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 Light Weight Deflectometer (LWD) is a portable device that measures the onsite dynamic or resilient modulus (Evd) of subgrade soils and pavements. It is suitable for cohesionless, cohesive and mixed types of soils with a maximum grain size of 63mm. It can be used on natural subgrade and subsoils, unbound base layers, granular layers, soil stabilised with lime or other additives. The LWD test method has been gaining in popularity following its use in European countries and the USA for over 30 years. The test method was originally designed to measure the resilient modulus (Evd) of an in-situ material which indicates the material’s stiffness. The LWD can be perceived as an on-site simulation of a laboratory cyclic triaxial test for unbound or bound materials. The tested soil volume by the LWD is 300mm in diameter and 600mm in depth. Numerous standards have been developed for the LWD and allow the characterisation of various materials based on Evd, which can facilitate the estimation of bearing capacity and the compaction degree of engineered fills. This is extremely beneficial for infrastructure design and testing as it allows a fundamental engineering property (Evd) to be measured and used in the design and construction process.
The resilient modulus (Evd) represents the elastic response of the soil specimen after many cycles of loading. Cyclic triaxial tests, used for measuring the resilient modulus (Evd) of untreated base/sub-base materials, are expensive and difficult to execute in New Zealand. This has caused the measurement of such an important geotechnical parameter to often be ignored during the design and construction process of infrastructure in New Zealand. The LWD test method therefore offers a great alternative for measuring the onsite resilient modulus, at a fraction of the laboratory cost. It should be noted that CBR characterises a soil based on its failure to penetration; this is an indirect measure of soil strength. In contrast to resilient modulus, CBR is not a fundamental material property. However, the use of CBR is well justified in the geotechnical community as it is widely accepted for pavement design.
2 LWD TEST METHOD, STANDARDS AND APPARATUS
2.1 LWD test method and standards
The principle of this test was developed in 1981. The test simulates a truck with a 10ton axle weight travelling on a road at 80km per hour. A steel plate of 300mm diameter is placed on the soil to be tested. A 10kg weight drops from a height of 72 cm onto the plate. The load pulse creates a soil pressure of 100kPa under the plate. The approximate duration of the load pulse is around 17ms and it is created by means of engineered springs located above the plate. An acceleration sensor is arranged on the load plate. The generated acceleration signal is recorded. From single and double integration of the acceleration signal, the velocity and the displacement (settlement) of the plate is calculated.
From this simple test method the recorded values can be used for determining:
- Resilient or dynamic soil modulus (Evd)
- Bearing capacity
- Dynamic spring stiffness (Kvd)
- Degree of compaction and compaction quality control
The resilient modulus Evd is given in MPa by the equation:
where s is the measured plate settlement in mm.
The specifications for the LWD apparatus are defined in both ASTM E2835-11 and ZTVE-StB 09 and conform to strict equipment production criteria. The apparatus is also required to be calibrated on an annual basis by an accredited calibration institute.
Numerous international standards have been produced following the inception of the LWD as a test procedure. The two main internationally accepted test methods are:
- ASTM E2835-11: American Standard Test Method for Measuring Deflections using a Portable Impulse Plate Load Test Device
- ZTVE-StB 09: German Engineering Code for Soil and Rock in Road Construction
While these test methods are the most widely accepted in industry, many other standards have been produced to complement more specific design codes and procedures. Some other available international standards on LWD include:
- TP BF-StB B 8.3 version 2012: German Engineering Code for Soil and Rock in Road Construction
- ZTV E-StB 09: German additional terms of contract and rules for earthwork in road construction
- ZTV T-StB 95: German additional terms of contract and rules for subbases in earthworks
- ZTV A-StB 97: German additional terms of contract and rules for excavation in traffic access
- RVS 08.03.04 March 2008: Austrian regulation – Compaction test by means of dynamic plate load test
- RIL 836, Deutsche Bahn AG: Guideline for the use of the Light Drop-Weight Tester in railway construction
- UNE 103807-2:2008: Spanish regulation – Plate Load Test by means of the Light Drop Weight Tester-Part 2
- TB 10102-2004, J338-2004: Chinese regulation – Standard for soil testing in railway construction
The number of standards available shows that the test procedure is widely accepted and utilised for a diverse range of engineering applications throughout the globe. This also provides a simple platform for the integration of the LWD into New Zealand design and construction procedures through the utilisation of these existing standards.
2.2 Description of LWD apparatus
There are numerous, well designed and robust LWD equipment apparatus available internationally (Zorn ZFG 2000, Keros LWD, Dynatest 3031 LWD, HMP LFG Pro). Figure 1 shows a schematic view of the LWD apparatus. This equipment is small and portable, allowing it to be easily transported and utilised on a construction site by one technician.
The apparatus is controlled during testing by means of an electronic recorder that provides a step by step guidance to the operator during the testing process. This removes the potential for any measurement errors.