Bender Element Research Articles

Experimental characterization of the small strain shear modulus of soft clay stabilized with cement and nano-SiO2 using bender element tests

Experimental characterization of the small strain shear modulus of soft clay stabilized with cement and nano-SiO2 using bender element tests

Optimizing modified triaxial testing for small strain zone using local displacement transducers and bender element for cement-treated soft soil

The settlement behavior is a common problem on the railway structure that can be optimized by applying cement-treated soil as ground restoration. However, the application of a high cement mixing content needs a proper estimation that can be achieved by adjusting the element testing. The strain measurement devices can estimate the deformation characteristics, such as secant modulus, Poisson ratio, and shear modulus that can describe the settlement behavior and stiffness of cement-treated soil. This research is focused on a static analysis of triaxial consolidated undrained (CU¯) testing that is improved by the axial and radial local displacement transducer (LDT) and bender element to increase the accuracy of measurement results. Furthermore, the secant modulus and shear modulus is more accurate when the combination of radial and axial LDT is used due to a small strain range. Lastly, the shear modulus measurement is improved by using a filler in the cement-treated soil for the bender element test. To conclude, this system of testing for the static condition can be utilized for the dynamic condition, because the measurement shows a reliable result for a small strain range which is the parameter of the dynamics condition.

Liquefaction Resistance of Different Size/Shape Sand-Clay Mixtures Using a Pair of Bender Element–Mounted Molds

Abstract The aim of this study is to assess the influences of both the size and shape of sand grains mixed with a clay on shear modulus (Gmax)– and shear wave velocity (vs)–based liquefaction resistance (cyclic resistance ratio, CRR) using an intensive series of bender element (BE) tests. A low plasticity clay (CL) was added to two specific particle gradations (0.15–0.30 mm, and 1.0–2.0 mm) of clean sands with particular shapes (angular and rounded) at the percentages of 5, 10, 15, 20, 30, and 40, by dry weight. The BE tests were performed on the remolded specimens of different size/shape sands containing various amounts of CL in a recently designed mold used specifically for cohesionless soils, and then the values of vs, Gmax, and CRR were investigated. In most of the cases, the testing results indicated that, up to 20 % fines, which is named as transition fines content (FCt), Gmax and CRR values decreased, and then they increased for the specimens tested. Eventually, Gmax and CRR values of the specimens were significantly changed by the physical features (size, shape) of the sand grains with all the clay inclusions and frequencies applied.

Sandy Soil Improvement Using MICP‐Based Urease Enzymatic Acceleration Method Monitored by Real‐Time System

This paper aims at monitoring the improvement of sandy soil properties with biocementation through the microbially induced calcite precipitation (MICP) method with reaction accelerations by self‐developed soybean urease enzymes. In this study, the concentration of calcium ions (Ca2+ ions as CaCl2) is varied at 50, 100, 250, and 500 mM to determine an optimum shear strength. The self‐developed soybean urease enzymes of 20% by volume (v/v) are used to accelerate the MICP reaction to finish within 7 days. Based on real‐time monitoring bender element system and direct shear tests, the optimum Ca2+ concentration is found as 250 mM. However, a detrimental effect occurs in case of high concentration of Ca2+ as CaCl2 (500 mM) because of solution acidification from high Cl− concentration. This condition lowers CaCO3 precipitation causing the reduction of biocementation process. At equivalent shear modulus, the biocementation time of MICP‐based sand with acceleration by urease enzymes is about 10 times faster than that without. Using spectrophotometer and pH meter, the ammonification rate and the solution pH of biocemented sand with acceleration by urease enzymes for 3 days are found relatively higher than those without urease enzymes for 40 days. The analyses by scanning electron microscopy (SEM) and X‐ray diffraction (XRD) confirm not only the occurrence of CaCO3 binding sand particles together but also the improvement of physical strengths of sandy soil samples with the MICP‐based urease enzymatic acceleration method. These results introduce an option to accelerate biocemented sandy soil improvement.

Assessment of Sulfate-Induced Heave in Chemically Treated Soils Using a Novel Hybrid Sensor

Abstract Highway agencies frequently encounter problems caused by sulfate-induced heave in lime- and cement-treated subgrades. A thorough evaluation of sulfate-related risks for subgrades involves time-consuming laboratory tests. A real-time monitoring of the sulfate heaving phenomenon in the test section in the real field environment would provide a quick performance evaluation of sulfate soils treated with chemical additives. This would help transportation agencies in producing resilient pavements with low maintenance issues. This article presents an innovative hybrid sensor comprising a bender element and time domain reflectometry that can evaluate sulfate-related heave in real time. The new sensor was calibrated in the laboratory by studying the strength and stiffness behavior of three chemically treated high-sulfate soils. Following laboratory calibration, the sensor was embedded in lime-treated field sections, where the changes in moisture and stiffness were monitored simultaneously in real time. Field testing proved that the sensor was able to provide data to assess sulfate-induced heaving in the field. This article details the development of this sensor that can save millions of dollars for transportation agencies in the better stabilization design for sulfate soils.

Correlation between motion of source bender element and feedback signal induced by self-monitoring

Since the introduction of bender element tests to soil testing, the reliability of the estimated travel time has been the most serious problem. The author has previously shown a potential solution whereby removing the response of the bender element subsystem from the whole response could dramatically improve the accuracy of the travel time estimation. In order to lay the foundation for estimating the response of the bender element subsystem, this paper examines the correlation between the displacements of the element and the induced feedback signals by employing a self-monitoring element. The response of the self-monitoring element is modeled as a transfer function involving two internal transfer functions that relate the input signals to the displacements and the displacements to the feedback signals, respectively. Using a laser displacement sensor, the displacements are directly measured through the entire surface and reveal the three-dimensional bending motion of the element oscillating in both longitudinal and width directions. The feedback signals are similar to, but inconsistent with, the tip displacements, and an attempt is made to correct the feedback signals. Finally, a conclusion is given on the potential for estimating the response of the bender element subsystem using the self-monitoring element.

Stiffness and strength anisotropy of overconsolidated Bootlegger Cove clays

This paper presents the evaluation of the stiffness and strength anisotropy of overconsolidated (OC) Bootlegger Cove Formation (BCF) clays at the Port of Alaska, formerly known as the Port of Anchorage. The stiffness and strength anisotropic material response was evaluated based on triaxial samples equipped with internal instrumentation including a submersible load cell and three subminiature linear variable displacement transducers (LVDTs). Three sets of bender elements were used in this research to measure shear wave velocities for different propagation and polarization directions. The effects of reproducing the stress history of the soil deposit on the stiffness cross-anisotropic behavior of the material are discussed. The laboratory test results are compared with in situ measurements of shear wave velocities based on suspension logging and crosshole and downhole soundings. The results of the experimental program showed that BCF clay is a cross-anisotropic material. Mean stiffness anisotropy ratios ranged from 0.90 to 1.22 and 0.93 to 1.46 for lightly OC and OC conditions, respectively. Strength anisotropy ratios, defined as the ratio of undrained shear strength under triaxial extension to compression, varied between 0.8 and 0.5. It is found that reproducing the stress history of the OC soil deposit during the laboratory reconsolidation stage did not have a significant impact on the initial stiffness anisotropy ratios of the BCF clay.

Liquefaction Resistance of Sand Improved with Enzymatically Induced Calcite Precipitation based on Laboratory Investigation

Liquefaction possibility assessment of soil in field is a prominent aspect on geotechnical engineering practice. Varieties of evaluations have been developed including penetration-based methods and shear wave velocity methods over the years since Seed & Idriss (1971) introduced the simplified method based on SPT data. With the focus on natural soil, those methods have been well evolved and have become the most frequent techniques used for evaluating liquefaction possibility of soil in situ. However, the capability of those empirical relations for evaluating liquefaction possibility of ground improved with EICP approach needs authentication. This research presents the capability of the available liquefaction evaluation charts for evaluating liquefaction possibility of ground improved with EICP approach. Investigations were conducted using cyclic triaxial apparatus integrated with bender element to evaluate liquefaction resistance, RL , of EICP-improved sand as a liquefaction countermeasure. Parameters used were confining pressures, degrees of saturation during curing, calcite contents, and particles size of the sand. The test results were compared with the field performance charts of natural soils introduced by investigators. The results show that slight amount of the precipitated calcite can improve notably the maximum shear modulus of soils directly associating to Vs improvement. The correlation between corrected resistance to liquefaction CRR (cyclic resistance ratio) and adjusted shear wave velocity Vs1 of EICP-improved sands, CRR-Vs1 correlation, tends to represent different tendency with that of unimproved sands. It appears that available liquefaction prediction curves are not capable for evaluating liquefaction possibility of EICP-improved ground.

Measuring soil bulk density from shear wave velocity using piezoelectric sensors

Bulk density and soil stiffness moduli are vital physical parameters related to soil compaction, porosity, moisture storage capacity, soil penetration resistance and structural integrity. Conventional methods for measuring soil density and stiffness moduli are destructive, time-consuming, complex, expensive and often require skilled operators to conduct the tests. A new soil density and stiffness moduli measurement technique that can evaluate soil density and stiffness moduli more rapidly, efficiently and precisely, at a low cost is introduced here. This study evaluated the use of shear wave velocity measurements using the piezoelectric extender and bender elements as a viable alternative to measure soil density and stiffness moduli of soil. To test this idea, soda-lime glass beads of <0.002, 0.04–0.07 and 1.00–1.30 mm in diameter were used to develop the empirical relationship between the shear wave velocity and the bulk density of soil in laboratory conditions. These empirical equations were then tested on sands and clayey soils for validation. Accuracy in terms of coefficient of determination (R2) and root mean squared error (RMSE) from the current and existing studies ranged within 0.91–0.93 and 0.073–0.177 g cm–3 respectively. Both shear and Young moduli were compared with the shear wave velocity of soil, with R2 and RMSE of 0.96–0.97 and 0.48–3.5 MPa respectively. The major advantage of this technique is that input and output signal data can be stored in a computer that can be used to calculate soil density and stiffness moduli automatically. This technique could play a vital role in improving crop yield and soil management practices.

A Portable Bender Element-Double Cone Penetration Testing Equipment for Measuring Stiffness and Shear Strength of In-Situ Soft Soil Deposits

A portable bender element-double cone penetration (BE-DCP) testing equipment, which is invented as a prototype model for measuring the small-strain stiffness (shear wave velocity, Vs) and shear strength of in-situ soft soil deposits (loose sand and soft clay) is developed in this study. The shear wave velocity in two orthogonal directions (Vs-hv and Vs-hh) of the in-situ soils is measured from paired bender elements (BE) installed on the side of the cone shaft. The bender elements are housed in a mechanically controlled sliding block in the shaft which not only protects bender elements while the BE-DCP apparatus is being pushed into the ground, but also ensures a good contact between adjacent soil particles and benders when the target depth is reached. The real-time relationship of the pushing force and penetration depth is recorded by force transducer and laser displacement sensor and graphed in LabVIEW. Based on the cone penetration testing (CPT) theory, two formulas are derived to calculate the shear strength parameters of the in-situ soils. Test results obtained from both the portable bender element-double cone penetration (BE-DCP) testing apparatus and conventional laboratory experiments on Ottawa sand 20/30, four different sizes of Dolomite sands in various packing densities and Kaolinite under different water contents and densities as well as a field testing were compared and validated, indicating that the novel designed portable bender elementdouble cone penetration (BE-DCP) apparatus is robust, fast, and cost-efficient for measurement of small-strain stiffness and shear strength of in-situ soils.

Effect of non-plastic fines on the anisotropic small strain stiffness of a calcareous sand

This study investigates the effect of non-plastic fines on the anisotropic smalls strain stiffness of a calcareous sand by conducting multidirectional bender element tests. GHH, GHV and GVH are evaluated on both horizontal and vertical planes and the stiffness anisotropy, considering the effect of fine content, is studied. The models for predicting small strain shear modulus G0 of the calcareous sands are modified. Test results show that G0 decreases with the increase of the fine content until a threshold fine content is reached. The stiffness anisotropy drops with the increases of the fine content. The modified Hardin equation provides a better prediction than the modified Wichtmann equation and the Payan equation.

Monitoring lime and cement improvement using spectral induced polarization and bender element techniques

Abstract Spectral induced polarization (SIP) and bender element (BE) techniques show a high sensitivity to particle size, particle distribution and content of generated hydration products, which essentially govern the efficiency of ground improvement. In this context, both SIP and BE were integrated on a column setup to monitor the processes of lime and cement stabilization. A 5 mmol/L Na2CO3 solution was injected into the sand-lime mixture to produce CaCO3 precipitation, while deionized water was injected into the sand-cement mixture to induce the hydration of cement. The average diameters of the precipitated particles or clusters were calculated from the relaxation time, which was a significant parameter of SIP signals, via the Schwarz equation. Two pairs of BE were used to demonstrate the heterogeneity of the product precipitation, which was probably caused by the location of the inflow and outflow on the SIP-BE column. SIP and BE showed the capability of nondestructively monitoring the spatiotemporal chemical evolution processes, which could be helpful for engineering applications.

Compressibility and monotonic shearing behaviour of Toronto peat

The shearing behaviour of organic soils, such as peat, under monotonic loading can be very different than those of inorganic soils due to the presence of fibers. A series of one-dimensional consolidation, direct simple shear, and bender element tests are conducted to investigate the compressibility and the monotonic shear strength of a peat deposit from the Port Lands area of Toronto, Ontario, Canada. Laboratory specimens are trimmed from block samples collected from a depth of about 4.0 to 4.5 m. The samples' organic content ranges from about 42% to 60%, with a fiber content of 20% to 30%. Because of their moderate organic and water (180%–237%) contents, the peat samples are found to be less compressible with a slower consolidation rate than several other surficial peat deposits reported in the literature. All samples exhibit a ductile shearing behaviour with undrained strength ratios ranging from 0.32 to 0.57 and a residual effective friction angle of about 30°. The monotonic tests are further examined within the framework of critical state soil mechanics. Bender element measurements are used to develop empirical relationships between several engineering parameters of Toronto peat with shear wave velocity.

Laboratory Investigation into the Effect of Particle Sizes on Shear Wave Parameters Using Bender Elements Test Results

Abstract Evaluating the influences of soil particle size on the dynamic behavior of soils during wave propagation has been an important issue in geotechnical engineering. Heretofore, the effects of particle size on shear wave velocities in soils have been examined using various experimental techniques. Most research in this regard so far have been carried out over a limited range of particle sizes, and the results have indicated contrasting effects of particle size on shear wave velocity (Vs). However, there has not been a comprehensive and unambiguous outcome describing the influences of particle size on Vs in soils. This research aims to experimentally investigate the effects of a wider range of particle sizes on Vs, wave shape, and frequency content of the shear waves transmitted in soil specimens. To this aim, a bender element apparatus embedded in a triaxial cell were used to minutely measure shear wave parameters of ten groups of local dry sandy soils, R1 (d50 = 0.225 mm) to R10 (d50 = 7.14 mm) samples. Moreover, dry tamping methods were employed to provide comparable samples at a similar initial void ratio of 0.8. Various amounts of frequency (5–12 kHz) and various confining pressures (50–500 kPa) were conducted on the identically prepared clean sand samples. Based on the results, it was observed that, for samples with mean particle sizes between 0.225 mm (R1) and 1.290 mm (R6), the Vs increased with the particle diameter, but for samples with mean particle sizes between 1.290 mm (R6) and 7.140 mm (R10), an increment in the particle diameter caused the Vs to decrease. It can be inferred that particle size influences the shape of the received signals such that the frequency content of the received signals, in both fine- and coarse-grained soils, are quite similar; however, medium-sized soils have a higher frequency content and a higher Vs, as well.

Monotonic and Cyclic Behavior of Salt-Encrusted Flat (Sabkha) Soil

Sabkha (salt-encrusted flat) soils are problematic because they lose strength due to wetting, and they have liquefaction potential. These soils are spread in North Africa, Australia, and most of the Eastern Province of Saudi Arabia. Owing to a lack of experimental studies, the cyclic behavior of sabkha soils is relatively unknown. The monotonic and cyclic behaviors of sabkha soil were studied based on effective stress (50, 100, and 150 kPa) and cyclic stress ratio (CSR) (0.15, 0.35, and 0.65) using cyclic triaxial and bender element tests. Results indicate that the sabkha exhibits ductile behavior with the cohesion value of 9.33 kPa and a friction angle of 33°. The maximum shear moduli are 18,900, 49,500, and 63,500 kPa for effective confining pressures of 50, 100, and 150 kPa, respectively. Furthermore, the shear modulus tended to decrease with shear strain for different cyclic stress ratios. On the other hand, the damping ratio depends on the level of the cyclic stress ratio. At a cyclic stress ratio of 0.15, the damping ratios remained constant with shear strain. For a cyclic stress ratio of 0.65, the damping ratios decreased with shear strain. However, at a cyclic stress ratio of 0.35, the damping ratio varied with shear strain depending on effective stress.

Normalised behaviour of a non-plastic silt–pumice sand mixture

The existence and failures of silty sand have been reported worldwide, and the behaviour of sand–silt mixtures at small to large strains has been intensively studied. Owing to the relatively low stress level achieved in laboratory testing, no unifying framework has yet been proposed for sand–silt mixtures. In the presented work, a crushable sand, made of pumice, was used as host sand so as to reach a unique normal compression line under pressures easily attainable in the laboratory. The sand was combined with non-plastic fines in proportions varying between 0 and 100%, and tested in compression and shearing with associated bender element testing for small-strain stiffness measurements. The unique relationship between pressure and volume on the normal compression line allowed for the normalisation of the data in a way that highlights the effects of fines irrespective of the density. It was found that once the effects of the fines on the index properties and compressibility have been cancelled, by normalising for volume, the behaviour of the sand–silt mixtures can be characterised as unique.

Spectral induced polarization study on enzyme induced carbonate precipitations: influences of size and content on stiffness of a fine sand

Heterogeneity in either chemically or microbiologically induced carbonate-based ground improvement methods is a major obstacle in engineering application. Spectral induced polarization (SIP), an innovative and nondestructive method, which has demonstrated promise in monitoring microbial activity, was used in this study to monitor enzyme induced carbonate precipitation (EICP). The complex conductivities, together with the shear wave velocities (Vs), of an EICP modified sand were monitored using a self-developed spectral induced polarization–bender element column. The mean precipitate size was calculated by relaxation time (τ) and the Schwarz equation. The precipitate contents were calculated by cumulative gamma distribution function on the global polarization magnitude (mn) with R2 = 0.989. The stiffness of the enhanced geomaterial, in terms of Vs, correlates to mn with a cumulative lognormal distribution function with R2 = 0.967. Contact cementation was postulated as the dominant association pattern. The possible mechanism for this may be the formation of eddies and the nucleation of CaCO3 crystals during precipitation. The results suggest that SIP can be used as an effective nondestructive monitoring tool to assess the stiffness of geomaterials.

Static liquefaction behaviour of gold mine tailings

Static liquefaction failure of tailings impoundments has been a persistent issue for the mining industry for many decades. In this study, the monotonic shearing response and instability of gold tailings are examined through a series of constant-volume and drained direct simple shear tests on slurry-deposited and moist-tamped specimens. The experiments were carried out on both the silt and the sand tailings produced at the mill and separated for use in dam construction. Laboratory shear wave velocity measurements made by means of bender element tests were also used to relate the shearing response and strength of the tailings to an in situ geophysical measurement. Specimen fabric differences produced by the different preparation methods do not translate into significant differences in the critical state line, liquefaction triggering or post-liquefaction strength for the sand tailings. Additionally, common trends of undrained yield and post-liquefaction strength ratios with state parameter were observed for both the sand and the silt tailings despite their different fines contents. An empirical method to evaluate the onset of instability and the post-liquefaction strength of the tailings using shear wave velocity is proposed.

Study on Characteristics of Shear Modulus of Zhanjiang Structured Clay During Triaxial Shearing

The small strain stiffness described by shear wave velocities was used to reveal the characteristics of soil structure. Bender element tests were performed on triaxial compression test samples to determine the shear wave velocity and maximum shear modulus of Zhanjiang structured clay and remolded clay, which could allow the stress–strain responses to be obtained concurrently. The results showed that the degradation of the shear modulus of the remolded clay was achieved by a reduction of the effective mean stress, while that of the structured clay was influenced by both the effective mean stress and structural damage. It was also determined that the difference between shear modulus calculated by effective mean stress and that calculated by shear wave velocity was caused by structural damage. Hence, the maximum shear modulus of structured clay is reasonably connected to soil structure, assessing the degradation of shear modulus can reveal the progression of structural damage, then the evolution of structural damage and stiffness degradation of structured clay during triaxial shearing is established.

Effect of sand on the stiffness characteristics of cement-stabilized clay

This study aims to investigate the effect of sand in the cemented clay matrix by understanding the limitations of conventional mixture design concepts. To characterize the behaviour of cemented sandy clay, an extensive experimental study was conducted based on bender element (BE) testing and ultrasonic pulse velocity (UPV) testing. Scanning electron microscope image showed that stiffer sand particle bonded to the cemented clay matrix caused the increase in stiffness. The velocity ratio and thereby dynamic Poisson’s ratio reduced with the increase in stiffness of cemented soil. Finally, a predication model is proposed to estimate the shear modulus and validated with the data obtained from literature for other soil and cement types.