Changes In Soil Fabric Research Articles

Effects of vegetation growth on soil microstructure and hydro-mechanical behaviour

Literature studies are presenting counterposed effects of vegetation on soil hydraulic behaviour. Besides, root impacts at the micro-scale are rarely correlated to phenomenological observations due to the complexity of adopting up-scaling factors. In this study, the microstructural causes behind observed changes in vegetated compacted clayey sand’s hydraulic and volume change behaviour were explored. Laboratory experiments were carried out on compacted samples seeded with Cynodon dactylon. Significant changes in soil water-saturated permeability, water retention and shrinkage upon drying were observed after root growth. Laboratory measurements were complemented by the quantification of root morphological features and observations at the micro-scale for different soil hydraulic states. X-ray scans and mercury intrusion porosimetry allowed to cover seven orders of magnitude of pore sizes, shedding light on soil fabric changes at the soil–root interface and the clay aggregate scale. The effects of roots on the soil pore size distribution consisted of Multiphysics phenomena: fissure generation, void clogging and soil aggregation due to roots’ chemical interaction. Furthermore, a good correlation was found between the normalised volume of roots and the micro-pores volume. This new expression was included in a framework to predict soil water retention behaviour considering the aggregated structure and soil–root hydro-chemical interactions.

Coupled effects of temperature and suction on the shear behaviour of saturated and unsaturated clayey sand–structure interfaces

The thermo-mechanical behaviour of saturated and unsaturated soil–structure interfaces plays a key role in analysing the performance of energy piles. Previous studies focused on saturated interfaces and did not investigate the coupled effects of temperature and suction on interface behaviour. In this study, a clayey sand–structure interface with a normalised roughness of one was tested through a new temperature- and suction-controlled direct shear apparatus. A variety of temperatures (8, 20 and 42°C), net normal stresses (25, 50, 100, 150, 225 and 300 kPa) and suctions (0, 50 and 200 kPa) were considered. The results show that temperature can have a minor impact on the friction angle, whose value at 42°C is smaller by about 2·2° than that at 8°C, probably because heating can reduce the shearing-induced contraction in the shear zone. More importantly, the interface strength increases non-linearly with increasing suction, and the incremental rate is temperature dependent. Heating the interface at a net normal stress of 50 kPa reduces this incremental rate due to reduction in surface tension and thermally induced changes in soil fabric. In contrast, this incremental rate increases at a net normal stress of 150 kPa with the same temperature increment, probably because the heated specimen has more small pores due to thermal contraction and more menisci water lenses, whose influence outweighs the effects of surface tension.

Apparent Hysteresis in Archie’s Law: Implications of Changing Pore Fluid Conductivity for Electrical Resistivity Monitoring of Infiltration

The electrical resistivity of the vadose zone is highly dependent on soil moisture content (VWC), making ERT a potential tool for non-intrusive infiltration monitoring. There’s a long history of using Archie’s law to convert resistivity values to VWC, often under the assumption that the pore-fluid conductivity is constant. Analysis of two years of soil conductivity and VWC data from sensors buried in a bioswale in Philadelphia, Pennsylvania showed that the characteristic curve of VWC versus resistivity exhibited hysteresis. The same value of resistivity was associated with different VWC during imbibition and drying. Accurate VWC could be estimated using Archie’s law only after incorporating changes in pore-fluid conductivity. Additionally, pore-fluid conductivity varied seasonally, with the highest values recorded during winter from road salt runoff. Archie exponents differed for sensors within 25 cm of each other despite both the uniform nature of the bioswale soil and fits to Archie’s Law for individual sensors, indicating considerable heterogeneity. This pattern was confirmed at two additional sites. Assuming the conductivity of the pore-fluid remains constant during infiltration not only produces erroneous soil moisture estimates, it can even reverse the apparent soil moisture trend. We conclude that to use ERT to monitor infiltration it is essential to independently record changes in pore-fluid conductivity, and that multiple sensors are required because changes in pore-fluid conductivity and Archie parameters can vary on a sub-meter scale for even apparently homogeneous sites. Additionally, long-term changes in Archie parameters, notably m, are possible, and may indicate long-term changes in soil fabric.

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.

Numerical analysis of slopes treated by nano-materials

Abstract Improvements in mechanical characteristics of soils treated by nano-materials (NMs) have been proved in the last three decades. The improvements are mainly attributed to changes in the soil fabric where a noticeable rise in shear strength has been obtained. This work, therefore, addressed a numerical study on the influence of the soil fabric changes due to the NMs enhancement on a slope stability problem using an upper bound discretization scheme. A parametric study was carried out at seven different inclination angles from 15 to 45° and with a variety of combinations of angle of shearing resistance (ϕ) and cohesion (c) values. This was carried out for two different types of slopes on purely frictional materials and c–ϕ materials. A noticeable increase in stability was obtained, based on a set of re-generated design charts, due to NMs enhancement (attributed to soil fabric changes). The re-generated design charts did not require iterative procedures and extended both x and y boundaries when compared with other available charts in the literature. Examination of the influence of the NM on the failure modes, to provide an insight into different failure mechanisms due to the soil fabric changes, was also considered.

Performance of tripod foundations for offshore wind turbines: a numerical study

Around 80% of offshore wind turbines (OWTs) are founded on deep seabeds using large-diameter monopiles that reach a length up to 60 m. In a race to seek cost-effectiveness, tripod foundations made with suction caissons represent a promising solution. The deformation mechanism induced by cyclic lateral loading on the tripod bucket embedded in sand reveals recovery of dynamic stiffness with the number of cycles that influences the subsequent dynamic response of the structure–foundation complex. This ‘self-healing’ and the coupled increase of settlement rate are caused by uneven change in soil fabric and density among the buckets. However, the capability of the recently developed constitutive models to predict this effect is limited. To address this issue, a series of centrifuge tests are herein back-analysed with a three-dimensional finite-element model. The implicit approach has been implemented using an advanced rate-independent hypoplastic model coupled with the conventional intergranular strain theory. The model captures the magnitude of the peak and residual cumulative rotation. The results indicate that the push–pull mechanism turns into a horizontal translation, with active wedges formed behind the pulled bucket. After about 100 cycles, due to significant redistribution of mean stresses, wedges are formed around both pushed and pulled buckets.

Dispersion characteristics of MgO-treated dispersive clay

The improper use of dispersive base soils has had far-reaching consequences in geotechnical structures but most significantly the cores of earth dams, road embankments, and irrigation channels. When exposed to water, the dispersive clay transforms into individual colloidal particles before getting washed. Dispersive clay is one with a large amount of sodium ions in its pore water. The laboratory tests included pinhole test, crumb test, and double hydrometer test on natural nondispersive clay prepared at three undrained shear strengths and dispersive clay prepared by adding 35% of Na2CO3 to the natural clay. Then, the soil treatment of the dispersive clay was made by mixing with different percentages of MgO. The MgO caused a change in soil fabric from a de-flocculated structure to a more flocculated one. The fines content generally decreased due to the MgO treatment, and when MgO increased, the sodium component in the dispersed soil decreased. Another criterion for identifying the dispersion of soil based on the output flow turbidity has been proposed through carrying out turbidity test on the output water. It is shown that in dispersive clay, there is a steady increase in turbidity with clay undrained shear strength after both tests (pinhole and crumb tests). A significant reduction in flow rate was recorded when the MgO increased from 0 to 6%. Another criterion for identifying the dispersion of soil based on the output flow turbidity has been proposed through carrying out turbidity test on the output water. It is shown that in dispersive clay, there is a steady increase in turbidity with clay undrained shear strength after both tests (pinhole and crumb tests).

Effect of relative density and biocementation on cyclic response of calcareous sand

Microbial-induced calcium carbonate precipitation (MICP) represents a promising approach to improve the geotechnical engineering properties of soils through the precipitation of calcium carbonate (CaCO3) at soil particle contacts and soil particle surfaces. An extensive experimental study was undertaken to investigate the influence of initial relative density on the efficiency of the biocementation process, the reduction of liquefaction susceptibility, and the cyclic response in biocemented calcareous soils. For this purpose, stress-controlled undrained cyclic triaxial shear (CTS) tests were carried out on untreated and MICP-treated calcareous sand specimens for different initial relative densities and magnitudes of biocementation. Improvement in the cyclic response was quantified and compared in terms of excess pore pressure generation, evolution of axial strains, and the number of cycles to liquefaction. The cyclic experiments show that MICP treatment can change the liquefaction failure mechanism from flow failure to cyclic mobility and can significantly change the excess pore pressure generation response of initially loose specimens. Scanning electron microscope (SEM) images indicate the CaCO3 crystals alter the characteristics of the sand particles and confirm the physical change in soil fabric that impacts the dynamic behavior and liquefaction resistance of MICP-treated specimens. Furthermore, the effect of biocementation was contrasted against the effect of relative density alone, and MICP treatment was shown to exhibit greater efficiency in improving the cyclic resistance than densification.

Improvement of Internal Stability of Alluvial Clay from Famagusta Bay, Cyprus, Using Copolymer of Butyl Acrylate and Styrene

ABSTRACT The internal stability of alluvial clays may be significantly compromised during a heavy rainfall due to infiltration of surface water causing sudden inundation, softening, and loss of erosion resistance or mechanical strength. Most of the available stabilization methods for clay soils employ pozzolanic or other cementitious binders, creating a chemically bound clay-admixture matrix. These admixtures commonly require a curing period after placement and compaction. Alternatively, aqueous polymers can be used in diluted form without any need for a curing period. Aqueous polymers can form agglomerations of clay particles enclosed in a matrix of polymer chains, held together by electrostatic and hydrogen bonding, improving erosion resistance. In this research, an aqueous polymer, namely, copolymer of butyl acrylate and styrene (CBAS), is mixed with alluvial clay sampled from Famagusta Bay, Cyprus, and the clay stability test is performed as a basis for assessing the degree of improvement on erosion resistance. A time-dependent approach for the evaluation of test results is followed to increase the accuracy of the analysis of the actual behavior observed during the test. A significant improvement in the erosion resistance is observed in treated test specimens. The mode of collapse of specimens during the clay soil stability test when aqueous polymer is used also changed from being gradual cracking and slaking to explosive. The swelling behavior and the effect of drying on the erosion resistance are also observed in the testing program. X-ray diffraction analysis and Fourier transform infrared spectroscopy are performed for observation of the effect of CBAS on microstructural interactions, such as electrostatic bonding and changes in soil fabric.

Micro-scale characterisation of clay at elevated temperatures

The main objective of this study is to use a suite of micro-scale tests to characterise the temperature effects on high plasticity clay. The unheated clay was examined using X-ray diffraction, field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectrometry to characterise the mineralogy, morphology and the elemental composition of natural soil. The clay was then cured under various temperatures ranging from 23 to 1000°C. The characteristics of heated clay were then evaluated using FESEM, cation-exchange capacity (CEC) tests, thermal gravimetric analysis (TGA) and Brunauer–Emmett–Teller (BET) surface area analyser. The results show minimal changes in soil fabric with increased temperature. However, specific surface area and CEC are shown to significantly decrease with temperatures beyond 100°C. TGA results can provide an insight into the drops of BET surface area and CEC. Depending on the range of temperature applied, the changes can be attributed to the temperature effects on the clay microstructure, loss of capillary and adsorbed water and organic matter, and the clay mineral's interaction with the displacement cation or nitrogen. The observed trends are used to discuss possible implications on the hydro-mechanical properties of soil such as the soil–water-retention curve, shear strength and volume change.

Investigation of the change in soil fabric during cone penetration in silt using 2D measurements

Interpretation of CPTU testing in silt is non-trivial because of the partially drained conditions that are likely to occur during penetration. A better understanding of the pore pressure generation/dissipation is needed in order to obtain reliable design parameters. Following a previous study using X-ray computed tomography (micro-CT) with volumetric digital image correlation (3D-DIC) that clearly showed the formation of distinct dilation and compression areas around the cone; the present work takes a closer look at those areas in order to link volumetric behavior to changes in soil fabric. High-resolution 2D backscattered electron images of polished thin sections prepared from frozen samples at the end of penetration are used. The images have a spatial resolution of 0.4 µm/pixel that allow a clear identification of grains and pore spaces. Image processing techniques are developed to quantify local porosity and obtain the statistical distribution of the particle orientation for the zones around the cone tip and shaft. It is shown that the formation of compaction regions is related to the ability of the grains to rearrange and align along a well-defined preferred orientation forming a more closed-fabric characterized by high anisotropy values, while zones of dilation are associated with a more open packing with grains randomly oriented and with large voids within. These observations suggested that for a saturated soil, water will move from a compressive zone to a neighboring dilative zone, creating a short drainage path. By shedding light on the link between soil fabric and drainage patterns, this study contributes toward a better understanding of the measured macro-response during CPTU tests on silt.

Seasonal effects on geophysical\u2013geotechnical relationships and their implications for electrical resistivity tomography monitoring of slopes

Current assessments of slope stability rely on point sensors, the results of which are often difficult to interpret, have relatively high costs and do not provide large-area coverage. A new system is under development, based on integrated geophysical–geotechnical sensors to monitor groundwater conditions via electrical resistivity tomography. So that this system can provide end users with reliable information, it is essential that the relationships between resistivity, shear strength, suction and water content are fully resolved, particularly where soils undergo significant cycles of drying and wetting, with associated soil fabric changes. This paper presents a study to establish these relationships for a remoulded clay taken from a test site in Northumberland, UK. A rigorous testing programme has been undertaken, integrating the results of multi-scalar laboratory and field experiments, comparing two-point and four-point resistivity testing methods. Shear strength and water content were investigated using standard methods, whilst a soil water retention curve was derived using a WP4 dewpoint potentiometer. To simulate seasonal effects, drying and wetting cycles were imposed on prepared soil specimens. Results indicated an inverse power relationship between resistivity and water content with limited hysteresis between drying and wetting cycles. Soil resistivity at lower water contents was, however, observed to increase with ongoing seasonal cycling. Linear hysteretic relationships were established between undrained shear strength and water content, principally affected by two mechanisms: soil fabric deterioration and soil suction loss between drying and wetting events. These trends were supported by images obtained from scanning electron microscopy.

Microstructure study of soft clayey soil under consolidation by vacuum preloading method

Vacuum preloading is a new technique for ground improvement. The vacuum drains out the pore-water, thus creating negative pore-water pressure and increasing the effective stress. The microstructure of soil samples after application of vacuum pressure has examined using scanning electron microscopy, and the orientation of soil aggregates and pores has been determined with GEOIMAGE software. The orientation of aggregates in fine-grained soil is an important factor influencing the anisotropy and engineering properties of the soil. Two quantitative indices to assess the orientation of the aggregates of fine-grained soil, alignment entropy () and frequency distribution function of clay aggregate orientation , are introduced in this paper. The difference between the anisotropy index and is discussed. The image analysis reveals that , , and other fabric data change with the method of consolidation; the isotropic and disordered soil fabric changes into anisotropic and ordered fabric. Therefore a quantitative analysis of the orientation of the aggregates in the soil fabric is necessary for an understanding of soil behaviour. In this paper it is shown that a combined vacuum–surcharge preloading method is a very useful and effective technique in geotechnical engineering.

Case study of a loess collapse field trial in Kent, SE England

Loess soils undergo collapse due to bond weakening under loading and, especially, wetting, and consequently constitute a major engineering geology hazard. To understand better the relationship between wetting and volume reduction in loess, a field collapse test was conducted at a ‘brickearth’ quarry, where a 5.0 × 5.0 × 1.5 m deep sample was isolated, flooded in a controlled manner and subjected to a surface stress of up to 210 kPa for 10 days. Geotechnical instrumentation, consisting of piezometers and rod extensometers, was complemented by geophysical instrumentation (resistivity arrays, shear wave transducers and a resistivity probe) to provide evidence of changes in interparticle bonding during the collapse process. Laboratory index and oedometer testing, together with SEM study of samples removed from the site, complemented the site monitoring. The field collapse test eliminated many problems associated with laboratory testing, notably small volumes of material and sample disturbance. This paper presents the geotechnical findings on ‘large-scale’ loess performance and relates them to the results of shear wave velocity and resistivity monitoring. The different behaviour of two distinct soil strata and the importance of the degree of saturation to soil fabric changes are demonstrated. The results identify how the soil in situ and oedometer samples respond under similar applied stresses.

Kaolinite – alkali interaction and effects on basic properties

The influence of type and amount of clays present in soils on their properties is well understood. The clays exert their influence through large specific surface area and charges on them. Their effect is mostly exhibited through inter particle bonding and subsequent particle associations. The mineralogical influence of soils in water is well documented. However, the change in soil water system because of presence some of the contaminants can greatly influence the soil behaviour. Some of the changes are due to formation of new compounds due to interactions between the soil and pollutant. The paper reports the effect of interaction of kaolinite mineral with alkali on the index properties of soils from which the geotechnical behaviour can be understood. Detailed X-ray diffraction studies have shown that sodium aluminum silicate hydroxide hydrate (NASH) is formed by clay alkali reactions. The type and amount of formation of the compound is influenced by the concentration of alkali solution. While the compound formed is in smaller quantities with 1 N NaOH solution, significantly high quantity is formed with 4 N NaOH solution. Presence of alumina is shown to play a significant role. It was observed that the formation of sodium aluminum silicate hydroxide hydrate is reduced in the presence of alumina. Specific gravity of contaminated clay soil was reduced which confirms the formation of new compounds. Water adsorption and specific surface area of soil are also influenced due to soil alkali interaction. The changes in the free swell and index properties of soil in the presence of alkali have been explained by the changes in soil fabric and the formation of new compound.

Use of a resistivity cone for detecting contaminated soil layers

A cone-penetration technique was developed to detect contaminated soil layers with electrolytes and NAPLs. In this study, laboratory and field experiments were performed to apply the technique. In the laboratory, the change in resistivity was quantitatively examined by adding salt or oil to soil samples. The results showed that the resistivity measurement was varied with an order of ppm for the electrolyte concentration in soil. It was found that the resistivity of sand increased with increasing oil concentration. The effect of oil content was stronger for lower water content of sand. The results obtained from field experiments showed that the resistivity cone can be used for detecting the contaminated layer in soils whose background values are known. Particularly, the instrument can be used effectively for examining the effects of remediation, by measuring the resistivity of the ground before and after remediation, unless the soil fabric changes significantly during remediation procedures.

Soil Fabric Changes During Consolidation

Abstract A method for characterizing the directional distribution of voids in soils at the microstructural level is presented. A scanning electron microscope (SEM) was used to obtain high-resolution, high-magnification images of the soil microstructure. These images were analyzed using an image analysis program to obtain parameters of the directional void ratio distribution function. Useful indices based on both solid and void phases for describing the changes of fabric in soils are proposed, and their evolution under one-dimensional compression of soils is studied. The analyses showed a high degree of preferred orientation attained by soil particles during consolidation.

Surface Thermodynamics and Some Engineering Properties of an Organo-Clay

Organo-clay minerals, in which surface inorganic cations have been exchanged by various organic ammonium cations, have the ability to adsorb many organic compounds. Organo-clays are of interest as potential substrates for the decontamination of sites containing organic fluids and as a barrier for plumes of organic contaminants. A factor that has not been examined in detail is the possible change in the fabric of a clay when converted to an organo-clay. A theoretical analysis of the interparticle forces, based on electrostatic and surface thermodynamic theory, indicates that the conversion of a normally hydrophilic swelling clay to an organo-clay should radically alter these forces, resulting in a change in fabric. To examine this, a smectite clay was converted to an organo-clay by treating it with hexadecyl trimethyl ammonium (HDTMA) cations. Scanning electron microscopy revealed a dramatic change in soil fabric, from an initial dense plate-like fabric to a more open granular fabric for the organo-clay. The granular fabric resulted in a rapid initial consolidation of the organo-clay and exhibited a much larger permeability, with water as the permeating fluid, than the unmodified clay. The permeability of the organo-clay decreases significantly when organic fluids are used as permeants.

Changes in soil fabric of Torripsamments under irrigated date palms, Saudi Arabia

Micromorphological observations were made of the individual layers in one soil profile in the desert and in four similar profiles in as many irrigated date palm orchards ranging in age from 10 to 230 years. Soluble silica in soil samples and ‘amorphous’ silica in clay fractions were also determined. Differences in the soil fabric among the profiles indicated changes over time such as modification of the types and distribution of voids, additions and transfers of CaCO 3 and silica and some transfers of clays. The differences in the soils could be related to the length of time under irrigation and to the quality of irrigation waters.

Swelling Pressures of Compacted Bentonite/Sand Mixtures

AbstractCompacted bentonitic clay/sand mixtures are being considered for use as buffer materials in the Canadian concept for nuclear fuel waste disposal. This paper describes a laboratory study of the swelling pressures that develop in statically compacted, air-dry specimens of mixtures of sodium bentonite and silica sand as they are saturated with double-distilled, deionized water. The results are interpreted with the aid of scanning electron microscope observations of the soils' structures.It is shown that the sand acts as an inert filler material, and swelling pressures are controlled by a parameter termed the effective clay dry density, γC, defined as the ratio of the mass of clay to the combined volume of the claq plus voids in the mixture. A threshold value of γC exists below which swelling pressures can be expected to be isotropic. Above the threshold value of γC, pressures parallel to the axis of compaction can be expected to be greatgr than those perpendicular to it. This is related to a change in soil fabric as γC is increased above the threshold value. For the Canadian disposal concept, γC would probably be below the limiting value and swelling pressures of 2.5 MPS or less are expected. The swelling pressures are likely to be isotropic within a saturated buffer mass.