Pile-net Composite Foundation Research Articles

Three-Dimensional Modeling on Load-Transferring Mechanism of Rigid Pile–Net Composite Foundation

Three-Dimensional Modeling on Load-Transferring Mechanism of Rigid Pile–Net Composite Foundation

Analysis of Load Transfer Law of Short Pile-Net Composite Foundation for High-Speed Railway

Although pile-net composite foundation has been widely used in the construction of high-speed railway, load transfer characteristics of its structural system during the filling of roadbed are seldom studied, so field studies are conducted in the Gan-Long (Ganzhou-Longyan) high-speed railway DK86 + 998.0–DK87 + 191.2 test section, aiming to explore the bearing characteristics of a short pile-net composite foundation over low to medium compressible soil. The study encompasses a field measurement of Earth pressure and pore water pressure which lasted almost two years, and a systematic analysis of variation law of soil pressure between piles and their tops and that of pore pressure in reinforcement zone and retaining layer in medium and low compressibility soil area. The results suggest that the soil in the test section bears the characteristic of a low water content, low porosity ratio, and high liquid limit, with compression factor being approximately 0.25, and test values of the natural soil pressure and the lateral distribution are close to the theoretical values. Soil pressure at the top of the piles is higher than that between the piles by approximately 2.4 times because of the differences in pile-net rigidity. Besides, the soil pressure of the pile-net composite foundation follows the pattern of “jagged” in the transverse direction of the roadbed. The filling load of the composite foundation soil arch is 80 kPa and the composite foundation reaction coefficient is 6.4 kN/m3 when the filling is stable. The pile-soil stress ratio at the shoulder and the center of the line is 3.2 and 2.8, respectively. The change of the hole pressure at the pile end point is larger than that at the reinforcement area, and the side verification short pile can effectively transmit the additional stress of the upper part to the pile end soil layer.

Experimental research on deformation of pile-net composite foundation with long-short piles under cyclic load

Experimental research on deformation of pile-net composite foundation with long-short piles under cyclic load

A simplified prediction method for additional stress on underlying layer of rigid pile-net composite foundation

To propose a simplified semi-empirical method for predicting the additional stress on the underlying layer of the rigid pile-net composite foundation, a finite element analysis, which verified by a series of field tests and model tests, are carried out. Focusing on the equivalent entity method recommended by Technical code for composite foundation, the equivalent frictional resistance coefficient, which is a key factor affecting the additional stress, is deduced through the static balance theory and β-method. Finite element analyses, including 432 tests, are conducted to analyse the effect of pile length, pile spacing, thickness of cushion layer, embankment height and geological conditions on the key coefficient. Then, a simplified semi-empirical model for predicting the key coefficient as well as the additional stress is proposed based on numerical results. The accuracy of the proposed model is verified by the good agreement between the experimental results and numerical results.

Engineering characteristics test on pile–net composite foundation in loess areas

To study the variation in pile–soil stress and settlement of a pile–net composite foundation in a collapsible loess area, a test section of the Gansu-Ningxia section of the Yinxi high-speed railway was analysed using an earth pressure box, a liquid level settlement system and a single-point settlement instrument. The monitoring results showed that for, the pile–net composite foundation, the earth pressure in the embankment was significantly affected by the geogrid and the soil arching effect. The geogrid enhanced the soil arching effect between the piles, resulting in an increase in earth pressure between the piles on the embankment. With an increase in the embankment filling height, the pile–soil stress ratio and load sharing ratio of the cement–fly ash–gravel (CFG) pile and cement–soil compaction (CSC) pile increased gradually and the pile–soil stress ratio of the CFG pile was greater than that of the CSC pile. The settlement patterns of the CFG pile top, CSC pile top and the soil between the piles was saddle-shaped. The settlement of the CSC pile was found to be much less than that of the CFG pile.

Study on characteristics of pile–net composite foundation at 500 kV substation in China

The overall requirements of substation engineering are very high, and the differential settlement control of substation engineering is relatively strict. On the soft foundation of the tidal flats at the Zhenhai 500 kV substation in China, a field test was carried out to study the working characteristics of the pile–net composite foundation during the soil filling and consolidation stage. Variation laws of the settlement, earth pressure, pile–soil stress ratio, geogrid tension and axial force of pile were obtained and analysed. When the height of the filled soil increased to 2 m, a plane of the equal settlement was formed. Because of the soil arch effect, the earth pressure on the pile top was always higher than that between the adjacent piles. The pile–soil stress ratio rapidly increased from 1.2 at the stage of soil filling to 6.0 at the initial stage of consolidation. The ability of the geogrid to transfer the load was limited, and the soil arch effect was the main reason that the load was transferred to the pile top. Due to the lateral negative friction resistance of the pile in the shallow soil, the axial force of the pile first increased and then decreased along the depth.

Theoretical Solution for Consolidation Settlement of Pile-Net Composite Foundation Under Embankment Load

With the development of the coastal cities in China, pile-net composite foundation, which contains the components of embankment fills, reinforced cushion, geogrid and piles, has been increasingly used for highways, railways, etc. because of its effectiveness in controlling the post-construction settlement with a relatively low cost. However, the pile-net composite foundation is so complicated that few studies provide the theoretical investigation on the composite foundation. In this study, a theoretical solution was developed by fully considering the soil arching effect, membrane effect and the interactions between component parts. The finite differential method was utilized to obtain the consolidated performances of pile-net composite foundation. According to the results, it is found that (a) the stress concentration ratio and axial force of the pile increases with consolidation of composite foundation; (b) the neutral point of the pile shifts down with the consolidation process; (c) membrane effect of the geogrid also has a obvious influence on the stress concentration ratio and (d) ignoring membrane effect will underestimate the pile stress ratio. Furthermore, the theoretical solution is used to compare with the finite element modeling to illustrate its feasibility.

The Dynamic Analysis of Soft Soil Subgrade in High-Speed Railway Strengthened by PHC Pile

The combination of PHC pile (prestressed concrete pile) with geogrid to reinforce the foundation has been widely used in high-speed railway, and the response of dynamic stress under the high-speed train is worth studying. By using Midas/GTS NX software to establish a 3-D model of pile-net composite foundation in one section of Hangzhou - Shenzhen line, the influence of pile-net composite foundation generated by high-speed train is studied. Moreover, this paper also studies the distribution and propagation of dynamic stress generated by train load in soft soil area. The results show that the dynamic stress reducing quickly in the embankment structure and the time –history curve of the dynamic stress behaves differently along the depth. With the depth increasing, there is phenomenon of peak superposition in the time-history curve of the vertical dynamic stress. In the meanwhile, the PHC pile is the main structure to propagate and sustain the dynamic stress in soft soil area, and with the tendency of ‘large in the middle, small on both ends’.

Research on Stress Distribution of CFG Pile-Net Composite Foundation in High-Speed Railway

Combined with the field tests of CFG (Cement Fly-ash Gravel) pile-net (geogrid) composite foundation in Danyang Test Section of the Beijing-Shanghai High-speed Railway, the settlement, pile-soil stress ratio and additional stress of composite foundation were analyzed by numerical simulation. It shows that the pile-soil stress ratio of CFG pile-net composite foundation is in the range of 4 to 6. And the reasonable distribution of additional stress in composite foundation is also discussed. The results can provide references for the design of the CFG pile-net composite foundation under embankment in high-speed railway.

An Analysis on Failure Pattern of CFG Pile-Net Composite Foundation of High-Speed Railway

CFG pile-net composite foundation has the advantages of small post construction settlement and uneven settlement, easy control of construction quality and fast construction speed. It`s widely used in subgrade engineering of high-speed railway. However, in coastal soft soil area, phenomena like breaking pile, collapsed embankment, cracking in embankment surface, undue settlement or convergence appeared in many places where railway CFG pile was improved during the foundation treatment, filling and preloading. Four numerical calculations were performed to investigate the failure pattern of CFG pile-net composite foundation with FLAC3D ( Fast Lagrangian Analysis of Continua, 3 dimensions) software. The paper describes four failure pattern, they are slip circle shear failure, pile tilt failure, roadbed sliding, soil sliding at pile end respectively through the analysis of the embankment and pile displacement as well as embankment deformation.

Research on the <i>In Situ</i> Test of Thick-Layer Soft Ground of High-Speed Railway Treated by PHC Pile

Based on the data obtained in the in-situ test of layered settlements in Wen-Fu Railway, the settlement regulation of the thick-layer soft soil treated by the composite structure of PHC pile-net is analyzed, and two calculation models are utilized to predict the final settlement. Since the predicted settlement is far less than the value required by the standard, the treatment method for the deep soft soils is proved to be reasonable. After that, according to the mechanism of the composite structure of PHC pile-net, the settlement values are calculated using various common calculation methods and the results are compared to the corresponding experimental data. The applicability of these methods is discussed and then the settlement calculation method of the thick-layer soft foundation treated by the composite structure of PHC Pile-net is obtained. The series of conclusions finally given provide the basis for the theoretical research and optimum design of pile-net composite foundation.

Model Test Research on Bearing Behavior of Pile-Net Composite Foundation in Super Large Area and Deep Soft Soil

Based on the treatment of in-situ foundation, two distinct model tests for pile-net composite foundation in super large area and deep soft soil with different pile spacing were designed to study the bearing behavior of the piles. The result shows: the axial force of pile shaft increases with the growth of embankment filling load and gradually rises down along pile shaft to a certain depth, then slowly decreases afterwards. The velocity of force changing varies from the correlated soil layers, which indicates that pile shaft resistance appears to be unlike in different pile sites; The ratio of pile shaft resistance area to the overall pile length is relatively high while of normal resistance area is smaller ,and when the upper load is mainly supported by bearing course on pile end, pile-net composite foundation turns to be friction and end-bearing pile; For each level of loads, both pile-soil stress ratio and load shearing ratio of pile increase modestly, then tend to be steady till the next load level is applying on.

Comparative Analysis on Neutral Point Distribution in Rigid Pile Composite Foundation on Collapsible Loess in High Speed Railway

To research and analyze the neutral point distribution of rigid pile composite foundation on collapsible loess ground in high speed railway, the centrifuge model tests are used to imitate pile raft composite foundation and pile net composite foundation in different pile spacing based on the experimental study at home and abroad. The research results show that the movement of negative skin friction and the position of neutral point which is drop conspicuously with larger pile spacing are a continuous changing process in composite foundation. In the pile spacing 2D,4D,6D of pile raft composite foundation, the stable range of neutral point position are 0.25(the pile length ratio), 0.59,0.63 separately in post-construction stage. In the pile spacing 2D(no pile-cap),4D(no pile-cap),4D(pile-cap),6D(pile-cap) of pile net composite foundation, the stable range of neutral point position are 0.37to0.43(the pile length ratio),0.50,0.80,0.81separately in post-construction stage. The pile-cap setting has more influence on Neutral point distribution. Suggestions are given that the waterproof and drainage measures in foundation must be considered when setting up composite foundation on collapsible loess.

Numerical Simulation Research on the Settlement Regularity in Pile-Net Composite Foundation

Based on the foundation treatment for deep soft soil in Chaoshan station district, this paper studied the engineering characteristics of soft soil in Chaoshan and the settlement regularity of pile-net composite foundation, etc. by numerical simulation. The results show: the reduction of pile spacing and the settings of pile cap can decrease the settlement of soil among the piles on foundation plane.