Soil nails & cutting slopes: Design to EC7 & BS 8006-2EC7 & BS 8006-2

Alan Phear

NIGG Seminar on Geotechnical design to EC7Thursday 11 April 2013

(Val Ferret, Tour de Mont Blanc, 2012)

Soil nails & cutting slopes – Design to EC7 & BS 8006-2BS 8006 2

CONTENT OF TALK

Unreinforced cutting slopes - design of slope stability to EC7.stability to EC7.

Design of soil nailing to BS 8006-2.

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Outline of 1st part of talk (on EC7)Multiple documents to refer to!pWhat we used to doG t h i l i k t i i EC7Geotechnical risk categories in EC7Headlines for overall stability in EC7New principles of designing to EC7Design requirements – Limit statesg qWater pressures & slope drainageDesign approach & partial factors for DA1Design approach & partial factors for DA1BS6031: 2009

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Multiple documents to refer to! (1)EC7EC7

BS EN 1997-1:2004. Geotechnical Design – Part 1: General Rules.BS EN 1997 2: 2007 Geotechnical DesignBS EN 1997-2: 2007. Geotechnical Design – Part 2: Ground investigation & testing.

Other EurocodesBS EN 1990. Basis of structural designBS EN 1991. Actions on structures

National Annex to EC7 (2006)National Annex to EC7 (2006)

Non-contradictory complementary InformationInformation

eg - BS 6031: 2009. Code of practice for earthworks.

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Multiple documents to refer to! (2)Execution standardsExecution standards

Published documents e.g. PD 6694-1 (traffic loading on g ( gstructures)

De facto standardsHighways Agency (HA) DMRBHA MCDHW (Series 100)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.

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What we used to doFor slope design, there is little change from what we p g , gused 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]

Wh h ?Why change?

Think more about inputs to design and therefore should get a more reliable designreliable design

Design to EC7 applies the partial factors as close to the source of the uncertainty as possible

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the uncertainty as possible

Structured risk management & reporting process D k t d i l diDesk study, including walkover (risk identification))Ground investigation (investigating risks)( g g )Interpretation of ground model & geotechnical parameters

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Design process – Cut slope X-sections

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Geometry (road or railway cuts) & risk categories Road or railwaycategories

1. Shallow Cut (EC7 Category 1)y

2. “Conventional” Cut - with no unusual characteristics (EC7 Category 2) Road or railwayCategory 2)

3. “Unconventional” Cut -with unusual characteristics (EC7 C t 3) Deep cut in

Unusual traffic conditions at toe

(EC7 Category 3) – an example

Unusual groundwater profile

Deep cut in Difficult ground

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Headlines for overall stability in EC7Overall stability is Section 11 of EC7 but should be the first ything checked for a site/structure

Covers soil and rock slopes

Satisfy the GEO and STR limit states for ULS and SLS

UK adopt DA1 for slopesUK adopt DA1 for slopes

DA1, combination 1 is A1 & M1 & R1

DA1 bi ti 2 i A2 & M2 d R1DA1, combination 2 is A2 & M2 and R1

For slopes, checking GEO and STR limit states, R1 is always unity for DA1always unity for DA1

For slopes, combination 1 is not usually relevant

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(Embankments are covered by Section 12 of EC7)

Some principles of Geotechnical design to EC7

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

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Design requirements – Limit statesULS and SLSULS and SLS

Ultimate limit states that apply to slope stability are GEO and STRand STR

Take into account all relevant modes of failure

GEO = failure or excessive deformation

f th dof the ground

• STR = internal failure or excessive deformation• STR = internal failure or excessive deformation of a structure (due to slope stability failure). Ground structure interaction shall be considered by allowing for the difference in relativeby allowing for the difference in relative stiffnesses [11.5.1(11)]

Deep and shallowfailuresfailures

BS EN 1990:2002+A1 permits variation ofBS EN 1990:2002+A1 permits variation of relevant partial factors where consequence of failure is either higher or lower than normal – does this apply here for shallow failures? -

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does t s app y e e o s a o a u esas the consequences of failure are usually only increased maintenance.

Water pressuresWater pressures in EC7 should not be factored [§A.2.1, NA]p [§ , ]

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]

C id d l id d d t N t dConsider seepage down slope, rapid drawdown etc. Noted 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]

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Slope drainageExamples of how pore pressures can be limited byExamples of how pore pressures can be limited by drainage: •Crest drain or ditch

t d i h l• toe drain or v channel•Slope drains•Deep sub-horizontaldrains

Drainage needs to be maintainedto be maintained (which is an operational cost & is only done ysometimes!)

EC7 §2.4.6.1 (11)

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addresses this topic.

Partial factors for GEO and STR, DA1ACTION Symbol Set

A1 A2

P t U f bl G 1 35 1 00Permanent Unfavourable γG 1.35 1.00Favourable γG 1.00 1.00

Variable Unfavourable γQ 1.50 1.30Favourable γQ 0 0

SOIL PARAMETER Symbol SetM1 M2

Angle of shearing resistance γφ 1.0 1.25

Effective cohesion γc’ 1.0 1.25Undrained shear strength γcu 1.0 1.4Unconfined strength γqu 1.0 1.4

RESISTANCEEarth resistance γR;e R1 = 1.0a es s a ce γR;e 0

Watch-its (1)Different partial factors are applied to cu and φ’ for p pp u φM2 material e.g. temporary works design for cut slope in Lambeth Clay over Thanet Sand (however this would be SLS design)would be SLS design)EC7 makes no distinction between temporary works or permanent works => the choice of partialworks or permanent works > the choice of partial factors is pertinent to the limit state being considered however risk assessment is recommended [§ 2.4.7.1 (4)(5)](4)(5)] Slope stability problems are in many ways about geometry; consider sensitivity analysis of designgeometry; consider sensitivity analysis of design values of geometrical data [§2.4.6.3] and consider 3D failure surface if appropriate [§11.5.1(9)]

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Watch-its (2)

Overall stability of specific structures (spread foundations, piles, anchorages, retaining walls and embankments) should be accounted for [§11 1 (2)]for [§11.1 (2)]

V if bili f l i l di i i ff d l dVerify 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 failure Partial factors normally used may not be appropriatefailure. Partial factors normally used may not be appropriate [§11.5.1(8)]

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Watch-its (3)

Favourable & unfavourable gravity loads: Since a distinction between favourable and unfavourable gravity loads is not possible in assessing the most adverse sliploads is not possible in assessing the most adverse slip 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)]

Acceptable analysis methods: A slope analysis should if th ll t d ti l t bilit f th lidiverify the overall moment and vertical stability of the sliding

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 [§11 5 1(10)]

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[§11.5.1(10)]

BS 6031: 2009

Code of Practice for EarthworksNon-contradictory complementary information (NCCI)§7 is called ‘Design of earthworks’ and gives guidance on how to apply EC7 to slope stability designapply EC7 to slope stability design.Lots of other useful advice.

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Soil nails & cutting slopes – Design to EC7 & BS 8006-2BS 8006 2

CONTENT OF TALK

Unreinforced cutting slopes - design of slope stability to EC7.

Design of soil nailing to BS 8006-2.

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Soil nails & cutting slopes – Design to EC7 & BS 8006-2BS 8006 2

BS 8006-2 was published in late 2011 and addresses the design f il iliof soil nailing.

It should be read in conjunction with the Execution standard for il ili BS EN 14490 2010soil nailing, BS EN 14490: 2010.

It has partial factors which are compatible with EC7.

Parts of it are based on the CIRIA book on soil nailing, CIRIAC637.

h f ll i lid ill di fIn the following slides, I will discuss some aspects of BS 8006-2.

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Topics in BS 8006-2 (soil nail design)A li i & iApplications & construction considerationsS it bilit f d &Suitability of ground & groundwater conditionsBasis for design (including soilBasis for design (including soil nails, durability, facings)Serviceability & movementsServiceability & movementsDesign verificationMaintenance

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Reinforced soil & soil nailingBS EN 1997-1 Geotechnical Design does not cover theBS EN 1997 1 Geotechnical Design does not cover the 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 and BS EN 14475 The partial factors set out in BS8006-1 and BS EN 14475. The partial factors set out in 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 14490 Th i l f i BS 8006 2 ibl14490. The partial factors in BS 8006-2 are compatible with EC7.

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Drilled then grouted nails

Photo courtesy of AD BarleyPhoto courtesy of AD Barley

Photo courtesy of Cementation Foundations Skanska Ltd

Self-drilled nailssacrificial hollow h d

Self-drilled nails

sacrificialdrill bit

hollowbar coupler

headplate nut

Self drilled nails

Photo courtesy of Dywidag Systems International

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Photos courtesy of Ischebeck TITAN Ltd

Basis of design•Design method

•Analysis of stabilityAnalysis of stability

•Soil nail pullout resistance

•Soil nail element design

•Durability & degradation

•Facing designg g

•Drainage design

Materials for soil nail tendonsUncoated steelU co ed s eeGalvanised steelC t d t lCoated steelStainless steelFibre reinforced plastic

Glass fibreCarbon fibreCarbon fibrePolyester compositesVinylester composites

Photos courtesy of Tony Barley & St i l St l Ltd

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& Stainless Steel Ltd

Corrosion protection guidance for soil nails

LOW RISK MEDIUM RISK HIGH RISKLOW RISK CATEGORY

MEDIUM RISK CATEGORY

HIGH RISK CATEGORY

T or P T in P in T or P T in P in T or P T in P in oin

SCEHCE HCE

oin

SCEHCE HCE

oin

SCEHCE HCE

Each category has temporary nails or permanent nails in a slightly corrosive or highly corrosive environmentin a slightly corrosive or highly corrosive environment

Summary of corrosion protection guidance for soil nailsfor soil nails•MOST corrosion protection systems are appropriate with mostsystems are appropriate with most risk categories and SLIGHTLY CORROSIVE ground conditions or

ienvironments.

•ONLY A FEW corrosion protection systems are appropriate for HIGH RISK category with HIGHLY CORROSIVE groundHIGHLY CORROSIVE ground conditions or environments

R f T bl 9 f BS 8006 2•Refer to Table 9 of BS 8006-2.

Guidance on design of facings

Hard

SoftSoft

FlexibleImage courtesy of Mott MacDonald

Photo courtesy of A & AMEC

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Arup & AMEC Image courtesy of Arup)

Design verificationApproachBS 8006-2 follows the approach to soil nail testing given in theto soil nail testing given in the Execution standard (BS EN 14490) Number of TestsNumber of TestsRelated to Geotechnical Risk CategoryCategory

PhilosophyTh h il i d dThe way the nail is tested needs to model the way it is actually loaded in practice.p

Summary

EC7 has resulted in only limited changes to the way slope stability design is carried out in the UK. p y g UI have talked about some of these changes & have have noted a few “Watch-its”.I have briefly introduced BS 8006-2 Soil nailing design.gI would welcome views in the discussion on some of the questions I have raised.

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