Design of soil nailed walls and slopes Part of lecture series I gave at QUT in 2016

1. The Design of
Earth-retaining Structures
ENB485
Chris Bridges
The Design of
Earth-retaining Structures
ENB485
Chris Bridges
The Design of
Earth-retaining Structures
ENB485
Chris Bridges
May 2016
Lecture 6
Design of Earth-retaining Structures
Soil Nailing

2. 17
Soil Nailing

3. What is soil nailing?
Anchors
Soil Nails
Soil Dowels
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4. What is soil nailing?
Anchor analogy – threaded bolt
Generates tensile force in
member
Normal force creates shear
stresses along failure plane
Requires pre-stress to be
maintained over life of structure –
(active structure)
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5. What is soil nailing?
Nail analogy
Shear stress around nail
generates tensile force in
member
‘Reinforces’ the slope
No pre-stress to be maintained
over life of structure – (passive
structure)
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6. Reinforcing the soil

7. Soil Nail – Main Components
HDPE Sheath
(for corrosion
protection)
External
centraliser
Internal
centraliser
Outer grout
tube
Inner grout
tube
Soil nail head
plate
(galvanised
for corrosion
protection)
Domed soil nail nut
(galvanised for
corrosion protection)
32mm diameter deformed soil
nail bar (galvanised for corrosion
protection)
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8. Nail Components

9. Nail Components

10. Applications
construction of new slopes and retaining walls;
construction of temporary works;
upgrading existing slopes with inadequate stability; and
the renovation of old retaining structures.
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11. Soil Nailing – Is
it suitable?
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12. Advantages Disadvantages
• construction flexibility
• cost
• environmental/aesthetic
considerations
• temporary conditions (cut must
stand vertically)
• groundwater
• clay soils
• land issues
• corrosion
• ground movement
What is soil nailing?

13. 18
Airport Link Project –
Kedron Park Hotel
Tunnels

14. Key
• Approx. A$5bn
• 15km of tunnelling including
5.7km of twin tunnels
• Connects CLEM 7 Tunnel to
Brisbane Airport
• Sections of new busway &
stations
• Upgrade of major airport
intersection (airport roundabout)
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15. Case Study - Kedron Park Hotel Tunnels
Contractors Requirements
Church and
Hall
Moreton Bay
Fig
Kedron Park
Hotel
Housing
Open Car
Park
Tunnel
through wall
Tunnel under
wall
• Cut-and-cover tunnel
• Cut required to in excess of
30m depth with final
structure built within the box
• Church hall within 5m of
wall, Church and Hotel
within 15m
• Access for construction
plant at the crest (large
mobile crane loads etc)
• Tunnels (roadheader, drill
and blast) required
underneath the excavation
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16. Kedron Park Hotel Tunnels
Contractors Preferences - Alternatives / Concepts
Initial Design – Stacked tunnel, internal
propping constructed top down. Secant
piles.
Issues with constructability and cost
Enquiry from TJH regarding suitability of
soil nails
Preparation of design for 30m+ high soil
nail wall (24m) / rock cut (6m)
Revised to 18m soil nail wall and 10m
contiguous piles due to mined tunnel
concerns
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17. Kedron Park Hotel Tunnels
Contractors Preferences – Adopted
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18. Kedron Park Hotel Tunnels
Design – Key Issues
Tunnelling through walls - constructability
Settlement of existing structures
Timescales
Suitability to gain approval through TJH design review process (PBA,
IV, CNI)
Appropriate modelling techniques
Construction plant as crest of wall
Tower crane
Existing services
Protected Trees
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19. Kedron Park Hotel Tunnels
Design - Layout
Extent of soil
nailed wall
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20. East
West
RS – ST CLAY
RS – H CLAY
VL-L STRENGTH
SANDSTONE
MS SANDSTONE
5mRL
22mRL Mobile crane
Kedron Park Hotel Tunnels
Design – Ground Model - South Wall
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21. Mobile Crane
Loading – 152kPa
Kedron Park Hotel Tunnels
Design – Wall Geometry - South Wall
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22. Kedron Park Hotel Tunnels
Design – Preliminary Sizing Recommendations
MODEL 1
• No soil nails
• Planar failure
Ref.: Zhang et al, 2001
MODEL 6
 Short soil nails
 L/H = 0.32
 Block failure
MODEL 8
 Long, widely spaced soil nails
 L/H = 1
 Planar failure

23. Kedron Park Hotel Tunnels
Design – Preliminary Sizing Recommendations
Ref.: Vucetic et al, 1996
TESTS 5-7
 L/H = 0.67 & 1.0
 Widely spaced at 0 deg inclination
 Planar failure
TESTS 1-4
• L/H = 0.67 (incl. 0-30 deg)
• Closely spaced
• Two-part wedge: no difference in
failure with nail inclination 0-30 deg.

24. Nail Spacing
Closer spaced nails provide greater internal stability
Wide spacing of nails can effect internal stability
Soil nail spacing should not exceed 1 nail per 6m2 of hard facing or 1
nail per 2m2 to 4m2 of flexible facing otherwise the soil nailed structure
will not act as a reinforced mass (CIRIA C637)
Max. spacing – typically 1 - 2m
Kedron Park Hotel Tunnels
Design – Preliminary Sizing Recommendations
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25. Nail Length
Long nails at top and shorter at bottom provides less stability
Shorter nails at top, longer at bottom provides “excellent” stability but
Shorter nails at the top will increase deformations (Shui & Chang,
2005)
As L/H increases, deformations decrease
Short nail lengths can effect external stability
Length of Nails - 0.8 - 1.2H (Clouterre)
Soil nail lengths above 15m should be avoided due to drilling
difficulties and greater deformation required to mobilise tensile
capacity.
Kedron Park Hotel Tunnels
Design – Preliminary Sizing Recommendations
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26. Nail Inclination
Nail inclination between 0 and 20 deg does not appear to affect
stability
>20 deg then rapid increase in wall deformation
Facing
Flexible facing provides similar overall stability to rigid facing, but more
localised deformation
Facing type (based on case studies) - face angle ≥70°= hard
face angle ≤70°= flexible
Kedron Park Hotel Tunnels
Design – Preliminary Sizing Recommendations
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27. vertical nail spacing, Sv = 1.2m
horizontal nail spacing, Sh = 1.5m
soil nail length, L / wall height, H = 15m/18m = 0.8
Kedron Park Hotel Tunnels
Design – Adopted Sizing
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28. Kedron Park Hotel Tunnels
Design – Adopted Sizing
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29. AS4678-2002 - Earth Retaining Structures
BS8006:1995 - Strengthened/ reinforced soils and other fills
BS8006:2011-2 - Strengthened/ reinforced soils and other fills
FHWA Geotechnical Engineering Circular No. 7 – Soil nail walls
(2003)
CIRIA C637 Soil nailing – best practice guidance (2005)
Kedron Park Hotel Tunnels
Design – Codes & Standards
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30. CIRIA Partial Factors
Kedron Park
Hotel Tunnels
Design –
Codes & Standards
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31. Kedron Park Hotel Tunnels
Design – Basis of Design

32. Kedron Park Hotel Tunnels
Design – Design Approach

33. Soil nailed wall
Kedron Park Hotel Tunnels
Design – Design Approach
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34. Modes of Failure
External Stability
Internal Stability
Kedron Park Hotel Tunnels
Design – Design Approach

35. Kedron Park Hotel Tunnels
Design – Analysis
1.333
Surcharge (Unit Weight): 32 kN/m³
Name: Stiff Clay Unit Weight: 21 kN/m³ Cohesion: 5 kPa Phi: 25 °
Name: Hard Clay Unit Weight: 21 kN/m³ Cohesion: 5 kPa Phi: 28 °
Name: VLS - LS Siltstone Unit Weight: 22 kN/m³ Cohesion: 22.5 kPa Phi: 30 °
Name: MS Siltstone Unit Weight: 22 kN/m³ Cohesion: 200 kPa Phi: 40 °
Stiff Clay
Hard Clay
VLS - LS Siltstone
MS Siltstone
South Wall
Eastern Side
Distance
0 5 10 15 20 25 30 35 40 45 50 55 60 65
-5
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Elevation(mRL)
-5
0
5
10
15
20
25
30
35
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36. SNAIL
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37. Kedron Park Hotel Tunnels
Design – Analysis
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38. Design pull-out capacity (AS4678)
T* = n .b ..D.Lf .u
Where: .
n = Structure classification design factor (Table 5.2)
= 0.9-1.1;
b = Bond reduction factor (Table B2) = 0.7;
D = Diameter of grout hole (m);
Lf = Fixed length (m);
u = Ultimate grout/ground resistance (bond stress)
(kN/m2).
Kedron Park Hotel Tunnels
Design – Analysis
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39. Design pull-out capacity (CIRIA C637)
T* = (1/gp)..D.Lf .u
Where: .
D = Diameter of grout hole (m);
Lf = Fixed length (m);
gp = Partial factor (1.25, temp; 1.5, perm; 2, perm,
highly plastic clays);
u = ultimate grout/ground resistance (bond stress)
(kN/m2).
Kedron Park Hotel Tunnels
Design – Analysis
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40. Bond Stress
Pull out tests
Empirical values
Effective stress design methods
Kedron Park Hotel Tunnels
Design – Analysis
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41. Design bond stress (effective stress design method)
u = (c* + σ‘v tanf*) (kN/m2)
where:
σ‘v = effective vertical stress at the mid-depth of nail behind
the failure surface;
f* = design angle of internal friction of soil (deg.);
c* = design value of cohesion of soil (kN/m2).
Kedron Park Hotel Tunnels
Design – Analysis
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42. Empirical values
Material Ultimate Bond Stress (kPa) Material Ultimate Bond Stress (kPa)
Residual soil 50 – 75 Class V Sandstone 150
Class V Shale 75 – 100 Class IV Sandstone 250 – 800
Class IV Shale 150
Typical values of ultimate bond stress for various materials in the
Sydney region are given below:
See also:
FHWA Geotechnical Engineering Circular No. 7 – Soil nail walls (2003)
CIRIA C637 Soil nailing – best practice guidance (2005)
Values should be confirmed by pull-out testing.
Kedron Park Hotel Tunnels
Design – Analysis
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43. Design pull-out capacity
Pull-out resistance is usually assumed to be uniform along
the length of the nail.
This is reasonable if nails are short and stiff (dia. of bar
>20mm, bond length <5m)
For more elastic nails (eg. fibre-composite nails) the ave.
bond stress reduces when bond length >3m
CIRIA Report C637
Kedron Park Hotel Tunnels
Design – Analysis
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44. 0
50
100
150
200
250
300
0 5 10 15 20 25 30
AverageBondStress(kPa)
Fixed Length (m)
GRP 22mm bar
GRP Strands
Steel 50mm bar
Steel 20mm bar
Kedron Park Hotel Tunnels
Design – Analysis
Effect of Fixed Length on Bond Stress
(Barley and Graham, 1997)
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45. Kedron Park Hotel Tunnels
Design – Analysis
Nail tensile capacity:
Bar strength (tension capacity) can be determined from equation B4(1) in
AS4678-2002
T = k n t fp Ap (kN)
where:
k = Importance category reduction factor (Table B1) =
0.8 (perm.) - 0.9 (temp);
n = structure classification design factor (Table 5.2, AS4678)
= 0.9 - 1.1
t = Material reduction factor (Table B2) = 0.9;
fp = Tensile strength of nail (kN/m2);
Ap = Cross sectional area of nail (m2).
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46. Deformation
Typical Movements of Soil Nailed Structures (Clouterre, 1991)
(after Murray, 1993)
Vertical or
horizontal
deformation
Coarse sand /
gravel
Sand Clay
dv = dh H/1000 2H/1000 4H/1000
do 4H/1000 to 5H/1000
F 0.8 1.25 1.5
l is the distance from the facing to the point where
deformations become negligible and is given by:
l = F H (1 – tan b)
Where
b = initial angle of the
face relative to the
vertical
H = perpendicular height
of structure
F = constant
Kedron Park Hotel Tunnels
Design – Analysis
US/UK now consider
3H/1000 for clays
(not high/v. high
plasticity)
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Parameter Unit Effect on Deformation
Wall Geometry
Nail length/Wall height - L/H ratio has greater effect when
L/H<0.97 for ≥ f’=38° and
L/H<0.83 for ≤ f’=38°
Nail inclination ° Deformation increases when inclination
>15°
Nail density m2/nail Deformation increases linearly with
increased nail spacing
Wall face angle ° Steeper wall face angle results in a
significant increase in wall deformation
Soil Properties
Soil modulus MPa Increasing deformation with decreasing
E, with greater increase in d when
E<20MPa
Cohesion kPa Has a significant impact on deformation,
especially when c’ ≤ 4kPa
Angle of internal friction ° Has a less impact than cohesion, but has
a greater effect when the other properties
start to take effect

48. Factor of safety for different soil nail
inclinations (90° wall face angle)
Factor of safety for different L/H ratios
(90° wall face angle)
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0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.80
1.20
1.60
2.00
2.40
-5 0 5 10 15 20 25 30
Deformation(m)
F.O.S
Nail Inclination (deg)
FOS Vertical Deformation
Horizontal Deformation
f'=36°, c’=6kPa
0.000
0.005
0.010
0.015
0.020
0.025
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
4
4.4
4.8
5.2
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
Deformation(m)
F.O.S
L/H Ratio
FOS Vertical Deformation
Horizontal Deformation
f'=36°, c’=6kPa

49. Effect of soil nail inclination on
settlement profile behind wall face
Effect of soil nail inclination on
nail force
(Nail 1 is top row of nails)
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-0.090
-0.080
-0.070
-0.060
-0.050
-0.040
-0.030
-0.020
-0.010
0.000
20 25 30 35 40
Settlement(m)
Distance from Wall Face (m)
10 deg
20 deg
25 deg
30 deg
Wall face
End of 5m
long nail
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
-10 -5 0 5 10 15 20 25 30 35 40
FactorofSafety
NailForce(kN)
Soil Nail Inclination below Horizontal (degrees)
Nail 1
Nail 2
Nail 3
Nail 4
Nail 5
Nail 6
FOS

50. Kedron Park Hotel Tunnels
Design – Analysis
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51. Kedron Park Hotel Tunnels
Design – Analysis
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52. Kedron Park Hotel Tunnels
Design – Analysis
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53. Kedron Park Hotel Tunnels
Design – Analysis
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54. Kedron Park Hotel Tunnels
Design – Analysis
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55. Kedron Park Hotel Tunnels
Design – Analysis
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56. FacingFHWA
Geotechnical Circular
No. 7
Kedron Park Hotel Tunnels
Design – Analysis
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57. FacingKedron Park Hotel Tunnels
Design – Analysis
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58. Kedron Park Hotel Tunnels
Design - Instrumentation
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59. MRTS03 Drainage, Retaining Structures and Protective Treatments
Project Specific
QA Specification R64 Soil nailing
CIRIA C637 Soil nailing – best practice guidance (2005)
Kedron Park Hotel Tunnels
Design – Specifications
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60. Kedron Park Hotel Tunnels
Design – Drawings
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61. Typical DTMR Permanent Nail
Kedron Park Hotel Tunnels
Design – Drawings
MIN. NOW 125mm
MIN TOTAL GROUT
COVER 30mm
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62. Kedron Park Hotel Tunnels
Design – Drawings
East Wall
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63. Kedron Park Hotel Tunnels
Design – Drawings
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64. Tree Roots
Kedron Park Hotel Tunnels
Design – Drawings
North Wall
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65. Mobile crane
Access ramp
Kedron Park Hotel Tunnels
Design – Drawings
South Wall
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66. Kedron Park Hotel Tunnels
Construction
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67. Kedron Park Hotel Tunnels
Construction
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68. Kedron Park Hotel Tunnels
Construction - Role
Full time
Record progress during
construction
Mapping of face to correlate
testing
Observe testing
Record groundwater
Site photography
ITP sign off
Site instructions
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69. Kedron Park Hotel Tunnels
Construction Sequence
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70. Kedron Park
Hotel Tunnels
Construction Sequence

71. Kedron Park Hotel Tunnels
Construction
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72. Kedron Park Hotel Tunnels
Construction
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73. Kedron Park Hotel Tunnels
Construction
East Wall
GRP Nails
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74. Kedron Park Hotel Tunnels
Construction
Western End
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75. Kedron Park Hotel Tunnels
Construction - Drainage
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76. Kedron Park Hotel Tunnels
Construction
East Wall – installing canopy tubes
GRP Nails
Piles at base of wall
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77. Spacers of inadequate strength
Damage to installed soil nail bars by
excavator
Lack of experience of contractors staff
Kedron Park Hotel Tunnels
Construction - Issues
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78. Kedron Park Hotel Tunnels
Construction - Testing
• Ultimate Load (Pullout) Test
• Acceptance Testing
• Exhumation of test nails

79. Kedron Park Hotel Tunnels
Construction – Tower Crane
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80. Kedron Park Hotel Tunnels
Construction – Tower Crane
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81. Kedron Park Hotel Tunnels
Construction

82. Kedron Park Hotel Tunnels
Construction

83. Kedron Park Hotel Tunnels
Construction - Monitoring
RS
VLS-LS
Siltstone
RS
VLS-LS
Siltstone
RS
VLS-LS
Siltstone
Mid-South Wall Corner North Wall Mid-North Wall
33mm 10mm 22mm
Prediction = 22mm Prediction = 55mm
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84. Kedron Park Hotel Tunnels
Construction - Monitoring
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85. Kedron Park Hotel Tunnels
Completion
• Two year design life, 4 month
construction (nail wall).
• 18m deep excavation with further 10m
deep piled wall below
• Over 1500 soil nails 10 to 15m long
(steel and GRP)
• Backfilled with flowable fill

86. Soil Nail Walls are not such a great idea in water charged sands and silts
(They also can’t be expected to have a 2 weeks stand up time)
Airport Link – Construction Issues
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87. 19
Other Projects

88. S E Queensland – Construction Issues

89. 1
2 3
4
5
6
7 8
9
10
11
12
13 14
15
1.51
Mudstone
Original Design
Sandstone
Claystone
Name: General Backfill
Model: Mohr-Coulomb
Unit Weight: 19 kN/m³
Cohesion: 5 kPa
Phi: 30 °
Name: Claystone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 40 °
Name: Mudstone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 42 °
Name: Sandstone
Model: Mohr-Coulomb
Unit Weight: 20 kN/m³
Cohesion: 2 kPa
Phi: 42 °
Sandstone
Directory: K:Q1215 - LMICoffeyDesignWallsRW08
Pressure (Unit Weight): 20 kN/m³
General Backfill
Offset (m)
0 5 10 15 20 25 30 35
Height(m)
0
5
10
15
20
25
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90. S E Queensland

91. S E Queensland

92. 1
2 3
4
5
6
7 8
9
10
11
12
13 14
15
1.53
Mudstone
Revised Design
Sandstone
Claystone
Name: General Backfill
Model: Mohr-Coulomb
Unit Weight: 19 kN/m³
Cohesion: 5 kPa
Phi: 30 °
Name: Claystone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 40 °
Name: Mudstone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 42 °
Name: Uncemented Sandstone
Model: Mohr-Coulomb
Unit Weight: 20 kN/m³
Cohesion: 0 kPa
Phi: 35 °
Name: Sandstone
Model: Mohr-Coulomb
Unit Weight: 20 kN/m³
Cohesion: 2 kPa
Phi: 42 °
Uncemented Sandstone
Directory: K:Q1215 - LMICoffeyDesignWallsRW08
Pressure (Unit Weight): 20 kN/m³
General Backfill
Offset (m)
0 5 10 15 20 25 30 35
Height(m)
0
5
10
15
20
25
Re-analysed wall with revised ground
model and design amended to
incorporate same length nails for full
height of wall over affected section
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93. Remediation
But, after wall completed:
Wall still showed signs of movement
Extra nails 10m in length were recommended along 20m of the
unstable section
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94. 1
2 3
4
5
6
7 8
9
10
11
12
13 14
15
1.59
Mudstone
Remediation Nail Design
Sandstone
Claystone
Name: General Backfill
Model: Mohr-Coulomb
Unit Weight: 19 kN/m³
Cohesion: 5 kPa
Phi: 30 °
Name: Claystone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 40 °
Name: Mudstone
Model: Mohr-Coulomb
Unit Weight: 21 kN/m³
Cohesion: 10 kPa
Phi: 42 °
Name: Uncemented Sandstone
Model: Mohr-Coulomb
Unit Weight: 20 kN/m³
Cohesion: 0 kPa
Phi: 35 °
Name: Sandstone
Model: Mohr-Coulomb
Unit Weight: 20 kN/m³
Cohesion: 2 kPa
Phi: 42 °
Uncemented Sandstone
Directory: K:Q1215 - LMICoffeyDesignWallsRW08
Pressure (Unit Weight): 20 kN/m³
General Backfill
Offset (m)
0 5 10 15 20 25 30 35
Height(m)
0
5
10
15
20
25
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95. S E Queensland
Over-excavation

96. 12 March 2015
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97. S E Queensland
Over-enthusiasm

98. Road over Rail Bridge

99. Existing Bridge

100. Existing RE Wall Drawings

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106. 12 March 2015
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107. Failures
Soil nail failures are fairly uncommon but the following are typical:
• hole collapse during installation of the soil nail, often due to loose fill
materials related to poorly backfilled utility trenches, for example;
• over excavation resulting in large unsupported lengths or heights of cut
slope;
• seepage;
• erosion/washout of slope surface;
• inappropriate soil - running sands;
• problems during drilling due to drill bits getting stuck or getting off line;
• grout loss during installation;
• poor construction of soil nail heads with voids beneath the soil nail head.
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108. Soil Nail Failures
Ref.: Tan. & Chow, 2004
Soil nail facing failure
Grout cover cracked and
peeled off from nail

109. Void within
cement grout
~250mm
Probed depth = 320 mm
into cement grout
Probed depth = 1590 mm
into cement grout
Cross-section
Soil Nail Failures
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110. Nail with
no grout
Plastic sheet
Short
column of
cement
grout
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111. Soil Nail Failures
Ref.: Sun & Tsui, 2005
Slope failure prior to soil nail
heads being installed during
heavy rain
Cause of failure:
• Ingress of water
Assisted by -
• Upper row of nails too low
below crest
• Wide nail spacing
• Lack of surface protection
• Poor construction of nails

112. Soil Nail Failures
6m3 failure in Completely Decomposed Tuff

113. References
• FHWA (1994). “Soil nailing field inspectors manual.” FHWA-SA-93-068, US
Department of Transportation, Washington.
• FHWA (1998). “Manual for design & construction monitoring of soil nail
walls.” FHWA-SA-96-069R, US Department of Transportation, Washington.
• FHWA (2003). “Soil nail walls.” Geotechnical Engineering Circular No.7,
FHWA0-IF-03-017, US Department of Transportation, Washington.
• Clouterre (1991) “French National Research Project Clouterre –
Recommendations Clouterre 1991”. (English Translation), Federal Highway
Administration, FHWA-SA-93-026, US Department of Transportation,
Washington.
• BSI (1989). “Code of practice for ground anchorages”, BS 8081:1989,
British Standards Institution, London.
• Highways Agency (1994). Design manual for roads and bridges- Advice
Note HA68/94, Design methods for the reinforcement of highway slopes by
reinforced soil and soil nailing techniques. The Stationary Office, London.
• Hong Kong Government Webpage:
http://www.cedd.gov.hk/eng/publications/index.htm
(Geoguide 7, Technical Guidance Notes, GEO Reports)