The aim of this work is to explore the influence of the end resistance and shaft resistance regarding the mechanism for jacked pile penetration and the load-transfer rule during the penetration process. A full-scale field test was conducted in an actual project located in Dongying, Shandong Province, China. In this test, the axial strain experienced by two closed Prestressed High-strength Concrete (PHC) pipe piles during jacking into layered soil was monitored successfully using Fiber Bragg Grating (FBG) sensors mounted on the pile shaft. The experimental results show that FBG sensors have a good stability, strong antijamming performance, and can effectively monitor the pile stress. The variation law of the jacking force reflects the distribution of the soil layer, and the hardness of the soil layer at the pile end limits the pile force. When the pile end enters the silt layer from the clay layer, the jacking force and shaft resistance increase by 2.5 and 1.7, respectively. The shaft resistance accounted for 44.99% of the jacking force. The end resistance is affected by the mechanical properties of soil, and the end resistance of the silt layer is approximately twice that of the clay layer. The end resistance of the silt layer accounted for 59.84% of the jacking force. When the pile end enters the soft soil layer from the hard soil layer, the impact of the pile driving speed and the tangential force on the surface of the pile body must both be considered. During the pile penetration process, as the penetration depth increases, the radial stress on the pile side at a given depth is gradually released, while the shaft resistance at the pile side degrades significantly.
A Prestressed High-strength Concrete (PHC) pipe pile is a hollow cylindrical precast concrete member produced by pretension prestressing, centrifugal techniques, and high temperature curing [
In the recent years, fiber optic sensing technologies have developed rapidly, and various sensors based on the fiber optic sensing technology are widely used in engineering monitoring [
To further improve the accuracy and resolution of the tests, Fiber Bragg Grating (FBG) sensors have been used to monitor the stress and strain of pile foundations. Lee et al. [
To investigate the capacity properties of closed PHC pipe piles during penetration into a layered stratum, a full-scale test was conduct based on an actual project located in Shandong province, China. The low-temperature sensitive, FBG strain sensors, and FBG temperature sensors were installed in the shallow grooves along opposite sides of the PHC pipe piles. Thus, the strain of the PHC pipe piles can be measured to avoid the effects of temperature under hydraulic driving. Thus, the axial force, end resistance, shaft resistance, and unit shaft resistance of the PHC pipe pile under hydraulic driving can be calculated. The research results can provide reference and guidance for jacked pile engineering in a cold region.
The test site is situated in Dongying City, Shandong Province, China, in the quaternary alluvial plain landform of the Yellow River Delta. The surface layer is backfill soil with a thickness of 0.9–5.3 m, and the buried depth of the groundwater level is 0.30–3.00 m. The soil parameters of the test site are as shown in Table
Soil parameter.
Soil type | Soil description | Thickness (m) | Water content, | Unit weight, | Void ratio | Cohesion, | Internal friction angle, | Compression modulus, |
---|---|---|---|---|---|---|---|---|
Fill | Plastic clay | 0.9–5.3 | 30.6 | 18.5 | 0.867 | 13.8 | 6.8 | 4.1 |
Silty soil A1 | Loose dense silty soil | 0.3–2.5 | 28.8 | 18.6 | 0.803 | 8.7 | 20.1 | 8.1 |
Silty clay B1 | Soft plastic clay | 0.3–1.9 | 30.9 | 18.3 | 0.876 | 17.6 | 7.4 | 4.8 |
Silty soil A2 | Medium dense silty soil | 0.3–4.0 | 28.3 | 18.7 | 0.794 | 8.7 | 21 | 8.7 |
Silty clay B2 | Soft plastic to flow plastic clay | 2.6–4.6 | 31.7 | 18.3 | 0.895 | 17.5 | 6.8 | 4.6 |
Silty soil A3 | Medium dense silty soil | 0.8–3.8 | 28.3 | 18.7 | 0.793 | 10.3 | 20.6 | 10.2 |
In the tests, the PHC-A400 (95) prestressed high-strength concrete pipe pile was used for testing. The pile length was 12 m, Young’ modulus was 36 GPa, axial compressive strength was 35.9 MPa, and the tested piles are denoted as P1 and P2. Before installing the sensors, a mark was placed on the pile body, and a slotting machine was used to make a shallow groove at a depth of 2 cm and a width of 4 cm. After slotting, the sensor position was leveled to avoid eccentrically compressing the sensors during pile pressing. The support of the sensor was fixed in the shallow groove using epoxy resin, and the FBG sensor was installed after the epoxy resin hardened. After the sensors were installed, the shallow groove was filled with epoxy resin and made flush with the surface of the pile body. The cable of the FBG sensor was led out from the drilled hole on the top of the pile. The procedures for the installation are shown in Figure
FBG sensor installation process: (a) marking the sensor locations; (b) slotting with a machining tool, (c) installing the sensor, and (d) sealing with epoxy resin.
In the test, six FBG strain sensors and FBG temperature sensors were installed in one side along the pile, and another seven FBG sensors were at the opposite side. The installation positions of the FBG sensors are as shown in Figure
Installation position of the FBG sensors.
The performance parameters of FBG sensors.
Sensor sleeve | Working temperature (°C) | Strain range ( | Gage (mm) | Strain resolution | Centre wavelength |
---|---|---|---|---|---|
Armor sheath | −30∼120 | ±1500 | 60 | 1 | 1510∼1570 |
The test used a 680-ton hydraulic pile driver with a maximum stroke of 1.8 m. It was difficult to control the penetration rate at a fixed value as the penetration depth increased during the process. The pile penetration speed was approximately 1.8–3 m/min. At the initial stage of pile penetration, the lower penetration resistance caused the pile penetration speed to be slightly larger. As the penetration depth increased, the resistance to penetration gradually increased and reduced the pressing speed. Bond et al. [
As shown in Figure
Jacking force versus penetration depth.
Through the wavelength difference of sensors measured, the axial strain
In the test, FBG strain sensors were installed on both sides of the pile, and the axial strain of the pile at level
The axial force
As shown in Figure
Axial force profiles along the piles at different penetration depths: axial forces of (a) P1 and (b) P2.
As shown in Figure
End resistance versus penetration depth.
As shown in Figure
Shaft resistance versus penetration depth.
The unit shaft resistance of the pile can be calculated as
As shown in Figures
Unit shaft resistance along piles at different penetration depths: unit shaft resistances of (a) P1 and (b) P2.
This paper monitored the strain and temperature state of PHC pipe piles during jacking using low-temperature sensitive, FBG, strain sensors, and FBG temperature sensors. The following conclusions are drawn. The low-temperature sensitive, FBG strain sensors, and FBG temperature sensors were installed on the surface of PHC pipe pile using the slot-embedding method. The installation method of the sensors improved their survival rate. The low-temperature sensitive, FBG, and strain sensors used in this study can better satisfy the monitoring requirements of PHC pipe pile penetration characteristics during jacking than traditional approaches. The distribution of the soil layers affects the jacking force and end resistance, and the change laws of the jacking force and end resistance with depth reflect the soil layer distributions. When the pile end penetrated the silty clay layer, the jacking force and end resistance increased by factors of 2.5 and 2, respectively. At a given depth, as the penetration depth of the PHC pipe pile increased, the pile and soil continuously sheared each other and the radial stress on the side of the pile gradually released. This resulted in a decreased unit shaft resistance. The shaft resistance gradually deteriorated, which caused the unit shaft resistance of the piles to gradually decrease and show a significant degradation.
The Microsoft Excel Worksheet data used to support the findings of this study are available from the corresponding author (
The authors declare that they have no conflicts of interest.
This research was supported financially by the National Natural Science Foundation of China (51708316, 51778312, and 51809146) and the Shandong Province Emphasis Research Program, China (2017GSF16107, 2018GSF117010, and 2018GSF117008).