Prediction of Ultimate Pile load from Axial load Tests and Penetration Tests

Transcription

1 Prediction of Ultimate Pile load from Axial load Tests and Penetration Tests Gehan E. Abdelrahman: Lecturerof Soil Mechanics and Foundations, Faculty of Engineering, Cairo University-Fayoum Branch. Abstract Pile load testing is the most acceptable method to determine the ultimate load of a pile. This paper presents a comparison between prediction methods to estimate the ultimate capacity load of an axially loaded pile. This study compared computed theoretical load capacity, using four different published techniques, and two semi-empirical methods with the results from forty four field load test database of driven and bored piles, constructed in different Egyptian soils. Introduction For pile foundation projects, it is usual to confirm capacity and verify that the behavior of the piles agrees with the assumption of the design. Frequently, this is achieved by means of performing a static-loading test, and normally, determining the capacity is the primary purpose of the test. The analysis and design of the load bearing capacity and settlement of an axially loaded pile are affected by several complex factors and, therefore, it is usually resorted to some assumptions and/or empirical approaches. In particular this is the case with regards to soil-pile structure interaction, and the distribution of soil resistance (skin friction) with depth. The methods available to predict the ultimate load of a single pile are: theory of bearing capacity for the soil foundation, pile load tests, empirical methods based on dynamic and driving formulas, and semi-empirical methods using in situ test results. Results of forty four axial static loading tests on driven and bored piles were analyzed, based on insitu tests data, standard penetration test, SPT, and cone penetration test, CPT, with comparison to empirical methods, Brinch-Hansen (1963), Chin-Kondner (1971), Chin (1980), and Decourt (1999). 1

2 Pile Load Tests The loading test generally followed the Egyptian code of deep foundation (part 4, 1995) procedures, reaching a maximum load of about one and half to two times the design load. Each increment is maintained until the rate of settlement measured at the pile head becomes equal to or less than 0.15 mm/hour or maintained a specified time according to Table (1) whichever occurs first. The pile is unloaded at a rate of 15 minutes each, the pile is left free without any loads for 4 hours. Table (1) Load increment-time LOAD INCREMENT, % Times, hrs Soil boring was obtained for each test site. Average SPT, N-values were measured along the length of the tested piles. Static cone penetration tests, CPT, carried out in the present investigation were adjacent to the pile load test. The results of those, in situ testing techniques, were used to predict the carrying load capacity of this pile. Interpretation of Test Results The Canadian Foundation Engineering lists the criteria for interpreting test loading results, which can be divided into two groups: 1. Criteria governing the acceptance of the tested pile with no consideration to the failure load. In most cases, a pile is deemed acceptable if the observed settlement of the pile head is within specified limits. These criteria overestimate or underestimate the pile capacity. 2. Criteria defining the failure load of the tested pile, from which the allowable load may be computed by applying the adequate factor of safety. These criteria have been used in this study because they provide a better understanding of pile capacity and behavior. Choice of Evaluation Methods In most common cases, the load test curves do not have well-defined failure loads. A large number of different failure criteria have been proposed for determining the ultimate capacity of a deep foundation element. In fact, 2

3 in most cases, a true ultimate capacity does not exist. Field test data are often available in the form of static or dynamic penetration resistance. It is easier to generate predictions of axial capacity directly from penetration resistance, rather than from more fundamental soil shear parameters. The aim of this study is to compare the two semi-empirical methods, using SPT results and CPT results with load capacity, using four different published techniques, and with the results from forty four field load test database of driven and bored piles, constructed in different Egyptian soils. 1-Standard Penetration Test Meyerhof (1976) had correlated shaft and base resistance of a pile with the results of a standard penetration test for displacement piles in saturated sand. Bromham and Styles, (1971), used the same correlation in stiff clays. The ultimate pile load, P u, is given by: P u = m N p A b + n Ñ A s / 50 (1) Where: N p = standard penetration number, N, at pile base Ñ = average value of N along pile shaft A b = net sectional area of pile toe (ft 2 ) A s = gross surface area of shaft (ft 2 ) m = equal 4 for driven piles and 1.2 for bored piles. n = equal 2 for driven piles and 1 for bored piles. In the above equation the recommended upper limit of the unit shaft resistance (Ñ /50) is 1 ton/ft 2 The Canadian manual, (1992), suggested factor of safety not less than 4 with Meyerhof equation for the reason of multitude of errors. The analysis in this study was according to the Egyptian code (1995) which used equation (1), with m value equal 2.25 and n value equal 1 for driven piles. For bored piles it recommends a reduction coefficient 0.5 to 1 the value of equation (1). In this study for board pile, a factor of reduction was chosen to be 0.7. Table (2) shows the ultimate pile base resistance, the ultimate pile shaft resistance, and the ultimate pile capacity, also shows the average percentage between SPT results and the working design load of the piles tested. 3

4 2-Cone Penetrometer Test The capacity of deep foundation in soils can be computed from the results of a static cone penetration test, CPT. The test is suitable to a large range of soils provided adequate pushing force is available for sufficient depth of penetration. The basis of the test is the measurement of the resistance to penetration of a 60 cone with a base area of 10 cm 2. The static cone has friction jacket point, of a length of 40 mm and 35.7 mm of diameter which allows both point resistance and local skin resistance to be measured (Begemann, 1965). The test data were obtained for every 200 mm depth interval. The ultimate cone resistance, q c and the ultimate sleeve friction, f s can be estimated using the following basic equation: P u = α q c A b + k f s A s (2) q c = measured cone-point resistance at base. f s = average shaft friction along pile length, as measured on the friction jacket. α = bearing capacity factor. k = friction coefficient. The Egyptian code of deep foundation (part 4, 1995) procedures suggested that the α value which depends on the ratio of pile diameter over cone diameter, is equal to 0.7, while the k value equals to one. The Egyptian code recommended that the average shaft friction along pile length f s is limited 1t/ft 2. In case of bored piles the previous equation should be multiplied by a factor from 0.5 to1. Van der veen (1957) suggested that the α factor is equal to one while the k factor is equals 2. The ultimate resistance of the pile point, of diameter d b, could be derived from the corresponding conepenetration curve by taking the average cone resistance over a distance d b below the pile point and 3.75 d b above the point. Bustamante and Gianeselli, (1982), Robertson et al.(1988), and Briand and Tucker, (1988) recommended that, the scaling effect and the method of installation are accounted for by the selection of α and k coefficient. The bearing coefficient α and the friction coefficient k, have values based on soil type and pile type, (Canadian Foundation Engineering manual, (1992)). Meyerhof (1956) suggested that for driven concrete piles, the ultimate skin friction f s could be estimated from the cone point resistance q c as follows: 4

5 f s = q c ( 3 ) The ultimate load capacity P u from CPT test was calculated according to the Egyptian code (1995). A reduction factor was chosen to be 0.7, for bored piles as an average value as considered in SPT analysis. Table (2) presents piles base resistance, and piles shaft resistance from CPT results. It is compared to the pile working load. A comparison was made between SPT and CPT tests results. 3-Brinch-Hansen Failure Criterion Brinch-Hansen, (1961, 1963) assumed that the shape of the pile load-movement curve is such that, when the movements are plotted against the square root of the movement divided by the corresponding load, the data plot in a straight line having a slope, C 1, and a y-intercept, C 2 as shown in Figure (1). The ultimate pile load Q u, corresponds to the ultimate movement S u, if the load 0.8 Q u gave the movement 0.25 S u, or if the load 0.9 Q u gave the movement 0.5 S u. The ultimate pile load Q u, and the ultimate movement S u, are calculated from following Equation: Q u = 1 / (2 C 1 C 2 ) (4) S u = C 2 / C 1 (5) Brinch-Hansen considered that the load-movement curve is a parabolic curve which can be calculated by using following equation: Q = S / (C 1 S + C 2 ) (6) Pile No. 25 Pile No "Measured" 150 Root sett / Load Linear ("Measured" y = x R 2 = Load Q ton Calculated Measured settlement mm Settlement mm Figure (1) Linear regression for B. Hansen method. Figurer (2)Ultimate pile load using B. Hansen method. Brinch-Hansen criterion has the advantage that it agrees well with the true failure value if the point 0.8Q u /0.25 S u 5

6 or 0.9Q u /0.5 S u plots on the test curve as shown in Figure (2). Fellenius (2001) recommended the Brinch-Hansen 80%-criterion method, which usually gives a good prediction to the true ultimate resistance, determined from the results of the static loading test. 4-Chin-Kondner Extrapolation Chin (1970; 1971) proposed an application to piles of the general work by Kondner, (1963). In this method each movement is divided by its corresponding load and the resulting value plotted against the movement. As shown in Figures (3 and 4), after some initial scatter, the plotted values will fall on straight line. The inverse slope of this line is the Chin-Kondner extrapolation of the ultimate load. Also the Chin-Kondner criterion can be used to determine the load-movement curve considering the a hyperbolic formula as follows: Q = S / [C 1 S + C 2 ] (7) Where: C 1 is the slope of the straight line, C 2 is the y-intercept of the straight line settle/q Pile No settle. mm "Measured" Linear ("Measured") y = 0.004x R 2 = Q ton pile No Settle. mm Series1 Series2 Figure (3) Linear regression for Chin-Kondner method Figure (4)Ultimate pile load using Chin-Kondner method 5-Modified Chin Failure Criterion The Chin criterion uses a plotting such that each movement is divided by its corresponding load S/Q and the resulting value plotted against the movement S. As shown in Figure (3), after some initial scatter, the plotted values will fall on a straight line when failure is approached. The inverse slope of the line defined as the ultimate pile load. The Egyptian code of deep foundation (part 4, 1995) suggested that the ultimate pile load 6

7 is the inverse slope C 1 multiplied by factor 1.2 as follows: Q u = 1 / (1.2 C 1 ) (8) Where: C 1 is the slope of the straight line. Chin method has the advantage that it can be used to indicate potential damage to the pile by sudden changes (i.e., curves or kinks) in the line. But the Chin criterion always results in a failure load that is greater than the maximum test load applied. 6-Decourt Extrapolation Decourt (1999) proposes a method with a construction similar to that used in Chin-Kondner. Decourt divided each load by its corresponding movement and plotted the resulting value against the applied load. Figure (5) shows that the curve tends to a line that intersects with the abscissa. A linear regression over the apparent line determines the line. As shown in Figure (6), similar to Chin-Kondner method, the load-movement curve can be calculated and compared to the actual load- movement curve of the test as follows: Q = C 2 S / [1- C 1 S] (9) The Decourt extrapolation load limit is equal to the ratio between the y intercept and the slope of the line as follows: Q u = C 2 / C 1 (10) Where: C 1 is the slope of the straight line, C 2 is the y intercept of the straight line, Q u is the ultimate load, Q is the applied load, and S is pile movement. Q/Sett Pile No Q ton "Measured" Linear ("Measured" y = x R 2 = Figure (5) Linear regression for Decourt method Load Q ton Pile No Settlement mm "Calculated" "Measured" Figure (6)Ultimate pile load using Decourt method 7

8 Data Base Evaluation Results The problem in all of the static load testing is to continue loading to failure. A distressing percentage of all pile load tests is performed to only one and half or twice the design load. If the failure does not occur, the test is deemed a success and production pile installation begins. In order to predict ultimate pile load, the results of 44 piles load tests, (21bored piles and 23 driven piles) were compared with the results calculated using six different published methods. It should be mentioned that, in general terms, the majority of the semi-empirical methods takes into account the following factors in the evaluation, the skin friction of pile, f s, the pile tip resistance, q c, soil type, method of pile installation, pile material, scale and depth effects, (Tanaka, (1993)). The SPT results and CPT were analyzed according to the Egyptian code. As shown in Table (2 and 3), the following results were observed: The measured skin friction of piles in CPT is twice the value predicted by the SPT for both bored and driven piles. The pile tip resistance measured by the CPT is bigger than that predicted by SPT by a factor of 1.3 in bored piles and the same in driven piles. From this study, reasonable assessments have been found for the load bearing capacity of pile in spite of that the last four methods allow the later part of the load-movement be continued beyond the maximum load applied, extrapolating the curve. From Tables (4 and 5) following results were obtained: The Brinch Hansen 80% method gives ultimate pile capacities very similar to those calculated using the Decourt method. Factor of safety of 2.5 is suitable for calculating working load using those methods. The Chin-Kondner extrapolation and modified Chin method gave similar results and tend to overestimate the ultimate pile capacity. Suggested Factor of safety is 3.5 for both methods for calculating working load. Conclusion The above study recommended the following conclusion: 1. All the methods have been shown to perform well. Decourt and B.Hansen methods appear reasonable and give a good estimation for ultimate pile capacity. 8

9 2. The ultimate pile load calculated using CPT is 1.8 times the ultimate pile load calculated by using Decourt method in both bored and driven piles. 3. The ultimate pile load calculated using SPT is the same as Decourt method in bored piles, while in driven piles it is bigger than that calculated using Decourt method by 1.7 as shown in Table (2). 4. SPT and CPT are the most reliable tests for calculating the working pile load with the factor of safety equal to 3.5 for SPT results and 4.5 for the CPT result. 5. The available extrapolation methods which are used for predicting working pile load, give a reasonable estimation with factor of safety equal 2.5 for Decourt and B.Hansen methods, and 3.5 for Chin-Kondner and Modified Chin methods. 6. All load tests should be carried to the highest load possible. Acknowledgment The author gratefully acknowledges the Arab Company for Foundation, VIBRO, especially Eng. Magada M. Shawky, for made the data available for this study. Results of a number of vertical pile load tests, together with soil reports were gratefully presented to the author as a part of the data base. References Begemann, H. K. S The Maximum Pulling Force on a Single Tension Pile Calculated on the Basis of Results of the Adhesion Jacket Cone Proc. 6 th Int. Conf. S.M& F.E.,Vol.2, pp Brinch-Hansen 1961, The Ultimate Resistance of Rigid Piles Against Transversal Forces, Danish Geotechnique Institute Brinch-Hansen 1963, Discussion on Hyperbolic Stress-Strain Response, American Society of Civil Engineering, ASCE, Journal for Soil Mechanics and Foundation Engineering, Vol.97, SM6, pp Bromham, S.B. & Styles, J. R. 1971, An Analysis of Pile Loading Tests in Stiff clay. Proc. 1 st Aus.-N.Z. Conf. Geomechs. Melbourne, Vol. 1, pp Canadian Geotechnical Society, 1992 "Canadian Foundation Engineering Manual. Chin, F.K Estimation of the Ultimate Load of Piles Not Carried to Failure, Proc.2 nd Southeast Asia. Conference on soil Engineering, pp Chin, F.K Discussion on Pile Test Arkansas River Project. American Society of Civil Engineering, ASCE, Journal for Soil Mechanics and Foundation Engineering, Vol.97, SM6, pp Decourt, L., 1999, Behavior of Foundations Under Working Load Conditions Proceedings of the 11 th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, Foz Dolguassu, Brazil, Vol.4, pp Egyptian Code of Soil Mechanics and Foundation Engineering, (1995), Part 4. Fellenius, B.H What Capacity Value to Choose from the Results of a Static Loading Test, Deep Foundation Institute, New Jersey, 9

10 Kondnr R.L. 1963, Hyperbolic Stress-Strain Response, in Cohesive Soils, American Society of Civil Engineering, ASCE, Journal for Soil Mechanics and Foundation Engineering, Vol.89, SM1, pp Poulos, G.H. and Davis, E.H. 1980, Pile Foundation Analysis and Design John Wiley & Sons, New York, USA. Tanaka, A. 1994, Prediction and Performance of Pile Using In Situ Test Data. Proceedings, International Conference on Design and Construction of Deep Foundation, Orlando, FL, USA, Vol. 3, pp Van der veen, C. & Boersma, L The Bearing Capacity of a Pile Predetermined by a Cone Penetration Test. Proc. 4 th Int. Conf. S.M. & F.E., Vol.2, pp

11 Table (2) Typical results of SPT and CPT test. Bored Piles SPT Q u CPT Q u Test W.L T.L code qc+0.7fs No. ton ton bearing friction sum bearing friction sum SPT Q u CPT Q u Driven Piles qc+fs W.L T.L bearing friction sum bearing friction sum Working load Test load 11

12 Table (3) Comparison between results of SPT and CPT tests and Decourt method. Bored Piles Test W.L T.L CPT/SPT CPT/SPT CPT/SPT Decourt SPT/Decourt CPT/Decourt No. ton ton friction 0.5Cp/0.7Sp SUM ton Average Driven piles CPT/SPT CPT/SPT CPT/SPT Decourt SPT/Decourt CPT/Decourt Qu friction 0.7Cp/Sp SUM ton W.L Working load Average T.L Test load Aver. All Table (4) Typical results different static methods 12

13 Bored Piles Pile Test W.L T.L F.O.S B.Hanse n Decourt Chin-kon M.Chin No. ton ton ton ton ton ton Driven Piles Table (5) Typical results different static methods compared to working pile load. 13

14 BoredPiles Chin.K/W M.Chin/W. Decourt/Hanse Decourt/M.Chi Decourt/Chin. B.Hansen/WL Decourt/WL Pile Test L L n n K Average Driven Average Aver..All

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