Subgrade Thickness

Engineers have the option of utilizing a geosynthetic to reduce overall road system thickness. The following is a quick overview of the benefits of cellular confinement and bi-axial, base stabilization geogrids in reducing overall required layer thickness.

Reducing System Thickness with EnviroGrid® Cellular Confinement

The following table shows the system thickness required to achieve a structural coefficient value of 0.35 for various materials, including EnviroGrid® filled with sandy soil*:

MATERIAL

TYPE

ENVIROGRID® FILLED WITH SANDY SOIL ASPHALTIC

CONCRETE

CRUSHED

STONE

SANDY

GRAVEL

LIME

STABILIZED

SOIL

SANDY

SOIL

REQUIRED LAYER

THICKNESS

FOR S.C. = 0.35

4 inches

(10 cm)

3.4 inches

(8.6 cm)

10 inches

(25 cm)

12.7 inches

(32 cm)

17.5 inches

(45 cm)

20 inches

(51 cm)

6 inches

(15 cm)

5.1 inches

(13 cm)

15 inches

(38 cm)

19.1 inches

(49 cm)

26.3 inches

(67 cm)

30 inches

(76 cm)

8 inches

(20 cm)

6.8 inches

(17 cm)

20 inches

(51 cm)

20 inches

(51 cm)

35 inches

(89 cm)

40 inches

(102 cm)

15” of crushed stone (.14 SC Value)* Utilizing AASHTO design methodology. A complete description of the AASHTO design procedure can be found at www.aashto.org.

Multiplying the SC of a material by the thickness of the layer determines the material layer’s SN. To achieve a SN of 2.90 with a top layer of 2” of asphalt concrete (.41 SC Value), the following are two possible options for the remainder of the base:

  1.  15″ of crushed stone (0.14 SC Value)
    • (15 x 0.14) + (2 x 0.41) = 2.92
  2. 6″ EnviroGrid® with sandy soil (0.35 SC Value)
    • (6 x 0.35) + (2 x 0.41) = 2.92

 

Alternatively, if the structural coefficient of a material in an existing design is known, an engineer can relate the structural coefficient of EnviroGrid® to the material in the design. For example, EnviroGrid® filled with sandy soil has an SC of 0.35 and sandy soil alone has an SC .07. Thus, 4” of EnviroGrid® filled with sandy soil has the same load bearing strength as 20” of sandy soil without EnviroGrid®. Therefore, a road design calling for 18” of a sandy soil fill could be reduced to 4” of EnviroGrid® with sandy soil.

 

Reducing System Thickness with BaseGrid Bi-Axial Geogrids

Bi-Axial base stabilization geogrids can also be used to reduce road system thickness. Utilizing the values calculated by the “RoadWorx, v1.0 Design Prescriptions for Better Roads” we can determine the required road system thickness for an unreinforced road system and one reinforced with a “Type 1” (BaseGrid 11) or a “Type 2” (BaseGrid 12) geogrid:

 

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Loading Conditions:

20,000 lb / axle load

100 psi tire pressure 1,200 passes

Subbase CBR = 20 (min) 1.5″ wheel rut depth

 

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Loading Conditions:

20,000 lb / axle load

100 psi tire pressure 1,200 passes

Subbase CBR = 20 (min) 3.0″ wheel rut depth

 

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