Laboratory-scale characterization of saturated soil samples through ultrasonic techniques

Abstract

the propagation of poroelastic waves in a soil specimen is dependent on the physical and mechanical properties of the soil. in the geotechnical practice, such properties are obtained through in-situ geotechnical testings or element soil testings in the laboratory. these methods require almost advanced equipment and both testing and sample preparation may be expensive and time-consuming. this paper aims to present an algorithm for a laboratory-scale ultrasonic non-destructive testing to determine the physical and mechanical properties of saturated soil samples based on the distribution of stress waves. the ultrasonic setup, in comparison to most conventional soil lab testing equipment, is low-cost and non-invasive such that it reduces the soil disturbance. for this purpose, a poroelastodynamic forward solver and differential evolution global optimization algorithm were applied to characterize the porosity, density, and other mechanical properties for a soil column. the forward solver was developed based on a semi-analytical solution which does not require intensive computational efforts encountered in standard numerical techniques such as the finite element method. It was concluded that the proposed high-frequency ultrasonic technique characterizes effectively the saturated soil samples based on the output stress wave measured by the receiver. this development makes geotechnical investigations time-efficient and cost-effective, and as such more suited to applications in remote areas.Characterizing foundation soils is the first step in design and construction of civil infrastructure. The measurement of physical and mechanical properties of soils (e.g shear wave velocity, compression wave velocity, density and porosity) requires intensive in-situ or laboratory tests, which can be time consuming and costly. Soil samples, especially from projects in remote areas, are required to be transported to a geotechnical laboratory for various tests. This can cause the disturbance of soil samples, and laboratory tests on disturbed samples may lead to erroneous conclusions.The laboratory methods for measuring the shear wave velocity of soil samples include the resonant column test, bender element test among others. However, there is no established standard developed for the interpretation of the dynamic test results 1 . The bender element method was developed in the 1980s and its simplicity is widely recognized: one transducer is placed at one end of a soil specimen for the generation of stress waves; one receiver is placed at the other end to record the induced stress waves. Various interpretation methods have been proposed in the past. The shear wave velocity can be calculated from the time difference between the input and output waves by assuming the absence of reflected or refracted waves 2 . However, it is well known that the identification of the arrival time of the output wave is subjective 1 . Other signal processing techniques such as the cross-correlation of the input and output stress waves 3 and the second arrival of...

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