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, industrial buildings, etc. 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 outer diameter of 324 mm. The test helical piles were either single or double-helix, with helix diameter of 610 mm and inter-helix 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 two weeks after installation, while the 9.0 m piles were tested nine 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 to 1.8 times that of 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 inter-helix spacing to helix diameter ratio. The mobilized soil strength parameters were back-calculated and compared with the estimated intact soil strength parameters.