EFFECT OF PILE LOAD TESTING SETUP ON THE ACCURACY OF THE RESULTS ABSTRACT

Transcription

1 EFFECT OF PILE LOAD TESTING SETUP ON THE ACCURACY OF THE RESULTS HOSSAM E.A. ALI 1 AND AHMED H. ABDEL-RAHMAN 2 ABSTRACT It is a common practice to axially load piles in order to test their load-settlement or timesettlement performance. Cycles of axial loads are applied on top of the tested pile by means of hydraulic jacks against reactions. Three types of testing setups are commonly known in the industry depending on the technique used to set up the reactions. A highly rigid steel plate tied with grouted anchors, steel girders restrained against tension piles, and a platform loaded by concrete blocks or sand bags are the common means to provide proper reactions against the hydraulic jacks. Naturally, these alternative loading setup techniques have unavoidable varying impact on the results. More importantly, for the same loading setup technique, the pile loading results are seriously influenced by the setup arrangement. This paper discusses the effect of each test setup on the accuracy of the results. A back analysis using the finite element method was performed to model the results of an actual pile load test including its testing setup. The same model was utilized to investigate the effect of other testing setups on the measured results. Based on this research study, recommendations for the ideal arrangement of different testing setups were provided. Recommended corrections for the measured settlement when using each loading setup configuration are also presented. Keywords: Pile foundation, pile load test, test setup, axial loading, settlement. 1 Assistant Professor, Structural Engineering Dept., Ain Shams University, Cairo, Egypt. 2 Assistant Professor, Civil Eng. Dept., Engineering Research Division, National Research Center of Egypt.

2 INTRODUCTION All foundation codes request performing axial loading tests on piles to ensure the designed working and ultimate load capacity as well as pile load-settlement curve. Static load tests are the most common and even the default technique of pile testing. Normally, the vertical load tests are carried out by subjecting the pile head to cycles of vertical static loads by means of a hydraulic jack against a reaction system. During loading/unloading the pile, the vertical movement of the pile top is monitored by means of dial gauges. There are three acceptable setups in the foundation codes (e.g. ASTM D1143, 1986; Egyptian Code of Foundation-Part 4, 2001; ACI 543R-74, 1986) to provide reaction against the loading jack as shown in Fig. 1. Fig. 1.a depicts a setup of loading against a rigid plate tied to wire tension anchors grouted at a depth deeper than the pile tip. In Fig. 1.b, tension piles are used as a reaction system, while in Fig. 1.c, a mass of heavy materials, such as concrete blocks, sand bags,..etc, is used to provide adequate source of loading. It has been addressed in the literature (Poulos and Davis, 1980 and NAVFAC DM-72) that the loading process of the reaction system can have a negative influence on the accuracy of the measured movement at the pile top. This is due to stress interference between the reaction system and the tested pile. Because of that fact, some foundation codes specified minimum distances between the tested piles and the reaction systems based on empirical or site observations. Alternatively, Poulos and Davis (1980) proposed a correction factor (F c ) based on theoretical elastic solutions that calibrate the field measured settlement (ä R ) to the true values (ä M ), where F c = δ δ R M In this study, the correction factors will be re-evaluated by a nonlinear finite element analysis performed on the results of a case history. INTERACTION BETWEEN THE TESTED PILE AND THE REACTION SYSTEM The mechanism in which the reaction system affects the settlement of the tested pile differs based on its configuration. The counter-weight system increases the confining pressure around the top part of the pile during the initial stage of loading, which reflects a stiffer pile response during the early stages of loading. That effect decreases with the increase of loading on the tested pile, which in turn reduces the net weight of the reaction system on the ground surrounding the tested pile. On the other hand, the tension piles reaction system tends to move upward during the downward movement of the tested pile, which in turn increases the friction force around the pile and hence causes relatively lesser measured settlement values for the pile than the true values for all load increments. Finally, the grouted anchor system develops, during loading of the tested pile, upward axial forces along the grouted part of the anchors that are normally located below the tested pile tip and arranged in a circle surrounding it. That response might be thought of inducing an upward movement to the soil below the pile tip, which in turn reduces the settlement of the tested pile. (1) 2

3 Due to the possible stress transfer mechanisms between the reaction system and the tested pile, ASTM-D1143 (1986) as well as many other codes provided limitations for the distances between the tested pile and the reaction systems. In this paper, the influence of these distances on the correction factor F c will be numerically addressed by performing a parametric study on a control model analyzed by the finite element method. Reaction Beam Jack Jack Anchor cable Tension Piles (a) Grouted Anchor (b) Loads Jack Tested Pile (c) Figure 1: Different loading setups a) Grouted anchors, b) Tension piles and c) Counter weights THE CASE HISTORY A bored cast-insitu pile of 1080 mm diameter and 22.0 m embedded depth was tested within the course of a quality control program of a bridge project in Giza, Egypt. The design capacity for that pile was 360 ton, while the test was performed up to 720 ton. The test setup was performed according to ASTM-D1143 (1986). The implemented reaction system consists of 8 anchors equally spaced and arranged in a circle around the pile. The loading hydraulic jack acted against a rigid steel plate tied to the anchors. Fig. 2 depicts the configurations and dimensions of the testing setup. 3

4 Jack Tested Pile (D = 1080mm) L= 22.0 m (-22.00) 12 (-32.00) 8 Grouted Anchor (-40.00) Figure 2: Setup of the field pile load test. Site Subsurface Condition The general subsurface soil condition at the location of the test pile is shown in Fig. 3. A top fill layer consisting of fine sand, silty clay pockets, limestone fragments, and plant roots is encountered from the ground surface and extended to a depth of 3.0 m. A medium stiff to stiff silty clay layer appeared after the top fill layer and extended to a depth of 11.0 m. A very dense fine to medium sand layer appeared after the clay layer and extended to the end of the boreholes. Test Results Figure 4 presents the measured load-settlement plot for the tested pile. The elastic line, which presents the axial deformation of the pile shaft itself, is also presented on the same plot. It can be seen that shaft friction resistance was the governing most of the pile capacity up to a load of about 600 ton. Afterwards, the pile settlement increased and exceeded the axial deformation of the pile which indicates control of the pile end bearing of the pile on its settlement behavior. 4

5 P 3. 0 m 8. 0 m Fill Medium stiff, silty CLAY q u = kpa γ b = 19.5 kn/m 3 Ground Surface GWT Very Dense, fine to medium, SAND SPT 50 Figure 3: Soil stratification at the pile load test location 0-4 Settlement (mm) Test Measurements Elastic Deformation of Pile Shaft (= PL/EA) Pile Axial Load (ton) Figure 4: Measured load-settlement curve of the pile load test 5

6 FINITE ELEMENT MODELING OF THE TEST RESULTS The configuration of the test setup shown in Fig. 2, was modeled in a finite element analysis. Fig. 5 presents the finite element mesh implemented in the analysis. The soil geometry was modeled by 6-node isoparametric axisymmetric triangular elements, and its behavior was modeled by the elasto-plastic Mohr-Coulomb model. Due to the geometry of the problem, the axisymmetric model is used to simulate the circular pile shaft as well as the loading scheme around the central axis of the pile, where the deformation and stress states are assumed identical in any radial direction. So doing, the problem can be considered as simplified 3D analysis. During modeling the test setup, similar load steps were imposed on the pile and the anchorage system up to the maximum load reached in the field test (i.e. 720 ton). As can be seen in Figure 6, the final settlement value predicted by the FE analysis at the ultimate load is so close to the value monitored in the field. Yet, for loads less than ultimate load, the FE analysis predicted higher values than recorded. This might be due to the fact that the loading steps in the field are applied in a relatively fast sequence that does not allow for the full undrained deformation to occur. This is especially observable in the first stages of loading when the friction resistance between the pile shaft and the clay layer represents the major part of pile capacity. To study the impact of the anchor system on the load-settlement results, the FE analysis was repeated without including the anchorage system. The results which are shown in Fig. 7 revealed not much change in the resulted load-settlement curve. Only a difference of about 4.0% only (i.e. F c = 1.04) was observed between the predicted settlement with and without the anchorage system. The small difference in the results might be due to the fact that the stress zone around the anchorage system is considerably far from the pile system itself. That in turn, confirms the applicability of the ASTM recommendations in that sense. It should be noted that the load-settlement curve of the no-anchorage case will be considered, herein, as the true load-settlement curve when analyzing the effect of other reaction systems. Moreover, the final value in this curve is used as the true measurement of settlement in this analysis. Therefore the correction factor F c is determined using the apparent final settlement exhibited by other reaction systems along compared to the true measured settlement (12.66 mm). PARAMETRIC STUDY ON THE ANCHORAGE REACTION SYSTEM From the above FE simulation of the field pile load test, it became feasible that utilizing anchorage system could have an effect on the accuracy of the results. Therefore, to study that effect, both spacing and depth ratios were utilized to represent the different anchorage configurations (reaction system). Based on this analysis, it became obvious that the anchorage reaction system yields variable values of correction factors when changing its layout. Plot shown in Fig. 8 presents the effect of the anchorage reaction system dimensions on the deviation of the measured values from the true ones. As can be seen from Fig. 8, the effect of embedded depth of the grouted anchors (H) diminishes when it become well below the pile tip (i.e. H/L >1.0). where L denotes for the pile embedded depth. In other words, the value of F c does not incomparably change for H/L = 1.5 to 2.0. On the other hand, the pattern and values of the correction factor completely differ when the grouted anchors placed above the pile tip (i.e. H/L < 1.0). 6

7 Similarly, the predicted settlement was re-evaluated by another FE analysis conducted for a pile installed in an extended dense sandy layer in order to investigate the effect of the soil profile on F c. Graph shown in Fig. 9 depicts the correction factors that represent the effect of the anchorage system on the deviation of the measured values from the true ones in sandy soils. Unlike the plot in Fig. 8, the maximum error reached its peak when the embedded depth of the grouted anchors located close to the pile tip (i.e. H/L 1.0). Similar to the previous analysis, the deeper the position of the grouted anchors, the less influence on the pile deformation (i.e. less value of F c ). Yet, unlike the previous analysis, the decrease in the values of F c continued even for H/L > 1.5. Accordingly, grouted anchors, in this case, shall be installed deeper than the previous case to maintain the same factor of safety In order to simplify the design procedure, Fig. 10 was produced as a design guide for determining the correction factor for piles installed in an extended uniform sandy soils. PARAMETRIC STUDY ON OTHER REACTION SYSTEMS In order to compare between the grouted anchors systems and other reaction setups, two other FE models were conducted to represent a similar pile load test using tension piles as well as the counter weight reaction system. To match practical conditions, similar piles to the tested one were used as tension piles (i.e. similar length and diameter). For the same reason, spacing/diameter (S/D) ratio was considered as 3. The results of the tension piles case revealed that the correction factor reaches to 1.80 comparing to 1.07 in the case of grouted anchors system. However by investigating the influence of other values of S/D ratio, the F c value dropped to 1.32 when S/D = 6 (not shown on a figure). This findings agrees with the charts proposed by Poulos and Davis (1980) using the elastic theory. On the other hand, in the case of counter weight model, it can be clearly noticed in Fig. 11 that the influence of the counter-weight has much less impact on the load settlement curve. Yet, it is still higher than grouted anchors system. Therefore, extreme care shall be taken when dealing with pile load test conducted using tension piles. 7

8 Flow Field Extreme velocity 0.00 m/day Total discharge 0. 0 m 3 /day/rad Figure 5: FE Model of the pile load test. 0 Axial Deformation (mm) Test Measurements FE Analysis Pile Axial Load (ton) Figure 6: FE simulation Results vs. pile load test measurments 8

9 0 Axial Deformation (mm) Test Measurements FE Analysis (with Anchorage System) FE Analysis (without Anchorage System) Pile Axial Load (ton) Figure 7: Results of FE Simulation with and without anchorage system 1.6 Correction Factor (F c ) H/L = 0.5 H/L = 1.0 H/L = 1.5 H S H/L = 2.0 L S/L Figure 8 Effect of the geometrical configuration of the anchor/pile spacing on the test results 9

10 1.4 Correction Factor (F c ) H/L = 1.5 H/L = 0.5 H/L = 1.0 H S L H/L = S/L Figure 9: Effect of the geometrical configuration of the anchor/pile spacing on the test results CONCLUSION REMARKS This paper discusses the effect of different pile load test setups on the accuracy of the results. A back analysis using the finite element method was performed in this study to model the results of an actual pile load test including its testing setup. The same model was utilized to investigate the effect of the most common two other testing setups on the measurements of settlement. Based on this research study, the settlement correction factor was determined for the different testing setups. As concluding remarks, the following finding were found: 1. The reaction system setup could remarkably affect the results of the load-settlement behavior of piles during testing. 2. Settlement correction factor ranges from 1.0 and could go up to 1.80 when using reaction systems of different configuration around the tested pile. This findings agrees with the charts proposed by Poulos and Davis (1980) using the elastic theory. 3. Errors in the pile load test results may be acceptable in grouted anchors system specially when using H/L over 1.5, while considerable higher errors are usually encountered when using tension piles even with large spacing/diameter ratio. The counter weight reaction system is relatively a primitive and simpler technique and induces more error than grouted anchorage system, yet it provides a substantial reduction in error comparing to the tension-pile system. 4. For the case of reaction system using anchors, detailed plots showing the effect of the configuration of the anchors around the tested pile were given. 10

11 REFERENCES 1. ASTM D (1986). "Method of Testing Piles Under Static Axial Compressive Load,", Annual Book of ASTM Standards, American Society For Testing and Materials, Vol , Philadelphia, PA. 2. Egyptian Code for Soil Mechanics and Foundation (2001). Deep foundation, Part 4, Egypt. 3. ACI 543R-74 (1986). "Recommendations For Design, Manufacture and Installation of Concrete Piles," American Concrete Institute, Detroit, MI. 4. Poulos, H.G. and Davis E. H., (1980). Pile Foundation Analysis and Design, John Wiley and Sons, New York. 5. NAVFAC DM-7.2 (1982). "Foundations and Earth Structures," Department of the Navy., Alexandria, VA, USA. 0 Axial Deformation (mm) FE Analysis Using Anchorage System FE Analysis Using Tension Piles FE Analysis Using Counter Weights Pile Axial Load (ton) Figure 11: Comparison between the FE Simulation Results for the grouted system, tension piles and counter weights as reaction systems 11

12 S/L H/L Figure 10: Reduction Factor for end bearing pile with grouted anchors in uniform sand layer 12