The shallow plate load test is the most widely used in-situ method for determining the bearing capacity and deformation parameters of shallow soil layers, compacted fill, and engineered subgrades. By applying incremental vertical loads to a rigid steel plate at the proposed foundation level and measuring the resulting settlement, the test delivers direct, reliable data for foundation design and construction quality acceptance. This article explains the technical principles, equipment, test procedures, applicable standards, data interpretation, and practical applications of the shallow plate load test.
Technical Overview
The shallow plate load test (PLT), also known as the shallow plate bearing test, is an in-situ geotechnical test used to evaluate the bearing capacity and settlement characteristics of soil within the stress-influence zone beneath shallow foundations. A rigid circular or square loading plate is placed at the base of a test pit excavated to the proposed foundation level, and vertical load is applied in controlled increments while the settlement of the plate is recorded at each stage. The resulting load-settlement (p-s) curve forms the basis for determining the ultimate bearing capacity, the characteristic value of subgrade bearing capacity, the deformation modulus, and the coefficient of subgrade reaction.
The test is classified as a "shallow" plate load test when the tested soil layer lies within approximately 3 to 5 meters below the excavation base, where the influence depth of the loaded plate covers the zone of practical interest for spread footings, rafts, and subgrade layers. For deeper strata or pile end-bearing layers, a deep plate load test performed at the bottom of a borehole is used instead. The shallow plate load test is recognized by major international and national standards, including ASTM D1194, ASTM D1195 and D1196, ISO 22476-13, BS 1377-9, GB 50021, GB 50202, and JGJ 340, and is routinely carried out by accredited third-party laboratories and geotechnical testing institutions worldwide.
Testing Samples
Because the plate load test is an in-situ test, the "sample" is the soil layer itself at the designated test location. The method is applicable to a wide range of near-surface materials:
Natural Cohesive Soils
Clays, silty clays, and clayey silts where undisturbed sampling is difficult and direct measurement of bearing behavior at foundation level is required.
Granular Soils
Sands, sandy gravels, and gravel deposits in which sample disturbance makes laboratory strength tests unreliable, and where the plate test provides a realistic measure of the in-place density and stiffness.
Compacted and Engineered Fill
Replacement cushions (sand, gravel, crushed stone), roller-compacted fill layers, and backfill behind structures, where the test verifies that compaction quality meets the specified bearing and modulus requirements.
Subgrade and Pavement Layers
Highway, railway, and airport subgrade layers, for which plate loading tests determine the modulus of subgrade reaction or resilient modulus used in pavement design.
Weathered Rock and Soft Rock
Highly to moderately weathered rock surfaces and soil-rock interfaces, where the test establishes allowable bearing pressures for lightly to moderately loaded structures.
Improved Ground
Ground treated by dynamic compaction, preloading, vibro-replacement, or other improvement techniques, tested to confirm that the specified post-treatment bearing capacity has been achieved.
Testing Items
The shallow plate load test provides the following principal results:
- Proportional limit load: the load at which the p-s curve departs from linearity, marking the onset of plastic deformation.
- Ultimate bearing capacity: the maximum pressure the soil can sustain before shear failure or excessive settlement occurs.
- Characteristic value of subgrade bearing capacity: the design value derived from the proportional limit, the ultimate load, or a settlement-based criterion.
- Deformation modulus (E0): the stiffness parameter of the tested soil layer calculated from the linear segment of the load-settlement curve.
- Coefficient of subgrade reaction (k): the pressure-to-settlement ratio used in the design of pavements, slabs-on-grade, and embedded structures.
- Resilient modulus: the recoverable stiffness parameter determined by repetitive loading for pavement and subgrade design.
- Settlement behavior: time-dependent settlement at each load increment, which reflects the consolidation and creep characteristics of the tested layer.
Testing Methods
Test Apparatus and Setup
A complete test assembly consists of a rigid steel loading plate (commonly 300 mm, 450 mm, 500 mm, 600 mm, or 750 mm in diameter, with an area of not less than 0.25 square meters for bearing capacity evaluation), a reaction system providing counterweight, a calibrated hydraulic jack with a capacity of at least 1.2 times the maximum test load, a load cell or calibrated pressure gauge for load measurement, and four dial gauges or displacement transducers mounted on a reference beam to measure plate settlement. The reaction may be supplied by stacked concrete blocks or steel ingots (kentledge) or by ground anchors, and the reference beam supports must be positioned well outside the zone of influence, typically at least 1.5 to 2 meters from the plate and reaction supports.
Test Pit Preparation
A test pit is excavated to the proposed foundation base level with a width of at least three times the plate diameter so that the stress bulb beneath the plate develops freely. Extreme care is taken to avoid disturbing the bearing surface; any loosened or puddled material is removed, and a thin layer of medium-dry sand or rapid-setting gypsum is spread beneath the plate to ensure full, level contact. The plate is then centered under the jack and reaction system, and the settlement gauges are symmetrically arranged on the plate rim.
Loading Procedure
The load is applied in 8 to 12 approximately equal increments, each typically one-tenth of the expected ultimate or design pressure, and the total applied load is carried to at least twice the design bearing pressure unless failure occurs earlier. In the standard slow maintained-load method, each increment is held until the settlement rate falls below 0.1 mm per hour before the next increment is added, with readings taken at prescribed time intervals. Where the standard requires it, the load is also removed in stages to measure elastic rebound. Quick-load and constant-rate-of-penetration variants are permitted by some standards for specific applications such as pavement evaluation.
Termination Criteria
Loading is terminated when one of the following conditions is observed:
- Settlement increases sharply and the p-s curve exhibits a pronounced steep drop segment.
- Soil around the plate visibly extrudes sideways, heaves, or develops radial cracking.
- Settlement under a given increment fails to stabilize within 24 hours.
- The total settlement reaches 0.06 times the plate diameter or width (s/b greater than or equal to 0.06).
- The maximum planned load, typically twice the design pressure, has been sustained successfully.
Data Interpretation
The characteristic value of subgrade bearing capacity is taken as the proportional limit load when it is clearly defined on the p-s curve; if the ultimate load is less than twice the proportional limit load, one-half of the ultimate load governs; otherwise the load corresponding to a relative settlement of s/b equal to 0.01 to 0.015 is adopted, subject to a limit of one-half of the maximum applied load. The deformation modulus is computed from the linear portion of the curve using the relation E0 = I × (1 - ν²) × d × p / s, where I is a plate shape factor (0.785 for a rigid circular plate and 0.886 for a rigid square plate), d is the plate diameter or width, ν is the Poisson's ratio of the soil (typically assumed between 0.25 and 0.45 depending on soil type), and p and s are the paired pressure and settlement values within the elastic range. Results from multiple test points on the same layer are evaluated statistically, and a minimum of three valid points per layer is normally required.
Applications
The shallow plate load test is applied across the full spectrum of geotechnical construction and design:
- Verification of bearing capacity for spread footings, rafts, and light industrial and residential buildings on shallow natural soils.
- Acceptance testing of replacement sand and gravel cushions and compacted fill layers in foundation construction quality acceptance.
- Confirmation of improvement effectiveness after dynamic compaction, vibro-compaction, preloading, and other ground treatment works.
- Determination of the coefficient of subgrade reaction and resilient modulus for highway, airport, and port pavement design.
- Design parameters for culverts, retaining wall bases, pipeline foundations, and machinery foundations requiring stiffness data.
- Forensic investigation of settlement or bearing failures in existing structures, providing measured values for comparison with design assumptions.
- Calibration and correlation of indirect in-situ methods such as standard penetration tests and cone penetration tests at project sites.
FAQ
What plate size should be selected for the test?
The plate should be large enough to engage a representative volume of soil and reflect particle size effects. For bearing capacity evaluation, a plate area of at least 0.25 square meters (approximately 565 mm diameter for a circular plate) is commonly specified, with larger plates such as 750 mm or 1000 mm preferred for gravelly or very heterogeneous deposits. Smaller plates of 300 mm to 500 mm are widely used for subgrade and pavement modulus testing under the relevant highway standards.
What is the difference between a shallow and a deep plate load test?
A shallow plate load test is performed in an open test pit at or near the proposed foundation level, typically within 3 to 5 meters of the surface, and measures the behavior of a broad zone beneath the plate. A deep plate load test is performed at the bottom of a borehole and is used to evaluate deeper strata or the end-bearing resistance of large-diameter piles, using specialized downhole loading apparatus.
How is the characteristic value of bearing capacity determined from the p-s curve?
The characteristic value is taken from the proportional limit when a clear linear segment is present; if the ultimate load is smaller than twice the proportional limit, one-half of the ultimate load is used; where neither boundary is distinct, the pressure at a relative settlement of 1.0 to 1.5 percent of the plate width is adopted, capped at one-half of the maximum test load. The selected value is then verified statistically across the required number of test points.
How long does a shallow plate load test take?
A single test typically requires one to three days depending on the soil type and load schedule. Free-draining sands and gravels usually stabilize within minutes to an hour per increment, while saturated clays may require many hours per increment because settlement is governed by consolidation. A full program with several test points at different locations may extend over one to two weeks.
How many test points are required on a project site?
Most standards, including GB 50021 and related acceptance codes, require a minimum of three valid test points in the same soil layer for each evaluation unit so that a statistical characteristic value can be established. Additional points are added for large sites, heterogeneous deposits, or where results show significant scatter.
Can the shallow plate load test be performed on rock?
Yes. On weathered rock or soft rock surfaces the test is used to confirm allowable bearing pressures, provided the plate is seated on sound, level material and rigid packing is used to ensure uniform contact. For hard intact rock with obviously high strength, the plate test is generally unnecessary and design values are assigned from empirical correlations instead.