Two phases of road construction Earthworks: preparation of

1
Highway Subgrade (2) Subgrade Construction

• Two phases of road construction
• Earthworks preparation of the natural soils or
  back-filling materials to the desired levels
  (formation level), gradients, and crossfalls
  (cross slope)
• Overlying pavement formation of the actual
  running surface (carriageway, curb, median,
  planting, etc.)
• II. Excavation of the cuttings
• Types of excavated materials (classified so as
  to serve payment purposes)
• Common excavation excavation in any material
  which is not solid rock as defined below
• Solid rock all material which would be very
  difficult to loosen with normal excavating
  machinery without some preliminary blasting and
  drilling, and including boulders and detached
  pieces of rock exceeding 6 cu. ft in size in
  general excavation and 3 cu. ft in trenches.
• 1 Wednesday, February 20, 2013

2
Highway Subgrade (2) Subgrade Construction

• 2. Procedures of excavation
• removing the topsoil over the width of the
  earthworks between the slope stakes and
  stockpiling it in separate heaps
• making sure a sufficient crossfall on the
  excavated surface as to shed water and prevent
  ponding
• proceeding from the lowest point of the cutting
  in the longitudinal section to prevent water from
  being trapped
• excavating the area between the extremities of
  the carriageways, including any hard shoulders to
  a level of 12 in. above the formation level
  (refer to p.34)
• when required, trimming off the excess of
  material in a single operation down to the
  formation level
• 2 Wednesday, February 20, 2013

3
Highway Subgrade (2) Subgrade Construction

• dressing with hot tar or bitumen within 24 hours
  of the preparation of the formation unless the
  formation is to be covered with a sealed pavement
  layer within that time (blinding the sealing coat
  with 3/16-in. gravel crushed rock, or slag at the
  rate of 200 sq. yd. per ton to prevent it being
  picked up by constructional traffic).
• III. Formation of Embankments
• It has been almost universal practice to
  build embankments up in relatively thin
  horizontal layers, each of which has been
  compacted at a moisture content near optimum.
  Typical practices can be summarized as follows
• 1) Density requirements
• Specifying of the end result the
  embankments for some or all projects be compacted
  to a stated minimum relative compaction.
• 3 Wednesday, February 20, 2013

4
Highway Subgrade (2) Subgrade Construction

• 2. Moisture control
• Stipulating moisture control, but in a
  qualitative way, which leaves control in the
  hands of the contractor, engineer, and inspector.
  Generally, aerating the overly wet soils or
  adding moisture to dry ones is common practice
  followed by all
• Employing nuclear devices for quick and often
  accomplishment of field-moisture determinations
  in order to be useful
• Other practices to dry soils or determine the
  moisture content oven-drying specially built
  forced draft microwave ovens, igniting an
  alcohol-covered sample the Proctor penetration
  needle a device utilizing the reaction of
  calcium carbide and water to produce acetylene
  gas
• 4 Wednesday, February 20, 2013

5
Highway Subgrade (2) Subgrade Construction

• General guides are that friable soils at optimum
  moisture content sufficient moisture to permit
  forming a strong cast by compressing the soils in
  the hand. For some clays, optimum moisture
  approximates the plastic limit, and can be judged
  by forming a ribbon, thread, and cube of a
  sample.
• 3. Compaction of embankments
• Rollers or other compaction devices increase soil
  density by expelling air from the voids in the
  soil and by arranging or forcing the soil grains
  into more intimate contact.
• In porous soils the individual particles are
  rearranged by pressure and vibration, but in
  cohesive soils more effort is required.
• 5 Wednesday, February 20, 2013

6
Highway Subgrade (2) Subgrade Construction

• Heavy soils must be placed in thin layers to
  overcome charges between individual particles.
• Repetitions of load are particularly effective in
  increasing the density of fine-grained soils but
  have less effect on coarse-grained materials.
• Heavier rollers or a different method rather than
  many load repetitions may offer a better way to
  produce increased densities.
• Heavy rolling equipment is not a cure-all. The
  cost and trouble of moving, the remolding and
  weakening of fine-grained soils under heavy
  rollers
• For fine-grained soils, lighter units are
  preferable to heavy tractor-scraper earth moving
  equipment.
• 6 Wednesday, February 20, 2013