When Science Goes Wrong (17 page)
ENGINEERING GEOLOGY: The Night the Dam Broke
THE URBAN TENTACLES of Los Angeles have not yet reached San Francisquito Canyon, an arid corridor that slices southward through the Transverse Ranges 40 miles northwest of downtown. A hiker can walk for miles there and see no life beyond the circling hawks and an occasional rattlesnake. But he couldn’t fail to notice the immense, eroded blocks of concrete, some weighing 10,000 tons or more, that lie half-hidden in the chaparral like the decaying ruins of a long lost civilisation. These monoliths serve as memorials to the victims of America’s worst civil engineering disaster of the 20th century – the failure of the St. Francis Dam on March 12, 1928.
The dam was the brainchild of William Mulholland, the famed water engineer whose projects made possible the explosive growth of Los Angeles during the early part of the 20th century. Born in Belfast in 1855, Mulholland arrived in Los Angeles at the age of 22 with little education and a few dollars in his pocket. He took employment as a ditch tender for the city’s water system, which was then dependent on the erratic local supply provided by the Los Angeles River. He quickly demonstrated an extraordinary aptitude for water engineering, and he rose through the ranks of the water company to become its superintendent. In 1902, when the city of Los Angeles turned the company into a public utility, Mulholland was appointed its chief engineer and general manager.
Mulholland’s most well-known achievement was the Los Angeles Aqueduct, begun in 1908, which brought water to the infant metropolis from the Owens River in the eastern Sierras. The aqueduct was an engineering marvel for its age, not only on account of its great length (233 miles) but also because, in spite of some very rugged terrain along the way, water moved from one end to the other entirely by the force of gravity. Where the aqueduct crossed a canyon, the water was carried down the slope, across the canyon floor, and up the other side within a V-shaped length of steel pipe called an inverted siphon; these siphons had to be tremendously strong to withstand the internal pressure generated by the drop. The aqueduct also had many tunnels: most notable among these was the five-mile long Elizabeth Lake tunnel, near the head of the Antelope Valley. Within this tunnel the aqueduct passed directly through the San Andreas Fault, and the torturous geology at the Fault posed special engineering problems. After emerging from the tunnel the aqueduct headed southward along the eastern flank of San Francisquito Canyon toward Saugus, terminating in a set of small reservoirs in the San Fernando Valley and nearby areas.
Thirty thousand Angelenos turned out in November 1913 to celebrate the opening of the aqueduct. The benefits to the city quickly became apparent, as irrigated orchards sprang up in the San Fernando Valley and plentiful water became available for homes and gardens in new subdivisions. Even today, the Los Angeles Aqueduct – extended in length and supplemented by an additional parallel pipe – supplies the bulk of the city’s water. The system brings almost half a billion gallons of water from the eastern Sierras to the thirsty metropolis every day.
Bill Mulholland became an almost godlike figure to the citizens of Los Angeles. He was urged to run for mayor, but he turned down the suggestion with a memorable comment. ‘Gentlemen,’ he told a crowd, ‘I would rather give birth to a porcupine backwards than be mayor of Los Angeles.’ Instead, he devoted himself to improving on what he had achieved. Over the following decade, he added four hydroelectric powerhouses to the aqueduct. Two of these were situated in San Francisquito Canyon: one was located at the head of the canyon, where the aqueduct dropped 1,000ft after exiting the Elizabeth Lake tunnel, and the other was sited six miles down the canyon. Over time, the powerhouses generated enough electricity to defray the entire costs of the aqueduct’s construction.
Godlike though Mulholland may have appeared to Angelenos, to the residents of Owens Valley he seemed more like the devil. The aqueduct sucked the lifeblood out of their soil; for every orchard that was planted in the San Fernando Valley, one had to be abandoned in Owens Valley. Agriculture came to a halt. Owens Lake, the natural terminus of the Owens River, dried up completely in 1924, and the lakebed gave off a cloud of arsenic-laced dust that choked the valley every time the wind blew. Meanwhile, the city’s agents began purchasing water rights farther to the north, enriching some settlers but threatening the rest with penury.
Infuriated by these developments, some of the settlers began a campaign of sabotage. In 1924 they seized the aqueduct’s headgates and diverted water back into the Owens River. This episode was followed by dynamite attacks on the inverted siphons and other parts of the aqueduct. The attacks led to interruptions in water delivery and necessitated expensive repairs.
The settlers’ campaign failed. Although the city’s acquisition of the local water rights involved some deception, it was done more or less in accordance with the legal requirements current at the time. The settlers gained little traction with the courts or with public opinion and eventually gave up. (The environmental problems in Owens Valley have remained unresolved to the present day: the city of Los Angeles is making some attempts at remediation, such as covering parts of the bed of Owens Lake with gravel and diverting a small portion of the Aqueduct water back into the Owens River.)
During the early 1920s, Mulholland came to realise that the city needed more water-storage capacity. One reason was the fear of drought. In fact, one three-year drought reduced flows to the point that the city had insufficient water to supply the needs of the farmers in the San Fernando Valley. In addition, there was the threat of sabotage. Finally, there was the always-looming danger of a rupture of the San Andreas Fault, an event that would block the aqueduct within the Elizabeth Lake tunnel. For all these reasons, Mulholland wanted to construct a set of reservoirs south of the fault that collectively could store a year’s supply of water for the city. The largest of these reservoirs, designed to hold half of the entire supply, was to be sited in San Francisquito Canyon, and construction of the necessary dam – its name anglicised to ‘St. Francis’ – began in 1924.
On the face of it, San Francisquito Canyon looked like an ideal location. In the northern part of the canyon lay a broad valley that could easily hold 32,000 acre feet (or about 10 billion gallons) of water. In fact, it had been the site of a large lake in prehistoric times. About halfway down the canyon, about a mile and a half north of the lower powerhouse, a rocky spur jutted out from the canyon’s western flank, constricting the canyon to a gorge barely 200ft wide at the canyon floor. Because the sides of the canyon were sloped, however, a dam would have to be considerably wider at its top – about 550ft if the dam were built straight across, and more if it were curved. Thus the greater portion of the dam would consist of its abutments, the sections that rested on the canyon’s sloping sides.
Mulholland had several geologists look at the proposed dam site before he made the decision to go ahead. These experts included John Branner, chairman of the geology department at Stanford University and the university’s second president. The geologists gave Mulholland their opinion that the site was suitable, but their inspections may not have been very detailed. After the disaster, Mulholland cited these geologists’ positive opinions but did not produce any written reports to back them up. Branner’s approval probably took the form of a verbal ‘looks OK to me’ after a single visit to the site, and he died before Mulholland made his final decision on the dam’s location.
One potentially troublesome feature of the site was already well known. A geological fault, the San Francisquito Fault, ran southward along the canyon, traversing the prospective site of the dam near the bottom of the sloping western wall of the gorge. Thus, if the dam were built, the fault would lie at the base of the dam’s west abutment (or ‘right’ abutment, according to the convention that the viewer is facing downstream). The fault was believed to be inactive, however, meaning that it was no longer subject to the slow accumulation of stress that might trigger an earthquake.
The reddish rock to the west of the fault, where the dam’s right abutment was to be located, is a conglomerate – that is, it consists of pebbles and cobbles in a matrix of hardened sand or silt. This particular example is known as the Sespe Formation, or Sespe Red Beds. The rock in this formation, especially in a zone near the fault, is rather crumbly and easily weathered, but its most startling behavior is evident when it becomes wet. When I visited the dam site in 2006, I pulled a loaf-size chunk of rock out of the slope where the right abutment once stood and placed it in the creek. The rock underwent a kind of slow-motion explosion: over a period of about 10 minutes, it gave off bubbles, started cracking, and then fell apart into a heap of stones and mud. In spite of this behaviour, Mulholland conducted tests that convinced him that the foundations of the right abutment would not fail or allow water to percolate through.
The rock to the east of the fault, which would carry the central section of the dam and its left abutment, is a mica schist. This is a sedimentary rock that has been altered by heat, pressure, and shearing forces so that its constituent grains are flattened, which gives the rock a laminated structure rather like slate. This particular formation is named the Pelona Schist. It is much harder than the rock of the Sespe Formation, but its layered structure gives it the tendency to split and slide along the plane of the layers, like a deck of cards. What was worse, the rock layers were tilted such that they roughly paralleled the slope of the east side of the gorge. At the fault itself, where the Sespe Formation and the Pelona Schist met, there was a layer of claylike ‘fault gouge’, about eight inches wide, that had been generated during innumerable ancient ruptures of the fault.
Undaunted by the problematic geological features of the site, Mulholland decided to go ahead. As the ‘chief’, his word was as good as law, and any review of his decision within or outside of the water department was perfunctory if it happened at all. As for environmental reviews or state safety inspections, such things did not happen in the 1920s.
Mulholland constructed the dam from concrete. Most of his prior dam-building experience involved earthen dams, but one year before starting the St. Francis Dam he had begun work on his first concrete dam. This was the dam now known as the Mulholland Dam, which confines the relatively small Hollywood Reservoir on the western fringe of Griffith Park. Apparently Mulholland chose concrete because of a lack of clay – needed to form the water-resistant core of an earthen dam – at the Hollywood and St. Francis sites. The design for the St. Francis Dam was very similar to that for the Mulholland Dam, and both were based on the then-current textbooks of dam design.
The two dams were gravity-arch dams. This means that they depended primarily on their weight to hold back the water in the reservoir. This weight – a quarter of a million metric tonnes in the case of the St. Francis Dam – thrusts directly downward, whereas the hydrostatic pressure of the reservoir water thrusts more or less horizontally downstream. The resulting combined thrust is aimed obliquely downward. For the dam to be stable against tilting, the combined thrust must be directed into the bedrock within the middle third of the dam’s footprint, not near the downstream edge or ‘toe’ of the dam or, even worse, downstream of the toe. The plans developed by Mulholland’s design engineers met this criterion, of course. An additional safety factor was added by the dam’s arched shape (curved convexly into the reservoir). This had the effect that some portion of the reservoir’s horizontal thrust was carried sideways into the dam’s abutments and thus into the canyon walls.
Conservative though this design was, Mulholland made it much less so by changes that he ordered after construction got under way. For one thing, he twice raised the height of the dam, from the original 175ft to 185ft and then to 195ft – an 11 per cent increase in height. His aim, of course, was to increase the holding capacity of the reservoir. But Mulholland did not order any compensatory thickening of the dam at its base. In fact, he actually omitted the lowermost portion of the dam’s toe, leaving the dam 20 feet (or 11 per cent) thinner at its base than called for in the design. This latter fact was only discovered decades later by Charles Outland, a local historian who made an extensive study of the disaster and wrote a detailed book about it. Outland spotted the shortened toe by closely examining photographs that had been taken during the dam’s construction. Because the dam was taller, and thinner at its base, than the original design had specified, it was significantly less stable against tilting.
The dam was completed in May 1926. Filling of the reservoir began during the construction phase but was not complete until March 7, 1928. At that point, the water lapped just a few inches below the spillway. Photographs taken at that time convey an image of graceful strength. The dam’s downstream face, rather than being smooth like most high dams that we’re familiar with today, was stepped. This gave the dam the look of a Roman amphitheatre and emphasised its curvature. Behind the dam, the reservoir spread out into the broad upper San Francisquito Canyon and its side-bays: it resembled a natural and serene lake.
Day-to-day inspections of the dam were left in the hands of the damkeeper, Tony Harnischfeger, who lived with his girlfriend, Leona Johnson, in a cottage located in the canyon a few hundred yards downstream from the dam. On the morning of March 12, 1928, five days after filling of the dam was complete, Harnischfeger telephoned Mulholland to tell him that muddy water was leaking from the base of the dam’s right abutment. High dams usually leak a certain amount of water, and the St. Francis Dam had already sprung several small leaks during the filling process, but the fact that this leak was muddy was novel and ominous. It suggested that water was not merely passing through the dam but was removing material as it did so.
