Large-scale Direct Shear Apparatus Research Articles

Characterization of embankment material using sand tire mix as additive and validating the results obtained from large scale direct shear test in Abaqus software

Laboratory direct shear tests often overlook larger particles due to limitations in shear box design, resulting in an inadequate portrayal of real-world soil conditions. To address this gap, the use of large-scale direct shear apparatus capable of accommodating larger particles is essential. However, conducting such tests with varied sizes of tire shreds, mixing ratios, and loads is intricate and resource-intensive. Therefore, the utilization of numerical modeling is suggested to simulate sand-tire mixtures under diverse conditions. This study presents conclusions derived from comparing numerical results with experimental observations, demonstrating a close correspondence between the two. It offers a comprehensive examination of the impact of sand and shredded tires mixtures on shear strength, indicating a consistent pattern: increased quantities of tire shreds correspond to heightened friction angles. Furthermore, specimens containing larger shredded tire particles display elevated friction angles, implying a possible connection between particle size and frictional behavior. This investigation underscores the importance of numerical modeling as a cost-efficient and effective approach to complement experimental inquiries, enriching our comprehension of soil-tire mixture dynamics and guiding practical applications in transportation and geotechnical engineering.

Effect of rubber-sand particle size ratio on shear properties of rubber-sand mixtures under normal cyclic loading

The mechanical properties of rubber-sand mixtures are influenced by the ratio of rubber-sand particle sizes and the rubber content. Rubber-sand mixtures used as roadbed fillers are primarily subjected to normal cyclic traffic loading. Therefore, a series of direct shear tests on rubber-sand mixtures under normal cyclic loading were conducted using a large-scale direct shear apparatus. The effects of the rubber-sand particle size ratio, rubber content, initial stress, and amplitude of normal cyclic loading on the shear strength of rubber-sand mixtures were evaluated. Based on the indoor tests, discrete element models of rubber-sand mixtures with different particle size ratios were established to explore the meso-mechanical mechanism of the mixed soil during the direct shear process. The results indicate that rubber-sand mixtures with low rubber content and a particle size ratio >1 have a positive effect on the shear strength of the mixed soil. The impact of initial stress on the shear stress of the mixed soil is greater than that of the normal cyclic load amplitude. As the particle size ratio increases, the inter-particle contact force becomes larger, resulting in more noticeable group anisotropy. Additionally, the main direction of the normal contact force anisotropy in the mixture consistently aligns with the deflection direction of the force chain. Rubber particles in rubber-sand mixtures with a large particle size ratio actively participate in the contact force chain network system, facilitating load transfer and positively influencing shear strength.

Performance of Geogrid-Reinforced Rubber-Coated Ballast and Natural Ballast Mix under Direct Shear Conditions

Rubber-coated ballast (RCB) is a new innovative ballast aggregate obtained by coating natural aggregates with recycled crumb rubber by using polyurethane binder. The objective of RCB is to reduce the ballast degradation, thereby reducing the deterioration of rail track geometry and also maintenance costs. Natural ballast; crumb rubber of sizes 0.4, 0.8, and 2 mm; Elastan polyurethane; and geogrid (triangular, 69 mm aperture size) were used in this study. The results of abrasion, impact value, and soundness tests show that RCB has a higher resistance to abrasion, impact, and weathering compared to natural ballast. Subsequently, the shear behavior and particle breakage of natural ballast, RCB, unreinforced, and geogrid-reinforced RCB–natural ballast mix was explored using the large-scale direct shear apparatus at different normal stresses and shearing rates (Sr). The test results indicate that due to rubber coating the peak friction angle (φ) of RCB samples reduces as compared to natural ballast. The reduction in φ of RCB samples is attributed to a change in surface texture. Moreover, 0.4-RCB (RCB made with rubber crumbs of size 0.4 mm) shows the least reduction in φ compared to 0.8-RCB and 2-RCB (RCB made with rubber crumbs of size 0.8 and 2 mm). It was further observed that with the increase in proportion of 0.4-RCB in geogrid-reinforced natural ballast both the peak friction angle and particle breakage (Bg) of ballast decreased significantly. The current study established the optimum content of 0.4-RCB to be mixed with geogrid-reinforced natural ballast to be in the range of 44%–53%.Practical ApplicationsThe problem of excessive degradation of ballast particles and the associated changes in the track geometry owing to the repeated application of heavy axle loads is addressed by coating the natural ballast with crumb rubber generated from waste tires. The rubber-coated ballast (RCB) is found to offer better abrasion, impact, and weathering resistance when compared to natural ballast. Moreover, it undergoes negligible particle breakage under direct shear conditions. The decrease in the extent of particle breakage helps in reducing the frequency of ballast replacement operations and hence tracks maintenance costs. Furthermore, RCB, when reinforced with geogrid, offers better shear strength than that of natural ballast alone. These observations imply that the tracks with geogrid-reinforced RCB can effectively reduce the risk of track instability.

Design and validation of a new temperature-controlled large-scale direct shear apparatus

To mitigate the size effect on the shear mechanical properties of geotechnical materials and address the limitations of conventional direct shear apparatus, we have designed a new temperature-controlled large-scale direct shear apparatus (TXTZJ-500) based on the existing ones. The innovative apparatus can be used to test the direct shear mechanical behaviors of different materials under freezing or high temperatures. By increasing the size of the shear box, this new direct shear apparatus significantly minimizes the impact of the size effect on the sample failure surface. Consequently, it represents the stress state and provides precise shear mechanical properties of the sample accurately. Subsequently, we conducted a series of direct shear tests on sandy soil and graded gravel materials at different temperatures and axial stress levels using the TXTZJ-500 apparatus. The obtained test results indicate that the TXTZJ-500 effectively replicates the strength and deformation characteristics of sandy soil, frozen soil, and graded gravel. The test data of the developed apparatus are stable and reliable, and can provide a test reference for assessing the mechanical properties of geotechnical materials.

Micro-macroscopic mechanical behavior of frozen sand based on a large-scale direct shear test

The mechanical properties of frozen soil are of utmost importance for infrastructure development in cold regions, and the microscopic characteristics play a vital role in revealing the mechanism of strength and deformation evolution. In this study, the mechanical behaviors of frozen sand at various negative temperatures are investigated by a self-developed temperature-controlled large-scale direct shear apparatus. The shear stress-shear displacement curve, dilation curve and shear strength characteristics at different negative temperatures and normal pressures are analyzed. According to the test results, as the cooling temperature decreases, the cohesion and internal friction angle of frozen sandy sample both increase significantly, and the shear dilatation tends to be larger. The brittle behavior of frozen sand becomes more obvious at lower temperatures. Then, a discrete element method (DEM) model is established to investigate the microscopic mechanical properties of frozen sand in the direct shear test. The particle movement, force chain evolution, and shear surface development during the shearing process are analyzed. The simulations of the DEM model can well explain the microscopic mechanism of the macroscopic strength and deformation law of frozen sandy soil. It is hoped that the research results can provide a reference for the DEM application in frozen soil mechanics.

Evaluating the Shear Strength of Subbase-subgrade Interface Using Large Scale Direct Shear Test

The inclusion of geogrid in road pavements can improve pavement performance through increasing the lateral confinement, bearing capacity, and overall rigidity of the pavement, as well as reducing the vertical and lateral pavement deformations. The materials used in the present study are: subbase granular materials Type B, two types of subgrade soil; clay and sandy soil, and two nonwoven biaxial geogrids (G1 and G2) used as reinforcing materials. Direct shear testing was adopted by manufacturing a large-scale direct shear apparatus consisted of an upper, square box of size 20 cm × 20 cm × 10 cm, and a lower, rectangular box of size 200 mm × 250 mm × 100 mm is used in the present study. The results show that, for the four normal stresses equal to 25, 50, 75 and 100 kPa, the interface shear stress curves increased and followed similar trend. For clay-subbase interface, installation of geogrid decreases the apparent cohesion of the material from 16.5 kPa (without reinforcement) to be 8 kPa and 13.5 kPa for G1 and G2, respectively. At sand-subbase interface, using geogrid leads to increase the cohesion of the material from 3.5 kPa to be 15.5 and 16 kPa for G1 and G2, respectively. The friction angle increases slightly from 30o (without reinforcement) to be 35o for G1 and G2 when the interface is subbase over clay. While, it decreased from 35.8o to be 32.1o for G1 and G2 at sand-subbase interface. The interaction coefficient for G1 and G2 increased when the normal strength increased at the clay-subbase interface. Otherwise, the behavior of interaction coefficient of the sand-subbase interface appears deferent trend, where increasing normal stress leads to decrease the interaction.

Investigation of Shear Strength of Subbase-Subgrade Interface with Geosynthetics Reinforcement Utilizing A Large-Scale Direct Shear Test

Geosynthetics are being used to strengthen road pavement. Geosynthetic inclusions improve pavement carrying capacity, maintenance costs, highway service life, reflective cracks, and undesirable large lateral and vertical deformations. The primary purpose of this research is to determine the effectiveness of geosynthetics (geogrids and geotextiles) in stabilizing the subgrade and reinforcing the base course layers in unpaved test sections. Determine the mechanical interaction of subgrade soils (clay and sand) and aggregate road base layers (subbase) with and without reinforcement. Compute the shear strength parameters (cohesion, friction angle, and interface coefficient factor). Therefore, Large-scale direct shear experiments in the laboratory were performed on subbase-subgrade materials with and without geosynthetics, under the applying normal of stresses (25, 50, 75, and 100) kPa, indicating the quantity overburden the pressure in paving. The present research uses a large-scale direct shear apparatus with an up square box (200 mm×200 mm×100 mm) and a bottom rectangular box (200 mm×250 mm×100 mm). A direct shear test was implemented by manufacturing this equipment. The results obtained from experiments showed that biaxial geogrid G1 has the best behavior for both (subbase-clay) and (subbase-sand) and has an interface shear coefficient factor more significant than unity and equal to 1.05 and 1.02, respectively.

Experimental Investigation of the Mechanical Properties of the Sand-Concrete Pile Interface Considering Roughness and Relative Density.

The aim of this study was to investigate the effect of roughness and relative density on the mechanical properties of sand–concrete pile interface. A series of direct shear tests were carried out on the interface using a large-scale direct shear apparatus with various relative densities of sand (73%, 47%, and 23%) and concrete blocks with four roughness values (I = 0, 10, 20, and 30 mm). Various mechanical properties (such as shear stress, volume change, peak shear strength, secant friction angle, and normalized friction coefficient) from the interface tests were compared with trends obtained from the pure sand direct shear test. For the smooth interface, the shear stress–horizontal displacement curves of the dense sand specimen exhibited a slight softening response, which became more apparent as the roughness increased. The curves of the loose sand specimen demonstrated a hardening response. The volumetric response was influenced by the combination of normal stress, relative density, and roughness. The peak shear strength demonstrated a nonlinear increasing trend as the normal stress increased. With an increase in the normal stress, the secant friction angle and peak friction coefficient decreased as exponential and power functions, respectively. Additionally, a critical roughness value Icr resulted from the tests, which halted the upward trend of the peak friction coefficient and normalized the secant friction angle when I exceeded Icr.

Effect of the particle size ratio on macro- and mesoscopic shear characteristics of the geogrid-reinforced rubber and sand mixture interface

As a new type of material for civil engineering projects, the rubber and sand mixture is widely used in roadbed fillers, offering environmental benefits over traditional tyre disposal methods. This study uses a large-scale direct shear apparatus to examine the interface shear properties of the geogrid-reinforced rubber and sand mixture, considering different particle size ratios (r), rubber contents, and normal stresses. Based on indoor tests, direct shear models of the mixture with different values of r are established in PFC3D, revealing the meso-mechanical mechanism of the mixture in the direct shear process. The results show that when r is greater than 1, incorporating a certain amount of rubber particles can increase the shear strength of the mixture. The r values of 15.78, 7.63, and 3.98 correspond to an optimal rubber content of 30%, 10%, and 20%, respectively. When r is less than 1, mixing rubber particles can only reduce the shear strength of the mixture. When the rubber content is low, the smaller the value of r, the greater is the thickness of the shear band. Furthermore, the normal and tangential contact forces are greater. The fabric anisotropy evolution law of the mixture is consistent with the change in the contact force distribution.

Experimental investigation of shear strength parameters of ballast encased with geogrid

So far, significant studies have been performed to increase the shear strength (τ) of ballast by reinforcing it with geogrid, though ballast encasement with geogrid, which could provide substantial confining pressure, has not yet been considered. Therefore, in this study, using a large-scale direct shear apparatus, effect of encasement of ballast with geogrid on shear behavior were investigated. For this purpose, two groups of ballast with average particle size (D50); 41.31 mm and 31.55 mm, and three types of biaxial geogrids GP35/35, GP40/20, and GP60/20, were considered. These two sets of experiments were carried out under normal stresses (σn) of 50, 100, and 150 kPa and with a shear rate of 1 mm/min. Experimental results showed that in the non-encased case, τ was affected by nominal maximum particle size (Dmax), uniformity coefficient (Cu) as well as the applied σn. According to the results σn was increased ​​from 50 to 150 kPa, the internal friction angle (φ) for group 1 decreased from 65.56° to 54.39° and for group 4 it decreased from 63.90° to 52.16°. Similarly, for the above-mentioned gradation groups, the dilation angle (ψ) decreased by 16.69% and 12.85% on average for all σn s, respectively. On the other hand, ballast encasement with geogrid improved its shear behavior, as for gradation group 1, the maximum effect of encasing on the φ corresponding to σn alteration from 50 to150 kPa was 74.5° for group 1 and 68.96° for group 4. While, the maximum reduction in ψ was 32.22% on average under these conditions, moreover for ballast group 4, the mentioned changes were 30.13%, 14.67%, and 27.86%, respectively. Also, in this study, the efficiency factor (α), varied in the range of 1.11–1.91. The effect of geogrid aperture size (A) on τ of encased ballast was investigated, too.

Empirical modelling of shear behavior of oil contaminated sand - geogrid interface

ABSTRACT Based on the experimental studies, a semi empirical predictive model has been presented and discussed in this paper to simulate the interface shear behavior of used diesel engine oil (UDEO) contaminated (0%, 2%, 4%, 6% and 8%) sand and geogrids. Experiments were conducted in a standard large-scale direct shear apparatus, which demonstrated that when the oil content increases from 0% to 8%, there is a drop in shear strength at the interface, with a marginal strain hardening trend. While the peak friction angle decreased as the UDEO content increased, the apparent cohesion showed an increasing trend. In addition to UDEO content, the normal stress is also an influencing parameter in the interface shear behavior. In order to simulate the interface shear behavior, a best-fit semi-empirical hyperbolic model has been suggested from the observed data using least square error method. The proposed model showed an excellent agreement with the experimentally observed behavior for both sand–sand and sand–geogrid interface.

Monotonic, cyclic and post-cyclic shearing behavior of sand-EPS geofoam interface

Sand-EPS geofoam is commonly used as a seismic buffer in a variety of geotechnical applications to alleviate damages imposed by seismic vibrations. Evaluation of the properties of the sand-EPS geofoam interface is an essential part of the design and installation of EPS geofoam in direct contact with the parent sand. In this study, a series of laboratory tests are carried out using a large-scale direct shear apparatus to evaluate the monotonic, cyclic and post-cyclic behavior of the sand-EPS geofoam interface. Adopting the relative density of the parent sand, the density of EPS geofoam, the applied normal stress, the cyclic shear strain semi-amplitude and the number of cycles as the variable parameters, their influence on the response of the sand-EPS geofoam interface is investigated. According to the experimental results, with the increase in the number of cycles, sand relative density, EPS geofoam density and the applied normal stress, the equivalent or secant stiffness of the sand-EPS geofoam interface increases while its damping ratio declines. The results of the monotonic (MDS) and post-cyclic monotonic (CMDS) shearing experiments also show that with an increase in the EPS density and normal stress, which offers greater resistance against sliding, the mobilized shear stresses generally increase.

Effects of the soil water content and relative roughness on the shear strength of silt and steel plate interface

Using a self-developed large-scale direct shear apparatus, 72 series of tests on the interface of a silt-steel structure and 18 series of direct shear tests of silt were performed. The effects of the water content of soil, interface roughness and normal stress on the shear mechanical behavior of silt-steel structure interface were studied. The test results confirm that the water content obviously affects the hardening–softening behavior of the silt-structure interface. The curve of the shear stress with shear displacement of the silt-steel interface was similar to the trend of the direct shear test results of the silt. The shear strength of the rough interface (Rmax > 0 mm) was significantly greater than the results of the direct shear test of silt, and the shear strength of the smooth interface (Rmax = 0 mm) was less than the shear strength of the direct shear of silt. In the process of the interface shear test, the deformation of the soil body gradually increased with the decrease in soil water content and increase in interface roughness. The contribution of matric suction of unsaturated silt to the interface peak shear stress was more prominent and hardly affected the residual shear stress. The interface shear strength for unsaturated soil was modeled. The comparison between model results and experimental results shows that the calculation model can accurately predict the shear strength of the unsaturated silt-steel interface under different interface roughness conditions.

Testing and modeling of frozen clay–concrete interface behavior based on large-scale shear tests

The shear behavior of the frozen soil−structure interface is important for accurately predicting the interface responses of structures adopted in the cold regions. The purpose of this study is to experimentally and theoretically investigate the shear behavior of frozen clay–concrete interface under engineering conditions. A large-scale direct shear apparatus with a temperature-controlled shear box is used to test the interface behavior. Test specimens consisting of a cement concrete block and frozen soil with initial water content ranging between 14.6% and 24.6% were prepared at different conditions of temperatures (15.4 to −9.8 °C), shear rates (0.03–0.9 mm min−1), and normal stresses (50–200 kPa). It is found that the peak shear strength is linear developing with increasing of normal stress, initial water content, and temperature. It increased from 67.7 to 133.3 kPa as the initial water content increased from 14.9% to 24.6% at temperature of −6.8 to −6.6 °C, and it increased from 51.2 to 80.6 kPa with temperature decreasing from 15.4 to −9.8 °C at initial water content of 14.6%–14.9%, furthermore it has a power law relationship with shear rate. The final vertical displacement increases with the decreasing temperature, and increasing initial water content. While, it is slight or could be ignored at lower shear rates (e.g. 0.03 mm min−1 and 0.15 mm min−1) and it is −0.25 mm and −0.28 mm at shear rate of 0.3 mm min−1 and 0.9 mm min−1, respectively. In addition, the evolution of vertical displacement also varies with test condition, the growth rate at beginning increases with increasing initial water content and decreasing temperature or ice content, which is because of the ice film effects the particle size. Moreover, a disturbed state concept model combined with linear and nonlinear characteristics is developed to describe the interface shear behavior. The disturbance D reflects the interface mechanical response and responds differently trend for different test conditions, increasing faster with increasing temperature and decreasing initial water content or shear rate. The testing results, including the test and model results, can be used to simulate the performance of engineered geotechnical assets such as earth dams or irrigation channels with concrete linings in cold regions.

Effect of Particle-Size Gradation on Coarse Sand-Geotextile Interface Response in Cyclic and Postcyclic Direct Shear Test

In order to investigate the influence of sand particle-size gradation on cyclic and postcyclic shear strength behaviour on sand-geotextile interfaces, a series of monotonic direct shear (MDS), cyclic direct shear (CDS), and postcyclic direct shear (PCDS) tests were performed using a large-scale direct shear apparatus. The influence of cyclic shear history on the direct shear behaviour of the interface was studied. The results indicated that cyclic shear stress degradation occurred at the sand-geotextile interface. Shear volumetric contraction induced by the cyclic direct shear increased with the increase in cycle number. The lowest final contraction value was observed in discontinuously graded sand. In the MDS tests, there were great differences in interface shear strength due to the different particle-size gradations, whereas the differences between shear volumes were negligible. In the PCDS tests, the shear stress-displacement curves exhibited postpeak stress hardening behaviour for different particle-size gradations, and differences in shear volumes were detected. The well-graded sand-geotextile interface had a higher value of shear stiffness and a higher damping ratio relative to the other interfaces. Postcyclic shear stress degradation was observed for the discontinuously graded sand-geotextile interface.

Effect of Sample Size on TDA Shear Strength Parameters in Direct Shear Tests

Tire-derived aggregate (TDA), a relatively new construction material, has been gaining acceptance as a backfill material for embankments, trenches, and earth-retaining structures because of its lightweight and excellent geotechnical properties. Type A TDA has a basic geometric shape, with particles approximately 12 to 100 mm in size. As a result of the simplicity and accuracy of the direct shear test, most laboratories choose this test in preference to more complex tests. However, TDA requires large-scale direct shear apparatus because of the consistently large size of its particles, and few facilities own this type of apparatus. Depending on the shear box dimensions, the aspect ratio of the particle size to the box dimensions may lead to variations in the shear strength results of the sample being evaluated. This research focuses on studying the effect of TDA sample size on the shear strength results of direct shear tests by using five different shear box sizes. The findings show that the angle of internal friction increases slightly as the dimensions of the shear box decrease. It was found that the maximum variation in the angle of internal friction and the cohesion results of the different shear boxes was only 1.9° and 2.4 kPa, respectively. These differences should be taken into consideration when TDA shear test results are used in the geotechnical design. It is recommended that a shear box with an aspect ratio (W/Dmax) greater than or equal to 4 should be used when evaluating the shear strength parameters of TDA.

Role of normal stiffness in 3D cyclic behavior of gravel–steel interface from large-scale direct shear tests

The shaft capacity of piles is essentially the shear strength response of soil–structure interfaces. The design of piles requires sound knowledge of the three-dimensional (3D) interface behavior under constant normal stiffness (CNS) condition, which has seldom been addressed. In this research, a series of 3D tests of a gravel–steel interface were conducted in two-way circular, cross and beeline cyclic shear paths at different normal stiffness using a large-scale direct shear apparatus. The 3D behavior and constitutive rules of the interface under CNS condition and the influence of normal stiffness were explored in detail. Test results indicate that the normal stiffness remarkably affects the magnitude of normal displacement, normal stress, shear stress, initial shear modulus and degradation of cyclic shear strength. Increased normal stiffness leads to accelerated reduction in the normal stress and therefore accelerated degradation of cyclic shear strength. The variation of normal stress results in “induced aeolotropy” in the shear strength, while it has no impact on the inherent isotropy of strength coefficient. The shaft capacity of the piles degrades slowly and might be lost incompletely if the surrounding soil is relatively hard and the normal stiffness is sufficiently small. By contrast, the normal stiffness is of poor significance in the relationship pattern between performance parameters, in addition to the friction angle of the interface. The resultant stress ratio versus resultant tangential displacement response presents perfect consistency characteristics and can be described using a hyperbola model, independent of normal stiffness and shear path, thereby considerably simplifying the 3D constitutive description of the interface under CNS condition.

Performance evaluation of polyurethane-stabilized railroad ballast under direct shear conditions

The behavior of Elastan-stabilized and unstabilized ballast was explored using the large-scale direct shear apparatus under different normal stresses (σn) and shearing rates (Sr). Fresh granite ballast and Elastan polyurethane having a density of 1100 kg/m3 was used for the study. Further, a triangular aperture geogrid was also used to compare its benefits with that of Elastan treatment of ballast. Elastan-stabilized ballast samples were tested after a curing period of 3 days. Test results established that the shear strength of ballast was profoundly affected by both the applied normal stress and shearing rate. The friction (φ) and dilation (ψ) angles of unstabilized ballast were found to decrease from 65° to 59° and from 21° to 10°, respectively with an increase of σn and Sr. The results further confirmed that the Elastan treatment significantly enhanced the shear strength of ballast. The friction (φ) and dilation (ψ) angles of Elastan-stabilized ballast increases from 65° to 75° and 21° to 22°, respectively. The stabilization efficiency factor (Sef), defined as the fraction of the shear strength of stabilized ballast to that of unstabilized ballast, varies from 1.6 to 1.75 for Elastan-stabilized ballast. The breakage of ballast (Bg) increased from 7.62 to 11.72% with an increase of σn and Sr. Further, the geogrid reinforcement reduced the Bg from 11.72 to 4.24% when compared to unstabilized ballast. On the other hand, visual inspection of the Elastan-stabilized ballast indicated no degradation of ballast particles.

Volume change behavior of root-permeated soils under partially saturated conditions

Vegetation, particularly roots, serves different functions in relation to increased shear strength under saturated and partially saturated conditions. Quantification of mechanical contribution of roots due to their tensile strength, and relationships of various vegetation parameters and plant-induced suction, as well as shear strength, have been widely studied. Although shear strength is directly related to the volume change characteristics of soil, dilative or contractive behaviour of root-permeated soils has not been of significant interest so far. This study investigates how volume change during shearing is related to the hydrological and mechanical characteristics of vegetated soils relevant to slope stability and shear strength of root-permeated soils under partially saturated conditions. Direct shear tests, on specimens planted with a mixture of species from different plant functional groups, were performed with an Inclinable Large-scale Direct Shear Apparatus (ILDSA). Matric suctions were monitored throughout the test with tensiometers. Vertical and horizontal displacement graphs were plotted to investigate the volume change behaviour. Maximum dilatancy angle was found to be positively correlated with plant-induced suction and net normalised stress, both of which were linked to root biomass and the root:shoot ratio. It was found that maximum dilatancy is controlled by matric suction and net normal stress.

Assessment of plant-induced suction and its effects on the shear strength of rooted soils

Shallow landslides, either on bare or vegetated slopes, can be triggered after a rainfall event due to loss of suction. An extensive laboratory programme was performed in this study to assess the relationships between water content, plant-induced suction, root biomass and shear strength parameters. Root-permeated soils, planted with combinations of different species, were tested in an inclinable large-scale direct shear apparatus. The effects of mycorrhizal fungi were also investigated with inoculated specimens. The results suggested that the root biomass, as well as root/shoot ratio, was an indicator of plant-induced suction and shear strength of root-permeated soils tested under laboratory conditions. Longer plant growth duration and more species yielded higher mean values of matric suction and normalised shear stress. Mycorrhizal fungi were found to be beneficial in improving the plant functions related to water uptake.