🔹 DAY 21 – Trial Pit Logging & CBR / Proctor Tests
Completely Explained in Simple Words
Not every geotechnical investigation depends only on boreholes.
For shallow foundations, pavements, roads, and utilities, trial pits and compaction tests are often more useful and economical.
Day 21 explains:
✔ What trial pits are
✔ How trial pit logging is done
✔ What CBR test is
✔ What Proctor test is
✔ How engineers use these results in real projects
🕳️ What Is a Trial Pit?
A trial pit (TP) is a shallow excavation made to directly see soil layers.
• Usually excavated using backhoe / excavator
• Depth: 1.5 m to 4.0 m (sometimes more)
• Shape: rectangular or square
👉 Unlike boreholes, trial pits allow you to see and touch the soil directly.
🧠 Why Trial Pits Are Important
Trial pits are very useful for:
• Shallow foundations
• Pavement and road projects
• Utility trenches
• Ground improvement checks
• Visual soil inspection
They help engineers understand:
✔ Soil layering
✔ Compaction condition
✔ Presence of fill or debris
✔ Groundwater seepage
✔ Construction difficulties
📝 Trial Pit Logging (Step-by-Step)
Trial pit logging is the systematic recording of what is observed inside the pit.
1️⃣ Trial Pit Identification
Each pit is labeled clearly:
• TP-01, TP-02, TP-03, etc.
• Location coordinates recorded
• Ground level noted
2️⃣ Depth and Layer Description
For each layer, record:
• Depth range (e.g., 0.0 – 0.6 m)
• Soil type
• Color
• Moisture condition
Example:
0.0 – 0.5 m: Brown silty sand, loose, dry
3️⃣ Soil Description (Visual & Manual)
Describe soil based on:
• Grain size (sand, silt, clay, gravel)
• Plasticity (low / medium / high)
• Consistency or density
Simple terms are used initially and later corrected using lab tests.
4️⃣ Groundwater Observation
Check and record:
• Any seepage
• Standing water
• Damp soil
⚠️ If water appears later, note the time delay.
5️⃣ Sampling from Trial Pits
Samples collected:
✔ Bulk samples (for CBR & Proctor)
✔ Disturbed samples
Samples are labeled properly with:
• TP number
• Depth
• Date
🚗 CBR Test (California Bearing Ratio)
🔍 What Is CBR Test?
CBR measures the strength of subgrade soil for:
• Roads
• Pavements
• Parking areas
• Airfields
👉 It tells how well soil can support traffic loads.
⚙️ How CBR Test Is Done (Simple)
Soil sample is compacted in a mold
Sample is soaked or unsoaked
A plunger penetrates the soil
Resistance is measured
CBR value (%) is calculated
📊 Typical CBR Values (Guide)
| Soil Type | CBR (%) |
|---|---|
| Soft clay | 2 – 4 |
| Silty clay | 4 – 8 |
| Sand | 8 – 20 |
| Well-compacted gravel | 20 – 60 |
👉 Higher CBR = stronger soil
🏗 Where CBR Is Used
• Pavement thickness design
• Road design
• Subgrade evaluation
• Ground improvement decisions
⚖️ Proctor Compaction Test
🔍 What Is Proctor Test?
The Proctor test determines:
✔ Maximum Dry Density (MDD)
✔ Optimum Moisture Content (OMC)
👉 It helps achieve proper compaction on site.
⚙️ Proctor Test Procedure (Easy Steps)
Soil is mixed with different water contents
Each sample is compacted in layers
Dry density is calculated
Graph is plotted
Peak point gives MDD & OMC
📈 Proctor Curve Explained Simply
• X-axis = Moisture content (%)
• Y-axis = Dry density
The highest point on the curve = best compaction condition.
🏗 Why Proctor Test Is Important
Used for:
• Earthworks
• Backfilling
• Road subgrade
• Embankments
Site compaction is usually specified as:
95% or 98% of MDD at OMC ± 2%
🆚 CBR vs Proctor (Easy Comparison)
| Feature | CBR Test | Proctor Test |
|---|---|---|
| Purpose | Soil strength | Soil compaction |
| Used for | Pavement design | Construction control |
| Output | CBR % | MDD & OMC |
| Sample | Compacted | Compacted |
❌ Common Mistakes Fresh Engineers Make
❌ Ignoring trial pits for shallow works
❌ Poor soil description
❌ Wrong sample labeling
❌ Confusing CBR with bearing capacity
❌ Not following Proctor results during site compaction
📝 How Day 21 Data Appears in Reports
Typical report statement:
“Trial pits revealed loose silty sand up to 1.2 m depth. Laboratory CBR values ranged from 6% to 10%. Proctor test indicated MDD of 1.85 g/cm³ at OMC of 11%.”
🎯 Key Takeaways – Day 21
✔ Trial pits give direct soil visibility
✔ Logging must be clear and systematic
✔ CBR controls pavement thickness
✔ Proctor controls site compaction quality
✔ Simple tests = big impact on design & construction
📘 Next – Day 22:
Structure of a Complete Geotechnical Report (From Cover to Conclusion)