Numerical Studies on Dynamic Behavior of Reinforced Soil Retaining Walls

Therefore, the dynamic behavior of reinforced earth retaining walls is of research interest for several researchers. Probable location of deformation zones based on the octahedral shear deformation that occurred on the backfill soil.

Fig. 6.8 Horizontal displacement, RMSA amplification factor and horizontal pressure for (a)rigid-faced wall (b)wrap-faced wall subjected to different scaled

Introduction

Reinforced Soil Retaining Structures
Seismic Performance of GRS Retaining Walls
Objectives of the Present Study
Organization of thesis

The aim of this research program is to investigate the dynamic response of reinforced earth retaining walls (RSRW) through numerical simulations. Numerical simulations of laboratory-scale shaking table tests on rigid reinforced earth-bearing walls are described.

Fig. 1.1 Application of reinforced soil structures: (a) highways; (b) bridge abutments;

Literature Review

Introduction

Reinforced Soil Structures: Behavior, Analysis and Design

RTA (2005) Design of Reinforced Earth Walls Based on Limit State Design Procedures within a Performance Based Framework. Design of reinforced earth retaining walls involves external stability (Fig. 2.1) and internal stability (Fig. 2.2).

Table 2.1 Design codes for seismic design of GRS walls (modified after Koseki et al.

Static Studies on GRS Retaining Walls

Dynamic Studies on GRS Retaining Walls

Physical model studies
Analytical studies
Numerical model studies

It was also observed that the extension of various upper reinforcements effectively improves the seismic stability of reinforced soil walls. They had studied six numbers of reinforced walls and emphasized on proper seismic design of the reinforced ground wall.

Fig. 2.3 Cross-section of geogrid wall suffered some damage during 1995 Hyogo-ken Nanbu Earthquake (Tatsuoka et al

Critical Appraisal

2014) investigated the similarities and differences in seismic performance between single- and multi-layer reinforced earth walls using a validated finite element model. It was found that the resonant frequency of a multi-level reinforced earth wall increases with increasing level offset.

Objectives and Detailed Scope of Work

There are very few studies available on the deformation behavior of backfill soil and the corresponding reinforcement deformations and deformation zones that occur when backfilling a reinforced earth wall. The second goal of the project is to evaluate the influence of different seismic ground motions, which are rich in frequency content, on the behavior of reinforced earth walls. To verify different modeling options for reinforced soil retaining wall components - backfill soil, reinforcements and soil-reinforcement and reinforcement-reinforcement interface.

Analysis of the strain behavior of reinforced earth retaining walls in terms of octahedral shear stresses developed in backfill soil and axial stresses developed in reinforcement.

Summary

Description of Numerical Programme and Model Properties

Introduction

Overview of FLAC 3D

FISH programming
Structural elements
Dynamic modeling in FLAC 3D

FISH programming language is one of the most useful features in FLAC3D that can be used to implement user-defined constitutive models and perform programming techniques. The users can define their own constitutive model by programming in FISH and can be implemented in FLAC3D. The structural elements and their interaction with rock or soil mass can be modeled in FLAC3D by built-in structural elements.

In FLAC3D, a region of material subjected to external and/or internal dynamic loading can be modeled by applying a dynamic input boundary condition at either the model boundary or at internal grid points.

Fig. 3.1 Basic calculation sequence in FLAC 3D (Itasca 2008) 3.2.1 FISH programming

Material Models for Numerical Simulation

Backfill soil
Geosynthetic reinforcement
Facing elements
Interface elements

Massing model (Masing 1926) is the simple cyclic model which can incorporate cyclic as well as hysteretic behavior of soil. Here, octahedral shear stress τoct and octahedral shear stress γoct are used to represent stress and strain ratio i. I_ !" is the octahedral shear strain at present state and +L_ !" and +M_!” are octahedral shear stresses at the starting points of unloading and reloading for that cycle, respectively.

3]^_ 3.11 Where the parameters (∆z)min are the smallest dimensions in normal direction, K and G are respectively bulk modulus and shear modulus of continuum zone adjacent to the interface.

Fig. 3.2 Numerical model of triaxial test sample (dia 38 mm & height 76 mm) The soil is simulated as elasto-plastic Mohr Coulomb material coded with hyperbolic soil modulus proposed by Duncan et al.(1980)

Soil-Reinforcement Interaction

Laboratory pullout tests
Numerical simulation of pullout tests

Therefore, interaction parameters obtained from pullout tests are used in the design of reinforced structures (BS. The materials required for numerical modeling of pullout tests are of three types: (i) soil, (ii) reinforcement and (iii) interfaces. The model parameters used for the numerical pullout test simulation is shown in table 3.3.

The pull-out tests are performed by applying a constant horizontal velocity to the reinforcement nodes along the box face.

Fig. 3.7 Grain size distribution of sand used in the study Table 3.1 Soil Physical Characteristics

Summary

Laboratory triaxial tests were performed on cohesionless soils at different confining pressures and the parameters of the hyperbolic model were evaluated and numerical simulation of the triaxial tests was performed to validate the behavior of the soil. The interaction behavior of the soil-geosynthetic system was determined by laboratory pullout tests and a numerical simulation of the same was performed to verify the geogrid structural element and its interaction with the soil in FLAC3D. Interfacial stiffness values ranging from 104 to 107 kN/m2/m are in close agreement with pullout test results with a 10% variation.

The geogrid structural elements used for simulation of reinforcement in pull-out test respond to the change in angle of internal friction of soil and confining stress.

Studies on Wrap-Faced Walls

Introduction

Developement of Numerical Models

Target physical model
Development of numerical grid
Selection of grid size
Sensitivity analysis
Validation of numerical model
Dynamic response of wrap-faced walls: displacements and strains

Physical model tests on wrap-faced reinforced soil wall models reported by Krishna and Latha (2007) and Latha and Krishna (2008) are considered for developing the numerical model. An additional pressure of 0.5 kPa is applied at the top and the model is solved to static equilibrium (Fig. 4.4e). Typical variations of displacements and accelerations with the number of cycles of dynamic loading (a = 0.2g and f = 3 Hz), at different heights of the wrapped wall are shown in Fig.

The numerical model developed to simulate the shaking table tests of wrapped wall is validated by comparing with the results of physical model tests as reported by Krishna and Latha (2007).

Fig. 4.1 Physical model configuration of wrap-faced wall (after Krishna and Latha, 2007)

Seismic Behavior of Full Scale model

Response of full scale wall after dynamic excitation

The maximum horizontal movements at an angle of 5.5 m are 168.8 mm at the lining, 94.3 mm at the end of the reinforcement and 0.8 mm at the deep backfill. Horizontal pressures at a height of 0.5 m above sea level are 30.2 kPa near the cladding; and 20.0 kPa near the end of the reinforcement. The vertical displacement (v) suddenly increases towards the end of the reinforcement in the higher layers, at the angle of 5.5 m and 4.5 m.

Therefore the zone of relative compression developed near the bottom of the reinforcement does not extend to the lower layers.

Fig. 4.21 Octahedral shear strain, horizontal displacement and vertical displacement along the length of backfill after 20 cycles of dynamic excitation (a =0.2g, f =5Hz,

Parametric Studies

Effect of length of reinforcement
Effect of number of reinforcing layers
Effect of backfill (angle of internal friction)
Effect of reinforcement stiffness
Effect of frequency of excitation
Effect of seismic acceleration

The horizontal displacements at the face, near the top of the wall, are 168.8 mm and 94.3 mm for walls with rebar lengths of 0.7H and 1.0H, respectively. a decrease in horizontal displacement is observed when the reinforcement length increases from 0.7H to 1.0H. The vertical displacement is 104.5 mm near the end of the reinforcement for a wall with a reinforcement length of 0.7H; but 61.8mm is close to the front. Near the flip side, a 27% decrease in vertical displacement is observed for increasing Lrein from 0.7H to 1.0H. The reduction in vertical settlement is 68% near the end of the rebar and 11% near the face for increasing the length of the rebar from 1.0H to 1.2H. The RMSA amplification factor is almost the same for all three wall types. 2005) also observed no significant changes in the increase in acceleration with increasing reinforcement length. The octahedral shear strains do not show any peak near the end of the reinforcement for a wall with a reinforcement length of 1.2H.

The vertical displacements near the end of the reinforcement are also the largest for the wall subjected to 3 Hz seismic excitation.

Fig. 4.27 Variation of ∆γ oct , u and v along the length of backfill after dynamic excitation for model wall with L rein /H=1.0 (a=0.2g, f=5 Hz, N L =6)

Response of Designed Wrap-Faced Wall Model

The maximum vertical displacement of 77.8 mm is seen near the end of the reinforcement (at a distance of 4.2 m from the face). Higher deformation values (in the range of 6-8%) are observed at the end of the reinforcement at angles +3.5 m (above). Increasing the number of reinforcement layers can reduce the displacement of the front surface, but will not affect the extent of the deformation zones within the unreinforced backfill.

So the extent of composite deformation can be significantly reduced by increasing the length of the top three layers of reinforcement to 1.0H.

Summary

Three different modes of deformation zones: shear deformation within reinforced zone, relative compaction near end of reinforcement and composite deformation zone extending to the backfill are observed. The influences of parameters such as length of reinforcement, number of layers of reinforcement, filling soil, and reinforcement stiffness on deformation zones formation are also studied. The formations of deformation zones are mainly influenced by reinforcement length, backfill soil and number of reinforcement layers.

The wall with longer reinforcement at higher height and minimum reinforcement length at lower height reduces the size of the composite deformation zone in retained backfill.

Studies on Rigid-Faced Walls

Introduction

Developement of Numerical Models of Rigid-Faced Walls

Target physical model
Development of numerical grid
Validation of numerical model
Sensitivity analysis
Dynamic response of rigid-faced walls: Displacements and Strains

Four layers of geotextile reinforcement of length (Lrein) 420 mm, (Krishna and Latha 2009) are used in the model. The boundary conditions applied to the model represent the actual boundary of the physical model tests (Krishna and Latha, 2009). The bottom boundary is completely fixed in the vertical direction to represent the rigid boundary between the model wall and the shaking table.

During the construction, the model wall is fixed in horizontal direction to represent the temporary supporting support.

Fig. 5.1 Test arrangement of rigid face reinforced retaining wall in physical model tests (after Krishna and Latha 2009)

Seismic Respnse of Full Scale Rigid-Faced Wall Models

Rigid-faced wall subjected to dynamic excitation

The response of model, after 20 cycles of excitation (a = 0.2g and f = 5 Hz), in the form of horizontal displacements (u), vertical displacements (v), RMSA amplification factors and horizontal pressures, near the direction and at the end of reinforcement is shown in Fig. The reinforcement strain increments showed maximum values, near the cladding which can be attributed to the ground movement (∆γoct in Fig. 5.17) near the cladding. The second zone (dashed line in Fig. 5.17) is the constant strain zone extending beyond the strengthened zone, formed as a result of shear deformation within the strengthened zone at higher elevation.

The contours of the octahedral shear deformation in the backfill soil of a wall with a rigid side, which was subjected to a dynamic excitation of an acceleration of 0.2 g and a frequency of 5 Hz, are shown in the figure.

Fig. 5.15 Typical displacement histories at different elevation of rigid-faced wall

Parametric Studies

Effect of backfill friction angle
Effect of reinforcement stiffness
Effect of length of reinforcing layers
Effect of number of reinforcing layers
Effect of facing stiffness
Effect of frequency of excitation
Effect of acceleration of dynamic excitation

Comparison of incremental octahedral displacement (∆γoct) developed within soil elements after dynamic excitation for wall with different backfill friction angles is shown in Fig. of reinforcement in fig. The variation of octahedral displacement along backfill shows a zone of relative settlement near the cladding and shear deformation within reinforced zone for wall with 4, 6 and 8 layers of reinforcement.

The incremental axial strain reduces to negligible value near the end of reinforcement for wall subjected to excitation of 0.2g and 0.4g.

Fig. 5.21 Response of model walls with backfill friction angles 30°, 38° and 43°

Comparison of Wrap-Faced and Rigid-Faced Walls Behaviour

The RMSA amplification factor is 1.46 and 3.12 at the top near the wall edge for the wrap-face and solid-face walls, respectively. However, the high-strain deformation zone in rigid-faced walls is very close to the surface. The variation in ∆εa_rein after dynamic excitation along the length of the reinforcement for the wrap face wall and a solid face wall is shown in Fig.

The maximum ∆εa_rein is within 1% near the face for solid-faced walls and within 2.5% at mid-length of the reinforcement for envelope-faced walls.