3.4.1. The shear strength behavior of silty soil unreinforced and reinforced by geotextiles in the unsaturatedcondition
a)Shear strength behavior of silty soil unreinforced and reinforced by geotextiles in the unsaturated condition
500 400 300 200 100 0
a) Unreinforement
0 2 4 6 8 10 121 4 Axial strain (%) c) Reinforced by 2 geotextilelayers
600 500 400 300 200 100 0
b) Reinforced by a geotextile layer
0 2 4 6 8 10 121 4 Axial strain (%) d) Reinforced by 3 geotextile 600
500 400 300 200 100 0
0 2 4 6 8 10 1214 Axial strain (%)
800 600 400 200 0
layers
0 2 4 6 8 10 121 4 Axial strain (%) Figure 3.10:Deviation stress versus axial strain of unreinforcement and
reinforcement with geotextile in the unsaturated condition
The relationship of deviation stress (=1-3) versus axial strain of soil andsoil reinforced by geotextile layers was shown in Figure 3.10. The results indicatedthat the deviation stress increased as the lateral pressure3and the number ofgeotextile layers increased.
Deviation stress(kN/mkPa) Deviation stress(kN/mkPa) Deviation stress(kN/mkPa) Deviation stress(kN/mkPa)
Unreinforcement 1 geotextile layer
geotextile layer geotextile layer 1000
800 600 400 200 0
0 50 100 150 200
Lateral pressureσ3(kN/mkPa)
Figure 3.11:The vertical versus lateral pressure of silty soil and geotextile soil at failure in unsaturatedcondition.
Therelationshipbetweenverticalandlateralpressuresofsoilandgeotextilesoil is shown in Figure 3.11 when the specimen failed at 15%strain.
Table 3.3:The cohesive (c) and internal friction angle () of soil and geotextile-soil at failure of this and previous studies
Condition Type of reinforcement (kN/mo) c(kPa) Reference
UU Unreinforced 65.6 19.8 Yang et al. [11]
UU 1 layer 68.3 13.9 Yang et al. [11]
UU 2 layers 65.0 183.6 Yang et al. [11]
UU 3 layers 68.0 226.0 Yang et al. [11]
CU Unreinforced 15.3 57.6 Yang et al. [42]
CU 1 layer 15.0 68.2 Yang et al. [42]
CU 2 layers 13.4 80.7 Yang et al. [42]
CU 3 layers 13.4 112.8 Yang et al. [42]
CU Unreinforced 29.9 11.8 Al-Omari et al. [41]
CU 1 layer 29.4 44.9 Al-Omari et al. [41]
UU Unreinforcement 23.4 60.9 This study
UU 1-geotextile layer 21.6 74.8 This study
UU 2-geotextile layers 20.8 83.3 This study
UU 3-geotextile layers 23.5 110.5 This study
Table 3.3 displays the results of calculating the total cohesive force (c) and the total internal friction angle () for the unreinforced and reinforced cases in the UU condition because excess pore water pressure cannot be assessed. The tableprovides thecohesionandinternalfrictionanglesfrompriorinvestigations.Theresultsshowed
Vertical pressureσ1(kPa)
1 geotextile layer 3 geotextile layers2 geotextile layers
thatinmostcases,thecohesionofreinforcedsoilincreaseddramatically,whereasthe internal friction angle changed without a general trend. In this study, the soil- geotextile was considered a heterogeneous material. Thus, the valuescandcan change without a general trend. UU shear strength should be used to evaluate the capacity of the soil-geotextile instead ofcand. In all cases, the UU shear strength ofreinforcedsoilincreaseddramaticallyasthenumberofgeotextilelayersincreased.
b)The shear strength increasement R uf in the unsaturated condition:
The shear strength increasementRufin the unsaturated condition was determined:
𝑅𝑢𝑓
= 𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑚𝑒𝑛𝑡
𝑢𝑛𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑚𝑒𝑛𝑡 (3.1)
Where:𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑚𝑒𝑛𝑡;𝑢𝑛𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑚𝑒𝑛𝑡: deviation at the failure of reinforced soil andsoil.
ResultsindicatedthatRufwasgreaterthan1atalllateralpressures,showingthat the reinforcement layers can increase the soil’s strength. TheRufvalue decreased as the lateral pressure increased. TheRufvalue increased as the number of fabric layersincreased. It is consistent with the conclusion that adding geotextile layers increases shear strength[11].
2 1.5 1 0.5
0 0 50 100 150 200 250
Lateral pressure3(kN/mkPa)
Figure 3.12:Shear strength increasement versus lateral pressure in the unsaturated condition.
3.4.2. The shear strength behavior of silty soil unreinforced and reinforced by geotextiles in the saturatedcondition.
Shear strength increasementRuf
1-geotextile layer 3-geotextile layer Unreinforcement 2-geotextile layer
Unreinforcement 2-geotextile layer1-geotextile layer 3-geotextile layer a)Shear strength behavior of silty soil unreinforced and reinforced by
geotextilein the saturated condition
The results indicated that deviation stress increased when the axial strain and the number of geotextile layers increased.
200 150 100 50 0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Axial strain (%)
Figure 3.13:The deviation stress and axial strain of soil and soil reinforced by geotextile in the saturated condition
80 60 40 20 0
0 1 2 3 4 5 6 7 8 9 101112131415 Axial strain (%)
Figure 3.14:The excess pore water pressure and axial strain of soil and soil reinforced by geotextile in the saturated condition
Figures 3.13 and 3.14 illustrated the deviation stress and excess pore water pressure with the axial strain of unreinforced and geotextile-reinforced saturated samples, respectively, in UU conditions. The saturated sample's undrained shearstrength,Suis determined to be fifty percent of the deviation stress at the failure.The sample's total shear resistance is calculated whencu= Suandu= 0. As thenumber
Excess pore water pressure(kPa)Deviation stress (kPa)
Rf
excess pore water pressure 58.7
42.4 2.2
11.1 3
8.
1.3 1.5 1
of geotextile layers increased, the UU shear strength and the excess pore water pressure increased.
Table 3.4:The excess pore water pressure and deviation pressure of soil and soil reinforced by geotextile in the saturated condition
Case Deviation
pressure (kPa)
Excess porewater pressureu(kN/mkPa)
Unreinforcement 83.02 8.30
1-geotextile layer 105.80 11.10
2-geotextile layers 126.26 42.40
3-geotextile layers 179.09 58.70
Yang et al. [11] confirmed that the excess pore water pressure increases as geotextiles can prevent lateral displacement or the potential tensile strain of the soil, thusraisingtheporewaterpressure.Inthestrainrangeof1%to3%,thesamplewith
reinforcementgeneratedahigherwaterpressurethantheunreinforcedsample,asthe
geotextile restrained the lateral deformation of the sample; thereby, the pore water pressure surged. 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 pore water pressure, and the excess water pressure dissipated due to the geotextile’s highpermeability.
b)The shear strength increasement R f in the saturated condition:
4 70
3 6050
2 40
30
1 20
10
0 0
Unreinforcement 1-geolayer 2-geo layers 3-geo layers Case
Figure 3.15:The shear strength increasementRfand excess pore water pressure ofsoil and soil reinforced by geotextile in the saturated condition.
Shear strength increasementRf Excess pore water pressure (kPa)
The shear strength increasement Rfin the saturated condition:
𝑅𝑓
= 𝑆𝑢reinforcement
𝑆𝑢 𝑢𝑛𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑚𝑒𝑛𝑡 (3.4)
Inwhich:Sureinforcement;Suunreinforcement:theshearstrengthofthesaturatedsoilandsoil reinforced with geotextile in UUconditions.
Figure 3.15 showed thatRfwas larger than 1, indicating the effect of geotextile onthereinforcementinthesaturatedcondition.TheRfindexincreasedasthenumberof
layerssurged.
3.4.3. Shear strength reduction of silty soil and geotextile soil due tosaturation Shear strength reductionTsheardue to saturation was determined as
𝑇𝑠ℎ𝑒𝑎𝑟
=∆𝜎 𝑢𝑛𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛− ∆𝜎 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛
∆𝜎𝑢𝑛𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛 (3.5)
In whichunsaturation;saturation(kPa): deviation stress of unsaturated andsaturated samples.
The results showed that the shear strengths of saturated samples were much lower than those of unsaturated ones, about 57% - 83%.
85 75 65 55 45
0 50 100 150 200 250
Lateral pressure3 (kPa)
Unreinforcement 1 geotextilelayer 2geotextilelayers 3 geotextilelayers
Figure 3.16:Shear strength reductionTsheardue to saturation of silty soil with andwithout geotextile