Applications of Ultrasonic Detection Technology in Bridge Concrete Pile Foundation Detection

In this paper,the application strategy of ultrasonic detection technology in the detection of concrete foundation piles is analyzed using a construction project as an example.It includes a basic overview of the project,an overview of ultrasonic testing technology in bridge concrete pile foundation testing,and an analysis of its practical application in the concrete pile foundation testing of this project.The objective of this analysis is to provide some reference for the application of ultrasonic testing technology and the improvement of the quality of bridge concrete pile foundation testing.

Quantitative analysis of magnetic anomaly of reinforcements in bored in-situ concrete piles

We quantitatively study magnetic anomalies of reinforcement rods in bored insitu concrete piles for the first time and summarized their magnetic anomaly character. Key factors such as measuring borehole orientation, borehole-reinforcement distance, and multiple-section reinforcement rods are discussed which contributes valid and quantitative reference for using the magnetic method to detect reinforcement rods. Through tests with model piles, we confirm the accuracy of theoretical computations and then utilize the law discovered in theoretical computations to explain the characteristics of the actual testing curves. The results show that the Za curves of the reinforcement rod reflect important factors regarding the reinforcement rods, such as rod length, change of reinforcement ratio, length of overlap, and etc. This research perfects the magnetic method for detecting reinforcement rods in bored in-situ concrete piles and the method has great importance for preventing building contractor fraud.

Deformation and failure modes of composite foundation with sub-embankment plain concrete piles

With the development of high-speed railway in China, composite foundation with rigid piles has become a stamdard solution of meeting the high requirements of stability and post-construction settlement of embankment on soft subgrade. Among several im- provement pattems, plain concrete piles have been extensively used to treat soft ground supported embankment. To investigate the deformation and failure modes of unimproved soft ground and soft ground reinforced by sub-embankment plain concrete piles, and to learn the influences of track and vehicle load, the effect of pile spacing, as well as the compression moduli of soil layers and upper load condition on the failure modes, a series of centrifuge model tests were performed. Test results indicate that the dis- placement of unimproved soft ground under the embankment increases continuously as embankment, track and train loading, and slip circle failure takes place. The deformation law of soft ground reinforced by sub-embankment plain concrete piles depends on pile spacing, compression modulus of the soft ground, and loading conditions. It was also found that plain concrete piles show displacement and failure patterns depending on its location, compression modulus of soft soil around the pile, and loading condi- tions. Furthermore, the evaluation of improved ground stability as well as the model test procedure is also presented.

Uplift behavior and load transfer mechanism of prestressed high-strength concrete piles

Prestressed high-strength-concrete (PHC) tube-shaped pile is one of the recently used foundations for soft soil. The research on uplift resistance of PHC pile is helpful to the design of pile foundations. A field-scale test program was conducted to study the uplift behavior and load transfer mechanism of PHC piles in soft soil. The pullout load tests were divided into two groups with different diameters, and there were three piles in each group. A detailed discussion of the axial load transfer and pile skin resistance distribution was also included. It is found from the tests that the uplift capacity increases with increasing the diameter of pile. When the diameter of piles increases from 500 to 600 mm, the uplift load is increased by 51.2%. According to the load-displacement (Q-S) curves, all the piles do not reach the ultimate state at the maximum load. The experimental results show that the piles still have uplift bearing capacity.

High-frequency interference waves in low strain dynamic testing of X-section concrete piles

Stress waves propagate along vertical,radial and circumferential directions when a non-uniformly distributed load is applied at one end of a three-dimensional shaft.As a result,the receiving signals are usually mixed with undesired interference components,often featuring as high-frequency fluctuations.Previous studies have revealed that sectional geometry(shape and size)greatly affects the high-frequency interference.In this study,low strain dynamic testing on full-scale X-section concrete is conducted in order to investigate the influences of high-frequency interference on velocity responses at the pile head.Emphasis is placed on the frequency and peak value of interference waves at various receiving points.Additionally,the effects of the geometrical,and mechanical properties of the pile shaft on high-frequency interference are elaborated on through the three-dimensional finite element method.The results show that the measured wave is obscured by interference waves superposed by two types of high-frequency components.The modulus and cross-sectional area are contributing factors to the frequency and peak value of the interference waves.On the other hand,the position with the least interference is determined,to some extent,by the accurate shape of the X-section.

Mechanical performance of the mixed post-installed connection in low-strength concrete

Due to inadequate bearing capacity of the chemically-planted steel bar in low-strength concrete, a new mixed post-installed connection is proposed using small diameter anchoring steel bars and grouting materials together to anchor the main steel bar. To investigate the feasibility of the proposed post-installed connections, a series of pull-out tests with different anchors were conducted for comparison,including fully adhesive anchors, partially adhesive anchors,grouting material anchoring connection and the new mixed post-installed connection. The experimental results of the single steel bar pull-out test show that the mixed post-installed connection can effectively enhance the bearing capacity of post-installed steel bars in low-strength concrete. The bearing capacity is increased by nearly two times with no cone-type concrete failure compared with the fully adhesive anchor. The results show that adopting the new mixed post-installed connection can ensure that joint performance meets the requirements if the space dimension is available.

Physical modeling of behaviors of cast-in-place concrete piled raft compared to free-standing pile group in sand

Similar to free-standing pile groups, piled raft foundations are conventionally designed in which the piles carry the total load of structure and the raft bearing capacity is not taken into account. Numerous studies indicated that this method is too conservative. Only when the pile cap is elevated from the ground level,the raft bearing contribution can be neglected. In a piled raft foundation, pileesoileraft interaction is complicated. Although several numerical studies have been carried out to analyze the behaviors of piled raft foundations, very few experimental studies are reported in the literature. The available laboratory studies mainly focused on steel piles. The present study aims to compare the behaviors of piled raft foundations with free-standing pile groups in sand, using laboratory physical models. Cast-in-place concrete piles and concrete raft are used for the tests. The tests are conducted on single pile, single pile in pile group, unpiled raft, free-standing pile group and piled raft foundation. We examine the effects of the number of piles, the pile installation method and the interaction between different components of foundation. The results indicate that the ultimate bearing capacity of the piled raft foundation is considerably higher than that of the free-standing pile group with the same number of piles. With installation of the single pile in the group, the pile bearing capacity and stiffness increase. Installation of the piles beneath the raft decreases the bearing capacity of the raft. When the raft bearing capacity is not included in the design process, the allowable bearing capacity of the piled raft is underestimated by more than 200%. This deviation intensifies with increasing spacing of the piles.

Key technology for construction of connection between steel pylonand concrete pile cap of middle pylon of Taizhou Bridge

Taizhou Bridge is a suspension bridge with three pylons and two 1 080 m main spans. The middle pylon is a steel frame with longitudinal herringbone shape and lateral gate shape. The connection between steel pylon and concrete pile cap is a key part to transfer the huge inner force from the pylon to the foundation. Its construction quality is a critical factor to the overall structural loading of the whole bridge ; therefore the contact ratio between the bearing steel plate of pylon and concrete pile cap is required to be over 75 %. The inclined joint surface in two directions, longitudinally at 39/1 920 and laterally at 1/4, posted a challenge to the construction work. A procedure test was carried out to find an optimal construction method by comparison, and finally the post-injection method was selected as it can meet the requirement of concrete strength and contact ratio at the connection. The successful application of the post-injection method in Taizhou Bridge can nrovide an examnle and reference for similar nroiects in the future.

Viscoelastic behavior of concrete pile

Based on constitutive theory of viscoelasticity,the viscoelastic behaviour of concrete pile was investigated.The influence of viscosity coefficient on the stress,displacement and velocity response was discussed.With the increase of viscosity coefficient,the amplitude of stress wave decreases,and the maximum value of the stress wave shifts to deeper position of the pile.In other words,the viscosity coefficient behaves as lag effect to stress wave.

Field study of plastic tube cast-in-place concrete pile

The compositions, technical principles and construction equipments of a new piling method used for ground improvement plastic tube cast-in-place concrete pile were introduced. The results from static load tests on single piles with different forms of pile shoes and on their composite foundations were analyzed. The distribution patterns of axial force, shaft friction and toe resistance were studied based on the measurements taken from buried strain gauges. From the point of engineering application, the pile has merits in convenient quality control, high bearing capacity and reliable quality, showing higher reasonability, advancement and suitability than other ground improvement methods. The pile can be adopted properly to take place of ordinary ground improvement method, achieving greater economical and social benefits.

Bearing Behavior of Cast-in-Place Expansive Concrete Pile in Coral Sand Under Vertical Loading

The low side friction of piles in coral sand results in the low bearing capacity of foundations.In this paper,expansive concrete pile is utilized to improve the bearing capacity of pile foundations in coral sand.Both model tests and numerical simulation are performed to reveal the bearing mechanism of expansive concrete pile in coral sand.Results showed that the lateral earth pressure near pile increases obviously and the side friction of piles is improved,after adding expansion agent to the concrete.The horizontal linear expansion is 1.11%and the bearing capacity increased 41%for the pile,when 25%expansion agent is added.Results in finite element numerical simulation also show that ultimate bearing capacity increases with the increase of the linear expansion ratio.Besides,the area for obvious increase in side friction is below the surface of soil about three times the pile diameter,and the expansion leads to a high side friction sharing of the pile.Therefore,the cast-in-place expansive concrete pile is effective in improving the bearing capacity of piles in coral sand.

Study on Interface mechanical behavior of steel tube reinforced concrete composite pile

Currently for the steel tube reinforced concrete composite pile research, although predecessors make a comprehensive research on the composite pile beating performance, design technology, but there are still many problems have not been solved, such as the steel tube reinforced concrete pile composite interracial force learn performance research is still in the initial stage. In this paper, we mainly discuss the research methods of several interface mechanical properties and propose the possibility of studying the mechanical properties of the steel tube reinforced concrete composite pile by using the principle of ultrasonic speckle.

Influence of the penetration of adjacent X-section cast-in-place concrete(XCC)pile on the existing XCC pile in sand

A series of small-scale 1g X-section cast-in-place concrete(XCC)pile-penetration model tests were conducted to study the effects of soil density and pile geometry on the lateral responses of an existing pile and the variations in surrounding soil stress.The results showed that the bending patterns of existing XCC piles varied with penetration depth.The lateral response of the existing pile was sensitive to the change in relative density and pile geometry.For example,the bending moment of the existing pile increased along with these parameters.The development of the radial stressσ′r/σ′v0 of the soil around an existing pile showed different trends at various depths during the penetration of the adjacent pile.Moreover,the change in radial stress during the penetration of the XCC pile did not exhibit the“h/R effect”that was observed in the free-field soil,due to the shielding effect of the existing piles.The peak value of radial stressσ′r_max/σ′v0 decreased exponentially as the radial distance r/R increased.The attenuation ofσ′r_max/σ′v0 with r/R in the loose sand was faster than in the medium-dense or dense sands.Theσ′r_max/σ′v0 at the same soil location increased with the cross-section geometry parameter.

Field observation of the thermal disturbance and freezeback processes of cast-in-place pile foundations in warm permafrost regions

The bearing capacity of pile foundations is affected by the temperature of the frozen soil around pile foundations.The construction process and the hydration heat of cast-in-place(CIP)pile foundations affect the thermal stability of permafrost.In this paper,temperature data from inside multiple CIP piles,borehole observations of ground thermal status adjacent to the foundations and local weather stations were monitored in warm permafrost regions to study the thermal influence process of CIP pile foundations.The following conclusions are drawn from the field observation data.(1)The early temperature change process of different CIP piles is different,and the differences gradually diminish over time.(2)The initial concrete temperature is linearly related with the air temperature,net radiation and wind speed within 1 h before the completion of concrete pouring;the contributions of the air temperature,net radiation,and wind speed to the initial concrete temperature are 51.9%,20.3%and 27.9%,respectively.(3)The outer boundary of the thermal disturbance annulus is approximately 2 m away from the pile center.It took more than 224 days for the soil around the CIP piles to return to the natural permafrost temperature at the study site.

High-Rise Residential Reinforced Concrete Building Optimisation

In the last few decades structure optimisation has become a main task in a civil engineering project. As a matter of fact, due to the complexity and particularity of every structure, the great amount of variables and design criteria to considerate and many other factors, a general optimisation’s method is not simple to formulate. As a result, this paper focuses on how to provide a successful optimisation method for a particular building type, high-rise reinforced concrete buildings. The optimization method is based on decomposition of the main structure into substructures: floor system, vertical load resisting system, lateral load resisting system and foundation system;then each of the subsystems using the design criteria established at the building codes is improved. Due to the effect of the superstructure optimisation on the foundation system, vertical and lateral load resisting system is the last to be considered after the improvement of floor. Finally, as a case example, using the method explained in the paper, a 30-story-high high-rise residential building complex is analysed and optimised, achieving good results in terms of structural behaviour and diminishing the overall cost of the structure.

温度与荷载共同作用下钢筋混凝土桩的细观损伤

为探究港口码头不同环境因素对码头桩身材料特性的劣化影响而降低桩身承载能力,通过颗粒流分析程序(particle flow code 3D,PFC3D)建立三维颗粒流离散元钢筋混凝土桩模型开展研究。对温度-应力模型进行建立,选取热学参数,构建了温度热管模型,通过分级加热,得到了钢筋混凝土桩的横向拉伸应变、纵向压缩应变以及微裂缝生长数量随温度变化曲线;调整模型混凝土粗骨料占比与强度等级,研究其对钢筋混凝土桩受热损伤的影响;通过施加温度场的模型进行水平加荷载后,得到的应力应变曲线;基于模型微裂隙数量的损伤变量,并构建了损伤变量-轴向应变曲线,进一步分析了温度-荷载共同作用的损伤规律。结果表明:混凝土轴向压缩应变和横向拉伸应变会随温度的增加而逐渐升高,细观微裂隙数量会随温度的升高而增长;粗骨料占比升高会使受热初始损伤增加;温度的升高,材料的宏观力学性能出现了不同程度的降低。

湿陷性黄土地基中桩基负摩阻力计算新方法

负摩阻力对湿陷性黄土地基中桩基的承载变形性状具有重要影响,现行桩基规范及黄土规范推荐的负摩阻力计算方法不能反映黄土桩基负摩阻力的实际性状。对近30年来在黄土地区开展的钢筋混凝土灌注桩现场浸水试验实测数据进行统计,分析黄土浸水完成后桩身中性点深度、桩身最大负摩阻力深度及负摩阻力系数与桩长径比的关系,通过线性拟合得到了中性点深度比、最大负摩阻力深度比及负摩阻力系数与桩长径比的经验表达式,提出湿陷性黄土地基中桩基负摩阻力计算的新方法。新方法用三角形表示负摩阻力沿深度的分布,能反映负摩阻力从桩顶向下先逐渐增大、达到最大值后随深度增加逐渐减小、最终在中性点处减小为零的分布特征。将提出的方法、桩基规范及黄土规范方法的计算结果与实测结果进行对比,结果表明,桩基规范预测的中性点深度比优于黄土规范,黄土规范预测的最大下拉荷载优于桩基规范,而用新方法计算的负摩阻力最接近实测结果。

钙质砂地基中群桩基础荷载传递特性研究

为研究钙质砂地基中群桩基础承载过程中的性能特征,开展不同桩距条件下群桩基础静载试验.得到不同桩距条件下群桩的Q-S曲线和S-lgt曲线,测得3d、4.5d、6d(d为桩径)群桩在静载条件下的极限承载力F_(u),研究各个桩段的桩身轴力、桩侧摩阻力以及桩端阻力的变化趋势.研究结果表明:群桩的Q-S曲线为缓变型,S-lgt曲线为渐变型,桩基承载变形大致分为稳定、渐进以及破坏3个阶段,相较于3d群桩,4.5d、6d群桩沉降发展更缓慢,F_(u)更高;同等桩距下,钙质砂地基表现出比陆源砂地基更强的群桩效应,钙质砂地基中群桩最佳桩距应在4.5d~6d之间.该研究为群桩在钙质砂地基中的应用提供数据支撑和理论参考.

地基中混凝土排桩主动隔振研究

为了探究地基中设置混凝土排桩后,排桩的各参数变化对近场主动隔振的影响规律,在有限元软件中建立与试验场地一致的三维有限元模型,通过计算与分析验证了有限元计算结果的有效性;参考TB 10621—2014《高速铁路设计规范》建立了足尺寸的有限元模型,分析在列车荷载作用下排桩的各参数变化对隔振效果的具体影响规律。研究结果表明:桩长参数对隔振效果影响较为明显,随着参数增大,隔振效果越理想,当桩长参数为10.5以后,继续增加比值,隔振效果提升不显著;随着桩间距参数越大,隔振效果越不理想,较小的桩间距可获得理想的隔振效果;埋深参数对隔振效果影响显著,参数越大,隔振效果越不理想,当参数为3.5时,隔振效果已很不理想,较浅埋深能获得较理想的隔振效果。

轴力对预应力高强混凝土管桩抗震性能的影响

目的研究轴力和往复荷载作用下预应力高强混凝土管桩的抗震性能,为预应力高强混凝土管桩的设计和实际工程应用提供参考依据。方法应用ABAQUS有限元软件对预应力高强混凝土管桩进行有限元模拟,并与已有试验结果对比,两者吻合良好,验证了模型的准确性。在此基础上,研究轴压比和有无普通钢筋及普通钢筋直径对预应力高强混凝土管桩滞回曲线、骨架曲线、刚度退化、延性及耗能能力的影响。结果配置普通钢筋可以提高预应力高强混凝土管桩的承载能力和延性,对刚度影响不大;随着轴压比的增加,预应力高强混凝土管桩的承载能力逐渐提高,但延性变差,轴压比为0.45时,预应力高强混凝土管桩发生脆性破坏。结论配置普通钢筋可以有效改善预应力高强混凝土管桩的抗震性能,预应力高强混凝土管桩的轴压比不宜超过0.3。