2.4 Soil Exploration, Earth Pressure and Retaining Structures

2.4 Soil Exploration, Earth Pressure and Retaining Structures

Introduction to Geotechnical Site Characterization and Stability

• Soil exploration is the systematic investigation of subsurface conditions to obtain critical information for safe and economical design of foundations, retaining structures, and earthworks.

• Earth pressure theories provide the analytical framework for estimating lateral forces exerted by soil on structures like retaining walls, basement walls, and sheet piles.

• Retaining structures are engineered systems designed to resist these lateral earth pressures and maintain the stability of soil masses at significantly different elevations.

• This unit integrates field investigation techniques with theoretical analysis and practical design, forming a complete workflow from site understanding to structural stability assurance.

---

1. Soil Exploration: Methods, Planning, and Execution

1.1 Objectives and Planning of Soil Exploration

• Primary Objectives:

  • Determine the nature and sequence of subsurface soil strata.

  • Obtain representative soil and rock samples for laboratory testing.

  • Determine the depth and location of groundwater table(s).

  • Identify problematic soils (expansive, collapsible, organic, liquefiable).

  • Assess in-situ engineering properties through field tests.

• Planning Considerations:

  • Project Requirements: Type, magnitude, and sensitivity of the proposed structure.

  • Site Topography and Geology: Review of existing maps, aerial photos, and geological reports.

  • Exploration Program: Deciding the number, location, depth, and spacing of borings/tests.

    • Spacing: Typically 10-30 m for buildings, closer for heavy or sensitive structures.

    • Depth: Should extend to a depth where stress increase from the structure is insignificant (typically 1.5-2 times the footing width for buildings, or to competent bearing stratum).

1.2 Methods of Subsurface Exploration

• Direct Methods (Visible Inspection):

  • Test Pits/Trenches: Provide direct visual examination of soil layers, stratification, and groundwater. Limited to shallow depths (3-5 m).

  • Open Excavations: Useful for large-scale projects like dams; allows large-scale sampling.

• Semi-Direct Methods (Borings):

  • Auger Boring:

    • Hand Auger: Suitable for shallow depths (< 5 m) and soft soils.

    • Mechanical Auger (Hollow/Continuous Flight): Can reach greater depths; suitable for most soils except hard rock or gravel.

  • Wash Boring: Uses a chopping bit and water jetting to advance a casing. Fast but provides highly disturbed samples.

  • Rotary Drilling: Uses a rotating drill bit with circulating fluid (air, water, mud) to cut through soil and rock. Essential for rock coring and deep investigations.

  • Percussion Drilling: Uses repeated lifting and dropping of a heavy chisel bit. Effective for boulders, gravel, and hard rock.

• Indirect Methods (Geophysical):

  • Seismic Refraction: Measures travel time of seismic waves to determine depth to bedrock and layer velocities.

  • Electrical Resistivity: Maps subsurface variations by measuring resistance to electrical current. Good for locating water tables, bedrock, and cavities.

  • Ground Penetrating Radar (GPR): Uses high-frequency radio waves to detect interfaces and objects in shallow subsurface.

1.3 Soil Sampling and Samplers

• Objective: To obtain a representative sample with minimal disturbance for laboratory testing.

• Types of Samples:

  • Disturbed Samples: Soil structure is destroyed. Used for index property tests (classification, water content, Atterberg limits). Obtained from split-spoon samplers or auger cuttings.

  • Undisturbed Samples: Soil structure, moisture content, and stress conditions are preserved as closely as possible. Required for strength, compressibility, and permeability tests.

Common Samplers

• Split-Spoon Sampler (SPT Sampler):

  • A thick-walled, split cylinder driven into the soil.

  • Provides disturbed samples.

  • Used in conjunction with the Standard Penetration Test (SPT).

  • Recovery Ratio: $\boldsymbol{R = \frac{\text{Length of Sample Recovered}}{\text{Length of Drive}}}$. Should be > 85% for good recovery.

• Thin-Walled Tube Sampler (Shelby Tube):

  • A seamless, thin-walled steel tube (Area Ratio < 10%).

  • Gently pushed (not driven) into cohesive soil to obtain undisturbed samples.

  • Area Ratio:

    $\boldsymbol{A_r = \frac{D_o^2 - D_i^2}{D_i^2} \times 100\%}$

    where, $\boldsymbol{D_o}$ is outer diameter and $\boldsymbol{D_i}$ is inner diameter. Lower $\boldsymbol{A_r}$ means less disturbance.

• Piston Sampler: Incorporates a stationary piston at the sampler tip to prevent soil from entering during lowering, reducing disturbance in soft clays.

1.4 Common Field Tests

• Standard Penetration Test (SPT):

  • Procedure: Drive a standard split-spoon sampler a distance of 450 mm (18 in) using a 63.5 kg hammer falling 760 mm. Record the number of blows (N) for the last 300 mm of penetration.

  • Corrected N-value ($\boldsymbol{N_{60}}$): Corrects for field efficiency, rod length, borehole diameter, and sampling method.

  • Uses: Correlates to relative density (sands), consistency (clays), and estimates bearing capacity and settlement.

• Cone Penetration Test (CPT):

  • Procedure: A conical tip (60°, 10 cm² area) is pushed continuously into the ground at a constant rate (2 cm/s). Measures tip resistance ($\boldsymbol{q_c}$) and sleeve friction ($\boldsymbol{f_s}$).

  • Friction Ratio: $\boldsymbol{R_f = \frac{f_s}{q_c} \times 100\%}$ (helps identify soil type).

  • Piezocone (CPTU): Also measures pore water pressure ($\boldsymbol{u}$).

  • Advantages: Continuous profile, fast, repeatable, no borehole required.

• Vane Shear Test (VST):

  • Procedure: A four-bladed vane is inserted into soft clay and rotated. The torque required to cause failure is measured.

  • Calculations: $\boldsymbol{s_u = \frac{T}{K}}$, where $\boldsymbol{K}$ is a vane constant based on its dimensions.

  • Purpose: Determines in-situ undrained shear strength ($\boldsymbol{s_u}$) of soft to medium clays.

• Plate Load Test (PLT):

  • Procedure: A rigid steel plate is placed at foundation level and loaded incrementally. Settlement is recorded for each load increment.

  • Purpose: Determines bearing capacity and settlement characteristics of the soil in-situ.

1.5 Site Investigation Report

• A comprehensive document summarizing all findings. Key components include:

  1. Introduction: Scope and purpose of investigation.

  2. Site Description: Location, topography, access.

  3. Field Exploration Program: Methods used, boring locations, depths, dates.

  4. Subsurface Profile: Description of soil/rock strata, graphical logs for each boring.

  5. Groundwater Conditions: Observed water levels, seasonal variations.

  6. Field Test Results: SPT N-values, CPT profiles, VST results.

  7. Laboratory Test Results: Summary tables of index, strength, and compressibility properties.

  8. Analysis and Interpretation: Discussion of soil behavior, design parameters recommended.

  9. Conclusions and Recommendations: Suitability of site, recommended foundation type (shallow/deep), bearing capacity, estimated settlement, special precautions (dewatering, excavation support).

---

2. Earth Pressure Theories

2.1 Lateral Earth Pressure States

• The lateral pressure ($\boldsymbol{\sigma_h}$) exerted by soil on a retaining structure depends on the wall movement relative to the backfill.

• Three Fundamental States:

  1. At-Rest Earth Pressure ($\boldsymbol{K_0}$):

     • Occurs when the wall is rigid and does not move (e.g., basement wall, bridge abutment).

     • Coefficient of Earth Pressure at Rest: $\boldsymbol{K_0 = \frac{\sigma_h'}{\sigma_v'}}$

     • For normally consolidated soils: $\boldsymbol{K_0 \approx 1 - \sin \phi'}$ (Jaky's formula).

     • For overconsolidated soils: $\boldsymbol{K_0(OC) \approx K_0(NC) \cdot (OCR)^{\sin \phi'}}$.

  2. Active Earth Pressure ($\boldsymbol{K_a}$):

     • Occurs when the wall moves away from the soil mass sufficiently.

     • Soil reaches a state of plastic equilibrium, mobilizing its shear strength.

     • This is the minimum possible lateral pressure.

  3. Passive Earth Pressure ($\boldsymbol{K_p}$):

     • Occurs when the wall moves into the soil mass sufficiently.

     • Soil is compressed and sheared, offering maximum resistance.

     • This is the maximum possible lateral pressure.

2.2 Rankine's Earth Pressure Theory (1857)

• Assumptions: Soil is homogeneous, isotropic, cohesionless, the wall is frictionless (smooth) and vertical, and the ground surface is horizontal.

• Active Earth Pressure Coefficient:

  $\boldsymbol{K_a = \frac{1 - \sin \phi}{1 + \sin \phi} = \tan^2 \left(45^\circ - \frac{\phi}{2}\right)}$

  Pressure Distribution (for a cohesionless soil): $\boldsymbol{\sigma_a = K_a \sigma_v = K_a \gamma z}$

• Passive Earth Pressure Coefficient:

  $\boldsymbol{K_p = \frac{1 + \sin \phi}{1 - \sin \phi} = \tan^2 \left(45^\circ + \frac{\phi}{2}\right)}$

  Pressure Distribution: $\boldsymbol{\sigma_p = K_p \sigma_v = K_p \gamma z}$

• For Cohesive Soils (c-$\boldsymbol{\phi}$ soil):

  • Active Pressure: $\boldsymbol{\sigma_a = K_a \gamma z - 2c \sqrt{K_a}}$

  • Passive Pressure: $\boldsymbol{\sigma_p = K_p \gamma z + 2c \sqrt{K_p}}$

  • Tension Crack Depth ($\boldsymbol{z_c}$): $\boldsymbol{z_c = \frac{2c}{\gamma \sqrt{K_a}}}$. In design, tension is usually ignored, and pressure is taken as zero from surface to $\boldsymbol{z_c}$.

2.3 Coulomb's Earth Pressure Theory (1776)

• Advantage over Rankine: Considers wall friction ($\boldsymbol{\delta}$) and sloping backfill ($\boldsymbol{\beta}$).

• Based on force equilibrium of a failing soil wedge.

• Active Earth Pressure Coefficient:

  $\boldsymbol{K_a = \frac{\cos^2 (\phi - \theta)}{\cos^2 \theta \cdot \cos(\delta + \theta) \left[ 1 + \sqrt{\frac{\sin(\delta+\phi)\sin(\phi-\beta)}{\cos(\delta+\theta)\cos(\theta-\beta)}} \right]^2}}$

  where, $\boldsymbol{\theta}$ is the wall inclination from vertical.

• Passive Earth Pressure Coefficient (Similar form, different sign conventions).

• Total Active Force per unit length:

  $\boldsymbol{P_a = \frac{1}{2} K_a \gamma H^2}$

  Acts at $\boldsymbol{H/3}$ from the base for triangular pressure distribution.

• Direction of Force: Inclined at an angle $\boldsymbol{\delta}$ to the normal of the wall.

---

3. Stability Analysis of Retaining Walls

3.1 Types of Retaining Walls

• Gravity Walls: Rely on their own mass for stability (stone masonry, plain concrete). Usually massive.

• Cantilever Walls: Reinforced concrete walls consisting of a vertical stem and a base slab (heel and toe). Most common for heights up to 6-8 m.

• Counterfort/Buttress Walls: Similar to cantilever but have thin vertical ribs (counterforts behind the wall or buttresses in front) to reduce bending moments. Used for greater heights (>8 m).

• Sheet Pile Walls: Thin, interlocking sections driven into the ground. Used for waterfront structures and temporary excavations.

• Anchored/Braced Walls: Walls supported by anchors (ties) or braces to increase stability against high pressures.

3.2 Failure Modes and Stability Checks

A retaining wall must be checked against the following potential failure modes:

1. Overturning Failure

• Mechanism: Wall rotates about its toe due to lateral earth pressure.

• Factor of Safety (FOS):

  $\boldsymbol{FOS_{overturning} = \frac{\sum \text{Resisting Moments about Toe}}{\sum \text{Overturning Moments about Toe}} \geq 1.5 - 2.0}$

2. Sliding Failure

• Mechanism: Wall translates horizontally due to lateral force.

• Resistance: Provided by base friction and/or passive resistance at the toe (shear key).

• Factor of Safety (FOS):

  $\boldsymbol{FOS_{sliding} = \frac{\sum \text{Resisting Horizontal Forces}}{\sum \text{Driving Horizontal Forces}} \geq 1.5}$

  $\boldsymbol{\sum \text{Resisting Forces} = (\sum V) \cdot \tan \delta_b + c_b \cdot B + P_p}$ where, $\boldsymbol{\sum V}$ is total vertical force, $\boldsymbol{\delta_b}$ is wall-soil base friction angle, $\boldsymbol{c_b}$ is soil-wall base adhesion, $\boldsymbol{B}$ is base width, $\boldsymbol{P_p}$ is passive force at toe.

3. Bearing Capacity Failure

• Mechanism: Soil beneath the toe fails in shear due to excessive vertical pressure.

• Procedure: Calculate the maximum and minimum pressure ($\boldsymbol{q_{max}, q_{min}}$) at the base using eccentricity formulas.

  • Eccentricity ($\boldsymbol{e}$):

    $\boldsymbol{e = \frac{B}{2} - \frac{\sum M_R - \sum M_O}{\sum V}}$

  • Must have $\boldsymbol{e \leq B/6}$ to avoid tension under the base.

  • Soil Pressure (for $\boldsymbol{e \leq B/6}$):

    $\boldsymbol{q_{max, min} = \frac{\sum V}{B} \left(1 \pm \frac{6e}{B}\right)}$

• Factor of Safety (FOS):

  $\boldsymbol{FOS_{bearing} = \frac{q_{ult}}{q_{max}} \geq 3.0}$

  where, $\boldsymbol{q_{ult}}$ is the ultimate bearing capacity of the foundation soil.

4. Deep-Seated Shear (Overall Stability) Failure

• Mechanism: A slip surface passes beneath the wall and behind it, involving a large soil mass. Common in weak soils.

• Analysis Method: Slope stability analysis (e.g., Method of Slices, Bishop's method, using software). Check for circular or non-circular failure surfaces.

5. Internal Failure (Structural Checks)

• Mechanism: Failure of the wall material itself (concrete crushing, steel yielding).

• Analysis: Structural design of stem, heel, and toe slabs for bending moment and shear force, following reinforced concrete design codes.

---

4. Techniques to Increase Stability of Retaining Walls

4.1 Geometric Modifications

• Increase Base Width ($\boldsymbol{B}$): Directly increases resisting moments and reduces base pressure.

• Use a Sloping Backfill: Reduces the active earth pressure coefficient ($\boldsymbol{K_a}$) according to Coulomb's theory. A surcharge slope ($\boldsymbol{\beta}$) is beneficial.

• Provide a Toe Projection: Extends the lever arm for resisting moments against overturning.

• Batter the Wall Face: Inclining the wall face into the backfill (negative batter) engages more wall friction and can reduce pressure.

4.2 Soil Improvement and Reinforcement

• Use of Cohesionless Backfill: Draining, free-draining materials (sand, gravel) have higher $\boldsymbol{\phi}$ and lower lateral pressure. They also prevent buildup of pore water pressure.

• Provision of Drainage: This is the single most effective measure.

  • Weep Holes: Perforated pipes through the wall stem near its base.

  • Drainage Blanket: A layer of granular material behind the wall with a perforated collector pipe at the base.

  • Prevents hydrostatic pressure, which can double the lateral force.

• Shear Key: A projection from the base slab into the underlying soil, engaging passive resistance to increase sliding resistance.

4.3 External Support Systems

• Counterforts/Buttresses: Connect the stem to the base slab, reducing the unsupported span of the stem and base, allowing for a thinner, more efficient section.

• Tiebacks/Ground Anchors: High-strength tendons drilled and grouted into the soil/rock behind the wall, then tensioned and connected to the wall. They provide an external resisting force.

• Struts/Braces: Horizontal members placed between opposing walls in excavations. Common in temporary works like braced sheet pile walls.

4.4 Specialized Wall Systems

• Reinforced Soil Walls (Mechanically Stabilized Earth - MSE): Uses horizontal layers of tensile reinforcement (geogrids, strips) within the soil mass to create a coherent, stable gravity structure.

• Gabion Walls: Wire mesh boxes filled with rock. Flexible, permeable, and good for erosion control.

• Crib Walls: Interlocking structural frames (timber, concrete) filled with granular material. Permeable and can tolerate some settlement.

4.5 Construction and Maintenance Considerations

• Proper Compaction: Backfill should be compacted in thin lifts to desired density. Over-compaction near the wall can induce excessive lateral pressures.

• Staged Construction: For high walls or weak foundations, construct and backfill in stages to allow for consolidation and strength gain.

• Joints and Waterproofing: Provide construction joints and proper drainage to prevent water ingress and frost damage.

• Monitoring: Install instrumentation (inclinometers, settlement markers) for critical walls to monitor performance during and after construction.

---

Summary and Design Synthesis

• Soil exploration provides the essential data (stratigraphy, parameters, groundwater) that feeds into earth pressure calculations.

• Earth pressure theory (Rankine/Coulomb) translates soil properties into design loads on the retaining structure.

• Stability analysis ensures the proposed wall geometry is safe against overturning, sliding, bearing, and overall failure.

• Stability enhancement techniques are integral to efficient design, often making an otherwise unstable section viable.

• The final design is an iterative process balancing geotechnical safety, structural efficiency, constructability, and cost.

• A well-investigated site and a rigorously analyzed design are the cornerstones of a safe and durable retaining structure.

---