3.1SUCTION EFFECT
Suction creates a good influence on slope stability with hydrological and mechanical effects. Due to limitation of pages, only significant results are described.
3.1.1 Site monitoring
3.1.1.1 Suction response under natural variation
On 30 October, 2012, slope subjected to 34mm rainfall. From Figure 9, there is a rapid decrease in suction. After significant rainfall, suction started to recover to a higher value until occurrence of rainfall. Similar circle of wetting and drying process often occur in monitoring period which wetting process will decrease suction and hence, induce slope instability.
3.1.1.2 Suction recovery rate increases with vegetation
From Figure 8, at O.lm depth, the suction recovery rate is obviously the highest in tree and followed by grass and bare. Vegetation obviously increases suction recovery rate.
The reason is vegetation helps to uptake water from soil by its root system (transpiration process) which added one more source of soil water outflow. Therefore, the vegetation helps to increase suction recovery rate after rainfall. Besides, there is also the reason that tree has a higher suction recovery rate than grass is, tree has comparatively high of plant height which induced a higher water potential difference in plant structure. Therefore, tree is more capable to uptake water from soil.
3.1.1.3 Suction retention
On 30 October, 2012, slope subjected to 34mm rainfall. From Figure 8 and Table 2, at 0.3m depth which is generally shallow failure zone, the suction retention of bare, grass and tree are 6.5, 5 (by interpolation) and 54.5 respectively. Tree has obvious retention of suction after rainfall.
The reason is, in 0.3m depth, it is within the root zone of tree and thus, significant transpiration effect occurred and retained a high value of suction. However, 0.3m depth is out of effective root zone of grass and thus, no significant transpiration effect occurred.
0
Figure 8: Suction before and after rainfall after subjected to 34mm rainfall on 30 October, 2012
Cover- 0.3m depth Maximum Suction before rainfall Minimum Suction after rainfall
(kPa) (kPa)
28-10-2012 to 31-10-2012 (Winter) (Subjected to 34mm rainfall)
Bare 136.4 6.5
Grass 125 (by interpolation) 5 (by interpolation)
Tree 133 54.5
Table 2 Maximum Suction before rainfall and Minimum Suction after rainfall after subjected to 34mm rainfall on 30 October, 2012
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Figure 9 Suction and Rainfall relationship on 0.1m depth during period of 111912012 to 28/2/2013
3.1.2 Laboratory Test
3.1.2.1 Suction response under natural variation
Under 100 mm rainfall for 1 hour, from Figure 10, suction obviously decreases with time and
Figure IO: Suction Changes in Bare Soil during rainfall for I hour at 30 degree slope
70 ~---,
Figure II: Suction Recovery in Bare soil at 30 degree slope after rainfall
3.1.2.2 Suction recovery rate increases with vegetation
From Figure 12, rate of inducing suction is the highest in Tree, followed by Grass and Bare which also makes a good conservation with site result.
80
Figure I2: Induced Suction Distribution of Bare, Grass and Tree
3.2 ROOT REINFORCEMENT EFFECT
Figure 13 Soil Matric Suctions for Pullout Tests
The suctions measured for all pullout tests are shown in Figure 13. T2 and T3 mean Test 2 in 12% water content soil and Test 3 in 20% water content soil respectively. The matric suction in Test 1 (dry soil) is approaching infinity and meaningless for measurement.
300
Figure 14 Pullout Resistance Curve in Different Soil Moisture Conditions The curves of pullout resistance of tap root models versus pullout displacement are shown in Figure 14.
In term of peak pullout resistance values, relatively dry soil (12%) provide pull-out resistance more than moist soil (20%) almost 300% as an effect of soil suction, while dry soil (0%) provide less resistance but still more than moist soil by about 70%.
Mobilised axial force (N)
0 50 100 150 200 250 300 -50 0 50 100 150 200 250 300 0 50 100 150 200 250 300
() 0
-50 50 - 50
-]oo
100 - 100-..c ~50 At8mm 50 150
-"0 ....
~00 0 200
-250 - ---AtO
- -At 100 At200
300 300
O%GWC 12%GWC 20%GWC
Figure 15 Mobilized Axial Force at Different Depths for Tap Root Tests
The mobilized axial forces were measured by the strain gauges installed at different depths. The forces are mobilized at similar displacements, about 6, 7 and 9mm for 20%,0% and 12% water content soil, respectively. Strictly speaking, root displaced farther to reach peak pullout resistance, or be fully mobilized, in 12% water content soil.
According to Figure 15, the slope of curve at bottom 85mm is smaller than that on the top. It indicated that the bonding of root and soil at root bottom was weaker than other positions.
Mobilized forces are reduced substantially after reaching peak for 12% water content soil but still well maintain above a half of peak resistance for 20% water content soil.
3.3 DISCUSSION
3.3.1 Suction response under natural variation
Suction decreases with rainfall because of water infiltration and recovers after rainfall with water outtake by natural evaporation and transpiration.
3.3.2 Suction recovery rate increases with vegetation
Vegetation creates significant and beneficial effect of suction recovering after rainfall.
3.3.3 Suction retention
Suction retained of tree vegetated cover on site after 34mm rainfall is 54.5kPa which helps to increases shear strength of soil and also creates beneficial effect on plant root pull out resistance. Therefore, tree cover is recommendedas the vegetation on slope.
3.3.4 Correlation to Mechanical Reinforcement
Root in relatively dry CDG (about 12% water content) with matric suction between 30 to 40 kPa has highest pullout resistance among three cases. When soil matric suction decreases to about 3kPa, root pullout resistance is the lowest. However, if soil matric suction is approaching infinity (0% water content), root pullout resistance is between the previous two cases.
Correlated to the suction retention value of 54.4kPa, the previous statement implies that plant root has higher pullout resistance in suction retention condition generated by vegetation than that in saturated condition caused by infiltration without vegetation protection.The mechanical reinforcement effect of plants is strengthened by the matric suction retention generated by their own during rainfall.