Threaded displacement piles – a comparison of the “Screwsol” system with continuous flight auger piles in Cork, Ireland-Paper

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The paper evaluates Screwsol threaded displacement piles against conventional continuous flight auger (CFA) piles at three sites in central Cork, Ireland. Both pile types were installed in deep, variable granular deposits, and static load tests were used to compare their performance.

Screwsol piles displace and compact the surrounding soil during installation, while a helical thread formed during extraction increases the pile’s effective diameter and soil interlock. Their design accounts for geotechnical capacity at the thread’s outer diameter and structural capacity at the smaller core diameter. An enhanced horizontal stress coefficient, <em>ks</em> (typically 1.20–1.40), is used to model the increased shaft friction.

Test results showed that Screwsol piles performed in line with predictions up to service loads and better than predicted at higher loads. CFA piles also broadly matched predictions. The Screwsol piles’ especially strong high-load performance suggests that displacement-induced soil compaction may increase toe capacity beyond current design assumptions. A 330/500 mm Screwsol pile developed almost the same shaft capacity as a 600 mm CFA pile, while a larger 430/600 mm Screwsol pile exceeded it.

The technique also reduced material use. For a comparable 1,800 kN service load, a 430/600 mm Screwsol pile was 18 m long, compared with 20.3 m for the 600 mm CFA pile, and required 43% less concrete. It generated negligible spoil. Carbon assessment estimated savings of about 380 kg CO₂ per pile, or roughly 24%, despite the Screwsol concrete mix having a higher cement content.

The study concludes that Screwsol piles are a viable alternative to CFA piles in Cork’s granular soils, with validated design methods, competitive load performance, and environmental benefits. They also avoid the noise and vibration associated with driven piles. Site-specific ground investigations and load testing remain important for design verification.

Keywords

Screwsol piles

CFA piles

threaded displacement piles

static load testing

granular soils

soil compaction

shaft friction

pile capacity

concrete savings

carbon emissions