BRITISH WATERWAYS TECHNICAL DIRECTORATECUTTING PRINCIPAL INSPECTION

NEW PRINCIPAL INSPECTION - SUMMARY DETAILSPrincipal Inspection PMO: 3069560

Principal Inspection Z2: 11115245

Maintenance Plant: 9600

Functional Location: GU-027-001-L

Structure Name: Cutting 22, Rowington

Object Type: 006

Grid Reference (centre): SP200692

GIS Object: Yes

BW Owned: Yes

Include in AIP: Yes

PI Date: 19/10/2011

PI Inspector: G Holland

Weather: Overcast, dry

Primary Asset - New Condition Grade: D

Primary Asset - New CoF Grade: 1

Primary Asset - New Serviceability Grade: 1

Next Principal Inspection End Date: 19/10/2031

Reason: As AIP cycle

PRIMARY ASSET CHARACTERISTIC DATADesign of Earth Structure: True Cutting

Main Construction Material: Rock

Full Length of Cutting (m): 700

Maximum Height (m): 14

Maximum Slope Angle (o): 90

Minimum Navigation Width (m): 12

PREVIOUS PRINCIPAL INSPECTION – SUMMARY DETAILSFunctional Location: GU-027-001-L

Principal Inspection Date: 11/04/2001

Condition Grade: D

CoF Grade: 1

Serviceability Grade: 1

Existing Outstanding Z4s: 10238023 – install timber bank protection

PRINCIPAL INSPECTION REPORT

1.0 General Note:Refer to the site plans for full details of this inspection. This report comprises text, associated photographs, annotated site plans and key. Photographs taken during this inspection are included in the Word document stored against the functional location in MOSS.

All chainages in this report have been referenced to canal chainage as displayed on ArcGIS and rounded to the nearest 5 m.

In writing this report reference was made to the following document – 2001 Principal Inspection Report ref J:/Geotech/ESPI/9043/GM_02576

2.0 Description (General Morphology, Adjacent Land, Crest, Face, Toe, Bank Protection, Drainage, Other Integral Structures):Cutting 22 is on the N side of the canal between GU-027-P0500 and immediately E of Rowington Hill Bridge at GU-028-P0200, and is 700m in length. The cutting was constructed to take the canal through a hill formed of the more resistant Arden Sandstone

The cutting slope rises slowly to 6m high and then more rapidly to 14m high at the crest of the hill before falling again to around 8m at the bridge and to zero just beyond. In the highest section the crest is formed of clearly defined spoil mounds up to 4m in height above the original ground level. The cutting face varies, from planar to stepped in a series of cut benches, and in a number of locations a buff to pink grey cross bedded fine grained sandstone (Arden Sandstone) is exposed. This forms vertical faces up to 3m high in places. The sandstone faces are stable. The remainder of the slope is at 250 to 560 and

Below the sandstone in 5 locations along the cutting there are remnants of old shallow slips into the canal. The two Easternmost have toe bulges into the canal of up to 1m although these do not appear to have moved since the last PI in 2001. It is known that in wet weather water issues onto and adjacent to these slips

The face and crest of the cutting is moderately to heavily vegetated with mature trees, shrubs and moderate to dense bramble cover making much of the mid and lower slope difficult to access except at a few locations.

There is evidence of old badger activity in both the inside and outside faces of the spoil mound at GU-027-P0800 but no evidence lower on the slope face. Another isolated badger sett exists at GU-028-P0025. All setts were full of leaf debris and appear disused.

To the North of the cutting is undulating arable land with two field drains draining towards the canal. These were dry on inspection but are known to flow in wet weather. The Eastern drain is carried in a culvert through the cutting crest before exiting the face at 4m aNWL via a high masonry headwall and onto a concrete apron. This was heavily overgrown and almost obscured. There are two minor face slips at this location.

At GU-028-P0125 is a 5m long brick structure with a quadrant cross-section built into the slope some 5m aNWL. The previous inspection suggested it was a stop plank store but this is highly unlikely given its position. It may be an original gun-powder store, used to keep the powder dry and cool during use in excavating the sandstone of the cutting.

The offside bank is obscured by vegetation but where visible is unprotected for the full length (except at Rowington Hill Bridge where there is masonry walling). The lower face is oversteepened to 700 in places and there are some overhanging trees. The previous PI recommended local offside timber bank protection to protect against scour caused by wash action, and oversteepening of the lower bank. Evidence from this inspection is that this does not appear to have deteriorated significantly since 2001 and is unlikely to be required in the medium term

3.0 Geology:The cutting is excavated through the Triassic Mercia Mudstone Group. At this location the Mercia Mudstones contain a significant number of sandstone horizons (skerries) the most significant of which is the Arden Sandstone. Most of the middle and upper cutting is through the Arden Sandstone Member, which forms a distinctive marker horizon in the Mercia Mudstone Group of central England. This comprises a massive cross bedded sandstone, and contains important fossils. The base of the sandstone is not exposed but the canal approaches at each end are in red marls so it is likely that the lowermost 2m to 3m of cutting and canal are in true Mercia Mudstone. This would explain the local face slips in the lower face

Superficial deposits are absent although mounds of Pleistocene till are recorded both N and S of the cutting.

4.0 Defects of Concern:At GU-028-P0000 is a crest parallel open drain ditch that has recently been excavated and an attempt made at cutting through the upper crest to drain water from the field down the face of the cutting. The landowner should be made aware of the inadvisability of discharging surface water onto the face of the cutting, particularly as this is immediately above an old slip on the lower face.

The historic slips on the face should be visually monitored for evidence of remobilisation. Should this occur it is possible that the slips could cause local obstructions in the canal although it is unlikely that a total blockage would occur

5.0 Geotechnical Assessment / Stability:

The upper and middle cutting is formed in benched sandstone and is likely to remain stable in the long term. However the geology of the cutting is such that groundwater will penetrate through the upper sandstone rock mass and along minor joints before meeting the upper surface of the underlying impermeable marl. Water will then discharge through the cutting face. It is unlikely that this could be prevented without significant drainage works. It is therefore likely that in the medium to long term minor slips will affect the face

6.0 100m Length Sub-Asset Grades Summary Table

GU-027-P0500 Condition C – 4m high with local face issues

CoF 1 – Rough pasture Serv. 1.

GU-027-P0600 Condition D – 5m high with minor slips

CoF 1 – Arable Serv. 1.

GU-027-P0700 Condition C – 6m high with drain and minor slips

CoF 1 – Arable Serv. 1.

GU-027-P0800 Condition D – 12m high with spoil mound and badger setts

CoF 1 – Arable Serv. 1.

GU-027-P0900 Condition C – Vertical sandstone bench and stable slopes

CoF 1 – Arable Serv. 1.

GU-028-P0000 Condition D – Major historic face slip

CoF 1 – Arable Serv. 1.

GU-028-P0100 Condition D – 2 No. historic face slips

CoF 1 – Arable Serv. 1.

7.0 RecommendationsDiscuss with landowner recent excavation of crest ditch and possible discharge of drainage onto cutting slope.

Monitor lower slope for evidence of remobilisation or new slips. If detected geotechnical advice should be sought

PHOTOGRAPHSGU-028-P0150 – View W showing leaning trees on lower slope and offside slip encroaching into canal

GU-028-P0050 – View W showing major offside slip with 1m toe bulge into canal

GU-027-P0575 – View N showing minor offside face slip

GU-027-P0700 – View E showing oversteepened offside lower slope and occasional leaning trees

GU-027-P0475 – View E showing start of offside cutting

GU-028-P0100 – View E showing offside cutting crest descending towards Rowington Hill Bridge

GU-028-P0100 – View W showing offside cutting crest and flat arable field to N

GU-028-P0025 – View NE showing disused badger sett in crest spoil mound

GU-027-P0725 – View E showing widening of cutting crest and start of crest spoil mounds

GU-027-P0900 – View E showing rounded crest to crest spoil mound

GU-028-P0000 – View W showing recently excavated crest ditch with exposed plastic field drains

GU-028-P0125 – View W showing quadrant shaped brick structure built into middle slope

GU-028-P0150 – View N showing 2m high vertical face of Arden Sandstone at crest

GU-028-0175 – View W showing leaning trees on offside middle slope