Axial compressive response of large-capacity helical and driven steel piles in cohesive soil

Abstract

The axial compression performance of large-capacity helical piles is of significant interest because they can offer an efficient alternative to conventional piling systems in many applications such as in oil processing facilities, transmission towers, and industrial buildings. This paper presents the results of seven full-scale axial compression load tests conducted on 6.0 and 9.0 m large-capacity helical piles and a 6.0 m driven steel pile. The results are considered essential to qualify and quantify the performance characteristics of large-capacity helical piles in cohesive soils. The test piles were close-ended steel shafts with an outer diameter of 324 mm. The test helical piles were either single or double helix, with a helix diameter of 610 mm and interhelix spacing that varied between 1.5 and 4.5 times the helix diameter. The subsurface soil properties at the test site were determined using field and laboratory testing methods. The 6.0 m piles were tested 2 weeks after installation, while the 9.0 m piles were tested 9 months after installation. The load–settlement curves were presented to better understand the behaviour of test piles. An ultimate capacity criterion was proposed to estimate the ultimate load of large-capacity helical piles. The test helical piles developed ultimate resistances up to 1.2–1.8 times that of the driven pile. The load-transfer mechanisms of large-capacity helical piles were studied, and it was found that soil disturbance during pile installation had a significant effect on the pile failure mechanism regardless the value of the interhelix spacing to helix diameter ratio. The mobilized soil strength parameters were back-calculated and compared with the estimated intact soil strength parameters.

Investigating performance of micropiled raft in foundation of power transmission line towers in cohesive soil: experimental and numerical study

“…Alternatively, in many of research works, the bearing capacity considered is not equivalent to the point at which the plunge failure occurs, either (Abd Elaziz and El Naggar 2014a; Abdelrahman et al 2003;Akgüner and Kirkit 2012;Décourt 2008;Elkasabgy and El Naggar 2014;Fellenius 1980;Gilchrist 1985).…”

Axial load testing of helical pile groups in glaciolacustrine clay

“…An axially loaded helical pile's failure surface can be described by the individual bearing model (IBM) or the cylindrical shear model (CSM) (Zhang 1999). The IBM predicts that bearing failure occurs at each helix and that there is negligible interaction between adjacent helices (Elkasabgy and El Naggar 2015). Past studies have found that the IBM dominates pile behaviour when the ratio of the vertical helix spacing (s h ) to D is greater than 1.5 or 3 (Zhang 1999;Tappenden 2007).…”

“…Perko (2009) suggests a lower ␣ for helical piles owing to poor soil-shaft contact caused by the shaft wobbling during pile installation. H eff is the length of shaft that contributes to Q shaft ; it can be estimated as the shaft length above the lower helix (H s ) minus 1D per helix, to account for a void forming above each helix (Elkasabgy and El Naggar 2015).…”

“…As differential pile settlement prevented groups from reaching the limit load, a settlement-based failure criterion was adopted for this study. Elkasabgy and El Naggar (2015) recommended that the ultimate load (Q u ) should fall within the nonlinear region of the load-settlement curve, where creep settlement is low. Based on this recommendation and from inspection of the load-settlement curves, Q u adopted for this study was defined as the load causing pile settlement of 5% of the helix diameter, or 15.2 mm.…”

“…The reduction in N t of the upper helices is likely due to soil disturbance in the interhelix region during pile installation. As the helices cut through the interhelix soil during installation the soil is partially destructured, whereas the soil below the lower helices remains undisturbed (Elkasabgy and El Naggar 2015). The large variability of the measured N t (between 5.4 and 10.4) suggests that the recommended value of 9.0 (Perko 2009;CFEM 2006) may not be representative of the bearing resistance of helical plates.…”