Background The Crag End Landslip project comprises the reconstruction of a 300m section of road near

Rothbury, for Northumberland County Council (NCC). The B6344 was closed in December

2012 following multiple landslips, causing dramatic socio-economic impacts and a major

inconvenience to residents.

This project follows a history of slope stability problems spanning several decades, with the

most recent failure triggered by one of the wettest periods on record. The 2012 landslips

caused part of an existing 90m length anchored sheet pile retaining wall to fail, initiating

significant cracking along the carriageway and distortion of the wall capping beam. In addition,

other sections of the road collapsed and slipped down slope over the 300m stretch.

NCC’s key requirements were to:

Open the road at the earliest opportunity: the road closure has had a dramatic social and economic effect on Rothbury.

Provide a robust solution within the available funding and requiring minimal maintenance: NCC requires a high quality structure.

Enhance the SSSI: the River Coquet is at the toe of the slope and the majority of the site is within a SSSI.

In order to achieve these aims and to solve such a complex geotechnical problem it was

essential that a multidiscipline team was mobilised that could work in together from start to

finish of the project.

NCC appointed VBA (a joint venture comprising VolkerStevin, Boskalis Westminster and

Atkins) was appointed as ECI contractor and subsequently for the construction phase of works.

The VBA team also included the design team of geologists, hydro geologists, geotechnical and

structural and highway engineers all essential for this project. Keller were appointed as the

specialist foundation contractors who were involved early in the ECI process to ensure

constructability. Other team members included ecologists, GI contractors and dewatering

contractors.

Geotechnical investigation and back analysis at ECI revealed a complex geological stratigraphy

and slope instability within shallow superficial deposits and that artesian groundwater in the

underlying Fell Sandstone was a key driving mechanism. Following an extensive value

engineering process involving the full project teams, the proposed solution was a combination

of a bespoke passive dewatering system which used deep wells to lower artesian

pressures and a new anchored bored pile retaining wall, a section of which has been

constructed behind the existing wall, to support the road in the event of potential slips

downslope of the road.

Technical engineering excellence and ingenuity

On site testing and extensive modelling to produce an efficient and reliable

design: During ECI, to gain an understanding of the local geology and hydrology,

downhole CCTV and geophysical testing as well as substantial field work was

planned by the geotechnical team and undertaken to produce 3D models which

were the basis of our design. Groundwater was identified as the major cause of

instability and extensive hydrogeological modelling was undertaken to understand

the groundwater regime and to predict its response to the proposed dewatering

scheme. Results from pumping trials were used for calibration. Sensitivity models

and studies were performed to assess the impact of varying permeability and

rainfall intensities.

Developing an innovative passive dewatering system to reduce groundwater

pressure: The initial slope drainage proposal was for an extensive counterfort system

which would have significant ecological, cost and programme implications. Following

extensive modelling and VE between the design teams de-watering was achieved

using a system of deep wells supplemented with minor modifications to existing

surface water features. The deep well solution required minimal import and export of

materials compared to counterfort drains and minimised impacts on the protected

SSSI environment. The wells work under gravity without a requirement for pumps,

minimising on-going maintenance - an important issue for NCC.

Innovative Ground Investigation (left) – managing the artesian water pressure and avoiding unplanned discharge into the river required forward planning using 4m of elevated borehole casing.

Passive dewatering system – two rows of dewatering well installed on the slope.

Crag End Land Slip Award Submission

British Geotechnical Association Fleming Award 2016

Buildability assessments: Due to restricted access constructability had a huge influence on

the wall design and due to unfavorable conditions; i.e. potential loss of ground on the passive

side, and the high active pressures associated with unstable sloping ground, the anchored

retaining wall required to withstand high bending moments.

Usually, large diameter piles would be needed to accommodate the steel required to resist

the high bending moments, however using the heavy plant could have potentially triggered

further landslides. Following meetings with the VBA construction and design team and Keller,

it was proposed to adopt small diameter (450mm) piles, reinforced with recycled high strength

circular hollow section tubes (API tubes) to provide the required moment resistance and

facilitate the use of 10 tonne piling rigs and a specialist overburden drilling system.

Klemm 709-2 (case and auger)

The requirement to construct part of the new wall close to the rear of the existing failed wall

added further complexity. To avoid existing ground anchors, the new piles could not be

constructed in the usual contiguous arrangement. Since the exact alignment of the anchor

tendons below ground was unknown, the anchors were exposed close to the capping beam

to get an indication of their alignment and thus determine a safe zone for piling. Generally a

series of triangular groups fitted into each safe zone was used but was modified to suit the

actual conditions. Owing to this pile configuration, additional retaining wall analysis

techniques were adopted and the wall also assessed using first-principles engineering analysis,

considering the provision of moment restraint at pile head to determine the worst case

combination of the newly induced axial loads and bending stresses.

Savings through value engineering of piling and design solutions: The construction and

design team worked closely together day to day on site in order to provide rapid value

engineering solutions to the ever changing site and ground conditions. For example the

retaining wall design required all piles to have a 3m socket into sandstone bedrock. Since the

exact rock-head level was not known and varied significantly, this was verified on site by the

designer and piling contractor during pile installation and fed back into the analysis. This was

essential owing to the risk of encountering large boulders within the superficial deposits and

the potentially serious consequences of founding a section of wall within a boulder rather than

in bedrock. In some areas this led to both piles and anchor free lengths being shortened, saving

NCC money and accelerating the construction programme while maintaining a robust design.

Sustainability and environmental merits

The presence of the SSSI over much of the site required careful management of

environmental and ecological issues: VBA worked closely with Natural England,

National Trust, Environment Agency and NCC’s ecological consultant (EcoNorth)

throughout design and construction to ensure minimal impact on the SSSI. High

environmental standards were maintained throughout the works for example by using

a Siltbuster® water treatment unit and Terra Firma Dura- base © matting to provide

safe access across the lower slopes. In total, 11 bird boxes, an owl box, an otter holt

and two bat boxes have been installed to encourage wildlife to the area.

Incorporating minimum maintenance and encouraging natural regeneration: The

passive dewatering system was designed to require no long term energy usage once

operational and to discharge to the natural wetlands by the river. Care was taken to

ensure the lower slope can be returned to its original use as wood pasture for example

by providing cattle crossing locations across drainage channels and reinstating

grasslands through natural regeneration and using a wild, local provenance seed mix,

as well as Oak tree planting to replace/enhance the sites veteran trees that have died

or are in very poor condition due to waterlogging.

Approach to safety

No reportable accidents with over 88,000 hours worked: Safety challenges

overcome included difficult topographic and ground conditions, restricted working

areas and complex site logistics. Well organised plant and people zones were created

during ECI, by designing an innovative and bespoke temporary scaffolding walkway

along the 350m length of the site to allow safe access for our operatives. This solution

eliminated the risk of plant/people interface and enabled the safe operation of up to

seven piling rigs working simultaneously along the narrow road. Piling operations

were safely completed one month ahead of schedule. Weekly safety meetings and

toolbox talks took place for all site staff, outlining safe use of plant and equipment.

Environmental tool box talks for all site staff were given by ecologists provide by

EcoNorth, to educate the team on the sensitive nature of the site and to promote

environmental awareness. Hazard spotting cards to promote a safety culture were

available and actively encouraged to be completed to improve safety and encourage

continuous improvement.

Adherence to programme and budget The project was completed within budget and prior to the agreed contract completion

date of April 2016: This is a significant accomplishment considering the technical

challenges, environmental constraints, pile layout design changes and increases in

work scope. It is a result of collaboration between team members, strong project

management and a resolute commitment to get the road re-open as early as possible.

Innovative and bespoke temporary scaffolding walkway along the 350m length of the site to allow safe access for our operatives.

Installing rock anchors (20-30m length) –fixed length socketed into Fell Sandstone

To achieve the programme extensive resources were deployed: This demanded careful time

and spatial management to plan around the relatively small working area and congested site.

The installation of 436 piles was an integral part of the stabilisation process and was

completed ahead of schedule, highlighting our team’s strength in managing the challenging

conditions faced.

Community benefit

Proactive engagement with the community: The B6344 is a key route to Rothbury and the

project has been of high public interest. Despite the inconvenience of the road closure, an

excellent relationship has maintained with the local community through attending

consultation events and working collaboratively with NCC to keep the project website

updated. It was important to keep the public informed of progression and the site team has

been upfront and honest about any issues.

VBA engaged with the local community prior to work commencing and also gave an

exhibition following completion of construction works.

Generating socio-economic benefits by engaging local people: Fifteen local workers were

employed and trained on site including four within our VBA site management team, general

operatives, plant operatives and joiners. Local companies were also utilised to undertake tree

surgery, supply aggregates, remove spoil and for specialist plant.

Sharing lessons and best practice with the civil engineering and local community:

VBA’s involvement with The Crag End Landslip has been shared amongst the

engineering community through a presentation at Leeds University, at the annual

Ground Engineering Slopes Conference in London, and evening lectures presented to

the Northern Geotechnical Group of the ICE and Geological Society North West

regional group.

Collaborative Working

Excellent team relationship: The key to the success of the project was fundamentally

down to the close working relationship between VBA construction and design team,

Keller, Dewatering Services Ltd and Northumberland County Council. During the

initial ECI phase of work VBA discussions took place with all the contractors as the

design was developed. By having this early liaison it meant that decisions could be

taken as an early stage and the client was able to have a better understanding of

anticipated costs.

Construction collaboration: VBA provided site support during construction and by

taking a proactive approach to producing detailed ground condition long-sections,

substantial cost savings to the client was achieved through revisions to pile lengths

and or rock anchor lengths. Weekly and sometimes daily progress meetings with

Keller took place to review production. As a result of challenging ground conditions

Keller needed to utilise additional rigs and drilling techniques in order to meet the

required construction programme. This required liaison with the design team and fast

track temporary and permanent works design checks.

Throughout the ECI design phase and construction phase VBA developed a good

relationship with AECOM who were appointed as Northumberland County Councils

Geotechnical advisors. Atkins geotechnical and hydrogeological project team

organised regular meetings with AECOM as the design progressed to present the

design. This worked well as AECOM was able to feed back to the client which gave the

client further confidence in the technically challenging design.

Rothbury Connected again!

Road reopened April 2016.

Regular media updates were given during the duration of the project.

Rothbury Exhibition – VBA and NCC met with residents of the local community and received excellent feedback.

Ground conditions cross section updated throughout piling – Value engineering

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Concrete R

TW

Concrete RTW

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SOP3

SOP4

T

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4

A85 A86A87 A88 A89 A90 A91

A92 A93 A94 A95 A96 A97 A98 A99 A100 A101 A102 A103 A104 A105 A106 A107 A108 A109 A110 A111 A112 A113 A114 A115 A116A117 A118 A119 A120

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A139 A140A141 A142 A143 A144

T

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0.75m

13.382m BEAM 1714.474m BEAM 18

12.831m BEAM 19 10.809m BEAM 2010.756m BEAM 21

10.688m BEAM 22 11.693m BEAM 23

13m BEAM 16

2m

2m

0.82m

12.96m BEAM 15

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A138

0.14

0.150.20

0.200.20

0.320.23

CONCRETE CAPPING BEAM

(ASSUME 110Kg/m²)

3000

(M

IN

IM

UM

R

OC

K S

OC

KE

T)

CONCRETE

TOP OF

FINISHED R.L

ASSUMED ROCK

HEAD PROFILE

APPROXIMATELY ≈ 3000 TO 4000 DEPTH TO EXISTING ANCHOR

VARIES, MINIMUM 1500

EXISTING

ANCHOR

4

2

.

5

°

EXISTING

SHEET

PILE WALL

EXISTING

CAPPING

BEAM

EGL

508mm DIA. BORED

PILES @ 600mm C/C

MINIMUM ANCHOR FREE LENGTH MUST

BE TO ROCK HEAD. THE ANTICIPATED

ANCHOR FREE LENGTH TO ROCK HEAD

AT EACH ANCHOR LOCATION WILL BE

AGREED WITH THE CONTRACTOR &

VBA AND VERIFIED DURING ANCHOR

INSTALLATION.

2000

VARIES, MINIMUM 2000

100

STAGGERED ANCHOR

ANGLE 27.5° & 32.5°

SINGLE FINE

GRAVEL 10-20

150Ø SLOTTED PVC

PIPE (DEPTH TO BE

CONFIRMED)

300 Min.

75

LEVEL

GROUND

300

1435

3000

(M

IN

IM

UM

R

OC

K S

OC

KE

T)

1000

1700

75

200

260mmØ RESERVATION

TUBE

NOTE:

REFER TO DRAWINGS

5129596-DRG-801 TO 811

FOR R.C. DETAILS

CAPPING BEAM RECESS

WIDTH 321 (32.5°) OR

291 (27.5°)

HEIGHT OF RECESS

504 (32.5°) OR

558 (27.5°)

Date

DesignedScale

Drawing Title

Project Title

Drawing Status

DO NOT SCALE

Date Date Date

Drawn Checked Authorised

Millim

etre

s

10

01

00

Client

Copyright C

VBA Joint Venture Limited (2013)

Original Size

Suitability

Tel:

Fax:

Drawing Number Revision

+44 (0)1524 599400

+44 (0)1524 599401

Whitegate

White Lund Trading Estate

Morecambe

Lancashire

LA3 3BY

Rev. Date

Description By

Chk'd

App'd

A1

PROPOSED ROCK ANCHOR POSITION AT VARIED CENTRES

PROPOSED ROCK ANCHOR LOCATIONS AT 1800mm CENTRES

LOCATION OF DEEP DE WATERING WELLS

EXISTING CONCRETE RETAINING WALL CAPPING BEAM

ASSUMED LOCATION OF EXISTING ANCHORS TENSION PILE

WITH CONSTRUCTION TOLERANCES APPLIED

} SURVEYED ANCHOR TENSION PILE

PILE

}

KEY

TEST PILE LOCATION

INSTALLING GROUND ANCHORS (OCT. 2015) WITH SCAFFOLD WALKWAY

PROVIDING SAFE ACCESS

CAPPING BEAM AND GROUND ANCHORS FOR

TRIANGULAR PILE ARRANGEMENT

SECTION A-A

TRIANGULAR PILE ARRANGEMENT TO REDUCE CONFLICT WITH EXISTING ANCHORS

LIVE RISK MITIGATION TOOL ADOPTED ON SITE TO ASSESS GROUND CONDITIONS ENCOUNTERED DURING PILING AND VALIDATE DESIGN - RESULTING IN SAVINGS IN PILE LENGTH IN SOME AREAS AND INCREASED PILE

LENGTH WHERE REQUIRED RESPONSE TO GROUND CONDITIONS

EXISTING GROUND ANCHORS EXPOSED AND

ALIGNMENT PROJECTED BACK TO PILE LOCATIONS

TO IDENTIFY SAFE ZONES FOR PILING

PILE ALIGNMENT MOVED FORWARD TO

MAXIMISE SPACE FOR PILING. CAPPING

BEAM WIDENED TO ACCOMMODATE

TRIANGULAR PILE ARRANGEMENT

PREPARED FOR SUBMISSION TO ICE

ROBERT STEPHENSON AWARD 2016

EXISTING ANCHORED SHEET

PILE WALL FAILED IN DEC. 2012

A

A

STANDARD CONTIGUOUS

PILE ARRANGEMENT

PILES RECONFIGURED TO TRIANGULAR ARRANGEMENT TO AVOID CONFLICT WITH EXISTING

ANCHORS - REQUIRING ADVANCED FIRST PRINCIPLES ANALYSIS TO VERIFY DESIGN

STANDARD CONTIGUOUS

PILE ARRANGEMENT

PILE INSTALLATION - MULTIPLE RIGS ON A CONSTRAINED SITE REQUIRED CAREFUL PLANNING

TO ENSURE SAFE WORKING

CONSTRUCTION OF CAPPING BEAM