Authored by Rami M Bakr*
Abstract
This paper studies the behavior of three continuous flight auger piles executed in unsaturated soil subjected to tension forces. Field tests and laboratory tests were conducted on representative samples collected from a borehole 17m deep. The water table does not exist until a depth of 17m. The author carried out slow static load tests to check the behavior of the piles when subjected to tension forces. The tension capacity of these piles was also investigated using theoretical methods and semi-empirical approaches. The author compared the values estimated from the theoretical and semi-empirical methods considered with those obtained using field load tests. One of the tested piles was loaded until removal from the soil to investigate its geometry. The results showed that the theoretical methods give higher values compared with static load tests, while semi-empirical methods give low estimates.
Keywords: Tension piles; Flight auger; Field tests
Introduction
Recently, the use of continuous flight auger piles has significantly increased. Higher productivity and greater capacity to transfer larger loads to the subsoil. The Continuous Flight Auger (CFA) pile, installed using a continuous helix auger, was first used in the United States during the fifties and in the seventies in Europe. In Egypt, the use of this type of piling has become more prevalent, especially in the Delta region. One of the factors that led to the spread of this type of piles is its suitability for the soil in the delta region, where the topsoil is mostly medium to stiff clay, followed by sandy soil. Since the use of this pile is increasing, it becomes imperative to understand its behavior.
Historical review
Since the beginning of the invention of CFA piles so far there has been a significant development in technology, and presently they can be installed up to 32m deep, 1200mm in diameter, and torque of up to 390 kN.m. CFA piles became very widespread due to their technical advantages combined with relatively low cost [1]. However, these authors warn from the adverse effects during the production process, which may need special attention, especially with pile continuity, soil disturbance due to auger extraction, and failure in weak soils due to high applied pressures causing a sig nificant waste of concrete. Operator skills play an essential role in controlling the construction of CFA piles is the most severe limitation of these piles [2]. The pilling contractor must take all necessary precautions during the installation process, including excavation, auger extraction, and positioning the reinforcements.
Advantages
Continuous flight auger piles are usually installed in most soil conditions. In addition to their efficiency to resist all types of loads including compressive, uplift, and lateral loads, they are also cost-efficient foundation solution. They can be installed up to depths of 32m and diameters of 300 mm to 1000mm, low noise level and no vibration and low noise level so ideal in built-up areas with weak soil conditions and high levels of groundwater, compared to bored piles, construction is rapid as temporary casings or support systems are not needed, and eliminates the soil relaxation. Marchetti dilatometer was used before and after installing a pile to investigate this effect. It was noted that the CFA piling construction method did not cause this relaxation. Bottiau [2] emphasizes that another critical advantage of the CFA pile is the possibility of continuous monitoring, which furnishes documentation on the piles installation.
Tension capacity of piles
The tension capacity of piles depends on several factors; these are construction methods, properties of the pile, and properties of soil. The design of piles to resist tension forces is widespread in the construction field. There are many situations in which this type of structures is mandatory, for example, foundations of power transmission towers, foundations that cross over extensive soils, foundations of lighting poles subjected to lateral wind forces. There are many theoretical methods to estimate the tension capacity for piles. However, the use of these methods is minimal because the parameters involved with these methods are complicated to obtain. Besides, these methods also may present a very optimistic and very conservative tension capacity. In Egypt, estimating tension capacity for piles using semi-empirical methods developed for compressive forces is a common practice among foundation engineers. In these cases, the tension capacity may be assumed as a percentage of the total skin friction resistance of the pile under compressive loading conditions. There are many methods to obtain the tension capacity of a pile. These methods are described by other researchers, such as [3,4].
Field Tests
Field tests, such as SPT, and refraction were performed.
Laboratory Tests
Laboratory tests were conducted out to determine the soil properties, on disturbed samples, and undisturbed samples, including; triaxial, unconfined compression, odometer, permeability tests, etc. Static and Dynamic Load tests also performed. The subsoil comprises of a superficial layer approximately 6.5m thick, composed of high porosity silty-sand clay, followed by clayey-sandy silt to a depth of 19m; the water table encountered at depth 17m. The upper layer is collapsible, presenting collapse ratios ranging from 2.4% to 24%, depending on the applied pressure, according to Vargas [10]. Some geotechnical characteristics of the experimental area presented in the following tables (Table 1 & 2).
Test Piles and Loading System
Three Continuous Flight Auger piles constructed in the study area. The CFA pile has the following dimensions: nominal diameter 0.40m and length 12m. The piles followed a predefined alignment and spacing between them was 4.80m. Three pile caps were constructed with dimensions of 0.70 x 0.70 x 0.70 𝑚3 for each pile studied. A concrete with strength, fcu= 25MPa, was used in the pile cap. Vertical steel reinforcement of 5Ø 16mm, 6m in length, was used.
Static Load Tests
For each pile, a static load test was carried out with a maintained load method. The static load tests were carried out ASTM guidelines. The test load was applied in steps of 120kN, up to the load at which the displacements indicated a rupture of the pile. Unloading was performed in four stages.
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