Chapter 4 Geotechnical Engineering: Unconsolidated Undrained (UU) Triaxial Test

1. Introduction: USCS Soil Classification

2. Simulation: USCS Soil Classification

3. Self-Assessment

1. Introduction

Fig. 19 **Figure 4.5 **: Triaxial test for Unconcolidated Undrained, Consolidated Undrained, and Consolidated and Drained test.

🧱 Soil Strength – Definition, Influencing Factors & Engineering Importance

Soil strength refers to the capacity of soil to resist deformation and failure under applied loads. It is governed by the internal resistance offered by particles through cohesion and friction. In geotechnical engineering, strength is assessed in terms of shear strength, typically represented as:

\[ \tau = c + \sigma \cdot \tan\phi \]

Where:

• \(( \tau \)) = shear strength

• \(( c \)) = cohesion

• \(( \sigma \)) = normal stress

• \(( \phi \)) = angle of internal friction

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🔍 Key Influencing Factors

Factor	Influence on Strength Behavior
Moisture Content	Increased water may weaken particle bonds
Drainage Conditions	Undrained behavior leads to pore pressure buildup
Soil Type	Clay = cohesive; Sand = frictional
Density / Compaction	Higher compaction enhances resistance
Loading Rate	Rapid loads may lead to undrained failure
Stress History	Overconsolidated soils show different responses

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💧 Role of Water & Drainage

• In Saturated Conditions: Excess pore water reduces effective stress → lowers shear strength

• In Drained Conditions: Water exits → pore pressure dissipates → strength stabilizes

• Undrained Behavior: Short-term loads (UU or CU tests) measure immediate strength before drainage occurs

Understanding water’s impact is essential in differentiating total stress vs. effective stress analysis and in choosing the right test method (UU, CU, CD).

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🏗️ Why Soil Strength Matters

🔨 Engineering Applications

Domain	Role of Soil Strength
Bearing Capacity	Determines allowable pressure under footings
Slope Stability	Prevents landslides or embankment failure
Retaining Structures	Guides lateral earth pressure estimation
Foundation Design	Ensures settlement stays within tolerable limits
Excavation Safety	Assesses trench stability and collapse risk

Reliable strength data is the foundation for safe and durable infrastructure—roads, buildings, bridges, dams—all depend on it.

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🧪 Typical UU, CU, and CD Strength Values – Regional Soil Profiles

Soil strength values from Unconsolidated Undrained (UU), Consolidated Undrained (CU), and Consolidated Drained (CD) triaxial tests vary significantly depending on soil type, saturation, consolidation history, and drainage conditions. These tests simulate different field scenarios:

• UU → Short-term construction safety (no drainage, no consolidation)

• CU → Intermediate-term stability (consolidated, undrained with pore pressure measurement)

• CD → Long-term settlement and failure (fully drained, consolidated)

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🌍 Representative Strength Parameters by Region & Soil Type

Region / Soil Type	UU Test – Su (kPa)	CU Test – φ′ (°), c′ (kPa)	CD Test – φ′ (°), c′ (kPa)
Virginia, USA – CL Clay	45–90	φ′ = 27–32°, c′ = 0–5	φ′ = 30–32°, c′ = 0–4.9
Indonesia – Soft Marine Clay	15–40	φ′ = 20–28°, c′ = 0–10	φ′ = 25–30°, c′ = 0–8
UK – London Clay	60–120	φ′ = 24–30°, c′ = 0–10	φ′ = 28–32°, c′ = 0–12
India – Black Cotton Soil	30–70	φ′ = 18–25°, c′ = 0–15	φ′ = 22–28°, c′ = 0–10
China – Silty Clay (NC)	40–80	φ′ = 26–30°, c′ = 0–5	φ′ = 28–33°, c′ = 0–6
Brazil – Residual Clayey Soil	50–100	φ′ = 25–30°, c′ = 0–8	φ′ = 27–32°, c′ = 0–10

📌 Note: UU test results are in terms of total stress (Su), while CU and CD provide effective stress parameters (φ′ and c′). CD tests often show slightly higher friction angles due to full drainage.

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🧭 Application Guidance

Test Type	Field Scenario Simulated	Use Case Examples
UU	No drainage, no consolidation	Excavation safety, embankment during construction
CU	Consolidated, undrained (pore pressure measured)	Foundation design, slope stability (short to medium term)
CD	Fully drained, long-term consolidation	Settlement prediction, long-term slope stability

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References

[Das, 2010] discusses the Unconsolidated Undrained (UU) triaxial test method as part of its coverage of shear strength testing. It typically explains the test setup, assumptions (no drainage, total stress analysis), and interpretation of results including Mohr circles and undrained shear strength.ASTM D2850 (UU Test) [ASTM International, 2023]: No drainage; total stress analysis; quick test for short-term conditions.

• ASTM D4767 (CU Test) [ASTM International, 2020]: Consolidated, undrained shearing; pore pressure measured; effective stress parameters obtained.

• ASTM D7181 (CD Test) [ASTM International, 2020]: Fully drained during shearing; slow test; used for long-term stability analysis.

• All tests use cylindrical soil specimens under axial loading to determine shear strength.

• Selection depends on field drainage conditions and design requirements (short-term vs long-term).

2. Simulation

🧪 Summary – Interactive UU Triaxial Test App

An interactive Python app built with ipywidgets, designed to organize and analyze data from the Unconsolidated Undrained (UU) triaxial test, a short-term strength evaluation method for cohesive soils under total stress conditions (ASTM D2850).

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⚙️ What It Does

Task	Description
Data Entry	Users input specimen dimensions and trial data (σ₃ and σ₁)
Calculation	Computes deviator stress (σ_d = σ₁ − σ₃) and (Su = σ_d / 2)
Report Generation	Displays tabulated results and interpretation notes
Mohr Envelope Plot	Visualizes σ₁ vs. σ₃ pairs in total stress space

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📥 Inputs

Input	Meaning
Sample ID	Identifier for the soil specimen
Diameter & Height	Used to compute cross-sectional area
Number of Trials	Defines how many σ₃–σ₁ stress pairs to evaluate
σ₃ (kPa)	Confining pressure during each test trial
σ₁ (kPa)	Axial failure stress during each trial

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📤 Outputs

Output	Interpretation
σ_d (kPa)	Deviator stress = σ₁ − σ₃
Su (kPa)	Undrained shear strength (total stress)
Mohr Envelope Plot	Stress-space visualization assuming φ = 0
Markdown Summary	Consolidated table with all trial results

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🧭 How to Interpret

• Su represents short-term strength for cohesive soils, assuming no drainage or consolidation

• Total Stress Analysis: friction angle (φ_{uu} = 0^\circ) implies horizontal failure envelope

• Results are used for stability evaluations such as slope failure or embankment design under rapid loading conditions

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# 🧪 Revised Interactive Unconsolidated Undrained (UU) Triaxial Test App

import pandas as pd
import numpy as np
import ipywidgets as widgets
from IPython.display import display, Markdown
import matplotlib.pyplot as plt

# 📐 Styling
style = {'description_width': '180px'}
layout = widgets.Layout(width='380px')

# 📋 Specimen Inputs
sample_ID = widgets.Text(value='Sample-001', description='Sample ID:', style=style, layout=layout)
specimen_diameter = widgets.FloatText(value=0.038, description='Diameter (m):', style=style, layout=layout)
specimen_height = widgets.FloatText(value=0.076, description='Height (m):', style=style, layout=layout)
num_trials = widgets.IntSlider(value=3, min=1, max=10, description='Number of Trials:', style=style, layout=layout)

# 📥 Trial Inputs (Interactive)
def create_trial_inputs(n):
    trials = []
    for i in range(n):
        sigma3 = widgets.FloatText(value=100 + i*50, description=f'Trial {i+1} – σ₃ (kPa)', style=style, layout=layout)
        sigma1 = widgets.FloatText(value=130 + i*50, description=f'Trial {i+1} – σ₁ (kPa)', style=style, layout=layout)
        trials.append((sigma3, sigma1))
    return trials

trial_inputs = create_trial_inputs(num_trials.value)

def update_trials(change):
    global trial_inputs
    trial_inputs = create_trial_inputs(change['new'])
    trial_box.children = [w for pair in trial_inputs for w in pair]

num_trials.observe(update_trials, names='value')
trial_box = widgets.VBox([w for pair in trial_inputs for w in pair])

# 🧮 Main Logic
run_btn = widgets.Button(description='Analyze UU Data', layout=widgets.Layout(width='250px'))

def on_run_click(b):
    D = specimen_diameter.value
    H = specimen_height.value
    A = np.pi * D**2 / 4

    results = []
    for i, (conf_widget, axial_widget) in enumerate(trial_inputs):
        sigma3 = conf_widget.value
        sigma1 = axial_widget.value
        sigma_d = sigma1 - sigma3
        Su = sigma_d / 2
        results.append({
            'Trial': f'Trial {i+1}',
            'σ₃ (kPa)': sigma3,
            'σ₁ (kPa)': sigma1,
            'σ_d (kPa)': sigma_d,
            'Su (kPa)': round(Su, 2)
        })

    df = pd.DataFrame(results)

    # 📄 Markdown Report
    md = f"""
### 🧱 UU Triaxial Test Report – Unconsolidated Undrained (Total Stress Analysis)

**Sample ID**: {sample_ID.value}  
**Diameter**: {D:.3f} m  
**Height**: {H:.3f} m  
**Area**: {A:.5f} m²  

---

| Trial | σ₃ (kPa) | σ₁ (kPa) | σ_d (kPa) | Su (kPa) |
|-------|----------|----------|-----------|-----------|
"""
    for row in results:
        md += f"| {row['Trial']} | {row['σ₃ (kPa)']} | {row['σ₁ (kPa)']} | {row['σ_d (kPa)']} | {row['Su (kPa)']} |\n"

    md += """
---

### 📘 Interpretation Notes

- UU tests are performed without allowing drainage or consolidation → reflects short-term behavior  
- Undrained shear strength is calculated as:  
  \[
  Su = \frac{σ_1 - σ_3}{2}
  \]
- Total stress analysis assumes φ = 0 → Mohr circles have horizontal failure envelope  
"""

    display(Markdown(md))

    # 📈 Mohr Envelope Plot
    fig, ax = plt.subplots(figsize=(6, 4))
    ax.plot(df['σ₃ (kPa)'], df['σ₁ (kPa)'], 'bo-', label='Stress Pairs')
    ax.set_xlabel('Confining Pressure σ₃ (kPa)')
    ax.set_ylabel('Axial Stress σ₁ (kPa)')
    ax.set_title('UU Test – σ₁ vs. σ₃ (Mohr Envelope)')
    ax.grid(True)
    ax.legend()
    plt.show()

run_btn.on_click(on_run_click)

# 📦 App Interface
display(widgets.VBox([
    sample_ID,
    specimen_diameter,
    specimen_height,
    num_trials,
    trial_box,
    run_btn
]))

<>:79: SyntaxWarning: invalid escape sequence '\['
<>:79: SyntaxWarning: invalid escape sequence '\['
C:\Users\satis\AppData\Local\Temp\ipykernel_86420\2334736742.py:79: SyntaxWarning: invalid escape sequence '\['
  md += """

3. Self-Assessment

🔍 Conceptual Questions

1. Why is the UU triaxial test suitable for evaluating short-term soil strength?

2. What assumptions justify the use of total stress analysis in the UU test?

3. Why is the internal friction angle (ϕ) typically assumed to be zero in UU conditions?

4. How does the deviator stress (σ₁ − σ₃) relate to the failure mechanism of cohesive soils?

5. How do UU test results inform decisions in rapid construction scenarios, such as embankments or slopes?

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💭 Reflective Questions

1. In your trials, how did changes in confining pressure (σ₃) affect the measured Su values?

2. What limitations might arise if UU test results are used for long-term design considerations?

3. If Su values vary widely across trials, what could be potential causes (sample disturbance, saturation, etc)?

4. How might your results differ if the soil specimen were overconsolidated or partially drained?

5. What visual trends are revealed in the Mohr envelope plot of σ₁ vs. σ₃, and how do they support the assumption of φ = 0?

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Theory & Method

Q1. What does the UU triaxial test assume about drainage and consolidation?

• A. Drainage allowed, no consolidation

• B. No drainage, full consolidation

• C. No drainage, no consolidation ✅

• D. Full drainage and consolidation

Q2. In UU conditions, undrained shear strength Su is calculated as:

• A. \(( Su = σ₁ × σ₃ \))

• B. \(( Su = \frac{σ₁ + σ₃}{2} \))

• C. \(( Su = \frac{σ₁ − σ₃}{2} \)) ✅

• D. \(( Su = σ₃ − σ₁ \))

Q3. Why is the Mohr failure envelope horizontal in a UU test?

• A. Cohesion is infinite

• B. Friction angle is zero ✅

• C. Sample is overconsolidated

• D. Stress-strain curve is linear

Application & Interpretation

Q4. Which soil type is most appropriate for UU testing?

• A. Clean sand

• B. Granular gravel

• C. Cohesive clay ✅

• D. Organic silt

Q5. A Su value of 90 kPa suggests:

• A. High shear strength

• B. Very weak soil

• C. Moderate undrained strength ✅

• D. Inappropriate test method