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Publication Number: FHWA-RD-97-148
An embankment refers to a volume of earthen material that is placed and compacted for the purpose of raising the grade of a roadway (or railway) above the level of the existing surrounding ground surface. A fill refers to a volume of earthen material that is placed and compacted for the purpose of filling in a hole or depression. Embankments or fills are constructed of materials that usually consist of soil, but may also include aggregate, rock, or crushed paving material.
Normally, the coarser fill materials are placed at or near the bottom or base of the embankment in order to provide a firm foundation for the embankment and also to facilitate drainage and prevent saturation. The top portion of an embankment usually is constructed of relatively high-quality, well-compacted subgrade material that is capable of supporting the overlying pavement layers and imposed wheel loadings without deflection or undesirable movement. The fill material used throughout the remainder of the embankment must be capable of meeting applicable specification quality requirements and be capable of being placed and compacted at or close to its maximum achievable density. The material is spread in relatively thin layers of 150 mm (6 in) to 200 mm (8 in) and each layer is compacted by rolling over it with heavy compaction equipment.
Many different types of soils may be suitable for use in the construction of an embankment or fill, ranging from granular soils (sand and gravel), which are highly desirable, to the more finely sized soils (silt and clay), which are usually somewhat less desirable. Certain types of soils (such as saturated clays and highly organic soils) are considered unsuitable for use as materials in embankment or fill construction. Regardless of the type(s) of soil(s) used to construct embankments or fills, the material should be well graded, capable of being well compacted, be within a proper range of moisture to optimize compaction, and be free of unsuitable or deleterious materials, such as tree roots, branches, stumps, sludge, metal, or trash.
Some oversize materials (over 100 mm (4 in) in size), such as rocks, large stones, reclaimed paving materials, or air-cooled slags, can be used for the construction of embankment bases. Although the use of oversize materials can result in a stable embankment base, the oversize materials should have strong particles that do not readily break down under the action of construction machinery, but which have a range of sizes so that void spaces are at least partially filled.
MATERIAL PROPERTIES AND TESTING METHODS
Some of the more important properties of materials that are used for the construction of embankments or fills include:
Gradation – well-graded fill materials that consist of two or more soil types, usually a mixture of granular and fine-grained soils, are most suitable for embankment construction. Because of the wide variety of soils that may be encountered, there is no universally recommended range of gradation for fill materials, although the maximum particle size should be less than 100 mm (4 in) so that it can be readily placed within a 200 mm (8 in) layer. Rock, or other oversize materials to be used as an embankment base, should consist of different size particles, with a specified maximum particle size.
Unit Weight and Specific Gravity – fill materials can vary in unit weight over a fairly wide range, depending on the type of material and its moisture content. Fill materials that are relatively low in unit weight offer the advantage of transmitting less dead load to the underlying soil that supports an embankment. There are usually no specified requirements for a minimum or maximum unit weight, either before or after compaction.
Moisture-Density Characteristics – the compaction characteristics (optimum moisture content and maximum dry density) of a soil fill material are the most important single property that affects embankment performance. Most specifications for embankment construction require the compacted fill material to have an in-place density that is within a certain percentage (usually 95 percent or greater) of the maximum dry density at a moisture content that is within a certain percentage (usually 3 percent or less) of optimum. The optimum moisture and maximum dry density of fill material(s) are determined in advance in the laboratory by means of either standard or modified moisture-density compaction tests. These tests methods are applicable for soils or earthen fill materials. Moisture-density characteristics cannot usually be determined for oversize (over 100 mm (4 in) materials) materials.
Shear Strength – the shear strength characteristics (cohesion and/or internal friction) are indicative of the ability of a fill material to support loads that are imposed upon it under given drainage conditions. Shear strength characteristics are not always specified for earthen fill materials, but are determined by triaxial compression or direct shear testing and are used to compute the slope stability of an embankment.
Compressibility – compressibility refers to the consolidation or settlement characteristics of a material under long-term loading conditions. The compressibility of a fill material is related to its shear strength, degree of compaction, void ratio, permeability, and degree of saturation. The settlement characteristics of an earthen fill material are determined by one-dimensional consolidation testing. Some settlement of an embankment or fill will occur during its construction, while the remainder of the settlement (if any) will occur in the postconstruction period.
Bearing Capacity – bearing capacity refers to the ability of a fill material to support the loadings imposed upon it over the life of the facility without undue settlement, volume change, or structural damage. Bearing capacity can be determined by laboratory testing and by field load tests.
Permeability – permeability or hydraulic conductivity refers to the ability of a soil (or an oversize material) to transmit water through the pore structure of the fill material at a given rate. This property is indicative of the ability of a compacted fill material to provide drainage for excessive moisture.
Table 24-9 provides a list of the standard test methods usually used to assess the suitability of conventional earthen fill materials for use in embankment or fill construction.
Table 24-9. Embankment or fill material test procedures.
|Gradation||Particle Size Analysis of Soils||ASTM D422|
|Sieve Analysis of Fine and Coarse Aggregate||ASTM D136|
|Unit Weight and Specific Gravity||Unit Weight and Voids in Aggregate||ASTM D29|
|Specific Gravity of Soils||ASTM D854|
|Relative Density of Cohesionless Soils||ASTM D2049|
|Maximum Index Density of Soils Using a Vibratory Table||ASTM D4253|
|Minimum Index Density of Soils and Calculation of Relative Density||ASTM D4254|
|Moisture Densiity Characteristics||Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 5.5 lb (2.49 kg) Rammer and 12 in. (305 mm) Drop||ASTM D698
|Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 10 lb (4.54 kg) Rammer and 18 in. (457 mm) Drop||ASTM D1557
|Density of Soil in Place by the Sand-Cone Method||ASTM D1556|
|Density and Unit Weight of Soil in Place by the Rubber Balloon Method||ASTM D2167|
|Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow-Depth)||ASTM D2922|
|Density of Soil in Place by the Sleeve Method||ASTM D4564|
|Shear Strength||Unconsolidated Undrained Compressive Strength of Cohesive Soils in Triaxial Compression||ASTM D2850|
|Direct Shear Test of Soils Under Consolidated Drained Conditions||ASTM D3080|
|Consolidated-Undrained Triaxial Compression Test on Cohesive Soils||ASTM D4767|
|Compressibility||One-Dimensional Consolidation Properties of Soils||ASTM D2435|
|One-Dimensional Consolidation Properties of Soils Using Controlled-Strain Loading||ASTM D4186|
|One-Dimensional Swell or Settlement Potential of Cohesive Soils||ASTM D4546|
|Bearing Capacity >||California Bearing Ratio (CBR) of Laboratory-Compacted Soils||ASTM D1883|
|Bearing Ratio of Soils in Place||ASTM D4429|
|Permeability||Permeability of Granular Soils by Constant Head||ASTM D2434|
|Corrosion Resistance||pH of Soil For Use in Corrosion Testing||ASTM G51|
|Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method||ASTM G57|
|Pore Water Extraction and Determination of the Soluble Salt Content of Soils by Refractometer||ASTM D4542|
REFERENCES FOR ADDITIONAL INFORMATION
Nichols, Herbert L. Moving the Earth. McGraw-Hill Publishing Company, New York, New York, 1976.
U.S. Bureau of Reclamation. Earth Manual. Washington, DC, 1991.