Applications of Light Falling Weight Defloctometer
Light Falling Weight Deflectometer ">click here to download
ENEC20003
List of figures
1. Introduction
1.1 Explanation of Light Falling Weight Deflectometer
2. Working Mechanism of Light Falling Weight Deflectometer (LFWD)
Figure: 2 working mechanism of LFWD (Source: Elhakim, Elbaz and Amer, 2014)
3. Applications of Light Weight Deflectometer (LWD)
3.1 The Light Falling Weight Deflectometer on geo synthesis base for testing aggregate layers
3.1.1 Modeling of Sub grade:
3.2 In situ evaluation of sand degree of compaction
3.3 Efficient quality control of base course materials
4. Conclusions and recommendations:
References
Geotechnical Engineering Design
Research Project Report
On
Application of Light Falling Weight Deflectometer for soil and exploration
ENEC20003
Team Members
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SANJIL ADHIKARI
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Abstract
This report typically focuses on application of Light Falling Weight Deflectometer (LFWD), working mechanism and its equipment descriptions. Generally, LFWD test is applicable to find out soil properties such as bearing capacity, dynamic modulus. This test is performed for the inspection of soil condition prior construction of structure. This test can be performed at any condition of soil without any extra support. The three applications of Light Falling Weight Deflectometer namely; Light Falling Weight Deflectometer on geo synthesis base to test aggregate, In situ assessment degree of compaction of sand and efficient quality control of base course material are explained.
Table of Contents
List of figures
Figures
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Page numbers
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Light falling weight deflectometer
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Working mechanism of LFWD
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layers of subgrade modelled on a testing rig
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1. Introduction
Light falling weight deflectometer (LFWD) is a transportable instrument utilized to measure stiffness of unbound or partly bound soil material. The test allows obtaining result in short time within reasonable accuracy at all conditions. Therefore, decision can made in straightway and it does not need any extra supportive equipment during the test (Insitu Test, 2018). Specifically, the light falling weight defelctometer measures bearing capacity of soil, thickness of pavement layer and quality of compactions which directly enable us to find out modulus of dynamic deformation of soil. The main advantages of this test are easy to use, accurate test result, non-destructive and lesser time requirement (Insitu Test, 2018).
Federal Highway Research Institute and HMP Company (Elhakim, Elbaz and Amer, 2014) developed the Light Falling weight Defelectometer (LFWD) device in 1981 at Magdeburg, Germany.
1.1 Explanation of Light Falling Weight Deflectometer
The LFWD instrument comprises following parts, which illustrated below with figure (Elhakim, Elbaz and Amer, 2014).
a) The uppermost parts of equipment clamp the falling mass at constant height. This mechanism allows the load to fall down on the resting plate on testing material.
b) LFWD comprises a rod of length 720mm is called guide rod. The weight of rod including falling weight is nearly 15kg.
c) For release and grip mechanism, there is a grip on falling weight.
d) The falling weight fluctuates from 10 to 20 kg. This weight is able to increase to grip bottom at a fixed height.
e) A rod of little resistance controls the falling weight when it allows falling on loading plate to transfer exact force.
f) LFWD has a pin for locking and unlocking of a falling weight.
g) It has sequences of rubber spring pad arrangement to provide precise transient pulse measurement to the force of impact having a variety of 16 to 30 ms.
h) It has anti tilting fixtures at the bottom part, which prevents the weight and vertical guide rod. The bridge between anti-tipping and load plate is load center ball.
i) It has a cup installed at a middle of the plate to take displacement record.
j) The loading plate of diameter is approximately 100 to 300 mm and the load is 5kg.
k) It is associated with cables to establish communication between loading plate to a process of data and storage. It uses GPS system to send data.
l) A printing device can link for self-regulation and documentation of data.
Figure 1: (LFWD) (Source: Elhakim, Elbaz and Amer, 2014)
2. Working Mechanism of Light Falling Weight Deflectometer (LFWD)
For long-term stability of soil underneath structure on either road or building, great inspection is required prior construction work. For that inspection of soil, a famous test Light Falling Weight Deflectometer can use. Through this device deflection curve, period of testing, mean values and dynamic modulus (Evd) can measure. For this test, a circular steel plate is use to establish contact with ground surface to find deformation (Insitu Test, 2018). Before recording a loading data, three pre loading is require to establish the smooth contact with ground surface (Elhakim, Elbaz and Amer, 2014).
1. The surface should be made smooth by removing all the loose materials.
2. Load plate should be kept on ground by making sure plate is directly in contact with surface to be tested.
3. The lightweight is drop from standard elevation though guide rod on dumping spring to transfer force to steel plate.
4. The loading plate is associated with accelerometer which measures movement and direct to control unit.
5. At first velocity is measured through integration of acceleration then deflection comes through double integration.
6. All three vales namely acceleration, velocity and deflection are usable to find average vale and deformation modulus (Evd).
7. All the result from tests can directly printout or can be stored on SD- card.
This test can perform within a very short time; it may take 2 to 3 minute. Therefore, before accomplishment of test following points should consider.
1. The ratio of diameters of test area and test plate should not be less than 1.5
2. Loading plates should be centered in place and arrangement of device is done to calculate deflection and amplitude.
3. Assembly of mechanical parts and electronic units is done through cable.
4. Removing lock pin from drop weight and dropping weight from required height.
Figure: 2 working mechanism of LFWD (Source: Elhakim, Elbaz and Amer, 2014)
The modulus of LWD is calculated by considering stationary plate load assessment (Elhakim, Elbaz and Amer, 2014).
Elwd = 1.5Rσ/ s
Where,
Elwd = deformation modulus
R= radius of plate
σ = stress at loading time
s = deflection of plate at loading condition
The stress below test plate is always constant that is 0.1 MN/m2 and diameter of plate is 30cm.
3. Applications of Light Weight Deflectometer (LWD)
3.1 The Light Falling Weight Deflectometer on geo synthesis base for testing aggregate layers
One of the applications of Light Falling Weight Deflectometer is to test aggregate layers on geosynthesis base. Accessing compaction and load bearing capacity of reinforced sub grades was necessary with the development of technology of earthworks using geosynthesis.
According to Bartnik (2015), laboratory study was conducted on aggregate layers laid on a weak subgrade reinforced with non-woven geotextile. The aim of the study was to analyze the possibility of applying the Light Falling Weight Deflectometer to access compaction of layers of soil laid on low bearing soils reinforced with geosynthesis. Measurements of geotechnical parameters were carried on the test bench in the laboratory. A weak sub grade was modeled by using various numbers of plastic sheets.
The control compaction of layers of soil embedded in embankments were carried out on the basis of the following criteria, degree of compaction Is, or the secondary deformation modulus E2 and deformation index Io = E2/E1, where E1 is the value of initial deformation modulus, from Plate Loading Test (PLT) . Investigated layer of embankment was properly compacted, and when the value of adopted compaction measure was reached, it became greater than or equal to the required value listed in the relevant standard on the earth works.Geosynthetics as reinforcements were applied to sub grades characterized by deformation modulus of E2 ≤ 30 MPa (Sulewska & Bartnik 2017).
3.1.1 Modeling of Sub grade:
An effort was completed to model fragile subgrade using a diverse material to gain a significance of the secondary modulus of deformation E2≤30 MPa. The study was passed on a test rig of the dimensions 1.50x1.50 m with the elevation of 1.25 m . Subsoil of adjustable load bearing capability was modeled by means of films of materials that is symbolized on Fig below (Sulewska & Bartnik 2017).
0.5 m thick medium sand (MSa) was placed at the lowest backing layer of modeled sub grades which was compacted very well to the degree of compaction Is = 0.98.Thin pieces of plastic were set on the surface of the backing layer of the ground to attain variable distortion moduli of the sub grade. As a primer for floor panels in construction, the 2mm thick sheets were used. A 5 cm thick non-compacted, medium sand leveling layer was formed on the surface of sheets. Leveling layer was not compacted in order to prevent slipping of non-woven geotextile on the backing sheets (Sulewska & Bartnik 2017).
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A non-woven geotextile such as polyster was laid as a single layer on the leveling surface and was used as a reinforcement for low bearing capacity modeled soil applying different factors such as weight 400 g/m2, specific weight 1.38 g/cm2, tensile strength MD 16 kN/m and tensile strength CD 21 kN/m, elongation at break 80%, CBR puncture test 2.6 kN (Sulewska & Bartnik 2017).
3.1.3 Test layer of aggregate:
Sulewska and Bartnik (2017) suggested that dolomite aggregates with a constant granulation of 0/31.5 mm to be placed on the upper layer of the modeled sub grade and kept directly on the geotextile which was later be treated with a plate compactor. An optimum moisture content Wopt = 8.0% and a maximum dry density ρd max = 2.16 g/cm3 on compaction of aggregate was obtained by Standard Proctor method.
After the examination of research results by Sulewska and Bartnik (2017), it was found that there is a possibility to manage compaction of aggregate layers with thickness in the range of 15 cm and 45 cm, placed on a weak sub grade and reinforced with non-woven geotextile such as polyester by the application of light falling weight deflectometer.The results of LFWD limits to obtain IS models that is significant in engineering practice. It is suggested that the layer of aggregate laid on non-woven geotextile is to be considered to the optimal thickness and the quality of compaction.
3.2 In situ evaluation of sand degree of compaction
The growing use of LWD in America and European countries as a means of testing the in situ dynamic or resilient modulus of soil shows how broadly accepted this device is. This modulus obtained from dynamic plate load test helps in the evaluation of subsoil degree of compaction. This test is best suited for coarse as well as mixed grained soil of 63 mm of highest size but is also helpful to find out the response of poorly classified sands either be calcareous or siliceous (Elhakim, Elbaz & Amer 2014).
The test procedure is same as working mechanism of equipment, which is already, explain in working mechanism of this report. LWD deformation modulus obtained in MPa is the ratio of 22.5 to the average settlement of the plate in mm, which can be automatically obtained from the LWD software.
In a unit volume chamber, a test was performed to evaluate the compaction of sand. Four different sands which were very loose, loose, medium dense and dense having relative densities 20%, 40%, 60% and 80% were taken and studied (Elhakim et al. 2014). The experiment showed the following results:
- The modulus of Light Weight Defloctometer is measured greater for the siliceous sand than the calcareous sand though they have identical values of relative density.
- The degree of compaction shows a direct relation with the LWD modulus. A high value of modulus indicates that the soil is more compacted. Here too the LWD modulus of siliceous sand is greater that the calcareous sand given the similar degree of compaction.
For same conditions of relative density, siliceous sand is more compacted since the LWD modulus is greater than that of calcareous sand. LWD is a quick test and is especially useful in measuring the resilient modulus by help of which we get to know the degree of compaction, thus helping in quality control of the subgrade soils.
3.3 Efficient quality control of base course materials
Light-weight deflectometer(LWD) is an equipment that is used to compact and control soil layers during construction of highway. It is widely used because of simple use and its movable in nature. Circular plates dropping from a certain height over the soil, which gives the vertical surface deflection below the plate, operate it. The main purpose of LWD is to find the stiffness of soil surface. LWD modulus (ELWD) is calculated for a soil using measured deflection. Elastic half-space theory is used to calculate ELWD where layers of soil are supposed to be in uniform elastic materials. Recycled pavement materials (RPM) and Natural aggregates (class 5 base) were used as a base course material.
Usually there are two method used in mechanistic quality control
1. Lightweight deflectometer
2. Large-scale model experiment
The vital elements of the construction of pavement are earthwork and another unbounded aggregate. ‘the primary tool for qualify management is currently the nuclear density gauge (NDG)’ (Mazari & Nazarian 2017, p. 92). Nuclear density gauge is widely used in industrial works in harsh condition. In-situ ‘nondestructive testing devices’ (NDT) evaluate the stiffness of pavement. The results obtained through NDT testing are more precise and represents the performance of pavement expected during mechanistic design. Stiffness and modulus are main parameters to design pavements. In-situ test devices are extensively used because it results rapid stiffness or modulus of soil layers. Implementing the NDT technique in a routine basis helps in measuring quality of pavement mostly for flexible pavements. It is necessary to transfer traditional based density specifications into mechanistic approach to give uniform construction, design and laboratory testing.
LWD has a vital potential control the quality compaction as well as identification of pavement layers resilient modulus. Because of its features like light in weight, portable in nature and easy procedure technique, work can be done within specific period. The modulus measured in conjunction will affect significantly because of differences in materials and moisture content. The resilient modulus of as-constructed pavement layer can be directly found by measurements obtained from LWD. Hence, systematic quality based ought to be enforced. ‘The AASHTO design procedure is still used in different parts of the world to design pavement structures using a layer coefficient and structural number’ (Ebrahimi & Edil 1993, p. 450). Here quality control was established from the relation between theoretical AASTHO (1993) layer coefficients of base course materials and measured deflections from LWD.
4. Conclusions and recommendations:
After going through all these things, no one can deny the fact that light weight deflectometer has vast applications in soil mechanics. It has gained worldwide popularity and is often used to characterize the soil and exploring their properties. LWD is easy, portable, light and also less time consuming device which means it reduces the construction timeframes. Used primarily during the construction of roads, this device helps in the estimation of modulus of soil. Lightweight deflectometer is also use for in situ calculation of sand compaction, calculate resilient modulus and test the aggregate layers placed on the Geosynthetic base. Thus, this LWD has a huge scope to know the characteristics of soil and knowing these properties will enhance the better stability of roads.
References
Ebrahimi, A., & Edil, T. (2013). Light-weight deflectometer for mechanistic quality control of base course materials. Proceedings of the Institution of Civil Engineers. Geotechnical Engineering, 166(5), 441.
Elhakim, A., Elbaz, K. and Amer, M. (2014). The use of light weight deflectometer for in situ evaluation of sand degree of compaction. HBRC Journal, 10(3), pp.298-307.
Insitu Test. (2018). Light Weight Deflectometer (LWD) Zorn ZFG 3000 for Compaction Control. [online] Available at: https://www.insitutest.com.au/light-weight-deflectometer/overview/ [Accessed 16 Dec. 2018].
Mazari, & Nazarian. (2017). Mechanistic approach for construction quality management of compacted geomaterials. Transportation Geotechnics, 13, 92-102.
Sulewska, M. and Bartnik, G. (2017). Application of the Light Falling Weight Deflectometer (LFWD) to Test Aggregate Layers on Geosynthetic Base. Procedia Engineering, 189, pp.221-226.
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