Search Results (6)

In this paper, the reinforced cement soil nailing support technology is adopted, and the soil nailing and surface layer of loess-based composite slurry are prepared by using loess and cement. A scale model box test is conducted to examine the changes in surface layer displacement and axial force in the soil nailing during excavation and loading. The step-by-step excavation process of the foundation pit, reinforced with a loess-based composite slurry soil nailing wall. It was simulated using ABAQUS finite element software (MATLAB R2022b). The results show that as the depth of the foundation pit continues to increase, the displacement of the surface layer increases first and then decreases, and the peak displacement appears in the middle of the foundation pit. During excavation, the axial force at the middle of each row of soil nails is greater than the axial force at the end, and the axial force will increase with the increase in depth. Throughout the loading process, the axial force in the soil nail diminishes as the depth of the foundation pit increases. Initially, the change is slow, but later it escalates considerably. As the excavation depth of the foundation pit increases, the safety factor of the foundation pit will gradually decrease, and finally stabilize at about 2.4, indicating that the loess-based cement slurry soil nailing wall support has high safety. Full article

Compared with the conventional horizontal conveyor-chain-type plastic-film-picking device, the longitudinal nail-tooth-chain-plate-type plastic-film-picking device developed by our team in the early stage has little tearing effect on the mulching film, and the separation effect of the plastic film and the impurity is better. With a view to further enhancing the performance of the plastic-film-picking device, this study optimized it and designed a friction-type nail-tooth-chain-plate plastic-film-picking chain that does not overload or slip and also facilitates the installation of functional components. The kinematic analysis of the picking nail teeth on the film-picking chain was carried out, and the motion equation and trajectory of the nail teeth during the operation were determined, as well as the requirements for the nail teeth to complete the mulching-film pickup. The key parameters of the plastic-film-picking system were determined by analyzing the no-leakage condition of the plastic film and the force. Moreover, the structural design and key parameters of the shovel-type film-lifting device were determined. According to the design results, a prototype was developed, and a multi-factor test of the operating parameters was carried out. The operating speed, spacing of the pickup nail teeth, depth of the film shovel into the soil, and distance between the tip of the film shovel and center of the picking drum were used as the experimental factors. The plastic-film pickup rate, impurity rate of the recovered mulching film, and traction resistance were used as the test indicators. A four-factor, five-level quadratic regression orthogonal combination experiment was conducted using the Central Composite Design (CCD). The effect of each test factor on the test index of the plastic-film-picking system was studied, the regression models were established, and the optimal parameter combination was acquired by using the multi-objective optimization method. When the working speed was 6 km/h, the pickup-nail-tooth spacing was 228.6 mm, the depth of the film-lifting shovel into the soil was 37 mm, and the distance between the tip of the film shovel and the center of the pickup drum was 130 mm, the field experiment shows that the plastic-film pickup rate was 90.12%, the impurity rate of the recovered mulching film was 8.96%, and the traction resistance was 19.905 kN. The relative errors between the test results and the predicted values of the regression models were less than 5%, indicating that the parameter optimization regression models were reliable, and the designed friction-type nail-tooth-chain-plate plastic-film-picking system met the technical requirements of agricultural-plastic-film recycling. The research results can provide a technical reference for the development of mulching-film collection machines. Full article

At present, the research results for the stress response and deformation characteristics of composite support structures are mostly based on ideal or standard working conditions. External disturbances often exist in practical engineering, which makes the monitoring data deviate from the calculation results. In order to analyze the causes of deviation and correct them in practice, it is necessary to consider the time-varying effect and study the construction mechanics behaviors of composite support structures. Based on in situ test data, the effects of soil predisturbance, excessive excavation, unloading on the surface of edges, the tensioning and lagging of the anchor, and continuous rainfall on the stress-time curves of soil nails were analyzed. On the basis of verifying the effectiveness of the model, ABAQUS finite element software (v.6.10) was used to simulate practical engineering based on ideal working conditions. Comparing the in situ test data and numerical simulation results, the development of mechanical response and deformation characteristics in the process of support structure installation and soil digging and filling were analyzed. Research shows that the time-varying effect has a significant impact on construction mechanics behaviors, especially on soil nailing combined with the use of prestressed anchors, due to layered excavation and support. Full article

Conventionally, a soil nail is a piece of steel reinforcement installed inside a hole drilled into the slope and filled with cement paste (CP) grout. Chloride penetration is a major deterioration mechanism of conventional soil nails as the CP grout is easy to crack with an uncontrollable crack opening when the soil nail is subject to loading or ground movements. Engineered Cementitious Composites (ECC) are a class of fiber-reinforced material exhibiting excellent crack control even when loaded to several percent of strain, and therefore, ECCs have great potential to replace traditional CP grout in soil nails for achieving a long service life. In this study, the chloride ion transport characteristics and electrically accelerated corrosion process of steel rebar in ECC and CP grouts are systematically studied. The rapid chloride ion penetration test results showed a reduction of 76% and 58% passing charges in ECC with 0.15% and 0.3% pre-loading strain, respectively, as compared to that in un-cracked CP. Furthermore, the accelerated corrosion experimental data showed that ECC under pre-loading strain still exhibited a coefficient of chloride ion diffusion that is 20–50% lower than CP grout due to the ability to control crack width. Service life calculations based on experimentally measured parameters showed that the predicted corrosion rate and corrosion depth of soil nails in ECC grout were much lower than those in CP grout. The findings can facilitate the design of soil nails with excellent durability and long service life. Full article

Soil nailing combined with prestressed anchors has a good workability and is relatively cheap in constraining the horizontal displacement. Current research on the technique, whether theoretical analyses, numerical simulations, or model tests, was conducted under ideal working conditions. However, in fact, external disturbances, such as tensioning-lagging of the anchor, are very common and play an important role on stress and displacement. Therefore, it is of great significance to carry out a field test considering the effects of external disturbances, which can obtain real and reliable data through real-time monitoring. In this paper, the impacts of the construction conditions on practical engineering are discussed based on in situ tests, and some reasonable suggestions for the upgrading of misbehaviors in the current construction situation are put forward. In particular, the influence features of soil predisturbance, excessive excavation, unloading on the surface of edges, tensioning-lagging of the anchor, and continuous rainfall on the stress–time curve of soil nails under practical working conditions are analyzed. Behaviors of three different retaining structures enhanced with (i) soil nails; (ii) soil nails and prestressed anchors without unbonded parts; and (iii) soil nails and prestressed anchors with a 2.5 m unbonded part were monitored during staged excavation to investigate the influences of (i) the prestressing force and (ii) the unbonded part of the prestressed anchors on the performance of the entire retaining system. Results show that (i) the prestressing force is the main factor affecting the stress and deformation of the composite retaining system, which is consistent with the existing literature; (ii) the variation of the magnitude and distribution of the soil nail force responding to the anchor prestressing force, however, showed no systematic trend; and (iii) the unbonded part of anchors, which was validated to be the main factor affecting the structural stability in dense materials in the existing literature, is found to have a minor influence in loose fill materials used in this study. Full article

In special geology conditions such as silt-soil, foundation pits are prone to instability and severe deformation. In this paper, a composite soil nailing structure was studied and its effect on a silt-soil symmetrical foundation pit investigated. The factors affecting the stability of the pit as well as its deformation characteristics were also explored. The results show that excavation depth of the foundation pit has a significant impact on its stability. The soil outside the foundation pit is in the form of a parabola, and the uplift of the soil mainly occurs at the bottom. The horizontal displacement of soil on the side wall of the foundation pit presents a “bulk belly” form. In addition, the axial force of soil nails is larger in the middle part, and smaller at both ends in the shape of a spindle. Moreover, the horizontal displacement is positively correlated with the inclination and spacing of the soil nails, but negatively correlated with the diameter and depth of the mixing pile inlay. Furthermore, the inclination and spacing of the soil nails, the diameter, and embedded depth of the mixing pile have their own critical values for stability of the foundation pit. Specifically, in this paper, with respect to soil nails, inclination should be below 30° and prestress value should not exceed 20 kN. With respect to the mixing pile, the diameter should be less than 1.5 m; when the embedded depth of the mixing pile exceeds the critical depth, the limiting effect of the mixing pile on horizontal displacement is not significant. This research provides important takeaways for the design of a composite soil nailing structure for symmetrical foundation pits. Full article