Calibration of Boundary-Type Earth Pressure Cells with Sand

Abstract

Abstract Earth pressure cells (EPCs) are a major part of any instrumentation project, whether in the field or laboratory. The reliability of EPC output mainly depends on its calibration before its installation. The calibration chart provided by manufacturers cannot be used directly because their charts are developed by applying fluid pressure, but when sand is used as contact material, EPCs’ response changes significantly. The major factor that causes deviation of EPC response in fluid and sand is that in fluid calibration, the applied load is fully transferred to EPC, irrespective of the size of the chamber, but for sand, the applied load and load reaching EPC are different because of arching, intergranular friction, and sidewall friction in a calibration chamber. In the present study, an in-house calibration chamber is developed to calibrate boundary-type EPC with Grade II sand as contact material. A load cell is attached just below the EPC to measure the actual load, which is reaching at EPC location. The output of EPC is correlated with load cell output rather than applied loading to determine calibration values of EPC for reliable measurements. A methodology for correcting the calibration factor of EPC is proposed and the effect of sidewall friction and sand layer thickness as well as the effect of load cell placement is discussed. The calibration factor of EPC is found to be dependent on the sand layer thickness and sidewall friction. It is recommended to calibrate boundary-type EPC by the proposed method along with the use of friction reduction measures.