Proceedings of Pile 2013, June 2-4 th 2013 THE STUDY OF t-z AND q-z CURVES ON BORED PILE BASED ON THE RESULTS OF INSTRUMENTED PILE LOAD TEST IN MEDIUM AND STIFF CLAYS Aswin Lim 1, Aksan Kwanda 2 and Paulus P. Rahardjo 3. 1 Faculty member, Parahyangan Catholic University, Jl. Ciumbuleuit No. 94 Bandung, INDONESIA 2 Graduate Student, Parahyangan Catholic University, Jl. Ciumbuleuit No. 94 Bandung, INDONESIA 3 Professor, Parahyangan Catholic University, Jl. Ciumbuleuit No. 94 Bandung, INDONESIA ABSTRACT : This paper presents the result of t-z and q-z curves based on instrumented bored pile which is constructed in layered soils consist of medium and stiff clay. There are fourteen vibrating wire strain gauges (VWSGs) installed at seven various level of bored pile and two tell-tale extensometers are installed inside the pile located at pile head and pile tip. Based on the axial loading test (ASTM D1143-07), the bored pile is tested until 250% of working load that is 1325 ton. Then the t-z and q-z curves are developed based on the data that are recorded from VWSGs and tell-tale extensometer. Two variation of pile elastic modulus are used, that are fc = 25 MPa (original modulus) and fc = 42 MPa (calibrated modulus), in order to fit the actual load settlement curve from axial load test. The result shows that calibrated modulus gives nearly similar with actual load settlement curve instead of original modulus. Although the load-settlement curve could be fitted with pile elastic modulus calibration, it has slight different with t-z curve for clay soil that proposed by Reese and O Neill,1988. Keywords : t-z curve, q-z curve, instrumented bored pile, medium and stiff clay, axial load test. INTRODUCTION The subgrade reaction, or t-z, method suggested by Seed and Reese (1957) is an expedient means of computing the axial movement of a pile under axial load. Procedures are available to generate the relationships between shear stress at the pile shaft ( load transfer, t ) and pile displacement, z, along the pile shaft. The most commonly used procedures, however, are empirical and based on data from tests on short piles, usually less than 30 meter long, with diameters less than 0,5 meter. Pile diameter, axial pile stiffness, pile length, and distribution of soil strength and stiffness along the pile are all factors that influence t-z behavior. The success in developing realistic t-z relationships for a pile depends on the accuracy of the ultimate load-transfer values of the soil (pile capacity), the distribution of those values along the pile, and the displacement characteristics of soil during load transfer ( Kraft et al, 1981). Finite difference equations are employed to achieve compatibility between pile displacement and the load transfer along a pile and between displacement and resistance at the tip of the pile. This method was first used by Seed and Reese (1957); other studies have been reported by Coyle and Reese (1966), Coyle and Sulaiman (1967), and Kraft et al (1981). The t-z difference method assumes the Winkler concept; that is, the load transfer at a certain pile section and the pile tip resistance are independent of the pile displacement elsewhere. The close agreement between the prediction and the loading test results in clays (Coyle and Reese, 1966) and the scattering of prediction values for the loading test in sands (Coyle and Sulaiman, 1967) may possibly be explained by the relative sensitivity of a soil to changes in patterns of stress (Reese et al, 2006). In this paper, the difference method is accommodated by t- z program which is developed by Rahardjo et al (1993) based on Reese s Method. SOILS CONDITION AND PILE INSTRUMENTATION DATA The soils stratification is classified based on pile boring record. The soil condition is dominated by cohesive soils ( medium and stiff clays). They start from elevation +8.00 until +3.00, then followed by cemented sand on elevation +3.00 until +6.50, and cohesive soils again until elevation -39.00. The length of the bored pile is 47.00 meter with 1.2 m diameter. Table 1. Soil stratification and pile sub section length Segment Elevation Pile Length Soil Type Depth (m) (m) A +8,00 - +3,00 5 Medium Clay B +3,00 - -6,50 9.5 Cemented Sand C1-6,50 - -13,00 6.5 Medium Clay C2-13,00 - -20,00 7 Medium Clay D -20,00 - -24,00 4 Stiff Clay E -24,00 - -29,00 5 Medium Clay F -29,00 - -31,00 2 Stiff Clay G -31,00 - -36,00 5 Medium Clay H -36,00 - -39,00 3 Stiff Clay Table 1 shows the soil stratification and pile sub section length. There are fourteen vibrating wire strain gauges (VWSGs) installed at seven various level of bored pile, which at elevation =6.50, -5.10, -13.10, - 20.10, -26.10, -32.10, and -37.60, respectively on both sides of pile, and two tell-tale extensometers are installed
inside the pile located at pile head (elevation +6.10)and pile tip (elevation -38.80). Figure 1 shows the bored pile with VWSGs diagram inside the soil to make clear understanding of the pile instrumentation location. The tell-tale rod extensometer assembly consisted of 9.5 mm diameter stainless stell rod, attached to a fixed anchor point in the pile, and placed within a protective 25 mm PVC pipe. The entire assembly is cast in the pile. As the pile compressed under specified test load, the steel rod remained free in the protective PVC pipe, which undergo compression with the concrete pile. Four dial gauges with accuracy 0.01 mm are used to measure the downward movement of the steel rod, relative to reference beam. The bored pile is design with concrete strength, fc, equal to 25 MPa and the reinforcement is 12D25. The measured of concrete cylinder strength is about 275 kg/cm 2 in day seventh. The bored pile is concreted by tremie method in order to make sure that the mud at the tip of bore hole will be pushed outside by the concrete. Thus, the bored pile waiting period is around 2 months. In this condition, the concrete strength has reached ultimate strength because the concreting period has passed 28 days. The axial load was applied using two hydraulic jacks of 1000 tons capacity on the pile head. Kentledge concrete blocks, primary and secondary beams were used for reaction system. Four dial gauges and survey instrument were used to measure the settlement at the pile top. The bored pile was designed for a working load (WL) of 530 ton and the loading schedule was conducted in five cycles. The maximum applied load for each cycle was 265 ton ( 50% WL), 530 ton (100%WL), 795 ton (150%WL), 1060 (200%WL), and 1325 ton (250%WL). In this paper, the fifth cycle data were used for analysis of t-z and q-z curves since the data have completed. Table 2 shows the load settlement data from axial loading at fifth cycle at pile head and pile tip recorded from tell-tale and Table 3 shows the average data recorded from VWSGs at each level. Table 2. The load settlement data from axial loading at fifth cycle Load (ton) Settlement (mm) Pile head Pile Tip 0 265 2.79 2.28 530 3.92 2.28 795 5.27 4.67 1060 6.85 4.73 1193 8.29 5.55 1325 20.35 12.08 Table 3. The average data recorded by VWSGs at each levels. Applied Level Load A B C D E F G (ton) 6.5-5.1-13.1-20.1-26.1-32.1-37.6 Fig 1. Bored pile section PILE AXIAL LOADING TEST The bored pile was installed on October 31, 2012, and the axial loading test was conducted on December 5 8, 2012 according to ASTM D1143-07. 0 0 0 0 0 0 0 0 265 69 50 45 14 9 5 2 530 135 96 88 38 25 15 2 795 204 145 142 72 44 25 6 1060 283 215 202 122 74 37 9 1192.5 322 256 239 156 94 56 16 1325 385 322 296 201 140 76 25 1060 376 239 167 93 58 38 11 795 305 225 160 87 55 35 35 530 126 81 24-13 -13-11 -17 265 71 35 19 4 1 1-7 0 17-11 13 20 15 12 4
DEVELOPMENT OF t-z CURVE AND q-z CURVE The development of t-z curve is divided to two major, that are, calculating t and calculating z. The principal of calculating t is adopted from Hooke s Law where stress is strain multiply with modulus. Then, when the stress divided with cross section area of pile, the axial force will be obtained. Finally, the t could be computed from the axial force divided with pile section peripheral area. Figure 2 shows the load transfer for each loadings along the pile (fc = 25 MPa). Moreover, the principal for calculating z is by subtracting the shortening of the pile from observed settlement. Here, the shortening of the pile is calculated from tell-tale data. segment C1. These two sections actually are stick together, however from this result, the soil shear strength of segment C2 should be larger than segment C1. The zm, maximum movement to fully mobilized shear strength, is around 4.0 6.7 mm. With the same procedure as above, the stiff clay sections, namely section D (4 meter), section F (2 meter), and section H (3 meter) t-z curves is shown in Figure 4. Fig. 4 The t-z curve for stiff clay with fc =25 MPa Fig. 2 The Load Transfer For Each Loadings Along The Pile (fc =25 Mpa) The original elastic modulus of the bored pile is 235000 kg/cm 2. The medium clay soil stratification is divided into 5 segments, namely segment A, segment C1, segment C2, segment E, and segment G, with length of each segment is 5 meter, 6.5 meter, 7 meter, 5 meter, and 5 meter, respectively. The t-z curve for medium clay with fc =25 MPa is shown in Figure 3. The largest t is at Section D, while the lowest t is at section H and the z m is around 3.5 to 6.0 mm. When the normalized t-z curves are plotted into Reese and O Neill s normalized t-z curves in clay, the results show that most of the curves, medium as well as stiff clays, laying below the lower boundary as shown in Figure 5. Fig. 5 The normalized t-z curves comparison (fc =25 MPa) Fig. 3 The t-z curve for medium clay with fc =25 MPa The dash lines represent the calculated t-z curve, while the solid lines represent the regression t-z curve into linear elastic fully plastic behavior (as input in t-z program). The computed t-z curves are relatively like hyperbolic function. From Figure 3, it shows that the highest t is on segment C2, while the lowest is on Finally, the load settlement curve prediction is calculated from the t-z curves, however, the load settlement curve is smaller than the actual ones. It seems that the pile elastic modulus is too small, beside Figure 2 also shows that the maximum axial load for final load is around 1000 ton. In order to get the closer result of load settlement curve, the trial-error method is used with changing the pile elastic modulus. The best fitted result of pile elastic modulus is 304600 kg/cm 2 (fc =42 MPa). Figure 6 shows the load transfer for each loadings along
the pile (fc = 42 MPa) with the maximum axial load at final loading is around 1325 ton. Figure 7 and Figure 8 also show the t-z curves for medium clay and stiff clay soils with the same trend line with Figure 3 and Figure 4, except that the t values are larger than previous one. normalized t-z curves laying inside the range of results, while the stiff clay normalized t-z curves remain the same as before. For the q-z curves, the q value is obtained from the axial load at pile tip divided with pile cross section area, and the z is obtained from the pile tip movement (tell-tale data). ). Figure 10 shows the q-z curves at pile tip. The straight line is for pile elastic modulus equal to 235000 kg/cm 2, and the dashed line reflects pile elastic modulus equal to 304600 kg/cm 2. The result is consistent with t-z curves where the larger axial load gives larger q values. Fig. 6 The Load Transfer For Each Loadings Along The Pile (fc =42 Mpa) Fig. 9 The normalized t-z curves comparison (fc =42MPa) Fig. 7 The t-z curve for medium clay with fc = 42 MPa Fig. 10 The q-z curves for stiff clay Finally, all of the load settlement results are summarized in Table 4 and Figure 11. The load settlement prediction result obtained from pile concrete strength equal to 42 MPa is nearly close to actual curve. Fig. 8 The t-z curve for stiff clay with fc = 42 MPa The z m values have slight different, where medium clay is around 4.2 6.5 mm, and stiff clay is around 3.7 6.0 mm. In addition, regarding with the comparison with Reese and O Neill s normalized t-z curves as shown in Figure 9, the medium clay Table 4. The load-settlement data comparison between field data and t-z prediction Field data fc' = 25 Mpa fc' = 42 Mpa Axial Load Settlement Axial Load Settlement Axial Load Settlement (ton) (cm) (ton) (cm) (ton) (cm) 0 0 0 0 0 0 265 2.79 783.929 6.3 1061.234 7.56
530 3.92 892.68 8.91 1182.33 10.19 795 5.27 893.338 8.94 1183.102 10.23 1060 6.85 908.129 9.8 1198.601 11.16 1192.5 8.29 1011.444 17.28 1301.916 18.57 1325 20.35 1059.128 20.7 1349.599 21.99 DISCUSSION Fig 11. Load Settlement curves The main purpose of this study obtains the t-z curve and q-z curve of medium and stiff clays based on instrumented bored pile data. Several things could be noted likes the calibration of pile elastic modulus, the shape of calculated t-z curves, and the maximum movement to mobilized maximum shear strength. The calibration of pile elastic modulus is done in order to get the closest t-z load settlement prediction curve compare to actual curve. The pile elastic modulus is become larger than original, it might be caused by the time of axial load testing. From pile recording data, the concrete strength at seventh days has reach design value, which is fc =25 MPa. The axial load testing is done after one month of concreting, of course the concrete strength will reach larger than original or design value because the strength of concrete increases by time. Regarding to the shape of calculated t-z curves, some of the curves do not reflect the ideal curve that is hyperbolic shape. It might be due to the instrumentation data are not recorded excellently. However, the error could be tolerated since the results of t-z curve could predict an excellent loadsettlement curve. For maximum movement to fully mobilize the shear strength, the results shows that fully agree with Reese and O Neill, which is around 2.4 7.2 mm for 1.2 meter pile diameter. 3. The movement to fully mobilize the shear strength, z m, concurs with Reese and O Neill previous study results which is around 0.2 0.6 % ratio of settlement to diameter of shaft. REFERENCES Coyle, H.M., and Reese, L.C. (1966). Load Transfer For Axially Loaded Piles In Clay. Journal of the Soil Mechanics and Foundation Division. ASCE. 92 : 1 26. Coyle, H.M., and Sulaiman, I.H. (1967). Skin Friction For Steel Piles In Sand. Journal of the Soil Mechanics and Foundation Division. ASCE. 93 : 261 278. Kraft, L.M., Ray, R.P. and Kagawa, T. (1981). Theoretical of t-z Curves. J. Geotech. Engrg. ASCE. 107(11):1543-1561. Rahardjo, P.P., Cosmas, R., and Rosnawati, I. (1992). TZ Program Komputer Untuk Analisis Pengalihan Beban Pada Pondasi Tiang Yang Dibebani Aksial. Parahyangan Catholic University. Geotechnical Engineering Centre. Reese, L.C., and M.W. O Neill. (1988). Field Load Test of Drilled Shaft.."Proceedings, Deep Foundations on Bored and Auger Piles, Ed. WF Van Impe, Balkema, Rotterdam: 145-192. Reese, L.C., Isenhower, W.M., Wang, S.T., (2006). Analysis of Design of Shallow and Deep Foundations. John Wiley and Sons. Seed, H.B., and Reese, L.C. (1957). The Action Of Soft Clay Along Friction Piles. Transactions. ASCE. 122 : 731 754. CONCLUSIONS Based on this study, several conclusions can be drawn, such as : 1. The load settlement prediction curve is successfully simulated with calculated t-z curves, though the pile elastic modulus should be calibrated. 2. The normalized t-z curves for medium clay is located inside Reese and O Neill range of result while stiff clay is located below the lower boundary.