Concept of the Plate Loading Test
The Plate Loading Test is an in-situ method used to determine the bearing capacity and settlement characteristics of soil and rock under load. It involves applying a load to a steel plate placed on the ground surface or at the bottom of an excavation and measuring the resulting settlements. This test provides direct information about the load-settlement behavior of the soil, which is crucial for designing foundations, pavements, and other structures. The standards governing this test include ASTM D1194 for bearing capacity and ASTM D1195/D1196 for pavements.
Figure 1: Plate Load Test.
Descriptions and Procedure
Apparatus Required
- Steel plate (typically circular or square, with a standard size of 300 to 760 mm in diameter)
- Hydraulic jack or loading apparatus
- Reaction load system (e.g., anchored beams or weighted platform)
- Settlement measuring devices (e.g., dial gauges or electronic displacement transducers)
- Load cells or pressure gauges
- Spirit level
- Excavation tools (if the test is conducted below the ground surface)
Procedure
1. Site Preparation
- Select a test location that is representative of the soil conditions.
- Clear the test area of any debris, vegetation, or loose material.
- If testing at a certain depth, excavate a pit to the desired depth and ensure the bottom is level and smooth.
2. Plate Placement
- Place the steel plate on the ground surface or at the bottom of the excavation.
- Ensure the plate is level using a spirit level. Shim the plate if necessary to achieve a level position.
3. Loading Setup
- Position the hydraulic jack or loading apparatus centrally on the steel plate.
- Set up the reaction load system to provide a counterforce for the hydraulic jack. This can involve anchoring beams or a weighted platform.
- Connect the load cell or pressure gauge to the hydraulic jack to measure the applied load.
4. Settlement Measurement Setup
- Arrange settlement measuring devices (dial gauges or electronic displacement transducers) around the plate to measure vertical displacements accurately.
- Ensure that the settlement measuring devices are securely fixed and zeroed before loading begins.
5. Loading Procedure
- Apply load to the plate incrementally using the hydraulic jack. Standard increments are typically one-fifth to one-tenth of the expected ultimate load.
- Maintain each load increment for a sufficient time (typically 1-15 minutes) to allow for settlement stabilization. Record the load and corresponding settlement at each increment.
- Continue loading until the soil fails (excessive settlement or no further load increase) or until the maximum expected load is reached.
6. Unloading Procedure
- Gradually unload the plate in decrements, similar to the loading increments.
- Record the rebound settlements at each decrement to understand the elastic recovery of the soil.
7. Data Collection and Analysis
- Collect load and settlement data throughout the loading and unloading phases.
- Plot the load-settlement curve to analyze the soil behavior under load.
Data Interpretation
1. Load-Settlement Curve
- Plot the applied load versus the measured settlement. The shape of this curve helps determine the bearing capacity and settlement characteristics of the soil.
- Identify the yield point (bearing capacity) where the curve shows a distinct change in slope, indicating the transition from elastic to plastic behavior.
2. Bearing Capacity
- The ultimate bearing capacity can be estimated from the load-settlement curve. It is the load at which the soil undergoes significant settlement without a corresponding increase in load.
- Use empirical or theoretical methods to interpret the data, such as Terzaghi's bearing capacity equations.
3. Modulus of Subgrade Reaction
- Calculate the modulus of subgrade reaction (k) as the ratio of the applied pressure to the corresponding settlement. This parameter is useful for pavement design.
𝑘=𝑞𝑠
where q is the applied pressure and s is the settlement.
4. Elastic Settlement
- Determine the elastic settlement component by analyzing the initial linear portion of the load-settlement curve.
Summary Table
Key Points to Remember
- Purpose: Determine bearing capacity and settlement characteristics of soil.
- Apparatus: Steel plate, hydraulic jack, reaction system, settlement measuring devices.
- Procedure: Prepare site, place plate, apply load, measure settlements, analyze data.
- Data Interpretation: Plot load-settlement curve, calculate bearing capacity, modulus of subgrade reaction, and analyze elastic settlement.
- Standards: ASTM D1194, ASTM D1195/D1196.
Final Summary
The Plate Loading Test is essential for assessing the load-bearing capacity and settlement behavior of soils in situ. It provides valuable data for the design of foundations, pavements, and other structures, ensuring safety and stability. Proper execution and interpretation of the test yield reliable geotechnical parameters crucial for engineering applications.