INSTRUMENTED FIELD DATA-BASED ASSESSMENT ON LOAD TRANSFER BEHAVIOR IN RAMMED AGGREGATE PIER® (RAP) ELEMENTS
Abstract
Among the methods of stone column installation, the use of Rammed Aggregate Pier® (RAP) elements offers an alternative to traditional methods like deep foundations or excavation/backfill for sites with challenging soil conditions that cannot meet the performance criteria of superstructures. The goal of using RAP elements is to reduce settlements to acceptable levels, increase load-bearing capacity, and minimize liquefaction potential. In this study, full-scale field load test was performed to identify the variation of shear resistance along RAP element installed by the Impact® System (displacement), making this research the first and only study conducted on RAP elements produced using the displacement system. More specifically, load cells were used to assess the mobilization of vertical load distribution along the column. By utilizing these load cells positioned at various levels along the column, the axial load distribution was monitored concurrently during the loading test. Full-scale load test results demonstrated that, under a 57.5 ton load, the displacement measured at the tell-tale element (reading bars mounted on load cells) varied relative to the displacement measured at the top level of the column, by approximately 15% at the 1 m level, and in the other levels (2 m, 4 m, and 6 m) by about 2-5%. When these results were evaluated in terms of the literature-defined settlement ratio (Rb >> 1), the measured displacements indicated that the response was governed by lateral expansion of the column. Furthermore, the load cells indicated that the applied load mobilized rapidly up to the 1 m level (~2D; D: pier diameter), with only about 10% of the load being transmitted; in other words, the load was accommodated predominantly by circumferential friction rather than transmission to the column tip. These findings support a deformation mechanism for RAP elements with L/D >> 3.5 (L: pier length) that is driven by circumferential friction rather than tip capacity.
Keywords
Rammed Aggregate Pier , Loading Test , Instrumentation , Load Transfer
Kaynakça
- ASTM D1143 – 81 (Reapproved 1994). Standard test methods for deep foundations under static axial compressive load. Annual Book of ASTM Standards.
- Barksdale, R.D., & Bachus, R.C. (1983). Design and construction of stone columns. Federal Highway Administration, Final Report SCEGIT, 83-104.
- Chen, J.F., Han, J., Oztoprak, S., & Yang, X.M. (2009). Behavior of single rammed aggregate piers considering installation effects. Comput. Geotech., 36(7), 1191-1199.
- Gamboa, W. (2022). Aggregate piers: stress transfer mechanism and construction effect. Master of Science in Civil Engineering, Montana State University, Montana.
- Greenwood, D.A. (1970). Mechanical improvement of soils below ground surfaces. Proceedings of the Ground Engineering Conference, Institution of Civil Engineers, London, 11-22.
- Handy, R.L., & White, D.J. (2006). Stress zones near displacement piers. II: radial cracking and wedging. J. Geotech. Geoenviron. Eng., 1O.1O61/(ASCE) 1090-0241 (2006) 132: 1(63), 63-71.
- Hughes, J.M.O. & Withers, N.J. (1974). Reinforcing of soft cohesive soils with stone columns. Ground Engineering, 7(3), 42-49.
- John, B., Tim., C., Hilary, S., & Michael, B. (2012). ICE manual of geotechnical engineering volume ll. geotechnical design construction and verification. Chapter 84: Ground Improvement.
- Lawton, E.C., & Fox, N.S. (1994). Settlement of structures supported on marginal or inadequate soils stiffened with short aggregate piers. Proc., Vertical and Horizontal Deformations of Foundations and Embankments, Geotechnical Special Publication No. 40, ASCE, College Station, Tex., Vol. 2, 962–974.
- Lawton, E.C., Fox, N.S., & Handy, R.L. (1994). Control of settlement and uplift of structures using short aggregate piers. Proceedings of ASCE National Convention, Atlanta, Georgia.