Pines Hotel Swanage – Cliff Stabilisation

Project Details

Location: Pines Hotel, Swanage, Dorset, UK
Value: £2.5M
Date of completion: May 2016
Duration: 11 Years; Final Construction Phase 30 Weeks
Client: Pines Hotel proprietor
Consultant and Contractor: CH2M; Knights Brown Construction
Project director: Professor Roger Moore

Project manager: Geoff Davis
Challenge summary: Cliff stabilisation project with very demanding physical, financial, organisational and human constraints
Challenge solution: An innovative facing structure which removed the cliff toe in order to provide space for later construction of beach houses as a means for the private client to recoup project costs.

Winner of 2017 Ground Engineering Awards in the category “UK Project with a Geotechnical Value of between £1M and £3M.

Finalist of the 2017 Fleming Awards

Introduction

The Pines Hotel is located in Swanage, Purbeck, a popular coastal town in southern England in which small and medium enterprise (SME) tourism businesses are a significant part of the local economy. The hotel is set back 10m from the edge of 30m-high cliffs which are prone to coastal erosion and landsliding; recession of the cliff top posed a serious threat to the hotel.

The Pines Hotel Cliff Stabilisation project was a complex technical problem. The solution was a counter-intuitive innovation to create a platform for beach buts cut into the toe of the slope. The conflicting objectives and needs of key stakeholders greatly exacerbated the technical and teamworking challenges.

Solving these challenges required particularly skilled teamworking, and also innovative solutions in the final design and during construction, under severe site constraints. Despite these challenges, the objective of the project was met, the hotel was saved, and the client now has a property of considerable value and development potential.

Main Body

The contracts’ supervisor for this project (Chris Evans), who had 35 years’ experience in coastal construction and landslide stabilisation, described this project as ‘The most challenging job I have ever done’. In order to highlight the nature of the many challenges involved, and how ‘joining forces’ was key to success, the project works are described from two perspectives: physical and technical constraints and organisational or human constraints.

Key physical and technical constraints at project start

Access to the site had to be from the beach for all but the smallest and lightest plant, and was possible for only a 2-3 hour window each day, and for only one week in two due to tides and weather constraints. Night working was not possible.

Japanese knotweed presented a potential showstopper in terms for the cost involved in removing large contaminated soil to a licensed tip.

Considerable temporary works were required to allow plant of reasonable size to access the cliff.

Active landsliding in winter 2012-2013 presented an immediate threat to the hotel and rendered detailed designs obsolete and required emergency stabilisation works.

Key organisational and human constraints

The client was a small independent business who commissioned the project in order to save their hotel business from collapsing into the sea, using private business reserves to finance the project. Consequently, the budget available was extremely tight.

Following a recommendation by Purbeck District Council, in 2005 the hotel owners approached Prof. Roger Moore, then of Halcrow Group Ltd (HGL), to carry out an assessment of cliff instability and recession risk to the property. By 2012 however, the situation was so perilous that the hotel was nearly lost due to a landslide. Following this, the client instructed HGL to design and implement stabilisation works in order to safeguard the future of the hotel and property. However, the owners of adjacent land were understandably wary about the impact of any works on their own property. Consequently, they did not give permission for the works to have any contact with their land. This necessitated design innovations and novel working methods during construction.

Gaining planning permission for cliff stabilisation works in a World Heritage site and area of landscape and environmental sensitivity was a long and involved process, complicated by the needs of the local authority to address separate issues about structural repair of a clock tower and shelters further along the beach, and the local authority’s financial limitations.

Due to the challenging nature of the site and works, contractors were reluctant to get involved.

Creating solutions through joining forces

Joining physical and technical forces in design

The design of the sidewall retaining structures presented a considerable technical challenge because of the limited size of piling plant that could access the site and the varied geology.

The cliffs are environmentally sensitive which required some geological exposures to be preserved.

A drone was used to scan the topography in an otherwise inaccessible site.

The originally specified natural-looking soil nail panel facing system did not meet the structural requirements so had to be replaced by a conventional mesh and sprayed concrete facing.

The counter-intuitive innovation of creating a platform for beach huts cut into the toe of the slope helped ensure the economic mobility of the solution.

Joining organisational and human forces in design and construction

Re-design and planning approval was given for a lower platform and cliff stabilisation scheme.

Continuous close collaboration between the client, local authority, stakeholders, designer, and contractor, and an excellent working relationship on site, allowed the efficient resolution of the challenges that arose during planning and construction, for example by making refinements to the design to resolve physical constraints.

Joining physical and technical forces in construction

Stabilising the cliffs proved to be a considerable challenge with many issues and potential ‘show stoppers’ arising throughout the course of the project.

A combination of standard geotechnical methods involving soil nailing, bored piling, and drainage, was used to solve the technical challenges through innovative design and construction. The full scheme comprises soil nails, a reinforce sprayed concrete soil nailed wall, bored pile walls, trench drains, and sub-horizontal drilled drains. A temporary rock buttress, using imported rock, provided to be a robust, flexible and effective temporary works solution, which also provided toe support to the cliff during construction.

As no engineering work could extend beyond the site boundaries, the design of the sidewall retaining structures, up to 6m high with no ties or props, and with limited size of plant which could access the site, presented a considerable technical challenge. An innovative technical solution was found comprising secant piles arranged in groups of three, to provide sufficient overturning and bending resistance for a retained height of up to 6m. This is close to the limit of what could be achieved technically with the size of piling rig that could access the site.

“Option C was the perfect choice as far as we are concerned” (Rob Hayman, Contracts Manager, Raymond Brown, Construction)

Conclusions

Despite the challenges, the objective of the project was met, the hotel was saved, and the client now has a property of considerable value and development potential rather than a site close to worthless.

The project was delivered under budget within a 30-week programme. Project value £2.5M including lead-in costs; geotechnical value £1.5M.

The project was recognised for its use of local knowledge and relationships with local constructors and local authorities to bring sustainable solutions that addressed initially conflicting needs.

“I’d like to thank everyone – a good team with everyone working well” (John Puddepha, client).