Wuming Leng

Seismic response of embankment slopes with different reinforcing measures in shaking table tests

In order to study the seismic response of the embankment slopes with different reinforcing measures, shaking table tests were performed on three embankment slope models (i.e., unreinforced embankment slope, 2-layer reinforced embankment slope and 4-layer reinforced embankment slope). Wenchuan earthquake motions and white noise excitations were performed to investigate the change of the model parameters, the horizontal acceleration response, the vertical acceleration response and the dynamic earth pressure response of embankment slopes. A comparison was made on the seismic response among the embankment slopes with different reinforcing measures. The results show that the natural frequency of reinforced embankment slope is larger than that of unreinforced embankment slope, and the reinforced embankment slope is less sensitive to seismic excitation. Horizontal acceleration response is obviously amplified by embankment slope. Horizontal acceleration magnification presents a decreasing trend with the increase of the peak value of input horizontal acceleration, and the decreasing ratio is higher for reinforced embankment slope. The vertical acceleration magnification of reinforced embankment slope is much smaller than that of unreinforced embankment slope, and the nonlinear characteristic of embankment slope in vertical direction is not as obvious as that in horizontal direction. Residual earth pressure is mainly induced at the upper part of embankment slope.

Experimental measurement of dynamic load parameters for pier pile caps of high-speed railway bridges

Train-induced dynamic responses and loads in bridge pier systems are not well understood. In this study, five different piers from two separate railway bridges were investigated experimentally. The dynamic responses and loads at the bridge beam ends, pier tops, and pile caps were measured by considering various train speeds. The frequencies of the dynamic loads on pile caps were also analyzed. The results show that the induced dynamic response decreases significantly with the descending pier height. The train-induced vertical dynamic displacements are related to the axle loads and the stiffness of the pile foundation and foundation soils. The accelerations at pier tops and pile caps are very similar but noticeably less than those at beam ends. The train-induced vertical dynamic loads on pile caps increase with the total span length. The measured change of dynamic forces (ΔQ) is approximately 20 − 30% of the measured peak loads (Q). In addition, the results show that the frequency (f) varies linearly with the train speed rather than with the bridge span length.

Effects of Pile Residual Loads on Skin Friction and Toe Resistance

In this paper, the causes and mechanisms of residual loads are described and discussed. Methods for measuring pile residual loads in the field are summarized and compared. Specifically, a driven pile and a bored pile were selected to show the effects of residual loads on skin frictions and toe resistances. The results indicate that the skin friction in the upper pile portion and the vertical compressive stiffness of the test piles are overestimated, while the toe resistance is underestimated, if the residual loads are ignored.

Comparison and evaluation of railway subgrade quality detection methods

Subgrade is a critical component of railway systems since it provides a stable platform for the track substructure. The implementation of quality assurance methods is very important during the construction of a new railway subgrade and in monitoring the working performance of operating railways. There are several indicators available to measure the compaction and mechanical performance of a completed subgrade system, including compaction degree (K), porosity (n), modulus of subgrade reaction (K30), basic bearing capacity ( σ 0 ), strain moduli (Ev), dynamic modulus of deformation (Evd), and light dynamic penetration (N10). The objective of this research was to compare and evaluate the frequently used engineering methods in order to select the important and reliable indicators of field parameters. Four soil models with different degree of compaction were considered in the common laboratory tests and detection methods in China to determine the construction quality. The implementation and test results of these methods were analyzed and compared. The study results indicate that K correlates with n linearly. There is also a good correlation among K30, Ev, and Evd. As a recommendation, K is considered as the controlling indicator for compaction performance, and Ev2 and Evd are considered as the controlling indicators for railway subgrade stiffness evaluation. The bearing capacity or strength indicators may be added to the quality detection work in heavy-haul railway construction.

Prediction of Lateral Behavior of Existing Bridge Pile Foundations due to Surcharge Load

Existing pile foundations supporting high-speed railways - especially those embedded in soft clays - are undoubtedly affected by the nearby embankment construction or excavation activity. The distance between the surcharge load point and an existing bridge pile foundation (d) is an important factor affecting the internal forces and displacements of the foundation system. Three-dimensional finite element models were created to investigate the behavior of a bridge pile foundation supporting a high-speed railway in China under the surcharge load. The analysis results compare well with the field data. The study results indicate that the internal forces and horizontal displacements of piles decrease with the increase of d. When d is less than 4 times of soft clay depth (h), the horizontal displacements of pile groups decrease linearly and rapidly with the increase of d/h ratio. When d is larger than 4h, the horizontal displacements of pile foundations decrease at a lesser degree with the increase of d/h ratio. Suitable d value can be determined based on the maximum allowable displacement of the superstructure under the normal operating condition.

Field Comparative Tests for Studying the Vertical Loading Performance of Piles Due to Water Level Periodical Fluctuation in Foundation Soils

The water level in the reservoir of the Three Gorge Dam rises from 145m to 175m since the dam is in full operation. The periodical fluctuation of the water level changes the water environment of the reservoir banks. The water saturation and drying cycles affects the strength and stiffness of original rock and soil mass. These changes have led to problems such as the instability of ancient landslides and the safety of buildings supported on pile foundations whose bearing stratums are affected by the changing water levels. Seven 8-story height buildings supported on pile foundations in Fengjie county in the Chongqing Municipality had been facing such threats. A series of field direct shear tests and pile loading tests were conducted to study the influence of capacity of foundation soils due to water level fluctuations. The results indicated that after water soaking, the cohesion strength of the foundation soil decreases significantly, while the internal friction angle is relatively stable. The weaken ratio of the internal friction angle which is the ratio of strength parameters before and after water soaking into the gravel soil is mainly between 0.78-1.0 and the weaken ratio of cohesion strength is around 0.4 to 0.8. The capacity of pile decreased when the water submerged the pile tip. Field calibration and indoor calibration have proved the availability of the O-cell boxes designed and produced by the authors in pile static loading tests. The observations provided important support for evaluating the safety of seven 8-story height buildings.

Experimental Study on Compression Modulus of Sandy Soil

The compression modulus is a key parameter for evaluating compression behavior of soil layer and for predicting foundation settlement. Highpressure confined compression test and the standard penetration test were performed to investigate the compression characteristics of sandy soil layers beneath pile toe, and to obtain the relationship of compression modulus with depth and overburden pressure. The experimental results indicated that the compression modulus of sandy soil increased with pressure, but its increasing rate gradually reduced. The compression modulus almost linearly increased with embedded depth. The ratio of compression modulus under the pressure ranging from 100kPa to 200kPa to that under the pressure in the range of overburden pressure to the additional stress plus overburden pressure (Es/Es0.1-0.2) ranged from 2.1 to 4.2. The empirical formula of the Es/Es0.1 -0.2 ratio with overburden pressure was established to give a reference to predict compression modulus of sandy soil under actual pressure, as its overburden pressure was known. At last, the basic principles of how to determinate the value of compression modulus were proposed for design of pile foundation.