2
The Geotechnical Aspects

G. Wayne Clough, James R. Martin, II, and Jean Lou Chameau

INTRODUCTION

Evidence obtained immediately following the Loma Prieta earthquake and in subsequent studies indicated a strong geotechnical influence on the observed behavior and damages. Much of the response could be termed "expected," but research following the earthquake has led to a refined understanding of previously defined problems and development of new areas of focus. For example, the earthquake allowed (1) a first-time "test" of sites that had been improved to resist liquefaction; (2) evaluation of soil density changes by comparing pre- and post-earthquake site test results; (3) direct measurement of effects of site amplification; (4) at least limited documentation of liquefaction-induced settlements and lateral movements; and (5) indirect measurement of the response of major landfills, underground structures, and reinforced earth retaining systems. 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. For example, even though up to 4,000 landslides occurred (Keefer, in press), such events were moderated by the effects of a four-year drought in Northern California. Other factors that should be considered in attempting to extrapolate lessons from the earthquake include the moderate size of the earthquake, the distance of the epicenter from large population centers and soils susceptible to liquefaction, the unique nature of the fault break and the relatively short duration



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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council 2 The Geotechnical Aspects G. Wayne Clough, James R. Martin, II, and Jean Lou Chameau INTRODUCTION Evidence obtained immediately following the Loma Prieta earthquake and in subsequent studies indicated a strong geotechnical influence on the observed behavior and damages. Much of the response could be termed "expected," but research following the earthquake has led to a refined understanding of previously defined problems and development of new areas of focus. For example, the earthquake allowed (1) a first-time "test" of sites that had been improved to resist liquefaction; (2) evaluation of soil density changes by comparing pre- and post-earthquake site test results; (3) direct measurement of effects of site amplification; (4) at least limited documentation of liquefaction-induced settlements and lateral movements; and (5) indirect measurement of the response of major landfills, underground structures, and reinforced earth retaining systems. 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. For example, even though up to 4,000 landslides occurred (Keefer, in press), such events were moderated by the effects of a four-year drought in Northern California. Other factors that should be considered in attempting to extrapolate lessons from the earthquake include the moderate size of the earthquake, the distance of the epicenter from large population centers and soils susceptible to liquefaction, the unique nature of the fault break and the relatively short duration

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council of strong shaking, and the presence of low reservoir levels behind earth dams and embankments. These conditions make it essential that care is taken in using the lessons learned from the earthquake. 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. It is notable that in the literature search for this paper many of the early, sometimes seemingly sensational, documents now seem dated. Their value for the future will not lie in the profundity of the insights developed but in the raw observations made of patterns of behavior. Clearly, the more recent publications, which reflect careful studies conducted in the intervening years since the event itself, are beginning to decipher properly the true causes of behavior. Also, with the publication of more research results, patterns are emerging that were not obvious before. This paper should be viewed as a summary of findings to date. Further useful results will undoubtedly be forthcoming. OVERVIEW It is useful to review some aspects of the Loma Prieta earthquake that are important to the geotechnical response associated with it. The Ms = 7.1 event (Mw = 6.9) was a moderate earthquake, with an epicenter located in the Santa Cruz Mountains, about 11 miles (18 km) from Santa Cruz and 60 miles (97 km) from the San Francisco Bay area. The causative fault rupture was bilateral, with a medial location of the epicenter. As a result, the strong shaking lasted only 8 to 15 seconds, shorter by as much as a factor of two relative to durations normally associated with an event of this magnitude. For the subsurface materials, this translates to a smaller number of cycles of loading than would have occurred otherwise. The map in Figure 2-1 shows the position of the epicenter, major population centers, and locations of liquefaction-induced damage and landslides. As expected, there is a concentration of damages and landslides near the epicentral area, which reflects the high level of accelerations and steep terrain in this vicinity. South of the epicenter, in the vicinity of Santa Cruz, Watsonville, and Moss Landing, certain land masses were particularly susceptible to ground movement due to liquefaction and landsliding. Damages were also concentrated to the north of the epicenter in the San Francisco Bay area, where the type of earthquake motions and the soil conditions combined to produce site amplification and liquefaction in a heavily populated area. These effects are explored in more detail subsequently. Table 2-1 lists peak horizontal accelerations and durations of strong shaking for a number of locations near the epicenter and in the San Francisco Bay area.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council FIGURE 2-1 Regional map of earthquake damage due to liquefaction and landsliding (after Seed et al., 1991 ).

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council TABLE 2-1 Peak Horizontal Accelerations at Selected Sites in the Loma Prieta Earthquake Location Epicentral Distance (miles)* Peak Ground Accelerations (gs) Ground Condition Epicenter 0 0.65 Rock Santa Cruz 11 0.47 Rock Watsonville 12 0.40 Rock San Jose 14 0.25 Stiff Soil San Francisco Airport 52 0.33 Fill/Soft Soil Ricon Hill, San Francisco 63 0.10 Rock Yerba Buena Island 64 0.07 Rock Treasure Island 64 0.16 Fill/Soft Soil Emeryville 65 0.24 Fill/Soft Soil * (1 mile = 1.6 km). The highest recorded accelerations were 0.6 g near the epicenter, and attenuation patterns of accelerations with distance from the epicentral region followed largely expected trends with some exceptions. Some 40 to 60 miles (64 to 97 km) from the epicentral region in the San Francisco Bay area, the peak accelerations varied from 0.05 g to 0.33 g, with the higher values associated with soft soil sites and the lower values recorded in rock and hard soil sites. LIQUEFACTION Occurrence And Recurrence 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. In a number of cases, liquefaction was accurately predicted prior to the earthquake (Clough and Chameau, 1983; Dupre and Tinsley, 1980). Recurrence of liquefaction in the same locations as in the 1906 San Francisco earthquake (Ms = 8.3) was not uncommon (Seed et al., 1991; O'Rourke et al., 1991). However, where damage patterns due to liquefaction in the earthquake mimicked those from the 1906 earthquake, the severity of liquefaction and damage from the earthquake was less than that associated with the 1906 event. Well-documented liquefaction failures were associated with uncompacted, saturated, sandy fills in the central San Francisco Bay region (EERI, 1990). Figure 2-2 indicates where sandy fills are present on the eastern side of San

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council 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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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Francisco. The majority of these fills were placed in the late 1800s or the early to mid-1900s and consist of sands that were dumped or dredged into place and allowed to settle in suspension (Dow, 1973). Fills placed after 1950 tended to have been subjected to some form of compaction. Table 2-2 provides a description of many of the major fills and their placement processes. It is important to note that the fills that were dumped into the bay consisted of a wide range of materials, including rubble from construction and demolition. Another commonly dumped fill material was dune sand, a soil that was abundant in the early days of filling of the waterfront areas. Dune sand has a uniform, medium gradation and is largely free of fines. With time, sands were also dredged from sediments in San Francisco Bay. These materials typically contained fines, in contrast to the clean dune sands, and they attained lower fill densities than the dumped clean sands. During the Loma Prieta earthquake, differences in responses of fills created by different placement techniques was exhibited in areas like the Marina District (Bonilla, 1992; O'Rourke et al., 1990, 1991). The dredged fills exhibited a tendency to liquefy more readily than the dumped fills. There were major fills around the bay that behaved well in the earthquake. In almost all cases, specific measures had been taken to compact the fills while they were being placed, or after placement. Those densified after placement are described in a subsequent section of this paper. The fills at the San Francisco Airport, and those at Foster City and Redwood Shores, were at least partially compacted during placement and exhibit medium to dense densities (EERI, 1990). In some cases, the fills also contain shells and are partially cemented. No significant liquefaction was found in these fills. The Loma Prieta earthquake provided the first opportunity to assess the accuracy of regional liquefaction susceptibility maps (Tinsley and Dupre, in press). The susceptibility map in Figure 2-3 was developed by Dupre and Tinsley (1980) for the Monterey Bay region using the procedures of Youd and Perkins (1987). Liquefaction susceptibilities were based on the occurrence of a large event like the 1906 earthquake. 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. This can be explained in terms of (1) the small size of the earthquake relative to the 1906 event, (2) lower water tables than those expected, and (3) local differences between grain sizes of deposits identified as liquefiable. The latter item was important in that flood plain deposits that were clay-rich did not fail, whereas areas of sand-rich tidal flat and abandoned channel deposits did fail. While broad mapping of liquefaction susceptible soils inherently has difficulty in capturing details, such as fines content or water table fluctuations, it is a valuable guide in identifying areas of potential problems.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council TABLE 2-2 Behavior of Fill Soils in Central San Francisco Bay Region in the 1989 Loma Prieta Earthquake Sites in City of San Francisco Fill Type Soil Type Fill Density Liquefaction Damage During Loma Prieta Earthquake* Embarcadero Unimproved End-Dumped Fine Sand Loose to Medium Moderate to Minor Hunter's Point Cofferdam Unimproved Hydraulic Fine, Silty Sand V. Loose to Loose Severe Manna District Unimproved Hydraulic, Fine, Silty Sand V. Loose to Loose Severe   Unimproved End-Dumped, Fine Sand Loose to Medium Moderate   Natural Ground Find Sand Medium to Dense None Mission District Unimproved End-Dumped Fine Sand, Rubble Loose to Medium Moderate to Minor Pier 80, 84 Improved Dumped Fine Sand Medium to Dense None Pier 45 Unimproved End-Dumped Fine Sand V. Loose to Loose Severe Sites Outside         San Francisco         Alameda Island Unimproved Hydraulic Fine, Silty Sand V. Loose to Loose Severe Bay Farm Island Unimproved Hydraulic Fine, Salty Sand V. Loose to Loose Severe Sites outside City of San Francisco Fill Type Soil Type Fill Density (V-Very) Liquefaction Damage During Loma Prieta Earthquake* Foster City Roller-Compacted Fine Sand, Some Cementation Medium to Dense None Oakland Airport Unimproved Hydraulic Fine, Silty Sand V. Loose to Loose Severe San Francisco Airport Roller - Compacted Fine Sand Medium to Dense None Seventh Street Terminal Unimproved Hydraulic Fine, Silty Sand V. Loose to Loose Severe Treasure Island Unimproved Hydraulic, Fine, Silty Sand V. Loose to Loose Severe   Improved Hydraulic Fine, Silty Sand Medium to Dense None *NOTES: Minor = Slight lateral spreading and/or settlements, little surficial evidence. Moderate = Minor lateral spreading and/or limited settlements, sand boils, etc. Severe = Large lateral deformation and/or settlements, large & numerous sand boils, etc. V. = Very

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council FIGURE 2-3 (a) Site map showing areas of Monterey Bay Region for which liquefaction susceptibility maps were developed;  (b) Liquefaction susceptibility map developed for area indicated by inset in (a) (adapted from Tinsley and Dupre, 1992).

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Settlements Differential settlements due to liquefaction were widespread during the Loma Prieta earthquake. In level ground areas, the settlements were generally caused by a loss of fill volume during sand boiling and consolidation of the fills following pore pressure build-up. However, in instances where liquefaction occurred in the presence of a slope (even a very mild slope), downslope lateral movements in the soils caused settlements in the upper reaches of the soil mass that moved (see next section). Magnitudes of the settlements were essentially a function of the thickness of the liquefiable soils and the liquefaction potential of the soils. Liquefaction-induced settlements were believed to have caused failures in the San Francisco Municipal Water Supply System (Scawthorn et al., 1991); structural damages in the Marina District (Mahin, 1991), the South of Market area (Seed et al., 1991), Fisherman's Wharf and the Embarcadero (Chameau et al., 1991), and Treasure Island and the Oakland Port (EERI, 1990; Egan and Wang, 1991; Seed et al., 1991); and damages to highways and runways, for example, the Oakland Airport (EERI, 1990). It should be noted that even at sites where seawall and containment dikes were present, large settlements still occurred in hydraulic fills behind these support systems. Accurate measured values of settlement due to liquefaction could not be derived from the information available to investigators. However, reasonable estimates could be made in some cases, and O'Rourke et al. (1991) were able to test existing methods of prediction of settlement caused by liquefaction. The procedures use results from Standard Penetration Tests (SPT) or Cone Penetration Tests (CPT) as input parameters. It was concluded that the methods worked well for clean sands but were not accurate in sands with fines unless corrections were applied to account for the effects of fines on the blow counts or cone penetration resistances. 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 2-2). These fills typically slope from 0.5 percent to 2 percent downhill toward the bay and are restrained by seawalls that run along the perimeter of the waterfront. Indications of lateral spreading in these fills during the earthquake were relatively minor. Much of the prominent pavement buckling in the central Marina District was attributed to oscillatory movements, not spreading. Except for an area near the marina where about 2 ft (0.6 m) of lateral movement occurred at St. Francis Spit (Taylor et al., 1992), permanent downslope (toward the bay) displacements of the fills were typically less than several inches. It must be kept in mind, however, that these movements were undoubtedly partially controlled by

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council the presence of seawalls. Interestingly, Mitchell et al. (1991) have presented the thesis that the presence of large box culverts—typically 25 ft (7.6 m) wide and 30 ft (9.2 m) deep—buried along the perimeter of Marina Green helped to control the lateral spreading of the fills that liquefied in this area. This seems reasonable, and the stabilizing effect could have been amplified by the densification of the fills around the culverts that occurred as the sheet piles for the excavation support were vibrated into place (Clough and Chameau, 1980). The relative lack of ground movements in the fills in the Old Mission Bay area was surprising, considering that lateral movements of up to 8 ft (2.4 m) occurred in this region during the 1906 earthquake (Youd and Hoose, 1978). 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. Possibilities include densification as a result of the 1906 earthquake, or that additional filling has occurred along the waterfront in this area since the 1906 earthquake. It is known that much of the rubble from buildings damaged during the 1906 event was pushed into the bay near the mouth of the old Mission Creek channel (Dow, 1973), and this could provide some buttressing support to the fills located farther inland. Lateral spreading in the central San Francisco Bay region also occurred at Treasure Island, the Oakland Port, and other areas along the eastern bay shore. Similar to the occurrences in San Francisco, movements at these sites were also restrained by seawalls and containment dikes. Because the influence of the containment structures upon the observed movements is difficult to quantify, the actual deformation behavior of the soils is somewhat moot, and the movement data from these sites would be of limited use in the development of methods to predict lateral movements in liquefied soils. Useful data on lateral spreading was obtained from the Monterey Bay area, where approximately 50 lateral spread sites were investigated by Tinsley and Dupre (1992). Lateral spreading throughout this region was strongly related to geologic facies. Approximately 95 percent of the spreads occurred in late Holocene fluvial point-bar deposits, fluvial channel deposits, and estuarine deposits. Beach and alluvial fan deposits rarely liquefied (see Figure 2-4). Geotechnical data from the field sites in the Monterey Bay area have yet to be sufficiently analyzed to develop relationships between the magnitude of the movements and the factors that controlled the movements. However, preliminary analyses suggest that the movements were not merely a function of simplified parameters such as slope angle, free-face height, etc. Although it was clear at all sites that the largest movements occurred near the free faces of laterally displaced slopes, there was no consistency in the ''safe setback'' distance from the free faces. At each site, the overall size of the soil mass that moved laterally was apparently controlled by the extent of the geologic unit in which the spread developed. Consistency for this selective behavior was confirmed by SPT and CPT measurements, which indicated soil conditions in the young fluvial and estuarine

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council FIGURE 2-4 Histogram showing distribution of lateral-spread ground failures according to sedimentary facies for the Loma Prieta earthquake (after Tinsley and Dupre, 1992). sediments to be the most favorable for liquefaction relative to older geologic units within the region. Tinsley (1993) compared measured lateral movements in sandy soils in the Monterey Bay region with movement predictions obtained using the Liquefaction Severity Index (LSI) method of Youd and Perkins (1987). The results indicated the upper-bound estimates for lateral movements from the LSI method were too low. The LSI method predicts lateral movements based on earthquake magnitude and distance to the vertical projection of the fault rupture; it does not consider soil parameters. A more rigorous technique, developed by Bartlett and Youd (in press), 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 (Clough and Chameau, 1983; Chameau et al., 1991). Two of the principal study areas, known as the TH and YBC sites, were located along the Embarcadero north of the Bay Bridge and Market

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Compacted or improved sand fills were made resistant to liquefaction in the Loma Prieta earthquake, but such improvements did little to reduce ground motions. The earthquake did not provide full design loading for the improved sites. There is a need for improved methods for predicting vertical and lateral movements caused by liquefaction, with allowances for the effects of fines on sand behavior. Broad mapping of liquefaction susceptibility identified areas of potential concern, but details of the subsoil conditions controlled the actual occurrence of liquefaction. Liquefaction, in a number of cases in the earthquake, represented a recurrence of liquefaction that occurred in the 1906 San Francisco event. Densification of sands after liquefaction occurred in a number of loose sandy fills. No densification was observed if the sands were of a medium density. Site Amplification Although site amplification was most prominent for soft soils, it also occurred in stiff soils and weak rocks under specialized conditions. Site effects not only amplified accelerations but also changed the frequency content of the motions. 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. One-dimensional site-response analysis tools were effective in explaining most ground motion issues, but more sophisticated methods were needed in cases where the underlying bedrock surface was non-uniform. Slopes, Fills, And Embankments Slope failures left major transportation arteries closed for extended periods of time. Prominent topographical features (bluffs, promontories, etc.) apparently amplified accelerations and in some cases were subject to rapid failure because of resonance effects, wave reflection, and lack of confinement. Coherent, deep-seated sliding occurred only near the seismic energy source zone. Predictions of the amount of movement of coherent landslides under the earthquake loading based on laboratory-derived material parameters were not successful; back-analysis of existing slides appeared to offer a better alternative to obtain material parameters.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Not all landslides induced by pre-Loma Prieta earthquakes were re-activated by the Loma Prieta earthquake, but incipient landslides were activated by the event. Well-designed earth dams sustained large levels of shaking, but localized damages could have led to problems if reservoir levels had been high. Valuable measurements were made of the acceleration response of earth dams that can be used in future studies. Sanitary landfills performed well in the earthquake, apparently in part due to their high damping capacity. Waterfront Containment Structures, Piers, And Retaining Structures Seawalls limited ground movements in the presence of moderate liquefaction but were less effective where liquefaction was extensive. Liquefaction of fills contained within, or underlying, pier facilities was the primary source of damages to retaining systems and piers. Conventional retaining structures and more-recently developed systems, such as soil nailed walls, performed well, even where subjected to large accelerations. Foundations In areas of prominent liquefaction, structures founded on shallow footings were damaged, while those supported on piles embedded in non-liquefiable materials performed satisfactorily. Notably, lateral movements of the soil surrounding the piles were typically less than one foot. Several bridges founded on alluvial soils in the Monterey Bay area suffered damage due to lateral movements of liquefied foundation soils. Structures that were constructed with batter piles to provide lateral restraint suffered heavier damage than adjacent structures with vertical piles. Seismic motions varied between the foundation supports of some long-span structures. 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. The positive performance of the CWP culverts and the BART tunnels maintained the viability of essential portions of the infrastructure in the Bay Area. The lack of damage for the CWP culverts and the BART tunnels at the

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council San Francisco Waterfront was in sharp contrast to the damages to the aboveground Embarcadero elevated freeway system. The CWP culverts performed well even in the presence of liquefied fills, because they were designed for this loading condition. 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; lack of consideration of site amplification effects in design of many older structures; potential problems that continue to exist with landslides along the Pacific Coast and in the Coastal Mountain Range; the need for better understanding of how to incorporate site amplification effects into design spectra; and the need for re-assessment of what was perceived to be satisfactory performance in the Loma Prieta earthquake relative to what might occur in a larger event. ACKNOWLEDGEMENTS The writers would like to gratefully acknowledge the assistance received in preparation of this paper. Information and advice was provided by T. L. Youd, D. Koutsoftas, M. M. Chiu. T. D. O'Rourke, M. S. Power, S. E. Dickenson, J. C. Tinsley, and D. K. Keefer. Assistance was also received from the Earthquake Engineering Research Institute, the Earthquake Engineering Research Center of U.C. Berkeley, and the National Center for Earthquake Engineering of the State University of New York at Buffalo. To all those who helped, we owe a significant vote of thanks. REFERENCES Bardet, J.P., M. Kapuskar, G.R. Martin, and J. Proubet. 1992. Site Response Analysis, The Loma Prieta California Earthquake of October 17, 1989—Marina District. U.S. Geological Survey Professional Paper, 155 1-F. Washington, D.C.: United States Government Printing Office. Bartlett, S.F., and T.L. Youd. In press. Empirical Prediction of Lateral Spread Displacement. Proceedings, 4th U.S.-Japan Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction," Honolulu, Hawaii, May, 1992. Boatwright, J., L.C. Seekins, T.E. Furnal, H. Liu, and C.S. Mueller. 1992. Ground-Motion Amplification, The Loma Prieta, California, Earthquake of October 17, 1989—Marina District. U.S. Geological Survey Professional Paper, 1551-F. Washington, D.C.: United States Government Printing Office.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Bonilla, M.G. 1992. Geological and Historical Factors Affecting Earthquake Damage. The Loma Prieta. California, Earthquake of October 17, 1989—Marina District. U.S. Geological Survey Professional Paper, 1551-F. Washington. D.C.: United States Government Printing Office. Borcherdt, R.D., and G. Glassmoyer. 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. Bulletin of the Seismological Society of America. 82(2):603-641. Buranek. D., and S. Prasad. 1991. Sanitary Landfill Performance During The Loma Prieta Earthquake. Pp. 1655-1660, in Proceedings, Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, Rolla, Missouri, Vol. II. Chameau, J.L., G.W. Clough, F. Reyna, and J.D. Frost. 1991. Liquefaction Response of San Francisco Bayshore Fills. Bulletin of the Seismological Society of America, 81 (5):2998-2018. Chiu, M.M. 1993. Personal communication. Clough, G.W., and J.L. Chameau. 1980. Measured Effects of Vibratory Sheet Pile Driving, Journal of the Geotechnical Engineering Division, ASCE, 106(GT10):108-1100. Clough, G.W., and J.L. Chameau. 1983. Seismic Response of San Francisco Waterfront Fills. ASCE Journal of the Geotechnical Engineering Division, ASCE, 109( ):491-506. Cole, W.F. 1991. Landslides Triggered by the Loma Prieta Earthquake, Implication for Zonation. Pp. 653-660 in Proceedings. Fourth International Conference on Seismic Zonation, Stanford, California, August, 1991. Dickenson, S.E., R.B. Seed, J. Lysmer, and C.M. Mok. 1991. Response of Soft Soils During the 1989 Loma Prieta Earthquake and Implications for Seismic Design Criteria. Proceedings, Pacific Conference on Earthquake Engineering, Auckland, New Zealand, November, 1991. Douglas, W.S., and R. Warshaw. 1971. Design of Seismic Joint for San Francisco Bay Tunnel. ASCE, Journal of the Structural Engineering Division, 7:1129-1141. Dow, G.R. 1973. Bay Fill in San Francisco: A History of Change. San Francisco State University, M.A., Thesis. Dupre, W.R., and J.H. Tinsley. 1980. Maps Showing Geology and Liquefaction Potential of Northern Monterey and Southern Santa Cruz Counties, California. U.S. Geological Survey, Miscellaneous Field Studies Map, MF-1199. EERI. 1990. Loma Prieta Earthquake Reconnaissance Report, Earthquake Spectra, Supplement Vol. 6. Earthquake Engineering Research Center. 1992. News. Vol. 13. No. 2. June. Egan, J.A., and Z.L. Wang. 1991. Liquefaction-Related Ground Deformation and Effects on Facilities at Treasure Island, San Francisco, During the 17 October 1989 Loma Prieta Earthquake. Pp. 57-76. Proceedings, 3rd Japan-U.S. Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction, Buffalo, NY, February, 1991. Harder, L.F. 1991. Performance of Earth Dams During the Loma Prieta Earthquake. Pp. 1613-1629 in Proceedings, 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, Rolla, Missouri, Vol. II, March, 1991. Idriss, I.M. 1990. Response of Soft Soil Sites During Earthquakes. Pp. 273-290 in Proceedings, H.B. Seed Memorial Symposium, Vol. 2. Johnson, M.E., M. Lew, J. Lundy, and M.E. Ray. 1991. Investigation of Sanitary Landfill Slope Performance During Strong Motion From the Loma Prieta Earthquake of October 17, 1989. Pp. 1701-1708 in Proceedings, 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, Rolla, Missouri, Vol. II. Keefer, D.K., ed. In press. The Loma Prieta, California, Earthquake of October 17, 1989—Landslides. NEHRP Report to Congress, U.S. Geological Survey, 1993. Kuesel, T.R. 1968. Structural Design of the Bay Area Rapid Transit System. Civil Engineering, ASCE, April, pp. 1-6. Mahin, S.A. 1991. The Loma Prieta Earthquake: Implications of Structural Damage LP03. Pp. 1587-1600 in Proceedings, 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, Vol. II, March, 11-15, Rolla, Missouri.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Makdisi, F.I., C.Y. Chang, Z.I. Wang, and C.M. Mok. 1991. Analysis of the Recorded Response of Lexington Dam During Various Levels of Ground Shaking. Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, Sacramento, Calif., May 30, pp. 10-1 to 10-10. Mitchell, D., R. Tinawi, and R.G. Sexsmith. 1991. Performance of Bridges in the 1989 Loma Prieta Earthquake—Lessons for Canadian Designers. Canadian Journal of the Geotechnical Engineering Division, No. 18, pp. 711-734, January. Mitchell, J.K., and F.J. Wentz. 1991. Performance of Improved Ground During the Loma Prieta Earthquake. Earthquake Engineering Research Center Report 91/12, University of California at Berkeley, October. Mitchell, J.K., T. Masood, R.E. Kayen, and R.B. Seed. 1990. Soil Conditions and Earthquake Hazard Mitigation in the Marina District of San Francisco. Earthquake Engineering Research Center Report 90/08, University of California, Berkeley. Nolan, J.M., and G.E. Weber. In press. Trenching Studies in the Summit Ridge Area. In Keefer, D.K., ed., The Loma Prieta, California Earthquake of October 17, 1989: Landslides and Stream Channel Change . U.S. Geological Survey. O'Rourke, T.D., T.E. Gowdy, H.E. Stewart, and J.W. Pease. 1990. Lifeline Performance and Ground Deformations in the Marina During the 1989 Loma Prieta Earthquake. Pp. 129-146 in Proceedings, 3rd U.S.-Japan Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction, San Francisco. O'Rourke, T.D., J.W. Pease, and H.E. Stewart. 1992. Lifeline Performance and Ground Deformation During the Earthquake. The Loma Prieta California, Earthquake of October 17, 1989— Marina District. U.S. Geological Survey Professional Paper 1551-F. Washington, D.C.: United States Government Printing Office. Plant, N., and G.B. Griggs. 1990. Coastal Landslides Caused by the October 17, 1989 Earthquake, Santa Cruz County, California. California Geology, 43(4):75-84. Sayed, H.S., A.M. Abdel-Ghaflar, and S.F. Masri. 1991. Parametric System Identification and Seismic Performance Evaluation of Earth Dams During the October 17, 1989, Loma Prieta Earthquake. Report No. CRECE-91-03, University of Southern California, Department of Civil Engineering, July. Scawthorn. C.R.. T.D. O'Rourke, M.M. Khater, and F. Blackburn. 1991. Loma Prieta Earthquake and The San Francisco AWSS: Analysis and Observed Performance. Pp. 527-540 in Proceedings, 3rd Japan-U.S. Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction, Buffalo, NY, February. Seed, H.B., F.I. Makdisi. and P. DeAlba. 1978. Performance of Earth Dams During Earthquakes. Journal of the Geotechnical Engineering Division, ASCE, GT-7, July. Seed, R.B., S.E. Dickenson, and I.M. Idriss. 1991. Principal Geotechnical Aspects of the 1989 Loma Prieta Earthquake. Soils and Foundations, Japanese Society of Soil Mechanics and Foundation Engineering, 31 (1):1-26. Sharma, H.D., and H.K. Goyal. 1991. Performance of a Hazardous Waste and Sanitary Landfill Subjected to Loma Prieta Earthquake. Pp. 1717-1725 in Proceedings, 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, Rolla, Missouri, Vol. II. Sitar, N., and G.W. Clough. 1983. Seismic Response of Steep Slopes in Cemented Soils. Journal of the Geotechnical Engineering Division, ASCE, 109(2):210-227. Spittler. T.E., and E.L. Harp, compilers. 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. U.S. Geological Survey Open-File Report 90-688, Scale 1:4,800. Taylor, H.T., J.T. Cameron, S. Vahdani, and H. Yap. 1992. Behavior of the Seawalls and Shoreline During the Earthquake. The Loma Prieta California Earthquake of October 17, 1989—Manna District, U.S. Geological Survey Professional Paper, 1551-F, Washington, D.C.: United States Government Printing Office.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council Tinsley, J.C. 1993. Personal communication. Tinsley, J.C., and W.R. Dupre. In press. Liquefaction Hazard Mapping, Depositional Facies, and Lateral Spreading Ground Failure in the Monterey Bay Area, Central California. Proceedings. 4th Japan-U.S. Conference on Earthquake Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction, Honolulu, Hawaii, May, 1992. Youd, T.L., and S.W. Hoose. 1978. Historic Ground Failures in Northern California Triggered by Earthquakes. U.S. Geological Survey Professional Paper 993. Youd, T.L., and D.M. Perkins. 1987. Mapping of Liquefaction Severity Index. Journal of the Geotechnical Engineering Division, ASCE, 113:1374-1392.

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council DISCUSSANTS' COMMENTS: GEOTECHNICAL ISSUES William Cotton, William Cotton & Associates As a geologist, these are the three things I would like to expand upon from Dr. Clough's paper: (1) seismic zonation maps, (2) landslide reactivation, and (3) risk communication. Concerning seismic zonation maps, the ground did behave as expected throughout the Bay Area and throughout the epicentral region. There were maps in place before the earthquake that predicted ground behavior. The predictive kinds of seismic hazards, for which I have maps, include liquefaction-induced ground failures, landslide reactivation, and ground rupture in the Santa Cruz mountains and along the San Andreas fault. There is work being done by Roger Borcherdt at the U.S. Geological Survey and others about trying to predict the response of the ground to certain types of earthquake excitation. Amplification capability maps will come into greater use in the future. From these maps, the state of California has a new law in effect called the Seismic Mapping Hazard Act, which will take the three other hazards—shaking, liquefaction, and landslides—and try to produce maps based upon seismic response. Good ground behavior prediction maps were available for the Marina District, the San Francisco Bay margins, the city of Santa Cruz, and the Watsonville and the Summit Road region of the Santa Cruz mountains. Concerning landslides, a large number of old deep-seated landslides in the epicentral region were reactivated during the earthquake. Most of these ''coherent'' landslides occurred in the Santa Cruz mountains. These slope failures provided a unique opportunity to evaluate the seismic slope stability and to test the dynamic slope stability methods currently being used in geotechnical practice. In addition, seismic displacement calculations were found to be very sensitive to selected yield coefficients, shear strength values, and acceleration-time history of the ground motion. The most challenging risk, as I see it, is the transfer of geohazard information to individuals or groups that are charged with mitigating. Engineers and scientists have a poor record of packaging their research results and transferring their knowledge to the public. Thank you. Maurice S. Power, Geomatrix Consultants I would like to elaborate on two locations of liquefaction during the Loma Prieta earthquake. These were the Port of Oakland's 7th Street Marine Terminal and Treasure Island. Both are areas where hydraulic fill was placed through bay waters to create land. During the earthquake the hydraulically placed sand fill liquefied at the marine terminal. As a result, the yard area settled by several inches, as did the rear crane rail, resulting in discontinuation of the crane operation. Lateral spread-

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council ing movements caused extensive damage to the rear batter piles. The Seed-Idriss correlation for liquefaction potential predicted the occurrence of liquefaction. The Port of Oakland has reconstructed the facility replacing the batter piles with octagonal vertical piles. Of particular interest from the geotechnical viewpoint is that the ground has been improved. Post-earthquake ground improvement (vibro-replacement) was used to densify the sand to resist future lateral spreading. Comparison of pre- and post-improvement blow counts in the sand indicates that the sand was effectively densified at the Port of Oakland site using the vibro-replacement technique. At the Treasure Island site, there was a general subsidence of about 4-6 inches (10.16-15.24 cm) during the earthquake, as well as spreading around the island perimeter. Again, the Seed-Idriss correlation agreed with the observations of liquefaction. Building sites on Treasure Island where pre-earthquake ground improvement had been implemented performed satisfactorily. There was evidence from pre- and post-earthquake survey measurements of an absence of lateral spreading movements at one location on the island perimeter where vibroflotation had been performed, whereas adjacent unimproved areas experienced spreading. Loma Prieta provided a wealth of recorded ground motion data. Improved attenuation relationships for estimating rock motions have been developed using these data. Correlations for assessing site response effects on ground motions have been developed. Knowledge of dynamic soil properties has been improved, and techniques for measuring dynamic properties have been evaluated. Analytical procedures for assessing site response have been found to give reasonable estimations of ground motions for the levels of excitation of the earthquake. Thank you. Thomas Hanks, U.S. Geological Survey The phrase "lessons learned" with respect to earthquakes first came into my consciousness 22 years ago, at the time of the 1971 San Fernando earthquake, when I was a graduate student at an epicentral distance of 62 miles (39 km). Back then, the National Academy of Sciences, which is sponsoring this meeting as well, put out a report entitled something like "Lessons Learned from the 1971 San Fernando Earthquake." We have been learning lessons from earthquakes ever since (and long before, of course), and this is appropriate for a scientific and engineering discipline that relies so heavily on observations, empiricism, and experience. And it is also true that a major earthquake in or near a major metropolitan area will be a learning experience for millions of people, most of whom don't know much about earthquakes. Nevertheless, there is a certain déjà 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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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council first stories, decayed timbers, bad foundations, hydraulic fill, and young bay mud hardly qualify as news here in San Francisco where these "lessons" had all been learned in 1906, if not before. These things keep happening, though, so we keep talking about them, but are we really making any progress in keeping these things from happening? And if not, why not? With the latter question in mind, I learned a few things as a result of my own Loma Prieta experience. The first is that the American public, even in very affluent, very well-educated neighborhoods in the heart of earthquake country, is surprisingly uninformed about even the basics of earthquake occurrence, hazards, and risk. The second is that the practice of earthquake hazards reduction is very different from the theory of earthquake hazards reduction. In reality, earthquake hazards reduction is an intensely local happening involving large sums of money that hardly anyone wants to spend unless one absolutely has to, regardless of whether the source of funds is a government agency at the federal, state, or local level, or a neighborhood organization, or a private citizen. Third, an ounce of prevention in this business, like so many others, is worth a pound of cure, a largely unappreciated nicety, because it means spending money when you "don't have to." I believe we should meet Tom Tobin's challenge: for all of us here and all that we represent, to take advocacy stands and more active roles to inform our citizen colleagues about what we know and to encourage our governing bodies to provide stronger incentives to practice earthquake hazards reduction in advance, so that potential hazards do not become real ones. 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. But selling the idea of potential earthquake damage to a community that is essentially built, a community that has predominately poor ground on what land is left is not so easy. Our ability as group of scientists and engineers to convince the body politic that earthquakes occur and that earthquake engineering allows one to prevent a disaster is more difficult than perhaps on the West coast. How does one translate the lessons learned from Loma Prieta to a different environment? One must transfer the context of the lesson as well as the craft itself. The context in the eastern United States and the western United States is quite different. For example, in the west, the sources of earthquakes are understood, but this is not so in the eastern United States. In the east, we deal with large segments of ground, and we call them seismic source zones. In the west, we deal with linear segments of ground, and we call them faults that produce earthquakes. Another issue that needs to be addressed is that the seismic building code

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Practical Lessons from the Loma Prieta Earthquake: Report from a Symposium Sponsored by the Geotechnical Board and the Board on Natural Disasters of the National Research Council provisions are primarily derived from the California experience. So the rate of seismicity is important as we translate the lessons to the eastern United States context. 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. So we may be designing for the same probability of occurrence of ground motion but nowhere near the same probability of being able to withstand the maximum event. In the eastern United States, the larger events that occur so infrequently may collapse structures, whereas in the western United States, the individual building performance will be significantly better. In the east, the forward-looking view of lessons learned must be tempered by the fact that the east is largely built and built with buildings that have had no seismic attention paid to them, so the issue is how to apply retrofit lessons. Given the design event, we need to find a way to look backwards to a community that's predominately built. Thank you.

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