NCHRP Web Doc 14 Laboratory Determination of Resilient Modulus for Flexible Pavement Design Final Report (1997) / Chapter Skim
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3 Resilient Modulus Evaluation of Aggregate Base and Subgrade Materials
3 Resilient Modulus Evaluation of Aggregate Base and Subgrade Materials
Pages 97-250
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From page 97... ...
Based on a Borough review of the literature, He repeated load, biaxial test was selected to characterize in this study the resilient modulus of unstabilized base and subgrade soils. RESILIENT MODULUS CONCEPI The resilient modulus is equal to the peak applied axial repeated stress (Figure 3a)
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From page 98... ...
The biaxial test offers three very important advantages in resilient modulus testing: 98
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From page 99... ...
In addition to the above advantages, undisturbed tube samples of the subgrade obtained from the field can be extruded and tested with a minimum amount of specimen preparation. Finally, the biaxial cell used for the repeated load biaxial test can also be employed in static testing.
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From page 100... ...
Deviator Stress Shear Stresses ~ = 0 tS3= Confining Pressure (minor principle stress)
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From page 101... ...
The general effects of principal stress axis rotation and shear stress reversal can be studied using the simple shear test [4l, 421. Resilient shear moduli determined on sand using the simple shear apparatus and the repeated load biaxial compression test have been found to be similar at comparable stress levels [411.
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From page 102... ...
Stresses Applied in a Cyclic Torsional Shear Test \^ is ~ ~ ~ ~1~" ~ ~ /~ (a) Shear to leD = Applied normal stress ~ = Applied shear stress y = Shearing strain Shear ModulUsG=7ly 'a r OTT -r 7 -' ~ ~ ~ '.
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From page 103... ...
The simple shear test has been used on a limited basis for a long time, but because of these disadvantages has not become well accepted for either static, cyclic or repeated load testing. The simple shear test does offer an attractive alternative for evaluating permanent deformation under more realistic stress reversal conditions than the repeated load biaxial test.
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From page 104... ...
_f 500 mm (19.7 in.) SteelFrame Serv - Controlled Hydraulic Actuator Slip Coupling - Smoothing Bush Load Cell Top Plate Top Platen (toothed)
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From page 105... ...
overcomes this problem by using as response the axial strain (ea) instead of the resilient modulus, and by using as independent predictors the deviator stress (~: and the confining pressure (~3~.
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From page 106... ...
Table IS. Resilient modulus models used in the experimental data analysis MODEL | MODEL | LINEAR REGRESSION MODEL PLOT EXPRESSION USED K-8 I M,` = KI8K2 I Log Me= LogK, + K:Log' Log MR VS .
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From page 107... ...
6 O ' ·_ C :' o ._ ._ Cat Is 3 2 1 O .
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From page 108... ...
to show that the length of stress paw followed influences the resilient modulus in addition to He mean normal stress and the shear stress. The application of the stress path concept requires a special biaxial testing apparatus Hat permits simultaneously varying He axial and confining pressure in a predetermined manner.
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From page 109... ...
For many slightly cohesive and cohesive fine-grained soils, the resilient moduli obtained from the repeated load biaxial test can be modeled as a bilinear function of the applied deviator stress. Confining pressure is held constant in this model.
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From page 110... ...
Resilient Modulus of Compacted A 6 Cohesive Soil 110
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From page 111... ...
is o A: · _1 · c~ \ L K14 Kl3 · l _ Kl2 Kls Deviator Stress, Od Figure 53 . General relationship between resilient modulus and deviator stress for fine-grained, cohesive soils 111
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From page 112... ...
[591 and is as follows: ME = Keg o~ (P°~9 `~' ad where: a, = deviator stress caused by the-wheel loading p'O = effective mean confining stress caused by the overburden, P 0= ((it + o2 + 031/3 K,~ and Kit = material constants evaluated from the repeated load test Both models take into account in a realistic way the effect of mean normal stress caused by the pressure of the overlying materials and also by the wheel loading. This approach minimizes We problem of increasing deviator stress (ad = ol-03)
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From page 113... ...
Resilient modulus tests simulating this condition by using a variable confining pressure have been performed on granular base and sands [6, 71-731. The theory of elasticity equations for calculating resilient moduli when a variable confining pressure is used have been given by Brown and Hyde [611.
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From page 114... ...
The vacuum biaxial test should be performed at (or near) the optimum moisture content where the moisture within the specimen is continuous.
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From page 115... ...
A 5000 lb. capacity flat load cell, non-fatigue rated and mounted inside the biaxial cell on top of the load piston, was used in the electrohydraulic feed-back of the MTS loading system to monitor the actual load applied to the sample.
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From page 116... ...
Table 19. Standard stress conditions used in resilient modulus test TEST ON FIN IN G DEVIATOR N O. BULK SEQUENCE PRESSURE STRESS LOAD STRESS NO.
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From page 117... ...
Axial Displacement Measurement As previously shown in Figure 47, axial displacement in most of Me resilient modulus tests was measured simultaneously by (~) a pair of EVDT transducers mounted outside the cell, (2)
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From page 118... ...
Resilient and permanent deformation behavior, however, is influenced by both particle breakage and aggregate structure. The aggregate compaction experiment involves comparing aggregate orientation and particle breakage of specimens prepared in the laboratory with the structure resulting from compacting a crushed stone base material in the field using a heavy vibratory roller.
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From page 119... ...
GT-BASE 4 Specimens compacted to ss % T-1~; dry density = 142.2 at 4.1 % water content C, 80 z 0 0 dL go at ~ ~0 J o lo.'- _.
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Table 22. Effect of compaction method~n particle fracturing and orientation COMPACTION t~ETEtOD NO.
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Effect of Compaction Mold Size on Maximum Dry Density Introduction. Resilient modulus tests must frequently be performed on base materials having a maximum aggregate size of I.5 in.
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maximum top size aggregate base 03ase I) the typical variation between moisture content and dry density obtained using 4 in., 6 in., and 12 in.
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From page 123... ...
Summary of moisture-density test results for 4, 6 and 12 in. compaction molds I COMPACTION MOLD SIZE (IN)
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DIAMETER MOLD O 1 2 ~ ~ S 6 WATER CONTENT, w (#) 7 8 9 Figure 55.
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Synthetic polymer specimen wtih axial deformation measurement system rigidly attached to specimen 125
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From page 126... ...
The test shows at the 95% confidence level that the resilient moduli measured using EVDTs rigidly mounted to plugs are statistically different than the value obtained using clamps mounted on the specimen at the same locations. This finding is true for both the low and high modulus synthetic specimens.
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From page 127... ...
Table 25. Statistical summary of resilient modulus from synthetic specimen experiment i ' ~ ~ SP ECIMEN STATISTIC E=E RNAL(1 )
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From page 128... ...
Resilient moduli using external, top to bottom, clamp and plug mounted EVDTs:, soft synthetic specimen U
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From page 129... ...
Bedding errors are particularly significant at low deviator stress levels [701. In the past bedding errors in repeated load testing have been assumed to be minimized by the application of repeated load cycles during the conditioning phase.
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From page 131... ...
A confining pressure (as) and a deviator stress (a,-a3)
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Approximate average stress state at the center of two aggregate bases due to compaction with a lO-ton vibratory roller 132
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From page 133... ...
The two different conditioning stress states used in the experiment gave resilient moduli for He materials tested that were on the average within 2.5% which is a negligible difference. For He evaluation of permanent deformation, conditioning using He principal stress ratio of 3.5 would more accurately depict the actual stress state and hence rive a more reliable indication of notentin1 permanent ~leformatinn behavior.
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From page 134... ...
Test ~Test Mr ~ 3rd PREC. ~Mr = 1 JOOO 1,500 1 JOOO CONDITIONING SINCE STRESS STATE: GT : ~1 = 52.5 ff3 SHRP: ~1 = 30.0 ~3 = 15 PSI Figure 61 .
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From page 135... ...
Table 27 . Conditioning test results - -- ado ~4 · he · ~S~cQ ~ { - · Eta -~ '- r' ~ 2" r ~lo_ - .
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600 500 '~ 400 JO ~_ 300 lo 200 100 fin ItN o ....
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From page 138... ...
This study, however, indicates Hat for some materials the shape of the load pulse does not have an effect upon the resilient moduli, particularly at low stress levels as defined by a small value of bulk stress. The haversine load pulse probably best approximates the loading to which a small element of base material is subjected due to traffic.
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From page 139... ...
Table 30 · Effect of pulse shape on resilient moduli calculated from K4 model: base 2 results FULL GR ADATION REPIACED GRAC ATION PULSE B EXTERNAL CLAMPS EXTERNAL CLAMPS (P81) ~ 2FS1 1 B2FS2 1 E 2FS1 1 B2FS2 1 B iRS1 1 B2RS2 1 B RS1 12 18,725 19,700 23,100 28,498 21,080 20,438 27,801 Haverelne60 5 1,086 53,203 53,198 58,768 5B,829 49,794 62,556 100 70,250 72,927 69,323 73,945 77,852 66,058 80,920 12 20,256 17,887 19,157 ~ 19,194 18,m 18,885 23,242 Trlang ular60 49,676 49,066 50,487 50,739 53,588 50,333 S9,503 1 00 66 040 67 590 68,669 69 078 74,753 68,704 80,t 90 .
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Difference in resilient moduli for haversine pulse compared with pneumatic, sinusoid and triangular pulses 141
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From page 142... ...
1 measurements, Base I and Base 2 filll gradation specimens give a regression mode} having He same slope but a different intercept as the scalped and replaced gradation model. Resilient moduli obtained using the internal LVDT measurements, for the full gradation, are on the average I8% and 24% higher in Base ~ and Base 2, respectively, than for the scalp and replace gradation for a range of bulk stresses (~)
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Table 32. Resilient moduli calculated from K-8 mode} based on two replicate specimens - scalp and replace study .
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Specimens of Base ~ and 2 Table 21) were prepared to 100% of TWO maximum dry density using vibratory compaction at the optimum moisture content.
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For stiff, low compressibility aggregate base materials, A B-value of 0.2 corresponds to a degree of saturation of about 98%. Flushing for 24 furs.
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From page 148... ...
SO Id to: 100 % __ _ T-180 density; dry density = 133.6 pc! ~ 8.0 % wstsr capers ~ I_ > n _ ~ m - -em 178 Rule Bushed Cone.
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From page 149... ...
Table37. Summary of SHRRP resilient modulus test results: K-8 mode} SPECIMEN EXTERNAL | TOP-BO;0 1CLAMPS rat K1 | K2 | rat | K1K2 rat 1 K1 0~22s7 ~,02t.040~S2710.9~71 ~,10S.
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From page 150... ...
To investigate the variation in resilient moduli predicted from measurements, values of resilient moduli for bulk stresses of ~ = 20, 35 and 50 psi were analyzed using the results from tests on 26 different specimens (13 replicate pairs) and 4 different materials.
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From page 151... ...
Variability of the K2 constant as indicated by the k statistic: SHRP base materials 151
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From page 152... ...
Variability of the K2 constant as indicated by the h statistic: SHRP base materials 152 MEASURMENT MOOD EXT T-S CLP
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From page 154... ...
By using two replicate specimens, the average coefficient of variation for the resilient modulus drops significantly to 6.6% and 7.~% for external and clamp measurements, respectively. These coefficients of variation are valid for typical unstabilized aggregate bases and the K-6 model.
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From page 155... ...
Figure 69 . Variation of resilient moduli for external and clamp based K-8 models: 99 percent confidence interval 155
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From page 157... ...
Results. A summary of the statistics for the K-8, Uzan and UTEP resilient modulus models are shown in Table 40 for both externally mounted EVDTs and EVDTs located on clamps.
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From page 158... ...
Table 41. Comparison of mode} fit statistics for K-O, Uzan and UTEP models: single and replicate specimens .
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From page 159... ...
Location of EVDT Displacement Measurements The memos usM to measure axial displacement in the resilient modulus test on granular materials potentially has a significant influence on the reliability of the results. This discussion is restricted to the three EVDT measurements used in this study: external, top to bottom, and clamps.
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From page 160... ...
C ~ arnp-measurecl ,0 60 :' ~ 0 ~so _ ~ 0 ._ _ ._ As ~ so 30 +~/ 20 10 Legend First Sample + Second Sample Rest ~ tend Modulus ++ ~ W __ 0 20 40 60 eo 100 Bulk Stress Figure 70. Comparison of K4 model linear and nonlinear fit of resilient modulus data: example with largest difference 160
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From page 161... ...
ASK(1) Table 42 .
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From page 162... ...
Observed error between clamp and externally measured resilient moduli 162
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From page 163... ...
For Figures 72 and 73, the UT-Austin resilient modulus mode] was used to calculate resilient moduli as a function of confining pressure for values of deviator stress (~-03)
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From page 165... ...
Even considering the large variability that occurs in resilient moduli caused by variable factors such as seasonal moisture changes, the use of inside deformation measurements, as previously discussed, is the most desirable technique. Lim+Fly Ash Stabilization A limited experiment was conducted to establish the feasibility of performing in the biaxial cell resilient modulus tests on lime-fly ash stabilized base materials.
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From page 166... ...
Table 43 . Summary of results for lime-fly ash stabilities base resilient modulus tests SrECIME Sr=~" M~.
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From page 167... ...
Specimens moist cured in Me oven for 72 hours following the Illinois method, had resilient moduli on the order of 10 times greater than the unbound granular specimens. In general measured axial displacements of the cured stabilized materials were about 0.0001 to 0.001 in.
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From page 168... ...
1 a, Load Duration l I l l l , _ 1 ~1 / / UNLOAD Strain Figure 74. Evaluation of resilient modulus from quasi-static loading-unIoading s~ess-strain curve 168
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From page 169... ...
The auasi-static resilient deformation was determined unon unloading by taking the absolute value of the difference between specimen deflection just before the end of the loading period and again at the end of the recovery period. Resilient moduli were then calculated following the same approach as for the repeated load test.
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From page 170... ...
Table 44. Comparison of resilient modulus results for haversine quasi-tatic and 5 Hz and 10 Hz Continuous Sine Loading - Single Data SPECIMEN BlSIC BlS2C PUMICE HaversIne SeO mla 2.S~ Have~s~e SO my 2.S mls ME4SUREME:NT I ODNT, EXTERNAL .
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From page 171... ...
Resilient Moduli for Selected Pulse Types - Bases 3 am 4 I 1 1 .
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From page 172... ...
quasi static test and repeated load haversine test 1' 1.16 ·= ·~ 1.12 3 ·~& 1.~ 1.04 1 .
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From page 173... ...
After 70,000 load repetitions, He rate of increase in permanent deformation is quite small for good quality base materials subjected to realistic repeated load biaxial stress conditions. Permanent deformation was measured using an externally mounted dial indicator graduated in 0.001 in.
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From page 175... ...
Permanent strain response at 70,000 load repetitions, by which time most base materials have stabilized, was therefore selected as the reference for comparison with the permanent strain behavior at 1700, at the end of the resilient modulus test, and 5000 load repetitions.
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From page 176... ...
Influence of number of load repetitions and material quality on permanent deformation 17S
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From page 177... ...
Because of the potential for unstable permanent deformation behavior, 1700 to 5000 load repetitions is not adequate to reliably define potential long term behavior for materials varying widely in quality. When only high quality aggregate base is used together with deviator stress ratios less Han about 5, 5000 load repetitions should generally be adequate to define permanent deformation behavior.
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From page 178... ...
| ~1/~3 Teq<8 1 6 1 6 Teq < 1 1 6 4 Teq> 11 4.5 2 LIGHT MEDIUM HEAVY 1. An alternative would be to use a confining pressure of 6 psi for all pavement strengths 2.
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From page 179... ...
Resilient modulus tests were also performed to determine Me benefits of using grouted specimens ends and to determine the effects of misalignment. Repeated load tests were performed on 52 clayey sand specimens in this phase.
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From page 180... ...
Standard proctor compaction curve for cohesive soils 180
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From page 181... ...
spring load hammer on each of 10 layers. This kneading compaction energy was selected to approximately match the AASHTO T-99 maximum dry density at the optimum moisture content.
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From page 182... ...
Static strength as a function of moisture content for Me clayey sand (A~) cohesive soil 182
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From page 183... ...
The specimens compacted by kneading at optimum moisture content have both a higher density and higher static shear strength than the specimens compacted by impact. In contrast, specimens prepared by kneading compaction at densities close to those for the T-99 impact test have, for moisture contents both above and below optimum, lower static shear strengths than the impact compacted specimens.
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From page 184... ...
Final configuration of the biaxial cell 184 Pressure ~t
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From page 185... ...
Grouted specimens subjected to 200 cycles at a deviator stress (a,) of 4 psi under a confining stress proof 6 psi have been found, based on a statistical analysis of the results, to behave the same during the first five cycles as during the last five cycles.
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From page 186... ...
specimens, compacted at both optimum and wet of optimum moisture contents, were tested with both grouted and ungrouted ends to verify the findings of Pezo et al. The specimens compacted wet of optimum moisture content were also used to identify whether excess pore pressures, which would decrease He resilient modulus, build up during repeated loading.
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From page 187... ...
The sand specimens were prepared and tested at a moisture content of about 9%. The loading sequences consisted of a constant confining stress of to 7.5 psi and 50 cycles at each deviator stress level.
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From page 188... ...
Dry song Creasing Short Level Short Decreasing Standard Level Short Increasing Long L,eve} Long Decreasing Standard Increasing Standard Decreasing Opt. Short increasing Short Decreasing Short Level L`ong Decreasing Standard Level Standard Increasing Standard Decreasing Long Level Long Increasing Wet Short Level Short Decreasing Short Increasing Standard Level Standard Increasing Long Level Long Increasing Starboard Decreasing Long Decreasing _ _ Opt.
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From page 189... ...
Shon Increasing Increasing Level [Decreasing Increasing Level Decreasing Decreasing Level Level Level Level Increasing Increasing Decreasing Increasing Decreasing Decreasing Level Decreasing Decreasing Increasing Increasing Increasing Level Decreasing Level Sloped Dense Regular Loose 189
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From page 190... ...
Soaking Specimens. Figure 84 shows measured resilient modulus as a function of deviator stress for To impact compacted specimens prepared at optimum moisture content.
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From page 191... ...
Effect of moisture conditioning on resilient moduli of A-6 cohesive soil compacted using impact method 191
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From page 192... ...
By having standard relationships of this type, resilient modulus tests need only be performed at He optimum moisture content. Soaking cohesive soil specimens to obtain a high degree of saturation is not considered to be practical because of the long time that can be required.
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From page 193... ...
Figure 85 . Effect of moisture conditioning on resilient moduli of clayey sand (A-6)
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From page 194... ...
Effect of moisture conditioning on resilient moduli of silty sand (A-S) cohesive soil 194
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From page 195... ...
Input variables with F values having probabilities of being exceeded less than 5% were considered to be correlated to MR. The only variable combinations for testing of cohesive soils found to be significantly correlated to MR were end type, moisture content, and end type/moisture content.
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From page 196... ...
50000 ~3 ` ~40000 at.= 30000 20000 I I Notes: Specimen Age = 131 days Deviator Stress = 10 psi Confining Stress = 6 psi . grouted 1- ungrouted 1 ~ 0 1000 2000 Load Repetitions Figure 87 .
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From page 197... ...
Variation in resilient modulus of two compacted subgrade soils with axial strain for different end conditions (After reference 80 197 10
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From page 198... ...
For the proposed unconfined compression test, 200 load repetitions are recommended for specimens without grouted ends at a deviator stress of 4 psi. Strength Gain with Time- Compacted Cohesive Soils After compaction in the laboratory, the resilient modulus of a specimen increases with time (Figures 89 and 90~.
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From page 199... ...
lo lo O 1........... Soil 7 - wet Moisture Content = 21.1 ~ 0.5 onto Dry Density = 103.6 + 0.6 pcf Confining Stress = 4 psi 34 days ~ 6 days 0 2 days Moo , .
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From page 200... ...
The specimens compacted wet of optimum moisture content were also used to identify whether excess pore pressures build up during load cycling which decreases the resilient modulus. All specimens were subjected to a load sequence combining He standard SHRP P46 (November, 1989)
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From page 201... ...
Measured resilient moduli of synthetic specimen 201 1 0000
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From page 202... ...
Both the grouted specimen tests and ungrouted tests were performed using the SHRP P46 (November, 1989) test procedure followed by 2000 load cycles at a confining pressure of 3 psi and deviator stress that gave SL = 50% of the static failure stress.
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From page 203... ...
, , , I 1111 . 0 5 10 15 Deviator Stress (psi)
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From page 204... ...
- r ~ _ 5 Deviator Stress (psi)
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From page 205... ...
(a) Optimum Moisture Content · Ungrouted & Conditioned ~% ~ 4- -it O- 1 1 1 1 .
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From page 206... ...
The Illinois DOT uses these resilient moduli in design. The unconfined test is realistic because the confining pressure existing in He upper portion of the subgrade is small.
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From page 207... ...
For a furler discussion of sensitivity, refer to the reliability discussion In Chapter 4 and also in Appendix F Since the correction is only applied for resilient moduli greater Han 7,500 psi, the effect on resulting pavement thickness should be small particularly when compared to variability of other factors such as seasonal variation in moisture content, stiffness variation along the route and increase in stiffness that occurs With time after specimen preparation.
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From page 208... ...
Figure 95. Preliminary correction factors for cohesive subgrade resilient modllli using axial deformation measured between platens - ungrouted specimen ends \ \ \ /' (a)
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From page 209... ...
Test findings -- Cohesive subgrade soils are usually compacted using a sheepsfoot compactor which causes a Pleading type of soil structure. The results of this study show that specimens prepared in the laboratory at and above the optimum moisture content by impact compaction exhibit similar resilient moduli as Pose cornl)
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From page 210... ...
Effect of impact and kneading compaction on resilient moduli: A-6 cohesive soil compacted at optimum 30 25 .m a, 20 :, ~ 15 ~_ 3) 10 G 5 O o l | ~ ~ Knead-SHRP \ ~ Impact-Varying \ · Knead-Constant \~'"''""'~'"'''''''''' '^~': ~ l l l .~ ~ ~ ~ 1 ~ ~ r ~ I I ~ ~ 1 l~7 -- ~ I-I 1 1 5 1 0 1 5 20 25 Deviator Stress (psi)
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From page 211... ...
Figure 99. Effect of impact and lading compaction on resilient moduli: A-S cohesive soil compacted at optimum 20 C=n 1 5 _' co ~ 10 ·_ cn ~ 5 a: o 0 Impact-SHRP ~ Knead-SHRP · - Impact-Constant · Knead-Constant -..~; o · ~o ~ Deviator Stress (psi)
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From page 212... ...
~Moisture Content ~Methods ; ~less than the moisture ~impact . ~ 80 content at time of static constru ction kneading greater Man or equal impact > SO to the moisture content kneading at time of construction greater Han the moisture <80 ~ consent et time of construction ~static STRESS SEQUENCE EFFECT AND FAILURE STRESS RATIOS - GRANULAR MATERIALS Past research has shown that for granular materials a single properly preconditioned specimen can be used to evaluate the resilient modulus over a reasonably wide range of stress levels [79, 92-951.
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From page 213... ...
Table 52. Approximate allowable and failure stress ratios Oi/~3 for twice Hangar base and subgrade mateirals Dynamic Principal Stress Material Quality ~ Ratio (C'~/~3)
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From page 214... ...
Moduli of this magnitude may be measured in granular materials at confining pressures greater Man about lO psi. Finally, We reliability of compliance calibration at deviator stresses less Man about 5 psi is also subject to important errors.
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From page 215... ...
Minimizing Misalignment Even after carefid system calibration, alignment errors should be evaluated during the conditioning phase of each repeated load test. For the two EVDTs used to measure axial displacement, illustrated in Figure 103, the ratio of maximum axial L`VDT displacement (Yma,`)
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From page 216... ...
Examples of connections typically used with biaxial cells with external tie bars (after reference 7 I)
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From page 217... ...
_ _~? mom _r.~ '1.` me, ~'-p it' Figurelo4 .CIamps used with 2 EVDTs to measure axial deformation on the specimen 217
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From page 218... ...
AMAL DEFORMATION MEASUREMENT Axial deformation measurements during repeated load testing have been made outside [38,96,97] and inside the biaxial cell directly on the specimen 13S,55,92,98-1001.
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From page 219... ...
synthetic specimens covered with a rubber membrane. For both the stiff and soft specimens the resilient modulus measured using clamps was only 6% lower Can measured using plugs.
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From page 220... ...
Figure 105. Selected methods used to support axial deformation measurement devices 220
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From page 221... ...
To use a modern optical extensometer, two lightweight reflective targets are attached on the sides of a specimen. The optical extensometer, which is located outside the biaxial cell, optically monitors the movement of the relative displacement between the two targets as shown in Figure 107.
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From page 222... ...
- Light Weight Target .
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From page 223... ...
Schematic layout of system Figure 107. Noncontact optical extensometer measurement system (compliments of the Cooke Corp., Tonawanda, N.Y.)
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From page 224... ...
Optical Extensometer Experiment. The repeated load resilient moduli were evaluated for 2 cylindrical, synthetic polymer specimens using a Mode!
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From page 225... ...
As a result, the presence of extraneous deformation, which is always present in a testing system, also plays an important role in repeated load biaxial testing. Table 54 gives a summary evaluation of available methods of deformation measurement.
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From page 226... ...
Compare with CV Dom optical extensometer tests; specimen stif~esses were similar but the same specimens were not used 1 Tabb S4. Summary of axial deformation measurement techniques for routine repeated load testing 10 0.6 Overal 1 Reproduci hi l ity, Method Accuracy CV (%)
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From page 227... ...
SPECIMEN DRAINAGE Repeated load tests have been performed on pavement materials in both the drained condition [~l, 103, 1071 and Me undrained condition [55,92,93,1001. Since present pavement design procedures are based on total stress concepts, the resilient modulus should be evaluated using total stress rather than effective stress regardless of whether a drained or undrained test is performed.
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From page 228... ...
For undisturbed specimens, back pressure saturation and testing will probably be required. For routine testing, the recommendation is given to perform the test at He optunum moisture content and correct He resilient modulus using empirical correction factors such as Pose given In Chapter 4.
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From page 229... ...
The difference between store and water stiffness is much less than for most cohesive soils usually tested ~ g~hnical laboratories. For example, at a back pressure of 40 psi, the theory indicates Tat We degree of saturation for a B pore pressure of 0.2 is 94, 97, 98 and 99% for resilient moduli of 10,000, 20,000, 30,000 and 60,000 psi, respectively (refer to Figures 108 through ~10~.
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From page 230... ...
Degree of salvation as a finch of B pore pressure parameter: Mat = 10,000 psi 1 m e ~ 0.8 .` 0.6 e 0.4 e i,' 0.2 o ..
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From page 231... ...
Degree of saturation as a fimction of B pore pressure parameter: MR = 60,000 psi '~)
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From page 232... ...
RESILIENT MODULUS AND PERMANENT DEFORMATION MODEIS FOR COMPACTED COHESIVE SOILS Basic Concepts Based on the experimental observations reported earlier in this chapter, generalized models are developed for He response of compacted cohesive soils under repeated loading. The key element in developing these models is in establishing appropriate normalized variables which give clearly defined relationships between resilient and permanent strain and pertinent independent variables such as stress level, moisture content, density and shakedown stress.
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From page 233... ...
stadc 1 jog Sample Failled After 3335 Cycles High Stress Level Unstable Condition i S=ss-S~ Loops at c~d=0.95 Bee 2.0 30 Axial Strain (%) Figure 113.
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From page 234... ...
O l ~ I -0.25 ~0.20 -0.15 _ ' -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 Norlllalized Water Content, [(w-wO)
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From page 235... ...
of compaction, whether the soil is stable or unstable depends primarily on the applied level of repeated axial stress and the compaction moisture content. Figure ~ 15 presents data from a number of different repeated load tests performed on different specimens of Me same AT soil.
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From page 236... ...
Cumulative permanent strain mode!
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From page 237... ...
Resilient strain versus permanent strain at apparent shakedown of compacted cohesive A-6 soil 237 4.0
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From page 238... ...
This component of Me mode} provides the resilient strain and hence the resilient modulus of a soil compacted at a known moisture content and subjected to a repeated loading at a given level of stress. The following steps are needed: a.
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From page 239... ...
The optimum moisture content (wO ~ must be determined from a compaction test and the failure deviator stress (~`f3 from a static biaxial test performed at the confining pressure of interest. Then use Figures I l7 and I l9 to predict the expected values of permanent and resilient strain (and hence resilient modulus.)
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From page 240... ...
it: ·m co c a.
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From page 241... ...
- a Z 0 0 ~ Zip O ~ ~ ~ 0 no ~ ~ Cal ~0 e.
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From page 242... ...
The curve upon which point 4 lies corresponds to a compaction curve that could be obtained using a lower constant energy than for the reference upper curve. The resilient modulus must now be determined at the different physical state (i.e., different density and moisture content)
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From page 243... ...
Desired: Constant Energy Compaction Curve (W = 22C%o, ad = 97 pcf) 1 1 1 1 19.0 21.0 23.0 25.0 Water Content, w(°/O)
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From page 244... ...
The resilient moduli selected should be based on the results of laboratory resilient modulus tests. Next, knowing the moisture content (22%)
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From page 245... ...
MR((,Pt) = Breakpoint resilient modulus at optimum moisture content and 95% of AASHTO T99 maximum dry density % clay = % particles finer than the 2 micron size PI = Atterburg plasticity index Equation (16)
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From page 246... ...
Correlatmn of resilient modulus ratio MD,'/M~.p`) moisture content change constant density (after reference 57 and 1 10)
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From page 247... ...
To obtain reliable, accurate results from the repeated load biaxial test, compliance in the testing system must be minimized and the system carefully calibrated, including testing synthetic specimens. Follow the procedures given in Appendix D
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From page 248... ...
Vacuum Shear Triaxial Test. Most aggregate base and granular subgrade resilient modulus tests are performed at optimum moisture content and maximum dry density.
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From page 249... ...
The effect of eliminating in He test the small confining pressure present in He upper part of He subgrade is only slightly conservative for cohesive soils. The proposed unconfined resilient modulus test procedure is given in Appendix E
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From page 250... ...
Specimens are tested at only He optimum moisture content and dry density. After once developing the generalized relationships, resilient moduli for site specific conditions can then be determined by performing a conventional static shear strength test and several over routine tests.
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Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.