BEHAVIOR OF SILTY SOIL WITH AND WITHOUT SAND CUSHION ON

CHAPTER 4: BEHAVIOR OF SILTY SOIL with and without SAND CUSHION

4.4. BEHAVIOR OF SILTY SOIL WITH AND WITHOUT SAND CUSHION ON

4.4.1. Theshearstrengthbehaviorofsiltysoilreinforcedwithasandcushionin the unsaturated condition.

a) Shear strength behavior of unsaturated soil reinforced by a sand cushion:

The relationship of deviation stress(kN/m=1-3) versus axial strain of soilreinforced by a sand cushion was shown in the below figure. The results indicatedthat the deviation stress increased as the lateral pressure3and the thickness of thesand cushion increased.

500 400 300 200 100 0

a) Unreinforement

0 2 4 6 8 10 1214 Axial strain (%)

b) 5 mm sand cushionthickness 800

600 400 200

0 0 2 4 6 8 10 12 14 Strain (%)

Deviation stress (kPa) CBRunsoaked/CBRsoaked Deviation stress (kPa)

200KPA 150KPA 100KPA 50KPA

200KPA 150KPA 100KPA 50KPA

Unreinforcement 5mm sand cushion 10mm sand cushion 20mm sand cushion 800

600 400 200 0

c)10 mm sand cushionthickness

0 2 4 6 8 10 1214

Strain (%)

800d) 20 mm sand cushion thickness 600

400 200 0

0 2 4 6 8 10 1214 Strain (%)

Figure 4.10:Deviation stress versus axial strain of sand cushion samples in the unsaturated condition

Therelationshipbetweenverticalandlateralpressuresofsoilandsandcushion- soil was shown in Figure 4.11 when the specimen failed at 15%strain.

900 800 700 600 500 400 300

0 50 100 150 200

Lateral pressureσ3(kN/mkPa)

Figure 4.11:The vertical versus lateral pressure of soil and sand cushion-soil at failure in unsaturatedcondition

Reinforcing theUUwith the sand cushion increased its shear strength substantially at failure. With a 5mm sand cushion, the sample's strength was approximately 1.2 to 1.4 times greater than without reinforcement. When the thickness of the sand increased, its strength increased slightly.

Table 4.2 displays the results of calculating the total cohesive force (c) and the total internal friction angle () for the reinforced cases in theUUcondition because excess pore water pressure cannot be measured.

Deviation stress (kPa) Vertical pressureσ1(kPa) Deviation stress (kPa)



Table 4.2:The cohesive (c) and internal friction angle () of sand cushion-soil at failure of this and previous studies

Condition Type of reinforcement (kN/mo) c(kPa) Reference

UU Unreinforcement 65.6 19.8 Yang et al. [11]

UU 5 mm sand cushion 63.8 69.8 Yang et al. [11]

UU 10 mm sand cushion 67.5 50.0 Yang et al. [11]

UU 15 mm sand cushion 67.2 74.7 Yang et al. [11]

UU 20 mm sand cushion 70.2 47.6 Yang et al. [11]

UU Unreinforcement 23.4 60.9 This study

UU 5 mm sand cushion 15.7 162.8 This study

UU 10 mm sand cushion 16.0 167.8 This study

UU 20 mm sand cushion 16.7 169.7 This study

In this study, when the 5 mm sand cushion was present, thecvalue increased rapidly(about2.7times),whereasthevaluedecreasedslightly.Whenthethickness of the sand cushion increased, these numbers increased slightly. However, Yang et al.

[11]showedthatthesenumberschangedwithoutageneraltrend.Inthisstudy,the sand cushion soil was considered a heterogeneous material. Thus, UU shear strength shouldbeusedtoevaluatethecapacityofthesandcushionsoilinsteadofcand.In all cases, the UU shear strength of reinforced soil increased dramatically as the thickness of the sand cushionincreased.

b)The shear strength increasement R uf in the unsaturated condition.

The shear strength increasementRufwas the ratio between the deviation at the failure of reinforced soil and unreinfroced soil. The shear strength increasementRufof sand cushion soil was shown in Figure 4.12:

2 1.5

1 0 50 100 150 200 250

Lateralpressure 3(kPa)

5mmsand cushion 10mm sand cushion 20mm sand cushion Figure 4.12:The shear strength increasement versus lateral pressure in the

unsaturated condition of sand cushion samples.

Shear strength increasementRuf(times)

Đệm cát 10mm Không giacường Đệm cát 20mm

Đệm cát 5mm

Đệm cát 10mm Không gia cường Đệm cát 20mm

Đệm cát 5mm

Excess pore water pressure (kPa)

10mm sand cushion unreinforcement 20mm sand cushion 5mm sand cushion

ResultsindicatedthatRufwasgreaterthan1atalllateralpressures,showingthat the reinforcement can increase the soil’s strength. TheRufvalue decreased as lateral pressure increased. TheRufvalue increased as the thickness of the sandincreased.

4.4.2. The shear strength behavior of silty soil reinforced by a sand cushion in the saturatedcondition.

a) Shear strength behavior of the saturated soil reinforced by the sand cushion.

The results indicated that deviation stress increased when the axial strain and the thickness of the sand cushion increased. The larger the strain and the sand thickness were, the higher the deviation was.

300 250 200 150 100 50 0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Strain (%)

Figure 4.13:The deviation stress and axial strain of soil and soil reinforced by the sand cushion in the saturated condition.

40 20mm sand cushion

30 5mm sandcushion

20 10 0

10mm sand cushion unreinforcement

-10 -20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Strain (%)

Figure 4.14:The excess pore water pressure and axial strain of soil and soil reinforced by a sand cushion in the saturated condition

Deviation stress (kPa)

1

-16.5 -10.9

1.87 -2.1 1.75

excess pore water pressure 3.3 8.3 Rf

As the thickness of the sand cushion increased, theUUshear strength and the excess pore water pressure increased. In the strain from 1% to 3%, the reinforced sample generated a higher water pressure than the unreinforced sample, as the sand cushionpreventedlateralexpansionofthesample;thereby,thewaterpressuresurged. As the strain increased, the soil sample developed lateral strain (sliding between the soil and geotextile) (1- and 2-layer reinforcement samples), which decreased the water pressure, and the excess pore water

pressure dissipated due to the sand

cushion’shighpermeability.Theexcessporewateruwaschangedintheporewater pressureatthesandcushion.Theexcessporewaterwasnegative,indicatingthatthere wasadecreaseintheporewaterpressure(16.5,10.9,2.1kPa)comparedtotheinitial

one.Inotherwords,thelateralexpansionandthehighpermeabilityofsanddissipated the pore waterpressure.

Table 4.3:The excess pore water pressure andUUshear strengthSuof soil and soilreinforced by a sand cushion in the saturated condition.

Case Deviation

pressure (kPa)

Excess porewater

pressureu(kN/mkPa) UU shear strength Su(kPa)

Unreinforcement 83.02 8.30 41.51

5 mm sand cushion 145.53 -16.50 72.77

10 mm sand cushion 154.83 -10.90 77.42

20 mm sand cushion 273.83 -2.10 136.91

b) The shear strength increasement R f in the saturatedcondition.

4 3 2 1 0

10 5 0 -5 -10 -15 -20 Unreinforcement 5mmsand

cushion Case

10mmsand

cushion 20mm sand cushion

Figure 4.15:The shear strength increasementRfand excess pore water pressure ofsoil and soil reinforced by sand cushion in the saturated condition.

Shear strengthincreasementRf Excess pore water pressure (kPa)

The strength increasement indexRfwas the ratio between the deviations of thesand cushion and the unreinforced samples at failure in the saturated condition.

Theresults showed that the strength increase indexRfincreased as the thickness of thesand increased when comparing the strength of unreinforced and reinforced soil.

4.4.3. Shear strength reduction of soil and sand cushion soil due tosaturation:

Shear strength reductionTshearwas defined as equation 3.5. When comparing thestrengthsofunsaturatedandsaturatedsoil,theshearstrengthreduction,Tshear,wassmallerthan1.It indicatedthat,aftersoaking,theshearstrengthdecreased.Thelarger

thelateralstressandthicknessofthesandwere,thehigherthestrengthreductionwas.

85 75 65 55 45

0 50 100 150 200 250

Lateral pressure3 (kPa)

Unreinforcement 5mm sand cushion 10mmsand cushion 20mm sandcushion Figure 4.16:Shear strength reductionTsheardue to saturation of soil and

soilreinforced by a sand cushion.