3.5. CONSOLIDATION BEHAVIOR OF SILTY SOIL UNDER EFFECTSOFSIDEFRICTION
3.5.1. The one-dimensional consolidation behavior under the effects of side friction pressure
a) The strain of specimens:
0%
1%
2%
3%
4%
5%
0.1 1 10 100
Time (minutes, log) 1000 10000
Strain (%)
Figure 3.17:Axial strain vs. time under 99.5kPa of compression
pressure(The specimens’ names exhibit the diameter,D, and the initial height,H0, in mm)
398.3 kPa 199.1 kPa 99.5 kPa 49.7 kPa 24.8 kPa
The temporal variation of the axial strain of the soil specimens under the compression pressure,P= 99.5 kPa, was presented in Figure 3.17. The smaller axial strainwasobservedinthesoilspecimenswiththehigherinitialheightandthesmaller diameter. As discussed previously, the side friction caused a decrease in the consolidationpressure.Asaresult,theaxialstrainofthespecimenswouldbesmaller due to the rise of side friction. Based on the test results, the lowest axial strain wasobservedinthespecimenswithD=50mmandH0=75mm,ofwhichthesidefrictionreached the highest level after 24h of the consolidationperiod.
Therequiredtimetocompletetheprimaryconsolidationofthespecimens,T100,wasdeterm inedusingthemethodofthelogtime-
deformationcurveasrecommendedinASTMD2435[13].AspresentedinFigure3.18,thevalueof T100wasproportionalto the square of the maximum drainage distance, which was
consistent with Terzaghi’sone-
dimensionalconsolidationtheory.Thisfindingagreeswiththeresults of several studies [17, 22]. The results also verified the adequacy of a 24h period to complete the primary consolidation of a loadincrement.
700 600 500 400 300 200 100
0 0 100 200 300 400 500 600
Hdrainage2(mm2)
Figure 3.18:Variation of time corresponding to 100% primary consolidation,T100, with square maximum drainage distance,Hdrainage2, under different
compressionpressures. The empty and solid symbols indicate the specimens with a 50 mm and 75 mm diameter, respectively.
b) The coefficient of consolidation
Figure 3.19 shows the variation of the coefficient of consolidation,Cv, of the clay specimens with the average consolidation pressure,Paverage. It was defined as the
T100(minutes)
20mm y = 0.0261x-0.922 R² = 0.90
10 mm 30 mm 40 mm 50 mm
averagevalueofthecompressionpressureandthereactionpressureactingonthetop and bottom of the soil specimens, respectively[17].
4.E-03 3.E-03 3.E-03 2.E-03 2.E-03 1.E-03 5.E-04 0.E+00
0 100 200 300 400
Average consolidation pressure (kPa)
Figure 3.19:Consolidation coefficient value with the average consolidation pressure.
The empty and solid symbols indicate the specimens with the diameters,D= 50 mm and 75 mm, respectively.
Itisobservedthatthegreatertheaverageconsolidationpressure,thesmallerthecoefficient of consolidation. The relationship betweenCvand consolidation pressurehas been reported differently in previous studies. Raju et al. [112] illustrated thesmallerCvvalueofthenormallyconsolidatedclayunderhigheroverburdenpressure.
Besides,Retnamony[113]concludedthatCvdecreasedwithahigherpressureforthemontmorillo nite mineral, in which physicochemical factors governed thecompression behavior.
In contrast,Cvwould increase with consolidation
pressureforkaolinite,illite,andpowderedquartz,whosecompressibilitybehaviorwascontrolle d by mechanical factors. For the remolded clay, Sridharan et al. [12] proposed that thedecreasing trend ofCvvs. consolidation pressure preferred to occur for more plasticsoilsduetothemobilizationofthediffusedoublelayerrepulsiveforceactingagainstthe external loading. That finding was supported by the results ofCvof the highplasticitysilt(i.e.,LL=91.5andPI=46.6)inthisstudy.Incontrast,forlowplasticityclay
(CL),Cvincreased with the increment in consolidation pressure[22].
The correlation between the two parameters is given below with a high coefficient of determination, R2= 0.90:
Consolidation coefficientCv(mm2 /min)
𝑎𝑣𝑒𝑟𝑎𝑔𝑒
𝐶𝑣=0.0261𝑃−0.922 (3.6)
0.85 0.80 0.75
150 D50H50 D50H40 D50H30
D50H20 D50H10 0.80
0.75 150
D75H50 D75H40 D75H30 D75H20
D75H10
c) The void ratio at the end of the primary consolidation (kN/mEOP):
1.1 1.0 0.9 0.8 0.7 0.6
10 100
Consolidation pressure at topofspecimens,P(kPa, log scale)
1.1 1.0 0.9 0.8 0.7
0.6 10 100
Consolidation pressure at top of specimens,P(kPa, log scale)
a) Diameter,D=75mm b) Diameter,D=50mm
Figure 3.20:Compression curves (eEOP-logP) without pressure correction forfriction pressure loss.
As expected, the compression curves of the soil specimens with initial heightsofH0=10mmand20mmwereidentical,illustratingthatthesidefrictionmarginallyaffect edthetestresults.So,theinfluenceofthefrictionalpressurelossonthe20mm thick specimens was negligible[17].
In contrast, the void ratio atEOPof the specimens with the initial height,H0≥ 30 mm, was significantly higher than those with a lowerH0. It illustrates that for the cases ofH0≥ 30 mm, the friction between the soil and the inner side of theconsolidation ring was high enough to cause a significant reduction in the actual consolidation pressure. In addition, the effects of friction loss were more visible for soil specimens with a smaller diameter. Several studies also provided a similar observation, which introduced that the side friction effect on one-dimensional consolidation test results was pronounced on the diameter-to-height ratio of the sample [18, 21].
d)Coefficient index
Voidratio atEOP,eEOP VoidratioEOP,eEOP
Based on the variation in the void ratio with the average consolidationpressure at the end of the primary consolidation, the results illustrated that the compression curves of all the soil specimens converged into a unique curve and wereindependent
D75H10 D75H20 D75H30D75H40 D75H50 D50H10
D50H20D50H30 D50H40 D50H50
of the dimensions of the samples and the friction pressure. It was because the effects ofsidefrictiononreducingthecompressionpressurewereeliminatedwhenusingthe average consolidation pressure to correct the compression curves (e-logP). In other studies [17, 18], this correction method was also applied to the consolidation test resultstorevealthetruee-logPcurvesofsoilswithnofrictionpressureloss.Thesoil specimens exhibit normal consolidation behavior with the coefficient index of thesoil,Cc0.32.
Since the clay specimens were remolded at a very high water content and void ratio, the pre-consolidation pressure would be too small to determine from the one- dimensional consolidation tests. However, it could be evaluated using the consolidation curves shown in Figure 3.21.
𝑃0 𝑃 𝑎𝑣𝑒𝑟𝑎𝑔𝑒
𝑒0−𝑒𝐸𝑂𝑃 (3.7)
10 𝐶𝑐
inwhicheEOPandPaveragearethevoidratioandequivalentaverageconsolidationpressure of all the specimens atEOP.
1.1 1.0 0.9 0.8 0.7 0.6
10 100
Average consolidation pressure,Paverage(kPa, log)
Figure 3.21:Compression curves (eEOP-logPaverage) of soil specimens after pressurecorrection for friction pressure loss.
The results ofP0are shown in Figure 3.22, in which the average pre-
consolidation pressure,P0_average, and the standard deviation,sd_P,are 7.92 kPa and 1.68 kPa, respectively. The evaluation of theP0_averagevalue would be verified whenpredicting the height of specimens and the friction pressure loss ratio.
Voidratio atEOP,eEOP
=
Preconsolidation pressure Fractional errorP0_average = 7.94 kPa
sd-Po = 1.68 kPa
50 mm 20 mm
30 mm 10 mm
40 mm
50 mm 40 mm 30 mm 20 mm 10 mm
15
12 4%
9
6 2%
3
0 0%
0 100 200 300 400
Average consolidation pressure,Paverage(kPa)
Figure 3.22:The estimated pre-consolidation pressure with fractional error/