Tactilus Cell Instrumentation-ERS

Evaluation of Using Tactilus Cells to Estimate Vertical and Horizontal Stresses in GeoStructures

Relating the measurements of pressures from the field instrumentation to estimated pressures through theory is never a straight forward process although relatively speaking it is slightly easier for vertical earth pressure than lateral earth pressures. Depending on the applications, sometimes determining the lateral earth pressure accurately has major impacts on design and estimating performance. This is especially true for earth retaining structures with frictional connection between the facing elements and the reinforcements. However, in most cases the instruments used in the field are about the same size as the facing block therefore even if pressure is measured, it is impossible to determine where the actual load is applied along the facing.

The purpose of this research is to experiment with a new instrument that is manufactured at sizes that are small enough to pin point where the actual load is applied along the facing. This instrument is referred by the manufacturer as "tactilus® sensors".  These sensors are manufactured at different sizes and when put together, the manufacturer claims that it can be used to map the pressure (http://www.sensorprod.com/tactilus.php). Individual sensors are constructed by series of interlaced lines that create a semiconductor matrix with a piezoresistive response. When the sensor is loaded that causes a mechanical strain, which changes the electrical resistivity of the semiconductor.  For the purpose of this research, three different sized sensors are purchased:

• Thirty two sensors at 25 mm by 25 mm
• Ten sensors at 10 mm by 10 mm
• Nine sensors at 8 mm by 8 mm (enclosed in water proof casing)

The goal of this research is to understand the suitability of these instruments to be used in geotechnical engineering applications to estimate accurate lateral earth pressures. This is an independent research and not sponsored by the manufacturer. Several of these sensors have been placed in the field during the construction of the GRS-IBS abutment and the remainder of the sensors is being tested in the laboratory. The laboratory testing program involves first relating the manufacturer's calibration with vertical pressures applied by different soil type and understanding the repeatability of the results as it relates to hysteresis. If successful, physical models  will be constructed in the laboratory to replicate field conditions as closely as possible and lateral earth pressures will be recorded. These findings will be correlated with field observations and theoretical numerical models. If successful, such a technology could provide significant advantages in geotechnical engineering and improve some of our design methodologies as it relates to lateral earth pressure distribution along the facing of earth retaining structures and potential boundary conditions due the vertical spacing between reinforcements.