A calibration technique to improve accuracy of the photogrammetry-based deformation measurement method for triaxial test
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RESEARCH PAPER
A calibration technique to improve accuracy of the photogrammetrybased deformation measurement method for triaxial testing Lin Li1
•
Yi Lu1 • Yang Cai1 • Peng Li2
Received: 26 January 2020 / Accepted: 12 September 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In 2015, a photogrammetry-based method was developed to measure soil deformation during triaxial testing with help of a handheld camera. Since this development, the photogrammetry-based method has been well-recognized due to its simplicity, low cost, and full-field deformation measurement capability. For better characterization of soil deformation during triaxial testing, a simple technique is presented to further improve the accuracy of the photogrammetry-based method through a calibration of four system parameters (i.e., cell wall thickness, refractive index of air, cell wall, and confining media). To evaluate this technique, several tests were performed on a 61 9 122 mm stainless-steel cylinder inside of a 152 9 200 mm triaxial cell. Measurement results indicate that the average point measurement accuracy of the photogrammetry-based was improved to 0.032 mm from 0.081 mm when the triaxial cell used was filled with water. Keywords Optical ray tracing Photogrammetry-based method Triaxial test
1 Introduction Both saturated and unsaturated soil triaxial tests require soil volume measurement during testing. The measurement of the unsaturated soil volume/deformation during triaxial testing used to be a great challenge for researchers. In the past, a great deal of research effort has been dedicated to developing different methods such as the double-wall cell (e.g. [3, 19]), local displacement transducers (e.g., [5, 6, 11]), laser [24], and volume controllers (e.g. [4, 13]) for soil volume/deformation measurement during triaxial testing. However, as summarized in Laloui et al. [12] and Zhang et al. [28], all of these methods have their limitations and accurate unsaturated soil deformation measurement during triaxial testing remained a great challenge for researchers. In 2015, a photogrammetry-based method was developed as presented in Zhang et al. [28] and Li et al. [16] and & Lin Li [email protected] 1
College of Civil Engineering, Nanjing Forestry University, No. 159 Longpan Road, Nanjing 210037, Jiangsu, China
2
School of Highway, Chang’an University, Xi’an 710064, Shaanxi, China
later successfully applied for unsaturated soil deformation measurement during triaxial testing as reported in Li and Zhang [14]. With this method, using a conventional saturated soil triaxial testing system, the unsaturated soil volume/deformation during testing can be easily measured with help of a handheld camera. Since this development, the photogrammetry-based method has been well-recognized for its simplicity, low cost, and full-field deformation measurement capability. The newly developed photogrammetry-based method overcomes most of the limitations associated with the other
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