Instrumented Load Testing and Construction Monitoring of Rigid Inclusions in Seattle-Paper

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The Georgetown Wet Weather Treatment Station in Seattle used unreinforced rigid inclusions (RIs) to support a process building containing water-holding basins and settlement-sensitive equipment. The 18-inch-diameter displacement augercast RIs were spaced 6 feet apart, extended about 40 feet below grade through loose silty sand and soft silt, and were bridged by a 2-foot-thick gravel and quarry-spall load distribution mat.<br /><br />Three production RIs instrumented with strain gages underwent static axial load testing. Their load-transfer behavior matched design expectations, with substantial load shed to shaft friction in denser soils. The remaining 588 RIs were installed in 18 working days. Quality control included grout-strength testing, installation-record review, and low-strain pile integrity testing (PIT).<br /><br />PIT identified anomalies in many RIs, typically 10 to 20 feet below grade. Excavation confirmed a full-depth fracture at one anomaly. The authors attributed the damage to soil densification and lateral or heave forces during installation of nearby RIs that had only partially set. Monitoring also showed tension developing in test RIs during adjacent installation. Although these fractures did not compromise the system’s intended settlement-control function, the authors note that reinforcement or other measures may be needed in different applications.<br /><br />The building’s predicted maximum total settlement was about 2.4 inches. Water preloading was used before installing grade-sensitive components; measured preload settlement was roughly two-thirds of the calculated value. Final monitored structural settlement reached 1.4 inches, excluding 2 feet of site fill not yet placed. Overall, the RI system and preloading provided effective settlement control and offered a faster, less costly, and lower-impact alternative to conventional deep foundations.

Keywords

Georgetown Wet Weather Treatment Station

Seattle

rigid inclusions

displacement augercast

load distribution mat

static axial load testing

pile integrity testing

settlement control

soil densification

water preloading