Canal & River Trust Fearns Wharf Neptune Street Leeds LS9 8PB T 0303 040 4040 E [email protected]www.canalrivertrust.org.uk Patron: H.R.H. The Prince of Wales. Canal & River Trust, a charitable company limited by guarantee registered in England and Wales with company number 7807276 and registered charity number 1146792, registered office address First Floor North, Station House, 500 Elder Gate, Milton Keynes MK9 1BB MEMO To Charlie Baker/Ian Lane Date 27 march 2013 Copy to John Ward Our ref GU Rowington & Shrewley Cutting memo – March 2013 From Andy Cawthorne Your ref Tel 0113 2005718 Email [email protected]GRAND UNION CANAL CUTTING FAILURES – 9 MARCH 2013 SITE REVIEWS During the winter of 2012/2013 following a prolonged wet winter slope failures occurred at Shrewley Cutting 24 GU-030-010-L in December 2012 as a result of overtopping of the adjacent fishing ponds on the crest. Slips also occurred to the nearby Rowington Cutting 22 GU-027-001-L in February 2013. The local Waterway Engineer Charlie Baker requested these structures be reviewed by Senior Earth Structures Inspector Andy Cawthorne on the 9 March 2013. The findings of the site visits are discussed below. GU-030-010-L Cutting 24, Shrewley - Offside This is a Principal Cutting which at the time of writing this report was flagged for deletion from SAP. The previous PI in 2001 pre-dates SAP but recorded this structure to be a D grade cutting up to 10 m in height that was bounded by large fishing ponds and Tunnel Barn Farm. In the area of recent concern the cutting was bounded by a hardcore access track associated with the embanked fishing ponds beyond. The BGS dataset in ArcGIS shows the cutting to be underlain by the Arden Sandstone Formation (sandstone & siltstone) part of the Mercia Mudstone Group of the Triassic Period. No Drift deposits were shown to overlie these deposits but exposed soil was observed to be a brown sandy silty Clay. Slope Failure In December 2012, the adjacent fishing pond apparently overtopped after a period of intense heavy rainfall. This water flowed down onto the cutting slope at GU-030-P0325L and a large slip developed.
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Canal & River Trust Fearns Wharf Neptune Street Leeds LS9 8PB
T 0303 040 4040 E [email protected] www.canalrivertrust.org.uk Patron: H.R.H. The Prince of Wales. Canal & River Trust, a charitable company limited by guarantee registered in England and Wales
with company number 7807276 and registered charity number 1146792, registered office address First Floor North, Station House, 500
Elder Gate, Milton Keynes MK9 1BB
MEMO To Charlie Baker/Ian Lane Date 27 march 2013
Copy to John Ward Our ref GU Rowington & Shrewley Cutting memo – March 2013
GRAND UNION CANAL CUTTING FAILURES – 9 MARCH 2013 SITE REVIEWS During the winter of 2012/2013 following a prolonged wet winter slope failures occurred at Shrewley Cutting 24 GU-030-010-L in December 2012 as a result of overtopping of the adjacent fishing ponds on the crest. Slips also occurred to the nearby Rowington Cutting 22 GU-027-001-L in February 2013. The local Waterway Engineer Charlie Baker requested these structures be reviewed by Senior Earth Structures Inspector Andy Cawthorne on the 9 March 2013. The findings of the site visits are discussed below. GU-030-010-L Cutting 24, Shrewley - Offside This is a Principal Cutting which at the time of writing this report was flagged for deletion from SAP. The previous PI in 2001 pre-dates SAP but recorded this structure to be a D grade cutting up to 10 m in height that was bounded by large fishing ponds and Tunnel Barn Farm. In the area of recent concern the cutting was bounded by a hardcore access track associated with the embanked fishing ponds beyond. The BGS dataset in ArcGIS shows the cutting to be underlain by the Arden Sandstone Formation (sandstone & siltstone) part of the Mercia Mudstone Group of the Triassic Period. No Drift deposits were shown to overlie these deposits but exposed soil was observed to be a brown sandy silty Clay. Slope Failure
In December 2012, the adjacent fishing pond apparently overtopped after a period of intense heavy rainfall. This water flowed down onto the cutting slope at GU-030-P0325L and a large slip developed.
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The slip was observed to run from GU-030-P0300L to P0330L which had regressed to the shoulder forming a large scarp up to 2 m in height with slipped material at the toe. It is understood from the local Engineer that the slip extended into the navigation and this was cleared to open the canal shortly thereafter. The timber post and wire mesh fence along the shoulder did not appear to be affected by the slip but further shallow regressions are anticipated of the slip as the slope moves to a more stable angle of repose. It is of note that the area of the recent slip appeared to encompass three smaller slips which had been identified in the last PI. Further old shallow slips were identified to the immediate north and south of the recent slip. All appeared stable but the southern ones between GU-030-P0350L and P0370L were observed to be slightly undercut and further movement is anticipated in the long term. Review of Condition Grades Cutting 24, Shrewley The previous PI recorded this structure as a Grade D due to instability. This is still deemed appropriate. Consequence of Failure is deemed a 2 and the Serviceability is a 1. Recommendations Cutting 24, Shrewley Poor condition Boundary fencing GU-030-P0300L to P0350L. Estimated cost £1k to £5k. It is recommended that the boundary fencing along the shoulder be replaced with a suitable timber replacement set back from the overstep shoulder. This will allow residual movement of the slope failure without impacting on the new fencing. This fencing should be set back 2 m from the edge subject to discussions with the adjacent land owner. It is also felt that parking of vehicles along this section of crest should be prohibited. Un-vegetated slope GU-030-P0300L to P0350L. Estimated cost £1k to £5k. Consideration should be given to the planting a young trees on the section of the recent slope failure. Tree growth will help stabilise the slope in the long term and reduce water ingress into the slope. Re-instate Structure into SAP and call a PI Cutting 24. No additional costs This structure has been flagged for deletion and this requires re-instatement into SAP. A new PI has been scheduled to call for this inspection in 2013/14 to access the full length of this structure.
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GU-027-001-L Cutting 22, Rowington - Offside In February 2013 after heavy periods of snow and wet wintry conditions large mature trees and associated root system and spoil slipped into the canal at two locations. Emergency tree works were undertaken and the tree trunk and root system were recovered up the slope creating a large section of bare earth up the slope and a small restriction of slipped material at the toe. The BGS dataset in ArcGIS shows the cutting to be underlain by the Arden Sandstone Formation (sandstone & siltstone) part of the Mercia Mudstone Group of the Triassic Period. No Drift deposits were shown to overlie these deposits but exposed soil was observed to be a brown sandy silty Clay. Site observations The following observations were made during the walkover: During the walk through, it was noted that numerous trees have been identified for felling as
part of the general vegetation management plan for this section of canal. As part of the walk through additional trees were identified to the local Engineer for inclusion. These were typically leaning trees or trees above areas of former instability where there loading on the slope could cause instability.
GU-027-P0640L – A field drain discharges onto the shoulder. This has scored out a gully down the slope to the canal and this could lead to instability in the future.
GU-027-P0760L – Site of recent slip/tree fall. This has left a 15 m to 20 m slip of exposed soil where the failed tree has been brought to the crest. The bulge of slipped material above water extended 0.5 m into the canal.
GU-027-P0760L to P0830L – A badger sett noted in the previous two Principal Inspections was noted on the field and cutting side of a spoil mound forming the upper part of the cutting. Recent tree trunk recovery works were within the vicinity of this sett.
GU-027-P0815L – A fallen mature tree stump associated root bowl and shallow spoil had slipped into the canal at this point restricting the channel by 1 m. Tree recovery works had dragged the material back to the crest in this location.
GU-027-P0950L – A shallow slip had developed on the lower half of the slope with two associated trees noted to have toppled.
GU-027-P0990L to GU-028-P0040L – A large historic slip area was noted to extend to the mid point of the cutting with the navigation reduced in width due to slipped material forming a bulge into the channel. This does not appear to be active but it is of note that the adjacent landowner had partially buried a drain which discharged into the base of this slip area. Leakage of this drain may be saturating the toe in this failed area which could cause further movement in the long term.
Review of Condition Grades Cutting 22, Rowington
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The previous PI recorded this structure as a Grade D due to instability. This is still deemed appropriate. Consequence of Failure is deemed a 1 and the Serviceability is a 1. However it is felt that the Serviceability should be a 2 due to the restrictions in the channel due to existing slips and associated material extending into the canal. Recommendations Cutting 22, Rowington Drainage discharges onto slope face GU-027-P0640L & GU-028-P0000L. Estimated cost £1k to £5k. Drainage water from adjacent land drains discharges onto the shoulder at GU-027-P0640L and has scoured a channel to the toe. At GU-028-P0000L a plastic land drain partially buried is noted to discharge into a large historic slip just above the toe. both issues are likely saturating the slope and these could lead to further instability. It is recommended that the Waterway Engineer discusses this discharges with the relevant landowners and measures are undertaken to ensure any agreed discharge occurs direct into the canal not the slope. This may require the formation of a lined channel or the repair/continuation of the drainage pipes to the cutting toe. Tree Works site review GU-027-001-L Cutting 22. No additional costs. Following the completion of the tree management works due to be undertaken February/March 2013, it is recommended that the local Engineer reviews the works to ensure all trees of concern were included in the works. These works should reduce the loading to the steep slope and reduce the amount of material/impact to navigation that further slips will create. Badger Sett on crest at GU-027-P0800L. No additional cost. It is recommended that the Ecology team records the location of this sett and periodically reviews the sett activity. This will help in future exclusion works to nearby Embankments should the need arise. If you have any queries regarding this communication please feel free to contact me. Regards Andy Cawthorne Senior Earth Structures Inspector ZX’s Shrewley – 12427494, 12427496 Rowington – 12427498, 12427508
