Practical Lessons from the Loma Prieta Earthquake (1994) / Chapter Skim
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2. The Geotechnical Aspects
2. The Geotechnical Aspects
Pages 29-68
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From page 29... ...
Thus, considerable useful experience can be derived from the Loma Prieta earthquake for geotechnical engineering. Although much has been learned from the earthquake, and more knowledge is to come, extrapolation of the information for the geotechnical community has to be tempered by the knowledge that special conditions ameliorated the damages.
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From page 30... ...
If the duration of the event had been longer, water tables and reservoir levels higher, or the epicenter closer to San Francisco damages could have been greater, and what appeared to be successful performance could have translated into unsuccessful behavior. The timing of this conference is well-matched to the discovery phase of the research on the Loma Prieta earthquake.
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From page 31... ...
1~ ~PaloAI~ {~ \ State Beach If/ \~/ State Peach ~ Sunnyvale JOSE`} | Gazos Beach Big Basin Redwoods of\ State Beach / San Grecoro , · Soil liquefaction, ground failure, sand boils Lateral spreading, significant settlements Scale: 0 5 10 miles .
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From page 32... ...
Liquefaction during the Loma Prieta earthquake was common near the shoreline of the San Francisco Bay and adjacent to rivers and bodies of water near the Pacific Ocean west of the epicentral region (Figure 2-1~. There were few surprises as to the locations of liquefaction, since most of the areas where it was evidenced fit expected criteria for liquefaction.
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From page 33... ...
Pee Break ~ ~3~;; lo tm Shotwell between \ 17th and 12th 33 1 ~ Piers 80, 94 FIGURE 2-2 Locations of waterfront fills in San Francisco and test sites TH and YBC. Liquefaction phenomena observed in Loma Prieta earthquake is noted (modified from Seed et al., 1991)
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From page 34... ...
In the Loma Prieta earthquake, the Dupre and Tinsley mapping accurately defined locations of major occurrences of liquefaction and lateral spreading. At the same time, broad regions within areas mapped as susceptible to liquefaction showed no response.
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From page 35... ...
At ct ct · s~ o Do .— so o .— o A)
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From page 37... ...
Lateral Spreading Lateral spreading was observed in most areas where significant liquefaction occurred during the Loma Prieta earthquake. In San Francisco, lateral movements were primarily associated with the sandy fills along the waterfront (Figure 2-21.
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From page 38... ...
occurred in this region during the 1906 earthquake (Youd and Hoose, 19781. The lack of significant lateral movements is partly attributed to the lesser magnitude of the Loma Prieta earthquake relative to the 1906 event, but other factors may have been at work.
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From page 39... ...
, as well as other prediction methods, have yet to be evaluated with the new field data. Fill Densification As A Result of Earthquake Shaking Following the Loma Prieta earthquake, SPT and CPT were conducted at several sites along San Francisco's waterfront, where similar tests had been performed in the late 1970s, which provided an opportunity to measure changes in density that were due to earthquake shaking (slough and Chameau, 1983; Chameau et al., 19911.
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From page 40... ...
-10 -12 -2 -4 a' a) -6 E Q -8 ~ -10 -12 FIGURE 2-5 Average CPT tip resistances measured at the YBC and TH sites before and after the Loma Prieta earthquake (after Chameau et al., 1991)
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From page 41... ...
SITE AMPLIFICATION AND RESPONSE SPECTRA Site amplification is a term used to define the occurrence of ground motions at the surface of sites that are larger than those that would occur if the site were composed of bedrock. In the Loma Prieta earthquake, amplification occurred in the San Francisco Bay area at sites underlain by weak rock, stiff soil and soft soil (Borcherdt and Glassmoyer, 19923.
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From page 42... ...
At instrumented sites that were underlain by soft bay mud, locations like Foster City, Treasure Island, or in the shoreline area of the East Bay, peak accelerations were 0.2 to 0.3 g. These accelerations were also rich in long period motions and, thus, possessed enhanced damage potential for weak structures (e.g., unreinforced masonry)
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From page 43... ...
However, for purposes of research studies of observed behavior, the more sophisticated analyses procedures would appear to warrant serious consideration. While the site amplification effects in soft soils in the Loma Prieta earthquake could have been anticipated, the one-dimensional analyses showed that the largest amplified accelerations were not generated at the fundamental frequencies of the sites (Seed et al., 19911.
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From page 44... ...
and Treasure Island (soft site) from the Loma Prieta earthquake with recommended S4 design code spectrum (adapted from Dickinson et al., 1991)
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From page 45... ...
. Trenches dug for purposes of identifying the slide planes for the Summit Road landslides showed that up to three displacement events in addition to that associated with the Loma Prieta earthquake had occurred over a period of 2,000 to 3,200 years (Nolan and Weber, in press)
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From page 46... ...
(1991) reported on the behavior of earth dams that were impacted by the Loma Prieta earthquake, and much of the material that follows is drawn from these sources.
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From page 47... ...
. FIGURE 2-8 Location map for earth and rockfill dams and major landfills (adapted from Harder, l 991)
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From page 48... ...
- ~ so 50 ct He Ad - ~ Ad 4— .
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From page 49... ...
Sanitary and Hazardous Waste Landfills A large number of major landfills and hazardous waste sites were shaken by the Loma Prieta earthquake. Table 2-4 presents a compilation of information on fifteen of these sites as derived from Sharma and Goyal (1991)
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From page 50... ...
c~ o in o o o ˘ - ~ cd id in · 50 in o an au as in A)
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From page 51... ...
They also suggested that the landfill would offer considerable damping and that random materials within the landfill, such as boards and metal parts, would act as reinforcment for the landfill slopes. WATERFRONT CONTAINMENT STRUCTURES, PIERS, AND LATERAL RETAINING STRUCTURES The Loma Prieta earthquake provided the opportunity to evaluate the performance of waterfront containment structures including seawalls, rockfill dikes, levees, cellular cofferdams, piers and wharves, and lateral retaining structures.
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From page 52... ...
Notably, this level of density may not be adequate to prevent damage in an event larger than the Loma Prieta earthquake. At Seventh Street Terminal in Oakland, lateral movement and failure of a reinforced concrete wharf occurred.
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From page 53... ...
Thus, it is uncertain what role, if any, liquefaction played in producing lateral movements at this site. The earthquake provided insight into the ability of battered pile foundation systems to resist lateral loadings.
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From page 54... ...
The differential support motions presumably induce forces that are larger than those induced during uniform support motions. This effect is suspected to have been the primary cause of several bridge failures during the Loma Prieta earthquake, although additional research is needed to fully understand this phenomenon.
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From page 55... ...
e CWP ct o cat \ San Francisco Daly City ~ l if: ; Scale: 0 _ _\ ~ ~ _' ~ =_-: Mission District Large CWP culverts Bart tunnels San Francisco Bay (1 mi = 1.6 Km) FIGURE 2-9 Locations of BART tunnels and large CWP culverts.
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From page 56... ...
{~1~ ~ Slicing ~1 I h EBxF°tae°ner A; - I 'TIP ~ 1~; 3~ ~ ~ Plat] 21: r ~ r l ~ Beanng Plate r Teflon—_ ~ _ Faced with Teflon Bracket ~ ~ l —Collar Wire Rope Assembly = ~ Inside Face of Tube FIGURE 2-10 Schematic of seismic joint to connect B ART tunnels to ventilation structures on San Francisco and Oakland sides of the San Francisco Bay (adapted from Douglas and Warshaw, 1971)
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From page 57... ...
The CWP culverts as structures also possess a bending resistance that helps to resist local movements. It is not clear whether the CWP culverts would perform as well if extensive liquefaction were to occur under an event larger than the Loma Prieta earthquake.
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From page 58... ...
· Primary amplification can occur by matching of the characteristic rock motion with the second fundamental period of the site. · Recent design spectra proposed for use with deep soft soil sites did not capture the maximum response periods in areas where site amplification occurred.
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From page 59... ...
· Differential support motions presumably induce forces larger than uniform support motions and apparently caused substantial damage to several bridges. Underground Structures · Underground structures other than pipelines were not damaged by the earthquake, because they largely moved with the ground or provided bending resistance to potential lateral movements.
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From page 60... ...
This has to be tempered by the fact that the Loma Prieta earthquake did not produce the large-scale liquefaction that might occur in a 1906-type event. Areas for Future Concern Areas for future concern include: · the existence of liquefiable sandy fills in the waterfront areas of San Francisco Bay; .
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From page 61... ...
1992. On the Characteristics of Local Geology and Their Influence on Ground Motions Generated by the Loma Prieta Earthquake in the San Francisco Bay Region, California.
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From page 62... ...
1990. Preliminary Map of Landslide Features and Coseismic Fissures in the Summit Road Area of the Santa Cruz Mountains Triggered by the Loma Prieta Earthquake of October 17, 1989.
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From page 63... ...
In press. Liquefaction Hazard Mapping, Depositional Facies, and Lateral Spreading Ground Failure in the Monterey Bay Area, Central California.
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From page 64... ...
Maurice S Power, Geomatrix Consultants I would like to elaborate on two locations of liquefaction during the Loma Prieta earthquake.
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From page 65... ...
Nevertheless, there is a certain deja vu about many of the lessons of the Loma Prieta earthquake that are before us at this symposium. The effects of earthquake strong ground motion, for example, on unreinforced masonry, soft
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From page 66... ...
C Thomas Statton, Woodward-Clyde Consultants The Loma Prieta earthquake had some interesting timing aspects for my career in that we were developing some seismic design provisions for the New York City building code; the earthquake gave greater impetus to the team.
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From page 67... ...
When we look at earthquake design in the west for specific structures we find that the design earthquakes represent 80-90 percent of the maximum expected values coming from the maximum expected earthquakes. This is not true in the eastern United States, where the design values that are currently being looked at may represent 50 percent of the ground motions of the maximum expected event.
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