Effects of cement-enhanced soil on the ultimate lateral resistance of composite pile in clayey soil

The composite pile consisting of core-pile and surrounding cement-enhanced soil is a promising pile foundation in recent years.However,how and to what extent the cement-enhanced soil influences the ultimate lateral resistance has not been fully investigated.In this paper,the ultimate lateral resistance of the composite pile was studied by finite element limit analysis(FELA)and theoretical upper-bound analysis.The results of FELA and theoretical analysis revealed three failure modes of laterally loaded composite piles.The effects of the enhanced soil thickness,strength,and pile-enhanced soil interface characteristics on the ultimate lateral resistance were studied.The results show that increasing the enhanced soil thickness leads to a significant improvement on ultimate lateral resistance factor(N P),and there is a critical thickness beyond which the thickness no longer affects the N P.Increasing the enhanced soil strength induced 6.2%-232.6%increase of N P.However,no noticeable impact was detected when the enhanced soil strength was eight times higher than that of the natural soil.The maximum increment of N P is only 30.5%caused by the increase of interface adhesion factor(a).An empirical model was developed to calculate the N P of the composite pile,and the results show excellent agreement with the analytical results.

3D finite elements analysis of vertically loaded composite piled raft

In recent years,a new type of foundation named composite piled raft foundation (also called long short composite piled raft) has been developed.Where designing shallow foundations would mean unacceptable settlement,or other environmental risks exist which could impair the structure in the future,composite piled raft foundations could be used.Finite element method was applied to study the behavior of this type of foundation subjected to vertical loading.In order to determine an optimal pile arrangement pattern which yields the minimum settlement,various pile arrangements under different vertical stress levels were investigated.Results show that with increasing the vertical stress on the raft,the effectiveness of the arrangements of short and long piles become more visible.In addition,a new factor named "composite piled raft efficiency" (CPRE) has been defined which determines the efficiency of long short piles arrangement in a composite piled raft foundation.This factor will increase when short piles take more axial stresses and long piles take less axial stresses.In addition,it is found that the changes in settlements for different long short piles arrangement are in a well agreement with changes in values of CPRE ratio.Thus,CPRE ratio can be used as a factor to determine the efficiency of piles arrangements in composite piled raft foundation from the view point of reducing raft settlements.

Consolidation analysis of composite foundation with partially penetrated cement fly-ash gravel(CFG) piles under changing permeable boundary conditions

Based on the double-layered foundation theory, the composite ground with partially penetrated cement fly-ash gravel(CFG) piles was regarded as a double-layered foundation including the surface reinforced area and the underlying untreated stratum. Due to the changing permeability property of CFG piles, the whole consolidation process of the composite ground with CFG piles was divided into two stages, i.e., the early stage(permeable CFG pile bodies) and the later stage(impermeable pile bodies). Then, the consolidation equation of the composite foundation with CFG piles was established by using the Terzaghi one-dimensional consolidation theory. Consequently, the unified formula to calculate the excess pore water pressure was derived with the specific solutions for the consolidation degree of composite ground, reinforced area and underlying stratum under instant load obtained respectively. Finally, combined with a numerical example, influencing rules by main factors(including the replacement rate m, the treatment depth h1, the permeability coefficient Ks1, Kv2 and compression modulus Es1, Es2 of reinforced area and underlying stratum) on the consolidation property of composite ground with CFG piles were discussed in detail. The result shows that the consolidation velocity of underlying stratum is slower than that of the reinforced area. However, the consolidation velocity of underlying stratum is slow at first then fast as a result of the transferring of effective stress to the underlying stratum during the dissipating process of excess pore water pressure.

A case study on behaviors of composite soil nailed wall with bored piles in a deep excavation

A complete case of a deep excavation was explored. According to the practical working conditions, a 3D non-linear finite element procedure is used to simulate a deep excavation supported by the composite soil nailed wall with bored piles in soft soil. The modified cam clay model is employed as the constitutive relationship of the soil in the numerical simulation. Results from the numerical analysis are fitted well with the field data, which indicate that the research approach used is reliable. Based on the field data and numerical results of the deep excavation supported by four different patterns of the composite soil nailed wall, the significant corner effect is founded in the 3D deep excavation. If bored piles or soil anchors are considered in the composite soil nailed wall, they are beneficial to decreasing deformations and internal forces of bored piles, cement mixing piles, soil anchors, soil nailings and soil around the deep excavation. Besides, the effects due to bored piles are more significant than those deduced from soil anchors. All mentioned above prove that the composite soil nailed wall with bored piles is feasible in the deep excavation.

Pile-soil stress ratio in bidirectionally reinforced composite ground by considering soil arching effect

To discuss the soil arching effect on the load transferring model and sharing ratios by the piles and inter-pile subsoil in the bidirectionally reinforced composite ground, the forming mechanism, mechanical behavior and its effect factors were discussed in detail. Then, the unified strength theory was introduced to set up the elastoplastic equilibrium differential equation of the subsoil under the limit equilibrium state. And from the equation, the solutions were derived with the corresponding formulas presented to calculate the earth pressure over and beneath the horizontal reinforced cushion or pillow, the stress of inter-pile subsoil and the pile-soil stress ratio. Based on the obtained solutions and measured data from an engineering project, the influence rules by the soil property parameters (i.e., the cohesion c and internal friction angle φ) and pile spacing on the pile-soil stress ratio n were discussed respectively. The results show that to improve the load sharing ratio by the piles, the more effective means for filling materials with a larger value of φ is to increase the ratio of pile cap size to spacing, while to reduce the pile spacing properly and increase the value of cohesion c is advisable for those filling materials with a smaller value of φ.

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.

Experimental Study of Dynamic Characteristics on Composite Foundation with CFG Long Pile and Rammed Cement-Soil Short Pile

Based on the idea of optimization design of pile type, the two kinds of the typical pile type are selected, which containing flexibility pile (e.g. rammed cement-soil pile is for short RCSP), and rigid pile (e.g. cement-flyash-gravel pile is for short CFGP). The three kinds of the composite foundation are designed, which are CFGP, CFG long pile and CFG short pile (for short CFGLP-CFGSP), CFG long-short pile and rammed cement-soil short pile (for short CFGLP-RCSSP). Natural earthquake is simulated by using the engineering blasting;the dynamic characteristics and dynamic response of the composite foundation are studied through field test. CFGLP-RCSSP is closed to linear relation. The bearing capacity of the four composite foundation of the CFGP, CFGLP-CFGSP, and CFGLP-RCSSP in the site are 225 kPa, 179 kPa, and 197 kPa, separately increases 150%, 98.8% and 119% compared to the natural foundation. The vibration main frequency is mainly depended on properties of foundation soil and piles between vibration source and measuring point, pilling load value. Horizontal vibration main frequency greater than the vertical vibration main frequency and the vertical vibration main frequency close to the first-order natural frequency of composite foundation. With the pilling load increasing, the CFGLP-RCSSP pile composite foundation combined frequency decreased. Under the same blast energy, the acceleration peak on the CFG pile composite foundation is less than CFGLP-CFGSP the corresponding values, as the load increases, the peak acceleration gently. CFG pile composite foundation is favorable on seismic. The distribution of peak acceleration is consistent within 4 m from pile top in the CFGLP_RCSSP composite foundation. The maximum of the horizontal acceleration peak along the pile body occurs at a distance of pile top 4 m or the pile top, and that of vertical acceleration peak occurred at a pile top.

Study of the Bearing Capacity at the Variable Cross-Section of A Riser- Surface Casing Composite Pile

Reducing the cost of offshore platform construction is an urgent issue for marginal oilfield development.The offshore oil well structure includes a riser and a surface casing.The riser,surface casing and oil well cement can be considered special variable cross-section piles.Replacing or partially replacing the steel pipe pile foundation with a variable cross-section pile to provide the required bearing capacity for an offshore oil platform can reduce the cost of foundation construction and improve the economic efficiency of production.In this paper,the finite element analysis method is used to investigate the variable cross-section bearing mode of composite piles composed of a riser and a surface casing in saturated clay under a vertical load.The calculation formula of the bearing capacity at the variable section is derived based on the theory of spherical cavity expansion,the influencing factors of the bearing capacity coefficient N_(c) are revealed,and the calculation method of N_(c) is proposed.By comparing the calculation results with the results of the centrifuge test,the accuracy and applicability of the calculation method are verified.The results show that the riser composite pile has a rigid core in the soil under the variable cross-section,which increases the bearing capacity at the variable cross-section.

The Dynamic Characteristic Experimental Method on the Composite Foundation with Rigid-Flexible Compound Piles

Based on the idea of optimization design of pile type, the composite foundations, which include cememt-flyash-gaavel (for short CFG) long piles and cement-soil (for short CS) short piles, and CS piles with CFG core as well, are formed. The method of the site dynamic characteristic tests of the composite foundations is discussed. The test results show that fireworks bomb may replace demolitions as the vibration resource. Vibration time is about 0.1 sec. Horizontal vibration major frequency is at 22.476 - 56.436 Hz, and vertical vibration major frequency is at 15.538 - 55.884 Hz. The pile arrangements of the composite foundation in the same site have more effect on the acceleration peak value. From the point of vibration, the anti-seismic effect of the CS piles with CFG core is better than others.

Numerical Analysis of the Settlement of Composite Foundation Reinforced with Crushed Stone Grouting Pile and Rigid Bearing Plate

This paper deals with a new type of crushed stone grouting pile with a rigid bearing plate. The load transfer characteristics were analyzed, and a settlement model of the composite foundation reinforced with crushed stone grouting pile and rigid bearing plate was built by FEM program. The effects of replacement ratio of capping plate, replacement ratio of pile, replacement ratio of grout diffusion zone, pile-soil modulus ratio, and serous-soil modulus ratio, on the composite foundation settlement were discussed. It is concluded that the proposed crushed stone grouting pile with a rigid bearing plate is effective in decreasing the settlement of composite foundation.

Calculation method of composite foundation sedimentation of grouting pile with cover plate under embankment load

Grouting pile is a new soft soil foundation treatment method with characteristics such as no vibration, no noise, no soil compaction, light construction machines and quick construction velocity and so on. At present, study on reinforcement mechanism and design calculation method of composite foundation of grouting pile is initially started without design specifications, so it is usually required to draw on design specifications of stump pile when designing composite foundation of grouting pile while grouting pile has its characteristics and difference although reinforcement mechanisms and construction processes of two types of piles are similar. Sedimentation formula of composite foundation of grouting pile with cover plate is educed and a suitable deformation mode is proposed by aiming to deformation characteristics of composite foundation of grouting pile with cover plate under embankment load on basis of relevant sedimentation theories of composite foundation by combination of characteristics of composite foundation of grouting pile. The sedimentation calculation formula of grouting pile with cover plate under embankment load is educed according to balance relation of force and displacement coordination conditions by elastic theory and sedimentation calculation model established is validated by sedimentation monitoring documents of one expressway in China.

Settlement of composite foundation with discrete material pile considering modulus change

Based on deeply discussing the deformation mechanism of composite foundation with discrete material pile, firstly, the settlement of composite foundation in rigid foundation conditions was assumed to consist of two parts, an expanding part and an un-expanding part. Then, in view of the differences of deformation properties between the expanding part and the un-expanding part, the relationships between the pile modulus and the applied load in these two parts were respectively developed. Thirdly, by introducing the above relationships into settlement analysis, a new method to calculate displacement of composite foundation with discrete material pile was proposed by using the multi-stage loading theory and the layer-wise summation approach. This method is effective not only for accounting for the effect of variations of pores on deformation modulus of the pile body in different depths, but also for describing the characteristics of different deformation mechanisms of the pile body with varying depth. Finally, the proposed method was used to a practical composite foundation problem, whose theoretical results were presented and compared to those of other methods. The rationality and feasibility of this method are identified through comparative analysis.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Stress Ratio-Strain Relation of Pile and Soil in Composite Foundation

A series of triaxial compression tests were arried out by means of composite-reinforced soil samples to simulate the interaction between soil and pile. The samples are made of gravel or lime-soil with different length at the center. The experiment indicates that the strength of the composite samples can not be obtained by superimposure of reinforcing pile and soil simply according to their replacement proportion. It also indicates the law for stress ratio of reinforcing column to soil. The stress ratio of reinforcing column to soil increases and reaches peak rapidly while load and strain is small. Then the ratio decreases. This law is in accordance with the measuring resuits in construction site.

Experiments of Multi-element Composite Foundation with Steel Pipe Pile and Gravel Pile

A set of serf-developed apparatus for foundation physical model were utilized to conduct model tests of the multi-element composite foundation with a steel pipe pile and several gravel piles. Some load-bearing characteristics of the multi-element Composite foundation, including the curves of foundation settlement, stresses of piles, pile-soil stress ratio, and load-sharing ratio of piles and soil, were obtained to study its working performances in silty sand soil. The experimental results revealed that the multi-element composite foundation with steel pipe pile and gravel pile contributed more than the gravel pile composite foundation in improving the bearing capacity of the silty fine sand.

Verification of Seismic Performance of Pile Foundation in Composite Ground through Experimental and Numerical Methods

A new construction method of pile foundation in composite ground, in which, prior to installing piles, the ground is improved around the heads of the piles in soft ground or ground subject to liquefaction, which is introduced in this paper. This construction method uses a combination of pile foundation construction together with common ground improvement methods, including deep mixing, preloading and sand compaction piling, and it is referred to as the composite ground pile method. Since an artificial ground with relatively high rigidity comparing with that of the original ground was formed around the pile in this method, and the seismic performance has not been made clear, thus the seismic performance of piles in composite ground was systematically analyzed through a series of centrifuge model tests and numerical analyses by using dynamic nonlinear finite element method, and a verification method for the seismic performance of piles in composite ground was proposed on the basis of the experimental and numerical results.

Parameter analysis of composite pile foundation in bridge foundation

In the current theory of bridge foundation design,all of the loads above the cap are loaded by the pile,and the bearing capacity of the soil among piles is not taken into account.In order to analyze the bearing capacity of the soil among piles in bridge pile foundation,a model of pile foundation is established based on a bridge foundation which is under construction,and by the finite element analysis software ANSYS.According to the results of finite element analysis(FEA)and current bridge foundation design theory,a feasible composite pile foundation which can be applied in the design of bridge foundation,is recommended.Additionally,a number of modifications are made to the original design.It was confirmed that these modifications derived from numerical simulations can improve the performance of the foundation.

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.

Settlement Characteristics of Cement Fly-Ash GravelPile-Plate Composite Foundation of High-Speed Railway

Field tests on settlement characteristics were carried out on the cement fly-ash gravel （CFG） pile-plate composite foundation on Beijing-Xuzhou section of Beijing-Shanghai high-speed railway. The settlements of the piles and the soil between pries were measured and analyzed. The results show that the settlement-time dependency experienced three phases： rapid development phase, stable development phase and stable phase. Therefore, surcharge preloading was necessary to reduce the settlement after construction. The finite element software Plaxis was used to calculate the deformations of the pile top and the soil between piles at the embankment center, as well as the settlements of CFG pile reinforcement area and the underlying stratum under surcharge preloading. The calculation results and the field test results were compared and analyzed. Both the results show that the settlement of the composite foundation mainly occured in underlying stratum. The settlement characteristics of pile-plate composite foundation under high embankment are also concluded.

Mechanical Properties of Three-Dimensional Fabric Sandwich Composites

Three-dimensional( 3 D) fabric composite is a newly developed sandwich structure,consisting of two identical parallel fabric decks woven integrally and mechanically together by means of vertical woven fabrics. In this paper,six types of 3 D fabric sandwich composites were developed in terms of compressive and flexural properties as a function of pile height( 10, 20 and30 mm) and pile distance( 16, 24 and 32 mm) in pile structures. The mechanical characteristics and the damage modes of the 3 D fabric sandwich composites under compressive and flexural load conditions were investigated. Besides,the influence of pile height and pile distance on the 3 D fabric sandwich composites mechanical properties was analyzed. The results showed that the compressive properties decreased with the increase of the pile height and the pile distance. Flexural properties increased with the increase of pile height, while decreased with the increase of pile distance.