Dynamic Soil Properties Research Articles

A weighted soil heath index approach for refined assessment of soil health in cropping systems

IntroductionPromoting sustainable crop production is enhanced by an effective method to assess soil health. However, soil health assessment is challenging due to multiple interactions among dynamic soil properties (i.e., soil health indicators) across management practices and agroecological regions. We tested several currently popular soil health assessment methods for cropping systems in Tennessee in the southeastern US and found that these methods failed to differentiate Tennessee soil health under long-term conservation and conventional management.Materials and methodsThis study developed a Tennessee weighted soil health index (WSHI) by: 1) selecting a set of management-sensitive soil health indicators, 2) assigning meaningful weights to indicators, and 3) normalizing the scores based on regionally relevant undisturbed natural reference sites. The tested cropping systems treatments were moldboard plow (MP) in continuous soybean (SS), no tillage (NT) in SS, NT with wheat cover (NTW) in SS, no cover and chisel plow (NCCT) in continuous cotton (CC), no cover and no tillage (NCNT) in CC, and hairy vetch cover and no tillage (VCNT) in CC. In addition, two woodlots and one grassland sites in the vicinity of the cropping systems were selected to represent undisturbed natural systems.Results and discussionOut of 22 indicators that proved to be management-sensitive, six were selected as a minimum dataset (MDS). These were particulate organic matter C (POM-C), soil respiration from 4-day incubation (4d CO2), small macroaggregate (0.250-2mm)-associated C (SMA-C), surface hardness (PR15), microbial biomass N (MBN), and bulk density (BD). Measured values of the MDS indicators were transformed into unitless normalized scores (based on the regional range of the indicator), and finally integrated into WSHI scores using a weighted-addition approach. Additionally, the soil health gap (SHG) between the soil health of the regional reference system and different cropping systems was calculated. Results revealed that WSHI strongly differentiated soil health between long-term conservation and conventional managements practices. The WSHI scores for southeastern cropland soils varied as follows: VCNT = NTW > NT > NCNT ≥ NCCT ≥ MP. The SHGs under MP, NCCT, NCNT, NT, NTW, and VCNT were 85.5, 79.9, 68, 45.1, 25.2, and 24.3, respectively, relative to the average WSHI of three undisturbed systems. Results showed that the WSHI approach is effective in more meaningful regional assessment of soil health and SHG can be a potential metric for comparing soil health across agroecological regions.

Experimental Study of Influence of Freeze–Thaw Cycles on the Dynamic Properties of Weathered Sand-Amended Expansive Soils

Expanded soils are widely distributed in Xinjiang, China, so roadbeds will inevitably pass through the areas of the expansive soil during road construction. While Xinjiang belongs to the seasonal frozen region, subjected to a freeze–thaw cycle, mud pumping is likely to occur in the subgrade under dynamical load. To study the dynamic properties of expansive soil for a range of freezing–thawing cycles and weathered sand contents, a series of dynamic triaxial tests were performed using a medium-scale true triaxial apparatus equipped with the cyclic loading device. And the maximum dynamic elastic modulus (Edmax) and ultimate dynamic stress amplitude (σdmax) were quantitatively analyzed by the modified Hardin–Drnevich three-parameter model, on which the expression of the damage degree under the joint action of both was established. The results indicate that the dynamic performance of expansive soil is greatly affected by freezing–thawing. Under the same weathered sand content, the maximum dynamic elastic modulus and the maximum dynamic stress amplitude of soil are inversely related to the number of freezing–thawing cycles. And, those two parameters tend to be stable after the sixth freeze–thaw cycle. Under the same number of freeze–thaw cycles, the maximum dynamic elastic modulus and the maximum dynamic stress amplitude firstly decrease and then increase with the increase in the weathered sand content. The critical dosage of weathered sand is 10%, and the strength of the improved soil reached the minimum value under this context. The damage degree model that integrates the effects of freeze–thaw cycles and the weathered sand can better predict the damage degree of soil.

Determination of Dynamic Properties of Fine-Grained Soils at High Cyclic Strains

Shear modulus (SM) and damping ratio (DR) are significant in seismic design and the performance of geotechnical systems. The evaluation of soil reactions to dynamic loads, such as earthquakes, blasts, train, and traffic vibrations, necessitates the estimation of dynamic SM and DR. The aim of this research is to determine the cyclic parameters of unsaturated soils in and around Peshawar, and how these properties depend upon the varied confining pressures and shear strains. Undisturbed samples were collected using Shelby tubes from five boreholes at different locations along Jamrud Road, Peshawar. The index properties (grain size distribution, plasticity index, and specific gravity) and dynamic properties of these samples were determined. Three samples of 100 mm in height and 50 mm in diameter were obtained from each Shelby tube. After preparing and mounting the sample in the triaxial cell, the sample is first saturated by increasing the cell and back pressures in increments of 50 kPa until the value of Skempton’s pore pressure parameter (B) reaches ≥ 0.96. Samples were consolidated at confining pressures of 150, 200, and 300 kPa, then subjected to cyclic shear strains of 0.2, 1, 2, 2.5, and 5%. Shear stress–strain hysteresis loops were plotted, and the values of SM and DR were calculated for each cycle. Generally, at shear strains of 0.2 and 1%, the slope of the loops is steep, and gradually becomes gentler at higher strains of 2, 2.5, and 5%. It is found that, with an increasing number of cycles, the SM and DR decrease. The SM decreases with increasing shear strain, whereas the DR increases at shear strains of 0.2–1%, then decreases for strains of 2, 2.5, and 5%. The confining pressure has more influence at a shear strain of 0.2–1%, while little effect has been observed at a shear strain of 2.2–5%. The values of SM are higher at higher confining pressures at a given shear strain. The results show higher stress values during the initial cycles because of the greater effective stress that developed in response to shear strain while, with an increase in the number of cycles, the pore water pressure gradually increases, thereby reducing the effective stress and weakening the bonds between soil particles. In dynamics, when the confining pressure increases, particles are closer to contact, so the travel paths of waves increase. The energy loss will increase, so DR will decrease.

Particle shape effects on dynamic properties of granular soils: A DEM study

Particle shape is an essential aspect of granular soils that affects their mechanical properties. It is challenging to isolate the effect of particle shape from other mechanical properties such as inter-particle friction coefficient and particle shear modulus in laboratory tests. The discrete element method (DEM) makes it feasible to investigate the effect of macroscopic and mesoscopic particle morphologies including sphericity and roundness. This study provides a systematical DEM investigation of the particle shape effect on the dynamic properties of granular soils by simulating cyclic undrained triaxial tests at various initial confining pressures and void ratios. The results indicate that specimen consisting of angular particles generally exhibit a higher secant shear modulus G than that of rounded particles. Additionally, particle shape has no significant effect on modulus ratio G/G0 but a noticeable impact on damping ratio D at relatively large strains due to dilatancy. A weighted coordination number CN' was proposed to reveal the microscopic effect of particle shape on G0. Contact sliding was found to be the main cause of modulus degradation and damping increase with increasing shear strain. The tangential shear modulus ratio Gtan/G0 is inversely correlated to the weighted contact sliding ratio fs'.

Soil–Structure Interactions for the Stability of Offshore Wind Foundations under Varying Weather Conditions

Fixed-bottom foundations of offshore structures, mainly monopiles, are subject to extreme events and other critical cyclic nature loads. Since offshore wind turbine structures are slender, the manufacturers of offshore wind turbines give a range of frequencies for safe operation during a structure’s life cycle. Highly reliable measurements and accurate determination of shear moduli and damping ratios are crucial to ensure the stability of these structures, for example, to avoid the resonance of the structures. Because foundation–soil properties change over a period of time due to various environmental factors, this should be taken into consideration for designs. In the current investigation, behaviours of dry sand under dynamic loads were explored. Cyclic loads of strain amplitudes of 0.05%, 0.1%, 0.25% and 0.5% were carried out in a cyclic simple shear apparatus to explore the evolution trend of the stiffness and damping ratio of the soil. Attempts were made to simulate varying weather conditions by conducting cyclic tests with different strain amplitudes representing normal weather conditions and extreme weather conditions. It was found that soil dynamic properties vary remarkably at first and then tend to stabilise under cyclic loading with the same strain amplitude. However, with varying strain amplitude, property variations continue further. From numerical analyses using the discrete element method, it was found that this is due to the disturbance of soil, causing further particle rearrangements and soil compactions, following a sudden change of strain amplitude, which leads to further property variations.

Multimode Free-Vibration Decay Column: Small-Strain Stiffness and Attenuation

This study presents a simplified resonant column testing method to obtain small-strain dynamic properties of soils in both torsional and flexural vibrations. The method exploits free vibration decay responses of the system produced by manual excitation while the specimen is subjected to an isotropic effective confining stress produced by a vacuum pressure. This method is readily applicable to standard resonant column and torsional shear devices and triaxial cells by attaching a metal bar with one or two accelerometers for manual excitation, but not using an electromagnetic driving plate. This paper describes the apparatus design, test procedure, system calibration, and data analyses, as well as the test results of dynamic properties of a dry sand, including small-strain elastic moduli and damping ratios obtained from the torsional and flexural modes. The results confirm that the suggested method can capture strain-dependent characteristics up to the strains of ∼10−4 beyond typical elastic threshold strains, although the isotropic effective confining stress is limited to ∼90 kPa. This unique testing method provides remarkably consistent and reliable measurement for the dynamic properties of soils, and it avoids any possible bias from the counterelectromotive force.

Effects of Drying–Wetting Cycle and Fines Content on Hysteresis and Dynamic Properties of Granite Residual Soil under Cyclic Loading

In southern China, granite residual soil (GRS) is widely used as road base material. Thus, it is important to study the effects of hot and rainy climates and cyclic loads generated by trains on the dynamic properties of GRS. In this work, by means of dynamic triaxial tests, the effects of the number of drying and wetting (D–W) cycles, fines content and number of load cycles on the hysteresis curve, dynamic shear modulus Gd and damping ratio λ of GRS are systematically investigated. The experimental results illustrate the changes in the morphology of the hysteresis curve and dynamic parameters with the numbers of load and D–W cycles, as well as the fines content. Namely, the area S, center offset d, and residual strain εsp of the hysteresis curve decrease with the increase of load cycle number, increase with the growth of fines content, and first decrease with the increase of D–W cycle number, then slowly increase to stabilized values. However, the major axis gradient k exhibits exactly the opposite relationships. Meanwhile, the dynamic shear modulus Gd increases with the growing load cycles and decreases with the addition of fines content, and the damping ratio λ shows the opposite behavior. It is also shown that Gd and λ vary linearly with respect to logN, where N is the number of D–W cycles. The dynamic properties of GRS are mostly affected by the number of load cycles, which is followed by the fines content and then the number of D–W cycles. The influence of the latter two factors on the dynamic properties of GRS may be primarily due to contact form changes between soil particles.

Probabilistic and deterministic-based approach for liquefaction potential assessment of layered soil

In the present study, deterministic and probabilistic approaches have been used for the assessment of liquefaction potential of ground during an earthquake. The deterministic approach was used to analyze and assess the liquefaction of loose saturated river bed deposit with emphasis on two benchmark locations. A wide range of earthquake data in the form of peak ground acceleration (PGA) values of 0.18 g, 0.37 g, 0.6 g and 0.75 g was used as input motions. The dynamic properties of soil were evaluated using standard penetration test (SPT) data obtained from the bore logs. The shear stress induced within soil deposit due to the seismic excitation was calculated in the form of cyclic stress ratio (CSR) and cyclic resistance ratio (CRR) in order to calculate the factor of safety (FOS) against liquefaction. In addition, liquefaction potential index (LPI) and probability of liquefaction (PL) were also calculated using input motion. It was observed, based on the probability analysis and liquefaction indices, that the shallow layer soil profile is safe against liquefaction, whereas deep layer soil profile is unsafe.

Effects of response spectrum of pulse-like ground motion on stochastic seismic response of tunnel

It is widely accepted that near-fault pulse-like ground motions potentially cause more severe damage to structures than ordinary ground motions. However, the effects of stochastic pulse-like ground motions on underground structures are not effectively considered. The impacts of response spectra of pulse-like ground motions on seismic response are also unclear. Hence, this study utilizes an underground tunnel to investigate the effects of response spectra of pulse-like ground motions on the stochastic seismic response by combining a pulse-like ground motion simulation method and a finite element method. The combination procedure enables stochastic seismic response analysis under spectrum-compatible pulse-like/ordinary ground motions. The finite element model considers the nonlinear dynamic properties of soil and soil–structure interaction. Monte Carlo simulations are carried out to quantify uncertainties of stochastic tunnel response. Results show that pulse-like ground motions cause larger bending moments for tunnel lining even when they match the same target spectrum of ordinary ground motions. Furthermore, the bending moment of tunnel lining is significantly amplified when the spectral acceleration of pulse-like ground motion contains bulges or multiple peaks in the ranges of 1 s–6 s. Therefore, the seismic risk of tunnels may be significantly underestimated in the near-fault regions without considering pulse-like ground motions and their spectral acceleration characteristics.

Dynamic Properties of a Compacted Residual Soil from the West Indies

This paper presents a laboratory investigation of the strain-dependent cyclic properties of a compacted tropical residual soil as measured in a resonant column and cyclic triaxial testing program. The mechanical properties were evaluated with respect to cyclic shear strain amplitude, initial void ratio, and confining pressure. It was shown that the existing models for the prediction of shear modulus reduction and damping ratio curves were not pertinent in the case of the compacted residual soil studied. Empirical equations were developed for the small-strain shear modulus and the normalized shear modulus, damping ratio, and pore water pressure ratio curves for void ratios between e = 1.00 and e = 1.50 and mean effective pressures of p′ = 50−300 kPa. The comparison of the models to the measured values suggest that the uncertainties associated with each of these models are lower than 20% of the predicted values. The results were established as part of a project for the construction of an embankment dam in the West Indies. However, the methodology as well as the model formulation framework presented in the article can be generalized to other residual soils and applied in all fields of geotechnical engineering.

Studying root-environment interactions in structured microdevices.

When interacting with the environment, plant roots integrate sensory information over space and time in order to respond appropriately under non-uniform conditions. The complexity and dynamic properties of soil across spatial and temporal scales pose a significant technical challenge for research into the mechanisms that drive metabolism, growth, and development in roots, as well as on inter-organismal networks in the rhizosphere. Synthetic environments, combining microscopic access and manipulation capabilities with soil-like heterogeneity, are needed to elucidate the intriguing antagonism that characterizes subsurface ecosystems. Microdevices have provided opportunities for innovative approaches to observe, analyse, and manipulate plant roots and advanced our understanding of their development, physiology, and interactions with the environment. Initially conceived as perfusion platforms for root cultivation under hydroponic conditions, microdevice design has, in recent years, increasingly shifted to better reflect the complex growth conditions in soil. Heterogeneous micro-environments have been created through co-cultivation with microbes, laminar flow-based local stimulation, and physical obstacles and constraints. As such, structured microdevices provide an experimental entry point into the complex network behaviour of soil communities.

Effects of Reduced Tillage on Dynamic Soil Properties and Winter Triticale Performance in Organic Farm at Wageningen, the Netherlands

There is an increased need of identifying reduced tillage method that improves soil organic matter and soil fertility and at the same time best crop performance in organic farming. This study assessed the effects of organically managed reduced tillage and conventional tillage on dynamic soil properties and winter triticale performance. Organic farm that has been treated with three different tillage systems for six consecutive years was used in this study at Drovendaal organic facility of Wageningen University and Research. The tillage systems included two reduced tillage NIT at 10 cm and ECO up to 20 cm deep and CON at a depth of 30 cm. The parameters evaluated are soil physico-chemical properties, leaf area index (LAI), plant dry matter, ear number per area, plant height and chlorophyll content (SPAD) at ear formation stage . Use of NIT led to higher SOM and N-NO3 of 13.03 mg kg-1 at 0-10 cm soil depth. Also, shallow soil depths had lower bulk density and penetration resistance compared to deeper depths but NIT had higher penetration resistance in all soil depths compared to ECO and CON. Based on crop performance, ECO and CON revealed a better crop performance than NIT. Therefore, ECO having intermediate potentials in soil properties and crop performance could be the right tillage system for organic crop production compared to NIT and CON. Moreover, ECO will be a suitable tillage system that improves soil fertility, mitigates greenhouse gases and at the same time influence high crop performance.

Shaking Table Tests of Seismic Response of Multi-Segment Utility Tunnels in a Layered Liquefiable Site

Damage to underground structures caused by liquefaction is one of the important types of hazards in the field of geotechnical engineering. Utility tunnels are the lifeline projects of cities. To ensure the sustainable and safe operation of utility tunnels over a design life of 100 years, this paper investigates the seismic response pattern of utility tunnels in the liquefied site. In this paper, shaking table tests were carried out on the utility tunnel in a layered liquefiable site. Based on the test data, the distribution law of acceleration field and pore pressure field in the model and the deformation of the soil were analyzed first. Then the soil-structure interaction, the strain and uplift of the structure were investigated. The results show that liquefaction of sand layers under strong earthquakes, resulting in seismic energy loss. The acceleration of the upper clay layer is attenuated by the seismic isolation of the liquefied soil. The utility tunnel affects the propagation of soil acceleration, which decays faster beneath the structure for the same height. The process of pore water pressure growth is a process of energy accumulation and the pore water pressure ratio curve and Arias intensity are significantly correlated. During the test, the phenomenon of sand boil appeared, and the cracks appeared on the ground surface and developed continuously. The utility tunnel in liquefied soil is lifted under the action of excess pore water pressure. There are vertical and horizontal displacement differences at the deformation joints. The strain in the utility tunnel at the stratigraphic junction is mainly influenced by the action of the bending moment, large shear deformation in the transverse section. The strain at the connection between the partition wall and the top slab is the largest and is the weak position of the structure, followed by the connection between the side walls and the top slab, and the bottom slab of the structure have a smaller strain. The results provide insights into the dynamic properties of soils and structures in liquefaction sites.

Experimental Study of Awash Soil under Static and Cyclic Shear Loading

The dynamic soil properties (shear modulus and damping ratio) are of great importance for the analysis and design of geotechnical structures subjected to dynamic loads such as earthquake. Cyclic simple shear tests were conducted to study the variation of shear modulus and damping ratio with a different number of factors for strain amplitudes of 0.01%, 0.1%, 1%, 2.5%, and 5% and for a frequency of 1 Hz at an axial stress of 150 kPa, 275 kPa, and 400 kPa. The result shows that the damping ratio decreases with an increase in confining pressure at different cyclic shear strains. The shear modulus increases with an increase in the void ratio at different cyclic shear strains. The damping ratio increases with a decrease in soil plasticity. The obtained values of shear modules were in the ranges of 0.292 MPa to 15.998 MPa and the damping ratio values from 0.146% to 30.851%. In concluding the major influencing factors that affect the dynamic properties of soils are confining pressure, void ratio, shear strain amplitude, and soil plasticity.

Seismic bearing capacity of shallow foundations subjected to inclined and eccentric loading using modified pseudo-dynamic method

In this paper, a comprehensive parametric study is conducted to evaluate the seismic bearing capacity of shallow strip foundations overlying dry and cohesionless granular soil under the actions of inclined and eccentric loadings. For this purpose, a systematic combination of the lower-bound theorems of the limit analysis, the finite element method, and the nonlinear programming is implemented. The seismic loading condition is simulated by the well-established modified pseudo-dynamic (MPD) approach by accounting for the significant influence of phase difference as well as the primary and shear waves propagation through applying non-uniform inertia forces along the vertical and horizontal directions, respectively. Unlike the pseudo-static approach, in the so-called modified pseudo-dynamic loading, a more realistic dynamic nature of the seismic excitation is taken into account using various soil dynamic properties and spectral parameters of the problem under study. In this way, a more viable response of the shallow foundation subjected to seismic loading could be captured by simulating a visco-elastic material through the well-defined Kelvin-Voigt model. The modified pseudo-dynamic (MPD) method considers the influences of shear and primary wave velocities, amplification of earthquake loading, and period of lateral shaking. The current MPD approach is capable of considering the local site effects and damping characteristics of the soil deposit underneath shallow footing by adopting a non-uniform and nonlinear acceleration field inside the soil medium. The results of the seismic bearing capacity are presented in the forms of spectral responses and failure envelopes for the eccentric and inclined loadings. The results show that the failure envelopes of the shallow foundation subjected to either inclined or eccentric loading significantly shrink with the increase in the earthquake accelerations and decrease in the material damping of the underlying soil mass. The amount of changes in the size of failure envelopes in the normalized V-H and V-M spaces due to the variation of seismic intensity and material damping depends directly on the non-dimensional frequency of the earthquake excitation.

Effect of coupled chloride salt erosion and Freeze-thaw on the dynamic mechanical properties of EPS cement soils

EPS cement soil is a type of material widely used in engineering, but it is often in a complex engineering environment. Freeze-thaw cycles and chloride salt erosion have a large impact on the strength of cement soils in engineering, and it is a very important subject to investigate the strength trends of cement soils under severe conditions of salt erosion and freeze–thaw environments. In this study, the apparent characteristics and dynamic mechanical property characteristics of the EPS cement soil after different numbers (0, 3, 6, 9, 12, 15) of freeze–thaw cycles in different concentrations of NaCl (0 g/L, 4.5 g/L, 18 g/L, 30 g/L) solutions were investigated by chlorine salt erosion and freeze–thaw cycle coupling test and SHPB test. The changes of appearance morphology, dynamic stress–strain curve, dynamic compressive strength and energy dissipation after impact were analyzed, and the mechanism of strength change of the cement soil was explained from the microscopic perspective. The results of the tests show that the cement soil mainly undergoes elastic deformation, plastic yielding and damage stages during the impact damage process; with the increase of chlorine salt concentration and the increase of the number of freeze–thaw cycles, the dynamic compressive strength of the cement soil specimens decreases continuously, and the rate of strength loss gradually slows down first, and when the number of freeze–thaw cycles is more than 9, the rate of strength loss begins to increase. In the 30 g / L chloride erosion environment, after 3,6,9,12, and 15 freeze–thaw cycles, the dynamic compressive strength decreased by 16.48 %, 22.35 %, 28.77 %, 34.36 %, and 55.87 %, respectively. Under the immersion environment of clear water and low concentration (4.5 g / L) chlorine salt, when the number of freeze–thaw cycles exceeds 12 times, the absorption energy and energy absorption efficiency of the sample decrease sharply. Under the immersion of high concentration (18 g / L and 30 g / L) chlorine salt, the absorption energy and energy absorption efficiency decrease rapidly after more than 9 freeze–thaw cycles.

Shaking Table Test of the Negative Skin Friction of a Single Pile Induced by Seismic Settlement of Model Soft Soil

The negative skin friction (NSF) of pile foundations is one of the most important factors in triggering building subsidence. To study the mechanism and development of the NSF over time, four methods have been used: theoretical calculations, field tests, model tests, and numerical modeling. To address the difficulty of sampling prototype soft soils in model tests, this study uses a dynamic triaxial instrument to select a similar material ratio that can effectively simulate the dynamic properties of the prototype soft soil as a soft soil layer, and input sine waves with different frequencies and peak ground acceleration (PGA) for shaking table tests to investigate the effects of frequency and PGA on the development of the distribution of NSF and the location of the neutral point of the pile foundation. The test results show that the pile NSF is mainly distributed in the upper part of the pile under horizontal sine wave loading, increases rapidly from 0 to 5 s, and stabilizes after 5 s. Its magnitude is influenced by the sine wave frequency and PGA, and the effect of PGA on the NSF is more significant. In addition, the pile neutral point position shifts downwards with increasing sine wave frequency and PGA, but the downwards shift is not significant.

Soil pore structure dynamics under steady-state wetting-drying cycle

The soil pore space is an arena for processes responsible for numerous critical soil functions. We sampled 15 soil samples from 3 horizons (A, E, Bt) and performed a detailed X-ray microtomography (XCT) imaging study of the full wetting-drying (W-D) curve. This way, we revealed the dynamics of soil pore structure under slow water changes for Albic Retisols. In total, our analysis is based on 135 3D tomography scans (9 soil moisture points for each sample). Using the obtained structural information in the form of binary pore-solid segmentations, we were able not only to visualize structural dynamics (which showed significant changes within the soil at ∼10 µm – 3 mm pore sizes range) but also computed major classical morphological metrics. The analysis of these parameters and conceptual model of structural behavior revealed that after the W-D cycle the studied soil degraded in general. This is contrary to the prevailing previous findings for mainly compacted soils where W-D cycles led to structural improvements. We discussed the major implications of these findings and outlined a possibility to deepen our understanding of soil structure-function relationships, including dynamic hydraulic soil properties and 3D soil structure digital model.

Evaluation of dynamic behavior of varved clays from the Warsaw ice-dammed lake, Poland

Abstract Soil stiffness degradation due to dynamic loading may result in damage to engineering structures. Therefore, dynamic soil properties gained importance in recent years in the areas where vibrations of different origins are transmitted to the ground. The aim of the study was to determine the stiffness properties of the Warsaw varved clay (central Poland), as a measure of its response to dynamic loading. A set of triaxial tests were conducted. Undisturbed samples were subjected to initial deviator stress of 100 kPa, reflecting vertical stress due to construction load, and then to three dynamic stages under axial displacement control conditions. Secant shear modulus G was adopted as a parameter of soil stiffness during dynamic loading. Main methodological result of the study is application of coefficient α (gradient of logarithmic change in stiffness with time) indicating type of soil behavior under dynamic loading. Negative α values (ranging from −0.48 to −0.91) indicated a weakening effect of the applied vibrations (decrease in stiffness), while positive α values (ranging from 0.11 to 0.21) indicated soil strengthening (increase in stiffness). Results of the study may be applied to compare an impact of several factors influencing soil stiffness during dynamic loading

Multidirectional Vibroseis Shaking and Controlled Blasting to Determine the Dynamic In Situ Response of a Low-Plasticity Silt Deposit

In this paper, efforts to characterize and compare the full-scale in situ three-dimensional (3D) dynamic response of a low-plasticity silt deposit to multidirectional loading from two different sources, a vibroseis shaker named T-Rex and controlled blasting, are presented. Horizontal vibroseis shaking at a frequency, f, of 10 Hz, produced dynamic responses in the silt that ranged from linear-elastic to nonlinear-inelastic, inducing maximum equivalent direct simple shear (DSS) shear strains, γDSS,max, up to 0.15% and residual excess pore pressure ratios, ru,r, of 14.1%. Blast-induced shear waves with predominant frequencies ranging from 9.6 to 14.6 Hz excited nonlinear-elastic and nonlinear-inelastic responses in the silt deposit, with γDSS,max of 1.14% and maximum ru,r of 61%. Importantly, these responses were observed to be minimally influenced by high frequency compression waves. Multidirectional loading, and excess pore pressure, ue, migration and impedance were identified as the predominant factors for achieving the large ru,r in the silt deposit from these two in situ testing techniques. The cyclic threshold shear strain, γtp, to trigger ru,r observed from the T-Rex shaking equaled 0.007% to 0.011% and varied with the initial soil stiffness. The two testing techniques demonstrated that the in-situ shear modulus, G, reduced to 90% of the maximum shear modulus, Gmax, at γDSS,max≈γtp, whereas by γDSS,max≈1%, G further reduced to 10 to 30% of Gmax corresponding to ru,r of ∼60%. Changes in soil fabric were quantified using small-strain shear-wave velocity measurements performed before and/or upon initiation and after each stage of dynamic testing, and were linked to the observed increase and decrease in γtp for the shallower and deeper 3D elements, respectively, following T-Rex shaking. The side-by-side comparison of the dynamic responses and soil properties derived from these two distinctly different field-testing techniques validate the use of controlled blasting for quantifying in situ dynamic soil properties and responses.