Interpretation of Pile Integrity Test (PIT) Results

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1 Annual Transactions of IESL, pp , 26 The Institution of Engineers, Sri Lanka Interpretation of Pile Integrity Test (PIT) Results H. S. Thilakasiri Abstract: A defect present in a pile will severely reduce the structural load carrying capacity of the pile. Therefore, post construction tests to verify the integrity of bored piles are extremely important. A most commonly used post construction integrity test is the low strain Pile Integrity Test commonly referred as PIT. In this type of integrity testing, a low strain dynamic impulse is given to the pile top by a hand held hammer and the velocity of the pile top is monitored using an accelerometer. The low strain impulse will generate a low strain wave that propagates through the pile and reflected at places where there are changes in the properties of concrete, cross sectional area of the pile or stiffness of the soil surrounding the pile. Such reflections are detected by the accelerometer at the pile top and the time Vs velocity plot recorded by the accelerometer is used to identify any defects in the pile shaft. There are two main methods of data analysis associated with the PIT namely, the Sonic Pulse Echo method and the Transient Response Analysis (TRA) method. In this research, analysis of PIT records related to the Sri Lankan soil conditions, construction and design practices are investigated using the Sonic Pulse Echo method. In the analysis, variation of the actual PIT records according to the commonly encountered soil conditions in Sri Lanka are presented. Furthermore, wave propagation through the pile is modeled using the Wave Equation Method. An MS Excel spreadsheet is developed to model the PIT using the wave equation method. The developed computer program is capable of dividing the pile into a maximum of 2 elements and incorporating varying soil stiffness along the pile shaft. An artificial velocity pulse is given to the top most element to simulate the hammer blow given in the PIT while monitoring the velocity of the same element for varying ground conditions and the defects commonly encountered in Sri Lanka. The location of the defects and its appearance in the velocity plot is also investigated. 1. Introduction Bored and cast in-situ concrete piles are very widely used in Sri Lanka for pile foundations. In this method of pile construction, a borehole is created in the ground by drilling and subsequently filling it with fresh concrete to form the pile. During the drilling process bentonite slurry is used to stabilize the sides of the borehole. The concreting is carried out from the ground surface using a tremie pipe and systematically bentonite is replaced by concrete during the concreting process. Common problems encountered due to such pile construction practice are (i) necking of the pile due to inflow of soil into uncased boreholes, (ii) mixing concrete and bentonite and (iii) collapsing of sides into the borehole and mixing of soil and concrete. If such a defect occurred in a pile the structural load carrying capacity of the pile will be severely reduced. Therefore, post construction tests to verify the integrity of bored piles are extremely important. If defective piles are identified such piles could be rectified or replaced with new piles to avoid failure of the foundation. But it should be emphasized here that even if a pile is proved to be a solid integral pile from integrity testing, it doesn t guarantee that the pile has the required load carrying capacity due to shear failure of the soil. Commonly used post constructional pile integrate testing methods are given below: i. Coring through the pile to obtain continuous concrete core samples of the pile and testing the obtained core samples to identify weak sections; ii. Load testing of piles using static or dynamic methods iii. Low strain dynamic testing of piles using Pile Integrity Tester (PIT); iv. Cross-hole Sonic Logging (CSL); Eng. (Dr.) H.S.Thilakasiri, C. Eng., MIE(SL), B.Sc. Eng. (Hons) (Moratuwa), M.Sc (Lond),PhD (USA), Senior Lecturer, Department of Civil Engineering, University of Moratuwa.. ENGINEER 78

2 Out of the above methods, coring and load testing are expensive and time consuming. Those methods cannot be applied to large number of piles in a site and sometimes are not conclusive, for example, coring through one side of the pile can miss defects on the other side of the pile. But these two methods have some advantages such as: (i) Ability to correct any defect through the hole created in the pile during coring; and (ii) Capacity is also assessed during load testing in addition to integrity testing. Other two methods are base on dynamic wave propagation through piles and are very widely used for integrity testing of piles. However, Cross-hole Sonic Logging (CSL) requires installation of PVC or steel tubes in the pile during casting stage and that method is used only in large project where such additional cost can be justified. Therefore, in this article most commonly used integrity testing of bored piles in Sri Lanka, the low strain dynamic testing using Pile Integrity Tester (PIT) is discussed and important factors related to interpretation of the results are presented. In the low strain Pile Integrity Test, commonly referred as PIT, a low strain dynamic impulse is given to the pile top by a hand held hammer and the velocity of the pile top is monitored using an accelerometer as shown in Figure 1. The low strain impulse will generate a low strain wave that propagates through the pile and reflected at places where there are changes in the properties of concrete, cross sectional area of the pile or stiffness of the soil surrounding the pile. Such reflections are detected by the accelerometer at the pile top and the time Vs velocity plot recorded by the accelerometer is used to identify any defects in the pile shaft. There are two main methods of data analysis associated with the PIT, namely, the sonic pulse echo method and the transient response method. The main difference between these two methods is that in the transient response method of analysis both velocity and force measurements during the hammer impact are needed. Therefore, in the transient response method, a hammer instrumented with a load cell is used instead of a normal hammer. In Sri Lanka, mostly Sonic Pulse Echo method is used to analyze Pit records and, therefore, the discussion in this paper is limited to that method of analysis. Figure. 1- PIT testing of bored and cast insitu concrete piles In the Sonic Pulse Echo method of analyzing PIT records using the velocity variation of the pile top with time, a normal hammer without a load cell is used to generate the stress wave. The low strain impulse generates a low strain wave that propagates through the pile with a velocity of, C, referred to as the wave velocity. For a linear elastic pile having a length one order of magnitude higher than its width, wave velocity is given by: C = E ρ Hammer Accelero meter Where E and ρ are Young s modulus and mass density of pile material respectively. As the stress wave passes through any section of the pile, it will impart an oscillation to the particles of that section and the particles of that section will move at a certain velocity referred to as particle velocity. The force, F, due to the stress wave at any section is related to the particle velocity, v, at that section during the propagation of the dynamic wave by: F = Zv Where Z is defined as the impedance of the pile at that section and it is given by: Z = EA/ C Where A is the cross sectional area of the pile. As the downward traveling stress wave, of magnitude F Down, propagates through the pile, if it encounters change in the impedance of the pile at a particular section, a reflected wave is generated. For example if the impedance varies from Z 1 to Z 2 a reflected wave of magnitude F up is generated. The magnitude of F Down and F up are related to impedance change by the following expression: [ Z2 Z1] FUp, = F Down [ Z2 + Z1] It is evident from the above equation that the magnitude of the upward traveling wave depends on the difference in the impedance encountered and depending on the sign of (Z 2 ENGINEER 79

3 Z 1) the generated upward traveling wave could be tension or compression. Since downward traveling wave is normally a compression wave, the upward traveling wave will be a tension wave if the wave travels from high impedance to low impedance (Z 1 > Z 2) and vise-versa. Therefore, it should be noted here that the PIT identifies change in the impedance, which is a combination of the pile cross sectional area, Young s modulus of the pile material and the density of the pile material. Apart from the reflections of the downward traveling wave due to the changes in the impedance of the pile section, soil resistance variations along the pile shaft can also generate upward traveling reflections. However, such reflections due to soil resistance variations are of low frequency and an experienced interpreter could separate the upward traveling waves generated due to impedance variations of the pile shaft from that generated due to soil resistance variations. Pile top velocity 2L/c 2X/c Depth Time T Figure 2 - Schematic diagram showing reflections from necking and toe The intensity of the defect is quantified in terms of the parameter β defined as Z 1/Z 2. Following classification of defects could be made based on the value of β (PDA User s manual). β=z 1/Z 2 Damage assessment 1. Uniform.8 1. Slight damage.6.8 Damage.6 > Pile with a major discontinuity If the pile is defect-free, the wave will travel down to the pile toe and will be reflected back to arrive at the pile top after a time period of 2L/C, where L and C are length of the pile and wave velocity through the pile respectively. Any other reflection before this time, if any, is due to changes in the pile impedance or the soil resistance. The above mentioned downward and upward traveling waves are illustrated in Figure 2 for a pile of length L. The pile cross section is reduced at a depth of X from the pile top and the downward traveling compression wave is reflected back and arrives the pile top after 2X/C time period from the initial input wave and the reflection from the pile toe arrives at the pile top after a time period of 2L/C. Even though the defect is easily identified from the above idealized PIT record, the interpretation of actual records can be very difficult and complicated due to: i. Attenuation of the stress wave as it travels down the pile shaft and as a result, inability to identify the reflections; ii. Presence of the reflections due to soil resistance variations along the pile shaft; iii. Masking of the reflections due to defects by the reflections due to buldged sections of the pile shaft; and iv. Other distortions of the collected data due to improper pile top cleaning, loose attachment of the accelerometer to the pile top etc. In this research, wave propagation through the pile is modeled using the Wave Equation Method. In the wave equation method, the pile is divided into small elements and the equation of motion of each element is considered during wave propagation using the finite difference method. An MS Excel spreadsheet is developed to model the PIT using the wave equation method. The developed computer program is capable of dividing the pile into a maximum of 2 elements and incorporating varying soil stiffness along the pile shaft. An artificial ENGINEER 8

4 velocity pulse is given to the top most element to simulate the hammer blow given in the PIT while monitoring the velocity of the same element for varying ground conditions and the defects commonly encountered in Sri Lanka. The ground conditions considered in this research consists of typical soil layering and bedrock conditions found at construction sites. The location of the defects and its appearance in the velocity plot is also investigated. Finally, some typical velocity plots obtained from actual PIT are also presented to demonstrate the practical applications of the analysis. 2. Wave equation method In the wave equation method, the entire pilesoil system is modeled as a series of masses supported and connected by set of springs and dashpots. The size of the individual mass elements and the stiffness of the springs reflect the mass and stiffness of various components of the real pile and the driving system. The soil is represented by a series of elasto-plastic springs and linear viscous dashpots. A schematic diagram of the entire system is shown in Figure 3. In the wave equation analysis, first introduced by Smith [1] to solve the 1-D wave propagation in a pile, the pile is divided into a number of elements and the mass of each of the elements is lumped at the nodal points as shown in Figure 3. The intermediate pile elements are connected by pile springs, of which the stiffness (k) estimated by AE/ L, Where A, E and L are the cross sectional area of the pile, Youngs modulus of pile material and length of an element respectively. The soil resistance, at the interface between pile element and the surrounding soil, during propagation of the stress wave consists of two parts: (i) a static resistance, proportional to the deformation and; (ii) a damping resistance, proportional to the velocity of the pile element. The static resistance by the surrounding soil on the pile elements is represented by an elastic perfectly plastic soil spring, in which the force is given by the axial compression (δ) of the spring multiplied by the stiffness (k / ) of the spring. According to the wave equation analysis proposed by Smith (196), the damping resistance is represented by a viscous dashpot, in which the force generated is estimated by multiplying the damping coefficient J skin of the dashpot, velocity of the pile element (v) and the axial force in the spring (k / δ). The stiffness (k / ) of the side soil spring is estimated as the ratio between the ultimate skin friction on the element and the limiting elastic displacement of the soil spring (skin quake). The last pile element is connected to the element above it by a pile spring. The soil resistance at the end is represented by a soil spring, with stiffness k(p), and a dashpot, with damping coefficient (J end). K(p) is estimated as the ratio between the ultimate end bearing capacity of the pile and the elastic limit of the end spring (end quake). The equilibrium of each of these pile elements is considered during a hammer blow and the resulting equations of motion in the time domain are solved using finite difference method. Interested users are referred to Thilakasiri et al [2] for complete formulation of wave equation method. Soil dashpots Elasto plastic Soil springs Pile springs Figure 3 - Pile-soil system as discretized by the Smith (196) 3. Simulation of PIT using the Wave Equation Method ENGINEER 81

5 Wave equation method, explained above, was programmed using MS Excel spread sheet. The developed program has the capability of discretizing a pile into 2 elements and carry out the finite difference time simulation of a 25m long pile using very small time step size of about.5 seconds until the toe reflection is reached at the pile top (after 2L/C from the initial impulse of the hammer). The input pulse of a PIT hammer is simulated using a triangular velocity impulse given to the top most element and the discretized pile-soil system is allowed to vibrate freely under the given velocity impulse. The response of the accelerometer attached at the top of the pile is obtained by recording the velocity of the top most pile element. The soil along the pile shaft could be divided into five layers and the skin friction resistance of the five layers could be changed independently. However, dynamic soil properties of the soil along the pile shaft cannot be changed for different soil layers along the pile shaft. The developed program can simulate the variation of the impedance of the pile shaft by changing the stiffness of the spring representing the stiffness of the pile elements. In the simulation process, a 25m long, 1m diameter pile is considered. The smith damping factors for the skin and toe are set at.1 and.2 m/sec respectively. The Young s modulus of the pile material is assumed to be 35x1 6 kpa and the unit weight of concrete is assumed to be 24 kn/m 3 resulting in a wave speed of 38 m/sec. The time taken for the toe reflection to observe at the pile top is about 14 msecs. 3.1 Observation of the toe reflection It is a very common observation that the toe reflection of a pile is observed as a positive or negative velocity pulse for similar piles from the same site. Figure 4 show PIT records from the same sites with toe reflection is observed as positive and negative velocity pulses in Figures 4(a) and 4(b) respectively. cm/s T (a) cm/s.6.3. T1 -.3 (b) 9: # Toe m 11: # m Toe MA: 1. MD: 5.8 LE: 9.12 WS: 3592 LO:. HI:. PV: 1 T1: 24 Vel MA: 1. MD: 4.3 LE: 9.6 WS: 3531 LO:. HI:. PV: 1 T1: 33 Figure 4 - Actual PIT records similar piles from the same site (a) showing a positive velocity reflection from the toe and (b) showing negative velocity reflection from the toe Velocit y Vs Time K-toe=5 K-toe=5 K-toe=25 K-toe=5 K-toe= Vel T ime (seconds) Figure 5 - Velocity records with different toe stiffness The above observation could be explained using the wave equation simulation program developed by varying the stiffness of the toe as shown in Figure 5. The ultimate capacity of the simulation pile was kept constant but the ratio between the toe resistance and the skin resistance was varied so that the stiffness of the spring representing the material at the pile toe is varied as shown in Figure 5. It is evident ENGINEER 82

6 from Figure 5 that the toe reflection could be either negative or positive depending on the stiffness of the material present at the toe of the pile. If the stiffness of the material at the toe is high, the toe reflection could be negative whereas when the stiffness of the material present at the toe is low the toe reflection could be a positive velocity pulse. 3.2 Variation of the velocity record depending on the variation of the stiffness of the soil along the pile shaft. It is commonly observed that the velocity records of the PIT tests show positive or negative velocity reflections that are not relevant to the variation of the impedance of the pile shaft. To demonstrate this the skin friction was varied along a uniform pile shaft. Figure 6 (a) shows a simulated velocity reflection of a pile having a 5m thick very soft layer, sandwiched between stiff layers, at about 5m depth from the top of the pile. Figure 6 (b) shows a similar velocity record of a pile having a 5m thick relatively stiffer layer present at about 5m from the top of the pile. (a) Velocity (m/sec) Velocity (m/sec) Time (seconds) Negative reflection Positive reflection It is evident from the above demonstration that the PIT records are influenced by the stiffness of the soil layers present along the pile shaft. If the person interpreting the records is not aware of the subsurface conditions at the site, the interpretation could be erroneous and the good piles may be identified as defective due to the reflections from the soil layers along the pile shaft. However, it should be noted here that the reflections due to the soil layer variations along the pile shaft are relatively small in magnitude and the pulse width is wider than the reflections due to defects of the shaft. 3.3 PIT records showing defective piles. Figure 7 shows a PIT record with a positive reflection indicative of a defect at about 7.5m below the top of the pile. In subsequent Pile Driving Analyzer (PDA) testing it was confirmed that the pile is defective and the β (ratio between the impedance) at the location of the defect is about 6%. cm/s T : # m Toe MA: 2. MD: 5.2 LE: WS: 416 LO:.88 HI:. PV: T1: 32 Figure 7- PIT record of a defective pile (Exponential magnification of 2 is applied to the toe reflection) The simulation of a defect at the middle of the simulation pile is shown in Figure 8. It should be noted here that the in both actual and simulated records, the positive reflection is between two negative reflections. However, the relative magnitude of the positive reflection of the defect is higher than that of the negative reflections. It should be noted here that the actual velocity record has a exponential magnification of 2 of the toe reflection. Vel -.4 Time (Seconds) (b) Figure 6 - Effect of the variation of the skin friction on the velocity record (a) presence of a very soft layer (b) presence of a stiff layer ENGINEER 83

7 Velocity (m/sec) Time (Seconds) Figure 8 - Simulated velocity record with Presence of a necking 3.4 Typical reflections of PIT records in Sri Lanka In most of the PIT records from Sri Lanka observed by the author indicate negative reflections immediately after the input velocity pulse and immediately before the toe reflection. This observation is clearly shown in Figure 4 (a) & (b) as well. This could be explained using the soil stiffness variations along the pile shaft. In most of the sites in Sri Lanka, there is a hard fill placed above soft soil deposits at the top level of the pile. Moreover, towards the lower end of the pile a very strong weathered rock layer is present. Due to the high stiffness of the fill at the top and the strong residual formation towards the toe of the pile, the positive reflections are generated and such a velocity record of a pile simulating this condition is shown in Figure 9. Velocity (m/sec) Time (Seconds) Figure 9 - Typical velocity record with stiff soil layer at the top and bottom of the pile 4. Conclusions Interpretation of the PIT results using the sonic pulse echo method was investigated in this paper. Typical problems associated with interpretation of the Time velocity record of the PIT test done on bored and cast insitu concrete piles were shown using typical velocity records from PIT tests. Special attention was paid to the toe reflections and reflections due to the variations of the stiffness of the soil present along the pile shaft. A computer programme capable of modeling pile response due to the hammer blow using the wave equation method was used to quantify the variations observed in the actual field PIT records. It was shown that the reflection of the toe could give rise to a positive or negative velocity reflection depending on the stiffness of the soil at the toe of the pile. If the soil at the toe of the pile is soft, the resulting velocity reflection could be positive while the presence of stiffer material at the pile toe could generate negative velocity reflection. However, it should be noted here that the toe reflection of the pile is a qualitative indication only and it should not be used to estimate the carrying capacity of the pile. Reflections due to the stiffness variation of the soil along the pile shaft were also qualitatively discussed using the wave equation model developed. It was shown that a relatively soft layer present along the pile shaft could give rise to a positive velocity reflection, which could be erroneously identified as a defect. Positive velocity reflection due to a defect present along the shaft of an actual pile and that of a simulated pile using the wave equation method was also presented. Finally, it could be concluded that the developed computer program using the wave equation method could be used to qualitatively explain the observations of the PIT. References 1. Smith, A. E. L. Pile Driving Analysis by Wave Equation, Journal of SMFD, ASCE, 86(4), pp 35-61, Thilakasiri, H. S., Abeyasinghe R. M. and Tennakoon, B. L. A study of ultimate carrying capacity estimation of driven piles using pile driving equations and the wave equation method, Proc. Annual Transactions of IESL, ENGINEER 84

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