Resilient Properties of Laboratory Compacted Subgrade Soils

Transcription

1 TRANSPORTATON RESEARCH RECORD Resilient Properties of Laboratory Compated Subgrade Soils LOUAY N. MOHAMMAD, ANAND J. PUPPALA, AND PRASAD ALAVLL The subgrade of road bed soils is generally onstruted near the opti-. mum moisture ontent-dry density ombination standard Protor tests. However, environmental and seasonal variations alter the design moisture ontents and the resilient properties of subgrade soils. These hanges influene strutural performane of pavements. Reent pavement design guides, whih use the resilient modulus for haraterizing pavement materials, require additional information on seasonal moisture ontent variations and their influene on resilient properties. This report presents results from an experimental study whih investigated the influene of moisture ontent variations on the resilient modulus of soils. Two soil types, sand and silty lay ompated at, above, and below optimum moisture ontent levels were tested using the AASHTO T-294 proedure. Two in-ell displaement measurement systems measured displaements with respet to the ends of the speimen and the middle one-third of the speimen. Results indiated that the influene of moisture ontent on moduli appears to be more evident in lays than in sands. The effet of moisture ontent on the resilient modulus omputed from both measurement systems is disussed. The measurement oeffiients used to onvert end system moduli to middle system moduli are provided. for various moisture ontent levels in both soils. The influene of onditioning onfining stresses and moisture ontents on aumulated plasti deformations is also evaluated. The regression model results for the tests onduted in the investigation are also inluded. The 1986 AASHTO design guide reommends using a resilient modulus as the property for haraterizing flexible pavement materials (1). The resilient modulus is defined as the ratio of repeated deviatori stress to reoverable axial strain. The subgrade soils are usually tested in repeated load triaxial tests at the optimum moisture ontent sine most embankment subgrade soils are ompated lose to the optimum moisture ontent and the maximum dry density levels determined from standard Protor tests. However, moisture ontent levels in the subgrade soils will vary as a result of seasonal and environmental flutuations. The hanges in moisture ontents in the field will result in the variation of soil saturation levels, whih in turn alter the strength and resilient properties of subgrade soils. Therefore, haraterization based on resilient modulus (M,.) values at optimum moisture ontents may not be aurate or reasonable enough to provide realisti designs for the design life of the pavement. n addition, it is reported that most pavement failures our as a result of wet and dry moisture ontent yles of subgrade layers (2). Saturation due to flooding of underlying soil layers results in erosion of sub grades (2). On the other hand, extreme drying in summer periods auses shrinkage raks in plasti lays. When wetted, the raks soften the subgrade layers and derease the strength of the pavement. Thus, it is important to identify the properties on the dry Louisiana Transportation Researh Center, Baton Rouge, La and wet side of the optimum moisture ontent levels whih are more representative of field onditions. This is the rationale for the use of relative damage fators whih represent moisture ontent flutuations due to seasonal temperature variations in determining the effetive resilient modulus. This modulus is used in the design of flexible pavements. Two types of soils were tested at three different moisture ontents. The soils were a blasting sand and a silty lay. The tests were onduted on the dry of optimum, at optimum and on the wet of optimum moisture ontent. This allowed to study of the influene of moisture ontent variation on the resilient modulus and plasti deformations developed during the onditioning and testing phases. Also, as a part of the laboratory investigations, the influene of the loation of internally plaed linear variable differential transformers (LVDTs) in providing aurate and realisti measurements is evaluated. BACKGROUND AND OBJECTVE Several investigators reognize moisture ontent as an important soil parameter for determining the resilient modulus bf solis :for the design and haraterization of ftexibie pavefuents (2-5). Eliiott et al. (5) listed drainability, hydrauli ondutivity, soil type, geometry of the road, topography, water table depth, preipitation, and temperature as important fators that affet moisture ontents in the field. Some of these fators and their influene on pavement performane are desribed below. Cohesive subgrade soils pose a problem in pratie, sine the pore pressure developed during traffi loading will not be dissipated immediately due to the low hydrauli ondutivity of soils. As a result, the effetive stresses and subsequently the strength of the subgrade soils will be dereased. The resilient properties also derease and may ause rutting failures in pavement when subjeted to higher traffi loads. The hydrauli ondutivity of the ompated soils depends on several fators like soil fabri, mineralogy, and moisture ontent or saturation level. The moisture ontent is the ontrolled variable in this study. Therefore, any fators that influene the moisture ontents of soils will also influene the performane of pavement setions. Several researh studies in the past have attempted to investigate the influene of moisture ontent on resilient modulus results. Thompson (3) provided regression equations relating the breakpoint resilient modulus values at a deviatori stress of 6 lb/in. 2 and the degree of saturation. These equations, whih showed the ombined effets of moisture ontents and densities, beome independent of the degree of saturation with an inrease in lay ontent and plastiity. Pezo et al. (6) investigated influene of moisture ontent and. plastiity indies by onduting M,. tests on several subgrade soils

2 88 TRANSPORTATON RESEARCH RECORD 154 from Texas. This investigation indiated that an inrease in moisture ontent resulted in a derease of the resilient moduli. t is also reported that the influene of moisture ontent is more signifiant on soils with lower plastiity indies than those with high plastiity indies. Conlusions drawn from previous studies regarding the role of moisture ontents are similar. This researh study is also aimed at understanding the resilient behavior of two soils at various moisture ontents. This investigation is, however, different from other studies. Previous studies utilized external or internal end measurements and the soil samples were tested under AASHTO proedures T-274 and T-292. This study utilized two types of internal measurement systems and the samples were tested using the reent AASHTO T-294 proedure (7). The results are used to understand the influene of moisture ontents on the resilient modulus and plasti deformations of soils, as well as to determine the influene of internal measurement systems in obtaining realisti measurements. t should be noted that the relative ompation level of dry, wet, and at optimum moisture ontents varied between 98 and 1 perent, whih indiates that the effet of dry density is not a ontrolling fator in this study. EQUPMENT AND SOL DESCRPTON Loading System An MTS model 81 losed loop servo-hydrauli material testing system was used for applying repeated loading. A detailed desription of the equipment is presented elsewhere (8-1). An automated test software was developed and used for onduting tests, and performing data aquisition, redution, and analysis tasks. Measurement Systems One of the aspets of interest in this study is the loation of L VDTs on the speimens. t should be mentioned that the T proedure, whih was used for onduting tests in the present program, suggests the use of an external L VDT system while the T proedure requires the use of both external and internal L VDT systems. The internal L VDT system loated inside the hamber is a better system than those used outside the hamber sine the measurements by internal systems are less influened by system ompliane errors. This study used two diametrially plaed internal L VDTs fixed on a Plexiglas lamp system. One system measured the deformations with respet to the ends of the speimen and the other measured the deformations in the middle one-third of the speimen. These systems are referred to as end system and middle systems, respetively. Desription of Soils and Speimen Preparation Two loally available soils, a uniform blasting sand and a silty lay, were used in this study. The blasting sand exhibited dry densities of "!max = kn/m 3 ( l l.9 lb/ft3) and "!min = 15.8 kn/m 3 (99. lb/ft 3 ). The silty lay had an optimum water ontent of 2.6 perent, a maximum dry density of 16.3 kn/m 3 ( 11.6 lb/ft 3 ) and a plastiity index (Pl) of 22. The silty lay and blasting sand were lassified as A-7-6 and A-3 using AASHTO lassifiation. Figure presents standard Protor density urves of these soils. Sand speimens were ompated in-plae in the triaxial ell to redue sample disturbane. Cohesive speimens were ompated in molds and arefully extruded for testing. Both speimens were 71.1 mm (2.8 in.) in diameter and mm in height. Speimens were ompated at three different moisture ontents and dry density ombinations whih over the optimum, dry of optimum, and wet of optimum ranges. A list of these levels is given in Table 1. Conditioning and Testing Proedure The speimens were first subjeted to a onditioning phase, followed by testing at various onfining and deviatori stress levels. The presribed deviatori load of haversine shape was applied in both onditioning and testing phases. The tests on both materials were performed at the onfining and deviatori stress levels reommended in the latest versions of AASHTO T Detailed desription of the AASHTO testing proedure an be found elsewhere (8-1). The data obtained from the aquisition were analyzed and redued to determine resilient strains, plasti strains, and the moduli values for eah of the onfining and deviatori stresses. ANALYSS OF RESULTS Sands Table 2, a, b, and, presents the resilient modulus test results for eah moisture ontent level. The data inlude the end and middle resilient modulus, along with their means, standard deviations (STD), and oeffiient of variations (CV) for eah set of five tests. The oeffiient of variation varied between.1 and 15. with most of the values being around 3.. This implies that the test results are highly repeatable. The higher oeffiients of variations (CV values around 1) were obtained for the end resilient moduli data from tests onduted above and below optimum moisture ontents at low onfining stresses. Figure 2 shows the variation of end resilient moduli at different moisture ontents and dry densities for various onfining stresses of 21 kpa (3 lb/in. 2 ), 7 kpa (1 lb/in 2 ), and 14 kpa (2 lb/in. 2 ), respetively. t should be noted that the resilient modulus inreases with an inrease in the onfining pressure. This is attributed to the stiffness properties whih inrease with an inrease in onfining pressure (9). The derease in dilational behavior at higher onfining pressures also results in lesser axial strains and higher moduli values. The influene of deviatori stress an also be dedued from Figure 2. The deviatori stress inrease resulted in a slight inrease in the end moduli values at eah onfining pressure. Overall, the deviatori stress has a minor influene on resilient properties when ompared with the onfining pressure. This is beause the deviatori load levels applied are lesser than the peak deviatori loads whih an indue signifiant hanges and deformations in the speimens. Therefore, the deformation responses in these tests are diretly proportional to the applied deviatori loading whih results in slightly higher M,. values. The ompation moisture ontent appears to have some influene on M,. results at low onfining pressures. Figure 2 also indiates that higher moduli values are obtained at dry and wet of optimum moisture ontent level. An inrease in strength at dry of optimum resulted in a slight inrease in M,. values over the optimum M,. values. This is in agreement with those reported by other investiga-

3 Max. Dry Density: pt Optimum Moisture: % 14 '------'-----'-----' '-----'-----'-----'----' Moisture Content(%) (a) Blasting Sand 14 Max. Dry Density: pt >-- Optimum Moisture: 2.58 % 12 ;::- (.) "' () 1 98 /, / / ""''\ \ \ \ \ Moisture Content(%) (b) Silty Clay FGURE 1 Standard protor urves.

4 9 TRANSPORTATON RESEARCH RECORD 154 TABLE 1 Density-Moisture Content Levels of the Soil Speimens Soil Desription Day Moisture Relative (Optimum) Density (pt) Content(%) Compation (%) Blasting Sand Silty Oay Day 15.5 Near Wet Day 96.5 Near Wet tions (2,3,5). On the other hand, the moduli at wet of optimum are higher than those of optimum at low onfining pressures. This trend annot be explained exept for the fat that leakage problems were experiened during the wet of optimum speimen tests. This may have dereased the moisture ontents in the soil speimens whih probably resulted in higher resilient properties. Results at higher onfining pressures appear to provide similar resilient properties at all moisture ontents. This is attributed to the small variations in relative ompation levels as well as the smaller role of moisture ontents in sands at higher onfining pressures. As mentioned earlier, the relative ompations at the moisture ontentdensity levels vary from 98 to 1 perent whih indiate that the orresponding relative densities of the sand do not vary onsiderably. This variation in relative density is not signifiant enough to provide distintly different M, values. Clays Table 3 presents the resilient modulus test results at dry of optimum, optimum and wet of optimum, respetively. The oeffiients of variation of lay test results are higher than those obtained for sands. However, this number is relatively small when ompared with the variations in modulus values. This indiates that the lay test results are also repeatable. Figure 3 presents lay speimen results at dry, near, and wet of optimum moisture ontent levels. The end measurement results are depited in this figure. An inrease in onfining pressure resulted in an inrease in moduli values. This is attributed to an inrease in stiffness with an inrease in onfining pressure. Results at three moisture ontent levels show that the M,. values at dry and optimum levels are loser but signifiantly higher than those at wet of optimum. TABLE 2 Sand Test Results al Orv of Ootimum '3 O'd Mr, ends {MPa} 11 kpa kpa Mean STD Mr,middle {MPa} CV Mean STD CV

5 TABLE 2 (ontinued) bl Ootimum "3 O"d Mr, ends {MPa} Mr, middle {MPa} 11 {kpa} {kpa} Mean STD CV Mean STD CV ) Wet of Ootimum "3 O"d Mr, ends {MPa} Mr, middle {MPa} 11 {kpa} {kpa} Mean STD CV Mean STD CV Note: en = Confining Pressure rd Deviatori Stress STD = Standard Deviation in MPa CV = Coeffiient of Variation in perent

6 5. Confining Pressure - 21 kpa 4. (i.. en..:! :: "'C 3. :\? -: u; Cl> ::: l QJ D dry of optimum near optimum wet of optimum Deviatori Stress (kpa) (a) Confining Pressure - 7 kpa 4. (i.. en..:! ::l "'C 3. :\? -: u; Cl> ::: 2. D dry of optimum near optimum wet of optimum 1. '-----'----' Deviatori Stress (kpa) (b) FGURE 2 nfluene of onfining stress on resilient modulus of sands at various moisture ontents.

7 Mohammad et al ,--1---r-----,--l ,--1--, , 4. - Confining Pressure - 14 kpa B : "' 2. - D dry of optimum near optimum wet of optimum 1. -l l l Deviatori Stress, (kpa) () FGURE 2 (ontinued) The derease in M, at wet of optimum is attributed to the strength derease at a higher saturation level as a result of smaller ohesion and frition angles and higher pore pressure developments. An inrease in the deviatori stress resulted in a redution of moduli values. This observation is onsistent with those reported in other studies (2-4). This phenomenon is attributed to fators suh as positive pore pressure development and fabri hanges due to stress yles. The pore pressures whih inrease with deviatori load magnitudes, yles and saturation levels of the speimens result in the redution of overall strength. The lower strength speimens provide lower moduli values. The fabri desribes the arrangement of partiles, partile groups, and pore spaes in a soil and its influene on mehanial properties of ohesive soils are well doumented (11). t is assumed that the TABLE 3 Silty Clay Test Results a) Orv of Optimum en O"d Mr 1 ends {MPa} {kpa} {kpa} ' Mean STD Mr, middle {MPa} CV Mean STD CV (ontinued on next page)

8 94 TRANSPORTATON RESEARCH RECORD 154 TABLE 3 (ontinued) bl Ootimum Mr, ends {MPa} Mr, middle {MPa}. {k;a} O'd {kpa} 11 Mean STD CV Mean STD CV : ) Wet of Ootimum '3 {kpa} Note: '3 = O'd = STD= CV= O'd {kpa} Mr, ends {MPa} Mr, middle {MPa} 11 Mean STD Confining Pressure Deviatori Stress Standard Deviation in MPa Coeffiient of Variation in perent CV Mean STD CV! fabri at the end of onditioning is far more dispersed (a state at whih no fae-to-fae assoiation in partiles exist) than at the beginning of onditioning. The tests whih start at the end of onditioning are therefore onduted on speimens with dispersed fabri. The fabri beomes further dispersed with the deviatori load and the number of yles. The inreasing dispersion results in the deline of resilient properties sine this type of orientation generally exhibits lower shear strength omponents, ohesion and frition angle of soils. Experimental verifiation of this assumption is beyond the sope of this investigation, but still needs to be assessed. Measurement Coeffiients Resilient deformations are onsiderably small and should be measured as aurately as possible by reduing soures of errors in the test. The influene of the loation of the L VDT systems in providing aurate measurements was evaluated by using two types of internal measurement systems plaed at different loations on the speimen. The influene of the measurement system is presented in the form of measurement oeffiients (MC) (9, 1). The measurement oeffi-

9 5. 4. s A Silty Clay - Dry of Optimum End System Middle System Ci.A Conf. Pressure Q.. A - 42 kpa s S - 21 kpa n A C - kpa -= ea ::,, s 3..l :!: : A 9A - es x s es es n es es e 2. e e e e Ck:: Deviatori Stress (kpa) (a) A End s Silty Clay - Near Optimum System Middle System Ci A Cont. Pressure Q.. A - 42 kpa 24. ea S - 21 kpa n s C - kpa -= ::,, t A es s :!: - : A 2. es '(ij s.a Ck:: e es - s es t 16. e es e e e Deviatori Stress (kpa) (b) FGURE3 nfluene of moisture ontent on resilient modulus test results of silty lay.

10 96 TRANSPORTATON RESEARCH RECORD 154 Ci D A Silty Clay - Wet of Optimum s End System Middle System A A "C 1e :ie - es A Conf. Pressure n 16. A - 42 kpa ::J S - 21 kpa :; e C - O kpa 12. e en t Cl> es ::: e 8. e e 4. L---'--'--'---' Deviatori Stress, (kpa) () FGURE 3 (ontinued) ient is defined as the ratio of the resilient modulus determined by the middle system to that determined by the end system. The oeffiient values are determined for various onfining and deviatori stresses. The oeffiients an be used to onvert the end measurement results to more realisti middle measurement system results. The middle system is assumed as a more appropriate and realisti method sine its measurements are not influened by end frition effets and system ompliane errors. Measurement oeffiients are alulated for the tests onduted in this study and are presented in Figures 4 (sand) and 5 (silty lay). Figure 4 shows the relationships between Mr values from the middle measurement system and the end measurement system. The three moisture ontent levels and their results are plotted in the same figure. The slopes of these lines represent the measurement oeffiients. The influene of the stresses on measurement oeffiients was not onsidered in this ase sine the stresses did not alter the MC values onsiderably. The MC values are presumed to depend on the relative stiffness variation between samples and the Plexiglas lamps used for holding middle measurement L VDT lamp system. n other words, a softer sample allows the lamps to slip whih indues signifiant errors into the measurements, while a stiff sample may not permit free movement of the L VDTs of middle system. Therefore, variations in soil stiffness during testing may affet the performane of middle measurement system and thus the measurement oeffiients. The measurement oeffiients of sands obtained from Figure 4 are 1.15 (at dry of optimum), 1.22 (at optimum), and 1.18 (at wet of optimum). The differenes in MC values obtained at various moisture ontents are small. This indiates that the moisture ontent of the sand does not signifiantly affet the measurement system apabilities. This is attributed to the permeable nature of sands whih immediately dissipates the developed pore pressures during testing, thereby keeping the stiffness properties intat during testing. Figure 5 ompares the measurement oeffiients of lays at different deviatori stresses for various onfining pressures ( to 42 kpa or to 6 lb/in. 2 ). The influene of onfining pressure appears to be more evident and, therefore, the stresses were inluded in the analysis to provide expressions for MC values. Higher MC values were obtained for an unonfined state and low deviatori stress. The values dereased with an inrease in the onning stress. This indiates that higher onfining pressures provide better ontat between L VDTs and the speimens whih allows more preise and aurate measurements. Linear regression analysis on the lay results provided the following measurement oeffiient equations as a funtion of onfining (e 3 ) and deviatori stresses (ed). MC= (.335 * CT3 -.51) CTt+ ( *CT3) (dry) (1) MC= (.32 * u ) CTt+ ( *u3) (opt) (2) MC= (.298 * u ) CTt+ ( *u3) (wet) (3) nreasing in moisture ontent levels dereased the MC values in the lay test results as opposed to the similar values obtained in the sand tests. Higher variation of MC values was obtained in the dry state than at the wet of optimum (Figure 5). This is attributed to pore

11 Mohammad et al Dry of Optimum ;;- a. Optimum... Wet of Optimum E... 4 tn >. Cl) = "C :i E 3... LL..:! "' ::J... 2 Measurement Coeffiients "C :11: Wet, MC= 1.18 = ;;, Opt, MC = 1.22 :::, Ory, MC= Resilient Modulus From End System (MPa) FGURE 4 Measurement oeffiients for sands. pressure developments and fabri hanges in the lay speimen. Although the tests were onduted in drained onditions, semidrained onditions prevailed during testing due to low hydrauli ondutivity of lays. This resulted in the development of pore pressures in the speimen. The wet of optimum state of a soil, whih has a higher degree of saturation than at dry and optimum states, produes higher and more uniform pore pressures in the speimen. The uniform distribution of pore pressures may have resulted in uniform measurements at wet of optimum and smaller measurement oeffiients. Also, the fabri at the wet of optimum state is more dispersed struture, whereas the fabri at the dry of optimum state is more of floulated (11). The floulated struture an undergo greater partile dispersions or orientations than the dispersed struture during repeated loading. Therefore fabri hanges in the speimen during testing are more signifiant at the dry of optimum than at the wet of optimum. Changes in partile orientations at dry level results in variations in measurements and the orresponding higher oeffiients. Plasti Deformation Development Figure 6 presents the plasti deformations of sands measured by the end system during the onditioning and the testing phases. All three moisture ontent levels are reported in this figure. The plasti deformations reported in the figure represent the aumulated deformations of one thousand yles during onditioning and four hundred yles for eah onfining stress during testing phase. The testing phase four hundred yles were obtained by summing the deformations from individual sets of yles (one hundred eah) for four sets of deviatori loads. These results provided signifiant understanding of the onditioning role in this kind of testing sine one of the objetives of onditioning, as reported by the AASHTO T-292 proedure, is to redue plasti deformation developments in the speimens. The deviatori stress influene is apparent sine higher deviatori loads usually resulted in larger plasti deformations. The influene of onfining pressure on plasti deformations is more intriate and requires further srutiny and attention. For sands, lower plasti deformations were obtained at all testing onfining stresses, 21, 7, 15, and 14 kpa with the exeption of 35 kpa. This indiates that onditioning not only redued the plasti deformations in the first test onfining pressure (whih is 21 kpa), but also in the ase of three upper onfining stresses, 7, 5, and 14 kpa whih are somewhat loser to the onditioning onfining stress of 14 kpa. This is a signifiant finding sine no speifi guidelines are available in the literature for determining the magnitudes of onditioning onfining stresses for granular field ore samples based on the plasti deformation riterion. The onditioning onfining stress for ores should be greater than the lateral onfining pressure of the depth at whih the soil samples are retrieved. n ases when the soil sample represents. a signifiant depth of subgrade (either a sample from a deeper depth or samples retrieved from a range of depths), the lateral pressure of the bottom layer of the subgrade or the deepest depth should be used as the onfining pressure for onditioning. Higher plasti deformations were observed at onfining stress of 35 kpa, possibly due to signifiant flutuations in the onfining pressures in the preeding two stages, 14 kpa (onditioning) and 21 kpa (first level of testing). Figure 6 also presents the plasti deformations developed during testing of the lay speimens. Results from three moisture

12 98 TRANSPORTA T/ON RESEARCH RECORD ,,----,------, ,-----, ,-----,--,----, Silty Clays - Dry of Optimum Silty Clays - Optimum... :; E... E :J /) a 1.2 "'--.- "'--.- "'-- Confining Pressure kpa 21 kpa 42 kpa : 1.4 :J /) a Confining Pressure kpa 21 kpa 42 kpa 1. '-----'----'----'------'----'------'-----''----'----'-----' Deviatori Stress, U d (kpa) (a) Dry of Optimum 1..,,,. ----'---'-----''----' Deviatori Stress, U d (kpa) (b) Optimum Silty Clays - Wet of Optimum Confining Pressure OkPa :; kpa E... E 1.4 :J /) a 42 kpa 1. '-----'----' Deviatori Stress, U d (kpa) () Wet of Optimum FGURE 5 Measurement oeffiients for silty lay. ontents and densities are depited in this figure. The lay samples were onditioned at 42 kpa and the first test was onduted at the same onfining pressure. This is followed by the testing at other onfining pressures of 21 and kpa. The figure suggests that the aumulated plasti deformations were larger at a onfining pressure of 42 kpa (6 lb/in. 2 ) and then dereased with dereasing onfining stresses. Even though onditioning did not result in the redution of plasti deformations at first testing onfining pressure (42 kpa), it signifiantly dereased the plasti deformations at other onfining pressures (21 and kpa). The purpose of onditioning in lays is probably ahieved at onfining stresses lower than the onditioning onfining stress. Reason for this is attributed to the stiffening or over-onsolidation of the speimen at a onditioning onfining pressure of 42 kpa. The stiffened speimen, therefore, appears to indue smaller strains at lower onfining pressures. This implies that the field ohesive ore samples_ require a onditioning onfining stress that is signifiantly higher than the lateral onfining pressure orresponding to the retrieval depth loation. Smaller plasti deformations were measured by the middle system than the end system, possibly due to the differenes in the lengths of the speimens that these systems were aounted for.

13 E' E... d (,) :. "'l:t' r.t) N >< : :,;::::; u E... 2.S? Cl> (,) :,;::::; 1 a: "C Cl> ::s E ::s (,) (,) <( Sand; T-294; End System C - Conditioning; T - Testing 8 - Dry of Optimum Optimum G - Wet of Optimum l_---1-=l_l L_---1:_j_---=3=""'=""-_J Confining Pressure (kpa) (a) Blasting Sand E' E... d. 6.. "'l:t' r.t) N >< : :,;::::; u E.S? Cl> (,) :,;::::; VJ a. "C Cl>... ::s E ::s (,) (,) <( 4 2 T 'f r -EJ- -e- -.() - Dry of Optimum Optimum Wet of Optimum Silty Clay; T-294; End System C - Conditioning; T - Testing <!>,T C -- T T / / Contration Swelling -2 '--J. --'---'-_,_ Confining Pressure (kpa) (b) Silty Clay FGURE 6 sands. Aumulated plasti deformations versus onfining stresses for

14 JOO TRANSPORTATON RESEARCH RECORD 154 Regression Model Analysis Regression models are generally used in the form of equations for prediting the moduli values. Theta (8) or bulk stress and deviatori stress are used in the models as preditors based on whether the soil is ohesionless or ohesive. These models are reommended in AASHTO proedures: Mr= k 1 * 8k2 for grandular soils (4) Mr= k 3 * uk 4 for ohesive soils (5) where k, and k 2 (granular soils) and k 3 and k 4 (ohesive soils) are regression oeffiients. The regression oeffiients are determined from the test results for both soils and are reported in Table 4a and b (sands and silty lays). Typial model results for sands and lays at wet of optimum are depited in Figure 7. t is interesting to note that k 2 and k 4, whih represent the slopes of the lines in the respetive models, appear to be dependent on the type of soil and the moisture ontent level. The onstants k, and k 3, whih represent the interepts in the figures, depend on the measurement systems, the moisture ontents and density levels. As expeted, higher k, and k 3 values are obtained for the middle system beause of higher resilient moduli measurements. Although these onstants varied with moisture ontent, no partiular trend is observed in both soils. SUMMARY AND CONCLUSONS The resilient modulus test results provided the following onlusions: 1. Sands exhibited higher resilient moduli at dry and wet of optimum than at optimum moisture ontent. Higher Mr values at dry of optimum over optimum is attributed to the higher strengths. The same at wet of optimum, on the other hand, did not follow a on- sistent trend possibly due to leakage problems assoiated during the wet of optimum tests. Overall, the statistial variation between the results at all three moisture ontent levels is insignifiant, probably due to very small range of relative ompations used and lesser influene of moisture ontents in sands. 2. The moduli values of lays derease with an inrease in moisture ontent. This is attributed to the inrease in positive pore pressure development with an inrease in moisture ontent or degree of saturation. Higher pore pressures derease the effetive stresses and the shear strength of the lay speimens, thereby resulting in smaller resilient moduli. 3. The measurement oeffiients of the sand tested are 1.15 (dry of optimum), 1.22 (optimum), and 1.18 (wet of optimum). The small variations in these oeffiients indiate that moisture ontents in sands did not influene the measurement systems. 4. The measurement oeffiients of lays for eah moisture ontent level are expressed as a funtion of onfining and deviatori stresses. Higher values are produed for the dry of optimum moisture ontent level. Lesser fabri hanges and uniform pore pressure developments at wet of optimum may have resulted in smaller variations between the end and middle measurements. 5. Conditioning resulted in smaller plasti deformations at most of the onfining stress levels for sands and at onfining stresses lower than the onditioning onfining stress for lays. 6. The regression model onstants appear to depend on the moisture ontent, density levels, and the measurement system. Though the model onstants varied with respet to moisture ontents, no partiular or signifiant trends between them are notied. ACKNOWLEDGMENTS This work was supported by the Louisiana Transportation Researh Center in ooperation with the Louisiana Department of Transportation and Development (LADOTD) state projet no. 92-2GT. The authors express their appreiation for this support. TABLE 4 Regression Constants for Sand and Silty Clay (a) Sand Moisture Dry End System Middle System Content Density (%) (pt) log(k,) "1 R1 log (k,) "1 R (b) Silty Clay Moisture Dry End System Middle System Content Density (%) (pt) log(k;s) R1 log(k:j) "" "" R1

15 5.8 ;; -= ::::s "C :!!E - ;; CZ> C') e- End System Middle System / / / / Sand - Wet of Optimum End : log k 1 = 4.15; k 2 =.52 (r 2 =.88) Middle : log k 1 = 4.39 ; k 2 =.46 (r 2 =.95) '----' ' log (Bulk Stress) (a) Blasting Sand , r----, , Silty Clay - Wet of Optimum End : log k 3 = 5.71; k 4 = -.41(r2 =.72) Middle : log k 3 = 5.88; k 4 = -.48 (r 2 =.82) ;; End ::::s A. - Middle :; "O :!!E 4.8. L,., L L J _,., log (Deviatori Stress) (b) Silty Clay FGURE 7 Typial regression model results.

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