Method for the quantitative description of discontinuities in the rock mass, Part 8: Seepage
1987 Edition

Overview of Scope

• Establishes parameters for describing rock mass structures and discontinuities to evaluate mechanical responses.
• Adopts terminology from IS 11358-1986 relating to rock mechanics.
• Highlights the significance of detecting irregular groundwater levels and perched water tables formed due to impermeable features like dykes or clay-filled fractures.
• Emphasizes criticality in tunneling projects where impermeable barriers can cause elevated water pressures.
• Specifies that rounding and reporting of results comply with IS 2-1960 standards.

Summary Table:

No explicit equations or tables are included in this section; it defines the foundational concepts for subsequent details.

flowchart LR
    RockMass --> Discontinuities
    Discontinuities --> ImpermeableFeatures
    ImpermeableFeatures --> IrregularWaterTables
    IrregularWaterTables --> EngineeringImplications

Summary of Key Definitions

• Terminology is drawn from IS 11358:1986 (Rock Mechanics Glossary).
• Clear understanding of rock mass composition and discontinuity types is essential for mechanical assessment.
• Important concepts include:
  • Rock Mass: Natural assemblage of rock blocks separated by various discontinuities.
  • Discontinuities: Breaks such as joints, faults, dykes that influence rock behavior.
  • Groundwater Tables: Includes irregular and perched water levels caused by impermeable barriers.

Notes:

• Impermeable features can cause pressure build-up during underground works.
• Numerical data rounding should adhere to IS 2-1960 guidelines.

Reference Table: Key Terms

flowchart LR
    RockMass --> Discontinuities
    Discontinuities --> ImpermeableBarriers
    ImpermeableBarriers --> IrregularWaterLevels
    IrregularWaterLevels --> ElevatedPressure

For full definitions refer to IS 11358:1986 as cited by this standard.

General Principles on Seepage

• Focuses on the quantitative description of water movement through rock mass discontinuities.
• Highlights that seepage flow is predominantly controlled by joints, faults, dykes, and clay-filled fractures.
• Notes occurrence of irregular groundwater and perched water tables due to impermeable barriers within the rock mass.
• Predicting these barriers is critical for underground excavations to mitigate high-pressure water inflows.

Key Parameters:

• Hydraulic conductivity (k) of discontinuities.
• Anisotropy in permeability due to discontinuity orientation and continuity.
• Identification of flow barriers caused by impermeable features.
• Variation in pressure heads across discontinuities.

Typical Seepage Flow Equation:

[ Q = k \times A \times \frac{\Delta h}{L} ]

Where:

• (Q): seepage discharge (m³/s)
• (k): hydraulic conductivity (m/s)
• (A): cross-sectional flow area (m²)
• (\Delta h): hydraulic head difference (m)
• (L): flow path length (m)

Practical Guidance:

• Use discontinuity spacing and aperture to estimate permeability.
• Account for layered flow and perched water effects.
• Incorporate impermeable barriers in groundwater modeling.

flowchart LR
    GroundwaterTable --> RockMass
    RockMass --> Discontinuities
    Discontinuities -->|Permeable| FlowPaths
    Discontinuities -->|Impermeable| PerchedWaterTable
    PerchedWaterTable --> PressureBuildUp
    PressureBuildUp --> TunnelFace

This part is fundamental for analyzing seepage related to safe underground construction.

Guidelines for Field Assessment of Seepage

• Seepage is defined as visible water or moisture along rock discontinuity surfaces.
• Visual inspection in well-lit underground areas is primary.
• Supplement with data from aerial photos, rainfall records, spring discharge, and temperature measurements.
• Quantify seepage as an attribute of discontinuities.

Field Procedure:

• Record presence, location, and intensity of seepage.

Seepage Intensity Scale:

• Document environmental variables like rainfall and temperature.

Example Quantitative Expression:

[ Q_s = A \times v ]

where:

• (Q_s): seepage discharge (volume/time)
• (A): seepage cross-sectional area (m²)
• (v): seepage velocity (m/s), estimated via flow observations or tracer tests

flowchart LR
    VisualInspection --> SeepagePresence
    SeepagePresence -- No --> RecordNone
    SeepagePresence -- Yes --> IntensityClassification
    IntensityClassification --> Slight
    IntensityClassification --> Moderate
    IntensityClassification --> Heavy
    IntensityClassification --> RecordEnvData
    RecordEnvData --> QuantitativeAnalysis

Focus is on combining qualitative and quantitative observations for comprehensive seepage evaluation.

Key Points for Presenting Results

• Final numerical results must be rounded following IS 2-1960 rules.
• Assess effects of frost and ice on seepage pathways, as ice blockages can distort seepage observations and impact surface and structural stability.
• Recognize that irregular and perched water tables arise from impermeable discontinuities such as dykes or clay-filled joints.
• Accurate prediction of these features is essential in tunneling to prevent unexpected high-pressure inflows.

Rounding Guidelines (IS 2-1960):

Suggested Reporting Table:

flowchart LR
    RockMass --> Discontinuities
    Discontinuities --> WaterBarriers
    WaterBarriers --> PerchedWaterTables
    Discontinuities --> SeepagePaths
    SeepagePaths --> FrostOrIceCheck
    FrostOrIceCheck -- Yes --> BlockedDrainage
    FrostOrIceCheck -- No --> NormalFlow
    BlockedDrainage --> SurfaceDamageAndInstability

This framework facilitates clear and precise communication of seepage data for engineering decisions.

Influence of Hydrogeology and Weather

• Irregular groundwater levels and perched water tables result from impermeable structures such as dykes and clay-filled fractures.
• Initial hydrogeological assessments depend on geological predictions regarding aquifers, barriers, and seepage directions.
• Additional investigations like boreholes, piezometer installations, and pumping tests may be necessary.
• Correlate local rainfall and temperature data with seepage observations for comprehensive understanding.
• Anticipate groundwater flow alterations due to construction, incorporating extreme weather, frost, and artificial drainage effects.

Practical Guidance:

Common Formula (Darcy’s Law):

[ Q = k \times A \times \frac{\Delta h}{L} ]

where parameters are as defined earlier.

flowchart LR
    GeologicalMapping --> IdentifyAquifersAndBarriers
    IdentifyAquifersAndBarriers --> PreliminaryAssessment
    PreliminaryAssessment --> SeepageObservation
    SeepageObservation --> HydrogeologicalTesting
    HydrogeologicalTesting --> WeatherDataIntegration

Assessment of Drainage Measures

• Seepage flow is categorized into five classes:

  • Class I: Dry surfaces with no seepage.
  • Class II: Minor seepage with dripping from discontinuities.
  • Class III: Moderate inflow with continuous flow, approximately 1 liter/min per 10 meters of excavation.
  • Class IV: Major inflow exceeding 1 liter/min per 10 meters.
  • Class V: Exceptionally high inflow, much greater than 1 liter/min per 10 meters, with source specification.

• Field evaluation includes checking surface drains, inclined boreholes, and drainage galleries, considering discontinuity orientation, spacing, and aperture.

• Borehole investigations with tracer tests, piezometers, and pumping tests are recommended.

• Permeability varies with rock type and fault zones, and anisotropic conditions are common.

• The highest inflow zones are critical for stability evaluations.

Flow Estimation:

[ Q = \frac{\text{Flow (l/min)}}{\text{Excavation length (10 m)}} ]

Flow Classification Table:

flowchart LR
    Excavation --> SeepageDetected
    SeepageDetected -- No --> ClassI
    SeepageDetected -- Minor --> ClassII
    SeepageDetected -- Medium --> ClassIII
    SeepageDetected -- Major --> ClassIV
    SeepageDetected -- Exceptional --> ClassV

Guidelines for Using Geological Maps and Air Photos

• Indicate general groundwater flow directions on maps and aerial photographs using arrows, based on hydrogeological information.
• Analyze aerial images for local drainage features and vegetation patterns indicative of groundwater presence.
• Depict impermeable barriers such as dykes, clay-filled fractures, and impermeable strata on simplified geological maps and cross-sections, along with groundwater levels.
• Designate optimal locations for investigative boreholes on maps and sections.
• Include rainfall and temperature data to support groundwater flow interpretations.

Recommended Workflow:

Note: Combine qualitative interpretations from images with quantitative hydrogeological data for comprehensive assessments.

flowchart TD
    AirPhotosAndMaps --> DrainagePatternIdentification
    DrainagePatternIdentification --> GroundwaterFlowMarking
    GroundwaterFlowMarking --> ImpermeableBarrierMapping
    ImpermeableBarrierMapping --> BoreholeLocationMarking
    BoreholeLocationMarking --> ClimaticDataIntegration
    ClimaticDataIntegration --> GroundwaterFlowInterpretation

Key Aspects of Engineering-Groundwater Interaction

• Preliminary hydrogeological evaluation uses geological data to predict aquifers, impermeable barriers, and seepage directions.
• Exploratory investigations such as boreholes, piezometers, and pumping tests are advised when data is inadequate.
• Prepare sketches showing phreatic surface levels before and after construction.
• Account for effects of extreme weather, frost, and artificial drainage systems.
• Summarize impacts of seepage and groundwater drawdown on structures like tunnels, slopes, and foundations.

Critical Considerations:

• Irregular groundwater levels caused by impermeable features can generate perched water tables and high-pressure inflows during tunneling.
• Predict drawdown effects on existing facilities and potential foundation settlements.

Typical Drawdown Equation (Simplified Theis):

[ s = \frac{Q}{4 \pi T} W(u) ]

where:

• (s): drawdown (m)
• (Q): pumping rate (m³/s)
• (T): transmissivity (m²/s)
• (W(u)): well function (dimensionless), with (u = \frac{r^2 S}{4 T t})
• (r): distance from pumping well (m)
• (S): storativity (dimensionless)
• (t): time since pumping started (s)

flowchart LR
    Surface --> PerchedWaterTable
    PerchedWaterTable --> ImpermeableBarrier
    ImpermeableBarrier --> MainAquifer
    MainAquifer --> TunnelExcavation
    TunnelExcavation --> DrawdownCone
    DrawdownCone --> ExistingFoundations

Seepage Rating System Overview

• Provides a qualitative to semi-quantitative scale (I to VI) to classify seepage through rock mass discontinuities.
• Ratings assist in characterizing seepage intensity for rock engineering applications.

Rating Scale:

• Apply ratings to individual discontinuities or groups.
• Useful for preliminary seepage assessment in tunnels, slopes, and foundations.
• Combine with structural and permeability measurements for design purposes.

flowchart LR
    DiscontinuityObservation --> SeepageFlowCheck
    SeepageFlowCheck -- None --> RatingI
    SeepageFlowCheck -- Slight --> RatingII
    SeepageFlowCheck -- Moderate --> RatingIII
    SeepageFlowCheck -- Considerable --> RatingIV
    SeepageFlowCheck -- Heavy --> RatingV
    SeepageFlowCheck -- VeryHeavy --> RatingVI
    Ratings --> DataVisualization
    DataVisualization --> MapsHistogramsSections

Use of Seepage Data in Stability Evaluation

• Seepage through discontinuities influences groundwater distribution, potentially creating perched water tables due to impermeable features.
• Predicting irregular groundwater profiles is vital for estimating tunnel inflow pressures and stability.
• Methods to quantify seepage parameters such as permeability and transmissivity are provided.

Key Equations:

1. Darcy’s Law:

[ Q = k \cdot A \cdot \frac{\Delta h}{L} ]

2. Perched water table prediction involves identifying impermeable discontinuities acting as barriers and estimating hydraulic gradients.

Typical Parameter Ranges:

Interpretation Flow:

flowchart TD
    FieldSeepageData --> IdentifyDiscontinuities
    IdentifyDiscontinuities --> EstimateConductivity
    EstimateConductivity --> CalculateFlowAndPressure
    CalculateFlowAndPressure --> PredictGroundwaterLevels
    PredictGroundwaterLevels --> StabilityAssessment

This analysis supports understanding seepage impact on rock mass stability essential for underground works.

Recommendations for Further Study

• Adhere to IS 2-1960 for rounding off numerical test and analysis results.
• Investigate irregular groundwater levels caused by impermeable features such as dykes and clay-filled fractures.
• Predict flow barriers critical for tunneling and seepage analyses.
• Evaluate frost and ice effects on seepage pathways as ice blockages can distort observations and affect excavation stability.
• Present results with clarity, maintaining proper rounding and contextual interpretation.

Additional Notes:

• Use Darcy’s law and flow net analyses for seepage and groundwater flow through discontinuities.
• Employ quantitative discontinuity characterization methods including spacing and persistence.

flowchart TD
    RockMassTesting --> GroundwaterCheck
    GroundwaterCheck -->|Impermeable Barriers| InvestigateBarriers
    GroundwaterCheck -->|No Barriers| StandardSeepageAnalysis
    RockMassTesting --> FrostIceAssessment
    FrostIceAssessment -->|Possible Ice Blockage| AdjustSeepageInterpretation
    FrostIceAssessment -->|No Ice| NormalSeepageReporting
    InvestigateBarriers & StandardSeepageAnalysis & AdjustSeepageInterpretation & NormalSeepageReporting --> FinalReport(IS2-1960)

Reference Documents and Related Codes

• Definitions and terminology are adopted from IS 11358:1986.
• Numerical rounding follows IS 2:1960 to ensure consistency.
• IS 11315 is part of a series of standards; Part 8 complements others focusing on rock mass evaluation.

Reference Table:

These references ensure uniform application and reporting standards.