GEOLOGIC AND HYDROGEOLOGIC CONTROLS OF BOUNDARIES OF LATERAL SPREADS: LESSONS FROM USGS LIQUEFACTION CASE HISTORIES Thomas L. Holzer1 & Michael J. Bennett2 1 2
U.S. Geological Survey (e-mail:
[email protected]) U. S. Geological Survey (e-mail:
[email protected])
Abstract: Predicting locations of boundaries of earthquake-induced lateral spreads is an important challenge in geotechnical earthquake engineering. These boundaries are where differential ground displacements cause damaging strains. Eight lateral spreads were analyzed to determine the extent to which subsurface conditions determined their boundaries. Either local subsurface geologic or hydrogeologic conditions determined the boundaries of most of the lateral spreads. An abrupt change in geologic facies was a common geologic boundary condition. These facies changes included edges of buried channel sands in both floodplain and alluvial fan environments and edges of sandy artificial fill. At one lateral spread, local changes in fines content of a sand bed determined the location of the boundary. Some boundaries were determined by where the water table passed beneath susceptible Holocene sediment into nonsusceptible Pleistocene sediment at the margins of these lateral spreads. This left Holocene sediment dry outside the area of the lateral spread. These geologic and hydrogeologic conditions generally determined the lateral margin of the liquefied interval. INTRODUCTION Lateral spreads involve predominantly horizontal displacements of large surficial blocks of soil; they are caused by earthquake-induced liquefaction in underlying layers (Committee on Earthquake Engineering 1985, p. 22). Predicting where lateral spreading will occur is an important challenge to engineering geologists and geotechnical engineers for engineering design of structures on liquefiable soils in areas subject to earthquake strong ground motion. An important question is “to what extent do local geologic conditions determine the boundaries of a lateral spread?” The answer is relevant to engineering design because damaging strains occur at these boundaries. Empirical and strain-based engineering methods for predicting ground displacements caused by lateral spreading generally ignore geologic considerations and rely only on geotechnical and seismological factors (e.g., Youd et al. 2002; Zhang et al. 2006). Rosinski et al. (2004), who considered geologic age as a predictive factor, is an exception. Here the extent to which local geologic factors determined the locations of boundaries of lateral spreads is examined. The analysis is based on comprehensive field investigations, which included both surface and subsurface exploration, at 8 lateral spreads (Fig. 1). The explorations were all conducted in California since 1971 by the U.S. Geological Survey (USGS) as part of its postearthquake investigation program. Each exploration typically included detailed surface mapping of ground deformation and subsurface exploration with borings and cone penetration tests (CPT) to determine both geotechnical and geologic conditions. The lateral spreads and their causative earthquakes are compiled in Table 1. The reader is referred to the references listed in Table 1 for more information about each lateral spread. These references discuss liquefaction analyses and contain other references to supporting geotechnical data. They also describe the geologic setting of each lateral spread in greater detail. Geologic criteria have long been used to evaluate liquefaction susceptibility of deposits. Youd and Perkins (1978) qualitatively estimated liquefaction susceptibility of different geologic 502
deposits for the purpose of regional mapping of liquefaction hazard. Holzer et al. (2006) recently used the liquefaction potential index to determine liquefaction probabilities of different geologic deposits. Whereas these classification schemes bear indirectly on where lateral spreads may occur, they do not help identify where boundaries are likely to be expressed.
Figure 1. Locations of USGS lateral spread case histories.
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Table 1. USGS case histories of lateral spreads in California Lateral Spread Earthquake Date M Reference Juvenile Hall San Fernando 1971 6.5 (Bennett 1989) Heber Road Imperial Valley 1979 6.5 (Bennett et al. 1981) Wildlife Superstition Hills 1987 6.5 (Holzer and Youd 2007) Miller Farm Loma Prieta 1989 6.9 (Holzer et al. 1994) Balboa Boulevard Northridge 1994 6.7 (Holzer et al. 1999) Wynne Avenue Northridge 1994 6.7 (Holzer et al. 1999) Juanita Avenue San Simeon 2003 6.5 (Holzer et al. 2005) Norswing Drive San Simeon 2003 6.5 (Holzer et al. 2005) CASE HISTORIES 1971 San Fernando Earthquake The 9 February 1971 M6.5 San Fernando earthquake was the first significant urban earthquake to shake California since the 1906 San Francisco earthquake. While shaking caused most of the damage and all of the fatalities, lateral spreading seriously damaged the San Fernando Valley Juvenile Hall.
Figure 2. Map of ground cracks, sand boils, and stream channels at Juvenile Hall in the San Fernando Valley caused by 1971 San Fernando earthquake (Bennett 1989, Fig. 3). The Juvenile Hall lateral spread is in a small alluvial basin in the north-central part of the San Fernando Valley (Fig. 2). The basin contains three small alluvial fans that originate from adjacent local hills. Two small natural drainage channels associated with the local fans also pass close by the lateral spread. One of the channels flows approximately through the area of
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permanent ground deformation. The northwest and southeast boundaries, respectively, of the lateral spread coincide with the North Olive View and Olive View Faults, but neither moved during the 1971 earthquake. The faults partially controlled sediment deposition. The Juvenile Hall lateral spread was 1,200-m long and 270-m wide (Fig. 2). Its axis trended northeast-southwest, and it extended from just east of the northeast wall of Juvenile Hall to the northeast edge of what was then Upper Van Norman Lake (see Upper Lake in Fig. 2). Maximum displacement within the lateral spread was 1.5 m on an average slope of 1.5%. Railroad tracks running by the south side of the Juvenile Hall indicated 9 to 10 cm of compression occurred within the lateral spread. Maximum measured differential subsidence within the lateral spread was 15 cm. A trench along the eastern wall of the Juvenile Hall intersected a crack at the surface with 18 cm of vertical movement. Sand boils occurred within the lateral spread as well as near the toe at the Upper Lake. Depth to the water table at Juvenile Hall was 4.3 m in April of 1971. Peak ground acceleration (PGA) was approximately 0.5 g.
Figure 3. Transverse cross section of the Juvenile Hall lateral spread with CPT profiles and liquefaction factors of safety for the 1971 San Fernando earthquake. Stippled part of B1 is inferred to be liquefied interval (Bennett 1989). See A-B in Figure 2 for location. View is to northeast. The geology of the lateral spread is illustrated in a transverse cross section in Figure 3. Four informal sedimentary units were identified (Bennett 1989). These complexly interbedded units were deposited in an alluvial-fan environment dominated by water-laid deposition. Unit A, which consists of subunits A1 and A2, is a poorly to moderately sorted, loose- to medium-dense sand; unit B, which consists of subunits B1 and B2, is a very loose to loose poorly sorted silty sand to silt; unit C is a medium-dense to dense, poorly sorted silty sand; and unit D is a stiff 505
clayey silt. Liquefaction factors of safety in soundings (Fig. 3) and grain-size analysis of a sand boil indicate liquefaction occurred primarily in unit B1 (Bennett 1989). Liquefaction factors of safety here and in subsequent analyses were computed using the methodology of Youd et al. (2001). The interval with factors of safety less than one is limited to the part of subunit B1 that is below the water table. At borings 4 and 6 (see Fig. 3), the water table intersected the middle of the subunit and limited the thickness of liquefied interval. Liquefaction was limited to the loose and saturated silty sand to sandy silt in unit B1. To the northwest sediments are dense and the water table is lower causing the effective stress and liquefaction resistance to increase; to the southeast, the liquefied interval thins and the sediments are too clayey to liquefy. 1979 Imperial Valley Earthquake The 15 October 1979 Imperial Valley M6.5 earthquake caused widespread liquefaction that damaged roads, canals, and fields in the Imperial Valley (Youd and Wieczorek 1982). The liquefaction provided excellent opportunities to examine the relation between subsurface geology and liquefaction (Bennett et al. 1981), including the Heber Road lateral spread (Figure 4).
Figure 4. Map of ground cracks, sand boil areas, and CPT soundings at Heber Road lateral spread (Bennett et al. 1981, Fig. 10). The Imperial Valley occupies the northern part of the Salton Trough, a major tectonic basin in southern California that is rapidly filling with sediment. Between 300 and 1600 years ago the Colorado River discharged flood waters into the closed depression that forms the valley and created a large prehistoric lake, Lake Cahuilla. After the lake water evaporated, smaller lakes continued to fill small depressions along river channels that dissected and meandered across the
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old lake bed. Deltas formed in these small lakes where streams discharged into them. The Heber Road site at Heber Dunes State Park is located within one of these small abandoned deltas. The Heber Road lateral spread disrupted road pavement and translated the road and an adjacent canal 1.2 m to the south into a depression (Fig. 4). The lateral spread was 160 m wide at the road and 100 m long. Arcuate ground cracks and scarps as high as 0.9 m defined its head. Sand boils formed on the lateral spread, especially along the northern end. No sand boils vented west of the lateral spread. The sand boils east of the lateral spread may have been associated with a buried drain (Youd and Wieczorek 1982). PGA at Heber Road was approximately 0.5 g.
Figure 5. East-west transverse cross section of Heber Road lateral spread with CPT profiles and liquefaction factors of safety for the 1979 Imperial Valley earthquake. Translation of lateral spread is out of the plane of the cross section. Stippled part of unit A2 is inferred to be the liquefied interval (Bennett et al. 1981). See Figure 4 for locations of CPT soundings. The lateral spread occurred in an abandoned channel that had been filled both artificially for agricultural purposes and naturally (see unit A2 in Fig. 5). The uppermost 1.5 m of unit A2 consists of loose fine sand. The average CPT tip resistance of unit A2 is 2 MN/m2; the average blow count is 4. Two laterally discontinuous natural sandy units abut and underlie the fill. On the western margin of the lateral spread, the area is underlain by a dense sandy point bar deposit (unit A1). Its average tip resistance is 15.7 MN/m2; its average uncorrected blow count is 31. Ground deformation was not associated with this unit. On the eastern margin of the lateral spread, the area is underlain by an overbank deposit of loose, fine sand, (unit A3). Its average tip resistance is 4.8 MN/m2; its average blow count is 11. Alternating dense sand beds and stiff clay beds underlie all three of these units at a depth of more than 5 m 507
Liquefaction factors of safety computed from CPT soundings at Heber Road indicate that the subsurface geology determined the location and boundaries of the lateral spread (Fig. 5). The lateral spread was restricted to the area underlain by channel fill (unit A2) in which factors of safety were less than one. Lateral spreading was not observed in the area underlain by units A1 and A3. In general, both units were less liquefiable than the channel fill. 1987 Superstition Hills Earthquake The 24 November 1987 Superstition Hills M6.6 earthquake in the western Imperial Valley provided the first recording of the build up of pore-water pressure in a sand layer undergoing liquefaction. Prior to the earthquake, the USGS installed strong motion instrumentation and porewater pressure transducers along the Alamo River at a site that had liquefied during the 1981 Westmorland earthquake (Bennett et al. 1984). The site, which is known as the Wildlife Liquefaction Array, was instrumented under the leadership of T. L. Youd following extensive subsurface exploration. The site was selected because of the regionally high rate of seismicity and the likelihood that the sediment would reliquefy in a future event. The prescience of the decision was confirmed in 1987 when liquefaction occurred during the 1987 earthquake (Holzer et al. 1989b).
Figure 6. Cross section through Wildlife Liquefaction Array with liquefaction factors of safety for the 1987 Superstition Hills earthquake and CPT profiles. Stippled unit B is inferred to be the liquefied interval (Holzer et al. 1989b). See Figure 7 for location. Sediments at the array consist of young (
