An embankment is an artificial mound constructed using earthen materials such as stone and soil, properly compacted, to support the raising of roadway or railways above the level of the existing surrounding ground surface.
In dams, embankment refers to successive layers of the earth such as soil, sand, clay, or rock, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides.
In this article, we discuss the characteristics, properties, types, and tests on embankment materials.
Types of Embankment Materials
1. Fine-Grained Soil
Fine-grained soil used in the embankment has a low permeability, low shear strength, and high compressibility. The pore pressure in this type of material is more due to rapid construction activities which result in reduced shear strength and potentially unstable conditions during construction.
Compressibility of fine-grained soil for embankments depends on the soil properties and the placement conditions. Dams and bunds constructed using fine-grained soil material have been proven to be highly resistant to earthquake damage.
2. Coarse-Grained Soil
Coarse-grained soils are used in structural fill zones, or shells, and in specialty filter and drain zones within embankments. Coarse-grained soils which usually consist of sand and gravel are also used in core zones, especially when the fines content is greater than 20 percent.
Sands and gravels that have less than about 5 percent fines by dry weight are pervious, easy to compact, and are minimally affected by changes in moisture. Coarse-grained soil tends to be highly vulnerable to surface erosion under wave action and surface runoff.
3. Broadly Graded Soils
Broadly graded soil deposits comprise a large range of particle sizes, and their engineering behavior is intermediate between fine-grained and coarse-grained soils. These type of soils typically exhibit properties of lower hydraulic conductivity, high shear strength, and lower compressibility in comparison with fine-grained soils.
Colluvial and bouldery alluvial deposits are also significant sources of broadly graded soils that are used as embankment material. Embankments constructed of broadly graded soils are, in general, highly resistant to earthquake damage.
Characteristics of Embankment Materials
The U.S. Bureau of Reclamation's Earth Manual (USBR, 1974, 1990) identifies the following characteristics of embankment material used in the construction of embankments.
1. Fine-Grained Soil
- The material must be formed into an essentially homogeneous mass, free from any potential paths of percolation through the zone or along with the contacts with the abutments or concrete structures.
- The soil mass must be sufficiently impervious to preclude excessive water loss through the dam.
- The material must not consolidate excessively under the weight of superimposed embankments.
- The soil must develop and maintain its maximum practicable shear strength.
- The material must not consolidate or soften excessively on saturation by water from the reservoir.
2. Coarse-Grained Soils
- The material must be formed into a homogeneous mass free from large voids.
- The soil mass must be free draining.
- The material must not consolidate excessively under the weight of superimposed fill.
- The soil must have a high angle of internal friction (i.e., high shear strength).
Material Properties of Embankment
A well-graded material consists of two or more soil types, usually a mixture of granular and fine-grained soils. There is no universally recommended range of gradation for fill materials, although the maximum particle size should be less than 100 mm.
2. Unit Weight and Specific Gravity
Materials that are relatively low in unit weight offer the advantage of transmitting the less dead load to the underlying soil that supports an embankment. Anyhow, there are no specified requirements for a minimum or maximum unit weight, either before or after compaction.
3. Shear Strength
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.
The compressibility of an embankment material is related to its shear strength, degree of compaction, void ratio, permeability, and degree of saturation.
This property of embankment material is indicative of the ability of compacted fill material to provide drainage for excessive moisture.
Test Procedure for Embankment Materials Properties
The table below provides the list of standard test methods usually used to assess the properties of embankment materials used in embankment construction as per ASTM Codes.
|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|
|Moisture Density 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 (Standard)|
|Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 10 lb (4.54 kg) Rammer and 18 in. (457 mm) Drop||ASTM D1557(Modified)|
|Compacted Density (In-Place Density)||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|