The screw plate load test (SPLT), also known as the helical plate load test, is an in-situ geotechnical testing method used to evaluate the bearing capacity and deformation parameters of foundation soils at depth. By rotating a helical steel plate to a target depth and applying controlled incremental loads, the test generates load–settlement curves from which bearing capacity values, deformation moduli, and subgrade reaction coefficients are derived. This article explains the working principle, applicable ground conditions, measured parameters, test procedures, and practical applications of the screw plate load test in foundation engineering.
Technical Overview
The screw plate load test is an in-situ loading test in which a steel helical (screw) plate, attached to a string of drill rods, is rotated into the ground to a predetermined depth so that it bears directly on undisturbed soil. Vertical loads are then applied in increments through the rods, and the settlement of the plate is recorded at each stage. The resulting load–settlement (p–s) relationship is used to evaluate the bearing capacity and compressibility of the soil at the actual depth of interest, typically between 2 m and 15 m below ground surface, with greater depths possible using specialized rigs.
Compared with the conventional shallow plate load test, which requires a large test pit and is limited to near-surface soils, the screw plate test eliminates excavation, reaches deeper strata, allows multiple test stages at different depths within a single borehole, and provides a loading condition that closely resembles a deep foundation element. Compared with laboratory compression tests, it avoids sample disturbance, which is particularly valuable in sands, soft silts, and highly sensitive clays where undisturbed sampling is difficult.
Key characteristics of the method include:
- Testing of undisturbed soil at depth, including strata below the groundwater table
- Determination of the characteristic value of subsoil bearing capacity, deformation modulus, compression modulus, and subgrade reaction coefficient
- Multistage testing at several depths in one investigation point
- Faster execution and lower cost than large-scale shallow plate load tests
- Consistency with recognized geotechnical standards such as GB 50021 (Code for Investigation of Geotechnical Engineering), GB 50007 (Code for Design of Building Foundation), BS 1377-9, EN 1997-2 (Eurocode 7, Part 2), and the ISO 22476 series of geotechnical field testing standards
The main limitations are that the method is unsuitable for gravelly, cobble-bearing, or cemented strata that obstruct screwing, that rod friction and rod elastic compression must be corrected for or measured directly, and that the relatively small plate diameter introduces scale effects that should be considered when extrapolating results to full-scale foundations.
Testing Samples
Fine-Grained Cohesive Soils
Soft to firm clays, silty clays, and clayey silts are the classic application of the screw plate test. The helical plate can be installed with minimal disturbance, and the test yields reliable bearing capacity and deformation parameters for settlement analysis. In sensitive or highly plastic clays, a waiting period after installation is recommended to allow torque-induced excess pore water pressure to dissipate before loading begins.
Sandy and Silty Soils
Loose to medium-dense sands, silty sands, and non-plastic silts, where undisturbed sampling is generally impractical, can be tested effectively. The test can be performed below the groundwater table without dewatering, which is a significant advantage over pit-based plate load tests.
Improved and Compacted Ground
Ground improved by dynamic compaction, preloading and surcharging, vibro compaction, or placement of engineered granular fill can be verified by screw plate load testing at the design depth, providing direct confirmation of post-improvement bearing capacity and stiffness.
Unsuitable Ground Conditions
The test is not applicable to soils containing abundant gravel, cobbles, boulders, construction debris, or very dense and hard cemented layers, because the plate cannot be screwed to depth without obstruction or excessive torque. Very stiff residual soils and weathered rock should be assessed with alternative in-situ methods such as pressuremeter or borehole jack testing.
Testing Items
The screw plate load test provides the following geotechnical parameters, each derived from the measured load–settlement response:
- Characteristic value of subsoil bearing capacity (fak) at the test depth
- Ultimate bearing pressure or limit pressure (pu) at failure or at the specified settlement limit
- Proportional (yield) limit pressure (p0) marking the transition from quasi-elastic to plastic soil behavior
- Deformation modulus (E0) of the soil at the test depth
- Compression (constrained) modulus derived from the slope of the p–s curve between load increments
- Subgrade reaction coefficient (k) for use in structural foundation interaction analysis
- Elastic rebound parameters from unload–reload cycles
- Indicators of stress history, such as estimated preconsolidation pressure, in fine-grained soils where required by the design brief
- Water-sensitivity (collapsibility) assessment in loess and other moisture-sensitive soils, using controlled inundation variants of the test
Testing Methods
Equipment and Instrumentation
A typical test system comprises a hardened steel helical plate, commonly 100 mm to 300 mm in diameter, coupled to drill rods; a drilling rig or torque head for installation; a reaction system using ground anchors or kentledge; a calibrated hydraulic jack with load cell; and electronic displacement transducers or dial gauges with a resolution of 0.01 mm mounted on an independent reference beam. A sleeve pipe or a load cell placed immediately above the plate may be used to isolate the applied load from rod friction. All load and displacement instruments must carry valid calibration certificates, and testing should be performed by accredited third-party laboratories or qualified geotechnical investigation units.
Test Preparation and Installation
A borehole is advanced to approximately 300 mm to 500 mm above the intended test depth, or the plate is screwed directly from the ground surface. The helical plate is then rotated smoothly into undisturbed soil until its bearing surface reaches the target depth. Verticality of the rod string is maintained, installation torque is recorded, and consecutive test depths are spaced at least 1.0 m apart, or as specified in the investigation plan. Zero readings are verified, and in cohesive soils an equalization period is observed before loading.
Loading Procedure
Load is applied in equal increments, typically one-tenth to one-eighth of the estimated ultimate pressure, with the maximum test load reaching at least twice the design bearing pressure. Under the relative stability (slow) method, each increment is maintained until the settlement rate falls below 0.1 mm per hour, with settlement readings taken at prescribed time intervals. A quick-loading schedule may be adopted for homogeneous strata when justified by the design engineer. Unloading is carried out in stages back to zero load, with rebound readings recorded at each stage.
Termination Criteria
The test is terminated when one of the following occurs: settlement reaches 0.06 times the plate diameter; the soil around the plate exhibits extrusion or heave indicating failure; the applied pressure cannot be maintained by the hydraulic system; or the stroke or capacity of the loading and reaction system is exhausted.
Data Processing and Interpretation
The primary output is the p–s curve, supported by s–log t or s–log p plots for each stage. The proportional limit pressure p0 is identified at the break in the p–s curve, and the ultimate pressure pu corresponds to the failure plateau or the pressure at the limiting settlement. The characteristic value of subsoil bearing capacity is taken as the lesser of p0 and the limit pressure divided by the applicable factor of safety, verified against the allowable settlement criterion, in accordance with GB 50021 and GB 50007. The deformation modulus is computed from the slope of the quasi-elastic portion of the curve using the elastic half-space relationship E0 = I × (1 − μ²) × d × Δp / Δs, where I is an empirical factor reflecting plate rigidity and embedment depth, μ is Poisson's ratio, d is the plate diameter, and Δp/Δs is the secant slope. Corrections for rod elastic shortening and rod friction are applied as required. The report presents all curves, derived parameters, installation records, and calibration data for review by the designer.
Applications
Screw plate load testing is widely used in foundation investigation, design verification, and construction quality control:
- Determination of bearing capacity and deformation parameters for shallow and deep foundation design in layered soil profiles
- Input parameters for settlement analysis of buildings, tanks, embankments, and machinery foundations on compressible soils
- Verification of ground improvement works, including dynamic compaction, preloading, and vibro compaction programs
- Correlation and calibration for the design of helical (screw) piles, whose load transfer mechanism closely matches the test configuration
- Assessment of collapsibility and water sensitivity in loess and other moisture-sensitive deposits through inundation variants
- Site characterization for reclamation and marine works where soft fine-grained deposits extend below the water table
- Confirmation of design assumptions where sampling disturbance would otherwise compromise laboratory test results
FAQ
What is the difference between a screw plate load test and a conventional plate load test?
A conventional plate load test is performed at the base of an excavation or test pit and evaluates near-surface soil, whereas the screw plate test rotates a helical plate down to the target depth and loads the soil in place. This allows the screw plate test to measure bearing capacity and stiffness at depth without excavation, including below the groundwater table, and to model the behavior of deep foundation elements more realistically.
To what depth can the test be performed?
The test is routinely performed to depths of about 2 m to 15 m. Greater depths are feasible with specialized drilling equipment, provided that the reaction system capacity, rod strength, and correction for rod friction are properly addressed.
Which soils are suitable and which are not?
Soft to firm clays, silts, silty sands, and loose to medium-dense sands are well suited to the test. Gravelly and cobble-bearing soils, fills containing debris, and very dense or cemented strata are unsuitable because the plate cannot be installed without obstruction.
How is the bearing capacity value determined from the test?
The characteristic value of subsoil bearing capacity is determined from the p–s curve, using the proportional limit pressure, the ultimate pressure divided by the specified factor of safety, or the pressure at the allowable settlement, in accordance with GB 50021 and GB 50007. The governing value is the lowest of the applicable criteria.
How long does a test take, and what corrections are needed?
Under the relative stability method, a single test at one depth typically takes several hours to about one day, depending on soil permeability and the number of load stages. Corrections for elastic compression of the rods and for rod friction, or direct load measurement above the plate, are required for accurate results, particularly at greater depths.