Summary of Soil Improvement Method under Precompression as per IS: 13094:

The following soil improvement methods under precompression are generally adopted in the different construction project as per the requirement depending upon the existing soil condition.


Vertical drain:



Advantage of Vertical Drain:

  • Acceleration of primary consolidation of Soft Soil
  • The gain in in-situ shear strength of soft soil
  • Applicable in case of non-replaceable large depth of Soft Soil


The efficiency of Vertical Drain:

  • May be maximum up to 80% depending upon the site condition


Types of Vertical Drain:

There are four types of vertical drain used for ground improvement technique.


Soil Improvement using Sand Drain:

  • Diameter – 150 mm to 500 mm
  • Spacing – 1.50 m to 6.00 m
  • Installation – Closed Mandrel or open mandrel method
  • The action of the granular pile in soft soil in case of large diameter sand drain is used to improve the soil condition.


Soil Improvement using Sand Wicks:

  • Small diameter sand drain prepacked in filter stocking (woven jute canvas)
  • Economic than large diameter sand drain
  • Simplest Method – Hand auger (for limited depth)


Soil Improvement using Cardboard Drains:

  • Dimension – 100 mm x 3 mm
  • Installation by mandrel which is removed after placing up to the desired depth
  • Little soil disturbance, long life and durable


Soil Improvement using Geosynthetic Drains:

  • Updated cardboard drain – Prefabricated vertical band drain (PVBD)
  • Dimension – 100 mm x 2.5 mm to 7.0 mm
  • Installation – Displacement method by the mandrel
  • Large depth installation – Maybe up to 25.00 to 30.00 m


Installation Rig for Geosynthetic Drain – STICHER (Mechanical & Hydraulic): 

  • It is a Sophisticated method of soil improvement
  • It is a fast installation process


Soil Improvement using Granular Drainage Blanket:

  • Laid over top of soft ground
  • Consists of clean medium to coarse sand in the layered, compacted manner (75 – 80 % Relative density)
  • Minimum thickness – 0.500 m
  • Minimum insertion of top of band drain inside drainage blanket – Half of the blanket thickness


Spacing and depth Of Vertical Drain:

  • Square or Triangular pattern – 1.00 m to 4.00 m
  • Economical depth of treatment – 5.00 m to 20.00 m


Theoretical Considerations:

  • Determination of drain spacing for the required degree of consolidation in a particular period of time (i.e. Waiting Period) for a known type of drain
  • Analysis based on the three-dimensional consolidation process
  • Consideration of the smearing effect
  • Required Drainage parameters – Coefficient of consolidation both horizontal and vertical direction of soft foundation soil in addition to the vertical drain characteristics (discharge capacity)


Summary of Reinforcement Soil Improvement Method as per IS: 13094:

The following soil improvement methods are generally adopted in the different construction project as per the requirement depending upon the existing soil condition.

Soil Improvement using stone column:

It is another method of ground improvement technique under in-situ soil reinforcement technique.

Stone column
Stone Column


Advantage of Stone Column:

  • Increase in load-carrying capacity of soft ground
  • Reduction in total and differential settlements
  • Increase in the resistance of soft foundation soil to liquefaction
  • Improvement in slope stability


Stone Column Installation and Construction Method:

  • Non-displacement Method 

-Bailer and casing method

-Direct mud circulation method

-Rotatory drill method

  • Displacement method
  • Vibro-replacement method

-Wet process

-Dry process


Salient features of Stone Column treatment:

  • Influence of soil type –

(i) Undrained shear strength of Clay 7.0-50.0 kPa and sensitivity < 4.0

(ii) Loose sandy soil

(iii) Not Suitable in the absence of desired bearing strata under the soft soil

  • Influence of construction methodology
  • Treatment Depth – Maximum 15.00 to 25.00 m in INDIA
  • Area of Treatment – Economic used for a large area of the soil mass
  • Termination – Preferably over or into the firm, dense strata
  • Arrangement Pattern – Triangular, square & hexagonal
  • Spacing – Generally 1.20 m to 2.50 m


The failure mechanism of the Stone Column:


The single stone column in a homogeneous soft layer:

  • Long Stone Column with Firm or Floating Support – Bulging Failure
  • Short Column with Rigid Base – Shear Failure
  • Short Floating Column – Punching Failure (Shear Failure could also occur)


The single stone column in a non-Homogeneous soft layer:

  • The soft layer at the surface – Bulging or shear failure
  • Thin very soft layer and or organics – Contained local bulging
  • Thick very soft layer and or organics – Local bulging failure


Design estimation of the load-carrying capacity of the Stone column:

  • Capacity based on “Bulging of Column.”
  • Capacity resulting from “Surcharge Effect.”
  • Bearing support provided by the intervening soil
  • The factor of safety over estimated Ultimate load – Minimum 2.50


Design estimation of settlement by the reduced stress method:

  • Estimation of the settlement of untreated ground as per IS:8009
  • Estimation of “Area Replacement Ratio (as)”
  • Estimation “Stress Concentration Factor (n)”
  • Estimation of “Reduction Ratio (b)”


Field Load Test:

  • Single Column test
  • Three column group test


Field Controls:


In “Non-Displacement Installation Method”

  • Boring operation
  • Set criteria of ramming for the measurement of its extent
  • Consumption of granular fill


In “Method using Vibrofloat.”

  • Penetration depth including the embedment in the firm strata
  • Monitoring volume of backfill for getting an indication of density achieved
  • Monitoring ammeter or hydraulic pressure gauge readings for the verification of maximum possible density achieved


Soil Improvement using Granular Blanket:

  • Laid over top of stone column treated ground
  • Consists of clean medium to coarse sand in the layered, compacted manner (75 – 80 % Relative density)
  • Minimum thickness – 0.500 m

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Vertical Drain and Stone Column for Ground Improvement
Article Name
Vertical Drain and Stone Column for Ground Improvement
Soil Improvement Method: (1) Pre-compression of in-situ soil: Vertical drain with or without stage construction (2) In-situ soil reinforcement: Stone column
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Construction Civil
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