Some current site monitoring and long term stability modelling on glacial till cuttings in Ireland.

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Alison Hopkins
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1 Some current site monitoring and long term stability modelling on glacial till cuttings in Ireland. What are we measuring and how can we predict or model behaviour? What effect is climate change having? What effect does vegetation have? Dr David Hughes (and Dr Gordon Clarke) School of Civil Engineering, Queen s University Belfast Canadian/Queen s University Belfast Collaborative research group Including University of British Columbia University of Saskatchewan University of Manitoba 1

Overview of four current cuttings (-25m) in glacial tills 1. Investigation of a failure of large road cutting in 1999 after 30 years. Dromore, (Roads Service). 2.

2 Overview of four current cuttings (-25m) in glacial tills 1. Investigation of a failure of large road cutting in 1999 after 30 years. Dromore, (Roads Service). 2. Instrumentation monitoring and hydrogeology of a new large road cutting excavated in 05. Loughbrickland, (Roads Service) 3. Monitoring (deep and near surface) of an usually steep old (150 year) railway cutting. Craigmore (NIR/Golder Associates). 4. Monitoring (deep and near surface) of a 30 year old road cutting to be oversteepened. Tullyhappy (Roads Service/Mouchel Parkman). The four sites we are working on are in the same region A drumlin swarm in south County Down. N Dromore a failed 30 year old road cutting Loughbrickland a new road cutting BELFAST Craigmore 150 year old rail cutting Tullyhappy Craigmore Dromore Loughbrickland Tullyhappy 30 year old road cutting 0 10 km 0 10 miles to Dublin 2

3 The quaternary geology of the region (Source: McCabe et al, 1999) 60W Lough Neagh County Down Drumlins Loughbrickland KIllard Point Morainal Bank Heavily overconsolidated stiff till 54N Dundalk Newry Mourne Mountains Dunmore Head Moraine Ballagan/Cranfield Moraine Dunany Point Moraine Summary of ice flow patterns and site specific evidence used to construct ice flow stages. Kells Moraine Main ice stream flows Tunnel valley Ice marginal moraines Drumlins Streamlined ridges, drumlins and drumlinised tranverse ridges Ice moulded bedforms associated with ice streams Main ice stream flows (Source: McCabe et al, 1999) Typical quaternary geology of the region. A Drumlin swarm in lodgement (glacial) tills Loughbrickland cutting 04 3

4 Typical monitoring used at Dromore and Loughbrickland On site weather station Deeper nested VW piezometers, to measure +ve pwp, with packers for better response to rainfall events -30m Additional instrumentation for Craigmore and Tullyhappy to be installed Near surface measurement of moisture content and suction both permanent real time and mobile measurement. Smethurst Clarke and Powrie, Geotechnique O Kane Consultants, EnviroSCAN Sensors 1-2 m 4

Dromore: The first failure of a cutting constructed 1970 Failed after heavy

3 days of heavy rain a large rotational slide was triggered on m high 1V to 2H

Can see carriageway heave Very poor drainage on many slopes (Temporarily)

5 Dromore: The first failure of a cutting constructed 1970 Failed after heavy rainfall Jan 1999 large rotational movement January 1999 after a wet autumn and 3 days of heavy rain a large rotational slide was triggered on m high 1V to 2H slope ( ). Can see carriageway heave Very poor drainage on many slopes (Temporarily) repaired Dromore Failed again heavy rainfall November 00 The slope had been regraded to 1 to 2.4 ( ) and permanent (small) berm placed, road opened. 5

November 00 Original berm placed to remediate initial failure

repaired Dromore Failed again heavy rainfall November 00 Note

6 Temporarily repaired but. failed again on existing slip plane after heavy rainfall November 00 Original berm placed to remediate initial failure Second berm to stabilise re mobilised material Temporarily repaired Dromore Failed again heavy rainfall November 00 Note two tension cracks on face illustrating possible wedge movement or graben in slumped material. 11 th March 01 6

Now have a slide mass resting (balancing) on a failure plane at residual (low) shear strength? What strength parameters? How stable? Equilibrium reached?

7 Now have a slide mass resting (balancing) on a failure plane at residual (low) shear strength? What strength parameters? How stable? Equilibrium reached? Schematic of mechanism of failure of temporarily repaired slope Graben feature with second tension crack Temporarily regraded Slope 1V:2.4H (22.6 0) 18.6m Temporary rockfill berm Deformation visible, at tension crack on regraded slope 3m 3m Continued movement along failure surface Regraded slope with temporary berm and graben after further movement (October 00 to March 01) 7

Stability analysis on original cutting using critical state values Elevation (m) 30 25 15 10 No Flow Pressure Head = 0m 22 18 16 Total Flow = 0 m3/s (potential seepage face) 14 12 10 8 Total Flow = 0

8 Stability analysis on original cutting using critical state values Elevation (m) No Flow Pressure Head = 0m Total Flow = 0 m3/s (potential seepage face) Total Flow = 0 m3/s (potential seepage face) No Flow 5 No No Flow Flow No Flow Distance (m) GeoStudio 04 Software Back analysis on original slope Using pore pressures from seepage analysis Critical state values from triaxial tests: c = 1kPa and Φ = 31 0 Elevation (m) Distance (m) FoS = 0.92 (can also generate suction from seepage files useful) Post-mortem of failure what we learnt Basal Slip surface here - coincident with pre-existing shear zone 8

60m thick Fractured rock The slip plane mid slope - basal slip plane through preexisting shear zone of fissured clay Slickensided basal slip plane

9 Two tills and a pre-existing shear zone or relict slip plane? Two types of tills with pre-existing shear zone between? Edge of carriageway 2.09m to top of brown fissured clay containing basal slip plane brown fissured clay containing basal slip plane 0.60m thick Fractured rock The slip plane mid slope - basal slip plane through preexisting shear zone of fissured clay Slickensided basal slip plane Striations Heavily fissured stone free brown clay showing slickensided basal failure plane obvious in left sample. Right hand sample taken from same block is matching left sample but inverted. Left sample Inverted right sample 9

10 Stability analysis on temporarily repaired cutting using residual strength values Original Slope 1V:2H ( ) Borehole 3 & 4 Failure surface Exposed horizon of brown heavily fissured, stone free, clay Estimated surface of fractured rock 18.6m 5.0 m 2.5m A-A (a) Estimated shape of failure surface (January 1999) Schematic of repair excavation below slip plane and granular material placed Granular backfill at 1V:2.4H (22.6 0) Culvert across carriageway to drain fill 45 o temporary back slope in excavation 3m (c) Final repair excavation and granular fill (December 01) Monitoring ongoing at Dromore - 2 boreholes with 4 piezometer - measure +ve pwp Can maybe look at seasonal variation and pore water pressure peaks with rainfall events? 10

Is this possible? Glacio tectonic shear of drumlins? Preferential fissures and cracking have been reported by McGown 140 Elevation (maod) 130 1 110 100 90 Ice movement?

11 Is this possible? Glacio tectonic shear of drumlins? Preferential fissures and cracking have been reported by McGown 140 Elevation (maod) Ice movement? Distance along Y-Y' axis (m) Low strength highly fissured slickensided residual strength and if it coincides with likely rotational failure plane? Next site is new road cutting at Loughbrickland Loughbrickland cutting 04 11

Sisters (drumlin) Bronze age burial site

12 Loughbrickland cutting Loughbrickland Village N Lough Brickland The Three Sisters (drumlin) Bronze age burial site (c.1,500-1,000 BC) Neolithic houses (c.4,000-2,500 BC) Bronze Age ring barrows Loughbrickland Typical borehole log Topsoil Stiff to very stiff dark grey sandy gravely silt with some cobbles and boulders Completely to highly weathered grey shale rock Bedrock Band of dense grey and fine medium and coarse sandy gravel with some cobbles Highly to moderately weathered grey shale rock 12

Loughbrickland Effect of rainfall on slope stability 1m Evapotranspiration infiltration Surface drainage Loughbrickland, Site photograph,

13 Loughbrickland Effect of rainfall on slope stability 1m Evapotranspiration infiltration Surface drainage Loughbrickland, Site photograph, November 04. View North BH1 BH2 BH 3&4 Tried to measure pwp change at toe of slope during stress relief of excavation with no success 18m 13

14 Pore water pressure response to excavation BH 1 BH 2 BH 3/4 CL ~ 5e -10 ms -1 5e -4 ms -1 Current findings Pore water pressure response to rainfall - near surface BH 2 Borehole Graph 2 pore 1 - Borehole water 1 pressure Head levels response Height / maod Precipitation / mm Summer 04 Autumn 04 Winter Spring 05 Summer 05 Season 0 14

15 Current findings Pore water pressure response to rainfall - near surface -1m -2m Cum. Rainfall / mm Days -10m Depth peizometer -1m -2m 231mm 154mm -10m 46mm How and what to model? Why are we measuring this? We get hydrogeologic response to environment but is that enough? Can we predict response using the Climate change scenarios? What ultimately do we need to model/know? 15

scenarios What soil model should be used? Simple strain softening model? Should we be using this in FE analysis?

16 Climate change. Increased frequency of failure - although drier in SE. Percent change in summer precipitation 80 UK regional variation Belfast Newcastle L borough Southampton Low Emissions scenarios High Emissions scenarios What soil model should be used? Simple strain softening model? Should we be using this in FE analysis? Source - Dr Nesha Kovacevic Geotechnical Consulting Group, London ϕ ', c' ϕ' p, c' p G = G (p, ε d ) K = K (p, ε v ) Soil strength reduced (over time) from critical to residual with longterm strain or creep ϕ' r, c' r (ε d p ) p (ε d p ) r ε d p 16

17 What suctions are generated in cutting during long dry periods? Soil Water Characteristic Curves for tills? Zone of partially saturated soil with high strength in summer, high suction and improved stability Summer lower moisture content - higher suction Winter higher moisture content - lower (or no) suction GeoStudio 07: Progressive Failure of strain-softening cut in London Clay (based on Potts et al Delayed Collapse of cut slopes in stiff clay. Geotechnique, Vol. 47, No. 5, pg ) We are currently looking at better ways to model delayed failure based on long term monitoring data across UK region Southampton, Loughborough, Newcastle, Belfast Elevation (m) Elements excavated in layers Free boundary: constant pore water pressure du = 0; impermeable Distance (m) du = 0; impermeable After Dr Curtis Kelln, QUB, now GeoSlope Inc 17

Strain Softening simulated via strength reduction (stress redistribution method) Case φ' c' SRF SRF = tanφ tanφ = c failure c failure 0 1 2 16.9 15

18 Strain Softening simulated via strength reduction (stress redistribution method) Case φ' c' SRF SRF = tanφ tanφ = c failure c failure After Dr Curtis Kelln, QUB, now GeoSlope Inc GeoStudio 07 coupled analysis preliminary modelling swelling back in 40 years to failure state After Dr Curtis Kelln, QUB, now GeoSlope Inc 18

Video of: pore water response during excavation followed by swelling and deviatoric straining Pore water pressure response: excavation followed by 40 years swelling Deviatoric straining (development

coupled Sigma and Seep importing stress into Slope Elevation (m) End of Swelling for ~ 40 1.

19 Video of: pore water response during excavation followed by swelling and deviatoric straining Pore water pressure response: excavation followed by 40 years swelling Deviatoric straining (development of rupture zone): excavation followed by 40 years swelling AND THEN stress redistribution After Dr Curtis Kelln, QUB, now GeoSlope Inc Strain Softening simulated via strength reduction After swelling coupled Sigma and Seep importing stress into Slope Elevation (m) End of Swelling for ~ Elements excavated in layers years Free boundary: constant pore water pressure du = 0; impermeable Distance (m) du = 0; impermeable Elevation (m) Strength Reduction Elements excavated in layers Free boundary: constant pore water pressure du = 0; impermeable Distance (m) du = 0; impermeable 19

Strain Softening simulated via strength reduction After swelling coupled Sigma and Seep importing stress into Slope Elevation (m) 1.

20 Strain Softening simulated via strength reduction After swelling coupled Sigma and Seep importing stress into Slope Elevation (m) Strength Reduction 2 Elements excavated in layers Free boundary: constant pore water pressure du = 0; impermeable Distance (m) du = 0; impermeable Elevation (m) Strength Reduction 3 Elements excavated in layers Free boundary: constant pore water pressure du = 0; impermeable Distance (m) du = 0; impermeable LOSS OF CONVERGENCE A winter field trip in Ireland Thanks

A case study: delayed failure of a deep cutting in lodgement till

A case study: delayed failure of a deep cutting in lodgement till Proceedings of the Institution of Civil Engineers Geotechnical Engineering 16 October 27 Issue GE4 Pages 193 22 doi: 1.168/geng.27.16.4.193 Paper 14777 Received 16/5/26 Accepted 16/2/27 Keywords: geotechnical