guyoster@bellsouth.net

  G E O S Y N T H E T I C S    

TerraGrid® geogrids are constructed of high tenacity

polyester yarns and coated with polyvinyl chloride. The open mesh structure allows superior interlocking with the surrounding soil. TerraGrid's high resistance to elongation makes it an ideal material for stabilizing bases, retaining walls, slopes, berms and dikes.

Retaining walls are structures built for the purpose of stabilizing very steep slopes using minimal land area. A common example is to infill the side of a hill to build or widen a road. The slope created is so steep as to be unstable and requires a retaining wall to prevent slope failure.

There are two basic types of retaining walls:

• A CANTILEVER retaining wall is usually made of reinforced concrete and acts as a structural member.
• A GRAVITY retaining wall has a vertical or near vertical face and its ability to retain soil behind it depends upon gravity (its own weight). This type of wall is usually composed of unreinforced concrete or soil. If made of soil, the wall generally consists of a facia and one or more layers of

reinforcing members extending from the facia back into the soil mass.  The functions of the facia  are to prevent soil from sloughing off the surface of the wall and to prevent erosion of the face of the wall.  The function of the reinforcing member (TerraGrid) is to increase the "effective" thickness of the wall which increases its stability.  The wall itself must be built to withstand pressures applied by the retained soil. This force, "lateral earth pressure', is dependent upon several parameters, including the geometry of the wall and the characteristics of the retained soil, such as its unit weight and angle of internal friction. In designing a retaining wall, it is necessary to consider potential forces that could cause wall failure and compare them to the stability calculations of the proposed wall, taking into account generally accepted safety factors.

When TerraGrid^® geogrid is used to construct a retaining wall, it functions as a reinforcing element. The grid and its interaction with the backfill causes the entire reinforced zone to act as one single mass.

This tying together of the reinforced zone is what is used for calculating the forces that the gravity wall can withstand.

TerraGrid® installation should always be carefully planned to ensure proper performance and efficient, speedy construction.

1.  Clear all construction debris, sharp objects, and large rocks from construction elevation.

2.  Properly compact base and level to desired elevation as specified by plans.

3.  Roll out TerraGrid® perpendicular to the face of the wall and cut to length specified in designs.

4. Tension the TerraGrid® by pulling so that there are no wrinkles.  The grid should lay flat and taut.

5.  The strength of TerraGrid® is in the roll length, therefore each section width should be one continuous piece.  (No overlapping is allowed to obtain the design length perpendicular to the wall face.)   In the parallel direction, the grid should be butted up to the adjacent grid section.

6.  Standard methods of backfill replacement are recommended.  NO tracked vehicles are to be allowed directly on the grid.  Care should be taken to ensure TerraGrid® remains taut and wrinkle-free during backfill placement.  Each lift thickness should be compacted as specified by the design.

Retaining walls are used whenever a change in grade is desirable or necessary. The alternatives to gravity retaining walls made of reinforced soil are gravity retaining walls made of concrete, cantilever retaining walls, and slopes. TerraGrid^® reinforced soil walls are typically much less expensive than other retaining wall alternatives. They are much more effective than slopes in effecting changes in grade, costing less in terms of land consumed.

Design of the retaining walls includes the selection of the proper strength TerraGrid^®, as well as the determination of the length and location of the grid layers. Please refer to the drawing below. There are four strengths of uniaxial TerraGrid^®, which are the type normally used in retaining walls. The parameter which defines the strength selected is long term design strength (LTDS).

The primary factors which determine the grid strength, spacing and length in the retaining wall design are; surcharge loads, the soil in each of the three zones (foundation soil zone, retained soil zone and reinforced soil zone) and the wall geometry. The wall geometry includes the height of the wall and the slope behind the wall. Additional factors which determine the vertical distance between layers are the connection strength between geogrid and the facia elements and the shear strength between courses of facia elements.

Retaining walls should always be designed by a professional engineer registered in the state of the project. Please call Southeast Railroad Supply Company with any questions regarding the values of TerraGrid^® properties recommended for use in design of retaining walls.

an angle less steep than the initial one. In many situations, it is desirable to have a slope steeper than the shear strength which the soil will allow. One method employed to achieve this objective is to provide reinforcement at specified increments back into the slope. The resulting reinforcement increases the effective failure angle, preserving the stability of the slope.

steeply than without geogrid reinforcement. This enables the engineer or designer to make the most of the land available on the site. The following illustrations indicate this process.

Use of TerraGrid® can result in economic savings in several ways. In cut situations where the foot of the slope is fixed, the use of TerraGrid^® can greatly reduce the amount of soil that would normally have to be removed. In fill situations, the use of TerraGrid^® can greatly reduce the amount of fill which must be imported and placed. In developing a commercial, residential, or industrial site, the amount of usable land can be increased by creating steeper slopes with the use of TerraGrid^®. By making the slopes steeper, TerraGrid^® can help the engineer or designer economically use his construction site. be

TerraGrid^® installation should always be carefully planned to ensure proper performance and efficient construction.

1.  Clear all construction debris, sharp objects, and large rocks from construction elevation.

2.  Properly compact base and level to desired elevation as specified by plans.

3.  Roll out TerraGrid^® perpendicular to the face of the slope and cut to length specified in designs.

4.  Tension the TerraGrid^® by pulling so that there are no wrinkles.  The grid should lay flat and taut.

5.  The strength of TerraGrid^® is in the roll length, therefore each section width should be one continuous piece.  (No overlapping is allowed to obtain the design length perpendicular to the wall face.)   In the parallel direction a minimum of four inches of overlap is recommended.

6.  Standard methods of backfill replacement are recommended.  NO  tracked vehicles are to be allowed directly on the grid.  Care should be taken to ensure TerraGrid^® remains taut and wrinkle-free during backfill placement.  Each lift thickness should be compacted as specified by the design.

The function of TERRAGRID® geogrids in ground stabilization application is to
spread the loads imposed upon the system. It accomplishes this by diverting the downward stresses laterally along the grid.

  Ground Stabilization

In a ground stabilization application, the function of the aggregate layer is to spread the loads imposed on it so that the pressure, or force per unit area, exerted on the subgrade is reduced below the critical value. The aggregate layer transfers the load outward as well as downward away from the source. Spreading of loads is accomplished by the interlocking and friction that occur between individual granular particles. The aggregate must be well compacted during construction and remain compacted and free from fines during the life of the project. In other words, for the aggregate layer to perform as designed throughout its intended life, the original quality and degree of the compaction must be preserved.

Aggregate support systems are improved with the use of a geogrid. TerraGrid^® geogrids contribute to the long-term performance of aggregate layers because of their ability to confine. A geogrid's high resistance to elongation and its ability to interlock with the aggregate allow it to reinforce the aggregate layer. TerraGrid^® also provides a high friction surface that resists lateral movement by the aggregate, thereby confining those particles in direct contact with the grid. The state of compaction of the aggregate determines how well this layer can withstand lateral pressure above the lowest level.

In many situations, TERRAGRID^® makes it possible to reduce the thickness of the base. Benefits include savings in aggregate and improved long-term performance. TERRAGRID^® can be used in a variety of stabilization projects including:

• STREETS    ROADS and HIGHWAYS
• PARKING LOTS, STORAGE   AREAS and  CONSTRUCTION ENTRANCES
• ACCESS LANES for FIRE/ EMERGENCY VEHICLES
• RUNWAYS, APRONS, TAXIWAYS, and OVERRUN  AREAS
• RAILROAD TRACK BEDS
• TEMPORARY ACCESS

TerraGrid^® geogrids provide a greater ability to prevent lateral movement of aggregate by the interlocking of the aggregate with the grid. The ability of the aggregate layer to spread surface forces and reduce

pressures on the subgrade is no longer dependent solely upon the properties of the aggregate itself. TerraGrid^® helps confine and preserve compaction.

During construction of roads, parking lots, sewer lines, etc., pockets of soft, poor soils are sometimes encountered. Typically soil is removed to a significant depth and replaced with expensive fill materials. This

procedure can be costly and time consuming. An effective alternative is to use TerraGrid^® placed near the bottom of the compacted granular fill (sometimes a separator geotextile is necessary).

TerraGrid^® is installed quickly and easily by semi-skilled labor and without special equipment.   Standard roll sizes are 12 feet by 150 feet for ease of transporting and handling in the field. 

1.  Remove sharp objects and large rocks from construction elevation.

2.  Properly compact base and level to desired elevation as specified by plans.

3.  Roll out TerraGrid^® on prepared subgrade.

4. Tension the TerraGrid^® by pulling so that there are no wrinkles.  The grid should lay flat and taut.

5.  When more than one width of grid is installed, the edges must be overlapped one to three feet (for extremely poor soils different measures may be needed).

6.  Backdump aggregate onto fabric.   (Do not drive tracked vehicles on the grid itself.)

7.  Spread aggregate, preferably with a tracked vehicle.  Compact aggregate as required using normal procedures.