Erol Guler

Reduced-Scale Shaking Table Tests on Geosynthetic-Reinforced Soil Walls with Modular Facing

AbstractReduced-scale shaking table testing is a useful tool for understanding the seismic behavior of geosynthetic-reinforced soil walls. This paper presents the results from a series of reduced-scale shaking table tests on eight different configurations. The effects of change in peak ground acceleration, reinforcement length and spacing, model scale, treatment of the top two facing block layers on the accelerations on a wall face, maximum displacements of the wall face during shaking, permanent displacements, and strains in reinforcement are investigated. Maximum accelerations measured on the wall face during shaking increased from bottom to top. Geotextile length and spacing did not affect the maximum accelerations and face displacements when the geotextile length met the minimum requirements of established design procedures. No significant permanent displacements were observed. Decreasing the geotextile length and increasing the geotextile spacing increased the geotextile strains when the geotextile w...

Numerical analysis of reinforced soil walls with granular and cohesive backfills under cyclic loads

Novel approaches to the dynamic analysis of the reinforced soil walls have been reported in the literature. Use of marginal soils reduces the cost of geosynthetic reinforced soil walls if proper drainage measures are taken. Therefore the affect of using cohesive marginal soils as backfill in geosynthetic reinforced retaining structures were investigated in this research. The dynamic response of reinforced soil walls was investigated in a similar focus, using finite element analysis. The results obtained from walls with cohesive backfill were compared to the results obtained from walls with granular backfill. The height of the wall was chosen as 6 m in the two-dimensional plane strain finite element model and the base acceleration was chosen to be a harmonic motion. The effects of various parameters like the backfill type, facing type, reinforcement stiffness, and peak ground acceleration on the cyclic response of reinforced soil retaining walls were investigated. After analyzing the wall response for end of construction and dynamic excitation phases, it was determined that the deformations and reinforcement tensile loads increased during the cyclic load application and that the amount of additional deformation that occurred during cyclic load application was strongly related to backfill soil type, facing type, reinforcement type and peak ground acceleration. It was determined that a cohesive backfill and geotextile reinforcement was a good combination to reduce the deformations of geosynthetic reinforced walls during cyclic loading for medium height walls.

Use of Perforated Base Pedestal to Simulate the Gravel Subbase in Evaluating the Internal Erosion of Geosynthetic Clay Liners

When geosynthetic clay liners (GCLs) are placed over coarse-grained gravel subgrades, the permittivity of the GCLs may increase because of internal erosion. To simulate this condition, geosynthetic clay liners typically are placed over gravel and tested in the laboratory under high hydraulic heads. In this study, a perforated base pedestal was used instead of gravel. The base pedestal was designed to have circular voids to represent the voids of a uniform and rounded gravel subgrade. Results obtained from tests where natural gravel and a perforated base pedestal were used were compared. To verify the effectiveness of the new approach, two different geosynthetic clay liners were tested over two different gravel subgrades. Tests also were conducted using rounded, uniform, coarse-grained gravels to compare to the results of the tests with the perforated base pedestals. The void diameter of the perforated base pedestals was chosen to be approximately the same as the maximum void size between the gravel particles. Test results indicated that a perforated base pedestal with uniform voids simulated a rounded, uniform, coarse-grained gravel subgrade in terms of internal erosion. The hydraulic heads that caused internal erosion were similar when a perforated base pedestal or a rounded gravel subgrade was placed beneath a GCL. When the same GCL was used over a base pedestal or over a gravel subgrade with equivalent void size, the difference in hydraulic heads at failure did not alter more than 5 m, except for one comparison. For most of the tests, the performance of the GCL placed over the gravel subgrade was slightly better than that of the perforated base pedestal in terms of internal erosion. These results indicated that the proposed technique of using perforated subbase to simulate gravel remains conservative for the GCLs and gravel subgrades considered as part of this study.

THE EFFECT OF UPWARD NAIL INCLINATION TO THE STABILITY OF SOIL NAILED STRUCTURES

In this paper, the performance efficiency and stability of two full-scaled nailed structures were monitored and compared. In the first setup the nails are inclined above horizontal where the nail inclination is below horizontal in the second setup. Thus, the efficiency of the soil nailed walls with nail orientations above horizontal is observed. In the literature, the installation of nails above horizontal were neither mentioned or applied. The analytical and field tests for the soil-nailed structures with nails installed at angles above horizontal gave better results in means of factor of safety with respect to the soil nailed structures with downward inclined nails.

Effect of repeated loading on the strength of clay

The behaviour of soils under dynamic loading conditions has been studied. There exists a large amount of data indicating that a reduction of strength in a soil takes place but that the amount of this reduction is limited. On the other hand some authors claim that such a reduction is not possible because the ultimate undrained shear strength of a given saturated soil depends only on its void ratio and structure. In this paper it is attempted to bring some clarification to this conflict and in doing so the Critical State Model is employed. Fifty-eight dynamic and nine static triaxial tests have been conducted on samples consolidated from a water content twice the liquid limit to different consolidation pressures. The soil used in experiments is ‘Arnavutkoy Kaolini’ with a liquid limit of wL = 65% and plastic limit wP = 30%. The results of the present series of tests indicate that the main effect of dynamic loading is on the increase of the pore water pressure which in turn causes the soil to behave like an overconsolidated soil. Previous research has shown that the void ratio of an overconsolidated soil sample decreases at the shear zone during shearing. All the results expressed above, as well as the results of tests conducted during this research, indicate that a reduction in the shear strength occurs in a normally consolidated clayey soil due to the repeated loading application.

Bearing capacity of strip footing on reinforced layered granular soils

In this study a limit equilibrium method is proposed to determine the bearing capacity of strip foundations on geosynthetic reinforced sand soils. A two-layered granular soil was foreseen to represent the loose in situ soil and the compacted fill above the reinforcement. First the modified bearing capacity factors Nq and Nγ were derived for the two layered granular reinforced soil. The bearing capacities were also calculated for different reinforcement geometries and soil properties using Finite Element analyses. The bearing capacities obtained from Finite Element and Limit Equilibrium analyses were compared, it was seen a good agreement. Therefore, it was concluded that the new limit equilibrium method proposed in this paper for reinforced two-layered soils can be successfully used in calculating the bearing capacities of geosynthetic reinforced soils.

Seismic Performance of Geosynthetic-Encased Stone Columns

The geotextile-encased columns (GECs) foundation system for embankments on soft or problematic soils was introduced in 1994. The GECs consist of compacted granular fill similar to common stone columns, but with a main decisive difference: GECs are confined in a high-strength woven geotextile cylinder (encasement). Consequently, they work properly even in extremely soft soils. Granular columns under compressive loads experience different failure modes, such as bulging, general shear failure, and sliding. However, the most common failure mode for stone columns in soft clays is bulging. With the help of GECs, the bulging failure can be prevented. The risk of bulging is even higher for earthquake loading conditions. A vast amount of numerical, analytical, and experimental research has been done to study different aspects of GECs behavior. Despite this available literature from several researchers, there is not much research, numerically or experimentally, about the seismic behavior of GECs. To analyze the effect of earthquakes on stone columns and to determine how the presence of GECs improves the soil behavior, three-dimensional finite element analysis was used. The results show that the encapsulation of the stone column by a geosynthetic reinforcement (i.e., instead of installing ordinary stone columns, using geosynthetic-encapsulated columns) helps with the integrity of the column and also improves the system behavior under earthquake loading conditions.

Comparison of Measured and Theoretical Pressure Distribution below Strip Footings on Sand Soil

AbstractThe paper presents vertical pressure values occurring under the center line of uniformly loaded laboratory model tests of a surface strip footing on a sand bed. The vertical pressure distribution in medium dense sand was measured using miniature pressure sensors. The experimental measures obtained from the model test program have been compared with values obtained from earlier theories. Tests were performed in plane strain conditions. Finally, the results obtained from experiments and theoretically obtained values were discussed. The results indicated that the distribution of pressure was not the same for every load level. It was found that the pressure distributions fit Westergaard distribution better at lower loads. They approach the pressure predicted by an earlier study as the applied loads reach bearing capacity values.

FACTORS AFFECTING FAILURE BY INTERNAL EROSION OF GEOSYNTHETIC CLAY LINERS USED IN FRESHWATER RESERVOIRS

Geosynthetic clay liners (GCLs) are often used as lining materials for freshwater reservoirs. To irrigate agricultural land without depleting groundwater, surface water is stored in these artificial ponds. In this study, hydraulic conductivity tests were performed on GCLs placed in flexible-wall permeameters under hydraulic heads of up to 50 m in order to investigate the risk of internal erosion. In these tests, base pedestals made of Plexiglas with uniform circular voids were placed beneath the GCLs instead of a typical gravel subgrade. The voids in the base pedestal represented the voids between uniform rounded gravel particles. Different types of GCLs were tested. GCL-1 was reinforced using needle-punching technology whereas GCL-2, GCL-3 and GCL-4 were unreinforced GCLs that were assembled in the laboratory. The effects on internal erosion of the void size in the subbase; the geotextile component that was in contact with the subbase; the bentonite component, and the manufacturing process of the GCLs were investigated. Test results indicated that internal erosion was directly related to the void diameter of the base pedestal. The resistance of the needle-punched GCL to internal erosion was better than that of the unreinforced GCL. The degree of internal erosion was also related to the engineering properties of the geotextile in contact with the base pedestal. Higher tensile strength of the GCL reduced the possible potential for internal erosion within it. The type of bentonite did not have a significant effect on internal erosion.

Effect of Bentonite Slurry Pressure on Interface Friction of Pipe Jacking

AbstractPipe jacking is a technique used to construct tunnels by pushing prefabricated pipes through the ground from an entrance shaft to an exit shaft. This technique is referred as microtunneling...