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When a Civil Engineering project encounters severe foundation condition, there are two possible alternative solutions are there; number one is avoiding the particular site, or number two is improving the existing ground. There are several methods of ground improvement techniques, as described below.

Types of Ground Improvement techniques:

1.1

Removal and replacement of poor in-situ sub-soil with competent backfill:

  • The said ground improvement technique is applicable for Replacement of Weak Compressible Cohesive Soil say Clayey/Silty Sub-Soil
  • Limited Depth of Application, Generally up to Top of GWT says up to 3.00 m from OGL
  • Involvement of Dewatering if require to extend beyond GWT
  • Replacement with Selective Compacted Cohesionless Non-Plastic (NP) Granular (Sand/Gravel) Soil
  • In case of in-situ Loose NP Cohesionless Sub-Soil, Loosening and Re-compaction of in-situ soil up to the desired depth
  • Obtained “Increased Strength and Stiffness” and also reduced the compressibility which is resulted in Uniform Controlled Foundation Bed
Ground Improvement techniques
Ground Improvement techniques

1.2

Ground Improvement techniques using In-situ soil densification:

  • By application of Shock and Vibration to the subsoil and thereby causing rearrangement of the soil structure from a loose to medium due to dense state
  • Applicable especially for Cohesionless Soils under high water table conditions
  • Methods namely Vibroflotation, Vibrocompaction, Compaction Piles, Blasting and Dynamic Compaction (in Cohesionless Soil) / Dynamic Consolidation (in Cohesive Soil)
  • Maximum Depth of Treatment maybe even beyond 30 m
  • Involvement of Special Equipment in case of Vibroflotation, Vibrocompaction Methods
  • Low to Moderate Level of Cost in these methods of Improvement

1.3

Ground Improvement techniques using Pre-compression / pre-consolidation of in-situ soil:

  • Expulsion of water from the pores causes consolidation of the Cohesive (Clayey/Silty) soil thereby resulting in the buildup of shear strength and substantially reduced values of final settlements of foundations
  • Achieved by pre-compression of the subsoil by subjecting the area to a preload
  • Application of Preloading may be in stages to allow the gradual buildup of soil strength enabling it to support further stages of preload safely
  • Acceleration of pre-consolidation by the provision of vertical drainage channels for Poorly Draining Soils such as Soft Clays
  • Methods namely Preloading with or without Vertical Drains, namely Prefabricated Vertical Band Drains (PVBD), Sand Column, Stone Column, Electro-osmosis etc.

1.4

Ground Improvement techniques using Injections and grouting into in-situ soil:

  • Injection of chemicals, lime, cement etc. into subsoils improve subsoil by the formation of bonds between soil particles
  • Mechanical compression of the subsoil is also achieved under certain conditions provided grout is pumped in under high pressure
  • Available methods are suitable for sands as well as fine-grained soils
  • Formed Impervious formation with increased strength and reduced compressibility which subsequently elimination of Liquefaction Danger
  • Methods namely Particulate Grouting, Chemical Grouting, Displacement Grouting, Pressure injected Lime, Jet Grouting etc.
  • Moderate to High Level of Cost in these methods of improvement.

1.5

Ground Improvement techniques using In-situ soil reinforcement:

  • Reinforcement introduced into the in-situ soil mass causes marked improvement in stiffness and consequently load carrying capacity and stability of soil mass
  • Reinforcements may be in the form of Dense Granular materials in the form of Stone Column for primary requirement of increased capability of carrying the vertical loads
  • Reinforcements may also be in the form of Horizontal or Vertical Strips and Membranes to increase the capacity of soil to withstand tensile, shear and compression loads and contribute towards improvement of stability of soil mass significantly
  • Methods namely Rammed or Vibroreplacement Stone Column, Sand Columns, Root Piles, Soil Nailing etc.
  • Low/Moderate to High Level of Cost in these methods of improvement.

1.6

Ground Improvement techniques using Admixtures:

Modification of any one or more of the soil properties to improve the desired performance of the soil (mostly Expansive Soil)

1.6.1

Objective:

  • To increase the strength of soil
  • To decrease shrink-swell characteristics of soil

1.6.2

Commonly used chemicals for ground improvement:

  • Lime
  • Cement
  • Combination of Lime & Cement
  • Use of Cohesive-Non-Swelling (CNS) Layer

1.6.2.1

Soil Stabilization with Lime:

  • Addition of Lime (Quick or Hydrated) – 2 nos. Reaction
  • First Reaction – Rapid within an hour or two results in an immediate reduction of plasticity index & correspondingly reduction in shrink-swell behaviour
  • The requirement of Lime for First Reaction – 2 to 3% (Lime Fixation Quantity)
  • Second Reaction – Slow within few weeks results in cementing and hardening action & correspondingly increase in strength
  • The requirement of Lime for Second Reaction – 6 to 9 % (Optimum) for a short curing period of 3 to 4 weeks

1.6.2.2

Soil Stabilization with Cement:

  • Suitable for Well Graded Granular Soil
  • Presence of Organic Matter (> 2%) in soil interferes with hydration of cement & hence weakens soil cement stabilization
  • Increase in strength with the more content of cement
  • Optimum Soil Cement Mixing Time – Few Minutes
  • Strength of Soil-Cement increases with curing period like Concrete
  • Rapid Strength Gain of Soil-Cement in moist and high-temperature condition
  • Gain in More Strength and Durability of Soil-Cement mixture, in somewhat higher moisture content (2 to 4 %) during field compaction

1.6.2.3

Soil Stabilization with Lime & Cement:

  • Combination Suitable for Expansive Soil
  • For a given lime content, strength increases with increasing cement content
  • Lime content is optimized for maximum strength with a specified amount of cement content and with specified curing period1

1.6.2.4

Soil Stabilization with CNS Soil layer:

  • CNS Soil – Clayey soil with certain cohesion but having no expansive characteristics, e.g. Locally Available Moorum
  • Very effective in counteracting shrink-swell characteristics of and retaining shear strength of expansive soil
  • Widely used in various Civil Engineering Construction including Canals, Roads etc.
  • Limited thickness of around 1.00 m CNS layer is required to eliminate the detrimental effects of expansive soil.
  • Reduction in Heave of underlying expansive soil with the increase in the thickness of CNS layer having constant cohesion
  • Reduction in requirement of the thickness of the CNS layer with an increase in cohesion
  • Governing criteria in counter-acting swelling and swelling pressure of expansive soil – Cohesion and thickness of the CNS layer

1.6.2.4.1

The tentative thickness of the CNS layer

The tentative thickness of the CNS layer Required to nullify the effect of Swelling Pressure of Expansive Soil are listed below-

Sl. No.Swelling Pressure of Expansive soil (Kg/Cm2)Tentative Thickness of CNS Soil Layer Required (cm)
11.0 – 1.575 – 85
22.0 – 3.090 – 100
33.5 – 5.0105 – 115

1.7

Thermals Treatment Technique:

Heating and Freezing

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Summary
Different Methods of Ground Improvement Techniques
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Different Methods of Ground Improvement Techniques
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Types of Ground Improvement Techniques: Replacement of poor in-situ sub-soil, In-situ soil densification, Pre-compression / pre-consolidation of in-situ soil, Injections and grouting into in-situ soil, In-situ soil reinforcement and Uses of admixtures.
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Construction Civil
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