Field Load Tests Research Articles

Evaluation of prediction models for tip resistances of rock-socketed drilled shafts

This study compiled a database called CYCU/RockTip/51 consisting of 51 rock-socketed drilled shafts installed at different sites worldwide, covering a wide variety of rock properties and shaft geometries. The tip resistances from seven representative prediction models were compared with the measured values. These measured values were obtained from the load–displacement curves of field load tests using three interpretation criteria. It was found that the prediction models of Teng, Coates, Rowe and Armitage, and ARGEMA overpredicted the measured capacity, while Zhang and Einstein, Vipulanandan et al. and Zhang are less biased. These tip prediction models also could be classified according to displacement requirements. The proposed tip prediction models of this study are presented based on different interpretation methods and the displacement ranges that they are mobilized. Finally, the normalized load–displacement curve was fitted to the hyperbolic curve with two model parameters ( a and b). The parameters a and b define the reciprocal of the initial slope and the asymptotic resistance, respectively. The statistics of ( b, a) for rock-socketed drilled shafts in CYCU/RockTip/51 are mean = (0.76, 1.09), COV = (0.13, 0.59), and correlation coefficient = −0.79. These statistics are useful for reliability-based design.

Final-State Structural Behavior of the Long-Span Kömürhan Cable-Stayed Bridge under Torsional and Bending Loadings

Final-state field load testing is an effective method for understanding the real behavior of a bridge before it opens to traffic. The paper presents the procedure and the results of final-state static field tests on the new long span Kömürhan cable-stayed bridge with a single pylon and back span anchorage block under torsional and bending loadings. A total of six static loading cases were conducted to investigate real structural behavior of the bridge having a back span anchorage block and main span deck connected to the pylon with 42 tensioned cables in the deck center. Measured results have been obtained from the surveys and sensors while theoretical results have been calculated from the three-dimensional finite element model of the bridge final-state geometry. The results of final-state static load tests include the main span deck deflections, pylon horizontal displacements, the strains of the main span deck and the pylon, and the cable forces. A good agreement has been achieved between the experimental and theoretical results. Both experimental and theoretical results have shown that the real bridge behavior is in the elastic state under the planned test loads and supplies the design requirements. It is also observed that the results from the torsional and bending load cases positioned in the same sections are close to each other.

New approach for predicting the load-displacement curve of axially loaded piles in sand

A relationship to predict the load-displacement curve for axially loaded piles in sandy soil is presented by using the tri-linear softening model to describe the shaft load-displacement relationships, as well as a bi-linear model is adopted to characterize the unit end resistance-displacement reaction in the region of the pile's tip soil. Subsequently, an analytical mathematical approach to a single pile-surrounding soil interaction relation is provided as a nonlinear analysis to obtain the load-displacement curves. The mechanical response of piles under compression loading was analysed, and thus, an analytical method is developed to establish a solution for the entire compression process of the piles. A combination of the prementioned models with the pile response under compression loading was employed to establish a fully analytical approach. Furthermore, additional analytical approaches and field-tested pile loading history cases were utilized to prove the degree of exactness of the suggested method by comparing the experimental load-displacement curves with the ones founded by the proposed theoretical method in this work. The results of this study illustrate that the proposed method could be considered as an analytical mathematical technique for utilizing both the tri-linear softening model and the bi-linear elastic-plastic model for the purpose of predicting the load-displacement curves of the axially loaded pile with a 98% degree of accuracy compared to field load tests results.

A database of installation monitoring and uplift load tests of round-shaft helical anchors in Brazil

Helical anchors have been widely used during the last 15 years to support electrical transmission line towers in Brazil; however, the prediction of the uplift capacity of these anchors is still a very difficult task. Typically, costly and time-consuming static load tests complemented with torque–capacity correlations are used for the determination of anchor capacity. One reason for the inability of the existing design methods is the degree of soil disturbance during installation, which is variable according to the soil type, number of helices, anchor configuration, and quality of installation. Therefore, an extensive database of field load tests and installation monitoring of helical foundations in different Brazilian sites would be valuable to guide the engineers for a better and optimum design, reducing project cost. The current paper presents a helical anchor database that includes data of soil characterization, anchor geometry, installation torque and load–displacement response of 107 anchor cases in 40 different sites in Brazil. Additionally, an analysis of the data presented here indicated that the torque-correlation method, used frequently for the quality control of helical anchors, needs to be improved by considering the influence of parameters such as relative embedment depth and soil type. Moreover, this database can contribute to a better understanding of helical anchor behaviour and be used for: (i) calibration of uplift capacity estimation methods and of resistance factors for ULS and SLS design, (ii) development of torque–capacity correlation models, (iii) correlations between ultimate helix bearing pressure and SPT blow count; (iv) characterization of the uncertainties of design methods; (v) improvement of numerical models; etc.

Base resistance of super-large and long piles in soft soil: performance of artificial neural network model and field implications

This study aims to examine the performance of artificial neural network (ANN) model based on 1137 datasets of super-large (1.0–2.5 m in equivalent diameter) and long (40.2–99 m) piles collected over 37 real projects in the past 10 years in Mekong Delta. Five key input parameters including the load, the displacement, the Standard Penetration Test value of the base soil, the distance between the loading point and pile toe, and the axial stiffness are identified via assessing the results of field load tests. Key innovations of this study are (i) use of large database to evaluate the effect that random selection of training and testing datasets can have on the predicted outcomes of ANN modelling, (ii) a simple approach using multiple learning rates to enhance training process, (iii) clarification of the role that the selected input factors can play in the base resistance, and (iv) new empirical relationships between the pile load and settlement. The results show that the random selection of training and testing datasets can affect significantly the predicted results, for example, the confidence of prediction can drop under 80% when an average R2 > 0.85 is required. The analysis indicates predominant role of the displacement in governing the base resistance of piles, providing significant implication to practical designs.

Scour diagnosis index based on cross-correlation analysis of bridge dynamic response under vehicle braking

Several innovative scour diagnostic indexes are proposed based on the cross-correlation analysis for bridge dynamic response excited by vehicle braking. Firstly, a vehicle-bridge coupled vibration model considering vehicle braking is established and its accuracy is verified by a field loading test. Secondly, the fixed shape characteristic of cross-correlation function amplitude vector of the braking-induced dynamic response is theoretically deduced and verified numerically with a continuous girder bridge example considering various test parameters, which is the basic for scour diagnosis. Further, two scour diagnostic indexes are proposed, one is the cross-correlation function amplitude change rate vector, which is used to identify the scour location with a threshold of 2%, and the other is the transversal cross-correlation function amplitude change rate vector, which is used to identify the scour uniformity. The good applicability and accuracy of proposed indexes for different scour states is verified by numerical simulation for above bridge example. The proposed diagnostic indexes are very sensitive to foundation scour and have strong error tolerance for test parameters. The dynamic response required for scour diagnosis can be obtained with regular bridge loading test.

Investigation into bearing performance of concrete expanded-plates piles: Field test and numerical modelling

Concrete expanded-plates (CEP) piles usually exhibit a higher bearing capacity due to the effect of plates. However, the full-scale field test and the simulation regarding the influence of plate parameters on the bearing performance of CEP piles have not been studied thoroughly. In order to promote the application of the novel, efficient and economic pile in road and railway foundations in practice, this study investigates the loading performance of CEP piles based on a field test and numerical simulation model. The field loading test on the bearing performance of CEP and straight piles is firstly performed to investigate the bearing capacities of two types of piles. A 3D simulation model of CEP pile subjected to vertical loads is then developed using the finite difference software FLAC 3D. The load-settlement curves and bearing capacities of piles obtained from the model are validated against the field test results. The bearing behaviours of CEP and straight piles are investigated and compared. In addition, the influence of plate positions, numbers, dimensions and spacing on the bearing performance of CEP piles has been studied. The novel insight from this study incites an original and better understanding of the bearing performance of CEP piles under vertical loads in engineering foundations.

Evaluation of a unified p-y method for lateral analysis of large-diameter drilled shafts using NVShaft

Lateral load analyses of large-diameter drilled shafts based on the conventional p-y method tend to result in unreliable responses. The paper highlights a unified p-y analysis that incorporates vertical side shear-induced moment and tip resistances, which becomes significant in laterally-loaded large-diameter drilled shafts. A simplified tip moment resistance model applicable to any well-defined soil or rock material is also proposed. To perform unified p-y analysis, a finite-difference-based comprehensive load analysis program, NVShaft, has been developed. The lateral shaft response using NVShaft was evaluated using two field load tests from Las Vegas in cemented soil, and two field load tests in sand and clay dominant sites from the PISA project in Europe. Comparison between measured responses and numerical predictions signifies the relative importance of added resistance components in the unified p-y analysis in different subsurface materials. Further investigation on diameter effects in the context of mentioned load test programs indicates that additional lateral resistance in large-diameter shaft comes mostly from side shear-induced resisting moment, followed by tip shear resistance.

Determination of Finite Element Modelling Errors for Box Culverts Using Field Load Tests

Buried culverts have been in service more than half a century and their age raises questions about their reliable serviceability for the new standard vehicles. The gross weight and axle spacing of these trucks play an important role on the performance of the buried culverts where the culvert is covered with shallow fill heights. Field live load tests were conducted by driving a loaded truck over some culverts at slow speed while recording data at the critical locations of these culvert. Finite element (FE) models were developed and calibrated using field load test results. Live load distribution formulations were adopted from AASHTO LRFD Bridge Design Specifications for different fill heights. Two- and three-dimensional FE models were modelled and loaded separately to compare the critical section forces. The results showed that the error using two different FE modelling approach yields more than 100% error for the maximum section forces when fill heights is between 60 cm and 120 cm. This study is aimed to highlighting the inherent error in numerical modelling of buried reinforced box culverts for design or rating purposes.

Seismic isolation technology for prestressed segmental precast piers

Prestressed segmental precast (PC) piers, with insufficient energy dissipation capacity, are seldomly adopted on bridges in seismic areas. To improve the seismic performance of PC piers, this study proposes a functional separation seismic isolation device (FSID), serving as a supplementary replaceable energy dissipation device. Full-scale field load tests on a real-world PC pier were carried out to qualify the seismic performance of this type of piers. Based on the results of tests and supplementary numerical analysis, an optimal design of the FSID was achieved. The numerical results show that the FSID has multi-stage statuses and reaction models. By optimizing the design parameters, FSID can enable the PC piers to satisfy the required seismic performance under various levels of earthquakes. Therefore, the performance-based seismic design of PC piers can be achieved using the FSID.

Universal Criterion for Interpreting Capacity from Load Tests on Piles

Interpretation of field load tests on piles has many important practical and financial implications in foundation engineering practice. However, identifying the ultimate capacity or resistance on a continuous load settlement curve remains elusive. The geotechnical engineering literature abounds with over 40 criteria to aid in interpreting the ultimate capacity from static load tests on deep foundations. These include settlement, settlement rate, offset, and creep criteria, among others. Since the 1990s criteria proposed by Davisson, AASHTO, and New York City Building Code as well as the 5% and 10% of diameter criteria have evolved to become the most routinely specified for all foundations, without assessing their performance or what they have been intended for. This study provides a side-by-side comparison of the performance of these methods in relation to usability, accuracy, precision, and length and diameter effects. In addition, the performance is also compared with the recently developed NYU criterion where the capacity is taken as the smallest of (i) a settlement corresponding to the elastic compression plus 0.75 in.; (ii) the capacity at plunging or strain-softening; or (iii) settlement corresponding to 5% of the pile diameter, unless the settlement threshold is modified by the structural engineer of record. Assessment is made possible using a database of 350 load tests conducted on square and round concrete piles, open and closed steel pipe piles, as well as H-piles. Comparison suggests that the NYU criterion is the most versatile, precise, and accurate among the examined methods.

Dynamic Stability Assessment of High-Speed Railway Bridges Using Numerical Model Updating

Numerical model updating using the data measured from the actual structure is required in order to minimize the error between the initial numerical model and the actual structure. Field load tests, which are conducted in order to assess the condition and safety of high-speed railway bridges, are generally expensive and restricted by railway control and weather conditions. Therefore, a method for evaluating the performance of high-speed railway bridges using updated numerical models without conducting field load tests is required. In this study, numerical model updating was performed by using the data measured from the ambient vibration test in order to assess the dynamic stability of high-speed railway bridges. In the ambient vibration test, the measurement point roaming method was applied in order to accurately measure high-speed railway bridges using a limited number of sensors. For numerical model updating, the univariate search method was used, and several measured parameters were updated and converted into the properties of the target bridges in the numerical models. The vertical and torsional modes of the updated numerical models differed by less than 5% from those estimated using the data measured from the target bridges. The responses of the updated numerical models were found to be similar to those measured from the high-speed railway bridges in operation. It was also shown that the updated numerical models could be used to assess the dynamic stability of the bridges.

Field test research and design of a new heliostat support structure in coarse gravel

A solar power tower plant includes a solar tower and tens of thousands of heliostats, with the latter accounting for 40–50% of the total cost. To reduce heliostat costs, a new type of heliostat support structure (HSS) is proposed, consisting of a prestressed high-strength concrete pipe pile and cast-in-place concrete. With the performance and applicability of this structure unknown, full-scale field load testing of the proposed HSS on coarse gravel, subjected to lateral loads or torsion–lateral combined loads, was conducted. The results indicated that lateral load amplifies the rotation under torque and the proposed HSS for mirror arrays can provide sufficient rigidity to limit structural deformations within tolerable values under severe wind loading. Together with three-dimensional modelling, the performance of the proposed HSS was thoroughly investigated to facilitate the development of modified prediction methods for its lateral and torsional load responses. A detailed design procedure is proposed for the new structure, considering foundation deformations, bearing capacity and survival requirements. This study proved that the proposed HSS can improve the economic performance of solar power tower plants.

Unsteady Aerodynamic Simulation Test Based on Ampere Force and Electromagnetic Field

Unsteady aerodynamic force refers to the aerodynamic force that the magnitude and direction of the load borne by the aircraft will change with time during flight. Usually, the simulation of unsteady aerodynamic force is completed by wind tunnel test. This paper introduces a new load simulation method based on electromagnetic field and Ampere force. Compared with the traditional wind tunnel test method, electromagnetic field load test will greatly reduce the test cost and make it possible to repeat multiple tests. In order to verify whether the test method is feasible and effective, this paper introduces a number of verification tests, including static load test and dynamic quasi modal test. By comparing the ground test results with those based on ABAQUS simulation, the reliability of the test method is verified. In this test technology, electromagnetic technology is used to study the force of structure, which is of great significance for multi physical field test and aircraft design test.

Development of Expanded Steel Pipe Pile to Enhance Bearing Capacity

An expanded steel pipe pile increases the cross-sectional area of conventional micropile by expanding the steel pipe to exhibit a higher bearing capacity owing to increased frictional resistance. However, construction cases of the expanded steel pipe pile are insufficient due to the absence of equipment for expanding steel pipes inside the ground. In this study, hydraulic expansion equipment was developed to verify the reinforcing impact on the bearing capacity and field applicability of the expanded steel pipe pile. A series of laboratory and test bed experiments was conducted to measure the expansion time and deformation of carbon steel pipes by using the developed equipment. The results of these experiments demonstrated that the developed equipment has sufficient ability and constructability to be used in the field for constructing expanded steel pipe piles. Then, field load tests were performed by constructing expanded and conventional steel pipe piles to confirm the improved bearing capacity of the expanded steel pipe pile compared to that of the conventional micropile. As a result, the expanded steel pipe pile exhibited a 20.88% increase in bearing capacity compared to that of the conventional steel pipe pile.

Performance Evaluation of a Long-Span Cable-Stayed Bridge Using Non-Destructive Field Loading Tests

As an important part of the transportation network, the reliability of bridge structures is of great significance to people’s personal safety, as well as to the national economy. In order to evaluate the performance of complex bridge structures, their mechanical behavior and fundamental characteristics need to be studied. Structural health monitoring (SHM) has been introduced into bridge engineering, and the structural response assessment, load effects monitoring, and reliability evaluation have been developed based on the collected SHM information. In this study, a performance evaluation method for complex bridge structures based on non-destructive field loading tests is proposed. The cable-stayed bridge in Guangxi with the largest span (Pingnan Xiangsizhou Bridge) was selected as the research object, and loading on the main girder was transferred to the piers and tower through the stay cables, whose structural responses are critical in the process of bridge operation. Therefore, the field loading tests—including deflection and strain testing of the main girder, as well as cable force tests—were also conducted for Pingnan Xiangsizhou Bridge by using non-destructive measurement techniques (multifunctional static strain test system, radar interferometric deformation measurement technology, etc.). Based on the numerically simulated results of a finite element model for Pingnan Xiangsizhou Bridge, reasonable field loading test conditions and loading arrangement were determined. Non-destructive field loading test results showed that the quality of the bridge’s construction is up to standard, due to a good agreement between the calculated and measured frequencies of the bridge. In addition, the calibration coefficients of displacement and strain were less than 1, indicating that Pingnan Xiangsizhou Bridge has satisfactory stiffness and strength.

Influence of Load Test Scale on Prediction of Ultimate Bearing Capacity of Aggregate Pier Reinforced Clay

Predicting the bearing capacity of spread footings on aggregate pier‐reinforced soil is particularly important for ground improvement. Various methods for predicting the bearing capacity of spread footings supported on aggregate pier‐reinforced clay have been proposed. In addition, many fields or laboratory load tests have been conducted to identify the improvement effects of aggregate pier. However, no study has quantitatively compared the effect of the experimental scale on the prediction of bearing capacity. In this study, multiple linear regression analyses were performed for field and laboratory load tests, and the effects of the load test scale on bearing capacity prediction were identified. The sensitivity analysis showed that the prediction model for the laboratory load test (MLRL) exhibited a higher sensitivity for the undrained shear strength than that of the field load test model (MLRF). However, the sensitivity of the area replacement ratio of MLRL was less than half that of the MLRF. As the undrained shear strength increased, the predicted bearing capacity of the MLRL was larger than that of the MLRF owing to the influence of boundary conditions on the experimental equipment.

Study on the Technical Status of Old Concrete Truss‐Arch Bridge Based on Vehicle‐Bridge Interaction

With the rapid development of the country’s economy, a large number of heavy vehicles are being used due to the expansion of transportation demand and increased transportation costs. For the old concrete truss‐arch bridges built in the last century, the bridges may hardly meet the modern traffic volume. With respect to old bridges in poor condition, it takes more time and resources to evaluate the technical status by field load tests, and there are some safety risks. A finite element method based on vehicle‐bridge coupled vibration theory was proposed for evaluating the technical status of concrete truss‐arch bridges in this paper. An old concrete truss‐arch bridge in the suburbs of Suzhou, China, was selected as the research object. The field static and dynamic load tests were conducted and the results are consistent with the results of the numerical simulation based on the method proposed in this study. It can be concluded that the proposed numerical method can be widely used to assess the technical state of the old concrete truss‐arch bridges in poor condition.

Health Monitoring-Based Assessment of Reinforcement with Prestressed Steel Strand for Cable-Stayed Bridge

This paper presents the method of reinforcing main girder of reinforced concrete cable-stayed bridge with prestressed steel strands. To verify the effectiveness of external prestressed strand reinforcement method. Static load tests and health monitoring-based assessment were carried out before and after reinforcement. Field load test shows that the deflection and stress values of the main girder are reduced by 10%~20% after reinforcement, and the flexural strength and stiffness of the strengthened beam are improved. The deflection and strain data of health monitoring of the specified section are collected. The deflection of the second span is 4 mm~10 mm, the strain range of the upper edge of the second span is -10 με~-40 με, and the strain range of the lower edge is 30 με~75 με. These values show the deflection and strain values fluctuate within a prescribed range, verifying the safety of the bridge. The reinforcement method of prestressed steel strand is feasible and effective. It can provide reference basis for the application of external prestressed strand reinforcement technology in similar projects.

A field study on the load sharing behavior of a micropiled raft underpinned by a waveform micropile

A waveform micropile (WMP) uses the jet-grouting method to generate shear keys along the pile shaft for improving shaft resistance and cost efficiency. In this study, field loading tests were performed to characterize the load sharing behavior upon inclusion of a WMP in a group of four micropiles. First, single-pile compressive loading tests were conducted on three WMPs and five type A micropiles (MP). Subsequently, a group-pile loading test was performed on a piled raft comprising 2 × 2 MPs and a central WMP. The load–settlements, axial stiffnesses, and load transfer mechanisms of individual MPs were analyzed during the tests, including the short- and long-term effects of the axial stiffnesses of the MPs on the load sharing ratio of the micropiled raft. The single-pile loading test results revealed that the shear keys along the WMPs caused its bearing capacities and axial stiffnesses to be 1.5 times and 2–5 times higher than those of MPs, respectively. In the micropiled-raft loading test, the load sharing ratios of the MPs increased with their axial stiffnesses, and the highest load sharing capacity was exhibited by the WMP, which constituted 30% of the total load and 2–3 times that of MPs. Moreover, the influence of raft on the load-sharing capacity should be considered as well.