Undrained Triaxial Compression Tests Research Articles

Undrained shear mechanical behaviors of dense gassy sand

Shallow gas can cause many disasters, and it is reported in many marine engineering constructions. For this, it is imperative to understand the impact of gas on the mechanical behaviors of soil. This study investigated the influence of undrained triaxial compression tests on dense gassy sand commonly encountered in coastal areas. Triaxial tests were performed on specimens with saturations of 100%, 99.8%, 95.9%, and 92.7% under confining pressures of 50 kPa and 200 kPa by a self-developed multi-purpose integrated triaxial apparatus (MITA) for gassy soil. The results are presented in terms of monotonic stress‒strain behavior, volumetric behavior, shear strength, and excess pore water pressure (EPWP). The occurrence of gas bubbles has different effects on loose and dense sands, augmenting the undrained shear strength of loose sand while concurrently diminishing that of dense sand. The deviatoric stress of dense sand increases during shear shrinkage, which is similar to the characteristics of loose sand under the influence of gas bubbles. However, following sand dilation, the effect of gas bubbles on deviatoric stress manifests in an antithetical manner. With elevated gas content, the shear strength of dense sand decreases, accompanied by a deceleration in the development of EPWP and a notable increase in volumetric changes. To this end, a microscopic explanation concerning the deformation and evolution of gas bubbles within sand during the shear process was presented to reveal the macroscopic laws governing the undrained shear attributes of dense gassy sand.

Investigation of strain-softening behaviours and development of constitutive model of red clays in Guiyang, China

Red clay is widely used in engineering projects; it is characterized by high moisture content, high plasticity, large porosity, high strength and low compressibility. The engineering properties and deformation modelling of red clay in Guiyang differ significantly from those of ordinary soils. Consequently, it is essential to conduct a detailed study of the stress–strain relationship of red clay in order to obtain accurate engineering parameters. Hence, a series of unconsolidated undrained triaxial compression tests (UU) was conducted on red clay in Guiyang under different water contents and confining pressures. The test results show that the stress–strain curve of the clay undergoes three stages: elastic deformation, plastic yielding and stress reduction after failure. After reaching peak deviatoric stress, the deviatoric stress decreases gradually and stabilizes, showing noticeable strain-softening characteristics. The non-linear model from the Nanjing Hydraulic Research Institute (NHRI) was introduced for fitting and obtaining relevant parameters. Then, based on the FLAC 3D platform, numerical simulation was carried out on a standard soil sample. The research results show that the calculated curve from the non-linear model fits the experimental curve quite well, and the numerical test curve also agrees with it. This indicates that the non-linear model from NHRI can effectively reflect the strain-softening behaviour of clay and is highly applicable to Guiyang red clay.

Enhanced Water Resistance of TiO2-GO-SMS-Modified Soil Composite for Use as a Repair Material in Earthen Sites.

Most earthen sites are located in open environments eroded by wind and rain, resulting in spalling and cracking caused by shrinkage due to constant water absorption and loss. Together, these issues seriously affect the stability of such sites. Gypsum-lime-modified soil offers relatively strong mechanical properties but poor water resistance. If such soil becomes damp or immersed in water, its strength is significantly reduced, making it unviable for use as a material in the preparation of earthen sites. In this study, we achieved the composite addition of a certain amount of sodium methyl silicate (SMS), titanium dioxide (TiO2), and graphene oxide (GO) into gypsum-lime-modified soil and analyzed the microstructural evolution of the composite-modified soil using characterization methods such as XRD, SEM, and EDS. A comparative study was conducted on changes in the mechanical properties of the composite-modified soil and original soil before and after immersion using water erosion, unconfined compression (UCS), and unconsolidated undrained (UU) triaxial compression tests. These analyses revealed the micro-mechanisms for improving the waterproof performance of the composite-modified soil. The results showed that the addition of SMS, TiO2, and GO did not change the crystal structure or composition of the original soil. In addition, TiO2 and GO were evenly distributed between the modified soil particles, playing a positive role in filling and stabilizing the structure of the modified soil. After being immersed in water for one hour, the original soil experienced structural instability leading to collapse. While the water absorption rate of the composite-modified soil was only 0.84%, its unconfined compressive strength was 4.88 MPa (the strength retention rate before and after immersion was as high as 93.1%), and the shear strength was 614 kPa (the strength retention rate before and after immersion was as high as 96.7%).

Undrained triaxial compression tests on normally consolidated bentonite considering temperature/confining pressure dependency

In the geological repository of high-level radioactive wastes (HLRW), one of the most important issues is the stability of artificial barrier basically using bentonite. However, due to the exist gaps between bentonite block and surround rock mass, the bentonite can swell relatively freely, whose constraining condition is quite different from the normal swelling pressure test. As an extreme case, the bentonite may engage completely into the surrounding ground and become a complete normal consolidated state. Therefore, in order to investigate the thermo-mechanical behavior of saturated bentonite at normally consolidated state, a newly proposed static compaction method for preparing specimen of saturated normally consolidated bentonite was proposed first. Then, a series of undrained triaxial compression tests were conducted under different confining pressure and temperature, by which the thermo-mechanical behavior of bentonite, used as an artificial barrier of geological repository of HLRW, is investigated systematically. The tests results reveal that the specimen prepared by the proposed method is confirmed to have sufficient saturation (> 0.92), meanwhile the prepared specimen is at slightly overconsolidated state that roughly equal to a K0-line consolidation. The deformation of the bentonite in undrained triaxial compression tests changes from plastic to brittle as the temperature increases. Simulation based on a sophisticated constitutive model considering water compressibility at high confining pressure was also discussed. The results are useful for assessing the mechanical behavior of bentonite after hundreds years of geological repository of HLRW.

A numerical model for assessing the effect of low clay content on wave-induced seabed liquefaction

In the marine environment, the seabed contains a certain amount of clay. Experimental studies show that the liquefaction susceptibility of the sandy seabed increases as clay content (CC) rises to a certain threshold, beyond which further increases in CC reduce liquefaction susceptibility. However, numerical models that describe the effect of CC on seabed liquefaction are very limited. This study proposed a dynamic poro-elasto-plastic finite element method model for analyzing liquefaction in the sandy seabed with CC below the threshold. Based on a series of undrained triaxial compression tests on sand-clay mixtures from existing literature, a unified constitutive framework was demonstrated to be effective for describing the liquefaction behavior of sand with low CC using one set of model parameters. Existing wave flume model tests validated the effectiveness of the proposed seabed model in describing the effect of low CC on excess pore water pressure (EPWP). Numerical results confirmed that adding a small amount of clay to the seabed increased the soil contraction and thus its liquefaction susceptibility. Wave-induced liquefaction was limited to a certain depth of the seabed, and the liquefaction depth was significantly affected by the CC. Adding a low content of clay the sandy seabed significantly increased both horizontal and vertical displacements under wave action, potentially leading to the instability of the seabed. This study provides a new method for accurately assessing the wave-induced stability of marine structures built on the sandy seabed containing certain amounts of clay.

Utilising the Ipzo parameter for more reliable estimates of undrained shear strength directly from CPTU data in saturated natural clay-like soils

Ramsey and Tho illustrated the ability of the Ipzo parameter to estimate plasticity index in a wide range of saturated natural soils and highlighted the potential of the Ipzo parameter for improving the reliability of estimations of other soil properties affected by plasticity index. In this note, an improved method is proposed for estimating undrained shear strength in clay-like soils using CPTU data, where clay-like is defined as Ipzo ≥ 12%. The improved method is validated using a geographically and geologically diverse “Nkt1-database” comprising 54 anisotropically consolidated undrained triaxial compression tests, with complementary CPTU measurements from 36 geological units at 18 globally distributed marine sites.

Experimental study of poromechanical behavior of Callovo-Oxfordian claystone in undrained triaxial compression and extension tests

The Callovo-Oxfordian (COx) claystone is selected as the host rock in the French project for geological disposal of radioactive waste. In its initial state, the host rock is fully saturated. Due to the low permeability, the pore fluid pressure can locally evolve in a quasi undrained condition due to subsequent mechanical and thermal loading, and then significantly affects deformation and cracking process of geological barrier. Most previous studies were carried out in quasi drained conditions or with constant pore pressure. Poromechanical behavior of COx claystone in undrained condition has been so far rarely investigated. In the present work, we carry out a new series of laboratory tests. Two representative loading paths are considered: triaxial compression with constant confining pressure and triaxial extension with constant mean stress. The variations of strain and pore pressure are measured during the tests. It is found that the poromechanical behavior of the claystone is clearly affected by loading path. The failure strength is higher in triaxial compression than in extension. The evolution of pore fluid pressure is correlated with volumetric strain. Compared with the results obtained in drained tests, it seems that the Terzaghi effective stress can be used for the description of pore fluid pressure effect on failure strength.

Experimental research on shear strength characteristics of cement-stabilized dredged silty clay reinforced with alginate fiber

In order to avoid uneven dispersion or agglomeration of traditional fibers after mechanical mixing, alginate fibers as natural fibers were selected in this study to reinforce the cement-stabilized dredged silty clay, forming alginate fiber-reinforced cement-stabilized dredged silty clay. The shear strength of cement-stabilized dredged silty clay reinforced with alginate fibers with the same cement content (9 % by wet weight) and cured for 7 and 28 days was investigated. Consolidated undrained (CU) triaxial compression tests were carried out on reinforced soil samples with varying fiber lengths (3 mm, 6 mm, and 9 mm) and fiber contents (0 %, 0.3 %, 0.6 %, and 0.9 % by weight of wet soil) to assess the shear strength characteristics. The results showed that, on average, the shear strength increased by 18.30 % and 18.56 % with the increasing content and length of alginate fibers, respectively. The variation in deviator stress and axial strain exhibited a shift from strain-softening to strain-hardening with increased confining pressure, alginate fiber content, and length. Hence, a double exponential curve model with different fiber content and length in three confined pressures was proposed, and the correlation coefficient of the fitting results was above 0.90. The secant modulus E50 increased and subsequently decreased with the length of fibers, with the largest secant modulus E50 observed for 6 mm fiber length. The strength parameters (cohesion c and internal friction angle φ) exhibited contrasting responses to variations in alginate fiber length and content. The cohesion of the fiber-reinforced soil specimens increased by 29.0 % and 13.1 %, respectively, for increased fiber content and length. Nevertheless, the angle of internal friction remained relatively unchanged. The findings of this study can serve as a reference guide for related engineering projects.

Constitutive modeling structural behaviors of Shanghai deep clay

The rapid development of underground engineering requires a comprehensive understanding of deep soils. Existing study about the deformation and strength behaviors of Shanghai clay is mostly focused on shallow soft clay (generally within 30 m depth), while counterpart for deep clay is needed. This work investigates the physical properties and mechanical behaviors of Shanghai deep clay (block samples of layer (8) obtained at a depth of around 50 m), via conducting a series laboratory tests including physical property tests, onedimensional consolidation tests and undrained triaxial compression tests, performing constitutive modelling. The results show: (1) Shanghai layer (8) clay can be classified as a low-plasticity silty clay, and the void ratio measured in the laboratory of the block samples is larger than remoulded samples; (2) The clay is a normally consolidated clay, and the block samples show greater compressibility and stronger initial structure; (3) The block samples show contraction behaviors in the undrained triaxial compression tests; (4) The structural behaviors is well captured by structural bounding surface model.

Small-strain characterization of Shanghai clays by triaxial compression tests

The mechanical properties of the upper Shanghai Layers 2-6 clays at small strain levels have been extensively investigated. Nonetheless, limited data exist on their small-strain characteristics through K 0-consolidated undrained triaxial compression tests, with even less information available for the deeper layers 8 and 10. This study employed a series of K 0-consolidated undrained TC tests on intact samples from both the upper layers 2-6 and the deeper layers 8 and 10 of Shanghai clay. We evaluated the undrained maximum secant stiffness (Eu max) and reference shear strain (γ0.7 ). An increase in over-consolidation ratio (OCR) results in a higher E u max/p’ and a more rapid decay of the G/G max with the increasing shear strain (γ). As the value of γ 0.7 increases with I p and decreases with OCR, a correlation between γ 0.7 and Ip can be established. This study observed significant correlations between the E u max, effective stress (p’), and the maximum deviatoric stress, as well as between E u max and the cone resistance obtained from field cone penetration tests. These correlations offer methods to estimate E u max and γ0.7 values based on parameters that are more readily measurable, which provide critical references for the calculations and design parameters in foundation engineering within the Shanghai region.

Undrained shear strength of artificially flocculated clays

The flocculation and sedimentation of suspended clay particles in marine environments are an important process to form natural marine clays. In this study, the process of flocculation and sedimentation is simulated using appropriate flocculating agents in the laboratory. A series of laboratory sedimentation tests are carried out on various floc samples prepared with different concentrations of flocculating agents, and the size and density of flocs are measured and are found to serve as good parameters to characterise the development of flocculation. Multiple series of undrained triaxial compression tests under normal consolidation, as well as under over-consolidation, are then carried out on a slightly flocculated sample and a largely flocculated sample, as well as an ordinary remoulded sample. The values of the undrained shear strength of the flocculated samples are compared with those of the ordinary remoulded sample. The largely flocculated sample exhibits a higher undrained shear strength under normal consolidation as well as under over-consolidation. A comparison with past studies on natural marine clays also indicates that the highly variable values of the undrained shear strength of marine clays can be attributed to the different levels of development of flocculated structures in clays.

Diatom-induced impact on shear strength characteristics of fine-grained soils

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom–kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37° and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils.

Mechanical behaviour and grain breakage characteristics of Tibetan glacial tills under triaxial compression

Deglaciation due to warming climate has left a great amount of glacial tills in ravine slopes and channels in southern Tibet, which can lead to destructive debris flows and pose great threats to downstream residents and infrastructures. The mechanical properties and grain size characteristics of glacial tills are key factors that influence the initiation and development of these debris flows. In this work, a series of consolidated undrained (CU) triaxial compression tests under different confining pressures were carried out on Zelongnong glacial tills at various relative densities of 9.2%, 49.5%, 69.0% and 88.7%. The grain size distribution change before and after the CU tests was also determined. Results show that the glacial tills at relative densities of 9.2%, 49.5% and 69.0% exhibited a remarkable strain-softening and contractive behaviour during shearing, while those at a relative density of 88.7% presented a dilatant behaviour. As the relative density increased, the shear strength, elastic modulus and shearing friction angle increased, reducing the liquefaction potential. Based on the increasing rate of mechanical resistance with the increasing relative density, a critical value of 69.0% was identified to appreciate the distinct mechanical responses of the specimens. From the grain size distributions before and after CU tests, the content of grains with a diameter larger than 0.85 mm decreased due to the grain breakage while that of grains smaller than 0.3 mm increased. The higher the specimen relative density, the larger the grain breakage ratio owing to the increasing friction resistance. This grain breakage in turn causes a downward shift of the critical state line on the plane of void ratio and mean effective stress.

ELASTOPLASTIC CONSTITUTIVE MODEL AND MECHANICAL BEHAVIOR OF UNSATURATED LOESS

To address the issue of the absence of a suitable loess constitutive model in the study of seismic response in loess tunnels, the elastoplastic constitutive model of unsaturated loess was established by replacing the fixed yield surface with the yield surface (i.e., the variable loading surface). This model improves the algorithm of integration since the shape and position change in the stress space due to the continuous loading of unsaturated loess after yielding. In this study, the elastoplastic constitutive model was used in isotropic compression, drained triaxial compression, and undrained triaxial compression tests. The test results were compared with the Cambridge model. The results revealed that the elastoplastic constitutive model achieved a smooth transition between elasticity and plasticity and can effectively simulate soil behavior. The numerical experiment of triaxial drainage compression conformed to the theoretical concept of the boundary surface model and provided ideal simulation properties. From the undrained dynamic triaxial compression numerical test, it was seen that the constitutive model can describe the elastoplastic deformation in the yield surface and better reflect the cyclic strength softening and cyclic stiffness degradation of soil.

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.

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.

Mechanical characteristics and microstructure of saturated sand under monotonic and cyclic loading conditions

To gain a deeper understanding regarding the intrinsic properties and correlations of macro mechanical responses, including critical states and large flow deformations under monotonic and cyclic loading conditions, with a view to revealing the fundamental characteristics of internal forces and deformations in granular materials. A series of undrained triaxial compression, triaxial extension and cyclic triaxial numerical tests are performed using discrete element method (DEM) software PFC3D. Under both monotonic and cyclic loading conditions, a comparable deformation mechanism of strain softening and strain hardening is observed, and the effective stress skeleton curves are in good agreement. As the progression from high stress ratio to the critical stress ratio, flow deformation and strain hardening are observed in saturated sand, where the mechanical coordination number gradually increases and the sliding ratio, suspended particle ratio decrease. The fabric anisotropy tends to increase in the flow deformation stage, decreases slightly in the strain hardening stage but remains highly anisotropic, and decreases rapidly in the stress decay (unloading) stage. As the cycle number increases after liquefaction of the sample, the percentage of deviatoric strain in the flow deformation stage to the total unidirectional shear strain increases, while the percentage in stress decay (unloading)/hardening stages decreases, which means that the strain development of samples is more fully developed in the flow deformation stage. The results serve as a reference for studying soil dilatancy and modifying constitutive models.

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.

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.