Developments in Liquefaction Analysis from Observations during Earthquakes

Liquefaction has occurred during most earthquakes, Niigata 1964, Tangshan 1976, Loma Prieta 1985, Kobe 1995, Chi-Chi Taiwan 1999, Turkey 1999, Haiti 2010, Mexicali 2010 and others. The performance of structures during the Niigata earthquake (1964) in Japan and the nature of damage suffered by them clearly brought out the fact that soil and soil foundation interaction play a significant role in controlling their performance. The extensive liquefaction of loose saturated sands during the Niigata earthquake provided a field verification of the liquefaction phenomenon. Several other earthquakes in Japan also induced liquefaction. It was mostly believed as a necessary consequence that only cohesionless soils are prone to liquefaction and silts and clay do not liquefy. The observations during the Tangshan earthquake and other earthquakes later on suggested that silts and low plasticity clays may also liquefy. The observations on performance of soils during earthquakes has, thus , contributed to better understanding of the susceptibility of different types of soils to liquefaction and development of methods of analysis of liquefaction as discussed in the paper. Introduction: Forensic engineering as applied to geotechnical engineering may be defined as investigation of geo-materials and soil-structures that fail or do not perform as intended resulting in damage or injury. Generally the purpose of Forensic engineering investigation is to determine the cause of failure with a view to improve performance in addition to other issues such as liability. From geo-hazard mitigation point of view forensic analysis should provide an answer to „what happened‟, „why it happened‟ and „how can it be prevented from happening‟. This is very relevant to the case of liquefaction phenomenon. Although liquefaction was known to have occurred in earlier times, the devastating effects of liquefaction during the March 27, 1964 Alaska (M=8.6) and June 16, 1964 Niigata (M= 7.5) earthquakes attracted the attention of geotechnical engineers. These two earthquakes occurred within period of about 90 days. Both these earthquakes caused extensive liquefaction and resulting damage which included slope failures, bridge and building foundation damage and floatation of buried structures. Some well known examples of liquefaction damage during these earthquakes are shown in Figures 1-3. Figure 1 shows the damage suffered by house displacement and tilting caused by liquefaction in the Turnagain Height area of Anchorage during the 1964 Alaska Earthquake. Figure 2a shows Collapse of the superstructure of the Showa Bridge by falling off its piers during Niigata Earthquake . Tilting of apartment buildings at Kawagishi-Cho, Niigata due to ground failure caused by liquefaction is shown in figure 2b.. Since the liquefaction in these earthquakes occurred in poorly graded sands at low to medium density, it was generally believed that only cohesionless soils are prone to liquefaction and fine grained soils do not liquefy. The Tangshan earthquake, July 28,1976 (M=7.5) provided evidence of liquefaction in low plasticity silts and several years later in the mid –eighties liquefaction aspects of silty soils were investigated. Later on , it has been recognized that all soils and including low plasticity clays should be considered liquefiable unless investigations prove otherwise. Liquefaction clayey deposits was observed in some earthquakes in Taiwan and Iran. It is , thus seen, that developments in liquefaction analysis have been strongly influenced by the evidence on their performance during the significant earthquakes. The paper discusses the developments in investigation of liquefaction analysis of sands and fine grained soils as they developed following several devastating earthquakes. If the liquefaction susceptibility of a soil can be ascertained before hand and it is found to be prone to liquefaction for the design earthquake, then measures can incorporated to mitigate the hazard. LIQUEFACTION INVESTIGATIONS: The liquefaction became important following the damage caused by the Alaska(1964) and the Niigata (1964) earthquakes. The studies were devoted to sands included the following: (a) Investigation of sites damaged by earthquakes (b) Laboratory tests using undarined cyclic triaxial and cyclic simple shear devices. (c) Vibration or shake table tests (d) Field tests such Standard Penetration tests (SPT) and Cone Penetration tests (CPT) and Shear Wave Velocity test. (e) Numerical analysis. STUDIES ON LIQUEFACTION OF SANDS: The laboratory studies helped identify the factors governing liquefaction of soils. Seed and Lee (1966) reported the first comprehensive data on liquefaction of sand using the cyclic triaxial test. Peacock and Seed (1968) used oscillatory shear device to study liquefaction in sand and a comparison was made of the shear stresses causing liquefaction in sand in the cyclic triaxial and the cyclic simple shear tests. It was observed that cyclic stresses causing liquefaction in loose saturated sands under cyclic simple shear conditions were only about 35 % of the cyclic stresses required to cause liquefaction under cyclic triaxial conditions. Since field conditions are more realistically duplicated in cyclic simple shear test but the cyclic triaxial tests are relatively easier to perform, therefore correction factors were proposed to correlate the cyclic triaxial data with the cyclic simple shear data. This resulted in the well known „simplified procedures‟ for liquefaction analysis of sand deposits. The sample size used in the cyclic triaxial and cyclic simple shear device being small, it was pointed out by Finn (1972) that testing large samples using shake table may better represent the liquefaction of field deposits. The results of the shake table studies were in general qualitative agreement with data obtained from cyclic triaxial and cyclic simple shear tests. Limited studies on liquefaction of undisturbed samples of sand were also attempted and it was observed that natural undisturbed samples were somewhat more resistant to liquefaction compared to laboratory made samples at the same relative density due to aging effect and strength increase in sand due to development of bond between sand particles . Because of difficulty in procuring undisturbed sand samples and the associated cost of performing such tests, they cannot be routinely used for liquefaction analysis. The same argument applies to shake table tests. This lead to the search for a field test which could be used for ascertaining liquefaction susceptibility at a site. The standard penetration test which is routinely used for sub-soil exploration showed promise for estimating the liquefaction also. Standard penetration data was collected for sites which had experienced major earthquakes and where liquefaction had or had not occurred (Seed et al, 1985). The SPT value (N1)60 has been adopted by the profession as an index for liquefaction of saturated sand deposits. The plot in Fig. 3 (Seed et al.; 1985) has been commonly used for this purpose. The plot (Fig. 3) with fines content of less than 5% is typical for the case of sands. The relationship between the cyclic stress ratios and (N1)60 in Fig. 4 is for an earthquake of magnitude 7.5. For an earthquake of magnitude different from 7.5, the cyclic stress ratio obtained from Fig. 1, should be modified by multiplying with the magnitude scaling factor (MSF) proposed by Seed et.al; 1975). Liquefaction potential is seen to decrease with an increase in the fine content in sand (Fig.3). Seed (1987) suggested the use of effective SPT value to account for the effect of fines in sand. The effective SPT value modifies the observed penetration resistance to equivalent clean sand penetration resistance and may be obtained as follows:       60 1 60 1 60 1 N N N eff    (1)   eff N 60 1 = Effective standard penetration resistance or equivalent clean sand penetration