The Basics of Embankment Construction

Rock/Shale/Slope Embankments and Compaction/Sediment Control

The information in this article focuses on the proper inspection of embankment construction, with the goal of ensuring a solid, good, lasting embankment. Not only will the embankment withstand the test of time, but it also ensures durability and safety for everyone using the highway.

Rock Embankment

If the rock can’t be removed without blasting, then it is known as rock excavation. Materials removed include all stones and boulders sized ½ yd3 or larger. When a rock fill is used, there are large voids between the rock chunks that must be filled with fines. While the fines can fill the sides and top of the embankment, large voids remain inside the embankment. Typically, deformations are not a concern due to the friction and interlocking between pieces because the rock pieces will remain intact.

  • Lift Requirements:
    • Voids are filled with small sized stones. Large stones are distributed to reduce the risk of pockets, but they shouldn’t be nested.
    • The top 2 feet should consist of suitable material that is compacted so that the target density is achieved. Shale or shake-like materials are not permitted in these 2 feet of embankment.
    • If the embankment depth is greater than 5 feet and is composed of rock, then the deposited rock should not exceed the size of the material being placed. It should never exceed 4 feet.
    • If the embankment is 5 feet or less, or the placed material isn’t composed of rock entirely, then the material shouldn’t exceed the top size of the placed rock. It should never exceed 2 feet.
    • The voids of the last lift should be sufficiently closed, or they must be chocked using suitable materials (such as small broken stones). Compaction is needed and varies based on the materials used.
    • Written permission is always required if shale is incorporated as a rock embankment.
  • Compaction Methods:
    • If the embankment construction uses rock that is large enough to perform density tests, then crawler-tread equipment should be used to compact the material. Another option is to use approved vibratory equipment. Or, both methods can be used.
    • All portions need to be compacted uniformly, achieved by successive passes while ensuring that the tread areas overlap.

Soft Rock and Shale Embankments

When the rock fill consists of rocks that weather rapidly, then there is the risk of the rock pieces becoming soil. As a result, the materials could fall into the voids between the rocks, causing sizable embankment settlement and slope failure in the future. This reason is why shale is not recommended because large pieces can settle down into the big voids. Avoiding these large settlements is essential to prevent slope instability.

  • Compaction and Lift Requirements
    • When soft rock, shale, or shale/soft rock mixtures are used, it should have a maximum 8-inch loose lifts. They should be compacted at least to a 95% maximum dry density. Optimum moisture content needs to be identified, and the control needs to keep these conditions within +1 and -2 percentage.
    • Approved equipment needs to be used for compaction. Usually, this equipment includes a static tamping-foot roller with each tamping foot projecting a minimum of 6 inches, as well as a vibrating tamping-foot roller with each tamping foot projecting a minimum of 4 inches.
    • A Light Weight Deflectometer is used for the compaction measurement.
    • The shale slaking action can be accelerated by applying water in the cut. Water should be applied again before compaction and disking. A multiple gang disk should be used so that the water is incorporated uniformly (the disk wheel should be a minimum diameter of 24 inches.
    • Embankment lifts should receive at least three passes using the vibratory roller, unless otherwise stated in writing. One complete coverage of the applicable area is considered a roller pass. Before the vibratory roller is used, the material should be bladed.

Embankments on Slopes and Hillsides

Preparation needs to be done before embankments are placed on natural soil slopes. These methods are also needed if the existing fill slopes are sharper than 4:1. Before the embankment fill is placed, benches are cut into the slopes with a minimum of 10 feet in width. If the natural soil slopes are 4:1 or flatter, then the surfaces must be deeply scarified or plowed.

When the bench cutting method is used, then the excavation is priced per yd3 based on the classes of excavation used. Direct payment is not needed for the scarifying or plowing.

Embankment Over Existing Roads

Specific precautions need to be followed when embankments are placed over existing roadways. These are the suggested treatments:

  • Existing pavement should be removed if the upper surface of the existing pavement is located 12 inches below the subgrade elevation of the new pavement.
  • Existing pavement should be broken if the upper surface of the existing pavement is located between 12 inches and 3 feet below the subgrade elevation of the new pavement. Individual pieces of the existing pavement should be broken to less than 1 yd3.
  • When the existing pavement consists of a concrete based with a hot mix asphalt surface, which is between 12 inches and 3 feet below subgrade elevation of the new pavement, then the hot mix asphalt should be removed and the concrete base needs to be broken.
  • When hot mix asphalt is 12 inches or less below or above the required subgrade elevation, then the existing pavement should be broken and removed.
  • When the top of the existing pavement is greater than 3 feet below the target subgrade elevation, then the existing pavement can be left in place.
  • Precautions are needed when an embankment is widened. First perishable material needs to be removed. Then, the plowing is completed for the existing shoulders 2 feet out from the existing pavement. Benches are cut into the old embankment with a minimum of 4 feet in width, unless otherwise specified. Direct payment is not needed for benching or plowing because the cost should be wrapped in the contract line items.

Compaction Control

These are the requirements to follow for compaction control:

  • Usually, a test strip or Geotech lab results determine the target density of the embankment compaction.
  • The control of moisture content should be within +1 and -2 percentage of the optimum moisture content. When the material is too wet, then excess moisture needs to be removed with aeration. When the material is too dry, then watering and disking is used to boost the moisture content.
  • Level, uniform layers should be placed for embankment material, then approved compacting equipment should be used. The compacting equipment must have at least three wheels, or the compacting equipment needs to be able to create an even, smooth surface.
  • Existing clods and lumps are treated or disked using mechanical techniques to ensure they are broken up.
  • The required compaction needs to be obtained for the loose depth of each lift. It should never exceed 8 inches.
  • The loose depth of lift shouldn’t surpass the tamper foot length when a tamping roller is used. Each tamping roller foot should be a minimum of 5 in2.
  • When silty loans, silts, or loessial type soils are used or encountered in embankment construction, then the moisture content should be controlled within -3 of the optimum moisture content.
  • Each lift should have field compaction tests. Before the next lift is placed, the required compaction needs to be obtained on the previous lifts.
  • When gravel soil or sandy soil is used (the materials have at least 80% or more gravel or sand particles), then the moisture level should be within a few percentage points below optimum. This requirement ensures the soils may be compacted to the specified density.

Settlement Control

Sometimes, preliminary investigation finds that existing soil beneath the embankment can settle over time due to the construction of a heavy embankment. So, settlement control is essential; which should include measurement of the settlement to make sure an acceptable rate of settlement is occurring.

Geotechnical instrumentation is placed at locations indicated on the plans. The measurement work includes provision of the equipment, installation, maintenance, and reading the results. Placement can be determined as follows:

  • Before embankment operations commence, installation needs to be completed with settlement plates (1/2 in. by 3 ft by 3 ft) as well as galvanized pipe (3/4 in. and 2 in.) and couplings for a cover.
  • Pipe sections that are ¾ inch should 4 feet in length for the settlement plates. The pipe can be vertically extended from the plate center through the new embankment and the joints should be spot welded. The cover pipe should not be welded to the plate, but should be slipped over the pipe.
  • Lateral stakes should also be installed as required in the plans. These stakes are usually ¾ inch by 4 ft steel rods, which are placed a minimum of 12 inches in the ground.
  • Settlement pipes and plates need to be protected during construction, so B Borrow is used for embankment material around the equipment.
  • When subgrade elevation is achieved for the embankment construction, then settlement stakes are installed (¾ inch by 4 ft steel rods).
  • Elevation readings should be taken on all settlement stakes and settlement plate extension pipes. These reading should occur at least every 7 days, or more depending on the conditions.
  • When subgrade elevation is achieved, readings should be obtained from the settlement plate extension pipes as well as the settlement stakes. Lateral stakes can be used to measure horizontal movement of the new fill or ground. When lateral movement is identified, then corrective measures must be used before work continues.
  • The new embankment is allowed a settlement period of 3 months after the subgrade elevation construction is complete (unless otherwise specified in the plans).
  • When subgrade elevation is achieved, readings should be obtained every 7 days until a ¼ inch or less settlement rate is measured for at least 4 weeks consecutively. The Office of Geothermal Engineering may reduce this measurement period. If the readings indicate more than ¼ inch settlement, then the monitoring period will be extended.
  • If serious settlement is identified while the embankment is being constructed or during the settlement period, then work needs to be halted so that corrective measures can be implemented as needed.

 

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