LWD Research Papers

The lightweight 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.

Water content is one of the most important properties that affects the modulus measurements of compacted soil. To explore the sensitivity of measured modulus-based in-situ test results to the effect of compaction water content, a field study was performed in the State of Delaware in the summer of 2008. Two Light Weight Deflectometers (LWDs) were used in the study to measure compacted soil modulus values, one with a 300 mm contact plate diameter and one with a 200 mm plate diameter. The fill material tested during this study was a poorly graded sand with silt (SP-SM). This paper demonstrates the sensitivity of the measured soil modulus values to fluctuation in soil moisture content in the field, and discusses possible approaches for interpreting this type of variable LWD data.

How to understand accelerometer based Light Weight Deflectometer graphical and numerical results

The dynamic plate loading test using the Light-Weight Deflectometer is an innovative field test designed to determine the dynamic deformation modulus of subsoils and fills in all types of earth working and ground engineering applications. In earth working, the test can be used for compaction control and for assessing the load-bearing capacity of the subsoil.

This article compares the results of numerical investigations on the Light-Weight Deflectometer obtained with the boundary element method against results from experimental tests.

Mechanical modelling of the dynamic load plate test with the Light Falling Weight Device (LFWD) is presented. The LFWD is employed on construction sites to verify the compaction degree of soil layers and to evaluate their bearing capacity. The mechanical models developed are intended to provide simple and efficient formulations, which allow a large number of numerical simulations at low expenses. The motion of the device is characterized by a mass- spring-dashpot system. Several one-dimensional linear and nonlinear representations of the soil are discussed and evaluated. Different phases of motion of the LFWD – soil interaction system are identified, and corresponding formulations of the equations of motion are given. In appendices efficient solution procedures of the governing equations of motion are proposed.

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.

The draft technical note provides guidance on implementation of Light Weight Deflectometers 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).

This study relates dynamic deflection modulus (from the Light Weight Deflectometer) with Field California Bearing Ratio (FCBR). The FCBR test was used to determine CBR on the field to counter practical problems associated with Lab CBR. The test results showed that there is a strong linear correlation between FCBR and Dynamic Deflection Modulus (Evd) for black cotton soil in saturated condition. By using LWD instead of FCBR would result in better quality control, significant cost and time saving.

The performance of pavements depends to a large extent on the strength and stiffness of the subgrades. Among the various methods of evaluating the subgrade strength, the use of portable falling weight deflectometers (PFWD) is gaining popularity in the recent years. This is due to its simplicity in design, portability, and the added advantages of providing quick and reliable estimates of the Young’s modulus of elasticity of pavement subgrades.

This study developed correlations between PFWD results and those obtained using the traditional approaches such as the California bearing ratio (CBR) test and the dynamic cone penetrometer (DCP) test. Regression models were developed as part of this study to enable the prediction of CBR values based on the average of observed values of the Young’s modulus obtained using the PFWD (Epfwd), and prediction of Epfwd from the average penetration-rates of DCPs performed for field density, and field-moisture content.

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.

Includes AASHTO Drafts:

Standard Method of Test for Laboratory Determination of Target Modulus Using Light-Weight Deflectometer (LWD) Drops on Compacted Proctor Mold – AASHTO Designation: TP 123-01 (2017)

Standard Method of Test for Compaction Quality Control Using Light Weight Deflectometer (LWD) – AASHTO Designation: TP 456-01 (2017)

Final Report

Apprendices

The objective of this research was to develop a modulus-based construction specification for acceptance of compacted geomaterials that considers the following constraints:

• The specification should be based on field measurement of modulus and moisture content.
• Acceptance criteria should be correlated with design moduli.
• The specification should be compatible with a variety of compacted geomaterials.
• The specification should consider the principles of unsaturated soil mechanics.
• Available models, devices, and methods should be incorporated in the specification.
• The validity and practicality of the proposed specification should be documented by its use as a shadow specification for a number of actual construction projects.

The primary objective of the project was to evaluate the suitability of using Foamed asphalt stabilized base (FASB) in high traffic volume pavements and to assess its fundamental engineering properties. FASB density, moisture content and hydraulic properties were evaluated in the field and its stiffness was monitored as the material dried and cured during the first week and at 4 to 6 months after placement. Field tests included nuclear moisture and density gauge readings, permeability assessment, and stiffness measurements using a lightweight deflectometer (LWD), a GeoGauge, and a falling weight deflectometer (FWD). Of particular interest was the increase in stiffness of the FASB with time during curing and the comparison of this increase with that observed in a companion GAB control section at the site.

This study compared results of field tests and laboratory tests on chemically stabilized soil at different curing times to assess whether a relationship exists between field and laboratory measurements. The goal was to determine if a field testing method could be used to assess whether the strength and stiffness in the field are consistent with laboratory measurements used for design.

Field testing included three devices that are portable, quick, and easy to use. These devices include: the Dynamic Cone Penetrometer (DCP), the PANDA penetrometer, and the Portable Falling Weight Deflectometer (PFWD). Laboratory testing was conducted to determine the unconfined compressive strength (UCS) and resilient modulus (MR) of laboratory specimens prepared using additive contents that were similar to samples taken from field test locations.

Correlations were examined and involved basic soil measurements (mineralogical, electrical, chemical and index properties) and mechanical properties (UCS and MR), and field test results (DCP, PANDA, and PFWD). The strongest trend was observed for the PFWD – MR comparison. The trend showed that both the PFWD modulus and MR increase with increasing curing time, as expected. These observations show that development of correlations between field and laboratory test results holds promise. However, development of such correlations will require that field and laboratory tests be performed on nearly identical soils and under identical curing conditions.

Compaction equipment and field tests are now available that can measure the properties used to design pavements and predict performance.

• LWDs and DCPs can be used during construction quality assurance to efficiently verify design target values.
• Several options exist to quantify moisture and more field measurement devices are coming.
• The time is now to accelerate implementation of performance based quality assurance so that our investments are well spent.

Pavement design requires knowledge of the performance of the foundation layers. The specific condition at a given site can be considered in the design; such as the use of locally available material, recycled or secondary aggregate in the pavement foundation, provided the specified end-product performance requirements are achieved.

Performance testing is required in order to ensure that the design assumptions for the material properties are met during construction. Both the capping and sub-base layers need to attain adequate strength and stiffness. Therefore there is a need to undertake complimentary compliance testing that would include target values based on the behaviour of the materials.

This paper describes the research into the development of performance testing for foundation layers based on case studies performed by Scott Wilson Pavement Engineering (SWPE). The performance testing was carried out by Falling Weight Deflectometer (FWD), German Dynamic Plate (GDP) and Prima 100. The results using these test methods were compared and correlated.

The results indicated that the relationships between FWD, GDP and Prima were material type and thickness dependent. The Prima usually gave broadly similar results to the FWD, but was significantly more variable from point to point. The GDP almost always gave a lower stiffness than the other devices, but to varying degrees.

Specification target values for granular materials and fine grained soils are proposed. For granular material, the grading number and field moisture content are used to select the dynamic cone penetrometer (DCP) and light weight deflectometer (LWD) target values. A sieve analysis is used to determine the grading number and an oven dry test to determine the field moisture content. For compacted fine grained soil, the plastic limit and field moisture content are used to determine the target values. The plastic limit is used to classify the soil and to estimate the optimum moisture content for compaction.

The DCP and LWD estimate the strength and modulus of compacted materials. More specifically, they measure the penetration and deflection. When measuring penetration and deflection, the moisture content remains a critical quality control parameter for all compaction operations. Therefore, the moisture content needs to be measured, or estimated confidently, at each location. The LWD and DCP are performance related construction quality assurance tests that are expected to: increase compaction uniformity, lower life cycle pavement costs, increase inspector presence at the construction site, improve documentation, and increase inspector safety and productivity.

Moduli and deflection target values (TVs) for the unbound pavement foundation materials have been proposed for use during pavement design. These TVs are estimated using the plastic limit (PL) for cohesive soils because the PL has been found to be a reasonably accurate predictor of the Soil Water Characteristic Curve (SWCC). Therefore, a family of SWCCs has been defined based on the PL. TVs are then verified during construction of the unbound pavement foundation using the lightweight deflectometer (LWD). Although the deflection values from LWD testing can be suitable to assess the performance of compacted soils, MnDOT has coupled the LWD response with laboratory resilient modulus testing and soil suction measurement to improve interpretation of the results.

Light drop-weight tester is a device for field tests and it is used for quick control of bearing capacity and compaction quality of built-in soils in different types of embankments. It is a modern device which is commonly used in Germany and now in Poland. The examples of calibration of the light drop-weight tester in laboratory and in-situ,
and its application in real embankment are presented.

The goals of this three year project included:

1. Demonstration of soils/subbase and Hot Mix Asphalt (HMA) IC technologies to department of transportation (DOT) personnel, contractors, etc.,
2. Develop an experienced and knowledgeable IC expertise base within DOT,
   Assisting DOT in the development of IC quality control (QC) specifications for the subgrade, subbase, and HMA pavement materials, and
3. Identification and prioritization of needed improvements and further research for IC equipment and data analysis.
4. Identification and prioritization of needed improvements and further research for IC equipment and data analysis.

Goal No. 1 was accomplished by demonstrating the abilities of the IC system such as: tracking roller passes, HMA surface temperatures, and intelligent compaction measurement values (ICMV).

Goal No. 2 was accomplished by building the IC knowledge base with extensive field experiences, data, and analysis/reports from diverse demonstration projects.

Goal No. 3 was accomplished by training DOT personnel and earthwork/paving contractors on the IC technologies via field demonstrations and open house activities. Continuous support was provided to the TPF State DOTs for the development of local, customized IC specifications during the project period.

Goal No. 4 was accomplished by compiling a comprehensive list of recommendations for the IC roller vendors to further improve their systems for widespread use of the technologies. Various IC systems were reviewed in-depth and gaps were identified for future research and engineering practices.

IC provides only an index, which is specific to the conditions associated with a particular site. An interpretation of comments provided the basis for the following recommendations:

• Use light weight deflectometers (LWD) for quality assurance of stiffness
• Establish a procedure to determine the target LWD value
• Eliminate calibration areas (control strips)
• Simplify IC data evaluation and presentation
• Calibrate the IC roller and related transducers
• Support development of alternative IC methodologies
• Simplify or eliminate moisture corrections

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)

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.

This study, from the Australian Rail Track Corporation (ARTC) Inland Rail project research, presents the resilient modulus from LWD for capping, structural fill, general fill and at foundation level, together with an example of near real-time reporting of the results using a geospatial platform. The LWD resilient modulus results are compared to cyclic triaxial test results at foundation level as well as Dynamic Cone Penetration (DCP) and Shear Vane Tests (SVT). The results indicate that the LWD may be used in combination with reduced traditional compliance testing frequency to save on time and laboratory testing effort.

A comparison of two methods of determining the stiffness of track-bed materials (dynamic penetration and dynamic plate load) comparing the PANDA / PANDOSCOPE and the Light Weight Deflectometer.

Trial to test the Bomag Vario 213 roller’s ability to suitably consolidate bottom ballast for a railway environment, and additionally, to compare methods of measuring the achieved stiffness, including the Light Drop Weight Tester and Plate Bearing Test.

Light drop-weight tester is a device for field tests and it is used for quick control of bearing capacity and
compaction quality of built-in soils in different types of embankments. It is commonly used in Germany and now in Poland. The examples of calibration of the light drop-weight tester in laboratory and in-situ, and its application in real embankment are presented.

The Building Research Establishment (BRE) produced a practice guide for working platforms for tracked plant, which has become a “standard” in the industry in the absence of any other widely published simple design method. The BRE design method does not apply for thick platforms or for soft subgrades, but continues to be used in those applications in the absence of an alternative document. A case study is discussed which applies the BRE in such a situation, but then compares with alternative methods (including the Light Weight Deflectometer) to assess the required working platform.

Proposed protocol for testing and assessing of piling work platforms using Ground Penetrating Radar (GRP) and the Light Weight Deflectometer (LWD)

Use of the LWD at the L-L Tailings Dam, the ore stockpile cover foundations, fuel tank station, and at the multiplate overpass foundations and backfill. A comparison of the LWD capabilities with alternative field testing measures is presented along with an evaluation of its effectiveness at Highland Valley Copper Mine.