PILE JACKING IN SAND AND SILT Andrew Jackson (
[email protected]) Cambridge University Engineering Department 13 February 2008
Abstract Pile jacking (also known as ‘press-in piling’) is an innovative technique for the installation of preformed piles, in which hydraulic rams are used to push the piles into the ground. Machines have been developed to allow this technique to be used quickly and efficiently. Advantages over traditional techniques such as dynamic pile driving include greatly reduced noise and vibration during installation. The use of pile jacking presents new challenges for the geotechnical engineer. Designers of jacked piles for any loading must predict the short-term axial force that will resist pile installation, so that a piling machine with a sufficiently high capacity to overcome this resistance can be selected. If unexpectedly high resistance is encountered, the piler may be unable to advance the piles to the required embedment. In many cases this installation resistance may be different from the long-term ultimate capacity which the pile can mobilise to resist applied axial loads. This paper describes a field investigation into the force required to jack piles into sand and silt. A series of field tests involving installing a 15m-long closed-ended tubular steel pile at a site in Japan is reported. During each test the total installation resistance was recorded and sensors in the pile measured the forces on the base and shaft. These resistances were found to be consistently less than standard predictions of the medium-term ultimate axial capacity from CPT (cone penetration test) data. This difference appears to be caused by rate-dependent partially-drained installation behaviour. Excess pore water pressure generated during installation reduces the installation resistance, making it lower than the drained medium-term capacity. The significance of this behaviour for designers of jacked piles is discussed, and techniques for predicting it are investigated.
The Jacked Piling Method The expansion of urban construction onto increasingly marginal sites is making the use of piled foundations an ever more attractive option for designers. One common form of piling, which can offer a higher ultimate capacity and stiffness than bored piling, is driven piling. However, pile driving produces levels of noise and ground-borne vibration which are unacceptable for many urban sites. A relatively new piling technique which offers the benefits of driven piling with additional advantages including reduced noise and vibration is jacked (or ‘press-in’) piling. In this method, a static jacking force is applied to a preformed pile to push it into the ground. The ‘Silent Piler’ type of pile jacking machine (Figure 1 overleaf) is capable of ‘self-walking’ along an advancing wall of piles, and gains reaction from previously-installed piles. The machine can be controlled by a single operator, with fresh piles being delivered by a single crane. Although developed in Japan, the technology has been used throughout the world (1), including on the Grand Arcade project in Cambridge (2).
Jackson, A. Pile Jacking in Sand and Silt. 2007. Previous research (3) has identified many advantages of jacked piling over traditional techniques, including:
very low levels of noise and vibration; enhanced capacity due to the low number of stress cycles experienced by the soil around the pile during installation; enhanced stiffness due to the full pre-loading experienced by the soil around the pile during installation; the potential for extraction and reuse of temporary piles, bringing cost savings and environmental benefits; and the ability to use the measured installation resistance as an indicator of the pile capacity.
Figure 1: A Silent Piler
However, the use of pile jacking presents new challenges for the geotechnical engineer. Designers of axially-loaded piles of any type require a prediction of the long-term ultimate axial strength that can be mobilised to support the applied load. Designers of jacked piles must also predict the shortterm axial force that will resist pile installation, so that a piling machine with a sufficiently high capacity to overcome this resistance can be selected. Much previous research has considered the ultimate axial strength of piles, but this is often not the primary factor controlling pile design; serviceability stiffness criteria or the strength required to resist other loading conditions may be more critical. Less research has considered the installation resistance, but this dictates piler selection irrespective of any stiffness criteria or later loading conditions. In many cases, the short-term axial force resisting pile installation is different from the longer-term pile capacity. If unexpectedly high installation resistance is encountered, the piler may be unable to advance the piles to the required embedment. As discussed later, the resistance and strength also vary with time after installation, in processes known as ‘set-up’ (increasing resistance) or ‘set-down’ (reducing resistance). If set-up occurs after initial installation it may be impossible to advance the piles after a delay or to extract temporary piles after use (Figure 2).
a) Pile Installation
b) Increased Pile Capacity Topples Piler During Extraction Figure 2: Problems with Jacked Piling
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Jackson, A. Pile Jacking in Sand and Silt. 2007.
Existing Methods and Theory The UWA-05 method (4) uses the results of cone penetration tests (CPTs) to predict pile capacity. Simplified forms of the UWA-05 expressions to relate the total base and shaft resistances 𝑄𝑏 and 𝑄𝑠 for a closed-ended tubular pile (Figure 3) to the modified CPT Area resistance 𝑞𝑡 are: Ab Ground Equation 1 𝑐
tan 𝛿 𝑑ℎ
Equation 2 Embedded length L
𝑄𝑏 = 𝑞𝑏 = 𝛼 𝑞𝑡 𝐴𝑏 𝑄𝑠 1 ℎ=𝐿 ℎ = 𝜏𝑠 = 𝑎 𝑞𝑡 max ,2 𝐿𝑃 𝐿 ℎ=0 𝐷
Shaft Perimeter resistance
P
Height h above tip
𝛿 is the angle of friction between the soil and the pile shaft. 𝛼, 𝑎 and 𝑐 are empirical parameters which require calibration. The UWA-05 method provides values of the parameters to predict the medium-term capacity of driven tubular piles in sand, calibrated against a large database of load tests. This is currently the only case for which the method is validated. However, the equations and parameters can be attributed to physical phenomena occurring during installation and loading, so it is expected that other installation, soil and loading conditions can be considered by appropriate variation of these parameters (5).
Qs
Diameter
D
Base resistance
Qb
Figure 3: Definition of Terms
One phenomenon which may cause the parameters to vary in soil conditions other than sand is partial drainage. In low-permeability soils, installation of a pile induces excess pore water pressure around the pile tip. The magnitude of the induced pressure is dependent on the installation velocity, and it affects the base and shaft resistance. For example, positive excess pore water pressures generated by partially drained compression of the soil under the pile base will reduce the effective stress in the soil around the pile, and thus reduce the shear stress which can be mobilised. Figure 4 shows how the pile installation resistance (point A) may therefore be different from the long-term drained capacity (point B), and may depend on the installation velocity 𝑣, the soil property 𝑐ℎ (the coefficient of horizontal consolidation), and the pile diameter 𝐷. Whilst other phenomena such as viscous behaviour may cause the installation resistance to differ from the medium-term resistance, partial drainage is expected to be the primary cause of such behaviour in the soils considered in this project. 1 Normalised resistance: qb / qb,drained qt / qt,drained τs / τs,drained 0
C
B
Drained load test after dissipation Pile installation resistance Pile mediumB term capacity CPT installation C resistance
A Set-up A
Partially drained installation
Normalised penetration velocity: vD/ch
Figure 4: Variation of Installation Resistance with Installation Velocity (after (6))
Partial drainage also provides a mechanism for the variation of capacity with time discussed above. After installation the induced pore water pressures dissipate away, reducing their affect on the pile capacity. This is represented by the transition from point A to point B in Figure 4. Again, other phenomena such as creep, aging and corrosion can cause the pile capacity to change with time, but 3/8
Jackson, A. Pile Jacking in Sand and Silt. 2007. partial drainage is expected to be the primary cause of such behaviour in the soils and relatively short timescales considered in this project. Finally, the cone penetration test itself may also be partially drained, affecting its resistance 𝑞𝑡 (point C on Figure 4). In general, a jacked pile designer therefore needs to be able to predict point A, point B and the behaviour in between, using information such as that provided by point C. A modified form of the UWA-05 method, adapted to account for the partially-drained behaviour shown in Figure 4, is suggested. This paper concentrates on predicting the partially-drained installation resistance (point A) from partially-drained CPT data (point C).
Test Methodology Field tests were conducted at the Takasu test site in Kochi, Japan during July and December 2006. 15m-long closed-ended cylindrical piles with an external diameter of 318.5mm were used. During each test, a pile was installed and extracted using an AT150 piler specially modified for these tests by Giken Seisakusho (Figure 5). The axial ground resistance was calculated using measurements from calibrated pressure cells in the hydraulic circuit driving the hydraulic cylinders of the piler, with a correction applied to account for the measured pile weight and hydraulic losses. Some of the tests used an instrumented pile in which a load cell in the pile base plate measured the base resistance separately from the shaft resistance, and pore pressure transducers measured the pore water pressures near the pile base.
Pressure cells in hydraulic circuit measure force exerted by piler Data to datalogger
Each test took place at an undisturbed part of the site. Figure 6 shows the results of ground investigations conducted at the site. All the data suggests a transition at approximately 10.5m below ground; above this level the soil is primarily sand, and below it is primarily silt. 0m BG
Wall of pre-installed sheet piles provides reaction to test pile loads
1.6 2.3 3.0
Made ground Silt Sandy gravel Silty sand
0 qt [MPa] 10
0 u2 [MPa] 0.6
5.2 Sand 9.4 10.5
Piler chuck grips pile Coiled wire extensometer measures penetration of pile Figure 5: Test Arrangement
Silty sand
Silt
15.8
16m Sandy BG gravel Figure 6: Ground Investigation Data
Further details of the test methodology and site conditions are given in (7) and (8). 4/8
Jackson, A. Pile Jacking in Sand and Silt. 2007.
Results Figure 7 shows the results of the field tests on the instrumented pile. These suggest the following observations:
The measured base resistance is very consistent between tests, suggesting that the site is laterally homogeneous. However, there is a much greater variation in the shaft resistance. Other research (7) has shown that the shaft resistance is sensitive to small discrepancies in the installation process. For example, the variation may be caused by horizontal movements of the piler chuck imposed by the piler operator during installation to correct any minor misalignment of the pile (8). Both the base and shaft resistance show a significant difference between the results in the sand and silt.
Tip Depth L [m]
0
0
50
100
150
200
Qb [kN] 250 300
Qs [kN] 0
0
2
2
4
4
6
6
8
8
10
10
12
12
14
14
100
200
300
400
500
Sand Silt
a) Base Resistance b) Shaft Resistance Figure 7: Results of Tests on Instrumented Pile
In the following section, the influence of soil condition is considered in more detail for the base and shaft resistance.
Tip Depth [m]
Figure 8 shows the results of tests on piles installed at different velocities. As suggested by Figure 4, faster installations (in which the induced pore water pressures are expected to be higher) appear to mobilise a lower resistance. The long-term capacity after the excess pressures have dissipated is therefore expected to be higher than the installation resistance (set-up). Other tests not documented here did indeed show set-up occurring at the site (7). Pore pressure transducers on the instrumented pile also showed positive Qb+Qs [kN] excess pressures; as described previously, 0 200 400 600 this is consistent with a reduced resistance 0 Average installation resistance during installation. 2 4 6 8
of eight monotonic tests installed at v=20mm/s.
Average installation resistance of eleven monotonic tests installed at v=30mm/s.
10 12
Figure 8: Results of Tests at Different Installation Velocities
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Jackson, A. Pile Jacking in Sand and Silt. 2007.
Analysis The suitability of the modified UWA-05 method for predicting the installation resistance can be assessed by back-analysing the test results.
White & Bolton
UWA-05
Sand average
Silt average
Tip Depth L [m]
Firstly, consider the base resistance. Figure 9 shows α=Qb/Aqt 0 0.2 0.4 0.6 0.8 1 1.2 a plot of a rearrangement of Equation 1 using the 0 measured installation base resistance. A value of 2 the parameter 𝑐 = −0.2 is assumed. Ignoring the data for very shallow installation, where the ground 4 is expected to be variable, the results are reasonably consistent with an assumption of a single value of 6 the parameter 𝛼 for the sand stratum above 8 10.5mBG and another for the silt stratum below 10.5mBG. In both strata, the ratio of pile base to Sand 10 CPT resistance 𝛼 is consistently less than predictions Silt of the medium-term base resistance in sand from 12 the UWA-05 method and by other researchers (9). 14 This agrees with the general behaviour shown in Figure 9: Back-Analysis of the Base Resistance Figure 4; partial drainage reduces the pile resistance in comparison with the CPT resistance. The difference in the ratios between the sand and the silt is caused by the different relative positions of their CPT and pile installations on Figure 4.
UWA-05
Back-analysis and predictions of the shaft resistance are more complicated than that of the base resistance, because the total shaft resistance is influenced by more than one stratum of soil. Each stratum has a different strength and drainage behaviour, and has had a different time for the dissipation of excess pore water pressures since their generation as the pile tip passed. However, an initial simplifying assumption is that the base and shaft resistances contributed by a given stratum of soil are influenced to the same extent by the local excess pore water pressure. This is logical, since it is the high stresses generated around the pile tip which cause both the base resistance and, to a large degree, the shaft resistance. Based on this approach, Equations 1 and 2 can then be combined to give Equation 3, which analyses the shaft resistance a tan δ/α in terms of the measured base resistance in an 0 0.01 0.02 0.03 0.04 0 attempt to eliminate the effects of differences between pile and CPT installation: 2
Tip Depth L [m]
4 6 8 Sand
10
Silt 12
c=-0.2 14 Figure 10: Back-Analysis of Shaft Resistance
𝑎 tan 𝛿 = 𝛼 𝑃 𝐴𝑏
𝑄𝑠 ℎ=𝐿 ℎ=0
ℎ 𝑄𝑏 max 𝐷 , 2
𝑐
𝑑ℎ
Equation 3
Figure 10 shows values of the parameter 𝑎 tan 𝛿 𝛼 derived from Equation 3 using the measured installation resistances. Scatter in the shaft resistance makes accurate analysis difficult. The ratio is close to that for medium-term resistances derived from the UWA-05 method, because the effect of partial drainage has been eliminated, leaving only the soil strength parameters. The small variation with depth is primarily due to a variation in soil strength rather than 6/8
Jackson, A. Pile Jacking in Sand and Silt. 2007. in drainage condition. The assumption that the shaft resistance is affected by partial drainage to the same degree as the base resistance seems to give reasonable results.
Conclusions and Recommendations Partially drained behaviour does appear to affect the installation resistance of jacked piles in sand and silt. Evidence for this is provided by observed differences between predicted medium-term capacities and measured installation resistances and by an observed influence of installation velocity on installation resistance. Whilst geotechnicians are used to considering the influence of partial drainage on silts, the influence on sands is less common and is caused by the relatively high speed of the installation process. In the soils tested, partial drainage during installation causes the installation resistance to be less than standard predictions of the medium-term pile capacity assuming drained sand conditions. A framework for understanding and predicting jacked pile installation resistance, based on the UWA-05 method, has been presented. A partial drainage curve, which is expected to look like Figure 4 but which must be measured for all the soils involved, is used to predict a modified value of 𝛼 at each tip depth. This allows each base resistance to be predicted from the corresponding CPT resistance using Equation 1. The shaft resistance is then predicted from an integration of this value along the pile shaft using Equation 3. As with the UWA-05 method, empirical calibration factors are required. Further research is needed to establish these values, but it is believed that much of the variation in the parameters can be obtained from site-specific ground investigations, leaving only CPT-to-pile conversion factors similar to those detailed in the UWA-05 method to be specified elsewhere. For example, measurement of the partial drainage behaviour is not currently common practice during site investigations, but it is readily achievable using modifications of CPT installations to include variable-rate or twitch tests (6). An understanding of the partially drained installation also helps predict the longer term behaviour. It should be recognised that the reduction of the installation resistance to less than longer-term values is dependent on the installation rate used; installing more slowly, or including a delay in installation during which excess pore water pressures can dissipate, will increase the installation resistance. In low permeability soils even the CPT resistance may have been reduced by partial drainage during its installation, so the medium-term pile resistance may be higher than standard predictions from the CPT data assuming drained conditions. Other research not described here (7) has shown that very significant increases in capacity can occur within a few hours of installation. Partial drainage curves could be used to predict the significance of this behaviour for a new site. To summarise the findings directly relevant for pile designers and piler operators:
Pile designers should be aware that the resistance to installing a pile is likely to be different from their predictions of the medium-term axial capacity. In soils undergoing set-up such predictions are likely to be a conservative (high) prediction of the installation resistance for specifying pilers, but they may be unconservative in some soils. Conversely, pile designers and operators should be aware that the practice of using the installation resistance as an indicator of the medium-term capacity may often be oversimplistic; in the soils tested it would be conservative (low), but in others it may not. Pile designers should be aware that further site investigation can be used to better understand the installation behaviour at sites where it is likely to be critical.
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Jackson, A. Pile Jacking in Sand and Silt. 2007.
Piler operators should be aware of the possibility for set-up during delays to cause refusal or trapped piles, and partial drainage curves could be used to predict the significance of this behaviour for a new site. Piler operators should be aware that, somewhat counter-intuitively, faster installation causes a lower installation resistance.
Acknowledgments The author would like to thank the following people for their contribution to the research: Dr D.J. White, Prof M.D. Bolton, Mr A.D. Deeks, Mr M. Gillard, Miss H. Dingle, Mr T. Nagayama, and all the staff and engineers at Giken who provided assistance, advice and support. This research was funded by Giken Seisakusho Ltd. as part of a long-term research collaboration with Cambridge University Engineering Department, which forms part of the activities of the International Press-in Association (www.press-in.org).
References 1. [Online] http://www.giken.com. 2. [Online] http://www.grandarcade.co.uk/Scheme/Construction/Vibration.htm. 3. White, D.J. and Deeks, A.D. Recent research into the behaviour of jacked foundation piles. Yokosuka . Proceedings of the International Workshop on Recent Advances of Deep Foundations. 2007. pp. 3-26. 4. Lehane, B.M., Schneider, J.A. and Xu, X. The UWA-05 method for prediction of axial capacity of driven piles in sand. Perth. Proceedings of the International Symposium on Frontiers in Offshore Geotechnics. A A Balkema, 2005. pp. 683-689. 5. White, D.J. A general framework for shaft resistance on displacement piles in sand. London : Proceedings of the International Symposium on Frontiers in Offshore Geotechnics (ISFOG). Taylor & Francis, 2005. pp. 697-703. 6. Silva, M.F. Numerical and physical models of rate effect in soil penetration. Cambridge University Engineering Department. 2005. PhD Thesis. 7. Jackson, A. The Setup of Jacked Piles. Cambridge University Engineering Department. 2007. MEng Thesis. 8. Dingle, H. The testing and analysis of jacked foundation piles. Cambridge University Engineering Department. 2006. MEng Thesis. 9. White, D.J. and Bolton, M.D. Comparing CPT and base resistance in sand. Proceedings of the Institution of Civil Engineers: Geotechnical Engineering, Vol. 158(GEI). 2005. pp. 3-14.
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