Axially Loaded Behavior of Driven PC Piles

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1 Axially Loaded Behavior of Driven PC Piles Shih-Tsung Hsu Associate Professor, Department of Construction Engineering, Chaoyang University of Technology, address: Abstract. To obtain a fair load-settlement curve of a driven pile, and to evaluate the ultimate pile capacity more accurately, a numerical model was created to simulate the ground movements during a pile being driven. After the procedure, the axially loaded behaviors of the piles in silty sand were analyzed. The numerical results are compared with those results by full scale pile load tests. It was found, although the loads added on the tested piles are different from those by the numerical analyses which applied displacement increments on piles, the load-settlement behaviors of piles calculated from the numerical model were close to those measured from field tests before the piles stressed to peak. Total load, shaft friction, and point bearing do not reach peak values at the same pile settlement; furthermore, the point bearing slowly increases all the while, with no peak. However, the point bearing only contributes 1~2% of ultimate pile capacity. No matter which relative density of silty sand, pile diameter, and pile length increased, ultimate pile capacity increased as well. Keywords: Driven piles, silty sand, ultimate loads, field test, numerical analysis INTRODUCTION Usually, a pile foundation is required to translate the upper structure load of a high raised building to a firm base through a low strength and high compressibility of soil stratum. To avoid noise and vibration, cast-in-place piles are often to be applied in an urban district. Thus, driving piles only can be adopted along the bank of river, such as the Keelung River lies in Taipei basin. The results of urbanization could not allow both noises and pollutions created by heaven industries, the heaven industries usually moved to the western coast of Taiwan. The lands which reside in western coast of Taiwan were gained by filling reclaimed soil. The problems of both bearing capacity and settlement will were raised in those reclaimed sites. In these scenarios, the piles may be the best choice to conquer above two problems. Because a reclaimed land is far from the urban region, noise and vibration created by a driven pile could be accepted mostly. In contrast to a cast-in-place pile, a driven pile meets the advantage of both a lower price and a shorter constructing time; therefore driven piles are usually to be served as a deep foundation for most buildings in Industrial Parks of Taiwan. The precast concrete (PC) pile with closed end is the common-used driven pile in Taiwan, which diameter is usually around 5cm or 6cm; a 1m in diameter was also be used. Due to the workmanship, the length of this pile is usually less than 5m. The load-increment-added method was commonly to be used for most pile load tests in Taiwan, thus the ultimate pile capacity could not be estimated accurately due to there is no evidently peak value can be found on the load-settlement curve. In this situation, if there is a numerical method which could exactly analyze the complete curves among total load, shaft friction, and point bearing versus settlement of a driven PC pile, the ultimate load can be clearly determined, the relations between ultimate load and pile length, pile diameter, and shear strength of soil may be educed accurately. It will highlight some helpful information on both calculation and research of driven PC piles. Hsu [1] had been proposed a continuous strain hardening-softening and volumetric dilatancy model for cohesionless soils. The sand model and a clay model which created by Lin [2] were applied to simulate the ground movement during pile driving, and then to analyze the axially loaded behavior of driven piles. The results by numerical piles are compared with the results by tested piles. After that, parametric studies on factors affecting the behavior of driven piles were performed. STRESS-STRAIN BEHAVIORS OF SOILS In this study, the pile was driven into alluvial soil, which consists of alternating layers of sand and clay. The sand has a D 1 of.12 mm, D of. mm, D 6 of.45 mm, a coefficient of uniformity equal to.92, a coefficient of gradation equal to 1.7, and a 1

2 fines content (< #2) of 6%, it is classified as SP-SM. The specific gravity G s is 2.69; maximum dry density of 15.8 kn/m, and minimum dry density of 1.7 kn/m. The constitutive model recommended by Hsu [1] was adopted for silty sand. The model is established based on the plasticity theory of the non-associated flow rule. The yield function f can be drawn as 1 sin f 1 (1) 1 sin p d ij and stress tensor ij, A plastic potential function g is required to describe the relationship between plastic strain increment and can be expressed as 1 sin g 1 (2) 1 sin The constitutive equations and the related parameters for the sand can be referred to Hsu[1]. The clayed soil has a liquid limit LL of 41, plastic index PI of 18, it is classified as CL. The nature water content n of 42%, of 18.7 kn/m. With regard to the constitutive model of the silty clay, this the specific gravity is 2.74, and the unit weight t study use the model suggested by Lin[2] to establish the stress-strain relations. Since undrained assumption are made for the clay, the yield function f equals to plastic potential function g, and can be represented as follows f g 1 2 S u () Addition information and model details are provided by Lin [2]. GROUND MOVEMENT INDUCED BY A DRIVING PC PILE To understand the suitability of driven PC piles been served as foundations of a HSR (High speed rail) system that lies in western Taiwan, a series of filed tests on driven PC piles was performed in Chiayi-Taipo district. Among the total, Hwang et al. [] exerted the inclinometers to measure the ground movement induced by a driving PC pile. The tested pile is made of precast concrete, with a hollow cross section of outer diameter 8cm and inner diameter 56cm. The pile tip is closed-ended and conical in shape, and was precast with each segment of length 17 m. During construction, two segments were driven in sequence and interconnected by welding to form a total length of 4 m. Measurements could be performed only after the driving was stopped. As show in Fig. 1, the measurements were taken when the PC pile reached the designated depths of 9, 17, 25, and 5 m, respectively. FIGURE 1. Subsoil condition and process of pile driving in the Chiayi-Taipo district (Redrawn from []) NUMERICAL ANALYSIS To establish a complete relationship between load and settlement of a driven pile, the both two constitutive models for silty soils before-mentioned were adopted in this study. In conjunction with the FLAC 2D software, a numerical program was established to investigate the loaded behavior of driven PC piles in silty soils. Assumptions made during the numerical analysis are as follows: 1. Pile body is elastic and homogeneous, the Young s modulus E of 22.8 GPa and poisons ratio of.17 were adopted; 2

3 2. Axial symmetrical condition is applicable for the driven pile;. The ground movements cause by pile driving can be simulated by a pseudo static assumption, which will be mentioned in next section; and 4. Load is uniformly applied on the top of a driven pile. To minimize the boundary effect, a distance of 6D (D is the diameter of a pile) between the pile and the side boundary and a net distance of 25D between the pile base and the bottom boundary are maintained. Rollers were used as the boundary constraint condition. A succession of numerical tests for simulating a PC pile being driven was performed. After the tests, a reasonable procedure is recommended herein: 1. Drawing the numerical mesh, and presetting the pile location (Fig. 2a); 2. Simulating the first driving stage (Fig. 2b): (1)The nodes locate at radial distance of 1/6 D from symmetrical axis which vertical region from ground surface to G.L.-8.6m were fixed their X directions; (2) The nodes reside on G.L. -8.6m which radial bound from symmetrical axis to 1/6 D were fixed their Y directions; () emptying out the space which locate in radial width of ~1/6 D, and vertical length of G.L. ~-8.6m; (4) Applying the radial displacement to 1/2 D on the nodes which their X directions were fixed, this process made the pile radius extend to 1/2D. In the meanwhile, applying the vertical displacement of 1/2 D on the nodes which their Y directions were fixed, this step simulated the pile being driven in ground to the destined depth of 9m.. As shown in Fig. 2c, 2d, and 2e, the procedures for simulating the second, the third, and the final driving stage are similar to the step 2 of simulating the first driving stage. Finally, the pile tip stopped at GL.-4m. FIGURE 2. Schematic diagram of numerical process for simulating pile driven into subsoil To measure the radial displacement which induced by pile being driven, the inclinometer was installed in the ground with radial distance of D (2.4m) from the pile center []. The numerical results are compared with measured results, and shown in Fig.. Dramatically, most of the numerical results are similar to those measured by the inclinometer for each stage. Since there are not too much discrepant between numerical analyses and filed tests, this study exerted the same procedures to simulate the other dimensions of driven piles. After the simulating processes of pile driving, replacing the empty region by a PC pile shaft, displacement increments were then applied on the pile top to study the axial loaded behavior of the driven PC piles in soil.

4 FIGURE. Comparison of measured and modeled radial ground movement at the location of D (2.4m) from pile center VERIFICATION OF NUMERICAL RESULTS To verify the suitability of the proposed model in analyzing the load-displacement behavior of driving PC piles in sandy soil, the calculated results are compared with those of the pile load tests. The dimensions of two tested piles and the subsoil conditions are shown in Fig. 4. According to the site investigation, the subsoil condition was a typically alternating layer of silty sand or silt along Keelung River of Taipei basin. The diameter of the PC piles is 6 cm and length of 27m. Two test piles were loaded following the fast loading test procedure that recommended by the code of ASTM D Both test results and numerical results on two same dimensions of driven PC piles are shown in Fig.4. It shows that the load-settlement curves calculated by numerical analyses were close to those measured by field tests. Yet the applied load near the peak, there are some distinctions on the load-settlement curve between field tests and numerical analyses. The phenomena are due to applying load increment on a tested pile but applying displacement increment on a numerical pile. Although so, the numerical model could consider the strain softening behavior of soils, thus the post-peak behavior of a driven PC pile can be calculated. Q G.L. SM, N=15 6-1m ML, N=16 5-2m -m SM, N=19 SM/ML, N=22 D=.6m Load (kn) 4 2 Nave=17, D=.6 m, L=27 m Symbol : Field tests Line : Numerical result -4m SM, N=18 1-5m Settlement (mm) FIGURE 4. Subsoil condition and location of the in-situ tested pile Comparison of measured and calculated load-settlement curve 4

5 PARAMETRIC STUDY After being calibrated with the field test results, the numerical method was used to investigate the effects of the following parameters on the axially loaded behavior of driven PC piles in silty sand: (1) relative density of silty sand (D r =% and 7%); (2) pile diameter (D=.5m,.6m,.8m, and 1.m); and () pile length (L=1m, 2m, and m). All piles are analyzed by both driving and loading procedures before-mentioned. General Description on Load- Settlement Behavior of a Driven PC Pile Usually, an ambiguous concept is regarding the post peak load-settlement behavior of a pile as same as the peak load-settlement behavior. Indeed, the friction load and end bearing do not reach peak value simultaneously. Generally, the friction load reaches peak value, the end bearing does not develop; when the end bearing develops, the friction load reaches residual state. To emphasis the advantage of this proposed model, the elastic-perfectly plastic model for analyzing a pile suggested by Nicola and Randoph [4] was compared with this research. Fig. 5(a) depicts that the numerical result by the proposed model could highlight the post peak behavior of a pile. Fig. 5(b) is the load-settlement behavior of a PC pile, which diameter D of.5m, length L of 1m, driven into silty sand with relative density D r of %. It can be found from Fig.6, the total load increases with an increase in pile settlement initially, the peak load occurs at settlement of about 4%D. There is a significant drop off in friction load from peak to residual. Contrastively, the point bearing slowly increases all the while, with no peak. This phenomenon results in the total load decreases first and then increases with an increase in settlement during post-peak stage; the load is even greater than the first peak load. To compare with the ultimate capacity of each pile conveniently, the first peak load was defined as the ultimate capacity herein. Fig. 5(b) shows that the point bearing solely contributes 1% of total load when a PC pile stressed to first peak, hence the friction load dominates the ultimate capacity. It also can be seen, the friction load increases with an increase in pile settlement initially, and drops from peak to residual; the residual friction load develops to a steady state with respect to a pile settlement of 6%D. 12 Load Total load Friction Load (kn) D=.5m,L=1m,Dr= % Total load Friction Point bearing Bearing 4 2 Settlement :Presented model :Elastic perfectly plastic model (a) Schema of model affecting on the load-settlement of a pile Settlement (mm) (b) Load-settlement of a driven pile FIGURE 5. Load-settlement curves of driven PC piles Parametric Effects on Ultimate Capacity of a Driven PC Pile To synthesize all parameters which affect pile behaviors, a brief summary of numerical studies on the pile diameter, the pile length and the relative density of sand was made to elucidate how these parameters affecting the ultimate pile capacities. Fig. 6 educes the ultimate pile capacity increases with an increase in pile length/diameter ratio, the larger the diameter, the more incremental the ultimate load. In addition, when the relative density of sand increased, the ultimate pile capacity also increased. Nevertheless, only 2% increase of ultimate pile capacity can be found as relative density changes from % to 7%. To date, dozens of pile-tested results have been accumulated in Taiwan. To compare with the numerical results, this study collected the field test results from Yun-Lin and Taipei Basin which pile settlement is more than 1%D, and adopted the suggestion by Terzaghi [5] to estimate the ultimate pile capacity. The soil properties of two test sites consist of silty sand and silt alternately which SPT-N values ranged from 15 to 22 and relative density D r is around %. Obviously, there are not much distinction the tendency between numerical results and field results not only for D=.5m but also for D=.6m of the driven PC pile (Fig. 7). Therefore, Fig.6 and Fig.7 can be used for a primary design of driven PC piles. CONCLUSIONS A numerical method was developed in this research to study the behaviors of a PC pile driven in soils consist of multiple layers. In-situ pile driven results were utilized to verify the reliability of numerical processes. After the procedure, parametric 5

6 studies were then carried on. Some conclusions can be drawn as following: 1. The radial deformation caused by the pile driven could be simulated through the numerical procedure that proposed by this study. The load-settlement of a pile calculated by numerical analyses are comparable to those measured by in-situ tests. 2. Since the in-situ loading on a PC pile is different from the numerical procedure, the post-peak behaviors estimated by field tests are different from those by numerical analyses. Although so, the proposed numerical method could highlight the post-peak behavior of a pile.. Total load, friction load, and end resistance do not reach peak value simultaneously. Friction load decrease with an increase in settlement during post-peak stage. In contrast to end resistance developing, no peak can be found during the pile stressed. 4. Ultimate load increase with increases in pile diameter, pile length, and relative density of sand. Ulti matepile capacity, Qu(kN) Length / Diameter ratio, L/ D Dr =% D=.5 m D=.6m D=.8m D=1.m Ulti mate pile capacity, Qu(kN) Length / Diameter ratio, L/ D Dr=7% D=.5m D=.6 m D=.8 m D=1. m (a) D r =% (b) D r =7% FIGURE 6. Ultimate pile capacities vs. length/diameter ratio under different pile diameter and relative density of sand 7 8 Ultimate pile capacity, Qu(kN) Filed tests from Yun- Lin Filed tests from Taipei basin Numerical results (Dr=%) Ultimate pile capacity, Qu(kN) Filed tests from Yun - Lin Filed tests from Taipei basin Numerical results (D r=%) Pile length, L (m) Pile length, L(m) (a) D=.5m (b) D=.6m FIGURE 7. Comparison of measured and calculated ultimate load development with pile length under different pile diameters ACKNOWLEDGMENTS The authors would like to thank the Professor Hwang, J. H. in Central University who provided a great deal of information on driving pile tests, Mr. Hung, C. J. and Mr. Chen, C. K. in Da-Chen Construction and Engineering Corp. for the help during the installation and testing of the PC piles and the National Science Council of the Republic of China for providing the financial support for the numerical study of this research (Grant No E-24--MY). REFERENCES 1. S. T. Hsu, A Constitutive Model for Uplift Behavior of Anchors in Cohesionless Soils, Journal of the Chinese Institute of Engineers, Vol. 28, No.2, pp. 5~17. (25) 2. Y. H. Lin, Uplift Behavior of Shaft Tension Anchors in a Layer Soil, MSc thesis, Chaoyang University of Technology, Taichung, Taiwan. (25). J. H. Hwang, N. Liang, and C.H. Chen, Ground Response during Pile Driving, Journal of Geotechnical and Geoenvironmental Engineering, Vol.127, No.11, pp (21) 4. A. D. Nicola, and M. F. Randoph, Tensile and Compressive Shaft Capacity of Pile in Sand, Journal of Geotechnical Engineering, vol. 119, no. 12, pp (199) 5. K. Terzaghi, Discussion on the Progress Report of the Committee on the Bearing Value of Pile Foundations, Proceeding, ASCE, Division of Soil Mechanics and Foundation, Vol. 68, No. 2, pp. 11~2. (1942) 6

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