Mini time domain reflectometry probe for monitoring local soil moisture of cut-off walls in centrifuge model tests

Moisture content is a critical geotechnical property of soils affecting the compressibility, shear strength and permeability. Time domain reflectometry (TDR) has been widely used to monitor the soil moisture content in field and unit cell and column tests in laboratory, while the typical TDR probes can not be used in laboratory scaled-down model tests since their needle lengths are no less than 5 cm. This paper presents a novel-designed and fabricated three-needle mini TDR probe, whose needles are 22 mm in length and 2 mm in diameter with a 10 mm needle spacing. The suitability and performance of the mini TDR probe have been evaluated in laboratory. The effectiveness of the TDR probe was validated through monitoring the consolidation process of a 6-cm thick soil-bentonite (SB) slurry-trench cut-off wall in the centrifuge model test (acceleration = 20g). The results indicate that the mini TDR probe is capable of real-time monitoring of soil moisture content variation in the supergravity condition. The proposed mini TDR probe is fabricated easily and quickly and has a great potential for monitoring local soil moisture content in laboratory model tests.