Contractors build on Capability’s legacy

15 June 2010



When modern safety requirements prompted spillway modifications at Blenheim Palace’s dam, civil engineers and reservoir inspectors had to find a solution to preserve this World Heritage Site and ensure structural integrity was not compromised. Kate Crawford reports.


It may be every designer’s dream to transcend a trend and create a timeless classic, but Lancelot Capability Brown is perhaps one of the few to achieve just that. When he was commissioned to redesign the landscape of England’s Blenheim Palace in Oxfordshire in the naturalistic style fashionable of the era, it’s unlikely he would have imagined that civil engineers and reservoir safety experts would be working together almost 250 years later to preserve it.

In the mid 1760s Brown began work on the ambitious project and within four years had flooded the Royal Park’s River Glyme Valley in Woodstock to create his largest ever lake, and what would later be described by Lord Randolph Churchill as, ‘The finest view in England’. Unfortunately, although he had considerable experience (being responsible for the design and construction of at least 40 earth dams), he left neither cross-sections nor specifications for his designs.

Specific information on the dam’s construction, vital for understanding its behaviour and designing required refurbishment works, had to be gleaned from researching the limited information that is available, and identifying the natural materials present in and around Blenheim Park that would have been used for its construction.

The meadows, then stretching along the valley floor from Woodstock Mill to the nearby village of Bladon, were on rich alluvial soils, whereas much of the park lies on rocks of the Great Oolite series. The only clay available was Oxford Clay, found on the high ground to the west of the dam.

Despite the limited supply of clay, Blenheim’s dam was constructed of a clay core mixed with limestone gravel, in keeping with Brown’s usual approach and the standard method used at this time.

Engineered as a single structure, the dam comprised a long earth fill embankment and an overflow cascade, with solid limestone bedrock foundations.

The modern design challenge

Some two centuries later, following the Reservoirs Act 1975, Brown’s large ornamental lake at Blenheim Palace became subject to regulatory inspections.

The dam, which is 200m long, has a 17m wide fixed crest spillway known as the cascade. It also has a low level outlet controlled by a 1.2x0.8m penstock which can regulate water levels.

Internal seepage and local settlement of Blenheim dam’s downstream face had taken place for a number of years and a variety of remedial works had been carried out to ensure the continued safety of the dam. However, settlement of the downstream face and formation of local erosion features had been occurring at an increasing rate. Leakage rates through one section of the dam, measured by v-notch, were worsening.

The supervising engineer, Richard Dawson of consultants Peter Brett Associates, was called to site in August 2007. He was brought in to inspect a local collapse of the crest which revealed steady leakage over the clay core. He called for an inspection under Section 10 of the Reservoirs Act and met on site with the appointed inspecting engineer, Martin Airey of Mott MacDonald, later the same week.

In addition to the leakage, previous studies had identified a deficit in spillway capacity. Blenheim dam is rated a category B reservoir, whereby a breach could endanger lives, although not in a community, and could result in extensive damage to property and infrastructure. The recommended design flood is a 10,000-year event.

Maintenance staff had also noted that the penstock on the low level outlet was becoming increasingly difficult to operate, and closing it completely was not achievable. There were also no facilities in place to allow inspection of the penstock or the low level outlet in the dry. As a result, these had not been inspected for some 25 years.

Consequently, the inspecting engineer’s report gave the following required measures in the interests of safety:

• Additional spillway capacity.

• Remedial works to the embankment and clay core to reduce leakage.

• Inspection and refurbishment of the low level outlet penstock and culvert.

During the remainder of 2007 and 2008, the team undertook ground investigation and analysis, further topographical survey and divers’ surveys, together with hydrological and hydraulic studies. A hydrodynamic model was constructed to assess options and also incorporated a smaller reservoir some 2km downstream. A feasibility study considered various options for coping with the 10,000-year flood, assessed at 83m3/sec.

Construction

The favoured solution was to reinforce the existing primary spillway (the cascade) sides, and create a new auxiliary 80m wide spillway across much of the dam crest in interlocking concrete blocks (Armorloc). This would be tied in with remediation and raising of the clay core.

Following detailed design and preparation of specifications later in 2008, JN Bentley Ltd, a North Yorkshire based civil engineering contractor, was awarded the construction and repair works contract in early 2009.

Work on Blenheim dam started in mid-May 2009 and was successfully completed in October with final planting works taking place in November. In detail, the construction works comprised:

• Remediation of the clay core by excavating a 0.6m wide trench to remove eroded and breached sections of the clay core and backfilling with cement bentonite slurry. Although extensive ground investigation had determined the extent of slurry wall required, due to the limited information available, work proceeded with caution. A geomembrane was also installed to protect the clay core. These measures aim to reduce the leakage flow through and over the core, consequently reducing the erosion of fine material that is causing localised settlement of the crest and downstream slope.

• Repairs and refurbishment to the existing low level outlet. Further underwater surveys were required to confirm exact dimensions of the draw off culvert. Due to uncertainty of the existing structural integrity of the culvert, and potential health and safety issues, the latest micron scanning sonar mounted on a remote operated submersible was used. Divers then inserted an HDPE pipe with a flange plate into the section of culvert upstream of the penstock and grouted it into position while underwater, allowing inspection and remedial work of the penstock and culvert in the dry. Some of the culvert was in very poor condition and repairs required considerable expertise and planning to ensure its future structural integrity. Future inspection will be much easier thanks to an old stop log facility brought back into commission, as well as the option to fit the flange plate on to the HDPE pipe.

• Strengthening the sides of the cascade with a reinforced concrete structure that supports, and is concealed behind, a series of limestone boulders. Limited drawdown of the reservoir was possible, so work needed to be planned and co-ordinated to allow continual use of the spillway. These works enhanced the existing cascade and allow for additional water levels and flow velocities associated with the provision of increased overflow capacity of the dam.

• Construction of an auxiliary 80m wide spillway on the dam crest also provides an increased spillway capacity to accommodate a 1 in 10,000-year flood. This involved the removal of all planting, stripping off the topsoil, and laying a bed of interlocking concrete blocks (Armorloc), before replacing the topsoil and reseeding. The auxiliary spillway will operate for events in excess of 25-50 year return periods. The removal and containment of isolated areas of Japanese Knotweed needed to be done in line with guidance from, and in close liaison with, the Environment Agency.

Remediation

Unfortunately, during restoration, three large plane trees and a number of deciduous and evergreen trees that were planted on the downstream face of the dam had to be felled. But once the works were complete the team worked with Andrew Nichols of landscape architect firm, Nichols Brown Webber, to restore the aesthetics of this historic landscape.

Almost 1200 tonnes of topsoil were brought in and the whole area re-seeded and planted with a selected grass and wild flower mix; the major focus being to re-establish the dense, mainly evergreen, woodland on either side of the cascade. Over 30,000 wild daffodils were also planted in the grassed areas.

Before the works, the only access to the bottom of the cascade was by a set of steep steps, but these were also replaced with new graded footpaths along the crest and up the downstream face. The new footpaths wind their way up to the top of the dam, with a new viewing point overlooking the cascade and down the River Glyme in a view made famous by photographer, Henry Taunt in 1900.

The construction contract was completed in November 2009 and celebrated with a ceremony led by His Grace the Duke of Marlborough, attracting media coverage.

Initial feedback and monitoring of the project has been very positive. Maintenance employees from Blenheim Park report that they are now able to operate the penstock easily and with more confidence.

As yet, the auxiliary spillway is untested, but the v-notch chamber measuring leakage through one section of the dam, which had been running copiously before the clay core remedial works, is now dry, indicating the remediation of the clay core has been successful.

The future

New legislation is currently going through the Parliamentary process which will mean reservoirs as small as 10,000m3 will have similar safety requirements to those currently in place for reservoirs of 25,000m3 or more.

Inevitably, this will mean that many more ornamental lakes will be subject to such inspection requirements in future. But for now, this particular Brown masterpiece and the finest view in England have been preserved for future generations.

Kate Crawford, Communications Officer at JN Bentley. Email: [email protected]

The author would like to thank the following for their help in producing this article:

Historical information kindly provided by: Jeri Bapasola – Archival Researcher at Blenheim Palace and author of ‘The Finest View in England - The Landscape & Gardens at Blenheim Palace’.

Technical information kindly provided by: Richard Dawson – Supervising Engineer, Peter Brett Associates; Martyn Higham – Associate, Peter Brett Associates; Rob Culledge – Contracts Manager, JN Bentley Ltd; Andrew Nichols – Landscape Architect, Nichols Brown Webber; Martin Airey – Inspecting Engineer, Mott MacDonald Limited.

Permission kindly granted by Roger File – Estates Director, Blenheim Palace.



Blenheim Dam

Catchment area: 127km2
Reservoir surface area: 47.34ha
Reservoir volume: 570,000m3
Primary spillway (Cascade): 17m wide at 76.7, AOD
(New) Auxiliary Armorloc spillway: 80m wide at 77.4m AOD
Top of dam: 78.3m AOD
Dam height (top of dam to downstream river bed): 8.1m
Total dam length: 200m
Crest width (typical): 8m
Downstream slope: 1:5
10,000 year peak inflow: 83m3/sec
1,000 year peak inflow: 48m3/sec



Crane Crane
Spillway Spillway
Geomembrane Geomembrane
Reservoir Reservoir
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