Laboratory Testing to Examine Deformations and Moments in Fiber-Reinforced Cement Pipe

Two 381 mm (15 in. nominal) diameter fiber reinforced cement pipes have been tested under embankment loading conditions to study pipe response in both low stiffness, fine grained backfill, and a high stiffness graded granular backfill. Pipe deformations and strains were measured and interpreted to provide insight into the effect of soil backfill on the deformations and moments that develop. Not surprisingly, the use of silty clay backfill resulted in greater pipe deflections while the stiffer granular backfill lead to greater load transfer to the surrounding ground. Calculations using elastic soil-pipe interaction theory were effective in estimating the observed changes in pipe diameter at typical service loads (overburden pressures of 100 kPa, i.e., 14.4 psi in the lower stiffness backfill and 200 kPa, i.e., 28.8 psi in the high stiffness backfill). Measured strain distributions show that the fiber reinforced pipe exhibited ovaling response similar to that seen for flexible and semiflexible pipes. As expected, tensile strains were observed on the outer surface at the springlines and the inner surface at the crown. Strains observed at the haunch were negligible, indicating that the bending moments within the pipe have conventional “hourglass” distribution, with negligible moments at shoulders and haunches. Differences in strain measured at the inner and outer surfaces were used with the elastic pipe modulus to calculate the experimental bending moments. Comparisons of those experimental bending moments with the bending moment calculated for a rigid pipe indicate that these FRC pipe structures are semirigid so that moments are reduced as a result of support provided by the surrounding soil. A design expression for moment arching factor (MAF or moment divided by the rigid pipe moments) developed in an earlier paper was found to provide reasonable estimates for the experimental moment values. Moment estimated using the design soil moduli of McGrath and MAF provide moment values that are reasonable and conservative relative to those that were observed.     

Minimum-Risk Bedding for Flexible Drain Pipes

The soil bedding for nonpressurized buried flexible pipes is critical for minimum-risk performance. Lack of adequate bedding reduces the strength of the pipes by a factor of 2 or more. To compensate, pipes must have high strength, at increased cost, in order to avoid the risk of failure. Structural performance and performance limits are compared for plastic drainpipes with a variety of beddings. Compared to a flat bedding, the risk of failure is greatly reduced by shaping the bedding. Shaped beddings perform essentially as well as full-contact embedment with select granular soil. Tests and analyses show that a 90° V-groove bedding that supports and aligns the pipe performs nearly as well as a 180° form-fit circular groove or a full-contact embedment of select soil. Comparison of beddings is important in the design and installation of drainpipes in order to assure minimum risk of failure.     

Earth Pressure due to Vibratory Compaction

This paper presents experimental data on the variation of lateral earth pressure against a nonyielding retaining wall due to soil filling and vibratory compaction. Air-dry Ottawa sand was placed in five lifts and each lift was compacted to achieve a relative density of 75%. Each compacted lift was 0.3?m thick. The instrumented nonyielding wall facility at National Chiao Tung University in Taiwan was used to investigate the effects of vibratory compaction on the change of stresses at the soil-wall interface. Based on the experimental data it has been found that, for a compacted backfill, the vertical overburden pressure can also be properly estimated with the traditional equation σv = γz. The effects of vibratory compaction on the vertical pressure in the backfill were insignificant. On the vertical nonyielding wall, extra horizontal earth pressure was induced by vibratory compaction. After compaction, the lateral earth pressure measured near the top of the wall was almost identical to the passive Rankine pressure. It is concluded that as the cyclic compacting stress applied on the surface of the backfill exceeded the ultimate bearing capacity of the foundation soil, a shear failure zone would develop in the uppermost layer of the backfill. For a soil element under lateral compression, the vertical overburden pressure remained unchanged, and the horizontal stress increased to the Rankine passive pressure. It was also found that the compaction-influenced zone rose with the rising compaction surface. The horizontal earth pressure measured below the compaction-influenced zone converged to the Jaky state of stress.     

Simulating Seismic Response of Cantilever Retaining Walls

Many failures of retaining walls during earthquakes occurred near waterfront. A reasonably accurate evaluation of earthquake effects under such circumstance requires proven analytical models for dynamic earth pressure, hydrodynamic pressure, and excess pore pressure. However, such analytical procedures, especially for excess pore pressure, are not available and hence comprehensive numerical procedures are needed. This paper presents the results of a finite-element simulation of a flexible, cantilever retaining wall with dry and saturated backfill under earthquake loading, and the results are compared with that of a centrifuge test. It is found that bending moments in the wall increased significantly during earthquakes both when backfill is dry or saturated. After base shaking, the residual moment on the wall was also significantly higher than the moment under static loading. Liquefaction of backfill soil contributed to the failure of the wall, which had large outward movement and uneven subsidence in the backfill. The numerical simulation was able to model quite well the main characteristics of acceleration, bending moment, displacement, and excess pore pressure recorded in the centrifuge test in most cases with the simulation for dry backfill slightly better than that for saturated backfill.     

Moment Capacities and Deflection Limits of PFRP Sheet Piles

The structural response of pultruded fiber-reinforced polymer (PFRP) sheet pile panels subjected to a uniform pressure load was investigated. Single, connected, and concrete-backfilled panels were tested to ultimate failure in an attempt to determine their moment capacities, deflection limits, and failure mechanisms. The load-carrying capacity of single-panel FRP piles was found to be 15% higher than that of three panels connected together. No pin and eye joint separation was observed at failure. The concrete backfilled hybrid panels exhibited significantly increased moment capacity. However, the increase in stiffness after the first concrete crack was, at best, only 76% over the pile without backfill. Bearing failure of a PFRP pile with a partially confined support created excessive deflection in the wall, but showed no significant reduction in the load capacity. On the other hand, with fully confined support, the ultimate failure of single, connected, and concrete-backfilled panels was dominated by local buckling, longitudinal tearing, and bearing crush at shear keys, upon reaching a deflection limit of span/50.     

Full-Scale Field Tests on Flexible Pipes under Live Load Application

This paper describes the procedure and results of the field tests on high-density polyethylene (HDPE), PVC, and metal large diameter pipes subjected to a highway design truck loading. Numerical simulations using finite element method are performed to determine pipe-soil system response under live load application. Comparisons of field test data with the predicted responses are made for soil pressures around and above the pipes, deformed cross-sectional pipe profiles, and pipe deflections. The field test results indicated that the buried flexible pipes, embedded with highly compacted graded sand with silt, demonstrated good performance without exhibiting any visible joint opening or structural distress. Under shallow burial conditions, the AASHTO specified deflection limit of 5% is found to be adequate for installation of the flexible pipes during the construction phase, and a vertical deflection limit of 2% is suggested for HDPE pipes based on the truck load response and repeated loading effect.     

Nonlinear Soil–Abutment–Bridge Structure Interaction for Seismic Performance-Based Design

Current seismic design of bridges is based on a displacement performance philosophy using nonlinear static pushover analysis. This type of bridge design necessitates that the geotechnical engineer predict the resistance of the abutment backfill soils, which is inherently nonlinear with respect to the displacement between soil backfill and the bridge structure. This paper employs limit-equilibrium methods using mobilized logarithmic-spiral failure surfaces coupled with a modified hyperbolic soil stress–strain behavior (LSH model) to estimate abutment nonlinear force-displacement capacity as a function of wall displacement and soil backfill properties. The calculated force-displacement capacity is validated against the results from eight field experiments conducted on various typical structure backfills. Using LSH and experimental data, a simple hyperbolic force-displacement (HFD) equation is developed that can provide the same results using only the backfill soil stiffness and ultimate soil capacity. HFD is compatible with current CALTRANS practice in regard to the seismic design of bridge abutments. The LSH and HFD models are powerful and effective tools for practicing engineers to produce realistic bridge response for performance-based bridge design.     

拉伸试验中充填体声发射特性及数值模拟研究

下向胶结充填采矿过程中, 胶结充填体顶板的破坏不仅受抗压强度的影响, 而且还表现为张拉破坏, 为了充分探索充填体顶板张拉破坏过程中的损伤演化规律, 对胶结充填体试件进行了单轴抗拉破坏声发射试验, 并利用RFPA2D软件对其拉伸破坏过程及声发射信号进行了数值模拟.模拟分析表明, 胶结充填体的抗拉破坏是始于试件圆盘中部, 沿着加载轴线方向出现裂隙萌生与扩展, 并汇聚成宏观的裂隙带, 最终导致整体失稳的破坏过程.模拟结果还再现了抗拉破坏时声发射的分布规律, 其结果对比室内试验的声发射特征规律, 具有很好的一致性.      

Validated Simulation Models for Lateral Response of Bridge Abutments with Typical Backfills

Abutment-backfill soil interaction can significantly influence the seismic response of bridges. In the present study, we provide numerical simulation models that are validated using data from recent experiments on the lateral response of typical abutment systems. Those tests involve well-compacted clayey silt and silty sand backfill materials. The simulation methods considered include a method of slices approach for the backfill materials with an assumed log-spiral failure surface coupled with hyperbolic soil stress-strain relationships [referred to as “log-spiral hyperbolic (LSH) model”] as well as detailed finite-element models, both of which were found to compare well with test data. Through parametric studies on the validated LSH model, we develop equations for the lateral load-displacement backbone curves for abutments of varying height for the two aforementioned backfill types. The equations describe a hyperbolic relationship between lateral load per unit width of the abutment wall and the wall deflection and are amendable to practical application in seismic response simulations of bridge systems.     

Pipe formation in Pb-Sn alloys

It has been shown that upward flowing pipes can be formed in the solid-liquid region of a Pb-Sn alloy when a density inversion exists in the liquid. Pipes do not form when there is no density inversion. These findings support proposed mechanisms for freckling and A segregate formation, based on observations of transparent ammonium chloride-water models.     

Failure Analysis of a Bridge Embankment with Cracked Approach Slabs and Leaking Sand

This paper describes a successful failure analysis to determine the causes of loss of backfill sand from a mechanically stabilized earth (MSE) wall, and cracks on the concrete approach slabs on top of it. The Texas Department of Transportation was concerned that the cracks on the approach slabs may be related to the excessive loss of backfill from behind the MSE walls, and that the embankment structure may be unsafe due to potential voids under the concrete slab. Several cubic meters of sugar sand had washed out of the wall and deposited adjacent to the paneled walls. A series of destructive and nondestructive tests were carried out to determine the causes of the problems. It was found that the cracking of the approach slab and the loss of backfill were unrelated. Suggestions for resolving both problems were made based on this study.     

Lateral Performance of Full-Scale Bridge Abutment Wall with Granular Backfill

Bridge abutments typically contain a backwall element that is designed to break free of its base support when struck by a bridge deck during an earthquake event and push into the abutment backfill soils. Results are presented for a full-scale cyclic lateral load test of an abutment backwall configured to represent the dimensions (1.7?m height), boundary conditions, and backfill materials (compacted silty sand) that are typical of California bridge design practice. An innovative loading system was utilized that operates under displacement control and that assures horizontal wall displacement with minimal vertical displacement. The applied horizontal displacement ranged from null to approximately 11% of the wall height (0.11H). The maximum earth pressure occurred at a wall displacement of 0.03H and corresponded to a passive earth pressure coefficient of Kp = 16.3. The measured force distribution applied to the wall from hydraulic actuators allowed the soil pressure distribution to be inferred as triangular in shape and the mobilized wall-soil interface friction to be evaluated as approximately one-third to one-half of the soil friction angle. Post-test trenching of the backfill showed a log-spiral principal failure surface at depth with several relatively minor shear surfaces further up in the passive wedge. The ultimate passive resistance is well estimated by the log-spiral method and a method of slices approach. The shape of the load-deflection relationship is well estimated by models that produce a hyperbolic curve shape.     

Seismic Displacement Criterion for Soil Retaining Walls Based on Soil Strength Mobilization

This paper presents a seismic displacement criterion for conventional soil retaining walls based on the observations of a series of shaking table tests and seismic displacement analysis using Newmark’s sliding-block theory taking into account internal friction angle mobilization along the potential failure line in the backfill. A novel approach that relates the displacement of the wall and the mobilized friction angle along the shear band in the backfill is also proposed. A range of horizontal displacement-to-wall height ratios (δ3h/H) between 2 and 5% representing a transitional state from moderate displacement to catastrophic damage were observed in the shaking table tests on two model retaining walls. This observation is supported by both Newmark’s displacement analysis and a new approach that relates the movement of the wall to the mobilization of the friction angle along the shear band in the backfill. A permissible displacement of the wall as defined by the displacement-to-wall height ratio, namely, δ3h/H, equal to 2% was found to be of practical significance in the sense that peak friction angle of the investigated sand is retained along the shear band in the backfill. It is also suggested that δ3h/H = 5% be used as a conservative indicator for the onset of catastrophic failure of the wall associated with fully softened soil strength along the shear band in cohesionless backfill.     

Reliability-Based Design for External Stability of Mechanically Stabilized Earth Walls

A two-phase approach was used to develop a reliability-based design (RBD) method for external stability of mechanically stabilized earth (MSE) walls. In the first phase, a parametric study was conducted using Monte Carlo simulation to identify parameters that affect the probability of external failure of MSE walls. Three modes of failure were considered: sliding, overturning, and bearing capacity. External stability was assessed by treating the reinforced soil as a rigid mass using the same procedures employed for conventional gravity-type wall systems. Results from the parametric study indicate that the mean and coefficient of variation of the backfill friction angle are significant for sliding, the mean and coefficient of variation of the friction angle of the backfill and coefficient of variation of the unit weight of the backfill are significant for overturning, and the mean and coefficient of variation of the friction angle of the foundation soil and the mean of the backfill friction angle are significant for bearing capacity. In the second phase, a series of additional simulations was conducted where the significant parameters identified in the parametric study were varied over a broad range. Results of these simulations were used to develop a set of RBD charts for external stability of MSE walls. A comparison indicates that similar reinforcement lengths are obtained using RBD and conventional methods and that the inherent probability of external failure in conventional deterministic design is ? 0.001. This probability of external failure is similar to inherent probability of failure reported by other investigators for similar geotechnical structures.     

Effect of Intermediate Principal Stress on Overconsolidated Kaolin Clay

The influence of intermediate principal stress on the mechanical behavior of overconsolidated kaolin clay is investigated using three-dimensional true triaxial testing on cubical specimens. A flexible boundary, true triaxial setup with a real-time feedback control system was used to test soil specimens under stress and strain-control modes. Undrained tests on kaolin clay show that the following vary with intermediate principal stress: the stiffness at small strains, excess pore pressure generated during shear, and strength and strain to failure. Failure occurred at peak deviator stress followed by shear band formations and localized bulging. Prior theoretical formulations of bifurcation and undrained instability support these experimental observations. Analysis of data in the octahedral plane indicates that kaolin clay follows a nonassociative flow rule, which is described by a constant third stress invariant failure criterion with von Mises plastic potential surface.     

Centrifuge Model Studies of the Seismic Response of Reinforced Soil Slopes

Centrifuge tests were used to study the dynamic behavior of soil slopes reinforced with geosynthetics and metal grids. The main objectives were to determine the failure mechanism and amount of deformations under seismic loading and to identify the main parameters controlling seismically induced deformations. Geosynthetically reinforced soil slopes (2V:1H) and vertical walls reinforced with metallic mesh strips were subjected to earthquake motions with maximum foundation accelerations of up to 1.08g. The experimental results show that slope movement can occur under relatively small base accelerations, and significant lateral and vertical deformations can occur within the reinforced soil mass under strong shaking. However, no distinct failure surfaces were observed, and the magnitude of deformations is related to the backfill density, reinforcement stiffness and spacing, and slope inclination.     

全尾砂膏体流变学研究现状与展望（下）:流变测量与展望

膏体充填是推动金属矿绿色开采发展的关键技术,并可为资源的深部开采提供安全、绿色、高效的技术支撑。全尾砂膏体流变学是膏体充填技术的基础理论,本文在综述膏体流变概念、特性与模型的基础上,进一步对流变测量技术现状进行了系统梳理,概述了现阶段常用的浆式旋转流变仪、坍落筒、L管、倾斜管及环管法进行流变测量的原理及应用,针对膏体这一屈服型非牛顿流体,重点分析了屈服应力的测量,并对以上方法的适用性进行了综合论述。流变测量深刻地影响着膏体流变理论及膏体充填工艺的发展,为此,对测量技术的关键问题进行了探讨,指出构建膏体流变测量标准及加强流变测量技术与充填工艺的结合是重点,并对膏体流变学研究的发展趋势进行了展望。      

Effects of Strain Rate on Low Cycle Fatigue Behaviors of High-Strength Structural Steel

 The low cycle fatigue (LCF) behavior of a high-strength structural steel was investigated in the strain rate range of 4×10-5-0. 12 s-1 (0. 001-3 Hz) under constant total strain (±1%) control. The cyclic stress response at all strain rates exhibited behavior of rapid softening in the early stage of fatigue life and subsequent saturation up to failure. It was found that the stress amplitude, the plastic strain amplitude, the plastic strain energy density and the fatigue life depend mainly on the strain rate. The strain rate of 0. 012 s-1 was found as a transition point where the LCF of the steel showed different behavior from low strain rate to high strain rate. The relationship between the time to failure and strain rate was expressed well by a power law relation. The fracture surfaces of the fatigue samples were characterized by using a scanning electron microscope (SEM) and the fracture mechanisms were discussed in terms of time-dependent deformation of the steel.     

三山岛海底金矿开采充填体与围岩变形规律的数值模拟

为合理解释三山岛金矿新立矿区海底开采充填体和围岩变形特征,建立了假二维的矿山开挖充填力学模型,并将其简化为平面应变问题。根据对充填体力学特性的研究,在模型建立过程中,对充填体采用了双屈服模型,对矿柱及围岩采用了应变硬化/软化塑性模型。利用FLAC3D数值模拟软件,并在模型中的相应位置设置位移监测点,分析了真实矿山开采过程中上下各采场充填体和围岩的移动变形规律。研究结果表明,新立矿区充填体变形主要为水平方向上的压缩变形,且具有累积效应,当充填体达到垂直方向的最大压缩量后,顶板围岩在水平构造应力作用下有向充填体上山方向滑动的趋势。     

Finite Element Analysis of Concrete Pavements Over Culverts

Accelerated distress of Portland cement concrete pavements (PCCP) over structures such as culverts, pipes, and tunnels beneath roadways is a common occurrence. In this article, finite element analysis is employed to analyze the response of concrete pavements over such structures. The factors that influence the overlying pavement slabs include: (1) cover depth, (2) pavement slab thickness and length, (3) cement concrete elastic modulus, (4) foundation modulus, and (5) backfill soil modulus. The tensile stresses at the bottom and top of the slab induced by wheel loads are predicted. In the traditional pavement design only the tensile stress at the bottom of the slab is considered to be significant. However, this study shows that the tensile stress at the top surface of pavement slabs over culverts may also cause the concrete pavements to fail. A laboratory model was employed to study the mechanical characteristics of Portland cement concrete pavement slabs over culverts and to verify the theoretical analysis.