The plate load test is an in-situ geotechnical test that measures the bearing capacity and settlement characteristics of soil or rock beneath a loaded plate. It directly verifies foundation design assumptions and is widely used for working platform certification, pavement subgrade assessment, and footing design verification.
What Is a Plate Load Test (PLT)?
A plate load test involves applying a controlled load to a steel plate placed on the ground surface or at foundation level. The load is increased incrementally while settlement is measured, producing a load-settlement curve that defines the ground's bearing behaviour.
When Is Plate Load Testing Used?
- Working platform certification — verify safe bearing capacity for cranes and piling rigs
- Foundation design verification — confirm assumed bearing capacity
- Pavement subgrade assessment — determine modulus of subgrade reaction (k-value)
- Compaction quality control — verify stiffness of engineered fill
- Rock bearing capacity — assess foundation bearing on rock
- Ground improvement verification — confirm improvement after stabilisation or compaction
Equipment Required
- Test plate (typically mild steel, 300 mm to 750 mm square or circular)
- Hydraulic jack with a pump
- Reaction frame or kentledge (counterweight system)
- Dial gauges or LVDTs (settlement measuring devices)
- Pressure gauge
- Loading column and supports
Test Procedure
Step 1: Preparation
- Excavate to test level (footing base, subgrade, or working platform surface)
- Level the test surface — sand or plaster bedding may be used
- Position the plate and seating load
Step 2: Seating
- Apply a small seating load (typically 1–5 kPa)
- Zero all gauges
Step 3: Loading
Incremental loading with settlement monitoring:
| Method | Load Increments | Hold Time | Measure |
|---|---|---|---|
| Maintained load | 6–8 increments to 1.5–2× design load | 15–60 min per increment | Settlement vs time |
| Constant rate of penetration (CRP) | Continuous loading | N/A | Continuous settlement |
| Cyclic loading | Load – unload – reload cycles | Variable | Elastic and plastic settlement |
Step 4: Unloading
- Unload in decrements, recording rebound
- Measure elastic recovery
Step 5: Reporting
The test report includes:
- Load-settlement curve
- Bearing capacity at specified settlement
- Modulus of subgrade reaction (k-value)
- Modulus of elasticity (E-value)
- Photographs of test setup
Interpretation
Bearing Capacity
The ultimate bearing capacity is interpreted from the load–settlement curve. Plot a load–settlement curve (load intensity on x-axis, settlement on y-axis).
- 10% of plate diameter settlement for sands
- 20% of plate diameter settlement for clays
- Or the load at which settlement accelerates rapidly
The allowable bearing capacity applies a factor of safety (typically 2–3) to the ultimate value.
Modulus of Subgrade Reaction (k-value)
Used for pavement and raft foundation design:
$$ k = \frac{q}{\delta} $$Where:
- q = applied stress (kPa)
- δ = settlement at that stress (mm)
Standard 300 mm plate k-value is converted to 750 mm (raft) or 1,500 mm (pavement) using:
$$ k_{750} = k_{300} \times \frac{300}{750} $$Elastic Modulus (E)
$$ E = \frac{qB(1-\nu^2)I}{\delta} $$Where I = 0.79 for rigid plate, B = plate diameter, ν = Poisson's ratio.
Calculating Allowable Load from Plate Load Test (PLT) Data
The plate load test applies a gradually increasing load to a steel plate (typically 300 mm to 760 mm square) placed at foundation level, while recording settlement at each load increment. The resulting load–settlement curve is used to estimate the allowable bearing capacity of the proposed foundation.
Identify the failure point on the load–settlement curve using one of the following:
| Method | Description |
|---|---|
| Tangent intersection method | Draw tangents to the initial and final portions of the curve; their intersection gives the ultimate load. |
| Log–log plot method | Plot load vs. settlement on log–log scale; the break in slope indicates failure. |
| Settlement limit method | If no clear failure, take the load corresponding to a specified settlement (e.g., 10% of plate width or 25 mm). |
where \(Q_{u(plate)}\) is the ultimate load on the plate and \(A_{plate}\) is the plate area.
3. Extrapolate to Full-Scale Foundation
The relationship between plate and foundation bearing capacity depends on soil type:
For Cohesionless Soils (Sand/Gravel)
\[ q_{u(foundation)} = q_{u(plate)} \times \frac{B_f}{B_p} \]where:
- \(B_f\) = width of the proposed foundation (m)
- \(B_p\) = width of the test plate (m)
For Cohesive Soils (Clay)
\[ q_{u(foundation)} = q_{u(plate)} \]The ultimate bearing capacity is approximately the same for plate and foundation in clay (independent of size).
4. Apply Factor of Safety
\[ q_{allowable} = \frac{q_{u(foundation)}}{F_s} \]where \(F_s\) is typically 2.5 to 3.0.
5. Check Settlement Criteria
Even if the bearing capacity is adequate, verify that the settlement at working load is within tolerable limits.
For settlement extrapolation:
| Soil Type | Settlement Ratio |
|---|---|
| Sand (dense) | \(\rho_f = \rho_p \left(\frac{B_f}{B_p}\right)^2\) |
| Sand (loose) | \(\rho_f = \rho_p \left(\frac{B_f}{B_p}\right)^{1.5}\) |
| Clay | \(\rho_f = \rho_p \left(\frac{B_f}{B_p}\right)\) |
where \(\rho_f\) = foundation settlement and \(\rho_p\) = plate settlement at the corresponding pressure.
Key Equations
\[ q_{u(foundation)} = q_{u(plate)} \times \frac{B_f}{B_p} \quad \text{(sand)} \] \[ q_{u(foundation)} = q_{u(plate)} \quad \text{(clay)} \] \[ q_{allowable} = \frac{q_{u(foundation)}}{F_s} \]Remember
- The test reflects only a shallow, limited zone of soil (approximately 1.5 to 2 times the plate width in depth).
- Results are not reliable for deep or layered soils.
- The test is short-term, so consolidation settlement in clays is not captured.
- Water table fluctuations are not accounted for.
Practical Workflow
- Plot the load–settlement curve.
- Determine \(q_{u(plate)}\) using the tangent or log–log method.
- Extrapolate to foundation size using the appropriate soil-type formula.
- Divide by the factor of safety (typically 3).
- Verify that settlement at the allowable load is within acceptable limits (e.g., 25 mm for isolated footings).
This gives you the allowable bearing pressure and corresponding allowable load:
\[ Q_{allowable} = q_{allowable} \times A_{foundation} \]Relationship Between Plate Load Test and Terzaghi's Ultimate Bearing Capacity
Terzaghi developed a theoretical equation to predict the ultimate bearing capacity of a shallow foundation:
General Equation (Strip Footing)
\[ q_u = cN_c + qN_q + \frac{1}{2}\gamma B N_{\gamma} \]Modified for Square/Circular Footings (same shape as test plate)
\[ q_u = 1.3cN_c + qN_q + 0.4\gamma B N_{\gamma} \]where:
| Symbol | Meaning |
|---|---|
| \(q_u\) | Ultimate bearing capacity (kPa) |
| \(c\) | Soil cohesion (kPa) |
| \(q\) | Surcharge = \(\gamma D_f\) (kPa) |
| \(\gamma\) | Unit weight of soil (kN/m³) |
| \(B\) | Width of foundation (m) |
| \(D_f\) | Depth of foundation (m) |
| \(N_c, N_q, N_{\gamma}\) | Bearing capacity factors (functions of \(\phi\)) |
The plate load test empirically measures \(q_u\), while Terzaghi's equation theoretically predicts \(q_u\). They are two independent paths to the same result.
| Approach | Basis |
|---|---|
| Plate load test | Field measurement of load–settlement behaviour |
| Terzaghi's equation | Theoretical limit equilibrium analysis using soil parameters (\(c, \phi, \gamma\)) |
The Plate Load Test Validates Terzaghi's Equation by comparing:
\[ q_{u(plate, measured)} \quad \text{vs.} \quad q_{u(plate, Terzaghi)} \]you can assess whether the soil parameters used in Terzaghi's equation are representative.
Using the plate dimensions (\(B_p\), typically 0.3 m) and soil parameters:
\[ q_{u(plate, Terzaghi)} = 1.3cN_c + \gamma D_f N_q + 0.4\gamma B_p N_{\gamma} \]Compare with Measured Plate Test Result
\[ q_{u(plate, measured)} = \frac{Q_{u(plate)}}{A_{plate}} \]If they agree closely, the soil parameters are validated.
Extrapolate Both to Full-Scale Foundation
Using Plate Load Test Data:
\[ q_{u(foundation)} = q_{u(plate, measured)} \times \frac{B_f}{B_p} \quad \text{(sand)} \] \[ q_{u(foundation)} = q_{u(plate, measured)} \quad \text{(clay)} \]Using Terzaghi's Equation Directly:
\[ q_{u(foundation)} = 1.3cN_c + \gamma D_f N_q + 0.4\gamma B_f N_{\gamma} \]Why the Size Effect Differs
The difference in extrapolation rules (sand vs. clay) comes directly from Terzaghi's equation:
For Sand (\(c = 0\)):
\[ q_u = \gamma D_f N_q + 0.4\gamma B N_{\gamma} \]The term \(0.4\gamma B N_{\gamma}\) is proportional to \(B\), so:
\[ \frac{q_{u(foundation)}}{q_{u(plate)}} \approx \frac{B_f}{B_p} \]This is why the plate load test extrapolation uses \(\frac{B_f}{B_p}\) for sand.
For Clay (\(\phi = 0\)):
\[ q_u = 1.3cN_c + \gamma D_f N_q \]There is no \(B\) term (since \(N_{\gamma} = 0\) when \(\phi = 0\)), so:
\[ q_{u(foundation)} = q_{u(plate)} \]This is why the plate load test extrapolation is 1:1 for clay.
Test Standards
| Standard | Title |
|---|---|
| AS 1289.6.2.3 | Determination of the bearing capacity of a soil — Plate load test |
| AS 1289.6.2.4 | Determination of the modulus of subgrade reaction — Plate load test |
| ASTM D1194 | Bearing capacity of soil for static load on spread footings |
| ASTM D1195 | Repetitive static plate load tests |
| TfNSW T113 | Plate load testing for working platforms |
Typical Results
| Ground Condition | Allowable Bearing Capacity (kPa) | k-value (MPa/m) for 300 mm plate |
|---|---|---|
| Very dense sand/gravel | 500–1,000 | 100–300 |
| Dense sand | 300–500 | 50–150 |
| Stiff clay | 200–400 | 40–100 |
| Firm clay | 100–200 | 20–50 |
| Loose sand | 50–150 | 10–30 |
| Cement-stabilised fill | 500–1,500 | 150–500 |
| Rock (competent) | 1,000–5,000+ | 200–1,000+ |
Limitations
- Scale effect — plate is smaller than the actual foundation, so settlement estimates are approximate
- Stress bulb — only tests ground to depth of 1.5× plate diameter
- Reaction system — requires heavy reaction load (up to 200 tonnes)
- Time — maintained load tests take 2–6 hours per test
- Temperature — hot weather can affect gauge readings (use sun shields)
Plate Load Testing for Working Platforms
For safe working platform certification:
| Plant Type | Typical Test Load | Settlement Limit |
|---|---|---|
| 50t crawler crane | 300–500 kPa | < 10 mm at test load |
| Piling rig | 400–600 kPa | < 15 mm at test load |
| 20t excavator | 150–250 kPa | < 15 mm at test load |
| Mobile crane outriggers | 500–800 kPa | < 10 mm at test load |
How many plate load tests are needed?
For working platforms: 1 test per 500–1,000 m². For foundation verification: 1 test per footing or per distinct soil zone.
Can plate load tests be performed at depth?
Yes. Tests can be performed at the base of excavations, within boreholes, or at foundation level using specialised equipment.
What k-value is needed for raft slabs?
AS 2870 specifies k-values for different site classes. Typical values range from 5–50 MPa/m for a 750 mm plate.
Is plate load testing the same as a bearing capacity test?
Plate load testing is one method of determining bearing capacity. It is the most direct method but has scale effect limitations.