The plate load test (PLT) is one of the most widely performed field tests in earthworks quality control. It directly measures the deformation behavior of compacted soils and granular layers under controlled loading conditions, providing the deformation moduli that characterize the stiffness and bearing quality of the tested material. In Italy, the reference standard is CNR 146/92 (Bollettino Ufficiale del CNR), which prescribes the test procedure, equipment specifications, and calculation methodology.
Test Equipment and Setup
The standard test setup consists of:
- Loading plate: A rigid circular steel plate, typically 300 mm in diameter (other sizes: 150, 450, 600, 762 mm may be used for special purposes). The plate must be thick enough to behave as a rigid body under load.
- Loading system: A hydraulic jack reacting against a loaded vehicle (truck, excavator) or a kentledge frame. The reaction system must provide at least twice the maximum test load.
- Settlement measurement: Two or three dial gauges (0.01 mm resolution) mounted on an independent reference beam, measuring the plate settlement relative to undisturbed ground at least 1.5 m from the plate center.
- Pressure measurement: A calibrated pressure gauge or load cell measuring the applied force, which is converted to contact pressure (N/mm² or MPa).
The test surface must be carefully prepared: leveled, free of loose material, and seated with a thin layer of fine sand or plaster to ensure uniform contact between the plate and soil.
Test Procedure
The standard CNR 146/92 procedure involves three phases:
First Loading Cycle (Cycle 1): Load is applied in increments (typically 6–10 steps) up to the maximum pressure specified for the layer type. At each step, the load is maintained until settlement stabilizes (rate < 0.02 mm/min), and the settlement is recorded. The pressure-settlement curve from this cycle reflects the total deformation: elastic plus plastic (irreversible).
Unloading (Cycle 1.5): Load is removed in decrements, recording the elastic rebound at each step. The difference between maximum settlement and residual settlement after unloading represents the elastic component of deformation.
Reloading (Cycle 2): Load is reapplied following the same increments as the first cycle. Since the plastic component has already occurred, the second cycle is steeper (stiffer), representing primarily elastic behavior. This cycle provides the reloading modulus.
Pressure Ranges by Layer Type
CNR 146/92 specifies different pressure ranges for calculating deformation moduli depending on the layer type:
- Subgrade (sottofondo): 0.05–0.15 N/mm² (50–150 kPa) for Md1; 0.05–0.10 N/mm² for alternative range.
- Foundation layer (fondazione): 0.15–0.25 N/mm² (150–250 kPa).
- Base layer (base): 0.25–0.35 N/mm² (250–350 kPa) or higher for heavily loaded pavements.
Using the correct pressure range is critical: the modulus is calculated from the secant slope within the specified range, not from the origin.
Calculating the Deformation Modulus
The deformation modulus Md is calculated from the pressure-settlement data within the applicable range using the Boussinesq elastic half-space solution:
Md = f × Δp × D / Δs
Where:
- f = shape factor (π/4 ≈ 0.785 for a rigid circular plate on an elastic half-space)
- Δp = pressure increment within the specified range (N/mm²)
- D = plate diameter (mm)
- Δs = corresponding settlement increment (mm)
Two moduli are calculated:
- Md1: From the first loading cycle — includes both elastic and plastic deformation. This is the “total” modulus and is always the smaller of the two.
- Md2: From the reloading cycle — primarily elastic deformation. This is always larger than Md1 and represents the “elastic” modulus.
The Compaction Ratio
The ratio Md2/Md1 is a key quality indicator. It measures how much of the total deformation is permanent (plastic) versus recoverable (elastic). A high ratio means significant plastic deformation occurred in the first cycle, indicating poor compaction. Typical acceptance criteria:
- Md2/Md1 ≤ 2.0: Good compaction — most deformation is elastic.
- 2.0 < Md2/Md1 ≤ 3.0: Marginal — additional compaction effort may be needed.
- Md2/Md1 > 3.0: Poor compaction — the layer should be reworked and recompacted.
Specifications typically require Md2/Md1 ≤ 2.0–2.5 depending on the layer type and project requirements.
Winkler Subgrade Modulus
The Winkler modulus (coefficient of subgrade reaction) K is also derived from the plate load test:
K = Δp / Δs (N/mm²/mm or MN/m³)
This parameter models the soil as a set of independent linear springs and is used in beam-on-elastic-foundation and slab-on-grade design (Winkler model). Note that K is scale-dependent — the value measured with a 300 mm plate must be corrected for the actual foundation size using the relationship: K_actual = K_plate × (D_plate / (D_plate + D_foundation))² for cohesionless soils, or K_actual = K_plate × D_plate / D_foundation for cohesive soils.
Automated Analysis with Geostru AI
Processing plate load test data manually involves interpolating between measurement points to extract settlements at the exact pressure range boundaries, performing linear regression within the specified range, and calculating moduli for both cycles. Errors in interpolation or range selection can significantly affect the reported moduli.
Geostru AI automates the entire workflow: input your measurement points (pressure and settlement for each step and cycle), specify the plate geometry and layer type, and the system returns Md1, Md2, compaction ratio, and Winkler modulus — all calculated according to CNR 146/92 with proper interpolation and range selection. You can also query the standard pressure ranges for any layer type before starting your analysis.
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