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Soil Testing (SBC): Plate load test and core cutter method verification checklist

 For a Civil Engineer, determining the Safe Bearing Capacity (SBC) is the most critical step before foundation casting. Below are the technical checklists for the Plate Load Test (Direct SBC) and the Core Cutter Method (Field Density/Compaction).

🏗️ 1. Plate Load Test Checklist (SBC Determination)

As per IS: 1888

This test determines the ultimate bearing capacity of the soil at the foundation level.

Pre-Test Setup

• [ ] Test Pit: Excavated to the actual proposed foundation level. Width of the pit should be at least 5 times the width of the test plate.

• [ ] Test Plate: Use a square or circular mild steel plate (standard sizes: $300\text{mm}$, $450\text{mm}$, $600\text{mm}$, or $750\text{mm}$ sq). Thickness must be $\ge 25\text{mm}$ to prevent bending.

• [ ] Seating Layer: A thin layer ($5\text{--}10\text{mm}$) of fine sand is spread to ensure the plate is perfectly level and in full contact with the soil.

• [ ] Reaction Loading: Ensure the kentledge (sandbags/bricks) or hydraulic jack setup is stable and can provide at least 25% more load than the expected ultimate load.

During Testing

• [ ] Seating Load: Apply a small seating load of $70\text{g/cm}^2$ and release it before the actual test starts.

• [ ] Loading Increments: Apply load in increments (usually $1/5$th of the estimated safe load or $10\text{kN/m}^2$).

• [ ] Settlement Recording: Use at least two (preferably four) dial gauges placed at corners.

• [ ] Observation Intervals: Record settlement at $1, 2.25, 4, 6.25, 9, 16, \text{ and } 25$ minutes, then hourly.

• [ ] Failure Criteria: Continue loading until the total settlement reaches $25\text{mm}$ or the soil fails.

🧪 2. Core Cutter Method Checklist (Field Density)

As per IS: 2720 (Part 29)

Used for finding the In-situ Dry Density of fine-grained/cohesive soils to verify compaction.

Equipment Check

• [ ] Cutter Dimensions: Standard cylindrical cutter (Internal diameter $100\text{mm}$, height $130\text{mm}$).

• [ ] Sharpness: Ensure the cutting edge is sharp and not dented; otherwise, it will disturb the soil structure.

• [ ] Dolly: A $25\text{mm}$ high steel dolly must be used to prevent damage to the cutter head during ramming.

Execution Steps

• [ ] Surface Preparation: Level a $300\text{mm} \times 300\text{mm}$ area. Remove any loose topsoil.

• [ ] Driving the Cutter: Drive the cutter into the soil using the rammer until the dolly is partially buried. Avoid excessive tilting.

• [ ] Extraction: Dig out the soil around the cutter and remove it carefully. Trim the top and bottom flush with a straight edge.

• [ ] Weight Record: Record the weight of the cutter + wet soil ($W_s$).

Laboratory Verification

• [ ] Moisture Content ($w$): Take a representative sample from inside the cutter for oven-drying to find the exact moisture percentage.

• [ ] Calculation:

  $$\text{Dry Density } (\rho_d) = \frac{\rho_{bulk}}{1 + w}$$

• [ ] Compaction Check: Compare the Field Dry Density with the Maximum Dry Density (MDD) from the Proctor test. It should usually be $\ge 95\%$.

📊 Summary Comparison Table