Study On The Anchorage Mechanism Of The Bamboo-Steel Composite Rockbolt

At present, China has paid more attention to protection of cultural relics, because a large number of earthen sites, especially those in the Silk Road of China, have being destroyed due to severe wind and rain erosion, structural fissures. And it has become one of the most issues that many experts focus on. As an effective and economical measure, rock-soil anchoring technique can increase the strength of soil and its self-stability. Therefore, it has been widely used by the departments of cultural relic protection.As a new type of rockbolt, Bamboo-steel composite rockbolts have been successfully applied in some soil relic projects. However, its anchorage mechanism and construction technology have not been studied systematically yet, and it will be much difficult to research the composite rockbolt because of its complicate structure and multi-interfacial feature. Therefore, a systematic study on this new rockbolts will not only fill the blank of the anchorage mechanism and technology, but also it can provide a scientific basis for applying this composite rockbolts in the steep slopes of the arid and semi-arid region, meanwhile, it has a reference meaning for learning other types of rockbolts.First of all, basing on collecting literature and field investigation, this paper demonstrates the geological environment of the study area, and anchorage techniques are optimized through investigating and comparing the present techniques. Then, combining with the theoretical and technological methods, such as the elastic-plastic mechanics, fracture mechanics, mathematics, deformation monitoring and numerical analysis and so on, the anchorage mechanism of the composite rockbolt is systematically studied by the theoretical, experimental and numerical means respectively. Finally, taking the 41-5 sub-region of Jiaohe ancient city as an example, the slope stability analysis before and after anchoring with composite rockbolts is respectively studied using the strength-reduction finite element method. The main results of the paper are listed as below: (1) The study area is located in the central uplift structure belt of the Turpan Basin, which is influenced by strong tectonic movement. The cliff is very steep with many tectonic fissures and unloading fissures, and its base has been eroded severely by the river. The weak strata at the cliff have also been eroded by the sand-carrying wind, which is formed in the special climate. As a result, many free faces have been formed. Therefore, the stability of the cliff has been greatly undermined by these unfavorable factors.(2) Combining with the anchor engineering of Jiaohe ancient city, the existing construction techniques have been investigated and compared. Considering the characteristics of the composite rockbolt and the earthen site, this paper summarizes and optimizes the construction techniques, including the location of anchor hole, hole-creating, lifting rock-bolts, installing rock-bolts, grouting, installing anchor devices, blocking the anchor holes, imitation of ancient surface, curing the rock-bolts etc.(3) Based on a realistic tri-linear bond-slip model with residual bond strength at the grout-bolt interface, this paper presents an analytical solution for predicting the full-range mechanical behaviour of grouted rockbolts in tension. The full-range behaviour consists of five consecutive stages: elastic stage, elastic-softening stage, elastic-softening-debonding stage softening-debonding stage and debonding stage. For each stage, closed-form solutions for the load-displacement relationship, interfacial shear stress distribution and bolt axial stress distribution along the bond length were derived. The ultimate load and the effective anchorage length were also obtained. The analytical model was calibrated and validated against two pullout experimental studies. The predicted load-displacement curves as well as the distributions of the interfacial shear stress and the bolt axial stress are in close agreement with test results. A parametric study is also presented, providing insights into the behaviour of the rockbolts:â‘ the ultimate load increases quickly with the bond length before the effective bond length l_e is reached, beyond which the ultimate load increases at a much slower rate.â‘¡the ultimate load clearly increases but the ductility decreases as the bolt axial stiffness increases.â‘¢a larger k increases both the ductility and the ultimate load.(4) Through fixing strain gauges on the surfaces of the strand and the bamboo, which are monitored by a static strain meter, some pull-out tests to the composite rockbolt are carried out. Some conclusions have been developed:a) The main failure mode occurs in the strand-composite material interface. As the load increases, the adhesive force at the proximal end will be overcame first and then the damage will move to the distal end until all interface are overcame.b) The axial stress is distributed exponentially along the bond length, and it increases with the increment of the load. When the pull-out force is small, the interfacial shear stress is also distributed exponentially along the bond length. With the load increasing, the peak shear stress is gradually transferred to the distal end.c) The axial stress in the strand is significantly higher than that in the bamboo. During the cycling pull-out process, the composite rockbolt has an apparent function of material memory. The value of the strain on the inner-surface is larger than that on the outer surface of the bamboo due to the difference of elastic modulus in these two surfaces.d) By contrast, through installing anchorage and anchor plate at the end of the composite rockbolt, the ultimate bearing capacity is greatly increased, which can provide a useful reference to improve the bamboo-steel composite rockbolt.(5) This study develops a finite element model to analyse the full-range nonlinear behaviour of the bamboo-steel composite rockbolt. The finite element model uses cohesive elements in Abaqus characterised by a tri-linear bond-slip model to simulate debonding along the grout-bolt interface. The parameters in the bond-slip model are calibrated using the analytical solution from the experimental pullout tests. On the one hand, its results verify the conclusions derived from the experiments. On the other hand, some new conclusions are also obtained about the positions which can not be monitored by the in-situ experiments.a) The shear stress on the bamboo-composite material and bamboo-cement mortar interface are also distributed exponentially along the bond length, and the peak shear stress is transferred to the distal end with the increment of the pull-out force. However, it is totally different from the strand-composite interface, the shear stress transfer is influenced by the peak shear stress development of the control interface, i.e. the strand-composite interface, other than their own softening and debonding. In addition, the axial stress distribution is different from the steel strand, and its peak is distributed in the middle of the bamboo.b) Due to different boundary conditions, the shear stress distribution along the cement mortar-soil interface is different from that of other interfaces, just the opposite direction, that is, the shear stress along the interface increases from the distal end to the proximal end.c) All of the interfaces are not softened or damaged except for the control interface, and the axial stress and shear stress along other interfaces adjust corresponding to the softening and debonding development of the control interface.d) The axial stress in the steel strand is significantly higher than that of the bamboo. At a same cross-section, the shear stress in the control surface is larger than the composite-bamboo, bamboo-mortar, mortar-soil interface, and the shear stress on these interfaces decreases in order.e) By comparing the numerical results and experimental data, they are in close agreement with the experimental results. Therefore, the conclusions derived from numerical simulation are reliable and credible.(6) Combined with the strength reduction elastic-plastic finite element method, the slope stability of the sub-region 41-5 before and after anchorage is analyzed using Abaqus. The results show that:â‘ the safety factor before anchoring is only 1.002, and the slope is in the limit equilibrium state. The failure mode is that the plastic zone develops from the bottom of the fissure to the free surface.â‘¡after anchoring, the stress distribution in the slope has been improved greatly. And the safety factor is up to 1.31, which meets the requirements of the engineering design. Meanwhile, the engineering practicability of the bamboo-steel composite rockbolts is also validated by this study.