The Chi Chi earthquake of September 21, 1999 triggered extensive soil liquefaction in Central Western Taiwan. The sand deposits in this area often contain various percentages of low to medium plasticity fines. The earthquake presented a rare opportunity to study the cyclic strength of silty sands. In the past six years, the authors performed a series of laboratory tests on reconstituted Mai Liao Sand specimens, which included monotonic/cyclic triaxial tests and cone penetration (CPT) calibration tests. A test site was established in Yuan Lin Township of Chang Hua County where various in situ testing and sampling techniques were experimented. We offer our views on the analyses that relate to cyclic behavior of the silty sands in this region based on our studies and preliminary verification using the Chi Chi earthquake as a reference. The paper describes the lessons learned from these post earthquake studies.
The current seismic design code stipulates that the peak ground acceleration (PGA) for structure design is either 0.23g or 0.33g in Taiwan area, pending on the geographic location of the construction site. Using the specified PGAs as a design basis, liquefaction potential analysis often found that most sandy materials may liquefy, even for strata with high SPT N values. As a countermeasure, ground improvement or pile foundation may be required. A less costly alternative, which is allowed within the context of the design code, is to adopt a reduced modulus of subgrade reaction for foundation design if the liquefaction hazard is considered minor. This paper presents a case history that uses reduced parameters for raft foundation design. The possibilities of using grouting techniques for liquefaction mitigation are also addressed.
Three case histories of failure of piles caused by ground lateral spreading are back studied using the beam on Winkler foundation method including nonlinear soil springs and a nonlinear moment-curvature relation for the pile. The lateral spreading effect is modeled by laterally displacing the soil springs support by an amount equal to the free field permanent deformation. Design procedures suggested by Tokimatsu（2003） and by JRA（1996） were also used for case histories evaluation and compared to available observation results.
To assess the liquefaction potential of soil during earthquakes, the simplified procedures by using the value of SPT-N，CPT－qc and seismic Vs are the most prevailing methods applied in the engineering practices. Basically, these methods were deduced from the worldwide data of liquefaction and non-liquefaction cases reported in the past major earthquakes. In Taiwan, large amount of liquefaction data were retrieved from the devastating Chi-Chi earthquake in 1999. Therefore, it is valuable to use the Taiwan data to evaluate the performances of those simplified methods commonly used, and to deduce an improved method, if possible, to take into account the effects of local soils. Based on investigations on all data, this paper proposed a method by using the hyperbolic functions to fit the critical cyclic resistance curves, a boundary curve for judging liquefaction or non-liquefaction, in the CSR-(N1)60, CSR-qc1N, and CSR-Vs plots, respectively. The advantage of using the hyperbolic function is that it is simple in form to have physical quantities as parameters. The performances of the Hyperbolic method along with other simplified methods are then thoroughly investigated in comparison studies. From the results of two assessment indices, the prediction success rate and the error of at-least safety factor, it is shown that the proposed Hyperbolic methods performed very well as compared to the other simplified SPT-N，CPT－qc and seismic Vs methods.
Seismic-induced soil liquefaction causes damages on soft ground. It is an important issue to decide peak ground acceleration(PGA) appropriately for liquefaction analysis considering site effects. These subjects are divided into three parts in this paper, including PGA attenuation law, empirical relationships and amplification coefficients in the code, and site-specific response analysis. A case study of Chianan Plain is implemented. PGA and liquefaction potential estimated from Design Earthquake and Maximum Considered Earthquake(MCE) according to newly-revised building codes in Taiwan are compared to those from seismic hazard analysis(SHA) and ground response analysis(GRA). It is shown that site effects estimated from codes may be un-conservative or over-conservative depending on seismic intensity. Performing a site-specific ground response analysis is the best way to decide PGA on soft ground for liquefaction analysis.
Geotechnical engineers usually deal with the natural materials, such as rocks and soils, and shall carry out their works according to very limited test results. A better sample quality becomes very important and essential in every aspect for a good test, no matter laboratory or field test. Especially for laboratory mechanics test, it is necessary to insure the samples are in “undisturbed” conditions. Disturbance will directly affect the test result, which might mislead the design or analysis results. This paper collects consolidation test results of some currently undergoing projects in Taipei basin and studies their specimen quality based on “Specimen Quality Designation”(SQD). It is anticipated to introduce a reasonable way to decide the specimen quality for so called “undisturbed” samples.