Drilled Shaft Load Testing Program for Design of Foundations for an Automated People Mover-Paper

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The PHX Sky Train Stage 2 project required more than 150 drilled shafts to support elevated guideways and stations at Phoenix Sky Harbor International Airport. The site is underlain by variable alluvial sand, gravel, and cobbles (SGC), with groundwater generally around 97–106 feet below ground. Large axial and lateral demands, along with difficult excavation conditions, made foundation design challenging.<br /><br />To verify performance under actual site and construction conditions, the project team tested an instrumented 84-inch-diameter drilled shaft. A bi-directional axial test applied a maximum load of 7,779 kips. Strain-gage measurements indicated substantial side resistance in the SGC, reaching 13.4 ksf near the load cell. Conventional AASHTO methods predicted notably lower resistance, so the design team adopted a site-specific unit side-friction value of 10.5 ksf.<br /><br />A subsequent lateral test reached 338 kips, with only 0.036 inches of lateral displacement. The shaft deflected less than predicted by LPILE using conventional p-y curves, indicating greater lateral soil resistance than the standard model suggested. Although the small movements prevented development of site-specific p-y curves, back-analysis supported increasing the subgrade modulus used in design from 275 to 500 pci.<br /><br />The load-test results enabled the team to reduce shaft sizes and avoid deeper foundations that conventional design methods would have required. The testing improved confidence in the design and construction approach and was estimated to save approximately $3 million, compared with a total testing cost of about $286,000. The case demonstrates the value of project-specific load testing in refining drilled-shaft design in coarse, cobbly soils.

Keywords

PHX Sky Train

drilled shafts

bi-directional load test

lateral load test

Phoenix Sky Harbor International Airport

alluvial sand gravel and cobbles

axial resistance

side friction

subgrade modulus

foundation design