Labuz, Joseph F.

Earth pressure cells, tiltmeters, strain gages, inclinometer casings, and survey reflectors were installed in fall 2002 during construction of a 26-ft (7.9-m) high Minnesota Department of Transportation (Mn/DOT) reinforced concrete cantilever retaining wall. A data acquisition system with remote access monitored some 60 sensors on a continual basis. Analysis of the data indicated the development of active earth pressure at the end of backfilling, with a resultant at about one-third of the backfill height. Translation of 0.45 in. (11 mm), or about 0.1% of the backfill height, was responsible for development of the active condition. The wall also rotated 0.03° into the backfill as a rigid body, while the top of the stem deflected 0.16 in. (4 mm) away from the backfill. Sensor readings showed the earth pressure distribution to be quite complex during the backfilling process. Evidence was found for residual lateral stresses from compaction. Translation of the wall overnight following the construction workday reduced the compaction-induced lateral stresses. Changes in earth pressure and wall deflection weeks after backfilling were attributed to changes in temperature and rainfall. The data showed that the wall design, while reasonable, could be made more efficient by removing the shear key, which was ineffective.

Resilient modulus, shear strength, dielectric permittivity, and shear and compressional wave speed values were determined for 36 soil specimens created from the six soil samples. These values show that the soils had larger stiffnesses at low moisture contents. It was also noted during testing that some non-uniformity was present within the axial displacement measurements; larger levels of non-uniformity were associated with low moisture contents, possibly due to more heterogeneous moisture distributions within these specimens. Lastly, the data collected during this study was used to recommend a relationship between granular materials' small strain modulus and their resilient modulus. This relationship was given in the form of a hyperbolic model that accurately represents the strain-dependent modulus reduction of the base and subgrade materials. This model will enable field instruments that test at small strains to estimate the resilient modulus of soil layers placed during construction.

In this study, researchers devised a scheme for calibration of earth pressure cells to observe their response to various loading configurations and to recommend a procedure for field installation. As a result of calibration tests, a field installation procedure was developed. Preliminary field data indicate that soil calibration and placement procedure provide reasonably accurate measurements.