SAFETY OF HIGH DAMS.

New Zealand Herald, Volume LXV, Issue 20136, 22 December 1928, Page 5 (Supplement)

SAFETY OF HIGH DAMS.

THE CALIFORNIAN DISASTER.

INHERENT DEFECTS OF SCHEME.

MR. HOLMES' INFERENCES EXAMINED.

The collapse of the St. Francis dam in California on March 13, 1928, is cited by Mr. Holmes as evidence that the Arapuni dam and contingent works are not safe, " as they possess soino features in common "ivith the dam that failed at Los Angeles and at many other places." The last phrase requires justification for, according to an American authority, there have not been failures of such dams "at many other places." Discussing the St. Francis disaster the Engineering News-Record (New York), observed that " for the first time in history, a high dam of masonry has failed." Mr. Holmes does not define the " features in common " to which he attributes the failure of. the St. Francis dam and which ho considers are weaknesses in the Arapuni scheme. The relevant passago of his report is as follows:—"The St. Francis dam impounded a lake about 18 miles long, or about the same length as the Arapuni lake. The dam was approximately the same height as the Arapuni dam, about 200 ft., and it was built without a proper engineering exploration of the strata under the foundations. The cause of the failure appears to be that water found its way into unstable material under the dam foundations, then a section of the dam slid bodily forward. Even though the dam was arched as at Arapuni, the arching effect did not prevent failure." This description. suggests that the Arapuni dam is virtually a replica of the St. Francis dam, whereas there were very considerable differences in the two designs. The maximum height of the Arapuni structure in midstream is 190 ft., as against 205 ft. in the St. Francis dam, while its base is- 162 ft. thick as against 175 ft. The length aiong the crest is, however, 300 ft., whereas the crest length of the St. Francis dam was 668 ft.—beyond the main structure there was " a low gravity conci'eto wall several hundred feet in length and from 30ft. to 40ft. in height according to terrain." No Preliminary Exploration, According to Mr. Holmes the St. Francis dam failed because " it was built without a proper engineering exploration of the strata undor the foundations." That feature is surely not " common " to Arapuni, since the explorations were most elaborate and comprehensive, and more exteiLaive than were considered necessary by Mr. Holmes himself in his advice to the chief electrical engineer in 1917. The failure of the St. Francis dam can be attributed principally to the unsatisfactory nature of the rocks cn whicli it was founded. "So far as can be ascertained, no geological examination was made of the dam site before construction began, and no crushing or immersion' tests were made of the conglomerate on which a considerable portion of it was founded." It was built across the San Francisqnito fault, which has long been known, and appears on the fault map of

California. Though some of the investigators, sinco the accident, stale that they were unable to detect any evidence of ircerit movement on this fault, other investigators," while confirming the conclusion of the Government's Commission that, the principal cause of the failure was the character of the rocks under the dam site, state that there is evidence of a movement on the fault resulting in an up-

lift at one end of the dam -which would have been sufficient to cause Ihe wreck. However, it is agreed that whether this uplift took place or not, the darn was | bound to fail from causes which wera guarded against in tho case of Arapuni by the most elaborate tests, both:laboratory and field, before a final decision was majje to adopt the scheme. The Question of Arching. Reference is made by Mr. Holmes to the arching of the Arapuni dam, as though that feature had been introduced as a safeguard against inherent defects in ? the scheme. In fact, the Arapuni dam does not depend for stability primarily on " arching." It has sufficient mass to ;j sustain the water pressure without ? "arching," but a moderate degree of "arching" was introduced as tending to | further stability and a? also tending to keep tightly closed the expansion joints which are carried through tho concrete at regular intervals. Though not neces- ;} sary to ensure the stability of the dam, ' the " arching," if the site were to any extent doubtful, would reduce the risk-, of failure. -■■■&£s Incidentally, it may be mentioned that the design of the dam has also been criticised on the' ground that while the | commission of engineers recommended that a dam of the gravity type should be adopted, instead of an arch dam as originally proposed, the engineers actesUy prepared designs for an arched gravity dam. But the design accords with the commissioner's recommendations. Its report stated that " the gravity dam should | be designed to be built oil a curve of moderate radius." Demonstration of Stability. |'S Because the St. Francis dam failed, Mr. 1 Holmes suggests that the Arapuni scheme J should be abandoned. He does not mention facts in connection with that failure, which suggest that extraordinary conditions were responsible. Two breaches, were made in the dam, while the central. section stood intact, " That tho violent wrenching and shock of this breaching,; superadded to the full pressure of the impounded water, did not displace the high middle section is a remarkable demonstration of stability;, it inspires thorough confidence in. tho principles of gravity dam construction," says the Engineering News-Record. ~ : " High dams can be built, safely, and dams now standing are safe. Engineers' universally recognise tho great truth, that ; care in building the foundations' i 3 the, weightiest, matter in dam construction.; Nor does the St. Francis failure furnish a; valid argument against building ou rock of less than the hardest kind. Many im-Jj portant dams resting on poor rock have | stood long and safely, because the quality;!! of the material was rightly appraised and | the construction planned with full con-1| sideration therefor. The presentaster, then, contains no fact or happening || that would Justify restriction of dam con-1 1 struction. It points the nesd, however, J: for an effective check on individual judg-Jj incn.t concerning foundations where many j| lives are at stake." . vj| To that it may be added that the fij Arapuni site was adopted only after the | most exhaustive examination; that thes favourable judgment was endorsed by_ a| commission of engineers and a geologist jj and confirmed by the offers of reputable j| engineering firms to construct the heart- |a works on the appointed site. Finally, it || may be stated that nothing has been dis-Jj covered since the actual work of construe-jj tion commenced to disturb confidence j that judgment. j