Study On Seismic Subsidence Of Loess And Negative Skin Friction Along Piles By Field Test

Seismic subsidence is a principal kind of ground disaster due to seismic loading, and seismic subsidence of loess (SSL) is the important one of damage behaviors for this special type of soil caused by earthquake. As we known, essential developing features in time domain of SSL, quantitative relations between SSL and structure strength of the soil mass, as well as primary characteristics of negative skin friction (NSF) along piles induced by SSL, are still indistinct. The crucial challenge, unfortunately, we are facing on the research works associated with SSL, lies in the relevant cases are so infrequent that NSF caused by SSL is difficult to investigate and the study of SSL remains hard to go on with in the field of earthquake engineering. Field tests are the effective methods to eliminate the defects, whereas the existing in-situ data is rare to meet the need of the study.Based on the field test by a short delay blasting, this dissertation takes major efforts on essential characteristics of SSL and NSF associated with the soil settlement. Using the data of in-situ exploding experiment and laboratory dynamic triaxial test, the author investigates behaviors of loess ground induced by the short delay blasting, distributional and developing characteristics in time domain of SSL, mathematical statistical model and probability-based prediction method for SSL, and essential features and generation mechanism of NSF caused by SSL. The creative achievements under the above-mentioned research works are summarized as the follows.(1) The analysis results of the exploding ground motions reveal that duration are the most important factor to influence design effects of a short delay blasting. Independent exploding events in the blasting process could be linked one to another and then the duration of exploding ground motion may be lengthen. These results would not influence the spectral characteristics of ground motion induced by the short delay blasting. Those actual time histories of ground shock with longer duration are better in meeting the demands for laboratory and field tests to understand response behaviors of soils and structures.(2) Grounding on the comprehensive analysis in distributional characteristics of exploding ground motion and SSL, the author figures out the coupling characteristics of PGA and H/V values could make an appreciable impact on the distribution of SSL in the loess field. The area with both higher PGA and H/V values of spectral features for exploding ground motion could suffer a greater SSL, and vice versa. Meanwhile, geographic and geomorphic conditions could strongly affect distributional features of soil seismic subsidence in a loess ground. In this experiment, the symmetry axis of SSL's distribution gradually rotates with time and directs towards the cliff that stands to the south of the field, where the soil bears a lesser settlement restriction, at the end of the in-situ observation.(3) By analyzing the developing regulation in time domain of SSL caused by the exploding ground motion, we can get that just at the end of the blasting, SSL could reach about 50% of the total observational settlement, while the other half would settle during a longer period later, i.e., SSL should experience a relative long-term-process during the field test, including two stages following one after the other, the rapid one and the tardily one. The rapid stage occurs along with the exploding event, whereas the tardily one is generated by the soil reconsolidation after structure strength is decreased due to the short delay blasting.(4) From the contrastive analysis results of field experiment, laboratory dynamic triaxial test and physical and mechanical mechanism of SSL, the dominant factors affecting SSL could be came down to as follows, cohesive strength (C), internal friction angle (φ), void ratio (e), ground motion intensity (PGA) and spectral features (e.g. H/V values) of ground motion, and geographic and geomorphic conditions. The factors of C and cp briefly represent the characteristics of soil structure strength, while e influences the final seismic subsidence magnitude of soil mass. Geographic and geomorphic conditions in the field are independent of soil's physical parameters or of ground shock on the influence mechanism of SSL. The geographic and geomorphic conditions could be analyzed separately during the analysis of dominant factors influencing SSL.(5) Based on the physical-mechanical mechanism of SSL, the relation of void ratio and seismic subsidence coefficient (SSC) is established through physical and mathematical analyses. It is clear that a stress ratio (defined by dynamic stress and consolidation stress) coincides with the compression values of void ratio. Consequently, both mathematical statistical model of SSL and three-dimensional theoretical matrix for regional assessment of SSL are educed. Moreover, curves of seismic probability within different reoccurrence periods of years for PGAs in the Loess Plateau are calculated and the probability-based prediction method for SSL in the region of Loess Plateau is provided. (6) Strain behaviors of pile body in response to exploding waves indicate that the maximum response magnitude and axial force along pile appearance at the same position, which is the buried depth of neutral point also, where NSF is OkPa. During the field experiment, the depth of neutral point increases gradually after the short delay blasting, however, the increment is very small, i.e. the neutral point could be considered as a fixed position with no change.(7) The field testing data show NSF rises gradually with increasing depth and the mounts up to the maximum value near the neutral point. Furthermore, the final average NSF reaches 54kPa approximately, with a corresponding total NSF along pile of 1654kN. These results are much greater than the previous NSF cases induced by loess settlement due to water soaking. The special characteristics of NSF, higher values, distributional features along pile and almost fixed position of neutral point, reveal that NSF observed in this field experiment might be associated with static friction.(8) By the comprehensive analysis of NSF's generating-and-developing features, pile body's eccentric stress (differences of NSF between two sides of each pile) and existing NSF data, it could be affirmed that NSF shares correlations with 3 aspects as follows, ground settlement, structure strength of soil mass, and static or kinetic friction. During the ground settlement, generally, structure strength of soil mass is the foremost factor that influences NSF.