Silverbirch Wind Farmsilverbirchwindfarm.com/wp...6-C-Landslide-Susceptibility-Review.pdf · Figure 1 Knocknageeha Peat Slide 1896 (Feehan et al (2008)) There are two schools of thought

Silverbirch Wind Farm

Landslide Susceptibility Review

March 2017

Job number Revision Prepared by Checked by Status Date 17924-6004 A Cormac Murphy Peter Barry Issued

17924 Landslide Susceptibility Review February 2017

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Table of contents

1 INTRODUCTION ............................................................................................................................ 2

2 HISTORY OF LANDSLIDE ................................................................................................................ 3

3 PEAT SLIDE RISK ASSESSMENT ...................................................................................................... 5

3.1 Peat Stability Hazard Ranking Assessment ............................................................................................ 7

3.1.1 Rainfall and Climate ................................................................................................................................................... 7

3.1.2 Presence of water on slope ........................................................................................................................................ 7

3.1.3 Rockhead or subsoil. .................................................................................................................................................. 7

3.1.4 Peat profile and depth ............................................................................................................................................... 7

3.1.5 Peat Strength .............................................................................................................................................................. 9

3.1.6 Slope and Slope regularity .......................................................................................................................................... 9

3.1.7 Geomorphology and Site History ............................................................................................................................... 9

3.1.8 Sub-profile drainage ................................................................................................................................................... 9

3.1.9 Peatslide History....................................................................................................................................................... 10

3.1.10 Potential Peatslide Severity ................................................................................................................................. 11

3.2 Calculation of Overall Peat Stability Hazard Ranking ........................................................................... 11

3.3 Peat Stability Risk Assessment ............................................................................................................ 12

4 CONCLUSION .............................................................................................................................. 13


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1 INTRODUCTION

This report presents an addendum to the Geotechnical Assessments completed for the proposed Silverbirch

Wind Farm site in Co. Kerry. The proposed wind farm development is located on elevated ground to the west of

the upper reaches of the River Blackwater, between the villages of Gneevguilla, Co. Kerry and Ballydesmond, Co.

Cork.

The original technical assessment was completed by MRG with inputs by Apex Geoservices. In completing the

Peat Stability Risk Assessment the Authors made reference to Historical failures at the site but concluded that

due to subsequent peat harvesting and forestry plantation it was difficult to confirm evidence for the purpose of

allocating a risk ranking to historical failures.

It is the intention of this technical note to apply the precautionary principle and address this difficulty using a

conservative risk allocation for Historical failures.


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2 HISTORY OF LANDSLIDE

The Geological Survey of Ireland lists the Knocknageeha Bog Slide of 1896 as occurring at the southern extremity

of the Silver Birch site.

A newspaper report from The Chronicle (Adelaide) in February 1897 tells of a 'A Great Irish Bog Slide'.

"A landslip of a disastrous character took place in the early hours of December 28 on the eastern side of the

Kenmare estate, on the confines of the counties of Kerry and Cork. A bog called Bogoghanimy, or the bog of the

Wale, comprising some 200 acres in extent, taking a southerly direction, swept everything before it for miles. A

family of eight persons living near the Quarry Lodge lost their live"

The article goes on to quote from an agent of Lord Kenmare.

"The cause of the slip is thus explained by an expert — 'The bog stretches over nearly a thousand acres. In wet

weather nearly the entire area is like a quagmire, and the water from it forms a stream which flows into the

River Flesk. The peasantry for years have been cutting turf at the verge of the bog, and the support, so to speak,

having been thus removed, the bulk moved away, and a further slip may be expected at any time."

An assessment of the Slide completed by the Natural Historian Robert Lloyd Praeger in 1937 attributed the likely

cause as "unwise turf cutting, by producing a high face without preliminary draining has frequently been the

cause of these accidents. Such was so in the fatal Kerry case".

There are no further recorded incidents of peat failure in the 121 years since that event.

Feehan et al (2008) presented an assessment of the extent of the Knocknageeha slide in 1896.


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Figure 1 Knocknageeha Peat Slide 1896 (Feehan et al (2008))

There are two schools of thought on the influence of historical slides on further development. One school of

thought is that the existence of a slide in an area is an indicator of likelihood of slides in adjacent areas of similar

nature in similar conditions.

Another school of thought is that the slide has now changed drainage conditions in that area and the remaining

peat has shown by the fact that it did not fail under the conditions which triggered the slide that it is resistant to

failure under the conditions which triggered the original slide.

It is reasonable to conclude that timeline plays a large part in this consideration and that the former is more

relevant to recent slide events but the latter should be considered in historical slides where there has been

significant subsequent use of the area in terms of drainage and turf extraction.


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3 PEAT SLIDE RISK ASSESSMENT

There are many and varied approaches to assessing peatslide risk for projects such as Silver Birch and the MRG

approach is consistent with established best practice. One of the more conservative approaches in terms of

incorporating historical land use risk is the Peatslide Hazard Rating System (Nichol, 2006), which provides a

pseudo-quantitative method of assessing the influence of the following hazards, which are widely acknowledged

to contribute to an increased risk of peat slide.

1. Rainfall and climate

2. Presence of water on the slope

3. Peat/Sub-strata interface

4. Peat profile and thickness

5. Shear strength of peat

6. Surface slope gradient and regularity

7. Geomorphology and Site History

8. The extent and condition of subterranean drainage pipes

9. Peatslide history

10. Potential impact of peatslides

The impact of each hazard factor is assessed against a cubic exponential scoring system, which reflects the

disproportionate increase in risk associated with adverse indicators for each category. Guidance on the

selection of scores for each category is provided in the technical paper entitled Peatslide Hazard Rating System

for Wind Farm Development Purposes (Nichol, 2006). A common scale of scores is adopted for each category,

as follows:

Low Risk – 3 points

Moderate Risk – 9 points

High Risk – 27 points

Very High Risk – 81 points

The rating system provides scope for the discretionary adjustment of scores in some instances. For any given

location, the overall risk rating is defined by the sum of the scores assigned to all hazard factors.

This approach is acknowledged as being systematic and compliant with industry best practice guidance, as

published by the Scottish Executive (2006).


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Table 1 Hazard Rating Criteria


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3.1 PEAT STABILITY HAZARD RANKING ASSESSMENT

The Hazard rankings for each of the headings in Table 1 Hazard Rating Criteria above are discussed below.

3.1.1 Rainfall and Climate

From the EIS the raifall data provided for the nearby town of Ballydesmond is given as 1252mm/Year

This represents a low to moderate hazard for this assessment.

3.1.2 Presence of water on slope

It can be assumed that prior to the 1896 event there was an extensive blanket bog/ raised bog at Knocknageeha

site. The remnant bog has been extensively cut with isolated pockets of turf banks with extensive man

made/natural drainage channels. Due to this it can be assumed that water will only occur occasionally on slopes.

3.1.3 Rockhead or subsoil

From the EIS the predominant subsoil is glacial till comprising shale and sandstone gravels, cobbles and boulders

in a silty clay matrix. The presence of gravels and boulders in the clay matrix breaks up the planar nature of the

peat/soil interface and is the rationale behind the selection of an intermediate risk score of 20 for this hazard.

3.1.4 Peat profile and depth

Extensive peat depth data was gathered for the assessment in the form of a GPR survey of the peat. This gives

detailed information on the surface and sub-surface profiles of the peat to a high degree of accuracy. The peat

depth ranking has been attributed to each area for this ranking. The peat depth data for the Southern section of

the site is presented below in Figure 2 Peat Depth Data (Apex GPR).


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Figure 2 Peat Depth Data (Apex GPR)


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3.1.5 Peat Strength

MRG completed a peat strength profiling exercise against peat depth data using conservative values for peat

strength. The hazard rating for this assessment has been based on that data for each of the locations.

3.1.6 Slope and Slope regularity

MRG provided slope analysis for each item of infrastructure assessed. The hazard rating for this assessment has

been based on that data for each of the locations.

3.1.7 Geomorphology and Site History

Natural erosion features such as hags, mounds, ridges, pools and incised streams, as well as disruption of the

ground surface by grazing, burning, forestry, drainage ditches, tracks, fence lines and man-made cuttings for

fuel, all affect the integrity of the near surface layers of peat and the tensile strength of the root-mat, in

particular. In addition, they may create localised over-steepening of slopes or unsupported blocks of peat.

The degree of hazard caused by erosion and degradation, and thus the score given in this category, should

reflect how quickly erosion and degradation are taking place, the size of the blocks or units being exposed, and

the amount of material being released.

In Knocknageeha there has been significant peat extraction in the intervening 121 years since the original slide

event. The wind farm infrastructure has been sited in areas of low peat depth where the peat has been

extracted. MRG did not find any indicators or evidence on the ground of the original peat slide head within the

development area. The nearest hags and mounds which are possibly associated with the run out of the original

slide are approx 250m west of T3.

3.1.8 Sub-profile drainage

As a blanket bog develops, over millennia, a network of peat pipes will also develop naturally, with new tributary

pipes forming as branches of the primary pipe. The principal pipes within a drainage network may grow to such

diameter that the peat forming the roof of the pipe is no longer able to bridge across the void, resulting in

collapse. If the debris resulting from roof collapse forms a blockage within a pipe network, groundwater

pressures upstream of the blockage may build to such levels that a new spring is formed, and porewater

pressures are redistributed within the peat mass, such that the continued development of the critical internal

drainage network takes on a new direction.

Within the downstream reaches of a bog drainage network, pipe collapses may join together, so that an open

drainage gulley is formed. Such gullies receive and convey both surface water runoff and shallow groundwater

flow, emerging from peat pipes. The network of pipes and gullies enable a blanket bog to remain stable under a

wide range of groundwater conditions. When a drainage network is interrupted, either due to a natural event,

such as pipe collapse or landslide, or due to construction works, an increase in the risk of peat instability will

result from the destabilising build-up of elevated porewater pressure within the peat mass.


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MRG did not note any peat pipes encountered in their peat assessment. This is most likely as a result of the

extensive harvesting of turf in the intervening years after the slide event. It must be noted that the peat pipes

are most often associated with intact blanket bog and they are rarely found in extensive cutaway bog such as

Silver Birch.

3.1.9 Peatslide History

Silver Birch Wind Farm is divided into 4 distinct geographical areas.

Ortho rectifying the image from Figure 1 Knocknageeha Peat Slide 1896 (Feehan et al (2008)) above and

superimposing the layout of Silver Birch Wind farm indicates that the southern section of the site interacts with

the historical slide. It is considered that the rest of the site is at such a remove from this event that no further

assessment is required beyond that already completed.

Turbine T3 and T4 and the borrow pit and peat repository are within the historical slide area with T1 and T2

adjacent.

For the purpose of this assessment the maximum hazard rating has been applied to all of the infrastructure of

Silver Birch South.


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3.1.10 Potential Peatslide Severity

The potential severity of a slide event at Knocknageeha is tempered by the fact that a significant slide has

already occurred. This significantly reduces the volume of material available to reach sensitive receptors in the

event of a failure. Consideration is also given to the broadscale removal of remnant turf in the intervening

period. The assessment for each part of the infrastructure reflects the remnant depth of peat and the distance

to a receptor.

3.2 CALCULATION OF OVERALL PEAT STABILITY HAZARD RANKING

The PHRS scores for the assessed locations range from 137 to 180 and average 166.

PHRS scores are intended as a means of comparing different sites and as a tool for prioritising mitigation works.

The PHRS system itself does not attach any particular significance to the total score for each site and leaves it to

the project engineers to draw their own conclusions, based on an understanding of the local conditions that

apply. However, as a rule of thumb, sites with an average rating of less than 200 are assigned a low priority,

while those with an average rating of more than 400 are identified for urgent attention. All of the PHRS scores

assessed for proposed wind turbine locations fall within the low priority range with regard to peatslide risk.


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3.3 PEAT STABILITY RISK ASSESSMENT

Risk Class Hazard Engineering Response Turbine Position

Risk Level 1

(0 to 70)

Negligible Do nothing. Acceptable.

Risk Level 2

(70 to 140)

Very Low Monitor and review. Manage by normal

slope maintenance procedures.

Borrow Pit, Peat

Repository

Risk Level 3

(140 to 200)

Low Further investigation of the peat slide

hazard may be required. Manage by

normal slope maintenance procedures.

T1, T2, T3, T4

Risk Level 4

(200 to 300)

Low-Moderate Peatslide stabilisation works may be

required.

Risk Level 5

(300 to 400)

Moderate Peatslide stabilisation works may be

required. Further studies required to

refine judgements.

Risk Level 6

(400 to 500)

High Peatslide stabilisation works likely to be

required. Further investigations will be

required, including a comprehensive

assessment of risks.

Risk Level 7

(>500)

Very High Large scale mitigation works will be

required. Urgent requirements for further

investigations, including a comprehensive

assessment of risks.

As can be seen from the PHRS table with the application of a conservative risk ranking for the historical slide the

risk ranking for the affected area of the site under this methodology remains Low.

As noted by MRG the extensive turf extraction and forestry plantation over the intervening years have removed

any potential indicators of the slide in the development footprint and it can be surmised that with the removal

of most of the peat, the significance of the risk of a peat slide has been largely mitigated through that process. If

the effect of the subsequent land use over the long timeline of 121 years were interpreted as having a mitigating

effect then the risk ranking for each location using this methodology would be Very Low.


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4 CONCLUSION

MRG conducted a peat stability assessment using a methodology consistent with industry best practice. In

completing the peat stability assessment they employed conservative parameters to model the stability of the

peat across the site using extensive peat data gathered using GPR. GPR surveying, in conjunction with

corroborative peat probing, provides the most complete dataset from which to conduct stability modelling on

peat.

MRG did not find any indicators associated with a peatslide history on the site. This is most likely as a result of

the broadscale removal of turf from the development footprint. This lends validity to their decision not to

attribute any ranking to historical failures since they were already using conservative, depth indexed shear

values for peat in carrying out stability analysis.

MRG concluded that T1 and T2 were Medium risk on their bespoke ranking output and T3 and T4 were Low risk

rankings

MWP have applied an alternate methodology, Peatslide Hazard Rating System (Nichol, 2006), which gives

prescribed weightings to historical land use and slide events.

The output of the MWP assessment while allocating the maximum hazard level to historical slides is a Low risk

level for all four turbine locations.


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Appendix 1

Overlay of Feehan et Al (2008) slide assessment

With Wind Farm Infrastructure

T3

T6

T5

T4

T2

T1

T3

T6

T5

T4

T2

T1

Borrow

Pit No.1

R

e

p

o

s

i

t

o

r

y

A

r

e

a

N

o

.

2

Borrow Pit No.1

T2 Coordinates:

E: 513629.562

N: 599149.404

JB1

JB4

L

-1

1

1

3

5

T

e

rtia

ry

L

o

c

a

l R

o

a

d

L

-

3

0

1

3

P

r

i

m

a

r

y

L

o

c

a

l

R

o

a

d

L

-

3

0

1

3

P

r

i

m

a

r

y

L

o

c

a

l

R

o

a

d

L

-

1

1

1

3

2

T

e

r

t

i

a

r

y

L

o

c

a

l

R

o

a

d

Widening to junction of

L-3013 and L-11135 required

New field gate

Borrow Pit No.2

Existing Watercourse Crossing No.5

New Watercourse

Crossing No.4

(Culvert)

New Site Entrance A

New Site Entrance C

New Site Entrance D

New Site Entrance E

New Site Entrance M

Existing track to

be upgraded

Public Road To

Be Widened

Existing track to

be upgraded

Existing track to

be upgraded

Existing track to

be upgraded

Existing track to

be upgraded

Borrow Pit No. 1

T1

T3

Repository

Area No. 1

be upgraded

T2 Coordinates:

E: 513629.562

N: 599149.404

New field gate

Existing bog road to be upgraded

Existing Drain

Widening to junction of

L-3013 and L-11135 required

MV cable circuit

from T5-T7



New Watercourse Crossing No.4 (Culvert)

New Site Entrance A

New Site Entrance D

New Site Entrance E

New Site Entrance M

Public Road To

Be Widened

Existing track to be upgraded

Existing track to

be upgraded

New Watercourse

Crossing No.4 (Culvert)

New Site Entrance B

New Site Entrance B

Temporary Construction

Compound No.1

L

-

2

0

3

2

P

r

i

m

a

r

y

L

o

c

a

l

R

o

a

d

L-3013 P

rim

ary Local R

oad

Widening to junction required

Widening to junction required

Documents

Silverbirch Wind Farmsilverbirchwindfarm.com/wp...6-C-Landslide-Susceptibility-Review.pdf · Figure 1 Knocknageeha Peat Slide 1896 (Feehan et al (2008)) There are two schools of thought