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Frontiers of Structural and Civil Engineering

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Front. Struct. Civ. Eng.    2020, Vol. 14 Issue (6) : 1462-1475    https://doi.org/10.1007/s11709-020-0674-8
RESEARCH ARTICLE
Effect of a less permeable stronger soil layer on the stability of non-homogeneous unsaturated slopes
Nabarun DEY(), Aniruddha SENGUPTA
Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
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Abstract

Slope failure occurs due to an increase in the saturation level and a subsequent decrease in matric suction in unsaturated soil. This paper presents the results of a series of centrifuge experiments and numerical analyses on a 55° inclined unsaturated sandy slope with less permeable, stronger silty sand layer inclusion within it. It is observed that a less permeable, stronger silty sand layer in an otherwise homogeneous sandy soil slope hinders the infiltration of water. The water content of the slope just above the stronger layer increases significantly, compared to elsewhere. No shear band is found to initiate in a homogeneous sandy soil slope, whereas for a non-homogeneous slope, they initiate just above the less pervious, stronger layer. A discontinuity of the shear zone is also observed for the case of a non-homogeneous soil slope. The factor of safety of a non-homogeneous, unsaturated soil slope decreases because of the less permeable, stronger layer. It decreases significantly if this less permeable, stronger soil layer is located near the toe of the slope.
Keywords non-homogeneous slope      stronger soil layer      factor of safety      centrifuge model test      unsaturated soils     
Corresponding Author(s): Nabarun DEY   
Just Accepted Date: 05 November 2020   Online First Date: 09 December 2020    Issue Date: 12 January 2021
 Cite this article:   
Nabarun DEY,Aniruddha SENGUPTA. Effect of a less permeable stronger soil layer on the stability of non-homogeneous unsaturated slopes[J]. Front. Struct. Civ. Eng., 2020, 14(6): 1462-1475.
 URL:  
https://academic.hep.com.cn/fsce/EN/10.1007/s11709-020-0674-8
https://academic.hep.com.cn/fsce/EN/Y2020/V14/I6/1462
Fig.1  Schematic diagram of the centrifuge used at BOKU.
Fig.2  (a) Centrifuge test box with (b) camera and lighting arrangements.
Fig.3  Procedure for constructing the laboratory model slope. (a) Soil compaction; (b) attaching supporting plate; (c) penetration test; (d) cutting and shaping; (e) attaching model box; (f) required model slope.
Fig.4  Geometry of the slopes. (a) Homogeneous; (b) non-homogeneous.
parameters scaling factor (model/prototype) unit model dimension prototype dimension
basic parameters
?length of slope 1/n mm 480 19200
?width of slope 1/n mm 155 6200
?height of slope 1/n mm 250 10000
?density of soil 1 g/mL 1.54 1.54
hydraulic parameters
?permeability of sand (k1) n m/s 1.1×10−4 2.75×10−6
?permeability of silty sand (k2) n m/s 4.8×10−6 1.2×10−7
?hydraulic gradient (i) 1 – – –
?suction 1 N/m2 – –
Tab.1  Scaling factors for centrifuge tests
Fig.5  Grain size distribution curves for the sand and silty sand.
properties unit sand silty sand
percentage of coarse, medium and fine sand % 0, 69.5, 30.4 0, 0.1, 90.1
percentage of gravel and fines content % 0, 0.1 0, 9.8
D10, D30, D60 mm 0.19, 0.3, 0.47 0.08, 0.14, 0.2
uniformity coefficient, Cu – 2.47 2.5
coefficient of curvature, Cc – 1.01 1.23
specific gravity, G – 2.62 2.71
maximum dry unit weight kN/m3 16.3 16.78
minimum dry unit weight kN/m3 13.9 12.95
maximum void ratio, emax – 0.86 1.05
minimum void ratio, emin – 0.58 0.58
permeability, k m/s 1.1×10−4 4.8×10−6
cohesion, C kN/m2 0.4 5.4
angle of internal friction, ϕ degree 34 32.5
Tab.2  Material properties of sand and silty sand
Fig.6  Discretized slope (with one stronger soil layer at mid-height).
Fig.7  Variation of (a) volumetric water content with suction pressure; (b) degree of saturation with matric suction.
Fig.8  A potential sliding zone with vertical slices for the case of a non-homogeneous layer at the base of the slope.
Fig.9  Water content variation of the slopes (in prototype scale).
Fig.10  Critical slip surfaces and the corresponding FOS. (a) Homogeneous slope; (b) non-homogeneous layer in the middle; (c) non-homogeneous soil layer at the base of the slope.
Fig.11  Deformation vectors within the slopes after centrifuge tests. (a) Homogeneous slope; (b) non-homogeneous layer at mid-height; (c) non-homogeneous layer at base.
Fig.12  Formation of shear strain fields in the homogeneous slope during (a) 35g; (b) 40g.
Fig.13  Formation of shear strain fields in non-homogeneous slope with one silty sand layer at mid-slope height during (a) 20g; (b) 30g; (c) 35g; (d) 40g.
Fig.14  Discontinuous shear zone in non-homogeneous slope with one silty sand layer at mid-height.
Fig.15  Formation of shear strain fields in non-homogeneous soil slope with a silty sand layer at the base of the slope during (a) 20g; (b) 30g; (c) 35g; (d) 40g.
The following symbols are used in this study:
A: ross-sectional area (m2)
a: curve-fitting parameter (-)
C?′: effective cohesion (kPa)
CΨ: correction function (-)
Dr: relative density (%)
g: centrifugal acceleration (m/s2)
I: slice index (-)
i: hydraulic gradient (-)
k: coefficient of permeability (m/s)
kw: unsaturated permeability (m/s)
ks: saturated permeability (m/s)
lbase: base length of each vertical slice (m)
m: curve-fitting parameter (-)
N: scaling factor (-)
n: curve-fitting parameter (-)
q: specific discharge (m3/s)
r: distance from centrifuge axis (m)
S: shear strength of soil (kPa)
∑ Sm: total driving shear force (kPa)
∑ Sr: total resisting shear force (kPa)
ua: pore air pressure (kPa)
uw: pore water pressure (kPa)
v: Darcian velocity of flow (m/s)
w: angular velocity (m/s)
Θr: residual volumetric water content (%)
Θs: saturated volumetric water content (%)
Θw: volumetric water content (%)
σn: total normal stress (kPa)
σs: suction stress (kPa)
φ: effective friction angle (°)
φb: angle defining the increase in shear strength for an increase in soil suction (°)
Ψ: suction pressure (kPa)
  
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