Prebored precast pile with an enlarged base (PPEB pile) is a new type of green and environmental protection pile foundation developed in China in recent years, which has complex bearing characteristics and many influencing factors. Based on the static load tests and key parameters’ tests in deep soft soil in Shanghai, a three-dimensional numerical analysis model was established using ABAQUS finite element software. The transfer law of load among the precast pile, cement soil, and soil around the pile and the action mechanism of the enlarged base were analyzed emphatically, and a sensitivity analysis of the main factors affecting the bearing performance was carried out. The calculation results show that the existence of the enlarged base can greatly improve the compressive bearing capacity, increasing the diameter and height of the enlarged base is beneficial to the bearing capacity, and the influence of the diameter expansion ratio is more effective. With the increase of the proportion of nodular piles, the ultimate bearing capacity increases slightly, but the deformation increases obviously. Under the condition of cement soil of the test piles, the spacing of the neighboring nodules of nodular piles has no obvious effect on the bearing capacity, and the 1 m spacing commonly used in engineering applications can be optimized. The increase of cement soil thickness is beneficial to the improvement of pile bearing capacity, but the efficiency is low. Finally, some improvement measures for the construction technology of the PPEB pile were put forward.
Prebored precast pile with enlarged base (PPEB pile) is a new type of green and environmental protection pile foundation developed in China in recent years [
Typical construction sequence of a PPEB pile. (a) Drilling. (b) Expanding base. (c) Grouting at the enlarged base. (d) Grouting along the pile shaft. (e) Inserting the precast pile (modified from [
Due to the special construction technology and complex material composition and structure, the bearing characteristics of the PPEB pile are complex and the influencing factors are numerous. On the basis of static load test and model test, many researchers [
Through field tests in Shanghai, the authors preliminarily discussed the bearing behavior of PPEB piles in deep soft soil stratum [
The field tests in this paper were carried out in the intersection of Jiang Yang South Road and Hong Wan Road of Shanghai. In general, there are eight soil layers within the upper 70 m of soil below the ground surface. The basic physical and mechanical properties of the soils are shown in Table
Summary of soil properties.
Soil layer | Depth (m) | |||||||
---|---|---|---|---|---|---|---|---|
① Fill | 0.0–3.0 | 25.0 | 17.0 | — | 25.0 | 20 | — | — |
② Silty clay with silt | 3.0–7.0 | 31.0 | 18.5 | 5 | 32.0 | 52 | 10.0 | 40 |
③ Soft organic clay | 7.0–19.5 | 50.4 | 16.7 | 4 | 26.7 | 31 | 3.3 | 28 |
④ Silty clay | 19.5–39.5 | 32.9 | 18.2 | 3 | 30.7 | 68 | 5.0 | 56 |
⑤ Silty clay with silt | 39.5–46.5 | 23.1 | 19.7 | 10 | 29.6 | 120 | 10.5 | 99 |
⑥ Silty clay | 46.5–51.0 | 31.0 | 18.4 | 7 | 28.0 | 113 | 5.5 | 89 |
⑦ Silt mixed silty clay | 51.0–59.5 | 28.2 | 18.7 | 2 | 32.5 | 178 | 9.0 | 89 |
⑧ Silty sand | 59.5–70.0 | 25.9 | 19.0 | — | 35.0 | — | 12.8 | — |
Three test piles, designated piles TP1, TP2, and TP3, were installed at the site. The length and shaft diameter of the test piles were 55 m and 750 mm, respectively. The inner precast concrete piles were assembled by a 40 m long, 600 mm diameter PHC pile with 110 mm wall thickness in the upper part and a 15 m long, 650 mm (500 mm) diameter nodular pile (i.e., 500 mm pile shaft diameter, which increases to 650 mm at the nodules) with 100 mm wall thickness in the lower part. The enlarged bases were 2750 mm long with a diameter of 1200 mm. To ensure the material strength of the precast piles, the test piles adopted C100 high-performance concrete with 100 mm cube compressive strength of 120.9 MPa and elastic modulus of 54.7 GPa.
Static load tests were carried out 43–45 days after pile installation. The maintained load method was adopted in accordance with the Chinese code JGJ106-2014 [
Obvious inflections can be observed on the load-displacement curves of TP1 and TP2. The displacements were small during the initial stages of loading and increased dramatically when reaching the ultimate bearing capacity of 8800 kN. As the pile head of TP3 was inclined during the test, the load test on TP3 was terminated at the applied load of 8000 kN. The load-displacement curve of TP3 had no obvious inflection. The displacements and applied loads of pile heads at several critical stages are summarized in Table
Summary of pile test results.
No. | Maximum load (kN) | Maximum displacement (mm) | Ultimate load (kN) | Ultimate displacement (mm) |
---|---|---|---|---|
TP1 | 10000 | 73.5 | 8800 | 36.7 |
TP2 | 9600 | 81.9 | 8800 | 35.7 |
TP3 | 8000 | 24.0 | >8000 | — |
Three-dimensional finite element analysis (FEA) method incorporated in the software ABAQUS was used to model the load of TP1. As the vertical loading of a single pile was axisymmetric, half of the model was taken for analysis to simplify the calculation. To minimize the influence of the FEA model boundary, the radius of soil around the pile was 20 m, a width more than 20 times the pile diameter in the plane, and the depth was 80 m, as shown in Figure
3D finite element mesh. (a) Soil. (b) Precast pile. (c) Cement soil around the precast pile.
The initial ground stress of the soil was considered in the model, while the displacement control method was used to apply the vertical load. The parameters of the soil were selected according to Table
Cement soil is an important part of PPEB pile. The characteristics and parameters of cement soil, precast pile-cement soil interface, and cement soil-soil interface are essential to obtain the correct numerical analysis results. The above parameters related to cement soil would be obtained through comprehensive analysis of field tests and laboratory tests.
Consistent with the requirements of the pile hole in the field test, the cement soil mixing pile K1 without precast pile was constructed. It could be considered that the cement soil property of K1 was the same as that of the test piles. Therefore, the core samples were obtained from the K1 for testing. After 48 days of curing, the unconfined compressive strength (
Due to the difficulty in drilling construction, the cement soil at the enlarged base could not be sampled and tested. Therefore, according to the composition of the actual enlarged base, the laboratory proportioning tests of cement soil were carried out. The typical silty clay and silty sand in Shanghai were selected as the soil materials, and P·O 42.5 Portland cement was selected as cementitious material. In the actual construction, the amount of cement grout at the pile toe was 100% of the enlarged base volume, and then the drill pipe was stirred up and down evenly for 1 to 5 times. After the completion of grouting, the volume ratio (cement grout/slurry) was about 1.0 to 1.5. In the laboratory test, the specific gravity of the slurry was 1.5, while the water-cement ratio of cement grout was 0.6. The volume ratio (cement grout/slurry) was set to 1, 1.5, and 2 to prepare the cement soil slurry of enlarged base. The rough samples were made by the hanging bag method (see Figure
Sampling by the hanging bag method.
After 28 days of curing in the standard curing room, the unconfined compressive strength test and shear test were carried out with the loading speed of 1 mm/min. The laboratory test results are shown in Table
Test results of cement soil at the enlarged base.
Soil | Water-cement ratio | Cement grout : slurry (volume ratio) | ||||
---|---|---|---|---|---|---|
Silty clay | 0.6 | 1 : 1 | 6.22 | 862 | — | — |
0.6 | 1.5 : 1 | 9.12 | 1149 | 857 | 43.9 | |
0.6 | 2 : 1 | 12.70 | 1745 | — | — | |
Silty sand | 0.6 | 1 : 1 | 11.60 | 901 | — | — |
0.6 | 1.5 : 1 | 13.46 | 1496 | 1121 | 45.7 | |
0.6 | 2 : 1 | 17.02 | 2206 | — | — |
Parameters of the precast pile and cement soil.
Pile material | Constitutive model | ||||
---|---|---|---|---|---|
Precast pile | — | — | 60000 | 0.15 | Linear elastic |
Cement soil around the precast pile | 400 | 40 | 150 | 0.30 | Mohr–Coulomb |
Cement soil at the enlarged base | 1000 | 45 | 1500 | 0.25 | Mohr–Coulomb |
In this FEA model, three interface elements were defined: precast pile-cement soil interface, precast pile-soil interface, and cement soil-soil interface. The contact behavior of all three interfaces was defined using an isotropic Coulomb friction model, which allowed for slippage and separation between the master and slave surfaces. The feasibility and accuracy of using the Coulomb friction model to simulate the pile-soil interaction had been well verified by many researchers [
To understand the characteristics of concrete-cement soil interface more clearly, the model tests were carried out using a cylindrical interface model device, as shown in Figure
Schematic diagram of concrete-cement soil interface test.
The friction coefficient (
Part of the cement soil slurry infiltrates into the soil around the PPEB pile, so the friction performance of the cement soil-soil interface is generally better than that of the concrete-soil interface. Combined with the results of the back analysis of the field test, the friction coefficient of the cement soil-soil interface was set to 0.53.
The load-displacement curve and axial force distribution of the test pile calculated by ABAQUS program are shown in Figure
Comparison between FEA and measured values. (a)
The development of the plastic zone of the PPEB pile-soil system during loading is shown in Figure
The development process of the yield zone around the pile and the enlarged base.
Before the initial loading stage
The comparison between the total axial force of PPEB pile and the axial force of the precast pile under representative loading conditions is shown in Figure
Axial force distribution of the nodular pile section.
The stress of the cement soil was much less than that of the precast pile, so that the load was first transferred from the precast pile to the cement soil and then to the soil around the pile. The vertical stress of cement soil in the upper pipe pile increased gradually along the depth. The vertical stress distribution of the lower nodular pile section was more complex. In the process of loading, each nodule was squeezed with its lower cement soil and had a tension trend with the upper cement soil. Therefore, within the range of each nodule, the vertical stress of the cement soil under the nodule was the largest, while the vertical stress of the cement soil above the next nodule was the smallest.
The vertical stress of the cement soil of the enlarged base was also clearly different above and below each nodule, as shown in Figure
Vertical stress change of cement soil at the enlarged base during loading.
The cement soil of the enlarged base was also subjected to a higher shear stress. Figure
Shear stress distribution of cement soil at the enlarged base and surrounding soil (
Horizontally, the structure of the PPEB pile is nonuniform, and longitudinally the size of the pile shaft is different (with enlarged base), as shown in Figure
Schematic diagram of the pile structure of the PPEB pile.
The setting of the enlarged base is an important feature that distinguishes the PPEB piles from other bored precast piles and concrete-cored deep cement mixing piles. The structure of the enlarged base has a great influence on the bearing performance. In the modeling calculation, the height of the enlarged base (
The calculation results are shown in Figure
Influence of the height of the enlarged base on bearing capacity. (a)
The diameter expansion ratio (
The numerical results under different conditions of diameter expansion ratio and pile end bearing layer are shown in Figure
Influence of the expansion ratio of the enlarged base and bearing layer. (a) End bearing layer, ⑦ silt mixed silty clay. (b) End bearing layer, ⑧ silty sand.
PHC nodular pile is generally used in the lower part of PPEB pile, and the nodules can effectively enhance the bonding force between the precast pile and cement soil. Especially at the end of the pile shaft, the PHC nodular pile and the enlarged base form a whole and bear the load together. In the numerical model, the length of the nodular pile segment was set to 0, 4, 15, 28, 40, and 55 m from the pile toe, accounting for 0%, 7%, 27%, 50%, 73%, and 100% of the total pile length, respectively.
With the proportion of nodular piles increased from 7% to 100%, the pile head displacement increased significantly while the ultimate bearing capacity increased only by about 3.5%, as shown in Figure
Influence of the proportion of the nodular pile on bearing capacity.
The spacing of the neighboring nodules (
Influence of the spacing of the neighboring nodules on bearing capacity.
The thickness of cement soil in PPEB pile varies with the diameter of the borehole. In the FEA model, the 650 mm (500 mm) diameter nodular pile was used in the lower part. When the diameter of cement soil borehole (
Under the condition that the size of the enlarged base was constant, the bearing capacity increased nonlinearly with the increase of the thickness of cement soil, as shown in Figure
Influence of the thickness of cement soil (diameter of the borehole) on bearing capacity. (a) Comparison of bearing capacity. (b) Efficiency analysis.
Based on the static load test and key parameters test in deep soft soil in Shanghai, a three-dimensional FEA model was established using ABAQUS. Then, the bearing characteristics of the PPEB pile were analyzed. The main conclusions are as follows: The Mohr–Coulomb elastic-plastic model was adopted for the cement soil around the pile shaft and at the enlarged base, and the elastic modulus was 150 MPa and 1500 MPa, respectively. The friction coefficients of precast pile-cement soil interface and cement soil-soil interface were 0.80 and 0.53, respectively. The numerical results well simulated the bearing and deformation behavior of the field test pile. At the initial stage of loading, the load had been transferred to the pile toe, and with the increase of the applied load, the shaft resistance was gradually mobilized from top to bottom. In the nonenlarged base part, the axial force of the PPEB pile was mainly controlled by the precast pile. In the enlarged base part, the load was transferred to the cement soil of the enlarged base through nodules and then to the soil around the enlarged base. The cement soil of the enlarged base was subject to higher vertical compressive stress and shear stress. The existence of an enlarged base could greatly improve the compressive bearing capacity of the PPEB pile. Increasing the diameter and height of the enlarged base was beneficial to the bearing capacity, and the effect of diameter expansion ratio was more effective. In practical engineering, it is reliable to control the enlarged base diameter at 1.6 times the borehole diameter and adopt the enlarged base height of 3 times the borehole diameter. With the increase of the proportion of nodular piles, the ultimate bearing capacity increased slightly, and the deformation increased obviously. Setting up 1 ∼2 section nodular piles in the lower part of the PPEB pile is a more reasonable measure for projects with higher deformation requirements. When the property of cement soil around the precast pile is good and the proportion of nodular piles is relatively small, the 1 m spacing commonly used in projects can be optimized. The bearing capacity increased nonlinearly with the increase of the thickness of cement soil. For deep soft soil areas, the efficiency of improving bearing capacity by increasing the thickness of cement soil is low.
The data used to support this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This study was supported by the Program of Shanghai Academic/Technology Research Leader (no. 18XD1422600), the China Postdoctoral Science Foundation (no. 2021M690784), and the Science and Technology Plan of Guangzhou Municipal Construction Group (no. 2020-KJ013).