Results Of Laboratory Model Tests Research Articles

Upper bound analytical solution for surrounding rock pressure of shallow unsymmetrical loading tunnels

By combining the results of laboratory model tests with relevant flow rules, the failure mode of shallow unsymmetrical loading tunnels and the corresponding velocity field were established. According to the principle of virtual power, the upper bound solution for surrounding rock pressure of shallow unsymmetrical loading tunnel was derived and verified by an example. The results indicate that the calculated results of the derived upper bound method for surrounding rock pressure of shallow unsymmetrical loading tunnels are relatively close to those of the existing “code method” and test results, which means that the proposed method is feasible. The current code method underestimates the unsymmetrical loading feature of surrounding rock pressure of shallow unsymmetrical loading tunnels, so it is unsafe; when the burial depth is less or greater than two times of the tunnel span and the unsymmetrical loading angle is less than 45°, the upper bound method or the average value of the results calculated by the upper bound method and code method respectively, is comparatively reasonable. When the burial depth is greater than two times of the tunnel span and the unsymmetrical loading angle is greater than 45°, the code method is more suitable.

Experimental and Numerical Studies of Circular Footing Resting on Confined Granular Subgrade Adjacent to Slope

Many situations exist where foundations must be constructed on or near slopes. As a result, the foundation’s bearing capacity is reduced, and the slope might collapse. This paper presents the results of laboratory and numerical model tests on the influence of soil confinement on the behavior of a model footing adjacent to slopes. Confining cylinders with different heights, each set back some distance from the slope crest, were adopted. The ultimate bearing capacity of a circular footing supported on a confined sand bed adjacent to a slope was studied. The studied parameters include the cell-embedment depth, slope angle, and subgrade density. The results indicate that the bearing load capacities of circular footings at any edge distance from the slope crest can be appreciably increased by soil confinement. The slope effect on the ultimate bearing capacity can be neglected when the cell-footing system is placed at setback distance with cell-embedment depth around two times the cell diameter. When the cell-footing system is placed directly on a steep slope crest, the improvement in the bearing capacity reached 4, 2.2, and 1.9 times the footing with subgrade relative densities of 80, 55, and 35%, respectively. The numerical analysis helped to explain the deformation behavior of cell-footing soil systems adjacent to a slope. It provides a realistic failure mechanism of the new system and illustrates how the cell can protect the slope from collapse by decreasing slope deformation.

DEM analysis of micro-structural events within granular shear zones under passive earth pressure conditions

Shear zones in initially medium dense cohesionless sand for quasi-static earth pressure problem of a small-scale retaining wall were analysed with a discrete element method (DEM) using spheres with contact moments. The passive sand failure for a very rough retaining wall undergoing horizontal translation was discussed. The DEM calculations were carried out with the different mean grain diameter. Micro-structural events appearing within granular shear zones such as: vortices, force chains, vortex structures and local void ratio fluctuations were investigated. Special attention was laid on a vortex-force chain correlation and frequency of the vortex appearance. The calculated geometry of shear zones was compared with experimental results of laboratory model tests analyzed using the DIC. DEM demonstrated its ability to describe the geometry of shear localization in sand behind the retaining wall and to follow the evolution of micro-structure in granular shear zones.

Study of a railway embankment reinforced with jute tassels

This paper presents the results of laboratory model tests and corresponding numerical analyses carried out on a model slope representing an actual railway embankment which failed on several occasions after overnight heavy rainfall. In the absence of any field data, the displacement behaviour and failure pattern of the model embankment slope are observed in the laboratory under fully saturated condition and under a static load at the crest simulating the actual rail loads. A study is also performed by reinforcing the slope with thin jute tassels of 1 mm diameter. The numerical simulation of the model tests is performed by a commercial program called FLAC. The responses of the model slope with and without jute tassels are observed in fully saturated condition. Significant improvement in deformation of the slope is observed both in the case of numerical analyses and laboratory experiments when jute tassels are utilized as a slope protection measure. The study indicates that the stability of the railway embankment can be improved significantly at low cost by reinforcing it with jute tassels.

BEHAVIOUR OF PRESTRESSED GEOTEXTILE-REINFORCED FINE SAND BED SUPPORTING AN EMBEDDED SQUARE FOOTING

This paper presents the results of laboratory model tests carried out on embedded square footing supported on geotextile reinforced sand bed. The effect of reinforcement with geotextile were studies through a series of laboratory model tests with different size of geotextile and depth of placement below footing. The effects of prestressing the geotextile on the strength improvement and settlement reduction of a reinforced sand bed are also being investigated. The model steel tank of size 120 cm x 50 cm x 50 cm and square footing of 10 cm are used. The study also highlights the effect of size of geotextile and placement of geotextile below footing on load-settlement characteristics.

Behavior of Compacted Lime — (Well-graded) Soil columns: Large scale tests and numerical modelling

This study presents the results of large scale laboratory model tests to investigate the behavior of Compacted Lime-Well-graded Soil (CL-WS) rigid stone columns in soft soils. Tests were carried out on composite specimens to evaluate the influence of different parameters such as: the diameter of the column, the slenderness ratio, area ratio and the shear strength of the surrounding soil. Finite element analysis has been also performed using PLAXIS software to compare the results of numerical and experimental modelling. In order to assess the real behavior of these columns, some tests have been carried out in the field. Based on the results, it was concluded that CL-WS columns increase the load carrying capacity of soft soils and reduce the settlement. In addition, the results show the influence of model size on the stiffness of the specimens which means that the load carrying capacity decreases by increasing the size of models. However, for specimens containing columns with diameter greater than 100 mm, the variations of stiffness become negligible and hence the results can be used to extrapolate and predict the full size behavior of these columns. A detailed comparison between the experimental and numerical modelling shows a very good agreement.

Behaviour Of Calcarious Soil Subjected to Oil Derivatives

Calcareous or salty soils are the soils which are containing highly dissolved sodium or calcium salts in natural conditions. The dissolution of salts increases with temperature, atmospheric pressure, in addition to the acidity of the dissolves solution. Calcareous soil is stiff and very hard if it is in a dry phase; it becomes collapsible and very week when wetted with water. It is very dangerous for structures when constructing on such soil especially when high stresses are applied on it. Oil tanks or pipes may damage from any reason, and the oil products may leak from these structures to the soil and infiltrate through soil skeleton and may cause leaching to CaCO3 salt particles in some regions in Iraq, as example the Baiji Oil Station or Al-Mosel Dam, CaCO3 percent reaches more than 40%.This study shines the lights about the behavior of Calcareous soil subjected to three oil derivatives (kerosene oil, crude oil, gas oil and a sample wetted with water to make good comparison ,and study effect of addition of this products on the collapsibility.A laboratory model included soil with 70% and 50% CaCO3 compacted to 11 kN/m3. Fix stress system was used which applies 50 kN/m2, the loading frame was manufactured in a way that keeping the weights over footing stable without tilting. Three oil derivatives (Kerosene, Gasoil and crude oil) were used for laboratory model tests; by wetting Calcareous soil with it. One sample was wetted with water for comparison; the settlement was recorded with soaking time at a constant stress level.The results of laboratory model tests shows that the settlements results from specimens soaked with lubricating oil, Gasoil and Kerosene, are much less than the settlements that belong to soaking with water (reduce settlement to about one third) and considered high improvement of such problematic soil by wetting with oil derivatives.

BEHAVIOR OF SQUARE FOOTING RESTING ON REINFORCED SAND SUBJECTED TO INCREMENTAL LOADING AND UNLOADING

The foundations of various structures are subjected to cyclic loading in addition to static loading in many situations. This paper presents the results of laboratory model tests on square footings supported on geogrid reinforced sand bed under incremental loading and unloading conditions for different densities of sand bed and U/B ratio. The incremental values of intensity of loads (loading, unloading and reloading) were applied on the footing to evaluate the response of a square footing and also to obtain the value of elastic rebound of the footing corresponding to each cycle of load. The effect of sand for the density 1.59 gm/cc, 1.69 gm/cc, and 1.79 gm/cc and for different U/B ratio of 0.2, 0.4, and 0.6were investigated on ultimate bearing capacity and the dynamic properties such as coefficient of elastic uniform compression Cu, coefficient of elastic uniform shear Cτ, coefficient of elastic non-uniform shear Cψ and the coefficient of elastic non uniform compression Cφ. The results shows that the value of ultimate bearing capacity and the value of Cu, Cτ, Cφ and Cψ of sand were increased by increasing the density of sand and with the increase of U/B ratio up to 0.4. The results of ultimate bearing capacity and values of dynamic properties (Cu, Cτ, Cφ and Cψ) for reinforced sand are greater than unreinforced sand bed.

Dynamic and Static Combination Analysis Method of Slope Stability Analysis during Earthquake

The results of laboratory model tests for simulating the slope failure due to vibration, including unreinforced slope and the slope reinforced by using geotextile, show that the slope failure occurs when a cumulative plastic displacement exceeds a certain critical value. To overcome the defects of conventional stability analysis, which evaluates the slope characteristics only by its strength parameters, a numerical procedure considering the stiffness and deformation of materials and geosynthetics is proposed to evaluate the seismic slope stability. In the proposed procedure, the failure of slope is defined when the cumulative plastic displacement calculated by a dynamic response analysis using actual seismic wave exceeds the critical value of displacement estimated by a static stability analysis considering seismic coefficient. The proposed procedure is applied to the laboratory model tests and an actual failure of slope in earthquake. The case study shows the possibility that the proposed procedure gives the realistic evaluation of seismic slope stability.

Behavior of Compacted Lime-soil Columns

This study presents the results of large scale laboratory model tests to investigate the behavior of Compacted Lime – Well-graded Soil (CL-WS) rigid stone columns in soft soils. The unit cell idealization is used for construction of composite specimens by evaluating the influence of different parameters such as the diameter of the column (D), the slenderness ratio (L/D) and the area ratio (Ar). Experiments were carried out on the both end bearing and floating columns. Load was applied over entire area of the specimens to find the stiffness of the improved ground. Based on the results, it was concluded that CL-WS columns increase the load carrying capacity and reduce the settlement of soft soils. In addition, the results show the influence of model size on the stiffness of the specimens which means that the load carrying capacity decrease by increasing the size of models. However, for specimens containing columns with diameter greater than 100 mm, the variations of stiffness become negligible and hence the results can be used to extrapolate and predict the full size behavior of these columns. The experimental results are compared with results reported in the literature for ordinary stone columns.

Bearing Capacity of Strip Footing on Sand Slopes Reinforced with Geotextile and Soil Nails

This paper presents the results of laboratory model tests on the behavior of a strip footing supported by a single geotextile layer and by a row of soil nails in a sandy slope. A comparison between bearing capacity improvements in the two cases were made and analyzed. Parameters varied include depth of the reinforcing layer, edge distance of the footing, location of soil nail row, and location of the footing relative to the slope crest. Bearing capacity of non-stabilized cases were initially determined and then compared with those of stabilized slopes. Results indicate that stabilized earth slope using a single geotextile layer or a row of soil nails significantly improves bearing capacity of strip footing. This improvement in bearing capacity increases as soil nail spacing decreases. Overall improvement is significantly better when using a single geotextile layer to stabilize earth slope than using a row of soil nails.

Experimental and numerical studies on footings supported on geocell reinforced sand and clay beds

This paper presents the results of laboratory model tests and numerical studies conducted on a square footing resting on geocell reinforced sand and clay beds. Using suitable scaling considerations, a model footing size was arrived from the prototype raft foundation. Commercially available geocells made up of polyethylene having an equivalent pocket diameter of 0⋅25 m and aspect ratio of 0⋅6 were used in the experimental investigation. Clean sand was used to fill the geocell pockets in both sand and clay bed tests. Test results of unreinforced, geocell reinforced, and geocell reinforced with additional planar geogrid at the base of the geocell cases are compared separately for sand and clay beds. Results reveal that the use of geocell increases the ultimate bearing capacity of the sand bed by 2⋅4 times and clay bed by 3⋅2 times. Provision of the planar geogrid at the base of the cellular mattress arrests the surface heaving and prevents the rotational failure of the footing. Moduli of subgrade reaction values indicate that the contribution of the geocell reinforcement exists even at very low settlements. Using the concept of equivalent composite model, the three-dimensional nature of the geocell is numerically simulated in the fast Lagrangian analysis of continua in 2D (FLAC2D). Experimental and numerical results are in good agreement with each other.

Numerical and experimental evaluation of bearing capacity factor N of strip footing on sand slopes

Results of laboratory model tests and numerical studies on the behavior of a strip footing adjacent to a sand slope are investigated and presented in this paper. The investigated parameters include the effects of depth of the first reinforcement layer, vertical spacing, number of reinforcement layers, and distance between the edges of footings on bearing capacity. Results were analyzed to determine the effects of each parameter. Using a strip footing located near a sand slope crest had a significant effect on improving bearing capacity. The improvement increased when relative density decreased. The depth of the first layer decreased with further improvement when the distance of the footing edge from the slope crest increased. Using a strip footing can be considered effective in controlling the horizontal movement of the subgrade and in decreasing slope deformation. Finite element analysis explained and identified the failure pattern of strip footings adjacent to a slope crest. The findings also confirmed the load transfer mechanism and showed how a slope can be protected when geotextile is used. Key words: Strip footing, slope stability, plaxis, bearing capacity, scale effects.

Experimental Investigation of the Bearing Pressure for Circular and Ring Footings on Sand

This paper presents the results of laboratory model tests of bearing pressure of circular and ring footing on sand. The effects of the embedment depth, internal friction of sand and the ratio of the inner to the outer diameter of the ring footing have been studied, in order to understand the behavior of the sand under the ring footing comparing with the circular one. An optimum ratio of the inner to outer diameter of the ring footing have been indicated which was (0.4), at which the bearing capacity will be greater than the circular footing. Also, the results indicated that there was no interested effect of the embedment depth on that optimum ratio.

Laboratory model tests on laterally loaded piles in plastic clay

This paper presents the results of laboratory model tests conducted to study the behavior of laterally loaded piles installed in plastic clay. The effects of different factors such as pile dimensions, pile material and method of installation were studied. The pile response is presented in terms of load–deflection curves, bending moment distribution, and the p–y curves. The load–deflection curves are generally non-linear specially for large length/diameter (L/d) ratios and large diameters. The ultimate lateral pile capacity increases with the increase in L/d ratio and the pile diameter. The maximum bending moment increases with the increase in L/d ratio and the pile diameter. The maximum bending moment occurs at a depth varying from 0·13L to 0·32L. The bending stresses at the ultimate load are much less than the yielding stress of steel and the flexural strength of concrete. This implies that the deflection controls the design of laterally loaded piles in plastic clays. The experimental p–y curves are different from the existing p–y curves for piles with small diameters and are approximately similar to the existing p–y curves for piles with larger diameters. The bored method of installation gives greater ultimate soil resistance than the preinstalled method for both concrete and steel piles. Overall this study showed that the laboratory model tests with relatively large diameter (>30 mm) can be used to give insight into the performance of laterally loaded piles installed in plastic clays.

Ultimate bearing capacity of shallow strip foundation under eccentrically inclined load, Part I

Results of laboratory model tests to determine the ultimate bearing capacity of shallow strip foundation supported by sand and subjected to eccentrically inclined load are presented. The tests were conducted in dense and medium dense sands. The embedment ratio (ratio of the depth of embedment <i>D_f</i> to the width of the foundation <i>B</i>) was varied from zero to one. The load eccentricity <i>e</i> was varied from zero to 0.15<i>B</i> as well as the load inclination from zero to 20 degrees. Based on the results of the present study, an empirical nondimensional reduction factor has been developed. This reduction factor is the ratio of the ultimate bearing capacity of the foundation subjected to an eccentrically inclined load to the ultimate bearing capacity of the foundation subjected to a centric vertical load.

A comparison of static and cyclic loading responses of foundations on geocell-reinforced sand

The results of laboratory-model tests on strip footings supported on unreinforced and geocell-reinforced sand beds under a combination of static and repeated loads are presented. The influences of various parameters are studied including reinforcement width, height of the geocell below the footing base and various amplitudes of repeated load. Mostly, a stable, resilient response is observed once incrementally accumulated plastic strain has ceased (usually during the first 10 cycles of loading). The reinforcement reduces the magnitude of the final settlement, acts as a settlement retardant, permits higher loads or increased cycling. The reinforcement’s efficiency in reducing the maximum footing settlement decreased as the height and width of geocell were increased. Plastic deformation was limited by geocells more under repeated loading than under a similar static loading, with the reduction being greatest when more reinforcement was present and when the loading rate was fastest. It is deduced that the greater resilient stiffness of a rapidly loaded polymeric geocell attracts load to itself thereby protecting the soil from some of the more challenging stress states and, hence, reduces deformation. Simple dimensional analysis showed the need for an increased stiffness of the geosynthetic components in order to deliver full-scale performance similitude.

The effect of deep excavation-induced lateral soil movements on the behavior of strip footing supported on reinforced sand

This paper presents the results of laboratory model tests on the influence of deep excavation-induced lateral soil movements on the behavior of a model strip footing adjacent to the excavation and supported on reinforced granular soil. Initially, the response of the strip footings supported on un-reinforced sand and subjected to vertical loads (which were constant during the test) due to adjacent deep excavation-induced lateral soil movement were obtained. Then, the effects of the inclusion of geosynthetic reinforcement in supporting soil on the model footing behavior under the same conditions were investigated. The studied factors include the value of the sustained footing loads, the location of footing relative to the excavation, the affected depth of soil due to deep excavation, and the relative density of sand. Test results indicate that the inclusion of soil reinforcement in the supporting sand significantly decreases both vertical settlements and the tilts of the footings due to the nearby excavation. However, the improvements in the footing behavior were found to be very dependent on the location of the footing relative to excavation. Based on the test results, the variation of the footing measured vertical settlements with different parameters are presented and discussed.

Behavior of confined square and rectangular footings under eccentric-inclined load

This paper presents the results of laboratory model tests on the effect of soil confinement on the behavior of square and rectangular model footings resting on Ganga sand under eccentric - inclined load. Confining cells with different heights and widths have been used to confine the sand. The studied parameters include the cell height, cell width and the depth from base of footing to the top of the confining cell, load eccentricity and load inclination. The ultimate bearing capacity improvement due to the soil confinement is represented using a non-dimensional factor, called the bearing capacity ratio (BCR). The results indicate that the ultimate bearing capacity of footings can be appreciably increased by soil confinement under axial load as well under eccentric-inclined load. It has been observed that such confinement resists the lateral displacement of soil underneath the footing leading to a significant decrease in the vertical settlement and hence improving the ultimate bearing capacity. For small cell width, the cell-soil footing behaves as one unit (deep foundation), while this pattern of behavior was no longer observed with large cell width. The recommended cell heights, depths, and widths that give the maximum ultimate bearing capacity improvement are presented and discussed.

Influence of Sidewall Friction on the Results of Small-Scale Laboratory Model Tests: Numerical Assessment

AbstractLaboratory model tests in geotechnical engineering are routinely performed in small concrete or steel test tanks to examine the bearing capacity of footings, lateral earth pressures on retaining walls, behavior of reinforced foundation soils, and more. Results from these model tests are influenced by the shear forces generated on the sidewalls. The influence of such extraneous forces may completely change the interpretation of the test results or the failure mechanism. This paper presents a methodology for assessing the influence of such sidewall forces through numerical simulations. Through systematic numerical simulations, the influence of factors such as the relative size of the test tank, soil properties, and sidewall properties as investigated. The influence of sidewall friction and relative size of the test tank on the results as also investigated. Sidewall friction was found to a lesser effect for a wider test tank or for soil with a greater friction angle. Some recommendations on the relat...