Prestressed High-strength Concrete Research Articles

Field and Numerical Study of the Bearing Capacity of Pre-Stressed High-Strength Concrete (PHC)-Pipe-Pile-Reinforced Soft Soil Foundations with Tie Beams

Pre-stressed high-strength concrete pipe piles (PHC pipe piles) have been widely used in actual soft foundation treatment projects due to their reliable quality, fast construction, assembly line production, and environmental friendliness. However, large-scale slip damage still occurs in construction projects. In order to reduce and avoid such accidents, a highway in Guangdong (section K31+100~K31+388) was taken as an example for this study. Plaxis 2D software (V22.01.00) was used to establish a PHC pipe pile composite roadbed model and investigate the effects of tie beam form, pile lengths, pile spacings, pile verticality, and embankment filling loading modes on the settlement and stability of the composite roadbed. The results show that the original treatment plan, which had the form of a PHC pipe pile with caps, had a low horizontal bearing capacity and a poor anti-disturbance ability, leading to the occurrence of a landslide accident. A comparison of different structural forms revealed that the longitudinal and transverse tie beam form was the most stable, followed by the transverse tie beam form, longitudinal tie beam form, PHC pipe pile form with caps, and PHC pipe pile form without caps. Compared to the structural form of PHC pipe piles with pile caps, the stabilities of the transverse tie beam form and the longitudinal tie beam form were improved by 42.47% and 38.61%, respectively, while that of the longitudinal and transverse tie beam form was improved by 50.87%. The application of longitudinal and transverse tie beams effectively reduced the settlement of the composite roadbed, as confirmed by both measured data and finite element analysis. This structure achieved the desired vertical settlement control and lateral anti-slip effects.

Multi-scale microstructure quantitative characterization and anti-erosion performance of PHC pipe pile

Pre-stressed high-strength concrete (PHC) pipe pile is widely used in many large-scale projects, thus their durability has attracted more and more attention. However, in most studies, PHC pipe pile is regarded as the same as ordinary reinforced concrete, ignoring the effect of special construction processes and hollow shape on its microstructure and anti-erosion performance. In this paper, the real PHC pipe pile was cored and cut to investigate the multi-scale microstructure and the anti-erosion performance of PHC pipe pile concrete layer. The results shown that pore content first increases and then decreases with PHC pipe pile from the outer layer to the inner layer (5F → 1F) by CT, BSE and LF-NMR results, and the pore content changes most significantly around the rebar in PHC pipe pile. In rebar interfacial transition zone (ITZ), the thickness of the voids in PHC pipe pile is greater than that of normal reinforced concrete under the same water-binder ratio condition. Additionally, the content of un-hydration products in aggregate ITZ is obviously greater than that in rebar ITZ. Furthermore, the chloride ion diffusion coefficient of the concrete on the outside of the PHC pipe pile is slightly larger than that on the inside, which is closely related to the higher pore connectivity of the concrete on the outside of the PHC pipe pile. Moreover, it was found that centrifugation can not only reduce the water-binder ratio of PHC pipe pile but also affect the radial pore structure of PHC pipe pile concrete, including porosity and pore connectivity. This work finally indicated that the microstructure of PHC pipe pile concrete is quite different from that of normal reinforced concrete, providing a theoretical basis for establishing the diffusion model of PHC pipe pile concrete.

Experimental Study on Bonding Performance between Prestressed Concrete Pipe Piles and Core Grout

To make the bearing capacity tests safer and more affordable for prestressed high-strength concrete (PHC) piles, this paper proposes a reaction device for anchor piles filled with core grout based on the geometric and mechanical characteristics of PHC piles. The proposed reaction device has the advantages of convenient construction, strong controllability of the connection quality and low cost. In addition, the pile will not experience prestress unloading or tensile stress under the effect of the upward pulling load. To promote the application of the reaction device developed for PHC pile bearing capacity tests, experimental studies are conducted on the bonding performance between the core grout and the inner wall of the PHC pile. The influence of various factors such as the strength of the core grout, the grouting length, the curing time, and the inner diameter of the PHC pile on the bond strength between the core grout and the inner wall of the PHC pile are investigated. Results show that as the inner diameter of the PHC pile increases, the bond strength between the core grout and the inner wall of the PHC pile decreases with a maximum difference of 5%. The bond strength decreases as the grouting length increases, and gradually stabilizes, with a difference of no more than 10% between the maximum and minimum bond strength values. The higher the strength of the grout is, the greater the bond strength between the core grout and the inner wall of the PHC pile is. The bond strength between the core grout and the inner wall of the PHC pile increases with the increase of the curing time within 28 days of curing, and the bond strength at 3 days meets the requirements of the PHC pile bearing capacity test.

Field testing of seismic performance of prestressed high strength concrete piles reinforced with non-prestressed tendons considering pile–soil interaction

Prestressed high-strength concrete (PHC) piles have been widely applied as foundations. However, the application of PHC piles was challenging under earthquake hazards. With the increasing demand for PHC piles in earthquake-prone areas, PHC piles reinforced with non-prestressed tendons (PRC piles) were proposed to address the problems. A full-scale field test for one PHC pile and four PRC piles with different non-prestressed tendon ratios (NPTRs) was conducted under horizontal cyclic loading, and their seismic performances were evaluated in terms of hysteretic behaviours, horizontal bearing capacity, ductility, pile deformation, and crack patterns. The results indicated that adding non-prestressed tendons led to an improvement in the hysteresis characteristics and deformation capacity but a decrease in the horizontal bearing capacity. The displacement ductility coefficients of all specimens reached or exceeded 2.8, and there was an optimal NPTR of 0.75% that maximized the ductility of PRC piles, which was 62.5% greater than that of the PHC pile. The pile-soil interaction was considered by analyzing the crack development of the surrounding soil and piles.

Shake-table test on dynamic response of prestressed high-strength concrete pipe piles under Soil–Structure interaction

This paper presents the results of a shake-table test on a superstructure model supported by a prestressed high-strength concrete (PHC) pipe pile in soft soil. The primary objective is to investigate the dynamic response of the PHC pipe pile in soft soil influenced by soil–structure interaction (SSI). Accordingly, a single PHC pipe pile with iron blocks is constructed and embedded in soft soil. First, the production of the physical model of the PHC pipe pile and experimental details are elucidated. Then, the study focuses on the dynamic response of the soil–pile–superstructure system considering different ground motion types and peak ground accelerations (PGAs). Finally, the inertial and kinematic effects on the dynamic response of the PHC pipe pile are analyzed. The results show that the SSI effect can increase the predominant period of the superstructure. With the SSI effect, the seismic design should consider the adverse effects of long-period and pulse-type ground motions on the pile. The SSI effect caused the rate of increase in the pile bending moment to be gradually higher than the increase in PGA. Moreover, the dynamic response of the PHC pipe pile is mainly dominated by the inertial effect.

Experimental research on the flow law of the grouting fluid in an annular gap between a non-soil-squeezing PHC pipe pile wall and a hole wall

The bearing capacity of drilling with pre-stressed concrete pile cased pile (hereafter referred to as DPC pile) is closely related to the grouting effect on an annular gap between a pre-stressed high-strength concrete (PHC) pipe pile wall and a hole wall (hereafter referred to as the an annular pile–soil gap). A physical grouting model testing system for the DPC pile based on a high-precision three-dimensional (3D) scanner and a multi-functional grouting box has been independently developed. In this method, 3D geometric size and spatial point information of the grouting stone have been quantitatively characterized. The influences of the water–cement ratio, grouting pressure, collapsed holes, and falling sand have been studied. The conclusions are obtained as follows: (1) a quantitative characterization method of the 3D geometric dimensions of the grouting stone based on a 3D scan is accurate and reliable and can overcome the shortcomings of traditional manual measurement. (2) In the same horizontal plane, grouting body thickness gradually decreases as its horizontal distance from the grouting outlet increases, and the higher the elevation, the greater the rate of decrease; conversely, the lower the elevation, the slower the rate of decrease. When the horizontal distance from the pulp outlet is equal, slurry thickness gradually decreases as the height increases. (3) For the grouting liquid with a water–cement ratio of 0.5, grouting pressure should not be less than 0.6 MPa. (4) When the falling sand is not far above the grouting outlet, a grouting root system can be formed, whose grout veins, complexity, and grout coverage area can be optimally improved by changing the fluidity of the grout and grouting pressure. When the falling sand is on the side of the grouting outlet or the surface of the grouting outlet but far away from the grouting outlet, it is easy to be avoided by the grout, which can greatly reduce the grouting effect.

Laboratory tests on the frictional capacity of core pile–cemented soil interface

Pre-bored grouted planted (PGP) piles are a new type of composite pile in which a core pre-stressed high-strength concrete (PHC) pile is surrounded by a cemented soil (CS) layer. The frictional capacity of the core PHC pile–CS interface has a significant effect on the bearing capacity of a PGP pile. In this work, a series of shear tests was carried out to determine the frictional capacity of pile–CS interfaces for both smooth square piles (SPs) and smooth pipe piles (PPs). The shear test results revealed the following. The peak skin friction (PSF) of the SP–CS and PP–CS interfaces both increased with the CS strength. The PSF of the SP–CS interface was smaller than the PSF of the PP–CS interface of identical pile (equivalent) diameter due to the occurrence of a stress concentration at the corners of the SP. The PSF of the SP–CS and PP–CS interfaces both decreased with the pile (equivalent) diameter. The occurrence of cracks in the CS caused brittle failure of both the SP–CS and PP–CS interfaces.

Research on Dynamic Pile-Driving Formula Parameters and Driving Feasibility of Extra-Long PHC Pipe Piles

Prestressed high-strength concrete (PHC) pipe pile has the advantages of high single pile bearing capacity, a wide range of applications, good driving resistance, fast construction speed, etc. It has been widely used in high-rise buildings, bridges, ports, and other industries. The application of extra-long PHC pipe piles with a length of more than 50 m is increasing. However, there are few studies on the drivability and hammering criteria of extra-long PHC piles. To analyze the drivability of extra-long piles and predict their bearing capacity, in this paper, high-strain dynamic tests were carried out on 14 test sections with the pile foundation of Temburong Bridge in Brunei as the research background. The hammer stop control criteria calculated according to the Hiley formula would lead to excessive hammering. Three types of damage occurred during construction: pile shaft breakage, weld tearing, and pile head breakage. The weight and drop height of the piling hammer selected for this project were appropriate, and the extra-long test piles can be hammered to the design depth. The values of Cp (Compression of the pile) and n (the efficiency of the blow) were fitted based on the dynamic test data, which provided a more accurate reference for the selection of subsequent piling parameters of the project. It provides a more accurate calculation method for predicting the bearing capacity of extra-long PHC piles and provides control criteria for pile stopping and a scientific basis for their design and construction.

Load-Bearing Characteristics of PHC Piles Constructed by the Inner Digging Method Based on Ultimate Load Testing and Numerical Simulation

This paper studies the load-bearing characteristics of two prestressed high-strength concrete (PHC) pipe piles constructed by the medium mid-digging and hammering methods. The ultimate load tests and numerical simulations of the pipe piles constructed by both methods were carried out to analyze the ultimate lateral resistance, and ultimate resistance performance characteristics of the two pipe piles and the influence of the wall thickness of the pipe piles on the bearing performance. The test results show that the pipe pile constructed by the middle inner digging method has a higher pile quality. The single pile bearing capacity of the pipe pile constructed by the middle inner digging method is 50% higher than that of the common hammering method. The enlarged part of the pile end has an obvious effect on improving the bearing capacity. The settlement of the pipe pile constructed by the middle inner digging method is smaller than that of the hammering method. The large diameter pipe pile constructed by the middle inner digging method usually shows characteristics of the end-bearing pile. The resistance of the pile end accounts for 40–50% of the top load. The numerical simulation results agree with the field test and are compared and discussed. The simulation results show that when the bearing capacity of the pile is provided by the pile side frictional resistance, the influence of the pile wall thickness on the bearing capacity is insignificant. When the top pile load is close to the bearing capacity of the pipe pile, the influence of the pipe pile wall thickness on the bearing capacity is greater.

Installation Effects and Behavior of a Driven Prestressed High-Strength Concrete Nodular Pile in Deep Saturated Soft Clay

This paper investigated the installation effects of a driven prestressed high-strength concrete (PHC) nodular pile in deep soft clay through full-scale tests, and the behavior of a driven PHC nodular pile is analyzed based on the field test results. The test results demonstrated that the driven PHC nodular pile installation process induced significant disturbance in the surrounding soil. The excess pore water pressures (Δu) that were induced by the installation of a driven PHC nodular pile were larger than the Δu that were induced by the installation of a driven PHC pipe pile. The driven PHC nodular pile installation caused significant Δu in the soil down to 2 m below the pile base, and the Δu became much smaller in the soil at 4 m below the pile base. The Δu dissipated slowly in the soft clay layers, and the Δu in the surficial soil layers dissipated faster than that in the deep soil layers. The nodules along the PHC nodular pile shaft could increase the pile shaft capacity. The measured shaft resistances of 350(400)-mm PHC nodular piles were 1.17–1.18 times the calculated shaft resistances of 350-mm PHC pipe piles in soft clay layers.

Studies of nano-SiO2 and subsequent water curing on enhancing the frost resistance of autoclaved PHC pipe pile concrete

Autoclave curing will cause severe deterioration on the frost resistance of pre-stressed high-strength concrete (PHC) pipe pile. To clarify the frost resistance degradation mechanism of autoclaved PHC pipe pile concrete and obtain effective improvement measures, nano-SiO2 (NS) with cement replacement ratios of 0.5%, 1.0%, and 1.5% were adopted, and concrete specimens were fabricated and subjected to standard curing, autoclave curing, and autoclave curing + one week of subsequent water curing, respectively. Then, a rapid freeze-thaw cycle experiment was utilized to test their frost resistance. In addition, the micro-morphology and pore structure of different concrete were studied through scanning electron microscopy and nuclear magnetic resonance. The results indicate that the mechanisms of the degradation of autoclaved concrete in frost resistance are the increase of concrete porosity, harmful pore ratio, and concrete brittleness. Compared with standard curing, autoclave curing can result in 68.5% loss in the frost resistance of concrete. Through playing the filling and pozzolanic effects, NS can improve the compactness of autoclaved concrete and increase its frost resistance to a certain extent. Subsequent water curing can replenish water into concrete, promote the hydration of the residual unhydrated cementitious materials in concrete, enhance the compactness of concrete, and play a positive role in improving the frost resistance of concrete. The incorporation of 1.0% NS can enhance the frost resistance of autoclaved concrete by 66.6%, while the combination of 1.0% NS incorporation and one-week subsequent water curing can enhance the frost resistance by 89.2%. Exponential function can be used to simulate the frost resistance degradation of the autoclaved PHC pipe pile and predict its service life based on frost resistance.

Flexural Test and Numerical Simulation of a New Combination Splice for Prestressed, Precast Concrete Piles Using High-Strength Steel Strands

Prestressed, precast concrete piles using High-Strength Steel Strands (PPCPs using HSSS) are a new type of precast pile. Compared with prestressed high-strength concrete (PHC) piles, the adoption of ultra-high-strength concrete and HSSS not only improves the load-bearing capacity, but also enhances the ductility of precast piles. The engineering application of PPCPs using HSSS requires not only a high bearing capacity of the pile segments, but also reliable splicing to ensure cooperation between pile segments. Based on the characteristics of strand anchorage plates, this paper proposes a new combination splice using the clamp ring and welding (Combination Splice). The theoretical analysis and design method of this Combination Splice is introduced. This research gives a thorough investigation into the flexural performance of PPCPs using HSSS with the Combination Splice. The flexural tests of PPCPs using HSSS with the Combination Splice were firstly conducted on eight full-scale pile specimens with three different pile diameters and four different steel reinforcement ratios. The flexural performances are evaluated in terms of crack resistance, flexural capacities, crack distribution, as well as strain development. The results indicate that the Combination Splice remain safe and intact when the piles reach the ultimate bending capacity. The ultimate bending moment of tested specimens with the Combination Splice is, on average, 10% larger than that of specimens using a theoretical formula. In light of the experimental data, a finite element analysis (FEA) model has been created to simulate the flexural performance of the piles with the Combination Splice. The FEA results show that the load–displacement curves and crack distribution regions are in good agreement with the experimental findings, which verifies the reliability and accuracy of the FEA model. The parameter analysis investigates the effects of the assembly gap and clamp ring corrosion on the flexural performance of PPCPs using HSSS. The results show that assembly gaps have a greater influence on the flexural capacity and deformation, while the influence of the clamp ring corrosion is negligible, indicating that the Combination Splice has certain advantages in terms of durability.

Long-Term Settlement Prediction of Ground Reinforcement Foundation Using a Deep Cement Mixing Method in Reclaimed Land

The greenhouse foundation method requires a lower allowable bearing capacity compared to general buildings, but the high-spec and expensive prestressed high-strength concrete (PHC) pile reinforcement method is mainly applied. Therefore, the deep cement mixing (DCM) method, which is one of the ground reinforcement foundations that replaces the PHC piles and secures structural safety suitable for the greenhouse foundation, was considered. To verify the structural safety of the DCM method, a geotechnical survey and soil test were conducted, and a long-term settlement monitoring system was established. The specifications of the DCM foundation were designed to be 0.8 m in diameter, 3 m × 3 m in width and length, and 3 m in depth. Based on the settlement monitoring data, long-term settlement was predicted considering the greenhouse durability of 15 years. For long-term settlement prediction, the Log S–T, hyperbolic, Asaoka method, Schmertmann theory, and the finite element method (FEM) analysis were performed. In the case of the Log S–T, hyperbolic, and Asaoka method based on actual measurement data, the settlement amount was predicted to be 12.18~20.43 mm, and in the case of the Schmertmann empirical formula, it was predicted to be 19.66 m. The FEM analysis result was 8.89 mm. As the most conservative result, the DCM foundation method designed in this paper had an allowable bearing capacity of 310 kN/m2 and a long-term settlement of 20.43 mm. This is the result of satisfying both the allowable bearing capacity of 100 kN/m2 and the allowable settlement range of 25.4 mm as a foundation. Through this study, it was proven that long-term structural safety can be sufficiently secured when the DCM foundation is constructed on a soft ground through a design that considers the required service life and allowable bearing capacity of the structure. In addition, it was confirmed that the Hyperbolic, Asaoka, and FEM analysis method adopted in this paper can be applied to the long-term settlement behavior analysis of the DCM foundation method.

Low-Cycle Reverse Loading Tests of the Continuous Basalt Fiber-Reinforced Polymer Column Filled with Concrete

The continuous basalt fiber-reinforced polymer column filled with concrete (BFRPC composite column) can well resist the erosion of the external environment, improve the durability of the structure, and reduce the life-cycle cost of the project. To evaluate the mechanical behaviors of the BFRPC composite column under cyclic lateral loading, laboratory low-cycle reverse loading tests were implemented on a BFRPC composite column specimen, a prestressed reinforced concrete (PRC) tube column specimen, and a prestressed high-strength concrete (PHC) tube column specimen. The failure features, hysteretic curve, and skeleton curve for these three types of column specimens were compared and analyzed through the load–displacement hysteretic curve. The results indicated that the BFRPC composite column possesses the better bearing capacity and deformation performance. The horizontal bearing capacity of the BFRPC composite column is at least three times better than that of PHC and PRC tube columns. Finally, the functional expression of the skeleton curves for the BFRPC composite column is fitted by the rational function fitting method.

Durable life assessment of centrifugal stratified PHC piles in marine environment

This study proposes an analytical approach to assess the durable life of pre-stressed high-strength concrete (PHC) pipe piles exposed to the marine environment with consideration of centrifugal stratification and different end types. The durable life of the PHC pile is divided into two stages: the initiation stage for chloride penetration and the propagation stage for rust accumulation. To simulate the two stages, a two-layer composite cylinder diffusion model and a thick-walled ring mechanical model are developed based on the governing mechanisms. The proposed diffusion model is verified via comparisons with the numerical models with respect to chloride concentration under different stages. Then, extensive parametric studies based on the proposed method are performed to investigate the effects of end type, cover thickness, and steel bar diameter on the durable life of a PHC pipe pile. The results indicate that both increasing the cover thickness and using the closed-ended pile can significantly extend the durable life of a PHC pipe pile. Furthermore, the open-ended pile with a thinner mortar layer has a longer durable life, while the durable life of a closed-ended pile only slightly increases with a larger thickness of the mortar layer.

Analysis of Influence Factors of Negative Skin Friction on PHC Pile in High-Backfill Sites

It is recognized that NSF on a pile affects the structural and geotechnical design. The formation of NSF on a pre-stressed high-strength concrete (PHC) pile under a control building in a Vietnamese project was investigated using a verified numerical model in FLAC3D. The height of backfills, the consolidation degree of soil, working loads on the PHC pile, and the pile diameter were analyzed in sensitivity analyses to investigate the influence on NSF. It was found that higher backfills and higher consolidation degree were dominant in the formation of NSF, causing an increase of the dragload and an upward movement of the neutral plane. The working load and the pile diameter affected the dragload and the position of the neutral plane less. A limit decrease of the dragload induced by NSF was observed when the working load increased. Accelerating the soil consolidation process would be suggested to avoid great displacement due to NSF.

Time Dependence of Neutral Plane Position in a PHC-Supported Soft Subgrade

This case study focuses on an International Circuit project in China where unmanned automobiles are to be tested in the future. The subgrade undergoes extensive settlement because of the widely distributed lacustrine deposits with high void ratio and compressibility. Prestressed high-strength concrete (PHC) pile-supported earth platforms are used for ground improvement in this area, but their effectiveness has not yet been confirmed. Associated with this adoption, the negative skin friction (NSF) of the piles and the related neutral plane (NP) position is of importance. Although NSF has been examined in detail, the NP position has not, especially with regard to its time dependence. This paper reports the settlement of the pile-supported soft soil subgrade as obtained from long-term observations of full-scale tests. The NP position is determined according to the layered settlement of the soil and the pile settlement, both of which are obtained from long-term field observations. The results reveal that the NP position has a clear time dependence that develops in stages. The NP is initially elevated with time, and then it gradually approaches a constant value. The existing models fail to predict this time dependence of the NP position in the studied sites. A exponential model is used, the fitting parameters of which have practical implications. The adopted model works well with the studied sites, especially in terms of reflecting the time dependence of the NP position and distinguishing the different stages of its development. This paper enhances the understanding of the NP and provides a reference data set for related engineering practices.

Static Load Test and Numerical Analysis of Influencing Factors of the Ultimate Bearing Capacity of PHC Pipe Piles in Multilayer Soil

Prestressed high-strength concrete (PHC) pipe piles have been widely used in engineering fields in recent years; however, the influencing factors of their ultimate bearing capacity (UBC) in multilayer soil need to be further studied. In this paper, a static load test (SLT) and numerical analysis are performed to obtain the load transfer and key UBC factors of pipe piles. The results show that the UBC of the test pile is mainly provided by the pile shaft resistance (PSR), but the pile tip resistance (PTR) cannot be ignored. Many factors can change the UBC of pipe piles, but their effects are different. The UBC of the pipe pile is linearly related to the friction coefficient and the outer-to-inner diameter ratio. Changes in the pile length make the UBC increase sharply. Low temperatures can produce freezing stress at the pile–soil interface. The effect of changing the Young modulus of pile tip soil is relatively small.

Experimental Study of Thermal Response of Vertically Loaded Energy Pipe Pile

Energy piles are a relatively new technology that have dual function as heat transferring and load bearing. Due to the influence of temperature cycles, additional thermal stress and relative displacement of the pile will be generated; this is different from the load transferring mechanism of the conventional pile. In order to study the thermodynamic characteristics of the energy pipe pile under dual working conditions and temperature cycles, field tests were carried out on the PHC (prestressed high-strength concrete) energy pipe pile without constraining on the top of the piles. Displacement gauges were arranged on the top of the pile, and concrete strain gauges (temperature, strain) were embedded in the pile. The variation laws of temperature, thermal strain, thermal stress, side friction resistance, and displacement of the pile top during the temperature cycling were analyzed. The test results show that the heat exchange system reached a stable state after being heated for 5 days in summer. The average temperature of the pile increased by 15.17 °C, to 34.68 °C; it was low at both ends and high in the middle part. After 5 days in the winter environment, the average temperature of the pile decreased by 10.09 °C, to 9.54 °C, which was high at both ends and low in the middle. The thermal stress was generated inside the pile, and the maximum compressive stress was 3.446 MPa and the maximum tensile stress was 2.69 MPa. The neutral point of the side friction resistance appeared 8 m below the pile top, about 2/3 of the pile length. The maximum negative side friction resistance under the summer condition was 42.06 KPa, the maximum positive side friction resistance under the winter condition was 29.93 KPa, and the lateral resistance of the pile degraded in winter. Under the influence of thermal load, the final pile top displacements in the summer and winter were −0.7 mm (0.175%D) and 0.77 mm (0.193%D), respectively.

Application of Fiber Bragg Grating Sensing in Bidirectional Tests of Pile Foundations

Abstract The fiber Bragg grating (FBG) technique has emerged as a new strain monitoring technique in civil engineering applications. In this paper, based on the principle of the FBG technique and the bidirectional test method, FBG sensors and conventional vibrating wire strain gauges (VWSGs) were employed to monitor the strain variation law of a pile under different loads and analyze the relationship between the side resistance and pile-soil relative displacement of soil layers in bidirectional field testing of a large-diameter cast in situ pile. Four field bidirectional test cases of cast in situ piles, prestressed high-strength concrete (PHC) and steel pipe pile, and pile-base post-grouting pile are presented. The applicability and stability of the FBG sensing technique were verified by analyzing the side resistance distribution and bearing capacity. The results showed that FBG sensors can remove the influence because of residual force and the effect of temperature change and concrete expansion photosensitive material provides an improved sensitivity to the external environment compared with a VWSG. The strain and side resistance variation tendency of the cast in situ pile were consistent between the two kinds of sensors. The FBG sensing technique had high test accuracy in the four bidirectional O-cell field test cases of the cast in situ pile, PHC and steel pipe pile, and pile-base post-grouting pile. The pile-base post-grouting technique is an effective procedure that was implemented to increase the ultimate bearing capacity. The research results of this paper reveal a new strain monitoring technique for the bidirectional test method of pile foundations and represent a new test method for measuring the bearing capacity of PHC and steel pipe piles that has a remarkable application value in practical engineering.