Characterization of the uncertainty of the Robertson and Wride model for liquefaction potential evaluation

As the profession moves toward the performance-based earthquake engineering design, it becomes more important and pressing to examine the uncertainty of the limit state model used for liquefaction potential evaluation. In this paper, the uncertainty of the Robertson and Wride model, a simplified model for liquefaction resistance and potential evaluation based on cone penetration test, is investigated in detail for its model uncertainty in the framework of first-order reliability analysis. The uncertainties of the parameters used in the Robertson and Wride model are also examined. The model uncertainty is estimated by calibration with a fairly large set of case histories. The results show that the uncertainty of the Robertson and Wride model may be characterized with a mean-to-nominal of 0.94 and a coefficient of variation of 0.15 based on the case histories examined.     

A numerical case study of the passage of a cold surge across Taiwan

Summary A numerical study of a cold surge that occurred from 19–23 December 2001 was conducted to better understand the cold surge characteristics over the Taiwan area. The National Centers for Environmental Prediction (NCEP) nested Mesoscale Spectral Model (MSM) was used for this study. Simulation results demonstrated that the nested NCEP MSM captured salient features of the selected cold surge case. We demonstrated that the local minimum center of the time change of virtual potential temperature (dVPT) can serve as the location of severe weather of the cold surge for all Taiwan regions. Furthermore, thermodynamic equation analyses revealed that the leading edge of the cold surge was maintained primarily by meridian thermal advection, while diabatic heating, vertical and zonal thermal advections were less important. The cold surge flows were blocked and lifted by the Central Mountain Range (CMR) on its windward side, which increased the cold surge’s vertical extent upward and increased northwestward tilting in the vertical structure. The flow eventually ascended, switched direction toward Taiwan, and descended over its northwestern and eastern coasts. The physical and circulatory characteristics of the cold surge differed noticeably on both sides of Taiwan. The cold surge’s leading edge over the Taiwan Strait was northeast-southwest oriented and had evident wind shear; on the east side of the island, it was stronger in intensity and faster-moving than its counterpart on the west side. Sensitivity experiments revealed that the CMR’s trapping of cold surge flows on its windward side affected cold surge leading edge characteristics in the following five ways: (1) weakening its intensity over the ocean but enhancing it inland, (2) decreasing its southward speed, especially over the western low plains region, but increasing its movement on the east coast, (3) increasing its vertical altitude and narrowing the horizontal extent of its vertical tilting, (4) altering its upper vertical circulatory structure, and (5) trapping a V-shape density current and enhancing its intensity in connection with the land-sea contrast.     

Setting up evaluate indicators for slope control engineering based on spatial clustering analysis

Rainfall-induced landslides (RILs) have been a source of social and economic disruption in the mountainous Baguio area in northern Philippines. Prolonged heavy rainfall usually happens during tropical cyclone and southwest monsoon activity. A pragmatic approach to RIL mitigation is to develop rainfall-based early warning. We implemented a modified regression method to derive the empirical minimum intensity (I)–duration (D) threshold I = 6.46 D ^?0.28 and a normalized ID threshold NI = 0.002 D ^?0.28 for rainfall duration ranging between 24 and 264 h. Using a separate data set to evaluate the applicability of the threshold, 93% of the landslide-triggering rainfall events fell above the derived threshold. RILs also occurred when 24-h rainfall was 0.02–28% of the mean annual precipitation or after accumulating at least 500 mm of rainfall from the onset of the rainy season. The thresholds may be further refined as more landslide data become available in the future.     

Applications of the Mild-Slope Equation to Tidal Computations in the Taiwan Strait

Employing harmonic analysis of tidal data in the Taiwan Strait, the cross-strait tidal characteristics are completely illustrated. Based on the two dimensional mild-slope equation which can be reduced to the shallow-water wave equation, a finite element model (Tsay et al., 1989) is applied to investigate the characteristics of tides in the Taiwan Strait. The co-range and equi-phase charts of major tidal constituents, such as M₂, S₂, N₂, and K₁, are reproduced. Anomalous amplification of semidiurnal tides in the Taiwan Strait is verified. With rotation effects neglected and by applying a non-reflective condition on the open boundaries, the numerical results of phase-lag and co-range distributions show very good agreement with observed data for semidiurnal tides in the Taiwan Strait. Due to crude representation of the topography at two ends along the China coast, computed tidal distributions deviate from the observations. However, both computed amplitudes and phase-lags compare very well with observed data along the central half of the China coast.     

Electrogenic capabilities of gram negative and gram positive bacteria in microbial fuel cell combined with biological wastewater treatment

The voltage and the power production of two gram negative and two gram positive bacteria in four identical continuous flow microbial fuel cells combined with biological wastewater treatment units were evaluated and compared in the present study. Each microbial fuel cell and biological treatment unit was operated at four different flow rates and four different external load resistances. The results show that overall removal efficiency of chemical oxygen demand for all four systems can reach more than 85.5 %. Each pure culture has different power generation performance that can be affected by some factors, such as wastewater characteristics, influent flow rate and hydraulic retention time of reactor. Good linear relationships between the flow rate and the potential and between the flow rate and the power density on four pure cultures at different external load resistances were found. Comamonas testosteroni has better power generation performance than Arthrobacter polychromogenes, especially at higher flow rate. Although Pseudomonas putida also showed higher power generation than Corynebacterium glutamicum, the difference was not statistically significant. It seems that gram negative bacteria could display higher power generation than gram positive bacteria at higher flow rate. However, more evidence is required to provide stronger proof for the difference of power generation between gram negative and gram positive bacteria.     

Microbial population structures in activated sludge before and after the application of synthetic polymer

Since Juang (2005) has found that the sludge settleability became much worse and the flocs displayed as pin flocs in only a few days after the addition of synthetic polymer was halted, it is hypothesized that the shift of microbial population in activated sludge before and after the addition of synthetic polymer might have occurred. Therefore, the identification of microbial population in the activated sludge of a lab-scaled continuous-flow type of treatment reactor was conducted at different phases of this study. The results of this study showed that the presence of synthetic polymer inhibited the growth of dominant bacteria and floc-formers in activated sludge, but gave a competitive advantage of growth to some other bacteria. The addition of synthetic polymer has caused the shift of microbial population and affected the growth of floc-formers in activated sludge. It took more than one month for the population structure of predominant microorganisms in activated sludge to return more closely to the initial population structure.     

Assessing CPT-based methods for liquefaction evaluation with emphasis on the cases from the Chi-Chi, Taiwan, earthquake

In the early morning (1:47 Taiwan time) of September 21, 1999, the largest earthquake of the century in Taiwan (M_w=7.6, M_L=7.3) struck this island country. The earthquake killed more than 2400 people and caused great destruction to buildings, bridges, dams, highways, and railways. One of the causes for heavy damages to the structures is soil liquefaction and ground settlement during the earthquake. In this paper, investigation of soil liquefaction and case histories of liquefaction are presented. Three CPT-based simplified methods, the Robertson method, the Olsen method, and the Juang method, are examined using the case histories derived from the Chi-Chi earthquake. The results of the comparison show that the Juang method is more accurate than the two methods in predicting liquefaction potential of soils based on the cases derived from the Chi-Chi earthquake, although all three methods are quite comparable in accuracy.     

Treatment of polluted river water by a gravel contact oxidation system constructed under riverbed

The objective of this study was to evaluate the treatment efficiency of a gravel contact oxidation treatment system which was newly constructed under the riverbed of Nan-men Stream located at the Shin Chu City of Taiwan. The influent and effluent water samples were taken periodically for the analyses of pH, temperature, dissolved oxygen, total suspended solids, five-day biological oxygen demand, NH₄ ⁺-N. The results showed that the average removal rates of five-day biological oxygen demand, total suspended solids and NH₄ ⁺-N were 33.6% (between ?6.7% and 82.1%), 56.3% (between ?83.0% and 93.4%) and 10.7% (between ?13.0% and 83.3%), respectively. The calculated mean first order reaction rate constant for five-day biological oxygen demand was 4.58/day with a standard deviation of 4.07/day and for NH₄ ⁺-N was 2.15/day with a standard deviation of 5.68/day. Therefore, it could be said that this gravel-contact-oxidation system could effectively remove biological oxygen demand, total suspended solids, and NH₄ ⁺-N in river water at a relatively short hydraulic retention time, although its pollutant treatment efficiency was not quite stable. However, to reach better or more stable treatment efficiency, aeration might sometimes be necessary to increase the dissolved oxygen in influent river water. And, longer hydraulic retention time of the system might also be required to increase NH₄ ⁺-N removal efficiency.     

Assessment of various code provisions based on optimum design of steel structures

This paper presents some comparative design results of typical structural systems in accordance with various code provisions, such as UBC and ATC-03 of the U.S. and TJ-11–78 of China. The solution comparisons are based on an optimization scheme for a given set of constraints and objective functions. Three levels of recursive procedures are employed in the algorithm to search for an optimum weight or construction cost including damages. Numerical examples are provided to compare the design results based on the aforementioned code provisions.     

Simplified procedure for reliability-based robust geotechnical design of drilled shafts in clay using spreadsheet

This paper provides a simplified procedure for reliability-based robust geotechnical design (RGD) using spreadsheet. In the RGD methodology, design robustness is achieved by adjusting “design parameters” without reducing the uncertainties in noise factors. This design approach generally involves a multi-objective optimisation, which is computationally challenging. To improve the efficiency of the RGD methodology, the design robustness is evaluated in terms of sensitivity index and the safety requirement is evaluated using mean value first order second moment (MFOSM). To ease the concern that the reliability index obtained with MFOSM may not be sufficiently accurate, a mapping function that relates MFOSM to a more accurate method such as first order reliability method is introduced. To further improve the efficiency of the proposed simplified RGD method, a new simplified procedure along with a more accurate robustness measure is developed that eliminates the need for multi-objective optimisation. With these modifications, the proposed simplified RGD method can efficiently be implemented in a single Excel spreadsheet. The proposed simplified method, which goes beyond any existing reliability-based RGD methods in terms of ease of use and computational efficiency, is illustrated in this paper with an example of robust design of drilled shaft in clay.