A Comparison of Vertical Earth Pressure and Settlement Data Collected from Two Long-Term Ground Improvement Monitoring Projects

Instrumentation efforts aimed at monitoring the stiffness and displacement characteristics of inclusions under surcharge load involve measurements of settlement and load to understand both the movement response and the portion of the surcharge load that is held by the inclusions. This paper compares two projects, one with stone columns and stiff surface soils (N > 15) to a 20-ft depth, and the other with 4.5-in. diameter helical piles and soft surface soils (N < 4). In both projects, the same types of vibrating wire (VW) earth pressure cells and VW settlement systems were used in similar arrangements, each installed in pairs with one on top of and one in between the inclusions. Both projects also had approximately the same level of loading (12 and 15 ft of fill at Site 1 and Site 2, respectively). The earth pressure cells on top of the helical piles showed they held nearly all of the surcharge loads at top of the stone column. Conversely, for the project with stiff clay surface soils and stone columns, most of the surcharge load was held by the clay at the ground surface. There was almost zero relative settlement between the stone columns and the stiff clay surfaces, and the clay held the full surcharge load (1,300 psf) without deflecting much at all, while the stone columns mobilized only 800 psf of vertical earth pressure at the top of the stone column. On the other hand, for the helical pile project with soft surface soils, the helical piles experienced less settlement (0.75 in.) compared to the adjoining soft soils (2.5 in.), indicating a stiffer load bearing response of the helical piles near the ground surface. Significant load concentration (approximately 11,000 psf) was measured by the earth pressure cells on top of the helical piles along with a commensurate amount of load relief on the surrounding soft soil (approximately 150 psf). In consideration of the above, it is the author's opinion that the planning of instrumentation programs using earth pressure cells to verify inclusion performance needs to consider the relative stiffness between the surface soils and the top of the inclusion elements before deploying earth pressure cells. With stiff surface soils and stone columns, it is unlikely to measure load concentration at the surface, whereas with helical piles and soft surface soils it is highly likely. In scenarios with stiff surface soils and stone columns, it may be practical to limit instrumentation efforts to measurements of settlement.