VTI rapport 1086A Utgivningsår 2021 vti.se/publications
Development of a simple field
method for measuring permanent
deformations in silty sand subgrade
Dina Kuttah
VTI rapport 1086A
Development of a simple field method
for measuring permanent deformations
in silty sand subgrade
Publikationsuppgifter – Publication Information
Titel/TitleUtveckling av en enkel fältmetod för mätning av permanenta deformationer hos siltig sandjord./ Development of a simple field method for measuring permanent deformations in silty sand subgrade
Författare/Author
Dina Kuttah (VTI, https://orcid.org/0000-0003-0478-1150)
Utgivare/Publisher
VTI, Statens väg- och transportforskningsinstitut/
Swedish National Road and Transport Research Institute (VTI)
www.vti.se/
Serie och nr/Publication No.
VTI rapport 1086A
Utgivningsår/Published
2021
VTI:s diarienr/Reg. No., VTI
2019/0449-9.2
ISSN
0347–6030
Projektnamn/Project
Utveckling av en enkel fältmetod för mätning av permanenta deformationer hos obundna väglager./ Development of a simple field method for measuring permanent deformations in unbound road layers.
Uppdragsgivare/Commissioned by
Trafikverket
Språk/Language
Engelska/English
Antal sidor inkl. bilagor/No. of pages incl. appendices
Kort sammanfattning
I denna studie genomfördes upprepade LWD-tester i en testgrop i Linköping. En packad siltig sandjord har valts för dessa tester. Serier av cykliska LWD-belastningar genomfördes på olika intervall av vattenkvot (nämligen 8 %, 10 % och 15 %) och olika tillämpade stressnivåer (nämligen 50 kPa, 100 kPa och 200 kPa).
Testresultaten visade att det testade materialets deformationsbeteende påverkades avsevärt av den tillämpade lastnivån, vattenkvoten och antalet belastningar. Det noterades att vid slutet av den femtionde LWD-belastningen, steg den permanenta deformationen mycket mindre jämfört med den elastiska deformationen. Från resultaten av de ackumulerade permanenta deformationer som utförts med upprepade LWD-tester, kan man utläsa att ökningen av permanenta skador inte beter sig på samma sätt under alla last- och vattenkvotsförhållanden. Vidare föreslås prediktionsmodeller för ackumulerade permanenta töjningar baserade på de cykliska LWD-mätningarna i denna studie. Generellt har alla utvecklade och antagna modellerna visat bra matchning mot de ackumulerade permanenta töjningarna (εp) uppmätta från de upprepade LWD-testerna, utom vid fallen p=100 kPa
samt 200 kPa med 15 % vattenkvot, på grund av de extrema ackumulerade permanenta skadorna som rapporterats för dessa fall.
Sammanfattningsvis visade denna studie att det upprepade LWD-tester skulle kunna ge en värdefull materialbedömning som skulle kunna utnyttjas för att fastställa risknivån för permanenta töjningar i undergrundslagret vid design- och konstruktionsstadier. Med hjälp av denna metod bör man kunna få en förbättring av byggkvaliteten som kan resultera i en ökad livslängd av vägen.
Nyckelord
Upprepad lätt fallvikt test, modellering, permanenta deformationer, siltig sandjord, spänningsnivåer, vattenhalt.
Abstract
In this study, repeated light weight deflectometer tests were conducted at a test pit located at the backyard of VTI in Linköping. A silty sand subgrade soil has been chosen for testing. At the beginning, the soil was compacted in the test pit, and then, series of in-situ repeated LWD were conducted at different ranges of water contents (namely 8%, 10% and 15%) and various applied stress levels (namely, 50 kPa, 100 kPa and 200 kPa).
The test results showed that the deformation behaviour of the tested material affected significantly by the applied stress level, water content, and the number of load repetitions. It was noted that at the end of the fiftieth LWD load application, the increment of nonrecoverable (plastic) deformation was much smaller compared to the resilient/recoverable deformation. From the results of the accumulated permanent strains (deformations) conducted by repeated LWD tests, it can be deduced that the increase in permanent strains does not behave in the same way under all load and water contents conditions. Furthermore, prediction models for accumulated permanent strains based on the repeated LWD measurements are suggested in this study. Generally, all the developed and adopted models have showed good matching to the accumulated permanent strain (εp) data measured by the repeated LWD
tests except for the cases of p=100 kPa and 200 kPa at 15% water content due to the excessive accumulated permanent deformations reported for these cases.
In summary, this study showed that the repeated light weight deflectometer test could be a useful material assessment tool that could be utilized to establish the risk level of permanent deformations in the subgrade layer during the design and construction stages. Using this methodology can lead to an improvement in construction quality that may result in an increase in pavement service life.
Keywords
Repeated light weight deflectometer tests, Modeling, Permanent deformations, Silty sand subgrade, Stress levels, Water content.
Sammanfattning
Trafikverket (TRV) har ägnat särskild uppmärksamhet åt att hitta en enkel och en tidsbesparande teknik för att utvärdera det permanenta deformationer hos obundna lager direkt i fält vid vägbyggen. För att uppfylla detta krav har den aktuella studien gjorts för att utvärdera möjligheten att använda VTI:s multifunktionella lätt fallvikt (LWD) för att fastställa elastiska och plastiska deformationer in-situ (orsakade av dynamiska krafter) på ett enkelt och noggrant sätt, vilket kan minska arbetsinsats och tidsåtgång för denna karaktärisering.
I denna studie genomfördes upprepade LWD-tester i en testgrop i Linköping. En packad siltig sandjord har valts för dessa tester. Serier av cykliska LWD-belastningar genomfördes på olika intervall av vattenkvot (nämligen 8 %, 10 % och 15 %) och olika tillämpade stressnivåer (nämligen 50 kPa, 100 kPa och 200 kPa).
Testresultaten visade att det testade materialets deformationsbeteende påverkades avsevärt av den tillämpade lastnivån, vattenkvoten och antalet belastningar. Det noterades att vid slutet av den femtionde LWD-belastningen, steg den permanenta deformationen mycket mindre jämfört med den elastiska deformationen. Från resultaten av de ackumulerade permanenta deformationer som utförts med upprepade LWD-tester, kan man utläsa att ökningen av permanenta skador inte beter sig på samma sätt under alla last- och vattenkvotsförhållanden. De mesta av de permanenta deformationerna har utvecklats vid de första belastningarna och sedan har ackumuleringen av de permanenta
deformationerna fortsatt långsammare med stadig nedgång under de sista cyklerna för de mätpunkter som testats vid de lägsta lastnivåerna och under 8 % och 10 % vattenkvot. Materialet med den högsta vattenkvoten som testades i denna studie (15 %) var mer benäget att ansamla permanenta skador när det utsattes för höga lastnivåer.
Vidare föreslås prediktionsmodeller för ackumulerade permanenta töjningar baserade på de cykliska LWD-mätningarna i denna studie. För den testade jorden har de modeller som ges i ekvationerna 7 och 9 visat god matchning mot den ackumulerade permanenta töjningen (εp) för de data som mäts
genom in-situ upprepade LWD-tester som en funktion av antalet belastningar, lastnivåer och
vattenkvot under testning. Den modell som ges i ekvation 9 med vattenhaltsvariabel (c = 2,58) gav den bästa matchningen till de ackumulerade permanenta töjningarna som uppmätts. Den modell som ges i ekvation 6 har också använts för att passa in-situ upprepade LWD-ackumulerade permanenta töjningar i form av dom elastiska töjningarna. Generellt har alla utvecklade och antagna modellerna visat bra matchning mot de ackumulerade permanenta töjningarna (εp) uppmätta från de upprepade
LWD-testerna, utom vid fallen p = 100 kPa samt 200 kPa med 15 % vattenkvot, på grund av de extrema ackumulerade permanenta skadorna som rapporterats för dessa fall.
Sammanfattningsvis visade denna studie att upprepade LWD-tester skulle kunna ge en värdefull materialbedömning som skulle kunna utnyttjas för att fastställa risknivån för permanenta töjningar i undergrundslagret vid design- och konstruktionsstadier. Med hjälp av denna metod bör man kunna få
Summary
It is well known that limiting the rutting in pavements should consider the actual load-associated plastic (permanent) strains. Therefore, the Swedish transport administration (trafikverket, TRV) have paid a particular attention to find a simple and a time saving technique to evaluate the permanent deformation behaviour of unbound layers directly in the field during roads construction.
To fulfil this requirement, the current study has been undertaken to evaluate the feasibility of using the VTIs multifunctional LWD to determine the in-situ elastic and plastic deformations components (caused by dynamic loading) separately, simply, and accurately. Something which can reduce the effort and time spent in characterizing the deformation behaviour of the tested material.
To fulfill the objective of the study, repeated light weight deflectometer tests were conducted at a test pit located at the backyard of VTI in Linköping. A silty sand subgrade soil has been chosen for testing. At the beginning, the soil was compacted in the test pit, and then, series of in-situ repeated LWD were conducted at different ranges of water contents (namely 8%, 10% and 15%) and various applied stress levels (namely, 50 kPa, 100 kPa and 200 kPa).
The test results showed that the deformation behaviour of the tested material affected significantly by the applied stress level, water content, and the number of load repetitions. It was noted that at the end of the fiftieth LWD load application, the increment of nonrecoverable (plastic) deformation was much smaller compared to the resilient/recoverable deformation.
From the results of the accumulated permanent strains (deformations) conducted by repeated LWD tests, it can be deduced that the increase in permanent strains does not behave in the same way under all load and water contents conditions. Most of the permanent deformations have been developed at the first few cycles and then the accumulation of the permanent deformations has continued its slow decline during the last cycles for the points tested at the lowest stress levels and under 8% and 10% water contents. The material in the wettest water content tested in this study was more prone to the accumulation of permanent strain when subjected to high stress levels.
Furthermore, prediction models for accumulated permanent strains based on the repeated LWD measurements are suggested in this study. For the tested subgrade soil, the models given in Eqs 7 and 9 have showed good matching to the accumulated permanent strain (εp) for the data measured by
in-situ repeated LWD tests as a function of loading cycles, stress levels and water content during testing. The model given in Eq.9 with power water content variable (of c=2.58) gave the best matching to the accumulated permanent deformations measured by in-situ repeated LWD tests, as compared to the model given in Eq. 7. The model given in Eq. 6 has also been used to fit the in-situ repeated LWD accumulated permanent strains in terms of the recoverable strains. Generally, all the developed and adopted models have showed good matching to the accumulated permanent strain (εp) data measured
by the repeated LWD tests except for the cases of p=100 kPa and 200 kPa at 15% water content due to the excessive accumulated permanent deformations reported for these cases.
In summary, this study showed that the repeated light weight deflectometer test could be a useful material assessment tool that could be utilized to establish the risk level of permanent deformations in the subgrade layer during the design and construction stages. Using this methodology can lead to an improvement in construction quality that may result in an increase in pavement service life.
Acronyms
a, b and c= Models parameters used in accumulative permanent deformation modeling BVFF= Bana Väg För Framtiden
LL= Liquid limit
LVDTs= Linear Variable Differential Transformers LWD= Light weight deflectometer
N= Number of load applications (cycles) NDG= Nuclear Density Gauge
p= Axial applied stress during repeated LWD tests pa= A reference pressure of about100 kPa
PD=Permanent deformation PL= Plastic limit
pm = Mean hydrostatic normal bulk stress
q = Deviator stress
R2= Coefficient of determination
RLT= Repeated load triaxial test
S = Normalized applied stress level = p/pa
Sf =A factor that takes into account the effect of stress state in permanent deformation modeling= {
𝑞𝑞 𝑝𝑝𝑎𝑎} {𝑝𝑝𝑚𝑚𝑝𝑝𝑎𝑎}𝛼𝛼 TRV= Trafikverket
UGMs= Unbound granular materials W= Water content (%)
α = A parameter obtained from regression analysis and used in calculating Sf
εp= Permanent (Plastic) strain in (%)
εr= Recoverable (Elastic) strain in (%)
θ = Sum of the principal stresses σz = Axial effective stress
Förord
This report is the final report for the project “Development of a simple field method for measuring permanent deformations in unbound road layers”, which is financed by the Swedish Transport Administration and VTI via the industry program BVFF (Bana Väg För Framtiden).
The author would like to thank Klas Hermelin, the Swedish Transport Administration, who in addition to being a project sponsor also followed the work with great interest. A big thank you also to Olle Tholén from KUAB AB who contributed with his experience, showed interest and developing further the VTI’s lightweight in cooperation with the author (project leader).
I myself have done the fields measurements in the project with the help of my colleague Håkan Arvidsson. I would like to thank Håkan Arvidsson, who have carried out with me the in-situ measurements and David Åberg from Tegneby Åkeri AB who helped to compact the test pit in the backyard of VTI with the subgrade soil tested in this project.
Thank you all for your contributions to the project! Linköping, December 2020
Dina Kuttah Project manager
Granskare/Examiner
Sigurdur Erlingsson, VTI
De slutsatser och rekommendationer som uttrycks är författarens/författarnas egna och speglar inte nödvändigtvis myndigheten VTI:s uppfattning./The conclusions and recommendations in the report are those of the author(s) and do not necessarily reflect the views of VTI as a government agency.
Innehållsförteckning
Publikationsuppgifter – Publication Information ...5
Kort sammanfattning ...6 Abstract ...7 Sammanfattning ...8 Summary ...9 Acronyms ...10 Förord ...11 1. Introduction ...13
2. The Permanent Strains Models ...15
3. Testing Methodology ...17
3.1. Testing plan and flowchart ...17
3.2. Equipment used and working principles ...18
3.2.1. The Nuclear Density Gauge ...18
3.2.2. Light weight deflectometer (LWD) ...19
3.2.3. Modified Proctor test ...21
4. Characteristics of the Tested Soil...22
5. In-situ Testing of the Selected Subgrade Soil ...24
5.1. Testing layout of the test pit with compacted soil ...24
5.2. In-situ Nuclear Moisture - Density Measurements ...26
5.3. In-situ repeated LWD Tests ...27
5.3.1. General ...27
5.3.2. Deformation measurements during LWD testing ...28
5.3.3. Effect of repeated loading on soil total deformations during LWD testing ...29
5.3.4. Effect of repeated loading, stress levels and water contents on soil permanent deformations during LWD testing ...30
5.3.5. Predicting the permanent strains measured during repeated LWD testing ...32
6. Conclusions ...39
7. Recommendations ...41
8. Recommendations for Future Studies ...42
1.
Introduction
According to Lekarp (1999), pavement unbound materials exhibit an elastoplastic behaviour under repeated repeated loads. The axial deformation of the material under repeated loading conditions consists of two parts, namely, the recoverable (resilient) strain and the plastic (permanent) strain (Lekarp, 1999).
Permanent deformation usually occurs in the geomaterials (base, subbase or subgrade soils) which are responsible for the surface rutting and that can lead to significant passenger discomfort and reduces ride quality (Puppala, 2009).
Many studies have been conducted to understand the permanent deformation (PD) behaviour of UGMs and the factors that affect the PD behaviour of UGMs under repeated traffic loading (Erlingsson and Rahman, 2013, Kuttah 2021).
Permanent deformation is a complex process that directly depends on many influencing factors, namely, the applied stress levels, number of load applications, the strength of material, the loading history, the effect of principal stress rotation, moisture content (degree of saturation), matric suction, fine content, density (degree of compaction), aggregate type, particle size distribution (gradation), and the amount and type of fines (plastic or non-plastic), (Lekarp et. al. 2000, Xiao et. al, 2015 and Alnedawi et. al. 2019). However, the stress level and number of load cycles emerge as the most important factors (Ramosa et al., 2020).
Among several factors that compromise the ability of flexible pavement structures to sustain mechanical loads without the accumulation of permanent deformation (PD), the moisture content of the unbound materials is one of the most relevant (Rahman and Erlingsson, 2015 and Silva et al, 2021).
The moisture content of the pavement layers varies with the infiltration of rainwater through cracks in the pavement or from the uncoated shoulders, variation of the level of the water table, and the
migration of moisture between the layers due to temperature variations, among other processes (Lima et al 2019).
There are a number of laboratory tests currently used to evaluate the permanent deformation of geomaterials; they attempt to reproduce in situ stress conditions in pavements. Laboratory
investigations using cyclic triaxial tests, simple and cyclic shear tests, resonant column and hollow cylinder tests, among others have been carried out (Ramosa et al., 2020).
The repeated load triaxial (RLT) test is the most widely used to study of geomaterials subjected to repeated loads (Rahman and Erlingsson, 2015, and Ramosa, 2020). However, the RLT test is relatively expensive and time consuming. Therefore, many road and transport administrations
including the Swedish transport administration (Trafikverket, TRV) have paid a particular attention to find a simple and a time saving technique to evaluate the unbound layers and subgrade soil quality and properties directly in the field during roads construction.
Nowadays, the light weight deflectometer (LWD) has widely been used overseas and become more and more popular in quality assurance of road works due to its light weight and time saving
procedures.
In spite of the fact that the setup and test times for LWD are relatively short (Siekmeier et al. 2009) there are some concerns about the effectiveness of the LWD in testing layered systems. This concern is mainly attributable to the fact that the LWD’s zone of influence may extend beyond the thickness of the tested layer.
Nazzal (2003) and Tompai (2008) found that the zone of influence of the LWD to vary between 1 and 2 times the plate diameter. Elhakim et al. (2014) investigated the zone of influence of the LWD by performing the LWD test on calcareous and siliceous sands placed in layers with varying thicknesses
resting on a rigid boundary (concrete floor). They found that the LWD reading reflects the stiffness of both the tested soil and the rigid boundary. The influence of the rigid base boundary on the LWD modulus diminishes with increasing the thickness of the sand layer. For soil layer thickness to plate diameter ratios of 1.5–2, the effect of the rigid layer is considered negligible something which goes well with Nazzal (2003) and Tompai (2008) findings.
In this study, the current VTI’s LWD has been developed further and used to measure and predict accumulated plastic deformations of a selected subgrade soil compacted at a test pit of 1.5 m depth. The plate of the used LWD has been 20 cm in diameter, therefore, there are no concerns about the effectiveness of the LWD in testing layered systems and the LWD’s zone of influence will not extend beyond the thickness of the tested layer.
The new LWD model has been used in estimating the compaction potential /deformation potential of compacted soil, something which will help in identifying the characteristics of the tested soil in early stages of construction, namely, after compaction.
As far as the author know, no study has been conducted yet to estimate the accumulated permanent deformation of compacted road material by simple in-situ tests, except those carried out by Kuttah (2020) and Kuttah (2021). The current study dealing with determining the accumulated permanent deformation of compacted soil using simple in-situ test, considering the influence of loading magnitude (stress levels), the number of load repetitions and water content on the permanent deformation accumulation for the silty sand subgrade tested in this study.
Predicting permanent deformation accumulation of a compacted road layer by simple field tests will give a good and early indication on the material behaviour under repeated loading, something which enable instance decision making during road construction. The decision may result in adopting early actions like, further compaction or stabilization requirement or some stress levels and moisture restrictions before continuing compacting the next road layer. Hence, an improvement in construction quality will be achieved resulting in an increase in pavement service life.
New models have been developed to correlate the measured accumulated permanent deformations using the new developed LWD to the number of loading cycles, stress levels and the water content. These models will provide supporting data for an increased understanding of the expected pavements ruts since the accumulated plastic deformations of subgrade soil contribute significantly to the pavements ruts and other deformations which require adequate and frequent pavements maintenance.
2.
The Permanent Strains Models
As highlighted previously, over the past decades, researchers have been concerned about permanent deformation and they continually search for the most accurate methods and models that will measure and predict these values (Monismith 1975; Ullidtz, 1993; Lekarp and Dawson, 1998; Erlingsson and Rahman, 2013; Rahman and Erlingsson, 2015; Kuttah, 2020 and Kuttah, 2021), among others. These methods were often used to determine shear stress–strain behaviour of geomaterials considering reversible and irreversible deformation under repeated loads (Gomes Correia, 2004).
The behaviour of geomaterials under repeated loads can be characterized by either using complex elastoplastic models (recoverable and permanent deformation are both considered) or by shakedown theory and mechanistic-empirical models (Hornych et al., 2004).
The focus of the current research will be on mechanistic-empirical models. These models can correctly simulate the response of materials; they are easy to implement, and they depend on fewer parameters than conventional elastoplastic models (Ramosa et al., 2020).
Mechanistic-empirical models can be divided into single-stage models and multi-stage models. A single-stage implies that the repetitive load tests are carried out at one stress level in one test; in this instance, multiple specimens are tested at different stress levels. Multistage models can test multiple levels of stress in one test on one specimen. This approach enables the effects that the stress level and stress history have on permanent deformation to be considered (Gregoire et al, 2011).
Mechanistic-empirical models often describe a relationship between the number of load cycles (N) and the accumulated permanent deformation. In fact, those models that only consider the value of N should not be used to predict permanent deformation because they are too simple and lacks accuracy (Ramosa et al., 2020).
One of the first simple models proposed, which relates plastic strains to the number of load
applications and other factors, was the model of Monismith et al. (1975), as shown in Eq. (1) below:
Eq. 1 where:
εp= Accumulated plastic strain (%)
N= Number of load applications
a, and b= Parameters that represent the influence of other factors.
A more developed permanent strain model which includes directly the influence of the axial applied stress level is the one developed by Ullidtz (1993) as given in Eq.2.
Eq. 2
where εp is the accumulated permanent strain, N is the total number of load cycles, a, b and c are
model parameters, σz is the axial effective stress; and pa is the atmospheric pressure, pa = 101.3kPa.
Puppala et al. (1999) performed dynamic triaxial tests adopting three types of soils. Then, it was found that the effect of force on deformation can be more fully reflected by mean normal bulk stress and an improved model based on the model in Eq. (2) was proposed, as shown in Eq. (3)
εp= a Nb
Eq. 3
where pm is the mean normal bulk stress, and a, b and c are regression coefficients.
More recent models depend on the mean (p) and deviatoric (q) stresses have been developed, as does the model developed by Rahman and Erlingsson (2015), see Eq.4.
Eq. 4
where εp is the accumulated permanent strain, N is the total number of load cycles, a and b are model
parameters associated with the material and the term Sf takes into account the effect of stress state in
permanent deformation accumulation given as:
Eq. 5
where pm is the hydrostatic stress (one third of the sum of the principal stresses, θ), q is the deviator
stress, pa is the reference stress here taken equal to the atmospheric pressure 100 kPa and α is a
parameter obtained from regression analysis.
Furthermore, Rahman et. al. 2020, proposed the model given in Eq.6 considering the resilient strain (εr) by simply replacing Sf given in Eq. 4 by εr as follows:
Eq. 6
where εr is the resilient (elastic) strain during one load cycle.
In this study, accumulative permanent strains models have been developed to match the permanent strains measured from a single stage in-situ repeated LWD as a function of the number of cycles (N), stress levels and water content. In addition, the model given in Eq. 6 that predict the permanent strains as a function of the number of cycles (N) and elastic strains has been applied to model the data
measured from the single stage repeated LWD conducted in this study as discussed in the following paragraphs. εp= a 𝑁𝑁𝑏𝑏 (pm /pa)c εp= a Sf 𝑁𝑁𝑏𝑏 𝑆𝑆𝑓𝑓 Sf = {𝑝𝑝𝑎𝑎𝑞𝑞} {𝑝𝑝𝑚𝑚𝑝𝑝𝑎𝑎}𝛼𝛼 εp= a εr 𝑁𝑁𝑏𝑏 𝜀𝜀𝑟𝑟
3.
Testing Methodology
3.1. Testing plan and flowchart
The project methodology and implementation steps of the project have been as follows:
KUAB Konsult och Utveckling AB in cooperation with the author has developed further the VTI’s LWD to measure the in-situ elastic and plastic deformations components (caused by dynamic loading) separately, simply, and accurately.
At the beginning, a commonly available silty sand subgrade soil has been chosen for testing. Materials classifications tests have been carried out at VTI to define the main physical and mechanical properties of the materials to be used for testing (e.g. particle size distribution tests, liquid limit (LL) and plastic limit (PL), compaction characteristics and specific gravity tests).
The in situ LWD tests have been carried out on a newly compacted subgrade layer in a test pit located at the backyard of VTI in Linköping. During the in-situ LWD tests, the elastic and plastic
deformations have been measures under repeated loading at different stress levels and water content conditions.
The subgrade compactability can be judged through a developed model which enables the prediction of accumulated plastic deformation (strain) of the tested compacted layer with respect to the applied load magnitude, number of loadings and in-situ water contents. Measuring the permanent deformation generated by repeated drops of a specified LWD drop loads under different water content conditions will give a good estimation to the expected deformations that may take place due to real traffic loading. Furthermore, these measurements will help in identifying the weak points in the tested road layers and the influencing factors that will likely resulted in more permanent deformations than others. The models developed to assess the material resistance to permanent deformation measured by in-situ LWD test with regard to the applied stress levels and water contents may lead to suitable actions that could reduce the future roads ruts and hence maintenance.
Figure 1 shows a detailed flow chart for the testing procedure. It can be seen from Figure 1 that the adopted in-situ repeated LWD tests have been carried out at stress levels of about 50 kPa, 100 kPa, and 200 kPa. These stress levels have been chosen to simulate the real ranges of applied stresses on the subgrade soil by moving vehicles for subgrade used for paved and unpaved roads.
mm. This method has been used to measure the moisture contents to a depth of 30 cm, something which goes well with the zone of influence of LWD tests, as will be discussed in the next paragraphs. Density and water content are determined with different radiation sources and detectors, which are integrated under a common outer shell and with a common display of results. With a built-in
microcomputer, the dry density and the water ratio are calculated from the measurement data, which are then displayed on a dashboard (Vägverket Publ. No. 1993: 26). The NDG used in this study is shown in Figure 2.
Figure 2. The Nuclear Density Gauge used in this study
. Photo: Dina Kuttah, VTI.
3.2.2. Light weight deflectometer (LWD)
The light weight deflectometers is usually used to determine the surface soil stiffness by means of geophones placed in direct contact with the ground surface. The operation of an LWD involves the measurement of deflections induced by dropping weight of up to 20 kg and detected by built-in geophones or accelerometers. The drop height can be easily and quickly adjusted by a movable release handle and hereby changing the maximum impact force.
A new multifunctional light weight deflectometer has been developed and used in the current research study, see Figure 3. The used LWD has been developed solely to fulfill the requirement of the current project. During the test, the falling mass (of 10 kg) impacts the plate, producing a load pulses that may be adjusted in magnitudes depending on the dropping heights and dropping masses. The center deflection of the tested material surface is measured through a hole in the loading plate by a highly accurate, seismic transducer (geophone) while the center deflection of the steel plate is measured by three geophones. Two more deflection geophones have been added at 30 cm and 60 cm distance from the loading centerline. The basic diameter of the loading plate used in this study was 200 mm. The drop height can be easily and quickly adjusted by a movable release handle and the peak value of the impact force is based on actual measurements from the load cell, see Figure 3.
Figure 4. Granular material behaviour under repeated loading.
This function enables instant determination of the number of drops (cycles) required during the LWD to bring, as much as possible, the soil to near elastic state. In other words, using the developed multifunctional LWD, the plastic (permanent) and elastic (recoverable) components of the
deformations can be measured separately. The data collection software is installed in a PC coupled to the LWD control panel. The time history graph from both the deflection sensors and the load sensor can be displayed in the PC screen in real time. Relevant information such as name, place, weather and comments can be added to the data file for each measuring point. The collected tests data can be printed as a report or can be exported to other software like Excel for further processing.
3.2.3. Modified Proctor test
The compaction characteristics of the tested soil has been determined using modified Proctor test according to ASTM D1557 (2012). The purpose of the test has been to determine the maximum dry density and the optimum moisture content of the tested subgrade soil.
4.
Characteristics of the Tested Soil
A silty sand subgrade soil has been selected to be tested in this study, as mentioned previously. Series of laboratory tests were carried out on the selected soil to determine its physical properties, namely, the particle size distribution, clay fraction, soil classification, specific gravity, liquid and plastic limits, and compaction characteristics.
The particle size distribution test on the selected soil has been carried out according to SS-EN 933-1 (2004) and the results of the test is illustrated in Table 1 below:
Table 1. Particle size distribution of the tested soil.
Sieve,
µm 22.4 16 11.2 8 5.6 4 2 1 0.5 0.25 0.125 0.063
% Finer
than 100 97 96 95 94 93 90 88 84 76 62 39.2
The clay content in the soil was tested according to VTI method for grain size distribution analysis with laser diffraction [10 nm - 2 mm] and was found to be 5%.
According to VVTK Väg (2008), the tested soil is of material type 4A (mixed-grained soils with frost danger class 3). With respect to SS-EN ISO 14688-2 (2004), the soil is classified as a silty sand with 5% clay, see Figure 5.
The liquid and plastic limits were determined at SGI (Statens geotekniska institut, Sweden) according to SS-EN ISO 17892-12 (2018). The tests results revealed a liquid limit (LL) of 18% and a plastic limit of (PL) 14.3% resulting in 3.7% plasticity index for the tested soil.
In this project, the compaction properties were determined by modified Proctor test as per ASTM D1557 (2012). The test was performed by compacting several soil samples using a cylindrical mold of 152.4 mm diameter. The soil samples were compacted at different molding water contents ranging between 0 to 16% to determine the water-density relationship.
The results of the compaction tests revealed that the tested soil has a maximum dry density of 2.03 g/cm3 at about 8.2% optimum moisture content, see Figure 6.
5.
In-situ Testing of the Selected Subgrade Soil
In this paragraph the tests data and results collected from in-situ tests carried out on the selected silty sand subgrade soil are illustrated and discussed in detail.
In-situ tests were carried out on the subgrade soil, namely, field density and moisture content tests, and repeated in-situ LWD. These tests were carried out on a test pit compacted with the selected soil under controlled conditions, as discussed in the following paragraph.
5.1. Testing layout of the test pit with compacted soil
A test pit at the backyard of the Swedish Road and Transport Research Institute (VTI) was constructed for in-situ testing. The test pit was instrumented with an electric drive motor roof panel. The roof panel can be opened and closed with the help of an electric motor to control as much as possible the testing conditions in the test pit, see Figure 7.
The test pit was approximately 9.5 m long x 5.7 m width x 1.5 m depth. The test pit was also equipped with a concrete well with a water discharging motor that was used to control the ground water level during testing.
At the beginning, the previously tested material of a previous project was removed from the test pit and the new selected subgrade material was placed and thoroughly compacted using a small vibrator, see Figure 7. When the compaction completed, the final subgrade surface was marked with circles representing the selected places of the points to be tested, as shown in Figure 8.
Figure 8. Position of the points tested on the final compacted soil.
5.2. In-situ Nuclear Moisture - Density Measurements
Table 3 shows a summary of the LWD applied stresses on the tested subgrade soil for each point using a steel plate of 200 mm in diameter together with the water content measurements during testing as conducted by NDG tests and calibrated using oven dried method for collected samples.
The tested points were grouped to three groups based on the convergence in testing time point and water content (W). Each group of closed testing water content shown in Table 3 includes three subgroups of points tested at target stresses of 50 kPa, 100 kPa and 200 kPa.
Table 3 shows also the degree of saturation and relative compaction values conducted for the tested points during the repeated LWD measurements.
Table 2. Summary of the results of NDG tests carried out on the compacted silty sand subgrade. Point Target applied stress (kPa) Actual applied load (kN) Actual applied stress (kPa) W
(%) Field dry density (kg/m3) Degree of saturation (%) Average W for a group (%) Average field dry density (kg/m3) Average relative compaction (%) 10 50 1.58 50.28 8.12 1887 54 8 1864 91.9 12 1.55 49.30 8.27 1858 52 11 100 3.26 103.60 8.12 1887 54 8 200 6.45 205.26 8.42 1830 51 9 6.48 206.15 8.27 1858 52 1 50 1.56 49.65 10.4 1739 53 10 1812 89.3 3 1.58 50.18 10.4 1833 63 5 1.65 52.52 10.4 1739 53 13 1.55 49.44 9.48 1789 53 2 100 3.19 101.52 10.4 1833 63 4 3.22 102.56 10.4 1833 63 14 3.28 104.32 9.48 1789 53 6 200 6.30 200.35 9.0 1875 59 7 6.35 202.18 9.0 1875 59 16 50 1.63 51.93 15.3 1810 89 15 1779 87.7 17 1.66 52.75 15.4 1746 80 21 1.61 51.23 15.3 1810 89 15 100 3.27 104.15 15.1 1812 88 22 3.23 102.69 15.1 1812 88 18 200 6.21 197.44 15.4 1746 80 19 6.25 198.78 15.4 1746 80 20 6.07 193.24 15.4 1746 80
5.3. In-situ repeated LWD Tests
5.3.1. General
The in-situ repeated LWD tests were divided into three series, as shown in Table 3 above. For each LWD testing series, at an average water content, the repeated LWD tests have been carried out on different points at three different target applied stresses namely 50 kPa, 100 kPa and 200 kPa. At the beginning, and after completing the compaction and marking the points on the compacted surface at the test pit, during which the testing water contents were around 10%, series of LWD tests were carried out at the three target applied stresses, namely, 50 kPa, 100 kPa and 200 kPa. During all the in-situ repeated LWD tests, the LWD plate diameter was 200 cm, but by changing the drop height of 10 kg drop weight, the applied stresses have been varied to achieve the target stresses.
In order to carry out LWD at water content drier than the initial water content of 10%, the steel cover of the test pit was left open for several hours in order to dry the soil under sunshine and then the LWD tests were carried out at a water content of about 8% under the similar target applied stresses.
Later, to carry out LWD tests on a water content wetter than the latest water content of 8%, the soil in the test pit was watered. The research team waited for few hours, to ensure consistence water content penetration and distribution through the soil layer, before carrying out the LWD tests. Then, the repeated LWD tests were performed at about 15% water content. For details of the exact stresses and water content of each individual point, see Table 3 above.
5.3.2. Deformation measurements during LWD testing
As mentioned previously, the multifunctional LWD can measure the tested layer permanent and recoverable deformations under the centerline of the drop weight. The central geophone (used to measure the total deformation) and a LVDT (used to measure the plastic deformation), both placed along the center of the dropping weight to measure the total and plastic deformation on the soil surface directly through a hole on the center of the steel plate. The LVDT was coupled also to a control beam to measure the absolute plastic deformation. The measurement of plastic deformations by a mounted LVDT is important because the plastic deformations cannot be conducted from the central geophone due to the general integration errors usually encountered.
Correspondingly the accumulated load-deformation loops could be collected and plotted for each tested point. Figure 9 A and B show typical examples of the effect of repeated loading on the accumulated permanent deformations for the case of point 4 and point 9, respectively. It can be seen from these figures that the tested subgrade soil is not truly elastic but experience some nonrecoverable deformation after each load application.
Also, it can be shown in Figure 9 A and B that at the end of the fiftieth LWD load application, the increment of nonrecoverable (plastic) deformation is much smaller compared to the
resilient/recoverable deformation. Note that each tested point has been loaded with several LWD applications at only one chosen stress level as given in Table 3.
0 1 2 3 4 0 200 400 600 800 1000 1200 1400 1600 Load (kN )
Accumulative soil deformation (µm)
Blow 1 Blow 2 Blow 3 Blow 4 Blow 5 Blow 6 Blow 7 Blow 8 Blow 9 Blow 10 Blow 11 Blow 12 Blow 13 Blow 14 Blow 15 Blow 16 Blow 17 Blow 18 Blow 19 Blow 20 Blow 21 Blow 22 Blow 23 Blow 24 Blow 25 Blow 26 Blow 27 Blow 28 Blow 29 Blow 30 Blow 31 Blow 32 Blow 33 Blow 34 Blow 35 Blow 36 Blow 37 Blow 38 Blow 39 Blow 40 Blow 41 Blow 42 Blow 43 Blow 44 Blow 45 Blow 46 Blow 47 Blow 48 Blow 49 Blow 50
Figure 9 B. Accumulated load-deformation loop for accumulated drops under about 200 kPa applied stress at 8% W (Point 9).
5.3.3. Effect of repeated loading on soil total deformations during LWD testing
For each point, the effect of the number of drops on the progress of total deformation has beenassessed. In addition, the effect of the number of drops on the progress of elastic (resilient) and plastic (permanent) deformation components has also been determined separately. Figure 10 shows typical curves for the effect of the number of drops on the total, resilient and permanent deformations during LWD testing on point 4 (under about 100 kPa applies stresses at 10% W).
Point 4 has been selected to illustrate the effect of repeated loading on total, resilient and permanent soil deformations during LWD testing. According to Figure 10, the permanent deformations reduce with increasing the number of drops and reach zero at around 22 LWD drops. The sudden increase of permanent deformation above zero at drop no. 16 may be attributed to a slight movement of the LWD during testing. The variation of the resilient deformations dies out with increasing the number of drops. Furthermore, a significant decrease trend of the total deformations curve with increasing the number of drops has been reported as well.
0 1 2 3 4 5 6 7 0 250 500 750 1000 1250 1500 1750 2000 2250 Load (kN )
Accumulative soil deformation (µm)
Blow 1 Blow 2 Blow 3 Blow 4 Blow 5 Blow 6 Blow 7 Blow 8 Blow 9 Blow 10 Blow 11 Blow 12 Blow 13 Blow 14 Blow 15 Blow 16 Blow 17 Blow 18 Blow 19 Blow 20 Blow 21 Blow 22 Blow 23 Blow 24 Blow 25 Blow 26 Blow 27 Blow 28 Blow 29 Blow 30 Blow 31 Blow 32 Blow 33 Blow 34 Blow 35 Blow 36 Blow 37 Blow 38 Blow 39 Blow 40 Blow 41 Blow 42 Blow 43 Blow 44 Blow 45 Blow 46 Blow 47 Blow 48 Blow 49 Blow 50
Figure 10. Effect of repeated loading on total, resilient and permanent soil deformations during LWD testing on point 4 (under about 100 kPa applies stresses at 10% W).
5.3.4. Effect of repeated loading, stress levels and water contents on soil
permanent deformations during LWD testing
A lot of effort has been made to develop models that can describe and predict the non-linear permanent deformation behavior of unbound granular materials under repeated loading. Many researchers have developed models to describe the stress and moisture dependency of the measured permanent deformation during RLT testing as described previously.
In this project, the effect of repeated loading on the permanent soil deformations during LWD testing is of particular interest, and therefore, it has been studied in detail. Figure 11 shows the effect of repeated loading on the permanent soil deformations during repeated LWD testing at different water contents and stresses levels. The missing data in Figure 11 for some of the LWD tests are because of
0
100
200
300
400
500
600
700
0
5
10
15
20
25
30
35
40
45
50
De
fo
rma
tio
ns
(
µm)
No. of LWD weight drops
Figure 11. Effect of repeated loading on the permanent deformations during LWD testing at different water contents and applied stress levels.
It can be noticed from Figure 11 that the lowest value of permanent axial deformation at the end of the 50th LWD drop was obtained for the points subjected to the lowest stress level of 50 kPa and tested at
lowest water content of 8%. At the same stress level of 50 kPa, increasing the moisture content to 10% and 15% resulted in 78% and 778% increase in permanent deformation values, respectively.
The highest value of permanent axial deformation was reported for the points subjected to the highest stress level of 200 kPa and tested at the highest water content of 15% at the end of the 23rd LWD drop.
Increasing the number of LWD drops further caused extreme deformations that were beyond the measuring limits of the used deformation measuring sensors, see Figure 11.
A similar trend was observed for points tested at the same water contents but exposed to increasing stress levels. Increasing the stress level from 50 kPa to 100 kPa and then 200 kPa at 8% water content resulted in 431% and 1183 % increase in the permanent deformation values, respectively.
y = 52.9x0.2398, R² = 0.917 y = 342.33x0.2267, R² = 0.99 y = 687.58x0.2695, R² = 0.996 y = 67.862x0.3507, R² = 0.99 y = 341.08x0.2666, R² = 0.984 y = 851.06x0.3347, R² = 0.979 y = 295.43x0.4064, R² = 0.984 y = 2028.7x0.1458, R² = 0.6524 y = 1448.7x0.5835, R² = 0.991 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 10 20 30 40 50 60 70 Ac cu mu la ted v er tic al p er ma nen t d efo rma tio n (µ m)
No. of LWD weight drops
p=50 kPa at 8% WC p=100 kPa at 8% WC p=200 kPa at 8% WC
p=50 kPa at 10% WC p=100 kPa at 10% WC p=200 kPa at 10% WC
Increasing the stress level from 50 kPa to 100 kPa and then 200 kPa at 15% water content resulted in 154% and 648 % increase in the accumulated permanent deformations, respectively.
This means that the percent increase in permanent deformations with increasing the stress levels, decrease with increasing the water content (i.e., 431% and 1183% increase in permanent deformation with increasing stress level from 50 kPa to 200 kPa for 8% WC this percent increase in permanent deformations with increasing the stress level, decreased to 154% and 648% when the WC increased to 15%). Note that the absolute increase in permanent deformation has been higher for each LWD drop at higher water content under the same stress level.
This observation is attributed to the fact that the permanent deformations at higher water content reached high values even under low stress levels.
Note that for the case of points tested at 15% moisture content under 100 kPa stress level, as seen from Figure 11, the recorded permanent deformations curve shows high values at the first few LWD loading cycles and then the trend of increase in permanent deformations dies out after few LWD drops. This observation is attributed to the loss of the plastic deformation data reported from point 22 after few drops and therefore the curve was continued based on the reported plastic deformation data from the other point only (point 15). This incident resulted in a nonuniform permanent deformations curve for this case with relatively poor power modeling (of R 2 = 0.65) as compared to the other groups shown in
Figure 11.
5.3.5. Predicting the permanent strains measured during repeated LWD testing
It is clear that the models shown in Eqs 2 and 6 require the permanent strains while in the developed repeated LWD test, the permanent deformations could be measured. Therefore, in order to convert the measured permanent and recoverable deformations to strains (the recoverable strains required in Eq.6), the LWD’s zone of influence should be estimated.As mentioned previously, Nazzal (2003) and Tompai (2008) found that the zone of influence of the LWD to vary between 1 and 2 times the plate diameter something which goes well with Elhakim et al. (2014) findings. Correspondingly, in this study, the zone of influence of the LWD was assumed to be 1.5-fold of the plate diameter of 20 cm (i.e. the zone of influence of the LWD was assumed to be 30 cm) in this study.
In order to consider the effect of number of load cycles, water content and the applied stresses on the prediction of the accumulated permanent strain measured by repeated in-situ LWD tests, the model given in Eq. 7 has been developed.
Eq. 7
where εp is the accumulated permanent strain, N is the total number of load cycles, a and b are
In order to adapt further the model given in Eq. 7 to match better the measured in-situ LWD accumulative plastic strains, a power water content (W) variable has been used as given in Eq. 9 below.
Eq. 9
where εp is the accumulated permanent strain, N is the total number of load cycles, a and b and c are
regression parameters associated with the material, W is the water content measured during LWD testing by NDG in (%), and S is as defined in Eq.8.
Based on the discussion above, a set of nonlinear parameters namely a, b and c that would provide a reasonable fit for a given model were determined, see Table 3.
The optimization of the model’s parameters given in Eqs.7 and 9 was carried out so that the sum of squares for error from the test data and the model predictions is minimized.
Table 3.The parameters that would provide a reasonable fit for the models given in Eqs. 7 and 9.
Model a b c
Eq. 7 0.0191 0.1115 …...
Eq. 9 0.00045 0.1435 2.5803
Figure 12 shows the accumulated permanent strain (εp) as a function of loading cycles for the data
measured by in-situ repeated LWD and the developed models given in Eqs 7 and 9. From the results of the accumulated permanent strains measured by repeated light weight
deflectometer tests, it can be deduced that the increase in permanent strains does not behave in the same way under all load and water contents conditions.
In general, εp increased with increasing water content and applied stress levels. It can be noticed that
most of the εp accumulation occurred in the initial part of the tests. Some results presented excessive εp
in some testing conditions, such as the cases of the largest stress levels in the wettest testing conditions.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Ac cum ul at ed pe rm ane nt st ra in ( % ) 1 10 20 30 40 50 1 10 20 30 40 50 1 10 20 30 40 50 W= 8% W= 10% W= 15%
p≈ 50 kPa p≈ 100 kPa p≈ 200 kPa
p≈ 50 kPa p≈ 50 kPa p≈ 100 kPa p≈ 100 kPa p≈ 200 kPa p≈ 200 kPa 1 10 20 30 40 50 1 10 20 30 40 50 1 10 20 30 40 50 1 10 20 30 40 50 1 10 20 30 40 1 10 20 34
For the εp curve of the points tested at 8% and 10% water contents under 50 kPa and 100 kPa, a very
slow increase in εp is produced. For the curve of points tested at 15% water contents under 50 kPa, the
increase in εp is a little more accentuated. Regarding the curves of points tested at 8% and 10% water
contents under 200 kPa, the increase in εp is even more accentuated.
On the other hand, for the curves of points tested at 15% water contents under 100kPa and 200 kPa, the increase in εp (%) is much more accentuated and the stabilization of the material is not observed.
Furthermore, it can be seen from Figure 12 that the model given in Eq.9 with power water content variable (with c=2.56) gives better matching to the permanent deformations measured by in-situ repeated LWD tests, as compared to the model given in Eq. 7. This observation emphasis the importance and influence of water content on the permanent deformations of the tested soil. Moreover, the model given in Eq. 6 has been applied to predict the accumulative permanent
deformations measured by in-situ repeated LWD tests. A set of nonlinear parameters namely a and b that would provide a best fit for a given water content and a stress level group were determined separately for each case and presented in Figure 13 and 14 respectively.
The values of a for the different points tested at the same stress level were plotted against w as shown in Figure 13. It shows that a can be expressed as a power function of W (within the ranges used in this study). The values of this parameter for the different stress levels and the corresponding R2 values can
be read in Figure 13.
Figure 13. Parameter a (Eq. 6) as a function of the water content for different applied stress levels. Similarly, the values of b for the different points tested at the same stress level were plotted against W as shown in Figure 14. It shows that b can be expressed as a linear function of W (within the ranges used in this study). The values of this parameter for the different stress levels and the corresponding R2
values can be read in Figure 14.
y = 7E-06x5.2055 R² = 0.9594 y = 0.0039x2.6634 R² = 0.8694 y = 0.0002x4.0451 R² = 0.8934 0 2 4 6 8 10 12 8 9 10 11 12 13 14 15 16 Pa ra met er a W (%)
Figure 14. Parameter b (Eq. 6) as a function of the water content for different applied stress levels. Figures 15, 16 and 17 show the measured versus modelled accumulated permanent strain using Eq. 6 as a function N for the tested soil with different stress levels at 8%, 10% and 15% moisture contents, respectively.
Furthermore, Figures 15, 16 and 17 show also the measured versus modelled accumulated permanent strain based on regression analysis as a function N for the tested soil with different stress levels at 8%, 10% and 15% moisture contents, respectively, together with the coefficient of determinations values (R2).
I t can be seen from Figures 15 to 17 that the model given in Eq. 6 fits reasonably well with the in-situ repeated LWD accumulated permanent strains except for the cases of p=100 kPa and 200 kPa at 15% water content and for N> 20 drops for the case of p= 200 kPa at 10% water content. For these cases, the model fails to fit the data or predict further observations reliably due to excessive observed accumulated permanent strains that could not be followed by the model. The missing data (showing N< 50 LWD drops) for some of the tests are because of termination of the tests due to excessive deformations that goes beyond the measurement limits of the used sensors.
y = -164.81x + 2642.7 R² = 0.9921 y = -41.7x + 684.33 R² = 0.8747 y = -21.374x + 369.89 R² = 0.9772 0 200 400 600 800 1000 1200 1400 1600 8 9 10 11 12 13 14 15 16 Pa ra met er b W%
Figure 15. Measured and modelled accumulated permanent strain using Eq. 6 for w=8%.
Figure 16. Measured and modelled accumulated permanent strain using Eq. 6 for w=10%.
y = 0,0002x0,2398 R² = 0,9167 y = 0,0011x0,2267 R² = 0,9902 y = 0,0023x0,2695 R² = 0,9957 0,0% 0,1% 0,2% 0,3% 0,4% 0,5% 0,6% 0,7% 0 10 20 30 40 50 60 Ac cu m ul at ed p er m an en t s tr ai n (% )
Number of LWD drops (loading cycles)
p=50 kPa p=100 kPa p= 200 kPa Eq. 6
y = 0,0002x0,3507 R² = 0,9897 y = 0,0011x0,2666 R² = 0,9843 y = 0,0028x0,3347 R² = 0,9792 0,0% 0,1% 0,2% 0,3% 0,4% 0,5% 0,6% 0,7% 0,8% 0,9% 1,0% 1,1% 1,2% 0 10 20 30 40 50 60 Ac cum ul at ed pe rm ane nt st ra in ( % )
Number of LWD drops (loading cycles)
Figure 17. Measured and modelled accumulated permanent strain using Eq. 6 for w=15%. The irregularity of Eq. 6 curves shown in Figures 15, 16 and 17 is attributed to the variable εr (the recoverable strain) used in Eq. 6 because this variable measured independently for each drop during LWD testing. y = 0,001x0,4064 R² = 0,9838 y = 0,0068x0,1458 R² = 0,6524 y = 0,0048x0,5835 R² = 0,9911 0,0% 0,1% 0,2% 0,3% 0,4% 0,5% 0,6% 0,7% 0,8% 0,9% 1,0% 1,1% 1,2% 1,3% 1,4% 1,5% 1,6% 1,7% 1,8% 1,9% 2,0% 2,1% 2,2% 2,3% 2,4% 2,5% 2,6% 2,7% 2,8% 2,9% 3,0% 3,1% 3,2% 0 10 20 30 40 50 60 Ac cum ul at ed pe rm ane nt st ra in ( % )
Number of LWD drops (loading cycles)
6.
Conclusions
Geomaterials exhibit elastoplastic behaviour during dynamic and repeated loading conditions. These loads are induced by the passage of a vehicle which then generates recoverable (resilient) deformation and permanent (plastic) deformation. Modelling this behaviour is still a challenge for geotechnical engineers as it implies the understanding of the complex deformation mechanism.
This work is part of a broader and comprehensive research in progress at VTI to provide input data to a future pavement design guide using simple in-situ technology. The proper understanding and characterization of the resistance to permanent deformation of road materials is a key part of any design guide. Correspondingly, this study was carried out to evaluate the effect of water content and stress levels on permanent deformation potential of silty sand subgrade soil by using in-situ repeated light weight deflectometer test. Furthermore, chosen mechanistic-empirical models used to predict the permanent strains of unbound materials were examined and modified to match better the permanent strains conducted in this study using in-situ repeated LWD tests.
From the results of this research the following conclusions may be withdrawn:
• The in-situ repeated LWD test procedure adopted in this study has the potentials to be used for characterizing the deformation behaviour of subgrade soils. This can considerably reduce the effort and time required for permanent deformation characterization as compared to more complicated tests.
• It was observed that the accumulated total deformations components depend largely on the number of loading cycles. At the end of the fiftieth LWD load application, the increment of nonrecoverable (plastic) deformation was much smaller compared to the resilient/recoverable deformation for the tested silty sand subgrade soil.
• It was found that the accumulated permanent deformations increase with increasing the loading cycles, applied stress levels and water content during testing. From the results of the accumulated permanent strains, it can be deduced that the increase in permanent strains does not behave in the same way under all load and water contents conditions. One can notice that most of the permanent deformations have been developed at the first few cycles and then the accumulation of permanent deformations (strains) has continued its slow but steady decline during the last cycles of LWD loading for the points tested at the lowest stress levels and under 8% and 10% water contents. The material in the wettest water content tested in this study was more prone to the accumulation of permanent deformation (strain) when subjected to larger stress levels.
• Also, it has been found that the percent increase in permanent deformations with increasing the stress levels, decrease with increasing the water content. This observation can be attributed to the fact that the permanent deformations at higher water content reached high values even under low stress levels. Note that the absolute increase in permanent deformation has been higher for each LWD drop at higher water content under the same stress level.
• For the tested subgrade soil, prediction models for accumulated permanent strains based on the repeated LWD measurements are suggested in this study. The models given in Eqs 7 and 9 have showed very good matching to the accumulated permanent strain (εp) for the data measured by in-situ repeated LWD tests as a function of loading cycles, stress levels and water content during testing. The model given in Eq.9 with power water content variable (of c=2.58) gave better matching to the permanent deformations measured by in-situ repeated LWD tests, as compared to the model given in Eq. 7. This observation emphasis the
• The model given in Eq. 6 has been applied to predict the accumulative permanent
deformations measured by in-situ repeated LWD tests in terms of the recoverable strains. This model has fitted reasonably well the measured in-situ repeated LWD accumulated permanent strains except for the cases of p=100 kPa and 200 kPa at 15% water content and for N> 20 drops for the case of p= 200 kPa at 10% water content. For these cases, the model has failed to fit the data or predict further observations reliably due excessive observed accumulated permanent strains.
7.
Recommendations
In practice, the engineers and designers desire information about the permanent deformations accumulated under structures. In this study, for the calculation of in-situ plastic strain development under repeated loading, the procedure of repeated LWD tests was proposed using a multifunction LWD capable of measuring the permanent deformations developed under repeated loading. It has been shown from this study that the repeated LWD testing could provide a powerful and quick material assessment tool for the accumulation of PD which could simplify the flexible pavement design by predicting the rutting behaviour. Information about the sum of plastic deformations provided by the repeated LWD tests could be adopted to determine the proper usage of construction materials and their performance.
It is suggested that the subgrade material tested in this study was demonstrated to be unsuitable for use under high water contents and stress levels. This is due to the fact that, under high stress levels that this subgrade may experience under high traffic loading, and frequent runny climate conditions, it suffered from high accumulated permanent already after a few dozen of loading cycles which indicates that plastic ruts would be produced soon after opening the road to traffic. The permanent deformation in the tested subgrade soil can be limited if a controlled combination between applied stress levels and moisture contents are maintained. It is recommended to keep the applied stress level on a compacted subgrade layer of this soil around 50 kPa and the water content lower than 10% to avoid early failure of the layer after traffic loading.
The predicting models adopted/developed in this study, namely Eqs. 6, 7 and 9, are sophisticated enough and easy to use. These models can be used to solve practical problems concerning the soil settlement subjected to repeated traffic loading for engineering purposes.
Nevertheless, it is important to mention that all the models developed in this study to fit repeated LWD test data are recommended to be used keeping in mind that they have been developed for specific materials and testing conditions (i.e. tested water contents, and stress levels). When using these equations for different testing materials and conditions as that they developed for, a combination of previous experience and engineering judgement should be considered.
8.
Recommendations for Future Studies
For future studies, the following research work is recommended:
1. The models given in Eqs 6, 7 and 9, could be verified further for its potential to predict the accumulated plastic strains that may encountered under long-term vehicle passes. This could be achieved by comparing the developed accumulated plastic strains models for the certain materials using repeated LWD testing with the data conducted from a heavy vehicle simulator (HVS) test carried out using the same testing material used to develop the models.
2. It is important to investigate the actual zone of influence of LWD for various materials subjected to different stress levels, water contents and using LWD plates of different
diameters. This research can be carried out using deformation and load sensors embedded in the compacted layer during the LWD testing.
3. The Nuclear Density Gauge (NDG) is the only device used in this study to assess the in-situ moisture contents. In future studies, it is recommender to measure the in-situ moisture content by other devices available in the market and compare between them in terms of judging the advantages and disadvantages of each device. This study will enable a better selection of the most suitable in-situ moisture content device that can be used in connection with the repeated LWD tests.
4. It is recommended to carry out studies to predict the accumulated permanent deformations using in-situ multistage repeated LWD in order to compare the results with the corresponding accumulated permanent deformations conducted by multistage RLT tests on the same
material.
5. It is also recommended to carry out similar studies to other types of road materials to predict the accumulated permanent deformations using in-situ repeated LWD.
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