Yit-Jin Chen

Critical Evaluation of Compression Interpretation Criteria for Drilled Shafts

This paper is a critical evaluation of the interpretation criteria of drilled shafts under axial compression loading. A wide variety of load test data are used for analysis, and these data are divided into drained and undrained databases. The interpretation criteria are examined from these load test results to establish a consistent compression interpretation criterion. Among these criteria, the range of each interpretation method presents approximately the same trend for both drained and undrained conditions. The statistical results show that the smaller the compression displacement, the higher the coefficient of variation. Moreover, the undrained load test results reveal less variability than the drained results. The load-displacement curve of a drained loading also demonstrates more ductility than that for undrained loading. Based on these analyses, the relative merits and interrelationships of these criteria are established, and specific design recommendations for the interpretation of compression drilled shaft load test, in terms of both capacity and displacement, are given.

Evaluation of Drained Axial Capacity for Drilled Shafts

The drained side and tip resistances of drilled shafts in cohesionless soils were evaluated from field load test case histories with axial uplift and compression loading. The side resistance was evaluated using the effective stress or beta (β) method, and the tip resistance in compression was evaluated using bearing capacity theory. For the side resistance, the results show that β values can reach 6.5 at shallow depth, but they decrease with depth. The β values in uplift and compression are essentially the same and generally vary by less than 4%. Comparing the measured and predicted (traditional) values of β, the predicted β values are low and overly conservative. For the tip resistance, the average ratio of the measured to predicted values is about 0.3 at a displacement/diameter of 4%. This ratio shows some scatter at shallow depths, but the scatter decreases with increasing depth. To mobilize the tip resistance fully, a displacement/diameter of about 10% is required. Based on these data and evaluations, design recommendations are proposed.

Evaluation of Lateral Interpretation Criteria for Drilled Shaft Capacity

Representative interpretation criteria are examined to evaluate the capacity of nonrigid drilled shaft foundations under lateral loading. A wide variety of lateral load test data are used for analysis, and these data are divided into drained and undrained databases. The interpretation criteria are applied to these load test data to establish consistent lateral interpretation criteria. Among these criteria, the results are generally comparable for both drained and undrained loading. The statistical results show that the smaller the displacement or rotation is, the higher the coefficient of variation. Moreover, the undrained load test results present somehow less variability than the drained results. Based on these analyses, the relative merits and interrelationships of these criteria are established, and specific design recommendations for the interpretation of lateral drilled shaft load test are given in terms of capacity, displacement, and rotation.

Some Characteristics of Ground Vibration as Induced by High-Speed Trains

This paper examines some characteristics of ground vibration as induced by high-speed trains using a wide variety of field-measured data. The measurements for analysis consist of different geological conditions, train speeds, foundation types, and superstructures. Based on these analyses, geological condition is found to be a very important factor which obviously influences initial mobilized vibration level and vibration attenuation. For the train speed factor, the vibration level increases with increasing train speed, but the incremental rate is more obvious on soft ground. The foundation type has an influence on the initial mobilized vibration level and its attenuation rate with distance. In addition, the trains on an embankment and a bridge with shallow foundation present relatively consistent vibration characteristics. Finally, some recommendations for further studies are proposed.

Evaluation of ground vibration induced by high-speed trains on embankments

This study uses a comprehensive measurement scheme to evaluate the behavior of ground vibration propagation induced by high-speed trains on embankment structures. Both near-field and far-field vibrations are examined. A wide variety of geological conditions are studied. In the evaluation of the differences in vibration attenuation, various frequency dependences of attenuation coefficient�including overall vibration, low-mid-high range, and 1/3 octave bands�are applied to the field measurement data. In general, the near-field vibration level and overall vibration attenuation decrease with increasing ground shear wave velocity. Furthermore, the vibration attenuation at low frequency is smaller compared to the attenuation at high frequency. Based on these analyses, the interrelationships of vibration level, vibration attenuation, ground shear wave velocity, and frequency are established. Specific analysis recommendations are proposed for the application of ground vibration assessment in engineering and related fields.

Evaluation of ground vibration induced by high-speed trains on bridge structures

A wide variety of field-measured data are used to evaluate the characteristics of ground vibration as induced by Taiwan high-speed trains on bridge structures. The measurements for analysis consist of various foundation types, geological conditions, and train speeds. Both near-field vibration (25 m from track center) and far-field vibration propagation are evaluated. Based on the results, the deep foundation in alluvium soils and the shallow foundation in rocks show higher near-field vibration level, whereas the foundation in gravelly soils has smaller vibration. The vibration level decreases with increasing concrete volume of structure and decreasing train speed. For vibration propagation, shallow foundations present higher vibration attenuation coefficients than those of deep foundations. Furthermore, the vibration attenuation is frequency dependent, and the vibration attenuation coefficient at low frequency range is much smaller compared with the coefficients at the middle and high frequency ranges. Based on these analyses, specific influence factors for ground vibration assessment are given.

Evaluation of Compression Tip Capacity for Drilled Shafts

This study examines representative analytical models to evaluate the tip capacity of drilled shaft foundations under axial compression loading. To ensure a reliable analysis, a wide variety of compression load test data were used for this purpose. Thereafter, these data were divided into drained and undrained databases based on the soil conditions along the shaft tip. For the evaluation of the bearing capacity theory, the predicted and measured results for undrained soils are reasonably consistent. However, the results showed that the predicted model greatly overestimates the tip capacity for drained soils. On the average, the measured capacity of the drained soils is about 10-20% of the predicted theory model under tolerable settlement. The possible influence factors are discussed and a simple suggested correction factor for shaft length is proposed. For the empirical method, the relationships between tip capacity and standard penetration test are directly developed from the measured results.

Evaluation of Side Resistance of Drilled Shafts in Multilayered Soils

This paper evaluates the behavior of the side resistance of drilled shafts constructed on the interlayer of drained and undrained soils. A wide variety of uplift and compression load test data are used for evaluation and all data have multilayered soils along shaft length. Two analysis models, including complete method ( model) and hybrid of and method ( - model), are applied to these load test data to compare the side resistance from the measurement results. Based on these analyses, the results show that the prediction of - model presents better statistical results than the model. Moreover, the mean ratio of measured side resistance/predicted side resistance of uplift loading is smaller than that of compression loading for both models and the difference is about 10-20 percents. Both analysis models also become less reliable at the shorter shafts.

Analysis Model of Ground Vibration Propagation for High-Speed Trains

Two simple analysis models for wave propagation, the Bornitz and Wiss methods, are examined in order to evaluate their reliability in measuring ground vibrations induced by high-speed trains. First, basic soil background vibrations for various geological conditions are established. Then, field measurement data for high-speed trains on bridge and embankment structures are used to evaluate the analysis models of wave propagation. The analysis models consider the effect of the basic soil background vibration. Based on these analyses, the analysis methods of Bornitz and Wiss can be used to achieve a reasonable prediction of ground vibration attenuation for high-speed trains. Soil background vibrations exhibit relative differences in measured locations, thereby significantly affecting the ground vibration attenuation of passing high-speed trains. The attenuation coefficients of both embankment and bridge structures become larger and appear to converge in a narrow range when the basic soil background vibration is omitted, especially in embankment structures.

Automatic Prediction Model of Ground Vibration for High-Speed Trains

This paper presents an automatic prediction model for ground vibration induced by Taiwan high-speed trains on embankment structures. The prediction model is developed using different field-measured ground vibration data. The main characteristics that affect the overall vibration level are established based on the database of measurement results. The influence factors include train speed, ground condition, measurement distance, and supported structure. Support vector machine (SVM) algorithm, a widely used prediction model, is adopted to predict the vibration level induced by high-speed trains on embankments. The measured and predicted vibration levels are compared to verify the reliability of the prediction model. Analysis results show that the developed SVM model can reasonably predict vibration level with an accuracy rate of 72% to 84% for four types of vibration level, including overall, low, middle, and high frequency ranges. The methodology in developing the automatic prediction system for ground vibration level is also presented in this paper.