Plate Load Test (PLT)

The plate load test can be used to estimate the vertical soil modulus Ev and the angle of friction φ for temporary working platforms and gravel rafts. This paper presents the results of plate load tests that have been undertaken on seven different sites across New Zealand. The paper discusses how to execute the test and interpret Ev and φ parameters when applying the DIN 18134 standard testing methodology. The paper provides some guidance on the correct interpretation of results in relation to the NZGS Specification NZGS_0510 Earthworks, dated 15/02/2024, Version 1.0 - Final.

The paper presents the estimated friction angle using the Corke et al. (2021) method along with five other methods and compares them with the associated Ev values. The results are discussed, with emphasis on the main conclusion, which is that the Ev values do not align with the estimated friction angles. The results indicate the paradox that it is very probable to estimate acceptably high friction angles accompanied by unacceptably low Ev values when tested to 500kPa as per DIN. The paper discusses the significance of this finding and the possible causes for this incompatibility.

The paper also provides awareness of this incompatibility to designers of temporary works and gravel rafts, which is relevant for the safe performance of temporary gravel platforms and the long-term performance of gravel rafts beneath shallow foundations. It is recommended that any gravel raft platform designer should conduct plate load tests and focus on both the stiffness and capacity results.

Finally, a new hybrid plate load test (PLT) procedure is proposed where a pressure of 1400kPa is exerted upon a 300mm (12in) plate in 14 stages, measuring Ev1 and Ev2 as per the German DIN standard (EFFC/DFI, 2025).

The modulus of subgrade reaction (ks), useful for designing flexible foundations, pavements, and slabs, is defined as pressure per unit settlement (kN/m³). While traditionally measured using a 762 mm plate in static Plate Load Testing, the Light Weight Deflectometer (LWD) with a 300 mm plate can also estimate ks. To account for size differences, a correction factor of 2.22 is applied. Accurate results require homogeneous material to a depth of 1.5x the plate diameter. By plotting pressure versus deflection and interpolating to a target settlement of 0.568 mm, ks can be calculated for dynamic field conditions using LWD.

This document outlines how to estimate the Resilient Modulus (Mr)—used in pavement design—from the Dynamic Deformation Modulus (Evd) measured by a Light Weight Deflectometer (LWD). While Evd represents stiffness under a single dynamic load, Mr reflects a material’s response to repeated traffic loading. Due to the high cost and complexity of direct Mr testing, indirect correlations using field tests like the LWD and Plate Load Test (PLT) are explored. These correlations must be used cautiously, as they are affected by factors like moisture content, density, and soil type. The document highlights recent methods such as LWD testing on Proctor-compacted lab samples to define target moduli. It also compares soil classification systems (DIN 18196 and AS1726) and provides tables linking soil types, compaction levels, and expected modulus values. While exact conversions are limited, combining field and lab data can produce reliable estimates for use in quality assurance and pavement design.

This document explores the relationships between various soil strength and stiffness parameters commonly used in geotechnical and pavement engineering: Ev1 and Ev2 (static deformation moduli from Plate Load Tests), Evd (dynamic modulus from the Light Weight Deflectometer), and the California Bearing Ratio (CBR). Each test offers insight into different aspects of soil behaviour, from small-scale material strength to large-scale structural performance. While the CBR test provides a measure of a soil's bearing capacity in a confined mold—primarily at the top 5 mm of the sample—the plate load tests (static or dynamic) assess the performance of a much larger volume of soil, capturing a more holistic view of stiffness under load.

Though no universal mathematical formula links CBR directly with Ev1, Ev2, or Evd, empirical correlations have been established through research, particularly in Germany. Pioneers like Prof. Dr. Weingart and Prof. Dr.-Ing. Floss developed reference tables and graphs suggesting relationships between these parameters for different soil types. For instance, one commonly used approximation is Ev2 ≈ 2 × Evd. These insights support more informed decision-making during earthworks and pavement design.

This paper outlines the methodology for determining the Modulus of Subgrade Reaction (ks)—also known as the Westergaard Modulus—and the Resilient Modulus (Mr) using the Plate Load Test (PLT), a field-based method for evaluating subgrade stiffness. The ks value, a critical parameter in the design of foundations, pavements, and temporary works, represents the pressure per unit of settlement and is typically expressed in kN/m³. While ks cannot be directly derived from initial PLT values (Ev1, Ev2), the document details how it can be extrapolated using a second-order polynomial equation in accordance with the DIN 18134 standard. For standardisation, a 762 mm diameter plate is used, with a reference settlement of 1.25 mm. If a 300 mm plate is employed, a correction factor of 2.22 is applied.

This paper also explains the distinction between Elastic Modulus (Es) and Resilient Modulus (Mr) and the relationship between them. While Es represents a one-time load-unload cycle, Mr reflects stiffness under repeated loading, making it especially relevant for pavement design.

Industry familiarity with density-based assessment for Quality Control (QC) purposes currently hampers the use of more accurate tests. An ARRB research project identified several alternative quality tests that have the potential to provide improved accuracy (as compared to density). These “new” methods (most are over 20 years old) provide a reduction in both the duration of onsite testing and turnaround of test results and provide the direct measurement of a stiffness (modulus) value. Overall, these alternative tests do not correlate well with the common density ratio (DR) test. Most alternative field equipment tests seem to be positioned between the Plate Load Test (PLT) and density in terms of accuracy and precision. The results and comparisons at the Smithfield Bypass project (approximately 11.5 km North-West of Cairns CBD) is presented. This site was tested in early 2019 and is one of several “live” test sites between 2017 and 2019. Field testing was undertaken upon an embankment being constructed over Avondale Creek and in parallel to standard tests for QC. Lessons learnt and comparisons between equipment at this site are provided. Dendrogram analysis is used to show the relative relationship between tests.

Geogrids are used to reinforced and stabilise the roads and improve the bearing capacity of pavements on soft subgrades. Additionally, geogrids installed within granular layer of a pavement formation are frequently used to control environmental distress and limit longitudinal cracking due to expansive subgrades by providing the stiffening, known as the new function of geosynthetic material to control deformations in the soil-geosynthetic composite. In order to quantify the effect of geogrid reinforcement / stabilisation, a full-scale pavement field trial was established upon a soft and expansive subgrade in 2018. Sections with variable geogrid arrangements were constructed in order to allow the quantitative assessment of geogrid reinforcement/stabilisation; both in terms of their initial contribution to composite insitu stiffness parameters and their benefit to the long-term performance of a pavement. On site testing was completed within all trial sections during the project’s construction phase, such that the initial state (strength and stiffness) of the subgrade and each pavement layer was adequately characterised. Post construction, ongoing performance monitoring has included the use of embedded strain gauges and pressure cells. An analysis of the first 3 years of monitoring of this full-scale field trial was conducted in January-February 2021, with selected results presented herein.

The data demonstrates that the inclusion of geogrids successfully improves the bearing capacity of the pavement profile by reducing the traffic imparted vertical pressures being exerted upon the underlying materials. The results also demonstrate that the presence of two geogrid layers (with one placed at the subgrade-subbase interface and one at the subbase-base interface) offer a greater improvement of the bearing capacity than the installation of a single geogrid at the interface of subgrade-subbase materials.

Additional observations relate to longitudinal cracking of the pavement, where greater control of cracking has been initially achieved in the test sections where geo-reinforcement was installed.

Results of compressive strength and resilient modulus measured in situ using a Variable Energy Dynamic Penetrometer (VEDP) – PANDA DCP, Light Weight Deflectometer – Portable Impulse (LWD-PI), and Plate Load Test (PLT) and laboratory Unconfined Compressive Strength (UCS) during a full-scale trial on the Australian Rail Track Corporation (ARTC) Inland Rail project. These alternative tests reduce the level of laboratory testing effort while the near real time display of results aids in construction time frames which is of particular benefit to projects in remote locations. The methods can be combined with traditional field testing methods to develop site-specific correlations and validate geotechnical parameters assumed in the design.

The current industry practice relies on the use of in situ dry density ratio (DDR) to control the quality of compaction during the construction of earthworks. The main reason is because the density measurements are relatively easy to undertake during construction, and the parameter itself is precise and with limited variability.

However, this approach has two major disadvantages, namely: (i) the in situ modulus of the layers is not directly measured, and (ii) there is a significant delay between the time of undertaking the DDR measurement and the delivery of the final test results. Such a (routine) delay in the provision of test results can lead to costly rework being required by the contractor if earthworks are found to be non conforming and require remediation after the works have further progressed. To address the above issues, this National Asset Centre of Excellence (NACOE) research project investigated the viability of using alternative testing techniques to control the quality of constructing earthworks.

This five-year study investigated a range of alternative testing devices, such as the light weight deflectometer (LWD), Clegg Hammer and PANDA probe with a focus to evaluate their effectiveness in assessing the quality of the earthworks constructed. A methodology has been developed to adopt the LWD as an alternative QA method. However, the methodology is equally applicable for other similar technologies.

The report presents the final research outcomes to allow the adoption or trialling of this alternative approach in future roadwork construction projects. The final deliverables include the proposed amendments to MRTS04 General Earthworks, and a technical note that details the technical basis and approach.

Appendix A of this report, includes the Queensland Department of Transport and Main Roads (QLD DTMR) Draft Technical Note – Guidance on Use of Light Weight Falling Deflectometers (LWDs) to be Accepted as an Alternative Method for Verification of Earthworks Compaction Requirements – June 2021. This provides guidance on moving from a density based specification for compaction control to a modulus based approach to testing in the field.

The draft technical note provides guidance on implementation of Light Weight Deflectometers as an alternative compaction control method including a flow chart showing key steps for assessment/derivation of equivalent acceptance thresholds for LWD use (in lieu of traditional (density) testing minimum thresholds included in MRTS04 – General Earthworks). We believe this “How to” guide will be very useful for industry on projects across the country.

Industry’s familiarity with density-based assessments for the quality assurance (QA) of pavement layers and subgrades currently favours conventional density measurements over other field tests that have the potential to better validate design parameters achieved during construction. Although such alternative tests (e.g. Prima 100 LWD, Geogauge or PANDA probe) are not necessarily as precise as conventional density testing, the density test itself has significant limitations.

These limitations include the time required to undertake the tests and its inability to reliably assess the in situ devices evaluated in this report.

The background to Phase 3 of the study is covered in Section 2 which discusses the interim findings from Phases 1 and 2. The field testing and analysis conducted on a live construction site to evaluate the alternatives test method is summarised in Section 3. A proposed technical specification is outlined in Section 4 and conclusions and recommendations are discussed in Section 5.

Advanced Methods for Compaction Quality Control Part 2 Question Answers

1. How is moisture ratio taken into account?
2. Modulus is influenced by moisture too; so, shouldn't a comparison should MDD v Modulus include consideration of moisture content?
3. Was the moisture content/ratio taken into account when comparing the Density vs Modulus?
4. The statistical analysis doesn't acknowledge some frustrating variables (1) – e.g. Density Ratio v Modulus not considering moisture content.
5. What was the moisture variations within all the testing?
6. Density Ratio can be performed within 1 day as per AS 1289 5.7.1.
7. Most Victorian testing for earthworks is using rapid HILF method which does not normally require curing if within the test method range.
8. He [the presenter] is referring to assigned value and not the HILF compaction.
9. Are there any case history recorded significant numbers of PLT vs LFWD?
10. PLT has shown vice-versa results and how can we take it PLT as reliability test?
11. The statistical analysis doesn't acknowledge some frustrating variables (2) – e.g. density ratio v CBR not considering material type
12. Embankments need to have a required modulus for strength as well as minimise settlement. Isn't density a measure of minimising settlement and therefore still important?
13. A dry material with low density will have a high modulus, but is this all we care about?
14. Slide 17 – These are awful results and not representative of road construction projects in NSW. What is the source of this data? PS. You are showing density (ratio) results and not quality test results!
15. Slide 19 – One crucial item is the location of the test results, that is layer and GPS / Chainage and Offset! Why is this mixing?
16. Slide 23 – If you are placing and compacting in 300 mm lifts, the LFWD and PLT will not give you reliable results as it influenced by the underlying layers. Therefore, why would you use this equipment?
17. Slide 25 – You have only tested and analysed one formation material. How can you draw any early conclusions?
18. Slide 29 – Work in the USA has shown the portable hand held FWD devices are susceptible to moisture content of the material being tested and have you taken this into consideration to the reliability of the test results from these devices?
19. Slide 50 - You would have to be a brave road agency engineer to allow a contractor to use 1000 mm lifts for embankments with a 18 T padfoot roller!

The Light Weight Deflectometer (LWD) is a portable device that measures the onsite dynamic or resilient modulus (Evd) of subgrade soils and pavements. The LWD, which has been used extensively in Europe and the United States, has become popular for assessing the stiffness of embankments, structural fills and other earth structures. It assesses the bearing capacity, the stiffness and the compaction degree of soils that have a maximum grain size of 63mm. The LWD assessment considers the stiffness (or compressibility) characteristics of the materials under testing to a depth of 600mm below plate level. The paper presents available correlations between Evd with the static soil modulus Ev obtained from static plate load tests. It also presents how Evd can be linked with CBR and thus be useful for pavement design, but also with the subgrade reaction modulus K of the assessed soils. It also discusses the fundamental principles behind the testing along with the benefits that may arise from its use on specific applications. Such applications include the design and construction monitoring of gravel rafts, the design of pavements, engineered and non-engineered fills, landfills, MSE walls, pipelines and services, evaluation of ground improvement effectiveness and soil stiffness mapping.

The paper presents available correlations between dynamic or resilient modulus (Evd) with the static soil modulus Ev obtained from static plate load tests. It also presents how Evd can be linked with CBR and thus be useful for pavement design, but also with the subgrade reaction modulus K of the assessed soils. Applications include the design and construction monitoring of gravel rafts, the design of pavements, engineered and non-engineered fills, landfills, MSE walls, pipelines and services, evaluation of ground improvement effectiveness and soil stiffness mapping.

Currently, the acceptance of construction works (e.g. earthwork embankments, subgrade and pavement granular layers) by Australian road regulatory bodies requires in situ density testing to be completed in order that a relative percentage of the laboratory-determined maximum dry density can be determined. Current earthworks specifications often rely on the assumption that there is a direct correlation between density and modulus parameters (i.e. the greater the density achieved, the higher the modulus of the compacted material). However, this assumption may not be valid and it is affected by many properties of the engineering material.

Over the last two decades, some alternative field assessment methods have been developed which either directly measure the in situ modulus or correlate with the resilient modulus. This project focuses on exploring the use of these new assessment methods. Such innovative testing methods can be grouped into the following four main categories:

• penetration test devices
• surface based impact devices
• geophysical methods
• in situ sensors.

Among these four categories, the ‘penetration test devices’ and ‘surface based impact devices’ were identified to have the potential to be adopted in TMR’s current Quality Assurance (QA) framework.

A literature review conducted in 2016/2017 highlighted that some similar international studies had been completed in recent years. Recent projects in Australia also demonstrated the advantages associated with these innovative, commercially-available field assessment techniques.

The project team attempted to rank the different QA methods using a weighted rating approach.

The traditional density test techniques were also evaluated as reference values in this assessment. The overall comparative assessment of the different QA test techniques is shown in the following table.

The Light Weight Deflectometer (LWD) is currently not standardized; as a result, there are a number of commercially available LWD designs that yield different deflection and elastic modulus values. This proves problematic because transportation agencies are beginning to prescribe target deflections and/or elastic modulus values during earthwork construction.

This paper presents the results of a comprehensive investigation into the influence of LWD design characteristics on measured deflection. The influence of the sensor type (accelerometer versus geo- phone), sensing configuration (measurement of plate versus ground surface), LWD rigidity, and applied load pulse were investigated through field testing and finite element analysis. The investigation revealed that the sensing configuration (i.e., the measurement of plate versus ground surface response) is the predominant cause of differences between the Zorn and Prima LWD responses (deflection normalized by peak force). Vertical plate deflection exceeded ground surface deflection by 65 % to 310 % on soils and by 20 % on asphalt. The relative influences of the sensor type (accelerometer versus geophone), plate rigidity, and load pulse each led to relatively small differences (<10 %) between Zorn and Prima LWD responses. The results of this investigation illustrate that each of the two LWD configurations will always produce different deflection and elastic modulus values for the same ground conditions, and that the differences will be difficult to predict.

Plate loading tests (PLTs) have been used to evaluate the compaction quality of the railroad subgrade in Korea. Two methods to determine the design modulus are being used together; one is an unrepetitive plate loading test (uPLT) that obtains the subgrade reaction modulus (K30) and the other is a repetitive plate loading test (rPLT) (e.g. Anix AX01a) that obtains the strain modulus (Ev). There are some differences between the two methods, such as, the way in which the design modulus is evaluated, the number of loading steps, and the test procedures. Firstly, this paper compares the two test methods and summarizes the differences between them. Secondly, the relationship between the two moduli was obtained by using the results of 30 field tests of uPLT test and rPLT test carried out on the subgrade under railroad construction. The comparisons show that the two tests give large differences in stress-displacement relationship and that the correlations between the two moduli didn’t indicate a good relationship. Consequently, it was found that corrections of two the moduli for stress and strain level are needed to evaluate the relationship between the two moduli, because the stress and strain level are different when K30 and Ev2 are evaluated. Therefore, if the relationship between the two moduli is developed from the correction procedure, it will help field engineers in the management of compaction control for railway embankments.

Five single drum IC rollers (smooth drum and padfoot) were assessed from Bomag USA, Case/Ammann, Caterpillar, Dynapac, Sakai America, and Volvo.

For IC correlation analysis, in-situ tests are used to directly obtain the response of the compacted materials under various loading situations and drainage/moisture conditions. Recommended in-situ test devices for soils/ subbase/ stabilized IC are as followings:

• Light Weight Deflectometer (LWD)
• Dynamic Cone Penetrometer (DCP)
• Calibrated Nuclear Moisture-Density Gauge for soils and subbase (NG)
• Falling Weight Deflectometer (FWD)
• Static Plate Loading Test (PLT)

Light Weight Deflectometer (LWD) replaces CBR Indicative Graph of Ev2 vs Evd vs CBR