A tool that scales the settlement measured with a small bearing plate up to predict how a real, full-size foundation will settle. Change the plate width, foundation width, applied pressure and soil type to see the settlement scale factor, predicted foundation settlement and subgrade modulus update in real time.
Parameters
Plate width B_p
m
Width of the rigid bearing plate (standard 30 to 75 cm)
Real foundation width B_f
m
Width of the actual footing that supports the building
Applied test pressure
kPa
Contact pressure applied to the bearing plate
Test plate settlement s_p
mm
Settlement of the plate measured at that pressure
Soil type
Sets the settlement scaling rule of the ground
Results
—
Settlement scale factor
—
Predicted foundation settlement (mm)
—
Test subgrade modulus k_p (kN/m³)
—
Foundation subgrade modulus k_f (kN/m³)
—
Applied test pressure (kPa)
—
Settlement check
—
Test plate vs real foundation — settlement and stress spread
Left is the small test plate, right is the large real foundation. At the same applied pressure, the wide foundation pushes its stress bulb much deeper and settles more. The shaded zones are the soil that deforms.
Ratio of the real foundation settlement s_f to the test plate settlement s_p (the scale factor). B_p: plate width, B_f: foundation width. Sand uses the Terzaghi-Peck relation; clay scales in proportion to width.
$$s_f = s_p \cdot \frac{s_f}{s_p}, \qquad k = \frac{p}{s}$$
Predicted foundation settlement s_f and subgrade modulus k (p: pressure, s: settlement). The foundation k_f equals the test k_p divided by the scale factor.
A large real foundation always settles more than the small test plate. This is the scale effect, and it is why the test result must never be used as-is.
What is the Plate Load Test Simulator?
🙋
A "plate load test" just puts a plate on the ground and stacks weight on it, right? What can you actually learn from that?
🎓
In short, it directly measures "if I apply this much pressure to the ground, how far does it sink". Before you found a building or a road on the soil, you want two things: how much load the ground can safely carry, and how much it will settle. The plate load test gives that answer right at the bottom of the excavation, at the depth the real foundation will sit. You place a rigid steel bearing plate about 30 to 75 cm across, apply load in steps with a hydraulic jack, and measure the settlement at each step. From that you build a load-settlement curve.
🙋
Got it! So if the test shows "200 kPa caused 8 mm of settlement", can I assume the real foundation will settle the same?
🎓
That is the single most important trap in this test. The test plate is small, the real foundation is large, and the two do not settle the same way. The bigger the loaded area, the deeper the stress reaches into the ground and the more soil it mobilises. So at the same applied pressure, a wide foundation settles far more than the small plate. We call this the "scale effect". Thinking "the test gave 8 mm, so the real foundation gives 8 mm" is a classic and dangerous mistake.
🙋
Oh, really? Then how do you get the real foundation's settlement?
🎓
You magnify the plate settlement by a "scale factor", and that factor depends on the soil. In sand, the Terzaghi-Peck relation says the factor grows as the foundation widens but levels off toward a limit of 4. In clay it is simpler — settlement scales almost in direct proportion to the width. Switch the "soil type" on the left to clay and raise the foundation width: you will see the factor climb steeply. For the default case in sand the scale factor is about 3, meaning the real foundation is predicted to settle three times as much as the test plate.
🙋
There are two subgrade moduli, k_p and k_f. What is the difference?
🎓
The subgrade modulus k is "pressure per unit settlement" — basically the spring stiffness of the ground. The k_p measured in the test is the spring stiffness under the plate. But the real foundation settles more at the same pressure, so as a spring it is "softer". So the foundation's k_f is the k_p divided by the scale factor — a smaller value. When you analyse a foundation with a spring model, feeding in the test k_p directly underestimates settlement. The rule is to always use a k_f corrected for the foundation width.
🙋
If only the soil right under the test plate is good, and there is a soft layer below it, that sounds dangerous.
🎓
That is exactly the point to watch. A plate load test can only evaluate to a depth of about 1.5 to 2 times the plate width. A 30 cm plate only sees a shallow layer 50 to 60 cm below the surface. But a 2 m wide foundation deforms the ground 3 to 4 m deep. If the soil is dense only under the plate and a soft layer lurks at that depth, the test passes but the real foundation settles badly. So a plate load test is never used alone — you confirm with borings that the ground is uniform to twice the foundation width first.
Frequently Asked Questions
No. The bearing plate used in the test (about 30 to 75 cm across) is far smaller than a real foundation, and at the same applied pressure the real foundation settles much more. A larger loaded area pushes its stress influence deeper into the ground and mobilises more soil. This is the scale effect: the test plate's settlement must be multiplied by a magnification factor that depends on the soil before it predicts the real foundation's settlement. This tool computes the scale factor for sand and for clay.
For sand the Terzaghi-Peck relation s_f/s_p = (2B_f/(B_f+B_p))² is used. The ratio grows as the foundation gets wider but levels off toward an upper limit of 4. For clay the settlement scales almost in direct proportion to width, so s_f/s_p = B_f/B_p and the ratio keeps growing with foundation width without bound. For the same test plate settlement, a large foundation on clay tends to suffer more severe settlement.
The subgrade modulus k is the pressure the ground carries per unit settlement (k = pressure / settlement) — essentially a spring stiffness. Because the real foundation settles more than the test plate at the same pressure, k_f = k_p / scale factor, so the foundation's k is smaller than the k_p measured in the test. When you model a foundation with springs, feeding in the test k_p directly underestimates settlement. Always use a k_f corrected for the foundation width.
Roughly to a depth of 1.5 to 2 times the plate width. Stress only reaches a zone scaled to the loaded width, so a 30 cm test plate only evaluates a shallow layer about 50 to 60 cm below the surface. A 2 m wide real foundation, by contrast, deforms the ground 3 to 4 m deep. If only the soil right under the test plate is good and a soft layer hides below, the test passes but the real foundation settles a lot. A plate load test should be used together with boring, after confirming the ground is uniform to about twice the foundation width.
Real-World Applications
Foundations of houses and small buildings: For the mat or strip footings of timber houses, site investigation is done with plate load tests or Swedish-weight soundings. The plate result is used to estimate the long-term allowable bearing capacity and settlement, and to decide the foundation type and whether ground improvement is needed. House footings are around 1 m wide — fairly close to the test plate — so the scale-effect correction is milder than for medium-rise buildings, but ignoring the factor still misjudges settlement.
Pavement design for roads and airports: The subgrade modulus K30 (30 cm plate) or K75 is used directly to assess the bearing capacity of road subgrades and bases. Because pavement design thickness depends on the spring stiffness of the ground, the K value from a plate load test is core design data for the pavement structure. A tyre contact patch is close in size to the test plate, so the scale effect matters relatively little in pavement work.
Quality control of fill and compaction: In land development and road embankments, a plate load test can confirm that each lift is adequately compacted. A lift that fails to reach the specified subgrade modulus or settlement is re-rolled. As a construction control test it is used for a relative pass/fail decision, which differs in purpose from an absolute foundation settlement prediction.
Setting the soil spring stiffness for CAE analysis: In FEM analysis where a foundation is modelled on spring supports, each spring is given a subgrade modulus. Using the k_p from a plate load test directly gives a value for a small loaded area, so a k_f corrected to the real foundation width must be entered. The scale correction in this tool helps make a first estimate of the soil spring stiffness fed into the analysis model.
Common Misconceptions and Pitfalls
The biggest misconception is treating "the test plate settlement" as "the real foundation settlement". As this tool repeatedly shows, the test plate is small and the real foundation is large, so at the same applied pressure the real foundation settles far more. In sand the scale factor reaches up to 4; in clay it grows by the ratio of foundation width to plate width. Believing "the test gave 8 mm so the real foundation will only settle 8 mm" is a classic and dangerous error that leads to differential settlement and serviceability problems. Always apply the scale correction to the test result.
Next is underrating the fact that the test plate only sees a shallow depth. A plate load test can evaluate only to 1.5 to 2 times the plate width — for a 30 cm plate that is just 50 to 60 cm below the surface. Even if that shallow layer is good, if a soft or consolidating layer hides in the several-metre depth where the real foundation spreads its stress, the test passes but the real foundation settles a lot, and long-term consolidation adds on top. Never conclude from a plate load test alone — always confirm the deeper stratigraphy with borings or soundings.
Finally, overlooking that the plate test settlement is an immediate settlement. The settlement measured over a short time in a plate load test is mainly immediate (elastic) settlement. In clay, consolidation settlement — which occurs slowly afterwards as water drains out — adds over a long period, and the final settlement is larger still than a simple scale-up of the test value. The scale factor in this tool addresses the width dependence of immediate settlement; the long-term consolidation settlement of clay needs separate study, such as a consolidation test.
How to Use
Enter plate width (typically 300–750 mm for standard PLT) and foundation width in millimeters using the slider or numeric input.
Set test pressure (50–500 kPa range) applied during the plate load test and measured plate settlement in millimeters.
Simulator calculates settlement scale factor using Terzaghi–Meyerhof extrapolation, derives test subgrade modulus k_p, then predicts full-size foundation settlement using foundation subgrade modulus k_f for your actual footing dimensions.
Worked Example
Square plate load test: 600 mm × 600 mm test plate at 200 kPa produces 8 mm settlement (k_p = 25 kN/m³). Scaling to a 2000 mm × 2000 mm foundation footing, the settlement scale factor = sqrt(600/2000) = 0.548, reducing k_f to approximately 13.8 kN/m³. Predicted foundation settlement = 200 kPa ÷ 13.8 kN/m³ = 14.5 mm. For clay-silt subgrades with higher compressibility, settlement values typically increase 20–40% versus granular soils.
Practical Notes
Always correct plate modulus k_p for soil type: fine sand (k_p 10–20 kN/m³), medium sand (20–50), gravel (50–150). Cohesive soils require undrained shear strength and OC ratio verification before extrapolation.
PLT settlement scale factors follow sqrt(B_foundation/B_plate); values under 0.4 indicate significant uncertainty—conduct multiple tests or geotechnical borings to validate subgrade homogeneity.
For mat foundations or raft systems, limit predicted settlement to 25–50 mm; differential settlement between zones must not exceed 1:300 (L/Δh ratio) for structural safety.