Green geosynthetic-reinforced soil walls

An option for ecological and environmental projects.

By Guerra Escobar and Roberto Madriz

For some projects, the environmental and landscaping requirements have obliged engineers and architects to look for novel methods of constructing retaining walls and slopes. Thus, the advent of “green” into these structures.

In this project, they are constructed from soil reinforced with geosynthetics with facades made from UV-degradable sacks filled with organic material and vegetation to create a natural appearance. This article provides a detailed description of the designs of the green reinforced soil walls and slopes.

These designs incorporate the use of various geosynthetics, including woven geotextiles to reinforce the soil, geodrains for the drainage both behind and within the walls, and permanent turf reinforcement mats (TRMs) to protect the wall facade from erosion. Also presented are details of the construction process needed to ensure the stability of the wall.

This article concludes with an analysis of the green reinforced soil walls highlighting the economic, technical, and environmental advantages.

1. Introduction

Retaining soil walls and slopes reinforced with geosyntheticmaterials present an alternative to traditional retaining wall solutions, such as walls of reinforced concrete or soil embankments in their natural angle of repose.

Reinforced soil walls are challenging more traditional constructions due to their economic competitiveness and their green environmental credentials. Further, the introduction of soil walls or slopes has permitted the construction of retaining walls in places where the load capacity of the foundation soil is not sufficient for rigid walls or where there are space restrictions preventing the construction of soil fills or soil embankments at their natural angle of repose.

Geosynthetic-reinforced soil walls or slopes are also attractive solutions because of the flexibility they provide, as their design can be adapted to suit a range of different loads, solicitations, geometries, landscape features, and environments. Increasingly urban housing complexes and new tourist developments are emphasizing ecological awareness and care for the environment in their construction, and geosynthetic-reinforced soil walls are used because they meet the construction goals.

For these types of projects we have developed a system for building green walls or slopes—geotextile-reinforced soil slopes that have a uniform covering of vegetation on their facade to give a natural appearance. The resulting slope is attractive and as strong and robust as traditional solutions that use concrete, block, or stone facades.

The facades of the so-called green walls are made from UV-degradable sacks filled with organic soil and vegetation. Once the facade is finished, these sacks are covered with a permanent erosion-control/turf-reinforcement mat (TRM) to guarantee the growth and development of the vegetation on the wall facade.

The facade inclination for this type of structure must be no more than 80® with respect to the horizontal, to guarantee the development of the vegetation on the wall facade. When the inclination is 70® or less, the structure is analyzed as a reinforced soil slope and for inclinations between 70®–80®, the structure is calculated as a reinforced soil wall.

The maximum height recommended for the green reinforced wall or slope is 12m (40ft) in one block. If the height is greater than 12m, the recommendation is to design a wall or slope with terraces and a berm of 1m (3.3ft) minimum between terraces. (For an 18m wall, the recommendation is to design three terraces, each 6m high with a 1m berm between each terrace.)

2. Design methodology

2.1 Overview

Soils have a high resistance to compressive forces but give way easily under the application of tensile forces. However, soils may be reinforced with other materials, such as geotextiles that are designed to absorb tensile forces.

With soil wall needing to resist both compressive and tensile stresses, a structure of much greater resistance is required by including a suitable geotextile within the soil mass. The extra strength provided is principally due to shear stresses produced by the friction between the geosynthetic material and the adjacent layers of soil.

Various design methodologies are available for the reinforcement of soil walls using geosynthetics, such as:

• Robert M. Koerner, “Designing with Geosynthetics”
• Robert D. Holtz, Barry R. Christopher, Ryan R. Berg, “Geosynthetic Engineering”

Fundamental to this methodology are the design principles of Whitcomb and Bell (1979), which state not to consider hydrostatic pressure in the design calculations and that the active failure surface should be a plane surface defined by ranking methodology.

2.2 Stages of the design methodology

The design methodology for soil walls or slopes reinforced with geosynthetics consists of three stages:

• Stage 1: Internal stability
  In this stage the vertical space between layers is calculated as well as the correct length of reinforcing required to achieve the necessary resistance. The calculations must be based on the technical specifications of the geosynthetic material used.
• Stage 2: External stability
  In this stage the design must be reviewed to ensure adequate external stability. This stage analyzes the overall structure using the limit equilibrium approach to verify the safety factors of base sliding, overturning and bearing capacity.
• Stage 3: External conditions
  In this stage the type of wall facade is specified and the conditions of drainage and subdrainage analyzed.

3. Design for a ‘green’ reinforced soil wall or slope

A case history to describe the process of design for a geosynthetic-reinforced soil retaining wall or slope was designed and constructed in Costa Rica in January 2008. This slope was designed to have an erosion resistant facade of vegetation and for this it is referred to as a “green” reinforced soil slope.

3.1 Initial conditions

For the example of the reinforced soil slope that is described in this document, a stability failure of the initial slope occurred in October 2007. (October is the rainiest month in Costa Rica).

Because of the failure, part of the road collapsed and some areas of the project had no access. The failure was a landslide, with typical soils of this region—a lateritic type with a red color. When pore pressure increases, the tear strength of this type of soil is reduced and it produces movement of the soil mass.

The landslide resulted because of the saturation of the soils due to the intense and nearly nonstop rain of the previous months. As a solution to the landslide, the proposal was to construct a geotextile-reinforced soil slope with a facade of vegetation, using the green wall methodology.

It was also important to construct a drainage system at the base and the rear of the slope. For this component of the project, a geocomposite drain was built, as later described in 3.5. A superficial drainage system over the retaining slope and the terrain beside the slope was necessary to control and avoid infiltration of water in the slope.

3.2 Design considerations

Geometry dimensions:

• Variable heights = 3.30m, 4.00m, 6.00m
• Maximum height = 6.00m. (This is the height critical for the design.)
• Base = 0.8H (80% of the height) = 4.80m
• Total length = 80m
• Facade inclination = 70® (with respect to the horizontal)

Loads:

• Surcharge load = 19.62 kN/m²
• Vehicle loads on the wall were considered

Fill material: reinforced soil

The fill material must meet or exceed these conditions:

• Cohesion = 0.98 kN/m²
• Friction angle = 28®
• Unit weight = 16.67 kN/m³

Minimum requirements for fill material (reinforced soil):

• Plasticity index < 10
• Maximum particle size 75mm
• Passing sieve #200 < 25% in weight
• Laboratory CBR > 10%
• Tested CBR expansion 0%
• Organic material content 0%

Minimum requirements for the compaction of the fill material:

• Determined optimum humidity and unit weight using the modified proctor
• Compaction minimum 95% of the modified proctor
• Seismic acceleration in order to analyze pseudostatic = 0.20g

Reduction factors and overall safety factors:

• FR_DI = 1.5 for installation damages
• FR_CR= 2.2 for creep
• FR_QD = 1.0 for chemical degradation
• FR_BD = 1.0 for biological degradation
• FS_G = 1.3 factor of overall safety

Reinforcement woven geotextiles used in design:

• Woven geotextile T2400:
  Class 1 – standard specification AASHTO M288-05
  Width resistance (ASTM D-4595) = 41 kN/m
• Woven Geotextile TR4000:
  Class 1 – standard specification AASHTO M288-05
  Width resistance (ASTM D-4595) = 64 kN/m

3.3 Analysis results for internal stability

The internal stability analysis is made using in-house software for the design of soil reinforced walls.

In this analysis, the space between layers is defined as well as the type of geotextile used and the length of geotextile necessary for each layer. The results for the exemplar wall are:

• 13 layers of 0.20m with geotextile TR4000
• 9 layers of 0.20m with geotextile T2400
• 4 layers of 0.40m with geotextile T2400

3.4 Analysis for external stability

For the analysis for external stability a number of factors must be taken into account, including the geotechnical characteristics of the foundation, backfill, and reinforced soils, as well as the static and dynamic conditions of the wall.

For the purposes of this article, which is to highlight the overall design and construction process, the details of this stage are less important and are not included here.

3.5 Wall drainage and subdrainage systems

Adequate wall drainage at the base and rear can be achieved using a geodrain at the rear of the wall and a drainage trench at the base.

The trench should have a transverse section of 0.40m x 0.30m, with a perforated drainage tube of 4in. in diameter. The geodrain should be hung in strips 2m wide, with 1m spacings between the strips to prevent a failure surface at the rear. The 2m wide strips of geodrain should cover the full height of the wall.

As with other walls, it is recommended to include internal drains to remove any water that infiltrates the wall. One option for the internal drains is to use strips 0.5m wide of geodrain, with a length that is approximately 66% that of the wall base. The drains should be put each 1.50m (both horizontally and vertically).

3.6 Facade of UV degradable sacks, vegetation, TRM

To achieve a wall facade of uniform vegetation the following steps are recommended:

• Polypropylene sacks filled with rich, organic soil are used as the base for the final covering of vegetation. The placement of these sacks in the wall facade should be carried out at the same time as the compaction of each layer of soil. This option is the most economical and is a major construction benefit because it is not necessary to use formwork at any stage in the construction process.
• The sacks should only be filled to one-third of their total volume so that the soil-filled part of the sack is 0.20m in height, 0.35m in width, and 0.30m in depth. The quantity of organic soil in each sack should be 0.021m³ (0.20 x 0.35 x 0.20 m³).

The sacks, however, need to be larger than this so that there is approximately 0.50m of sack length at one end without filling that can be used as an anchorage when the soil is compacted over it. The connection between the geotextile and the sacks is provided by gravity that is guaranteed by the soil compacted over the empty part of the sack.

To use this method of construction successfully, it is important to install vegetation as soon as the wall construction is complete. Sacks must not be left exposed (without the benefit of a vegetative covering) for more than a week.

To ensure the correct growth of the vegetative covering, a permanent erosion control mat (TRM) must be placed on the outer surface of the wall. The principal functions of this mat are to improve the growing conditions for the vegetation and to prevent the erosion of the organic material in the sacks.

Permanent turf reinforcement mat (TRM435):

• Tensile strength (ASTM D-6818) = 2.6 kN/m
• Thickness (ASTM D-6525) = 8.9mm
• Color = green

4. Conclusions

We have shown how “green” reinforced soil walls can be constructed from soil reinforced with geosynthetics.

An attractive vegetative facade is created by the use of UV-degradable polypropylene sacks filled with organic rich soil and a permanent erosion control mat to protect the wall facade. These walls are technically and economically attractive solutions for all types of retaining walls, but are particularly suited for projects where the landscaping and the natural appearance of the walls is important.

From an economic perspective, the walls are attractive because the cost of a reinforced soil wall is generally lower in comparison with traditional structures. Natural fill soil walls or embankments use a large amount of material in their construction but reinforced soil walls can be constructed with steeper slopes substantially reducing the amount of material needed, and thus, a lower cost. Compared to rigid structure walls, reinforced soil walls are usually lower in cost due to the relative prices of the materials used.

From a technical perspective, the walls are attractive because all the construction materials are easily obtainable and the construction techniques do not require specialized equipment or workers. Following the steps outlined in this article, a green reinforced wall can be built by anyone with basic construction skills.

Further, the reinforcement of the soil with geosynthetic materials guarantees an improvement of the wall’s factor of safety in static and dynamic conditions, over that of a natural fill soil wall or embankment.

Guerra Escobar, M.P., AMANCO–PAVCO, Geosistemas Pavco S.A., San José, Costa Rica.
Roberto Madriz, AMANCO technical advisor for geosystems
Ron Bygness, editor of Geosynthetics, also contributed to this article.

References

AASHTO (American Association of State Highway and Transportation Officials), “Guide for Design of Pavement Structures” (1993).

AASHTO, “Geotextile Specification for Highway Applications,” AASHTO Designation: M 288-00, Washington, D.C. (2000).

EPA (Environmental Protection Agency), “Storm Water Management for Construction Activities,” USA (1992).

Holtz, R.D., Christopher, B.R. and Berg, R.R., “Geosynthetic Engineering,” BiTech Publishers Ltd. (1997).

Holtz, R.D., Kovacs, W.D., “An Introduction to Geotechnical Engineering” (1981).

IFAI, “A Design Primer: Geotextiles and Related Materials,” Industrial Fabrics Association International, USA (1992).

Koerner Robert, “Designing with Geosynthetics, Fifth Edition” (2005).

PAVCO, Departamento de Ingenier“a, “Manual de Diseño, Séptima Edición” (2006).