The Plate Load Test (PLT) is a crucial field test used to determine the bearing capacity and settlement of soil under a given load. This test provides direct measurements of soil behaviour and is essential for designing foundations, particularly in shallow soils. This guide details the procedure, equipment, data interpretation, and applications of the plate load test.
Equipment Required
- Loading Plate: A rigid circular or square plate, typically made of steel, with diameters ranging from 300 mm to 750 mm.
- Loading Jack and Reaction System: Hydraulic or mechanical jacks to apply the load, and a reaction frame or dead weights to counteract the load.
- Settlement Measuring Devices: Dial gauges or electronic displacement transducers with a sensitivity of 0.01 mm.
- Load Measuring Devices: Load cells or proving rings to measure the applied load accurately.
- Levelling Instruments: For accurate placement and levelling of the plate.
- Excavation Tools: For preparing the test pit.
Test Procedure
1. Site Selection and Preparation
- Location Selection: Choose a site representative of the area where construction is planned.
- Test Pit Excavation: Excavate a pit at the desired depth, usually at the level of the foundation or subgrade. The pit should be large enough to accommodate the loading plate and provide space for the settlement measuring devices.
- Plate Placement: Place the loading plate at the centre of the pit. Ensure the plate is level and in full contact with the soil.
2. Setting Up the Reaction System
- Reaction Frame: Position a reaction frame over the plate. This frame should be stable and capable of withstanding the applied load.
- Loading Jack: Place the hydraulic or mechanical jack between the reaction frame and the loading plate.
3. Loading Procedure
- Initial Load: Apply a small seating load to the plate to ensure contact with the soil.
- Incremental Loading: Apply load in increments, typically 1/5th of the estimated ultimate bearing capacity or as specified in standards.
- Recording Settlements: After each load increment, allow the load to stabilise and record the settlement readings at regular intervals (e.g., every 1, 2, 4, 8, 15, 30, and 60 minutes) until the settlement rate is less than a specified value (e.g., 0.02 mm/minute).
4. Unloading Procedure
- Load Removal: Gradually remove the load in decrements, recording the rebound settlements similarly to the loading phase.
- Final Readings: After complete unloading, record the final settlement readings to determine the elastic recovery.
Data Interpretation
- Load-Settlement Curve: Plot the applied load versus the corresponding settlement to obtain the load-settlement curve.
- Bearing Capacity: Determine the bearing capacity of the soil from the curve. The ultimate bearing capacity is typically identified by the point where the curve sharply increases (indicating failure).
- Settlement Analysis: Calculate the allowable bearing capacity considering the permissible settlement for the structure.
Applications
- Foundation Design: Helps in designing foundations for buildings, bridges, and other structures by providing the bearing capacity and settlement characteristics of the soil.
- Soil Investigation: Assists in assessing the suitability of a site for construction projects.
- Quality Control: Used during construction to ensure that the soil properties meet the design requirements.
Advantages and Limitations
Advantages:
- Direct measurement of soil bearing capacity.
- Provides immediate results.
- Applicable for various soil types.
Limitations:
- Labour-intensive and time-consuming.
- Requires heavy equipment and skilled personnel.
- The results are specific to the test location and may not represent the entire site.
Conclusion
The Plate Load Test is a fundamental procedure in geotechnical engineering for determining the bearing capacity and settlement behaviour of soils. Its accuracy and direct approach make it invaluable for foundation design and soil investigation. By following the outlined procedure, engineers can ensure reliable data for safe and efficient structural design.