Strain Measurements of Concrete Piers in Expansive Soils

Foundation systems in expansive soils within the western US commonly utilize drilled concrete piers and grade beams. A field test site was established to investigate the behavior of full scale drilled concrete piers. Five concrete piers were instrumented and installed to depths ranging from 3 to 7.6 m in a known expansive soils deposit on the Colorado State University campus. Four reinforced concrete piers were instrumented with vibrating wire strain gauges at 1.8, 3, 4.3 and 5.5 m depths. One pier was not instrumented. Two of the piers were installed beneath a simulated slab-on-grade and three were placed in an uncovered area. The uncovered area was irrigated beginning a year after installation of the piers. A vibrating wire strain gauge was installed in a laboratory test cylinder. The laboratory cylinder was used to compare baseline readings with the field cylinders, and to monitor environmental factors (e.g. temperature and shrinkage) influencing strain. Strain in the concrete piers was monitored over a two year period and compared with soil water content changes, free field heave and displacement of the piers at the ground surface. Performance of the drilled concrete piers was compared with that of a patented helical pier system installed in the same environment. Skin friction mobilized in the heaving soil zones has not been sufficient to overcome the anchoring capacity of the deep piers, but the 3 m pier heaved approximately 30 mm over the 2 year period. Significant tensile strains have been measured, and have continued to increase. It is believed that strain in the concrete piers can be correlated with movement of the wetting front downward through the soil.