The Light-Weight Deflectometer is a light weight portable tool used for measuring the bearing capacity (deflection) of:

subgrade/subsoils

granular layers

unbound base layers

backfilling materials.

The zone of influence for the test typically extends to between 1 and 1.5 times the plate diameter.

The apparatus comprises of:

• A loading plate (300mm) capable of approximately uniform distribution of the impulse load

• A load cell, used to measure the applied load of each impact to a precision of 0.1kN

• A deflection sensor, capable of measuring the maximum vertical movement

• A 10kg drop weight, capable of being raised to variable height

• A data capture system

The device gives an indication of bearing capacity, deformation and compaction, and can calculate a surface modulus (MPa) value based on the force required to generate a given deflection (mm) for that soil type. Surface modulus is the most accurate and independent means for judging deformation (stiffness) and, thus, a material’s level of compaction.

By understanding the pulse shape/pattern along with site observations from each test, the LWD can provide an indication of poor compaction and/if excessive water is present within the material.

A CBR (California Bearing Ratio) value can be calculated using the mean surface modulus.

Based on the static plate load test requiring a load vehicle, the LWD provides a simple, fast and repeatable test providing an accurate assessment of compaction parameters and along with its integrated GPS interface it records the coordinates of each test location.

The Light Weight Deflectometer (LWD) meets the ASTM 2583, the IAN73 (UK) standard

The benefits using the portable device include:

• No requirement for kentledge so it’s not tying up any plant

• Lightweight and portable meaning we can test in small spaces where a traditional static plate test is impossible

• Rapid assessment

• GPS interfaced test locations

• Light weight and portable

• Can test on a range of material types including: treated soils, stabilised soils, unbound mixtures, hydraulically bound mixtures and cold recycled mixtures which contain bitumen.

 

How it Works

Principles of Measuring Compaction

For a highway to perform and have a long life, its soil and aggregate layers need to provide a stiff, stable foundation. Inspections are required during construction to ensure that pavement foundation materials have been compacted enough to ensure this condition. Light Weight Deflectometer’s (LWD’s) are used to determine the stiffness of unbound materials (subgrade/subsoils and base layers, granular layers & backfilling materials) during construction.

 

The device measures a deflection and estimates a modulus value (Evd) based on the force required to generate a given deflection for that soil type. Modulus is the most accurate and independent means for judging deformation (stiffness) and, thus, a material’s level of compaction. By measuring the modulus value, the Light Weight Deflectometer (LWD) provides the direct link between the design specification (modulus value) and the actual site condition (modulus value).

 

Stiffness is the relationship between stress and strain in the elastic range in other words how well a material is able to return to its original shape and size after being stressed.

In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value.  Three basic subgrade stiffness/strength characterizations are California Bearing Ratio (CBR),

 Resistance Value (R-value)

elastic (resilient) modulus.

Resilient Modulus (Mr)

Load bearing capacity

The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by the degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good.

Sub-grade materials are typically characterized by (1) their resistance to deformation under load, in other words, their stiffness or (2) their bearing capacity, in other words, their strength. In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value.

How the Light Weight Deflectometer Equipment Works

A mechanical impact on a circular steel plate produces a deformation on the soil surface. A light weight is dropped from a standard height and, after release, moves down a rod to a dumping spring that transfers the force to the centre of the load plate.

The load plate contains an accelerometer that measures movement and sends a signal to the control unit. The first integration of acceleration shows the velocity of the plate. And the second integration gives the deflection. The deflection and the velocity of the plate are calculated by double integration of the acceleration. The dynamic deformation modulus is calculated from these results provided that all further parameters like the contact pressure are constant. This simple approximation leads to stable results.

Using the 10kg falling weight, the Light Weight Deflectometer (LWD) measures in the range of 15 -70 MN / m². Using the 15kg falling weight (1.5 times the impact load), the measuring range extends from 70-105 MN / m².

The measuring procedure starts with three pre-loading drops for good contact with the ground. Then, three measuring drops are executed for registration and calculation of the average value and dynamic deformation modulus Evd. The results are stored on a USB Stick or printed out directly. With the aid of a USB Stick and Bluetooth the data can transferred to a PC quickly and easily.