Chapter 2. Literature Review
2.2 Reinforced Soil Structures: Behavior, Analysis and Design
The behavior of reinforced soil structures depends on three of its basic components: soil, reinforcement and facing and their interaction characteristics.
Among these, backfill soil and its engineering behavior governs internal stress distribution, pullout resistance and failure surface shape (Federal Highway Administration, FHWA 2001). Based on the engineering properties and its interaction with reinforcement and drainage properties, granular soils are ideally suited for reinforced soil retaining wall structures (FHWA 2001, BS8006 2010). Major functions of the reinforcement members in GRS retaining structures are to sustain
Literature Review 8 tensile loads and deformation, if any developed, in the fill. The reinforcements are classified as extensible reinforcement like polymer products and inextensible reinforcement like metallic mat and strip etc. (FHWA 2001). In case of the extensible reinforcement, deformation of reinforcement is comparable or even greater than soil deformation, while in inextensible reinforcement deformation of reinforcement is much less than soil deformation of soil (FHWA 2001). Different types of facing components are being used, basically, to prevent the soil to slide away from the rows of reinforcements; and also to contribute in stability of the structure by maintaining reinforcement members to function together.
In reinforced soil structure design, soil-reinforcement interaction is an important factor, which governs the composite behavior of soil and reinforcement.
The soil reinforcement interactions are controlled by two interaction mechanism namely pullout of reinforcement from soil (pullout mechanism) and soil sliding over the reinforcement (direct shear mechanism). Internal stability of reinforced soil retaining walls will be contributed by strength of geosynthetics and length of reinforcement required to prevent pullout (FHWA 2001). The pullout resistance offered by reinforcement is due to frictional force developed between the soil and reinforcement. Many researchers conducted pullout tests on geosynthetic reinforcement for the determination of interaction properties (Bergado et al. 1986;
Juran et al. 1988; Farrag et al. 1993; Gurung and Iwao 1999; Palmeira 2004; Shahu 2007; Subaida et al. 2008).
Koseki et al. (2006) reviewed the design codes for seismic design of geosynthetic reinforced soil walls as listed in Table 2.1. Different agencies developed design guidelines for reinforced soil retaining walls. They are: Federal Highway
Administration (FHWA 2001 and 2010); British Standard (BS8006 2010) and National Concrete Masonry Association (NCMA, 1997 and 1998) etc.
Table 2.1 Design codes for seismic design of GRS walls (modified after Koseki et al.
2006) Reference
codes
Title Summery
BS8006 (2010) Code of practice for strengthened/reinforced soils and other fills
Based on limit states design format and guidelines for partial material factors and load factors for various applications NCMA, 1997 Design manual for segmental
retaining walls.
Based on allowable stress design method
AASHTO (2002)
Standard specification for highway bridges
Based on allowable stress design
AASHTO (2004)
LRFD bridge design
specifications,
Based on limit state design factor called “load and resistance factor design – LRFD”
FHWA (2001) Mechanically stabilized earth wall and reinforced soil slope:
design and construction guide lines
Based on allowable stress design
FHWA (2010) Based on limit state design
CHBDC (2000) Canadian highway bridge design code
Gives no guidance on GRS wall for static and seismic design and recommends use of analytical limit state design methods and should be checked against accepted working stress design methods
CFEM (2006) Canadian Foundation
Engineering Manual
A description of Tie-back Wedge (Simplified)method for geosynthetic reinforced soil wall but no guidance on use of limit state design
RTA (2005) Design of reinforced soil walls Based on limit state design procedures within performance based framework
2.2.1 RTRI (2006) Design standard for railway
earth structures
PWRC(2000) Design and construction manual for geotextile reinforced soil structures
Design of reinforced soil retaining walls involves external stability (Fig. 2.1) and internal stability (Fig. 2.2) considerations. External stability issues consider
Literature Review 10 sliding and overturning of the structure as a monolithic block; bearing capacity of the foundation soil against increased normal pressure near the toe; and a potential deep seated failure surface. External design ensures that the reinforced block provides enough gravity resistance against the external forces. Internal stability aspects verify geosynthetic performance against tie-back and pullout failure. One should estimate the anticipated reinforcement forces and the geometry of the potential sliding surface at limit state. Internal design ensures that the reinforced block is strong enough and maintains its structural integrity against external forces (i.e. horizontal pressure from the retained fill, traffic surcharge) and self-weight (FHWA, 2001).
Fig. 2.1 Potential external failure mechanism for reinforced soil wall (FHWA 2001)
Fig. 2.2 Potential internal failure mechanism of reinforced soil wall (FHWA 2001) (a) Tie-back failure (b) Pullout failure
The static and dynamic design of reinforced soil structures involves the estimation of external forces acting on the structures to evaluate external and internal stability. In static analysis, the force exerted on the reinforced block, by the retained soil is estimated using fundamental soil mechanics principles of lateral earth pressure (Coduto 2002). The soil retained behind the mechanically stabilized earth (MSE) wall is assumed to be in limit equilibrium. The force exerted, on the reinforced soil zone, by the retained soil is calculated by well known Rankine (1857) or Coulomb (1776) earth pressure theories. This force is assumed to be acting at one-third the wall height (H/3, where H is the total height of wall), corresponding to a triangular earth pressure distribution.
For areas with seismic hazards, stability under seismic forces is evaluated by considering two additional forces: the seismic thrust and horizontal inertia force.
During an earthquake, the retained fill exerts a dynamic horizontal thrust in addition to static thrust on the reinforced soil wall, using Mononobe-Okabe (1929) method.
The reinforced soil mass is also subjected to horizontal inertia force that arises from part of the reinforced soil block due to the horizontal acceleration. These two forces are evaluated using site specific peak horizontal ground acceleration value, obtained from relevant codal provisions (IS 1893; EC8). The external and internal stabilities of the reinforced soil wall, with the selected reinforced configurations (strength, length, spacing etc.), are evaluated from the lateral forces considered and corresponding resisting forces developed. Relevant factors of safety for various mechanisms considered in the stability analysis (FHWA 2001) must be ensured to complete the design process.
Literature Review 12