Experience is the best teacher but not the kindest. Failures demand attention and always hold lessons about what not to do again. Learning from failures— hopefully from other people’s failures—provides the most reliable basis for anticipating what might go wrong in other cases. In this chapter we describe 10 cases of slope failure and recount briefly the circumstances under which they occurred, their causes, and their consequences. The examples are followed by an examination of the factors that influence the stability of slopes, and the causes of instability, as illustrated by these examples.
EXAMPLES OF SLOPE FAILURE
The London Road and Highway 24 Landslides The London Road landslide in Oakland, California, occurred in January 1970 during a period of heavy rainfall. Front-page headlines in the January 14, 1970, Oakland Tribune exclaimed ‘‘Storm Hammers State— Slide Menaces 14 Homes—The Helpless Feeling of Watching Ruin Approach.’’ Figure 2.1 shows houses in the slide area that were destroyed by the slide and had to be abandoned by their owners. The slide covered an area of about 15 acres, and the sliding surface was estimated to be as deep as 60 ft (20 m) beneath the surface of the ground. As evident from the height of the headscarp in comparison with the houses on the right in Figure 2.1, the slide movements were very large. Some 14 houses were destroyed, and a jet fuel pipeline at the bottom of the hill was never used again because of the danger that it would be ruptured by slide movements. Because the cost of stabilizing the massive slide was greater than the economic benefit, it was not repaired, and an entire neighborhood was lost permanently.
Not only was there heavy rainfall during January 1970, the entire preceding year had been unusually wet, with about 140% of the average rainfall recorded at the nearest rain gage station. As discussed later, these prolonged wet conditions played a significant role in the occurrence of the massive landslide.
The Highway 24 landslide shown in Figure 2.2 occurred in January 1982, when a storm blew in from the Pacific Ocean and stalled over the San Francisco Bay area. In a 24-hour period in early January, the storm dumped nearly 10 in. of rain on the area, where the normal yearly rainfall is about 25 in. The sudden enormous deluge resulted in literally thousands of landslides in the San Francisco Bay area. Typically, these slides were shallow. The intense rainfall saturated the upper few feet of the ground on the hillsides, which came sliding and flowing down in many places, knocking down trees, destroying houses, and blocking roads. Figure 2.2 shows Highway 24 near Orinda, California, partially blocked by a flow of sloppy, saturated soil that flowed onto the roadway, just one of the thousands of slides that occurred during the storm.
The London Road landslide and the flow slide on Highway 24, less than 10 miles apart, illustrate two very different types of slope failures that occur in the same area. The London Road landslide was very deepseated and followed two successive years of abovenormal rainfall. Although detailed soil exploration was not carried out at the London Road site, some interesting facts may be surmised based on what could be seen at the ground surface. The slide movement exposed serpentine rock in one area. Serpentine is a metamorphic rock that can be hard and strong but is subject to rapid deterioration to a weak powdery mass when exposed to air and water. Although the exposed serpentine retained its rocklike appearance, it could be penetrated several inches with a bare hand and had essentially no strength or stiffness. It can be surmised that the strength of the serpentine, and other soils and rocks underlying the London Road area, had been deteriorating slowly over a period of many tens, hundreds, even thousands of years since the hillside was formed. Such deterioration results from chemical and physical processes that can gradually change the properties of earth materials. Eventually, this reduction in strength, combined with two years of heavy rainfall and resulting high groundwater levels, led to the very deep-seated landslide.
In contrast, the slide that blocked Highway 24 was very shallow, probably no more than 3 ft (1 m) deep. This type of slide develops very quickly as a result of relatively brief, extremely intense rainfall. Infiltration within a brief period affects only the upper few feet of soil. Within this depth, however, the soil may become saturated and lose much of its strength. In the area east of San Francisco Bay, the hillsides are blanketed by silty and sandy clays of low plasticity over the top of less weathered and stronger rock. The thickness of the soil cover ranges from zero to 15 ft (5 m). The soil has formed from the underlying rocks and has reached its present condition through processes of weathering, erosion, shallow sliding, and deposition farther downhill. When dry, the soils that blanket the hillsides are stiff and strong, and the slopes they form are stable. During intense rains, however, water infiltrates the ground rapidly because the ground contains many cracks that provide secondary permeabilty. Although the processes leading to this type of slide are still the subject of research study, it is clear that conditions can change very quickly, and that the transition from stable ground to a fluid mass in rapid motion can take place within minutes. The high velocities with which these slides move makes them very dangerous, and many lives have been lost when they flowed down and crushed houses without warning.
The London Road and the Highway 24 slides illustrate a relationship between rainfall and landslides that has been observed in many places: Long periods of higher-than-average rainfall cause deep-seated, slowmoving slides, with shear surfaces that can extend tens of feet below the ground surface. One or two days of very intense rainfall, in contrast, tend to cause shallow slides involving only a few feet of soil, which move with high velocity once they are in motion.
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