Analysis of Axial Pile Load Test(s) on Large Bored Grouted and Instrumented Piles M. Shahien Structural Engineering Department, Tanta University, Egypt mshahien@hamzaconsult.org H. M. El-Naggar Housing and Building National Research Centre (HBRC), Egypt, hayelelnaggar@gmail.com ABSTRACT Large capacity piles are important type of foundation elements that are needed in case of high column loads such as in tall buildings with large span between columns. In many projects, especially if the designer uses one pile supporting one columnconcept (Hamza, 3). The axial capacity of the large bored piles can be further increased by grouting the shaft skin and the base of the pile. The increase in the capacity of the piles due to grouting can be tremendous and can reach to as high as almost twice the capacity of non-grouted piles. A data base of pile load tests of instrumented grouted piles is under development in a major study aiming to propose a simple method to estimate the axial capacity of large bored grouted piles. The data base include pile load tests from projects constructed in Egypt and abroad. The main goal of this paper is to analyze pile load test(s) from one project. The instrumentation of the tested piles allowed for the separation of skin friction base contribution to the axial load. Further, the contribution of the skin friction of every layer along the pile shaft could be identified. The results of the analysis clarified how base and shaft grouting increased the axial capacity of the pile and decreased the settlements for certain load level compared to conventional un-grouted piles installed under similar conditions. KEYWORDS: Pile Load Tests, Instrumented Piles, Grouted Piles, Large Bored, Axial Capacity 1. INTRODUCTION In recent years, significant research has been carried out to develop new techniques which are suitable to withstand problems in the major project such as high applied loads in high rise building projects. Base and shaft grouting of piles are one of these new techniques which aim at decreasing the settlements and increasing the capacity of the pile under axial load. Base grouting consists of injection grout under pressure at the base of the pile after the concrete is sufficiently hard. Suckling and Eager (1) made a comparison between base-grouted and non-basegrouted bored piles bearing in Thanet sand. They showed that the non-base-grouted bored pile bases have ultimate en-bearing capacity ranged from 1, kn/m to 17, kn/m and 17, to 1, kn/m for base- grouted pile. On the other hand, shaft-grouting causes increase in the soil density in the zone which had been disturbed by the pile construction and thereby improves the shaft resistance of the pile. Based field measurement, Littlechild et al () found that the shaft resistance of grouted pile were approximately double that of non-grouted piles.
Depth (m) 9. m International Conference on Advances in Structural and Geotechnical Engineering (ICASGE 15( Depth (m) Depth (m) This paper is a part of a major study that aims to the development of a simple method to estimate the axial capacity of large bored grouted piles. The data base includes pile load tests from projects constructed in Egypt and abroad. This paper presents and discusses the results of pile load test of a base and shaft grouted bored pile that was carried out for a project in Egypt. The test pile was instrumented. The instrumentation of the pile allowed the collection of the base resistance and shaft resistance of all the layers along the shaft of the pile. The study focuses on calculating the resistance from different static methods, including the Egyptian Code (1) method, the Bustamente (), the AASHTO (7) and the FHWA (1) methods. Furthermore, the interpretation of the results included the extrapolation of the load settlement data of the entire test, as well as, those of base and shaft data of every layer along the shaft of the pile. The extrapolation of the load versus settlement data were carried out utilizing the Chin and Kondner (197) and Decourt(1999) and () methods. Such interpretation allowed for a comparison between the shaft resistances computed from the static methods and those obtained from the pile load test. The comparison aimed the development of a design approach for grouted piles.. SOIL CONDITIONS A comprehensive geotechnical investigation was conducted. The investigation comprised of four boreholes that were drilled in the site down to a depth of 5 m from the ground surface. The ground water table (G.W.T.) was measured in piezometers at.5 m below ground surface. Figure (1) shows the stratification and geotechnical parameters of the different layers in the site. 1 1 1 1 1 3 3 3 3 3.5 Test Pile1 Ground Surface Made Ground G.W.T (.5) LEmbedded=.3 m Dia. = 1 mm Cutt off Level (.3) Stiff Brown CLAY Meidum dense to dense SAND Top ofgrout (17.3) L grouted Tip of Pile (.3) Medium dense to very dense SAND Figure (1): (A) Genereral soil profile, (B) NSPT, (C) Undrained Shear Strength (kpa) 3. TESTED PILE 3.1 Pile Geometry and Construction 1 1 1 1 1 3 3 3 N SPT 1 3 BH.1 3 (A) 3 (B) BH. BH.3 3 (C) BH. 1 1 1 1 1 3 3 3 Su(Pocket), kpa 5 5 75 1 B.H.1 B.H.3
The tested pile is a trial pile that was bored with a diameter of 1 mm and with depth of.3 m. The bottom nine meters were shaft grouted and the pile was also base grouted. The used concrete strength fcu= 5 MPa and E = 1, MPa. The pile was reinforced along the entire length of the pile. The cutoff level is (-.3). The bottom nine meters were shaft grouted and the pile was also base grouted. As the pile should extend to platform level, a double wall casing extending from platform level to cut-off level was installed to eliminate the skin friction along this part during the loading test as shown in Figure (1-A). 3. Instrumentation The pile is instrumented with the following Figure (): 1) Five sets of vibrating wire sister bar strain gauges to be used to measure the strain at different levels of the pile. The levels include; cut off level, top of sand level, top of grout length, middle of grout length and 1 m above toe level. ) Three tell-tales extensometer were fixed to the pile to measure the displacement at three levels; top of sand, top of shaft grouting and toe of pile. 1 1 1 1 1 3 3 3 3 3.... 1 TELL TALES (TO TOE, TOP OF GROUT, TOP OF SAND PLATFORM (-3.) CUT-OFF (-.3) (-.3) A B C VW STRAIN GAUGE @each level (-.5) A VW STRAIN GAUGE @ each level (-11.) B VW STRAIN GAUGE @ each level (-17.3) VW STRAIN GAUGE @each level (-1.) C VW STRAIN GAUGE @ each level (-5.3) A-A B - B Figure (): Pile Instrumentation Scheme C - C 3.3 Shaft grouting Figure (3) shows the construction steps of the pile including those of shaft and base grouting. For the purpose of shaft grouting, bundles of poly-ethylene pipes are fixed to reinforcement cage. Each bundle consists of a number of grout pipes (mm diam.) and each pipe covers m of grout length. Cement grout is pumped under pressure through the grouting pipes approximately two days after concreting so that the green concrete cover is easily fractured allowing the
grout to flow freely around the shaft along the grouted length. The grout initially fills the interface between the shaft and soil, and then penetration occurs (Hamza, 3 and Wahba, 1). 3. Base grouting Base grouting is carried out via a flat jack that is installed at the bottom of the pile with the steel reinforcement cage. The jack is connected with grout pipes. After two days and after completion of the shaft grouting, the flat jack is charged with grout at a pressure. The circumferential seal between the steel top plate and the thin steel membrane of the jack is broken and grout initially compacts and then flows into the soils beneath and around the toe. Excessive uplift of the pile should be avoided (Hamza, 3 and Wahba, 1). Figure (3): Construction steps for pile and shaft / base grouting procedure (Wahba 1).
. ESTIMATION OF AXIAL PILE CAPACITY USING STATIC METHODS The pile static axial capacity was estimated for base, shaft and total resistances using Egyptian Code (1), Brown et al. (1), ASHTTO (7), and the Bustamante () methods. The Bustamente () is originally developed for grouted piles. The estimated ultimate shaft, base, and total resistance are shown in Table (1). 5. PILE LOAD TEST AND COLLECTED DATA Figure (-A) shows static field test setup. The axial Pile head movement was measured by four dial gauges with a precision of.1mm. A precise level, equipped with plane plate micrometer with.1 mm resolution was used to measure and backup check the axial movement at three points fixed to the pile head. The shaft movements at the top of sand layer, at the top of grout length and at the toe of the pile; were measured using three telltales that were fixed inside the pile. The axial load was applied using three,kn hydraulic jacks that were placed between the pile head and the reaction system that consisted of a crown restrained by twelve ground anchors distributed around the pile Figure (-B). The hydraulic jacks were capable of holding the axial load for enough time. Load cells located between the jacks and the reaction system were used for axial load measurements. The applied load was checked by obtaining the force that results from multiplying the piston area of the jack by the applied hydraulic pressure recorded by a pressure gauge mounted on the pumping unit connected to the jacks. The hydraulic jack load was increased if any reduction in load cell was observed. Maximum pile load applied was 11, kn which is % of design load with one cycle of loading and unloading. The test was carried out in accordance with ASTM D 113. During the test, continuous observations were made of load readings, hydraulic pressure, dial gauges, the level, till tales readings, and dial gauges during the test. Figure(5-A), shows the axial load versus pile head settlements relationship of the test, and Figure (5-B) shows skin friction and end bearing separately. Figure() shows axial head load versus settlements of pile head, top of sand layer, top of shaft grout and at pile toe. Strain measurements at different levels by set of four vibrating wire strain gauges, allowed the average strain and thus load distributions along the shaft length to be determined during the test. Figure (7) shows average strain distribution along Pile Shaft, while Figure() shows average estimated Load Distribution along Pile Shaft. Furthermore, skin friction for each layer (CLAY, SAND, GROUT1, and GROUT layer) is measured at each applied load increment as shown in Figures (9). 3.3 (b) Figure () (A) Pile Load Test setup, (B) Distribution of ground anchors
Load (kn) 5 5 75 1 15 P working Cycle 1 Cycle Load (kn) 5 5 75 1 Friction Bearing Settlement (mm) P working Settlement (mm) 1 1 (A) 1 Figure (5): (A) Pile load vs displacemnet, (B) friction/end-bearing vs displacemen 1 (B) Settlement (mm) 1 3 5 7 9 1 5 5 75 1 15 Head Settlement Top of sand Top of grout Teo Toe Load (kn) Figure () Pile head load versus settlements of pile head, top of sand layer, top of shaft grout and at pile toe Figure (7): Average strain along pile shaft (negative strains are compressional strain)
Figure () Average load distribution along pile shaft Figures (9): Predicted Skin friction Load v displacement for each Layer. INTERPRETATION OF DATA The ultimate capacities of the piles are determined from the load test results using two methods Chin- Kondner, and Decourt Extrapolation..1 Chin-Kondner Extrapolation Chin and Kondnor (197) proposed a method to determine the ultimate capacity. To apply this method, divide each movement with its corresponding load and plot the resulting value against the movement Figure (1).
Figure (1): Chin-Kondner Extrapolation curves for ultimate capacities (skin friction for every layer, total skin friction, end bearing, and total axial capacity)
The values will fall along a straight line after some initial variation. The inverse slope of this line is the Chin-Kondner Extrapolation of the ultimate load, Qu, which is given by: C1= is the slope of the linear regression. Decourt Extrapolation Q u = 1 c 1 The Decourt extrapolation (1999) & () is employed to determine the ultimate capacities. In this method, each load is divided by the corresponding movement. The ratio is plotted versus the applied load. The extrapolations of the total capacity, base resistance, total frictional resistance, and the frictional resistance of the clay, sand, grouted 1 and grouted layers are shown in Figure (11). The extrapolated load limit, Qu is determined by the expression: : C1 = Slope of straight line C = y-intercept of the straight line 7. RESULTS AND DISCUSSION Q u = c c 1 Table (1) summarizes the estimated capacities using the methods mentioned in section of this paper. The extrapolations of the resistances of the total capacity end bearing resistance, total skin resistance, and the mobilized skin resistances of clay, sand, grouted 1 and grouted layers; are shown also in Table (1). The ECDF (1), the AASHTO (7) and the Brown et al (1) methods estimated very close and comparable values of the resistances for skin resistances of each layer with the exception of the grouted layers 1 and, total skin resistance, end bearing resistance and total resistance of the pile. The interpreted resistances using both the Chin-Kondner and the Decourt methods are in excellent agreement. The average interpreted resistances of each component in Table (1) are considered to be the measured mobilize resistances. The measured resistances are used to estimate the ratio of measured resistance to estimated resistance. The calculated ratios are shown in Table (). The conventional ECDF (1), the AASHTO (7) and the Brown et al (1) methods under estimated the resistances of total skin, end bearing and total resistances of the pile. The ratios of underestimation reached to as low as.9 to.. Such underestimation is due to the high mobilized resistances of the grouted layers and grouted base resistances. The mentioned methods do not take into account the influence of grouting on the capacities. For non-grouted clay and sand layers, the above mentioned conventional methods slightly underestimate the skin resistances. The ratios of underestimation reached to as low as. to.. The underestimation is higher in case of sand as compared to clay.
Figure (11) Decourt Extrapolation curves for ultimate capacities (skin friction for every layer, total skin friction, end bearing, and total axial capacity)
Resistance Total ultimate Resistance Total ultimate skin friction Total ultimate end bearing Skin Friction Table.(1) Measured and estimated eesistances ECDF (1) AASHTO (7) Qu Estimated(kN) Brown et al. (1) FHWA Bustamante () For grouted part QuMeasured(kN) Chin Decourt 93.9 1.33 911.3 ----- 5 719.5 559.7 99.7 59. ----- 1.7 1739.9 35.31 351.55 351.55 1 13.5 395. CLAY 11.3 1.9 1.9 ----- 1.7 155.31 SAND 117.5 13.7 15. ----- 19.39 GROUT1 19. 117. 11. 919.5 5 575.7 GROUT 19. 119. 175. 57.1 1 759.97 Table.() Ratios of measured resistance to estimated resistance Resistance ECDF1 AASHTO7 FHWA1 Total ultimate resistance Total ultimate skin friction Total ultimate end bearing Skin Friction Bustamante For grouted part.7 3.1. ----- 3.7 3.5.91 -----.3 3.11 3.11.55 CLAY 1.59 1.7 1.7 ----- SAND 1.73 1.5 1.3 ----- GROUT1 3.17.5 3.33 1.9 GROUT 5. 7.5 5. 1.53 As mentioned above, the under estimation of the skin resistance of non grouted clay layer is reasonable. The influence of soil disturbance of the clay samples on the measured undrained shear strength can be easily used to explain the slight underestimation. On the other hand the significant under estimation of the skin resistance of the non-grouted sand layer could be attributed to the influence of energy level during carrying out the SPT and thus the obtained N values used in the estimation process. As expected the conventional ECDF (1), the AASHTO (7) and the Brown et al (1) methods significantly underestimate the skin resistances of grouted sand layers named as GROUT 1 and GROUT. The underestimation reached as low as. to.3 and.13 to. for GROUT 1 and GROUT, respectively.
As expected, the grouted base resistance is significantly underestimated by the conventional methods. The underestimation reached as low as.3 to.3. The Bustamante () method that is originally developed for grouted piles underestimated the skin resistances of the grouted layers with different degrees as well as the base resistance. The estimate in case of GROUT 1 layer is considered to be excellent. While the underestimation ratios of the skin resistance of GROUT and base resistance are.5 and.39, respectively.. CONCLUDING REMARK The paper presents results of instrumented large diameter bored pile load test on a pile that is partially grouted through the skin along the lower 9m of the shaft and grouted at the base. The instrumentation allowed the separation of the mobilized resistances of the different layers non-grouted and grouted along the shaft of the pile. The instrumentation, allows also the measurement of the base resistance of the grouted base of the pile. The capability of conventional methods of estimating the different capacities as well as that of the Bustamante () method that is originally developed for grouted piles; to estimate the capacities of skin and base resistances is assessed in this paper. The assessment indicated that there is a need to develop or modify a method estimate the axial capacity of grouted piles. ACNOWLEDGEMENT The authors express gratitude for the data provided by both Prof. Dr. M. Hamza of Hamza Associates and Eng. A. Wahby, for the data and help provided. Without such assistance, this paper would not be possible to appear in this shape. REFERENCES AASHTO (7) LRFD Bridge Design Specifications. Customary U.S. Units, th edition(13 Interim), Washington, D.C. ASTM D 113 Standard Test Methods for Deep Foundations Under Static Axial Compressive Load Bustamante, M. (). Les colonnes de jet grouting. Report of the Seminar: Pathologies des Sols et des Foundations, http://www.keller-france.com/rechercheet-developpement/theses-et-publications: p [in French] Beadman, D., Pennington, M. and Sharratt, M. Pile test at the Shard London Bridge, Ground Engineering, 5 (1), 1, 9 Brown, D.A., Turner, J.P., and Castelli, R.J., (1). Drilled Shafts: Construction Procedures and LRFD Design Methods. Publication FHWA-NHI-1-1, FHWA, Washington, D.C. Chin, FV, and Kondner, A. (197) Estimation of the ultimate load of piles notcarried to failure.proceedings of the nd Southeast AsianConference on Soil Engineering, Singapore, pp. 1 9. Decourt, L., (). Loading tests: interpretation and prediction of their results, ASCE GeoInstitute Geocongress New Oreleans, March 9-1, "Housing John from Reaserch to Practice in Geotechnical Engineering", Geotechnical Special Publication, GSP1, Edited by J.E. Laier, D.K. Crapps, and M.H. Hussien, PP. 5-.
Egyptian Code of Deep Foundations - ECDF (1). Soil Mechanics and Foundation Engineering. Part - Deep Foundations, th addition, HBRC. Hamza, M. M. (3). Personal Communication. Suckling, T.P. and Eager, D.C. Non base grouted piled foundations in Thanetsandsfor a project in East India Dock, London, Underground Construction Symposium,Brintex, London, UK,1, pp. 13 Wahby, A. (1) Personal Communications