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High and low strain dynamic pile testing - Independent Geoscience Pty Ltd

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High and low strain dynamic pile testing - Independent Geoscience Pty Ltd

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  1. High Strain and Low Strain Dynamic Pile Testing at the New Blanchetown Bridge, SA Jonathan Cannon Director, Independent Geoscience Pty Ltd, Australia This paper describes the use of dynamic pile testing during the construction of a new bridge over the Murray River at Blanchetown, SA. The bridge carries the Sturt Highway, which is the main road link between Adelaide and Sydney. The bridge foundations comprise groups of driven precast prestressed concrete piles and dynamic pile testing was used to establish driving criteria and to confirm pile resistance. At Pier 1, on the Adelaide side of the river, the first piles did not stop at the expected founding level and dynamic testing was used to investigate their behaviour. The testing indicated all piles were broken. A program of high strain and low strain dynamic testing was then used to assess the piles and to investigate alternative methods for successfully driving piles at the required locations. The paper describes the testing methods and an interpretation of the cause of breakages and the remedial methods adopted. 1 INTRODUCTION “Osterberg Cells,” where the jack is placed near the toe of the pile and a “quasi-dynamic” method. Dynamic testing has been shown to be by far the quickest and most cost effective of these methods and is the most frequently used method for verification of pile foundations. The dynamic method relies on the transmission of stress “waves” along an elastic “rod.” When an impact is applied to the top of a long slender elastic “rod,” ie a pile, the whole rod does not move as a single rigid object but rather the top section is compressed. This section then compresses the adjacent section, which in turn compresses the next adjacent section etc etc and a compression wave travels along the rod. When the “wave” reaches the toe a reflection is generated, which is controlled by the resistance and damping of the ground at the toe. The behaviour has been described mathematically by the wave equation: A new $15 million bridge has been constructed over the Murray River at Blanchetown, SA. It carries the Sturt Highway, which is the main road link between Adelaide and Sydney, and consequently it carries a high proportion of heavily loaded truck traffic. The client was the State highway authority, Transport SA, and the Main Contractor on the project was York Civil Construction Pty Ltd. Geotechnical advisor to Transport SA was Rust-PPK Pty Ltd. There is an existing bridge roughly parallel to and at a similar level to the new bridge but defects have been found in the structure and it was not designed for current highway loadings. A decision was made to construct a completely new bridge. The new bridge comprises 7 main spans of 50m and 2 shorter end spans. Foundations for all but the west abutment comprise groups of driven precast prestressed concrete piles, which are octagonal and 457mm across. Lengths varied from 12 to 19m overall and they were prestressed to about 10MPa. The pile toes were pointed with a steel pin about 75mm diameter and about 100mm in length at the tip of the point. The main contractor drove the piles with a recently re-conditioned Kobe K35 diesel hammer fitted to a fixed leader assembly. For work over water the crane and leader were supported on a segmental floating barge, which was kept in position using “spud” piles together with winches attached to fixtures on the riverbank. The foundation conditions across most of the site were similar and comprised the valley of the Murray River. There were variable depths of soft recent sediments and fill over the local soft rock, which is described in the boreholes as Marl and appears to be a weakly cemented calcareous sand or silty sand. On the west side of the river between the Adelaide abutment and Pier 1, there was a reasonably steep cliff, in which interbedded layers of weakly cemented and strongly cemented layers could be seen in the Marl. ( 2 u / t 2 ) E ( 2 u / x 2 ) (1) where u is the displacement of a point at time t and at a location x in an elastic rod, and is the density and E is Young’s Modulus for the material of the rod. Changes in pile section, or resistance along the pile return reflections to the top of the pile. As noted above, well-proven equipment and software is available to conduct testing and analysis using dynamic methods. High strain testing equipment requires the pile to be struck with a full sized piling hammer or a drop weight with sufficient energy to “set” the pile. This strains the ground and thus mobilises resistance from the ground around the pile. Pile force and velocity are measured near the top of the pile. As both force and velocity are measured the downward, or input wave can be separated from the upward, or reflected wave. As resistance is mobilised in the ground surrounding the pile a capacity can be demonstrated. If the pile can be struck with sufficient energy then plastic movement of the ground resistance can be reached and the ultimate capacity of the pile can be determined. If less energy is applied and a small “set” is achieved then the method can demonstrate a certain resistance but it will probably be less than the ultimate capacity. Typically plastic movement of skin friction is achieved at a set of about 3mm/bl. End bearing requires much greater displacement and general geotechnical theory suggests the toe displacement needs to reach 10% or more of the pile diameter to reach plunging failure. Consequently it is 2 DYNAMIC PILE TESTING Dynamic pile testing has been in commercial use for over 25 years and has been in use in Australia since 1978. Both high strain and low strain methods are recognised in the Australian Piling Code as methods of assessing pile foundations for resistance (high strain only) and integrity. Other methods presently available within Australia for assessing pile foundations include conventional static testing, a new static method using 1

  2. unlikely that dynamic testing would be able to demonstrate the ultimate resistance of a pile with a high proportion of end bearing. However, in most cases more than enough resistance can be demonstrated and most structures placed on piles will not tolerate very high deflections and so plunging failure of an end- bearing pile is of esoteric interest only. A photo of a current model PAK Pile Driving Analyzer is shown in Figure 1 below. Readers should see reference 1 for a description of the theories used by the method and reference 2 for a description of the “State of the Art” in high strain dynamic pile testing. determination of a pile top stiffness. However, it should be noted that this is only applicable at very low strains and may not be indicative of the pile behaviour at high strains (ie at “real” deflections) and as noted previously the method cannot determine the ultimate resistance of the pile. 2.1 Damage Detection Theory 2.1.1 High Strain The Author previously described this theory in reference 3. Consider a pile with a reduction in area or modulus at some point. Let Z represent the “impedance” EA/c with Z1 being the impedance of the top section and Z2 being the impedance of the bottom section. When a downward traveling stress wave Fi arrives at this point part of the wave is reflected upward Fu and part is transmitted across the section change and continues downward Fd such that both continuity and equilibrium are satisfied. It has been shown in reference 4 that: Fd Fi (2Z 2 /( Z 2 Z1 )) Fu Fi (( Z 2 Z1 ) /( Z 2 Z1 )) (2) (3) Thus for a uniform pile where Z2=Z1, Fu=0 and Fd =Fi and the wave passes unchanged. At a complete break in the pile where Z2=0, Fd=0 and Fu=-FI and the wave is completely reflected but an initial downward traveling compression wave is reflected upwards as a tension wave. Likewise, any decrease in area or modulus will cause an upward tension reflection and any increase in area or modulus will cause an upward compression reflection. If we now introduce an integrity factor, being the proportion of section change, Z 2 / Z1 (4) then it can be shown that Figure 1 Pile Driving Analyser Low strain dynamic testing requires the top of the pile to be struck only with a hand held hammer and a small wave of high acceleration but very low strain is generated. Pile top velocity is monitored by a very sensitive accelerometer attached to the top of the pile. The impact wave and the reflection are measured and displayed on a screen for interpretation by a trained and experienced operator. As the strains are very small the technique cannot mobilise and demonstrate any significant resistance. Fu Fi (1 ) /(1 ) (5) (6) And if we define Fu / Fi (7) then (1 ) /(1 ) and there can be some quantification of the change in impedance from top measurements. An arbitrary classification of “damage” has been used in reference 5 as follows: “Severity” 1.0 Undamaged 0.8-1.0 Slight damage 0.6-0.8 Damage <0.6 Broken If there are cracks in a pile or “slacks” at mechanical pile joints high strain testing allows the width to be quantified in an approximate manner. The quantification is based on the displacement required to close the gap so that the downward wave can continue. This is the integration of the relative velocity change caused by Figure 2 Pile Integrity Tester Some models, such as the PIT Collector from Pile Dynamics Inc., as shown in Figure 2, include an instrumented hammer to measure the impact force. The impact force can be compared with the measured velocity to provide a better indication of possible defects at the top of the pile. It also allows for the the reflector.

  3. ( v ) dt next day and all 3 were considered by the Author to be severely damaged about 4m below ground level. In only one case was it possible to detect any response from the pile toe. Following consideration of the ground conditions by the Contractor, his Client and the Client’s advisors another 4 piles were driven with dynamic testing from the first blow. Senior representatives of all the above parties inspected the driving. Three of these piles were also damaged. Damage appeared to commence when each of the piles reached a penetration of about 7 to 8m. There was no detectable change in driving of the piles when the dynamic testing indicated the piles to be damaged. The Contractor was reluctant to believe the piles were damaged and deliberately drove one of the piles until the PDA indicated very severe damage ( =35%). One of the other piles was considered significantly damaged ( =79%) and one pile was considered by the Author to be starting to show damage but the calculated section change remained 100%. The contractor remained skeptical and so employed a very large excavator to dig around the piles and a crane was used to pull the piles out for visual inspection. The visual inspection showed the piles were damaged at the locations and to the extent suggested by the dynamic testing. The following figures and photographs show the correlation between dynamic testing and the visual inspection. ∫ (8) F R t 2 t 1 I where t1 and t2 are the beginning and end of the crack or slack and R is the difference between force and proportional velocity at time t1. I would strongly suggest that this quantification only be conducted after you are confident that the reflector is a crack or gap at a splice location and not more significant damage. Generally damage gets worse with further driving whereas gaps at splices or a crack do not. More significant damage can start with a crack. Low Strain Low strain testing involves very small impact waves. After the initial impact force will be zero at the pile top as this is a definition of a free pile end. The upward traveling wave produces a difference between top force and velocity 2Fu Ftop Z1Vtop (9) If we now express the waves in terms of velocities from (6) above Fu / Fi Z i Vtop / 2Fi 1 / 2(Vtop / Vi ) (10) This suggests that we should adopt ½ of the velocity increase caused by the impedance change to calculate . This may be true near the pile top but if there has been considerable damping of the signal along the shaft, which is common in practical applications, then the return velocity Vtop is often amplified to about the magnitude of Vi in order to identify the response. At the toe of the pile we would expect =0 hence =1 and if the toe response is amplified to be the same magnitude as Vi then the ½ factor should not be applied. Hence at any “significant” distance down the pile the application of the ½ is questionable. BLANCHTOWN TESTING Initial testing for the project was at pier 8 at the east (Waikeri) end of the bridge and this was uneventful with all piles remaining intact and demonstrating the required resistance. A driving criterion was determined for driving of other piles at the pier. Testing started at Pier 1 after 3 piles had been driven. Pier 1 is immediately adjacent to a 20m cliff at the west (Adelaide) end of the bridge as described earlier. Piles had been expected to found at 7 to 10m penetrations but two 12m piles and one 19m pile had been driven approximately to ground level and none had shown the required resistance. Testing was requested by the Contractor to find out what resistance the piles would provide. The experienced piling foreman suspected damage but it was not considered to be a strong possibility. Testing was conducted the Figure 3 PDA screen of badly damage pile Note the very significant reflection from damage shown in negative “waveup” ie Fu. There is no clear response from the pile toe. BTA=35. Figure 4 Photo of badly damaged pile after removal

  4. The photo shows that the badly damaged pile was completely broken with exposed and bent reinforcing being the only connection between the 2 sections of pile. There is a reflection from damage just starting to occur above the toe. The toe is still clearly detected. BTA=100. Figure 8 Photo of slightly damaged pile The photo shows cracks that had been closed by prestress after the pile was removed from the ground. The cracks were only on one side of the pile. A low strain test was conducted on this pile after it was removed from the ground, as shown in the photo, and the low strain testing, which is very sensitive to even small cracks, did not detect any problem with the pile. The damage to all piles appeared to be linked to lateral deflection of the pile toe. As the piles were driven adjacent to a steeply sloping rock surface it is the Author’s opinion that the sloping surface also continued below the ground and as the piles met the sloping surface the pointed toe was deflected sideways, despite the steel pin projecting from the pointed toe. It appears the designers also agreed that the damage was related to deflection of the pile toe. Driving at pier 1 continued with the original piles after each location had been pre-drilled. Interestingly the pre-drill was only 100mm diameter. However, it would appear that in almost all cases this was sufficient for the pile to follow the pre-drilled hole and “toe into” the soft rock. Low strain integrity testing was subsequently conducted on all Pier 1 piles after driving was completed together with most piles at Piers 2 and 3. At Piers 2 and 3 the rock surface was not steeply dipping and all of the piles that were tested appeared to be undamaged. One further broken pile was detected at Pier 1 and this was one of the extra piles installed to replace broken piles. The low strain test data for this pile is shown below. Figure 5 PDA screen of damaged pile Note there is still a significant reflection from damage above the toe shown in negative “wave up” ie Fu. The reflection from the toe is still visible. BTA=79. Figure 6 Photo of damaged pile after removal The photo shows the end of the pile is permanently deflected. Even the 10MPa prestress did not pull the pile straight again. However the toe section is still firmly attached to the upper section hence the toe reflection detected by the PDA. The toe detail is also shown clearly in this photo. Figure 9 PIT data replacement pile C The data shows a strong reflection at 4.5m, a second reflection from the same defect at 2x4.5=9m and a third reflection at 3x4.5m=13.5m. No response from the toe can be detected at 14m, which suggests severe damage. Figure 7 PDA screen for slightly damaged pile

  5. CONCLUSIONS The use of pointed piles should be questioned when driving to sloping rock surfaces. If the slope is steeper than the angle of the point then the tip will not touch the rock and the pile will certainly be deflected. When such conditions are expected a flat toe with a projecting steel pin ie a “Balkan Point” will probably provide a better chance of avoiding deflection of the pile toe. The Author believes a flat toe is better in all circumstances. Driving behavior does not necessarily provide any indication of pile damage. It is suggested that all piles driven to a sloping rock surface are checked for integrity using some form of dynamic testing. Dynamic testing has proved useful at this project in demonstrating the required pile resistance, determining required driving criteria, and proving pile integrity. The correlations presented in this paper confirm that interpretation of dynamic test results by an experienced operator can correctly identify the location and approximate extent of pile damage. REFERENCES 1. J Cannon (1990). “Dynamic Pile Testing for Marine Construction.” Proceedings of Third Australian Ports and Harbours Conference, Melbourne Australia, August Instit’nEng’s Aust pp 66-72 2. G. Goble and G Likins (1996). “On the Application of PDA Dynamic Pile Testing.” Proceedings of Fifth International Conference on the Application of Stress Wave Theory to Piles. Orlando, Florida USA, September Townsend, Hussein, McVay Eds pp 263- 273 3. J Cannon (1990). “Integrity Testing of Piled Foundations” Proceedings of 2nd National Structural Engineering Conference, Adelaide Australia, October Instit’n of Eng’s Aust pp 450-455 4. F Rausche and GG Goble (1986). “Determination of Pile Damage by Top Measurements” Special Technical Publication 670 ASTM pp500-506 5. PDA-W Manual of Operation (2001) Pile Dynamics Inc.

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