Design and construction for ground improvement - Guidelines, Part 1: Stone columns

Scope of IS 15284 Part 1: Stone Columns for Ground Improvement

Key Points from Clause 3.1 and Annex A:

• Column Spacing Determination (Table A-3):

• Load Capacity Estimation (Annex A, Clause 9.3.2):

Load capacity of stone column treated ground =
a) Lateral resistance of surrounding soil against bulging +
b) Additional resistance from soil surcharge +
c) Bearing support from soil between columns

• References for Foundation Design:

Summary:

• Determine column spacing based on plan area and pattern.
• Use combined soil-column interaction to estimate load capacity.
• Refer to Annex A for detailed load capacity formulas.
• Use referenced IS codes for foundation design and settlement.

flowchart TD
    A[Plan Area & Number of Columns] --> B[Calculate Area per Column]
    B --> C{Column Pattern}
    C -->|Triangular| D[Area = 0.866 × S²]
    C -->|Square| E[Area = 1.0 × S²]
    D --> F[Determine Effective Spacing S]
    E --> F
    F --> G[Estimate Load Capacity]
    G --> H[Lateral Resistance]
    G --> I[Surcharge Resistance]
    G --> J[Bearing Support]

This concise framework guides the scope and application of IS 15284 Part 1 for stone column design.

IS 15284 Part 1: Key References & Formulas

References (Table 2)

Column Spacing (Clause 3.1, Table A-3)

Load Capacity Estimation (Annex A, Clause 9.3.2)

For stone columns in cohesive soils, total load capacity =

[ Q = Q_{\text{column bulging}} + Q_{\text{surcharge}} + Q_{\text{intervening soil}} ]

• (Q_{\text{column bulging}}): Resistance from lateral soil confinement.
• (Q_{\text{surcharge}}): Additional resistance due to surcharge load.
• (Q_{\text{intervening soil}}): Bearing from soil between columns.

Additional Notes

• Settlement estimation procedures are in Annex B.
• Use alternate proven formulas based on soil and installation methods.
• Refer Annex D for typical stone column data sheet format.

flowchart TD
    A[Load Capacity of Stone Column] --> B[Resistance by Surrounding Soil (Bulging)]
    A --> C[Resistance due to Surcharge]
    A --> D[Bearing Support by Intervening Soil]

This summary provides essential references, spacing formulas, and load capacity components as per IS 15284 Part 1.

IS 15284 Part 1: Key Definitions & Specifications Summary

References (Table 2):

• IS 1892:1979 – Subsurface investigations for foundations
• IS 6403:1981 – Bearing capacity of shallow foundations
• IS 8009 (Part 1):1976 – Settlement calculation for shallow foundations

Column Spacing (Clause 3.1, Table A-3)

• S = effective spacing between columns
• A = plan area per column

Load Capacity Estimation (Annex A, Clause 9.3.2)

For stone columns in cohesive soils, total load capacity (Q) is:

[ Q = Q_{bulging} + Q_{surcharge} + Q_{soil} ]

Where:

• (Q_{bulging}) = resistance from soil against lateral bulging of stone column
• (Q_{surcharge}) = increased resistance due to surcharge load
• (Q_{soil}) = bearing capacity of soil between columns

Practical Notes

• Use Annex A for detailed calculation procedures.
• Alternate reliable formulas may be applied based on site-specific soil conditions.
• Refer Annex D for standardized stone column data recording sheets.

flowchart TD
    A[Plan Area & Number of Columns] --> B[Calculate Area per Column]
    B --> C{Column Pattern}
    C -->|Triangular| D[Area = 0.866 × S²]
    C -->|Square| E[Area = 1.0 × S²]
    D --> F[Calculate Spacing S]
    E --> F
    F --> G[Estimate Load Capacity Q]
    G --> H[Q = Q_bulging + Q_surcharge + Q_soil]

This concise summary covers the definitions, key formulas, and references for stone column design per IS 15284 Part

IS 15284 Part 1 - Necessary Information (Clause 4 & related)

For design and installation of stone columns, the following key data is essential:

• Soil Investigation Data
  As per IS 1892, including borelogs, penetration tests, in-situ tests to identify soil profile, index properties, shear strength, and compressibility.

• Groundwater Level
  Accurate groundwater table and its seasonal variations.

• Structural Layout
  Foundation system, loading pattern, and intensity from structural analysis.

• Nearby Structures
  Information on adjacent constructions to assess interaction effects.

Key Formula from Clause 3.1 (Table A-3): Area per Column

Where:

• ( S ) = effective spacing between columns

References for Further Design

Summary Diagram: Input Data Flow for Stone Column Design

flowchart TD
    A[Soil Investigation Data (IS 1892)] --> D[Design & Installation]
    B[Ground Water Level] --> D
    C[Structural Layout & Load] --> D
    E[Nearby Structures Info] --> D

This ensures comprehensive assessment for safe, effective stone column design and installation.

IS 15284 Part 1 (2003) - Equipment and Accessories: Key Formulas & Specifications

Load Capacity Estimation (Annex A, Clause 9.3.2)

For stone columns in cohesive soils, the total load capacity (Q_total) is the sum of:

• Q1: Capacity from lateral soil resistance against column bulging
• Q2: Capacity from surcharge-induced soil resistance increase
• Q3: Bearing support from intervening soil between columns

[ Q_{total} = Q_1 + Q_2 + Q_3 ]

Typical Data Sheet (Annex D, Clause 14.2)

• Records stone column dimensions, installation method, soil properties, and load test results.
• Essential for quality control and design verification.

Practical Notes:

• Alternate formulas may be used if proven reliable for specific subsoil and installation methods.
• Settlement estimation is detailed in Annex B.

Summary Table: Load Capacity Components

flowchart TD
    A[Stone Column Load Capacity] --> B[Q1: Lateral Soil Resistance]
    A --> C[Q2: Surcharge-induced Soil Resistance]
    A --> D[Q3: Bearing Support from Soil Between Columns]

This approach ensures a comprehensive evaluation of stone column performance under load per IS 15284 Part 1.

Important Features of Stone Column Treatment (IS 15284 Part 1)

Key Points:

• Stone columns improve soil bearing by replacing weak soil with compacted granular columns.
• Load capacity depends on:
  • Lateral soil resistance against bulging,
  • Increased surcharge resistance,
  • Bearing support from intervening soil.
• Settlement reduction uses stress concentration factor (n) and replacement ratio (a).

Key Formulas (Annex A):

Let:

• ( A_c ) = area of stone columns,
• ( A_g ) = area of untreated soil,
• ( \sigma ) = applied vertical stress,
• ( \sigma_c ), ( \sigma_g ) = stress in column and soil respectively,
• ( a = \frac{A_c}{A_c + A_g} ) (replacement ratio),
• ( n = \frac{\sigma_c}{\sigma_g} ) (stress concentration factor),
• ( S ) = settlement,
• ( m ) = modulus of volume decrease of soil,
• ( H ) = thickness of treated soil.

Stress sharing: [ (A_c + A_g) \sigma = A_c \sigma_c + A_g \sigma_g ] [ \Rightarrow \sigma_c = n \sigma_g ]

Settlement of treated soil: [ S = \frac{H \sigma_g}{m} = \frac{H \sigma}{m (1 + a(n - 1))} ]

Settlement reduction ratio: [ \beta = \frac{S}{S_0} = \frac{1}{1 + a (n - 1)} ] where ( S_0 ) is settlement of untreated soil.

Tables & Data:

• Use soil investigation data as per IS 1892.
• Refer to Annex D for typical stone column data sheets.
• Installation techniques detailed in Annex C (e.g., bailer and casing for non-displacement).

Summary Diagram:

flowchart TD
    A[Applied Load] --> B[Stress Sharing]
    B --> C[Stone Column Stress (σ_c)]
    B --> D[Surrounding Soil Stress (σ_g)]
    C & D --> E[Settlement of Treated Soil]
    E --> F[Reduced Settlement (β)]
``

IS 15284 (Part 1) - Design Considerations: Key Points

1. Environmental Factors (Clause 9.4)

• Consider aggressive chemicals in soil/groundwater.
• Account for artesian conditions.
• Assess soil type: very soft clay/silt prone to shear or bulging failure.
• Refer to Fig. 4 for stone column failure mechanisms in non-homogeneous cohesive soils:
  • Localized bulging in soft layers.
  • Shear failure in thick very soft layers.

2. Load Capacity & Settlement (Clause 9.3.2)

• Use procedures in Annex A (load capacity) and Annex B (settlement).
• Alternate validated formulae may be used based on soil and installation.

3. Mixed Soils (Clause 8.5)

• Check failure for both sandy and clayey soils.
• Use the lower value of the two for design.

Typical formula for load capacity of a single stone column (from Annex A):

[ Q_{sc} = A_{sc} \times q_{sc} ]

Where:

• (Q_{sc}) = load capacity of stone column
• (A_{sc}) = cross-sectional area of stone column
• (q_{sc}) = allowable bearing pressure of stone column material

Failure Mechanisms Illustration (Fig. 4 summary):

graph TD
  A[Very Soft Clay/Silt] --> B[Shear Failure]
  A --> C[Bulging Failure]
  C --> D[Localized Bulge]
  C --> E[Contained Local Bulge]
  C --> F[Local Bulging Failure]

Summary:
Design must integrate soil aggressiveness, soil layering, and dual soil-type failure checks, using IS 15284 Annex procedures for capacity and settlement.

IS 15284 Part 1: Failure Mechanisms Key Points

1. Failure Mechanisms in Stone Columns (Clause 8 & Fig. 4)

• Failure modes depend on soil type and layering, especially in soft/very soft clays or silts.
• Typical failures:
  • Shear Failure: Occurs in soft clay/silt where columns shear off.
  • Bulging Failure: Localized lateral expansion of stone columns.
  • Localized Bulge: In thin very soft layers, bulging is contained.
  • Limited Bulge: In very soft clay with organics, bulging is limited.

Fig.4 illustrates failure types:

• 4A: Soft surface layer - bulging or shear.
• 4B: Thin very soft layer - contained bulge.
• 4C: Thick very soft layer - local bulging.

2. Stress Concentration Factor, n (Clause 7.6.3)

• Calculated using elastic theory considering modular ratio (stone/clay).
• Modular ratio, ( m = \frac{E_{stone}}{E_{clay}} ).
• ( n ) depends on ( m ) and vertical displacement compatibility.
• Values should be selected from Table 7.6.2 (not provided here).

3. Factor of Safety (Clause 9.6)

• Must consider soil strength, column strength, and environmental factors.
• Typical FoS ranges from 1.5 to 3 depending on conditions.

4. Environmental Factors (Clause 9.4)

• Consider aggressive chemicals, artesian pressure, groundwater conditions.
• These affect durability and design of stone columns.

Summary Table (Conceptual)

IS 15284 Part 1: Design Considerations - Key Points

1. Environmental Factors (Clause 9.4)

• Account for aggressive chemicals in soil/groundwater.
• Consider artesian pressure and soil variability.
• Design must ensure durability against chemical attack and water conditions.

2. Load Capacity & Settlement (Clause 9.3.2)

• Use procedures in Annex A (Load Capacity) and Annex B (Settlement).
• Alternate validated formulas may be applied based on soil and installation.

3. Failure Modes in Cohesive Soils (Fig. 4)

• Very Soft Clay/Silt: Shear or bulging failure.
• Soft Clay/Silt: Localized bulging or shear failure.
• Failure depends on thickness and softness of layers:
  • 4A: Soft surface layer — bulging/shear.
  • 4B: Thin very soft layer — localized bulge.
  • 4C: Thick very soft layer — bulging failure.

4. Mixed Soils (Clause 8.5)

• Check failure for both sandy and clayey soils.
• Use the lower of the two calculated failure values for design.

Typical Formula for Stone Column Load Capacity (from Annex A):

[ Q = A_c \times q_c + A_s \times q_s ] Where:

• ( Q ) = Load capacity
• ( A_c ) = Area of improved soil
• ( q_c ) = Bearing capacity of improved soil
• ( A_s ) = Area of stone column
• ( q_s ) = Bearing capacity of stone column

Summary Diagram: Failure Modes in Cohesive Soil

graph TD
  A[Very Soft Clay/Silt] -->|Shear or Bulging| B[Failure]
  C[Soft Clay/Silt] -->|Localized Bulging or Shear| B
  D[Soft Surface Layer (4A)] -->|Bulging or Shear| B
  E[Thin Very Soft Layer (4B)] -->|Localized Bulge| B
  F[Thick Very Soft Layer (4C)] -->|Bulging Failure| B

Ensure detailed soil investigation and environmental assessment for reliable design.

IS 15284 Part 1: Construction Procedures for Stone Columns

Key Points & References:

• Load Capacity & Settlement Estimation

  • Refer Annex A for load capacity formulas.
  • Refer Annex B for settlement estimation methods.
  • Alternate reliable formulas may be used based on soil and installation.

• Factor of Safety

  • Clause 9.6.3: Apply minimum factor of safety as recommended in soil mechanics-based capacity formulas.

• Installation Techniques (Clause 10 & Annex C)

  • Create a borehole, fill with granular material (stone/stone-sand mixture).
  • Compact the fill to form a dense column with required strength.
  • Installation methods include vibro-replacement, vibro-displacement, or other approved techniques.

• Quality Control (Clause 12.1)

  • For boring methods, monitor boring criteria and granular fill consumption strictly.
  • Ensures column integrity and uniformity.

Typical Load Capacity Formula (Annex A example):

[ Q_u = A_c \times q_c + A_s \times q_s ]

• (Q_u): Ultimate load capacity
• (A_c): Cross-sectional area of stone column
• (q_c): Allowable stress in stone column
• (A_s): Area of soil surrounding column influenced
• (q_s): Allowable stress in soil

Simplified Installation Process Flowchart

flowchart TD
    A[Start: Site Investigation] --> B[Mark Column Locations]
    B --> C[Drill Borehole]
    C --> D[Fill with Granular Material]
    D --> E[Compact Material]
    E --> F[Check Quality Control]
    F --> G[Complete Column Construction]

For detailed formulas and proportions, refer to Annex A, B, and C of IS 15284 (Part 1): 2003.

IS 15284 Part 1: Field Controls Summary

Key Field Controls for Vibroflots (Clause 12.2)

• Penetration Depth: Measure total depth including embedment in firm strata.
• Backfill Volume Monitoring: Track volume added to estimate achieved density.
• Ammeter/Hydraulic Pressure Monitoring: Use readings to confirm maximum density in vibrofloted columns.

Additional Field Controls (Clause 11.3)

• Remove slush after boring.
• Compact minimum 0.5 m (preferably 0.75 m) below granular blanket by rolling/tamping to meet densification criteria.

Load Capacity & Settlement Estimation (Clause 9.3.2)

• Use procedures in Annex A (Load capacity) and Annex B (Settlement).
• Alternate reliable formulas may be used based on soil and installation method.

Column Diameter Measurement (Clause 1.1)

• Measure diameter at uppermost part over a depth of 4 times the diameter.
• Average these measurements for column diameter used in fill consumption calculations.

Typical Formula (from Annex A/B - simplified):

[ Q_{ult} = q_c \times A_c + q_s \times A_s ]

Where:

• (Q_{ult}) = Ultimate load capacity
• (q_c) = End bearing resistance
• (A_c) = Cross-sectional area of column base
• (q_s) = Skin friction resistance
• (A_s) = Surface area of column shaft

flowchart TD
    A[Vibroflot Installation] --> B[Measure Penetration Depth]
    B --> C[Monitor Backfill Volume]
    C --> D[Check Ammeter/Pressure Readings]
    D --> E[Confirm Max Density Achieved]
    E --> F[Estimate Load & Settlement (Annex A/B)]

IS 15284 (Part 1) - Load Testing Key Points

Load Test Intensity & Kentledge (Clause 1.1 k)

• Test 1 column per 625 m² area.
• Test load intensity = 1.1 × Design load intensity.
• Minimum kentledge = 1.3 × Design load.

Load Application & Duration (Clause 13.7 f, g, h)

• Maintain design & max test load for ≥12 hours after settlement stabilizes.
• Settlement readings at intervals:
  1, 2, 4, 8, 16 min; 0.5, 1, 1.5, 2, 3, 4 h ... until stable.
• Unload in 5 stages, allowing settlement stabilization at each.

Acceptance Criteria (Clause 13.7 j)

Estimation Procedures (Clause 9.3.2)

• Refer Annex A (Load capacity) & Annex B (Settlement).
• Alternate formulas allowed if proven reliable.

Typical Setup (Fig. 5 & 6)

• 3.36 m PCC footing or stiff steel plate.
• Dial gauges (4 Nos) for settlement measurement.
• 30 cm sand blanket over gravelly clay.
• Load applied via kentledge or hydraulic jack.

Summary Table: Load Test Parameters

flowchart TD
    A[Start Load Test] --> B[Apply Load in Increments]
    B --> C[Record Settlement at intervals]
    C --> D{Settlement Stabilized?}
    D -- No --> C
    D -- Yes --> E[Maintain Load for 12

IS 15284 Part 1: Key Points on Adjacent Structures & Environmental Factors

1. Adjacent Structures (Clause 9.2 & 9.2.1)

• Precaution: Take care to avoid damage to existing nearby structures during construction.
• Measures: Use suitable construction methods, monitoring, and protective systems to prevent settlement or lateral displacement.

2. Environmental Factors (Clause 9.4)

• Consider aggressive chemicals in soil/groundwater, artesian conditions, etc.
• These factors influence material durability and design safety.

3. Stone Column Spacing (Clause 3.1 & Table A-3)

• Use: Determine effective spacing based on plan area and number of columns.

4. Failure Mechanisms (Fig. 4)

• In non-homogeneous cohesive soils, stone columns may fail by:
  • Shear failure
  • Bulging (localized or general)
• Soil types like very soft clay/silt or organics are critical for design.

flowchart LR
    A[Adjacent Structures] --> B[Damage Prevention]
    B --> C[Monitoring]
    B --> D[Protective Measures]
    E[Environmental Factors] --> F[Aggressive Chemicals]
    E --> G[Artesian Conditions]
    H[Stone Column Spacing] --> I[Triangular Pattern]
    H --> J[Square Pattern]

Summary:

• Protect adjacent structures by careful construction (Clause 9.2.1).
• Account for environmental aggressiveness (Clause 9.4).
• Use spacing formulas from Table A-3 for stone columns.
• Understand failure modes in soft soils for safe design.

IS 15284 (Part 1) - Recording of Data for Stone Columns

Key Specifications (Clause 14.1)

A competent inspector must record:

• Sequence of installation of stone columns in the group
• Spacing between stone columns
• Type and size of rammer and its stroke
• Dimensions of stone columns (diameter, length)
• Depth of installation
• Blow counts for each stone charge and total blows for compaction
• Final set for last 5 blows (or as specified) per stone charge
• Time taken for each stone column installation including casing withdrawal
• Any other important observations during and after installation

Typical Data Sheet (Annex D)

• Includes fields for all above parameters to ensure systematic data recording.

Load Capacity Estimation (Annex A)

For stone columns in cohesive soils, total load capacity ( Q ) is sum of:

[ Q = Q_c + Q_s + Q_i ]

• ( Q_c ): Capacity from lateral soil resistance against bulging
• ( Q_s ): Capacity from surcharge-induced soil resistance
• ( Q_i ): Bearing support from intervening soil between columns

References for Further Design & Testing

Summary Diagram of Data Recording Process

flowchart TD
    A[Start Installation] --> B[Record Sequence & Spacing]
    B --> C[Record Rammer Type & Stroke]
    C --> D[Measure Dimensions & Depth]
    D --> E[Record Blow Counts per Stone Charge]
    E --> F[Record Final Set for Last Blows]
    F --> G[Record Time Taken]
    G --> H[Note Other Observations]
    H --> I[Complete Data Sheet]

This systematic recording ensures quality control and reliable design based on IS 15284 (Part 1).

Estimation of Load Capacity of a Stone Column
(IS 15284 Part 1, Annex A, Clause 9.3.2)

Key Formulas

1. Limiting axial stress in stone column due to bulging:

[ \sigma = \sigma_{HL} + K_{Pcol} \sigma_o = (\sigma_{0} + 4cL) K_{Pcol} ]

• (\sigma) = limiting axial stress in column
• (\sigma_0 = K_0 \sigma'_v) = initial radial effective stress
• (cL) = undrained shear strength of surrounding clay
• (K_0 = 0.6) (for clays) or (K_0 = 1 - \sin \phi) (for frictional soils)
• (\sigma'_v = \gamma' \times 2D) (average vertical effective stress at depth (2D))
• (K_{Pcol} = \tan^2(45^\circ + \phi_c/2)) (passive earth pressure coefficient of column)
• (\phi_c) = internal friction angle of column material (typically 38°–42°)

2. Safe load on column:

[ Q_1 = \frac{\sigma \times \pi D^2 / 4}{\text{Factor of Safety}} = \frac{\sigma \times \pi D^2 / 4}{2} ]

• (D) = diameter of stone column
• Factor of Safety = 2 (minimum recommended)

Notes:

• (\gamma') = effective unit weight of soil within influence zone
• Average bulge depth considered = (2D)
• Ultimate capacity should ideally be verified by load test with FOS ≥ 2.5 (Clause 9.6.2)
• Total bearing capacity includes contributions from:
  • Stone column bulging resistance
  • Surcharge effect on surrounding soil
  • Bearing of intervening soil between columns

Typical Data Sheet Reference:

See Annex D of IS 15284 Part 1 for stone column recording format.

flowchart TD
    A[Stone Column Load Capacity] --> B[Bulging Resistance]
    A --> C[Surcharge