Field Test of Excess Pore Water Pressure at Pile–Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor
<p>Silicon piezoresistive pressure sensors.</p> "> Figure 2
<p>Wheatstone bridge circuit.</p> "> Figure 3
<p>Photos of micro-silicon piezoresistive pressure sensors.</p> "> Figure 4
<p>Soil layer distribution.</p> "> Figure 5
<p>Schematic diagram of pore water pressure sensor along pile of S1 test pile (unit: mm).</p> "> Figure 6
<p>Pore water pressure sensor: (<b>a</b>) Hole opener, (<b>b</b>) transmission line, (<b>c</b>) epoxy coating, (<b>d</b>) sensor seal.</p> "> Figure 7
<p>Total pore water pressure of pile–soil interface during pile-jacking (the data came from the sensor test results with h/B = 1 from the pile end): (<b>a</b>) Test pile S1 jacking process, (<b>b</b>) test pile S2 jacking process.</p> "> Figure 8
<p>Curves of excess pore water pressure of pile–soil interface during pile jacking (the data came from the sensor test results with h/B = 1 from the pile end): (<b>a</b>) Test pile S1 pile jacking process, (<b>b</b>) test pile S2 pile jacking process.</p> "> Figure 9
<p>Variation curves of excess pore water pressure of pile–soil interface with depths during pile-jacking: (<b>a</b>) h/B = 1, (<b>b</b>) h/B = 3, (<b>c</b>) h/B = 7, (<b>d</b>) h/B = 15, (<b>e</b>) h/B = 27.</p> ">
Abstract
:1. Introduction
2. Silicon Piezoresistive Sensor
3. Engineering Geological Survey of the Test Site
4. Test Pile Conditions and Measuring Point Arrangement
4.1. Conditions of Test Pile
4.2. Arrangement of Measuring Points
5. Test Results and Analysis
5.1. Development Law of Pore Water Pressure at Pile–Soil Interface during Pile Jacking
- (1)
- At the initial stage of pile jacking, the excess pore water pressure at the pile–soil interface increased sharply, indicating that the soil was extruded and expanded when shearing occurred at the pile–soil interface, which resulted in the increase in excess pore water pressure at the pile–soil interface with the burial depth. Through field observation, Tang et al. [32,33] also found the same change law.
- (2)
- During pile jacking, the excess pore water pressure at the pile–soil interface increased to the peak and then began to decline, but the excess pore water pressure was still large after the decline. At the end of the S1 pile jacking, the excess pore water pressure from the pile end position h/B = 1 decreased from 201.4 to 86.3 kPa. After the end of pile jacking in the upper section of test pile S2, the excess pore water pressure from the pile end position h/B = 1 decreased from the maximum 324.6 to 39.2 kPa. It can be seen that the excess pore water pressure dissipates rapidly in the interval of pile jacking, and, after the end of pile jacking in the under section of test pile S2, the pressure drops from 374.1 to 114.3 kPa. The pore water pressure sensors measured at different positions h/B from the pile end have the same variation law as above.
- (3)
- After the completion of pile jacking, there is a hydraulic gradient between the pile–soil interface and the soil around the pile, and the pore water pressure in the soil around the pile will transfer to the pile–soil interface under the action of seepage. Therefore, the pore water pressure at the pile–soil interface will be stable within 24 h in the early stage of consolidation and will rise before the disappearance of the hydraulic gradient. After the hydraulic gradient disappeared completely, the pore water pressure began to dissipate, and the pore water pressure curve showed a downward trend.
5.2. Dissipation Law of Pore Water Pressure at Pile–Soil Interface during Pile Jacking
- (1)
- Unlike the dissipation law of pore water pressure in the soil around the pile, the dissipation law of excess pore water pressure at the pile–soil interface after the pile jacking end first goes up and then down. The reason is that the pressure difference between the pile–soil interface and the soil around the pile occurs after the construction, that is, a hydraulic gradient exists between the pile–soil interface and the soil around the pile, and the pore water pressure in the soil around the pile will transfer to the pile–soil interface under the action of seepage. Therefore, the excess pore water pressure at the pile–soil interface rises first at the initial stage of consolidation, and the pressure difference between the pile–soil interface and the soil surrounding the pile does not exist until the hydraulic gradient disappears.
- (2)
- The excess pore water pressure of the test pile S1 and S2 pile–soil interface measured within 24 h after becoming basically stable was 41 and about 65 kPa, respectively, and at this time, the excess pore water pressure dissipation degree had reached 75%–85%, and the dispersal rate is obviously more than the soil consolidation theory calculation speed, namely on the field test of the pile–soil interface excess pore water pressure dissipation rate and soil consolidation theory calculation speed. The permeability coefficient of the viscous soil layer is about 1.0 × 10–7 cm/s and the coefficient of consolidation Cv is about 6.0 × 10–3 cm2/s. The field-measured pore water pressure dissipation rate is obviously faster than the calculation rate based on the consolidation theory around the pile. Some scholars have made corresponding studies [34]. It is generally believed that the hydraulic splitting effect occurs in the soil around the pile due to the excessive pore water pressure generated in the process of continuous pile jacking, resulting in a large number of fractures. As a result, the consolidation speed of soil around the pile was accelerated at the beginning of the end of pile jacking. With time, cracks were gradually bridged, the permeability of soil around the pile was reduced, and the consolidation speed gradually slowed down. Different pore water pressure sensors from the pile to the pile end h/B were tested, which had the same variation law as above [35].
5.3. Change Law of Excess Pore Water Pressure at the Pile–Soil Interface with the Burial Depth
6. Conclusions
- (1)
- At the initial stage of pile jacking, the excess pore water pressure at the pile–soil interface increased sharply, and the excess pore water pressure increased with the increase in burial depth. At the end of pile jacking, the excess pore water pressure at the pile–soil interface dissipated rapidly, and the excess pore water pressure of test pile S1 decreased from 201.4 to 86.3 kPa, while the excess pore water pressure of test pile S2 decreased from 324.6 to 39.2 kPa.
- (2)
- After the end of pile jacking, the dissipation law of excess pore water pressure at the pile–soil interface is that it rises first and then falls. Due to the existence of a hydraulic gradient, the pore water pressure in the soil around the pile will transfer to the pile–soil interface under the action of seepage. The excess pore water pressure at the pile–soil interface will first rise at the initial stage of consolidation and then dissipate when the hydraulic gradient disappears at the pile–soil interface. With the extension of time, the fracture of the pile–soil interface under the action of hydraulic fracturing increases the dissipation rate of the pile–soil interface.
- (3)
- The excess pore water pressure at the pile–soil interface has an influence on the shear strength change at the pile–soil interface during the process of pile jacking, and the shear strength change at the pile–soil interface during the process of pile jacking can reflect the change trend of excess pore water pressure. Research on the generation and dissipation of excess pore water pressure at the pile–soil interface in the process of pile jacking plays an important role in the pile’s mechanical characteristics and bearing capacity and has important engineering application value.
- (4)
- The dissipation trend of pore water pressure at different positions of the pile body is consistent. After a period of steady state, the pore water pressure dissipates gradually with the increase in time. With the increase in the sensor position, the pore water pressure decreases and the dissipation rate of pore water decreases.
Author Contributions
Funding
Conflicts of Interest
References
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Soil Type | Soil Thickness (m) | Severe r/(kN/m2) | Moisture Content w(%) | Void Ratio e | Liquid Limit wL(%) | Plastic Limit wP(%) | Cohesion C(kPa) | Internal Friction Angle φ(°) | Compresson Modulus Es(MPa) | Characteristic Value of Bearing Capacity fak(kPa) |
---|---|---|---|---|---|---|---|---|---|---|
Plain fill | 2.44 | |||||||||
Silt | 1.45 | 18.6 | 27.7 | 0.792 | 28.5 | 19.6 | 10.0 | 18.9 | 6.0 | 95 |
Silty clay | 0.54 | 18.4 | 31.1 | 0.881 | 32.5 | 20.0 | 16.9 | 7.3 | 3.0 | 80 |
Silty clay | 0.77 | 18.4 | 30.5 | 0.873 | 32.1 | 19.7 | 18.5 | 10.0 | 3.0 | 80 |
Silt | 2.39 | 18.8 | 28.0 | 0.796 | 28.9 | 20.0 | 9.5 | 19.9 | 6.5 | 100 |
Silty clay | 3.66 | 18.3 | 31.5 | 0.892 | 32.3 | 19.8 | 18.8 | 7.2 | 3.0 | 80 |
Silt | 2.39 | 18.8 | 28.0 | 0.793 | 28.9 | 19.9 | 10.0 | 20.3 | 7.0 | 120 |
Silty clay | 0.8 | 18.5 | 30.8 | 0.875 | 32.6 | 19.8 | 17.9 | 9.9 | 3.5 | 85 |
Silty clay | 6.37 | 18.5 | 30.8 | 0.862 | 32.6 | 19.9 | 18.6 | 10.7 | 4.0 | 90 |
Silt | 1.49 | 18.8 | 27.7 | 0.789 | 28.6 | 19.7 | 10.7 | 20.3 | 7.0 | 120 |
Silt | 1.54 | 19.0 | 27.0 | 0.754 | 28.7 | 20.2 | 11.7 | 22.0 | 7.5 | 160 |
Silty sand | 17.68 | 19.3 | 24.0 | 0.692 | 5.0 | 34.0 | 10.3 | 220 |
Sensor Position | h/B = 1 | h/B = 3 | h/B = 7 | h/B = 15 | h/B = 27 | |
---|---|---|---|---|---|---|
Time | ||||||
12 h | 105 | 96 | 88 | 45 | 28 | |
24 h | 98 | 86 | 70 | 38 | 19 | |
36 h | 92 | 75 | 65 | 33 | 15 | |
48 h | 85 | 70 | 61 | 28 | 12 |
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Wang, Y.; Liu, X.; Zhang, M.; Yang, S.; Sang, S. Field Test of Excess Pore Water Pressure at Pile–Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor. Sensors 2020, 20, 2829. https://doi.org/10.3390/s20102829
Wang Y, Liu X, Zhang M, Yang S, Sang S. Field Test of Excess Pore Water Pressure at Pile–Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor. Sensors. 2020; 20(10):2829. https://doi.org/10.3390/s20102829
Chicago/Turabian StyleWang, Yonghong, Xueying Liu, Mingyi Zhang, Suchun Yang, and Songkui Sang. 2020. "Field Test of Excess Pore Water Pressure at Pile–Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor" Sensors 20, no. 10: 2829. https://doi.org/10.3390/s20102829