Real-time ultrasonic testing for air-dried bimrock under triaxial deformation


Soil and rock mixture (SRM), also known as “bimrocks” (block-in-matrix rocks), is defined as structurally complex formation made up of a mixture of competent rock blocks with various sizes and high strength, fine-grained soil, random cracks, and pores that formed in the Quaternary period area (Lindquist, 1994; Xu et al., 2011; Wang et ah, 2014). SRMs originate from a variety of geological processes; the three most common processes are submarine landslides, tectonic melanges, and weathering eluvial aggregates, which are widespread all over the world and often represent a challenge in engineering practice. With the development of rock and soil mechanics and various kinds of large-scale projects, the study of the mechanical behavior of SRM has become an attractive direction in the field of geomechanics. The mechanical and physical properties of SRM are characterized by the extreme nonhomogeneity, looseness, and environmental sensitivity. To study the physical and mechanical properties of SRM, many scholars have carried out lots of significant studies, such as in situ large- scale horizontal push-shear tests (Xu et al., 2011), in situ shear test (Lindquist, 1994; Medley and Lindquist, 1995; Medley, 2001), laboratory shear test (Iannacchione, 1997; Iannacchione and Vallejo, 2000; Vallejo and Mawby, 2000), laboratory uniaxial test (Wang et al., 2014,2015), laboratory triaxial test (Fragaszy et al., 1992; Lindquist, 1994; Springman et al., 2003; Riicknagel et al., 2013), and meso-mechanical test (Wang et al., 2015; Lopera Perez et al., 2015). Unfortunately, one of the challenges in investigating the mechanical properties and meso-structure is the nondestructive inspection in real time during triaxial deformation, which includes the detection of rock-soil interaction, shear band, movement of rock blocks, cracks, and defects.

Ultrasonic techniques—as they are nondestructive and easy to apply for both in situ and laboratory conditions—are commonly used for establishing soil, rock, and rock like strength through ultrasonic pulse velocity (UPV) measurement (Kahraman, 2001; Yasar and Erdogan, 2004; Meglis et al., 2005). In addition, ultrasonic information (e.g., UPV, amplitude, quality factor) can reflect the characteristics of internal structure of soil and rock under various geological environments. These techniques have been used for determining the various properties of soil, rock, and rock-like material. However, the application of the real-time ultrasonic technique to SRM is not common. Wang et al. (2014) investigated the meso-damage mechanism and the cracking characteristics of SRM by ultrasonic test at the uniaxial compressive state. In fact, the stress state of SRM in geological environment is triaxial flexible boundary in situ stress. Therefore, the study on the mechanical and ultrasonic properties of SRM under triaxial compression is significant. Because of this, the real-time ultrasonic test under triaxial deformation exposed to the flexible confining boundary was conducted to investigate the mechanical and ultrasonic properties of SRM, with different rock block percentages (RBPs) (i.e., 20%, 30%, 40%, and 50%). The results of the study can be helpful in the geophysical exploration, geological reinforcement, and dynamic response prediction of SRM.

Materials and specimen preparation

Specimens were cylindrical in shape with a diameter of 50 mm and a height of 100 mm, according to the Soil Specimen Preparation Standard (Standard for Soil Test Method, GB/T 50123-1999); the diameter of blocks should not be greater than 10 mm, and the threshold value for rock and soil is 2 mm. Ten sieving tests of soil indicated that the soil belonged to high-plasticity clay. The soil contained lots of strong hydrophilic clayed minerals. The liquid limit of the hard clay can reach up to 40%, and the plastic limit can reach up to 36%; the plasticity index was about 48, and the liquidity index was about 0.05-0.127. Scanning electron microscopic (SEM) and X-ray diffraction (XRD) studies were conducted to identify the composition and contents of minerals. From the results of SEM studies, the rodlike and irregular quartz grains with a grain size of about 0.01-0.03 mm and that are probably surrounded by clay minerals can be observed. The XRD studies revealed that the soil has higher percentage of clay minerals, such as kaolinite (26.73%), montmorillonite (61.52%), and illite (6.25%). The rock blocks used in this study are marble stones with a size of 2-4 and 6-8 mm, and they were mixed in the ratio of 1:1. The natural density of the rock blocks is 2.67 g/cm3, whereas the compressive strength is about 94.5 MPa.

Compaction test was used to prepare the specimens (Riicknagel et al., 2013; Wang et ah, 2015); the optimal hammer count was determined based on the relationship between the density and the number of compaction tests. To keep the same soil matrix density (i.e., void ratio) in SRM specimens, the optimal hammer count for SRM specimens was determined as 14, 16, 18, 24, and 32 times, respectively. During the preparation of SRM specimens, the required amount of water was added into the mixture; the optimal water content was determined to be 9.5%, by compaction test. The required amount of rock blocks and soil material for each specimen was mixed and homogenized in a mixer. Then, the mixtures were poured into the cast iron test module cylinders of 50 mmx 100 mm in diameterx height. The specimens were then sealed with a plastic film to prevent water evaporation.

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