Gongcheng Kexue Yu Jishu/Advanced Engineering Science

Landslide dams are usually formed instantaneously by natural forces, the accumulation bodies have the characteristics of complex space structure, wide gradation of dam materials, poor dam stability, and they are easy to fail under the flow erosion. As a major natural disaster of flood and drought, safety evaluation and disaster prediction of landslide dams have been the focus of attention by the scholars around the world, but many questions remain unanswered, which are mainly manifested in: 1) Accumulation bodies are composed of natural wide-graded rockfill materials with significant state-dependent correlation, there is a lack of the state-dependent dilatancy theory and constitutive model of wide-graded rockfill materials. 2) After the formation of landslide dams, they would be affected by external loads, such as the rise of upstream water level of dammed lake, continuous unsteady seepage, landslide surge in the dammed lake, and earthquake, there is also a lack of standards and methods for stability evaluation. 3) Due to the lack of necessary flood relief facilities, the landslide dams are prone to fail; under the action of outburst flow, obvious nonlinear characteristics are manifested during the breach development, as well as strong unsteady flow characteristics of the hydraulic elements; there is a lack of numerical models for landslide dam breaching which can reflect the erosion mechanisms of wide graded materials. Therefore, it is necessary to conduct integrated scientific measures, such as field explorations, multi-scale physical model tests, and numerical simulation methods, so as to reveal the physical description, internal structure, macroscopic mechanical properties of the landslide dams and their spatial and temporal variations; and then, a state-dependent (i.e., gradation, pore ratio, and stress level) dilatancy equation for wide-graded landslide deposit will be presented, and a generalized elastic-plastic constitutive model that can adapt to complex stress paths and the limit equilibrium analysis method of landslide dam body will be established. Large-scale hydraulic model tests and centrifugal model tests of dam breaching will be conducted to reveal the dynamic erosion characteristics of landslide dam materials and the evolution law of breaches under the action of unsteady flow. Subsequently, the erosion equation of sand-laden flow under the dynamic boundary condition by the action of unsteady flow will be established, and the numerical model for dam breach process considering the fluid-solid interaction will be put forward to realize the numerical simulation of the characteristics of water flow movement, the law of dam material transport, the evolution process of breaches, and the structural instability of landslide dam in the whole process of overtopping and seepage failure. Integrating the reliability theory and numerical simulation method of dam breach process, an integrated numerical simulation platform for seepage, deformation, stability, and failure process of landslide dams considering fluid-solid will be developed; consequently, the theoretical system and method of safety evaluation and disaster prediction of the full-life cycle of the landslide dams will be established. The expected results achieved in the project will provide scientific theory and key technological support for improving the decision-making level of disaster prevention and reduction of landslide dams in China.

The mechanical properties of the interface between soil and structure have always been a hot topic in geotechnical engineering. In order to explore the interfacial shear characteristics of non-water reaction polymer and concrete, the effects of vertical stress and shear rate on shear strength and shear modulus of polymer concrete interface were studied based on monotonic direct shear test. The experimental results showed that: under the given vertical stress and shear rate, with the increase of shear displacement, the polymer concrete interface presented shear softening phenomenon. The shear rate had little effect on the interfacial shear strength, cohesion and friction angle, but had a significant impact on the interfacial shear modulus, and the shear modulus value decreased with the increase of shear rate, and the decrease amplitude was obvious; the vertical stress had a significant impact on the shear strength and shear modulus of polymer concrete interface, and the shear strength and shear modulus of polymer concrete interface changed with the increase of shear rate. The vertical stress increased continuously. At the same time, the hyperbolic constitutive model formula of polymer concrete interface was systematically deduced, and the validity of the model was preliminarily verified according to the relevant experimental results.

With the rise of development and utilization of marine resources, inclined pile foundation with good horizontal bearing capacity has been widely used as marine structures such as cross-sea bridge, offshore platform and high pile wharf. Under the action of strong nonlinear superposition of wave and current, severe local scour will occur around the inclined pile, which will reduce the safety of the structure and even lead to structural damage. In order to understand the influence of inclination angle on the scour characteristics of downstream inclined piles under different wave heights and flow velocities, and the difference from that of vertical piles, local scour tests under combined waves and current and pure current were carried out in the wave-current flume. By conducting tests on α=0°, 10°, 20° and 30° downstream inclined piles, the influence of inclination angle on scour duration, relative time scale and scour hole morphology was studied. The influence of dimensionless parameters Fr, KC and Ucw on maximum scour depth of downstream inclined pile was analyzed, and detailed comparison was made with previous vertical pile data. The results showed that when the pile was inclined to downstream, inclination angle had an important influence on the scouring characteristics of pile under combined waves and current. Compared with the vertical pile, with the increase of inclination angle, development rate of scour depth, scale and depth of scour hole gradually decreased, while ralative time scale increased gradually. For the 20° downstream inclined pile, when d50=0.403 mm，Uc=0.206 m/s，H=7 cm, the time for maximum scour depth point to be transferred from lateral front to front was about 60 min, which was much longer than that of a vertical pile. The dune of downstream inclined pile was distributed symmetrically along the central axis and had a bimodal structure, and the scour hole shape was obviously different from that of a vertical pile. Under the condition of moveable bed, regular sand waves were formed on the bed surface, the scour hole scale was further reduced, and the double peaks were more obvious. The relative scour depth of downstream inclined pile was the function of dimensionless parameters Fr, KC and Ucw, which was consistent with the trend for a vertical pile’s fitting curve.

Deflection is the basic data of bridge health monitoring. It can provide quantitative information for both structural safety evaluations and maintenance purposes. By analyzing the relationship between the displacement and its first and second derivatives (inclination and curvature), a method to detect the local stiffness damage of the structure by using the area difference of curvature of the deflection curve before and after the damage was proposed. The area enclosed by the curvature curves before and after damage was divided into several elements, and the ratio of the square of each element area to the sum of the square of all elements area was used as the damage location parameter ΔAκ. The peak value of ΔAκ can locate the damage. As a theoretical example, a simply supported T–beam bridge model with different degrees (5%～50%) and different numbers of local stiffness damage was simulated. By calculating ΔAκ value of each element, the damage in the structure was accurately detected. The method was also applied on a PMMA model bridge in the laboratory. The results showed that ΔAκ value at the damage location was much larger than that at the undamaged location, which could accurately locate the minor local stiffness damage in the structure, and ΔAκ was independent of the damage degree. The damage could be clearly located even in the case of measurement noise. This method is helpful to improve the efficiency of daily safety inspection of bridge, quantify the inspection data, objectify the identification results, and promote the application in actual bridge engineering.

Pipe jacking synchronous grouting is an important method to reduce formation disturbance and final settlement. In order to obtain the relationship between the grouting pressure and the final settlement, find out the reasonable grouting pressure and grouting amount in the actual project to reduce the disturbance of the surrounding stratum, and provide theoretical and experimental supports for the improvements of the synchronous grouting technology, the stratum–mud interaction and the mechanism of settlement from the perspective of the structural characteristics of soil particles and bentonite molecules were analyzed, theoretical assumptions for the four stages of settlement were put forward. Then, the geotechnical engineering centrifuges and self-developed pipe jacking projects were used to simulate the pipe jacking under different grouting pressures on-site. Through the comparative analyses of the settlement curves, the conclusions of the theoretical analysis part were verified. Finally, the relevant parameters such as the grouting pressure in the experiment were applied to the pipe jacking project in Suzhou Donghui Park and the settlement measurement points were arranged on-site, further verified the conclusions. Researches showed that the settlement after grouting can be divided into four stages: soil collapse stage, seepage loss stage, mud skin formation stage and grout uplift stage. The soil collapse stage was short and the settlement speed was fast. Seepage loss stage lasted for a long time and the total settlement was large, which was the main part of the formation settlement. At the mud skin formation stage, the bentonite molecules accumulated on the mud–stratum contact surface to form mud skin, and the slurry was not permeated into the stratum and the settlement of the stratum slowed down. Finally, at the grout uplift stage, the grouting pressure of the mud acted on the mud skin to push the overlying soil and produce settlement compensation. The size of grouting pressure and grouting amount had a great influence on the final settlement. Small grouting pressure and grouting amount would increase the settlement caused by soil collapse and seepage loss, while large grouting pressure would cause excessive settlement compensation effect, even could cause surface uplift. Selecting a reasonable grouting pressure and grouting amount was very important and effective for controlling stratum settlement. It could be used in various pipe jacking projects on-site.