Uplift capacity of horizontal anchor plate in geocell reinforced sand

The paper investigates the uplift performance of horizontal anchor plate in geocell reinforced sand through a series of model tests. It is noted that the unreinforced anchor plate undergoes a clear failure at a displacement of about 3% of its width, whereas with the provision of geocell and a layer of geotextile right below the geocell mattress significantly increases the uplift capacity by about 4.5 times higher than that of unreinforced sand and could sustain anchor displacement of more than 60%. Results indicates that the geocell mattress by virtue of its rigidity distributes the uplift load in the lateral directions to a larger area, thereby reducing the stress in the overlying soil mass and hence increases the performance of anchor plate system. The provision of the additional geotextile layer right below the geocell mattress is found to be very effective in increasing the stiffness as well as load carrying capacity of anchor plate system. The optimum size (i.e., width and length) of geocell mattress giving adequate load carrying capacity of anchor plate is found to be 5.4 times of anchor width (5.4B). The comparison of model tests results with 3D numerical analysis shows good agreement, indicating that the proposed model is able to capture the uplift load-displacement behaviour of geocell reinforced anchor plate system.     

Geocell reinforcement for performance improvement of vertical plate anchors in sand

In this paper influence of geocell reinforcement on performance of vertical plate anchors is studied. A series of model tests were carried out in a test bed-cum-loading frame assembly. The anchor used was a steel plate of size 100 mm × 100 mm. With geocell reinforcement the anchor could sustain deformations as high as 60–70% of its height when the load carrying capacity was increased by four fold. The optimum length, width, and height of geocell mattress giving maximum performance improvement are found to be 5, 3 and 2.8 times the anchor height respectively. For adequate performance improvement size of geocell pocket opening should be close to the anchor size. The load dispersion angle that depicts the rigidity of the geocell mattress tends to increase with increase in its width, height and reduction in pocket size. A numerical study using fast Lagrangian analysis of continua was carried out. The agreement between observed and computed results is found to be reasonably good.     

Experimental and numerical study on reinforcement effect of plate anchors or flip anchors on model slopes

Flip anchors are a kind of ground anchor that rotate and open in the ground to attain pull-out resistance without the use of grout. Compared to ordinary grouted ground anchors, flip anchors can be driven into the existing ground quickly and are suitable for the emergency reinforcement of slopes. However, little research has been done on the slope reinforcement effect of flip anchors. In this paper, experiments on a model slope reinforced by plate anchors or flip anchors were conducted. During the experiments, vertical loading with a rigid loading plate was applied to the shoulder of the model slope to investigate its stability. Experiments were firstly conducted with and without model plate anchors under a plane strain condition. Then, experiments were conducted using actual flip anchors under a three-dimensional condition. In these experiments, the depth of the anchor plates, h, and the installation state of the anchor heads of the flip anchors (open or closed anchor head condition) were varied. After the experiments, corresponding numerical simulations (FEM) were conducted, and a subloading t_ij model was applied to describe the soil behaviour. The numerical method used in this research successfully reproduced the reinforcing effect of the flip anchors. According to the test and calculated results, compared with the cases without reinforcement and with plate anchors, the effectiveness of the flip anchors for slope stability was verified. Moreover, the flip anchors installed under the closed anchor head condition required a larger displacement to produce a reinforcing effect than the anchors installed under the open anchor head condition.     

不排水黏土中深埋锚板的抗拔承载力

针对不排水均质黏土中深埋条形锚板和圆形锚板,分别构造两种简单的机动容许速度场,利用塑性力学极限分析的上限法求解锚板的极限抗拔承载力。采用非线性有限元方法对构造的速度场和承载力计算结果进行了验证。与现有的相关解答对比可知:本文上限解与已有的上限解或滑移线场解答较吻合。提出的速度场合理、简单,并严格符合机动容许场的要求,便于实际应用,能为工程设计提供参考依据。     

Physical and numerical modelling of strip footing on geogrid reinforced transparent sand

This paper presents the results of laboratory scale plate load tests on transparent soils reinforced with biaxial polypropylene geogrids. The influence of reinforcement length and number of reinforcement layers on the load-settlement response of the reinforced soil foundation was assessed by varying the reinforcement length and the number of geogrid layers, each spaced at 25% of footing width. The deformations of the reinforcement layers and soil under strip loading were examined with the aid of laser transmitters (to illuminate the geogrid reinforcement) and digital camera. A two-dimensional finite difference program was used to study the fracture of geogrid under strip loading considering the geometry of the model tests. The bearing capacity and stiffness of the reinforced soil foundation has increased with the increase in the reinforcement length and number of reinforcement layers, but the increase is more prominent by increasing number of reinforcement layers. The results from the physical and numerical modelling on reinforced soil foundation reveal that fracture of geogrid could initiate in the bottom layer of reinforcement and progress to subsequent upper layers. The displacement and stress contours along with the mobilized tensile force distribution obtained from the numerical simulations have complimented the observations made from the experiments.     

CFRP筋复合式锚具锚固性能的试验研究

当复合材料FRP(Fiber Reinforced Polymer/Plastics)筋或拉索应用到斜拉桥的拉索体系时,黏结式锚具和夹片式锚具均有其自身的局限性。对此,根据普通拉索锚固体系的特点,并结合CFRP筋夹片式锚具和黏结式锚具的研究成果,提出了锚固CFRP筋的复合式锚具。复合式锚具由楔紧锚固端和黏结锚固端组成,其中楔紧锚固端包括锚杯、带有凹齿的夹片、铝套管以及塑料薄膜,黏结锚固端包括钢套筒和黏结介质活性粉末混凝土RPC(Reactive Power Concrete)。静载试验研究了锚杯长度、钢套筒长度、夹片预紧力、筋材预张拉力等试验参数对复合式锚具锚固性能的影响。试验结果表明,复合式锚具的试件极限荷载最大为208kN,相应的锚固效率系数为104%,大于95%,满足规范要求。复合式锚具两种锚固形式的锚固长度较合理组合为:对不预张拉锚具,锚杯长度40mm,黏结锚固长度100mm;对预张拉锚具,锚杯长度60mm,黏结锚固长度60mm。提出的复合式锚具极限荷载计算式具有较好的适用性。     

Effect of reinforcement form on the bearing capacity of square footings on sand

This paper presents the results of laboratory model loading tests and numerical studies carried out on square footings supported on geosynthetic reinforced sand beds. The relative performance of different forms of geosynthetic reinforcement (i.e. geocell, planar layers and randomly distributed mesh elements) in foundation beds is compared; using same quantity of reinforcement in each test. A biaxial geogrid and a geonet are used for reinforcing the sand beds. Geonet is used in two forms of reinforcement, viz. planar layers and geocell, while the biaxial geogrid was used in three forms of reinforcement, viz. planar layers, geocell and randomly distributed mesh elements. Laboratory load tests on unreinforced and reinforced footings are simulated in a numerical model and the results are analyzed to understand the distribution of displacements and stresses below the footing better. Both the experimental and numerical studies demonstrated that the geocell is the most advantageous form of soil reinforcement technique of those investigated, provided there is no rupture of the material during loading. Geogrid used in the form of randomly distributed mesh elements is found to be inferior to the other two forms. Some significant observations on the difference in reinforcement mechanism for different forms of reinforcement are presented in this paper.     

Experimental and theoretical studies on the ultimate bearing capacity of geogrid-reinforced sand

Geosynthetic reinforced soil (GRS) structures have gained popularity in replacing concrete rigid piles as abutments to support medium or small-spanned bridge superstructures in recent years. This study conducted 13 model tests to investigate the ultimate bearing capacity of the GRS mass when sand was used as backfill soil. The GRS mass was constructed and loaded to failure under a plane strain condition. Test results were compared with two analytical solutions available in literature. This study also proposed an analytical model for predicting the ultimate bearing capacity of the GRS mass based on the Mohr-Coulomb failure criterion. The failure surface of the GRS mass was described by the Rankine failure surface. The effects of compaction and reinforcement tension were equivalent to increased confining pressures to account for the reinforcing effects of the geosynthetic reinforcement. The proposed model was verified by the results of the model tests conducted in this study and reported in literature. Results indicated that the proposed model was more capable of predicting the ultimate bearing capacity of the GRS mass than the other two analytical solutions available in literature. The proposed model can be used to predict the ultimate bearing capacity of GRS structures when sand was used as backfill material. In addition, a parametric study was conducted to investigate the effects of friction angle of backfill soil, reinforcement spacing, reinforcement strength, and reinforcement stiffness on the ultimate bearing capacity of the GRS mass calculated with and without compaction effects. Results showed that the ultimate bearing capacity of the GRS mass was significantly affected by the friction angle of backfill soil, reinforcement spacing and strength. Compaction effects resulted in an increase in the ultimate bearing capacity of the GRS mass.     

Experimental study on uplift mechanism of pipeline buried in sand using high-resolution fiber optic strain sensing nerves

Reliable assessment of uplift capacity of buried pipelines against upheaval buckling requires a valid failure mechanism and a reliable real-time monitoring technique. This paper presents a sensing solution for evaluating uplift capacity of pipelines buried in sand using fiber optic strain sensing (FOSS) nerves. Upward pipe-soil interaction (PSI) was investigated through a series of scaled tests, in which the FOSS and image analysis techniques were used to capture the failure patterns. The published prediction models were evaluated and modified according to observations in the present study as well as a database of 41 pipe loading tests assembled from the literature. Axial strain measurements of FOSS nerves horizontally installed above the pipeline were correlated with the failure behavior of the overlying soil. The test results indicate that the previous analytical models could be further improved regarding their estimations in the failure geometry and mobilization distance at the peak uplift resistance. For typical slip plane failure forms, inclined shear bands star from the pipe shoulder, instead of the springline, and have not yet reached the ground surface at the peak resistance. The vertical inclination of curved shear bands decreases with increasing uplift displacements at the post-peak periods. At large displacements, the upward movement is confined to the deeper ground, and the slip plane failure progressively changes to the flow-around. The feasibility of FOSS in pipe uplift resistance prediction was validated through the comparison with image analyses. In addition, the shear band locations can be identified using fiber optic strain measurements. Finally, the advantages and limits of the FOSS system are discussed in terms of different levels in upward PSI assessment, including failure identification, location, and quantification.     

Experimental and numerical analysis of large scale pull out tests conducted on clays reinforced with geogrids encapsulated with coarse material

The pullout test is one of the methods commonly used to study pullout behavior of reinforcements. In the current research, large pullout tests (i.e. 100 × 60 × 60 cm) have been conducted to investigate the possibility of pullout resistance enhancement of clays reinforced with HDPE geogrid embedded in thin layers of sand. Pullout tests on clay–geogrid, sand–geogrid and clay–sand–geogrid samples have been conducted at normal pressures of 25, 50 and 100 kPa. Numerical modeling using finite element method has also been used to assess the adequacy of the box and geogrid sizes to minimize boundary and scale effects. Experimental results show that provision of thin sand layers around the reinforcement substantially enhances pullout resistance of clay soil under monotonic loading conditions and the effectiveness increases with increase in normal pressures. The improvement is more pronounced at higher normal pressures and an optimum sand layer thickness of 8 cm has been determined for maximum enhancement. Results of numerical analysis showed the adequacy of the box and geogrid length adopted as well as a relatively good agreement with experimental results.     

Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.     

锚拉桩三维土拱效应数值分析与试验研究

针对锚拉桩设计中土压力计算模式存在的问题,提出了三维土拱效应模型这种新的计算方法,通过有限元方法,对该模型的正确性进行了验证。在此基础上,讨论了锚头间距、土体粘聚力、内摩擦角及弹性模量对三维土拱效应的影响。结果表明:随着锚头间距增大,土拱效应减小;土体粘聚力和内摩擦角增大,土拱效应增强,但增大到一定值后,荷载分担比例变化较小,且粘聚力值变化对土拱效应的影响比内摩擦角更显著;土体弹性模量增大,荷载分担比例也增大,最大可达到100%,此时结构体系可采用点锚,三维土拱效应存在于整个锚索自由段。为进一步验证模型的正确性,进行了室内模拟试验,试验结果表明,挡板后水平及竖向均存在土拱效应。     

Experimental and numerical investigation of the airflow around a raised permeable panel

Analysis of the airflow around an elevated permeable panel is presented in this paper. The airflow was studied by both a 3D computational simulation and a full scale experiment using two kinds of cladding material, namely an impermeable plastic film and permeable nets. The air velocity at different locations around the panel was measured by rotary cup anemometers in order to investigate the airflow. A three-dimensional numerical simulation (CFD) was employed to analyze the edge effects. In the numerical model, the net was simulated as a porous medium obeying Forchheimer’s law. Both numerical results and full-scale experiments indicate important differences between the airflow around the panel covered by impermeable material (film) and the airflow around and through the permeable panels (nets). Airflow around the elevated experimental panel was found to become smoother when the plastic film is replaced by permeable nets. The numerical results derived by the 3D computational model show good qualitative and quantitative agreement with the full scale experimental data in the case of permeable (net-covered) panels.     

Experimental investigation and numerical simulation of a furnished office fire

Experiments were conducted in a full-scale model room equipped with both movable and fixed fire loads to explore fire growth and spread via heat release rates, indoor air temperature and species concentration. The room space is a brick structure that measures 5.7 m in interior length, 4.7 m in width and 2.4 m in ceiling height. The northeast and southeast corners each feature a 2.1 m × 0.9 m open doorway. Numerical simulations with parameter adaptation were carried out using FDS software to predict the fire features and were compared with the experimental results. In this study, the material properties and oxygen limit settings in the FDS software were tested to explore their influence on the tendency of heat release rate and on the total amount of heat release. The results show that the heat release rate from the FDS simulations is comparable to the full-scale experiment results during the fire growth period. Temperature profile near ceiling can be modeled well. In the full-involvement burning and decaying periods, the qualitative trends were identical, although the simulated value differed greatly from the experimental result.     

A numerical investigation into the response of free end tubular composite poles subjected to axial and lateral loads

This paper presents a numerical investigation of cantilevered glass fiber-reinforced polymer (GFRP) tubular poles subjected to lateral and axial loads. A 3D finite element analysis was conducted to establish the lateral load–deflection responses under different axial loads and the axial load–bending moment interaction curves at ultimate. The model accounts for geometric nonlinearities and the composite laminate structure. Failure modes were established based on either material failure according to the Tsai-Wu failure criterion, or stability failure. The model was validated by using experimental results. A parametric study was then carried out on poles with various angle-ply and cross-ply laminates as well as different diameter-to-thickness (D/t) and length-to-diameter (L/D) ratios. The study showed that the reduction in axial strength as (L/D) ratio increases becomes more severe as (D/t) ratio is reduced. The GFRP laminate structure has a considerable effect on axial and flexural strengths of the poles for certain (D/t) ratios. It was also shown that axial load–moment interaction curves are generally linear. Increasing the fraction of longitudinal fibers in cross-ply laminates or reducing the fiber angle with the longitudinal direction in angle-ply laminates results in a larger interaction curve. A simplified design approach for the poles has been proposed.     

充气锚杆数值单元的建立与分析

为了研究充气锚杆的受力特征及相应参数对于锚杆锚固力的影响,利用数值方法建立了充气锚杆的计算模拟单元,分别改变锚杆充气长度和充气压力,得到相应锚固力的变化规律,结果表明:随着橡胶膜长度的增大,锚杆的极限锚固力不断增大,二者符合线性关系;由于橡胶膜充气压力的不同,导致了不同的极限锚固力,锚杆极限锚固力与充气压力之间的关系,以及锚杆达到极限锚固力所需的位移与充气压力之间的关系,均可用线性方程进行描述。     

Laboratory investigation of the behavior of a geosynthetic encased stone column in sand under cyclic loading

This paper presents the results of a laboratory investigation into the behavior of a geosynthetic encased stone column (GESC) installed in sand under cyclic loading using a reduced-scale model. A number of test variables were considered, such as the geosynthetic encasement stiffness and the cyclic loading characteristics, including loading frequency and amplitude. The results indicate among other things that the overall benefit of the encasement to the performance of the stone column is greater under cyclic loading than under static loading. It is shown that the degree of load transfer to the column becomes smaller when subjected to cyclic loading than under static loading, leading to a 25% decreased stress concentration ratio. The encasement is found to be more effective in improving the stone column performance when subjected to lower frequency and/or smaller amplitude loading. The lateral bulging zone of the GESC under cyclic loading tends to extend beyond the reported critical encasement length for an isolated static loading case, and therefore full encasement is recommended. Practical implications of the findings are discussed in detail.