Vertical Reinforcement Spacing Research Articles

IMPROVEMENT OF SOIL USING GEOGRIDS TO RESIST ECCENTRIC LOADS.

IMPROVEMENT OF SOIL USING GEOGRIDS TO RESIST ECCENTRIC LOADS.

Secondary reinforcement effect on the value and location of maximum reinforcement load

This paper numerically evaluates the effect of secondary reinforcement on the value and location of the maximum reinforcement load along the primary reinforcement layers (Tmax) in geosynthetic-reinforced soil (GRS) walls under working stress conditions. Data from three instrumented sections of a well-instrumented GRS wall were used for model validation. A parametric study was carried out considering different controlling factors (i.e. the vertical reinforcement spacing, facing type and secondary reinforcement stiffness and length). The results show that for a constant relative soil-reinforcement stiffness index, the variation of the vertical reinforcement spacing and stiffness may not affect the location and normalised value of Tmax. In general, for the conventionally used type of reinforcement, the secondary reinforcement inclusion reduces Tmax to values lower than those corresponding to the active condition (Ka). For a given facing type, the combined effect of the secondary reinforcement length and stiffness is the main factor that controls the Tmax location. In general, increasing the secondary reinforcement stiffness and length moves the location of Tmax from the back of the facing to a distance corresponding to the length of the secondary reinforcement layers. In addition, for this condition, a flexible face model performs similarly to a block face model.

Numerical Investigation of Reinforced Soil Structures with GRS-IBS Design Features

This paper focuses on investigation of the effects of the presence of a bearing bed and reinforced soil foundation (RSF), the vertical reinforcement spacing and reinforcement stiffness, and subgrade compressibility on design components, including the lateral displacement of facing, maximum tension in reinforcement (Tmax), connection strength (To), and differential settlement in reinforced soil structures reinforced with woven geotextile. The investigation was conducted through numerical simulation of reinforced soil structures using the finite element program Plaxis 2D under plain strain condition. The results from the numerical analyses showed that the vertical reinforcement spacing has more effect on the design components than the reinforcement stiffness. The effect of slab load in widely spaced structures showed that the Tmax decreased linearly with depth. However, for closely spaced structures with a bearing bed, the reduction in Tmax with depth was bilinear. The existence of To in closely spaced structures was evident. The observed values of To were very close to values of Tmax. Inclusions of the bearing bed and RSF were found to have beneficial effects and can be implemented in design and construction of widely spaced reinforced soil structures to improve the performance.

A new generation of soil-geosynthetic interaction experimentation

A new device was developed to comprehensively assess the interaction between soil and reinforcement as well as the interaction between neighboring reinforcement layers in a reinforced soil mass, under both working and ultimate interface shear stress conditions. An understanding of these two interactions is required to assess the mechanical behavior of a geosynthetic-reinforced soil mass considering varying vertical reinforcement spacings. Specifically, the new device allows direct visualization of the kinematic response of soil particles adjacent to the geosynthetic reinforcement layers, which facilitates evaluation of the soil displacement field via digital image analysis. Evaluation of the soil displacement field allows quantification of the extent of the shear influence zone around a tensioned reinforcement layer. Ultimately, the device facilitates investigating the load transfer mechanisms that occur not only at the soil-reinforcement interface, but also at distances farther from the interface, thereby providing additional insight into the effect of vertical reinforcement spacing on a reinforced soil mass. Finally, the device allows monitoring of dilatancy within the reinforced soil mass upon shear stress generation at the interface between soil and reinforcement. Overall, the device was found to provide the measurements needed to adequately predict the strains developing both in reinforcement layers tensioned by direct application of external loads as well as in reinforcement layers tensioned by the shear transfer induced by adjacent geosynthetic reinforcements. Ultimately, the proposed experimentation technique allows generation of data required to evaluate the load transfer mechanisms amongst soil and reinforcement layers in reinforced soil structures. The strain magnitude in the neighboring reinforcements was found to exceed a magnitude of 10% of the strain magnitude obtained in the active reinforcement. The zone of shear stress transfer from the soil-reinforcement interface was found to exceed 0.2 m on each side of the active reinforcement.

Numerical analysis of field geosynthetic-reinforced retaining walls with secondary reinforcement

Geosynthetic-reinforced retaining (GRR) walls typically have vertical reinforcement spacing of 0·6 m, and this relatively large spacing has been known to cause comparatively high connection forces. To reduce this connection force, short geosynthetic reinforcement layers (referred to as secondary reinforcement layers) are installed between blocks where there are no primary reinforcement layers. This paper presents two-dimensional numerical simulations that were developed to analyse an instrumented GRR wall with secondary reinforcement layers in the field. A finite differential software was employed to develop the numerical model. In addition to the Mohr–Coulomb model, the cap yield model based on the theory of hardening plasticity was used to represent the behaviour of backfill. Inclinometer casings, earth pressure cells and strain gauges were installed in the instrumented GRR walls to measure the facing deflections, lateral earth pressures, vertical earth pressures and geogrid strains. The measured results and numerical predictions were compared and discussed, and reasonable agreement between these results was found. Compared to the measured results, the numerical predictions slightly underestimated the maximum wall facing deflections and vertical earth pressures, and slightly overestimated lateral earth pressures and strains in primary and secondary reinforcement layers. For comparison, a numerical model without secondary reinforcement was developed as well. This comparison revealed that the GRR wall with secondary reinforcement resulted in smaller facing deflections and maximum strains in primary reinforcement layers. Overall, the numerical analysis indicated that secondary reinforcement could provide clear benefits in improving the performance of GRR walls.

Behaviour of Bamboo–Geogrid Reinforced Fly Ash Wall Under Applied Strip Load

This technical note presents results of model tests performed on bamboo geogrid reinforced fly ash walls under applied strip load. The effect of vertical spacing and length of bamboo geogrid mattresses and strips on the deformation behavior of the model walls was studied and discussed. The results demonstrated the effect of increase in length and coverage ratios and decrease in vertical spacing of reinforcements on the improved performance of the model walls, including increased failure surcharge pressure, reduced facing displacement and backfill settlement. The failure surcharge pressure compared with the unreinforced model wall was improved by 185.76 and 311.84% for bamboo geogrid strips and mattresses respectively when vertical spacing and length of reinforcement used were 20 and 65% of the wall height respectively.

Seismic Response of WasteTyre Reinforced Embankment Slopes Based on Shaking Table Test

The seismic behavior of unreinforced and waste tyre-reinforced embankment slopes was investigated through 1-g shaking table tests. This study examined the effects of various factors on the stability of embankment slopes. The factors include the vertical waste tyre reinforcement spacing, seismic waves, peak ground accelerations. The influence of reinforcement methods on acceleration amplification factor, crest settlement and internal failure mechanism are also investigated. The results suggest that the beneficial effects of tyre-reinforcement are more pronounced in embankment slopes with smaller vertical reinforcement spacing and larger peak ground acceleration. Tyre-reinforced slopes can decrease the crest acceleration amplification factor by more than 32% compared to unreinforced slopes when subjected to a 0.4g Wenchuan earthquake input motion. In addition, with Wenchuan earthquake input motion, the average acceleration amplification factor reduction of slopes reinforced by triaxial geogrids is roughly 1.4 to 2 times greater than that of slopes reinforced by a composite of waste tyres and triaxial geogrids, or by a composite of waste tyres and tyre shreds. Along the surface of the slopes, from about one-third of the slope height, the acceleration amplification factor of all embankment slopes increases gradually with elevation, and the maximum accelerations were observed on the top of the slope. The unreinforced slope displaced more significantly compared to the reinforced slope when subjected to the same peak ground acceleration. The relative displacement of an unreinforced slope is almost 1.25 times greater than that of a tyre-reinforced slope, for a 0.1g Wenchuan earthquake input motion. The corresponding multiples are 1.4 and 1.58 for 0.2g and 0.4g accelerations, respectively. The captured failure pattern on embankment slopes further indicates that a tyre-reinforced slope has smaller cracks with shallow depth and more uniform deformation than those in an unreinforced slope, when subjected to a 0.4g Wenchuan earthquake input motion. Therefore, the tyre-reinforced soil, which has the potential to be more widely used and the benefit of reducing waste disposal costs, could improve some engineering properties of slopes much like the triaxial geogrids.

Numerical analysis of instrumented mechanically stabilized gabion walls with large vertical reinforcement spacing

The paper describes numerical models that were developed to simulate the performance of two instrumented mechanically stabilized earth walls constructed in Izmir, Turkey. These walls were constructed with gabion facing, hybrid reinforcement layers, and fill on a rigid foundation. The hybrid reinforcement layers comprised primary reinforcement (geogrid) and secondary reinforcement (wire mesh). The vertical spacing between the primary reinforcement changed from 1 m to 2 m in two walls while other properties were kept the same. The responses of the field walls at the end of construction were simulated and compared with the numerical results. The results calculated from the numerical models showed generally good agreement with the measured wall facing displacements, horizontal fill displacements, and tensile forces in the geogrid and in the wire mesh. The maximum calculated facing displacements for the walls with 1 m and 2 m reinforcement spacing were 30.7 and 36.4 mm, respectively. The maximum tensile forces in the geogrid layers were increased by 1.5 times in the 2 m spacing wall as compared with the 1 m spacing wall due to the increase of primary reinforcement spacing. However, the spacing change did not have an obvious effect on the increase of tensile forces in the secondary reinforcement (the wire mesh). The calculated results were also compared with theoretical results relating to the earth pressure distributions and the location of the maximum tensile strains in the primary reinforcement. The horizontal earth pressures against the wall facing were close to the active earth pressures for both walls. The maximum tensile strain line of the reinforcement was close to the Rankine's failure line.

Deformations of geosynthetic reinforced soil under bridge service loads

This paper evaluates the results of 13 large scale Geosynthetic Reinforced Soil (GRS) column load tests, also known as performance tests (PT) or mini-pier tests, to study the effect of tensile strength (Tf), vertical reinforcement spacing (Sv), facing elements, and backfill properties on the deformations of GRS at 200 kPa, typical bridge bearing pressures, and also at 400 kPa. The results indicate that GRS performs well under service conditions. A semi-empirical expression is proposed for prescribed bearing pressures to limit vertical strain to 0.5% of the abutment height. In addition, recommendations for estimating lateral deformation for GRS bridge abutments are also provided. At 200 kPa surcharge for this series of tests, vertical settlements ranged from 8.3 to 33.9 mm (or from 0.4% to 1.7% axial strain); lateral deformations ranged from 3.0 mm to 10.1 mm (or 0.6%–2.0% lateral strain); and reinforcement strain ranged from less than 1% during construction to less than 3% during loading. The lateral deformation results indicate that the maximum displacement occurs in the top third region of the wall face. Comparing the vertical and lateral displacement data shows that most GRS models experienced negligible positive volume changes up to about 1% under typical bridge service loads.

Bearing Capacity and Settlement Characteristics of Sand Subgrades with Vertical Reinforcement Supporting a Square Footing

The restraining effects in the sand subgrade under the footing can be provided by inserting the vertical reinforcing bars beyond the footing base without disturbing the subgrade below the footing. This technique of soil reinforcement can be quite beneficial for the footings where improvement is necessary. This paper presents the results of model square footing tests with vertical reinforcements inserted into the sand subgrade beyond the footing base only. The variables selected for the investigation were spacing of vertical reinforcement, extent of reinforcement from the edge of footing, length of reinforcement, diameter of reinforcement, width of footing and surface characteristics of the reinforcement. The values of theoretical bearing capacity of reinforced sand subgrade were determined using the method of slices and compared with experimental results. It was found that results were comparable particularly when the length of reinforcement and extent of reinforcement were two times the footing width.

보강재 간격 및 길이가 강성벽 일체형 보강노반의 거동에 미치는 영향

국내에 적용되고 있는 블록식 보강토 옹벽의 전면벽체는 보강재의 고정 및 뒷채움재의 유실방지 역할을 하지만, 벽체에 작용하는 하중에 대한 역학적 기능은 부족하다. 보강토체를 구성하는 벽체에 휨 및 전단에 대한 저항 성능을 부여하기 위하여 개발된 강성벽 일체형 보강노반은 다양한 장점에도 불구하고, 기존 공법과 비교하여 경제성 및 시공성 측면에서 단점이 부각되어 현장에의 적용은 미루어져 왔다. 본 연구에서는 강성벽 일체형 보강노반의 국내 적용성 향상을 목적으로 강성벽의 성능은 확보하면서 경제성 및 시공성을 향상시키기 위한 보강재 연직 배치 간격 및 길이 변화가 보강토체 전체의 거동에 미치는 영향을 평가하였다. 이를 위하여 강성벽을 갖는 1/10 축소모형 보강토체 및 높이 3m의 실내 실대형 시험체를 조성하고, 모사 열차하중 하에서의 변형 특성을 평가하였다. 시험 결과 비록 짧은 보강재를 사용함에도 불구하고 강성벽 보강노반에서 벽체 수평 발생변위 및 침하는 허용한계 수준 이하이었으며, 보강재 연직 배치간격이 30cm에서 40cm로 증가하여도 사용성 측면에서의 성능 수준에는 큰 변화가 없는 것을 확인할 수 있었다. Facings of mechanically stabilized earth retaining walls have function to fix the reinforcement and prevent backfill loss, but the walls are lack of structural rigidity capable of resisting applied loads. The reinforced subgrade with rigid wall was developed to have the structural functions under train loading. Though it has lots of advantages such as small deformation after construction, its negative side effects of economics and difficult construction were mainly mentioned and not practically used. To apply it for railroad subgrade, this study focus on the construction cost down and the enhancement of constructability without functional loss. To do so, the behaviors of reinforced subgrade with rigid wall were evaluated with the change of the vertical spacing and length of reinforcement. Small scale model tests (1/10 scale) and 3 m full scale tests were performed to evaluate deformation characteristics of reinforced subgrade under simulated train loading. Even though it uses short reinforcement, it showed small horizontal displacement of wall and plastic settlement of subgrade. Also, it was verified that not only 30 cm but also 40 cm of vertical spacing of reinforcement had good performance in serviceability aspects.

Earthquake Response of Reinforced Segmental Retaining Walls Backfilled with Substantial Percentage of Fines

AbstractThis paper reports on the seismic performance of three geosynthetic-reinforced segmental retaining wall systems backfilled with a silty sand mixture, using a shaking table excited by 1995 Kobe earthquake loadings. The preparation of the backfill mixture and its properties, the tested wall configurations, the reinforcement layouts and instrumentations, and the observed wall performance are described. Visual observations and test results indicate that walls having 0.4-m vertical reinforcement spacing, backfilled with soil containing a large percentage of fines, performed better than those having good-quality sandy soil under otherwise identical conditions. Vertical spacing of 0.8 m with removal of interlocking facing blocks in one of the walls did not lead to global collapse under repeated applications of the Kobe earthquake loadings. Only localized shear failure behind the top block layer was observed as the top facing blocks tended to topple. The good performance was attributable to apparent cohes...

Evaluation of Geosynthetic Forces in GRSRW under Dynamic Condition

Geosynthetics have proved to be suitable for reinforced soil retaining walls. Based on the increasing uses of geosynthetic reinforced soil systems in the regions, which bear frequent earthquakes, the study of dynamic behavior of structures seems necessary. Determining the reinforcement forces is; therefore, one of the most important and main points of discussions in designing retaining walls, by which we prevent from conservative planning. Thus, this paper intended to investigate the effects of such parameters as wall height, acceleration type, vertical spacing of reinforcement, type of reinforcement and soil type on forces and deformation through numerical modeling of the geosynthetic reinforced soil retaining walls (GRSRW) under dynamic loading with finite difference method by using FLAC. The findings indicate rather positive results with each parameter.

Centrifuge modeling test of a geotextile-reinforced wall with a very wet clayey backfill

A clayey vertical geotextile-reinforced earth wall (VGREW) in a wet state due to poor drainage conditions after several consecutive days of heavy rainfall was simulated by a series of centrifuge VGREW models. The models were constructed using clayey soil very close to its liquid limit. Through centrifugal tests on these models, the effectiveness of various reinforcement arrangements, on the alleviation of deformation of the clayey VGREW subjected to the previously mentioned adverse conditions, was examined. For the reinforcement length, there exists a critical beyond which no further improvement can be attained, while smaller vertical reinforcement spacing leads to shorter critical reinforcement length. The improvement ratio, I r, is defined as a relative measure for evaluating the efficiency of the improvement and is presented as contour plots to facilitate the consideration of the design. Four failure modes are observed in this study and their evolution for each reinforcement arrangement is also demonstrated.