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22 tigated through a back calculation (to match the measured The response of the modeled pile-soil system (e.g., force at static capacity). The results (see section 2.5.3.2.) suggest that the pile top) under a given boundary condition (e.g., mea- there is no correlation between the soil type and the Case- sured velocity at the pile top) is compared to the measured damping coefficient. The recommended practice is to use the response (force measured). The modeled pile-soil system or, Case method based on a specific site/area calibration (GRL, more accurately, the modeled soil that brings about the best 1999). This approach, in conjunction with the application of match (visual graphical match) between the calculated and the method for maximum resistance (RMX), has proven measured responses, is assumed to represent the actual soil worthwhile. Accumulated experience on extensive jobs in resistance. The static component of that resistance is assumed the Boston area (e.g., Geosciences Testing and Research, Inc. to be the pile's capacity and reflects that time of driving. The 1997, 1998) has demonstrated the effectiveness of the Case signal matching procedure was first suggested by Goble et al. method, when calibrated. A statistical examination of local (1970), utilizing the computer program CAPWAP. Others calibration was performed in Florida by McVay et al. (2000). developed similar analyses, (e.g., Paikowsky, 1982; Paikow- The results of this analysis suggest that for 48 case histories, sky and Whitman, 1990) utilizing the computer code the ratio of the static pile capacity to the Case method pre- TEPWAP. The TNO program was developed by Midden- diction at EOD was 1.344 0.443 (mean 1 SD). drop and van Weel (1986), which led to improvements and As no generic conditions exist for the use of the Case to the CAPWAPC program, which is used to date. method, international or national calibrations are unrealistic. Because the projection of local calibration (based on good experience and practice) beyond the geographical location 2.5.3 The Controlling Parameters may be unwise or unsafe, the Case method was excluded from the dynamic analyses examined for this project. 2.5.3.1 Overview Preliminary examination of the parameters controlling the 2.5.2.5 Dynamic Measurement: performance of the dynamic analyses was carried out prior to The Energy Approach a final detailed evaluation of these methods, leading to the calculation of appropriate resistance factors. Such examina- The Energy Approach uses basic energy relations in con- tion influenced the subcategorization of the dynamic meth- junction with dynamic measurements to determine pile ods (according to the important controlling parameters), hence capacity. The concept was first presented by Paikowsky directing the user to utilize the appropriate resistance factor (1982) and was examined on a limited scale by Paikowsky according to the relevant conditions of the employed method. and Chernauskas (1992). Extensive studies of the Energy For example, if soil type is a controlling factor and the accu- Approach method were carried out by Paikowsky et al. racy of the signal matching method is largely affected by soil (1994) and Paikowsky and LaBelle (1994). The underlying type, evaluation of the method for different soil types will assumption of this approach is the balance of energy between result in the development of different resistance factors the total energy delivered to the pile and the work done by depending on the soil type. Conversely, if soil type does not the pile/soil system. The basic Energy Approach equation is control the accuracy of the specific dynamic method, cate- gorization based on soil type is neither desired nor pursued. Emax Ru = The following sections outline the logic used for the pre- ( Dmax - Set ) (28) Set + liminary examination of the controlling parameters, the 2 analyses, and the results. The rationale presented in this sec- tion follows previous studies by Paikowsky et al. (1994), where Ru = maximum pile resistance, Emax = measured maxi- Paikowsky (1995), Paikowsky et al. (1995), and Paikowsky mum energy delivered to the pile, Dmax = measured maximum and Chernauskas (1996). Paikowsky and Stenersen (2000, pile top displacement, and Set = permanent displacement of 2001) present more detailed results related to the dynamic the pile at the end of the analyzed blow, or 1/measured blow analyses of this study and are provided in Appendix B. count. For further details regarding the Energy Approach The evaluation of static capacity through data derived from method see Paikowsky et al. (1994) and Paikowsky (1995). pile driving is based on the concept that the driving operation induces failure in the pile-soil system, (i.e., a very fast load test 2.5.2.6 Dynamic Measurement: is carried out under each blow). Dynamic analyses encounter The Signal Matching Techniques three fundamental difficulties: (1) correct formulation of the penetration process (e.g., soil motion, soil plugging etc.), The signal matching technique is often referred to as post- (2) separation of the static resistance out of the total resis- driving analysis or the office method. With the availability of tance overcome during penetration, and (3) time dependent faster, portable computers, it became reasonably simple to pile capacity (Paikowsky, 1995). The parameters controlling conduct the analysis in the field, although the field method the accuracy of the dynamic predictions reflect, therefore, the analyses cannot be carried out for each blow during driving. ability of each method to address the above difficulties.

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23 Based on the concept of a pile loading to failure under each Smith Tip Damping (sec/m) blow, it has traditionally been assumed that during high driv- 0 0.5 1 1.5 2 2.5 3 ing resistance (i.e., refusal) there is not sufficient pile pene- tration to mobilize the full pile capacity (Chellis, 1961). Clay Therefore the dynamic methods are deficient under high driv- Silty Clay ing resistance, categorized as equal or above 12BPI (Blows Sandy Clay Per Inch) or approximately 5BPcm (Blows Per cm) (Massa- Clayey Silt chusetts Highway Department, 1988). Silt Soil type is also believed to have a major effect on the 10.102 dynamic analyses because soil damping parameters are com- Sandy Silt Soil Type 4.380 monly employed to represent viscous resistance in the model- Clayey Sand ing of the soil's dynamic behavior. This viscosity is assumed Silty Sand 5.492 to be soil type dependent and associated with intrinsic soil Sand properties. High viscosity values are expected for cohesive Gravelly Sand soils and low viscosity values are expected, therefore, for 3.412 Sandy Gravel cohesionless soils. Naturally, under a given velocity, high vis- Gravel cous values are associated with higher dynamic resistance Rock and logically should prove more difficult to accurately define Till the static resistance. 1 sec/m = 0.3048 sec/ft The effect of time is well recognized but poorly quantified. With time, piles undergo a decrease or increase of capacity, Figure 8. Soil type at the pile's tip versus Smith tip known as relaxation and set-up, respectively. While the resis- damping coefficients used in CAPWAP for 372 PD/LT2000 tance during driving and its static component represent the pile-cases. conditions encountered during penetration, the major inter- est remains the long-term ability of the pile to carry load dur- soil type control the value that should be used as a damping ing its service life. The examination of the dynamic-method factor. predictions with static load tests (often carried out long after A summary of the statistics obtained when examining the driving) therefore remains valid. The predictions can be the accuracy of the signal matching technique (specifically assessed in relation to the time at which the data have been CAPWAP) based on soil type is presented in Table 10. The sta- obtained (i.e., EOD or BOR). tistics shown are the mean and standard deviation of a normal The following sections provide a short summary of the process in which the importance of each of the above assumed controlling parameters was examined. The results are used to Smith Side Damping (sec/m) evaluate additional possible controlling factors, laying down 0 0.5 1 1.5 2 2.5 3 the framework for the detailed evaluation of the dynamic methods and the resulting resistance factors. More details are Clay provided by Paikowsky and Stenersen in Appendix B. Silty Clay Sandy Clay Clayey Silt 2.5.3.2 The Effect of Soil Type Silt The effect of soil type was examined in two ways: (1) the Sandy Silt Soil Type 3.048 correlation between the parameters assumed to be soil type Clayey Sand dependent and soil type, i.e., damping parameters; and (2) the Silty Sand accuracy of the predictive methods relative to the soil type. Sand Figures 8 and 9 present the relationship between soil type Gravelly Sand and Smith-damping parameters (Smith, 1960) used in approx- Sandy Gravel imately 370 CAPWAP analyses from PD/LT2000 for the tip Gravel and side pile resistances, respectively. Figure 10 presents the Rock back-calculated Case-damping coefficient required to obtain Till a match between the predicted capacity and the measured sta- 1 sec/m = 0.3048 sec/f t tic capacity for 290 case histories from the PD/LT database (Paikowsky et al., 1994). All three figures clearly indicate Figure 9. Soil type at the pile's side versus Smith side that no unique relationship exists between soil type and damp- damping coefficients used in CAPWAP for 371 PD/LT2000 ing parameters, suggesting that mechanisms other than the pile-cases.

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24 Case Damping Coeff icient, Jc EOD and last BOR, (i.e., in the case of multiple restrikes, -1.5 -1.00 -0.50 0.0 0.50 1.00 1.5 only the last restrike is considered for the analysis). The results suggest that the time of driving significantly affects Clay the performance of the CAPWAP prediction, regardless of Silty Clay soil type. The mean values for the BOR sets are closer to one, Sandy Clay while the mean values for the EOD are closer to two. The Clayey Silt COVs show values of 0.33 and 0.39 for BOR, while the EOD Silt ratios are 0.55 and 0.85, indicating the existence of a sub- stantial scatter. Again, the cases examined for piles in rock Sandy Silt are not indicative and are excluded from being meaningful in Soil Type Clayey Sand -1.86 -5.04 7.04 respect to soil type effect. Silty Sand -2.25 1.51 Further evaluation of the records was carried out on the Sand basis of driving resistance. The division between cases for Gravelly Sand which the driving resistance is smaller or greater than 5BPcm Sandy Gravel (5 blows per centimeter), examines the aforementioned Gravel notion of refusal and the expected accuracy of the dynamic Rock methods. The results, shown in Table 10, suggest that analy- Till 1.56 ses were less accurate and had larger scatter in cases for which no. of cases = 290 2.22 the driving resistance was smaller than 5BPcm than when driving resistance was above 5BPcm. Though driving resis- Figure 10. Soil type at the pile's tip versus back tance seems to be an important factor, clear understanding of calculated Case-damping coefficient (Jc) based on static its influence on the accuracy of the dynamic methods calls load test results for 290 PD/LT pile-cases (Paikowsky for additional investigation, which is briefly presented in sec- et al., 1994). tion 2.5.3.4. In summary, while the performance of the signal matching analysis (CAPWAP) is not well correlated to soil type, other distribution function for the ratio of the pile's static capacity factors associated with soil type may be important (e.g., low (based on Davisson's failure criterion) to the pile capacity driving resistance in soft cohesive soils or gain of capacity obtained in the CAPWAP analysis. There are no significant with time); but soil type itself does not appear to be impor- differences between clay and till versus sand and silt that jus- tant. The data presented in Table 10 suggests that time of driv- tify analysis categorization based on soil type. Although the ing must be considered and driving resistance needs to be case histories for piles found on rock provide different val- further examined. ues, the numbers are based on a small subset of 15 pile case histories, compared to 100 and 265 pile case histories for the other soil type categories. 2.5.3.3 The Effect of Time on Tested Capacity Table 10 provides further examination of time of driving and driving resistances as subsets of the soil type categoriza- Penetration of piles into fine-grained soils causes compres- tion. Two sets are examined based on the time of driving: sion and disturbance, resulting in soil strength during driving TABLE 10 Statistical parameters of the ratio between static capacity (Davisson's Criterion) and signal matching analysis (CAPWAP) categorized according to soil type, time of driving and driving resistance Clay & Till Sand & Silt Rock Mean 1.352 1.517 0.930 Standard Deviation 0.723 1.085 0.172 Number of Cases 100 265 15 Time of Driving EOD BOR(last) EOD BOR(last) EOD BOR(last) Mean 1.634 1.133 2.068 1.193 0.968 0.925 Standard Deviation 0.899 0.444 1.765 0.391 0.132 0.203 Number of Cases 45 40 77 116 7 7 Blow Count (BPcm) <5 5 <5 5 <5 5 <5 5 <5 5 <5 5 Mean 1.127 1.725 0.750 1.315 2.191 1.458 1.126 1.283 1.070 0.952 0.671 0.879 Standard Deviation 0.637 0.807 0.241 1.160 1.901 0.512 0.386 0.355 ----- 0 .136 0.163 0.230 Number of Cases 35 35 11 10 64 13 74 40 1 6 3 3 NOTES: EOD = End of Driving; BOR(last) = Beginning of the last restrike; BPcm = Blows per centimeter

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25 that differs from its long-term strength, thus affecting pile for time evaluation (i.e., actual loading in construction or capacity. Although factors such as thixotropy and aging con- dynamic testing as part of quality control), and (3) at present, tribute to this phenomenon, the migration of pore water is the the dynamic methods evaluation should concentrate on the most significant cause of capacity gain with time. Measure- long-term pile capacity. ments carried out on a model (Paikowsky and Hart, 2000) and full-scale piles (Paikowsky and Hajduk, 1999, 2000) show that pore pressure at magnitudes similar to the total soil pres- 2.5.3.4 The Effect of Soil Motion sure creates in clays around the pile's shaft zones of about zero effective stress, resulting in almost a complete loss of Overview. Paikowsky and Chernauskas (1996) show that the frictional resistance. Paikowsky et al. (1995, 1996) examined stationary soil assumption, under which the soil/pile interac- the static and dynamic gain of capacity with time based on tion models were developed, does not reflect the physical radial consolidation; a normalization process was followed, phenomenon that occurs during pile driving. Pseudo-viscous allowing for comparison between different pile sizes. damping serves as a mechanism to absorb energy; but, as it Table 11 presents a summary of parameters describing does not reflect the actual phenomenon, it cannot be corre- the pile capacity gain with time based on static and dynamic lated to physical properties (e.g., soil type) or time of driving. If the motion of the displaced soil is a major factor con- testing. The slope of the relation between the static capac- tributing to energy loss during driving, a substantial portion ity and the maximum static capacity (scale of 0 to 1) to the of the dynamic resistance should be a function of two param- elapsed time after driving (logarithm scale) for a 152.4 mm eters: (1) acceleration of the displaced soil (especially at the radius (1 ft diameter) pile is denoted as Cgt. Similar relations tip) that can be conveniently examined as a function of the for the ratio of dynamic capacity (with time) to the maximum driving resistance, and (2) mass/volume of the displaced soil static capacity result in a slope denoted by the parameter Cgtd. that is a function of the pile geometry, namely, small vs. large The time required for the standard pile to gain 75% of its max- displacement piles. A brief summary of the findings imum capacity is denoted as t75. The time extrapolation for described by Paikowsky and Stenersen regarding the above any desired pile size is achieved through the relationship of two factors follows. Further details of their research are pro- vided in Appendix B. t75 (pile) = 4r2 t75 (table) (29) Soil Acceleration/Driving Resistance. The energy loss For which r = the desired pile radius (ft.) or its equivalent through the work performed by the displaced soil mass at for a pile of different shape. the tip is directly related to the acceleration of this mass. The The data in Table 11 show that while the rate of capacity detailed evaluation of the soil's motion at the tip is beyond the gain is similar according to both analyses (Cgt = 0.389, Cgtd = scope of the present research and is described by Hlscher 0.348), the associated time for achieving 75% of the maxi- (1995), Hlscher and Barends (1996), and Hajduk et al. mum capacity (normalized for all piles to 304.8 mm diame- (2000). The indirect evaluation of these accelerations can be ter) is about 20 times greater when analyzed by static meth- performed through analysis of the driving resistance, which ods than when analyzed by dynamic methods. In other words, is the measure of the pile's final displacement under each dynamic testing and analyses (namely CAPWAP), while fol- hammer blow. With low driving resistance (easy driving), lowing the physical behavior of capacity gain, exhibit this high acceleration and velocity (i.e., free-end analogy) are gain much faster than the actual gain monitored by the static developed at the tip. In the case of high driving resistance load test results. The ramifications of these conclusions are (hard driving), there is small acceleration at the tip, resulting that (1) actual gain of capacity is much slower than that exhib- in little, if any, mobilization of the soil mass beyond a radi- ited by the dynamic methods, (2) scheduling of construction ating elastic wave. The corresponding energy loss due to soil or testing based on capacity gain should consider the reason motion is, therefore, small. TABLE 11 Summary of static-and-dynamic-based capacity gain with time parameters based on data sets (Paikowsky et al. 1996) Static Data Sets LTT and Dynamic Data ALL DATA PUT/LTT Set PD/LTT Cgt t75* Cgtd t75** Cgt t75** No. of Cases 15 5 7 6 22 11 Average for all 0.389 385.0 0.348 21.3 0.376 186.6 piles in set. Standard De- 0.119 226.3 0.068 7.9 0.106 237.9 viation Notes: *closed-ended pipe piles only; **t75 = time for a standard pile (0.3048m radius) to gain 75% of its maximum capacity; Cgt = rate of pile capacity gain with the logarithm of time

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26 To evaluate the blow count that identifies the transition when pile plugging takes place (Paikowsky and Whitman, between easy and hard driving (high and low soil acceleration) 1990). The piles, therefore, can be classified as small (e.g., H the ratio between the static capacity and the CAPWAP pre- and unplugged open pipe) and large (e.g., closed pipe and diction (KSW) by blow count for all pile case histories in square concrete) displacement piles. Additional classification PD/LT2000 was determined, as presented in Figure 11a. Fig- of open-pipe piles can be made according to a tip-area ratio ure 11b presents the data separated into intervals of 8 BP10cm similar to that used for soil samplers (Paikowsky et al., 1989). (2BPI), with the mean and standard deviation of each group As most soil displacement takes place at the tip area, the graphed as a point and an error bar against the mid point blow classification of piles can be better served by looking at the count of the interval. For example, for driving resistance ratio between the pile's embedded surface area and the area between 0 and 8BP10cm there were 42 case histories with a of the pile tip (Paikowsky et al., 1994): mean of 2.506 and a standard deviation of 2.217 plotted at the center of the interval, i.e., at 4BP10cm. The data pre- Askin Surface area in contact with soil AR = = (30) sented in Figure 11b show that for the first two intervals (up Atip Area of pile tip to 16BP10cm) the predicted capacity was substantially lower than for all other intervals with a significantly higher scatter. Using this ratio, a pile traditionally referred to as a "large dis- After approximately 16 blows per 10cm, the mean and stan- placement" pile can behave like a "small displacement pile" if dard deviation of the individual intervals fall within the range it is driven deeply enough. A quantitative boundary of AR = of all case histories. The boundary of the dynamic method 350 between "small" and "large" displacement piles was pro- evaluation based on driving resistance was defined, there- posed by Paikowsky et al. (1994). fore, as 16BP10cm (4BPI). Figure 12a presents the relationship between AR and the ratio of the static capacity over CAPWAP prediction (KSW) Displaced Soil/Pile Area Ratio. The volume of the displaced for all pile case histories in PD/LT2000. The data are sepa- soil is identical to the volume of the penetrating pile, except rated into AR intervals of 175, with the mean and standard deviation of each group graphed as a point and error bar at Blow Count (blows/in) 0 10 20 30 40 50 60 70 80 90 100 KSW = CAPWAP or TEPWAP Predictions KSW = CAPWAP or TEPWAP Predictions 4 4 6.85, 11.26, 5.97, 9.38, 5.26, 4.75, 4.41 no. of cases in Total no. of cases = 382 no set LG SM intervals of 175 Load Test Results Load Test Results DISP DISP (a) 3 Sand & Silt 3 Clay & Till mean for all cases = 1.452 Rock (a) 2 2 139 1 no set 111 76 1 18 3 12 11 5 7 0 standard deviation for all cases = 0.985 0 40 80 120 160 200 240 280 320 360 400 0 Blow Count (blows/10cm) 0 175 350 525 700 875 1050 1225 1400 1575 Area Ratio, AR Blow Count (blows/inch) 0 2 4 6 8 10 12 14 16 18 20 4 KSW = CAPWAP or TEPWAP Predictions 4.72 4 Total no. of cases = 382 no. of cases Total no. of cases = 71 no. of cases in in intervals 3 (b) Load Test Results 8 blows/10cm interval 3 mean for all cases = 1.452 42 mean for all cases = 1.460 KSW 2 (b) 2 66 25 64 38 32 15 16 8 16 60 16 21 1 10 1 6 8 10 standard deviation for all cases = 0.985 0 standard deviation for all cases = 0.734 0 8 16 24 32 40 48 56 64 72 80 88 0 Blow Count (blows/10cm) 0 175 350 525 1050 2100 3150 Area Ratio, AR Figure 11. The ratio of static capacity to dynamic signal matching prediction, KSW versus blow count for all pile- Figure 12. KSW versus area ratio, (a) for all pile-cases in cases in PD/LT2000 (a) all data points, and (b) data PD/LT2000 and (b) for 71 pile-cases with driving grouped in intervals of 8 blows/10cm (2BPI). resistance exceeding 16 BP10cm (4BPI) at the EOD.