The Constructor

Embankment Materials – Types, Characteristics, Properties, and Tests

Embankment Material

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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.

Fig 1: Embankment Construction.

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

  1. 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.
  2. The soil mass must be sufficiently impervious to preclude excessive water loss through the dam.
  3. The material must not consolidate excessively under the weight of superimposed embankments.
  4. The soil must develop and maintain its maximum practicable shear strength.
  5. The material must not consolidate or soften excessively on saturation by water from the reservoir.

2. Coarse-Grained Soils

  1. The material must be formed into a homogeneous mass free from large voids.
  2. The soil mass must be free draining.
  3. The material must not consolidate excessively under the weight of superimposed fill.
  4. The soil must have a high angle of internal friction (i.e., high shear strength).

Material Properties of Embankment

1. Gradation

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.

4. Compressibility

The compressibility of an embankment material is related to its shear strength, degree of compaction, void ratio, permeability, and degree of saturation.

5. Permeability

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.

PropertyTest MethodReference
GradationParticle Size Analysis of SoilsASTM D422
Sieve Analysis of Fine and Coarse AggregateASTM D136
Unit Weight and Specific GravityUnit Weight and Voids in AggregateASTM D29
Specific Gravity of SoilsASTM D854
Relative Density of Cohesionless SoilsASTM D2049
Maximum Index Density of Soils Using a Vibratory TableASTM D4253
Moisture Density CharacteristicsMoisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 5.5 lb (2.49 kg) Rammer and 12 in. (305 mm) DropASTM D698 (Standard)
Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 10 lb (4.54 kg) Rammer and 18 in. (457 mm) DropASTM D1557(Modified)
Compacted Density (In-Place Density)Density of Soil in Place by the Sand-Cone MethodASTM D1556
Density and Unit Weight of Soil in Place by the Rubber Balloon MethodASTM D2167
Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow-Depth)ASTM D2922
Density of Soil in Place by the Sleeve MethodASTM D4564
Shear StrengthUnconsolidated Undrained Compressive Strength of Cohesive Soils in Triaxial CompressionASTM D2850
Direct Shear Test of Soils Under Consolidated Drained ConditionsASTM D3080
Consolidated-Undrained Triaxial Compression Test on Cohesive SoilsASTM D4767
CompressibilityOne-Dimensional Consolidation Properties of SoilsASTM D2435
One-Dimensional Consolidation Properties of Soils Using Controlled-Strain LoadingASTM D4186
One-Dimensional Swell or Settlement Potential of Cohesive SoilsASTM D4546
Bearing CapacityCalifornia Bearing Ratio (CBR) of Laboratory-Compacted SoilsASTM D1883
Bearing Ratio of Soils in PlaceASTM D4429
PermeabilityPermeability of Granular Soils by Constant HeadASTM D2434
Corrosion ResistancepH of Soil For Use in Corrosion TestingASTM G51
Field Measurement of Soil Resistivity Using the Wenner Four-Electrode MethodASTM G57
Pore Water Extraction and Determination of the Soluble Salt Content of Soils by RefractometerASTM D4542

Read More : Factors Affecting Embankment Dams Design and Construction

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