Surcharge Pressure Research Articles

Modeling the Response of Wrap-faced Reinforced Lime Treated Soil Retaining Walls under Static Condition

This paper presents model studies conducted on geotextile wrap faced reinforced soil walls under static conditions in a rectangular model box. Untreated and lime treated clay was used as backfill soils. Numerical modeling has been done by PLAXIS software. The surcharge pressure, angle of the slopes, and different number of reinforcement layers were varied in model tests. The influence of these various parameters at different elevations of the retaining wall, settlement at the crest, and horizontal face displacement are presented in this paper. The tests were conducted at 45° and 60° slopes. It was seen that vertical crest settlement and horizontal displacement of reinforced soil wall decrease with an increase in reinforcing layers.

Rigid diaphragm wall with a relief shelf to mitigate the deformations of soil and shallow foundations subjected to normal faulting

Previous studies developed several types of weak and soft walls to deviate reverse fault ruptures and to absorb the compressional soil deformation. However, different strategies are required to address the extensional soil deformation caused by normal faulting. This study proposes a rigid diaphragm wall with a relief shelf (RDWRS) to mitigate soil and shallow foundation deformations in normal fault. The relief shelf is applied to keep the stability of the diaphragm wall and to protect the shallow foundation on the ground surface. Numerical modeling is performed to investigate the diaphragm wall-normal fault interaction mechanisms and the effects of RDWRS on deformation mitigation of shallow foundations. The RDWRS shows promising effects concerning the mitigation of soil and shallow foundation deformations. The protection effects depend on soil depth, foundation width, foundation location, surcharge pressure, wall depth relative to the rupture, the width of the relief shelf, and the distance between the wall and the foundation. As this paper presents the results of numerical modeling, experimental studies are suggested for future studies. Besides, the economy of the RDWRS should be comprehensively considered when applying this method.

A Ring-Shear Radial-Seepage Apparatus for Evaluating the Permeability of Shear Bands in Compacted Clay

Compacted clay cores are extensively used in rockfill dams to cut off water flow because of their low permeability and wide availability. Large differential settlement often occurs at certain locations in a dam, causing large shear concentrations in the compacted clay core in the form of shear bands. This raises concern about the permeability evolution in the shear bands in compacted clay during a large shear process. Thus, a ring-shear radial-seepage apparatus was developed for evaluating the permeability of the shear bands in compacted clay. The apparatus tests hollow cylindrical specimens that are held in a shear box, which consists of an upper box, lower box, and inner chamber. The specimen is compressed in the height direction and sheared in the circumferential direction to form a concentrated shear band. The permeability of the shear band is measured by radial-seepage from the inner side to the outer side of the hollow cylinder. To demonstrate the performance of the apparatus, a series of tests was conducted on compacted specimens of a clay used for the core wall of a high rockfill dam. The test results showed that the change in permeability of the shear bands in compacted clay was dependent on the surcharge pressure. If the current surcharge pressure was far less than the preconsolidation pressure caused by the previous compaction process, the permeability of the shear band might increase significantly. This experimental finding can be useful for identifying high-risk leakage zones in a dam with a compacted clay core.

Load bearing and settlement characteristics of Geosynthetic Encased Columns under seismic loads

Ordinary stone columns and Geosynthetic Encased Stone columns are used in remediation of soft soils with low bearing capacities. Primary objective of granular columns is to provide structural support to the overlying foundation layers. Given that the granular columns implemented in soft soil conditions support a large portion of the surcharge pressure, developing an understanding of settlement response and pressure bearing characteristics of granular columns under seismic loading conditions is of paramount importance. This study aims to present experimental findings of granular column load bearing response under simulated seismic loads and the effects of lateral boundary conditions on column behavior. In order to mimic the seismic behavior of columns supporting a sustained surcharge load, two different large-scale experimental assemblies are used in 1-g shaking table tests. The assemblies used were a laminar box and a rigid box which were intended as physical analogies for free-field and rigid boundary conditions. The reinforcement strains, pressure concentration on column heads, and settlement of the unit cells enhanced with granular columns with varying reinforcement stiffnesses are studied. The relationship between unit cell settlement and two important earthquake intensity measures namely, Arias Intensity and Shaking Intensity Rate, are also investigated.

Influence of Surcharge Load on the Adjacent Pile Foundation in Coastal Floodplain

<p><strong> </strong>In this paper, in order to investigate the behavior of existing piles caused by the horizontal and compression deformation of soft substratum due to backfill surcharge on coastal floodplain, three-dimensional finite element models of piles adjacent to surcharge load were established. The deformation and migration law of soft soil was analyzed. The behavior of single pile and double row pile adjacent to surcharge load were studied, in which the influence of surcharge load location, surcharge pressure, pile stiffness, and pile top constraint conditions were considered. The results show that as the position of surcharge load is closer and the surcharge pressure increases, the response (e.g. deformation and bending moment) is more obvious. With the increase of pile stiffness, the range of passive load is increased. The deformation behavior of pile body under different constraints of pile cap is significantly different. The effect of secondary bending moment caused by pile axial force is obvious and cannot be ignored. If there is a thick soft substratum, it is beneficial to improve the behavior of adjacent piles by using cement mixing pile reinforcement.</p>

Development of a fault simulator for soils under large vertical stress in a centrifuge

Severe damages of structures observed in previous earthquakes due to faulting, especially reverse faulting with large discontinuous ground displacement under large earth pressure, have triggered studies on the failure mechanism of structures within fault zone. However, most of centrifuge tests focused on normal faulting due to the lack of apparatus simulating faulting under large earth pressure. In this research study, a fault simulator was designed and manufactured for centrifuge study of buried structures in soils over dip–slip fault, by which a large overburden pressure can be applied to the model soil. The design specifications are: (a) both normal and reverse faulting can be generated, (b) the simulator can function for reverse faulting condition under 50g with the surcharge pressure of 500 kPa, (c) the maximum offset of 30 mm with the dip angle of 60° can be achieved at the bottom of model soil, (d) the front face is transparent to observe the model ground during centrifuge tests. Using the fault simulator, a preliminary test series on buried pipe are conducted. The developed fault simulator shows very promising results with respect to rupture propagation, ground deformation and pipe behaviour.

Effect of Acceleration on Wrap Faced Reinforced Soil Retaining Wall on Soft Clay by Performing Shaking Table Test

This research incorporates shaking table testing of scale wrap faced soil wall models to evaluate the seismic response of embankment. Currently the seismic designs of highway or railway embankment rely on little or no empirical data for calibrating numerical simulations. This research is working towards filling that empirical data gap. The specific purpose of the study was to evaluate the seismic response of constructed embankment model regarding the different input base accelerations with fixed frequency. A series of one-dimensional (1D) shaking table tests (0.05g, 0.1g, 0.15g and 0.2g), were performed on a 0.4 meters high wrap faced reinforced-soil wall model. Additionally, it was placed over 0.3 meters high soft clayey foundation. Predominantly, the influence of the base acceleration on the seismic response was studied in this paper. The physical models were subjected to harmonic sinusoidal input motions at a fixed frequency of 1 Hz, in order to assess the seismic behavior. The effects of parameters such as acceleration amplitudes and surcharge pressures on the seismic response of the model walls were considered. The relative density of the backfill material was kept fixed at 60%. The results of this study reveal that input accelerations and surcharge load had significant influence on the model wall, pore water pressure, and changes along the elevation. Acceleration response advances with the increase in base acceleration, so the difference being more perceptible at higher elevations. The pore water pressures were found to be high for high base shaking and low surcharge pressures at higher elevations. The results obtained from this study are helpful in understanding the relative performance of reinforced soil retaining wall under different test conditions resting on soft clay.

Dynamic response of wrap-faced cement treated reinforced clayey soil retaining walls

This paper presents model studies conducted on geotextile wrap faced reinforced soil walls under dynamic conditions mounted on a shake table. Untreated and cement-treated clay soils were used as backfill soils. Numerical modeling has been done by PLAXIS software. The surcharge pressure, frequency of shaking, base acceleration, and a different number of reinforcement layers were varied in model tests. The influence of these various parameters at different elevations of the retaining wall, settlement at the crest, and horizontal face displacement are presented in this paper. All model walls were subjected to 20 cycles of sinusoidal shaking, and the slope was kept at 45°. It was seen that vertical crest settlement and horizontal displacement of reinforced soil wall decrease with an increase in reinforcing layers.

Yield acceleration of reinforced soil slopes using the modified pseudo-dynamic method

This study presents the yield acceleration of reinforced soil slopes using the horizontal slice approach. Yield acceleration plays a key role in calculating the permanent seismic displacement of slopes. The modified pseudo-dynamic method, in conjunction with the equivalent linear site response analysis, is used. Unlike the pseudo-static analysis, the pseudo-dynamic method takes into account the effects of time and phase difference of waves propagation. The developed computer code is used to compute the yield acceleration. Using the planar and log-spiral slip surfaces, this code considers the distribution of reinforcement layers as uniform and variable spacing. The effect of surcharge pressure is also considered. According to the results, the modified pseudo-dynamic method using the equivalent linear ground response analysis led to more critical results than other methods.

Large-strain consolidation model considering radial transfer attenuation of vacuum pressure

This paper proposes a large-strain consolidation model that can consider radial transfer attenuation of vacuum pressure. In order to facilitate the solution, this proposed model is rewritten to take hydraulic head as the independent variable that can reduce the computational complexity. The universality of the proposed model is discussed based on the comparisons with several existing typical models. It is found that the proposed large-strain consolidation model has better applicability and can incorporate the attenuation of vacuum pressure. That’s to say, the model is more universal and can be simplified to typical models under restrictive conditions. In addition, the comparisons between numerical results and laboratory measurements indicate that the proposed model considering radial attenuation of vacuum pressure can better describe the dissipation law of excess pore-water pressure. Meanwhile, the proposed model can be utilized for the different combinations of surcharge pressure and vacuum pressure.

Stability analysis of twin horse-shoe shaped tunnels in rock mass

This study examines the stability of a single and twin horse-shoe shaped unlined tunnels laid in rock media and loaded with surcharge pressure on ground surface. The tunnels are assumed to be sufficiently long so that the plane strain analysis remains applicable. The failure of rock mass is governed by the generalized Hoek-Brown (GHB) yield criterion. The analysis has been performed by using the lower bound finite elements limit analysis in conjunction with the power cone programming (PCP). No assumption needs to be associated with either the smoothing of the yield surface or fixing the value of the exponent in the GHB criterion. For a given cover to diameter ratio of the tunnels, the maximum permissible value of the surcharge pressure on ground has been computed ensuring the stability of the tunnels. Apart from the different basic material yield parameters (GSI, mi and σci), the effect of clear spacing (S) between the twin tunnels on the results has also been studied. Failure patterns have also been explored for a number of cases to identify the region of the plastic shear zone. The computations reveal that the optimal spacing (Sopt), beyond which the tunnels behave like isolated ones, remains almost independent with respect to the material parameters and increases continuously with an increase in H/B; the value of Sopt/B increases from 2.5–3.0 for H/B = 1 to 13.0–14.0 for H/B = 6. The computational results have been duly validated and even compared with the different available solutions for square and circular twin tunnels in rock media.

Advanced analysis of masonry retaining walls using mixed discrete–continuum approach

A study was carried out into extending the use of the mixed discrete–continuum approach for advanced numerical analysis of masonry retaining walls. This included incorporation of soil plasticity, irregular wall morphology and application to a real-life scenario to identify the causes of observed damage patterns. Backfill soil was simulated by way of a deformable continuous medium, while masonry units were represented as polyhedral rigid blocks interacting with each other. Upon validation, the approach was used to simulate the behaviour of an historical masonry retaining wall suffering from severe cracks, large deflections and partial collapse due to material degradation and differential soil settlement. To understand the progressive response of the structure better, a parametric analysis was performed on the tensile strength of masonry when the system was subjected to differential settlement. Further static analyses were performed to assess the capacity of the wall under uniform surcharge pressure. The results demonstrated the sensitivity of the macro behaviour of the wall to the tension capacity of masonry. The study also showed that the proposed modelling approach can provide useful performance information on existing retaining walls that are vulnerable, and that such validated numerical models should be used to determine any interventions and repairs.

A Semianalytical Solution for Passively Loaded Piles Adjacent to Surcharge Load

Piles adjacent to a surcharge load commonly support not only active loads from superstructures but also the passive loads caused by soil lateral movement. To investigate the influence of passive load and the response along pile shafts of existing actively loaded piles, a load transfer model for analyzing the soil‐pile interaction was developed based on plastic deformation theory and the triparameter soil model. An analytical solution for the deformation and internal force of such piles was proposed using the transfer matrix method, in which the transfer matrix coefficients for piles in free, plastic, and elastic zones were analytically obtained by considering the second‐order axial force effect caused by lateral loading and soil yielding based on the triparameter soil model. The proposed methodology was validated by comparing its predictions with field measurements and previously published results. A good match between model predictions, field measurements, and previously published results implies that the proposed method can be used to evaluate the response of passive piles adjacent to a surcharge load. Parametric studies were also carried out to investigate the influence of surcharge pressure, soil resistance, and boundary conditions on the behavior of passively loaded piles adjacent to a surcharge load.

SLOPE STABILITY ANALYSIS CONSIDERING WEIGHT OF TREES AND ROOT REINFORCEMENT

We study the effect of roots of alder trees on soil reinforcement and slope stabilization. Two types of soil, i.e. Marl and Clayey soils and alders of three ages are considered. The slope stability is studied according to the tree indices based on tree age and soil type. The effect of root reinforcement on slope stability is considered by an additional cohesion. The stability analyses are carried out by the FEM. We perform parameter studies considering tree age, soil type and surcharge. The results indicate that soil type is effective on cohesion. The results also showed that with increasing age of trees from 7 to 15 years, the amount of additional root cohesion increased and with the increase of the age of trees to 20 years this amount slightly decreased. Also, with regard to a constant slope geometry, the type of soil and the uniform surcharge pressure, 7-year-old trees have shown better performance in slope stabilization. It has been observed that as the age of alder trees grows, although the amount of additional root cohesion increases, however, due to increased surcharge pressure, the overall slope stability factor decreases.

Effect of glycerol on the behaviour of an expansive soil during wetting and drying cycles

ABSTRACT The behaviour of expansive soil with different flooding fluid, i.e. distilled water and solution of glycerol with different percentages of glycerol (10%, 20%, 30% and 40%) was studied through a number of cyclic wetting and drying tests. Experimental tests were conducted on compacted samples of the expansive soil in a modified oedometer with different flooding fluids under a surcharge pressure of 10 kPa. The vertical and radial deformations of the samples were determined during different stages of wetting and drying. The results showed that during cycles of wetting and drying, the deformation of the soil decreased with increasing the number of cycles for all flooding fluids. The magnitude of deformation for samples flooded with a solution of glycerol is less than distilled water and this reduction is a function of glycerol concentration. The equilibrium condition of samples with glycerol solution is reached in fewer cycles than distilled water. The results also indicated that the swelling-shrinking deformation was nearly the same when the equilibrium condition was attained. Comparison of the results showed that by increasing the percent of glycerol, these curves were contracted. Furthermore, the paths of wetting and drying converged to an S-shaped curve at equilibrium condition.

Swelling stress effects on shear strength resistance of subgrades

ABSTRACT Pavements across Free State in South Africa deteriorate as a result of swelling and shear failure of the subgrade materials, and cyclic loads associated with moving vehicles. This study investigated the causes of distresses and cracks identified on the pavements across the province. Material samples were collected from three representative sites by core drilling. Series of consolidated undrained (CU) triaxial tests, unconfined compressive strength (UCS) tests, free swell index (FSI) tests, zero swelling tests (ZSTs) and California bearing ratio (CBR) tests were performed to determine shear strength resistance and swelling index of the subgrade material, respectively. X-Ray Diffraction (XRD) tests were employed to identify the minerals responsible for swelling activities in the subgrades. The results revealed that the investigated pavements deteriorate by swelling of the subgrades and were significantly affected by traffic loads that triggered resistance failure to moisture from the asphalt layer. The asphalt thickness layer failed to provide enough surcharge pressure to confine swelling stress from the subgrades. In comparison, swelling stress values from the CBR mould were 1.6% lower than that of the swelling stress values obtained from oedometer ring cell on the average. The result further revealed that swelling stress was the major cause of the pavements' failure. Thus, as swelling stress increased with an increase in moisture; this consequently led to a considerable decrease in shear resistance of the subgrades.

Influence of moisture loss on swell pressure and volumetric strain behaviour of desiccated kaolinitic residual soil specimens

ABSTRACT The present study examines the influence of variations in swell pressure due to moisture loss on the swell and collapse behaviour of residual kaolinitic soil specimens. Double oedometer test (DOT) results revealed that moisture loss from an initially dry soil reduces the swell pressure and renders it more collapsible; conversely, it increases the swell pressure of an initially wet soil and renders it expansive. Furthermore, the ratio of inundation pressure (P) to swell pressure (Ps ) [(P/Ps )] governs the nature of wetting induced volumetric strains. The swell and collapse magnitudes of the unsaturated kaolinitic soils at a given surcharge pressure are predicted from a unified equation (based on P/Ps ratio). Likewise, a proposed equation predicts the swell pressure of desiccated kaolinitic soils (compacted soils that have experienced moisture loss).

Parametric studies on two-tiered model fly ash wall

ABSTRACT The multi-tiered wall is earth-retaining structure, similar to a reinforced wall, constructed by providing offsets between the tiers at a certain height of the reinforced wall. The current study makes an effort to employ fly ash as a backfill material for the multi-tiered reinforced wall. The behaviour of laboratory models has been studied. Single and two-tiered models were prepared for the study. The effect of the reinforcement length, reinforcement spacing and the offset distance between the tiers on the failure pattern, horizontal displacement and normalized surcharge pressure was determined. From the experimental investigation, it was observed that the critical offsets between the tiers of the wall increase due to the introduction of reinforcement, as offsets between the tiers, D was observed 0.4L and 0.6L for the wall model without any reinforcement and the wall model with reinforcement, respectively, where L is the lower tier height. The failure patterns were found to agree with the Federal Highway Administration (FHWA) guidelines for tiered wall except for the unreinforced wall model at the 0.4L offset distance. There was an increase in normalized surcharge pressure due to the introduction of jute geotextile reinforcement up to 68.75% at the 0.7H length of reinforcement, where H = total wall height. As the jute geotextile reinforcement length increases, the horizontal displacement of the wall facing reduces up to 98% as compared to unreinforced model whereas, the increase in spacing between the reinforcing layers increased the horizontal displacement of the wall facing.

Vertical uplift resistance of close-spaced shallow rectangular group anchor plates embedded in sand

This study describes an analytical solution for determining the ultimate vertical uplift resistance of a group of two and multiple number of close-spaced shallow rectangular anchor plates embedded horizontally in sand. The analysis was performed by using an upper bound theorem of limit analysis with the employment of the kinematically admissible three-dimensional (3D) rigid wedge collapse mechanisms. Results are obtained in terms of dimensionless uplift factors Fγ and Fq due to the components of soil self-weight and surcharge pressure acting on the ground surface for a wide range of parameters. It was found that the magnitude of uplift factors decreases substantially with a decrease in the clear spacing between the anchors, soil friction angle, and embedment ratio, and an increase in the aspect ratio of anchors. Further, it was noticed that when the clear spacing between the anchors is greater than or equal to the certain critical value, the interaction effect of anchors vanishes and the magnitude of uplift factors associated with a group of anchors becomes equal to that of single isolated anchors. The present solutions are found to compare reasonably well with those theoretical, numerical, and experimental results available in the literature.

Computational Model for the Evaluation of Reinforced Concrete Silos Subjected to Thermal Load

Silos are special structures subjected to many different unconventional loading conditions like temperature differences which result in unusual failure modes. So, it is necessary for many codes to maintain and study the effect of thermal loads in design. The evaluation of design and construction practices is an essential step in the development of the design code for reinforced concrete (RC) silos, especially in arid zones like Saudi Arabia. This work evaluates the effect of thermal loads on silo wall design in terms of applied forces and stresses. These thermal loads affect the silo walls in two main manners, tangential oriented stresses (circumferential stress) due to thermally induced surcharge pressure during cooling of a filled silo structure and stresses due to differences of temperature across the wall thickness. A computation analytical finite element model (FEM) has been applied in a commercial analyzing program (SAP 2000 version 16). Various code provisions were used with comparison with the FEM results. For hoop forces, EU regulation, German standard, and Polish norm provisions were compared with a linear FEM with two parameters, wall thickness and temperature difference. For oriented stresses in silo wall, the American concrete institute (ACI) provisions were used in comparison with linear and nonlinear FEM with the same two parameters, wall thickness and temperature difference. This work showed that the nonlinear analysis of FEM has good matching with the corresponding values in ACI, leading to the conclusion that nonlinear analysis is more accurate than linear analysis. Moreover, the study results of hoop forces showed a distinct pattern with the temperature difference, silo radii, and insignificant silo wall thickness for each of FEM, EU, and Poland codes. This study is used for the rapid determination of critical areas of concern for critical loading combinations and for varying silo configurations.