Soil Improvement and Ground Modification

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Chapter 4-1
• Soil Improvement and Ground Modification

Muzamir bin Hasan, M.Eng. Lecturer
2
Introduction

• The existing soil at a construction site may not
  always be totally suitable for supporting
  structures.
• For example, granular soil may be very loose and
  indicate large elastic settlement. So the soil
  needs to be densified to increase its unit weight
  and thus shear strength.

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Introduction

• Sometimes the top layer of the soil are
  undesirable and must be removed and replaced with
  better soil.
• Soft saturated clay layers are often encountered
  at shallow depths below foundation large
  consolidation may occur.

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Introduction

• Various tecniques for improving soil are used-
• Reduce the settlement of structures.
• Improve the shear strength of soil and thus
  increase the bearing capacity of shallow
  foundations.
• Increase the factor of safety against possible
  slope failure of embankments and earth dams.
• Reduce the shrinkage and swelling of soils.

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Introduction

• If unsuitable soil conditions are encountered at
  the site, one of the following 4 procedures may
  be adopted to ensure satisfactory performance of
  the structure (Mitchell, 1976)
• Bypass the unsuitable soil by means of deep
  foundations extending to a suitable bearing
  material.
• Redesign the structure and its foundations for
  support by the poor soil, a procedure that may
  not be either feasible or economical.
• Remove the poor material and either treat it to
  improve and replace it, or substitute it by a
  suitable material.
• Treat the soil in place to improve its properties.

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Improvement Techniques

• The factors that must be considered in the
  selection of the best technique in any case
  include the following (Mitchell, 1976)
• Soil type sand, clay, organic, etc.
• Area and depth of treatment required depend on
  the geometric characteristics of the soil deposit
  and load distribution.
• Type of structure and load distribution.
• Soil properties strength, compressibility,
  permeability, etc.

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Improvement Techniques

• Permissible total and differential settlements.
• Material availability stone, sand, water,
  admixture, stabilizers, etc.
• Availability of skills and equipment.
• Environmental considerations waste disposals,
  erosion, water pollution, etc.
• Local experience and preferences.
• Economics.

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Compaction

• If a small amount of water us added to a soil
  that is then compacted, the soil will have a
  certain unit weight.
• If the moisture content of the same soil is
  gradually increased and the energy of compaction
  is the same, the dry unit weight of the soil will
  gradually increase.
• The reason is that water acts as a lubricant
  between the soil particles, and under compaction
  it helps rearrange the solid particles into a
  denser state.

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Compaction

• The standard lab test-
• Standard Proctor test (ASTM designation D-698)
• Modified Proctor test (ASTM designation D-1557

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Compaction
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Compaction
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Compaction

• Compaction in the field depends on several
  factors, such as-
• Type of compactor
• Soil type
• Moisture content
• Lift Thickness
• Towing speed of the compactor
• The number of the roller passes

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Field Compaction

• Ordinary compaction in the fields is done by
  rollers.
• Of the several types of roller used, the most
  common are-
• Smooth wheel roll
• Pneumatic rubber-tired rollers
• Sheepsfoot rollers
• Vibratory rollers

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Field Compaction
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Field Compaction
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Field Compaction
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Smooth Wheel Roller

• Can create vertical vibration during compaction.
• Suitable for proof-rolling subgrades and for
  finishing the construction of fills with sandy or
  clayey soils.
• Provide 100 coverage under the wheels, and the
  contact pressure can be as high as 300-400kN/m2.
• However, they do not produce uniform unit weight
  of compaction when used on thick layers.

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Smooth Wheel Roller
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Smooth Wheel Roller
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Smooth Wheel Roller
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Smooth Wheel Roller
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Smooth Wheel Roller
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Pneumatic Rubber-Tired Rollers

• Better in many respects than smooth wheel
  rollers.
• May weight as much as 2000 kN, consist of a
  heavily loaded wagon with several rows of tires.
• These tires are closely spaced four to six in a
  row.
• The contact pressure under the tires may range up
  to 600-700kN/m2, and they produce about 70-80
  coverage.
• Can be used for sandy and clayey soil compaction,
  produce a combination of pressure and kneading
  action.

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Hamm Model GRW 18 Rubber Tired (Pneumatic)
Roller
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Pneumatic Rubber-Tired Rollers
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Sheepsfoot Rollers

• Consist basically of drums with large numbers of
  projections.
• The area of each of the projections may be 25-90
  cm2.
• Most effective in compacting cohesive soils.
• The contact pressure under the projections may
  range from 1500-7500 kN/m2.
• During compaction in the field, the initial
  passes compact the lower portion of a lift.
• Later, the middle and top of the lift are
  compacted.

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Sheepsfoot Rollers
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Vibratory Rollers

• Efficient in compacting granular soils.
• Vibrators can be attached to smooth wheel,
  pneumatic rubber-tired, or sheepsfoot rollers to
  send vibrations into the soil being compacted.

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Dynamic Compaction

• Is a technique that is beginning to gain
  popularity in United States for densification of
  granular soil deposits.
• This process primarily involves dropping a heavy
  weight repeatedly on the ground at regular
  intervals.
• The weight of the hammer used varies from 8-35
  metric tons and the height of the hammer drop
  varies between 7.5 and 30.5 m. The stress waves
  generated by the hammer drops help in the
  densification.

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Dynamic Compaction

• The degree of compaction achieved depends on
  the-
• a) weight of the hammer
• b) height of the hammer
• c) spacing of the locations at which the hammer
  is dropped

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Dynamic Compaction

• Densification by dynamic compaction is performed
  by dropping a heavy weight of steel or concrete
  in a grid pattern from heights of 30 to 100 ft.
  It provides an economical way of improving soil
  for mitigation of liquefaction hazards.
• Local liquefaction can be initiated beneath the
  drop point making it easier for the sand grains
  to densify. When the excess pore water pressure
  from the dynamic loading dissipates, additional
  densification occurs.
• As illustrated in the photograph, however, the
  process is somewhat invasive the surface of the
  soil may require shallow compaction with possible
  addition of granular fill following dynamic
  compaction.

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Dynamic Compaction
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Dynamic Compaction
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• Techniques Dynamic compaction
• The dynamic compaction technique is used to
  densify the ground to great depths thanks to the
  creation of very high energy waves. The technique
  was invented and developped by Mr. Louis MENARD
  and MENARD .

Source www.menard-soltraitement.com/
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• It requires the use of pounders weighing 15 to 40
  tons released in free fall from a height of 10 to
  40 meters. The arrangement of the impact points
  and the other parameters of the treatment
  (energies, phasing, rest periods) depend on the
  characteristics of the soil to be treated and on
  the results of the trial zone. This ground
  treatment process is used for the foundations of
  buildings, or to stabilise large areas of
  embankment work or loose soil.

Source www.menard-soltraitement.com/
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• Saint-Etienne Industrial and Business Park
• The site is an old mining discovery zone filled
  in over around sixty meters deep. A total
  surface of six hectares are foreseen as building
  area. The settlements monitored over a six month
  period thanks to 21 measuring points ranged from
  5 to 43 cm.

Source www.menard-soltraitement.com/
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• The ground improvement solution chosen was deep
  dynamic compaction with an energy of 700 t.m,
  combined with standard 300 t.m compaction The
  soil characteristics of the worst zone were as
  follows up to 12 meters deep 0.37 MPa lt Pl-Po
  lt 0.86 MPa 3.0 MPa lt Ep lt 8.8 MPa

Source www.menard-soltraitement.com/
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Vibroflotation

• Is a technique developed in Germany in 1930s for
  in-situ densification of thick layers of loose
  granular soil deposits.
• The process involves the use of a vibroflot
  (called vibrating unit), which is about 2 m in
  length.
• The vibrating unit has an eccentric weight inside
  it and can develop a centrifugal force.

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Vibroflotation

• The weight enables the vibrating unit to vibrate
  horizontally.
• There are openings at the bottom and top of the
  vibrating unit for water jets.
• The vibrating unit is attached to a follow up
  pipe.

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Vibroflotation

• Vibroflotation involves the use of a vibrating
  probe that can penetrate granular soil to depths
  of over 100 feet. The vibrations of the probe
  cause the grain structure to collapse thereby
  densifying the soil surrounding the probe. To
  treat an area of potentially liquefiable soil,
  the vibroflot is raised and lowered in a grid
  pattern. Vibro Replacement is a combination of
  vibroflotation with a gravel backfill resulting
  in stone columns, which not only increases the
  amount of densificton, but provides a degree of
  reinforcement and a potentially effective means
  of drainage.

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Vibroflotation
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Vibroflotation
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Vibroflotation

• The entire compaction process can be divided into
  4 stages-
• The jet at the bottom of the vibroflot is turned
  on, and the vibroflot is lowered into the ground.
• The water jet creates a quick condition in the
  soil, which allows the vibrating unit to sink.
• Granular material is poured into the top of the
  hole. The water from the lower jet is transferred
  to the jet at the top of the vibrating unit. This
  water carries the granular material down the
  hole.
• The vibrating unit is gradually raised in about
  0.3 m lifts and held vibrating for about 30
  seconds at a time. This process compacts the soil
  to the desired unit weight.

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Vibroflotation

• The capacity of successful densification of in
  situ soil depends on several factors, the most
  important of which is the grain-size distribution
  of the soil and also the nature of the backfill
  used to fill the holes during the withdrawal
  period of the vibroflot.

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Vibroflotation
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• Techniques Vibroflotation
• The compaction of a loose soil or a loose
  backfill can be achieved at high depth by
  penetration of a vibrator. In granular soils,
  this penetration results in a liquefaction of the
  surrounding soil and a quasi immediate
  settlement. Further to the results of a trial
  zone, Menard defines - the vibrator frequency
  - the power of the vibrator - the requested
  period of vibration for each different soil layer

Source www.menard-soltraitement.com/
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• PASIR PAJANG CONTAINER TERMINAL (Singapore)
• In 1996, Menard was awarded the soil improvement
  works of a reclamation area made of hydraulic
  fill on a depth of about 10 m. The aim of this
  soil improvement was to densify the hydraulic
  backfill, increase the bearing capacity and
  reduce the liquefaction potential.

Source www.menard-soltraitement.com/
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• The technical solution adopted by Menard was a
  combined treatment made of dynamic compaction for
  the first 5 m and vibroflotation for the layer 5
  to 10 m. The acceptance criteria defined by the
  client was a value of qc higher than 15 MPa over
  the 10 m to be densified. A geotechnical campaign
  made of 46 CPTs before the works and 60 CPTs
  after the works has shown the improvement of the
  hydraulic backfill and has confirmed this
  acceptance criteria. .

Source www.menard-soltraitement.com/
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Source www.menard-soltraitement.com/