Geosynthetics reach new heights

September 2010 » Features » PROGRESSIVE ENGINEERING
Design considerations for the use of geogrids in tall earth-retaining structures.
Aderson M. Vieira, Ph.D.

Since the early 1970s, geosynthetic materials have been developed and used as reinforcement in earth-retaining structures. Since then, the use of geosynthetics as reinforcing material for reinforced soil structures has proven to be economically attractive based on the many thousands of mechanically stabilized earth (MSE) retaining walls and reinforced soil slopes (RSS) successfully completed.

Recently, advancement in geosynthetic technology and a better understanding of reinforced soil structure behavior has allowed design engineers to use geosynthetic products — geogrids in particular — as reinforcement for tall MSE retaining walls and RSS.

An 80-foot-tall MSE retaining wall was designed with polyester geogrid as reinforcement to support up to 653 tons of mining truck traffic in a seismically active region.

Some of the challenges design engineers face in using geogrids in these structures are excessive deformation of the geogrids, poor interaction between soil and geogrid, degradation with time, and construction techniques. Some of these challenges can be mitigated with the appropriate choice of the geogrid and soil properties. Engineers must consider geogrid properties, such as mechanical, endurance, and degradation characteristics, during the design process.

Although the advancement of geosynthetic technology has allowed successful use of geogrids in tall earth-retaining structures, their use is still not widespread. Geogrid characteristics, such as those presented here, may impact the long-term performance of tall MSE retaining walls and slopes and should be carefully considered to account for geosynthetic technology limitations.

MSE techniques and advantages
MSE retaining walls are basically a geotechnical system consisting of alternating layers of engineered fill soil and tensile reinforcing elements, such as geogrids, sometimes connected to a facing element. In the case of reinforced soil slopes, geogrids or geotextiles are used to improve the soil strength and steepen the slope. Rapid acceptance of MSE wall and RSS systems can be mainly attributed to the following key benefits:

  • cost-effectiveness of the construction technique;
  • simplicity of the construction technique;
  • reduction of construction time;
  • overall quality of performance and reliability;
  • improvement of soil shear strength;
  • flexibility and relative tolerance to differential settlement;
  • minimization of land acquisition; and
  • aesthetic appeal.

Development of geosynthetic materials as reinforcement for earth structures is still progressing and improvements are being made in the way MSE walls and RSS are designed and constructed. The construction technique and flexibility of reinforced soil structures make the technology very attractive. In addition, MSE walls can be built quickly from prefabricated materials, such as precast concrete panels or modular blocks.

Although gravity or cantilever cast-in-place (CIP) concrete walls have performed well and are economical for some applications, in most cases these concrete walls cannot compete with the economics of geosynthetic-reinforced soil structures. In particular, for tall earth-retaining structures and structures over soft soils, MSE retaining walls and RSS are more cost-effective than cantilever reinforced concrete or gravity wall types. In these cases, the use of geosynthetic to improve the soil strength capacity can eliminate the need for CIP reinforced concrete elements and subsurface improvements such as deep foundation structures.

The Federal Highway Administration (FHWA) reported that the cost savings in some completed projects is more than 50 percent. The cost of constructed MSE walls can generally be 30 to 50 percent less expensive than conventional CIP concrete walls (Alzamora and Barrow, 2007), depending on the wall height. Click here to view a typical cross section for an 80-foot tall MSE retaining wall reinforced with geogrids.

Designing with geosynthetics
Geogrid characteristics should be carefully considered during the design process to account for geosynthetic/geotechnical technology limitations, which can impact the long-term performance of tall MSE retaining walls and slopes. The design engineer should consider geogrid mechanical, endurance, and degradation properties. Among several mechanical properties, the geogrid ultimate tensile strength is, in general, the most relevant property for the design of MSE retaining walls and RSS. In the case of endurance, the two main relevant properties are installation damage and creep behavior. In particular for tall earth-retaining structures, where higher stress levels are imposed to the geogrids, creep behavior can be critical in successful performance of the structure. In general, the most significant degradation properties that should be considered in a typical design of reinforced soil structures are:

  • oxidation effects, in the case of high-density polyethylene (HDPE) and polypropylene (PP) polymers; and
  • hydrolysis and ultraviolet (UV) effects, in the case of high-tenacity polyester geogrids (PET).

Other thermo-chemical effects may have to be taken into consideration for special project conditions. One of the key elements in the design of tall reinforced soil structures is the geogrid long-term strength. The geogrid long-term strength is directly derived from the mechanical, endurance, and degradation polymer characteristics. Long-term design strength is usually governed by tensile rupture for polyester geogrids, and either strain or rupture for polyethylene geogrids (Kaliakin and Dechasakulsom, 2005).

Tall retaining walls, in general, require strong uniaxial geogrids with high tensile strength capacity because of high stress levels developed at their base, good chemical degradation resistance, and low deformation. The ultimate tensile strength capacity of geogrids used to construct tall MSE retaining walls and slopes may be greater than 20 kips/foot.

Advances in soil reinforcement
Soil-geogrid interaction is one of the main components of the MSE retaining wall design. The current standard practice consists of specifying a select granular backfill to be used with the geogrids in the reinforced zone. Cohesionless, well-graded granular soils are typically specified as backfill soil within the reinforced zone of the wall for most commercial, federal, and state highway projects. Cohesionless granular materials are generally desirable to provide: good durability; high permeability, enabling better drainage and less pore pressure build-up; ease in moisture conditioning and compaction; good frictional strength for good soil-reinforcement interaction; and, relatively high pullout resistance (FHWA, 2009). Where cohesionless granular soils are not readily available, the use of cohesive materials may be desirable to improve the cost-effectiveness of the system. But cohesive soil presents a more complex material behavior and its use is not an industry standard practice.

A 108-foot-tall soil slope with HDPE geogrid reinforcement was successfully designed and constructed with slope ratios as great as 1.4 horizontal to 1 vertical.

Nevertheless, some studies have included cohesive soils as reinforced backfill material for MSE walls. One of the major constraints with these studies is the modeling of the cohesive soil behavior and the sophistication needed to generate an acceptable design (e.g. time-dependent finite-element analyses). The interaction between geogrid and reinforcement is hard to predict if cohesive soils are used, mainly because these soils present time-dependent (elastic-viscoplastic) behavior. In addition, geosynthetics can also exhibit relevant time-dependent behavior because of the physical and chemical nature of the polymer material, which should be considered in the design. The combined time-dependent effects of both soil and geogrid are not considered in the standard design practice because of the complexity.

Consequently, there is still a great concern within the engineering design community in using geosynthetics with cohesive soil in reinforced soil structures. This concern is based on earlier cases that incorporated geogrids into cohesive soil that resulted in poor performance and excessive deformation. Some of these poor performance cases can be attributed to the presence of cohesive soils in the reinforced backfill area, leading to drainage issues, strength loss of the backfill during saturation, decrease in geogrid soil interaction, and relatively greater construction and post-construction movements.

Despite the lack of a simple and suitable model to simulate the behavior of geosynthetics in cohesive soils and several technical uncertainties that need to be answered, successful cases of reinforced earth structures using geogrids with cohesive soils have been reported. In addition, some studies have been performed to experimentally assess the interaction between geogrid and cohesive soils to subsequently improve and advance the use of geosynthetics in reinforced soil structures.

Advances in geosynthetic technology
Technological developments in the polymer industry have been continuously incorporated into new geosynthetic products, enhancing geosynthetic materials properties used in geotechnical applications. Geogrids can be manufactured with “new” polymers exhibiting considerably less deformation under permanent loading than regular polyester/HDPE geogrids of equivalent nominal strength. These geogrids are manufactured from high-modulus, low-creep synthetic yarns and have a protective polymer coating with standard strengths that may vary in the range of 1,300 to 27,500 lb/ft. In addition, geogrids with strength of more than 68,000 lb/ft have reportedly been manufactured, supplied, and used in special applications. These geogrids are manufactured from the new generation of polymers such as aramide and polyvinyl alcohol and open new perspectives for project-specific reinforcement. These new-generation geogrids have resulted from close cooperation between manufacturers, consulting engineers, scientists, and contractors.

However, there are still many technical questions about the use of geosynthetics in the construction of reinforced soil structures, particularly concerning excessive deformation and soil-geogrid interaction. These technical uncertainties impose constraints in the design of tall reinforced soil structures and especially in the use of cohesive soil as reinforced backfill. As engineers, manufacturers, scientists, and researchers continue to improve design and construction techniques and develop new polymer technologies, many of these technical questions are being addressed. Based on past experiences, the use of geosynthetics in tall reinforced walls and the use of cohesive soils in reinforced soil structures can be successfully achieved if proper design considerations and materials are selected, in addition to quality construction methods. Successful projects have proven that the use of geogrids can be a feasible and economical solution for tall reinforced soil structures.

References

  • Kaliakin, V. N.; and Dechasakulsom, M., 2005, Modeling the Time-Dependent Behavior of Geosynthetically Reinforced Soil Structures with Cohesive Backfill, in Reinforced Soil Engineering: Advances in Research and Practice.
  • Alzamora, D., and Barrow, R.J., 2007, Mechanically Stabilized Earth Walls on the Interstate Highway System — Thirty Years of Experience, TR NEWS 249, Transportation Research Board March-April 2007.
  • Koerner, R. M., 2005, Designing with Geosynthetics, 5th edition, editor Pearson-Prentice Hall.
  • FHWA, 2009, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Vol. 1 and 2, FHWA-NHI-10-024, GEC 011.

Aderson M. Vieira, Ph.D., EIT, and Tom Mando Kapita, P.E., work with reinforced soil structures in Terracon’s Phoenix office. Terracon is an employee-owned engineering consulting firm providing geotechnical, environmental, construction materials, and facilities services from more than 100 offices nationwide with more than 2,700 employees. Contact Vieira and Kapita at amvieira@terracon.com and tmkapita@terracon.com, respectively.


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