⚡ electrical engineering

Estimating the allowable bearing pressure of soils from small-scale plate load and direct shear tests: laboratory-prepared mixtures and natural subgrade soils

This study demonstrates that combining small-scale plate load tests (evaluating settlement) with direct shear tests (evaluating shear strength) on laboratory-prepared and natural soils provides a reliable, conservative method for estimating allowable bearing pressure, serving as an effective alternative when full-scale field testing is unavailable.

Original authors: Kakasaheb D. Waghmare, Harsh Rajput, Dipraj Bachchhav, Archana Attarde, K. K. Tripathi

Published 2026-11-03

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CC BY 4.0

Original authors: Kakasaheb D. Waghmare, Harsh Rajput, Dipraj Bachchhav, Archana Attarde, K. K. Tripathi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Every structure built on the ground, from a simple garden path to a massive highway overpass, relies on the soil beneath it to hold its weight. Before engineers can design a foundation, they must answer a critical question: how much pressure can this specific patch of earth safely support before it fails or sinks too far? If the ground gives way, the structure above it cracks or collapses; if it settles unevenly, doors jam and walls split. The standard way to find this limit is to perform a plate load test, where a heavy metal plate is pressed into the soil in the field until it reaches its breaking point. While this method is the most direct and reliable, it requires enormous, expensive machinery and takes days to complete, making it impractical for many routine projects. Engineers often turn to cheaper, faster alternatives, but these rely on guesswork and correlations that can sometimes be misleading.

A team of researchers in India set out to find a middle ground that is both affordable and trustworthy. They asked whether a small-scale version of the heavy test, combined with a different laboratory measurement of soil strength, could predict the safe limits of the ground without needing the massive field equipment. To test this, they gathered soil from five different natural sites in Pune and also created six specific mixtures of sand, clay, and gravel in the lab. They compacted these soils into steel boxes to mimic the density found in the real world. Inside these boxes, they placed metal plates of two different sizes—one roughly the size of a large dinner tray and another about the size of a small coffee table—and slowly loaded them with weights to see how much the soil squished under the pressure. Simultaneously, they took samples of the same soil and sheared them apart in a separate machine to measure how much friction held the soil particles together.

The researchers discovered that the two tests told different stories, and the smaller story was the one that mattered. The test that measured how much the soil sheared or slid apart suggested the ground could hold a tremendous amount of weight, often predicting safe limits between 450 and 1,000 kilonewtons per square meter. However, the test that measured how much the soil settled or sank under the load told a much more cautious tale. In five out of the six laboratory mixtures, the settlement test showed that the soil would compress too much long before it actually failed. This meant that the safe limit was not determined by the soil's strength, but by how much it was willing to squish. When they applied this same method to the natural soils from the field, the results were even more revealing. For the sandy and gravelly sites, the small-scale test with the larger plate predicted a safe pressure that was about 88 to 95 percent of what the standard field tests suggested. The smaller plate was even more cautious, predicting only 62 to 82 percent of the standard value.

The most striking validation came from a site with soft, silty soil where the researchers could not use the standard heavy machinery. Here, they compared their small-scale results against a single, full-scale field test performed on the actual ground. The full-scale test showed the soil could safely hold 33 kilonewtons per square meter. The researchers' small-scale method, using the two different plate sizes, produced results of 29 and 36 kilonewtons per square meter. These two numbers effectively bracketed the real-world result, capturing the true limit with surprising accuracy. The study concluded that relying on the shear test alone would have been dangerously optimistic, overestimating the soil's capacity by a factor of nearly three in some cases. Instead, the most reliable approach is to use the small-scale settlement test to find the limit of how much the ground will sink, and to use the shear test only as a backup check. This combination offers a conservative, safe, and practical way to estimate ground strength for roads and foundations, especially in locations where bringing in heavy, expensive testing equipment is impossible.

Technical Summary: Estimating Allowable Bearing Pressure from Small-Scale Plate Load and Direct Shear Tests

Problem Statement
The plate load test (PLT) is widely regarded as the most reliable method for determining the allowable bearing pressure of foundation and subgrade soils because it simulates foundation loading without relying on empirical correlations. However, full-scale PLTs are often impractical due to high costs, long durations, and the requirement for heavy reaction platforms to mobilize ultimate capacities. Conversely, alternative in-situ methods like the Standard Penetration Test (SPT) are cost-effective but rely on empirical correlations between blow counts, friction angles, and bearing pressures, which introduce uncertainty. Furthermore, in granular soils where undisturbed sampling is difficult, settlement criteria often govern design, yet standard laboratory tests may not adequately capture scale effects. There is a need for a practical, reliable, and conservative estimation method for allowable bearing pressure that can be executed where standard field equipment is unavailable.

Methodology
This study investigates a hybrid laboratory procedure combining a Small-Scale Plate Load Test (SPLT) and a Direct Shear Test (DST) to estimate allowable bearing pressure. The research was conducted in two series:

Series A (Laboratory-Prepared Mixtures): Six controlled mixtures of sand, kaolin, and coarse aggregate were prepared. Soils were compacted in steel containers to their Maximum Dry Density (MDD) and Optimum Moisture Content (OMC).
Series B (Natural Subgrade Soils): Five natural soil sites in Pune, India, were sampled. Soils were compacted in the laboratory to their in-situ field density and moisture content.
Testing Apparatus: Both series utilized a large-box direct shear apparatus modified for SPLT. Two plate sizes were employed: 0.10 m × 0.27 m and 0.20 m × 0.30 m. The plates were positioned at the edge of the container to minimize boundary confinement effects.
Procedures:
- DST: Performed on specimens at field/MDD to determine shear strength parameters (cohesion $c$ and friction angle $\phi$ ).
- SPLT: Load was applied in increments, and settlement was recorded until failure or apparatus capacity. The ultimate plate capacity ( $q_{up}$ ) was identified using the tangent-intersection method.
- Evaluation: The allowable bearing pressure ( $q_a$ $q_{a}$ ) was defined as the minimum of two criteria:
  1. Shear Criterion ( $q_{ns}$ ): Calculated from DST parameters using Terzaghi's equation (with a factor of safety $F=3$ ).
  2. Settlement Criterion ( $q_p$ ): Determined from the SPLT load-settlement curve at the plate settlement corresponding to a permissible 25 mm footing settlement, scaled using Terzaghi–Peck relations for cohesionless soils and linear relations for cohesive soils.
Validation: Results for natural soils were compared against SPT-based estimates (using various empirical correlations) and, at one site (Site 4), against a full-scale field PLT.

Key Results

Laboratory Mixtures (Series A):
- Replacing 20% of sand–kaolin mass with aggregate increased ultimate plate capacity by 31–75% and maximum dry density by approximately 0.2 g/cm³. However, higher ultimate capacity did not guarantee higher safe bearing pressure; Soil 6 exhibited the highest ultimate capacity but a lower settlement-based pressure (360 kN/m²) compared to Soil 5 (850 kN/m²) due to a softer load-settlement response.
- The settlement criterion governed the allowable pressure in five of the six mixtures. The shear-based net safe capacity derived from DST (451–1024 kN/m²) was 1.4 to 2.8 times higher than the settlement-based pressure, indicating that relying solely on DST is unconservative for these soils.
- Direct extrapolation of plate ultimate load to footing capacity using width ratios yielded erratic results (ranging from 133 to 1533 kN/m²) and was deemed unreliable.
Natural Soils (Series B):
- For natural granular soils, the allowable pressure derived from the 0.20 m plate combined with DST ranged from 88% to 95% of SPT-based values. The 0.10 m plate yielded more conservative values, ranging from 62% to 82% of SPT estimates.
- At Site 4 (soft, high-plasticity silt), the SPLT results (29 kN/m² for the 0.10 m plate and 36 kN/m² for the 0.20 m plate) bracketed the full-scale field PLT safe pressure of 33 kN/m².
- In all natural soil cases, the SPLT-based estimates were lower than or equal to SPT-based estimates, confirming the procedure's conservative nature.

Key Contributions and Significance
The paper proposes a practical, low-cost methodology for estimating allowable bearing pressure when full-scale field testing is not feasible. The primary contribution is the demonstration that combining SPLT (for the settlement criterion) with DST (for the shear criterion) provides a reliable and conservative estimate.

Conservatism: The study validates that this hybrid approach yields conservative results compared to standard SPT correlations and aligns closely with field PLT data in soft soil conditions.
Settlement Dominance: It reinforces the established view that in granular and mixed soils, the settlement criterion typically governs the allowable pressure, a factor often missed if only shear strength parameters are considered.
Plate Size Effect: The research highlights that while smaller plates (0.10 m) are highly conservative, larger plates (0.20 m) provide estimates closer to SPT-based values, offering a balance between safety and economic design.
Applicability: The procedure is presented as a viable screening or preliminary design tool for transportation infrastructure geotechnics, particularly for earthworks, subgrades, and abutments where heavy reaction frames cannot be mobilized.

Limitations and Future Work
The authors acknowledge that the tests were conducted on remoulded soil in rigid containers, meaning natural soil structure, layering, and saturation effects are not fully represented. The extrapolation from small plates to footings relies on empirical scaling that may be uncertain for gravelly soils. Additionally, the SPT-based benchmarks used for comparison are themselves empirical. The authors conclude that while the method is promising for preliminary design, results should be corroborated by field tests before final design. Future work is suggested to compare SPLT results on undisturbed samples with field PLTs, evaluate saturated conditions, and test a broader range of soil types using a newly designed portable instrument capable of DST, CBR, and SPLT.

Technical Summary: Estimating Allowable Bearing Pressure from Small-Scale Plate Load and Direct Shear Tests

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