1822 Landslides

When a steeply inclined mass of soil is suddenly shaken, a slip-plane can form and the material slides downhill. During a landslide, structures sitting on the slide move downward and structures below the slide are hit by falling debris (Figure 18.12).

Landslides frequently occur in canyons, along cliffs, on mountains, and anywhere else where unstable soil exists. Landslides can occur without earthquakes (they often occur during heavy rains that increase the weight and reduce the friction of the soil) but the number of landslides is greatly increased wherever large earthquakes occur. Landslides can move a few inches or hundreds of feet. They can be the result of liquefaction, weak clays, erosion, subsidence, ground shaking, etc.

During the 1999 Ji Ji, Taiwan, earthquake, many of the mountain slopes were denuded by slides, which continued to be a hazard for people traveling on mountain roads in the weeks following the earthquake. The many RC gravity retaining walls that supported the road embankments in the mountainous terrain were all damaged: either from being pushed downhill by the slide (Figure 18.13) or in some cases broken when the retaining wall was restrained from moving downhill (Figure 18.14).

One of the more interesting retaining wall failures during the Ji Ji Earthquake was in a geogrid fabric and mechanically stabilized earth (MSE) wall at the entrance to the Southern International University (Figure 18.15). This wall was quite long and tall and its failure was a surprise since MSE walls have a good performance record during earthquakes. It was speculated that the geogrid retaining system had insufficient embedment into the soil and also it was unclear why an MSE wall would be used in a cut roadway section.

One of the best-known and largest landslides occurred at Turnagain Heights in Anchorage during the 1964 Great Alaska Earthquake. The area of the slide was about 8500 ft wide by 1200 ft long. The average drop was about 35 ft. This slide was complex, but the main cause was the failure of the weak clay layer and the unhindered movement of the ground down the wet mud flats to the sea. Figure 18.16 and Figure 18.17 provide a section and plan view of the slide.

The soil failed due to the intense shaking, and the whole neighborhood of houses, schools, and other buildings slid hundreds of yards downhill, many remaining intact during the fall (Figure 18.18).

Bridges are also severely damaged by landslides. During the 1999 Ji Ji, Taiwan, earthquake, landslides caused the collapse of two bridges. The Tsu Wei Bridges were two parallel three-span structures that crossed a tributary of the Dajia River near the city of Juolan. The superstructure was simply supported "T" girders on hammerhead single-column bents with "drum"-type footings and seat-type abutments. The girders sat on elastomeric pads between transverse shear keys. The spans were about 80 ft long by 46 ft wide, and had a 30° skew. The head scarp was clearly visible on the hillside above the bridge. During the earthquake, the south abutment was pushed forward by the landslide, the first spans fell off the bent caps on the (far) north side, and the second span of the left bridge also fell off of the far bent cap (Figure 18.19).

Also, the tops of the columns at Bent 2 had rotated away from the (south) Abutment 1. Therefore, it appears that both the top of Abutment 1 and the top of Bent 2 had moved away from the slide, while the remaining spans, restrained by Bent 3 and Abutment 4, had remained in place. Perhaps the landslide originally had pushed against Bent 2, rotating the columns forward, and the debris had

Copyright 2005 by CRC Press

FIGURE 18.9 Elevation view of the Webster Street Tube. 8

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FIGURE 18.9 Elevation view of the Webster Street Tube. 8

Copyright 2005 by CRC Press

FIGURE 18.11 Liquefaction induced damage to the Webster Street Tube tunnel.

Before landslide

Before landslide

After landslide

since been removed by the current or by a construction crew. Perhaps the skew had rotated the spans to the right as they fell, pushing them against the shear keys at Bent 2, which rotated the top of the columns forward and eventually pushed the spans off the tops of Bent 2 and Bent 3. Or perhaps there was an element of strong shaking that combined with the landslide to create the column rotation and fallen spans.

Dams are particularly vulnerable to landslides since they are frequently built to hold back the water in canyons and mountain streams. Moreover, inspection of the dam after an earthquake is often difficult when slides block the roads leading to the dam. When the Pacoima concrete arch dam was built in the

FIGURE 18.13 Gravity retaining wall pushed outward by landslide.

1920s, a covered tunnel was constructed to allow access to the dam. However this tunnel, along with roads and a tramway to the dam, were damaged by massive landslides during and for several days after the 1971 San Fernando Earthquake (Figure 18.20).

The Lower San Fernando Dam for the Van Norman Reservoir was also severely damaged during the 1971 San Fernando Earthquake. It was fortunate that water levels were low as the concrete crest on this earthen dam collapsed due to a large landslide along both the upstream (Figure 18.21) and the downstream (Figure 18.22) faces. Considering the vulnerability of thousands of residences in the San Fernando Valley below (Figure 18.23), a dam failure can be extremely costly in terms of human lives and property damage.

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