Undrained Triaxial Tests Research Articles

Contractancy and shear behavior of extremely loose structure soils with particle breakage in saturated and unsaturated conditions

Pumices with high pore voids of volcanic origin are distributed throughout Japan and are the causal layer of slope failures. In many cases of surface failures, it is difficult to assume that the resulting layers are fully saturated. The high waterholding capacity of the pumice suggests that they were deposited in an unsaturated state with a high degree of saturation. In this study, saturated triaxial compression tests and fully undrained unsaturated triaxial compression tests were conducted on artificially produced pumice and natural pumice while measuring the amount of crushing. This is to clarify the relationship between the crushing and mechanical properties of pumice with porous particles, which are often the cause of such disasters, and their behaviour under unsaturated conditions. The results showed that the pumice stones have an ultra-high pore structure. Moreover, pumice with porous particles reached a steady state under both saturated and highly saturated unsaturated conditions, and the amount of crushing increased under highly saturated unsaturated conditions.

Gradation and state effects on the strength and dilatancy of coarse-grained soils

Well-graded soils can be found in nature and in engineered structures, such as dams and embankments. Prediction of their behavior is still an engineering challenge in part due to the lack of data in the literature, arguably due to difficulties associated in testing these soils in the laboratory and in situ. Particularly, there is still debate over the effect of the increased range of particle sizes (i.e., widening gradation) on the shear strength and dilatancy of coarse-grained soils. This paper presents the results of drained and undrained isotropically-consolidated triaxial compression tests on six soil mixes of varying gradation. These soils were sourced from a single natural deposit and selectively sieved and mixed to isolate the effects of gradation from those of particle shape and mineralogy. The results indicate that the critical state lines in void ratio – mean effective stress space move downward as the gradation becomes wider. For the same state parameter, the soils with a wider gradation exhibit greater dilatancy and generate negative excess pore pressures with greater magnitudes than the poorly-graded soils. In drained conditions, the greater dilatancy of the well-graded soils leads to greater peak friction angles, while in undrained conditions it leads to greater undrained shear strengths. The results show that these differences in behavior can only be captured when interpreting the results in terms of the state parameter and normalized state parameter, while comparing the results in terms of the void ratio or relative density obscures the effect of differences in gradation.

Monotonic and cyclic behaviour of sand-silt mixtures through the equivalent state parameter

This paper presents the results of a laboratory investigation into the effect of non-plastic fines on the monotonic and cyclic behaviour of sand-silt mixtures. For this purpose, drained and undrained triaxial monotonic and undrained stresscontrolled cyclic triaxial tests were performed on clean sand and its mixtures with non-plastic silt. The Critical State theory known as a characteristic state of soil behaviour and the equivalent state concept were used to the interpretation of the laboratory tests results. By estimating parameter b, which recognizes that different percentages of fines contribute differently to the strength of the sand and consequently the equivalent intergranular void ratio, (eg)eq, a single Critical State Line, CSL, is determined in the (eg)eq-log(p΄) plane, and a single liquefaction resistance curve at the CRR15-(eg)eq, independently of fines content, fc, based on the monotonic and cyclic tests results, respectively. It is shown that parameter b depends on the fines content, fc, and on the loading type of the laboratory test conducted, while (eg)eq proves to be a suitable parameter for the estimation of the monotonic behavior and undrained critical state strength as well as the liquefaction resistance of granular mixtures up to the threshold fines content value, fcth, independently of their fines content. The effectiveness of state parameter, ψ, and equivalent state parameter, (ψg)eq, in the estimation of the undrained critical state strength and liquefaction resistance of sand-silt mixtures is confirmed.

The friction-weakening cause by localized wear-induced damage and its effect on liquefaction resistance of sandy soil using DEM

Localized wear-induced damage caused by stress concentration at the contact point can lead to the decrease of the frictional coefficient (μ) of sandy soil, further affecting the liquefaction resistance. In this study, a bi-linear the friction-weakening model is proposed to describe the decreasing trend of μ with the increase of normal contact force (fn). Through a series of undrained cyclic triaxial tests, the influence of localized wear-induced damage on the characteristics of cyclic liquefaction is investigated from both macroscopic and microscopic perspectives. The macroscopic results indicate that samples have weaker liquefaction resistance due to the friction-weakening. Under low confining pressure (P0), the influence of the friction-weakening on the liquefaction resistance is relatively limited. With the increase of P0, the decrease of liquefaction resistance gradually becomes significant. However, the effect of friction-weakening on the liquefaction resistance does not remain constant, but shows a decreasing trend with the accumulation of excess pore water pressure ratio (EPWPR). From the microscopic perspective, the reduction of liquefaction resistance can be explained by the coordination number (Z) and the contact sliding ratio. Although higher Z caused by the friction-weakening may lead to greater liquefaction resistance, higher sliding contact ratio have been found to weaken the liquefaction resistance, and its influence exceeds Z.

The Influence of Preconsolidation Pressure on Undrained Shear Strength Characteristics of Peaty Clay in Sri Lanka

This research study investigated the effect of preconsolidation pressure on the shear strength parameters of very soft peaty clay based on a series of laboratory tests. The study included 15 Consolidated Undrained (CU) triaxial tests on peaty clay extracted from the Nilwala river basin, with preconsolidation pressures ranging from 0 to 100 kPa in 20 kPa intervals and cell pressures of 50, 100 and 150 kPa. The results indicated that the peak deviator stress of peaty clay decreased with increasing preconsolidation pressure, indicating decrease in its shear strength parameters with preconsolidation pressure. It was observed that peaty clay is highly sensitive to disturbance and requires sufficient time to regain its shear strength once its natural microstructure is altered. The study has significant implications for the construction of embankments on soft soils, particularly peaty clays. It highlights the need for accurate estimation of the undrained shear strength parameters of peaty clay, considering soil composition, structure and type of loading. It also emphasizes the importance of minimizing disturbance during construction activities to avoid compromising the stability of embankments.

Use of recycled ceramic beads to enhance mechanical behavior of low-plastic clays

Due to the huge amount of annual non-renewable ceramic waste disposal, it is in favor of eco-system to reuse the ceramic waste in earthwork structures. Thus, among several different applications of ceramic components, they can also be used to replace aggregates in the core of embankment dams, waste disposal projects and to form subgrade of roads and railways. The main issues addressed in this study are the effect of the volumetric content and size of the ceramic inclusions on the mechanical properties of ceramic-clay mixtures. Using an experimental approach, strain-controlled monotonic, stress-controlled cyclic, and post-cyclic monotonic triaxial undrained tests were made. The innovation of this study is that sphere beads of ceramic with different diameters were added to low-plastic clay to explore the role of bead size on the mechanical properties of bead and clay mixtures under monotonic, cyclic and post-cyclic loadings. Thus, we tested four types of specimens consisting of merely pure clay, 80% clay–20% bead, 60% clay–40% bead, and 40% clay–60% bead. Scanning electron microscope (SEM) images were additionally used to investigate the fabric of the clay between the ceramic beads. Test results revealed, firstly, that effective friction angle and undrained shear strength increase with the bead content. Secondly, dilative behavior was captured in all the bead and clay mixtures. Further, both dynamic shear modulus and damping ratio increase with the bead content. Similarly, post-cyclic shear strength increases with the bead content. Finally, different bead diameters did not change the observed trend.

Suffusion in densely compacted Satozuka pumice sand and its impact on static loading undrained shear strength and dilation behaviour

Pumice sand of volcanic origin contains a high fraction of non-plastic fines (>40 % for Satozuka pumice sand in Sapporo, Japan). Suffusion in such soil can wash away a portion of the fine particles and alter the soil microstructure. The moisture content and degree of compaction can affect the suffusion characteristics of soil deposits, however their effect has not yet been evaluated. Future construction sites in growing Sapporo City, consisting of pumice sand, will require a high degree of compaction (over 90 % and preferably over 95 %) as this sand is prone to suffusion in spite of its dense state. The aim of this study is to assess the impact of suffusion on densely compacted pumice sand with a high proportion of fines, based on its mechanical properties, with an emphasis on shear strength and dilatancy. Firstly, the suffusion characteristics of Satozuka pumice sand were evaluated. Subsequently, undrained triaxial tests (CU¯ tests) under monotonic loading were conducted on high-density specimens, with suffusion and without suffusion, to study the impact of suffusion. It is seen in the results that the hydraulic conductivity, shear strength, stress paths, and dilatancy are all noticeably affected by suffusion. The specimens with suffusion exhibit an increase in residual shear strength and maximum deviator stress under shearing and experience an earlier occurrence of phase transformation from contraction to dilation during shearing. This tendency implies that suffusion has no significant negative impact on the deterioration of earth fill made from pumice sand and non-plastic fines, and that it persists at degrees of compaction between 80 % and 100 %.

An extended hypoplastic model for sands with additions of highly plastic fines formulated under the ISA framework

In the present work, the hypoplastic model for sands by Wolffersdorff, (1996) coupled with Intergranular Strain Anisotropy (ISA) by Fuentes et al., (2020) is extended to account for the influence of fines content on the undrained cyclic response and liquefaction resistance of the material. A set of undrained cyclic triaxial tests on clean Ottawa C778 sand and on the same sand with additions of bentonite or laponite were simulated to evaluate the model’s capabilities. The comparison between experiments and element test simulation results suggest that the proposed extended model is able to accurately describe the behavior of clean Ottawa C778 sand and the significant influence of plastic fines in the response of the mixtures, such as the reduction in the accumulation rates of strains and pore water pressure under cyclic loading, and therefore, the improvement in the liquefaction resistance with increasing fines content.

Microstructure study of natural marine clay in loading and unloading processes

The microstructure evolutions of natural marine clay during loading and unloading processes were investigated using undrained triaxial tests. The pore distribution and arrangement of soil particles and aggregates were measured by mercury intrusion porosimetry (MIP) and field emission scanning electron microscope (FESEM) tests, respectively. The results show that pores do not always collapse into smaller ones with stress levels increasing; the destruction of cemented soil structure will lead to the connection of small pores into large pores. In the consolidation process, as the consolidation pressure increased from 0 to 100 kPa, the total specific surface decreased from 20.01 mm2/g to 17.42 mm2/g. Conversely, the total specific surface increased from 17.42 mm2/g to 19.25 mm2/g as the confining pressure surpassed the preconsolidation pressure, indicating the formation of new surfaces. The rebound effect during the unloading process is another factor causing the increase in pore volume, but the destruction of the soil structure can significantly reduce this effect. The microscopic morphology at the point of soil failure under high stress levels in both loading and unloading processes tended to be similar, as the soil structure was completely destroyed.

Calibration Method and Material Constants of an Anisotropic, Linearly Elastic and Perfectly Plastic Mohr–Coulomb Constitutive Model for Opalinus Clay

AbstractNagra, the cooperative for developing and implementing a long-term radioactive waste depository in Switzerland, identified Opalinus Clay as the most suitable host rock for deep geological containment. This paper deals with those features of Opalinus Clay that are important for the design and construction of the underground structures. Consolidated drained (CD) and consolidated undrained (CU) triaxial compression tests on specimens from deep boreholes revealed that Opalinus Clay exhibits pronounced stiffness and strength anisotropy, dependency of stiffness on the initial confining pressure, slightly non-linear pre-failure stress–strain behaviour, and a drop in axial resistance after a certain amount of shearing. Within the scope of establishing a rigorous—yet practical—design approach for the repository tunnels and caverns, the simplest possible constitutive model capable of reproducing the main aspects of the Opalinus Clay behaviour is adopted. The non-associated linear elastic and perfectly plastic MC model is chosen as a starting point, on account of its wide use in tunnel engineering practice, its simplicity, and the clear physical meaning of its parameters. This paper presents a systematic and robust calibration method for an extended version of this model, which considers the pronounced strength and stiffness anisotropy of Opalinus Clay. The paper additionally provides the full suite of the equations that describe the model behaviour under triaxial CU or CD testing conditions and for any bedding orientation relative to the specimen axis. The equations are employed to determine ranges of material constants for two varieties of Opalinus Clay, based upon the results of 73 CU and CD tests. A thorough comparison between the model predictions and the experimental response is conducted, to demonstrate the versatility and limitations of the constitutive model and of the proposed calibration approach.

Strain rate effects on intact deep-sea sediments under different confining pressures and overconsolidation ratios

Underwater construction has gone to greater depths below the sea level for the extraction of resources such as oil and minerals. During different construction stages of deep-sea structures, different loading rates may be applied to deep-sea sediments. Generally, the strength of soil usually increases with the loading rate, and the sensitivity to strain rate effects may be more pronounced in deep-sea sediments. Previously, the strain rate effects were mainly studied on shallow marine deposits, but rarely on deep-sea sediments due to the difficulty of sampling. In this study, the strain rate effects on intact deep-sea sediments were investigated by a series of undrained triaxial tests with different strain rates of 0.5%/h, 2.6%/h and 13.2%/h under normally and over-consolidated conditions. In addition, scanning electron microscopy (SEM), nitrogen adsorption (NA) and mercury intrusion porosimetry (MIP) tests were conducted to illustrate the unique microstructure and pore size distributions of the deep-sea sediments, which showed a flocculent structure with abundant mesopores and macropores. It is found that higher strain rate leads to higher undrained shear strength due to the delay of increase of excess pore pressure as a result of low permeability. Strain rate effects are more obvious with the increase of confining pressure and then become less significant when the confining pressure is increased further. This change of trend is attributed to the increase of particle contact and more particle rearrangements when the confining pressure is first increased, but the particle rearrangement is suppressed under higher confining pressure. The sensitivity to strain rate effects increases with the overconsolidation ratio as more particle rearrangement is enabled under less confinement.

Mechanism of the variation in axial strain of sand subjected to undrained cyclic triaxial loading explained by DEM with non-spherical particles

The mechanical response of sand is affected by the axial strain accumulated during undrained cyclic triaxial loading tests. However, the large deformation that occurs during the post-liquefaction stage is difficult to measure using testing apparatus. To address this limitation, this research utilizes the numerical method of DEM with non-spherical particles to simulate the undrained cyclic triaxial loading test on clean sand material. The validity of DEM is confirmed through laboratory test. A four-stage scheme for the development of axial strain in the liquefaction process is proposed and verified by analyzing the deformation features of sand specimens with different densities. Meanwhile, the macroscopic behavior is also described in conjunction with microscopic characteristics of sand particles, such as coordination number, force chain network and anisotropy degree of contact normal, which are calculated using DEM. In addition, the central value of variating axial strain at each cycle is analyzed to determine its variation feature via loading cycle. Due to the capability of DEM in describing the microscopic behavior of soil particle aggregation, it is found that the difference in the proposed four-stage scheme between compression and extension sides is the main cause of the central value of axial strain shifting monotonically.

A new empirical model for the undrained shear strength of undisturbed marine fine-grained soil based on piezocone penetration tests

Simple, precise, and economical evaluation of the undrained shear strength Su of undisturbed marine soil can facilitate the construction of offshore projects. In this study, piezocone penetration tests (CPTu) were conducted at the site of a wind power project, and consolidated undrained triaxial tests were performed on specimens of undisturbed marine fine-grained soil taken from the site. Analysis shows that the Eslami–Fellenius and Chinese soil classification methods based on CPTu data are more applicable for marine soils. The test results indicate that the stress–strain curve exhibits strain softening if the clay content is less than 30 %, otherwise it exhibits either strain hardening or strain stability. Furthermore, the specimens exhibit two deformation failure modes: softening contraction and hardening dilatancy. From the correlation between the consolidated undrained triaxial test data and the CPTu data, Su can be determined simply via an explicit expression involving the measured pore pressure behind the cone (u2), the corrected cone tip resistance (qt), and the soil depth H, and this expression is significantly more precise than the existing empirical formulas. The proposed method offers a sound basis for designing the foundations of marine engineering and infrastructure projects in different sea region.

Comparison of a tailings material and a natural sand in seismic site response analyses

This article investigates the seismic response of a tailings sand material in the context of site response analysis. The computed response is contrasted with that obtained for an equivalent deposit consisting of a well-characterised natural sand. Initially, the fundamental properties of both materials are used to interpret and compare their behaviour in the framework of Critical State Soil Mechanics (CSSM). Subsequently, both materials' monotonic and cyclic responses are calibrated for an advanced bounding surface plasticity model. Finally, the impact of different calibration approaches is examined through site response analyses under a strong motion. This process identifies fundamental differences between the two geo-materials, impacting the single elements simulations, especially the accuracy of undrained cyclic triaxial tests. The site response analyses examined herein show the impact of the calibrations adopted for both materials in terms of acceleration response spectra, displacements, and excess of pore water pressures (PWP). Although a relatively deep phreatic surface is adopted, the cyclic resistance and permeability of both materials plays a dominant role in the site response characteristics, controlling the excess PWP generation and hence the non-linear behaviour in the soil deposit. Finally, parametric studies are also conducted to explore the impact of permeability of the sands in the site response.

Study on Mechanical Properties of Deep Expansive Soil and Coupling Damage Model of Freeze–Thaw Action and Loading

This study is primarily intended to present a damage constitutive equation under the combined action of confining pressures and freeze–thaw cycles by subjecting deep expansive clay to the consolidated undrained triaxial tests. We study the influence of the numbers of freeze–thaw cycles on various mechanical indexes of soil by using the TSZ-2 instrument (fully automatic triaxial instrument). As the number of freeze–thaw cycles increases, the ultimate peak stress of the soil decreases, and then, the effect of the freeze–thaw effect on the shear strength gradually weakened. By combining the expression method of the damage variable under the action of loading alone with the expression method under the action of freeze–thaw cycles alone, we brought in the damage evolution equation to obtain the damage constitutive equation under the combined action of confining pressures and freeze–thaw cycles. The stress values under three confining pressures (100 kPa, 200 kPa, and 300 kPa) can be determined by using the final damage constitutive model. The measured data with a water content of 17% and six freeze–thaw cycles were compared with the theoretical data. The actual strength values were 118.4 kPa, 152.3 kPa, and 184.1 kPa, and the theoretical strength values were 120 kPa, 150 kPa, and 186 kPa. The fitting degree of the strength value was as high as 99%, which verifies the feasibility of this model. This study can serve as an available reference for well wall construction and disaster prediction in deep coal mining.

Weakening of mechanical parameters of ion-absorbed rare-earth ores subjected to leaching

Ion-adsorbed rare-earth ores are mined using in-situ leaching, and their mechanical properties significantly affect the efficient and safe recovery of rare earth elements. However, the mechanism of the change in the mechanical properties of the ore body due to the physicochemical processes caused by leaching remains unclear. To explore the strength evolution characteristics of the ore body during the leaching process, unconsolidated undrained triaxial tests were conducted to confirm how the stress–strain curve and shear strength of rare-earth samples change during leaching. Magnetic resonance imaging and T2 spectral characterizations were obtained by using nuclear magnetic resonance technology to measure the interior pore structure of samples during leaching. A scanning electron microscope equipped with an energy dispersive spectrometer was used to investigate the morphology evolution and the composition changes of the internal micro-area of the samples, to demonstrate the correlation between the microstructural change and the macroscopic mechanical properties. The results show that when a 2% ammonium sulfate solution is employed for mineral leaching, the effective leaching duration is 0–3 h. During this time, ion exchange occurs along the direction of solution seepage, resulting in the dispersion and migration of fine particles from the top to the bottom of the sample, which further triggers a change in the sample's pore structure and pore size. In addition, the local loss of fine particles resulted in a reduced bond strength between minerals, forming an unstable soil structure with a loose upper part and a dense lower part, which is macroscopically expressed as a declining shear strength parameter of the rare-earth sample.Graphical abstract

Shear strength characteristics of basalt fiber-reinforced loess

Loess owns the characteristics of collapsibility, disintegration and solubility, which pose a challenge to engineering construction. To examine the shear strength of basalt fiber-reinforced (BFR) loess, consolidated undrained (CU) triaxial tests were conducted to explore the impacts of water content (w), fiber length (FL), fiber content (FC) and cell pressure (σ3) on the shear strength. According to the results, the shear strength model was established taken into account the impacts of FL, FC, and fiber diameter (d). The results showed that the peak strength of BFR soils enhanced as FL, FC, and σ3 increasing, whereas it decreased with increasing of w. Compared to unreinforced soil, the peak strength of BFR loess improved 64.60% when FC was 0.2% and FL was 16 mm. The optimum reinforcement condition for experimental loess was that of FL was 16 mm and FC was 0.8%. The reinforcing mechanism of fibers was divided into a single tensile effect and spatial mesh effect. The experimental and calculated results agreed well, which suggested the model is suitable for predicting the shear strength of BFR loess. The research results can offer a guideline for the application of BFR loess in the subgrade and slope engineering.

Constitutive modeling of clay shales in undrained conditions and its experimental verification for Opalinus Clay

Clay shales are low-permeable sedimentary rocks that exhibit inherent cross-anisotropic stiffness, strength, and permeability. Accurate design of tunnels excavated in clay shales requires appropriate constitutive models that account for the anisotropic properties of the host rock, especially under undrained conditions. This paper presents a transversely isotropic elastoplastic-damage model for fully-saturated clay shales in undrained conditions. The model describes four deformation regimes observed during consolidated undrained experiments, i.e. (1) transversely isotropic elastic deformation, (2) anisotropic plastic deformation with cross-anisotropic peak strength, (3) semi-brittle post-peak softening due to both strength and stiffness degradation, and (4) residual strength state. The performance of the model was evaluated using available laboratory tests for Opalinus Clay, the selected host rock for underground disposal of radioactive waste in Switzerland. A large experimental data set, consisting of 18 undrained triaxial compression tests, was used to back-calculate the required model parameters. The adopted experimental data include the effective stress path and the stress–strain curves of specimens with bedding orientations between 0° and 90° with respect to the loading and effective consolidation stresses between 0.76 and 16 MPa. It is shown that the proposed model adequately captures the key material features of Opalinus Clay observed in the undrained experiments.

Study of Mechanical Properties of Saline Soils under Different Stress Paths

The stress path is a critical factor affecting the mechanical properties of saline soils. In order to study the mechanical properties of saline soils under different stress paths, in situ saline soils in the Qian’an area of western Jilin province were selected for this study, and triaxial shear tests under six different stress paths were conducted, including the consolidated undrained triaxial test under the conventional stress path under the isobaric consolidation condition; the consolidated drained triaxial test under the conventional, equal p, reduced p, and increased p stress paths under the isobaric consolidation condition; and the consolidated drained triaxial test under the conventional stress path under the K0 consolidation condition. The effects of the consolidation conditions, drainage conditions, and stress paths on the mechanical properties of in situ saline soils were investigated. The results reveal that the stress–strain relationship curves of soil samples decrease continuously in the order of increased p, conventional, equal p, and decreased p, and they all show the characteristics of strain hardening. The stress path curves have the same slope under the same stress path. For different confining pressures, only the relative positions of the curves are different. Under the conventional stress path, the slope is 1; under the increased p stress path, the slope is 1/3; under the equal p stress path, the slope is 3; and under the decreased p stress path, the slope of the curve is −1. For the same confining pressure, the magnitude relationships of shear strength under the different stress paths are as follows: increased p > conventional > equal p > decreased p. For the cohesion c and internal friction angle φ, the consolidation condition has a greater effect on the cohesion c and a smaller effect on the internal friction angle φ; the drainage condition has a smaller effect on the cohesion c and a larger effect on the internal friction angle φ; and the stress paths have a greater effect on both cohesion c and internal friction angle φ.

Mechanical Behavior of Fiber-Reinforced Soils under Undrained Triaxial Loading Conditions

The design of fiber-reinforced soil structures, such as embankments and pavements, can be carried out using the results of unconsolidated, undrained triaxial compression tests conducted on specimens at their “as-compacted” water content and analyzed in terms of total stresses. The effects of soil and fiber type on the mechanical behavior of fiber-reinforced soils have not been methodically or adequately examined in the past under these conditions, and the effects of fiber length and content on the shear strength parameters of fiber-reinforced soils need further experimental documentation. Accordingly, five soils ranging from “excellent” to “poor” materials for use in earthwork structures were tested in the present study, in combination with five types of polypropylene fibers having lengths ranging from 9 to 50 mm. Unconsolidated undrained triaxial compression tests were conducted on specimens at their “as-compacted” water content, with fiber contents ranging from 0.5 to 2% by weight of dry soil. Fiber reinforcement reduces the stiffness and increases the deformability of the soil. The fiber-reinforced soils exhibit a more ductile behavior in comparison with the unreinforced soils. A Mohr–Coulomb type linear failure criterion satisfactorily describes the shear strength behavior of fiber-reinforced soils in total stress terms. The cohesion values of the fiber-reinforced soils range between 61 kPa and 301 kPa and increase up to seven times in comparison with the cohesion values of the unreinforced soils. The variations of the angle of internal friction of soils due to fiber reinforcement are generally limited to ±25%. The cohesion improvement due to fiber reinforcement is increased with increasing fiber content and fiber length up to 30 mm and is inversely proportional to the fine-grained fraction and the cohesion of the unreinforced soil.