Soil nails & cutting slopes: Design to
Alan Phear NIGG Seminar on Geotechnical design to EC7 Thursday 11 April 2013
(Val Ferret, Tour de Mont Blanc, 2012)
Soil nails & cutting cutting slope slopess – Desig Design n to EC7 & CONTENT OF TALK
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Unreinforced Unrein forced cutti cutting ng slopes slopes - design of slope slope
Design of soil nailing to BS 8006-2.
Outline of 1st part of talk (on EC7)
Mult Multii le docu docume ment ntss to to ref refer er to! to!
What we used to do eo ec n ca r s ca egor es n
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Headlines for overall stability in EC7
New principles of designing to EC7
Desi n re uirements – Limit st states
Water pressures & slope drainage
BS6031: 2009
Multiple documents to refer to! (1)
BS EN 1997-1:2004. Geotechnical Design – Part 1: General Rules. . – Part 2: Ground investigation & testing.
Other Eurocodes BS EN 1990. Basis of structural design BS EN 1991. Actions on structures
Non-contradictory complementary Information eg - BS 6031: 2009. Code of practice for earthworks.
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Multiple documents to refer to! (2)
Published documents e. . PD 6694-1 traffic loadin on structures)
De facto standards
Highways Agency (HA) DMRB
HA MCDHW (Series 100)
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HA IAN 124/11. Use of Eurocodes for the design of highway structures. (This doesn’t cover earthworks) Network Rail. NR/L3/CIV/071 . Geotechnical Design. Issue 4.
What we used to do For slo e desi n there is little chan e from what we used to do!
Design was governed by BS6031 in which a global factor of safety was applied to cover overall uncertainty… For first time slides with a good standard of investigation…a factor of safety between 1.3 and 1.4 should be designed for. For a slide involving an entirely pre-existing slip surface… a factor of safety of 1.2 should be provided [§ 6.5.1.2 BS6031: 1981]
y c ange
6
Think more about inputs to design and therefore should get a more
Design to EC7 applies the partial factors as close to the source of
Structured risk management & reporting process
7
es s u y, nc u ng walkover (risk identification Ground investigation (investigating risks) Interpretation of ground model & geotechnical parameters
Design process – Cut slope X-sections
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Geometry (road or railway cuts) & risk Road or railway
1. Shallow Cut (EC7 Category 1) 2. “Conventional” Cut - with no unusual characteristics (EC7
3. “Unconventional” Cut with unusual characteristics a egory – an example
Road or railway
Unusual traffic conditions at toe Difficult ground Unusual groundwater profile
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Headlines for overall stability in EC7
Overall stabilit is Section 11 of EC7 but should be the first thing checked for a site/structure
Covers soil and rock slopes
Satisfy the GEO and STR limit states for ULS and SLS
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DA1, combination 1 is A1 & M1 & R1 , com na on
s
an
For slopes, checking GEO and STR limit states, R1 is
For slopes, combination 1 is not usually relevant
(Embankments are covered by Section 12 of EC7)
Some principles of Geotechnical design to EC7
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Be aware of the distinction between permanent/variable actions and favourable/unfavourable actions Use of characteristic values with partial factors to form design values Application to several aspects of uncertainty rather than a single lumped factor of safety applied to cover all uncertainty
Design requirements – Limit states
Ultimate limit states that apply to slope stability are GEO
Take into account all relevant modes of failure
GEO = failure or excessive deformation o
e groun
of a structure (due to slope stability failure). Ground structure interaction shall be considered stiffnesses [11.5.1(11)]
Deep and shallow
relevant partial factors where consequence of failure is either higher or lower than normal – does this a l here for shallow failures? as the consequences of failure are usually only increased maintenance. 13
Water pressures Water ressures in EC7 should not be factored
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A.2.1 NA
For ULS design values of groundwater pressure shall represent the most unfavourable values (~condition) that could occur during the design lifetime of the structure [§2.4.6.1 (6)P] For SLS design values shall be the most unfavourable values (~condition) which could occur in normal circumstances [§2.4.6.1 (6) P] ons er seepage own s ope, rap raw own e c. o e in code w.r.t slopes along waterfronts [§11.3 (5)], choice of calculation method [§11.5.1 (3)] and joints and fissures in rock [§11.5.2 (1) P]
Slope drainage drainage: •Crest drain or ditch • oe ra n or v c anne •Slope drains •Deep sub-horizontal drains Drainage needs (which is an operational cost & is onl done sometimes!)
EC7 §2.4.6.1 (11) addresses this topic. 15
(from CIRIA C591)
Partial factors for GEO and STR, DA1 Symbol
ACTION
ermanen Variable
SOIL PARAMETER
n avoura e
γ
Set A1
A2
.
.
Favourable
γG
1.00
1.00
Unfavourable
γQ
1.50
1.30
Favourable
γQ
0
0
Symbol
Set M1
M2
γφ
1.0
1.25
Effective cohesion
γc’
1.0
1.25
Undrained shear strength
γcu
1.0
1.4
Unconfined strength
γqu
1.0
1.4
Angle of shearing resistance
RESISTANCE Earth resistance
R1 = 1.0
Watch-its (1)
Different artial factors are a lied to c and ’ for M2 material e.g. temporary works design for cut slope in Lambeth Clay over Thanet Sand (however this EC7 makes no distinction between temporary = factors is pertinent to the limit state being considered however risk assessment is recommended [ § 2.4.7.1 Slope stability problems are in many ways about
values of geometrical data [ §2.4.6.3] and consider 3D failure surface if appropriate [ §11.5.1(9)] 17
Watch-its (2)
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Overall stability of specific structures (spread foundations, piles, anchorages, retaining walls and embankments) should be accounted .
Verify stability of slope including existing, affected or planned structures in ULS for GEO and STR [§11.5.1(1)]
For existing failed slopes, consider circular as well as non-circular . [§11.5.1(8)]
Watch-its (3)
Favourable & unfavourable gravity loads: Since a distinction between favourable and unfavourable gravity
surface, any uncertainty about weight density of the ground should be considered by applying upper and lower characteristic values of it [§11.5.1(12)]
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Acceptable analysis methods: A slope analysis should ver y e overa momen an ver ca s a y o e s ng mass. If horizontal equilibrium is not checked, inter-slice forces should be assumed to be horizontal => Swedish Circle Method (Fellenius) (1927) and Janbu (1957) with horizontal interslices forces are NOT acceptable . .
BS 6031: 2009
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Code of Practice for Earthworks Non-contradictory complementary information (NCCI)
§7 is called ‘Design of earthworks’ and gives guidance on how to . Lots of other useful advice.
Soil nails & cutting slopes – Design to EC7 & CONTENT OF TALK
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Unreinforced cutting slopes - design of slope stability to EC7. Design of soil nailing to BS 8006-2.
Soil nails & cutting slopes – Design to EC7 &
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BS 8006-2 was published in late 2011 and addresses the design o so na ng. It should be read in conjunction with the Execution standard for so na ng, : . It has partial factors which are compatible with EC7. Parts of it are based on the CIRIA book on soil nailing, CIRIA C637. In the following slides, I will discuss some aspects of BS 80062.
Topics in BS 8006-2 (soil nail design)
pp cat ons construct on considerations u a y o groun groundwater conditions as s or es gn nc u ng so nails, durability, facings)
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Design verification
Maintenance
Reinforced soil & soil nailing
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design and execution of reinforced soil structures nor soil nailing. In the UK, the design and execution of reinforced fill structures should be carried out in accordance with BS . 8006-1 should not be replaced by similar factors from Eurocode 7. ( NA.4) In the UK, the design and execution of soil nailing should be carried out in accordance with BS 8006-2 and BS EN e part a actors n - are compat e . with EC7.
Drilled then grouted nails
Photo courtesy of Cementation Foundations Skanska Ltd
Self-drilled nails drill bit
bar
coupler
ea plate
nut
Self-drilled nails
Photo courtesy of Dywidag Systems International
Photos courtesy of Ischebeck TITAN Ltd 26
Basis of design •Design method •Anal sis of stabilit •Soil nail pullout resistance •Soil nail element design •Durability & degradation •Facin desi n •Drainage design
Materials for soil nail tendons
Uncoated steel
Galvanised steel oa e s ee
Stainless steel
Fibre reinforced plastic
Glass fibre Polyester composites Vinylester composites
Photos courtesy of Tony Barley a n ess ee
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Corrosion protection guidance for soil nails
CATEGORY
CATEGORY
CATEGORY
T or P
T in
P in
T or P
T in
P in
T or P
T in
P in
in
HCE
HCE
in
HCE
HCE
in
HCE
HCE
SCE
SCE
SCE
Each category has temporary nails or permanent nails
Summary of corrosion protection guidance •MOST corrosion protection risk categories and SLIGHTLY CORROSIVE ground conditions or env ronments. •ONLY A FEW corrosion protection systems are appropriate for HIGH RISK category with conditions or environments • e er to a e
o
- .
Guidance on design of facings Hard
Flexible Image courtesy of Mott MacDonald
Photo courtesy of rup
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Image courtesy of Arup)
Design verification Approach BS 8006-2 follows the approach
Execution standard (BS EN 14490)
Related to Geotechnical Risk Philosophy
e way t e na s teste nee s to model the way it is actually loaded in ractice.
