Soil Improvement and Ground Modification

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Chapter 4-2
• Soil Improvement and Ground Modification

Muzamir bin Hasan, M.Eng. Lecturer
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Sand Drains

• Is another way to accelerate the consolidation
  settlement of soft, normally consolidated clay
  layers and achieve precompression before the
  construction of desired foundation.
• Sand drains are constructed by drilling holes
  through the clay layers in the field at regular
  intervals. The holes are then backfilled with
  sand.

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Sand Drains

• This can achieved by several means, such as-
• Rotary drilling and then backfilling with sand.
• Drilling by continuous flight auger with hollow
  stem and backfilling with sand.
• Driving hollow steel piles.

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Sand Drains

• The soil inside the pile is then jetted out after
  which backfilling with sand is done.
• After backfilling the drill holes with sand, a
  surcharge is applied at the ground surface.
• This surcharge will increase the pore water
  pressure in clay.
• The excess pore water pressure in the clay will
  be dissipated by drainage both vertically and
  radially to the sand drains which accelerates
  settlement of the clay layer.

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Sand Drains
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Sand Drains
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Prefabricated Vertical Drains (PVDs)

• Also known as wick or strip drains, were
  originally developed as substitute for commonly
  used sand drain.
• With the advent of material science, these drains
  are manufactured from synthetic polymers such as
  polypropylene and high density polyethylene.
• PVDs are normally manufactured with a corrugated
  or channeled synthetic core enclosed by a
  geotextile filter.

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Prefabricated Vertical Drains (PVDs)

• Installation rates reported in the literature are
  on the order of 0.1 to 0.3 m/s, excluding
  equipment mobilization and setup time.
• PVDs have been used extensively in the past for
  expedient consolidation of low permeability soils
  under surface surcharge.
• The main advantage of PVDs over sand drain is
  that they do not require drilling and thus,
  installation is much faster.

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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)

• The Consolidation Problem
• The design of structures on soft compressible
  soils (clays) has created problems for owners,
  engineers and contractors. Construction without
  soil treatment is usually impractical due to
  unpredictable long-term settlement. Simple
  surcharging as a soil consolidation method can
  take many years. Soil consolidation using PVDs
  can rapidly increase settlement rates and cut
  project durations drastically.

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Prefabricated Vertical Drains (PVDs)

• What are PVDs?
• PVDs are installed vertically to depths
  exceeding 200 feet (65 meters). The water, under
  pressure in excess of hydrostatic, flows through
  the filter fabric of the PVDs and into the
  channels of the PVDs core where it can flow
  vertically out of the soil. This flow may be
  either up or down to intersecting natural sand
  layers or to the surface where a sand drainage
  blanket or prefabricated horizontal strip drains
  are provided. The water in the soil has only to
  travel the distance to the nearest PVDs to reach
  a free drainage path.

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Prefabricated Vertical Drains (PVDs)

• The PVDs core is made of high quality flexible
  polypropylene which exhibits a large water flow
  capacity in the longitudinal direction of the
  core via preformed grooves or water channels on
  both sides of the core. Each PVDs can provide a
  greater vertical discharge capacity than a 6 inch
  diameter sand column.

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Prefabricated Vertical Drains (PVDs)

• The PVDs core is tightly wrapped in a geotextile
  filter jacket of spun-bonded polypropylene which
  has a very high water permeability while
  retaining the finest of soil particles. Both the
  core and geotextile filter jacket have high
  mechanical strength, a high degree of durability
  in most environments, and high resistance to
  chemicals, micro-organisms, and bacteria.

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Prefabricated Vertical Drains (PVDs)

• How are PVDs designed?
• The PVDs are usually placed in a triangular
  configuration of 3 to 12 feet (1 to 4 meters) -
  depending on the desired consolidation time. As a
  result of this method of accelerating the
  consolidation process, uneven post-construction
  settlements can be virtually eliminated.

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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)
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Prefabricated Vertical Drains (PVDs)