Abstract: Common problems in engineering projects that involve artificial ground freezing include i
nadequate thickness,
strength and continuity of artificial frozen walls. But it is difficult to monitor artificial frozen walls using only a few thermometer holes in fixed positions or with other existing approaches. Here we report a novel experimental design that investigates changes in ultrasonic properties (P-wave velocity, head-wave amplitude, frequency spectrum) measured during progressive upward freezing compared with those of a falling series of different but spatially uniform temperatures in order to determine the freezing state and frozen-section thickness in 150 mm cubic blocks of Ardingly Sandstone. Water content, porosity and density were estimated during upward freezing to ascertain water migration, porosity and density change at different stages of freezing. The period of the receiving wave increased substantially and coda waves changed from loose to compact during both upward freezing and at different temperatures. The trend of increasing wave velocity can be divided into three stages during a series of falling temperatures: (1) little increase at temperatures above –1℃, (2) substantial increase between –1 and –5℃, and (3) slow increase between –5 and –20℃. During upward freezing,
Vp increased more or less uniformly. Head-wave amplitude decreased continually at first，but later increased gradually during upward freezing and during the falling temperature series. The frequency spectrum gradually became more regular and single-peaked as temperature decreased during the latter. In contrast, a multiple-peak frequency spectrum always characterized upward freezing, and, as the cryofront reached the top of the block, the frequency components revealed the evolving cryotic regions. Wave velocity
Vp and the centroid frequency
fc were adopted as ultrasonic parameters to evaluate quantitatively the temperature (
T) of frozen sandstone. Determination of
Vp provides a convenient method to evaluate the freezing state and to calculate the thickness of the frozen section of upward freezing sandstone, with accuracies of 73.37 to 99.23%.