Piezo Disk Elements Research Articles

Compressibility and small strain stiffness characteristics of silt-hematite mixtures

Iron mining waste is disposed of near mines and forms iron ore (hematite) tailings dams. Furthermore, the distinctive characteristics of hematite particles can affect the geotechnical properties of soil. This study aimed to experimentally examine the compressibility and small-strain stiffness of silt-hematite mixtures. Silt-hematite mixtures were prepared in an oedometer cell at a fixed relative density with different hematite contents (0% to 100% in weight). Vertical displacements and loads were measured with the oedometer, and shear and compression waves were measured using bender and piezo-disk elements, respectively. The results showed that the void ratio and compressibility increased, while the constrained modulus decreased with increasing hematite content as hematite formed porous aggregate owing to high van der Waals forces. The elastic wave velocities (Vs and Vp) and small-strain moduli (Gmax and Mmax) increased with increasing hematite content owing to hematite's high stiffness and coordination number. Furthermore, the elastic wave velocities of high hematite-content specimens were practically constant at low-stress levels (<20 kPa), because of the van der Waals forces being greater than the skeletal forces. This study suggests that the geotechnical properties of hematite-containing soils, hematite-coated soils, and iron ore tailings soils can be evaluated by the hematite ratio.

Impact of diatoms on the load-deformation response of marine sediments during drained shear: Small-to-large strain stiffness

A comprehensive characterization of the seafloor is difficult, as natural marine sediments comprise granular materials. In particular, the presence of diatoms in granular mixtures significantly alters the engineering properties and behavior. Herein, we investigate the critical role of diatoms in the load-deformation response, shear strength, and small-to-large strain stiffness in sand-silt-diatom mixtures during the drained shear process. Mixtures with different fines contents (FCs) of 0%, 10%, 20%, and 30% were prepared in a triaxial cell incorporated with bender and piezo disk elements for elastic wave measurements. Triaxial tests were conducted under four different confining effective stresses, and elastic wave signals were monitored at every 0.5% of the axial strain. Test results show that a distinct transition exists in the stress-strain response and shear strength characteristics at FC = 20%. Using volumetric gravimetric analysis, we could clearly anticipate percolation threshold FC, where the specimen with 20% FC appeared to be on the boundary between the sand-controlled and transition zones in the textural triangular chart. For coarse-dominant mixtures (FC < 20%), silts enhanced particle interlocking and contributed to an increase in the contact area. In contrast, the internal friction of transitional mixtures (FC = 20%, 30%) decreased when a diatom with an excessively low shear modulus participated in load-carrying mechanisms. Using large-strain stiffness, we assessed fabric changes that reflect friction and the coordination number. Using small-strain stiffness from elastic waves, we estimated the state for a constant fabric. The characteristics of the moduli at large strain and those at small strain were distinctly different.

Resilient modulus estimation using in-situ modulus detector: performance and factors

ABSTRACT The resilient modulus of subgrade should be estimated for pavement designs that undergo repetitive loading from vehicular movement. In this study, an in-situ modulus detector (IMD) with flat, wedge, and cone tips is employed to estimate the resilient modulus. The specimens with three different levels of water content are compacted, and the dynamic responses for two hammer drop heights are analysed. For all specimens, elastic waves are acquired to estimate the shear modulus and Poisson’s ratio using bender and piezo-disk elements. The resilient modulus estimated from the IMD is compared with the lightweight deflectometer (LWD) modulus. The experimental results show a stronger relationship between the flat and wedge tips than between the cone and wedge tips. Furthermore, higher drop heights result in strong relationships between the resilient, shear, and LWD moduli. This study proposes an intrinsic design and verification of an IMD for reliable resilient modulus estimation in pavement design.

Characteristics of elastic waves passing through a flowable fill at early age

Elastic waves obtained from transducers are widely used to monitor the strength and integrity of cementitious materials. A pair of bender elements (BEs) and piezo disk elements (PDEs) embedded in a cuboid mould are applied to monitor elastic waves passing through a cementitious material known as a flowable fill at early age. The flowable fills used in this study are mixed with calcium sulfoaluminate cement, sand, silt, fly ash, water, and an accelerator. Compressional and shear waves acquired from the PDEs and BEs, respectively, are observed for up to 72 hrs. Results show that the compressional and shear wave velocities increased with curing time. The sensors can be used effectively to monitor the hardening characteristics of flowable fills at early age, and the BEs are particularly suitable for evaluating the hardening conditions of flowable fills as a result of the wide variation in shear wave velocities.

Evaluation of the freezing–thawing effect in sand–silt mixtures using elastic waves and electrical resistivity

The purpose of this study is to investigate the effects of the freezing–thawing process on soils using elastic waves and electrical resistivity. Sand–silt mixtures with different silt fractions of 0%, 10%, 20%, 30%, 40%, 60%, 80%, and 100% weight at a fixed degree of saturation of 40% are placed in a nylon freezing cell. The temperature of the sand–silt mixtures decreases from 20°C to −13.5°C during freezing and increases from −13.5°C to 20°C during thawing. Bender elements and piezo disk elements are used to continuously measure shear and compressional waves, respectively, during freezing and thawing. Four circular electrodes made of stainless steel are used to measure electrical resistivity. For the temperature measurement, a thermocouple is inserted into the specimen. The shear and compressional wave velocities as well as electrical resistivity dramatically change near 0°C during freezing and thawing for all sand–silt mixtures. After one cycle of freezing–thawing, the elastic wave velocities decrease and the electrical resistivity increases due to the fabric change of the specimens. During the thawing process, the electrical resistivity displays hysteresis behaviors (not elastic wave velocities) from −3°C to 0°C. This study demonstrates that elastic waves and electrical resistivity may effectively capture the property changes of the sand–silt mixtures during freezing–thawing.

탄성파를 이용한 모래-실트 혼합토의 동결-융해 특성

겨울철의 낮은 기온으로 인하여 지반 내부의 간극수는 동결과 융해를 반복한다. 지반에 이러한 동결-융해작용이 반복되면 흙의 입자구조 변형이 발생하며 이는 지중 기반시설에 손상을 가져올 수 있다. 본 연구는 흙의 동결-융해 과정에서의 탄성파 속도변화를 통하여 흙의 강성 변화 양상을 알아보기 위하여 수행되었다. 40 %, 60 %, 80 %의 3가지 모래-실트의 무게비를 가진 모래-실트 혼합토를 포화도 40 %, 상대밀도 70 %로 동일하게 조성하였다. 각 시료를 동결-융해를 위해 제작된 사각형 형태의 셀에 다짐법으로 조성하였다. 탄성파를 측정하기 위하여 한 쌍의 벤더 엘리먼트와 피에조 디스크 엘리먼트를 시료 양편에 설치하였으며, 시료의 온도 변화 양상을 관찰하기 위하여 탄성파 트랜스듀서와 같은 깊이의 중앙부에 열전대를 설치였다. 조성한 시료에 대하여 시료를 <TEX>$20^{\circ}C$</TEX>에서 <TEX>$-10^{\circ}C$</TEX>까지 동결시킨 후 <TEX>$-20^{\circ}C$</TEX>를 18시간 동안 유지하였으며, 다시 실험실 상온(<TEX>$20^{\circ}C$</TEX>)까지 온도를 서서히 올려 융해시켰다. 이 과정에서 온도, 전단파, 그리고 압축파를 측정하였다. 연구결과, 융해 이후의 탄성파 속도는 같은 온도의 동결 이전보다 감소하였다. 이때 전단파의 속도가 압축파의 속도보다 더 큰 비율로 감소하는 모습을 보였다. 실트의 함량이 40 %에서 80 %까지 증가함에 따라 탄성파의 속도는 증가하는 양상을 보였다. 본 연구를 통해 동결-융해가 불포화토의 입자구조를 느슨하게 만들며, 그 영향은 압축파에 비해 전단파 속도의 변화에서 잘 나타남을 알 수 있었다. In winter season, the pore water inside the ground freezes and thaws repetitively due to the cold air temperature. When the freezing-thawing processes are repeated on the ground, the change in soil particle structure occurs and thus the damage of the infrastructure may be following. This study was performed in order to investigate the stiffness change of soils due to the freeze-thaw by using elastic waves. Sand-silt mixtures are prepared with in the silt fraction of 40 %, 60 % and 80 % in weight and in the degree of saturation of 40 %. The specimens are placed into the square freezing-thawing cell by the temping method. For the measurement of the elastic waves, a pair of the bender elements and a pair of piezo disk elements are installed on the cell, and a thermocouple is inserted into soils for the measurement of the temperature. The temperature of the mixtures is decreased from <TEX>$20^{\circ}C$</TEX> to <TEX>$-10^{\circ}C$</TEX> during freezing, is maintained at <TEX>$-20^{\circ}C$</TEX> for 18 hours, is gradually increased up to the room temperature of <TEX>$20^{\circ}C$</TEX> to thaw the specimens. The shear waves, the compressional waves and the temperature are measured during the freeze-thaw process. The experimental result indicates that the shear and the compressional wave velocities after thawing are smaller than those of before freezing. The velocity ratio of after thawing to before freezing of shear wave is smaller than that of the compressional wave. As silt fraction increases from 40 % to 80 %, the shear and compressional wave velocities are gradually increased. This study suggests that the freezing-thawing process in unsaturated soil loosens the soil particle structure, and the shear wave velocity reflects the effect of freezing-thawing more sensitively than the compressional wave velocity.

Characteristics of elastic waves in sand–silt mixtures due to freezing

The objective of this paper is to characterize the properties of the compressional and shear waves resulting from the freezing of soil. After the sand–silt mixtures with different degrees of saturation (5%, 10%, 20%, 40% and 100%) are prepared in nylon freezing cells, the temperature of the specimens decreased from 20°C to −10°C. During the freezing of the soil, three pairs of piezo disk elements and bender elements, installed at three different locations along the depth, are used for the continuous measurement of the compressional and shear waves. The three properties of elastic waves, represented by velocities, resonant frequencies and amplitudes, are monitored continuously. Significant increases are observed in both of the elastic waves near the temperature of 0°C. As the specimens freeze, the influent factor of the compressional wave velocity changes from the degree of saturation to the ice bonding. After soil freezing, the critical factor that affects the shear wave velocity turns from the confining stress into ice bonding and contact size. The variation of the resonant frequency of the elastic waves according to the temperature and degree of saturation is similar to the variation of the velocity of elastic waves. After soils are frozen, the Poisson's ratio decreases with an increase in the degree of saturation with ice. This study provides fundamental information about the change of properties of elastic waves in soils due to freezing.

포화도에 따른 동결토의 탄성파 특성 변화

The strength of frozen soils is one of the significant design parameters for the construction in frozen ground. The properties of frozen soils should be investigated to understand the strength of frozen soils. The objective of this study is to figure out the characteristics of elastic waves in frozen soils, which reflect the constituent and physical structure of frozen soils in order to provide fundamental information of those according to the degree of saturation. Freezing cell is manufactured to freeze specimens, which are prepared with the degree of saturation of 10%, 40%, and 100%. Piezo disk elements are used as the compressional wave transducers and Bender elements are used as the shear wave transducers. While the temperature of specimens changes from 20℃ to -10℃, the velocities, resonant frequencies and amplitudes of the compressional and shear waves are investigated based on the elastic wave signatures. Experimental results reveal that the elastic wave velocities increase as the degree of saturation increases. The variation of resonant frequencies coincide with that of elastic wave velocities. A marked discrepancy in amplitudes of compressional and shear waves are observed at the temperature of 0℃. This study renders the basic information of elastic waves in frozen soils according the degree of

압축파와 전단파를 이용한 불포화토의 간극비 산정

Soils are commonly unsaturated in the near surface. The stiffness of soils is affected by the amount of air and water. The objective of this study is to evaluate the porosity of the unsaturated soils by using the elastic waves including compressional and shear waves. The elastic waves are measured at different degrees of saturation by controlling the matric suction. Thus, the unsaturated soils are characterized at different levels of the matric suction. Shear and compressional waves are measured by using the bender elements and the piezo disk elements, respectively. Both transducers are installed on the walls of the rectangular cell. The unsaturated soils are prepared by using uniform size sands and silts. Test results show that both compressional and shear wave velocities change according to the matric suction. The elastic modulus, the shear modulus, and the Poisson`s ratio are estimated based on the measured elastic wave velocities. In addition, the void ratio of the unsaturated soils estimated using elastic wave velocities matches well with the volume based void ratio. This study demonstrates that the elastic waves can be effectively used for the characterization of unsaturated soils.

Nongrouted Ratio Evaluation of Rock Bolts by Reflection of Guided Ultrasonic Waves

The objective of this study is to develop and apply reflection methods for guided ultrasonic waves used in evaluating the grout condition of rock bolts installed in a rock mass. Three types of rock bolts with different grouting ratios are prepared. First, piezoelectric disk elements are used as the source of guided ultrasonic waves to investigate grouted nonembedded rock bolts. Second, reflection methods with piezo disk elements and hammer impacts are used with rock bolts embedded in concrete columns. Third, a hammer impact with a center punch is applied for investigating rock bolts embedded in a rock mass in the field. The measured guided waves were analyzed using wavelet transforms. The peak magnitudes of the wavelet transform are used for group velocity calculations. Although piezo disk elements are sufficient as sources for nonembedded rock bolts and rock bolts installed in concrete columns, they do not provide sufficient energy in the field. However, a hammer impact with a center punch can generate guided ultrasonic waves with enough energy to evaluate the nongrouted ratio in rock bolts embedded in a rock mass. The group velocities of the guided ultrasonic waves increase with increasing nongrouted ratio. This study demonstrates that the suggested hammer impact method is effective for evaluating the nongrouted ratio of rock bolts in the field.

Characteristics of Elastic Waves According to Cementation of Dissolved Salt

Natural cementation influences the mechanical behavior of granular media. The goal of this study was to investigate the phenomenon of the low strain stiffness of granular media during precipitation and cementation of dissolved salt in the pore fluid. Granular specimens of glass beads with three different sizes (0.26, 0.5, and 1.29 mm) were cemented by NaCl aqueous solutions (0, 0.1, 0.50, 1.0, and 2.0 mol L−1). The small‐strain stiffness was investigated by measuring compressional (P) and shear (S) waves using bender elements and piezo disk elements during the cementation process. The change of P‐ and S‐wave velocities during cementation due to drying shows three distinct stages based on the degree of saturation, S: (i) S‐wave velocity increases, but P‐wave velocity decreases at a degree of saturation of 100 to ∼90%; (ii) the elastic wave velocities are stable at 90 to ∼10% saturation; and (iii) the elastic wave velocities dramatically change (increase and decrease) at a degree of saturation of 10 to ∼0% due to salt cementation and capillary force. Without cementation, the three grain size particles are controlled by capillary forces. The cementation occurs at contact points of particles for low salt concentrations and at both pores and contact points for high salt concentrations. The measured small‐strain stiffness agreed with the predicted stiffness based on the micromechanical model. This study demonstrates the elastic wave characteristics of granular media in a vadose zone according to the cementation of dissolved salt.

Active vibration control for the identification‐based model of a circular plate

An active vibration control system is proposed for suppressing the small amplitude vibration of circular plate. An experimental set‐up consists of a hard‐walled cylinder with a thin metallic plate at one end. Primary excitation is provided by a low frequency loudspeaker installed centrally at the bottom of the cylinder. The vibrations of the plate are measured by the application of strain sensors and accelerometers. Intelligent materials such as 2‐layer piezo disk elements are used as the actuators. For the considered system the OE (Output Error) method of discrete‐time model identification for real‐time active vibration control have been applied. The mathematical model obtained by this method identification is then employed for the linear pole placement controller design. Numerical simulations describing the attenuation effects are presented and discussed.