Synthesis of Geosynthetic Reinforced Soil (GRS) Design Topics

CHAPTER 2. EMBEDMENT LENGTH

Uniform reinforcement lengths of 0.6 to 0.7H (H is the wall height at face) have commonly been used for reinforced soil walls and abutments. This is a result of three major design guidelines: the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges, the Federal Highway Administration (FHWA) National Highway Institute (NHI) manual, and the National Concrete Masonry Association (NCMA) manual.^(1,2,3) The NCMA design manual, widely used in the private sector, requires a minimum reinforcement length of 0.6H. A minimum length of 0.7H, on the other hand, has been specified in the FHWA and AASHTO guidelines that are used routinely in the public transportation sector. The standard of practice in Europe and Asia uses a similar criterion for minimum reinforcement length: 0.7H for routine applications and 0.6H for low lateral load applications with a minimum length of 9.84 ft (3 m).^(4,5) Brazilian guidelines, perhaps the most conservative of all, require a minimum length of 0.8H.

Despite the popularity of the 0.6 to 0.7H rule, uniform reinforcement lengths as small as 0.3 to 0.4H have been employed with good success for construction of GRS walls.⁽⁶⁾ The subject of short reinforcement lengths is important because such lengths can result in significant cost savings, especially in situations where construction involves excavation of existing ground. Another important design issue with reinforcement length is nonuniform reinforcement lengths (truncated base walls). In situations where excavation into rock or stiff deposits to allow for uniform reinforcement lengths involves significant costs, the use of nonuniform reinforcement lengths in the lower part of the wall may prove beneficial. There are, however, design concerns with this practice.

RESEARCH AND CASE HISTORIES

This synopsis addresses the issues of short reinforcement length and design implications as they relate to GRS walls and abutments with short reinforcement lengths. The synopsis includes a review of research and case histories on short reinforcement and a discussion of the issues. Both uniform and nonuniform reinforcement lengths are addressed. GRS walls involving a "constrained fill" zone (i.e., where rock, heavily overconsolidated soil, or a nail wall is present behind a wall) are also discussed.

Reinforced Soil Walls With Uniform Reinforcement Lengths

When considering the subject of short reinforcement lengths, the case histories and research by Tatsuoka and his associates in collaboration with Japan Railwayare perhaps the most noteworthy. ^(7,8,9) Beginning in the mid-1980s, Tatsuoka and his associates developed a GRS wall system with full-height rigid facing, referred to as the reinforced railroad/road with rigid (RRR) facing system. Because of its superior performance during earthquakes and heavy rainfalls compared with conventional cantilever walls and metallic-reinforced soil walls, the RRR system has become the "default" retaining wall type in Japan railway construction. To date, more than 74.5 mi (120 km) of RRR walls have been built for railway embankments in Japan with great success.

The RRR system involves a two-stage construction procedure. First, a GRS wall with geosynthetic reinforcement of approximately 0.3H in length, at typical spacing of .98 ft (0.3 m), is constructed using gravel gabions as facing. The wall is allowed to deform under its self-weight before the second-stage construction is begun, which involves installing full-height rigid concrete facing (cast-in-place) over the gabion face. The rigid concrete facing is attached to the reinforced soil mass via extruded steel bars that are embedded in the soil mass. In spite of the very short reinforcement lengths of 0.3 to 0.4H, no failure has ever been observed during construction. It is important to note that both granular and cohesive backfills have been used in the construction of the RRR system. (Nonwoven-woven composite geotextiles were used for cohesive backfills to facilitate drainage while maintaining sufficiently high tensile capacity.)⁽⁷⁾ Tatsuoka et al. suggest that overturning may be the most critical mode of failure for an GMSE wall with short reinforcement lengths of 0.3 to 0.4H, while sliding typically governs for conventional GMSE walls with reinforcement lengths of 0.6 to 0.7H.⁽⁷⁾

Segrestin of Terre Armée International, in a rebuttal to Tatsuoka's many comments regarding RRR walls versus reinforced earth walls, stated that a number of reinforced earth walls with reinforcement lengths as low as 0.45H had been constructed successfully. ⁽¹⁰⁾ Segrestin also cited a 34.4-ft- (10.5-m-) high instrumented reinforced earth wall with reinforcement length as short as 0.48H. Segrestin maintained that the basic mechanism and behavior of reinforced soil structures with reinforcement lengths between 0.4H and 0.7H are identical, as has been suggested by a finite element model (FEM) parametric study conducted by Terre Armée.⁽¹⁰⁾

Bastick conducted FEM analysis of MSE walls undergoing changes due to reduced reinforcement length.⁽¹¹⁾ It was found, similar to Terre Armée's study, that the performance of an MSE wall would remain quite similar as long as the reinforcement lengths were kept above 0.4 to 0.5H and the reinforcement spacing stayed the same.⁽¹⁰⁾ The finding was confirmed by a full-scale experiment with reinforcement length of 0.48H loaded to an average surcharge of 121.83 psi (840 kPa).

Other cases of using short reinforcement lengths have typically been associated with (a) walls with "constrained fill zone," where there is the presence of a rock or heavily over-consolidated soil outcrop, or an existing nailed wall, and the space constraint makes commonly used reinforcement lengths of 0.6 to 0.7H impractical, and (b) walls with reinforcement anchored by metal plates or geosynthetic loops.^(12,13,14) Case (a) is in line with the scope of this synopsis and will be elaborated in the following discussion.

Lawson and Yee proposed a design-and-analysis method for reinforced soil retaining walls involving a constrained fill zone.⁽¹²⁾ Within the constrained reinforced fill zone, the full active failure wedge is unable to develop because of the relative close proximity of the rigid zone behind the reinforced fill. In the method, the magnitude of horizontal thrust acting on the wall face, Ph, is evaluated by the following equation: Ph = 0.5 K H2. For walls with reinforcement lengths greater than 0.5H, the Rankine active wedge can fully develop within the granular fill zone; hence, K is equal to Ka. However, for reinforcement lengths less than 0.5H, the full active wedge cannot develop fully, and the magnitude of K decreases with decreasing reinforcement lengths.

An extreme example of reduced lateral thrust for walls with a constrained fill zone is a very tall wall constructed by Lin et al. of Taiwan, a region with very heavy seasonal rainfall.⁽¹⁵⁾ The wall was 129.6 ft (39.5 m) high, constructed in six tiers (with three 26.2-ft- (8-m-) high tiers and three shorter tiers). The geogrid reinforcement length was 4.9 ft (1.5 m), or 0.19H. All tiers, despite having to carry the soil-weight from upper tiers, performed satisfactorily even with the very short reinforcement length.

It is interesting to note that the reinforcement length of 0.19H happens to agree with the finding of a numerical study for a reinforcement spacing of 1.3 ft (0.4 m).⁽¹⁶⁾ Vulova's numerical study also agrees with Tatsuoka's assertion that overturning will be the controlling failure mode for an GMSE wall with short reinforcement lengths.

Morrison et al. performed centrifuge tests on shored GMSE walls.⁽¹⁷⁾ The results were the following: (a) reinforcement lengths in the range of 0.25 to 0.6H generally produced stable wall systems; (b) reinforcement lengths of 0.25H or shorter generally produced outward deformation followed by an overturning collapse of the GMSE mass under increasing gravitational levels; (c) at reinforcement lengths less than 0.6H, deformation produced a "trench" at the shoring interface, interpreted to be the result of tension because the trench was observed with reinforcement lengths of 0.6H or longer; and (d) a conventional GMSE wall with retained fill and a reinforcement length of 0.3H was stable up to an acceleration level of 80 g, which represented a prototype height of approximately 88.6 ft (27 m).

It has been suggested that a shorter reinforcement length may result in larger lateral displacements and likely more settlement as well. A FEM study conducted by Chew, et al. showed that shortening reinforcement length from 0.7 to 0.5H caused about a 50-percent increase in lateral deformation.⁽¹⁸⁾ Ling and Leshchinsky reported that, with a reinforcement length of 0.5H, a reinforced soil wall would give satisfactory performance considering the maximum displacement mobilized in the reinforcement layers.⁽¹⁹⁾ A recent study by Liu suggests that the larger lateral displacement is due to larger lateral deformation of the soil behind the reinforced zone.⁽²⁰⁾

It is of interest to note that a soil mass reinforced by closely spaced reinforcement (i.e., spacing not more than 0.82 to 0.98 ft (0.25 to 0.30 m)) will likely behave as a coherent mass. This behavior is clearly evidenced in two loading tests of full-scale segmental facing GRS bridge abutment walls, referred to as the National Cooperative Highway Research Program (NCHRP) GRS test abutments.^(21,22) Two different woven geotextiles were used as reinforcement, each 10.33 ft (3.15 m) long and at 0.66-ft (0.2-m) spacing. The backfill was a nonplastic silty sand, and the abutment was loaded by applying increasing vertical loads via a strip footing near the wall face. A tension crack was observed on the wall crest in both tests. The tension crack was first detected exactly where the reinforced zone ended (i.e., 10.33 ft (3.15 m) from the back face of the facing) under an applied pressure of 21.8 to 29.0 psi (150 to 200 kPa). The location of the tension cracks suggests that the reinforced soil mass behaves as a coherent mass. The coherent soil mass behavior is particularly prevalent for GRS walls with closely spaced reinforcement. This also explains why overturning has been the most critical failure mode with short reinforcement lengths of 0.3 to 0.4H, especiallywith closely spaced reinforcement. The exception is when a constrained fill zone is involved.

Table 1 summarizes highlights of the case histories and case studies with uniform short reinforcement lengths (reinforcement lengths ≤ 0.5H).

Table 1 . Research: walls and abutments with uniform "short" reinforcement lengths.

FEM = Finite Element Model
FDM = Finite Difference Model

Reinforced Soil Walls With Nonuniform Reinforcement Lengths

Nonuniform reinforcement lengths in a reinforced soil wall, also known as a truncated base wall, typically have reduced reinforcement lengths in the lower part of the wall. A truncated base wall is employed when costs of excavation to allow for uniform reinforcement lengths are significant and construction may be difficult. The reduction in length takes the following two forms: stepped wall (reducing lengths in groups of two to four layers) and trapezoidal wall (reducing lengths at almost every layer).

Nonuniform reinforcement lengths for GMSE walls are allowed in the FHWA NHI manual.⁽²⁾ The manual provides general design guidelines for a truncated-base wall and states that this provision should only be considered if the base of the GMSE wall is founded on rock or competent soil; competent soil is soil that will exhibit minimal post-construction settlement. For foundation soil that is less than competent, ground improvement techniques may be used prior to GMSE construction.

The British standard BS 8006 design manual for GMSE walls also allows the use of a truncated base.⁽⁵⁾ It states that a trapezoidal wall should only be considered where foundations are formed by excavation into rock or where other competent foundation conditions exist. The manual prescribes a minimum reinforcement length of 0.4H for the lower portion of the wall. In Asia, Hong Kong's Geoguide 6 follows the FHWA NHI manual, except it also stipulates that soil arching needs to be accounted for in design.⁽⁴⁾

Japan Railway has employed reinforcement with truncated lengths in the lower part of a GRS wall when the costs of excavation to allow for full length reinforcement are high.^(7,23,24) The walls constructed with truncated base have performed satisfactorily during heavy rainfalls and severe earthquake events.

Segrestin reported applications of truncated base walls with steel-strip reinforcement in constrained fill situations.⁽¹⁰⁾ It was reported that wider metal strips, or a more closely spaced, increased number of strips, have been employed in truncated base walls.

The Colorado Department of Transportation (CDOT) constructed a 24.9-ft- (7.6-m-) high GRS wall with a truncated base in DeBeque Canyon along Interstate 70.⁽²⁵⁾ For comparison purposes, a 32.8-ft (10-m) section was constructed with full-length reinforcement. A road base material was used for backfill, and the wall was situated over a firm foundation. Measurements of lateral displacements along a full-length reinforcement section (reinforcement length = 16.4 ft (5 m)) and along a truncated-base section (reinforcement length at base = 3.61 ft (1.1 m), or 0.14H), taken 6 months after construction, were very similar-both on the order of 0.12 to 0.23 inches (3 to 6 mm), with a maximum displacement of about 0.31 inches (8 mm). Adams et al. of the FHWA reported a number of GRS abutments with a truncated base.⁽⁶⁾ The GRS abutments have performed satisfactorily.

Thomas and Wu conducted FEM analysis on the behavior of GRS walls with a truncated base.⁽²⁶⁾ Major findings of the study were threefold. First, when designing a GRS wall with a truncated base, external stability should be thoroughly checked. Truncated-base walls are more likely to experience sliding failure, and the length of reinforcement at the lowest level should be at least 0.35H or 0.9 m. Second, soil type and compaction of the backfill play a significant role in the performance of a GRS wall with a truncated base. The use of cohesive backfill should be avoided for a truncated-base wall. Third, the foundation soil needs to be sufficiently stiff for a truncated-base wall. Other numerical studies have also indicated that truncated base walls can perform equally well as full-length walls under certain conditions. (See references 11, 16, 26, 27, 28, and 29.)

Lee et al. conducted a forensic study on a series of failed walls founded on rock with rock forming the back-slope for the lower reinforcements, and concluded that the resistance against sliding failure is reduced by a truncated base due to a smaller base area.⁽³⁰⁾ They noted that soil arching due to the rock behind the fill would reduce vertical stress above the back of lower reinforcements and hence lead to overestimation of resistance to pullout failure.

Table 2 summarizes highlights of the case histories and case studies with "truncated base" reinforcement lengths (reinforcement lengths ≤ 0.5H).

Table 2 . Research: walls and abutments with "truncated base" reinforcement lengths.

GRS = Geosynthetic Reinforced Soil
FEM = Finite Element Model
DACSAR = Deformation Analysis Considering Stress Anisotropy and Reorientation

DISCUSSION

A minimum reinforcement length of 0.6 to 0.7H (H = height of wall at wall face) has been used in most designs of GRS/GMSE walls. However, reinforcement lengths as short as 0.3 to 0.4H have been shown to be stable under certain conditions, and studies have suggested that GRS walls with reinforcement lengths between 0.4 and 0.7H behave approximately the same. For situations where a uniform reinforcement length is to be employed, a minimum reinforcement length of 0.6H is well justified. Care must be exercised to prevent tension crack on the wall crest if the wall is to carry significant vertical loads near the wall face (e.g., bridge abutments). Extending the top one or two reinforcement layers well beyond the assumed failure plane based on a design analysis should help reduce the tension cracks.

Use of uniformly shorter reinforcement, with a reinforcement length of 0.35 to 0.5H, may result in larger lateral movement and is acceptable only if (a) the wall will not be subject to heavy edge loads such as a bridge abutment, (b) a granular backfill is employed and well compacted, (c) the foundation is competent (to minimize post-construction settlement), and (d) external stability, especially against overturning, is satisfied.

Use of a truncated base wall is a viable approach when costs of excavation to allow for uniform reinforcement lengths are significant and the foundation material is competent. There is strong evidence offered by case histories and research that a truncated base wall will perform satisfactorily as long as the base of a reinforced soil wall is founded on a competent foundation, and external stability, especially against sliding, is assured. The reinforcement length at the lowest level should generally be at least 0.3H.