optimistic, even though the Vir-gin River does carry a high percentage of silt most of the time.

Construction was started in April of this year and was to have been com-pleted in June, in order to work during the three months of minimum flow, hence no provisions were made to care for water during the period of construc-tion. Delays caused the construction period to extend into July and August, during which flash floods of dangerous magnitude are not unexpected In this connection, however, it must be men-tioned that the dam failed before it was overtopped, although it would have been overtopped had it held a few hours long-er.

The fill had been extended, on a level with the spillway floor to a point 65 feet from the south abutment, as shown by the dotted lines on Fig. 1. Lowest point on crest was 25 feet below spill-way floor, or 100 feet above stream bed. Top width at spillway floor was 40 feet, and side slopes were running about 1.4 horizontal to 1 vertical.

The Virgin River had been at flood stage intermittently for several weeks prior to the failure of the dam, and had caused the level of the lake to rise grad-ually. Leakage through the dam had al-so increased with the additions in the reservoir, and on the morning of Aug-ust 5 was estimated to be 200 second feet.

This leakage was discharging principally from the downstream slope at an elevation 25 feet above stream bed, although smaller leaks were in evidence at other elevations. Heavy rains on the watershed above the dam brought a sharp rise in reservoir level during the

ARIZONA HIGHWAYS

SEPTEMBER, 1929 Page Nine morning of August 5, until at noon a depth of 86 feet existed.

This rise increased the velocity of the water passing through the structure, with a resulting increase in volume. At 12:30 P. M. August 5 a slide occurred on the downstream face of the dam, and a slightly greater volume of water was thus permitted to pass through the structure. It was inevitable that other slides should follow the first, and the dam was gradually eaten away, during which the amount of water passing through increased to about 300 second feet, and the level of the reservoir rose four feet, or to a point ten feet below the lowest point on the crest.

At 1:30 P. M. only a triangular section of dam was all that remained, the apex of which was only a few feet above the water line, and which gave away before the horizontal pressure of the water in the reservoir. The reservoir was emptied in thirty minutes, and by two o'clock not more than 10bio vards of rock and earth remained of the 85,000 cubic yards which had been in place a short hour and a half before.

Residents below the dam had ample warning of the failure and there was no loss of life, and little property damage. The property loss in Arizona was estimated to be $2,000, but was probably greater in Nevada. There was no damage to highways or bridges, although the flood went over the floor of a highway bridge two miles west of Mesquite, Nevada, on U.S. Highway 91.

Imperfections in design and construction of the Littlefield Dam are at once apparent to the engineer with experience on dams, but will be discussed briefly. A dam was being constructed to store water to a maximum height of 133 feet, and yet no foundation preparations whatever were made. The lessons which might have been learned from the St. Francis disaster were overlooked entirely by the owners and builders of the Littlefield Dam. A rock fill dam is a very porous structure, hence it is necessary and customary to construct an impervious structure upstream face from crest of dam to lowest bed rock.

If the dam had been finished, and stood, its usefulness during dry periods would have been naught as more water would have passed through the structure than naturally flows in the Virgin River during periods of minimum flow.

Of greater importance than this, however, is the stability of such a structure which is dependent upon the ability of the upstream facing to prevent the passage of water through the fill in quantities sufficient to carry any particles of the fill. Any attempt to construct a dam which will surely fail if overtopped, without some means of caring for floods during construction, can only succeed if the weather conditions are ideal. The extremely flashy nature of Southwestern streams makes an attempt of this sort practically doomed to certain failure.

The failure of the Littlefield Dam is further proof of the great need for the recently enacted laws in Arizona and California pertaining to dam supervision and control by the State.

The writer is indebted to Mr. G. W. Holmes, construction foreman at the dam, for valuable information regarding the dam, which has been incorporated into this article.