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Evaluating the Capacity of Helical Piles in Clay Tills Using Pile Load Tests

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Evaluating the Capacity of Helical Piles in Clay Tills Using Pile Load Tests Evaluating the Capacity of Helical Piles in Clay Tills Using Pile Load Tests Ivanna Montani Stantec Consulting Ltd, Winnipeg, Manitoba, Canada ABSTRACT This thesis analyzes seven static load tests conducted on helical piles installed in clay till at a site in Northern Manitoba, Canada. Four methods, the Davisson Offset Limit, the Hansen Ultimate Load, the Chin-Kondner Extrapolation, and the Decourt Extrapolation are used to determine the ultimate capacity using the pile load test results. The ultimate capacities obtained were then used to determine the empirical parameter Kt for helical piles in clay tills, this parameter relates the pile capacity of helical piles to the installation torque of the pile. The Davisson Offset Limit and the Hansen Ultimate Load provided consistent and conservative ultimate capacities based on the pile load test results and showed lesser variability in results compared with the Chin-Kondner Extrapolation and Decourt Extrapolation methods. The ultimate loads based from the Hansen and Davisson methods were used to calculate a Kt value. It was determined that a Kt value ranging from 9 m-1 to 11 m-1 is appropriate for evaluating the capacity of a helical pile in clay tills using the installation torque of the pile. RÉSUMÉ Cette thèse fait l’analyse de sept essais de mise en charge statique effectués sur des pieux vissés installés dans une formation de moraine (till) argileuse dans le nord du Manitoba, au Canada. Quatre méthodes ont été utilisées pour la détermination de la capacité ultime utilisant les résultats des essais de mise en charge : la méthode de la charge limite décalée (Davisson), la méthode de la charge ultime (Hansen), ainsi que les méthodes d’extrapolation de Chin-Kondner et de Decourt. Les capacités ultimes obtenues ont ensuite été utilisées pour déterminer le paramètre empirique Kt pour pieux vissés en moraine argileuse. Ce paramètre relie la capacité du pieux vissé au couple d’installation du pieu. Les méthodes de la charge limite décalée (Davisson) et de la charge ultime (Hansen) ont fourni des capacités ultimes constantes et conservatrices par rapport aux résultats des essais de mise en charge et ont démontré des résultats moins variables en comparaison avec les méthodes d’extrapolation de Chin-Kondner et de Decourt. Les charges ultimes obtenues à partir des méthodes de Hansen et Davisson ont été utilisées pour calculer une valeur Kt. Il a été établi qu’une valeur Kt se situant entre 9 m-1 et 11 m-1 est convenable pour l’évaluation de la capacité d’un pieu vissé en moraine argileuse en utilisant le couple d’installation du pieu. 1 INTRODUCTION load capacity to installation torque recommended in the Canadian Foundations Engineering Manual (CFEM) to 1.1 Objective determine a range for the empirical parameter, Kt that engineers use to estimate the capacity of helical piles. This thesis presents and compares four different methods of evaluating the ultimate pile capacity of helical piles from 1.2 General full scale load test data. It determines the parameter that engineers use to estimate the capacity of helical piles. The Seven helical piles were installed and tested at a site in four methods presented are: the Davisson Offset Limit, the northern Manitoba. The seven piles are all steel helical Hansen Ultimate Load, the Chin-Kondner Extrapolation, piles with an outer shaft diameter (d) of 244 millimeters and and the Decourt Extrapolation. These methods were a single helix located at the toe of the pile with a 457- obtained from the Basics of Foundation design book by millimeter diameter (D). The piles were installed to a depth Bengt H. Fellenius, latest edition published in 2009. Using (H) below the frost sensitive soils line, ranging from 16.71 the ultimate capacities estimated with the four different meters to 18.97 meters below the surface. All piles were methods, it is possible to utilize the relationship of helical installed with the helix embedded in the clay till, below the frost penetration line. The installation torque (T) was recorded for each pile. A schematic for a steel helical pile is provided in Figure 1. Figure 2. Static Load Test Summary 1.3 Site Conditions The test site is located in a region of sporadic Figure 1. Schematic of Steel Helical Pile (James P. discontinuous permafrost. The permafrost was found Hambleton, 2014) approximately 10 to 15 meters below the surface, with thaw sensitive soils extending up to 17 meters below the The seven pile load tests were conducted following the surface. Frozen and thaw sensitive soils may settle upon Quick Method in accordance with ASTM D1143/D1143M- thawing thereby inducing a drag load (negative skin 07 (Reapproved2013) “Standard Test Methods for Deep friction) on the helical piles, for this reason the skin friction Foundations Under Static Axial Compressive Load”. The capacity of the pile is ignored, and the pile is assumed to static load tests for the piles were limited in deflection as all derive all its capacity from the bearing resistance of the seven piles were intended to be production piles and later helix at the toe of the pile. The pre-development site used in construction. For this reason, the tests were not stratigraphy consisted of peat/organic materials followed carried to failure. A summary of the dimensions and by a grey clay layer overlying clay till until the end of the installation torque for Pile No. 1 through Pile No. 7 is given boreholes. The peat and organics were observed to be in Table 1. A summary of all seven Load (Q) vs. Deflection fibrous, wet, and extend from the ground surface to a depth (δ) curves from the load test date are shown in Figure 2. of 1.6 meters. The grey clay layer was observed to be low to medium plasticity, moist, silty and contained some sand and trace gravel. The grey clay layer extends from a depth Table 1. Summary of Test Piles of 1.6 meters to a depth of 7.4 meters. Clay till was encountered below the clay layer to the depths explored Pile Embedment Shaft Helix Installation during the geotechnical investigation. The clay till was grey No. Below Surface Diameter Diameter Torque and low in plasticity, contained silt, some sand, and gravel. (m) (mm) (mm) (kN-m) The clay till was confirmed to be very dense in 1 16.71 244 457 116 compactness through Standard Penetration Test (SPT) at 2 18.32 244 457 142 the site. 3 18.64 244 457 133 4 18.16 244 457 153 2 LITERATURE REVIEW 5 17.50 244 457 220 6 18.97 244 457 174 2.1 Helical Piles 7 16.78 244 457 246 Steel helical piles, also known as screw piles or screw anchors, are a type of deep foundation used to provide stability against compressive, tensile, and lateral loads for a structure (Tappenden & Sego, 2007). A screw pile consists of a circular pile shaft with one or more helices attached to the shaft of the pile by welding to increase the bearing capacity of pile. Due to their varying sizes helical piles may be used for a variety of applications with a wide range of loading scenarios ranging from large piles to support bridges and lighthouses to smaller piles used as foundations for cabins and decks (Koulack, 2016). Helical piles are installed by screwing the pile into the ground by applying a turning moment at the head of the pile which causes the helices to penetrate the ground (Tappenden & Sego, 2007). There are several methods of installation usually carried to failure or a predetermined maximum load depending on the size of the pile and the soil in which it is and typically lasts 3 to 6 hours (CFEM, 2006). For this test being installed. Smaller piles are installed using a manually method, the duration of each load increment should not powered wrench while larger piles are installed with a exceed 15 minutes and should not be shorter than 5 hydraulic wrench (Koulack, 2016). Figure 3 shows an minutes. It is recommended that the load increments to example of helical pile installation with a hydraulic wrench. failure or maximum predetermined load should not be less than 25 increments, 35 to 40 preferred (CFEM, 2006). Figure 4 below provides a schematic for a typical static load test set up. Figure 4. Schematic of Hydraulic Jack Acting Against Anchored Reaction Frame (ASTM Designation: D1143/D1143M-07, 2013) A typical static load test is composed of the pile being tested, a load cell that applies the load to the test pile, as well as a load transfer beam and anchor piles that hold the Figure 3. Helical Pile Installation with Hydraulic Wrench system in place as the load is applied. Four dial gauges are (courtesy of Stantec Consulting Ltd.) used to measure the pile head deflection measured relative to a fixed reference beam, the average of the four readings is the used for the load-deflection curve. Installation for helical piles usually requires minimal to no excavation which makes this foundation type well suited 2.3 Pile Capacity for installation near other structures, as well they are a quick and economical option where no delay in The design capacity of a foundation element, such as a construction is important (Koulack, 2016). Screw piles are pile, communicated to the structural engineer is determined not typically well-suited for installation in very dense by applying a resistance factor to an ultimate capacity gravelly soils that may cause damage to the helices or value. The ultimate pile capacity can be predicted using have shallow refusal, but there has been commercial screw theoretical methods and is verified by interpreting the pile piles fabricated with thick helices, up to 25 mm, to prevent capacity from the static load test of the pile or using the damage during installation in dense soils (Tappenden & installation torque and capacity relationship.
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