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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.

1.0

Vertical drain:

Vertical-drain
Vertical-drain

1.1

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

1.2

The efficiency of Vertical Drain:

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

1.3

Types of Vertical Drain:

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

1.3.1

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.

1.3.2

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)

1.3.3

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

1.3.4

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

1.4

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

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

1.5

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

1.6

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

1.7

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)

2.0

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

2.1

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

2.2

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

2.3

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

2.4

The failure mechanism of the Stone Column:

2.4.1

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)

2.4.2

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

2.5

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

2.6

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)”

2.7

Field Load Test:

  • Single Column test
  • Three column group test

2.8

Field Controls:

2.8.1

In “Non-Displacement Installation Method”

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

2.8.2

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

2.9

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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Summary
Vertical Drain and Stone Column for Ground Improvement
Article Name
Vertical Drain and Stone Column for Ground Improvement
Description
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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